Grain size is an important determinant of grain yield in rice. Although dozens of grain size genes have been reported, the molecular mechanisms that control grain size remain to be fully clarified. Here, we report the cloning and characterization of GR5 (GRAIN ROUND 5), which is allelic to SMOS1/SHB/RLA1/NGR5 and encodes an AP2 transcription factor. GR5 acts as a transcriptional activator and determines grain size by influencing cell proliferation and expansion. We demonstrated that GR5 physically interacts with five Gγ subunit proteins (RGG1, RGG2, DEP1, GS3, and GGC2) and acts downstream of the G protein complex. Four downstream target genes of GR5 in grain development (DEP2, DEP3, DRW1, and CyCD5;2) were revealed and their core T/CGCAC motif identified by yeast one-hybrid, EMSA, and ChIP–PCR experiments. Our results revealed that GR5 interacts with Gγ subunits and cooperatively determines grain size by regulating the expression of downstream target genes. These findings provide new insight into 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.

粒径是水稻产量的重要决定因素。尽管已经报道了数十个晶粒尺寸基因，但控制晶粒尺寸的分子机制仍有待完全阐明。在这里，我们报道了 GR5 （GRAIN ROUND 5） 的克隆和表征，GR5 是 SMOS1/SHB/RLA1/NGR5 的等位基因，编码 AP2 转录因子。GR5 作为转录激活剂，通过影响细胞增殖和扩增来决定颗粒大小。我们证明 GR5 与五个 Gγ 亚基蛋白 （RGG1 、 RGG2 、 DEP1 、 GS3 和 GGC2） 物理相互作用，并作用于 G 蛋白复合物的下游。GR5 在籽粒发育中的四个下游靶基因 （DEP2、DEP3、DRW1 和 CyCD5;2） 揭示了其核心 T/CGCAC 基序，并通过酵母单杂交、EMSA 和 ChIP-PCR 实验鉴定了它们的核心 T/CGCAC 基序。我们的结果表明，GR5 与 Gγ 亚基相互作用，并通过调节下游靶基因的表达协同决定晶粒大小。这些发现为 G 蛋白信号通路在控制粒径中的遗传调控网络提供了新的见解，并为高产水稻育种提供了潜在的靶点。