Evolution of Epigenetic Mechanisms in Plants: Insights from H3K4 and H3K27 Methyltransferases

Histone posttranslational modifications are a component of epigenetic mechanisms essential to the dynamic control all DNA-templated processes in different eukaryotic lineages. Many of these modifications, and the enzymes responsible for their implementation, are conserved among plant and animals; whereas, others may have changed and/or acquired new functions during eukaryotic evolution. To illustrate the principles by which histone-modifying systems have been shaped in the green lineage, here we follow the evolutionary history of the methyltransferases responsible for histone H3 lysine 4 (H3K4) and lysine 27 (H3K27) methylation. Recent analyses of these chromatin-modifying proteins have not only provided evidence for conserved, but also functional, divergence in plants. A pattern of ancient and lineage-specific gene duplications followed by functional diversification appears to be the norm during the evolution of these methyltransferases. We discuss how changes in the function of H3K4 and H3K27 methyltransferases relate to their domain architecture and how the expansion of the gene families encoding these enzymes in the plant lineage is associated with the recruitment of their chromatin-modifying activities for the regulation of new genes and/or pathways.