The Rho-family GTPase controls rice grain size and yield by regulating cell division.

Grain size is a key factor for determining grain yield in crops and is a target trait for both domestication and breeding, yet the mechanisms underlying the regulation of grain size are largely unclear. Here we show that the grain size and yield of rice () is positively regulated by ROP GTPase (Rho-like GTPase from plants), a versatile molecular switch modulating plant growth, development, and responses to the environment. Overexpression of rice not only increases cell numbers, resulting in a larger spikelet hull, but also accelerates grain filling rate, causing greater grain width and weight. As a result, OsRac1 overexpression improves grain yield in by nearly 16%. In contrast, down-regulation or deletion of OsRac1 causes the opposite effects. RNA-seq and cell cycle analyses suggest that OsRac1 promotes cell division. Interestingly, OsRac1 interacts with and regulates the phosphorylation level of OsMAPK6, which is known to regulate cell division and grain size in rice. Thus, our findings suggest OsRac1 modulates rice grain size and yield by influencing cell division. This study provides insights into the molecular mechanisms underlying the control of rice grain size and suggests that OsRac1 could serve as a potential target gene for breeding high-yield crops.