SMALL GRAIN 11 Controls Grain Size, Grain Number and Grain Yield in Rice

Background: Grain size is one of key agronomic traits that determine grain yield in rice. Several regulators of grain size have been identified in rice, but the mechanisms that determine grain size and yield remain largely unknown. Results: Here we characterize a small grain (smg11) mutant in rice, which exhibits small grains, dense panicles and the increased number of grains per panicle. Cloning and sequence analyses of the SMG11 gene reveal that smg11 is a new allele of DWARF2 (D2), which encodes a cytochrome P450 (CYP90D2) involved in brassinosteroid biosynthetic pathway. Overexpression of D2/SMG11 increases grain size and grain weight of wild-type plants. Overexpression of D2/SMG11 at a suitable level also significantly increases grain yield in rice. Cellular analyses indicate that D2/SMG11 controls grain size by promoting cell expansion. Further results reveal that D2/SMG11 influences expression of several known grain size genes involved in the regulation of cell expansion, revealing a novel link between D2/SMG11 and known grain size genes. Conclusions: SMG11 controls grain size by promoting cell expansion in grain hulls. SMG11 regulates cell expansion, at least in part, by influencing expression of several grain size genes involved in the regulation of cell expansion. The smg11 is a new allele of DWARF2/D2. The suitable expression of SMG11 increases grain size, grain weight and grain yield. Our findings reveal the functions of D2/SMG11 in grain size and grain yield, suggesting that the suitable expression of D2/SMG11 is a promising approach to improve grain yield in rice.