Grain size is a key agronomic traits that influence grain yield in crops. The transcription factor GS2/OsGRF4 can improve grain size and yield, but its underlying mechanism remains unclear. Here we report a suppressor of the gain-of-function allele GS2AA (SUG1) that encodes a plant-specific protein DEP2/SRS1/EP2/OsRELA and acts as a transcriptional regulator. The sug1 mutants form short grains, while overexpression of SUG1 results in long grains. GS2 directly activates the expression of SUG1. SUG1 associates with transcription factors OsBZR1, OsMADS56 and OsSPL13 to control grain size through GA and BR signaling as well as growth pathways. Natural variation in SUG1 contributes to grain size diversity, and the SUG1Hap2 allele from indica varieties can be used to improve grain size and yield of japonica varieties with the SUG1Hap3 allele. Thus, our findings uncover that the GS2-SUG1 module controls grain size by integrating multiple growth signals, providing the potential targets for crop improvement.
