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

This study is reported by Jin-long Qiu’s group at the Institute of Microbiology, Chinese Academy of Sciences. As the primary source of food for humans and feed for animals, cereal endosperm plays a decisive role in determining seed size, viability, and overall grain yield. Rice endosperm forms through double fertilization, where one sperm cell fuses with the central cell to produce a triploid endosperm. Early endosperm development comprises two critical stages: coenocyte formation (rapid nuclear division without cytokinesis) and cellularization (cell wall formation and cytokinesis to form functional cells). Disruptions in cellularization lead to seed abortion, but the regulatory networks governing this process remained elusive.

Using CRISPR/Cas9 gene editing, Zengqian Wang and their colleagues in Qiu’s group generated knockout mutants of OsMKK6 and OsMPK4. Loss-of-function mutations in either gene were homozygous lethal; selfed progeny of heterozygotes showed approximately 50% endosperm abortion, a maternally inherited phenotype. Detailed cellular observations confirmed that the endosperm development failure stemmed from impaired endosperm cellularization. The authors found that OsMKK6 formed a cascade with OsMPK4, and OsMPK4 interacted and phosphorylated OsFIE1 and OsFIE2—core components of the Polycomb Repressive Complex 2 (PRC2). This phosphorylation enhances the protein stability of OsFIE1 and OsFIE2, ensuring that the PRC2 complex effectively mediates histone H3 lysine 27 trimethylation (H3K27me3) at imprinted gene loci. In osmkk6 and osmpk4 mutants, the levels of H3K27me3 modification at imprinted gene loci were significantly reduced, causing the abnormal upregulation of endosperm development-related imprinted genes (e.g., OsYUCCA11, OsFBX365, and OsPEG1). Notably, the osfie1osfie2 double mutant exhibited similar endosperm cellularization defects, further confirming that OsFIE1 and OsFIE2 are downstream targets of the OsMKK6-OsMPK4 cascade.

This study not only uncovers a novel regulatory pathway (MKK6-MPK4-PRC2-imprinted genes) governing early rice endosperm development but also provides new insights into the cross-talk between MAPK signaling and PRC2-mediated epigenetic regulation. Given the evolutionary conservation of MAPK signaling pathways and PRC2-mediated gene silencing across eukaryotes, this regulatory module might be applicable to other cereal crops and even non-cereal plants, offering valuable insights for understanding seed development broadly.

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