2.2. Oleanolic acid, β-amyrin, and sapogenin monomers content analysis in P. grandiflorus

Oleanolic acid, β-amyrin and three saponin monomers (platycoside E, platycodin D3, and platycodin D) standards were purchased from Chengdu Desite Biotechnology Co., Ltd. (Chengdu, China). Metabolites were extracted from 0.5 g of P. grandiflorus root treated with MeJA concentrations of 0, 10, 50, 100, or 200 μM by adding 1 ml of methanol, sonicating for 30 min, centrifuging at 12 000 rpm for 10 min at 4°C, and filtering through a 0.22-μm syringe membrane for High-Performance Liquid Chromatography (HPLC) analysis. The HPLC detection was performed on the Agilent Series1260 system (Agilent Technologies Inc., USA), with Techmate C18 column (250 × 4.6 mm, 5 μm). The mobile phase for oleanolic acid and β-amyrin was methanol (A)-water (B) (90:10 v/v), with detection wavelengths of 203 and 215 nm, respectively. The mobile phase for saponin monomers was acetonitrile (A)-water (B) (27:73 v/v), with a detection wavelength of 215 nm. The sample injection volume was 10 μl, the flow rate was 1 ml/min, column temperature was 30°C [51]. Each sample was performed with three biological replicates.