Exogenous abscisic acid reduces saikosaponin accumulation by inhibiting saikosaponin synthesis pathway gene expression under drought stress in Bupleurum chinense DC

Abscisic acid (ABA) plays an important role in stress resistance and secondary metabolism of plants under drought stress. Bupleurum chinense DC. is a medicinal plant cultivated in arid areas in China. In this study, the effects of progressive drought stress (22 days) and exogenous ABA (5 mg L–¹) treatment on plant physiological characteristics and saikosaponin secondary metabolism were investigated in 2-year-old B. chinense. The samples were collected on 6, 10, 14, 18 and 22 days after drought stress (DADS). In drought stressed B. chinense plants treated with ABA, root weight loss and malondialdehyde accumulation was reduced, while drought resistance increased, as indicated by the accumulation of soluble protein and soluble sugar content, and by the increased activities of superoxide dismutase and peroxidase. At the early stage of drought stress (first 6 days), saikosaponin increased more sharply in plants exposed to drought stress and its accumulation indicated increased sensitivity to this condition. At this time, saikosaponin content reached 31.83 mg g⁻¹, which was 33.6 % higher than that in control plants. Using principal component analysis and fuzzy membership function calculation, comprehensive evaluation values of gene expression in the drought (D) and D + ABA treated groups at different drought exposure time were evaluated. Exogenous ABA inhibited the expression of saikosaponin biosynthesis-related genes from 6 to 14 DADS of drought stress. However, gene expression was up-regulated at later stages of drought stress (the 22 DADS), while genes involved in saikosaponin content were down-regulated in the D group compared with those in the D + ABA group. Indeed, saikosaponin content in both of these groups was significantly higher than that in the control group. Correlation analysis suggested that the expression of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) gene was positively correlated with total saikosaponin (SSs) (p < 0.01), and saikosaponin a (SS-a) and saikosaponin d (SS-d) (p < 0.05) content, while that of farnesyl pyrophosphate synthase (FPS) gene was positively correlated with SSs (p < 0.05), and SS-a and SS-d (p < 0.01) content in the D group. These results indicated that HMGR and FPS play a key role in saikosaponin biosynthesis during drought stress. Moreover, the negative correlations between multiple genes and saikosaponin after ABA treatment [particularly that of cytochrome P450−7 with SS-d (p < 0.01) and glycosyl transferase-8 with SS-d (p < 0.05)], indicated that exogenous ABA reduced saikosaponin accumulation by inhibiting the expression of saikosaponin biosynthesis-related genes in B. chinense exposed to drought stress. This study provides new insight into the effect of ABA on secondary metabolism and it will be of great importance to cultivators and researchers aiming to understand how ABA application affects medicinal plant cultivation.