Six Uridine-Diphosphate Glycosyltransferases Catalyze The Glycosylation Of Bioactive C-13-Apocarotenols

Six transferases in Nicotiana benthamiana and Mentha x piperita catalyze the glycosylation of a range of hydroxylated alpha- and beta -ionone/ionol derivatives. C-13-apocarotenoids (norisoprenoids) are carotenoid-derived oxidation products that perform important physiological functions in plants. Although their biosynthetic pathways have been extensively studied, their metabolism including glycosylation remains poorly understood. Candidate uridine-diphosphate glycosyltransferase genes (UGTs) were selected based on their high transcript abundance in comparison with other UGTs in vegetative tissues of Nicotiana benthamiana and peppermint (Mentha x piperita), as these tissues are rich sources of apocarotenoid glucosides. Hydroxylated C-13-apocarotenol substrates were produced by P450-catalyzed biotransformation and microbial/plant enzyme systems were established for the synthesis of glycosides. Natural substrates were identified by physiological aglycone libraries prepared from isolated plant glycosides. In total, we identified six UGTs that catalyze the glucosylation of C-13-apocarotenols, where Glc is bound either to the cyclohexene ring or the butane side chain. MpUGT86C10 is a superior novel enzyme that catalyzes the glucosylation of allelopathic 3-hydroxy-alpha -damascone, 3-oxo-alpha -ionol, 3-oxo-7,8-dihydro-alpha -ionol (Blumenol C), and 3-hydroxy-7,8-dihydro-beta -ionol, whereas a germination test demonstrated the higher phytotoxic potential of a norisoprenoid glucoside in comparison to its aglycone. Glycosylation of C-13-apocarotenoids has several functions in plants, including increased allelopathic activity of the aglycone, facilitating exudation by roots and allowing symbiosis with arbuscular mycorrhizal fungi. The results enable in-depth analysis of the roles of glycosylated norisoprenoid allelochemicals, the physiological functions of apocarotenoids during arbuscular mycorrhizal colonization, and the associated maintenance of carotenoid homeostasis.