Although grain size is one of the most important components of grain yield, little is known about the mechanisms that determine final grain size in crops. Here we characterize rice small grain1 (smg1) mutants, which exhibit small and light grains, dense and erect panicles and slightly short plants. The short grain and panicle phenotypes of smg1 mutants are caused by a defect in cell proliferation. The smg1 mutations are identified, using a map-based cloning approach, in a mitogen-activated protein kinase kinase 4 (OsMKK4). Relatively higher expression of OsMKK4/SMG1 is detected in younger organs than in older ones, consistent with its role in cell proliferation. GFP-OsMKK4/SMG1 fusion proteins appear to be ubiquitously distributed in plant cells. Further results reveal that OsMKK4 influences brassinosteroid (BR) responses and expression of BR-related genes. Thus, our findings identify OsMKK4 as a factor for grain size, and suggest a possible link between the MAPK pathways and BRs in grain growth.

Keywords:Rice,SMG1/OsMKK4,cell proliferation,grain size,panicle size