Abscisic acid enhanced ajmalicine biosynthesis in hairy roots of Rauvolfia verticillata by upregulating expression of the MEP pathway genes

Abstract

Hairy root cultures of Rauvolfia verticillata were established and used to analyze the expression of the eight ajmalicine-pathway genes (the five MEP pathway genes, including DXR, MCT, MECS, HDS, and HDR}; the three ajmalicine-pathway-specific genes, including TDC, STR, and SGD) and ajmalicine accumulation in the cultures treated with ABA. In the ABA treatment, the five MEP-pathway genes were simultaneously up-regulated at the transcriptional level, while the three ajmalicine-pathway-specific genes, including TDC, STR, and SGD were not up-regulated. The statistical analysis showed that the MEP pathway gene expression levels in the ABA treatment were significantly different from those in the control. TDC showed no response to ABA treatment, and SGD expression decreased slightly. Surprisingly, the well-known rate-limiting enzyme gene STR showed dramatically decreased expression in the ABA treatment. The HPLC analysis showed that the content of ajmalicine in the ABA-treated hairy roots of R. verticillata (196.9 ± 3.6 μg/g dry wt) was significantly higher than that in the control (171.0 ± 7.6 μg/g dry wt). This suggests that the higher expression levels of the MEP pathway genes could enhance ajmalicine biosynthesis and the MEP pathway plays an important role in ajmalicine biosynthesis in R. verticillata. Finally, STR might not be the rate-limiting enzyme involved in ajmalicine biosynthesis of R. verticillata.