The maternally expressed polycomb group gene OsEMF2a is essential for endosperm cellularization and imprinting in rice

Cellularization is a key event in endosperm development.Polycomb group(PcG)genes,such as Fertilization-Independent Seed 2(FIS2),are vital for the syncytium-to-cellularization transition in Arabidopsis plants.In this study,we found that OsEMF2a,a rice homolog of the Arabidopsis PcG gene Embryonic Flower2(EMF2),plays a role similar to that of FIS2 in regard to seed development,although there is limited sequence similarity between the genes.Delayed cellularization was observed in osemf2a,associated with an unusual activation of type I MADS-box genes.The cell cycle was persistently activated in osemf2a caryopses,which was likely caused by cytokinin overproduction.However,the overaccumulation of auxin was not found to be associated with the delayed cellularization.As OsEMF2a is a maternally expressed gene in the endosperm,a paternally inherited functional allele was unable to recover the maternal defects of OsEMF2a.Many imprinted rice genes were deregulated in the defective hybrid seeds of osemf2a(♀)/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.

水稻PcG类基因OsEMF2b T-DNA插入杂合突变体的细胞学研究

采用石蜡切片法对水稻OsEMF2b的T-DNA插入杂合突变体的花器官形态、雌雄蕊发育和种子形成等过程进行了细胞学研究。结果表明,OsEMF2b杂合突变体花器官形态和各部分的数量有变异,但花药和胚珠发育以及花粉和胚囊的育性正常。OsEMF2b杂合突变体出现一定比例的受精卵不分裂、胚胎发育延迟、胚胎畸形,胚乳游离核不能细胞化和胚乳细胞退化解体等异常现象,从而导致其结实率明显低于野生型。OsEMF2b基因可能在水稻的花器官形成和种子发育过程中具有重要的调控作用。