The maternally expressed polycomb group gene is essential for endosperm cellularization and imprinting in rice.

Cellularization is a key event in endosperm development. Polycomb group (PcG) genes, such as (), are vital for the syncytium-to-cellularization transition in plants. In this study, we found that , a rice homolog of the PcG gene (), plays a role similar to that of in regard to seed development, although there is limited sequence similarity between the genes. Delayed cellularization was observed in , associated with an unusual activation of type I MADS-box genes. The cell cycle was persistently activated in caryopses, which was likely caused by cytokinin overproduction. However, the overaccumulation of auxin was not found to be associated with the delayed cellularization. As is a maternally expressed gene in the endosperm, a paternally inherited functional allele was unable to recover the maternal defects of . Many imprinted rice genes were deregulated in the defective hybrid seeds of (♀)/9311 (♂) (m9). The paternal expression bias of some paternally expressed genes was disrupted in m9 due to either the activation of maternal alleles or the repression of paternal alleles. These findings suggest that OsEMF2a-PRC2-mediated H3K27me3 is necessary for endosperm cellularization and genomic imprinting in rice.