MicroProtein OsPAR1 regulates grain size through OsBCL1-mediated cell expansion control in rice

Grain size is a crucial agronomic trait that influences the grain weight and yield of rice (Oryza sativa L.). Here, we identified and characterized OsPAR1, an HLH-type miP (microProtein), as a positive regulator of grain size and grain weight. Loss-of-function Ospar1 mutants exhibited reduced cell size of the rice spikelet hulls, resulting in significantly smaller grains and lower grain weight. Conversely, OsPAR1-overexpressing lines displayed enlarged hull cells, leading to considerably larger grains and increased grain weight. Biochemical analyses showed that OsPAR1 interacts with the bHLH transcription factor OsBCL1 and functions as a transcriptional co-activator. OsPAR1 promotes OsBCL1 binding to the promoter of OsEXPA5, a key gene involved in cell expansion. Meanwhile, Osbcl1 and Osexpa5 mutants showed smaller grains with decreased grain length, grain width, grain thickness, and 1,000-grain weight. We also demonstrated that the maize (Zea mays) ZmPAR1, which is homologous to OsPAR1, is also involved in the regulation of grain size, implying a conserved role in the grain size of cereal crops. Notably, a critical SNP in the OsPAR1 coding region, causing a Glu-to-Gly substitution, is associated with grain size variation in cultivated rice. Collectively, we uncover the crucial role of OsPAR1 in grain size control, which offers a genetic basis for rice breeding in the future.