DEMETER-mediated DNA Demethylation in Gamete Companion Cells and the Endosperm, and its Possible Role in Embryo Development in Arabidopsis

Seed development begins upon double fertilization, producing the embryo and endosperm, which are genetically identical, except for their ploidy level. DEMETER (DME), a member of the DNA glycosylase family, functions as a DNA demethylase via the base excision repair pathway. DME is specifically expressed prior to fertilization in two gamete companion cells, central cell of the female gametophyte and vegetative cell of the male gametophyte, but not in the heritable gamete cells or embryo. Mutations in the DME gene cause hypermethylation in the endosperm, leading to endosperm overproliferation and seed abortion after fertilization. DME-mediated DNA demethylation preferentially targets euchromatic transposable elements (TEs), resulting in TE activation and initiation of de novo methylation through RNA-directed DNA methylation, and provides FERTILIZATION-INDEPENDENT SEED 2 (FIS2)-Polycomb Repressive Complex 2-binding sites, resulting in histone modifications and genomic imprinting during reproduction. The global demethylation of TEs in gamete companion cells and active de novo methylation in the embryo suggest a new role of sexual companion cells in reinforcing the genome integrity of the heritable tissue. In this review, we provide an overview of demethylation in sexual companion cells and the endosperm, and discuss its evolutionary effect on the heritable gamete cells and embryo.