Functional Characterization of β-Glucuronidase Genes Involved in Baicalein Biosynthesis from Scutellaria baicalensis Based on Transcriptome Analysis

Baicalein is a unique flavonoid compound with important pharmacological activities, derived from Scutellaria baicalensis Georgi. Baicalein, as the aglycone of baicalin, is a key form for exerting pharmacological activity in vivo. β-glucuronidases (GUSs) are the enzymes involved in the conversion of baicalin to baicalein. In this study, the content of baicalein in S. baicalensis was significantly increased by 20.44% after treatment with 5% PEG6000. Seven GUSs from the glycoside hydrolase 79 family were identified through comparative transcriptome analysis. Among them, GUS1 and GUS2 were confirmed to have catalytic activity in converting baicalin to baicalein in prokaryotic and eukaryotic systems. The correlation analysis further revealed a significant positive correlation of 0.962 (p < 0.01) between the expression of GUS2 and baicalein content in six different sources of S. baicalensis. Interestingly, the presence of variable sites in the GUS1 and GUS2 genes significantly affected their catalytic efficiency in the S. baicalensis samples from the six geographic origins. These findings also provide valuable GUS biological enzyme resources for the effective synthesis of baicalein and offer new insights into the accumulation pattern of baicalein in S. baicalensis.