Impaired terpenoid backbone biosynthesis reduces saponin accumulation in Panax notoginseng under Cd stress.

Panax notoginseng saponins (PNS) are major secondary metabolite of Panax notoginseng (Burk.) F.H. Chen. Previous studies identified that P. notoginseng planting soil usually with high content of Cd. However, the effects of Cd stress on the accumulation of PNS and the corresponding regulation mechanisms have yet to be reported. In the present study, the impact of Cd stress on the PNS accumulation of P. notoginseng was studied in pot culture experiments. The effect of Cd stress on antioxidant enzyme activity was studied using hydroponics. In addition, transcriptase sequencing analysis was used to study the effect of Cd stress on the expression of PNS metabolism transcripts in hydroponic experiments. Cd treatments significantly decreased the accumulation of PNS in the rhizome and main root. The sensitive concentration of antioxidant enzyme activity for both leaf and stem was 2.5µM, whereas the sensitive concentration for the root was 5.0µM. Transcriptome analysis showed that 5132 genes (2930 up- and 2202 downregulated) were regulated by 5.0µM Cd stress in the root of P. notoginseng. Among them, six upregulated differentially expressed genes (DEGs) were related to the methylerythritol 4-phosphate (MEP) pathway, whereas three of the downregulated DEGs were mevalonate kinase (MVK), phosphomevalonate kinase (PMK), and geranylgeranyl diphosphate synthase (type II, GGPS). Of the 15 transcripts selected for real-time quantitative-PCR, 13 were expressed in the same manner as identified using RNA-seq. In conclusion, Cd stress inhibited the accumulation of PNS in the root of P. notoginseng by reducing the expression of MVK, PMK, and GGPS in the terpenoid backbone biosynthesis pathway, and also caused by the removal of reactive oxygen species.