A willow UDP-glycosyltransferase involved in salicinoid biosynthesis

The salicinoids are phenolic glycosides that are characteristic secondary metabolites of the Salicaceae, particularly willows and poplars. Despite the well-known pharmacology of salicin, that led to the development of aspirin over 100 years ago, the biosynthetic pathways leading to salicinoids have yet to be defined. Here, we describe the identification, cloning and biochemical characterisation of SpUGT71L2 and SpUGT71L3, - isozymic glycosyl transferases from Salix purpurea – that function in the glucosylation of ortho-substituted phenols. The best substrate in vitro was salicyl-7-benzoate. Its product, salicyl-7-benzoate glucoside, was shown to be endogenous in poplar and willow. Together they are inferred to be early intermediates in the biosynthesis of salicortin and related metabolites in planta. The role of this UGT was confirmed via the metabolomic analysis of transgenic plants produced by RNAi knockdown of the poplar orthologue (UGT71L1) in the hybrid clone Populus tremula x P. alba, INRA 717 1-B4.