A MAPK cascade regulates nuclear migration during female gametogenesis to establish specific cell identity and rice female fertility

During the development of the rice female gametophyte (FG, embryo sac), there exists a free-nuclear stage wherein eight nuclei, originating from the same mother cell nucleus, respectively migrate to specific positions to acquire distinct cell identities after cellularization and develop into the egg cell, central cell, or accessory cells. Therefore, this nuclear migration stage is essential for fertilization and rice fertility. How these nuclei can find their own way to exactly move to different predetermined positions and how the two polar nuclei are selected have been intriguing questions for decades. Here, we show that a mitogen-activated protein kinase cascade, composed of OsMEKK2/OsMEKK21, OsMKK6, and OsMPK4, precisely regulates free-nuclear migration and determines the number and identities of polar nuclei. Specifically, OsMKK6 phosphorylates OsMPK4 to restrict nuclear migration at the chalazal pole, ensuring the accurate number and positioning of antipodal cells and polar nuclei. Loss-of-function mutations in OsMKK6 and OsMPK4 result in an excess of polar nuclei at the expense of antipodal cells. Additionally, OsMEKK2 and OsMEKK21 function as upstream regulators, phosphorylating and modulating downstream substrates to control nuclear migration at both poles. This regulation is essential for FG fertility and seed production. Our findings reveal a hierarchical model for the regulation of nuclear migration during embryo sac development and provide valuable insights into how an embryo sac orchestrates the migration of eight nuclei in response to different positional cues, thereby establishing female fertility in rice.