DNA methylation and Polycomb-mediated repression at plant transposable elements: Antagonism, cooperation and a continuum of epigenetic silencing states

Transposable elements (TEs) are primarily silenced by DNA methylation and H3K9 methylation in plants, but can also be targeted by Polycomb-mediated H3K27me3. Increasing evidence shows that these pathways can antagonize or cooperate in a context- and DNA methylation level-dependent manner. Here, we review emerging evidence for this crosstalk and discuss potential mechanisms underlying interactions between DNA methylation and Polycomb repression in plants. We propose that DNA methylation, Polycomb and their interactions define a continuum of TE silencing states through which TEs may transition during development and across generations. Comparative genomic studies further indicate that the relative contribution of Polycomb- and DNA-methylation-mediated silencing has diversified during plant evolution.