FEMALE GAMETOPHYTE GUARD interacts with OsERECTA2 to regulate embryo sac development by affecting the number of megaspore in rice

The development of embryo sacs, a process regulated by an array of genes, significantly impacts seed setting rate and yield production in rice (Oryza sativa L.). The establishment of the single archesporial cell and single functional megaspore are crucial events during embryo sac development. Nevertheless, the molecular mechanisms controlling the number of archesporial cell and functional megaspore number remain poorly understood. In this study, we identified the FEMALE GAMETOPHYTE GUARD gene (OsFGG), encoding a protein in the kinesin family, whose mutation displayed increased archesporial cells, degenerated megaspores, and increased megaspores, finally leading to degenerated or double-female-gametophyte embryo sacs (two nonholonomic embryo sacs coexisting within a single ovule). RNA-seq revealed dysregulated expression levels of genes relative to female reproduction (such as OsAGG1, OsMSP1) and protein processing in endoplasmic reticulum (like OsFes1C and OsDER1) in fgg mutants. Physical interactions between OsFGG and OsERECTA2 (OsER2) was demonstrated by using yeast two-hybrid, bimolecular fluorescence complementation, and luciferase complementation imaging assays. The OsFGG proteins and interacted complex of OsFGG and OsER2 mainly localized at endoplasmic reticulum. Additionally, oser2 and fgg oser2 mutants exhibited similar abnormalities in archesporial cells and functional megaspores as observed in fgg mutants. These findings present cytological characteristics and molecular insights into female reproduction and underscore the cooperative role of OsFGG and OsER2 in sustaining single archesporial cell and single functional megaspore and subsequent embryo sac development in rice.