Interplay of the histone methyltransferases SDG8 and SDG26 in the regulation of transcription and plant flowering and development

Histone methylations play fundamental roles in epigenetic regulation of transcription in many eukaryotes. In the model plant Arabidopsis thaliana as in human, a same histone lysine residue (e.g. H3K4 or H3K36) can be a potential target substrate for various different histone lysine methyltransferases (HKMTs). Using powerful genetic tool in Arabidopsis, we here investigate the interplay between two close HKMT homologues, SET DOMAIN GROUP 8 (SDG8) and SDG26, in regulating transcription and plant growth and development. We show that the sdg8 mutation is epistatic to the sdg26 one, leading to a sdg8 sdg26 double mutant exhibiting defects similar to sdg8 (reduced level of H3K36me3, increased level of H3K36me1, reduced plant body size, early flowering associated with reduced expression of FLC and MAFs and increased expression of FT and SOC1), but opposite to sdg26 (increased rosette size, late flowering associated with increased FLC and MAF5 expression and reduced FT and SOC1 expression). In parallel to the finding of the epistasis of SDG8, our study also unravels novel functions of SDG26 in H3K36me1 deposition and in the interplay with SDG8 in regulating the genome-wide gene expression. The implication of various HKMTs in establishing different forms of histone methylation and gene-context-specific chromatin modifications likely provides an advantageous mechanism for the regulation of transcription to cope with complex and plastic growth and developmental programs in plants and possibly also in other organisms.