Udp-Glycosyltransferases of Plant Hormones

UDP-glycosyltransferases (GTases, UGT) catalyze the transfer of the sugar moiety from the uridine-diphosphate-activated monosaccharide (e.g. uridine-diphosphate-5’-glucose, UDPG) molecule to the specific acceptor. Glycosides contain aglycons attached by a β-glycosidic bond to C1 of the saccharide moiety. Glycosylation is one of the mechanisms maintaining cellular homeostasis through the regulation of the level, biological activity, and subcellular distribution of the glycosylated compounds. The glycosides play various functions in plant cells, such as high-energy donors, or signalling molecules, and are involved in biosynthesis of cell walls. Plant cells exhibit structural and functional diversity of UGT proteins. The Arabidopsis thaliana genome contains more than 100 genes encoding GTases, which belong to 91 families, and are deposited in the CAZY (Carbohydrate Active enzyme) database (www. cazy.org/GlycosylTransferases.html). The largest UGT1 class is divided into 14 subfamilies (A-N), and includes proteins containing highly conserved 44-amino acid PSPG (Plant Secondary Product Glycosyltransferase) motif at the C-terminus. The PSPG motif is involved in the binding of UDP-sugar donors to the enzyme. UGT1’s catalyze the biosynthesis of both ester-type and ether-type conjugates of plant hormones (phytohormones). Conjugation of the phytohormones is an important mechanism that regulates the concentration of physiological active hormone levels during growth and development of plants. Glycoconjugation of phytohormones is widespread in the plant kingdom and all known phytohormones are able to form these conjugates. Most plant hormone conjugates do not indicate physiological activity, but rather are involved in transport, storage and degradation of the phytohormones. UDPG-dependent glycosyltransferases possess high substrate specificity, even within a given class of phytohormones. In many cases, the phenotype of plants is strongly affected by loss-of-function mutations in UGT genes. In this paper, advances in the isolation and characterization of glycosyltransferases of all plant hormones: auxin, brassinosteroids, cytokinin, gibberellin, abscisic acid, jasmonates, and salicylate is described