G proteins regulate rice grain size by modulating the Hippo signaling pathway

G-protein pathways play critical roles in many aspects of plant development. Among the 5 Gγ proteins in rice (Oryza sativa), DENSE AND ERECT PANICLE 1 (DEP1) and GRAIN SIZE 3 (GS3) have been shown to determine grain size, but the exact mechanism by which G proteins regulate the process is not fully understood. Here, we demonstrate that the G proteins modulate the stability of MPS ONE BINDER KINASE ACTIVATOR-LIKE 1A (OsMOB1A), a core component in the Hippo pathway. In the Hippo signaling pathway, the protein kinase Serine/Threonine Kinase 1 (OsSIK1) phosphorylates the scaffold protein OsMOB1A and thus regulates its stability. We discovered that disruption of either OsSIK1 or OsMOB1A leads to smaller grains, whereas overexpression of either gene results in larger grains. DEP1 and GS3 physically interact with OsSIK1, thus affecting the OsSIK1-OsMOB1A interaction and the phosphorylation of OsMOB1A by OsSIK1. OsMOB1A stability was decreased in vivo in dep1-1 mutant and GS3 overexpression lines, but increased in gs3 mutants and DEP1 overexpression lines. Moreover, the impact of Gγ proteins on the Hippo pathway was Gγ dose-dependent. Genetic analysis indicated that Gγ proteins act upstream of the Hippo pathway. Our results uncovered a molecular mechanism by which G proteins regulate rice grain size by modulating the Hippo signaling pathway and provide potential targets for improving grain yield.