The <i>Arabidopsis</i> PRC2 subunit MSI1 interacts with HY5 to repress hypocotyl elongation through H3K27 tri-methylation

Polycomb repressive complex 2 (PRC2) is a conserved multi-protein complex that catalyzes histone H3 trimethylation at lysine 27 (H3K27me3), an epigenetic mark associated with transcriptional repression. However, the mechanistic link between PRC2-mediated H3K27me3 and light-regulated hypocotyl elongation during photomorphogenesis remains largely unexplored. In this study, we identify the MULTICOPY SUPPRESSOR OF IRA1 (MSI1), a core component of PRC2, as a negative regulator of hypocotyl elongation under light conditions in Arabidopsis thaliana. MSI1 physically interacts with ELONGATED HYPOCOTYL 5 (HY5), a basic leucine zipper (bZIP) transcription factor central to photomorphogenic signaling, in vitro and in vivo. Genetic and molecular analyses reveal that MSI1 is indispensable for HY5-mediated repression of hypocotyl growth, suggesting a cooperative function between chromatin-based silencing and transcriptional regulation. Chromatin immunoprecipitation followed by high-throughput sequencing (ChIP-seq) showed that MSI1 and HY5 co-occupy numerous genomic loci. HY5 facilitates PRC2 complex recruitment to the target genes by interacting with MSI1, thereby promoting H3K27me3 deposition and repressing gene expression. Furthermore, transcriptomic profiling reveals that MSI1 and HY5 jointly suppress a subset of genes involved in auxin signaling, providing mechanistic insight into their role in photomorphogenesis. Together, our findings uncover an epigenetic mechanism by which HY5 recruits PRC2 through MSI1 to modulate light-responsive growth.