Abstract
4-Hydroxy-2,5-dimethylfuran-3(2H)-one (HDMF) is an important odorant in some fruits, and is proposed to play a crucial role in the caramel-like notes of some teas. However, its biosynthesis and metabolism in tea plants are still unknown. Here, HDMF glucoside was unambiguously identified as a native metabolite in tea plants. A novel glucosyltransferase UGT74AF3a and its allelic protein UGT74AF3b specifically catalyzed the glucosylation of HDMF and the commercially important structural homologues 2 (or 5)-ethyl-4-hydroxy-5 (or 2)-methylfuran-3(2H)-one (EHMF) and 4-hydroxy-5-methylfuran-3(2H)-one (HMF) to their corresponding β-D-glucosides. Site-directed mutagenesis of UGT74AF3b to introduce a single A456V mutation resulted in improved HDMF and EHMF glucosylation activity and affected the sugar donor preference compared with that of the wild-type control enzyme. The accumulation of HDMF glucoside was consistent with the transcript levels of UGT74AF3 in different tea cultivars. In addition, transient UGT74AF3a overexpression in tobacco significantly increased the HDMF glucoside contents, and downregulation of UGT74AF3 transcripts in tea leaves significantly reduced the concentration of HDMF glucoside compared with the levels in the controls. The identification of HDMF glucoside in the tea plant and the discovery of a novel-specific UDP-glucose:HDMF glucosyltransferase in tea plants provide the foundation for improvement of tea flavor and the biotechnological production of HDMF glucoside.