MONENSIN SENSITIVITY1 (MON1)/CALCIUM CAFFEINE ZINC SENSITIVITY1 (CCZ1)-Mediated Rab7 Activation Regulates Tapetal Programmed Cell Death and Pollen Development

The vesicle trafficking gene, OsRab7, is critical for pollen development and male fertility in cytoplasmic male-sterility rice

Rice cytoplasmic male sterility (CMS) provides an exceptional model for studying genetic interaction within plant nuclei given its inheritable trait of non-functional male gametophyte. Gaining a comprehensive understanding of the genes and pathways associated with the CMS mechanism is imperative for improving the vigor of hybrid rice agronomically, such as its productivity. Here, we observed a significant decrease in the expression of a gene named OsRab7 in the anther of the CMS line (SJA) compared to the maintainer line (SJB). OsRab7 is responsible for vesicle trafficking and loss function of OsRab7 significantly reduced pollen fertility and setting rate relative to the wild type. Meanwhile, over-expression of OsRab7 enhanced pollen fertility in the SJA line while a decrease in its expression in the SJB line led to the reduced pollen fertility. Premature tapetum and abnormal development of microspores were observed in the rab7 mutant. The expression of critical genes involved in tapetum development (OsMYB103, OsPTC1, OsEAT1 and OsAP25) and pollen development (OsMSP1, OsDTM1 and OsC4) decreased significantly in the anther of rab7 mutant. Reduced activities of the pDR5::GUS marker in the young panicle and anther of the rab7 mutant were also observed. Furthermore, the mRNA levels of genes involved in auxin biosynthesis (YUCCAs), auxin transport (PINs), auxin response factors (ARFs), and members of the IAA family (IAAs) were all downregulated in the rab7 mutant, indicating its impact on auxin signaling and distribution. In summary, these findings underscore the importance of OsRab7 in rice pollen development and its potential link to cytoplasmic male sterility.

A root-knot nematode effector targets the Arabidopsis cysteine protease RD21A for degradation to suppress plant defense and promote parasitism

Meloidogyne incognita is one of the most widely distributed plant-parasitic nematodes and causes severe economic losses annually. The parasite produces effector proteins that play essential roles in successful parasitism. Here, we identified one such effector named MiCE108, which is exclusively expressed within the nematode subventral esophageal gland cells and is upregulated in the early parasitic stage of M. incognita. A yeast signal sequence trap assay showed that MiCE108 contains a functional signal peptide for secretion. Virus-induced gene silencing of MiCE108 impaired the parasitism of M. incognita in Nicotiana benthamiana. The ectopic expression of MiCE108 in Arabidopsis suppressed the deposition of callose, the generation of reactive oxygen species, and the expression of marker genes for bacterial flagellin epitope flg22-triggered immunity, resulting in increased susceptibility to M. incognita, Botrytis cinerea, and Pseudomonas syringae pv. tomato (Pst) DC3000. The MiCE108 protein physically associates with the plant defense protease RD21A and promotes its degradation via the endosomal-dependent pathway, or 26S proteasome. Consistent with this, knockout of RD21A compromises the innate immunity of Arabidopsis and increases its susceptibility to a broad range of pathogens, including M. incognita, strongly indicating a role in defense against this nematode. Together, our data suggest that M. incognita deploys the effector MiCE108 to target Arabidopsis cysteine protease RD21A and affect its stability, thereby suppressing plant innate immunity and facilitating parasitism.

Recent advances in plant endomembrane research and new microscopical techniques

The endomembrane system consists of various membrane-bound organelles including the endoplasmic reticulum (ER), Golgi apparatus, trans-Golgi network (TGN), endosomes, and the lysosome/vacuole. Membrane trafficking between distinct compartments is mainly achieved by vesicular transport. As the endomembrane compartments and the machineries regulating the membrane trafficking are largely conserved across all eukaryotes, our current knowledge on organelle biogenesis and endomembrane trafficking in plants has mainly been shaped by corresponding studies in mammals and yeast. However, unique perspectives have emerged from plant cell biology research through the characterization of plant-specific regulators as well as the development and application of the state-of-the-art microscopical techniques. In this review, we summarize our current knowledge on the plant endomembrane system, with a focus on several distinct pathways: ER-to-Golgi transport, protein sorting at the TGN, endosomal sorting on multivesicular bodies, vacuolar trafficking/vacuole biogenesis, and the autophagy pathway. We also give an update on advanced imaging techniques for the plant cell biology research.

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.

The triticale mature pollen and stigma proteomes - assembling the proteins for a productive encounter

Triticeae crops are major contributors to global food production and ensuring their capacity to reproduce and generate seeds is critical. However, despite their importance our knowledge of the proteins underlying Triticeae reproduction is severely lacking and this is not only true of pollen and stigma development, but also of their pivotal interaction. When the pollen grain and stigma are brought together they have each accumulated the proteins required for their intended meeting and accordingly studying their mature proteomes is bound to reveal proteins involved in their diverse and complex interactions. Using triticale as a Triticeae representative, gel-free shotgun proteomics was used to identify 11,533 and 2977 mature stigma and pollen proteins respectively. These datasets, by far the largest to date, provide unprecedented insights into the proteins participating in Triticeae pollen and stigma development and interactions. The study of the Triticeae stigma has been particularly neglected. To begin filling this knowledge gap, a developmental iTRAQ analysis was performed revealing 647 proteins displaying differential abundance as the stigma matures in preparation for pollination. An in-depth comparison to an equivalent Brassicaceae analysis divulged both conservation and diversification in the makeup and function of proteins involved in the pollen and stigma encounter. SIGNIFICANCE: Successful pollination brings together the mature pollen and stigma thus initiating an intricate series of molecular processes vital to crop reproduction. In the Triticeae crops (e.g. wheat, barley, rye, triticale) there persists a vast deficit in our knowledge of the proteins involved which needs to be addressed if we are to face the many upcoming challenges to crop production such as those associated with climate change. At maturity, both the pollen and stigma have acquired the protein complement necessary for their forthcoming encounter and investigating their proteomes will inevitably provide unprecedented insights into the proteins enabling their interactions. By combining the analysis of the most comprehensive Triticeae pollen and stigma global proteome datasets to date with developmental iTRAQ investigations, proteins implicated in the different phases of pollen-stigma interaction enabling pollen adhesion, recognition, hydration, germination and tube growth, as well as those underlying stigma development were revealed. Extensive comparisons between equivalent Triticeae and Brassiceae datasets highlighted both the conservation of biological processes in line with the shared goal of activating the pollen grain and promoting pollen tube invasion of the pistil to effect fertilization, as well as the significant distinctions in their proteomes consistent with the considerable differences in their biochemistry, physiology and morphology.

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.

Barley endosomal MONENSIN SENSITIVITY1 is a target of the powdery mildew effector CSEP0162 and plays a role in plant immunity

Encasements formed around haustoria and biotrophic hyphae as well as hypersensitive reaction (HR) cell death are essential plant immune responses to filamentous pathogens. In this study we examine the components that may contribute to the absence of these responses in susceptible barley attacked by the powdery mildew fungus. We find that the effector CSEP0162 from this pathogen targets plant MONENSIN SENSITIVITY1 (MON1), which is important for the fusion of multivesicular bodies to their target membranes. Overexpression of CSEP0162 and silencing of barley MON1 both inhibit encasement formation. We find that the Arabidopsis ecotype No-0 has resistance to powdery mildew, and that this is partially dependent on MON1. Surprisingly, we find the MON1-dependent resistance in No-0 not only includes an encasement response, but also an effective HR. Similarly, silencing of MON1 in barley also blocks Mla3-mediated HR-based powdery mildew resistance. Our results indicate that MON1 is a vital plant immunity component, and we speculate that the barley powdery mildew fungus introduces the effector CSEP0162 to target MON1 and hence reduce encasement formation and HR.

Whole-Transcriptome Analysis Reveals Autophagy Is Involved in Early Senescence of zj-es Mutant Rice

Senescence is a necessary stage of plant growth and development, and the early senescence of rice will lead to yield reduction and quality decline. However, the mechanisms of rice senescence remain obscure. In this study, we characterized an early-senescence rice mutant, designated zj-es (ZheJing-early senescence), which was derived from the japonica rice cultivar Zhejing22. The mutant zj-es exhibited obvious early-senescence phenotype, such as collapsed chloroplast, lesions in leaves, declined fertility, plant dwarf, and decreased agronomic traits. The ZJ-ES gene was mapped in a 458 kb-interval between the molecular markers RM5992 and RM5813 on Chromosome 3, and analysis suggested that ZJ-ES is a novel gene controlling rice early senescence. Subsequently, whole-transcriptome RNA sequencing was performed on zj-es and its wild-type rice to dissect the underlying molecular mechanism for early senescence. Totally, 10,085 differentially expressed mRNAs (DEmRNAs), 1,253 differentially expressed lncRNAs (DElncRNAs), and 614 differentially expressed miRNAs (DEmiRNAs) were identified, respectively, in different comparison groups. Based on the weighted gene co-expression network analysis (WGCNA), the co-expression turquoise module was found to be the key for the occurrence of rice early senescence. Furthermore, analysis on the competing endogenous RNA (CeRNA) network revealed that 14 lncRNAs possibly regulated 16 co-expressed mRNAs through 8 miRNAs, and enrichment analysis showed that most of the DEmRNAs and the targets of DElncRNAs and DEmiRNAs were involved in reactive oxygen species (ROS)-triggered autophagy-related pathways. Further analysis showed that, in zj-es, ROS-related enzyme activities were markedly changed, ROS were largely accumulated, autophagosomes were obviously observed, cell death was significantly detected, and lesions were notably appeared in leaves. Totally, combining our results here and the remaining research, we infer that ROS-triggered autophagy induces the programmed cell death (PCD) and its coupled early senescence in zj-es mutant rice.

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

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

Selection and validation of reference genes for RT-qPCR normalization in different tissues of milk thistle (Silybum marianum, Gaert.)

Quantitative reverse transcription PCR is a sensitive technique for evaluating transcriptional profiles in different experimental datasets. To obtain a reliable quantification of the transcripts level, data normalization with stable reference genes is required. Stable reference genes are identified after analysis of their transcripts profile in every new experiment and species of interest. In Silybum marianum, a widely cultivated officinal plant, only few gene expression studies exist, and reference genes for RT-qPCR studies in the diverse plant tissues have never been investigated before. In this work, the expression stability of 10 candidate reference genes was evaluated in leaves, roots, stems and fruits of S. marianum grown under physiological environmental condition. The stability values for each candidate reference gene were calculated by four canonical statistical algorithms GeNorm, NormFinder, Bestkeeper and ΔCt method in different subsets of samples, then they were ranked with RefFinder from the most to the least suitable for normalization. Best combinations of reference genes are finally proposed for different experimental data sets, including all tissues, vegetative, and reproductive tissues separately. Three target genes putatively involved in important biosynthetic pathway leading to key metabolites in the fruits of milk thistle, such as silymarin and fatty acids, were analyzed with the chosen panels of reference genes, in comparison to the ones used in previous papers. To the best of our knowledge, this is the first report on a reliable and systematic identification and validation of the reference genes for RT-qPCR normalization to study gene expression in S. marianum.

COPII Components Sar1b and Sar1c Play Distinct Yet Interchangeable Roles in Pollen Development

The development of pollen is a prerequisite for double fertilization in angiosperms. Coat protein complex II (COPII) mediates anterograde transport of vesicles from the endoplasmic reticulum to the Golgi. Components of the COPII complex have been reported to regulate either sporophytic or gametophytic control of pollen development. The Arabidopsis (Arabidopsis thaliana) genome encodes five Sar1 isoforms, the small GTPases essential for COPII formation. By using a dominant negative approach, Sar1 isoforms were proposed to have distinct cargo specificity despite their sequence similarity. Here, we examined the functions of three Sar1 isoforms through analysis of transfer DNA insertion mutants and CRISPR/Cas9-generated mutants. We report that functional loss of Sar1b caused malfunction of tapetum, leading to male sterility. Ectopic expression of Sar1c could compensate for Sar1b loss of function in sporophytic control of pollen development, suggesting that they are interchangeable. Functional distinction between Sar1b and Sar1c may have resulted from their different gene transcription levels based on expression analyses. On the other hand, Sar1b and Sar1c redundantly mediate male gametophytic development such that the sar1b;sar1c microspores aborted at anther developmental stage 10. This study uncovers the role of Sar1 isoforms in both sporophytic and gametophytic control of pollen development. It also suggests that distinct functions of Sar1 isoforms may be caused by their distinct transcription programs.

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

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

Vacuole Biogenesis in Plants: How Many Vacuoles, How Many Models?

Vacuoles are the largest membrane-bounded organelles and have essential roles in plant growth and development, but several important questions on the biogenesis and dynamics of lytic vacuoles (LVs) remain. Here, we summarize and discuss recent research and models of vacuole formation, and propose, with testable hypotheses, that besides inherited vacuoles, plant cells can also synthesize LVs de novo from multiple organelles and routes in response to growth and development or external factors. Therefore, LVs may be further classified into different subgroups and/or populations with different pH, cargos, and functions, among which multivesicular body (MVB)-derived small vacuoles are the main source for central vacuole formation in arabidopsis root cortical cells.

βVPE is involved in tapetal degradation and pollen development by activating proprotease maturation in Arabidopsis thaliana

Vacuolar processing enzyme (VPE) is responsible for the maturation and activation of vacuolar proteins in plants. We found that βVPE was involved in tapetal degradation and pollen development by transforming proproteases into mature protease in Arabidopsis thaliana. βVPE was expressed specifically in the tapetum from stages 5 to 8 of anther development. The βVPE protein first appeared as a proenzyme and was transformed into the mature enzyme before stages 7-8. The recombinant βVPE protein self-cleaved and transformed into a 27 kDa mature protein at pH 5.2. The mature βVPE protein could induce the maturation of CEP1 in vitro. βvpe mutants exhibited delayed vacuolar degradation and decreased pollen fertility. The maturation of CEP1, RD19A, and RD19C was seriously inhibited in βvpe mutants. Our results indicate that βVPE is a crucial processing enzyme that directly participates in the maturation of cysteine proteases before vacuolar degradation, and is indirectly involved in pollen development and tapetal cell degradation.

Interaction between VPS35 and RABG3f is necessary as a checkpoint to control fusion of late compartments with the vacuole

Vacuoles are essential organelles in plants, playing crucial roles, such as cellular material degradation, ion and metabolite storage, and turgor maintenance. Vacuoles receive material via the endocytic, secretory, and autophagic pathways. Membrane fusion is the last step during which prevacuolar compartments (PVCs) and autophagosomes fuse with the vacuole membrane (tonoplast) to deliver cargoes. Protein components of the canonical intracellular fusion machinery that are conserved across organisms, including Arabidopsis thaliana, include complexes, such as soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), that catalyze membrane fusion, and homotypic fusion and vacuole protein sorting (HOPS), that serve as adaptors which tether cargo vesicles to target membranes for fusion under the regulation of RAB-GTPases. The mechanisms regulating the recruitment and assembly of tethering complexes are not well-understood, especially the role of RABs in this dynamic regulation. Here, we report the identification of the small synthetic molecule Endosidin17 (ES17), which interferes with synthetic, endocytic, and autophagic traffic by impairing the fusion of late endosome compartments with the tonoplast. Multiple independent target identification techniques revealed that ES17 targets the VPS35 subunit of the retromer tethering complex, preventing its normal interaction with the Arabidopsis RAB7 homolog RABG3f. ES17 interference with VPS35-RABG3f interaction prevents the retromer complex to endosome anchoring, resulting in retention of RABG3f. Using multiple approaches, we show that VPS35-RABG3f-GTP interaction is necessary to trigger downstream events like HOPS complex assembly and fusion of late compartments with the tonoplast. Overall, our results support a role for the interaction of RABG3f-VPS35 as a checkpoint in the control of traffic toward the vacuole.

A whole-cell electron tomography model of vacuole biogenesis in Arabidopsis root cells

Plant vacuoles are dynamic organelles that play essential roles in regulating growth and development. Two distinct models of vacuole biogenesis have been proposed: separate vacuoles are formed by the fusion of endosomes, or the single interconnected vacuole is derived from the endoplasmic reticulum. These two models are based on studies of two-dimensional (2D) transmission electron microscopy and 3D confocal imaging, respectively. Here, we performed 3D electron tomography at nanometre resolution to illustrate vacuole biogenesis in Arabidopsis root cells. The whole-cell electron tomography analysis first identified unique small vacuoles (SVs; 400-1,000 nm in diameter) as nascent vacuoles in early developmental cortical cells. These SVs contained intraluminal vesicles and were mainly derived/matured from multivesicular body (MVB) fusion. The whole-cell vacuole models and statistical analysis on wild-type root cells of different vacuole developmental stages demonstrated that central vacuoles were derived from MVB-to-SV transition and subsequent fusions of SVs. Further electron tomography analysis on mutants defective in MVB formation/maturation or vacuole fusion demonstrated that central vacuole formation required functional MVBs and membrane fusion machineries.

A whole-cell electron tomography model of vacuole biogenesis in Arabidopsis root cells

Plant vacuoles are dynamic organelles that play essential roles in regulating growth and development. Two distinct models of vacuole biogenesis have been proposed: separate vacuoles are formed by the fusion of endosomes, or the single interconnected vacuole is derived from the endoplasmic reticulum. These two models are based on studies of two-dimensional (2D) transmission electron microscopy and 3D confocal imaging, respectively. Here, we performed 3D electron tomography at nanometre resolution to illustrate vacuole biogenesis in Arabidopsis root cells. The whole-cell electron tomography analysis first identified unique small vacuoles (SVs; 400-1,000 nm in diameter) as nascent vacuoles in early developmental cortical cells. These SVs contained intraluminal vesicles and were mainly derived/matured from multivesicular body (MVB) fusion. The whole-cell vacuole models and statistical analysis on wild-type root cells of different vacuole developmental stages demonstrated that central vacuoles were derived from MVB-to-SV transition and subsequent fusions of SVs. Further electron tomography analysis on mutants defective in MVB formation/maturation or vacuole fusion demonstrated that central vacuole formation required functional MVBs and membrane fusion machineries.

Salicylic acid reverses pollen abortion of rice caused by heat stress

BACKGROUND

Extremely high temperatures are becoming an increasingly severe threat to crop yields. It is well documented that salicylic acid (SA) can enhance the stress tolerance of plants; however, its effect on the reproductive organs of rice plants has not been described before. To investigate the mechanism underlying the SA-mediated alleviation of the heat stress damage to rice pollen viability, a susceptible cultivar (Changyou1) was treated with SA at the pollen mother cell (PMC) meiosis stage and then subjected to heat stress of 40 °C for 10 d until 1d before flowering.

RESULTS

Under control conditions, no significant difference was found in pollen viability and seed-setting rate in SA treatments. However, under heat stress conditions, SA decreased the accumulation of reactive oxygen species (ROS) in anthers to prevent tapetum programmed cell death (PCD) and degradation. The genes related to tapetum development, such as EAT1 (Eternal Tapetum 1), MIL2 (Microsporeless 2), and DTM1 (Defective Tapetum and Meiocytese 1), were found to be involved in this process. When rice plants were exogenously sprayed with SA or paclobutrazol (PAC, a SA inhibitor) + H2O2 under heat stress, a significantly higher pollen viability was found compared to plants sprayed with H2O, PAC, or SA + dimethylthiourea (DMTU, an H2O2 and OH· scavenger). Additionally, a sharp increase in H2O2 was observed in the SA or PAC+ H2O2 treatment groups compared to other treatments.

CONCLUSION

We suggest that H2O2 may play an important role in mediating SA to prevent pollen abortion caused by heat stress through inhibiting the tapetum PCD.

Transcription Factor OsTGA10 Is a Target of the MADS Protein OsMADS8 and Is Required for Tapetum Development

This study aimed at elucidating regulatory components behind floral organ identity determination and tissue development. It remains unclear how organ identity proteins facilitate development of organ primordia into tissues with a determined identity, even though it has long been accepted that floral organ identity is genetically determined by interaction of identity genes according to the ABC model. Using the chromatin immunoprecipitation sequencing technique, we identified OsTGA10, encoding a bZIP transcription factor, as a target of the MADS box protein OsMADS8, which is annotated as an E-class organ identity protein. We characterized the function of OsTGA10 using genetic and molecular analyses. OsTGA10 was preferentially expressed during stamen development, and mutation of OsTGA10 resulted in male sterility. OsTGA10 was required for tapetum development and functioned by interacting with known tapetum genes. In addition, in ostga10 stamens, the hallmark cell wall thickening of the endothecium was defective. Our findings suggest that OsTGA10 plays a mediator role between organ identity determination and tapetum development in rice stamen development, between tapetum development and microspore development, and between various regulatory components required for tapetum development. Furthermore, the defective endothecium in ostga10 implies that cell wall thickening of endothecium is dependent on tapetum development.