A UDP glucose: thiohydroximate glucosyltransferase from Tropaeolum majus L

Arabidopsis glucosyltransferase UGT74B1 functions in glucosinolate biosynthesis and auxin homeostasis

Glucosinolates are a class of secondary metabolites with important roles in plant defense and human nutrition. Here, we characterize a putative UDP-glucose:thiohydroximate S-glucosyltransferase, UGT74B1, to determine its role in the Arabidopsis glucosinolate pathway. Biochemical analyses demonstrate that recombinant UGT74B1 specifically glucosylates the thiohydroximate functional group. Low Km values for phenylacetothiohydroximic acid (approximately 6 microm) and UDP-glucose (approximately 50 microm) strongly suggest that thiohydroximates are in vivo substrates of UGT74B1. Insertional loss-of-function ugt74b1 mutants exhibit significantly decreased, but not abolished, glucosinolate accumulation. In addition, ugt74b1 mutants display phenotypes reminiscent of auxin overproduction, such as epinastic cotyledons, elongated hypocotyls in light-grown plants, excess adventitious rooting and incomplete leaf vascularization. Indeed, during early plant development, mutant ugt74b1 seedlings accumulate nearly threefold more indole-3-acetic acid than the wild type. Other phenotypes, however, such as chlorosis along the leaf veins, are likely caused by thiohydroximate toxicity. Analysis of UGT74B1 promoter activity during plant development reveals expression patterns consistent with glucosinolate metabolism and induction by auxin treatment. The results are discussed in the context of known mutations in glucosinolate pathway genes and their effects on auxin homeostasis. Taken together, our work provides complementary in vitro and in vivo evidence for a primary role of UGT74B1 in glucosinolate biosynthesis.

A radioassay of enzymes catalyzing the glucosylation and sulfation steps of glucosinolate biosynthesis in Brassica species

A new method for assaying the enzymes uridine diphosphoglucose (UDPglucose):thiohydroximate glucosyltransferase and 3'-phosphoadenosine-5'-phosphosulfate:desulfoglucosinolate sulfotransferase has been designed. The assay system is based on the separation of nonionic [14C]desulfobenzylglucosinolate from anionic [14C]UDPglucose and anionic [14C]benzylglucosinolate, respectively, by differential adsorption to DEAE-ion-exchange disks. The procedure eliminates elaborate chromatographic techniques. The method was used to measure both enzymes in several Brassica spp. In addition, sulfotransferase activity was monitored during partial purification from seedlings of Brassica napus (cv Westar).