Cell-cell interactions during patterning of the Arabidopsis anther

OsEDM2L mediates m6 A of EAT1 transcript for proper alternative splicing and polyadenylation regulating rice tapetal degradation

N⁶ -methyladenosine (m⁶ A) modification affects the post-transcriptional regulation of eukaryotic gene expression, but the underlying mechanisms and their effects in plants remain largely unknown. Here, we report that the N⁶ -adenine methyltransferase-like domain-containing protein ENHANCED DOWNY MILDEW 2-LIKE (OsEDM2L) is essential for rice (Oryza sativa L.) anther development. The osedm2l knockout mutant showed delayed tapetal programmed cell death (PCD) and defective pollen development. OsEDM2L interacts with the transcription factors basic helix-loop-helix 142 and TAPETUM DEGENERATION RETARDATION to regulate the expression of ETERNAL TAPETUM 1 (EAT1), a positive regulator of tapetal PCD. Mutation of OsEDM2L altered the transcriptomic m⁶ A landscape, and caused a distinct m⁶ A modification of the EAT1 transcript leading to dysregulation of its alternative splicing and polyadenylation, followed by suppression of the EAT1 target genes OsAP25 and OsAP37 for tapetal PCD. Therefore, OsEDM2L is indispensable for proper messenger RNA m⁶ A modification in rice anther development.

Auxin guides germ-cell specification in Arabidopsis anthers

Germ cells (GCs) are the key carriers delivering genetic information from one generation to the next. In a majority of animals, GCs segregate from somatic cells during embryogenesis by forming germlines. In land plants, GCs segregate from somatic cells during postembryonic development. In a majority of angiosperms, male GCs (archesporial cells) initiate at the four corners of the anther primordia. Little is known about the mechanism underlying this initiation. Here, we discovered that the dynamic auxin distribution in developing anthers coincided with GC initiation. A centripetal auxin gradient gradually formed toward the four corners where GCs will initiate. Local auxin biosynthesis was necessary for this patterning and for GC specification. The GC determinant protein SPOROCYTELESS/NOZZLE (SPL/NZZ) mediated the effect of auxin on GC specification and modified auxin biosynthesis to maintain a centripetal auxin distribution. Our work reveals that auxin is a key factor guiding GC specification in Arabidopsis anthers. Moreover, we demonstrate that the GC segregation from somatic cells is not a simple switch on/off event but rather a complicated process that involves a dynamic feedback circuit among local auxin biosynthesis, transcription of SPL/NZZ, and a progressive GC specification. This finding sheds light on the mystery of how zygote-derived somatic cells diverge into GCs in plants.

Comparative transcript profiling and cytological observation of the newly bred recessive genic male sterility non-heading Chinese cabbage (Brassica rapa ssp. chinensis) line WS24-3A

BACKGROUND

WS24-3A is a newly bred non-heading Chinese cabbage genic male-sterile line, in which sterility is controlled by a recessive gene, designated as Bra2ms. WS24-3A has been used for hybrid breeding.

OBJECTIVE

To reveal the underlying molecular mechanisms responsible for the sterility of WS24-3A.

METHODS

Cytological observation of the process of sterile/fertile anther development was performed to determine the tissue and stage in which sterility occurs. Phenotyping and transcriptomic analyses were performed to identify differentially expressed genes (DEGs) between sterile and fertile flower buds at different stages.

RESULTS

Cytological analysis revealed no tetrads at stage 7 or at later stages of anther development, and the degradation of callose was delayed. Abnormal meiocytes were surrounded by sustaining callose that degenerated gradually in WS24-3A. Comparative transcript profiling identified 3282 DEGs during three anther developmental stages, namely, pre-meiotic anther, meiotic anther, and anthers with single-celled pollen stage. The difference in DEG percentage between up-regulated and down-regulated at meiotic anther stage was obviously larger than at the other two stages; further, most DEGs are important for male meiosis, callose synthesis and dissolution, and tapetum development. Ten DEGs were found to be involved in anther and pollen development, which were analyzed by quantitative PCR.

CONCLUSION

Bra2ms affected gene expression in meiocytes and associated with callose synthesis, degradation and tapetum development. Our results provide clues to elucidate the molecular mechanism of genic male sterility in non-heading Chinese cabbage.

Gene Regulatory Network for Tapetum Development in Arabidopsis thaliana

In flowering plants, male gametophyte development occurs in the anther. Tapetum, the innermost of the four anther somatic layers, surrounds the developing reproductive cells to provide materials for pollen development. A genetic pathway of DYT1-TDF1-AMS-MS188 in regulating tapetum development has been proven. Here we used laser microdissection and pressure catapulting to capture and analyze the transcriptome data for the Arabidopsis tapetum at two stages. With a comprehensive analysis by the microarray data of dyt1, tdf1, ams, and ms188 mutants, we identified possible downstream genes for each transcription factor. These transcription factors regulate many biological processes in addition to activating the expression of the other transcription factor. Briefly, DYT1 may also regulate early tapetum development via E3 ubiquitin ligases and many other transcription factors. TDF1 is likely involved in redox and cell degradation. AMS probably regulates lipid transfer proteins, which are involved in pollen wall formation, and other E3 ubiquitin ligases, functioning in degradating proteins produced in previous processes. MS188 is responsible for most cell wall-related genes, functioning both in tapetum cell wall degradation and pollen wall formation. These results propose a more complex gene regulatory network for tapetum development and function.

Cytological characterization of anther development in Panax ginseng Meyer

Ginseng (Panax ginseng), a valued medicinal herb, is a slow-growing plant that flowers after 3 years of growth with the formation of a solitary terminal umbel inflorescence. However, little is known about cytological events during ginseng reproduction, such as the development of the male organ, the stamen. To better understand the mechanism controlling ginseng male reproductive development, here, we investigated the inflorescence and flower structure of ginseng. Moreover, we performed cytological analysis of anther morphogenesis and showed the common and specialized cytological events including the formation of four concentric cell layers surrounding male reproductive cells followed by subsequent cell differentiation and degeneration of tapetal cells, as well as the formation of mature pollen grains via meiosis and mitosis during ginseng anther development. Particularly, our transverse section and microscopic observations showed that the ginseng tapetal layer exhibits obvious nonsynchronous cell division evidenced by the observation of one or two tapetal layers frequently observed in one anther lobe, suggesting the unique control of cell division. To facilitate the future study on ginseng male reproduction, we grouped the anther development into 10 developmental stages according to the characterized cytological events.

Specification of tapetum and microsporocyte cells within the anther

Flowering plants form male reproductive cells (microsporocytes) during sporophytic generation, which subsequently differentiate into multicellular male gametes in the gametophytic generation. The tapetum is a somatic helper tissue neighboring microsporocytes and supporting gametogenesis. The mechanism controlling the specification of the tapetum and microsporocyte cell fate within the anther has long been a mystery in biology. Recent investigations have revealed molecular switches and signaling pathways underlying the establishment of somatic and reproductive cells in plants. In this review we discuss common and diversified signaling molecules and regulatory pathways including receptor-like protein kinases, redox status, glycoprotein, transcription factors, hormones and microRNA implicated in the specification of tapetum and microsporocytes in plants.

Cytological characterization and allelism testing of anther developmental mutants identified in a screen of maize male sterile lines

Proper regulation of anther differentiation is crucial for producing functional pollen, and defects in or absence of any anther cell type result in male sterility. To deepen understanding of processes required to establish premeiotic cell fate and differentiation of somatic support cell layers a cytological screen of maize male-sterile mutants has been conducted which yielded 42 new mutants including 22 mutants with premeiotic cytological defects (increasing this class fivefold), 7 mutants with postmeiotic defects, and 13 mutants with irregular meiosis. Allelism tests with known and new mutants confirmed new alleles of four premeiotic developmental mutants, including two novel alleles of msca1 and single new alleles of ms32, ms8, and ocl4, and two alleles of the postmeiotic ms45. An allelic pair of newly described mutants was found. Premeiotic mutants are now classified into four categories: anther identity defects, abnormal anther structure, locular wall defects and premature degradation of cell layers, and/or microsporocyte collapse. The range of mutant phenotypic classes is discussed in comparison with developmental genetic investigation of anther development in rice and Arabidopsis to highlight similarities and differences between grasses and eudicots and within the grasses.

Maize multiple archesporial cells 1 (mac1), an ortholog of rice TDL1A, modulates cell proliferation and identity in early anther development

To ensure fertility, complex somatic and germinal cell proliferation and differentiation programs must be executed in flowers. Loss-of-function of the maize multiple archesporial cells 1 (mac1) gene increases the meiotically competent population and ablates specification of somatic wall layers in anthers. We report the cloning of mac1, which is the ortholog of rice TDL1A. Contrary to prior studies in rice and Arabidopsis in which mac1-like genes were inferred to act late to suppress trans-differentiation of somatic tapetal cells into meiocytes, we find that mac1 anthers contain excess archesporial (AR) cells that proliferate at least twofold more rapidly than normal prior to tapetal specification, suggesting that MAC1 regulates cell proliferation. mac1 transcript is abundant in immature anthers and roots. By immunolocalization, MAC1 protein accumulates preferentially in AR cells with a declining radial gradient that could result from diffusion. By transient expression in onion epidermis, we demonstrate experimentally that MAC1 is secreted, confirming that the predicted signal peptide domain in MAC1 leads to secretion. Insights from cytology and double-mutant studies with ameiotic1 and absence of first division1 mutants confirm that MAC1 does not affect meiotic cell fate; it also operates independently of an epidermal, Ocl4-dependent pathway that regulates proliferation of subepidermal cells. MAC1 both suppresses excess AR proliferation and is responsible for triggering periclinal division of subepidermal cells. We discuss how MAC1 can coordinate the temporal and spatial pattern of cell proliferation in maize anthers.

AMS-dependent and independent regulation of anther transcriptome and comparison with those affected by other Arabidopsis anther genes

BACKGROUND

In flowering plants, the development of male reproductive organs is controlled precisely to achieve successful fertilization and reproduction. Despite the increasing knowledge of genes that contribute to anther development, the regulatory mechanisms controlling this process are still unclear.

RESULTS

In this study, we analyzed the transcriptome profiles of early anthers of sterile mutants aborted microspores (ams) and found that 1,368 genes were differentially expressed in ams compared to wild type anthers, affecting metabolism, transportation, ubiquitination and stress response. Moreover, the lack of significant enrichment of potential AMS binding sites (E-box) in the promoters of differentially expressed genes suggests both direct and indirect regulation for AMS-dependent regulation of anther transcriptome involving other transcription factors. Combining ams transcriptome profiles with those of two other sterile mutants, spl/nzz and ems1/exs, expression of 3,058 genes were altered in at least one mutant. Our investigation of expression patterns of major transcription factor families, such as bHLH, MYB and MADS, suggested that some closely related homologs of known anther developmental genes might also have similar functions. Additionally, comparison of expression levels of genes in different organs suggested that anther-preferential genes could play important roles in anther development.

CONCLUSION

Analysis of ams anther transcriptome and its comparison with those of spl/nzz and ems1/exs anthers uncovered overlapping and distinct sets of regulated genes, including those encoding transcription factors and other proteins. These results support an expanded regulatory network for early anther development, providing a series of hypotheses for future experimentation.

AMS-dependent and independent regulation of anther transcriptome and comparison with those affected by other Arabidopsis anther genes

BACKGROUND

In flowering plants, the development of male reproductive organs is controlled precisely to achieve successful fertilization and reproduction. Despite the increasing knowledge of genes that contribute to anther development, the regulatory mechanisms controlling this process are still unclear.

RESULTS

In this study, we analyzed the transcriptome profiles of early anthers of sterile mutants aborted microspores (ams) and found that 1,368 genes were differentially expressed in ams compared to wild type anthers, affecting metabolism, transportation, ubiquitination and stress response. Moreover, the lack of significant enrichment of potential AMS binding sites (E-box) in the promoters of differentially expressed genes suggests both direct and indirect regulation for AMS-dependent regulation of anther transcriptome involving other transcription factors. Combining ams transcriptome profiles with those of two other sterile mutants, spl/nzz and ems1/exs, expression of 3,058 genes were altered in at least one mutant. Our investigation of expression patterns of major transcription factor families, such as bHLH, MYB and MADS, suggested that some closely related homologs of known anther developmental genes might also have similar functions. Additionally, comparison of expression levels of genes in different organs suggested that anther-preferential genes could play important roles in anther development.

CONCLUSION

Analysis of ams anther transcriptome and its comparison with those of spl/nzz and ems1/exs anthers uncovered overlapping and distinct sets of regulated genes, including those encoding transcription factors and other proteins. These results support an expanded regulatory network for early anther development, providing a series of hypotheses for future experimentation.

Stamen development and winter dormancy in apricot (Prunus armeniaca)

BACKGROUND AND AIMS

In temperate woody perennials, flower bud development is halted during the winter, when the buds enter dormancy. This dormant period is a prerequisite for adequate flowering, is genetically regulated, and plays a clear role in possibly adapting species and cultivars to climatic areas. However, information on the biological events underpinning dormancy is lacking. Stamen development, with clear differentiated stages, appears as a good framework to put dormancy in a developmental context. Here, stamen developmental changes are characterized in apricot (Prunus armeniaca) and are related to dormancy.

METHODS

Stamen development was characterized cytochemically from the end of August to March, over 4 years. Developmental changes were related to dormancy, using the existing empirical information on chilling requirements.

KEY RESULTS

Stamen development continued during the autumn, and the flower buds entered dormancy with a fully developed sporogenous tissue. Although no anatomical changes were observed during dormancy, breaking of dormancy occurred following a clear sequence of events. Starch accumulated in particular places, pre-empting further development in those areas. Vascular bundles developed and pollen mother cells underwent meiosis followed by microspore development.

CONCLUSIONS

Dormancy appears to mark a boundary between the development of the sporogenous tissue and the occurrence of meiosis for further microspore development. Breaking of dormancy occurs following a clear sequence of events, providing a developmental context in which to study winter dormancy and to evaluate differences in chilling requirements among genotypes.

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.