RGG1, involved in the cytokinin regulatory pathway, controls grain size in rice

Abstract Heterotrimeric GTP binding proteins (G proteins) and cytokinin play important roles in regulating plant growth and development. However, little is known about the mechanism by which they coordinate the regulation of grain size in rice. We functionally characterized one gene, RGG1, encoding a type-A Gγ subunit. Strong GUS staining were detected in young panicles and spikelets, suggesting the role of this gene in modulating panicle-related traits development. By regulating cell division, over-expression of RGG1 in Nipponbare (NIP) and Wuyunjing 30 (WYJ30) significantly decreased plant height, panicle length and grain length. However, rgg1 mutants generated by CRISPR/Cas9 system exhibited no obvious phenotypic differences, which may be due to its extremely low expression level in vivo. Transcriptomic analysis showed that differentially expressed genes (DEGs) were enriched in the cytokinin biosynthesis pathway. We confirmed this result by measuring the endogenous cytokinin levels and showed that the content of cytokinin was lower in OE lines. Additionally, the results of 6-benzylaminopurine (6-BA) treatment showed that increased expression of RGG1 decreased the sensitivity to 6-BA under low concentration. Our results reveal a novel G proteins—cytokinin module controlling grain size in rice, which will be beneficial for understanding the mechanisms of G proteins regulating grain size and plant development.