GR5 acts in the G protein pathway to regulate grain size in rice

Grain size is an important determinant of grain yield in rice. Although dozens of grain size genes have been reported, the molecular mechanism in controlling grain size remains to be further elucidated. Here, we report the cloning and characterization of GR5 (GRAIN ROUND 5), which is allelic to SMOS1/SHB/RLA1/NGR5 and encodes a member of an AP2 transcription factor. GR5 acts as a transcription activator and determines grain size by influencing cell proliferation and expansion. We demonstrated that GR5 physically interacts with five Gγ subunit proteins including RGG1, RGG2, DEP1, GS3 and GGC2, and acts downstream of G protein complex. Moreover, four downstream target genes DEP2, DEP3, DRW1 and CyCD5;2 of GR5 in grain development were gained and the core T/CGCAC motif was identified by yeast one-hybrid, EMSA and ChIP-PCR analysis. Our results further revealed that GR5 interacts with Gγ subunits and cooperatively determines grain size by regulating the expression of downstream target genes. These findings enrich the genetic regulatory network of the G protein signaling pathway in the control of grain size, and provide a potential target for high-yield rice breeding.