Mechanism of Anther Dehiscence in Rice (Oryza sativa L.)

This paper presents a new explanation of the mechanism of antherdehiscence in rice during the period from floret opening topollen dispersal. The theca dehisced on the stomium in the apicalpart and the anther wall in the basal part of the large locule.Comparison of the anther dehiscence process under various airhumidity conditions showed that the process, until the splittingat the apical and basal parts, was a moisture-requiring processwhereas the widening of the splits in both parts was a desiccatoryprocess. Observation of the anther transverse section, revealedthe marked development of the U-shaped thick cell wall in theendothecium adjacent to these two splits. From these observations,the anther dehiscence mechanism may be explained as follows.At the time of anthesis, pollen grains swell rapidly in responseto the floret opening and cause the theca to bulge, rupturingthe septum. The pollen pressure combined with the inward bendingof the locule walls adjacent to the stomium causes splittingof the stomium in the apical part of the theca. At the sametime, the septum rupture extends to the bottom of the largelocule supported by the pollen pressure. After these processes,the locule walls adjacent to both splits straighten probablydue to their water loss. This straightening widens the splitsand the swollen pollen grains overflow from the widened splits.Copyright1999 Annals of Botany Company Anther dehiscence, Oryza sativa L., pollen grain swelling, rice, septum, stomium, theca.     

A novel cell ablation strategy blocks tobacco anther dehiscence.

We utilized a new cell ablation strategy to ablate specific anther cell types involved in the dehiscence process. The tobacco TA56 gene promoter is active within the circular cell cluster, stomium, and connective regions of the anther at different developmental stages. We introduced a cytotoxic TA56/barnase gene into tobacco plants together with three different anticytotoxic barstar genes. The anticytotoxic barstar genes were used to protect subsets of anther cell types from the cytotoxic effects of the TA56/barnase gene. The chimeric barstar genes were fused with (1) the tobacco TP12 gene promoter that is active at high levels in most anther cell types; (2) the soybean lectin gene promoter that is active earlier in the connective, and at lower levels in the circular cell cluster and stomium, than is the TA56 promoter; and (3) the tobacco TA20 gene promoter that is active at high levels in most anther cell types but has a different developmental profile than does the TP12 promoter. Normal anther development and dehiscence occurred in plants containing the TA56/barnase and TP12/barstar genes, indicating that barstar protects diverse anther cell types from the cytotoxic effects of barnase. Anthers containing the TA56/barnase and lectin/barstar genes also developed normally but failed to dehisce because of extensive ablation of the circular cell cluster, stomium, and contiguous connective regions. Anthers containing the TA56/barnase and TA20/barstar genes failed to dehisce as well. However, only the stomium region was ablated in these anthers. The connective, circular cell cluster, and adjacent wall regions were protected from ablation by the formation of barnase/barstar complexes. We conclude that anther dehiscence at flower opening depends on the presence of a functional stomium region and that chimeric barnase and barstar genes containing promoters that are active in several overlapping cell types can be used for targeted cell ablation experiments.     

The final split: the regulation of anther dehiscence

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.     

The arabidopsis DELAYED DEHISCENCE1 gene encodes an enzyme in the jasmonic acid synthesis pathway

delayed dehiscence1 is an Arabidopsis T-DNA mutant in which anthers release pollen grains too late for pollination to occur. The delayed dehiscence1 defect is caused by a delay in the stomium degeneration program. The gene disrupted in delayed dehiscence1 encodes 12-oxophytodienoate reductase, an enzyme in the jasmonic acid biosynthesis pathway. We rescued the mutant phenotype by exogenous application of jasmonic acid and obtained seed set from previously male-sterile plants. In situ hybridization studies showed that during the early stages of floral development, DELAYED DEHISCENCE1 mRNA accumulated within all floral organs. Later, DELAYED DEHISCENCE1 mRNA accumulated specifically within the pistil, petals, and stamen filaments. DELAYED DEHISCENCE1 mRNA was not detected in the stomium and septum cells of the anther that are involved in pollen release. The T-DNA insertion in delayed dehiscence1 eliminated both DELAYED DEHISCENCE1 mRNA accumulation and 12-oxophytodienoate reductase activity. These experiments suggest that jasmonic acid signaling plays a role in controlling the time of anther dehiscence within the flower.     

The Mechanics of the Grass Flower: Anther Dehiscence and Pollen Shedding in Maize

In this paper on the flower mechanics of the grasses, the openingmechanism of the maize anther is studied. Both the septum betweeneach two locules and the stomium of these porate-dehiscing anthersappear to be opened due to lysis of the middle lamellae of theircells. Additional mechanical force of the expanding pollen mightbe necessary to completely dissociate the parenchyma cells ofthe septum. A number of hours before anthesis the anther isstructurally able to dehisce. At anthesis the dehydrating endotheciumcells bend the locule walls bordering the pore in outward direction.Presumably evaporation is not the only cause for this dehydration. Poaceae; Zea mays ; flower; anther; dehiscence; endothecium; pollen     

Features related to anther opening in Solanum species (Solanaceae)

The mode of anther opening and the morphological and histological variability of the stomium are described in 30 Solanum species. Poricidal, poricidal‐longitudinally dehiscing and longitudinally dehiscing anthers are observed. In the three types, the stomium may be diverse with regard to shape and histological characteristics before opening, but is always composed of small epidermal cells as the sole anther wall layer; the stomial cells may be differentiated only in part of the anther length. Particular crescent‐shaped structures in the epidermis, called ‘ridges’, are observed to line the stomium in most species. These ridges may be related to the stomium opening, working together with the cells with thickened walls of the anther. Cells with thickened walls are developed in the endothecium, middle layers and/or connective tissue at the apical end of the anther, surrounding the pore; only in the longitudinally dehiscing anthers of S. nitidum does an endothecium with thickened cell walls develop along its entire length. At least two histological features (the differentiation of small stomial epidermal cells as a unique layer, and the distribution of cells with thickened walls) seem to constrain the form of the open stomium. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158 , 344–354.     

WUSCHEL regulates cell differentiation during anther development

During anther development a series of cell specification events establishes the male gametophyte and the surrounding sporophytic structure. Here we show that the homeobox gene WUSCHEL, originally identified as a central regulator of stem cell maintenance, plays an important role in cell type specification during male organogenesis. WUS expression is initiated very early during anther development in the precursor cells of the stomium and terminates just before the stomium cells enter terminal differentiation. At this stage the stomium cells and the neighboring septum cells that separate the pollen sacs undergo typical cell wall thickening and degenerate which leads to rupture of the anther and pollen release. In wus mutants, neither stomium cells nor septum cells differentiate or undergo cell death and degenerate. As a consequence, the anther stays intact and pollen is not released. CLAVATA3 which is activated by WUS in stem cell maintenance, is not activated in anthers indicating a novel pathway regulated by WUS. Comparing WUS function in stem cell maintenance and sexual organ development suggests that WUS expressing cells represent a conserved signaling module that regulates behavior and communication of undifferentiated cells.     

Anther structure and pollen development in Melicoccus lepidopetalus (Sapindaceae): An evolutionary approach to dioecy in the family

Anther and pollen development in staminate and pistillate flowers of dioecious Melicoccus lepidopetalus (Sapindaceae) were examined by light and electron microscopy. Young anthers are similar in both types of flowers; they consist of epidermis, endothecium, two to four middle layers and a secretory tapetum. The microspore tetrads are tetrahedral. The mature anther in staminate flowers presents compressed epidermal cells and endothecium cells with fibrillar thickenings. A single locule is formed in the theca by dissolution of the septum and pollen grains are shed at two-celled stage. The mature anthers of pistillate flowers differ anatomically from those of staminate flowers. The epidermis is not compressed, the endothecium does not develop fibrillar thickenings, middle layers and tapetum are generally persisting, and the stomium is nonfunctional. Microspore degeneration begins after meiosis of microspore mother cells. At anthesis, uninucleate microspores and pollen grains with vegetative and generative nuclei with no cytokinesis are observed. Some pollen walls display an abnormal exine deposition, whereas others show a well formed exine, although both are devoid of intine. These results suggest that in the evolution towards unisexuality, the developmental differences of anther wall tissues and pollen grains between pistillate and staminate flowers might become more pronounced in a derived condition, such as dioecy.     

The diversity of anther structures and dehiscence patterns among Hamamelididae

HUFFORD, L. D. & ENDRKSS, P. K., 1989. The diversity of anther structures and dehiscence patterns among Hamamelididae. This survey of anther structures and dehiscence patterns focuses on the range of diversity among extant Hamamelididae. The definition and structure of the anther stomium are considered in detail to provide a basis for characterizing dehiscence patterns. We are concerned particularly with the structural basis and distribution of so-called valvate dehiscence, which we define here as occurring only in those anthers that possess stomial bifurcations or markedly eccentric stomia. Valvate dehiscence is restricted to Trochodendrales and Hamamelidales among Hamamelididae, although some Hamamelidaceae possess only linear, not markedly eccentric stomia that lead to longitudinal dehiscence patterns. Anther forms are somewhat variable and do not appear to be highly correlated with stomial patterns, although stomial bifurcations occur most frequently in anthers with broad, thick connectives that extend for the full length (or nearly so) of the thecae. Valvate dehiscence usually occurs in anthers in which the pollen sacs are embedded in bulky superficial tissues. An evolutionarily secondary extension of the stomium around the thecal shoulders seems to have occurred in taxa with a nonextensive connective and may facilitate a broader anther opening in cases of longitudinal dehiscence. An endothecial-like connective hypodermis is a notable characteristic among examined 'Lower Hamamelididae' (except Disanthus) and is also present in Daphnipfiyllum and Eucommia. We hypothesize that this specialized connective hypodermis facilitates a broader opening of the anther.     

Partial silencing of the NEC1 gene results in early opening of anthers in Petunia hybrida

The NEC1 gene, previously isolated from Petunia hybrida, is expressed at high levels in nectaries, and in a very localized fashion in stamens, particularly in the anther stomium cells and the upper part of the filament. To elucidate the function of the NEC1 gene, co-suppression was employed for down-regulation of NEC1 expression, and transposon insertion mutagenesis was used to knock out the NEC1 function. Among the transgenic plants and plants carrying dTph1 inserted in the NEC1 gene, an "early open anther" phenotype was observed. In this mutant phenotype, the anthers already open in young flower buds (1.8 cm) that still contain immature pollen, resulting in poor pollen quality and impaired pollen release. The results obtained indicate that NEC1 might be involved in the development of stomium cells, which are ruptured during the normal process of anther dehiscence to release mature pollen. Southern analysis revealed the presence of a highly homologous NEC1-like gene, named NEC2, in the P. hybrida genome. The presence of NEC2 was confirmed by segregation analysis and sequencing of genomic clones. The implications of these results and possible reasons why no visually obvious phenotype in nectaries could be produced by co-suppression or transposon insertion mutagenesis are discussed.     

Ethylene regulates the timing of anther dehiscence in tobacco

We investigated the involvement of ethylene signaling in the development of the reproductive structures in tobacco ( Nicotiana tabacum L.) by studying flowers that were insensitive to ethylene. Ethylene-insensitivity was generated either by expression of the mutant etr1-1 ethylene-receptor allele from Arabidopsis thaliana or by treatment with the ethylene-perception inhibitor 1-methylcyclopropene (MCP). Development of ovaries and ovules was unaffected by ethylene-insensitivity. Anther development was also unaffected, but the final event of dehiscence was delayed and was no longer synchronous with flower opening. We showed that in these anthers degeneration of the stomium cells and dehydration were delayed. In addition, we found that MCP-treatment of detached flowers and isolated, almost mature anthers delayed dehiscence whereas ethylene-treatment accelerated dehiscence. This indicated that ethylene has a direct effect on a process that takes place in the anthers just before dehiscence. Because a similar function has been described for jasmonic acid in Arabidopsis, we suggest that ethylene acts similarly to or perhaps even in concurrence with jasmonic acid as a signaling molecule controlling the processes that lead to anther dehiscence in tobacco.     

An Arabidopsis aspartic protease functions as an anti-cell-death component in reproduction and embryogenesis

The components and pathways that regulate and execute developmental cell death programmes in plants remain largely unknown. We have found that the PROMOTION OF CELL SURVIVAL 1 (PCS1) gene in Arabidopsis, which encodes an aspartic protease, has an important role in determining the fate of cells in embryonic development and in reproduction processes. The loss-of-function mutation of PCS1 causes degeneration of both male and female gametophytes and excessive cell death of developing embryos. Conversely, ectopic expression of PCS1 causes the septum and stomium cells that normally die in the anther wall to survive instead, leading to a failure in anther dehiscence and male sterility. PCS1 provides a new avenue for understanding the mechanisms of the programmed cell death processes that are associated with developmental pathways in plants and makes available a useful tool for engineering the male sterility trait for hybrid seed production.     

The diversity of stamen structures and dehiscence patterns among Magnoliidae

ENDRESS, P. K. & HUFFORD, L. D., 1989. The diversity of stamen structures and dehiscence patterns among Magnoliidae . Structure of stamens, particularly the patterns of anther dehiscence were studied over a wide range of families of the Magnoliidae with emphasis on the Magnoliales and Laurales as the most conservative orders of the angiosperms. Valvate dehiscence by proximal and distal stomial bifurcation was found (in addition to the already known Sarcandra and Polyalthia) for the first time in Degeneriaceae, Himantandraceae, Eupomatiaceae, in some additional Annonaceae, and in Peumus of the Monimioideae sensu lata. At least proximal bifurcations of the stomia occur in some Magnoliaceae and Ranunculaceae. An endothecial-like connective hypodermis was found (in addition to the already known Chloranthaceae and Magnoliaceae) in some Annonaceae, in Pseudowintera (Winteraceae), and in Thalictrum (Ranunculaceae). In the Annonaceae an endothecial-like connective hypodermis is partly correlated with valvate dehiscence by stomial bifurcations (as in many Hamamelididae). However, in many Magnoliidae stamens with this valvate pattern the anther is massive, especially in ‘laminar’ stamens, and the counterforce to the opening valves is therefore provided on the morphological and not on the histological level. Concomitant with valvate dehiscence by circular or elliptic flaps in the Laurales is often structural and functional dissocation of the two pollen sacs of a thcca, which is expressed by: (1) independent opening of each pollen sac, (2) lack of disruption of the interlocular zone of a theca, (3) frequent occurrence of asymmetry of the two pollen sacs of the theca, (4) frequent loss of one pollen sac per theca. In Berberidaceae with similar flaps asymmetry of the two pollen sacs of a theca is also common. These finds, together with the detection by paleobotanists of valvate anthers from the Lower Cretaceous, point to the probability that valvate anthers were more common in primitive angiosperms than previously thought.     

Receptor-like protein kinase 2 (RPK 2) is a novel factor controlling anther development in Arabidopsis thaliana

Receptor-like kinases (RLK) comprise a large gene family within the Arabidopsis genome and play important roles in plant growth and development as well as in hormone and stress responses. Here we report that a leucine-rich repeat receptor-like kinase (LRR-RLK), RECEPTOR-LIKE PROTEIN KINASE2 (RPK2), is a key regulator of anther development in Arabidopsis. Two RPK2 T-DNA insertional mutants (rpk2-1 and rpk2-2) displayed enhanced shoot growth and male sterility due to defects in anther dehiscence and pollen maturation. The rpk2 anthers only developed three cell layers surrounding the male gametophyte: the middle layer was not differentiated from inner secondary parietal cells. Pollen mother cells in rpk2 anthers could undergo meiosis, but subsequent differentiation of microspores was inhibited by tapetum hypertrophy, with most resulting pollen grains exhibiting highly aggregated morphologies. The presence of tetrads and microspores in individual anthers was observed during microspore formation, indicating that the developmental homeostasis of rpk2 anther locules was disrupted. Anther locules were finally crushed without stomium breakage, a phenomenon that was possibly caused by inadequate thickening and lignification of the endothecium. Microarray analyses revealed that many genes encoding metabolic enzymes, including those involved in cell wall metabolism and lignin biosynthesis, were downregulated throughout anther development in rpk2 mutants. RPK2 mRNA was abundant in the tapetum of wild-type anthers during microspore maturation. These results suggest that RPK2 controls tapetal cell fate by triggering subsequent tapetum degradation, and that mutating RPK2 impairs normal pollen maturation and anther dehiscence due to disruption of key metabolic pathways.     

Activation tagging of an Arabidopsis SHI-RELATED SEQUENCE gene produces abnormal anther dehiscence and floral development

The tapetum is a layer of cells covering the inner surface of pollen sac wall. It contributes to anther development by providing enzymes and materials for pollen coat biosynthesis and nutrients for pollen development. At the end of anther development, the tapetum is degenerated, and the anther is dehisced, releasing mature pollen grains. In Arabidopsis, several genes are known to regulate tapetum formation and pollen development. However, little is known about how tapetum degeneration and anther dehiscence are regulated. Here, we show that an activation-tagged mutant of the S HI-R ELATED S EQUENCE 7 (SRS7) gene exhibits disrupted anther dehiscence and abnormal floral organ development in addition to its dwarfed growth with small, curled leaves. In the mutant hypocotyls, cell elongation was reduced, and gibberellic acid sensitivity was diminished. Whereas anther development was normal, its dehiscence was suppressed in the dominant srs7-1D mutant. In wild-type anthers, the tapetum disappeared at anther development stages 11 and 12. In contrast, tapetum degeneration was not completed at these stages, and anther dehiscence was inhibited, causing male sterility in the mutant. The SRS7 gene was expressed mainly in the filaments of flowers, where the DEFECTIVE-IN-ANTHER-DEHISCENCE 1 (DAD1) enzyme catalyzing jasmonic acid (JA) biosynthesis is accumulated immediately before flower opening. The DAD1 gene was induced in the srs7-1D floral buds. In fully open flowers, the SRS7 gene was also expressed in pollen grains. It is therefore possible that the abnormal anther dehiscence and floral development of the srs7-1D mutant would be related with JA.     

The DEFECTIVE IN ANTHER DEHISCIENCE 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

The Arabidopsis mutant defective in anther dehiscence1 (dad1) shows defects in anther dehiscence, pollen maturation, and flower opening. The defects were rescued by the exogenous application of jasmonic acid (JA) or linolenic acid, which is consistent with the reduced accumulation of JA in the dad1 flower buds. We identified the DAD1 gene by T-DNA tagging, which is characteristic to a putative N-terminal transit peptide and a conserved motif found in lipase active sites. DAD1 protein expressed in Escherichia coli hydrolyzed phospholipids in an sn-1-specific manner, and DAD1-green fluorescent protein fusion protein expressed in leaf epidermal cells localized predominantly in chloroplasts. These results indicate that the DAD1 protein is a chloroplastic phospholipase A1 that catalyzes the initial step of JA biosynthesis. DAD1 promoter::beta-glucuronidase analysis revealed that the expression of DAD1 is restricted in the stamen filaments. A model is presented in which JA synthesized in the filaments regulates the water transport in stamens and petals.