Characterization of UDP-glycosyltransferase family members reveals how major flavonoid glycoside accumulates in the roots of Scutellaria baicalensis

Flavonoid glycosides extracted from roots of Scutellaria baicalensis exhibit strong pharmaceutical antitumor, antioxidative, anti-inflammatory, and antiviral activities. UDP glycosyltransferase (UGT) family members are responsible for the transfer of a glycosyl moiety from UDP sugars to a wide range of acceptor flavonoids. Baicalin is the major flavonoid glycoside found in S. baicalensis roots, and its aglycone baicalein is synthesized from a specially evolved pathway that has been elucidated. However, it is necessary to carry out a genome-wide study of genes involved in 7-O-glucuronidation, the final biosynthesis step of baicalin, which might elucidate the relationship between the enzymes and the metabolic accumulation patterns in this medicinal plant.

This study demonstrated that glucosylation and glucuronidation are two major glycosylated decorations in the roots of S. baicalensis. Higher expression level and affinity to substrate of SbUGAT4, and expansion of this gene family contribute high accumulation of baicalin in the root of S. baicalensis.

We reported the phylogenetic analysis, tissue-specific expression, biochemical characterization and evolutionary analysis of glucosyltransferases (SbUGTs) and glucuronosyltransferases (SbUGATs) genes based on the recently released genome of S. baicalensis. A total of 124 UGTs were identified, and over one third of them were highly expressed in roots. In vitro enzyme assays showed that 6 SbUGTs could use UDP-glucose as a sugar donor and convert baicalein to oroxin A (baicalein 7-O-glucoside), while 4 SbUGATs used only UDP-glucuronic acid as the sugar donor and catalyzed baicalein to baicalin. SbUGAT4 and SbUGT2 are the most highly expressed SbUGAT and SbUGT genes in root tissues, respectively. Kinetic measurements revealed that SbUGAT4 had a lower Km value and higher Vmax/Km ratio to baicalein than those of SbUGT2. Furthermore, tandem duplication events were detected in SbUGTs and SbUGATs. Conclusions: This study demonstrated that glucosylation and glucuronidation are two major glycosylated decorations in the roots of S. baicalensis. Higher expression level and affinity to substrate of SbUGAT4, and expansion of this gene family contribute high accumulation of baicalin in the root of S. baicalensis.