Metabolic Conversion of Castasterone and Brassinolide into Their Glucosides in Higher Plants

Castasterone (CS) and brassinolide (BL) were administered to mung bean (Vigna radiata) explants, Arabidopsis thaliana seedlings, and cultured Catharanthus roseus cells, and the glucosylated metabolites were analyzed using LC/MS/MS. In mung bean and C. roseus, CS-2-O-glucoside (CS-2G), -3-O-glucoside (CS-3G), -22-O-glucoside (CS-22G), and -23-O-glucoside (CS-23G) were identified as metabolites of CS, whereas BL-2G, BL-3G, and BL-23G were identified as metabolites of BL. In A. thaliana, CS and BL were converted into their respective 2-O- and 23-O-glucosides. Of the metabolites identified with BL and CS administration, BL-23G was the predominant metabolite in mung bean and A. thaliana, whereas the 3-O-glucoside of BL was abundant in C. roseus. This is the first report of the metabolic conversion of CS into CS-2G, CS-3G, CS-22G, and CS-23G, and of BL into BL-2G and BL-3G. Our results indicate that the glucosylation profiles of BL and CS vary with plant species, and that the glucosylation of CS is rather limited quantitatively, compared with that of BL.