Abstract
Steroid hormones are essential for development, and the precise control of their homeostasis is a prerequisite for normal growth. UDP-glycosyltransferases (UGTs) are considered to play an important regulatory role in the activity of steroids in mammals and insects. This study provides an indication that a UGT accepting plant steroids as substrates functions in brassinosteroid (BR) homeostasis. The UGT73C5 of Arabidopsis thaliana catalyses 23-O-glucosylation of the BRs brassinolide (BL) and castasterone. Transgenic plants overexpressing UGT73C5 displayed BR-deficient phenotypes and contained reduced amounts of BRs. The phenotype, which was already apparent in seedlings, could be rescued by application of BR. In feeding experiments with BL, wild-type seedlings converted BL to the 23-O-glucoside; in the transgenic lines silenced in UGT73C5 expression, no 23-O-glucoside was detected, implying that this UGT is the only enzyme that catalyzes BL-23-O-glucosylation in seedlings. Plant lines in which UGT73C5 expression was altered also displayed hypocotyl phenotypes previously described for seedlings in which BR inactivation by hydroxylation was changed. These data support the hypothesis that 23-O-glucosylation of BL is a function of UGT73C5 in planta, and that glucosylation regulates BR activity.