DYT1 directly regulates the expression of TDF1 for tapetum development and pollen wall formation in Arabidopsis

Cloning and functional verification of the male sterile gene BrQRT3 in Chinese cabbage

Chinese cabbage is a cross-pollinated crop with significant heterosis, and male sterile lines are an important way to produce hybrid seeds. In this study, a male sterile mutant msm0795 was identified in an EMS-mutagenized population of Chinese cabbage. Cytological observations revealed that the microspores failed to separate after the tetrad stage, and thus developed into abnormal pollen grains, resulting in anther abortion. MutMap combined with Kompetitive Allele Specific PCR genotyping showed that BraA01g011280.3.5 C was identified as the candidate gene, which encodes polygalacturonase QRT3 and plays a direct role in the degradation of pollen mother cell wall during microspore development, named BrQRT3. Subcellular localization and expression analyses demonstrated that BrQRT3 was localized in the cell membrane and was ubiquitously expressed in roots, stems, leaves, flower buds, and flowers, but the expression of BrQRT3 was gradually suppressed with the anther development. Ectopic expression confirmed that over-expression of BrQRT3 in qrt3 background Arabidopsis mutant can rescue the pollen defects caused by loss of AtQRT3 function. It is the first time to achieve a male sterile mutant caused by the mutation of BrQRT3 in Chinese cabbage. These findings contribute to elucidate the mechanism of BrQRT3 in regulating stamen development of Chinese cabbage.

The characteristic analysis of TaTDF1 reveals its function related to male sterility in wheat (Triticum aestivum L.)

BACKGROUND

The male sterile lines are an important foundation for heterosis utilization in wheat (Triticum aestivum L.). Thereinto, pollen development is one of the indispensable processes of wheat reproductive development, and its fertility plays an important role in wheat heterosis utilization, and are usually influencing by genes. However, these key genes and their regulatory networks during pollen abortion are poorly understood in wheat.

RESULTS

DEFECTIVE IN TAPETAL DEVELOPMENT AND FUNCTION 1 (TDF1) is a member of the R2R3-MYB family and has been shown to be essential for early tapetal layer development and pollen grain fertility in rice (Oryza sativa L.) and Arabidopsis thaliana. In order to clarify the function of TDF1 in wheat anthers development, we used OsTDF1 gene as a reference sequence and homologous cloned wheat TaTDF1 gene. TaTDF1 is localized in the nucleus. The average bolting time of Arabidopsis thaliana overexpressed strain (TaTDF1-OE) was 33 d, and its anther could be colored normally by Alexander staining solution, showing red. The dominant Mosaic suppression silence-line (TaTDF1-EAR) was blue-green in color, and the anthers were shrimpy and thin. The TaTDF1 interacting protein (TaMAP65) was confirmed using Yeast Two-Hybrid Assay (Y2H) and Bimolecular-Fluorescence Complementation (BiFC) experiments. The results showed that downregulated expression of TaTDF1 and TaMAP65 could cause anthers to be smaller and shrunken, leading to pollen abortion in TaTDF1 wheat plants induced by virus-induced gene-silencing technology. The expression pattern of TaTDF1 was influenced by TaMAP65.

CONCLUSIONS

Thus, systematically revealing the regulatory mechanism of wheat TaTDF1 during anther and pollen grain development may provide new information on the molecular mechanism of pollen abortion in wheat.

The regulatory mechanism of rapid lignification for timely anther dehiscence

Anther dehiscence is a crucial event in plant reproduction, tightly regulated and dependent on the lignification of the anther endothecium. In this study, we investigated the rapid lignification process that ensures timely anther dehiscence in Arabidopsis. Our findings reveal that endothecium lignification can be divided into two distinct phases. During Phase I, lignin precursors are synthesized without polymerization, while Phase II involves simultaneous synthesis of lignin precursors and polymerization. The transcription factors MYB26, NST1/2, and ARF17 specifically regulate the pathway responsible for the synthesis and polymerization of lignin monomers in Phase II. MYB26-NST1/2 is the key regulatory pathway responsible for endothecium lignification, while ARF17 facilitates this process by interacting with MYB26. Interestingly, our results demonstrate that the lignification of the endothecium, which occurs within approximately 26 h, is much faster than that of the vascular tissue. These findings provide valuable insights into the regulation mechanism of rapid lignification in the endothecium, which enables timely anther dehiscence and successful pollen release during plant reproduction.

MAPK20-mediated ATG6 phosphorylation is critical for pollen development in Solanum lycopersicum L

In flowering plants, male gametogenesis is tightly regulated by numerous genes. Mitogen-activated protein kinase (MAPK) plays a critical role in plant development and stress response, while its role in plant reproductive development is largely unclear. The present study demonstrated MAPK20 phosphorylation of ATG6 to mediate pollen development and germination in tomato (Solanum lycopersicum L.). MAPK20 was preferentially expressed in the stamen of tomato, and mutation of MAPK20 resulted in abnormal pollen grains and inhibited pollen viability and germination. MAPK20 interaction with ATG6 mediated the formation of autophagosomes. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis showed that ATG6 was phosphorylated by MAPK20 at Ser-265. Mutation of ATG6 in wild-type (WT) or in MAPK20 overexpression plants resulted in malformed and inviable pollens. Meanwhile, the number of autophagosomes in mapk20 and atg6 mutants was significantly lower than that of WT plants. Our results suggest that MAPK20-mediated ATG6 phosphorylation and autophagosome formation are critical for pollen development and germination.

Gene co-expression network analysis in areca floral organ and the potential role of the AcMADS17 and AcMADS23 in transgenic Arabidopsis

Areca catechu L., a monocot belonging to the palm family, is monoecious, with female and male flowers separately distributed on the same inflorescence. To discover the molecular mechanism of flower development in Areca, we sequenced different floral samples to generate tissue-specific transcriptomic profiles. We conducted a comparative analysis of the transcriptomic profiles of apical sections of the inflorescence with male flowers and the basal section of the inflorescence with female flowers. Based on the RNA sequencing dataset, we applied weighted gene co-expression network analysis (WGCNA) to identify sepal, petal, stamen, stigma and other specific modules as well as hub genes involved in specific floral organ development. The syntenic and expression patterns of AcMADS-box genes were analyzed in detail. Furthermore, we analyzed the open chromatin regions and transcription factor PI binding sites in male and female flowers by assay for transposase-accessible chromatin sequencing (ATAC-seq) assay. Heterologous expression revealed the important role of AcMADS17 and AcMADS23 in floral organ development. Our results provide a valuable genomic resource for the functional analysis of floral organ development in Areca.

Developmentally dependent reprogramming of the Arabidopsis floral transcriptome under sufficient and limited water availability

BACKGROUND

Environmental stresses negatively impact reproductive development and yield. Drought stress, in particular, has been examined during Arabidopsis reproductive development at morphological and transcriptomic levels. However, drought-responsive transcriptomic changes at different points in reproductive development remain unclear. Additionally, an investigation of the entire transcriptome at various stages during flower development is of great interest.

RESULTS

Here, we treat Arabidopsis plants with well-watered and moderately and severely limiting water amounts when the first flowers reach maturity and generate RNA-seq datasets for early, middle, and late phases during flower development at 5, 6, and 7 days following treatment. Under different drought conditions, flowers in different developmental phases display differential sets of drought-responsive genes (DTGs), including those that are enriched in different GO functional categories, such as transcriptional regulation and response to stresses (early phase), lipid storage (middle phase), and pollen and seed development and metabolic processes (late phase). Some gene families have different members induced at different floral phases, suggesting that similar biochemical functions are carried out by distinct members. Developmentally-regulated genes (DVGs) with differential expression among the three floral phases belong to GO terms that are similar between water conditions, such as development and reproduction, metabolism and transport, and signaling and stress response. However, for different water conditions, such similar GO terms correspond to either distinct gene families or different members of a gene family, suggesting that drought affects the expression of distinct families or family members during reproductive development. A further comparison among transcriptomes of tissues collected on different days after treatment identifies differential gene expression, suggesting age-related genes (ARGs) might reflect the changes in the overall plant physiology in addition to drought response and development.

CONCLUSION

Together, our study provides new insights into global transcriptome reprogramming and candidate genes for drought response, flower development, aging and coordination among these complex biological processes.

Environmental regulation of male fertility is mediated through Arabidopsis transcription factors bHLH89, 91, and 10

Formation of functional pollen and successful fertilization rely on the spatial and temporal regulation of anther and pollen development. This process responds to environmental cues to maintain optimal fertility despite climatic changes. Arabidopsis transcription factors basic helix-loop-helix (bHLH) 10, 89, and 91 were previously thought to be functionally redundant in their control of male reproductive development, however here we show that they play distinct roles in the integration of light signals to maintain pollen development under different environmental conditions. Combinations of the double and triple bHLH10,89,91 mutants were analysed under normal (200 μmol m-2 s-1) and low (50 μmol m-2 s-1) light conditions to determine the impact on fertility. Transcriptomic analysis of a new conditionally sterile bhlh89,91 double mutant shows differential regulation of genes related to sexual reproduction, hormone signal transduction, and lipid storage and metabolism under low light. Here we have shown that bHLH89 and bHLH91 play a role in regulating fertility in response to light, suggesting that they function in mitigating environmental variation to ensure fertility is maintained under environmental stress.

Acyl-CoA synthetase 1 plays an important role on pollen development and male fertility in tomato

The development of pollen is critical to male reproduction in flowering plants. Acyl-CoA synthetase (ACOS) genes play conserved functions in regulating pollen development in various plants. Our previous work found that knockout of the SlACOS1 gene in tomato might decrease fruit setting. The current study further revealed that SlACOS1 was important to pollen development and male fertility. The SlACOS1 gene was preferentially expressed in the stamen of the flower with the highest expression at the tetrad stage of anther development. Mutation of the SlACOS1 gene by the CRISPR/Cas9-editing system reduced pollen number and viability as well as fruit setting. The tapetum layer exhibited premature degradation and the pollen showed abnormal development appearing irregular, shriveled, or anucleate in Slacos1 mutants at the tetrad stage. The fatty acid metabolism in anthers was significantly impacted by mutation of the SlACOS1 gene. Furthermore, targeted fatty acids profiling using GC-MS found that contents of most fatty acids except C18:1 and C18:2 were reduced. Yeast complementation assay demonstrated that the substrate preferences of SlACOS1 were C16:0 and C18:0 fatty acids. Male fertility of Slacos1 mutant could be slightly restored by applying exogenous palmitic acid, a type of C16:0 fatty acid. Taken together, SlACOS1 played important roles on pollen development and male fertility by regulating the fatty acid metabolism and the development of tapetum and tetrad. Our findings will facilitate unraveling the mechanism of pollen development and male fertility in tomato.

Time-Course Transcriptomic Analysis Reveals Molecular Insights into the Inflorescence and Flower Development of Cardiocrinum giganteum

Cardiocrinum giganteum is an endemic species of east Asia which is famous for its showy inflorescence and medicinal bulbs. Its inflorescence is a determinate raceme and the flowers bloom synchronously. Morphological observation and time-course transcriptomic analysis were combined to study the process of inflorescence and flower development of C. giganteum. The results show that the autonomic pathway, GA pathway, and the vernalization pathway are involved in the flower formation pathway of C. giganteum. A varied ABCDE flowering model was deduced from the main development process. Moreover, it was found that the flowers in different parts of the raceme in C. giganteum gradually synchronized during development, which is highly important for both evolution and ecology. The results obtained in this work improve our understanding of the process and mechanism of inflorescence and flower development and could be useful for the flowering period regulation and breeding of C. giganteum.

bHLH-regulated routes in anther development in rice and Arabidopsis

Anther development is a complex process essential for plant reproduction and crop yields. In recent years, significant progress has been made in the identification and characterization of the bHLH transcription factor family involved in anther regulation in rice and Arabidopsis, two extensively studied model plants. Research on bHLH transcription factors has unveiled their crucial function in controlling tapetum development, pollen wall formation, and other anther-specific processes. By exploring deeper into regulatory mechanisms governing anther development and bHLH transcription factors, we can gain important insights into plant reproduction, thereby accelerating crop yield improvement and the development of new plant breeding strategies. This review provides an overview of the current knowledge on anther development in rice and Arabidopsis, emphasizing the critical roles played by bHLH transcription factors in this process. Recent advances in gene expression analysis and functional studies are highlighted, as they have significantly enhanced our understanding of the regulatory networks involved in anther development.

The transcription factors and pathways underpinning male reproductive development in Arabidopsis

As Arabidopsis flowers mature, specialized cells within the anthers undergo meiosis, leading to the production of haploid microspores that differentiate into mature pollen grains, each containing two sperm cells for double fertilization. During pollination, the pollen grains are dispersed from the anthers to the stigma for subsequent fertilization. Transcriptomic studies have identified a large number of genes expressed over the course of male reproductive development and subsequent functional characterization of some have revealed their involvement in floral meristem establishment, floral organ growth, sporogenesis, meiosis, microsporogenesis, and pollen maturation. These genes encode a plethora of proteins, ranging from transcriptional regulators to enzymes. This review will focus on the regulatory networks that control male reproductive development, starting from flower development and ending with anther dehiscence, with a focus on transcription factors and some of their notable target genes.

Genome-Wide Identification and Characterization of MYB Gene Family and Analysis of Its Sex-Biased Expression Pattern in Spinacia oleracea L

The members of the myeloblastosis (MYB) family of transcription factors (TFs) participate in a variety of biological regulatory processes in plants, such as circadian rhythm, metabolism, and flower development. However, the characterization of MYB genes across the genomes of spinach Spinacia oleracea L. has not been reported. Here, we identified 140 MYB genes in spinach and described their characteristics using bioinformatics approaches. Among the MYB genes, 54 were 1R-MYB, 80 were 2R-MYB, 5 were 3R-MYB, and 1 was 4R-MYB. Almost all MYB genes were located in the 0-30 Mb region of autosomes; however, the 20 MYB genes were enriched at both ends of the sex chromosome (chromosome 4). Based on phylogeny, conserved motifs, and the structure of genes, 2R-MYB exhibited higher conservation relative to 1R-MYB genes. Tandem duplication and collinearity of spinach MYB genes drive their evolution, enabling the functional diversification of spinach genes. Subcellular localization prediction indicated that spinach MYB genes were mainly located in the nucleus. Cis-acting element analysis confirmed that MYB genes were involved in various processes of spinach growth and development, such as circadian rhythm, cell differentiation, and reproduction through hormone synthesis. Furthermore, through the transcriptome data analysis of male and female flower organs at five different periods, ten candidate genes showed biased expression in spinach males, suggesting that these genes might be related to the development of spinach anthers. Collectively, this study provides useful information for further investigating the function of MYB TFs and novel insights into the regulation of sex determination in spinach.

Transcription Factors and Their Regulatory Roles in the Male Gametophyte Development of Flowering Plants

Male gametophyte development in plants relies on the functions of numerous genes, whose expression is regulated by transcription factors (TFs), non-coding RNAs, hormones, and diverse environmental stresses. Several excellent reviews are available that address the genes and enzymes associated with male gametophyte development, especially pollen wall formation. Growing evidence from genetic studies, transcriptome analysis, and gene-by-gene studies suggests that TFs coordinate with epigenetic machinery to regulate the expression of these genes and enzymes for the sequential male gametophyte development. However, very little summarization has been performed to comprehensively review their intricate regulatory roles and discuss their downstream targets and upstream regulators in this unique process. In the present review, we highlight the research progress on the regulatory roles of TF families in the male gametophyte development of flowering plants. The transcriptional regulation, epigenetic control, and other regulators of TFs involved in male gametophyte development are also addressed.

MYB transcription factors and their roles in the male reproductive development of flowering plants

As one of the largest transcription factor families with complex functional differentiation in plants, the MYB transcription factors (MYB TFs) play important roles in the physiological and biochemical processes of plant growth and development. Male reproductive development, an essential part of sexual reproduction in flowering plants, is undoubtedly regulated by MYB TFs. In this review, we summarize the roles of the MYB TFs involved in the three stages of male reproductive development: pollen grains formation and maturation, filament elongation and anther dehiscence, and fertilization. Also, the potential downstream target genes and upstream regulators of these MYB TFs are discussed. Furthermore, we propose the underlying regulatory mechanisms of these MYB TFs: (1) A complex network of MYB TFs regulates various aspects of male reproductive development; (2) MYB homologous genes in different species may be functionally conserved or differentiated; (3) MYB TFs often form regulatory complexes with bHLH TFs.

Barley TAPETAL DEVELOPMENT and FUNCTION1 (HvTDF1) gene reveals conserved and unique roles in controlling anther tapetum development in dicot and monocot plants

The anther tapetum helps control microspore release and essential components for pollen wall formation. TAPETAL DEVELOPMENT and FUNCTION1 (TDF1) is an essential R2R3 MYB tapetum transcription factor in Arabidopsis thaliana; however, little is known about pollen development in the temperate monocot barley. Here, we characterize the barley (Hordeum vulgare L.) TDF1 ortholog using reverse genetics and transcriptomics. Spatial/temporal expression analysis indicates HvTDF1 has tapetum-specific expression during anther stage 7/8. Homozygous barley hvtdf1 mutants exhibit male sterility with retarded tapetum development, delayed tapetum endomitosis and cell wall degeneration, resulting in enlarged, vacuolated tapetum surrounding collapsing microspores. Transient protein expression and dual-luciferase assays show TDF1 is a nuclear-localized, transcription activator, that directly activates osmotin proteins. Comparison of hvtdf1 transcriptome data revealed several pathways were delayed, endorsing the observed retarded anther morphology. Arabidopsis tdf1 mutant fertility was recovered by HvTDF1, supporting a conserved role for TDF1 in monocots and dicots. This indicates that tapetum development shares similarity between monocot and dicots; however, barley HvTDF1 appears to uniquely act as a modifier to activate tapetum gene expression pathways, which are subsequently also induced by other factors. Therefore, the absence of HvTDF1 results in delayed developmental progression rather than pathway failure, although inevitably still results in pollen degeneration.

MIR159 regulates multiple aspects of stamen and carpel development and requires dissection and delimitation of differential downstream regulatory network for manipulating fertility traits

Unravelling genetic networks regulating developmental programs are key to devising and implementing genomics assisted trait modification strategies. It is crucial to understand the role of small RNAs, and the basis of their ability to modify traits. MIR159 has been previously reported to cause defects in anther development in Arabidopsis; however, the complete spectrum and basis of the defects remained unclear. The present study was therefore undertaken to comprehensively investigate the role of miR159 from Brassica juncea in modulating vegetative and reproductive traits. Owing to the polyploid nature of Brassica, paralogous and homeologous copies of MIR159A, MIR159B, and, MIR159C were identified and analysis of the precursor uncovered extensive structural and sequence variation. The MIR159 locus with mature miR159 with perfect target complimentarily with MYB65, was cloned from Brassica juncea var. Varuna for functional characterization by generating constitutively over-expressing lines in Arabidopsis thaliana Col-0. Apart from statistically significant difference in multiple vegetative traits, drastic differences were observed in stamen and pistil. Over-expression of miR159a led to shortening of filament length and loss of tetradynamous condition. Anthers were apiculate, with improper lobe formation, and unsynchronized cellular growth between connective tissue and another lobe development. Analysis revealed arrested meiosis/cytokinesis in microspores, and altered lignin deposition pattern in endothecial walls thus affecting anther dehiscence. In the gynoecium, flaccid, dry stigmatic papillae, and large embryo sac in the female gametophyte was observed. Over-expression of miR159a thus severely affected pollination and seed-set. Analysis of the transcriptome data revealed components of regulatory networks of anther and carpel developmental pathway, and lignin metabolism that are affected. Expression analysis allowed us to position the miR159a-MYB65 module in the genetic network of stamen development, involved in pollen-grain maturation; in GA-mediated regulation of stamen development, and in lignin metabolism. The study, on one hand indicates role of miR159a-MYB65 in regulating multiple aspects of reproductive organ development that can be manipulated for trait modification, but also raises several unaddressed questions such as relationship between miR159a and male-meiosis, miR159a and filament elongation for future investigations. Accession numbers: KC204951-KC204960. Project number PRJNA1035268.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-023-01377-7.

ARF3-Mediated Regulation of SPL in Early Anther Morphogenesis: Maintaining Precise Spatial Distribution and Expression Level

Early anther morphogenesis is a crucial process for male fertility in plants, governed by the transcription factor SPL. While the involvement of AGAMOUS (AG) in SPL activation and microsporogenesis initiation is well established, our understanding of the mechanisms governing the spatial distribution and precise expression of SPL during anther cell fate determination remains limited. Here, we present novel findings on the abnormal phenotypes of two previously unreported SPL mutants, spl-4 and spl-5, during anther morphogenesis. Through comprehensive analysis, we identified ARF3 as a key upstream regulator of SPL. Our cytological experiments demonstrated that ARF3 plays a critical role in restricting SPL expression specifically in microsporocytes. Moreover, we revealed that ARF3 directly binds to two specific auxin response elements on the SPL promoter, effectively suppressing AG-mediated activation of SPL. Notably, the arf3 loss-of-function mutant exhibits phenotypic similarities to the SPL overexpression mutant (spl-5), characterized by defective adaxial anther lobes. Transcriptomic analysis revealed differential expression of the genes involved in the morphogenesis pathway in both arf3 and spl mutants, with ARF3 and SPL exhibited opposing regulatory effects on this pathway. Taken together, our study unveils the precise role of ARF3 in restricting the spatial expression and preventing aberrant SPL levels during early anther morphogenesis, thereby ensuring the fidelity of the critical developmental process in plants.

Spatiotemporal profiles of gene activity in stamen delineate nucleo-cytoplasmic interaction in a male-sterile somatic cybrid citrus

Cytoplasmic male sterility (CMS) has long been used to produce seedless fruits in perennial woody crops like citrus. A male-sterile somatic cybrid citrus (G1 + HBP) was generated by protoplast fusion between a CMS callus parent 'Guoqing No. 1' Satsuma mandarin (Citrus unshiu, G1) and a fertile mesophyll parent Hirado Buntan pummelo (Citrus grandis, HBP). To uncover the male-sterile mechanism of G1 + HBP, we compared the transcriptome profiles of stamen organ and cell types at five stages between G1 + HBP and HBP, including the initial stamen primordia, enlarged stamen primordia, pollen mother cells, tetrads, and microspores captured by laser microdissection. The stamen organ and cell types showed distinct gene expression profiles. A majority of genes involved in stamen development were differentially expressed, especially CgAP3.2, which was downregulated in enlarged stamen primordia and upregulated in tetrads of G1 + HBP compared with HBP. Jasmonic acid- and auxin-related biological processes were enriched among the differentially expressed genes of stamen primordia, and the content of jasmonic acid biosynthesis metabolites was higher in flower buds and anthers of G1 + HBP. In contrast, the content of auxin biosynthesis metabolites was lower in G1 + HBP. The mitochondrial tricarboxylic acid cycle and oxidative phosphorylation processes were enriched among the differentially expressed genes in stamen primordia, meiocytes, and microspores, indicating the dysfunction of mitochondria in stamen organ and cell types of G1 + HBP. Taken together, the results indicate that malfunction of mitochondria-nuclear interaction might cause disorder in stamen development, and thus lead to male sterility in the citrus cybrid.

Anatomy and Comparative Transcriptome Reveal the Mechanism of Male Sterility in Salvia miltiorrhiza

Salvia miltiorrhiza Bunge is an important traditional herb. Salvia miltiorrhiza is distributed in the Sichuan province of China (here called SC). Under natural conditions, it does not bear seeds and its sterility mechanism is still unclear. Through artificial cross, there was defective pistil and partial pollen abortion in these plants. Electron microscopy results showed that the defective pollen wall was caused by delayed degradation of the tapetum. Due to the lack of starch and organelle, the abortive pollen grains showed shrinkage. RNA-seq was performed to explore the molecular mechanisms of pollen abortion. KEGG enrichment analysis suggested that the pathways of phytohormone, starch, lipid, pectin, and phenylpropanoid affected the fertility of S. miltiorrhiza. Moreover, some differentially expressed genes involved in starch synthesis and plant hormone signaling were identified. These results contribute to the molecular mechanism of pollen sterility and provide a more theoretical foundation for molecular-assisted breeding.

Biosynthesis and transport of pollen coat precursors in angiosperms

The pollen coat is a hydrophobic mixture on the pollen grain surface, which plays an important role in protecting male gametes from various environmental stresses and microorganism attacks, and in pollen-stigma interactions during pollination in angiosperms. An abnormal pollen coat can result in humidity-sensitive genic male sterility (HGMS), which can be used in two-line hybrid crop breeding. Despite the crucial functions of the pollen coat and the application prospect of its mutants, few studies have focused on pollen coat formation. In this Review, the morphology, composition and function of different types of pollen coat are assessed. On the basis of the ultrastructure and development process of the anther wall and exine found in rice and Arabidopsis, the genes and proteins involved in the biosynthesis of pollen coat precursors and the possible transport and regulation process are sorted. Additionally, current challenges and future perspectives, including potential strategies utilizing HGMS genes in heterosis and plant molecular breeding, are highlighted.

Ascorbic acid-mediated reactive oxygen species homeostasis modulates the switch from tapetal cell division to cell differentiation in Arabidopsis

The major antioxidant L-ascorbic acid (AsA) plays important roles in plant growth, development, and stress responses. However, the importance of AsA concentration and the regulation of AsA metabolism in plant reproduction remain unclear. In Arabidopsis (Arabidopsis thaliana) anthers, the tapetum monolayer undergoes cell differentiation to support pollen development. Here, we report that a transcription factor, DEFECTIVE IN TAPETAL DEVELOPMENT AND FUNCTION 1 (TDF1), inhibits tapetal cell division leading to cell differentiation. We identified SKEWED5-SIMILAR 18 (SKS18) as a downstream target of TDF1. Enzymatic assays showed that SKS18, annotated as a multicopper oxidase-like protein, has ascorbate oxidase activity, leading to AsA oxidation. We also show that VITAMIN C DEFECTIVE1 (VTC1), an AsA biosynthetic enzyme, is negatively controlled by TDF1 to maintain proper AsA contents. Consistently, either knockout of SKS18 or VTC1 overexpression raised AsA concentrations, resulting in extra tapetal cells, while SKS18 overexpression in tdf1 or the vtc1-3 tdf1 double mutant mitigated their defective tapetum. We observed that high AsA concentrations caused lower accumulation of reactive oxygen species (ROS) in tapetal cells. Overexpression of ROS scavenging genes in tapetum restored excess cell divisions. Thus, our findings demonstrate that TDF1-regulated AsA balances cell division and cell differentiation in the tapetum through governing ROS homeostasis.

Molecular regulation of tomato male reproductive development

The reproductive success of flowering plants, which directly affects crop yield, is sensitive to environmental changes. A thorough understanding of how crop reproductive development adapts to climate changes is vital for ensuring global food security. In addition to being a high-value vegetable crop, tomato is also a model plant used for research on plant reproductive development. Tomato crops are cultivated under highly diverse climatic conditions worldwide. Targeted crosses of hybrid varieties have resulted in increased yields and abiotic stress resistance; however, tomato reproduction, especially male reproductive development, is sensitive to temperature fluctuations, which can lead to aborted male gametophytes, with detrimental effects on fruit set. We herein review the cytological features as well as genetic and molecular pathways influencing tomato male reproductive organ development and responses to abiotic stress. We also compare the shared features among the associated regulatory mechanisms of tomato and other plants. Collectively, this review highlights the opportunities and challenges related to characterizing and exploiting genic male sterility in tomato hybrid breeding programs.

Rice anther tapetum: a vital reproductive cell layer for sporopollenin biosynthesis and pollen exine patterning

The tapetum is the innermost layer of the four layers of the rice anther that provides protection and essential nutrients to pollen grain development and delivers precursors for pollen exine formation. The tapetum has a key role in the normal development of pollen grains and tapetal programmed cell death (PCD) that is linked with sporopollenin biosynthesis and transport. Recently, many genes have been identified that are involved in tapetum formation in rice and Arabidopsis. Genetic mutation in PCD-associated genes could affect normal tapetal PCD, which finally leads to aborted pollen grains and male sterility in rice. In this review, we discuss the most recent research on rice tapetum development, including genomic, transcriptomic and proteomic studies. Furthermore, tapetal PCD, sporopollenin biosynthesis, ROS activity for tapetum function and its role in male reproductive development are discussed in detail. This will improve our understanding of the role of the tapetum in male fertility using rice as a model system, and provide information that can be applied in rice hybridization and that of other major crops.

The full-length Auxin Response Factor 8 isoform ARF8.1 controls pollen cell wall formation and directly regulates TDF1, AMS and MS188 expression

Auxin Response Factor 8 plays a key role in late stamen development: its splice variants ARF8.4 and ARF8.2 control stamen elongation and anther dehiscence. Here, we characterized the role of ARF8 isoforms in pollen fertility. By phenotypic and ultrastructural analysis of arf8-7 mutant stamens, we found defects in pollen germination and viability caused by alterations in exine structure and pollen coat deposition. Furthermore, tapetum degeneration, a prerequisite for proper pollen wall formation, is delayed in arf8-7 anthers. In agreement, the genes encoding the transcription factors TDF1, AMS, MS188 and MS1, required for exine and pollen coat formation, and tapetum development, are downregulated in arf8-7 stamens. Consistently, the sporopollenin content is decreased, and the expression of sporopollenin synthesis/transport and pollen coat protein biosynthetic genes, regulated by AMS and MS188, is reduced. Inducible expression of the full-length isoform ARF8.1 in arf8-7 inflorescences complements the pollen (and tapetum) phenotype and restores the expression of the above transcription factors. Chromatin immunoprecipitation-quantitative polymerase chain reaction assay revealed that ARF8.1 directly targets the promoters of TDF1, AMS and MS188. In conclusion, the ARF8.1 isoform controls pollen and tapetum development acting directly on the expression of TDF1, AMS and MS188, which belong to the pollen/tapetum genetic pathway.

Comprehensive Insight into Tapetum-Mediated Pollen Development in Arabidopsis thaliana

In flowering plants, pollen development is a key process that is essential for sexual reproduction and seed set. Molecular and genetic studies indicate that pollen development is coordinatedly regulated by both gametophytic and sporophytic factors. Tapetum, the somatic cell layer adjacent to the developing male meiocytes, plays an essential role during pollen development. In the early anther development stage, the tapetal cells secrete nutrients, proteins, lipids, and enzymes for microsporocytes and microspore development, while initiating programmed cell death to provide critical materials for pollen wall formation in the late stage. Therefore, disrupting tapetum specification, development, or function usually leads to serious defects in pollen development. In this review, we aim to summarize the current understanding of tapetum-mediated pollen development and illuminate the underlying molecular mechanism in Arabidopsis thaliana.

The genome of Areca catechu provides insights into sex determination of monoecious plants

The areca palm (Areca catechu) has a monoecious spadix, with male flowers on the apical side and females on the basal side. Here, we applied multiomics analysis to investigate sex determination and floral organ development in areca palms. We generated a chromosome-level reference genome of A. catechu with 16 pseudochromosomes, composed of 2.73 Gb and encoding 31 406 genes. Data from RNA-seq and ATAC-seq (assay for transposase accessible chromatin sequencing) suggested that jasmonic acid (JA) synthesis and signal transduction-related genes were differentially expressed between female and male flowers via epigenetic modifications. JA concentration in female flowers was c. 10 times than that in males on the same inflorescence, while JA concentration in hermaphroditic flowers of abnormal inflorescences was about twice that in male flowers of normal inflorescences. JA promotes the development of female flower organs by decreasing the expression of B-function genes, including AGL16, AP3, PIb and PIc. There is also a region on pseudochromosome 15 harboring sex-related genes, including CYP703, LOG, GPAT, AMS and BiP. Among them, CYP703, AMS and BiP were specifically expressed in male flowers.

The tapetal tissue is essential for the maintenance of redox homeostasis during microgametogenesis in tomato

The tapetum is a specialized layer of cells within the anther, adjacent to the sporogenous tissue. During its short life, it provides nutrients, molecules and materials to the pollen mother cells and microsporocytes, being essential during callose degradation and pollen wall formation. The interaction between the tapetum and sporogenous cells in Solanum lycopersicum (tomato) plants, despite its importance for breeding purposes, is poorly understood. To investigate this process, gene editing was used to generate loss-of-function mutants that showed the complete and specific absence of tapetal cells. These plants were obtained targeting the previously uncharacterized Solyc03g097530 (SlTPD1) gene, essential for tapetum specification in tomato plants. In the absence of tapetum, sporogenous cells developed and callose deposition was observed. However, sporocytes failed to undergo the process of meiosis and finally degenerated, leading to male sterility. Transcriptomic analysis conducted in mutant anthers lacking tapetum revealed the downregulation of a set of genes related to redox homeostasis. Indeed, mutant anthers showed a reduction in the accumulation of reactive oxygen species (ROS) at early stages and altered activity of ROS-scavenging enzymes. The results obtained highlight the importance of the tapetal tissue in maintaining redox homeostasis during male gametogenesis in tomato plants.

Global transcriptome analysis reveals potential genes associated with genic male sterility of rapeseed (Brassica napus L.)

Rapeseed is the third leading source of edible oil in the world. Genic male sterility (GMS) lines provide crucial material for harnessing heterosis for rapeseed. GMS lines have been widely used successfully for rapeseed hybrid production. The physiological and molecular mechanism of pollen development in GMS lines of rapeseed (Brassica napus L.) need to be determined for the creation of hybrids and cultivation of new varieties. However, limited studies have focused on systematically mining genes that regulate the pollen development of GMS lines in B. napus. In the present study, to determine the stage at which pollen development begins to show abnormality in the GMS lines, we performed semi-thin section analysis of the anthers with five pollen development stages. The results indicated that the abnormal pollen development in DGMS lines might start at the meiotic stage, and abnormal pollen development in RGMS lines probably occurred before the tetrad stage. To investigate the critical genes and pathways involved in pollen development in GMS lines, we constructed and sequenced 24 transcriptome libraries for the flower buds from the fertile and sterile lines of two recessive GMS (RGMS) lines (6251AB and 6284AB) and two dominant GMS (DGMS) lines (4001AB and 4006AB). A total of 23,554 redundant DEGs with over two-fold change between sterile and fertile lines were obtained. A total of 346 DEGs were specifically related to DGMS, while 1,553 DEGs were specifically related to RGMS. A total of 1,545 DEGs were shared between DGMS and RGMS. And 253 transcription factors were found to be differentially expressed between the sterile and fertile lines of GMS. In addition, 6,099 DEGs possibly related to anther, pollen, and microspore development processes were identified. Many of these genes have been reported to be involved in anther and microspore developmental processes. Several DEGs were speculated to be key genes involved in the regulation of fertility. Three differentially expressed genes were randomly selected and their expression levels were verified by quantitative PCR (qRT-PCR). The results of qRT-PCR largely agreed with the transcriptome sequencing results. Our findings provide a global view of genes that are potentially involved in GMS occurrence. The expression profiles and function analysis of these DEGs were provided to expand our understanding of the complex molecular mechanism in pollen and sterility development in B. napus.

De Novo Assembly of a Sarcocarp Transcriptome Set Identifies AaMYB1 as a Regulator of Anthocyanin Biosynthesis in Actinidia arguta var. purpurea

The kiwifruit (Actinidia arguta var. purpurea) produces oval shaped fruits containing a slightly green or mauve outer exocarp and a purple-flesh endocarp with rows of tiny black seeds. The flesh color of the fruit results from a range of anthocyanin compounds, and is an important trait for kiwifruit consumers. To elucidate the molecular mechanisms involved in anthocyanin biosynthesis of the sarcocarp during A. arguta fruit development, de novo assembly and transcriptomic profile analyses were performed. Based on significant Gene Ontology (GO) biological terms, differentially expressed genes were identified in flavonoid biosynthetic and metabolic processes, pigment biosynthesis, carbohydrate metabolic processes, and amino acid metabolic processes. The genes closely related to anthocyanin biosynthesis, such as phenylalanine ammonia-lyase (PAL), chalcone synthase (CHS), and anthocyanidin synthase (ANS), displayed significant up-regulation during fruit development according to the transcriptomic data, which was further confirmed by qRT-PCR. Meanwhile, a series of transcription factor genes were identified among the DEGs. Through a correlation analysis. AaMYB1 was found to be significantly correlated with key genes of anthocyanin biosynthesis, especially with CHS. Through a transient expression assay, AaMYB1 induced anthocyanin accumulation in tobacco leaves. These data provide an important basis for exploring the related mechanisms of sarcocarp anthocyanin biosynthesis in A. arguta. This study will provide a strong foundation for functional studies on A. arguta and will facilitate improved breeding of A. arguta fruit.

Ethylene Activates the EIN2-EIN3/EIL1 Signaling Pathway in Tapetum and Disturbs Anther Development in Arabidopsis

Ethylene was previously reported to repress stamen development in both cucumber and Arabidopsis. Here, we performed a detailed analysis of the effect of ethylene on anther development. After ethylene treatment, stamens but not pistils display obvious developmental defects which lead to sterility. Both tapetum and microspores (or microsporocytes) degenerated after ethylene treatment. In ein2-1 and ein3-1 eil1-1 mutants, ethylene treatment did not affect their fertility, indicating the effects of ethylene on anther development are mediated by EIN2 and EIN3/EIL1 in vivo. The transcription of EIN2 and EIN3 are activated by ethylene in the tapetum layer. However, ectopic expression of EIN3 in tapetum did not induce significant anther defects, implying that the expression of EIN3 are regulated post transcriptional level. Consistently, ethylene treatment induced the accumulation of EIN3 in the tapetal cells. Thus, ethylene not only activates the transcription of EIN2 and EIN3, but also stabilizes of EIN3 in the tapetum to disturb its development. The expression of several ethylene related genes was significantly increased, and the expression of the five key transcription factors required for tapetum development was decreased after ethylene treatment. Our results thus point out that ethylene inhibits anther development through the EIN2-EIN3/EIL1 signaling pathway. The activation of this signaling pathway in anther wall, especially in the tapetum, induces the degeneration of the tapetum and leads to pollen abortion.

bHLH010/089 Transcription Factors Control Pollen Wall Development via Specific Transcriptional and Metabolic Networks in Arabidopsis thaliana

The pollen wall is a specialized extracellular cell wall that protects male gametophytes from various environmental stresses and facilitates pollination. Here, we reported that bHLH010 and bHLH089 together are required for the development of the pollen wall by regulating their specific downstream transcriptional and metabolic networks. Both the exine and intine structures of bhlh010 bhlh089 pollen grains were severely defective. Further untargeted metabolomic and transcriptomic analyses revealed that the accumulation of pollen wall morphogenesis-related metabolites, including polysaccharides, glyceryl derivatives, and flavonols, were significantly changed, and the expression of such metabolic enzyme-encoding genes and transporter-encoding genes related to pollen wall morphogenesis was downregulated in bhlh010 bhlh089 mutants. Among these downstream target genes, CSLB03 is a novel target with no biological function being reported yet. We found that bHLH010 interacted with the two E-box sequences at the promoter of CSLB03 and directly activated the expression of CSLB03. The cslb03 mutant alleles showed bhlh010 bhlh089–like pollen developmental defects, with most of the pollen grains exhibiting defective pollen wall structures.

Mutation of glucose-methanol-choline oxidoreductase leads to thermosensitive genic male sterility in rice and Arabidopsis

Thermosensitive genic male sterility (TGMS) lines serve as the major genetic resource for two-line hybrid breeding in rice. However, their unstable sterility under occasional low temperatures in summer highly limits their application. In this study, we identified a novel rice TGMS line, ostms18, of cultivar ZH11 (Oryza sativa ssp. japonica). ostms18 sterility is more stable in summer than the TGMS line carrying the widely used locus tms5 in the ZH11 genetic background, suggesting its potential application for rice breeding. The ostms18 TGMS trait is caused by the point mutation from Gly to Ser in a glucose-methanol-choline (GMC) oxidoreductase; knockout of the oxidoreductase was previously reported to cause complete male sterility. Cellular analysis revealed the pollen wall of ostms18 to be defective, leading to aborted pollen under high temperature. Further analysis showed that the tapetal transcription factor OsMS188 directly regulates OsTMS18 for pollen wall formation. Under low temperature, the flawed pollen wall in ostms18 is sufficient to protect its microspore, allowing for development of functional pollen and restoring fertility. We identified the orthologous gene in Arabidopsis. Although mutants for the gene were fertile under normal conditions (24°C), fertility was significantly reduced under high temperature (28°C), exhibiting a TGMS trait. A cellular mechanism integrated with genetic mutations and different plant species for fertility restoration of TGMS lines is proposed.

The methyl-CpG-binding domain family member PEM1 is essential for Ubisch body formation and pollen exine development in rice

Pollen exine is composed of finely-organized nexine, bacula and tectum, and is crucial for pollen viability and function. Pollen exine development involves a complicated molecular network that coordinates the interaction between pollen and tapetal cells, as well as the biosynthesis, transport and assembly of sporopollenin precursors; however, our understanding of this network is very limited. Here, we report the roles of PEM1, a member of methyl-CpG-binding domain family, in rice pollen development. PEM1 expressed constitutively and, in anthers, its expression was detectable in tapetal cells and pollen. This predicted PEM1 protein of 240 kDa had multiple epigenetic-related domains. pem1 mutants exhibited abnormal Ubisch bodies, delayed exine occurrence and, finally, defective exine, including invisible bacula, amorphous and thickened nexine and tectum layer structures, and also had the phenotype of increased anther cuticle. The mutation in PEM1 did not affect the timely degradation of tapetum. Lipidomics revealed much higher wax and cutin contents in mutant anthers than in wild-type. Accordingly, this mutation up-regulated the expression of a set of genes implicated in transcriptional repression, signaling and diverse metabolic pathways. These results indicate that PEM1 mediates Ubisch body formation and pollen exine development mainly by negatively modulating the expression of genes. Thus, the PEM1-mediated molecular network represents a route for insights into mechanisms underlying pollen development. PEM1 may be a master regulator of pollen exine development.

The SlHB8 acts as a negative regulator in tapetum development and pollen wall formation in Tomato

Pollen development is crucial for the fruit setting process of tomatoes, but the underlying regulatory mechanism remains to be elucidated. Here, we report the isolation of one HD-Zip III family transcription factor, SlHB8, whose expression levels decreased as pollen development progressed. SlHB8 knockout using CRISPR/Cas9 increased pollen activity, subsequently inducing fruit setting, whereas overexpression displayed opposite phenotypes. Overexpression lines under control of the 35 s and p2A11 promoters revealed that SlHB8 reduced pollen activity by affecting early pollen development. Transmission electron microscopy and TUNEL analyses showed that SlHB8 accelerated tapetum degradation, leading to collapsed and infertile pollen without an intine and an abnormal exine. RNA-seq analysis of tomato anthers at the tetrad stage showed that SlHB8 positively regulates SPL/NZZ expression and the tapetum programmed cell death conserved genetic pathway DYT1-TDF1-AMS-MYB80 as well as other genes related to tapetum and pollen wall development. In addition, DNA affinity purification sequencing, electrophoretic mobility shift assay, yeast one-hybrid assay and dual-luciferase assay revealed SlHB8 directly activated the expression of genes related to pollen wall development. The study findings demonstrate that SlHB8 is involved in tapetum development and degradation and plays an important role in anther development.

Cytochemical and comparative transcriptome analyses elucidate the formation and ecological adaptation of three types of pollen coat in Zingiberaceae

BACKGROUND

The pollen ornate surface of flowering plants has long fascinated and puzzled evolutionary biologists for their variety. Each pollen grain is contained within a pollen wall consisting of intine and exine, over which the lipoid pollen coat lies. The cytology and molecular biology of the development of the intine and exine components of the pollen wall are relatively well characterised. However, little is known about the pollen coat, which confers species specificity. We demonstrate three types of pollen coat in Zingiberaceae, a mucilage-like pollen coat and a gum-like pollen coat, along with a pollen coat more typical of angiosperms. The morphological differences between the three types of pollen coat and the related molecular mechanisms of their formation were studied using an integrative approach of cytology, RNA-seq and positive selection analysis.

RESULTS

Contrary to the 'typical' pollen coat, in ginger species with a mucilage-like (Caulokaempferia coenobialis, Cco) or gum-like (Hornstedtia hainanensis, Hhn) pollen coat, anther locular fluid was still present at the bicellular pollen (BCP) stage of development. Nevertheless, there were marked differences between these species: there were much lower levels of anther locular fluid in Hhn at the BCP stage and it contained less polysaccharide, but more lipid, than the locular fluid of Cco. The set of specific highly-expressed (SHE) genes in Cco was enriched in the 'polysaccharide metabolic process' annotation term, while 'fatty acid degradation' and 'metabolism of terpenoids and polyketides' were significantly enriched in SHE-Hhn.

CONCLUSIONS

Our cytological and comparative transcriptome analysis showed that different types of pollen coat depend on the residual amount and composition of anther locular fluid at the BCP stage. The genes involved in 'polysaccharide metabolism' and 'transport' in the development of a mucilage-like pollen coat and in 'lipid metabolism' and 'transport' in the development of a gum-like pollen coat probably evolved under positive selection in both cases. We suggest that the shift from a typical pollen coat to a gum-like or mucilage-like pollen coat in flowering plants is an adaptation to habitats with high humidity and scarcity of pollinators.

Analysis of Floral Organ Development and Sex Determination in Schisandra chinensis by Scanning Electron Microscopy and RNA-Sequencing

S. chinensis is a typical monoecious plant, and the number and development of female flowers determines the yield of S. chinensis. Due to a lack of genetic information, the molecular mechanism of sex differentiation in S. chinensis remains unclear. In this study, the combination of scanning electron microscopy (SEM) and RNA sequencing (RNA-seq) was used to understand the way of sex differentiation of S. chinensis and to mine the related genes of sex determination. The result shows the development of male and female S. chinensis flowers was completed at the same time, the unisexual S. chinensis flowers did not undergo a transition stage between sexes, and sex may have been determined at an early stage in flower development. The results of the gene function analysis of the plant hormone signaling pathway and sucrose metabolism pathway suggest that auxin and JA could be the key hormones for sex differentiation in S. chinensis, and sucrose may promote pollen maturation at the later stage of male flower development. Two AGAMOUS (GAG) genes, 10 AGAMOUS-like MADS-box (AGLs) genes, and the MYB, NAC, WRKY, bHLH, and Trihelix transcription factor families may play important roles in sex determination in S. chinensis. Taken together, the present findings provide valuable genetic information on flower development and sex determination in S. chinensis.

Comprehensive analysis of the MYB transcription factor gene family in Morus alba

BACKGROUND

The V-myb myeloblastosis viral oncogene homolog (MYB) family of proteins is large, containing functionally diverse transcription factors. However, MYBs in Morus are still poorly annotated and a comprehensive functional analysis of these transcription factors is lacking.

RESULTS

In the present study, a genome-wide identification of MYBs in Morus alba was performed. In total 166 MaMYBs were identified, including 103 R2R3-MYBs and four 3R-MaMYBs. Comprehensive analyses, including the phylogenetic analysis with putative functional annotation, motif and structure analysis, gene structure organization, promoter analysis, chromosomal localization, and syntenic relationships of R2R3-MaMYBs and 3R-MaMYBs, provided primary characterization for these MaMYBs. R2R3-MaMYBs covered the subgroups reported for R2R3-MYBs in Arabidopsis and Populus, and had two Morus-specific subgroups, indicating the high retention of MYBs in Morus. Motif analysis revealed high conservative residues at the start and end of each helix and residues consisting of the third helix in R2 and R3 repeats. Thirteen intron/exon patterns (a-m) were summarized, and the intron/exon pattern of two introns with phase numbers of 0 and 2 was the prevalent pattern for R2R3-MaMYBs. Various cis-elements in promoter regions were identified, and were mainly related to light response, development, phytohormone response, and abiotic and biotic stress response and secondary metabolite production. Expression patterns of R2R3-MaMYBs in different organs showed that MaMYBs involved in secondary cell wall components and stress responsiveness were preferentially expressed in roots or stems. R2R3-MaMYBs involved in flavonoid biosynthesis and anthocyanin accumulation were identified and characterized based on functional annotation and correlation of their expression levels with anthocyanin contents.

CONCLUSION

Based on a comprehensive analysis, this work provided functional annotation for R2R3-MYBs and an informative reference for further functional dissection of MYBs in Morus.

Genetic Structure and Molecular Mechanisms Underlying the Formation of Tassel, Anther, and Pollen in the Male Inflorescence of Maize (Zea mays L.)

Maize tassel is the male reproductive organ which is located at the plant’s apex; both its morphological structure and fertility have a profound impact on maize grain yield. More than 40 functional genes regulating the complex tassel traits have been cloned up to now. However, the detailed molecular mechanisms underlying the whole process, from male inflorescence meristem initiation to tassel morphogenesis, are seldom discussed. Here, we summarize the male inflorescence developmental genes and construct a molecular regulatory network to further reveal the molecular mechanisms underlying tassel-trait formation in maize. Meanwhile, as one of the most frequently studied quantitative traits, hundreds of quantitative trait loci (QTLs) and thousands of quantitative trait nucleotides (QTNs) related to tassel morphology have been identified so far. To reveal the genetic structure of tassel traits, we constructed a consensus physical map for tassel traits by summarizing the genetic studies conducted over the past 20 years, and identified 97 hotspot intervals (HSIs) that can be repeatedly mapped in different labs, which will be helpful for marker-assisted selection (MAS) in improving maize yield as well as for providing theoretical guidance in the subsequent identification of the functional genes modulating tassel morphology. In addition, maize is one of the most successful crops in utilizing heterosis; mining of the genic male sterility (GMS) genes is crucial in developing biotechnology-based male-sterility (BMS) systems for seed production and hybrid breeding. In maize, more than 30 GMS genes have been isolated and characterized, and at least 15 GMS genes have been promptly validated by CRISPR/Cas9 mutagenesis within the past two years. We thus summarize the maize GMS genes and further update the molecular regulatory networks underlying male fertility in maize. Taken together, the identified HSIs, genes and molecular mechanisms underlying tassel morphological structure and male fertility are useful for guiding the subsequent cloning of functional genes and for molecular design breeding in maize. Finally, the strategies concerning efficient and rapid isolation of genes controlling tassel morphological structure and male fertility and their application in maize molecular breeding are also discussed.

Effects of Different Chemicals on Sexual Regulation in Persimmon (Diospyros kaki Thunb.) Flowers

Research on crop sexuality is important for establishing systems for germplasm innovation and cultivating improved varieties. In this study, androecious persimmon trees were treated with various concentrations of ethrel (100, 500, and 1,000 mg/L) and zeatin (1, 5, and 10 mg/L) to investigate the morphological, physiological, and molecular characteristics of persimmon. Ethrel at 1,000 mg/L and zeatin at 10 mg/L both significantly reduced the stamen length and pollen grain diameter in androecious trees. Ethrel treatment also led to reduced stamen development with degenerated cellular contents; zeatin treatment promoted the development of arrested pistils via maintaining relatively normal mitochondrial morphology. Both treatments altered carbohydrate, amino acid, and endogenous phytohormone contents, as well as genes associated with hormone production and floral organ development. Thereafter, we explored the combined effects of four chemicals, including ethrel and zeatin, as well as zebularine and 5-azacytidine, both of which are DNA methylation inhibitors, on androecious persimmon flower development. Morphological comparisons showed that stamen length, pollen viability, and pollen grain diameter were significantly inhibited after combined treatment. Large numbers of genes involving in carbohydrate metabolic, mitogen-activated protein kinase (MAPK) signaling, and ribosome pathways, and metabolites including uridine monophosphate (UMP) and cyclamic acid were identified in response to the treatment, indicating complex regulatory mechanisms. An association analysis of transcriptomic and metabolomic data indicated that ribosomal genes have distinct effects on UMP and cyclamic acid metabolites, explaining how male floral buds of androecious persimmon trees respond to these exogenous chemicals. These findings extend the knowledge concerning sexual differentiation in persimmon; they also provide a theoretical basis for molecular breeding, high-yield cultivation, and quality improvement in persimmon.

Gene silencing, knockout and over-expression of a transcription factor ABORTED MICROSPORES (SlAMS) strongly affects pollen viability in tomato (Solanum lycopersicum)

BACKGROUND

The tomato (Solanum lycopersicum L.) is an economically valuable crop grown worldwide. Because the use of sterile males reduces the cost of F1 seed production, the innovation of male sterility is of great significance for tomato breeding. The ABORTED MICROSPORES gene (AMS), which encodes for a basic helix-loop-helix (bHLH) transcription factor, has been previously indicated as an essential gene for tapetum development in Arabidopsis and rice. To determine the function of the SlAMS gene (AMS gene from S. lycopersicum) and verify whether it is a potential candidate gene for generating the male sterility in tomato, we used virus-induced gene silencing (VIGS), CRISPR/Cas9-mediated genome editing and over-expression technology to transform tomato via Agrobacterium infection.

RESULTS

Here, the full-length SlAMS gene with 1806 bp from S. lycopersicum (Accession No. MK591950.1) was cloned from pollen cDNA. The results of pollen grains staining showed that, the non-viable pollen proportions of SlAMS-silenced (75%), -knockouted (89%) and -overexpressed plants (60%) were significantly higher than the wild type plants (less than 10%; P < 0.01). In three cases, the morphology of non-viable pollen grains appeared tetragonal, circular, atrophic, shriveled, or otherwise abnormally shaped, while those of wild type appeared oval and plump. Furthermore, the qRT-PCR analysis indicated that SlAMS in anthers of SlAMS-silenced and -knockouted plants had remarkably lower expression than in that of wild type (P < 0.01), and yet it had higher expression in SlAMS-overexpressed plants (P < 0.01).

CONCLUSION

In this paper, Our research suggested alternative approaches to generating male sterility in tomato, among which CRISPR/Cas9-mediated editing of SlAMS implied the best performance. We also demonstrated that the downregulation and upregulation of SlAMS both affected the pollen formation and notably led to reduction of pollen viability, suggesting SlAMS might be essential for regulating pollen development in tomato. These findings may facilitate studies on clarifying the SlAMS-associated molecular regulatory mechanism of pollen development in tomato.

Transcriptome profiling of flower buds of male-sterile lines provides new insights into male sterility mechanism in alfalfa

BACKGROUND

The use of heterosis to produce hybrid seeds is a challenge to breeding for improved crop yield. In previous studies, we isolated a male sterile alfalfa hybrid and successfully obtained a genetically stable alfalfa male sterile line through backcrossing, henceforth named MS-4. In this study, we used RNA-seq technology to analyze the transcriptome profiles of the male sterile line (MS-4) and the male fertile line (MF) of alfalfa to elucidate the mechanism of male sterility.

RESULTS

We screened a total of 11,812 differentially expressed genes (DEGs) from both MS-4 and MF lines at three different stages of anther development. Gene Ontology (GO), and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses revealed that these DEGs are mainly involved in processes such as energy metabolism, lipid and amino acid metabolism, carbohydrate metabolism, in addition to cell synthesis and aging. The results from protein-protein interaction (PPI) network analysis showed that the ribosomal protein (MS.Gene25178) was the core gene in the network. We also found that transcriptional regulation was an influential factor in the development of anthers.

CONCLUSIONS

Our findings provide new insights into understanding of the fertility changes in the male sterile (MS-4) of alfalfa.

Cytological Analysis and Fine Mapping of paa1 (Post-meiosis Abnormal Anther 1) Mutant with Abnormal Tapetum and Microspore Development

To further understand the molecular mechanism for rice male reproduction, a rice male sterile mutant paa1 was screened from the rice mutant library generated by treatment with ⁶⁰Coγ-rays. Genetic analysis revealed that paa1 is controlled by a single- recessive nuclear gene, and the anthers of the paa1 mutant were smaller than those of WT plants with a white color. Histological analysis demonstrated that the anthers of the paa1 mutant began to turn abnormal at the microspore stage after meiosis, with abnormal degradation of tapetum, deformed Ubisch bodies, and defective pollen exine. TUNEL assay results also confirmed the delay of tapetum PCD in paa1. Map-based cloning was performed for the PAA1 location. As a result, PAA1 was located in a 88-kb region at the end of chromosome 10, which comprises a total of seven candidate genes, and no genes related to anther development have been reported in this region. The results indicate that PAA1 is an essential gene in regulating tapetum development and pollen/microspore formation after rice meiosis.

MYB2 Is Important for Tapetal PCD and Pollen Development by Directly Activating Protease Expression in Arabidopsis

Tapetal programmed cell death (PCD) is a complex biological process that plays an important role in pollen formation and reproduction. Here, we identified the MYB2 transcription factor expressed in the tapetum from stage 5 to stage 11 that was essential for tapetal PCD and pollen development in Arabidopsis thaliana. Downregulation of MYB2 retarded tapetal degeneration, produced defective pollen, and decreased pollen vitality. EMSA and transcriptional activation analysis revealed that MYB2 acted as an upstream activator and directly regulated expression of the proteases CEP1 and βVPE. The expression of these proteases was lower in the buds of the myb2 mutant. Overexpression of either/both CEP1 or/and βVPE proteases partially recover pollen vitality in the myb2 background. Taken together, our results revealed that MYB2 regulates tapetal PCD and pollen development by directly activating expression of the proteases CEP1 and βVPE. Thus, a transcription factor/proteases regulatory and activated cascade was established for tapetal PCD during another development in Arabidopsis thaliana. Highlight: MYB2 is involved in tapetal PCD and pollen development by directly regulating expression of the protease CEP1 and βVPE and establishes a transcription factor/proteases regulatory and activated cascade.

The N-terminal acetyltransferase Naa50 regulates tapetum degradation and pollen development in Arabidopsis

The N-terminal acetylation of proteins is a key modification in eukaryotes. However, knowledge of the biological function of N-terminal acetylation modification of proteins in plants is limited. Naa50 is the catalytic subunit of the N-terminal acetyltransferase NatE complex. We previously demonstrated that the absence of Naa50 leads to sterility in Arabidopsis thaliana. In the present study, the lack of Naa50 resulted in collapsed and sterile pollen in Arabidopsis. Further experiments showed that the mutation in Naa50 accelerated programmed cell death in the tapetum. Expression pattern analysis revealed the specific expression of Naa50 in the tapetum cells of anthers at 9-11 stages during pollen development, when tapetal programmed cell death occurred. Reciprocal cross analyses indicated that male sterility in naa50 is caused by sporophytic effects. mRNA sequencing and quantitative PCR of the closed buds showed that the deletion of Naa50 resulted in the upregulation of the cysteine protease coding gene CEP1 and impaired the expression of several genes involved in pollen wall deposition and pollen mitotic division. The collective data suggest that Naa50 balances the degradation of tapetum cells during anther development and plays an important role in pollen development by affecting several pathways.

Delayed callose degradation restores the fertility of multiple P/TGMS lines in Arabidopsis

Photoperiod/temperature-sensitive genic male sterility (P/TGMS) is widely applied for improving crop production. Previous investigations using the reversible male sterile (rvms) mutant showed that slow development is a general mechanism for restoring fertility to P/TGMS lines in Arabidopsis. In this work, we isolated a restorer of rvms-2 (res3), as the male sterility of rvms-2 was rescued by res3. Phenotype analysis and molecular cloning show that a point mutation in UPEX1 l in res3 leads to delayed secretion of callase A6 from the tapetum to the locule and tetrad callose wall degradation. Electrophoretic mobility shift assay and chromatin immunoprecipitation analysis demonstrated that the tapetal transcription factor ABORTED MICROSPORES directly regulates UPEX1 expression, revealing a pathway for tapetum secretory function. Early degradation of the callose wall in the transgenic line eliminated the fertility restoration effect of res3. The fertility of multiple known P/TGMS lines with pollen wall defects was also restored by res3. We propose that the remnant callose wall may broadly compensate for the pollen wall defects of P/TGMS lines by providing protection for pollen formation. A cellular mechanism is proposed to explain how slow development restores the fertility of P/TGMS lines in Arabidopsis.

The GA-DELLA-OsMS188 module controls male reproductive development in rice

Pollen protects male sperm and allows flowering plants to adapt to diverse terrestrial environments, thereby leading to the rapid expansion of plants into new regions. The process of anther/pollen development is coordinately regulated by internal and external factors including hormones. Currently, the molecular mechanisms underlying gibberellin (GA)-mediated male reproductive development in plants remain unknown. We show here that rice DELLA/SLR1, which encodes the central negative regulator of GA signaling, is essential for rice anther development. The slr1-5 mutant exhibits premature programmed cell death of the tapetum, lacks Ubisch bodies, and has no exine and no mature pollen. SLR1 is mainly expressed in tapetal cells and tetrads, and is required for the appropriate expression of genes encoding key factors of pollen development, which are suggested to be OsMS188-targeted genes. OsMS188 is the main component in the essential genetic program of tapetum and pollen development. Further, we demonstrate that SLR1 interacts with OsMS188 to cooperatively activate the expression of the sporopollenin biosynthesis and transport-related genes CYP703A3, DPW, ABCG15 and PKS1 for rapid formation of pollen walls. Overall, the results of this study suggest that the GA hormonal signal is integrated into the anther genetic program and regulates rice anther development through the GA-DELLA-OsMS188 module.

Tapetal 3-Ketoacyl-Coenzyme A Synthases Are Involved in Pollen Coat Lipid Accumulation for Pollen-Stigma Interaction in Arabidopsis

Pollen coat lipids form an outer barrier to protect pollen itself and play essential roles in pollen-stigma interaction. However, the precise molecular mechanisms underlying the production, deposition, regulation, and function of pollen coat lipids during anther development remain largely elusive. In lipid metabolism, 3-ketoacyl-coenzyme A synthases (KCS) are involved in fatty acid elongation or very-long-chain fatty acid (VLCFA) synthesis. In this study, we identified six members of the Arabidopsis KCS family expressed in anther. Among them, KCS7, KCS15, and KCS21 were expressed in tapetal cells at anther stages 8-10. Further analysis demonstrated that they act downstream of male sterility 1 (MS1), a regulator of late tapetum development. The kcs7/15/21 triple mutant is fertile. Both cellular observation and lipid staining showed pollen coat lipid was decreased in kcs7/15/21 triple mutant. After landing on stigma, the wild-type pollen grains were hydrated for about 5 min while the kcs7/15/21 triple mutant pollen took about 10 min to hydrate. Pollen tube growth of the triple mutant was also delayed. These results demonstrate that the tapetum-localized KCS proteins are involved in the accumulation of pollen coat lipid and reveal the roles of tapetal-derived pollen coat lipid for pollen-stigma interaction.

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.

Iron insufficiency in floral buds impairs pollen development by disrupting tapetum function

Reduction of crop yield due to iron (Fe) deficiency has always been a concern in agriculture. How Fe insufficiency in floral buds affects pollen development remains unexplored. Here, plants transferred to Fe-deficient medium at the reproductive stage had reduced floral Fe content and viable pollen and showed a defective pollen outer wall, all restored by supplying floral buds with Fe. A comparison of differentially expressed genes (DEGs) in Fe-deficient leaves, roots, and anthers suggested that changes in several cellular processes were unique to anthers, including increased lipid degradation. Co-expression analysis revealed that ABORTED MICROSPORES (AMS), DEFECTIVE IN TAPETAL DEVELOPMENT AND FUNCTION1, and BASIC HELIX-LOOP-HELIX 089/091/010 encode key upstream transcription factors of Fe deficiency-responsive DEGs involved in tapetum function and development, including tapetal ROS homeostasis, programmed cell death, and pollen outer wall formation-related lipid metabolism. Analysis of RESPIRATORY-BURST OXIDASE HOMOLOG E (RBOHE) gain- and loss-of-function under Fe deficiency indicated that RBOHE- and Fe-dependent regulation cooperatively control anther reactive oxygen species levels and pollen development. Since DEGs in Fe-deficient anthers were not significantly enriched in genes related to mitochondrial function, the changes in mitochondrial status under Fe deficiency, including respiration activity, density, and morphology, were probably because the Fe amount was insufficient to maintain proper mitochondrial protein function in anthers. To sum up, Fe deficiency in anthers may affect Fe-dependent protein function and impact upstream transcription factors and their downstream genes, resulting in extensively impaired tapetum function and pollen development.

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.