DNA Demethylation and Its Consequences in the Germline

The companion cells of the Arabidopsis thaliana egg and sperm, the central and vegetative cells, respectively, undergo active DNA demethylation prior to fertilization. However, its biological significance, extent of conservation, and targeting preferences are not yet clear. Here we show that localized demethylation of interspersed, small transposable elements is a common feature of A. thaliana companion cells. The DEMETER DNA glycosylase is required for the DNA demethylation. DME-mediated DNA demethylation in the central cell is required to establish imprinted gene expression in the endosperm. However, the similarity among DME targets in the central and vegetative cells, despite their different functions and developmental fates, suggests that establishment of genomic imprinting may not be the basal function of DME. Lack of DEMETER in vegetative cells causes reduced methylation of transposons in sperm. This suggests that the basal function of companion cell demethylation is to reinforce transposon silencing in plant gametes.