Association of the PWWP-domain protein HUA2 and the H3K36 methylation in flowering time control

Trimethylation of histone H3 at lys36 (H3K36me3) promotes gene transcription and governs plant development and plant responses to environmental cues. Yet, how H3K36me3 is translated into specific downstream events remains largely uninvestigated. Here, we report that the Arabidopsis PWWP-domain protein HUA2 binds methyl-H3K36 in a PWWP motif-dependent manner. Mutations of the PWWP motif impeded HUA2 function to successfully rescue the hua2-7 mutant phenotype. Genetic interaction analysis revealed that HUA2 is hypostatic to the H3K36-methyltransferase gene SDG8, albeit both hua2-7 and sdg8-1 mutants display early-flowering phenotypes under long-day, medium-day, or short-day photoperiod. The mutant early-flowering phenotypes were found primarily associated with the reductions of expression of the transcriptional repressor genes FLC and MAF1. Chromatin immunoprecipitation revealed that H3K36me3 levels at FLC and MAF1 are SDG8-dependent but not HUA2-dependent. In contrast, histone acetylation (H3ac, H3K9ac) levels at FLC and MAF1 were found reduced in hua2-7 and the reductions were largely SDG8-dependent. Collectively, our results suggest that HUA2 functions as a H3K36me3 reader to promote H3ac/H3K9ac in the transcriptional activation of FLC and MAF1 to prevent precocious plant flowering.