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 a Panax ginseng squalene 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 ( PgSE1 and PgSE2 ) of P. ginseng were expressed in a yeast erg1 mutant (ergosterol auxotroph). The yeast mutant expressing PgSE1 or PgSE2 can restore growth on medium lacking ergosterol. Transgenic ginseng roots overexpressing the PgSE1 gene were constructed by Agrobacterium -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 β-sitosterol). qPCR analysis revealed that overexpression of PgSE1 in 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 in P. ginseng . This result indicates that the P. ginseng squalene epoxidase gene ( PgSE1 ) encodes an efficient enzyme responsible for not only phytosterol production but also ginsenoside production in P. ginseng .