Involvement of UDP-glycosyltransferase of Kalanchoe pinnata (Lam.) Pers in quercitrin biosynthesis and consequent antioxidant defense

Plants in the Crassulaceae family are rich in flavonoids, which may contribute to their high antioxidant activity and strong tolerance to abiotic stresses. Kalanchoe spp. are known to have high antioxidant activity, but the main components responsible for this activity and the mechanisms that control antioxidant accumulation in these plants are unknown. In this study, we identified the dominant flavonoid and investigated the mechanisms of its accumulation in Kalanchoe leaves. A wide metabolomics analysis showed that flavonoids were the most enriched metabolites in the leaves of Kalanchoe pinnata (Lam.) Pers, Kalanchoe delagoensis (Eckl. et Zeyh.) Druce, and Kalanchoe fedtschenkoi ‘Rosy Dawn’. Among these metabolites, levels of quercitrin were significantly higher in K. pinnata (Lam.) Pers than in the other two species. RNA-seq and targeted metabolomics analyses conducted on four different K. pinnata (Lam.) Pers tissues indicated that UDP-glycosyltransferase regulates the synthesis of quercitrin. Using in vitro enzyme assays and overexpression in tobacco we demonstrated that KpUGT74F catalyzes glycosylation of quercetin to produce quercitrin. This study identified the major antioxidant substances and their key regulatory mechanisms in Kalanchoe pinnata (Lam.) Pers to provide an important theoretical basis for understanding the high stress tolerance of these plants.