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

Abstract

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.

Endoplasmic reticulum stress pathway mediates the early heat stress response of developing rice seeds

“…Heat stress accelerates the suppression of these genes at 2 DAF time points, which likely accelerates the endosperm cellularization. These results are consistent with endosperm development in rice Embryonic Flower2 (OsEMF2a) mutant, wherein increased expression of cell-cycle genes leads to overproliferation of coenocytic endosperm and delays the cellularization, and this process has been linked to over-production of cytokinin (Cheng et al, 2021;Tonosaki et al, 2020). These results along with previous reports suggest that cytokinin-modulated suppression of cell-cycle genes likely accelerates the endosperm cellularization under heat stress.…”

“…Recent evidence suggests an increase in JA levels and JA response during light and heat acclimation (Balfag on et al, 2019). In Arabidopsis, enhanced heat resilience by JA treatment and hyperheat-sensitivity of JA mutants suggest that JA positively regulates heat tolerance (Clarke et al, 2009;Kazan, 2015). Consistent with these findings, we found that JA responds to heat stress in young seeds.…”

Dynamics of H3K27me3 Modification on Plant Adaptation to Environmental Cues

“…This scenario suggests gene duplication is a major evolutionary force shaping plant PRC2, most likely in a species-or genus-specific manner (Table 1). Nevertheless, the distinct PRC2 complexes and their functions in cereal growth and development remain to be elucidated [24,25,[40][41][42][43][44]. To maintain H3K27me3 homeostasis, it is reasonable to have opposite players to remove the mark on targets (eraser).…”

Rice LEAFY COTYLEDONE1 regulates embryonic envelope development and chlorophyll biogenesis in embryo

“…CLSM was performed as described previously (Cheng et al, 2020). Images of the PI-stained embryos were taken using an LSM710 (Zeiss) microscope with excitation/emission wavelengths of 559/619 nm.…”

“…The maize used in this study was grown in the experimental field of Biogle GeneTech in Xishuangbanna, Yunnan Province, China. Because the seeds produced by osnf-yb7 were inviable, the mutants used were propagated asexually by ratooning as described previously (Cheng et al, 2020). The rice and maize seeds were germinated in a growth chamber.…”