- Enzymatic Activation Of Alkanes Constraints And Prospects Uk 2017
- Enzymatic Activation Of Alkanes Constraints And Prospects Ukulele Chords
- Enzymatic Activation Of Alkanes Constraints And Prospects Ukulele
- Constraints And Prospects Of Yam Production In Nigeria
Abstract
Controlling male fertility is an important goal for plant reproduction and selective breeding. Hybrid vigour results in superior growth rates and increased yields of hybrids compared with inbred lines; however, hybrid generation is costly and time consuming. A better understanding of anther development and pollen release will provide effective mechanisms for the control of male fertility and for hybrid generation. Male sterility is associated not only with the lack of viable pollen, but also with the failure of pollen release. In such instances a failure of anther dehiscence has the advantage that viable pollen is produced, which can be used for subsequent rescue of fertility. Anther dehiscence is a multistage process involving localized cellular differentiation and degeneration, combined with changes to the structure and water status of the anther to facilitate complete opening and pollen release. After microspore release the anther endothecium undergoes expansion and deposition of ligno-cellulosic secondary thickening. The septum separating the two locules is then enzymatically lysed and undergoes a programmed cell death-like breakdown. The stomium subsequently splits as a consequence of the stresses associated with pollen swelling and anther dehydration. The physical constraints imposed by the thickening in the endothecium limit expansion, placing additional stress on the anther, so as it dehydrates it opens and the pollen is released. Jasmonic acid has been shown to be a critical signal for dehiscence, although other hormones, particularly auxin, are also involved. The key regulators and physical constraints of anther dehiscence are discussed.
Psalm one the death of frequent flyer rar. The Interaction between Plants and Bacteria in the Remediation of Petroleum Hydrocarbons: An Environmental Perspective. An AlkS-like activator seems to be involved in the activation of alkB1 in response to the presence of alkanes. Prospects and applications for. Enzymatic breakdown of the septum. At the cellular level, anther dehiscence is similar to silique dehiscence and, like microspore separation, is thought to involve cell wall-degrading enzymes which break down the pectin between cells (Roberts et al., 2002).
Anther development, Arabidopsis, dehiscence, endothecium, male sterility, pollen
Introduction
The development and release of functional pollen is critical for plant reproduction and selective breeding. Hybrid vigour, heterosis, is often determined by non-mutually exclusive mechanisms that result in the superiority of a hybrid over its parents, with respect to rapid growth rate and increased yields (Lippman and Zamir, 2007). However the emasculation process required for the generation of hybrids can be extremely time consuming and labour intensive, thus male-sterile lines are often favoured in commercial hybrid seed production (Wilson and Zhang, 2009). There is therefore a need for a better understanding of the processes involved in anther development that may provide effective mechanisms for the efficient generation of hybrids and for the general control of fertility. Providing a mechanism for rescuing fertility is also an essential requirement for hybrid production. Therefore, approaches for controlling male fertility that rely upon pollen release, rather than pollen generation, may provide good opportunities for future hybrid development.
The final stages of anther development and pollen release involve a switch from cellular differentiation to degeneration. Specialized cell types within the anther determine the site of anther opening, and synchronized development of the pollen with the anther wall tissues coordinates the timing of dehiscence. Previously there has been much focus on the development of functional pollen (Scott et al., 2004; Ma, 2005; Feng and Dickinson, 2007); however, there has been less discussion about the relatively uncharacterized events associated with anther dehiscence and pollen release. This review will therefore concentrate on these later events.
Anther and pollen development
Understanding of the basic process of anther and pollen development has increased over recent years, with much information coming from detailed analyses of male-sterile mutants in Arabidopsis (Sanders et al., 1999; Scott et al., 2004; Ma, 2005; Wilson and Zhang, 2009; Feng and Dickinson, 2010).
Pollen is formed within specialized organs, the stamens, in the flower. These comprise a wider upper region that forms the anther that contains the pollen, and a stalked region, the filament, containing the vasculature, which extends to ensure that pollen is released away from the flower (Smyth et al., 1990; Goldberg et al., 1993). The major events in Arabidopsis anther development have been placed into 14 stages by Sanders et al. (1999). The initiation of floral organ primordia from pleuripotent cells within the floral meristem involves AGAMOUS (AG), alongside other homeotic genes, to form organ initials according to the ABCDE model (Robles and Pelaz, 2005). Many of the key genes regulating the subsequent development of the different cell layers in anther and pollen formation have been identified (Fig. 1).
Anther and pollen development pathway. WUSCHEL (WUS) activates AGAMOUS (AG) at the centre of the floral apex, the number of pluripotent cells increases, then AG represses WUS to promote differentiation. Stamen initiation is controlled by the homeotic genes APETALA3 (AP3), PISTILLATA (PI), and AG, with the primordia forming as a tetrad of archesporial cells. AG induces microsporogenesis via activation of NOZZLE/SPOROCYTELESS (NZZ/SPL) (Ito et al., 2004). The transcription factors JAGGED (JAG) and NUBBIN (NAB) are also involved in the process of defining stamen structure (Dinneny et al., 2006). Two CLAVATA 1-related leucine-rich repeat receptor-like protein kinases, BARELY ANY MERISTEM1 (BAM1) and BAM2, act in a regulatory loop with NLZ/SPL to promote somatic cell types and to restrict NZL/SPL expression to the inner region of the locule (Hord et al., 2006; Feng and Dickinson, 2010). Archesporial cell number and tapetal cell fate is controlled by a leucine-rich repeat receptor kinase EXTRA SPOROGENOUS CELLS/EXCESS MICROSPOROCYTES1 (EXS/EMS1) (Canales et al., 2002; Zhao et al., 2002) and its ligand TAPETAL DETERMINANT1 (TPD1) (Jia et al., 2008). The SERK1 and SERK2 complex is thought to form a receptor complex with EMS1 in the tapetal plasma membrane (Albrecht et al., 2005; Colcombet et al., 2005), which then binds TPD1 (Yang et al., 2003, 2005) (indicated by the red dotted line). Tapetal development is initiated by DYSFUNCTIONAL TAPETUM1 (DYT1) (Zhang et al., 2006) and DEFECTIVE IN TAPETAL DEVELOPMENT AND FUNCTION1 (TDF1) (Zhu et al., 2008), with tapetal maturation, pollen wall formation, and tapetal programmed cell death (PCD) involving ABORTED MICROSPORES (AMS), MALE STERILITY1 (MS1) (Wilson et al., 2001), and ABCWBC27 (Sorensen et al., 2003; C. Yang et al., 2007; Xu et al., 2010) with the MALE GAMETOGENESIS IMPAIRED ANTHERS (MIA) gene, encoding a type V P-type ATPase, MALE STERILITY2 (MS2), and LEUCINE AMINOPEPTIDASE (LAP3). The final stage of dehiscence involves jasmonic acid (JA)-induced gene expression and transcription factors associated with endothecium secondary thickening, MYB26 (Steiner-Lange et al., 2003; Yang et al., 2007) and the NAC SECONDARY WALL THICKENING PROMOTING FACTOR1 (NST1) and NST2 double mutant (Mitsuda et al., 2007). Regulation is indicated by arrows; green arrows indicate proven direct regulation; inhibition is shown by a T-bar.
Anther and pollen development pathway. WUSCHEL (WUS) activates AGAMOUS (AG) at the centre of the floral apex, the number of pluripotent cells increases, then AG represses WUS to promote differentiation. Stamen initiation is controlled by the homeotic genes APETALA3 (AP3), PISTILLATA (PI), and AG, with the primordia forming as a tetrad of archesporial cells. AG induces microsporogenesis via activation of NOZZLE/SPOROCYTELESS (NZZ/SPL) (Ito et al., 2004). The transcription factors JAGGED (JAG) and NUBBIN (NAB) are also involved in the process of defining stamen structure (Dinneny et al., 2006). Two CLAVATA 1-related leucine-rich repeat receptor-like protein kinases, BARELY ANY MERISTEM1 (BAM1) and BAM2, act in a regulatory loop with NLZ/SPL to promote somatic cell types and to restrict NZL/SPL expression to the inner region of the locule (Hord et al., 2006; Feng and Dickinson, 2010). Archesporial cell number and tapetal cell fate is controlled by a leucine-rich repeat receptor kinase EXTRA SPOROGENOUS CELLS/EXCESS MICROSPOROCYTES1 (EXS/EMS1) (Canales et al., 2002; Zhao et al., 2002) and its ligand TAPETAL DETERMINANT1 (TPD1) (Jia et al., 2008). The SERK1 and SERK2 complex is thought to form a receptor complex with EMS1 in the tapetal plasma membrane (Albrecht et al., 2005; Colcombet et al., 2005), which then binds TPD1 (Yang et al., 2003, 2005) (indicated by the red dotted line). Tapetal development is initiated by DYSFUNCTIONAL TAPETUM1 (DYT1) (Zhang et al., 2006) and DEFECTIVE IN TAPETAL DEVELOPMENT AND FUNCTION1 (TDF1) (Zhu et al., 2008), with tapetal maturation, pollen wall formation, and tapetal programmed cell death (PCD) involving ABORTED MICROSPORES (AMS), MALE STERILITY1 (MS1) (Wilson et al., 2001), and ABCWBC27 (Sorensen et al., 2003; C. Yang et al., 2007; Xu et al., 2010) with the MALE GAMETOGENESIS IMPAIRED ANTHERS (MIA) gene, encoding a type V P-type ATPase, MALE STERILITY2 (MS2), and LEUCINE AMINOPEPTIDASE (LAP3). The final stage of dehiscence involves jasmonic acid (JA)-induced gene expression and transcription factors associated with endothecium secondary thickening, MYB26 (Steiner-Lange et al., 2003; Yang et al., 2007) and the NAC SECONDARY WALL THICKENING PROMOTING FACTOR1 (NST1) and NST2 double mutant (Mitsuda et al., 2007). Regulation is indicated by arrows; green arrows indicate proven direct regulation; inhibition is shown by a T-bar.
Initially divisions in the L1, L2, and L3 layers of the floral meristem bring about the formation of the stamen primordium. Divisions in the L1 layer increase the surface area of the anther and form the epidermis, whilst L3 cells divide to form connective and vasculature tissues. Periclinal divisions of the L2 cells result in the formation of four clusters of archesporial cells in the anthers (Fig. 1). These then divide to form the sporogenous layer (Sp) and the primary parietal layer (PP), which goes through further divisions to form the maternal layers of the anther (Fig. 2A). This results in a final anther structure comprised of four maternal cell layers: the outer epidermis, endothecium, middle cell layer, and tapetum surrounding the inner sporogenous cells (Fig. 2B) (Sanders et al., 1999; Scott et al., 2004; Feng and Dickinson, 2007).
Diagram of anther lineage and structure. (A) A widely accepted cell lineage model has been suggested for the origin of the cell layers in the anther (Scott et al., 2004; Feng and Dickinson, 2007). Four clusters of archesporial cells (Ar) in the anthers divide to form the primary parietal layer (PP) and the primary sporogenous layer (Sp). The PP layer then goes through a further division to form two secondary parietal layers, the inner secondary parietal layer (ISP) and the outer secondary parietal layer (OSP). The OSP then divides again and differentiates to form the endothecium layer (En), whereas the ISP divides and develops to form the tapetum (T) and middle cell layer (M). (B) This results in the four cell layers of the anther: the outer epidermis (E; yellow), endothecium (En; green), middle cell layer (M; blue), and tapetum (T; red); and the inner sporogenous cells (Sp; purple).
Diagram of anther lineage and structure. (A) A widely accepted cell lineage model has been suggested for the origin of the cell layers in the anther (Scott et al., 2004; Feng and Dickinson, 2007). Four clusters of archesporial cells (Ar) in the anthers divide to form the primary parietal layer (PP) and the primary sporogenous layer (Sp). The PP layer then goes through a further division to form two secondary parietal layers, the inner secondary parietal layer (ISP) and the outer secondary parietal layer (OSP). The OSP then divides again and differentiates to form the endothecium layer (En), whereas the ISP divides and develops to form the tapetum (T) and middle cell layer (M). (B) This results in the four cell layers of the anther: the outer epidermis (E; yellow), endothecium (En; green), middle cell layer (M; blue), and tapetum (T; red); and the inner sporogenous cells (Sp; purple).
The process of anther dehiscence
Pollen release requires careful timing and regulation so that there is synchronized development of the anther and the flower, to ensure that pollen release occurs at the optimal time to maximize either cross- or self-fertilization. Anther dehiscence is a multistage process that involves localized cellular differentiation and degeneration, combined with changes in the structure and water status of the anther to facilitate complete anther opening and pollen release.
Anther opening involves two types of specialized cells, the cells separating the two lobes of the anther (septum) that break down to form a single locule, and the stomium which is made from modified epidermal cells which splits to facilitate anther opening (Fig. 3). The differentiation of these cells occurs early during anther development stage 4, before pollen mother cell meiosis when the endothecium and connective walls are formed (Sanders et al., 1999, 2005; Ma, 2005). Initially the septum degenerates, resulting in a bilocular anther, which is followed by stomium cell breakage and pollen release (Sanders et al., 1999; Scott et al., 2004).
Septum and stomium region of an Arabidopsis anther. (A) The stomium region after microspore release; bands of secondary thckening (arrows) can be observed in the endothecium. (B) The stomium region at anther dehiscence, after septum and stomium lysis. En, endothecium; St, stomium, S, septum. Bar=50 μm.
Septum and stomium region of an Arabidopsis anther. (A) The stomium region after microspore release; bands of secondary thckening (arrows) can be observed in the endothecium. (B) The stomium region at anther dehiscence, after septum and stomium lysis. En, endothecium; St, stomium, S, septum. Bar=50 μm.
The endothecium and the localized secondary thickening within this cell layer also play a critical role in anther opening and pollen release. Endothecium development is coordinated with pollen maturation and the degeneration of the anther tapetum and middle layer. Free download pangya bonus pang hack programs for minecraft pc. This process involves the expansion and subsequent deposition of ligno-cellulosic thickening in the endothecial cells, the breakdown of the septum in the anther wall, combined with anther dehydration and pollen swelling, resulting in stomium breakdown and anther opening (Keijzer, 1987; Bonner and Dickinson, 1989; Scott et al., 2004).
Stomium structure
The stomium is formed by the differentiation of epidermal cells within the anther, to form a single cell region, the location of which determines the position of anther opening (Fig. 3). Detailed microscopy in tomato revealed that the development of intersporangial septa and epidermal cell differentiation are early events occurring as the archesporial cells and tapetum differentiate, with subsequent development of the endothecium (Bonner and Dickinson, 1989). The endothecium then undergoes selective deposition of secondary thickening, whilst the stomium and septum region between the locules does not undergo thickening. This localized thickening is critical for subsequent anther opening. Prior to dehiscence the stomium undergoes cell death and splitting; this does not appear to require viable pollen to be present since splitting is still seen in male-sterile lines generated by tapetal cell ablation (Koltunow et al., 1990; Mariana et al., 1990).
The importance of a functional stomium for dehiscence has been demonstrated by cell ablation studies in tobacco (Beals and Goldberg, 1997). Beals and Goldberg (1997) expressed the cytotoxic barnase gene under the control of the TA35 promoter, which is expressed in the circular cell cluster, stomium, and connective tissues, and combined this with expression of the anti-cytotoxic barstar gene in subsets of anther cell types to protect these cells. They showed that extensive ablation of the circular cell cluster, stomium, and connective tissues caused a failure of dehiscence. However, the same effect was observed if the surrounding tissues were protected by barstar expression, with only specific ablation of the stomium, indicating the importance and independent role of the stomium in anther opening, compared with the surrounding cells.
The majority of dehiscence research has been carried out on a limited number of species including Arabidopsis, Lilium, rice, maize, and members of the Solanaceae, including tobacco, tomatoes, and aubergine/eggplant. The basic process of anther dehiscence appears quite conserved across different plant species, with the dicot model plant Arabidopsis thaliana (Sanders et al., 1999) showing a similar dehiscence process to the Solanaceae (Bonner and Dickinson, 1989; Sanders et al., 2005) and to the monocots rice (Matsui et al., 1999) and maize (Keijzer et al., 1996). However, subtle differences exist between the species that influence anther structure and thus final opening position.
Analysis of anther dehiscence and stomium structure in 30 Solanum species identified variation in the shape and histological features of the stomium, with three main types of dehiscence mechanism observed: (i) poricidal, where dehiscence occurs through a small apical pore: (ii) poricidal-longitudinally dehiscing, where there is an apical pore but the opening continues down in a longitudinal split; and (iii) longitudinally dehiscing anthers where the stomium forms along the entire anther length (Garcia et al., 2008). However, in all cases the stomium consisted of small epidermal cells that serve as the only anther wall layer. The differentiation of these stomial cells and the distribution of thickening in the endothecium around the stomium determine the form of stomium opening (Garcia et al., 2008). Variation has also been observed in the structure of the endothecial layer; for example, in tomato the endothecium only develops in the distal one-third of the anther in the region adjacent to the stomium (Bonner and Dickinson, 1989), whereas in maize the endothecium completely surrounds the locule (Cheng et al., 1979).
In solanaceous species, specialized cell types are found in the ‘notch’ region under the stomium; these have been referred to by various names including the circular cell cluster, intersporangial septum, or hypodermal septum. Their development has been characterized in detail in tobacco (Sanders et al., 2005). They originate as subepidermal cells derived from the L2 layer that differentiate early during anther formation; these expand and accumulate small vesicles, whereas the flanking ‘pre-stomium’ cells do not divide and remain cytoplasmically dense. The circular cell clusters are typically 2–3 cells deep as a consequence of periclinal divisions; these then expand, the small vesicles fuse to form a large central vacuole, and calcium oxalate crystals accumulate. During stomium degeneration the calcium oxalate is released from the circular cell cluster onto the pollen and is subsequently transferred to the stigma upon pollination. The role of calcium oxalate has not been fully established, but it has been suggested that it provides calcium ions required for the pollen germination process (Iwano et al., 2004). Cell-specific ablation studies have, however, shown that the stomium functions independently from the circular cells to control dehiscence (Beals and Goldberg, 1997). The presence of the circular cells and calcium oxalate accumulation appear to be a feature of the Solanaceae, and do not occur in Arabidopsis or Lilium.
Degeneration of cells in the anther
Enzymatic breakdown of the septum
At the cellular level, anther dehiscence is similar to silique dehiscence and, like microspore separation, is thought to involve cell wall-degrading enzymes which break down the pectin between cells (Roberts et al., 2002). Several hydrolytic enzymes and proteins linked to cell wall loosening are thought to be involved, including polygalacturonases (PGs), β-1,4-glucanases, and expansins (Bonghi et al., 1993; Taylor et al., 1993, 1994; Lashbrook et al., 1994; del Campillo and Bennett, 1996; Cho and Cosgrove, 2000).
These enzymes are part of large gene families that have not been fully characterized but have been shown to act redundantly in specific cell types; for example, in the Arabidopsis and rice genomes there are at least 69 and 59 predicted PGs, respectively (Kim et al., 2006; Gonzalez-Carranza et al., 2007). It has been suggested that one group of related PGs tend to be expressed in flowers and flower buds (Torki et al., 2000; Kim et al., 2006). A number of these have been characterized in Arabidopsis and linked to changes in pollen wall development; QUARTET1 (QRT1) and QRT2 (Rhee and Somerville, 1998) and QRT3 (Rhee et al., 2003) are required for degradation of the pollen mother cell wall as microspores are released from their tetrads. Three endo-PGs have been identified as involved in anther dehiscence, silique dehiscence, and floral abscission; ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE1 (ADPG1) and ADPG2 are both required for silique dehiscence, whilst all three (ADPG1, ADPG2, and QRT2) are required for anther dehiscence (Ogawa et al., 2009). Floral abscission is regulated by jasmonic acid (JA), ethylene, and abscisic acid (ABA); all three of these PGs have been shown to be regulated by JA, ADPG2 by ethylene (Gonzalez-Carranza et al., 2007) and QRT2 by ethylene and ABA (Ogawa et al., 2009). This suggests that anther dehiscence-related PG activity is also likely to be regulated by JA, ethylene, and ABA. Recently a PG gene, PS-2, which is expressed concurrently with anther dehiscence, has also been described in tomato, which when mutated results in a failure of anther dehiscence (Gorguet et al., 2009).
Programmed cell death (PCD) of the septum and stomium
The importance of tapetal PCD for successful pollen formation has been highlighted by a number of male-sterile mutants that fail to go through normal tapetal breakdown (Kawanabe et al., 2006; Li et al., 2006; Vizcay-Barrena and Wilson, 2006; Parish and Li, 2010). However, the anther septum and stomium go through a process of degeneration and cell death to facilitate pollen release, and this is also thought to be via a PCD-related process (Kuriyama and Fukuda, 2002; Sanders et al., 2005).
There have been a number of reports of dehiscence mutants resulting from changes to endothecium and stomium degeneration (Sanders et al., 1999). For example, anthers in the non-dehiscence1 mutant undergo an abnormal cell death programme, which results in endothecium degeneration and indirectly causes failure of stomium region breakage, although the pollen appears normal in this mutant (Sanders et al., 1999). Work in Lilium suggests that PCD appears to commence in the tapetal tissues and then extends to the outer anther tissues, including the middle cell layer, stomium region, and later, after dehiscence, to the endothecium and connective tissue (Varnier et al., 2005).
Ultrastructural analysis of the interlocular septum, the connective tissue, middle layer, and epidermis surrounding the stomium of Solanum lycopersicum revealed features consistent with PCD, including nuclear condensation, shrinkage of the plasma membrane, and aberrant mitochondrial morphology (Senatore et al., 2009). They also observed the accumulation of ricinosomes, precursors of protease vesicles, in these cells during the dehiscence process. Ricinosomes have been shown to harbour KDEL-tailed cysteine proteases, which are involved in the final stages of cell death. Senatore et al. (2009) demonstrated that one of these cysteine proteases (SlCysEP) appeared to be an early predictor of dehiscence-associated cell death in the anther. SlCysEP could be detected early during tomato anther development in the interlocular septum and epidermal cells surrounding the stomium, and then later as dehiscence commenced it accumulated in the sporophytic tissues surrounding the locules (Senatore et al., 2009). Previous work by Xu and Chye (1999) in Solanum melongena (aubergene/eggplant) indicated the presence of a cysteine protease (SmCP) linked to PCD that localized to the anther epidermis and endothecium during dehiscence. Koltunov et al. (1990) also identified a cysteine proteinase which was specifically expressed in the anther stomium, circular cells, and subsequently the connective and endothecium in Nicotiana tabacum; however, it is not known if this was associated with the PCD processes. These data suggest that there may be a number of cysteine proteases that are active during stomium breakdown that may be involved in regulating PCD processes in these tissues.
In Arabidopsis the progression of PCD degeneration in the anther appears to be halted when PROMOTION OF CELL SURVIVAL1 (PCS1) is ectopically expressed under regulation of the cauliflower mosaic virus (CaMV) 35S promoter (Ge et al., 2005). In these lines, dehiscence fails; however, the pollen appears to be functional. Mutations in pcs1 appear to show the reverse effect, of premature gametophyte cell death. PCS1 encodes an aspartic protease that appears to act during reproduction and embryogenesis as an anti-cell death component. It is speculated that PCS1 acts in the endoplasmic reticulum either by processing and activating a ‘survival factor’, or by processing and inactivating a pro-PCD component that prevents PCD (Ge et al., 2005).
There are also reports of precocious endothecial breakdown in Arabidopsis anthers, defects associated with irregular callose formation during intine formation, and a failure of pollen desiccation as a consequence of overexpression of the plantacyanin gene (Dong et al., 2005). Plantacyanin belongs to the phytocyanin family of blue copper proteins and is expressed at very low levels in the endothecium and tapetum. Plantacyanins are strongly expressed in the pistils and appear to be involved in pollen tube attraction; however, no phenotype has been observed in the T-DNA insertional knockout (Higashiyama, 2010). They have been shown to bind copper and to display a high redox potential; therefore it is likely that the Arabidopsis plantacyanin gene has the ability to produce reactive oxygen species (ROS). Overexpression of the plantacyanin gene brings about premature PCD of the endothecium during anther stage 14, at which point the wild-type endothecium is still active (based on staging by Sanders et al., 1999), and indehiscent anthers (Dong et al., 2005). Increased expression of the plantacyanin gene is also seen in the receptor-like protein kinase2 (rpk2) mutant, which displays a lack of the middle cell layer, tapetal hypertrophy of the anthers, and defects in endothecial thickening (Mizuno et al., 2007). These effects may therefore be a consequence of increased ROS activity in specific anther cell types, which alter the normal PCD processes within the anther.
Types of endothecial secondary thickening
In general the process of dehiscence is similar in many species; however, there are subtleties associated with differences in anther structure (Garcia, 2002b) and types of anther endothecium thickening (Garcia, 2002a). Four main types of endothecium thickening have been observed, and these tend to occur in a species-specific manner: (i) annular rib types, which are single radial rings which are unconnected and run in parallel to each other; (ii) helical rib types, which form as a helix along the periclinal cell axis, which is also described as a U-shaped thickening; (iii) reticulate ribs, where irregular thickening forms on every face with multiple sites of branching and anastomising sites to form a network; and (iv) palmate ribs, where ribs and a solid plate form in the inner periclinal wall of every cell.
The structure of the endothecial cell layer has also been shown to be critical for dehiscence. The insertional mutant of the maize HD-ZIP IV gene OCL4 (OUTER CELL LAYER4) shows a partial male sterility that is subject to environmental changes; this reduced fertility is thought to result from the presence of an extra subepidermal cell layer with endothecium characteristics in the anther wall (Vernoud et al., 2009). During early development the outer secondary parietal layer divides periclinally in addition to the normal anticlinal division, resulting in a double endothecium-like layer. These periclinal divisions are restricted to the distal part of the anther (Vernoud et al., 2009). The epidermis-specific OCL4 expression in immature anthers is restricted to the region of the anther locule where the extra cell layer differentiates. The epidermal expression of OCL4 suggests that the epidermis can therefore regulate the events in the subepidermal cell layers associated with endothecium differentiation.
Regulation of endothecium secondary thickening
In the Arabidopsis anther, the endothecium is first established during anther stage 5. It undergoes expansion during anther stages 6–10 and develops secondary cell wall thickening during anther stage 11, at which point bar-like ligno-cellulosic fibrous bands are deposited (Sanders et al., 1999; Scott et al., 2004); however, the regulatory network controlling the biosynthesis and selective deposition of this thickening has not yet been fully elucidated.
Endothecium secondary thickening is essential for providing the mechanical force for anther dehiscence (Keijzer, 1987; Bonner and Dickinson, 1989). This has been demonstrated experimentally by analysis of Arabidopsis male-sterile mutants myb26 (Dawson et al., 1999; Steiner-Lange et al., 2003) and the NAC secondary wall thickening promoting factor1 (nst1)nst2 double mutant (Mitsuda et al., 2007). Disruption of endothecial thickening in these mutants results in a failure of anther dehiscence and male sterility. Phloroglucinol and ethidium acridine orange staining indicate that this thickening is composed of lignin as well as cellulose (Dawson et al., 1999; Yang et al., 2007). The composition of this thickening also seems important since disruption of monolignol biosynthesis, in the triple mutant (ccc), which carries mutations in cinnamoyl CoA reductase1 (CCR1), cinnamyl alcohol dehydrogenase c (CAD), and CAD d, results in a reduction of stem lignin by 50% and male sterility due to abnormal endothecial secondary thickening (Thevenin et al., 2010).
Many of the reported Arabidopsis dehiscence mutants have defects linked to JA biosynthesis; however, JA does not appear to regulate MYB26 expression (Mandaokar et al., 2006) and methyl jasmonate treatment does not rescue the myb26 mutation (Dawson et al., 1999), indicating that this pathway is distinct from the JA pathway. In the myb26 mutant, anther development appears normal up to anther stage 11 (Sanders et al., 1999); however, during the later stages, the characteristic band-like ligno-cellulosic wall thickenings seen in the wild-type anther endothecium wall do not form. Degradation of the septum and formation of stomium take place normally in the mutant; however, the endothecium cells fail to expand, they collapse and the subsequent shrinkage of the anther walls does not occur, which results in failure of anther opening and pollen release (Dawson et al., 1999). Overexpression of MYB26 results in ectopic secondary thickening in various above-ground tissues (Yang et al., 2007). MYB26 therefore appears to be a critical factor regulating endothecium expansion and secondary thickening in a cell-specific manner in the anther.
Two NAC domain transcription factors, NST1 and NST2, have also been shown to be involved in the regulation of endothecium wall thickening (Mitsuda et al., 2005). NST2 is expressed predominantly in anther tissues, whilst NST1 is also expressed in vegetative tissues. The double mutant nst1nst2 has an anther-indehiscent phenotype due to lack of secondary thickening in the endothecium, which is similar to the myb26 mutant (Mitsuda et al., 2005). Overexpression of MYB26 induces ectopic secondary thickening and expression of NST1NST2, suggesting that they act downstream of MYB26 (Yang et al., 2007).
The control of secondary wall formation in interfascicular fibres and secondary xylem also involves NST1 and SECONDARY WALL ASSOCIATED NAC DOMAIN PROTEIN1 (SND1, also called NST3) (Mitsuda et al., 2007; Zhong et al., 2007). NST1 and SND1 function redundantly in the regulation of secondary thickening in fibres (Mitsuda et al., 2007; Agalou et al., 2008) and siliques (Mitsuda and Ohme-Takagi, 2008). Dominant repression of NST1 or SND1 and the double knockout caused a severe reduction in the secondary thickening of fibres, whilst overexpression of either gene induced ectopic secondary thickening in various above-ground tissues (Zhong et al., 2006; Mitsuda et al., 2007). This and the phenotypes of ectopic secondary thickening as a consequence of overexpression of MYB26 (Yang et al., 2007) and NST1 or NST2 (Mitsuda et al., 2005) suggest that there is high conservation in the secondary thickening pathways of floral and vegetative tissues.
The Arabidopsis UPCURLED LEAF 1 (UCL1) gene may act upstream to control secondary thickening by regulating the expression of MYB26. Dehiscence fails to occur in the ucl1 mutant; this mutation is thought to be due to the up-regulation of one of the 16 Arabidopsis Class IV HD-ZIP genes, HOMEODOMAIN GLABROUS 3 (HDG3) (Li et al., 2007). Overexpression of HDG3 resulted in a down-regulation of MYB26, NST1, and NST2 expression, suggesting that HDG3 plays a negative role in regulating anther dehiscence. No phenotype was observed in the single hdg3 mutant, suggesting possible redundancy with other members of the HD-ZIP family (Li et al., 2007). However, expression of HDG3 is low in wild-type anthers; therefore, the effect on dehiscence could also be a consequence of ectopic expression.
The RECEPTOR-LIKE PROTEIN KINASE 2 (RPK2) gene has also been found to be involved in secondary cell wall thickening in the endothecium, although the expression of RPK2 was not detected in this tissue during anther development (Mizuno et al., 2007). The rpk2 mutant is male sterile and exhibits a number of defects associated with pollen development, including abnormal tapetal development, a lack of middle cell differentiation, and a failure of anther dehiscence due to the endothecial cell collapsing because of reduced secondary thickening (Mizuno et al., 2007). It is thought that RPK2 functions as a plasma membrane-bound receptor controlling anther development (Mizuno et al., 2007), which may be required for endothecial thickening, or else the presence of an enlarged tapetum, or a lack of middle layer means that the endothecium in the rpk2 mutant fails to form normally. Expression analysis in the rpk2 mutant identified down-regulation of many genes linked to pollen development and secondary thickening, including a number of PGs, genes involved in lignin biosynthesis (e.g. phenylalanine ammonia-lyase and peroxidase), and a number of stress-related genes, particularly those associated with ABA (Mizuno et al., 2007). It has also previously been shown that the abscisic acid-insensitive 8 (abi-8) mutant is male sterile, exhibits defects linked to secondary thickening and cellulose biosynthesis, and is associated with cross-talk between the JA and ethylene pathways (Brocard-Gifford et al., 2004). This implies a direct, or indirect, link between these final events of anther development and stress responses.
Cytokinins have also been implicated in the regulation of secondary wall formation, since ARABIDOPSIS HISTIDINE-CONTAINING PHOSPHOTRANSFER4 (AHP4) has been shown to negatively regulate secondary thickening in the endothecium (Jung et al., 2008). The AHP proteins are mediators of the multistep phosphorelay pathway of cytokinin signalling. Overexpression of AHP4 results in a reduction of endothecial thickening, while the ahp4 mutant showed a slight reduction in thickening. AHP4 expression also correlated to levels of IRREGULAR XYLEM1 (IRX1), 6, and 8 gene expression, suggesting a link to the cellulose biosynthesis pathway (Jung et al., 2008).
Opening the anther
Anther opening involves differential forces on the anther walls to facilitate rupture of the septum and stomium and then subsequent opening of the anther (Fig. 4). Observations of the process of dehiscence in Gasteria verrucosa suggest that as pollen intine formation occurs, the epidermal and endothecium cells of the anther lose some of their starch and start tangential and radial expansion, which is then followed by endothecial secondary thickening (Keijzer, 1987). However the stomium and septum region between the locules does not undergo secondary thickening. The septum undergoes enzymatic lysis of the middle lamellae, with the mechanical swelling of the bordering epidermal cells (Keijzer, 1987) facilitating stomium opening. Tangential swelling of the epidermis and endothecium increases the circumference of the locule wall; however, because the endothecium walls have secondary thickening the inner locule wall dimensions are fixed. This outer enlargement combined with the inner fixed dimensions causes the locule wall to bend inwards, resulting in disruptions to the stomium cells (Fig. 4). The small epidermal cells facing the septum (stomium) are then mechanically broken by inward bending of the adjacent locule walls.
Anther dehiscence. (A, C, E) Transverse sections of the anther; (B, D, F) diagram of the proposed forces on the anther during dehiscence. (A) Microspore release: the tapetum starts to break down, the endothecium expands, and secondary thickening is deposited. (B) As the pollen expands there is an outward pressure (red arrows) exerted from the inside of the locule on the anther which also increases in size; however, the ‘spring-like’ bands of secondary thickening in the endothecium restrict expansion, causing tension to develop. (C) Enzymatic lysis of the stomium combined with the pressure from the expansion of the pollen causes the septum to break to form a single locule. (D) At this point the anther walls begin to dehydrate due to evaporation and active water transport (blue arrows), causing shrinkage of the epidermal cells, resulting in an increased tension on the stomium region (green arrows). (E) As this pressure increases, the stomium splits and the anther walls retract (F). St, stomium; S, septum.
Anther dehiscence. (A, C, E) Transverse sections of the anther; (B, D, F) diagram of the proposed forces on the anther during dehiscence. (A) Microspore release: the tapetum starts to break down, the endothecium expands, and secondary thickening is deposited. (B) As the pollen expands there is an outward pressure (red arrows) exerted from the inside of the locule on the anther which also increases in size; however, the ‘spring-like’ bands of secondary thickening in the endothecium restrict expansion, causing tension to develop. (C) Enzymatic lysis of the stomium combined with the pressure from the expansion of the pollen causes the septum to break to form a single locule. (D) At this point the anther walls begin to dehydrate due to evaporation and active water transport (blue arrows), causing shrinkage of the epidermal cells, resulting in an increased tension on the stomium region (green arrows). (E) As this pressure increases, the stomium splits and the anther walls retract (F). St, stomium; S, septum.
In rice it has been shown that the force created by anther desiccation does not appear sufficient for stomium and septum rupture; however, the swelling of the pollen grains generates the force that is required for septum lysis (Matsui et al., 1999) (Fig. 4). On the other hand, the later stages of locule opening appear dependent upon desiccation of the anther, with shrinkage of the tangential outer wall, which is limited by secondary thickening, providing the force for opening (Keijzer, 1987).
Dehydration of the anther wall
The final stages of anthesis involve the dehydration of the endothecium and epidermal cells, which cause the locule to bend outwards. It has been suggested that this occurs, at least in part, as a consequence of evaporation via stomata on the adaxial side of the anthers, since anther dehydration can be affected by relative humidity (Keijzer, 1987). Keijzer (1987) observed in G. verrucosa that evaporation was more rapid from older anthers, and suggested that this may be as a consequence of low relative humidity inside the bud. This could also be due to active removal of water, since depletion of starch from the anther filaments in G. verrucosa coincided with osmotic retraction of water from the anthers (Keijzer, 1987).
Observations of the water status in tomato anthers has revealed differential regions of anther dehydration, suggesting relocation of water within the anthers and petals, and that desiccation is unlikely to be a major factor in the dehydration of the anther walls (Bonner and Dickinson, 1990). At the time of anther wall dehydration, Bonner and Dickinson (1990) also observed the depletion of starch from the connective tissue surrounding the vasculature and proposed that the conversion of starch to sugar would serve to increase the osmotic potential of the anther tissues and could provide a mechanism for dehydration within specific cell types and regions of the anther. More recent work in Arabidopsis has shown localized accumulation of the H+-sucrose transporter, AtSUC1, around the connective tissues of anthers, which may serve to increase osmotic potential and induce dehydration of the surrounding regions in the anther (Stadler et al., 1999).
Analysis of dehiscence in Petunia suggests that this dehydration may partly be caused by retraction of water from the anthers to the nectaries. It has been proposed that the Petunia NECTARY1 (NEC1) and NEC2 genes may function in the upper part of the filament and stomium cells to alter the starch to sugar balance and regulate water potential in the stomium and nectaries, resulting in anther dehydration (Ge et al., 2000, 2001). It therefore seems likely that an active process of dehydration is occurring within the anther to provide the final force for anther opening (Fig. 4).
Active water movement in the anther may also be due to localized accumulation of cations; Matsui et al. (2000) observed the transfer of potassium ions from the anther locule onto barley pollen grains post-anthesis. They proposed that the swelling of pollen, which is at least partly responsible for septum rupture, was due to the osmotic effect of the potassium ion accumulation in the pollen grains. This has also been supported by observations of high levels of potassium in the stomium area of barley anthers (Rehman and Yun, 2006), suggesting that the potassium ions may play a role in attracting water from the surrounding regions and causing the swelling of the endothecium and pollen prior to anther opening. Potassium ion accumulation has also be suggested to play a role in filament extension in lily (Heslop-Harrison et al., 1987). High levels of potassium have also been identified in mature pollen of other species, for example Lilium and Tradescantia paludosa (Bashe and Mascarenhas, 1984; Heslop-Harrison et al., 1987), and the presence of potassium channels and changes in potassium level have been linked to subsequent pollen tube germination (Bashe and Mascarenhas, 1984).
Other factors may also be involved in anther dehydration. Flavonoids are known to play a wide variety of roles in plants, from protection from ultraviolet (UV) light, to the pigmentation of flowers to attract pollinators (Shirley, 2006) and in the regulation of auxin transport (Jacobs and Rubery, 1998). In Arabidopsis the absence of FLOWER FLAVONOID TRANSPORTER (FFT) affects flavonoid levels, with the fft-1 mutant exhibiting altered root growth, seed and pollen development, and anthers that fail to dehisce (Thompson et al., 2010). The FFT transcript is localized to epidermal guard cells and it has been proposed that it may control dehiscence by playing a role in anther dehydration (Thompson et al., 2010).
Water translocation frequently occurs via plasmodesmata connections; however, the large aquaporin gene family has been shown to mediate the passive movement of water across membranes. Many of these have not yet been characterized; however, PIP1 and PI2, two tobacco aquaporins, are specifically expressed in the anther and stylar tissues (Bots et al., 2005a). In tobacco, PIP2 protein expression is altered during anther dehiscence, with PIP2 RNAi (RNA interference) plants showing retarded dehiscence, suggesting that aquaporins are also involved in anther dehydration (Bots et al., 2005b).
Roles of phytohormones in anther dehiscence
Jasmonic acid
Studies on a large number of dehiscent mutants have shown that jasmonates contribute to the control of anther dehiscence, filament elongation, and pollen viability (Scott et al., 2004). JA is derived principally from the 18-carbon fatty acid linolenic acid (LA), and its biosynthesis is catalysed by several enzymes, including phospholipase A1 (PAL1), 13-lipoxygenase, allene oxide synthase, allene oxide cyclase, and 12-oxo-phytodienoic acid reductase (Fig. 5).
Jasmonic acid (JA) biosynthesis pathway. Linolenic acid (18:3) is released from membrane phospholipid by a lipolytic enzyme (DAD1); this is subsequently converted to an allene oxide (12, 13-epoxy-octadecatrienoic acid) by a lipoxygenase (LOX) and allene oxide synthase (AOS), a member of the cytochrome P450 enzyme family (CYP74A). JA is generated after one round of cyclization, one reduction, and three rounds of β-oxidation. An alternative pathway can also occur, resulting in cis-3-hexenal and traumatin via HPL (CYP74B). Green boxes show mutants that have been characterized in this pathway: defective in anther dehiscence 1 (dad1) (Ishiguro et al., 2001), allene oxide synthase (AOS) (Ishiguro et al., 2001; Park et al., 2002), opr3 (mutation in 12-oxophytodienoic acid reductase) (Stintzi and Browse, 2000) and delayed-dehiscence1 (dde1) (Sanders et al., 2000).
Jasmonic acid (JA) biosynthesis pathway. Linolenic acid (18:3) is released from membrane phospholipid by a lipolytic enzyme (DAD1); this is subsequently converted to an allene oxide (12, 13-epoxy-octadecatrienoic acid) by a lipoxygenase (LOX) and allene oxide synthase (AOS), a member of the cytochrome P450 enzyme family (CYP74A). JA is generated after one round of cyclization, one reduction, and three rounds of β-oxidation. An alternative pathway can also occur, resulting in cis-3-hexenal and traumatin via HPL (CYP74B). Green boxes show mutants that have been characterized in this pathway: defective in anther dehiscence 1 (dad1) (Ishiguro et al., 2001), allene oxide synthase (AOS) (Ishiguro et al., 2001; Park et al., 2002), opr3 (mutation in 12-oxophytodienoic acid reductase) (Stintzi and Browse, 2000) and delayed-dehiscence1 (dde1) (Sanders et al., 2000).
Delayed dehiscence or non-dehiscence phenotypes have been observed in mutants defective in JA biosynthetic enzymes, including the fatty acid desaturation (fad) mutants (McConn and Browse, 1996), opr3 (mutation in 12-oxophytodienoic acid reductase) (Stintzi and Browse, 2000), delayed-dehiscence1 (dde1) and dde2 (Sanders et al., 2000; von Malek et al., 2002), defective in anther dehiscence 1 (dad1) (Ishiguro et al., 2001), and allene oxide synthase mutants (Park et al., 2002) (Fig. 5).
In general, defects in all stages of the JA pathway (Fig. 5) appear to cause similar phenotypes of reduced filament elongation and a lack of dehiscence, although detailed morphological analysis of indehscent anthers has only been conducted the coi1, opr3, and dad1 mutants (Feys et al., 1994; Sanders et al., 2000; Ishiguro et al., 2001). In the Arabidopsis dad1 mutant the stomium fails to open and elongation of the filament is delayed; however, other features required for dehiscence, such as tapetal degeneration, septum breakdown, endothecium thickening, and formation of trinucleate pollen, occur normally (Ishiguro et al., 2001). In the dde1 mutant, which carries a defect in OPDA-reductase 3, dehiscence occurs but is delayed; however, this may be due to an incomplete block in the JA pathway and the gradual accumulation of jasmonates via alternative OPR gene family members (Sanders et al., 2000). Stomium opening and fertility can be rescued by exogenous application of jasmonates; however, specific treatments of dde1 suggest that the anthers are responsive to jasmonate treatment only during stages 9–11 (Sanders et al., 2000).
The involvement of JA in anther dehiscence is also supported by the JA signal transduction mutant coronatine insensitive (coi1); its sterility is due to non-dehiscence that cannot be rescued by exogenous jasmonate (Feys et al., 1994; Xie et al., 1998; Devoto et al., 2002). COI1 is an F-box protein, which recruits JAs and forms a complex with JAZ proteins (denoted for their ZIM and Jas motifs) in the presence of JAs. JAZ proteins repress transcription of JA-responsive genes; in the COI1–ligand–JAZ ternary complex they are polyubiquitinated and subsequently degraded by the 26S proteasome, and this allows JA signal transduction (Devoto et al., 2002; Katsir et al., 2008). The coi1 mutant fails to respond to JA and coronatine because JAZ proteins are not degraded in the presence of these signals (Katsir et al., 2008).
JA synthesized in the filaments is thought to act partly by regulating water transport in the stamens and petals, which subsequently plays a role in filament extension and dehiscence (Ishiguro et al., 2001). The DAD1 gene encodes a PLA1 that catalyses the production of free LA from cellular lipids as the first step in JA biosynthesis that is produced within the stamen filament prior to anther dehiscence. Ishiguro et al. (2001) suggest that DAD1 acts by regulating JA levels to control water transport into the vascular tissues from the endothecium, connective tissue, and anther locules. This in turn influences the development of the flowers and anthers to facilitate the correct timing of petal opening and anther dehiscence. They propose that stamen filament extension and petal elongation are initiated by production of JA via DAD1 induction in the upper part of the filament; this promotes water uptake from the locules, endothecium, and connective tissue into this region. During the later stages of development, cells in the upper and lower parts of the filament express DAD1, inducing JA and causing water removal from the anther cell walls into the filament, resulting in filament extension and subsequent petal elongation and flower opening (Ishiguro et al., 2001). This may be, at least partly, responsible for the dehydration and expansion of the anther cell layers required for anther opening. Ishiguro et al. (2001) propose that JA acts by inducing the expression of genes required for water transport in the anther, for example the plasma membrane H+-sucrose transporter, AtSUC1, which has been found in the parenchymatous tissues surrounding the connective tissues of anthers and has been proposed to facilitate water removal from the walls of the anther (Stadler et al., 1999). Keijzer (1987) reported that the stamen filament remains hydrated during anther wall dehydration, and this may be explained by the observed cell-specific modification of water potential in the anther prior to dehiscence (Stadler et al., 1999).
The activation-tagged mutant of SHI-RELATED SEQUENCE7 (SRS7) also shows disrupted anther dehiscence; viable pollen is produced but tapetal breakdown and anther opening do not occur, indicating a link between tapetal degeneration and anther dehiscence (Kim et al., 2010). SRS7 is predominantly expressed in the filament at the same stage as DAD1 and has similar fertility defects to JA mutants, and Kim et al. (2010) propose that SRS7 may be involved in JA signalling.
JA-induced gene expression
JA synthesis and signalling have important functions in the temporal coordination of late stamen development. Studies show that in addition to acting early in the floral primordial initiation and initiation of microsprogenesis, AGAMOUS (AG) (Bowman et al., 1991) plays a regulatory role in controlling late-stage stamen development (Ito et al., 2007), including anther morphogenesis and dehiscence, as well as filament formation and elongation. During late-stage development it has been shown that AG acts at least partly by directly regulating the transcription of DAD1 and therefore JA biosynthesis (Ito et al., 2007).
Expression analysis in the opr3 mutant has identified 821 genes in the stamen that are regulated by JA application and, of these, 13 are transcription factors (Mandaokar and Browse, 2009). Pollen from the opr3 mutant is inviable if manually extracted from the anther, indicating that JA is playing a role in pollen development as well as release. Two of these transcription factors, MYB21 and MYB24, act in an overlapping manner to regulate anther dehiscence. The myb21myb24 double mutant has short filaments, and the anthers and petals fail to open, but when manually extracted the pollen is viable; these defects could not be rescued by JA treatment (Mandaokar and Browse, 2009). AtMYB21 has also been shown to be repressed by the light signalling factor CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1), and COP1 is required for correct tissue-specific expression of MYB21 (Shin et al., 2002). MYB21 has also been shown to directly regulate PHENYLALANINE LYASE (PAL) and ALTERNATIVE OXIDASE (AOX) expression (Shin et al., 2002). Expression of AtMYB24 is tightly regulated during anther development, and overexpression results in various floral defects including retarded anther development and non-dehiscence. Stomium and septum lysis do not occur in the AtMYB24 overexpression lines, and reduced amounts of endothecial secondary thickening are seen, with disruption in the expression of genes in the phenylalanine pathway (Yang et al., 2007). MYB21 and MYB24 appear to act downstream as transcription factors that mediate the JA response.
Microarray analysis of gene expression in anther development in rice showed that 314 genes responded to either gibberellin (GA) or JA treatment, and 24 GA- and 82 JA- responsive genes showed significant changes in expression between meiosis and the mature anther stages (Wang et al., 2005). This suggests significant cross-talk between the JA and GA pathways.
Gibberellins
GAs have a major role in male fertility, with GA-deficient mutants showing abnormal stamen development (Chhun et al., 2007; Hu et al., 2008; Rieu et al., 2008b). GA mutants show reduced filament elongation, which is thought to be due to reduced cell extension (Cheng et al., 2004). GA is needed for the coordination and synchrony of floral organ development, as shown by analysis of the five Arabidopsis C19-GA 2-oxidases mutants (Rieu et al., 2008a). GA is also required for pollen formation, with development halting in the ga1-3 mutant post-meiosis, but prior to pollen mitotic divisions (Cheng et al., 2004). The tapetum appears to be a major site for GA production (Itoh et al., 1999; Kaneko et al., 2003; Hu et al., 2008), with a peak of GA3ox3 and GA3ox4 expression detected prior to tapetal breakdown and the initiation of dehiscence (Hu et al., 2008). GA biosynthesis also appears to occur in the filament, with a number of GA3oxidases showing expression there during late anther development. However, one of the early single-copy genes involved in GA biosynthesis, AtCPS, appears only to be expressed in the anther (Silverstone et al., 1997), suggesting that the anther may exert regulation over GA biosynthesis in the filaments (Mutasa-Gottgens and Hedden, 2009).
GA and GAMYB are known to be involved in the regulation of anther development (Murray et al., 2003; Cheng et al., 2004; Kaneko et al., 2004). HvGAMYB is a transcription factor shown to be up-regulated by GA, and anthers of transgenic barley overexpressing HvGAMYB are male sterile due to a failure in dehiscence with the stomium remaining intact (Murray et al., 2003). This is thought to be at least partly due to a lack of pollen expansion in these lines, resulting in reduced pressure on the stomium and a resulting failure of stomium lysis (Murray et al., 2003). GA promotion of floral organ expansion and anther development is mediated, in part, by these homeotic transcription factors. GA induction of their expression, however, did not occur in the presence of the translation inhibitor cycloheximide, indicating that it is indirect (Yu et al., 2004). Regulation of these homeotic transcription factors may also be mediated by miRNA159 and MYB33, which have been shown to be involved in GA-regulated anther development. GA enhances miR159 levels by opposing DELLA function. Interestingly, overexpression of miRNA159 caused a reduction in MYB33 transcript in Arabidopsis flowers, and anthers failed to release pollen (Achard et al., 2004). The role of MYB33 is currently unclear, except that it is known to act redundantly with MYB65, and has a facultative role during the earlier stages of tapetal development (Millar and Gubler, 2005).
Auxin
There is increasing evidence suggesting that other phytohormones are also involved in anther dehiscence. Auxin plays a key role in general floral development, organ formation, and pollen development, and also has a major effect on coordinating the maturation of pollen and the dehiscence of the anther (Cheng et al., 2006; Cecchetti et al., 2008).
Targeted expression of the rolB (root loci B), an Agrobacterium oncogene that increases auxin sensitivity, in a cell-autonomous manner in tobacco suggested that local increases in auxin caused a delay in anther dehiscence (Cecchetti et al., 2008). The regulatory role of auxin on anther development was later confirmed by analysis of Arabidopsis auxin receptor mutants. In the tir1 afb1,2,3 auxin receptor multiple mutant, early pollen maturation and anther dehiscence was seen. Endothecial lignification occurred prematurely before tapetal degeneration, and septum and stomium degeneration were early and simultaneous, rather than sequential; pollen mitotic divisions were also premature, due to induction of the cell cycle by auxin, and stamen filament elongation was reduced (Cecchetti et al., 2008). Accumulation of auxin, as a consequence of glyphosate treatment, also resulted in problems in anther dehiscence in cotton due to reorientation of the cytoskeleton and alterations in secondary wall thickening in the anther endothecium. The orientation of thickening changed from longitudinal to transverse, thus hindering septum breakdown, resulting in male sterility; this effect could also be induced by altering auxin transport in cotton anthers (Yasuor et al., 2006).
Early during floral development in Arabidopsis, prior to differentiation of the pollen mother cells, expression of the auxin biosynthesis genes (YUCCA flavin monooxygenases) YUC2 and YUC6 was detected in the anthers (Cheng et al., 2006; Cecchetti et al., 2008). Free auxin levels, based upon expression of the DR5::GUS reporter, were seen in the tapetal tissues during stages 8 and 9 (Aloni et al., 2006) and were shown to be very high in the anther from stage 10 (tapetal degeneration and the first pollen mitotic division) to stage 12 (bilocular anther) (Aloni et al., 2006; Feng et al., 2006; Cecchetti et al., 2008). It is thought that this auxin is synthesized within the anther, since YUC2 and 6 expression also occurs at this stage (Cheng et al., 2006; Feng et al., 2006; Cecchetti et al., 2008) and auxin transport inhibitors/mutants (mdr1 pgp1) have minimal effects on the late events of anther dehiscence and pollen maturation (Noh et al., 2001). Stamen filament elongation and pollen viability were reduced in the mdr1 pgp1 auxin transport mutant (Noh et al., 2001), but was not significantly affected by the premature dehiscence observed in the tir1 afb multiple mutant (Cecchetti et al., 2008). These data suggest that anther dehiscence and pollen maturation are coordinated and regulated by endogenously anther-synthesized auxin, and that their development is independent of the pre-anthesis anther filament elongation, which requires auxin transport into the vasculature (Cecchetti et al., 2008). Auxin levels are highly regulated during anther development, with auxin serving to limit precocious pollen maturation and dehiscence, and to coordinate the timing of these events (Cecchetti et al., 2008).
Auxin is unlikely to act in isolation from the other hormonal pathways and is thought to regulate anther dehiscence through JA. Loss of AUXIN RESPONSE FACTOR 6 (ARF6) and ARF8 disrupts JA production and thus causes delayed or non-dehiscence, and reduced filament and petal elongation as seen in the dad1 mutant (Tabata et al., 2010), which can be rescued by exogenous application of JA (Nagpal et al., 2005; Cecchetti et al., 2007). ARF6 and 8 have been shown to be required for activation of DAD1 expression and therefore to regulate JA biosynthesis (Tabata et al., 2009).
However, it is also likely that another auxin-mediated pathway may affect anther dehiscence. Alterations in local auxin homeostasis, as a consequence of the up- or down-regulation of the GT trihelix DNA-binding transcription factor PETAL LOSS-D (PTL-D) gene resulted in a failure of septum and subsequent stomium degeneration and consequential failure of dehiscence (Li et al., 2008). However, the ptl mutant, which is fully sterile, cannot be rescued by JA treatment, suggesting that PTL is acting via an alternative auxin-mediated pathway to the JA pathway.
Ethylene
Retardation of dehiscence has also been observed by alteration of ethylene levels. Studies in tobacco have revealed that ethylene has an effect on the final events of dehiscence, degeneration of the stomium cells and dehydration (Rieu et al., 2003). Ethylene-insensitive tobacco flowers, resulting from mutation of the ethylene receptor1 (etr1) mutant, or by treatment with the ethylene-perception inhibitor 1-methyl-cyclopropene (MCP), resulted in loss of dehiscence synchrony, with flower opening and delay in stomium cell degeneration and dehydration (Rieu et al., 2003). Two Petunia genes, PhERS1 and PhERS2, encoding ethylene receptor homologues are proposed to regulate the timing/synchronization of stomium degeneration and anther dehiscence. Antisense suppression of PhETR2 in Petunia led to stomium cell degeneration and anther dehiscence before anthesis (Wang and Kumar, 2007). The role of ethylene in regulating dehiscence in tobacco flowers has also been supported by analysis of ethylene-insensitive plants, or treatment with the ethylene perception inhibitor MCP, which resulted in delayed dehiscence, while ethylene treatment accelerated dehiscence (Rieu et al., 2003). It is therefore proposed that ethylene may act as a signal regulating anther dehiscence in tobacco and Petunia, in a manner similar to JA in Arabidopsis (Rieu et al., 2003).
Morphological effects of stress
Anther dehiscence has been shown to be a highly regulated process, which appears to be particularly sensitive to abiotic stress. Environmental conditions, particularly increased temperature, have a dramatic effect on pollen fertility and anther dehiscence (Sakata and Higashitan, 2008; Jagadish et al., 2010; Zinn et al., 2010). This reduction in fertility appears to be at least partly due to tissue-specific auxin reduction, since auxin treatment can at least partly compensate for high temperature stress (Sakata et al., 2010); however it is currently not known whether this is acting via alterations in endothecium thickening. Heat stress has been shown to result in protein expression changes, particularly associated with dehiscence (Jagadish et al., 2010). High temperature stress has also been linked to a reduction in the swelling of pollen, which provides the force for anther opening (Matsui et al., 2000).
There are also indications that secondary thickening in the anther endothecium may involve localized stress-related transcriptional responses. Overexpression of NST1 Arabidopsis resulted in ectopic secondary thickening and up-regulation of genes associated with abiotic stresses, including drought, wounding, temperature, and osmotic stress (Mitsuda et al., 2005). Expression analysis of the rpk2 mutant, which shows altered endothecium secondary thickening, also identified differential expression of a number of stress-related genes, particularly those associated with ABA (Mizuno et al., 2007).
Conclusions
Molecular understanding of anther dehiscence has increased over recent years. It has been shown to involve a number of key developmental processes including cellular differentiation, cell expansion, degradation, and PCD, alongside selective modifications of water status in specific cell types to facilitate pollen dispersal. The key events of this process are shown in Fig. 6. Many of these processes involve pathways similar to those found in other tissues, for example secondary thickening in the anther and vegetative tissues, and enzymatic lysis of abscission and dehiscence zones. There are many aspects of the dehiscence process which have still not been fully characterized, for example the regulation of stomium and septum formation, the precise delimitation of thickening within the anther, and the regulatory networks that determine differentiation and water movements in the anther. This means that currently it is also difficult to predict the molecular effects that physical and environmental stresses have upon the anther during opening. Providing greater understanding of such processes will provide valuable insight into controlling crop fertility in a world of changing environmental stresses.
Key developmental events in anther dehiscence.
Key developmental events in anther dehiscence.
We would like to thank the BBSRC for funding.
References
P
, A
, DC
, NP
. Modulation of floral development by a gibberellin-regulated microRNA
, , 2004
, vol. (pg. 3357
-)A
, S
, E
, et al. A genome-wide survey of HD-Zip genes in rice and analysis of drought-responsive family members
, , 2008
, vol. (pg. 87
-)C
, E
, V
, E
, S
. The Arabidopsis thaliana SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASES1 and 2 control male sporogenesis
, , 2005
, vol. (pg. 3337
-)R
, E
, M
, CI
. Role of auxin in regulating Arabidopsis flower development
, , 2006
, vol. (pg. 315
-)D
, JP
. Changes in potassium-ion concentrations during pollen dehydration and germination in relation to protein-synthesis
, , 1984
, vol. (pg. 55
-)TP
, RB
. A novel cell ablation strategy blocks tobacco anther dehiscence
, , 1997
, vol. (pg. 1527
-)ZC
, G
, A
, N
. Abscission in leaf and fruit explants of Prunus persica (L.) Batsch
, , 1993
, vol. (pg. 555
-)L
, H
. , New Phytologist
, , vol. 113
(pg. -115
)LJ
, HG
. Anther dehiscence in Lycopersicon esculentum II. Water relations
, , 1990
, vol. (pg. 367
-)M
, R
, N
, K
, R
, T
. PIP1 and PIP2 aquaporins are differentially expressed during tobacco anther and stigma development
, , 2005
, vol. (pg. 113
-)M
, F
, M
, K
, H
, C
. Aquaporins of the PIP2 class are required for efficient anther dehiscence in tobacco
, , 2005
, vol. (pg. 1049
-)JL
, GN
, EM
. Expression of the Arabidposis floral homeotic gene AGAMOUS is restricted to specific cell types late in flower development
, , 1991
, vol. (pg. 749
-)I
, TJ
, ME
, B
, RR
. The Arabidopsis thaliana ABSCISIC ACID-INSENSITIVE8 encodes a novel protein mediating abscisic acid and sugar responses essential for growth
, , 2004
, vol. (pg. 406
-)C
, AM
, R
, H
. EXS, a putative LRR receptor kinase, regulates male germline cell number and tapetal identity and promotes seed development in Arabidopsis
, , 2002
, vol. (pg. 1718
-)V
, MM
, G
, P
, M
. Auxin regulates Arabidopsis anther dehiscence, pollen maturation, and filament elongation
, , 2008
, vol. (pg. 1760
-)V
, MM
, G
, M
, G
, P
, M
. ROX1, a gene induced by rolB, is involved in procambial cell proliferation and xylem differentiation in tobacco stamen
, , 2007
, vol. (pg. 27
-)H
, L
, S
![Enzymatic Activation Of Alkanes Constraints And Prospects Uk Enzymatic Activation Of Alkanes Constraints And Prospects Uk](/uploads/1/2/4/7/124790772/542223862.jpg)
X
, DE
, D
, D
, NP
, J
. Gibberellin regulates Arabidopsis floral development via suppression of DELLA protein function
, , 2004
, vol. (pg. 1055
-)PC
, RI
, DB
. Comparison of anther development in genic-male-sterile and in male-fertile corn (Zea mays) from light microscopy and scanning microscopy
, , 1979
, vol. (pg. 578
-)Y
, X
, Y
. Auxin biosynthesis by the YUCCA flavin monooxygenases controls the formation of floral organs and vascular tissues in Arabidopsis
, , 2006
, vol. (pg. 1790
-)T
, K
, K
, E
, Y
, M
, H
, M
, M
, M
. Gibberellin regulates pollen viability and pollen tube growth in rice
, , 2007
, vol. (pg. 3876
-)HT
, DJ
. Altered expression of expansin modulates leaf growth and pedicel abscission in Arabidopsis thaliana
, Proceedings of the National Academy of Sciences, USA
, , vol. 97
(pg. -9788
)J
, A
, R
, CE
, JI
. Arabidopsis SOMATIC EMBRYOGENESIS RECEPTOR KINASES1 and 2 are essential for tapetum development and microspore maturation
, , 2005
, vol. (pg. 3350
-)J
, E
, I
, S
, ZA
, BJ
. Characterization and genetic mapping of a mutation (ms35) which prevents anther dehiscence in Arabidopsis thaliana by affecting secondary wall thickening in the endothecium
, , 1999
, vol. (pg. 213
-)E
, AB
. Pedicel breakstrength and cellulase gene expression during tomato flower abscission
, , 1996
, vol. (pg. 813
-)A
, M
, D
, C
, R
, E
, J
, L
, M
, JG
. COI1 links jasmonate signalling and fertility to the SCF ubiquitin–ligase complex in Arabidopsis
, , 2002
, vol. (pg. 457
-)JR
, D
, MF
. NUBBIN and JAGGED define stamen and carpel shape in Arabidopsis
, , 2006
, vol. (pg. 1645
-)J
, ST
, EM
. Plantacyanin plays a role in reproduction in Arabidopsis
, , 2005
, vol. (pg. 778
-)X
, HG
. , Trends in Genetics
, , vol. 23
(pg. -510
)X
, HG
. Cell–cell interactions during patterning of the Arabidopsis anther
, , 2010
, vol. (pg. 571
-)XL
, WM
, S
, B
, ZH
, HW
. Auxin flow in anther filaments is critical for pollen grain development through regulating pollen mitosis
, , 2006
, vol. (pg. 215
-)B
, CE
, CN
, JG
. Arabidopsis mutants selected for resistance to the phytotoxin coronatine are male sterile, insensitive to methyl jasmonate, and resistant to a bacterial pathogen
, , 1994
, vol. (pg. 751
-)CC
. An approach to the diversity of endothecial thickenings in Solanaceae
, , 2002
, vol. (pg. 214
-)CC
. , Annals of Botany
, , vol. 90
(pg. -706
)CC
, M
, G
. Features related to anther opening in Solanum species (Solanaceae)
, , 2008
, vol. (pg. 344
-)X
, C
, M
, G
, H
, Y
. An Arabidopsis aspartic protease functions as an anti-cell-death component in reproduction and embryogenesis
, , 2005
, vol. (pg. 282
-)YX
, GC
, PE
, et al. NEC1, a novel gene, highly expressed in nectary tissue of Petunia hybrida
, , 2000
, vol. (pg. 725
-)YX
, GC
, E
, J
, J
, GJ
, J
. Partial silencing of the NEC1 gene results in early opening of anthers in Petunia hybrida
, , 2001
, vol. (pg. 414
-)R
, T
, P
. Anther development: basic principles and practical applications
, , 1993
, vol. (pg. 1217
-)ZH
, KA
, JA
. Expression of polygalacturonases and evidence to support their role during cell separation processes in Arabidopsis thaliana
, , 2007
, vol. (pg. 3719
-)B
, D
, A
, RG
, AW
. ps-2, the gene responsible for functional sterility in tomato, due to non-dehiscent anthers, is the result of a mutation in a novel polygalacturonase gene
, , 2009
, vol. (pg. 1199
-)JS
, Y
, BJ
. Anther-filament extension in Lilium – potassium-ion movement and some anatomical features
, , 1987
, vol. (pg. 505
-)T
. , Plant and Cell Physiology
, , vol. 51
(pg. -189
)CL
, C
, BJ
, SE
, H
. The BAM1/BAM2 receptor-like kinases are important regulators of Arabidopsis early anther development
, , 2006
, vol. (pg. 1667
-)J
, MG
, N
, et al. Potential sites of bioactive gibberellin production during reproductive growth in Arabidopsis
, , 2008
, vol. (pg. 320
-)S
, A
, J
, I
, K
. The DEFECTIVE IN ANTHER DEHISCENCE gene encodes a novel phospholipase A1 catalyzing the initial step of jasmonic acid biosynthesis, which synchronizes pollen maturation, anther dehiscence, and flower opening in Arabidopsis
, , 2001
, vol. (pg. 2191
-)T
, KH
, TS
, H
, EM
. The homeotic protein AGAMOUS controls late stamen development by regulating a jasmonate biosynthetic gene in Arabidopsis
, , 2007
, vol. (pg. 3516
-)T
, F
, H
, P
, N
, M
, JL
, EM
. The homeotic protein AGAMOUS controls microsporogenesis by regulation of SPOROCYTELESS
, , 2004
, vol. (pg. 356
-)H
, M
, H
, X
, Y
, M
. The gene encoding tobacco gibberellin 3beta-hydroxylase is expressed at the site of GA action during stem elongation and flower organ development
, , 1999
, vol. (pg. 15
-)M
, T
, H
, S
, A
. Calcium crystals in the anther of Petunia: the existence and biological significance in the pollination process
, , 2004
, vol. (pg. 40
-)M
, PH
. , Science
, , vol. 241
(pg. -349
)SV
, R
, R
, TR
, S
, J
, PQ
. Physiological and proteomic approaches to address heat tolerance during anthesis in rice (Oryza sativa L.)
, , 2010
, vol. (pg. 143
-)G
, X
, HA
, D
. Signaling of cell fate determination by the TPD1 small protein and EMS1 receptor kinase
, Proceedings of the National Academy of Sciences, USA
, , vol. 105
(pg. -2225
)KW
, SI
, YY
, KS
, MH
, JS
. Arabidopsis histidine-containing phosphotransfer factor 4 (AHP4) negatively regulates secondary wall thickening of the anther endothecium during flowering
, , 2008
, vol. (pg. 294
-)M
, H
, Y
, T
, M
, M
, M
. Where do gibberellin biosynthesis and gibberellin signaling occur in rice plants?
, , 2003
, vol. (pg. 104
-)M
, Y
, M
, et al. Loss-of-function mutations of the rice GAMYB gene impair alpha-amylase expression in aleurone and flower development
, , 2004
, vol. (pg. 33
-)L
, HS
, AJ
, GA
. Jasmonate signaling: a conserved mechanism of hormone sensing
, , 2008
, vol. (pg. 428
-)T
, T
, M
, H
, K
. Abolition of the tapetum suicide program ruins microsporogenesis
, , 2006
, vol. (pg. 784
-)C
. The processes of anther dehiscence and pollen dispersal. I. The opening mechanism of longitudinally dehiscing anthers
, , 1987
, vol. (pg. 487
-)CJ
, HB
, MC
. The mechanics of the grass flower: anther dehiscence and pollen shedding in maize
, , 1996
, vol. (pg. 15
-)J
, SH
, S
, WH
, SE
. Patterns of expansion and expression divergence in the plant polygalacturonase gene family
, , 2006
, vol. pg. R87
SG
, S
, YS
, DJ
, JC
, CM
. Activation tagging of an Arabidopsis SHI-RELATED SEQUENCE gene produces abnormal anther dehiscence and floral development
, , 2010
, vol. (pg. 337
-)AM
, J
, KH
, M
, RB
. Different temporal and spatial gene expression patterns during anther development
, , 1990
, vol. (pg. 1201
-)H
, H
. , Current Opinion in Plant Biology
, , vol. 5
(pg. -573
)CC
, C
, AB
. Two divergent endo-beta-1,4-glucanase genes exhibit overlapping expression in ripening fruit and abscising flowers
, , 1994
, vol. (pg. 1485
-)N
, DS
, HS
, et al. The rice tapetum degeneration retardation gene is required for tapetum degradation and anther development
, , 2006
, vol. (pg. 2999
-)QJ
, B
, X-Y
, L-J
. The effect of increased expression of an Arabidopsis HD-ZIP gene on leaf morphonlogy and anther dehiscence
, , 2007
, vol. (pg. 567
-)X
, G
, Z
, H
, LJ
. A gain-of-function mutation of transcriptional factor PTL results in curly leaves, dwarfism and male sterility by affecting auxin homeostasis
, , 2008
, vol. (pg. 315
-)ZB
, D
. , Trends in Genetics
, , vol. 23
(pg. -66
)H
. Molecular genetic analyses of microsporogenesis and microgametogenesis in flowering plants
, , 2005
, vol. (pg. 393
-)A
, B
, B
, BM
, G
, YJ
, YD
, J
. Transcriptional regulators of stamen development in Arabidopsis by transcriptional profiling
, , 2006
, vol. (pg. 984
-)A
, J
. MYB108 acts together with MYB24 to regulate jasmonate-mediated stamen maturation in Arabidopsis
, , 2009
, vol. (pg. 851
-)C
, MD
, J
, J
, RB
. Induction of male sterility in plants by a chimeric ribonuclease gene
, , 1990
, vol. (pg. 737
-)T
, K
, T
. Mechanism of anther dehiscence in rice (Oryza sativa L.)
, , 1999
, vol. (pg. 501
-)T
, K
, T
. High temperature at flowering inhibits swelling of pollen grains, a driving force for thecae dehiscence in rice
, , 2000
, vol. (pg. 430
-)M
, J
. The critical requirement for linolenic acid is pollen development, not photosynthesis, in an Arabidopsis mutant
, , 1996
, vol. (pg. 403
-)AA
, F
. The Arabidopsis GAMYB-like genes, MYB33 and MYB65, are microRNA-regulated genes that redundantly facilitate anther development
, , 2005
, vol. (pg. 705
-)N
, A
, H
, M
, M
, K
, M
. NAC transcription factors, NST1 and NST3, are key regulators of the formation of secondary walls in woody tissues of Arabidopsis
, , 2007
, vol. (pg. 270
-)N
, M
. NAC transcription factors NST1 and NST3 regulate pod shattering in a partially redundant manner by promoting secondary wall formation after the establishment of tissue identity
, , 2008
, vol. (pg. 768
-)N
, M
, K
, M
. The NAC transcription factors NST1 and NST2 of Arabidopsis regulate secondary wall thickenings and are required for anther dehiscence
, , 2005
, vol. (pg. 2993
-)S
, Y
, K
, T
, K
, T
, K
, K
. Receptor-like protein kinase 2 (RPK 2) is a novel factor controlling anther development in Arabidopsis thaliana
, , 2007
, vol. (pg. 751
-)F
, R
, J
, F
. , The Plant Journal
, , vol. 33
(pg. -491
)E
, P
. Gibberellin as a factor in floral regulatory networks
, , 2009
, vol. (pg. 1979
-)P
, CM
, H
, et al. Auxin response factors ARF6 and ARF8 promote jasmonic acid production and flower maturation
, , 2005
, vol. (pg. 4107
-)B
, AS
, EP
. Multidrug resistance-like genes of Arabidopsis required for auxin transport and auxin-mediated development
, , 2001
, vol. (pg. 2441
-)M
, P
, S
, SM
. ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE1 (ADPG1), ADPG2, and QUARTET2 are polygalacturonases required for cell separation during reproductive development in Arabidopsis
, , 2009
, vol. (pg. 216
-)RW
, SF
. Death of a tapetum: a programme of developmental altruism
, , 2010
, vol. (pg. 73
-)JH
, R
, HB
, IT
, KA
, R
. A knock-out mutation in allene oxide synthase results in male sterility and defective wound signal transduction in Arabidopsis due to a block in jasmonic acid biosynthesis
, , 2002
, vol. (pg. 1
-)S
, SJ
. Developmental regulation of K accumulation in pollen, anthers, and papillae: are anther dehiscence, papillae hydration, and pollen swelling leading to pollination and fertilization in barley (Hordeum vulgare L.) regulated by changes in K concentration?
, , 2006
, vol. (pg. 1315
-)SY
, E
, PD
, CR
. Microspore separation in the quartet 3 mutants of Arabidopsis is impaired by a defect in a developmentally regulated polygalacturonase required for pollen mother cell wall degradation
, , 2003
, vol. (pg. 1170
-)SY
, CR
. Tetrad pollen formation in quartet mutants of Arabidopsis thaliana is associated with persistence of pectic polysaccharides of the pollen mother cell wall
, , 1998
, vol. (pg. 79
-)I
, S
, SJ
, et al. Genetic analysis reveals that C19-GA 2-oxidation is a major gibberellin inactivation pathway in Arabidopsis
, , 2008
, vol. (pg. 2420
-)I
, O
, N
, et al. The gibberellin biosynthetic genes AtGA20ox1 and AtGA20ox2 act, partially redundantly, to promote growth and development throughout the Arabidopsis life cycle
, , 2008
, vol. (pg. 488
-)I
, M
, J
, C
, K
. Ethylene regulates the timing of anther dehiscence in tobacco
, , 2003
, vol. (pg. 131
-)JA
, KA
, ZH
. Abscission, dehiscence, and other cell separation processes
, , 2002
, vol. (pg. 131
-)P
, S
. Flower and fruit development in Arabidopsis thaliana
, , 2005
, vol. (pg. 633
-)T
, A
. male sterility accompanied with abnormal anther development in plants—genes and environmental stresses with special reference to high temperature injury
, International Journal of Plant Developmental Biology
, , vol. 2
(pg. -51
)T
, T
, S
, M
, Y
, N
, Y
, H
, M
, A
. Auxins reverse plant male sterility caused by high temperatures
, Proceedings of the National Academy of Sciences, USA
, , vol. 107
(pg. -8574
)PM
, AQ
, K
, KN
, YC
, PY
, MT
, TP
, RB
. Anther developmental defects in Arabidopsis thaliana male-sterile mutants
, , 1999
, vol. (pg. 297
-)PM
, AQ
, BH
, RB
. Differentiation and degeneration of cells that play a major role in tobacco anther dehiscence
, , 2005
, vol. (pg. 219
-)PM
, PY
, C
, JD
, TP
, EW
, RB
. The arabidopsis DELAYED DEHISCENCE1 gene encodes an enzyme in the jasmonic acid synthesis pathway
, , 2000
, vol. (pg. 1041
-)RJ
, M
, HG
. , The Plant Cell
, 16 Suppl 1
(pg. -S60
)A
, CP
, JS
. Ricinosomes predict programmed cell death leading to anther dehiscence in tomato
, , 2009
, vol. (pg. 775
-)B
, G
, H
, S
, I
, J
, S
, NC
, JH
, PS
, G
. AtMYB21, a gene encoding a flower-specific transcription factor, is regulated by COP1
, , 2002
, vol. (pg. 23
-)BW
. Flavonoid biosynthesis: ‘new’ functions for an ‘old’ pathway
, , 2006
, vol. (pg. 377
-)AL
, C
, E
, TP
. Developmental regulation of the gibberellin biosynthetic gene GA1 in Arabidopsis thaliana
, , 1997
, vol. (pg. 9
-)DR
, JL
, EM
. , The Plant Cell
, , vol. 2
(pg. -767
)AM
, S
, US
, P
, K
, H
. The Arabidopsis ABORTED MICROSPORES (AMS) gene encodes a MYC class transcription factor
, , 2003
, vol. (pg. 413
-)R
, E
, M
, N
. The AtSUC1 sucrose carrier may represent the osmotic driving force for anther dehiscence and pollen tube growth in Arabidopsis
, , 1999
, vol. (pg. 269
-)S
, US
, L
, C
, ZA
, E
, K
, H
. Disruption of Arabidopsis thaliana MYB26 results in male sterility due to non-dehiscent anthers
, , 2003
, vol. (pg. 519
-)A
, J
. The Arabidopsis male-sterile mutant, opr3, lacks the 12-oxophytodienoic acid reductase required for jasmonate synthesis
, Proceedings of the National Academy of Sciences, USA
, , vol. 97
(pg. -10630
)R
, M
, T
, M
, CE
, Y
, KT
, Y
, K
, S
. Arabidopsis auxin response factor6 and 8 regulate jasmonic acid biosynthesis and floral organ development via repression of class 1 KNOX genes
, , 2010
, vol. (pg. 164
-)JE
, SA
, S
, JA
. Characterization and accumulation pattern of an mRNA encoding an abscission-related beta-1,4-glucanase from leaflets of Sambucus nigra
, , 1994
, vol. (pg. 961
-)JE
, STJ
, SA
, GA
, JA
. Changes in polygalacturonase activity and solubility of polyuronides during ethylene-stimulated leaf abscission in Sambucus nigra
, , 1993
, vol. (pg. 93
-)J
, B
, B
, L
, L
, A
, C
, L
. The simultaneous repression of CCR and CAD, two enzymes of the lignin biosynthetic pathway, results in sterility and dwarfism in Arabidopsis thaliana
, , 2010
EP
, C
, V
, JM
, BJ
. An Arabidopsis flavonoid transporter is required for anther dehiscence and pollen development
, , 2010
, vol. (pg. 439
-)M
, P
, R
, D
. Characterization of a ubiquitous expressed gene family encoding polygalacturonase in Arabidopsis thaliana
, , 2000
, vol. (pg. 427
-)AL
, F
, RS
, C
. Programmed cell death progressively models the development of anther sporophytic tissues from the tapetum and is triggered in pollen grains during maturation
, , 2005
, vol. (pg. 118
-)V
, G
, F
, RB
, P
, PM
. The HD-ZIP IV transcription factor OCL4 is necessary for trichome patterning and anther development in maize
, , 2009
, vol. (pg. 883
-)G
, ZA
. Altered tapetal PCD and pollen wall development in the Arabidopsis ms1 mutant
, , 2006
, vol. (pg. 2709
-)B
, E
, K
, B
. The Arabidopsis male-sterile mutant dde2-2 is defective in the ALLENE OXIDE SYNTHASE gene encoding one of the key enzymes of the jasmonic acid biosynthesis pathway
, , 2002
, vol. (pg. 187
-)Y
, PP
. Characterization of two ethylene receptors PhERS1 and PhETR2 from petunia: PhETR2 regulates timing of anther dehiscence
, , 2007
, vol. (pg. 533
-)Z
, Y
, C
, Y
, L
, D
, C
, Z
, Y
, K
. Microarray analysis of gene expression involved in anther development in rice (Oryza sativa L.)
, , 2005
, vol. (pg. 721
-)ZA
, SM
, J
, R
, PJ
. The Arabidopsis MALE STERILITY1 (MS1) gene is a transcriptional regulator of male gametogenesis, with homology to the PHD-finger family of transcription factors
, , 2001
, vol. (pg. 27
-)ZA
, DB
. From Arabidopsis to rice: pathways in pollen development
, , 2009
, vol. (pg. 1479
-)DX
, BF
, S
, M
, JG
. COI1: an Arabidopsis gene required for jasmonate-regulated defense and fertility
, , 1998
, vol. (pg. 1091
-)FX
, ML
. Expression of cysteine proteinase during developmental events associated with programmed cell death in brinjal
, , 1999
, vol. (pg. 321
-)J
, C
, Z
, D
, MY
, Z
, W
, D-B
, ZA
. The ABORTED MICROSPORES regulatory network is required for postmeiotic male reproductive development in Arabidopsis thaliana
, , 2010
, vol. (pg. 91
-)C
, Z
, J
, K
, G
, ZA
. Arabidopsis MYB26/MALE STERILE35 regulates secondary thickening in the endothecium and is essential for anther dehiscence
, , 2007
, vol. (pg. 534
-)SL
, L
, CS
, LF
, XQ
, LQ
, WC
, D
. Overexpression of TAPETUM DETERMINANT1 alters the cell fates in the Arabidopsis carpel and tapetum via genetic interaction with excess microsporocytes1/extra sporogenous cells
, , 2005
, vol. (pg. 186
-)SL
, LF
, HZ
, CS
, WC
, L
, V
, D
. TAPETUM DETERMINANT1 is required for cell specialization in the Arabidopsis anther
, , 2003
, vol. (pg. 2792
-)XY
, JG
, M
, H
Enzymatic Activation Of Alkanes Constraints And Prospects Uk 2017
,ZL
, LJ
. Over-expression of a flower-specific transcription factor gene AtMYB24 causes aberrant anther development
, , 2007
, vol. (pg. 219
-)H
, M
, E
, A
, E
, J
, B
. Glyphosate-induced anther indehiscence in cotton is partially temperature dependent and involves cytoskeleton and secondary wall modifications and auxin accumulation
, , 2006
, vol. (pg. 1306
-)H
, T
, Y
, J
, P
, EM
. Floral homeotic genes are targets of gibberellin signaling in flower development
, Proceedings of the National Academy of Sciences, USA
, , vol. 101
(pg. -7832
)W
, YL
, L
, C
, U
, H
. Regulation of Arabidopsis tapetum development and function by DYSFUNCTIONAL TAPETUM (DYT1) encoding a putative bHLH transcription factor
, , 2006
, vol. (pg. 3085
-)DZ
, GF
, B
, H
. The excess microsporocytes1 gene encodes a putative leucine-rich repeat receptor protein kinase that controls somatic and reproductive cell fates in the Arabidopsis anther
, , 2002
, vol. (pg. 2021
-)R
, T
, ZH
. SND1, a NAC domain transcription factor, is a key regulator of secondary wall synthesis in fibers of Arabidopsis
, , 2006
, vol. (pg. 3158
-)R
, ![Enzymatic Activation Of Alkanes Constraints And Prospects Uk Enzymatic Activation Of Alkanes Constraints And Prospects Uk](/uploads/1/2/4/7/124790772/488644954.png)
EA
, ZH
. Enzymatic Activation Of Alkanes Constraints And Prospects Ukulele Chords
The MYB46 transcription factor is a direct target of SND1 and regulates secondary wall biosynthesis in Arabidopsis
, , 2007
, vol. (pg. 2776
-)J
, H
, H
, JF
, H
, C
, YF
, ZN
. Defective in Tapetal development and function 1 is essential for anther development and tapetal function for microspore maturation in Arabidopsis
, , 2008
, vol. (pg. 266
-)KE
, M
, JF
. Temperature stress and plant sexual reproduction: uncovering the weakest links
, , 2010
, vol. (pg. 1959
-)© The Author [2011]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved. For Permissions, please e-mail: [email protected]
0 Comments
You have entered an invalid code
Thank you for submitting a comment on this article. Your comment will be reviewed and published at the journal's discretion. Please check for further notifications by email.
Philos Trans R Soc Lond B Biol Sci. 2008 Aug 27; 363(1504): 2755–2765.
Published online 2008 May 16. doi: 10.1098/rstb.2008.0024
PMID: 18487128
This article has been cited by other articles in PMC.
Associated Data
Kirschvink & Kopp supplemental information Table S1 and figure S1
GUID: 8F13D644-C552-45A6-AA42-5C142C5CD193
rstb20080024s15.doc (26K)
Abstract
Two major geological problems regarding the origin of oxygenic photosynthesis are (i) identifying a source of oxygen pre-dating the biological oxygen production and capable of driving the evolution of oxygen tolerance, and (ii) determining when oxygenic photosynthesis evolved. One solution to the first problem is the accumulation of photochemically produced H2O2 at the surface of the glaciers and its subsequent incorporation into ice. Melting at the glacier base would release H2O2, which interacts with seawater to produce O2 in an environment shielded from the lethal levels of ultraviolet radiation needed to produce H2O2. Answers to the second problem are controversial and range from 3.8 to 2.2 Gyr ago. A sceptical view, based on the metals that have the redox potentials close to oxygen, argues for the late end of the range. The preponderance of geological evidence suggests little or no oxygen in the Late Archaean atmosphere (less than 1 ppm). The main piece of evidence for an earlier evolution of oxygenic photosynthesis comes from lipid biomarkers. Recent work, however, has shown that 2-methylhopanes, once thought to be unique biomarkers for cyanobacteria, are also produced anaerobically in significant quantities by at least two strains of anoxygenic phototrophs. Sterane biomarkers provide the strongest evidence for a date 2.7 Gyr ago or above, and could also be explained by the common evolutionary pattern of replacing anaerobic enzymes with oxygen-dependent ones. Although no anaerobic sterol synthesis pathway has been identified in the modern biosphere, enzymes that perform the necessary chemistry do exist. This analysis suggests that oxygenic photosynthesis could have evolved close in geological time to the Makganyene Snowball Earth Event and argues for a causal link between the two.
Keywords: Great Oxygenation Event, sterol biosynthesis, Makganyene Snowball Earth
1. Introduction
The debate about the history of atmospheric oxygen is most probably the longest-running and still unresolved controversy in the history of modern science. It began in the mid-nineteenth century with some of the earliest publications in biogeochemistry, when Ebelman (1845) and Bischof (1854), considering the balance of oxygen release by organic carbon and pyrite burial and oxygen consumption by iron and manganese oxidation, speculated that atmospheric oxygen levels might have changed over time with changes in biota (Berner & Maasch 1996). In 1856, Koene (1856/2004) argued, based on the presence of reduced matter in early rocks, that the Earth's initial atmosphere had been high in carbon dioxide and free of oxygen, and that over geological time the action of photosynthetic plants had resulted in a decline in carbon dioxide and a rise in oxygen. These early works apparently fell upon deaf ears, as few successor publications appeared until late in the nineteenth century (see Stevenson (1900), van Hise (1904) and Clarke (1924) for reviews). Koene's work had by this time vanished into obscurity; Stevenson (1900) knew of it only through a synopsis published in 1893–1894 by T. L. Phipson, one of Koene's former students.
Later, the Rhodesian geologist MacGregor (1927) complemented the chemical models with observations, both in the field and in the laboratory, of the Precambrian of southern Africa. He argued that the quantity of carbon buried in shales was sufficient to account for the quantity of oxygen in the atmosphere and therefore suggested that oxygen had accumulated over the course of the Earth's history. Consequently, he wrote, ‘the assumption [that the most ancient rocks were themselves formed under an atmosphere of oxygen] can only be justified by the clearest field evidence of contemporary oxidation in the most ancient rocks themselves’ (p. 158).
MacGregor turned to the rocks of Rhodesia and found evidence that the pre-Lomagundi metasediments of the Archaean Basement Schists, as he called them, were deposited under an oxygen-free atmosphere. In particular, he noted that (i) chemical analyses showed high ferrous/ferric ratios in Archaean sediments, (ii) the deposition of banded ironstones and their confinement to the Precambrian could be explained if they had been formed by the action of either iron-oxidizing bacteria or oxygen-producing organisms in an anaerobic ocean, and (iii) rounded pyrite-bearing clasts in Archaean conglomerates suggested fluvial transport without exposure to oxidizing conditions.
The post-World War II era saw the addition of more sophisticated chemical theory and measurements to the discussion. introduced considerations of equilibrium thermodynamics and discussed some of the earliest sulphur isotope evidence. Holland (1962) expanded Urey's calculations and considered, among other lines of evidence, the conditions necessary to oxidize uranium and prevent the deposition of detrital uraninite. By the mid-1960s, many of the arguments raised in modern discussions of the timing of the rise of oxygen were in place. In 1965, the National Academies of Science hosted one of the first symposia devoted to the issue (Cloud et al. 1965).
The past 42 years have seen numerous revolutions in geology and biology—from the molecular revolution in microbiology, to the development of theories of plate tectonics and abrupt climate change, to the recognition of the role of non-uniformitarian events in the Earth's history—as well as the development of at least one new technique, the study of mass-independent fractionation (MIF) of sulphur isotopes, that may directly reveal anoxia in the ancient atmosphere (). Combined with high-resolution data of critical intervals in the Earth's history from the ongoing continental drilling projects, these advances hold out the promise of a fourth stage in our understanding of the evolution of oxygenic photosynthesis and the rise of oxygen—one that can build a strong connection between the predictions of theory and specific geological observations.
In this paper, we first review a recent development that provides a solution to the long-standing ‘chicken-and-the-egg’ problem concerning the evolution of photosystem II (PSII): O2-evolving processes require O2-mediating enzymes to limit toxicity, while O2-mediating enzymes are unlikely to evolve without a source of O2. A similar problem arises from the fact that chlorophyll synthesis in green plants has three steps that require O2, which is produced by oxygenic photosynthesis. Next, we review the biomarker controversy, which arises from the apparent conflict between pieces of geological evidence for planetary anoxia contemporary with lipid biomarkers interpreted as suggesting the existence of oxygenic photosynthesis. Finally, we will summarize a simple and plausible sequence of events that, in our view, accounts for all of the biological and geological constraints on the geological history of oxygenic photosynthesis.
2. Glacial peroxides and the evolution of oxygen-mediating enzymes
The ability of dissolved O2 to accept an electron from a suitable donor (e.g. Fe+2) and form the superoxide radical, , is a severe threat to the organisms living in an aerobic environment. The superoxide radical reacts spontaneously with water to form the hydroxyl radical (·OH), which attacks the sugar/phosphate backbone of DNA. The need to control this toxic process fostered the evolution of O2-mediating enzymes, such as superoxide reductase () and superoxide dismutase (which convert to H2O2; Wolfe-Simon 2005), catalase (which converts H2O2 to water) and various oxygen-binding domains such as haem that help prevent superoxide formation (). Oxygen-mediating pathways had to evolve prior to the metabolic usage of O2, including the production of oxygen by the O2-evolving complex of PSII. Moreover, the O2-evolving complex appears to derive oxygen from the Mn cluster of catalase, also implying that the O2-mediating enzymes came first (; ).
But while there must be a source of superoxide radicals to act as an adaptive pressure for the evolution of oxygen-mediating pathways, most geological and geochemical processes in the Earth's atmosphere and hydrosphere are relatively reducing (figure 1). Volcanic gases are buffered by redox reactions on the reducing end of this spectrum, more reducing than the ferrous–ferric redox couplet and far from the highly oxidized water/oxygen couplet. In fact, photolytic reactions involving ultraviolet (UV) radiation and water vapour provide the only known abiotic pathway for producing biologically significant concentrations of molecular O2. However, the environments under which this process can happen are lethal to all living organisms, as the same UV radiation is destructive to complex organic molecules, including DNA.
Electron activity (pϵ) of typical redox couples in water at pH 7 and 25°C (adapted from ). Half-reactions on the left couple spontaneously with those below them on the right, and most pairs are suitable for driving biological metabolism. The primary donor of PSII (P680) has the largest redox potential change known for any organic molecule, and in its oxidative (rest) state, it is capable of oxidizing water, producing oxygen. The thick bracket signifies the range of gases emitted from volcanic eruptions. Subaerial eruptions tend to be more oxidizing than subaqueous eruptions, so the shift in eruptive style towards subaerial vulcanism noted by would have helped the eventual oxidation of the atmosphere, even though all volcanic gases are on the reducing side of the ferrous/ferric couple. Note that the term ‘oxidizing’ appears to have quite separate meanings in the geological and biological sciences. Redox couples near the quartz–fayalite–magnetite buffer in rocks are considered oxidizing by the Earth scientists, but are actually on the reducing side of the diagram and clearly faraway from the and O2 couples considered oxidizing by biologists.
One solution to this puzzle was provided by , who noted that Antarctic ice cores preserve interannual variations in H2O2 concentration which reflect the history of the Antarctic ozone hole (Frey et al. 2005, 2006). As ozone is the major filter for short-wavelength UV, reductions in stratospheric ozone facilitate photochemical reactions involving H2O which generate H2O2 and H2 gas. H2 diffuses away and is lost, whereas H2O2, with a freezing point near that of water, condenses and accumulates in the ice.
noted that the Late Archaean Pongola glaciation and Early Palaeoproterozoic glaciations occurred in atmospheres with little oxygen, lacking an ozone screen and bathed in UV radiation strong enough to produce the MIF observed in sulphur compounds (figure 2). During these glaciations, the same photochemical processes acting today in Antarctica would have acted over entire ice sheets, building up H2O2 concentrations. While similar photochemistry would occur in the liquid ocean, H2O2 produced there would diffuse away rather than accumulating. During a normal (non-Snowball) glaciation, peroxide-laced snow would follow normal glacial dynamics, being compressed into glacial ice, flowing in glaciers to the ocean and melting either there or along the wet-base portions. Upon melting, H2O2 would disproportionate into O2 and water (2H2O2→2H2O+O2), thereby producing an environment protected from lethal UV radiation but ‘poisoned’ with trace amounts of oxygen, in which oxygen-mediating enzymes might evolve. The small ‘whiffs of oxygen’ reported recently by and at two nearby localities on the palaeocontinent of Vaalbarra at ca 2.5 Gyr ago could be the geochemical fingerprint of oxic meltwaters mixing with glacial flour (powdered rock) at the base of a polar ice field. Although the oldest confirmed glacial unit in the Palaeoproterozoic occurs sometime after 2.45 Gyr ago (Evans 2000; Young et al. 2001), we do not know the full duration of the Late Archaean and Palaeoproterozoic glacial epochs, as the preservation of glacial deposits depends upon having continents in the correct position, having sufficient accommodation space and the fate of deposits over geological time. The Earth may well have experienced glacial ice-house conditions, akin to those of the Late Palaeozoic and Cenozoic, during the time gap between the preserved Pongola and Huronian glacial deposits.
The Earth's glacial history, with intervals of atmospheric anoxia indicated by the light shading (adapted from a cartoon of Lovelock 1979). Major Precambrian glaciations are indicated by ‘icicles’ dangling from a temperature versus time curve through the Earth's history, and include the Pongola of southern Africa at ca 2.9 Gyr ago, the Palaeoproterozoic glaciations between ca 2.5 and 2.22 Gyr ago (the three major units of the Huronian series in Laurentia and the Makganyene of southern Africa), the Cryogenian glaciations in the Neoproterozoic (ca 800–600 Myr ago) and the Permian and Neogene glaciations in the Phanerozoic. The UV–peroxide generating mechanism of is expected to operate on any ice sheet formed in an atmosphere without an ozone or other UV screen. The question mark indicates the possibility of earlier glacial intervals, not yet recognized.
3. Are Archaean lipid biomarkers contaminants?
As of 2004, at least 473 biochemical reactions were known in which molecular O2 was a substrate (Raymond & Blankenship 2004; ). Hence, one way to constrain the history of oxygen is to look in sediments for biomarkers that are the products of oxygen-dependent reactions. As with any geobiological tracer, one must verify that the materials being studied are of the same age as the rock; and as with many promising new techniques, the application of organic geochemistry to the Precambrian had a rocky, controversial start. During the 1960s and early 1970s, a flurry of studies reported traces of Precambrian porphyrins, fatty acids, alkanes, acyclic isoprenoids and even amino acids (Hayes et al. 1983; Brocks et al. 2003a). Follow-up studies revealed that virtually all the organic compounds were contaminants, usually petroleum-derived hydrocarbons from the laboratory and field environments. In response, and to their great credit, workers in the field evolved into their own most severe critics.
Despite these discouraging initial problems, great progress has been made in identifying the proper target molecules to study and in elucidating the diagenetic processes operating on them (Knoll et al. 2007). The lipid fraction from biological membranes is by far the best, as lipids can preserve the topology of their skeletal backbones despite various changes during diagenesis (such as the loss and replacement of hydroxyl groups, linear side-chain degradation and saturation of double bonds). Of particular importance as potential constraints on the history of oxygen are the degradation products of polycyclic isoprenoids, especially hopanols and sterols, which preserve enough of their structural information to identify the biochemical pathways through which they have formed. Although not unknown, production of the same isoprenoid compound through different pathways is rare. The Precambrian fossil records of hopane and sterols have been the primary focus of a series of papers (Brocks et al. , 2003a,b; Summons et al. , ). For background, we recommend two excellent reviews on sterol biosynthesis and the structure and function of the family of triterpene cyclase enzymes by and , and on the oxygen dependence for sterol synthesis in yeast by .
This body of work employs vastly superior techniques and internal controls for reproducibility than were used in the discredited studies of the 1960s. However, concerns about contamination have not evaporated, as Brocks et al. (2003b, p. 4322) noted: ‘The age of the molecules that contain the biologic information is not fully resolved. Hence, paleobiological interpretation… should be cited cautiously and with reference to the remaining uncertainty of syngeneity.’ Possible ‘red flags’ include the presence of characteristically Phanerozoic biomarkers (such as dinosteranes, generally associated with dinoflagellates, a Mesozoic–Cenozoic taxon), as well as the puzzling lack of any geological trace of intermediate evolutionary stages in the long sterol biosynthetic pathway. identified one extant planctomycete, Gemmata obscuriglobus, which has just such an intermediate sterol synthesis pathway and is possibly on the ancestral eukaryote lineage (), but it is unknown whether the pathway is an immature relict or has been truncated. Regardless, as Brocks (2005) noted, the incremental pattern of eukaryotic radiation observed in the Proterozoic fossil record ought to be matched by similar changes in lipid biomarkers, but instead Archaean sterols with a fully modern fingerprint materialize nearly a billion years before the first fossils with possible eukaryotic affinity. New analytical methods, such as the examination of hydrocarbons in fluid inclusions (Dutkiewicz et al. 2006; ), may soon resolve this contamination controversy.
4. Is O2 a fundamental requirement for hopane and sterol formation?
For organic biomarkers to provide strong constraints on palaeo-oxygen levels, it is important to verify that the fundamental chemistry involved requires oxygen, precluding any possibility of anaerobic mechanisms, and that the basic processes on which the inferences are based have not changed over geological time. Even if biosynthetic pathways leading to hopanol and sterol synthesis evolved during Hadean or Archaean time, we argue that recent work has weakened the case considerably linking them to the presence of oxygenic photosynthesis. Brocks et al. (2003b, p. 4331) summarized their four lines of molecular evidence for free O2 in the surface waters as follows.
1. The bitumens contain molecular fossils of bacteriohopanoids. Although hopanoid biosynthesis does not require oxygen, these lipids have never been isolated from strict anaerobes (Ourisson et al. 1987).
2. The Archaean shales also contain high relative concentrations of cyanobacterial 2-methylhopanes. These biomarkers are indirect evidence for oxygen release within the photic zone.
3. The side-chain degradation pattern of the 2-methylhopane series (Fig. 5) indicates oxic conditions during earliest diagenesis of cyanobacterial organic matter.
4. The bitumens contain molecular fossils of sterols. Sterol biosynthesis in extant eukaryotes requires dissolved oxygen in concentrations equivalent to 1% PAL (Jahnke and Klein 1983).
Recent work suggests that all of these can be explained by mechanisms that do not require free oxygen in the environment; each will be considered in turn.
(a) Bacteriohopanoids and 2-methylhopanes
As Brocks et al. (2003b) noted, the production of hopanoids does not involve O2, and subsequent work has identified multiple anaerobic bacteria that produce hopanoids. Fischer et al. (2005) found that at least one bacterium, Geobacter sulphurreducens, can synthesize diverse hopanoids (not including 2-methylhopanoids) when grown under strictly anaerobic conditions. Moreover, reported that copious quantities of 2-methylhopanoids are produced under anaerobic conditions mimicking those presumed to exist during Archaean time by Rhodopseudomonas palustris. As they state (p. 15 102), ‘… because 2-MeBHPs may be produced by organisms that do not engage in oxygenic photosynthesis and because their biosynthesis does not require molecular oxygen, 2-methylhopanes cannot be used as de facto evidence for oxygenic photosynthesis’.
(b) Side-chain degradation pattern
Brocks et al. (2003b, p. 4330) stated:
The side-chain degradation pattern of the C30 to C36 2-methylhopane homologous series relative to the corresponding C30 to C36 3-methylhopanes suggests that late Archaean cyanobacteria lived in an oxygenated micro-environment. The C30 to C36 2-methylhopane series has in all analyzed samples a characteristic even-over-odd carbon number predominance. The elevated abundance of the C32-homologue relative to C31 and C33 indicates oxidative side-chain cleavage of a bacteriohopanetetrol to a C33 carboxylic acid and subsequent decarboxylation under non-reducing conditions.
The even-over-odd carbon number predominance of 2-methylhopanes relative to 3-methylhopanes can also be explained by anaerobic processes. isolated an anaerobic sulphate-reducing bacterial strain from a petroleum-contaminated sediment that metabolizes straight-chain saturated alkanes with a similar two-carbon removal process. If grown on pure Ceven alkanes, their total fatty acids were predominantly even in carbon number, while if grown on Codd alkanes, their total fatty acids were predominantly odd in carbon number. Although this example is from the degradation of alkanes, similar chemistry ought to work for the degradation of isoprenoid side chains. While post-depositional degradation would affect side chains of the 2- and 3-methylhopane fractions equally, it is clear that anaerobic organisms have a variety of biochemical pathways involving the addition of small carbon compounds that can produce such even/odd patterns, the complexity and mechanisms of which have yet to be resolved (see and Berthe-Corti & Fetzner (2002) and references therein).
(c) Sterols
Sterols and their diagenetic derivatives, steranes, are potentially more relevant to the oxygen question. In yeast, the modern biosynthetic pathway to ergosterol (the simplest sterol required for strictly anaerobic growth; ) is a long chain of biochemical reactions that use the following three O2-dependent enzymes: (i) squalene monooxygenase, (ii) lanosterol 14-α-methyl demethylase cytochrome P450, and (iii) sterol-4-methyl oxidase (Risley 2002). Figure 3 shows the first biosynthetic steps of this pathway, from the linear squalene molecule to lanosterol. Molecular oxygen is first used by enzyme (i) to form an epoxide bridge between the C2 and C3 carbons in the linear squalene molecule, which is then cyclized and converted to lanosterol as shown, leaving the oxygen as an hydroxyl on C3. To convert lanosterol to ergosterol, enzymes (ii) and (iii) use oxygen to remove three methyl groups, two on C4 and one on C14.
Sterol biosynthetic pathway, with proposed modifications for anoxic operation. In eukaryotes, the oxygen-dependent steps include the conversion of squalene to squalene epoxide, and the removal of two methyl groups on the C4 carbon and one on the C14 carbon (indicated by faint circles). As indicated by the faint dotted circles, we suggest that an ancestral anaerobic eukaryote may have directly done the initial cyclization reaction on squalene rather than its epoxide, in the fashion of many bacterial squalene–hopane cyclase (SHC) enzymes that will cyclize either form. The C3 hydroxyl (if it was really there in Archaean time) could be added after the cyclization reaction as described in the text. Similarly, a variety of demethylation reactions are known from sulphate-reducing organisms that could, in principle, remove the three methyl groups from lanosterol to form ergosterol (not shown), which is the simplest sterol from which eukaryotes can grow anaerobically.
Owing to the inertness of the C–H bond in hydrocarbons and the resonance energy of aromatic compounds, it is often assumed that oxygen must be involved in the activation, rearrangement and metabolism of molecules like these. For demethylation, oxygen is used to introduce hydroxyl groups by replacing one of the carbon-bound hydrogens, after which other enzymes can proceed through a series of steps including dehydrogenation and eventual decarboxylation (see for a recent overview of the oxidative mechanism for enzyme (iii)). It was long assumed that a direct hydrogen atom transport (HAT) reaction, where an activated radical removed a stably bound H atom from a stable carbon substrate leaving a reactive intermediate, was impossible under anaerobic conditions (see Suflita et al. (2004) for an excellent review). As noted below, this is no longer the case.
recognized over 40 years ago that many anaerobic enzymes have been replaced by aerobic equivalents. Raymond & Blankenship (2004) discovered that, of the 473 reactions using O2 as a substrate, there were more than 80, in at least 20 metabolic pathways, for which there was a direct anaerobic-to-aerobic substitution. Hence, the substitution of an O2-dependent enzyme for an anaerobic one appears to be a common evolutionary occurrence. In at least one lipid pathway, that of haem/chlorophyll biosynthesis, 3 out of 17 steps were replaced on a one-to-one basis with oxygen-dependent enzymes; two of these are in the nine-step haem portion of the pathway. If three swaps happened in only 17 steps in the chlorophyll pathway, it is not unreasonable to suggest that four swaps might have happened in the more than 25-step sterol pathway.
Most enzymes involved in the sterol synthesis pathway are membrane bound. Rather than being handed directly from one enzyme to another, substrates passively diffuse within this membrane layer between enzymes, which do not need to interact directly with each other. Hence, from an evolutionary perspective, it is easy to modify intermediate steps within a long biosynthetic pathway of this sort without ‘rebuilding’ the entire chain from scratch. Squalene monooxygenase might have been a late addition to this pathway, and the cyclase enzyme could have evolved its specificity to 2,3-oxidosqualene sometime after the Great Oxygenation Event.
The question then focuses on whether or not there are plausible anaerobic substitutes for the steps catalysed by the oxygen-dependent enzymes (i), (ii) and (iii) listed above, particularly the HAT reactions. The answer appears to be yes, and the relevant discoveries were triggered from two surprising sources: economic geology and the discovery of anaerobic methane oxidation. Prior to 1990, it was thought that aerobic processes were the major cause of petroleum degradation in nature, yet it has since become clear from the study of deep oil reservoirs that extensive biological degradation occurred when sulphate-bearing (but anoxic) waters made contact with the hydrocarbons. A variety of sulphate-reducing bacteria were eventually implicated (Aeckersberg et al. 1991; ; ), and a series of previously unknown but powerful biochemical pathways were discovered (; ). Despite the stability and inertness of C–H and C–C bonds, these bacteria are able to perform all structural rearrangements needed to oxidize saturated and aromatic hydrocarbons to CO2 anaerobically, including demethylation. Although, owing to environmental and health concerns, the major biochemical work has been focused on the aromatic hydrocarbons such as benzene, fully saturated hydrocarbons such as alkanes and isoprenoids such as pristane are also degraded and altered (Bonin et al. 2004; Suflita et al. 2004). A comparison of relevant bond energies for the first hydrogen atom extraction in the radical formation process RH⇒R·+H· is illuminating; removal of an H atom from benzene requires approximately 113 kcal mol−1, compared with only 105 and 101 kcal mol−1 from methane and ethane, respectively. The example of anaerobic methane oxidation, done by an archaeon working in concert with a sulphate-reducing bacterium (Orphan et al. , ), shows that these direct HAT reactions are indeed possible even without the participation of electrons in adjacent π orbitals, as may happen in aromatic compounds.
We will consider possible anaerobic precursors for the squalene monooxygenase (i) and demethylation (ii) and (iii) enzymatic steps separately.
(d) Squalene monooxygenase
Brocks et al. (2003b) referred to the kinetic work of on the squalene monooxygenase from the yeast Saccharomyces cerevisiae to argue that O2 concentrations equivalent to 1% of the present atmospheric level would have been required for Archaean sterol synthesis; these levels would either demand the presence of photosynthetic oxygen or operate at the bottom of a melting, peroxide-rich ice sheet noted earlier.
Enzymatic Activation Of Alkanes Constraints And Prospects Ukulele
However, the evidence permits alternative interpretations. First, the putative Archaean steranes do not have a hydroxyl group on the C3 carbon, which is the oxygen fingerprint of the squalene monooxygenase enzyme. Diagenesis would have removed the hydroxyl group had it originally been present, but without it there is no physical evidence that the squalene monooxygenase enzyme was ever involved in the formation of the particular sterol biomarkers in question. Although a clever organic geochemist might be able to infer indirectly whether or not a functional group was once present on the C3 carbon, to our knowledge no evidence of this sort has yet been reported. Using the O2 constraint for squalene monooxygenase as a palaeoenvironmental indicator is an example of extrapolating the modern biosphere back ca 3 Gyr, and must be done with caution.
Nevertheless, it is worth considering the possibility that the Archaean steranes might have originally been sterols, and to determine if there are plausible anaerobic alternatives to get the hydroxyl on the C3 carbon. As noted by Fischer & Pearson (2007), it is not possible to hydroxylate the linear squalene molecule prior to cyclization and have the OH group wind up on the proper carbon, so an anaerobic hydroxylation step of this sort most probably came after the cyclization reaction. Conversion to the present epoxide-based sterol synthesis pathway would have happened after O2 became abundant by an enzyme swap. This scenario is made more plausible by the fact that the oxidosqualene cyclase in eukaryotes evolved from a larger class of less substrate-specific bacterial squalene–hopane cyclase enzymes (; Fischer & Pearson 2007). Squalene cyclization reactions are highly exothermic, and the presence or absence of the oxygen in the epoxide moiety has little to do with the reaction kinetics (; Fischer & Pearson 2007). It also appears that the resulting stereochemical conformation of the product is controlled by how a methyl group on C8 is positioned (Fischer & Pearson 2007), not by the configuration of the epoxide moiety as was once thought. These bacterial enzymes will even today cyclize either squalene or 2,3-oxidosqualene, whereas the more specific eukaryotic version accepts only the 2,3-oxidosqualene.
For this scenario to work, there must have been an anaerobic hydroxylase capable of removing a C3 hydrogen from the sterene and replacing it with hydroxyl. There are several possibilities for this. As noted above, this could be done by one of the direct sulphate-driven hydrogen atom extraction processes (RH⇒R·+H·; Bonin et al. 2004; Suflita et al. 2004). Extraction energies for this would be well below that required for benzene (approx. 113 kcal mol−1), a reaction that does proceed anaerobically. Alternatively, purified and characterized a molybdenum/iron–sulphur/haem enzyme, ethylbenzene dehydrogenase, that catalysed the anaerobic hydroxylation of the –CH2– of ethylbenzene. The reaction puts an oxygen atom from water onto the substrate as a hydroxyl while reducing an electron acceptor. Kniemeyer and Heider reported similar activity for other related substrates, indicating that this is simply the first of a potentially large family of dehydrogenase enzymes that can activate otherwise stable hydrocarbons without the use of O2.
(e) Demethylation steps (enzymatic reactions (ii) and (iii) above)
To understand the mechanism for the oxidative removal of methyl groups, developed a micelle-based system for characterizing the enzymological properties of the yeast sterol-C4-methyl-oxidase (enzyme (iii)), and using various mutants and information from the genome were able to confirm the reaction scheme. O2, NADH and cytochrome b5 are used in the first step, converting the C4-α-methyl groups into the alcohol, –CH2OH, which they were able to isolate from the micelles as a stable intermediary product. The same enzyme then catalyses the removal of two more hydrogens to form the acetate, –COOH, presumably by the addition of the remaining oxygen not used in the first step. The subsequent removal of the carbon by the enzyme 4αCD does not require oxygen, but it does involve the temporary conversion of the hydroxyl on C3 mentioned earlier into a ketone, making its presence essential for this particular O2-dependent demethylation pathway. In yeast, this same enzyme removes both the C4 methyl groups.
In contrast, reviewed some completely anaerobic pathways for demethylation in sulphate- and nitrate-reducing bacteria. The best characterized of these is for the metabolism of toluene, which involves the initial addition of a fumarate co-substrate to the methyl group. A novel glycyl radical enzyme, (R)-benzylsuccinate synthase, removes a hydrogen from the methyl group and adds the fumarate to the methyl radical to yield benzylsuccinate. (This is the difficult activation step that avoids the use of molecular oxygen.) Following this, the benzylsuccinate is converted to benzoyl CoA and succinate via β-oxidation, and the succinate is recycled to fumarate via another enzyme, succinate dehydrogenase. As Boll et al. noted, ‘Thus, the overall pathway affords an oxygen-independent six electron oxidation of the methyl group of toluene to the carbonyl-CoA group of benzoyl-CoA.’ These are representatives of a family of previously unknown enzymes that can activate internal carbons by the addition of products that can be oxidized by conventional β-oxidation pathways, sidestepping the need for activation by molecular oxygen (). They also observed further that variants of the fumarate pathway appear to be ‘everywhere’ in the anaerobic metabolism of hydrocarbons in a large variety of sulphate- and nitrate-reducing organisms. Hence, a similar mechanism tailored to the removal of the C4 (and C14) methyl groups of lanosterol is not unreasonable, and in addition would not need an adjacent hydroxyl group on C3 to proceed, which the oxidative pathway of sterol-C4-methyl-oxidase (enzyme (iii) above) apparently does require. As the complete genome sequences are known for several of these sulphate-reducing organisms as well as the yeast, S. cerevisiae, it may be possible to recreate completely anaerobic sterol biosynthesis by reintroducing these fumarate pathways into yeast or other model eukaryotes.
An additional category of powerful anaerobic enzymes capable of doing structural rearrangements on hydrocarbon skeletons includes the radical S-adenosylmethionine (SAM) reactions (; see figure S1 in the electronic supplementary material). This superfamily of enzymes can produce SAM radicals energetic enough to catalyse the removal of a single hydrogen atom from a carbon backbone. In all these enzymes studied to date, the reaction mechanism appears to proceed by causing an exposed iron in a [4Fe–4S] cluster to move from tetrahedral-like coordination to the octahedral geometry. In the process, the sulphur atom bound to the adenosylmethionine moiety is released in a highly activated form, and can be focused enzymatically to perform dehydrogenation, demethylation or other structural rearrangements. Oxygen, however, poisons the Fe–S clusters by oxidizing the iron atoms in all these enzymes.
In summary, although enzymes with the proper protein scaffolding have not yet been discovered, the biosphere has preserved the chemistry necessary to perform anaerobic sterol synthesis. The absence of the enzymes themselves is unsurprising given the rapidity with which the scaffolding portion of enzymes evolve compared with the functional groups and the selection pressure to swap entire enzymes for ones that are O2 tolerant. Hence, we remain sceptical that the removal of these methyl groups can be used as a firm constraint on the existence of abundant oxygen supplies in the Archaean oceans.
5. When does geological evidence demand the presence of oxygenic photosynthesis?
The question of when oxygenic photosynthesis and environmental free oxygen first arose can be addressed from two directions. One could, on the grounds of uniformitarianism, assume that the Earth's system processes resembled modern processes as far back as the slightest bit of geological evidence suggests a kinship; one could assume that any oxidation or biochemical process that involves oxygen today has always involved oxygen, and any evidence of such a process is therefore evidence for oxygenic photosynthesis. This has been done many times (e.g. Rosing & Frei 2004), and a great deal of elegant intellectual effort has been spent trying to explain how oxidation of the Earth's surface could be delayed many hundreds of million years after the advent of oxygenic photosynthesis (e.g. Canfield 2005; Catling & Claire 2005; Fennel et al. 2005).
Alternatively, following MacGregor (1927), one could take a sceptical approach and, working forward from the Earth's origin, question the essence of oxygenic photosynthesis until the rock record permits no alternative explanation. As with all dichotomies, the most correct answer probably lies in-between. The sceptical approach is a fruitful one for identifying the limits of current scientific understanding and generating new hypotheses to direct testing of those limits, but a strictly sceptical approach would render it nearly impossible to investigate many historically contingent events.
Our sceptical hypothesis to explain the Great Oxygenation Event (figure 4) rejects, for the reasons discussed above, the uniformitarian interpretation of the biomarker record and accepts the direct geological indicators of the redox transition (using the redox tower indicated in figure 1). In the Archaean, the presence of detrital uraninite, detrital pyrite and sulphur MIF argue for a dominantly anaerobic surficial environment, kept clement by a combination of greenhouse gases including H2O, CO2, CH4 and perhaps SO2. The redox levels present in the environment were governed by reducing gases such as CH4, sulphate produced by photo-oxidation (Farquhar et al. 2001) and microbial disproportionation () of S0, volcanogenic SO2 and ferric iron produced by anaerobic iron-oxidizing photosynthetic bacteria (; Widdel et al. 1993; Kappler et al. 2005).
History of the Earth's oxygenation, showing important constraints described in the text. Adapted from Kirschvink & Weiss (2002). Question marks indicate uncertainty in the relative levels of oxygen present.
During glacial intervals, such as the Pongola at ca 2.9 Gyr ago and the Huronian at ca 2.5–2.3 Gyr ago, H2O2 would accumulate in ice and be released in the basal meltwaters. O2 released by H2O2 disproportionation could then act as a local agent for oxic poisoning and drive the evolution of oxygen-mediating enzymes. Observed decreases in sulphur MIF associated with these glacial intervals may be the products of oceanographic changes. Both enhanced physical mixing, associated with the larger pole–equator thermal gradient present in ice-house conditions (), and the presence of more oxidizing electron acceptors, associated with the glacial peroxide source, would lead to greater mixing of oxidized and reduced sulphur reservoirs and thus diminish the MIF preserved in the sedimentary record.
Constraints And Prospects Of Yam Production In Nigeria
Several redox-sensitive trace elements have been used in an attempt to constrain palaeoredox conditions, notably U, Mo and Re (e.g. Rosing & Frei 2004; ). Uranium is soluble in its oxidized form and insoluble in its reduced form, so evidence for uranium mobility has been taken as evidence for oxidizing conditions. The redox potential of the U(VI)/U(VIII) couplet is, however, comparable to that of the ferrous/ferric couplet, and so does not provide much additional constraint on oxygen levels. Mo(IV) similarly forms insoluble sulphide minerals in its reduced form and is soluble in its oxidized Mo(VI) state. Owing to the stability of MoS2, increased Mo input through oxidative weathering provides a better constraint on redox conditions, but the redox potential of the MoS2/Mo(VI) couplet is still below that of other redox-sensitive metals, such as Mn(II)/Mn(IV), below that of the P870 photosystem complex of purple bacteria, and well below that of H2O/O2 (see table S1 in the electronic supplementary material).
The oldest evidence for an environment containing massive amounts of free molecular oxygen of which we are aware is the ca 2.22 Gyr ago Kalahari Manganese Member of the Hotazel Formation, Transvaal Supergroup, South Africa (Cairncross et al. 1997). As indicated in figure 1 and in the electronic supplementary material, table S1, nitrate and molecular O2 are the only environmentally significant oxidants capable of converting soluble Mn2+ into insoluble Mn+4, and nitrate itself requires oxygen to form in the modern oceans. Hence, the deposition of the BIF-hosted manganese in this unit is a firm oxygen constraint. The Hotazel Formation was also deposited in the aftermath of the Makganyene Snowball Earth Event (; ), the only confirmed low-latitude glaciation in the Palaeoproterozoic (Evans et al. 1997; Hilburn et al. 2005).
It is hard to imagine a more dramatic step in biochemical evolution than the final tinkering with the manganese–calcium cluster in the oxygen-evolving complex of PSII that led to the splitting of water and the release of molecular O2. Novel innovations that confer large selective advantages to a species lead to their rapid, exponential growth, and the creation and dominance of ecological niches. Such a profound evolutionary innovation ought to have left a clearly legible mark on the planet. Two dramatic events punctuate the Earth's history in the critical period ca 2.3 Gyr ago, when our sceptical interpretation suggests the first appearance of oxygenic photosynthesis: the Makganyene Snowball Earth and the deposition of the Kalahari Manganese Field. As we have argued elsewhere (), the simplest explanation of these occurrences is that the sudden release of oxygen from the evolution and radiation of oxygenic phototrophs destroyed reduced greenhouse gases such as methane and initiated Snowball conditions.
An additional implication of this scenario is that the formation of glacial peroxides is an essential step in the development of oxygen-mediating enzymes, and ultimately oxygenic photosynthesis. The Earth-like planets that orbit too close to their parent star for ice to form are therefore unlikely to evolve the aerobic metabolism essential for animal life.
6. Conclusions
As noted by , production and accumulation of hydrogen peroxide on the surface of polar ice caps provides a straightforward mechanism for generating trace but substantial concentrations of H2O2 and free O2 in the ocean below melting glaciers, away from the lethal influence of UV radiation. This environment is capable of driving the evolution of oxygen-mediating enzymes, and paving the way for the evolution of the oxygen-evolving cluster of PSII.
We find no chemical requirement for molecular oxygen in the biosynthesis of the lipid biomarkers of presumed Archaean age, which cannot be met by known anaerobic biochemical mechanisms. As the anaerobic enzymes that perform analogous chemical steps to oxygen-requiring ones use redox-sensitive metal cofactors that are poisoned by oxygen (), there is intense evolutionary pressure to swap them out for those that are not oxygen sensitive. As three such substitutions in 17 enzymatic steps are documented in the biosynthesis of chlorophyll, we argue that four substitutions in a more than 25-step sterol synthesis pathway are not unreasonable.
The evolution of the oxygen-releasing complex of PSII as measured by environmental redox indicators is correlated with the Makganyene Snowball Earth and the deposition of the Kalahari Manganese Field. We suggest that the destruction of reduced greenhouse gases such as methane () did not take 400 Myr, but was rapid enough to trigger the Makganyene Snowball (e.g. ).
Acknowledgments
We thank John Abelson, Mel Simon, John H. Richards, Ann Pearson, Woodward Fischer, Roger Summons, Jochen Brocks, Timothy D. Raub and David Fike for their helpful discussions. They bear no responsibility for the opinions expressed herein, but have helped us greatly to strengthen our arguments. The Agouron Institute, the NASA Astrobiology and Exobiology Programs and the NSF Graduate Fellowship Programs provided support during the development of these ideas. We thank Jim O'Donnell and Tony Diaz of the Caltech Geology Library for helping us to track down some of the now-obscure early references we discuss.
Footnotes
One contribution of 15 to a Discussion Meeting Issue ‘Photosynthetic and atmospheric evolution’.
Supplementary Material
Kirschvink & Kopp supplemental information:Table S1 and figure S1
Questions and answer from the discussion:
References
- Aeckersberg F, Bak F, Widdel F. Anaerobic oxidation of saturated-hydrocarbons to CO2 by a new type of sulfate-reducing bacterium. Arch. Microbiol. 1991;156:5–14. doi:10.1007/BF00418180[Google Scholar]
- Aeckersberg F, Rainey F.A, Widdel F. Growth, natural relationships, cellular fatty acids and metabolic adaptation of sulfate-reducing bacteria that utilize long-chain alkanes under anoxic conditions. Arch. Microbiol. 1998;170:361–369. doi:10.1007/s002030050654 [PubMed] [Google Scholar]
- Aitken C.M, Jones D.M, Larter S.R. Anaerobic hydrocarbon biodegradation in deep subsurface oil reservoirs. Nature. 2004;431:291–294. doi:10.1038/nature02922 [PubMed] [Google Scholar]
- Anbar A.D, et al. A whiff of oxygen before the Great Oxidation Event? Science. 2007;317:1903–1906. doi:10.1126/science.1140325 [PubMed] [Google Scholar]
- Andreasen A.A, Stier T.J.B. Anaerobic nutrition of Saccharomyces cerevisiae. I. Ergosterol requirement for growth in a defined medium. J. Cell. Comp. Physiol. 1953;41:23–36. doi:10.1002/jcp.1030410103 [PubMed] [Google Scholar]
- Berner R.A, Maasch K.A. Chemical weathering and controls on atmospheric O2 and CO2: fundamental principles were enunciated by J. J. Ebelman in 1845. Geochim. Cosmochim. Acta. 1996;60:1633–1637. doi:10.1016/0016-7037(96)00104-4[Google Scholar]
- Berthe-Corti L, Fetzner S. Bacterial metabolism of n-alkanes and ammonia under oxic, suboxic and anoxic conditions. Acta Biotechnol. 2002;22:299–336. doi:10.1002/1521-3846(200207)22:3/4<299::AID-ABIO299>3.0.CO;2-F[Google Scholar]
- Bischof K.G. Cavendish Society; London, UK: 1854. Elements of chemical and physical geology. [Google Scholar]
- Blankenship R.E, Hartman H. The origin and evolution of oxygenic photosynthesis. Trends Biochem. Sci. 1998;23:94–97. doi:10.1016/S0968-0004(98)01186-4 [PubMed] [Google Scholar]
- Boll M, Fuchs G, Heider J. Anaerobic oxidation of aromatic compounds and hydrocarbons. Curr. Opin. Chem. Biol. 2002;6:604–611. doi:10.1016/S1367-5931(02)00375-7 [PubMed] [Google Scholar]
- Bonin P, Cravo-Laureau C, Michotey V, Hirschler-Rea A. The anaerobic hydrocarbon biodegrading bacteria: an overview. Ophelia. 2004;58:243–254.[Google Scholar]
- Brocks J.J. The evolution of steroids and eukaryotes in the proterozoic. Eos Trans. AGU. 2005;86:B12A-5.[Google Scholar]
- Brocks J.J, Logan G.A, Buick R, Summons R.E. Archean molecular fossils and the early rise of eukaryotes. Science. 1999;285:1033–1036. doi:10.1126/science.285.5430.1033 [PubMed] [Google Scholar]
- Brocks J.J, Buick R, Logan G.A, Summons R.E. Composition and syngeneity of molecular fossils from the 2.78 to 2.45 billion-year-old Mount Bruce Supergroup, Pilbara Craton, Western Australia. Geochim. Cosmochim. Acta. 2003a;67:4289–4319. doi:10.1016/S0016-7037(03)00208-4[Google Scholar]
- Brocks J.J, Buick R, Summons R.E, Logan G.A. A reconstruction of Archean biological diversity based on molecular fossils from the 2.78 to 2.45 billion-year-old Mount Bruce Supergroup, Hamersley Basin, Western Australia. Geochim. Cosmochim. Acta. 2003b;67:4321–4335. doi:10.1016/S0016-7037(03)00209-6[Google Scholar]
- Cairncross B, Beukes N.J, Gutzmer J. Associated Ore & Metal Corporation, Ltd; Johannesburg, Republic of South Africa: 1997. The manganese adventure: the South African manganese fields. [Google Scholar]
- Canfield D.E. The early history of atmospheric oxygen: homage to Robert M. Garrels. Annu. Rev. Earth Planet. Sci. 2005;33:1–36. doi:10.1146/annurev.earth.33.092203.122711[Google Scholar]
- Catling D.C, Claire M.W. How Earth's atmosphere evolved to an oxic state: a status report. Earth Planet. Sci. Lett. 2005;237:1–20. doi:10.1016/j.epsl.2005.06.013[Google Scholar]
- Clarke, F. W. 1924 The data of geochemistry Bulletin 770. Washington, DC: US Geological Survey.
- Cloud P.E, Holland H.D, Commoner B, Davidson C.F, Fischer A.G, Berkner L.V, Marshall L.C. Symposium on the evolution of the Earth's atmosphere. Proc. Natl Acad. Sci. USA. 1965;53:1169–1226. doi:10.1073/pnas.53.6.1169[Google Scholar]
- Darnet S, Rahier A. Enzymological properties of sterol-C4-methyl-oxidase of yeast sterol biosynthesis. Biochim. Biophys. Acta Mol. Cell Biol. Lipids. 2003;1633:106–117. [PubMed] [Google Scholar]
- Dutkiewicz A, Volk H, George S.C, Ridley J, Buick R. Biomarkers from Huronian oil-bearing fluid inclusions: an uncontaminated record of life before the Great Oxidation Event. Geology. 2006;34:437–440. doi:10.1130/G22360.1[Google Scholar]
- Ebelman J.J. Sur les produits de la décomposition des especes minérales de la familie des silicates. Ann. Mines. 1845;7:3–66.[Google Scholar]
- Evans D.A.D. Stratigraphic, geochronological, and paleomagnetic constraints upon the Neoproterozoic climatic paradox. Am. J. Sci. 2000;300:347–433. doi:10.2475/ajs.300.5.347[Google Scholar]
- Evans D.A, Beukes N.J, Kirschvink J.L. Low-latitude glaciation in the Paleoproterozoic. Nature. 1997;386:262–266. doi:10.1038/386262a0[Google Scholar]
- Farquhar J, Bao H, Thiemens M. Atmospheric influence of Earth's earliest sulfur cycle. Science. 2000;289:756–758. doi:10.1126/science.289.5480.756 [PubMed] [Google Scholar]
- Farquhar J, Savarino J, Airieau S, Thiemens M.H. Observation of wavelength-sensitive mass-independent sulfur isotope effects during SO2 photolysis: implications for the early atmosphere. J. Geophys. Res. 2001;106:32 829–32 839. doi:10.1029/2000JE001437[Google Scholar]
- Fennel K, Follows M, Falkowski P.G. The co-evolution of the nitrogen, carbon and oxygen cycles in the Proterozoic ocean. Am. J. Sci. 2005;305:526–545. doi:10.2475/ajs.305.6-8.526[Google Scholar]
- Fischer W.W, Pearson A. Hypotheses for the origin and early evolution of triterpenoid cyclases. Geobiology. 2007;5:19–34.[Google Scholar]
- Fischer W.W, Summons R.E, Pearson A. Targeted genomic detection of biosynthetic pathways: anaerobic production of hopanoid biomarkers by a common sedimentary microbe. Geobiology. 2005;3:33–40. doi:10.1111/j.1472-4669.2005.00041.x[Google Scholar]
- Frey M.M, Stewart R.W, McConnell J.R, Bales R.C. Atmospheric hydroperoxides in West Antarctica: links to stratospheric ozone and atmospheric oxidation capacity. J. Geophys. Res. 2005;110:D23301. doi:10.1029/2005JD006110[Google Scholar]
- Frey M.M, Bales R.C, McConnell J.R. Climate sensitivity of the century-scale hydrogen peroxide (H2O2) record preserved in 23 ice cores from West Antarctica. J. Geophys. Res. 2006;111:D21301. doi:10.1029/2005JD006816[Google Scholar]
- Fuerst J. Intracellular compartmentation in planctomycetes. Annu. Rev. Microbiol. 2005;59:299–328. doi:10.1146/annurev.micro.59.030804.121258 [PubMed] [Google Scholar]
- Gaidos E.J, Nealson K.H, Kirschvink J.L. Biogeochemistry—life in ice-covered oceans. Science. 1999;284:1631–1633. doi:10.1126/science.284.5420.1631 [PubMed] [Google Scholar]
- Goldfine H. Evolution of oxygen as a biosynthetic reagent. J. Gen. Physiol. 1965;49:253–268. doi:10.1085/jgp.49.1.253[PMC free article] [PubMed] [Google Scholar]
- Hayes, J. M., Kaplan, I. R. & Wedeking, K. W. 1983 Chemical fossils in Precambrian sediments. In Earth's earliest biosphere, its origin and evolution (ed. J. W. Schopf), ch. 5-3-1, pp. 100–101. Princeton, NJ: Princeton University Press.
- Hilburn I.A, Kirschvink J.L, Tajika E, Tada R, Hamano Y, Yamamoto S. A negative fold test on the Lorrain Formation of the Huronian Supergroup: uncertainty on the paleolatitude of the Paleoproterozoic Gowganda glaciation and implications for the Great Oxygenation Event. Earth Planet. Sci. Lett. 2005;232:315–332. doi:10.1016/j.epsl.2004.11.025[Google Scholar]
- Holland H.D. Model for the evolution of the Earth's atmosphere. In: Engel A.E.J, James H.I, Leonard B.F, editors. Petrologic studies: a volume in honor of A.F. Buddington. Geological Society of America; Boulder, CO: 1962. pp. 447–477. [Google Scholar]
- Imlay J.A. Iron–sulphur clusters and the problem with oxygen. Mol. Microbiol. 2006;59:1073–1082. doi:10.1111/j.1365-2958.2006.05028.x [PubMed] [Google Scholar]
- Jahnke L.L, Klein H.P. Oxygen requirements for formation and activity of the squalene epoxidase in Saccharomyces cerevisiae. J. Bacteriol. 1983;155:488–492.[PMC free article] [PubMed] [Google Scholar]
- Jenney F.E, Verhagen M, Cui X.Y, Adams M.W.W. Anaerobic microbes: oxygen detoxification without superoxide dismutase. Science. 1999;286:306–309. doi:10.1126/science.286.5438.306 [PubMed] [Google Scholar]
- Kappler A, Pasquero C, Konhauser K.O, Newman D.K. Deposition of banded iron formations by anoxygenic phototrophic Fe(II)-oxidizing bacteria. Geology. 2005;33:865–868. doi:10.1130/G21658.1[Google Scholar]
- Kaufman A.J, et al. Late Archean biospheric oxygenation and atmospheric evolution. Science. 2007;317:1900–1903. doi:10.1126/science.1138700 [PubMed] [Google Scholar]
- Kirschvink J.L, Weiss B.P. Mars, panspermia, and the origin of life: where did it all begin? Palaeontol. Electron. 2002;4:8. editorial 2. [Google Scholar]
- Kirschvink J.L, Gaidos E.J, Bertani L.E, Beukes N.J, Gutzmer J, Maepa L.N, Steinberger R.E. Paleoproterozoic Snowball Earth: extreme climatic and geochemical global change and its biological consequences. Proc. Natl Acad. Sci. USA. 2000;97:1400–1405. doi:10.1073/pnas.97.4.1400[PMC free article] [PubMed] [Google Scholar]
- Kniemeyer O, Heider J. Ethylbenzene dehydrogenase, a novel hydrocarbon-oxidizing molybdenum/iron–sulfur/heme enzyme. J. Biol. Chem. 2001;276:21 381–21 386. doi:10.1074/jbc.M101679200 [PubMed] [Google Scholar]
- Knoll A.H, Summons R.E, Waldbauer J.E. The geological succession of primary producers in the oceans. In: Falkowski P.G, Knoll A.H, editors. Evolution of primary producers in the sea. Elsevier; Amsterdam, The Netherlands: 2007. pp. 133–163. [Google Scholar]
- Koene C.J. Mellen Press; New York, NY: 1856/2004. An English translation of the chemical constitution of the atmosphere from Earth's origin to the present, and its implications for protection of industry and ensuring environmental quality. [Google Scholar]
- Kopp R.E, Kirschvink J.L, Hilburn I.A, Nash C.Z. The Paleoproterozoic Snowball Earth: a climate disaster triggered by the evolution of oxygenic photosynthesis. Proc. Natl Acad. Sci. USA. 2005;102:11 131–11 136. doi:10.1073/pnas.0504878102[PMC free article] [PubMed] [Google Scholar]
- Kump L.R, Barley M.E. Increased subaerial volcanism and the rise of atmospheric oxygen 2.5 billion years ago. Nature. 2007;448:1033–1036. doi:10.1038/nature06058 [PubMed] [Google Scholar]
- Lesburg C.A, Caruthers J.M, Paschall C.M, Christianson D.W. Managing and manipulating carbocations in biology: terpenoid cyclase structure and mechanism. Curr. Opin. Struct. Biol. 1998;8:695–703. doi:10.1016/S0959-440X(98)80088-2 [PubMed] [Google Scholar]
- Liang M.C, Hartman H, Kopp R.E, Kirschvink J.L, Yung Y.L. Production of hydrogen peroxide in the atmosphere of a Snowball Earth and the origin of oxygenic photosynthesis. Proc. Natl Acad. Sci. USA. 2006;103:18 896–18 899. doi:10.1073/pnas.0608839103[PMC free article] [PubMed] [Google Scholar]
- Lovelock J.E. Oxford University Press; Oxford, UK; New York, NY: 1979. Gaia, a new look at life on Earth. [Google Scholar]
- MacGregor A.M. The problem of the Precambrian atmosphere. S. Afr. J. Sci. 1927;24:155–172.[Google Scholar]
- McKay C.P, Hartman H. Hydrogen-peroxide and the evolution of oxygenic photosynthesis. Orig. Life Evol. Biosph. 1991;21:157–163. doi:10.1007/BF01809444 [PubMed] [Google Scholar]
- Niyogi K.K. Photoprotection revisited: genetic and molecular approaches. Annu. Rev. Plant Physiol. 1999;50:333–359. doi:10.1146/annurev.arplant.50.1.333 [PubMed] [Google Scholar]
- Orphan V.J, House C.H, Hinrichs K.U, McKeegan K.D, DeLong E.F. Methane-consuming archaea revealed by directly coupled isotopic and phylogenetic analysis. Science. 2001;293:484–487. doi:10.1126/science.1061338 [PubMed] [Google Scholar]
- Orphan V.J, House C.H, Hinrichs K.U, McKeegan K.D, DeLong E.F. Multiple archaeal groups mediate methane oxidation in anoxic cold seep sediments. Proc. Natl Acad. Sci. USA. 2002;99:7663–7668. doi:10.1073/pnas.072210299[PMC free article] [PubMed] [Google Scholar]
- Pavlov A.A, Kasting J.F, Brown L.L, Rages K.A, Freedman R. Greenhouse warming by CH4 in the atmosphere of early Earth. J. Geophys. Res. 2000;105:11 981–11 990. doi:10.1029/1999JE001134 [PubMed] [Google Scholar]
- Pearson A, Budin M, Brocks J.J. Phylogenetic and biochemical evidence for sterol synthesis in the bacterium Gemmata obscuriglobus. Proc. Natl Acad. Sci. USA. 2003;100:15 352–15 357. doi:10.1073/pnas.2536559100[PMC free article] [PubMed] [Google Scholar]
- Philippot P, Van Zuilen M, Lepot K, Thomazo C, Farquhar J, Van Kranendonk M.J. Early Archaean microorganisms preferred elemental sulfur, not sulfate. Science. 2007;317:1534–1537. doi:10.1126/science.1145861 [PubMed] [Google Scholar]
- Rashby S.E, Sessions A.L, Summons R.E, Newman D.K. Biosynthesis of 2-methylbacteriohopanepolyols by an anoxygenic phototroph. Proc. Natl Acad. Sci. USA. 2007;105:15 099–15 104. doi:10.1073/pnas.0704912104[PMC free article] [PubMed] [Google Scholar]
- Raymond J, Blankenship R.E. Biosynthetic pathways, gene replacement, and the antiquity of life. Geobiology. 2004;2:199–203. doi:10.1111/j.1472-4677.2004.00037.x[Google Scholar]
- Raymond J, Segre D. The effect of oxygen on biochemical networks and the evolution of complex life. Science. 2006;311:1764–1767. doi:10.1126/science.1118439 [PubMed] [Google Scholar]
- Risley J.M. Cholesterol biosynthesis: lanosterol to cholesterol. J. Chem. Educ. 2002;79:377–384.[Google Scholar]
- Rosenfeld E, Beauvoit B. Role of the non-respiratory pathways in the utilization of molecular oxygen by Saccharomyces cerevisiae. Yeast. 2003;20:1115–1144. doi:10.1002/yea.1026 [PubMed] [Google Scholar]
- Rosing M.T, Frei R. U-rich Archaean sea-floor sediments from Greenland—indications of >3700 Ma oxygenic photosynthesis. Earth Planet. Sci. Lett. 2004;217:237–244. doi:10.1016/S0012-821X(03)00609-5[3700 Ma+oxygenic+photosynthesis&author=M.T+Rosing&author=R+Frei&volume=217&publication_year=2004&pages=237-244&' target='pmc_ext' ref='reftype=other&article-id=2606766&issue-id=174759&journal-id=136&FROM=Article%7CCitationRef&TO=Content%20Provider%7CLink%7CGoogle%20Scholar'>Google Scholar]
- Rueter P, Rabus R, Wilkes H, Aeckersberg F, Rainey F.A, Jannasch H.W, Widdel F. Anaerobic oxidation of hydrocarbons in crude-oil by new types of sulfate-reducing bacteria. Nature. 1994;372:455–458. doi:10.1038/372455a0 [PubMed] [Google Scholar]
- So C.M, Young L.Y. Initial reactions in anaerobic alkane degradation by a sulfate reducer, strain AK-01. Appl. Environ. Microbiol. 1999;65:5532–5540.[PMC free article] [PubMed] [Google Scholar]
- Stevenson J. The chemical and geological history of the atmosphere. I. The history of free oxygen. Philos. Mag. 1900;50:312–323. , 399–407. [Google Scholar]
- Suflita, J. M., Davidova, I. A., Gieg, L. M., Nanny, M. & Prince, R. C. 2004 Anaerobic hydrocarbon biodegradation and the prospects for microbial enhanced energy production. In Petroleum biotechnology: developments and perspectives, vol. 151 (eds R. Vazquez-Duhalt & R. Quintero-Ramirez), pp. 283–305. Amsterdam, The Netherlands: Elsevier.
- Summons R.E, Jahnke L.L, Hope J.M, Logan G.A. 2-Methylhopanoids as biomarkers for cyanobacterial oxygenic photosynthesis. Nature. 1999;400:554–557. doi:10.1038/23005 [PubMed] [Google Scholar]
- Summons R.E, Bradley A.S, Jahnke L.L, Waldbauer J.R. Steroids, triterpenoids and molecular oxygen. Phil. Trans. R. Soc. B. 2006;361:951–968. doi:10.1098/rstb.2006.1837[PMC free article] [PubMed] [Google Scholar]
- Ueno Y, Yamada K, Yoshida N, Maruyama S, Isozaki Y. Evidence from fluid inclusions for microbial methanogenesis in the Early Archaean era. Nature. 2006;440:516–519. doi:10.1038/nature04584 [PubMed] [Google Scholar]
- Urey H.C. On the early chemical history of Earth and the origin of life. Proc. Natl Acad. Sci. USA. 1952;38:351–363. doi:10.1073/pnas.38.4.351[PMC free article] [PubMed] [Google Scholar]
- van Hise, C. 1904 A treatise on metamorphism US Geological Survey Monograph. Washington, DC: US Geological Survey.
- Walker J.C.G. Was the Archean biosphere upside down? Nature. 1987;329:710–712. doi:10.1038/329710a0 [PubMed] [Google Scholar]
- Wendt K.U, Poralla K, Schulz G.E. Structure and function of a squalene cyclase. Science. 1997;277:1811–1815. doi:10.1126/science.277.5333.1811 [PubMed] [Google Scholar]
- Wendt K.U, Schulz G.E, Corey E.J, Liu D.R. Enzyme mechanisms for polycyclic triterpene formation. Angew. Chem. 2000;39:2812–2833. doi:10.1002/1521-3773(20000818)39:16<2812::AID-ANIE2812>3.0.CO;2-# [PubMed] [Google Scholar]
- Widdel F, Rabus R. Anaerobic biodegradation of saturated and aromatic hydrocarbons. Curr. Opin. Biotechnol. 2001;12:259–276. doi:10.1016/S0958-1669(00)00209-3 [PubMed] [Google Scholar]
- Widdel F, Schnell S, Heising S, Ehrenreich A, Assmus B, Schink B. Ferrous iron oxidation by anoxygenic phototrophic bacteria. Nature. 1993;362:834–836. doi:10.1038/362834a0[Google Scholar]
- Wolfe-Simon F. The role and evolution of superoxide dismutases in algae. J. Phycol. 2005;41:453–465. doi:10.1111/j.1529-8817.2005.00086.x[Google Scholar]
- Young G.M, Long D.G.F, Fedo C.M, Nesbitt H.W. Paleoproterozoic Huronian Basin: product of a Wilson cycle punctuated by glaciations and a meteorite impact. Sediment. Geol. 2001;141:233–254. doi:10.1016/S0037-0738(01)00076-8[Google Scholar]
Articles from Philosophical Transactions of the Royal Society B: Biological Sciences are provided here courtesy of The Royal Society