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-Oglucosylation 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.glucosylation ͉ glycosyltransferase ͉ homeostasis ͉ plant ͉ steroid

Despite its importance in plant growth and development, the auxin biosynthetic pathway has remained elusive. In this study, we analyzed hormone series transcriptome data from AtGenExpress in Arabidopsis and found that aminoethoxyvinylglycine (AVG) had the strongest anti-auxin activity. We also identified other effective compounds such as L-amino-oxyphenylpropionic acid (AOPP) through additional screening. These inhibitors shared characteristics in that they inhibited pyridoxal enzymes and/or aminotransferases. They reduced endogenous IAA levels in both monocots and dicots. L-AOPP inhibited root development of Arabidopsis in main root elongation, gravitropism, root skewing and root hair formation. This inhibition was generally recovered after exogenous IAA treatment, and the recovery was almost completely to the level of non-inhibited seedlings. The compounds inhibited conversion from tryptophan to indole-3-pyruvic acid in enzyme extracts from Arabidopsis and wheat. Our data collectively suggest that the inhibitors directly blocked auxin biosynthesis, and that the major target site was tryptophan aminotransferase. This enzyme probably makes up one of the major biosynthesis pathways conserved among higher plants. Each inhibitor, however, demonstrated a different action spectrum in shoot and root of rice and tomato, indicating diversity in biosynthesis pathways between organs and species. Our results provide novel insights into auxin biosynthesis and action, and uncover structural characteristics of auxin biosynthesis inhibitors.