A dihydrochalcone-specific O-methyltransferase from leaf buds of Populus trichocarpa implicated in bud resin formation

Production of secreted leaf bud resin is a mechanism for temperate trees to protect dormant leaf buds against frost damage, dehydration, and insect herbivory. Bud resins contain a wide variety of special metabolites including terpenoids, benzenoids, and phenolics. The leaf bud resins of Populus trichocarpa and P. balsamifera contain high concentrations of O-methylated dihydrochalcones, but the enzymes for methylating these compounds remain enigmatic. We used transcriptomics and differential gene expression analyses to identify a gene encoding a P. trichocarpa dihydrochalcone-specific O-methyltransferase, which we named PtDOMT1. Detailed enzymatic analyses demonstrated PtDOMT1 to be a highly selective and regiospecific O-methyltransferase which methylates 4 and 4' positions of dihydrochalcones using S-adenosyl-l-methionine as a methyl donor. PtDOMT1 did not methylate any other flavonoid or phenolic substrate tested. These findings establish the final step in the biosynthesis of O-methylated dihydrochalcones in poplar and represent the first molecular analysis of leaf bud resin biosynthesis in plants.