Rice OsMKK6-OsMPK4 cascade regulates endosperm cellularization via Polycomb Repressive Complex 2

Cereal endosperm, derived from the fertilized central cell in the process of double fertilization, is a primary source of human food and animal feed. Early endosperm development involves rapid nuclear division, followed by cellularization, and disruptions to this process affect seed size, viability, and yield. However, the regulatory mechanisms governing the early cereal endosperm development remain largely unclear. Here, we demonstrate that the OsMKK6-OsMPK4 cascade plays an important role in early endosperm development of rice. Knockout of either OsMKK6 or OsMPK4 causes maternally inherited endosperm abortion due to impaired cellularization. Mechanistically, the OsMKK6-OsMPK4 cascade directly interacts and phosphorylates OsFIE1 and OsFIE2, core components of the Polycomb Repressive Complex 2 (PRC2), thereby modulating H3K27me3 at imprinted gene loci. Consistently, the osfie1osfie2 mutant exhibits endosperm cellularization defects similar to those of osmkk6 and osmpk4. Our work links MAPK signaling to PRC2 function in endosperm cellularization, providing new insights into rice endosperm development.