SummaryGlucosinolates 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 μm) and UDP‐glucose (approximately 50 μm) 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.
摘要葡萄糖苷酸盐是一类在植物防御和人类营养中具有重要作用的次级代谢产物。在这里,我们描述了一种推定的 UDP-葡萄糖:硫代羟基 S-葡萄糖基转移酶 UGT74B1 的特性,以确定其在拟南芥葡萄糖苷酸途径中的作用。生化分析表明,重组的 UGT74B1 能特异性地对硫代羟基功能基团进行葡萄糖基化。苯乙酰硫代羟肟酸(约 6 μm)和 UDP-葡萄糖(约 50 μm)的 Km 值较低,这有力地表明硫代羟肟酸是 UGT74B1 的体内底物。 插入功能缺失的 ugt74b1 突变体显示出葡萄糖苷酸积累的显著下降,但并未消失。此外,ugt74b1 突变体还表现出类似于辅助素过量产生的表型,如附生子叶、光照生长植株下胚轴变长、不定根过多以及叶片维管束化不完全。事实上,在植物早期发育过程中,突变体 ugt74b1 幼苗积累的吲哚-3-乙酸几乎是野生型的三倍。然而,其他表型,如叶脉萎黄,很可能是由硫代羟肟酸毒性引起的。对植物发育过程中 UGT74B1 启动子活性的分析表明,其表达模式与葡萄糖苷酸代谢和辅助素处理的诱导相一致。我们结合葡萄糖苷酸通路基因的已知突变及其对植物生长素平衡的影响对这些结果进行了讨论。总之,我们的工作为 UGT74B1 在葡萄糖苷酸生物合成中的主要作用提供了互补的体内外证据。