The role of OsMSH4 in male and female gamete development in rice meiosis.

Meiosis is essential for gametogenesis in sexual reproduction in rice (Oryza sativa L.). We identified a MutS-homolog (MSH) family gene OsMSH4 in a trisomic plant. Cytological analysis showed that developments of both pollen and embryo sacs in an Osmsh4 mutant were blocked due to defective chromosome pairing. Compared with the wild type, the Osmsh4 mutant displayed a significant ~21.9% reduction in chiasma frequency, which followed a Poisson distribution, suggesting that class I crossover formation in the mutant was impaired. Temporal and spatial expression pattern analyses showed that OsMSH4 was preferentially expressed in meiocytes during their meiosis, indicating a critical role in gametogenesis. Subcellular localization showed that OsMSH4-green fluorescent protein was predominantly located in the nucleus. OsMSH4 could interact with another MSH member (OsMSH5) through the N-terminus and C-terminus, respectively. Direct physical interaction between OsMSH5, OsRPA1a, OsRPA2b, OsRPA1c, and OsRPA2c was identified by yeast two-hybrid assays and further validated by pull-down assays. Our results supported the conclusion that the OsMSH4/5 heterodimer plays a key role in regulation of crossover formation during rice meiosis by interaction with the RPA complex.

BRITTLE SHEATH1 encoding OsCYP96B4 is involved in secondary cell wall formation in rice.

KEY MESSAGE: Mutation of BSH1 leads to brittle sheath phenotype and reduction of very-long-chain fatty acids and their derivatives in wax. The cell wall plays an important role in plant mechanical strength. Several brittle culm mutants have been identified and characterized in rice. Here, we characterized an anther culture-derived rice brittle sheath mutant, named bsh1 and isolated BSH1 via map-based strategy. BSH1 encodes OsCYP96B4 protein, which was localized on ER membrane in the protoplast transient assay. BSH1 is mainly expressed in developing vascular tissues and the cells in which cell wall secondary thickening is occurring. Mutation in bsh1 causes changes in cell wall composition by affecting the expression of cell wall-related genes. Moreover, bsh1 shows reduced amounts of very-long-chain fatty acids and their derivatives in wax rather than the medium-chain fatty acids. In summary, BSH1 functions mainly in secondary cell wall formation, and probably in wax biosynthesis in an unidentified mechanism.

Identification and characterization of an epi-allele of FIE1 reveals a regulatory linkage between two epigenetic marks in rice.

DNA methylation and histone H3 Lys 9 dimethylation (H3K9me2) are important epigenetic repression marks for silencing transposons in heterochromatin and for regulating gene expression. However, the mechanistic relationship to other repressive marks, such as histone H3 Lys 27 trimethylation (H3K27me3) is unclear. FERTILIZATION-INDEPENDENT ENDOSPERM1 (FIE1) encodes an Esc-like core component of the Polycomb repressive complex 2, which is involved in H3K27me3-mediated gene repression. Here, we identify a gain-of-function epi-allele (Epi-df) of rice (Oryza sativa) FIE1; this allele causes a dwarf stature and various floral defects that are inherited in a dominant fashion. We found that Epi-df has no changes in nucleotide sequence but is hypomethylated in the 5' region of FIE1 and has reduced H3K9me2 and increased H3K4me3. In Epi-df, FIE1 was ectopically expressed and its imprinting was disrupted. FIE1 interacted with rice Enhancer of Zeste homologs, consistent with its role in H3K27me3 repression. Ectopic expression of FIE1 in Epi-df resulted in alteration of H3K27me3 levels in hundreds of genes. In summary, this work identifies an epi-allele involved in H3K27me3-mediated gene repression that itself is highly regulated by DNA methylation and histone H3K9me2, thereby shedding light on the link between DNA methylation and histone methylation, the two important epigenetic marks regulating rice development.

OsARG encodes an arginase that plays critical roles in panicle development and grain production in rice.

Nitrogen is a crucial nutrient for plant growth and development. Arginine is considered to be an important amino acid for nitrogen transport and storage, playing a crucial role during plant seedling development. However, little is known about the role of arginine in nitrogen remobilization at the reproductive stage. We isolated a rice mutant nglf-1 with reduced plant height, small panicle and grain size, and low seed-setting rate (10% in nglf-1 compared to 93% in wild-type). Map-based cloning revealed that the mutant phenotype was caused by loss of function of a gene (OsARG) encoding an arginine hydrolysis enzyme, which is consistent with arginine accumulation in the mutant. The phenotype was partially corrected supplying exogenous nitrogen, and fully corrected by expression of a wild-type OsARG transgene. Over-expression of OsARG in rice (cv. Kitaake) increased grain number per plant under nitrogen-limited conditions. OsARG was ubiquitously expressed in various organs, but most strongly in developing panicles. The OsARG protein was localized in the mitochondria, consistent with other arginases. Our results suggest that the arginase encoded by OsARG, a key enzyme in Arg catabolism, plays a critical role during panicle development, especially under conditions of insufficient exogenous nitrogen. OsARG is a potential target for crop improvement.

Rice APC/C(TE) controls tillering by mediating the degradation of MONOCULM 1.

Rice MONOCULM 1 (MOC1) and its orthologues LS/LAS (lateral suppressor in tomato and Arabidopsis) are key promoting factors of shoot branching and tillering in higher plants. However, the molecular mechanisms regulating MOC1/LS/LAS have remained elusive. Here we show that the rice tiller enhancer (te) mutant displays a drastically increased tiller number. We demonstrate that TE encodes a rice homologue of Cdh1, and that TE acts as an activator of the anaphase promoting complex/cyclosome (APC/C) complex. We show that TE coexpresses with MOC1 in the axil of leaves, where the APC/C(TE) complex mediates the degradation of MOC1 by the ubiquitin-26S proteasome pathway, and consequently downregulates the expression of the meristem identity gene Oryza sativa homeobox 1, thus repressing axillary meristem initiation and formation. We conclude that besides having a conserved role in regulating cell cycle, APC/C(TE) has a unique function in regulating the plant-specific postembryonic shoot branching and tillering, which are major determinants of plant architecture and grain yield.