Earlier Degraded Tapetum1 (EDT1) Encodes an ATP-Citrate Lyase Required for Tapetum Programmed Cell Death

CTB6 Confers Cold Tolerance at the Booting Stage by Maintaining Tapetum Development in Rice

Rice is highly sensitive to cold stress, particularly at the booting stage, which significantly threatens rice production. In this study, we cloned a gene, CTB6, encoding a lipid transfer protein involved in cold tolerance at the booting stage in rice, based on our previous fine-mapped quantitative trait locus (QTL) qCTB10-2. CTB6 is mainly expressed in the tapetum and young microspores of the anther. CTB6 interacts with catalases (CATs) to maintain their stability, thereby scavenging reactive oxygen species (ROS) accumulation in anthers and facilitating tapetum development under cold stress conditions. Additionally, CTB6 has lipid-binding ability and affects the lipid content in anthers to regulate cold tolerance at the booting stage. Haplotype analysis and promoter activity assay revealed a specific single nucleotide polymorphism (SNP)-489 variation in the promoter of CTB6, which enhances its expression and results in improved cold tolerance in Hap1-K varieties. The CTB6 near-isogenic line (NIL) exhibited enhanced cold tolerance at the booting stage, with no significant effects on other agronomic traits. Our findings uncover a natural variation of CTB6 for cold tolerance at the booting stage and provide new genetic resources for cold tolerance breeding in rice.

ATP-CITRATE LYASEB1 supplies materials for sporopollenin biosynthesis and microspore development in Arabidopsis

Acetyl-CoA is the main substrate of lipid metabolism and functions as an energy source for plant development. In the cytoplasm, acetyl-CoA is mainly produced by ATP-citrate lyase (ACL), which is composed of ACLA and ACLB subunits. In this study, we isolated the restorer-4 (res4) of the thermo-sensitive genic male sterile mutant reversible male sterile-2 (rvms-2) in Arabidopsis (Arabidopsis thaliana). RES4 encodes ACLB1, and res4 harbors a point mutation (Gly584 to Arg) in the citryl-CoA lyase domain. Both the ACLA and ACLB subunits are expressed in the tapetal layer of anthers. RES4 is regulated by MS188, and the res4 point mutation leads to pollen with a defective exine structure. In res4, lipid accumulation was significantly reduced within the tapetum and locules. These results indicate that acetyl-CoA synthesized by ACL is used for sporopollenin biosynthesis in the tapetum. Microspore diameter was significantly smaller in res4 than in wild type, indicating that acetyl-CoA from the tapetum supplies microspore development. Previous studies have shown that delayed degradation of the tetrad wall in res2 and res3 provides additional protection for rvms-2 microspores. The reduced volume of res4 microspores may lessen the requirement for cell wall protection to restore rvms-2 fertility. This study reveals the function of ACL in anther development and the mechanisms of fertility restoration in photoperiod- and thermo-sensitive genic male sterile lines.

Identification of the new allele ptc1-2 and analysis of the regulatory role of PTC1 gene in rice anther development

Anther development involves a series of important biological events that are precisely regulated by many genes. Although several important genes involved in rice anther development have been identified, the regulatory network involved in tapetal development and pollen wall formation is still largely unclear. PERSISTENT TAPETAL CELL 1 (PTC1) encodes a PHD-Finger protein, which plays a critical role in the regulation of tapetal cell death and pollen development in rice. Here, we report the isolation and characterization of a new allele ptc1-2 with 2-base deletion in the third exon, causing the absent of the PHD domain due to the sequence change. Cytological analysis revealed delayed tapetal PCD, defective pollen exine formation and abnormal ubisch bodies development. Transcriptome analysis revealed that genes related to pollen wall formation (secondary metabolism, phenylalanine synthesis, and cutin and wax biosynthesis pathways), cell death (cysteine and methionine metabolism and DNA repair pathways), and carbohydrate synthesis (starch and sucrose metabolism pathways) were significantly altered in ptc1-2 mutant. A total of 13 reported anther development genes exhibited significant expression changes in the ptc1-2 mutant. Yeast two-hybrid and BiFC analyses showed that PTC1 could interact with API5, an inhibitor of apoptosis, and the citrin-binding enzyme EDT1. This work is helpful in deepening the understanding of the regulatory network of male reproductive development in rice.

OsALKBH9-mediated m6A demethylation regulates tapetal PCD and pollen exine accumulation in rice

The N6-methyladenosine (m6A) mRNA modification is crucial for plant development and stress responses. In rice, the male sterility resulting from the deficiency of OsFIP37, a core component of m6A methyltransferase complex, emphasizes the significant role of m6A in male fertility. m6A is reversible and can be removed by m6A demethylases. However, whether mRNA m6A demethylase regulates male fertility in rice has remained unknown. Here, we identify the mRNA m6A demethylase OsALKBH9 and demonstrate its involvement in male fertility regulation. Knockout of OsALKBH9 causes male sterility, dependent on its m6A demethylation activity. Cytological analysis reveals defective tapetal programmed cell death (PCD) and excessive accumulation of microspores exine in Osalkbh9-1. Transcriptome analysis of anthers shows up-regulation of genes involved in tapetum development, sporopollenin synthesis, and transport pathways in Osalkbh9-1. Additionally, we demonstrate that OsALKBH9 demethylates the m6A modification in TDR and GAMYB transcripts, which affects the stability of these mRNAs and ultimately leads to excessive accumulation of pollen exine. Our findings highlight the precise control of mRNA m6A modification and reveal the pivotal roles played by OsALKBH9-mediated m6A demethylation in tapetal PCD and pollen exine accumulation in rice.

Abnormal Degraded Tapetum 1 (ADT1) is required for tapetal cell death and pollen development in rice

The timely degradation of tapetum, the innermost somatic anther cell layer in flowering plants, is critical for pollen development. Although several genes involved in tapetum development have been characterized, the molecular mechanisms underlying tapetum degeneration remain elusive. Here, we showed that mutation in Abnormal Degraded Tapetum 1 (ADT1) resulted in overaccumulation of Reactive Oxygen Species (ROS) and abnormal anther development, causing earlier tapetum Programmed Cell Death (PCD) and pollen abortion. ADT1 encodes a nuclear membrane localized protein, which is strongly expressed in the developing microspores and tapetal cells during early anther development. Moreover, ADT1 could interact with metallothionein MT2b, which was related to ROS scavenging and cell death regulation. These findings indicate that ADT1 is required for proper timing of tapetum PCD by regulating ROS homeostasis, expanding our understanding of the regulatory network of male reproductive development in rice.

mRNA cleavage by 21-nucleotide phasiRNAs determines temperature-sensitive male sterility in rice

Temperature-sensitive male sterility is one of the core components for hybrid rice (Oryza sativa) breeding based on the 2-line system. We previously found that knockout of ARGONAUTE 1d (AGO1d) causes temperature-sensitive male sterility in rice by influencing phased small interfering RNA (phasiRNA) biogenesis and function. However, the specific phasiRNAs and their targets underlying the temperature-sensitive male sterility in the ago1d mutant remain unknown. Here, we demonstrate that the ago1d mutant displays normal female fertility but complete male sterility at low temperature. Through a multiomics analysis of small RNA (sRNA), degradome, and transcriptome, we found that 21-nt phasiRNAs account for the greatest proportion of the 21-nt sRNA species in rice anthers and are sensitive to low temperature and markedly downregulated in the ago1d mutant. Moreover, we found that 21-nt phasiRNAs are essential for the mRNA cleavage of a set of fertility- and cold tolerance-associated genes, such as Earlier Degraded Tapetum 1 (EDT1), Tapetum Degeneration Retardation (TDR), OsPCF5, and OsTCP21, directly or indirectly determined by AGO1d-mediated gene silencing. The loss of function of 21-nt phasiRNAs can result in upregulation of their targets and causes varying degrees of defects in male fertility and grain setting. Our results highlight the essential functions of 21-nt phasiRNAs in temperature-sensitive male sterility in rice and suggest their promising application in 2-line hybrid rice breeding in the future.

ACL and HAT1 form a nuclear module to acetylate histone H4K5 and promote cell proliferation

Acetyl-CoA utilized by histone acetyltransferases (HAT) for chromatin modification is mainly generated by ATP-citrate lyase (ACL) from glucose sources. How ACL locally establishes acetyl-CoA production for histone acetylation remains unclear. Here we show that ACL subunit A2 (ACLA2) is present in nuclear condensates, is required for nuclear acetyl-CoA accumulation and acetylation of specific histone lysine residues, and interacts with Histone AcetylTransferase1 (HAT1) in rice. The rice HAT1 acetylates histone H4K5 and H4K16 and its activity on H4K5 requires ACLA2. Mutations of rice ACLA2 and HAT1 (HAG704) genes impair cell division in developing endosperm, result in decreases of H4K5 acetylation at largely the same genomic regions, affect the expression of similar sets of genes, and lead to cell cycle S phase stagnation in the endosperm dividing nuclei. These results indicate that the HAT1-ACLA2 module selectively promotes histone lysine acetylation in specific genomic regions and unravel a mechanism of local acetyl-CoA production which couples energy metabolism with cell division.

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 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.

Comparative Transcriptome Analysis Reveals Key Insights into Fertility Conversion in the Thermo-Sensitive Cytoplasmic Male Sterile Wheat

Thermo-sensitive cytoplasmic male sterility (TCMS) plays a crucial role in hybrid production and hybrid breeding; however, there are few studies on molecular mechanisms related to anther abortion in the wheat TCMS line. In this study, FA99, a new wheat thermo-sensitive cytoplasmic male sterility line, was investigated. Fertility conversion analysis showed that FA99 was mainly controlled by temperature, and the temperature-sensitive stage was pollen mother cell formation to a uninucleate stage. Further phenotypic identification and paraffin section showed that FA99 was characterized by indehiscent anthers and aborted pollen in a sterile environment and tapetum was degraded prematurely during the tetrad period, which was the critical abortion period of FA99. The contents of O^2-, H₂O₂, MDA and POD were significantly changed in FA99 under a sterile environment by the determination of physiological indexes. Furthermore, through transcriptome analysis, 252 differentially expressed genes were identified, including 218 downregulated and 34 upregulated genes. Based on KOG function classification, GO enrichment and KEGG pathways analysis, it was evident that significant transcriptomic changes in FA99 under different fertility environments, and the major differences were "phenylalanine metabolism", "phenylpropanoid biosynthesis", "cutin, suberine and wax biosynthesis", "phenylalanine, tyrosine and tryptophan biosynthesis" and "citrate cycle (TCA cycle)". Finally, we proposed an intriguing transcriptome-mediated pollen abortion and male sterility network for FA99. These findings provided data on the molecular mechanism of fertility conversion in thermo-sensitive cytoplasmic male sterility wheat.

Identification of QTNs, QTN-by-environment interactions and genes for yield-related traits in rice using 3VmrMLM

Rice, which supports more than half the population worldwide, is one of the most important food crops. Thus, potential yield-related quantitative trait nucleotides (QTNs) and QTN-by-environment interactions (QEIs) have been used to develop efficient rice breeding strategies. In this study, a compressed variance component mixed model, 3VmrMLM, in genome-wide association studies was used to detect QTNs for eight yield-related traits of 413 rice accessions with 44,000 single nucleotide polymorphisms. These traits include florets per panicle, panicle fertility, panicle length, panicle number per plant, plant height, primary panicle branch number, seed number per panicle, and flowering time. Meanwhile, QTNs and QEIs were identified for flowering times in three different environments and five subpopulations. In the detections, a total of 7~23 QTNs were detected for each trait, including the three single-environment flowering time traits. In the detection of QEIs for flowering time in the three environments, 21 QTNs and 13 QEIs were identified. In the five subpopulation analyses, 3~9 QTNs and 2~4 QEIs were detected for each subpopulation. Based on previous studies, we identified 87 known genes around the significant/suggested QTNs and QEIs, such as LOC_Os06g06750 (OsMADS5) and LOC_Os07g47330 (FZP). Further differential expression analysis and functional enrichment analysis identified 30 candidate genes. Of these candidate genes, 27 genes had high expression in specific tissues, and 19 of these 27 genes were homologous to known genes in Arabidopsis. Haplotype difference analysis revealed that LOC_Os04g53210 and LOC_Os07g42440 are possibly associated with yield, and LOC_Os04g53210 may be useful around a QEI for flowering time. These results provide insights for future breeding for high quality and yield in rice.

Killing me softly - Programmed cell death in plant reproduction from sporogenesis to fertilization

Regulated or programmed cell death (RCD or PCD) is a fundamental biological principle integral to a considerable variety of functions in multicellular organisms. In plants, different PCD processes are part of biotic and abiotic stress responses, but also occur as an essential aspect of unperturbed plant development. PCD is particularly abundant during plant reproduction, eliminating unwanted or no longer needed cells, tissues, or organs in a precisely controlled manner. Failure in reproductive PCD can have detrimental consequences for plant reproduction. Here we shed a light on the latest research into PCD mechanisms in plant reproduction from sex determination over sporogenesis to pollination and fertilization.

Grass-specific ABERRANT MICROSPORE DEVELOPMENT 1 is required for maintaining pollen fertility in rice

Pollen development includes a series of biological events that require precise gene regulation. Although several transcription factors (TFs) have been shown to play roles in maintaining pollen fertility, the major regulatory networks underlying tapetum development and pollen wall formation are largely unknown. Herein, we report that ABERRANT MICROSPORE DEVELOPMENT1 (AMD1), a protein annotated previously as unknown protein, is required for tapetum development and pollen exine patterning in rice (Oryza sativa L.). AMD1 encodes a grass-specific protein exhibiting transactivation activity in the nucleus and is spatiotemporally expressed in the tapetum and microspores during pollen development. Further biochemical assays indicate that AMD1 directly activates the transcription of DEFECTIVE POLLEN WALL (DPW) and POLYKETIDE SYNTHASE2 (OsPKS2), which are both implicated in sporopollenin biosynthesis during exine formation. Additionally, AMD1 directly interacts with TAPETUM DEGENERATION RETARDATION (TDR), a key TF involved in the regulation of tapetum degradation and exine formation. Taken together, we demonstrate that AMD1 is an important regulatory component involved in the TDR-mediated regulatory pathway to regulate sporopollenin biosynthesis, tapetum degradation, and exine formation for pollen development. Our work provides insights into the regulatory network of rice sexual reproduction and a useful target for genetic engineering of new male-sterile lines for hybrid rice breeding.

The gene TaPG encoding a polygalacturonase is critical for pollen development and male fertility in thermo-sensitive cytoplasmic male-sterility wheat

Thermo-sensitive cytoplasmic male sterility is of great significance to heterosis and hybrid seed production in wheat. Consequently, it is worthwhile to research the genes associated with male sterility. Although polygalacturonases (PGs) have been studied to play a crucial role in male reproduction of many plants, their functions in the reproductive development of wheat remain unclear. Here, TaPG (TraesCS7A02G404900) encoding a polygalacturonase was isolated from the anthers of KTM3315A, a wheat thermo-sensitive cytoplasmic male sterile with Aegilops kotschyi cytoplasm. Expression pattern analyses showed that TaPG was strongly expressed in fertile anthers and its protein was localized in the cell wall. Further verification via barley stripe mosaic virus revealed that the silencing of TaPG exhibited abnormal anthers, premature degradation of tapetum, pollen abortion, and defective pollen wall formation, resulting in the declination of fertility. Conclusively, our research suggested that TaPG contributed to the pollen development and male fertility, which will provide a novel insight into the fertility conversion of thermo-sensitive cytoplasmic male sterility in wheat.

The RNA binding protein OsLa influences grain and anther development in rice

La proteins are found widely in eukaryotes and play a variety of vital roles. AtLa1 has been identified as an La protein that is necessary for embryogenesis in Arabidopsis; however, the existence and biological functions of La proteins in rice (Oryza sativa L.) remain unclear. In this study, we identified and characterized two La proteins in rice that are homologous to AtLa1 and named them OsLa1 and OsLa2. Both the OsLa1 and OsLa2 genes encode RNA-binding proteins with an La domain and two RNA-binding domains. Mutant OsLa1 reduced grain length and pollen fertility, whereas OsLa1 overexpression caused the opposite phenotypes. Further experiments indicated that OsLa1 modulates grain size by influencing cell expansion. Interestingly, mutant OsLa2 resulted in thin grains with decreased weight and a low seed-setting rate. We also found that OsLa1 interacted with OsLa2 and that both OsLa1 and OsLa2 interacted with OseIF6.1, a eukaryotic translation initiation factor involved in ribosome biogenesis. In addition, OsLa1 was able to bind to OseIF6.1 mRNA to modulate its expression. Complete OseIF6.1 knockout caused lethality and OseIF6.1/oseif6.1 heterozygous plants displayed low fertility and low seed setting. Together, our results enrich our knowledge of the role of La proteins in rice growth and development, as well as the relationship between La and eIF6 in rice.

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.

Rice Cell Division Cycle 20s are required for faithful chromosome segregation and cytokinesis during meiosis

Chromosome segregation must be under strict regulation to maintain chromosome euploidy and stability. Cell Division Cycle 20 (CDC20) is an essential cell cycle regulator that promotes the metaphase-to-anaphase transition and functions in the spindle assembly checkpoint, a surveillance pathway that ensures the fidelity of chromosome segregation. Plant CDC20 genes are present in multiple copies, and whether CDC20s have the same functions in plants as in yeast and animals is unclear, given the potential for divergence or redundancy among the multiple copies. Here, we studied all three CDC20 genes in rice (Oryza sativa) and constructed two triple mutants by clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9-mediated genome editing to explore their roles in development. Knocking out all three CDC20 genes led to total sterility but did not affect vegetative development. Loss of the three CDC20 proteins did not alter mitotic division but severely disrupted meiosis as a result of asynchronous and unequal chromosome segregation, chromosome lagging, and premature separation of chromatids. Immunofluorescence of tubulin revealed malformed meiotic spindles in microsporocytes of the triple mutants. Furthermore, cytokinesis of meiosis I was absent or abnormal, and cytokinesis II was completely prevented in all mutant microsporocytes; thus, no tetrads or pollen formed in either cdc20 triple mutant. Finally, the subcellular structures and functions of the tapetum were disturbed by the lack of CDC20 proteins. These findings demonstrate that the three rice CDC20s play redundant roles but are indispensable for faithful meiotic chromosome segregation and cytokinesis, which are required for the production of fertile microspores.

TMT-based comparative proteomic analysis of the male-sterile mutant ms01 sheds light on sporopollenin production and pollen development in wucai (Brassica campestris L.)

A spontaneous male-sterile mutant ms01 was discovered from the excellent high-generation inbred line 'hx12-6-3' in wucai. Compared with wild-type 'hx12-6-3', ms01 displayed complete male sterility with degenerated stamens and no pollen. In this study, cytological observation revealed that the tapetum of the anthers of ms01 had degraded in advance, and microspore development had stagnated in the mononuclear stage, ultimately resulting in completely aborted pollen. Genetic analysis indicated that the sterility of ms01 was controlled by a single recessive nuclear gene. In the differential proteomic analysis of 'hx12-6-3' and ms01 flower buds using a tandem mass tags-based approach, a comparison of two stages (stage a and stage e) revealed 1272 differentially abundant proteins (DAPs). The abnormal variation of the anther cuticle, pollen coat, and sporopollenin production were effected by lipid metabolism and phenylpropanoid biosynthesis in the mutant ms01. Further analysis elucidated that pollen development was associated with amino acid metabolism, protein synthesis and degradation, carbohydrate metabolism, flavonoid biosynthesis and glutathione metabolism. These results provide novel insights into the molecular mechanism of GMS (genic male sterility) in wucai. SIGNIFICANCE: ms01, as the first indentified spontaneous male-sterile mutant in wucai, plays a significant role in the initial study of GMS (genic male sterility). In our study, the key DAPs related to anther and pollen development were obtained by TMT-based comparative proteomic analysis. We found that the abnormal accumulation of H₂O₂ might induce premature degradation of the tapetum, causing anther metabolism disorder and pollen abortion. This process involved multiple DAPs and formed a complex regulatory network that generated a series of physiological metabolic alterations, ultimately leading to male sterility. Our results provide a theoretical foundation for further research on the complex anther and pollen development process.

Nuclear variations and tapetum polyploidy related to pollen grain development in Passiflora L. (Passifloraceae)

Tapetal cells comprise an anther tissue fundamental to pollen grain development. They are associated with endoreduplication events, which culminate in polyploid and multinucleated cells, high metabolic activity, and different organelle arrangements to support all the development of the pollen grains. Passiflora species present a secretory tapetum, with diversity in the number and size of nuclei. Tapetal cells undergo numerous changes in a short period of development when compared to the plant's life span. To improve our knowledge of tapetum development, tests assessing ploidy levels, anatomy, cytochemistry, transmission electron microscopy, flow cytometry, as well as conventional and molecular cytogenetics were used in Passiflora actinia and P. elegans. The current data show striking differences in nuclear organisation during tapetal cell development, including mono to quadrinucleate cells, and ploidy levels from 2n to 32n. One of the most peculiar features was the atypical behaviour of the ER, which accumulated in the cell border, similar to a 'cER', as well as large dictyosomes. This endomembrane configuration may be related to the tapetum nutritional network and secretion of compounds at the end of meiosis. Another atypical feature of the ER was the formation of an invagination to establish 'binucleated' polyploid cells. This membrane projection appears when the nuclei form two lobes, as well as when it organises a nucleoplasmic reticulum. These data demonstrate that there are important ultrastructural changes in tapetal cells, including organelle arrangements, ploidy levels, and nuclear activity, common to P. actinia and P. elegans, but different from the plant model A. thaliana. This article is protected by copyright. All rights reserved.

POLLEN STERILITY, a novel suppressor of cell division, is required for timely tapetal programmed cell death in rice

Timely programmed cell death (PCD) of the tapetum supplying nutrients to microspores is a prerequisite for normal pollen development. Here we identified a unique mutant of rice (Oryza sativa L.), pollen sterility (post), which showed aborted pollens accompanied with extra-large husks. Due to failure of timely PCD of tapetal cells, post exhibited abnormal pollen wall patterning and defective pollen grains. By map-based cloning, we identified a causal gene, POST, encoding a novel protein which is ubiquitously localized in cells. RNA in situ hybridization showed that POST is highly detected in the tapetum and microspores at stages 8 and 9. Transcriptome analysis indicated that POST could function as an important regulator of the metabolic process involved in tapetal PCD. Compared with wild-type rice, post mutant has an increased cell number resulting from elevated expression of cell cycle associated genes in grain husks. Overexpression of POST inhibits grain size in wild type, while appropriate expression of POST in post mutant can recover the seed fertility but has little effect on the large grains, illustrating that fine-tuning of POST expression could be a potential strategy for rice yield improvement. The connection between cell division and cell death conferred by POST provides novel insights into the understanding of the tapetal PCD process.

PsEND1 Is a Key Player in Pea Pollen Development Through the Modulation of Redox Homeostasis

Redox homeostasis has been linked to proper anther and pollen development. Accordingly, plant cells have developed several Reactive Oxygen Species (ROS)-scavenging mechanisms to maintain the redox balance. Hemopexins constitute one of these mechanisms preventing heme-associated oxidative stress in animals, fungi, and plants. Pisum sativum ENDOTHECIUM 1 (PsEND1) is a pea anther-specific gene that encodes a protein containing four hemopexin domains. We report the functional characterization of PsEND1 and the identification in its promoter region of cis-regulatory elements that are essential for the specific expression in anthers. PsEND1 promoter deletion analysis revealed that a putative CArG-like regulatory motif is necessary to confer promoter activity in developing anthers. Our data suggest that PsEND1 might be a hemopexin regulated by a MADS-box protein. PsEND1 gene silencing in pea, and its overexpression in heterologous systems, result in similar defects in the anthers consisting of precocious tapetum degradation and the impairment of pollen development. Such alterations were associated to the production of superoxide anion and altered activity of ROS-scavenging enzymes. Our findings demonstrate that PsEND1 is essential for pollen development by modulating ROS levels during the differentiation of the anther tissues surrounding the microsporocytes.

The regulatory framework of developmentally programmed cell death in floral organs: A review

Developmentally programmed cell death (dPCD) is a tightly controlled biological process. In recent years, vital roles of dPCD on regulating floral organ growth and development have been reported. It is well known that flower is an essential organ for reproduction and a turning point of plants' life cycle. Hence, uncovering the complex molecular networks which regulates dPCD processes in floral organs is utmost important. So far, our understanding of dPCD on floral organ growth and development is just starting. Herein, we summarize the important factors that involved in the tapetal degeneration, pollen tube rupture, receptive synergid cell death, nucellar degradation, and antipodal cell degradation. Meanwhile, the known factors that involved in transmitting tract formation and self-incompatibility-induced PCD were also introduced. Furthermore, the genes that associated with anther dehiscence and petal senescence and abscission were reviewed as well. The functions of various types of factors involved in floral dPCD processes are highlighted principally. The regulatory panorama described here can provide us some insights about flower-specific dPCD process.

Gene Expression Dynamics in Rice Peduncles at the Heading Stage

Improving grain yield in the staple food crop rice has been long sought goal of plant biotechnology. One of the traits with significant impact on rice breeding programs is peduncle elongation at the time of heading failing which leads to significant reduction in grain yield due to incomplete panicle exsertion. To decipher transcriptional dynamics and molecular players underlying peduncle elongation, we performed RNA sequencing analysis of elongating and non-elongating peduncles in two Indian cultivars, Swarna and Pokkali, at the time of heading. Along with genes associated with cell division and cell wall biosynthesis, we observed significant enrichment of genes associated with auxins, gibberellins, and brassinosteroid biosynthesis/signaling in the elongating peduncles before heading in both the genotypes. Similarly, genes associated with carbohydrate metabolism and mobilization, abiotic stress response along with cytokinin, abscisic acid, jasmonic acid, and ethylene biosynthesis/signaling were enriched in non-elongating peduncles post heading. Significant enrichment of genes belonging to key transcription factor families highlights their specialized roles in peduncle elongation and grain filling before and after heading, respectively. A comparison with anther/pollen development-related genes provided 76 candidates with overlapping roles in anther/pollen development and peduncle elongation. Some of these are important for carbohydrate remobilization to the developing grains. These can be engineered to combat with incomplete panicle exsertion in male sterile lines and manipulate carbohydrate dynamics in grasses. Overall, this study provides baseline information about potential target genes for engineering peduncle elongation with implications on plant height, biomass composition and grain yields in rice.

The mitochondrial aldehyde dehydrogenase OsALDH2b negatively regulates tapetum degeneration in rice

Timely degradation of anther tapetal cells is a prerequisite for normal pollen development in flowering plants. Although several genes involved in tapetum development have been identified, the molecular basis of tapetum degeneration regulation remains poorly understood. In this study, we identified and characterized the nucleus-encoded, conserved mitochondrial aldehyde dehydrogenase OsALDH2b as a key regulator of tapetum degeneration in rice (Oryza sativa). OsALDH2b was highly expressed in anthers from meiosis to the early microspore stage. Mutation of OsALDH2b resulted in excess malonaldehyde accumulation and earlier programmed cell death in the tapetum, leading to premature tapetum degeneration and abnormal microspore development. These results demonstrate that OsALDH2b negatively regulates tapetal programmed cell death and is required for male reproductive development, providing insights into the regulation of tapetum development in plants.

Tapetum-Dependent Male Meiosis Progression in Plants: Increasing Evidence Emerges

In higher plants, male meiosis is a key process during microsporogenesis and is crucial for male fertility and seed set. Meiosis involves a highly dynamic organization of chromosomes and cytoskeleton and specifically takes place within sexual cells. However, studies in multiple plant species have suggested that the normal development of tapetum, the somatic cell layer surrounding the developing male meiocytes, is indispensable for the completion of the male meiotic cell cycle. Disrupted tapetum development causes alterations in the expression of a large range of genes involved in male reproduction. Moreover, recent experiments suggest that small RNAs (sRNAs) present in the anthers, including microRNAs (miRNAs) and phased, secondary, small interfering RNAs (phasiRNAs), play a potential but important role in controlling male meiosis, either by influencing the expression of meiotic genes in the meiocytes or through other unclear mechanisms, supporting the hypothesis that male meiosis is non-cell autonomously regulated. In this mini review, we summarize the recorded meiotic defects that occur in plants with defective tapetum development in both Arabidopsis and crops. Thereafter, we outline the latest understanding on the molecular mechanisms that potentially underpin the tapetum-dependent regulation of male meiosis, and we especially discuss the regulatory role of sRNAs. At the end, we propose several outstanding questions that should be addressed in future studies.