Glycosylation of bioactive C13-apocarotenols in Nicotiana benthamiana and Mentha x piperita

C13-apocarotenoids (norisoprenoids) are carotenoid-derived oxidation products, which perform important physiological functions in plants. Although their biosynthetic pathways have been extensively studied, their metabolism including glycosylation remains elusive. Candidate uridine-diphosphate glycosyltransferase genes (UGTs) were selected for their high transcript abundance in comparison with other UGTs in vegetative tissues of Nicotiana benthamiana and Mentha x piperita, as these tissues are rich sources of apocarotenoid glucosides. Hydroxylated C13-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 unprecedented glucosylation of C13-apocarotenols, where glucose is bound either to the cyclohexene ring or butane side chain. MpUGT86C10 is a superior novel enzyme that catalyzes the glucosylation of allelopathic 3-hydroxy--damascone, 3-oxo--ionol, 3-oxo-7,8-dihydro--ionol (Blumenol C) and 3-hydroxy-7,8-dihydro-{beta}-ionol, while a germination test demonstrated the higher phytotoxic potential of a norisoprenoid glucoside in comparison to its aglycone. Glycosylation of C13-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 analyses of the roles of glycosylated norisoprenoid allelochemicals, the physiological functions of apocarotenoids during arbuscular mycorrhizal colonization and the associated maintenance of carotenoid homeostasis.