The 5-methylcytosine DNA glycosylase ROS1 antagonizes parent-of-origin specific DNA methylation in Arabidopsis endosperm

DNA methylation patterning is a consequence of opposing activities of DNA methyltransferases and DNA demethylases. In flowering plants, two distinct female gametes, the egg cell and the central cell, are fertilized, producing what will become the embryo and the endosperm of the seed. In Arabidopsis, a 5-methylcytosine DNA glycosylase, DME, demethylates regions in the central cell genome, leading to methylation differences between maternally- and paternally-inherited endosperm genomes after fertilization. DME is required for endosperm gene imprinting. Homologues of DME include ROS1, DML2 and DML3. It is unknown whether any of these DNA glycosylases are required for endosperm methylation patterning. We show that ROS1 prevents hypermethylation of paternally-inherited alleles in the endosperm at regions that lack maternal or paternal-allele methylation in wild-type. Thus, ROS1 promotes epigenetic symmetry between genomes in the endosperm by preventing paternal genome hypermethylation. We propose that ROS1 and DME act in a parent-of-origin-specific manner at shared endosperm targets, and consider implications for the evolution of imprinting mechanisms.