Methyl jasmonate induces the biosynthesis and accumulation of triterpenoid saponins in Polygala tenuifolia hairy roots.

Polygala tenuifolia Willd. is an important medicinal plant widely applied in traditional Chinese medicine. Its pharmacological activities are primarily derived from triterpenoid saponins and other specialized metabolites. Nevertheless, the regulatory mechanisms governing methyl jasmonate (MeJA)-triggered accumulation of bioactive constituents in P. tenuifolia remain poorly characterized. In this study, to evaluate corresponding changes in biomass and characteristic metabolite production, hairy root cultures of P. tenuifolia were subjected to MeJA treatment at concentrations spanning 0 to 400µM over a 1-6 day period. MeJA modulated hairy root growth and secondary metabolite accumulation in a concentration-dependent and time-dependent manner. Compared with the control group, the contents of onjisaponin B and polygalasaponin III increased by 1.81-fold and 2.60-fold, respectively, following treatment with 50µM MeJA for 3 days. A total of 447 differentially expressed transcription factors (TFs) that belonged to 64 families were identified through transcriptome analysis. MeJA suppressed the expression of most TF families, whereas the NAC family exhibited a global upregulation trend. The above results indicated that MeJA activated transcriptional reprogramming of secondary-metabolism-associated genes via targeted regulation of specific TFs. Correlation analysis involving ten key enzyme genes and six characteristic metabolites demonstrated that the majority of biosynthetic genes were positively associated with bioactive metabolite levels. Of these genes, UGT74B1 and UGT73B2 showed strong positive correlations with multiple saponin components; CYP94A2 served as a critical node responsible for oxidative modification. In contrast, MK and HDS displayed negative correlations with saponins, which suggested precursor competition across distinct terpenoid pathways. As indicated by RT-qPCR validation for ten candidate biosynthetic genes, MeJA time-dependently upregulated key genes participating in the MVA/MEP pathways, squalene cyclization, CYP450-mediated oxidation, and UGT-catalyzed glycosylation, thereby facilitating precursor supply and subsequent structural modification. Accordingly, triterpenoid saponin biosynthesis and accumulation in P. tenuifolia hairy roots were enhanced. This study offers fresh understanding of the molecular basis underlying MeJA-modulated secondary metabolism in P. tenuifolia, and identifies candidate genes applicable to metabolic engineering for elevating yields of bioactive compounds in hairy root cultures.