Effect of light intensity on the accumulation of protostane triterpenes in Asian water plantain (Alisma orientale)

Alismatis rhizoma is an essential medicine in clinical practice. Asian water plantain (Alisma orientale (Sam.) Juzep) is one of the original plants of Alismatis rhizoma. Previous studies have identified the optimal light intensity range for the growth and development of A. orientale, but the mechanism by which light intensity affects the accumulation of secondary metabolites of A. orientale is unknown. The aim of this study was to investigate the effect of light intensity on the accumulation of triterpenoids in A. orientale saplings and its potential molecular mechanism. The dry weight and contents of total triterpenes and indicative components (alisol B 23-acetate and alisol C 23-acetate) as well as the expression of key enzyme genes in the triterpene biosynthesis pathway under different light intensities (50–600 μmol m⁻²·s⁻¹) were determined. The results showed that the accumulation of dry matter and the contents of total triterpenes, alisol B 23-acetate, and alisol C 23-acetate increase first and then decrease with increasing light intensity, with the maximum values of 31.65 g, 18.35 mg·g⁻¹, 1.91 mg·g⁻¹, and 0.13 mg·g⁻¹ recorded at light intensities of 400, 200, and both of 300 μmol m⁻²·s⁻¹, respectively. Light intensities of 200–400 μmol m⁻²·s⁻¹ promote the expressions of key enzyme genes and the accumulation of total triterpenes significantly. Correlation analysis showed that the expression levels of key enzyme genes are significantly correlated with total triterpene and indicative component contents, and these correlations are strongest under moderate light intensities. Overall, our results reveal that a moderate light intensity of 200–400 μmol m⁻²·s⁻¹ is beneficial for the growth and synthesis of protostane triterpenes in A. orientale seedlings, and that its probable mechanism involves the upregulated expression of enzymes that are key in the synthesis of triterpenoid ingredients. This study clarified the suitable light intensity range for the synthesis and accumulation of protostane triterpenes of A. orientale, which provided scientific basis for the production of high-quality superior forms of A. orientale.