Key enzymes of triterpenoid saponin biosynthesis and the induction of their activities and gene expressions in plants.

Triterpenoid saponins are one of the key active components of many medicinal plants. The biosynthetic pathway of triterpenoid saponins in higher plants and a lot of experimental results both indicated that the key enzymes involved in triterpenoid saponin synthesis are squalene synthase (SS), squalene epoxidase (SE), lupeol synthase (LS), dammarenediol synthase (DS), beta-amyrin synthase (beta-AS), cytochrome P450-dependent monooxygenase (PDMO), and glycosyltransferase (GT). The activities and coding genes of the key enzymes could be induced by a range of factors in various plant species. However, the effects of the factors on the content and composition of the triterpenoid saponins in specific plants are not certainly coincident, and different factors appear to induce the gene expressions of the key enzymes by different signal pathways and at different levels. This paper could provide a reference for strengthening the triterpenoid saponin-synthesizing capability of specific medicinal plants at enzyme and/or gene expression levels in order to improve the plants' commercial values.