Platycodon grandiflorus subjected to full- and restricted-water regimes show differential biosynthesis of triterpenoid saponins

Platycodon grandiflorus (Jacq.) A.DC. (PG) is a traditional herb, and triterpenoid saponins are the main bioactive components of PG, including platycodin D, deapioplatycodin D, and platycodin D3. Our study aimed to explore the biosynthesis of triterpenoid saponins in responses to water regulation. PG samples were collected at different stages under full water (FW) and restricted-water (RW) conditions. Soil moisture, root weight, osmoregulatory substances including soluble sugar, soluble protein, and proline, as well as malondialdehyde were measured. Activities of superoxide dismutase, peroxidase, and catalase were detected. Expressions of key genes involved in the platycodin metabolism pathway and platycodin contents were determined, followed by correlation analysis. Significantly lower soil moisture as well as increased root dry matter content and malondialdehyde were observed in RW group. Soluble protein and proline were significantly higher in the RW group at the day 30, and 45, and higher levels of proline were also found at day 60, and 75. Soluble sugar content was rapidly increased in the RW group at day 30 and 60. Not only higher activities of peroxidase, catalase, and superoxide dismutase, but also high expressions of AACT , HMGR and HMGS were found in the late water regulation in RW group. Platycodin D content was rapidly increased in both groups, and the RW group had significantly higher platycodin D content. Linear mixed effects modeling analysis based on restricted maximum likelihood estimation revealed that proline, malondialdehyde, HMGR and platycodin D were negatively correlated with soil moisture. Platycodin D and deapioplatycodin D were positively correlated with AACT , HMGR , HMGS , SS , and UGT-1 , and negatively correlated with β-AS . Platycodin D3 was positively correlated with IPPI , MVD and SS , while negatively correlated with β-AS . Our results presented here provide insights into the effects of water regulation on the biosynthesis of triterpenoid saponins in PG.