Standing up against the silencer: Chromatin factors antagonizing Polycomb

Polycomb group proteins are central regulators of plant development, maintaining transcriptional repression through H2AK121 monoubiquitination and H3K27 trimethylation. However, Polycomb-mediated silencing must remain reversible to allow developmental transitions and environmental responses. Recent studies have uncovered a diverse repertoire of chromatin factors that restrict, remove, or counterbalance Polycomb activity in plants. These include histone deubiquitinases and demethylases, trithorax-related proteins, adenosine triphosphate (ATP)-dependent chromatin remodelers, transcription factors, and additional regulatory proteins that protect gene expression programs and chromatin architecture from inappropriate Polycomb recruitment or spreading. Their actions range from the direct removal of repressive marks to the inhibition of Polycomb recruitment and the remodeling of nucleosome accessibility. Importantly, some of these factors display context-dependent dual roles, acting either as Polycomb antagonists or facilitators depending on the locus, protein associations, or developmental context. Evolutionary analyses suggest that Polycomb regulatory networks and their antagonistic pathways have undergone substantial diversification and rewiring during Viridiplantae evolution. Extending functional studies beyond Arabidopsis thaliana will be essential to determine how these regulatory networks contribute to developmental plasticity and environmental adaptation in plants with agronomically-relevant traits.