GRAIN SIZE AND WEIGHT 3 affects rice grain size and weight through interaction with SR45a and its transcriptional efficiency

Grain size is a critical factor determining rice yield. We previously identified the gene GSW3, derived from Oryza rufipogon, which negatively regulates rice grain size and weight; however, the regulatory mechanism remained unclear. GSW3 encodes two polypeptides of different lengths: a 147-aa peptide GSW3HY3 and a 166-aa peptide GSW3KJ01. In this study, we characterized the functional differences between the two peptides and investigated the mechanism by which GSW3 regulates grain size. The two polypeptides were colocalized in the nucleus. The function of the GSW3KJ01 allele was similar to that of the GSW3HY3 allele, and the gsw3KJ01 line showed an increase in grain length and width, although the degree of increase was significantly lower than that of the gsw3HY3 line. GSW3HY3 and GSW3KJ01 physically and genetically interacted with SR45a, but their interaction sites with SR45a were different. SR45a positively regulated grain size in rice and acted upstream of GSW3. The promoter activity of GSW3KJ01 was 0.62 times that of GSW3HY3, and the overexpression of GSW3HY3 and haplotype analysis indicated that the transcriptional efficiency affected grain size and weight. Core promoter element analysis revealed that GSW3 might play a role in plant hormone signaling pathways. Compared with the wild-type HY3, the gsw3HY3 line was more sensitive to exogenous IAA and ABA. GSW3 influences grain size and weight through its interaction with SR45a and the transcription efficiency of its promoter. Our studies elucidate a pathway for regulating rice grain size and weight.