Rice Tapetum Research Articles

Rice tapetum differentiation is sensitive to downregulation of OsUCH3, a ubiquitin C-terminal hydrolase.

The tapetum is a special tissue within the stamen that supplies nutrients to support microspore differentiation. Many genes have been identified to function in tapetum differentiation (Astrand et al., 2021). Ubiquitin C-terminal hydrolase (UCH) is a type of cysteine protease involved in protein deubiquitination process (Mevissen and Komander, 2017) and plays roles in gonadal transformation and spermatogenesis in animals (Luo et al., 2009; Kwon et al,. 2004; Wang et al., 2006). OsUCH3 (Os02g43760) was highly expressed in early rice stamen cells (Chen et al., 2015), the OsUCH3 expression was first observed in all cells of anther primordium and then gradually concentrated into the germ cells and tapetum cells, and OsUCH3 protein exhibits stronger ubiquitin C-terminal hydrolase enzymatic activity (Wang et al., 2018). These data suggested that OsUCH3 is likely associated with stamen development. To explore whether OsUCH3 plays roles in rice stamen development, we constructed transgenic lines to down-regulate OsUCH3 expression through RNAi vector as described previously with primers (ccACTAGTATGGAGGATGCTCATTCC and TcGGATCCCACAACTTTCGAAAGAGC) (Liu et al., 2005) (Figure 1a–g) and the OsUCH3-targeting CRISPR-Cas9 mutant lines (Figure 1h). CRISPR-Cas9 osuch3 mutant lines exhibited the same male-sterile phenotype as OsUCH3R. In observation of the overall morphology, there was no significant difference at vegetable development (Figure 1a). However, low seed setting rate was observed in the RNAi lines (Figure 1b) and CRISPR-Cas9 osuch3 mutant (Figure 1h). Alexander staining showed that OsUCH3R lines had abnormal pollen (Figure 1d) with the latter being indicated by the significant reduction in pollen grain number (Figure 1e, g). The RT-qPCR results revealed that OsUCH3 expression levels were significantly reduced in OsUCH3R transgenic rice (Figure 1f). To examine whether OsUCH3 affected stamen development, we pollinated OsUCH3R pistils with wild-type pollen grains, which rescued the sterility of OsUCH3R. The result suggested that the low seed setting rate resulted mainly from down-expression of OsUCH3 (Figure 1i). Morphological analysis revealed abnormal tapetum differentiation and locule collapse were observed in the OsUCH3R lines from stage 5 (S5) through 12 (S12) (Figure 1j). This observation suggested that the downregulation of OsUCH3 caused a defect in the tapetum differentiation and stamen development. In contrast to the degeneration of the tapetum that was observed in the WT from stages 7 (S7) to 9 (S9) (Figure 1k–m), the tapetum in the OsUCH3R lines was not properly degenerated during these stages (Figure 1n–p) as revealed by transmission electron microscopy (TEM) analysis. In addition, TEM observations revealed that the secondary cell walls of the endothecium cells were thinner in the OsUCH3R lines than in the wild type (Figure 1p, red arrowhead). We found green fluorescence in wild-type tapetal cells from stages 7 to 9 (Figure 1q-t) using the terminal deoxynucleotidyl transferase-mediated TdT-mediated dUTP Nick-End Labeling (TUNEL) assay (Chen et al., 2018). No such TUNEL signals were detected in OsUCH3R tapetal cells during the same developmental period (Figure 1u–x). These results suggested that DNA degradation during tapetum differentiation after stage 7 was abnormal in OsUCH3R. Numerous genes were known being involved in tapetum differentiation (Chen et al., 2018). GAMYB4 was up-regulated while the expression of YY1, OsCP1 and UDT1 exhibited no change in the OsUCH3R lines (Figure 1y). OsUCH3 is lower expressed in shoot tips and leaf primordia. While no differential expression of SAM genes OsWOX3, the expression of OSH1 and OsWUS was significantly decreased in the OsUCH3R lines (Figure 1y). These changes demonstrated the link between abnormal tapetum differentiation in the OsUCH3R lines with the changes in the expression levels of known tapetum genes. The phenotypes we found in OsUCH3R lines and CRISPR mutants demonstrated that tapetal differentiation is sensitive to the change of expression level of OsUCH3 and protein deubiquitination is an importance factor for proper tapetum differentiation. This finding provided a new opportunity not only in deciphering the regulatory network of tapetum differentiation at protein level, but in manipulating male sterility for agricultural applications. Sequence data from this article can be found in the GenBank/EMBL data library. This work was supported by grants from the Chinese Transgenic Project (grant no. CARS-01-06, 2016ZX08010001 and 2016ZX08009-003) and the National Natural Science Foundation of China (grant no. 31630006). The authors declare no competing interests. D.H.W., Z.H.X. and S.N.B. designed the research and wrote the manuscript. D.H.W. performed most of the experiments. N.L. and S.D.Y. provided an assistance for mutant genotyping and plant material fixation. Z.S.C and Y.N.L provided technical assistance for transgenic plant analyse. Z.H.L helped the semithin sectioning.

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.

OsMYB103 is essential for tapetum degradation in rice.

OsMYB103 positively regulates tapetum degradation, and functions downstream of TDR and upstream of EAT1 and PTC1. The precise regulation of programmed cell death (PCD) of the tapetum is crucial for the development of anthers and pollen in rice. In this study, we isolated and identified a male-sterile mutant of rice, osmyb103, which exhibited delayed tapetum degradation and defective mature pollen. Map-based cloning and genetic complementation revealed that OsMYB103 corresponded to the gene LOC_Os04g39470 and encoded a R2R3 MYB transcription factor. OsMYB103 was localized in the nucleus and was expressed preferentially in the tapetal cells and microspores of the anther. OsMYB103 regulated the expression of two transcription factors, ETERNAL TAPETUM 1 (EAT1) and PERSISTENT TAPETAL CELL 1 (PTC1), both of which regulated tapetum degradation positively. Moreover, the expression of OsMYB103 was directly regulated by the additional positive regulator of tapetum degradation TAPETUM DEGENERATION RETARDATION (TDR) and was able to interact with it. Genetic evidence confirmed that OsMYB103 acted upstream of EAT1. The results show that OsMYB103 is a positive regulator of tapetum degradation in rice. These findings provide a better understanding of the regulatory network that underlies degradation of the tapetum in rice.

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

BackgroundDuring anther development, the tapetum provides essential nutrients and materials for pollen development. In rice, multiple transcription factors and enzymes essential for tapetum development and pollen wall formation have been cloned from male-sterile lines.ResultsIn this study, we obtained several lines in which the MYB transcription factor OsMS188 was knocked out through the CRISPR-Cas9 approach. The osms188 lines exhibited a male-sterile phenotype with aberrant development and degeneration of tapetal cells, absence of the sexine layer and defective anther cuticles. CYP703A3, CYP704B2, OsPKS1, OsPKS2, DPW and ABCG15 are sporopollenin synthesis and transport-related genes in rice. Plants with mutations in these genes are male sterile, with a defective sexine layer and anther cuticle. Further biochemical assays demonstrated that OsMS188 binds directly to the promoters of these genes to regulate their expression. UDT1, OsTDF1, TDR, bHLH142 and EAT1 are upstream regulators of rice tapetum development. Electrophoretic mobility shift assays (EMSAs) and activation assays revealed that TDR directly regulates OsMS188 expression. Additionally, protein interaction assays indicated that TDR interacts with OsMS188 to regulate downstream gene expression.ConclusionOverall, OsMS188 is a key regulator of tapetum development and pollen wall formation. The gene regulatory network established in this work may facilitate future investigations of fertility regulation in rice and in other crop species.

The MYB transcription factor Baymax1 plays a critical role in rice male fertility.

Key message Rice male fertility gene Baymax1, isolated through map-based cloning, encodes a MYB transcription factor and is essential for rice tapetum and microspore development.Abstract The mining and characterization of male fertility gene will provide theoretical and material basis for future rice production. In Arabidopsis, the development of male organ (namely anther), usually involves the coordination between MYB (v-myb avian myeloblastosis viral oncogene homolog) and bHLH (basic helix-loop-helix) members. However, the role of MYB proteins in rice anther development remains poorly understood. In this study, we isolated and characterized a male sterile mutant (with normal vegetative growth) of Baymax1 (BM1), which encodes a MYB protein. The bm1 mutant exhibited slightly lagging meiosis, aborted transition of the tapetum to a secretory type, premature tapetal degeneration, and abnormal pollen exine formation, leading to ultimately lacks of visible pollens in the mature white anthers. Map-based cloning, complementation and targeted mutagenesis using CRISPR/Cas9 technology demonstrated that the mutated LOC_Os04g39470 is the causal gene in bm1. BM1 is preferentially expressed in rice anthers from stage 5 to stage 10. Phylogenetic analysis indicated that rice BM1 and its homologs in millet, maize, rape, cabbage, and pigeonpea are evolutionarily conserved. BM1 can physically interacts with bHLH protein TIP2, EAT1, and PHD (plant homeodomain)-finger member TIP3, respectively. Moreover, BM1 affects the expression of several known genes related to tapetum and microspore development. Collectively, our results suggest that BM1 is one of key regulators for rice male fertility and may serve as a potential target for rice male-sterile line breeding and hybrid seed production.

Monitoring autophagy in rice tapetal cells during pollen maturation.

We have previously shown that autophagy is required for post meiotic anther development including programmed cell death-mediated degradation of the tapetum and pollen maturation in rice. However, the spatiotemporal dynamics of autophagy in the tapetum remain poorly understood. We here established an in vivo imaging technique to analyze the dynamics of autophagy in rice tapetum cells by expressing green fluorescent protein-tagged AtATG8, a marker for autophagosomes. 3D-imaging analysis revealed that the number of autophagosomes/autophagy-related structures is extremely low at the tetrad stage (stage 8), and autophagy is dramatically induced at the uninucleate stages (stage 9-10) throughout the tapetal cells during anther development. The present monitoring system for autophagy offers a powerful tool to analyze the regulation of autophagy in rice tapetal cells during pollen maturation.

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

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

Proteomic analysis of lysine acetylation provides strong evidence for involvement of acetylated proteins in plant meiosis and tapetum function.

Protein lysine acetylation (KAC) is a dynamic and reversible post-translational modification that has important biological roles in many organisms. Although KAC has been shown to affect reproductive development and meiosis in yeast and animals, similar studies are largely lacking in flowering plants, especially proteome-scale investigations for particular reproductive stages. Here, we report results from a proteomic investigation to detect the KAC status of the developing rice anthers near the time of meiosis (RAM), providing strong biochemical evidence for roles of many KAC-affected proteins during anther development and meiosis in rice. We identified a total of 1354 KAC sites in 676 proteins. Among these, 421 acetylated proteins with 629 KAC sites are novel, greatly enriching our knowledge on KAC in flowering plants. Gene Ontology enrichment analysis showed chromatin silencing, protein folding, fatty acid biosynthetic process and response to stress to be over-represented. In addition, certain potentially specific KAC motifs in RAM were detected. Importantly, 357 rice meiocyte proteins were acetylated; and four proteins genetically identified to be important for rice tapetum and pollen development were acetylated on 14 KAC sites in total. Furthermore, 47 putative secretory proteins were detected to exhibit acetylated status in RAM. Moreover, by comparing our lysine acetylome with the RAM phosphoproteome we obtained previously, we proposed a correlation between KAC and phosphorylation as a potential modulatory mechanism in rice RAM. This study provides the first global survey of KAC in plant reproductive development, making a promising starting point for further functional analysis of KAC during rice anther development and meiosis.

Functional analysis of a rice tapetum specific (RTS) gene promoter variant identified in rice cultivar IR64

Functional analysis of a rice tapetum specific (RTS) gene promoter variant identified in rice cultivar IR64

An anther development F-box (ADF) protein regulated by tapetum degeneration retardation (TDR) controls rice anther development.

In this study, we reported that a F-box protein, OsADF, as one of the direct targets of TDR , plays a critical role in rice tapetum cell development and pollen formation. The tapetum, the innermost sporophytic tissue of anther, plays an important supportive role in male reproduction in flowering plants. After meiosis, tapetal cells undergo programmed cell death (PCD) and provide nutrients for pollen development. Previously we showed that tapetum degeneration retardation (TDR), a basic helix-loop-helix transcription factor, can trigger tapetal PCD and control pollen wall development during anther development. However, the comprehensive regulatory network of TDR remains to be investigated. In this study, we cloned and characterized a panicle-specific expression F-box protein, anther development F-box (OsADF). By qRT-PCR and RNA in situ hybridization, we further confirmed that OsADF expressed specially in tapetal cells from stage 9 to stage 12 during anther development. In consistent with this specific expression pattern, the RNAi transgenic lines of OsADF exhibited abnormal tapetal degeneration and aborted microspores development, which eventually grew pollens with reduced fertility. Furthermore, we demonstrated that the TDR, a key regulator in controlling rice anther development, could regulate directly the expression of OsADF by binding to E-box motifs of its promoter. Therefore, this work highlighted the possible regulatory role of TDR, which regulates tapetal cell development and pollen formation via triggering the possible ADF-mediated proteolysis pathway.

OsMYB103 is required for rice anther development by regulating tapetum development and exine formation

The Arabidopsis AtMYB103 gene is required for anther development, but whether the homologous gene in rice has the same role is unclear. Sequence analysis indicated that the rice OsMYB103 gene shares a high sequence similarity with AtMYB103. Therefore, we investigated the functional role of OsMYB103 in anther development using an RNAi approach. The OsMYB103 RNA transcript was expressed most abundantly in flowers, specifically in the tapetum, premeiotic pollen mother cells, and meiotic PMCs. OsMYB103-RNAi transgenic lines grew normally during their vegetative phase but displayed reduced male fertility, a phenotype that was associated with downregulated OsMYB103 transcript levels. Expression of OsMS2, an ortholog of the Arabidopsis AtMS2 gene, was also dramatically reduced in the transgenic plants. Knockdown of OsMYB103 led to defects in tapetum development, and most of the microspores in mature anthers lacked exines. Moreover, OsMYB103 could partially rescue the male sterility phenotype of an AtMYB103 knockout mutant ms188. Taken together, these results indicate that OsMYB103 does have an important role in rice tapetum and microspore development.

Separated Transcriptomes of Male Gametophyte and Tapetum in Rice: Validity of a Laser Microdissection (LM) Microarray

In flowering plants, the male gametophyte, the pollen, develops in the anther. Complex patterns of gene expression in both the gametophytic and sporophytic tissues of the anther regulate this process. The gene expression profiles of the microspore/pollen and the sporophytic tapetum are of particular interest. In this study, a microarray technique combined with laser microdissection (44K LM-microarray) was developed and used to characterize separately the transcriptomes of the microspore/pollen and tapetum in rice. Expression profiles of 11 known tapetum specific-genes were consistent with previous reports. Based on their spatial and temporal expression patterns, 140 genes which had been previously defined as anther specific were further classified as male gametophyte specific (71 genes, 51%), tapetum-specific (seven genes, 5%) or expressed in both male gametophyte and tapetum (62 genes, 44%). These results indicate that the 44K LM-microarray is a reliable tool to analyze the gene expression profiles of two important cell types in the anther, the microspore/pollen and tapetum.

Ultrastructural study of rice tapetum under low-temperature stress

The development of male reproductive organs in rice is very sensitive to various environmental stresses. For example, exposing plants to low temperatures during the heading stage leads to a reduction in grain yield. Here, we grew rice under normal conditions and also at three different temperatures -- 16, 18, and 20°C. Treatment at a low temperature significantly decreased pollen viability and grain production. Cytological observations of the anther showed that the tapetum was the most sensitive to low-temperature stress, resulting in male sterility due to functional loss of the tissue. Detailed observations by transmission electron microscopy suggested that this abnormality was restricted primarily to the ER structures. The endoplasmic reticulum, a highly vulnerable sub-cellular organelle, showed two typical morphological aberrations, one in its pattern of arrangement, the other in the formation of ER bodies. Of our three experimental chilling temperatures, the most severe abnormalities were observed in tapetal cells exposed to 16°C.

The RiceTapetum Degeneration RetardationGene Is Required for Tapetum Degradation and Anther Development

In flowering plants, tapetum degeneration is proposed to be triggered by a programmed cell death (PCD) process during late stages of pollen development; the PCD is thought to provide cellular contents supporting pollen wall formation and to allow the subsequent pollen release. However, the molecular basis regulating tapetum PCD in plants remains poorly understood. We report the isolation and characterization of a rice (Oryza sativa) male sterile mutant tapetum degeneration retardation (tdr), which exhibits degeneration retardation of the tapetum and middle layer as well as collapse of microspores. The TDR gene is preferentially expressed in the tapetum and encodes a putative basic helix-loop-helix protein, which is likely localized to the nucleus. More importantly, two genes, Os CP1 and Os c6, encoding a Cys protease and a protease inhibitor, respectively, were shown to be the likely direct targets of TDR through chromatin immunoprecipitation analyses and the electrophoretic mobility shift assay. These results indicate that TDR is a key component of the molecular network regulating rice tapetum development and degeneration.