The GW2-WG1-OsbZIP47 pathway controls grain size and weight in rice

Regulation of seed size is a key strategy for improving crop yield and is also a basic biological question.However,the molecular mechanisms by which plants determine their seed size remain elusive.Here,we report that the GW2-WG1-OsbZIP47 regulatory module controls grain width and weight in rice.WG1,which encodes a glutaredoxin protein,promotes grain growth by increasing cell proliferation.Interestingly,WG1 interacts with the transcription factor OsbZIP47 and represses its transcriptional activity by associating with the transcriptional co-repressor ASP1,indicating that WG1 may act as an adaptor protein to recruit the transcriptional co-repressor.In contrary,OsbZIP47 restricts grain growth by decreasing cell proliferation.Further studies reveal that the E3 ubiquitin ligase GW2 ubiquitinates WG1 and targets it for degradation.Genetic analyses confirm that GW2,WG1,and OsbZIP47 function in a comm on pathway to control grain growth.Taken together,ourfindi ngs reveal a genetic and molecular framework for the control of grain size and weight by the GW2-WG1-OsbZIP47 regulatory module,providing new targets for improving seed size and weight in crops.

OspTAC2 encodes a pentatricopeptide repeat protein and regulates rice chloroplast development

Functional chloroplast generation depends on the precise coordination of gene expression between the plastid and the nucleus and is essential for plant growth and development. In this study, a rice（Oryza sativa） mutant that exhibited albino and seedling-lethal phenotypes was isolated from a60Co-irradiated rice population. The mutant gene was identified as an ortholog of the Arabidopsis plastid transcriptionally active chromosome protein 2（p TAC2） gene, and the mutant strain was designated osptac2. Sequence and transcription analyses showed that Osp TAC2 encodes a putative chloroplast protein consisting of 10 pentratricopeptide repeat（PPR） domains and a C-terminal small Mut S-related（SMR） domain. Cytological observations via microscopy showed that the Osp TAC2-green fluorescent fusion protein is localized in the chloroplasts. Transmission electron microscopy revealed that the chloroplast of the osptac2 mutant lacks an organized thylakoid membrane. The transcript levels of all investigated PEP（plastid-encoded RNA polymerase）-dependent genes were dramatically reduced in the osptac2 mutant, whereas the transcript levels of NEP（nuclear-encoded polymerase）-dependent genes were increased. These results suggest that Osp TAC2 plays a critical role in chloroplast development and indicate that the molecular function of the Osp TAC2 gene is conserved in rice and Arabidopsis.

Characterization and molecular cloning of a serine hydroxymethyltransferase 1(OsSHM_1) in rice

Serine hydroxymethyltransferase (SHMT) is important for one carbon metabolism and photorespiration in higher plants for its participation in plant growth and development, and resistance to biotic and abiotic stresses. A rice serine hydroxymethyltransferase gene, OsSHM1, an ortholog of Arabidopsis SHM1, was isolated using map-based cloning. The osshm1 mutant had chlorotic lesions and a considerably smaller, lethal phenotype under natural ambient CO2 concentrations, but could be restored to wild type with normal growth under elevated CO2 levels (0.5% CO2), showing a typical photorespiratory phenotype. The data from antioxidant enzymes activity measurement suggested that osshm1 was subjected to significant oxidative stress. Also, OsSHM1 was expressed in all organs tested (root, culm, leaf, and young panicle) but predominantly in leaves. OsSHM1 protein is localized to the mitochondria. Our study suggested that molecular function of the OsSHM1 gene is conserved in rice and Arabidopsis.