Grain size is one of the most important determinants of crop yield in cereals. Here, we identified a dominant mutant, big grain2 (bg2-D) from our enhancer-trapping population. Genetic analysis and SiteFinding PCR revealed that BG2 encodes a cytochrome P450, OsCYP78A13. Squence search revealed that CYP78A13 has a paralogue Grain Length 3.2 (GL3.2, LOC_Os03g30420) in rice with distinct expression patterns, analysis of transgenic plants harboring either CYP78A13 or GL3.2 showed that both can promote grain growth. Sequence polymorphism analysis with 1,529 rice varieties showed that the nucleotide diversity at CYP78A13 gene body and the 20-kb flanking region in the indica varieties were markedly higher than those in japonica varieties. Further, comparison of the genomic sequence of CYP78A13 in the japonica cultivar Nipponbare and the indica cultivar 9311 showed that there were 3 InDels in the promoter region and 8 SNPs in its coding sequence. Detailed examination of the transgenic plants with chimaeric constructs suggested that variation in CYP78A13 coding region is responsible for the variation of grain yield. Taken together, our results suggest that the variations in CYP78A13 in the indica varieties hold potential in rice breeding for application of grain yield improvement.

Keywords:
cytochrome P450;grain size;grain yield;cell-cycle;sequence polymorphisms