Chemical and Transcriptomic Analyses Provide New Insights into Key Genes for Ginsenoside Biosynthesis in the Rhizome of Panax japonicus C. A. Meyer

<i>Panax japonicus</i> C. A. Meyer is renowned for its significant therapeutic effects and is commonly used worldwide. Its active ingredients, triterpenoid saponins, show variation in content among different tissues. The tissue-specific distribution of saponins is potentially related to the expression of vital genes in the biosynthesis pathway. In this study, the contents of five saponins (ginsenoside Ro, chikusetsusaponin IV, chikusetsusaponin IVa, ginsenoside Rg1, and ginsenoside Rb1) in three different tissues were determined by HPLC. Transcriptome sequencing analysis identified differentially expressed genes (DEGs) involved in triterpenoid saponin biosynthesis, highlighting significant correlations between saponin contents and the expression levels of 10 cytochrome p450 monooxygenase (CYP) and 3 UDP-glycosyltransferase (UGT) genes. Cloning, sequencing, and prokaryotic expression of UGT genes confirmed the molecular weights of UGT proteins. Gene sequence alignment and phylogenetic analysis provided preliminary insights into UGT gene functions. Meanwhile, the function of one UGT gene was characterized in the yeast. These findings advance our understanding of the triterpenoid saponin biosynthesis in <i>P. japonicus</i> and support future research in traditional Chinese medicine (TCM) and synthetic biology.