Overexpression of the squalene epoxidase gene (PgSE1) resulted in enhanced production of ginsenosides and phytosterols in transgenic ginseng

Squalene epoxidase (also called squalene monooxygenase) catalyses the conversion of squalene into 2,3-oxidosqualene by epoxidation and is regarded as the rate-limiting enzyme for sterol and saponin biosynthesis. However, the role of the squalene epoxidase gene in saponin biosynthesis in plants is not yet well understood. Here, we investigated the effects of overexpression of aPanax ginsengsqualene epoxidase gene (PgSE1) on the production of phytosterols and ginsenoside saponins in ginseng adventitious roots. For the functional complementation test, the two squalene epoxidase sequences (PgSE1andPgSE2) ofP. ginsengwere expressed in a yeasterg1 mutant (ergosterol auxotroph). The yeast mutant expressingPgSE1orPgSE2can restore growth on medium lacking ergosterol. Transgenic ginseng roots overexpressing thePgSE1gene were constructed byAgrobacterium-mediated genetic transformation. The transgenic ginseng roots resulted in the enhanced production of both ginsenosides (ginsenoside Rg1, Re, Rf, Rc, Rb1, Rb2, and Rd) and phytosterols (campesterol, stigmasterol, and beta-sitosterol). qPCR analysis revealed that overexpression ofPgSE1in transgenic ginseng roots clearly enhanced the expression of dammarenediol-II synthase (PgDDS) and cycloartenol synthase (PgPNX), which are key enzymes for ginsenoside and phytosterol biosynthesis inP. ginseng. This result indicates that theP. ginsengsqualene epoxidase gene (PgSE1) encodes an efficient enzyme responsible for not only phytosterol production but also ginsenoside production inP. ginseng.