A genetic pathway for tapetum development and function in Arabidopsis

Arabidopsis ADP-RIBOSYLATION FACTOR-A1s mediate tapetum-controlled pollen development

Propagation of angiosperms mostly relies on sexual reproduction, in which gametophytic development is a pre-requisite. Male gametophytic development requires both gametophytic and sporophytic factors, most importantly early secretion and late programmed cell death of the tapetum. In addition to transcriptional factors, proteins at endomembrane compartments, such as receptor-like kinases and vacuolar proteases, control tapetal function. The cellular machinery that regulates their distribution is beginning to be revealed. We report here that ADP-RIBOSYLATION FACTOR-A1s (ArfA1s) are critical for tapetum-controlled pollen development. All six ArfA1s in the Arabidopsis genome are expressed during anther development, among which ArfA1b is specific to the tapetum and developing microspores. Although the ArfA1b loss-of-function mutant showed no pollen defects, probably due to redundancy, interference with ArfA1s by a dominant negative approach in the tapetum resulted in tapetal dysfunction and pollen abortion. We further showed that all six ArfA1s are associated with the Golgi and the trans-Golgi network/early endosome, suggesting that they have roles in regulating post-Golgi trafficking to the plasma membrane or to vacuoles. Indeed, we demonstrated that the expression of ArfA1b^DN interfered with the targeting of proteins critical for tapetal development. The results presented here demonstrate a key role of ArfA1s in tapetum-controlled pollen development by mediating protein targeting through post-Golgi trafficking routes.

Morphological characteristics and transcriptome analysis at different anther development stages of the male sterile mutant MS7-2 in Wucai (Brassica campestris L.)

BACKGROUND

The discovery of male sterile materials is of great significance for the development of plant fertility research. Wucai (Brassica campestris L. ssp. chinensis var. rosularis Tsen) is a variety of non-heading Chinese cabbage. There are few studies on the male sterility of wucai, and the mechanism of male sterility is not clear. In this study, the male sterile mutant MS7-2 and the wild-type fertile plant MF7-2 were studied.

RESULTS

Phenotypic characteristics and cytological analysis showed that MS7-2 abortion occurred at the tetrad period. The content of related sugars in the flower buds of MS7-2 was significantly lower than that of MF7-2, and a large amount of reactive oxygen species (ROS) was accumulated. Through transcriptome sequencing of MS7-2 and MF7-2 flower buds at three different developmental stages (a-c), 2865, 3847, and 4981 differentially expressed genes were identified in MS7-2 at the flower bud development stage, stage c, and stage e, respectively, compared with MF7-2. Many of these genes were enriched in carbohydrate metabolism, phenylpropanoid metabolism, and oxidative phosphorylation, and most of them were down-regulated in MS7-2. The down-regulation of genes involved in carbohydrate and secondary metabolite synthesis as well as the accumulation of ROS in MS7-2 led to pollen abortion in MS7-2.

CONCLUSIONS

This study helps elucidate the mechanism of anther abortion in wucai, providing a basis for further research on the molecular regulatory mechanisms of male sterility and the screening and cloning of key genes in wucai.

CRISPR/Cas9-based discovery of maize transcription factors regulating male sterility and their functional conservation in plants

Identifying genic male-sterility (GMS) genes and elucidating their roles are important to unveil plant male reproduction and promote their application in crop breeding. However, compared with Arabidopsis and rice, relatively fewer maize GMS genes have been discovered and little is known about their regulatory pathways underlying anther and pollen development. Here, by sequencing and analysing anther transcriptomes at 11 developmental stages in maize B73, Zheng58 and M6007 inbred lines, 1100 transcription factor (TF) genes were identified to be stably differentially expressed among different developmental stages. Among them, 14 maize TF genes (9 types belonging to five TF families) were selected and performed CRISPR/Cas9-mediated gene mutagenesis, and then, 12 genes in eight types, including ZmbHLH51, ZmbHLH122, ZmTGA9-1/-2/-3, ZmTGA10, ZmMYB84, ZmMYB33-1/-2, ZmPHD11 and ZmLBD10/27, were identified as maize new GMS genes by using DNA sequencing, phenotypic and cytological analyses. Notably, ZmTGA9-1/-2/-3 triple-gene mutants and ZmMYB33-1/-2 double-gene mutants displayed complete male sterility, but their double- or single-gene mutants showed male fertility. Similarly, ZmLBD10/27 double-gene mutant displayed partial male sterility with 32.18% of aborted pollen grains. In addition, ZmbHLH51 was transcriptionally activated by ZmbHLH122 and their proteins were physically interacted. Molecular markers co-segregating with these GMS mutations were developed to facilitate their application in maize breeding. Finally, all 14-type maize GMS TF genes identified here and reported previously were compared on functional conservation and diversification among maize, rice and Arabidopsis. These findings enrich GMS gene and mutant resources for deeply understanding the regulatory network underlying male fertility and for creating male-sterility lines in maize.

Characterization and functional analysis of LoUDT1, a bHLH transcription factor related to anther development in the lily oriental hybrid Siberia (Lilium spp.)

Lily (Lilium spp.), with its beautiful flower, is an important horticultural crop and a popular ornamental plant, but because the abundant pollen pollutes the flowers and surroundings, its use is restricted. To solve this problem, the mechanism of pollen development in lily needs to be analyzed. However, the complex and delicate process of anther development in lily remains largely unknown. In this study, LoUDT1, a bHLH transcription factor (TF), was isolated and identified in lily. LoUDT1 was closely related to OsUDT1 of Oryza sativa and AtDYT1 of Arabidopsis. It was localized in the cytoplasm and nucleus and showed no transcriptional activation in yeast cells. LoUDT1 interacted with another bHLH TF, LoAMS, and the interaction depended on their BIF domains. LoUDT1 and LoAMS were both expressed in the anthers but showed different expression patterns. LoUDT1 was continuously expressed during the entire development of anthers, whereas LoAMS was only highly expressed early in anther development. With overexpression of LoUDT1 in Arabidopsis, normal anther development was affected and defective pollens were produced, which caused partial male sterility of transgenic plants. These defects depended on the level of LoUDT1 accumulation. By contrast, with the appropriate expression of LoUDT1 in a dyt1-3 mutant, normal pollen grains were produced, showing partial fertility. Thus, LoUDT1 might be a key regulator of anther development in lily. By further increasing the understanding of anther development, the results of this study can provide a theoretical basis for the molecular breeding of pollen-free lilies.

A dye combination for the staining of pollen coat and pollen wall

The pollen coat, which forms on the pollen surface, consists of a lipid-protein matrix. It protects pollen from desiccation and is involved in adhesion, pollen-stigma recognition, and pollen hydration during interactions with the stigma. The classical methods used for pollen coat observation are scanning and transmission electron microscopy. In this work, we screened a collection of fluorescence dyes and identified two fluorescent brighteners FB-52 and FB-184. When they were used together with the exine-specific dye, Basic fuchsin, the pollen coat and the exine structures could be clearly visualized in the pollen of Brassica napus. This co-staining method was applied successfully in staining pollen from Fraxinus chinensis, Calystegia hederacea, and Petunia hybrida. Using this method, small pollen coat-containing cavities were detected in the outer pollen wall layer of Oryza sativa and Zea mays. We further showed these dyes are compatible with fluorescent protein markers. In the Arabidopsis thaliana transgenic line of GFP-tagged pollen coat protein GRP19, GRP19-GFP was observed to form particles at the periphery of pollen coat. This simple staining method is expected to be widely used for the studies of the palynology as well as the pollen-stigma interaction.

ZmMs25 encoding a plastid-localized fatty acyl reductase is critical for anther and pollen development in maize

Fatty acyl reductases (FARs) catalyse the reduction of fatty acyl-coenzyme A (CoA) or -acyl carrier protein (ACP) substrates to primary fatty alcohols, which play essential roles in lipid metabolism in plants. However, the mechanism by which FARs are involved in male reproduction is poorly defined. Here, we found that two maize allelic mutants, ms25-6065 and ms25-6057, displayed defective anther cuticles, abnormal Ubisch body formation, impaired pollen exine formation and complete male sterility. Based on map-based cloning and CRISPR/Cas9 mutagenesis, Zm00001d048337 was identified as ZmMs25, encoding a plastid-localized FAR with catalytic activities to multiple acyl-CoA substrates in vitro. Four conserved residues (G101, G104, Y327 and K331) of ZmMs25 were critical for its activity. ZmMs25 was predominantly expressed in anther, and was directly regulated by transcription factor ZmMYB84. Lipidomics analysis revealed that ms25 mutation had significant effects on reducing cutin monomers and internal lipids, and altering the composition of cuticular wax in anthers. Moreover, loss of function of ZmMs25 significantly affected the expression of its four paralogous genes and five cloned lipid metabolic male-sterility genes in maize. These data suggest that ZmMs25 is required for anther development and male fertility, indicating its application potential in maize and other crops.

JASMONATE-ZIM DOMAIN proteins engage Polycomb chromatin modifiers to modulate Jasmonate signaling in Arabidopsis

Jasmonate (JA) regulates various aspects of plant growth and development and stress responses, with prominent roles in male reproductive development and defenses against herbivores and necrotrophic pathogens. JASMONATE-ZIM DOMAIN (JAZ) proteins are key regulators in the JA signaling pathway and function to repress the expression of JA-responsive genes. Here, we show that JAZ proteins directly interact with several chromatin-associated Polycomb proteins to mediate repressive chromatin modifications at JA-responsive genes and, thus, their transcriptional repression in Arabidopsis. Genetic analyses revealed that the developmental defects, including anther and pollen abnormalities, resulting from loss or block of JA signaling were partially rescued by loss of Polycomb protein-mediated chromatin silencing (Polycomb repression). We further found that JAZ-mediated transcriptional repression during anther and pollen development requires Polycomb proteins at four key regulatory loci. Analysis of genome-wide occupancy of a Polycomb factor and transcriptome reprogramming in response to JA revealed that Polycomb repression is involved in the repression of various JA-responsive genes. Taken together, our study reveals an important chromatin-based mechanism for JAZ-mediated transcriptional repression and JA signaling in plants.

Formation pattern and regulatory mechanisms of pollen wall in Arabidopsis

In angiosperms, mature pollen is wrapped by a pollen wall, which is important for maintaining pollen structure and function. Pollen walls provide protection from various environmental stresses and preserve pollen germination and pollen tube growth. The pollen wall structure has been described since pollen ultrastructure investigations began in the 1960s. Pollen walls, which are the most intricate cell walls in plants, are composed of two layers: the exine layer and intine layer. Pollen wall formation is a complex process that occurs via a series of biological events that involve a large number of genes. In recent years, many reports have described the molecular mechanisms of pollen exine development. The formation process includes the development of the callose wall, the wavy morphology of primexine, the biosynthesis and transport of sporopollenin in the tapetum, and the deposition of the pollen coat. The formation mechanism of the intine layer is different from that of the exine layer. However, few studies have focused on the regulatory mechanisms of intine development. The primary component of the intine layer is pectin, which plays an essential role in the polar growth of pollen tubes. Demethylesterified pectin is mainly distributed in the shank region of the pollen tube, which can maintain the hardness of the pollen tube wall. Methylesterified pectin is mainly located in the top region, which is beneficial for improving the plasticity of the pollen tube top. In this review, we summarize the developmental process of the anther, pollen and pollen wall in Arabidopsis; furthermore, we describe the research progress on the pollen wall formation pattern and its molecular mechanisms in detail.

The impaired biosynthetic networks in defective tapetum lead to male sterility in watermelon

Heterosis has been widely applied in watermelon breeding, because of the higher resistance and yield of hybrid. As the basis of heterosis utilization, genic male sterility (GMS) is an important tool for facilitating hybrid seed production, while the detailed mechanism in watermelon is still largely unknown. Here, we report a spontaneous mutant Se18 exhibited complete male sterility due to the uniquely multilayered tapetum and the un-meiotic pollen mother cells during pollen development. Using TMT based quantitative proteomic analyses, a total of 348 differentially abundant proteins (DAPs) were detected with the overwhelming majority down-regulated in mutant Se18. By analyzing the putative orthologs/homologs of Arabidopsis GMS related genes, the biosynthesis and transport of sporopollenin and tryphine precursors were predictably altered in mutant compared to its sibling wild type. Moreover, the general phenylpropanoid pathway as well as its related metabolisms was also expectably impaired in mutant, coincident with the pale yellow petals. Notably, some key transcriptional factors regulating tapetum development, together with their down-regulated targets, offered potentially valuable candidates regarding of male sterility. Collectively, the disrupted regulatory networks underlying male sterility of watermelon was proposed, which provide novel insights into genetic mechanism of male reproductive process and rich gene resources for future research. SIGNIFICANCE: Watermelon is an importantly economical cucurbit crop worldwide, with high nutritional value. Although several male sterile mutants have been identified in watermelon, the underlying molecular mechanism is poorly elucidated. Comparative cytological analysis revealed that the defective development of tapetum was responsible for male sterility in mutant Se18. Combined with the morphological comparison, male floral buds at 2.0-2.5 mm in diameter were confirmed with no obvious phenotypic differences but distinct cytological defects, which were in turn sampled for TMT based proteomic analyses. Referring to functionally characterized GMS related genes, the genetic pathway DYT1-TDF1-AMS-MS188-MS1 regulating tapetum development, together with some downstream targets, were considerably altered in mutant Se18. Moreover, enrichment analyses illustrated the general phenylpropanoid related metabolisms, as well as the biosynthesis and transport of sporopollenin and tryphine precursors, were significantly disrupted in defective anther development. Collectively, the proposed regulatory networks in watermelon not only contribute to a better understanding of molecular mechanisms underlying male sterility, but also provide valuable GMS related candidates for future researches.

IMPERFECTIVE EXINE FORMATION (IEF) is required for exine formation and male fertility in Arabidopsis

KEY MESSAGE

IEF, a novel plasma plasma membrane protein, is important for exine formation in Arabidopsis. Exine, an important part of pollen wall, is crucial for male fertility. The major component of exine is sporopollenin which are synthesized and secreted by tapetum. Although sporopollenin synthesis has been well studied, the transportation of it remains elusive. To understand it, we analyzed the gene expression pattern in tapetal microdissection data, and investigated the potential transporter genes that are putatively regulated by ABORTED MICROSPORES (AMS). Among these genes, we identified IMPERFECTIVE EXINE FORMATION (IEF) that is important for exine formation. Compared to the wild type, ief mutants exhibit severe male sterility and pollen abortion, suggesting IEF is crucial for pollen development and male fertility. Using both scanning and transmission electron microscopes, we showed that exine structure was not well defined in ief mutant. The transient expression of IEF-GFP driven by the 35S promoter indicated that IEF-GFP was localized in plasma membrane. Furthermore, AMS can specifically activate the expression of promoterIEF:LUC in vitro, which suggesting AMS regulates IEF for exine formation. The expression of ATP-BINDING CASSETTE TRANSPORTER G26 (AGCB26) was not affected in ief mutants. In addition, SEM and TEM data showed that the sporopollenin deposition is more defective in abcg26/ief-2 than that of in abcg26, which suggesting that IEF is involved in an independent sporopollenin transportation pathway. This work reveal a novel gene, IEF regulated by AMS that is essential for exine formation.

Development of the Middle Layer in the Anther of Arabidopsis

The middle layer is an essential cell layer of the anther wall located between the endothecium and tapetum in Arabidopsis. Based on sectioning, the middle layer was found to be degraded at stage 7, which led to the separation of the tapetum from the anther wall. Here, we established techniques for live imaging of the anther. We created a marker line with fluorescent proteins expressed in all anther layers to study anther development. Several staining methods were used in the intact anthers to study anther cell morphology. We clarified the initiation, development, and degradation of the middle layer in Arabidopsis. This layer is initiated from both the inner and outer secondary parietal cells at stage 4, stopped cell division at stage 6, and finally degraded at stage 11. The neighboring cell layers, the epidermis, and endothecium continued cell division until stage 10, which led to a thin middle layer. The degradation of the tapetum cell wall at stage 7 lead to its isolation from the anther wall. This work presents fundamental information on the development of the middle layer, which facilitates the further investigation of anther development and plant fertility. These live imaging methods could be useful in future studies.

OsMS188 Is a Key Regulator of Tapetum Development and Sporopollenin Synthesis in Rice

BACKGROUND

During anther development, the tapetum provides essential nutrients and materials for pollen development. In rice, multiple transcription factors and enzymes essential for tapetum development and pollen wall formation have been cloned from male-sterile lines.

RESULTS

In this study, we obtained several lines in which the MYB transcription factor OsMS188 was knocked out through the CRISPR-Cas9 approach. The osms188 lines exhibited a male-sterile phenotype with aberrant development and degeneration of tapetal cells, absence of the sexine layer and defective anther cuticles. CYP703A3, CYP704B2, OsPKS1, OsPKS2, DPW and ABCG15 are sporopollenin synthesis and transport-related genes in rice. Plants with mutations in these genes are male sterile, with a defective sexine layer and anther cuticle. Further biochemical assays demonstrated that OsMS188 binds directly to the promoters of these genes to regulate their expression. UDT1, OsTDF1, TDR, bHLH142 and EAT1 are upstream regulators of rice tapetum development. Electrophoretic mobility shift assays (EMSAs) and activation assays revealed that TDR directly regulates OsMS188 expression. Additionally, protein interaction assays indicated that TDR interacts with OsMS188 to regulate downstream gene expression.

CONCLUSION

Overall, OsMS188 is a key regulator of tapetum development and pollen wall formation. The gene regulatory network established in this work may facilitate future investigations of fertility regulation in rice and in other crop species.

Potential Association of Reactive Oxygen Species With Male Sterility in Peach

Male sterility is an important agronomic trait for hybrid vigor utilization and hybrid seed production, but its underlying mechanisms remain to be uncovered. Here, we investigated the mechanisms of male sterility in peach using a combined cytology, physiology, and molecular approach. Cytological features of male sterility include deformed microspores and tapetum cells along with absence of pollen grains. Microspores had smaller nucleus at the mononuclear stage and were compressed into belts and subsequently disappeared in the anther cavity, whereas tapetum cells were swollen and vacuolated, with a delayed degradation to flowering time. Male sterile anthers had an ROS burst and lower levels of major antioxidants, which may cause abnormal development of microspores and tapetum, leading to male sterility in peach. In addition, the male sterility appears to be cytoplasmic in peach, which could be due to sequence variation in the mitochondrial genome. Our results are helpful for further investigation of the genetic mechanisms underlying male sterility in peach.

Overexpression of Two CCCH-type Zinc-Finger Protein Genes Leads to Pollen Abortion in Brassica campestris ssp. chinensis

The pollen grains produced by flowering plants are vital for sexual reproduction. Previous studies have shown that two CCCH-type zinc-finger protein genes in Brassica campestris, BcMF30a and BcMF30c, are involved in pollen development. Due to their possible functional redundancy, gain-of-function analysis is helpful to reveal their respective biological functions. Here, we found that the phenotypes of BcMF30a and BcMF30c overexpression transgenic plants driven by their native promoters were similar, suggesting their functional redundancy. The results showed that the vegetative growth was not affected in both transgenic plants, but male fertility was reduced. Further analysis found that the abortion of transgenic pollen was caused by the degradation of pollen contents from the late uninucleate microspore stage. Subcellular localization analysis demonstrated that BcMF30a and BcMF30c could localize in cytoplasmic foci. Combined with the studies of other CCCH-type genes, we speculated that the overexpression of these genes can induce the continuous assembly of abnormal cytoplasmic foci, thus resulting in defective plant growth and development, which, in this study, led to pollen abortion. Both the overexpression and knockout of BcMF30a and BcMF30c lead to abnormal pollen development, indicating that the appropriate expression levels of these two genes are critical for the maintenance of normal pollen development.

Increased expression of the MALE STERILITY1 transcription factor gene results in temperature-sensitive male sterility in barley

Understanding the control of fertility is critical for crop yield and breeding; this is particularly important for hybrid breeding to capitalize upon the resultant hybrid vigour. Different hybrid breeding systems have been adopted; however, these are challenging and crop specific. Mutants with environmentally reversible fertility offer valuable opportunities for hybrid breeding. The barley HvMS1 gene encodes a PHD-finger transcription factor that is expressed in the anther tapetum, which is essential for pollen development and causes complete male sterility when overexpressed in barley. This male sterility is due at least in part to indehiscent anthers resulting from incomplete tapetum degeneration, failure of anther opening, and sticky pollen under normal growth conditions (15 °C). However, dehiscence and fertility are restored when plants are grown at temperatures >20 °C, or when transferred to >20 °C during flowering prior to pollen mitosis I, with transfer at later stages unable to rescue fertility in vivo. As far as we are aware, this is the first report of thermosensitive male sterility in barley. This offers opportunities to understand the impact of temperature on pollen development and potential applications for environmentally switchable hybrid breeding systems; it also provides a 'female' male-sterile breeding tool that does not need emasculation to facilitate backcrossing.

Arabidopsis FAX1 mediated fatty acid export is required for the transcriptional regulation of anther development and pollen wall formation

The mutation of FAX1 (Fatty Acid Export 1) disrupts ROS homeostasis and suppresses transcription activity of DYT1-TDF1-AMS-MS188 genetic network, leading to atypical tapetum PCD and defective pollen formation in Arabidopsis. Fatty acids (FAs) have multiple important biological functions and exert diverse cellular effects through modulating Reactive Oxygen Species (ROS) homeostasis. Arabidopsis FAX1 (Fatty Acid Export 1) mediates the export of de novo synthesized FA from chloroplast and loss of function of FAX1 impairs male fertility. However, mechanisms underlying the association of FAX1-mediated FA export with male sterility remain enigmatic. In this study, by using an integrated approach that included morphological, cytological, histological, and molecular analyses, we revealed that loss of function of FAX1 breaks cellular FA/lipid homeostasis, which disrupts ROS homeostasis and suppresses transcriptional activation of the DYT1-TDF1-AMS-MS188 genetic network of anther development, impairing tapetum development and pollen wall formation, and resulting in male sterility. This study provides new insights into the regulatory network for male reproduction in plants, highlighting an important role of FA export-mediated ROS homeostasis in the process.

MS1, a direct target of MS188, regulates the expression of key sporophytic pollen coat protein genes in Arabidopsis

Sporophytic pollen coat proteins (sPCPs) derived from the anther tapetum are deposited into pollen wall cavities and function in pollen-stigma interactions, pollen hydration, and environmental protection. In Arabidopsis, 13 highly abundant proteins have been identified in pollen coat, including seven major glycine-rich proteins GRP14, 16, 17, 18, 19, 20, and GRP-oleosin; two caleosin-related family proteins (AT1G23240 and AT1G23250); three lipase proteins EXL4, EXL5 and EXL6, and ATA27/BGLU20. Here, we show that GRP14, 17, 18, 19, and EXL4 and EXL6 fused with green fluorescent protein (GFP) are translated in the tapetum and then accumulate in the anther locule following tapetum degeneration. The expression of these sPCPs is dependent on two essential tapetum transcription factors, MALE STERILE188 (MS188) and MALE STERILITY 1 (MS1). The majority of sPCP genes are up-regulated within 30 h after MS1 induction and could be restored by MS1 expression driven by the MS188 promoter in ms188, indicating that MS1 is sufficient to activate their expression; however, additional MS1 downstream factors appear to be required for high-level sPCP expression. Our ChIP, in vivo transactivation assay, and EMSA data indicate that MS188 directly activates MS1. Together, these results reveal a regulatory cascade whereby outer pollen wall formation is regulated by MS188 followed by synthesis of sPCPs controlled by MS1.

Characterization and transcriptome analysis of a dominant genic male sterile cotton mutant

BACKGROUND

Male sterility is an efficient trait for hybrid seed production and germplasm innovation. Until now, most studies on male sterility were on cytoplasmic and recessive genic sterility, with few on dominant genic male sterility, especially in cotton, due to lack of such mutant.

RESULTS

We discovered a natural male sterile (MS) Sea Island cotton (G. barbadense) mutant. Genetic analysis showed the mutation was caused by a dominant mutation in a single nuclear gene. Comparative cytological observation of anther sections from MS and wild-type (WT) uncovered cellular differences in anther at and after the tetrad stage of pollen mother cells (PMC). In the MS anthers, the outer wall of pollen grains was free of spinules, the tapetum was vacuolated and showed delayed degradation, consequently, no functional pollen grains. Comparison of transcriptomes from meiosis, tetrad, mononuclear and binuclear pollen, and pollen maturation stages identified 13,783 non-redundant differentially expressed genes (DEGs) between MS and WT. Based on the number of DEGs, analyses of enriched GO terms and KEGG pathways, it was evident that significant transcriptomic changes occurred at and after the tetrad stage, consistent with cytological observation, and that the major differences were on metabolism of starch, sucrose, ascorbate, aldarate, alanine, aspartate and glutamate, and biosynthesis of cutin, suberine and wax. WGCNA analysis identified five modules containing 920 genes highly related to anther development, especially the greenyellow module with 54 genes that was highly associated with PMC meiosis and tetrad formation. A NAC transcription factor (Gh_D11G2469) was identified as a hub gene for this module, which warrants further functional characterization.

CONCLUSIONS

We demonstrated that the MS trait was controlled by a single dominant nuclear gene and caused by delayed tapetum degradation at the tetrad stage. Comparative transcriptome analysis and gene network construction identified DEGs, enriched GO terms and metabolic pathways, and hub genes potentially associated with anther development and the MS trait. These results contribute to our understanding of dominant genic male sterility (DGMS) and provided source for innovation of cotton germplasm.

Brassinosteroid-mediated reactive oxygen species are essential for tapetum degradation and pollen fertility in tomato

Phytohormone brassinosteroids (BRs) are essential for plant growth and development, but the mechanisms of BR-mediated pollen development remain largely unknown. In this study, we show that pollen viability, pollen germination and seed number decreased in the BR-deficient mutant d^{^im} , which has a lesion in the BR biosynthetic gene DWARF (DWF), and in the bzr1 mutant, which is deficient in BR signaling regulator BRASSINAZOLE RESISTANT 1 (BZR1), compared with those in wild-type plants, whereas plants overexpressing DWF or BZR1 exhibited the opposite effects. Loss or gain of function in the DWF or BZR1 genes altered the timing of reactive oxygen species (ROS) production and programmed cell death (PCD) in tapetal cells, resulting in delayed or premature tapetal degeneration, respectively. Further analysis revealed that BZR1 could directly bind to the promoter of RESPIRATORY BURST OXIDASE HOMOLOG 1 (RBOH1), and that RBOH1-mediated ROS promote pollen and seed development by triggering PCD and tapetal cell degradation. In contrast, the suppression of RBOH1 compromised BR signaling-mediated ROS production and pollen development. These findings provide strong evidence that BZR1-dependent ROS production plays a critical role in the BR-mediated regulation of tapetal cell degeneration and pollen development in Solanum lycopersicum (tomato) plants.

The temporal regulation of TEK contributes to pollen wall exine patterning

Pollen wall consists of several complex layers which form elaborate species-specific patterns. In Arabidopsis, the transcription factor ABORTED MICROSPORE (AMS) is a master regulator of exine formation, and another transcription factor, TRANSPOSABLE ELEMENT SILENCING VIA AT-HOOK (TEK), specifies formation of the nexine layer. However, knowledge regarding the temporal regulatory roles of TEK in pollen wall development is limited. Here, TEK-GFP driven by the AMS promoter was prematurely expressed in the tapetal nuclei, leading to complete male sterility in the pAMS:TEK-GFP (pat) transgenic lines with the wild-type background. Cytological observations in the pat anthers showed impaired callose synthesis and aberrant exine patterning. CALLOSE SYNTHASE5 (CalS5) is required for callose synthesis, and expression of CalS5 in pat plants was significantly reduced. We demonstrated that TEK negatively regulates CalS5 expression after the tetrad stage in wild-type anthers and further discovered that premature TEK-GFP in pat directly represses CalS5 expression through histone modification. Our findings show that TEK flexibly mediates its different functions via different temporal regulation, revealing that the temporal regulation of TEK is essential for exine patterning. Moreover, the result that the repression of CalS5 by TEK after the tetrad stage coincides with the timing of callose wall dissolution suggests that tapetum utilizes temporal regulation of genes to stop callose wall synthesis, which, together with the activation of callase activity, achieves microspore release and pollen wall patterning.

To develop into mature pollen grains, microspores require formation of the pollen wall. To date, pollen wall developmental events, including production and transportation of pollen wall components, synthesis and degradation of the callose wall, and deposition and demixing of primexine, have been studied in Arabidopsis, and a number of anther- or tapetum-specific genes involved in pollen wall formation have been uncovered. However, whether the specific expression patterns of these genes contribute to pollen wall development or patterning remains unclear. Here, we show that TEK, a transcription factor that specifies formation of nexine (the inner layer of the pollen wall exine), represses the expression of the callose synthase CalS5 after the tetrad stage, which accurately fits with the timing of callose wall dissolution causing microspore release. Moreover, we show that premature expression of TEK in the wild-type anthers disturbs callose wall synthesis and pollen wall patterning. This work reveals that a pollen wall regulator must be kept under a strict temporal control to perform its functions, and that these temporal controls are coordinated with other pollen wall developmental events to determine pollen wall formation and patterning.

OsMYB80 Regulates Anther Development and Pollen Fertility by Targeting Multiple Biological Pathways

Pollen development is critical to the reproductive success of flowering plants, but how it is regulated is not well understood. Here, we isolated two allelic male-sterile mutants of OsMYB80 and investigated how OsMYB80 regulates male fertility in rice. OsMYB80 was barely expressed in tissues other than anthers, where it initiated the expression during meiosis, reached the peak at the tetrad-releasing stage and then quickly declined afterward. The osmyb80 mutants exhibited premature tapetum cell death, lack of Ubisch bodies, no exine and microspore degeneration. To understand how OsMYB80 regulates anther development, RNA-seq analysis was conducted to identify genes differentially regulated by OsMYB80 in rice anthers. In addition, DNA affinity purification sequencing (DAP-seq) analysis was performed to identify DNA fragments interacting with OsMYB80 in vitro. Overlap of the genes identified by RNA-seq and DAP-seq revealed 188 genes that were differentially regulated by OsMYB80 and also carried an OsMYB80-interacting DNA element in the promoter. Ten of these promoter elements were randomly selected for gel shift assay and yeast one-hybrid assay, and all showed OsMYB80 binding. The 10 promoters also showed OsMYB80-dependent induction when co-expressed in rice protoplast. Functional annotation of the 188 genes suggested that OsMYB80 regulates male fertility by directly targeting multiple biological processes. The identification of these genes significantly enriched the gene networks governing anther development and provided much new information for the understanding of pollen development and male fertility.

A Silent Exonic Mutation in a Rice Integrin-α FG-GAP Repeat-Containing Gene Causes Male-Sterility by Affecting mRNA Splicing

Pollen development plays crucial roles in the life cycle of higher plants. Here we characterized a rice mutant with complete male-sterile phenotype, pollen-less 1 (pl1). pl1 exhibited smaller anthers with arrested pollen development, absent Ubisch bodies, necrosis-like tapetal hypertrophy, and smooth anther cuticular surface. Molecular mapping revealed a synonymous mutation in the fourth exon of PL1 co-segregated with the mutant phenotype. This mutation disrupts the exon-intron splice junction in PL1, generating aberrant mRNA species and truncated proteins. PL1 is highly expressed in the tapetal cells of developing anther, and its protein is co-localized with plasma membrane (PM) and endoplasmic reticulum (ER) signal. PL1 encodes an integrin-α FG-GAP repeat-containing protein, which has seven β-sheets and putative Ca²⁺-binding motifs and is broadly conserved in terrestrial plants. Our findings therefore provide insights into both the role of integrin-α FG-GAP repeat-containing protein in rice male fertility and the influence of exonic mutation on intronic splice donor site selection.

Phenotypic, genetic, and molecular function of msc-2, a genic male sterile mutant in pepper (Capsicum annuum L.)

Bulked segregant analysis and fine mapping delimited the pepper genic male sterile (msc-2) locus into a 336 kb region on chromosome 5. A strong candidate gene, Capana05g000766, a homolog of AtMS1, was indentified in this region. Genic male sterility (msc-2) is used to produce hybrid seeds in Northern China. However, no co-segregated markers have been reported or candidate genes controlling this trait have been cloned. Here, bulked segregant analysis and genotyping of an F₂ population and a 18Q5431AB line were employed to fine map msc-2, which was delimited to a 336 kb region. In this region, Capana05g000766 was a homolog of AtMS1, which encodes a plant homeodomain finger involved in tapetum development. A "T" deletion in the Capana05g000766 locus leads to a premature stop codon, which may cause a loss-of-function mutation. Real-time PCR analysis revealed that Capana05g000766 was an anther-specific gene and down-regulation of the gene resulted in male sterility. Therefore, Capana05g000766 was identified as the strongest candidate gene for the msc-2 locus. Allelism tests showed that msc-1 and msc-2 were nonallelic, and bimolecular fluorescence complementation analysis indicated that the two genes did not interact directly with each other at the protein level. As msc-1 and msc-2 are homologs of AtDYT1 and AtMS1 in Arabidopsis, they may play similar roles in tapetum development in genic male sterile peppers, and Msc-1 might be up stream of Msc-2 in the regulation of other genes involved in tapetum development.

Abnormal tapetum development in hermaphrodites of an androdioecious tree, Tapiscia sinensis

Tapiscia sinensis Oliv. (Tapisciaceae) has been proven to be a functional androdioecious species with both male and hermaphroditic individuals, and the pollen viability of males is far higher than that of hermaphrodites. To better understand the causes of the low pollen viability in hermaphroditic flowers, different stages of anther development were observed. We found that hermaphroditic flowers exhibit abnormal tapetum development, resulting in low pollen viability. To clarify the underlying molecular mechanism of abnormal tapetum development in hermaphrodites, quantitative real-time PCR analyses were performed. The results revealed that the expression levels of an important transcription factor for tapetum development and function, T. sinensis DYSFUNCTIONAL TAPETUM1 (TsDYT1), and its potential downstream regulatory genes T. sinensis DEFECTIVE in TAPETAL DEVELOPMENT and FUNCTION1 (TsTDF1), T. sinensis ABORTED MICROSPORE (TsAMS) and T. sinensis MALE STERILITY 1 (TsMS1) were all significantly downregulated in hermaphrodites compared with males at some key stages of anther development. The amino acid sequence similarity, expression pattern, gene structure and subcellular localization of these genes were analyzed, and the results indicated functional conservation between T. sinensis and homologues in Arabidopsis thaliana. Next, rapid amplification of cDNA end and thermal asymmetric interlaced PCR were employed to clone the full-length cDNA and promoter sequences of these genes, respectively. In addition, results of yeast two-hybrid analysis showed that TsDYT1 can form heterodimers with TsAMS, and yeast one-hybrid analysis demonstrated that TsDYT1 directly binds to the promoter regions of TsTDF1 and TsMS1. TsTDF1 can directly regulate expression of TsAMS, suggesting that a functionally conserved pathway exists between A. thaliana and T. sinensis to regulate tapetum development. In conclusion, the results suggest that abnormal expression of core transcription factors for tapetum development, including TsDYT1, TsTDF1, TsAMS and TsMS1, plays an important role in the abnormal development of the tapetum in T. sinensis hermaphrodites. Furthermore, a hermaphroditic tapetum with abnormal function causes the low pollen viability of hermaphroditic trees. Our results provide new insight into our understanding of the underlying mechanism of why pollen viability is much higher in males than hermaphrodites of the androdioecious tree T. sinensis.

Differential gene expression among three sex types reveals a MALE STERILITY 1 (CpMS1) for sex differentiation in papaya

BACKGROUND

Carica papaya is a trioecious plant species with a genetic sex-determination system defined by sex chromosomes. Under unfavorable environmental conditions male and hermaphrodite exhibit sex-reversal. Previous genomic research revealed few candidate genes for sex differentiation in this species. Nevertheless, more analysis is still needed to identify the mechanism responsible for sex flower organ development in papaya.

RESULTS

The aim of this study was to identify differentially expressed genes among male, female and hermaphrodite flowers in papaya during early (pre-meiosis) and later (post-meiosis) stages of flower development. RNA-seq was used to evaluate the expression of differentially expressed genes and RT-qPCR was used to verify the results. Putative functions of these genes were analyzed based on their homology with orthologs in other plant species and their expression patterns. We identified a Male Sterility 1 gene (CpMS1) highly up-regulated in male and hermaphrodite flower buds compared to female flower buds, which expresses in small male flower buds (3-8 mm), and that might be playing an important role in male flower organ development due to its homology to MS1 genes previously identified in other plants. This is the first study in which the sex-biased expression of genes related to tapetum development in the anther developmental pathway is being reported in papaya. Besides important transcription factors related to flower organ development and flowering time regulation, we identified differential expression of genes that are known to participate in ABA, ROS and auxin signaling pathways (ABA-8-hydroxylases, AIL5, UPBEAT 1, VAN3-binding protein).

CONCLUSIONS

CpMS1 was expressed in papaya male and hermaphrodite flowers at early stages, suggesting that this gene might participate in male flower organ development processes, nevertheless, this gene cannot be considered a sex-determination gene. Due to its homology with other plant MS1 proteins and its expression pattern, we hypothesize that this gene participates in anther development processes, like tapetum and pollen development, downstream gender specification. Further gene functional characterization studies in papaya are required to confirm this hypothesis. The role of ABA and ROS signaling pathways in papaya flower development needs to be further explored as well.

Investigation of the genes associated with a male sterility mutant (msm) in Chinese cabbage (Brassica campestris ssp. pekinensis) using RNA-Seq

In Chinese cabbage, hybrid seed production is performed using male sterility lines, an important approach to heterosis utilization. In this study, a stably inherited male sterile mutant msm was obtained from the 'FT'-doubled haploid line of Chinese cabbage using isolated microspore culture combined with ⁶⁰Co γ-ray mutagenesis. The genetic backgrounds of 'FT' and msm were highly consistent; however, compared with wild-type 'FT', msm exhibited completely degenerated stamens and no pollen phenotype. Other characters showed no significant differences. Cytological observations revealed that stamen abortion in msm begins during the tetrad period and that tapetum cells were abnormally expanded and highly vacuolated, leading to microspore abortion. Genetic analysis indicated that the msm mutant phenotype is controlled by a single recessive nuclear gene. Comparative transcriptome analysis of 'FT' and msm flower buds using RNA-Seq technology revealed 1653 differentially expressed genes, among which, a large number associated with male sterility were detected, including 64 pollen development- and pollen tube growth-related genes, 94 pollen wall development-related genes, 11 phytohormone-related genes, and 16 transcription factor-related genes. An overwhelming majority of these genes were down-regulated in msm compared with 'FT'. Furthermore, KEGG pathway analysis indicated that a variety of carbohydrate metabolic and lipid metabolic pathways were significantly enriched, which may be related to pollen abortion. The expression patterns of 24 male sterility-related genes were analyzed using qRT-PCR. In addition, 24,476 single-nucleotide polymorphisms and 413,073 insertion-deletion events were specifically detected in msm. These results will facilitate elucidation of the regulatory mechanisms underlying male sterility in Chinese cabbage.

Different expression pattern of flowering pathway genes contribute to male or female organ development during floral transition in the monoecious weed Ambrosia artemisiifolia L. (Asteraceae)

The highly allergenic and invasive weed Ambrosia artemisiifolia L. is a monoecius plant with separated male and female flowers. The genetic regulation of floral morphogenesis is a less understood field in the reproduction biology of this species. Therefore the objective of this work was to investigate the genetic control of sex determination during floral organogenesis. To this end, we performed a genome-wide transcriptional profiling of vegetative and generative tissues during the plant development comparing wild-growing and in vitro cultivated plants. RNA-seq on Illumina NextSeq 500 platform with an integrative bioinformatics analysis indicated differences in 80 floral gene expressions depending on photoperiodic and endogenous initial signals. Sex specificity of genes was validated based on RT-qPCR experiments. We found 11 and 16 uniquely expressed genes in female and male transcriptomes that were responsible particularly to maintain fertility and against abiotic stress. High gene expression of homologous such as FD, FT, TFL1 and CAL, SOC1, AP1 were characteristic to male and female floral meristems during organogenesis. Homologues transcripts of LFY and FLC were not found in the investigated generative and vegetative tissues. The repression of AP1 by TFL1 homolog was demonstrated in male flowers resulting exclusive expression of AP2 and PI that controlled stamen and carpel formation in the generative phase. Alterations of male and female floral meristem differentiation were demonstrated under photoperiodic and hormonal condition changes by applying in vitro treatments.

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.

Acyl-CoA synthetases from Physcomitrella, rice and Arabidopsis: different substrate preferences but common regulation by MS188 in sporopollenin synthesis

ACOS5, OsACOS12 and PpACOS6 are all capable of fatty acyl-CoA synthetase activity but exhibit different substrate preferences. The transcriptional regulation of ACOS for sporopollenin synthesis appears to have been conserved in Physcomitrella, rice and Arabidopsis during evolution. Sporopollenin is the major constituent of spore and pollen exines. In Arabidopsis, acyl-CoA synthetase 5 (ACOS5) is an essential enzyme for sporopollenin synthesis, and its orthologues are PpACOS6 from the moss Physcomitrella and OsACOS12 from monocot rice. However, knowledge regarding the evolutionary conservation and divergence of the ACOS gene in sporopollenin synthesis remains limited. In this study, we analysed the function and regulation of PpACOS6 and OsACOS12. A complementation test showed that OsACOS12 driven by the ACOS5 promoter could partially restore the male fertility of the acos5 mutant in Arabidopsis, while PpACOS6 did not rescue the acos5 phenotype. ACOS5, PpACOS6 and OsACOS12 all complemented the acyl-CoA synthetase-deficient yeast strain (YB525) phenotype, although they exhibited different substrate preferences. To understand the conservation of sporopollenin synthesis regulation, we constructed two constructs with ACOS5 driven by the OsACOS12 or PpACOS6 promoter. Both constructs could restore the fertility of acos5 plants. The MYB transcription factor MS188 from Arabidopsis directly regulates ACOS5. We found that MS188 could also bind the promoters of OsACOS12 and PpACOS6 and activate the genes driven by the promoters, suggesting that the transcriptional regulation of these genes was similar to that of ACOS5. These results show that the ACOS gene promoter region from Physcomitrella, rice and Arabidopsis has been functionally conserved during evolution, while the chain lengths of fatty acid-derived monomers of sporopollenin vary in different plant species.

A VPE-like protease NtTPE8 exclusively expresses in the integumentary tapetum and is involved in seed development

Programmed cell death (PCD) is an essential process for development, and shows conserved cytological features in both plants and animals. Caspases are well-known critical components of the PCD machinery in animals. However, currently few typical counterparts have been identified in plants and only several caspase-like proteases are known to be involved in plant PCD, indicating the existence of great challenge for confirming new caspase-like proteases and elucidating the mechanisms regulating plant PCD. Here, we report a novel cysteine protease, NtTPE8, which was extracted from tobacco seeds and confirmed as a new caspase-like protease. Recombinant NtTPE8 exhibited legumain and caspase-like proteolytic activities, both of which could be inhibited by the pan-caspase inhibitor (Z-VAD-FMK). Notably, NtTPE8 possessed several caspase activities and the capacity to cleave the cathepsin H substrate FVR, indicating a unique character of NtTPE8. NtTPE8 was exclusively expressed in the integumentary tapetum and thus, is the first specific molecular marker reported to date for this cell type. Down-regulation of NtTPE8 caused seed abortion, via disturbing early embryogenesis, indicating its critical role in embryogenesis and seed development. In conclusion, we identified a novel caspase-like cysteine protease, NtTPE8, exclusively expressed in the integumentary tapetum that is involved in seed development.

Complementary Transcriptomic and Proteomic Analysis Reveals a Complex Network Regulating Pollen Abortion in GMS (msc-1) Pepper (Capsicum annuum L.)

Pepper (Capsicum annuum L.) is a globally important horticultural crop. Use of the genic male-sterile (GMS) line enables efficient commercial hybrid pepper seed production. However, the mechanisms of pepper GMS functioning remain unclear. In this study, we used proteomic and transcriptomic analysis to identify proteins and genes related to genic male sterility. A total of 764 differentially expressed proteins (DEPs) and 1069 differentially expressed genes (DEGs) were identified in the proteomic and transcriptomic level respectively, and 52 genes (hereafter “cor-DEGs-DEPs” genes) were detected at both levels. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis identified 13 DEPs and 14 DEGs involved in tapetum and pollen development. Among the 13 DEPs identified, eight were involved in pollen exine formation, and they were all up-regulated in the fertile line 16C1369B. For the 14 DEGs identified, ABORTED MICROSPORES (AMS) and DEFECTIVE IN TAPETAL DEVELOPMENT AND FUNCTION1 (TDF1) were involved in tapetum development, and both are possibly regulated by Msc-1. All of these genes were detected and confirmed by qRT-PCR. The presence of these genes suggests their possible role in tapetum and pollen exine formation in GMS pepper. Most key genes and transcription factors involved in these processes were down-regulated in the sterile line 16C1369A. This study provides a better understanding of GMS (msc-1) molecular functioning in pepper.

Progressive programmed cell death inwards across the anther wall in male sterile flowers of the gynodioecious plant Plantago lanceolata

A cell death signal is perceived and responded to by epidermal cells first before being conveyed inwards across the anther wall in male sterile Plantago lanceolata flowers. In gynodioecious plants, floral phenotype is determined by an interplay between cytoplasmic male sterility (CMS)-promoting factors and fertility-restoring genes segregating in the nuclear background. Plantago lanceolata exhibits at least four different sterilizing cytoplasms. MS1, a "brown-anther" male sterile phenotype, segregates with a CMSI cytoplasm and a non-restoring nuclear background in P. lanceolata populations. The aim of this study was to investigate the cytology of early anther development in segregating hermaphrodite and male sterile flowers sharing the same CMSI cytoplasm, and to determine if the sterility phenotype correlates with any changes to the normal pattern of programmed cell death (PCD) that occurs during anther development. Cytology shows cellular abnormalities in all four anther wall layers (epidermis, endothecium, middle layer and tapetum), the persistence and enlargement of middle layer and tapetal cells, and the failure of microspore mother cells to complete meiosis in male sterile anthers. In these anthers, apoptotic-PCD occurs earlier than in fertile anthers and is detected in all four cell layers of the anther wall before the middle layer and tapetal cells become enlarged. PCD is separated spatially and temporally within the anther wall, occurring first in epidermal cells before extending radially to cells in the inner anther wall layers. This is the first evidence of a cell death signal being perceived and responded to by epidermal cells first before being conveyed inwards across the anther wall in male sterile plants.

Functional Disruption of the Tomato Putative Ortholog of HAWAIIAN SKIRT Results in Facultative Parthenocarpy, Reduced Fertility and Leaf Morphological Defects

A number of plant microRNAs have been demonstrated to regulate developmental processes by integrating internal and environmental cues. Recently, the Arabidopsis thaliana F-box protein HAWAIIAN SKIRT (HWS) gene has been described for its role in miRNA biogenesis. We have isolated in a forward genetic screen a tomato (Solanum lycopersicum) line mutated in the putative ortholog of HWS. We show that the tomato hws-1 mutant exhibits reduction in leaflet serration, leaflet fusion, some degree of floral organ fusion, and alteration in miRNA levels, similarly to the original A. thaliana hws-1 mutant. We also describe novel phenotypes for hws such as facultative parthenocarpy, reduction in fertility and flowering delay. In slhws-1, the parthenocarpy trait is influenced by temperature, with higher parthenocarpy rate in warmer environmental conditions. Conversely, slhws-1 is able to produce seeds when grown in cooler environment. We show that the reduction in seed production in the mutant is mainly due to a defective male function and that the levels of several miRNAs are increased, in accordance with previous HWS studies, accounting for the abnormal leaf and floral phenotypes as well as the altered flowering and fruit development processes. This is the first study of HWS in fleshy fruit plant, providing new insights in the function of this gene in fruit development.

A putative bHLH transcription factor is a candidate gene for male sterile 32, a locus affecting pollen and tapetum development in tomato

The tomato (Solanum lycopersicum) male sterile 32 (ms32) mutant has been used in hybrid seed breeding programs largely because it produces no pollen and has exserted stigmas. In this study, histological examination of anthers revealed dysfunctional pollen and tapetum development in the ms32 mutant. The ms32 locus was fine mapped to a 28.5 kb interval that encoded four putative genes. Solyc01g081100, a homolog of Arabidopsis bHLH10/89/90 and rice EAT1, was proposed to be the candidate gene of MS32 because it contained a single nucleotide polymorphism (SNP) that led to the formation of a premature stop codon. A codominant derived cleaved amplified polymorphic sequence (dCAPS) marker, MS32D, was developed based on the SNP. Real-time quantitative reverse-transcription PCR showed that most of the genes, which were proposed to be involved in pollen and tapetum development in tomato, were downregulated in the ms32 mutant. These findings may aid in marker-assisted selection of ms32 in hybrid breeding programs and facilitate studies on the regulatory mechanisms of pollen and tapetum development in tomato.

OsPKS1 is required for sexine layer formation, which shows functional conservation between rice and Arabidopsis

The sporopollenin precursors, as a general constituent of sexine, are synthesized in the tapetum and deposited on the pollen surface after transportation and processing. The polyketide synthase condenses the acyl-CoA into a hydroxyalkyl α-pyrone, which is predicted to be a component of the sporopollenin precursors. In this study, we found that the rice POLYKETIDE SYNTHASE 1 (OsPKS1) was the orthologue of Arabidopsis POLYKETIDE SYNTHASE A/LESS ADHESIVE POLLEN 6 (PKSA/LAP6) through sequence alignment. The OsPKS1 knockout mutants obtained by Crispr-Cas9-mediated editing exhibited a complete male sterile phenotype. Cytological observations revealed that abnormal bacula deposition and ubisch body structures for sexine formation led to pollen rupture in ospks1. The expression analysis showed that the OsPKS1 was highly expressed in tapetal cells and anther locules from stage 9 to stage 11 during anther development in rice. Subcellular localization demonstrated that the OsPKS1 protein was preferentially localized to the ER. The genomic sequence of OsPKS1 driven by the PKSA/LAP6 promoter restored the sexine pattern of Arabidopsis pksa/lap6. These results indicated that OsPKS1 is required for sexine layer formation in rice and functionally conserved in the sporopollenin synthesis pathway.

The key genes and pathways related to male sterility of eggplant revealed by comparative transcriptome analysis

BACKGROUND

Male sterility (MS) is an effective tool for hybrid production. Although MS has been widely reported in other plants, such as Arabidopsis and rice, the molecular mechanism of MS in eggplant is largely unknown. To understand the mechanism, the comparative transcriptomic file of MS line and its maintainer line was analyzed with the RNA-seq technology.

RESULTS

A total of 11,7695 unigenes were assembled and 19,652 differentially expressed genes (DEGs) were obtained. The results showed that 1,716 DEGs were shared in the three stages. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis indicated that these DEGs were mainly involved in oxidation-reduction, carbohydrate and amino acid metabolism. Moreover, transcriptional regulation was also the impact effector for MS and anther development. Weighted correlation network analysis (WGCNA) showed two modules might be responsible for MS, which was similar to hierarchical cluster analysis.

CONCLUSIONS

A number of genes and pathways associated with MS were found in this study. This study threw light on the molecular mechanism of MS and identified several key genes related to MS in eggplant.

Identification of candidate genes underlying genic male-sterile msc-1 locus via genome resequencing in Capsicum annuum L

Based on genome resequencing, a strong candidate gene Capana02g002096 was identified in this study. Capana02g002096 encodes a homolog of AtDYT1 which is a bHLH transcription factor and involves in the early tapetal development. Genic male-sterile line is an efficient tool for commercial hybrid seed production in pepper; however, so far, only few genes controlling this trait have been cloned. A spontaneous genic male-sterile mutant, msc-1, had been identified and widely used in China, of which the male-sterile trait was proved to be controlled by a single recessive locus. For cloning the gene(s) underlying the msc-1 locus, genome resequencing and comparison analyses were performed between male-sterile and male-fertile lines. According to the genomic variations and genes' annotations, Capana02g002096 was selected as a candidate gene underlying the msc-1 locus. Capana02g002096 encodes a homolog of AtDYT1, which is a bHLH transcription factor and involves in the early tapetal development. Moreover, a 7-bp deletion was identified in the exon of Capana02g002096, which led to a premature stop codon and may cause a loss-of-function mutation. Further genotyping in the 16C1369AB population containing 1110 plants, a F₂ population consisting of 510 plants and 46 inbreed lines revealed that the male-sterile phenotype was co-segregated with the 7-bp deletion. Additionally, real-time PCR analysis revealed that Capana02g002096 was an anther-specific gene and repression of the gene's expression through VIGS led to male-sterile phenotype. Therefore, based on the evidence at genetic, genomic, transcriptional and posttranscriptional levels, Capana02g002096 was considered as a strong candidate gene underlying the msc-1 locus in pepper and was renamed Msc-1.

The zinc-finger transcription factor BcMF20 and its orthologs in Cruciferae which are required for pollen development

Brassica campestris Male Fertility 20 (BcMF20) is a typical zinc-finger transcription factor that was previously isolated from flower buds of Chinese cabbage (Brassica campestris ssp. chinensis). By applying expression pattern analysis, it can be known that BcMF20 was specifically and strongly expressed in tapetum and pollen, beginning from the uninucleate stage, and was maintained during the mature-pollen stage. As BcMF20 was highly conserved in Cruciferae, it can be indicated that this zinc-finger transcription factor is important during the growth of Cruciferae. In this study, 12 C2H2-type zinc-finger TFs which shared high homology with BcMF20 were found from NCBI via BLAST. A new molecular phylogenetic tree was constructed by the comparison between BcMF20 and these 12 C2H2-type zinc-finger TFs with NJ method. By analyzing this phylogenetic tree, the evolution of BcMF20 was discussed. Then, antisense RNA technology was applied in the transgenesis of Arabidopsis thaliana to get the deletion mutants of BcMF20, so that its function during the pollen development can be identified. The results showed: BcMF20 are in the same clade with three genes from Arabidopsis. The inhibition of BcMF20 expression led to smaller amounts of and lower rate in germination of pollen and lower rate in fruit setting in certain transgenetic plants. This also led to the complete collapse of pollen grains. By SEM and TEM, pollen morphology and anther development processes were observed. In the middle uninucleate microspore stage, a relatively thin or even no primexine was formed in microspores. This may result in the malformation of the pollen wall and finally cause the deformity of pollens. Above all, it can be indicated that BcMF20 may act as a part of regulation mechanisms of TAZ1 and MS1. Together they play a role in a genetic pathway in the tapetum to act on proliferation of tapetal cells and keep the normal development of pollens.

Proteomes and Phosphoproteomes of Anther and Pollen: Availability and Progress

In flowering plants, anther development plays crucial role in sexual reproduction. Within the anther, microspore mother cells meiosis produces microspores, which further develop into pollen grains that play decisive role in plant reproduction. Previous studies on anther biology mainly focused on single gene functions relying on genetic and molecular methods. Recently, anther development has been expanded from multiple OMICS approaches like transcriptomics, proteomics/phosphoproteomics, and metabolomics. The development of proteomics techniques allowing increased proteome coverage and quantitative measurements of proteins which can characterize proteomes and their modulation during normal development, biotic and abiotic stresses in anther development. In this review, we summarize the achievements of proteomics and phosphoproteomics with anther and pollen organs from model plant and crop species (i.e. Arabidopsis, rice, tobacco). The increased proteomic information facilitated translation of information from the models to crops and thus aid in agricultural improvement.

Positive regulation of AMS by TDF1 and the formation of a TDF1-AMS complex are required for anther development in Arabidopsis thaliana

Tapetum development and pollen production are regulated by a complex transcriptional network that consists of a group of tapetum-specific Arabidopsis transcription factors (TFs). Among these TFs, DEFECTIVE IN TAPETAL DEVELOPMENT AND FUNCTION 1 (TDF1) encodes an R2R3 MYB factor, and ABORTED MICROSPORE (AMS) encodes a basic helix-loop-helix (bHLH) factor. However, knowledge regarding the regulatory role of TDF1 in anther development remains limited. Here, we discovered that TDF1 directly regulates AMS via an AACCT cis-element. We found the precocious AMS transcript and absence of AMS protein in ams^-/- gpTDF1:AMS-FLAG lines, suggesting the timing of the TDF1-regulated AMS expression is a prerequisite for AMS functioning. We found that TDF1 interacts with AMS. Additionally, the TDF1-AMS complex additively promotes the expression of AMS-regulated genes, suggesting that TDF1 and AMS regulate the downstream genes through a feed-forward loop. EPXB5, encoding a beta-expansin family protein, is another direct target of TDF1, and it is highly expressed in the tapetum and pollen grains. The TDF1-AMS complex acts in concert to activate EXPB5 expression through a feed-forward loop. The identification of the regulatory pathway between TDF1 and AMS provides an interlocked feed-forward loop circuit that precisely regulates the transcriptional cascades that support anther development.

SHI/STY Genes Affect Pre- and Post-meiotic Anther Processes in Auxin Sensing Domains in Arabidopsis

In flowering plants, mature sperm cells are enclosed in pollen grains formed in structures called anthers. Several cell layers surrounding the central sporogenous cells of the anther are essential for directing the developmental processes that lead to meiosis, pollen formation, and the subsequent pollen release. The specification and function of these tissues are regulated by a large number of genetic factors. Additionally, the plant hormone auxin has previously been shown to play important roles in the later phases of anther development. Using the R2D2 auxin sensor system we here show that auxin is sensed also in the early phases of anther cell layer development, suggesting that spatiotemporal regulation of auxin levels is important for early anther morphogenesis. Members of the SHI/STY transcription factor family acting as direct regulators of YUC auxin biosynthesis genes have previously been demonstrated to affect early anther patterning. Using reporter constructs we show that SHI/STY genes are dynamically active throughout anther development and their expression overlaps with those of three additional downstream targets, PAO5, EOD3 and PGL1. Characterization of anthers carrying mutations in five SHI/STY genes clearly suggests that SHI/STY transcription factors affect anther organ identity. In addition, their activity is important to repress periclinal cell divisions as well as premature entrance into programmed cell death and cell wall lignification, which directly influences the timing of anther dehiscence and the pollen viability. The SHI/STY proteins also prevent premature pollen germination suggesting that they may play a role in the induction or maintenance of pollen dormancy.

OsCER1 Plays a Pivotal Role in Very-Long-Chain Alkane Biosynthesis and Affects Plastid Development and Programmed Cell Death of Tapetum in Rice (Oryza sativa L.)

Cuticle waxes, which are primarily comprised of very-long-chain (VLC) alkanes, play an important role in plant reproductive development. ECERIFERUM1 (CER1) is recognized as the core element for VLC alkane biosynthesis in Arabidopsis (Arabidopsis thaliana). However, genes involved in the VLC alkane biosynthesis in rice remain unclear, and the alkane-form pathway in rice has still to be further explored. Here, we show that OsCER1, a homology of CER1, functions in VLC alkanes biosynthesis, which also could regulate anther development and plastids differentiation in rice. OsCER1 was highly expressed in the tapetum (stage 10) and bicellular pollen cells (stage 11). The decreased content of VLC alkanes (C25 and C27) in the OsCER1 knocked down plants as well as the increased content of C27 alkanes in the OsCER1 overexpression plants indicates that OsCER1 participates in VLC alkane biosynthesis. Downregulation of OsCER1 in rice led to sterility, and fewer amyloplasts within the mature pollen grains. In addition, the downregulation of OsCER1 in rice caused delayed tapetal programmed cell death and abnormal development of plastids in the tapetal cells. Furthermore, significantly altered levels of expression of genes involved in the pollen development were exhibited in the OsCER1 knocked down plants. These results indicate that OsCER1 is critical for VLC alkanes biosynthesis, plastids differentiation, and pollen development. This work provides insights into the VLC alkanes biosynthesis in anther development in rice.

De novo transcriptome reconstruction of a thermo-sensitive male sterility mutant in rapeseed (Brassica napus; Brassicaceae)

PREMISE OF THE STUDY

SP2S is a spontaneous thermo-sensitive genic male sterility (TGMS) mutation that facilitates two-line hybrid breeding in Brassica napus (Brassicaceae). De novo assembly of the floral bud transcriptome of SP2S can provide a foundation for deciphering the transcriptional regulation of SP2S in response to temperature change.

METHODS

mRNAs of the young floral buds of SP2S and its near-isogenic line SP2F grown under cool (16°C)/warm (22°C) conditions were sequenced on an Illumina Solexa platform, producing 239.7 million short reads with a total length of 19.95 Gbp.

RESULTS

The reads were assembled de novo using the Trinity program, resulting in 135,702 transcripts with an average length of 784 bp, an N50 value of 1221 bp, and a total length of 107 Mbp. We identified 24,157 cDNA-derived simple sequence repeats in the assembly. We found 137 and 195 single-nucleotide polymorphisms and 49 and 51 differentially regulated KEGG orthology groups when comparing sample SP2S at 22°C vs. SP2S at 16°C and sample SP2S at 22°C vs. SP2F at 22°C, respectively.

DISCUSSION

The numerous differentially expressed genes and the derived single-nucleotide polymorphisms show abnormal transcriptional regulation in the TGMS system. These results outline an intricate transcriptional regulation that occurred in the rapeseed TGMS SP2S when the temperature changed.

Down-Regulation of OsEMF2b Caused Semi-sterility Due to Anther and Pollen Development Defects in Rice

Anther and pollen development are crucial processes of plant male reproduction. Although a number of genes involved in these processes have been identified, the regulatory networks of pollen and anther development are still unclear. EMBRYONIC FLOWER 2b (OsEMF2b) is important for rice development. Its biological function in floral organ, flowering time and meristem determinacy have been well-studied, but its role, if only, on male reproduction is still unknown, because null mutants of OsEMF2b barely have anthers. In this study, we identified a weak allele of OsEMF2b, osemf2b-4. The T-DNA insertion was located in the promoter region of OsEMF2b, and OsEMF2b expression was significantly decreased in osemf2b-4. The osemf2b-4 mutant exhibited much more normal anthers than null mutants of OsEMF2b, and also showed defective floret development similar to null mutants. Cytological analysis showed various defects of anther wall and pollen development in osemf2b-4, such as slightly or extremely enlarged tapetum, irregular or normal morphology microspores, and partial or complete sterility. OsEMF2b was highly expressed in tapetum and microspores, and the protein was localized in the nucleus. The expression of 15 genes essential for anther and pollen development was investigated in both WT and osemf2b-4. Fourteen genes including GAMYB was up-regulated, and only PTC1 was down-regulated in osemf2b-4. This suggests that up-regulated GAMYB and down-regulated PTC1 might contribute to the defective anther and pollen development in osemf2b-4. Overall, our work suggests that OsEMF2b plays an essential role during post-meiotic anther and pollen development.

TGMS in Rapeseed (Brassica napus) Resulted in Aberrant Transcriptional Regulation, Asynchronous Microsporocyte Meiosis, Defective Tapetum, and Fused Sexine

The thermo-sensitive genic male sterility (TGMS) line SP2S is a spontaneous rapeseed mutation with several traits that are favorable for the production of two-line hybrids. To uncover the key cellular events and genetic regulation associated with TGMS expression, a combined study using cytological observation, transcriptome profiling, and gene expression analysis was conducted for SP2S and its near-isogenic line SP2F grown under warm conditions. Asynchronous microsporocyte meiosis and abnormal tapetal plastids and elaioplasts were demonstrated in the anther of SP2S. The tetrad microspore did not undergo mitosis before the cytoplasm degenerated. Delayed degradation of the tetrad wall, which led to tetrad microspore aggregation, resulted in postponement of sexine (outer layer of pollen exine) formation and sexine fusion in the tetrad. The nexine (foot layer of exine) was also absent. The delay of tetrad wall degradation and abnormality of the exine structure suggested that the defective tapetum lost important functions. Based on transcriptomic comparisons between young flower buds of SP2S and SP2F plants, a total of 465 differentially expressed transcripts (DETs) were identified, including 303 up-regulated DETs and 162 down-regulated DETs in SP2S. Several genes encoding small RNA degrading nuclease 2, small RNA 2'-O-methyltransferase, thioredoxin reductase 2, regulatory subunit A alpha isoform of serine/threonine-protein phosphatase 2A, glycine rich protein 1A, transcription factor bHLH25, leucine-rich repeat receptor kinase At3g14840 like, and fasciclin-like arabinogalactan proteins FLA19 and FLA20 were greatly depressed in SP2S. Interestingly, a POLLENLESS3-LIKE 2 gene encoding the Arabidopsis MS5 homologous protein, which is necessary for microsporocyte meiosis, was down-regulated in SP2S. Other genes that were up-regulated in SP2S encoded glucanase A6, ethylene-responsive transcription factor 1A-like, pollen-specific SF3, stress-associated endoplasmic reticulum protein 2, WRKY transcription factors and pentatricopeptide repeat (PPR) protein At1g07590. The tapetum-development-related genes, including BnEMS1, BnDYT1, and BnAMS, were slightly up-regulated in 3-mm-long flower buds or their anthers, and their downstream genes, BnMS1 and BnMYB80, which affect callose dissolution and exine formation, were greatly up-regulated in SP2S. This aberrant genetic regulation corresponded well with the cytological abnormalities. The results suggested that expression of TGMS associates with complex transcriptional regulation.

Reactive oxygen species mediate tapetal programmed cell death in tobacco and tomato

BACKGROUND

Hybrid vigor is highly valued in the agricultural industry. Male sterility is an important trait for crop breeding. Pollen development is under strict control of both gametophytic and sporophytic factors, and defects in this process can result in male sterility. Both in the dicot Arabidopsis and in the moncot rice, proper timing of programmed cell death (PCD) in the tapetum ensures pollen development. Dynamic ROS levels have been reported to control tapetal PCD, and thus pollen development, in Arabidopsis and rice. However, it was unclear whether it is evolutionarily conserved, as only those two distantly related species were studied.

RESULTS

Here, we performed histological analyses of anther development of two economically important dicot species, tobacco and tomato. We identified the same ROS amplitude during anther development in these two species and found that dynamic ROS levels correlate with the initiation and progression of tapetal PCD. We further showed that manipulating ROS levels during anther development severely impaired pollen development, resulting in partial male sterility. Finally, real-time quantitative PCR showed that several tobacco and tomato RBOHs, encoding NADPH oxidases, are preferentially expressed in anthers.

CONCLUSION

This study demonstrated evolutionarily conserved ROS amplitude during anther development by examining two commercially important crop species in the Solanaceae. Manipulating ROS amplitude through genetic interference of RBOHs therefore may provide a practical way to generate male sterile plants.

Transcriptional regulation of male-sterility in 7B-1 male-sterile tomato mutant

The 7B-1 tomato (Solanum lycopersicum L. cv Rutgers) is a male-sterile mutant with enhanced tolerance to abiotic stress, which makes it a potential candidate for hybrid seed breeding and stress engineering. To underline the molecular mechanism regulating the male-sterility in 7B-1, transcriptomic profiles of the 7B-1 male-sterile and wild type (WT) anthers were studied using mRNA sequencing (RNA-Seq). In total, 768 differentially expressed genes (DEGs) were identified, including 132 up-regulated and 636 down-regulated transcripts. Gene ontology (GO) enrichment analysis of DEGs suggested a general impact of the 7B-1 mutation on metabolic processes, such as proteolysis and carbohydrate catabolic process. Sixteen candidates with key roles in regulation of anther development were subjected to further analysis using qRT-PCR and in situ hybridization. Cytological studies showed several defects associated with anther development in the 7B-1 mutant, including unsynchronized anther maturation, dysfunctional meiosis, arrested microspores, defect in callose degradation and abnormal tapetum development. TUNEL assay showed a defect in programmed cell death (PCD) of tapetal cells in 7B-1 anthers. The present study provides insights into the transcriptome of the 7B-1 mutant. We identified several genes with altered expression level in 7B-1 (including beta-1,3 glucanase, GA2oxs, cystatin, cysteine protease, pectinesterase, TA29, and actin) that could potentially regulate anther developmental processes, such as meiosis, tapetum development, and cell-wall formation/degradation.

An auxin maximum in the middle layer controls stamen development and pollen maturation in Arabidopsis

Here, we investigated the role of auxin distribution in controlling Arabidopsis thaliana late stamen development. We analysed auxin distribution in anthers by monitoring DR5 activity: at different flower developmental stages; inhibiting auxin transport; in the rpk2-3 and ems1 mutants devoid of middle layer (ML) or tapetum, respectively; and in the auxin biosynthesis yuc6 and perception afb1-3 mutants. We ran a phenotypic, DR5::GUS and gene expression analysis of yuc6rpk2 and afb1rpk2 double mutants, and of 1-N-naphthylphthalamic acid (NPA)-treated flower buds. We show that an auxin maximum, caused by transport from the tapetum, is established in the ML at the inception of late stamen development. rpk2-3 mutant stamens lacking the ML have an altered auxin distribution with excessive accumulation in adjacent tissues, causing non-functional pollen grains, indehiscent anthers and reduced filament length; the expression of genes controlling stamen development is also altered in rpk2-3 as well as in NPA-treated flower buds. By decreasing auxin biosynthesis or perception in the rpk2-3 background, we eliminated these developmental and gene expression anomalies. We propose that the auxin maximum in the ML plays a key role in late stamen development, as it ensures correct and coordinated pollen maturation, anther dehiscence and filament elongation.