Control of grain size and number by MAPK signaling in rice

Grain size, a main component of grain yield, is regulated by a complex network. The mitogen-activated protein kinase (MAPK) cascade participates in multiple signaling pathways to regulate various biological processes. Recent studies indicate that MAPK signaling plays key roles in regulating grain size. For instance, OsERECTA1(OsER1)–OsMKKK10–OsMKK4–OsMPK6 signaling regulates grain size and grain number per panicle. Grain size is also affected by the OsMKKK70–OsMKK4–OsMPK6 module, which functions upstream of OsWRKY53. In addition, MITOGEN-ACTIVATED PROTEIN KINASE PHOSPHATASE1 (OsMKP1), the GSK3/SHAGGY-like kinase GSK2, and the Rho-family GTPase OsRac1 controls grain size in rice by modulating MAPK signaling. Here, we discuss recent findings on the importance of MAPK signaling in rice grain-size control and examine mechanisms by which MAPK signaling coordinates grain size, grain number and stress responses.