Abstract
Heterotrimeric G proteins, which consist of Gα , Gβ and Gγ subunits, function as molecular switches that regulate a wide range of developmental processes in plants. In this study, we characterised the function of rice RGG2, which encodes a type B Gγ subunit, in regulating grain size and yield production. The expression levels of RGG2 were significantly higher than those of other rice Gγ -encoding genes in all tissues tested, suggesting that RGG2 plays essential roles in rice growth and development. By regulating cell expansion, overexpression of RGG2 in Nipponbare (NIP) led to reduced plant height and decreased grain size. By contrast, two mutants generated by the clustered, regularly interspaced, short palindromic repeat (CRISPR)/CRISPR-associated protein 9 (Cas9) system in the Zhenshan 97 (ZS97) background, zrgg2-1 and zrgg2-2, exhibited enhanced growth, including elongated internodes, increased 1000-grain weight and plant biomass and enhanced grain yield per plant (+11.8% and 16.0%, respectively). These results demonstrate that RGG2 acts as a negative regulator of plant growth and organ size in rice. By measuring the length of the second leaf sheath after gibberellin (GA3 ) treatment and the GA-induced α-amylase activity of seeds, we found that RGG2 is also involved in GA signalling. In summary, we propose that RGG2 may regulate grain and organ size via the GA pathway and that manipulation of RGG2 may provide a novel strategy for rice grain yield enhancement.