Abstract Background: Grain size is a direct determinant of grain weight and yield in rice; however, the genetic and molecular mechanisms determining grain size remain largely unknown. Findings: We identified a mutant, wide grain 3 (wg3), which exhibited significantly increased grain width and 1000-grain weight. Cytological analysis showed that WG3 regulates grain size by affecting cell proliferation. MutMap-based gene cloning and a transgenic experiment demonstrated that WG3 encodes a GRAS protein. Moreover, we found that WG3 directly interacts with DWARF AND LOW-TILLERING (DLT), a previously reported GRAS protein, and a genetic experiment demonstrated that WG3 and DLT function in a common pathway to regulate grain size. Additionally, a brassinosteroid (BR) sensitivity test suggested that WG3 has a positive role in BR signaling in rice. Collectively, our results reveal a new genetic and molecular mechanism for the regulation of grain size in rice by the WG3-DLT complex, and highlight the important functions of the GRAS protein complex in plants. Conclusion: WG3 functions directly in regulating grain size and BR signaling in rice.
