Ginseng (Panax ginseng C.A. Meyer) has been used as an important oriental medicine since ancient times. Ginseng roots, one of the most famous and expensive crude drugs, have been commonly used to promote the quality of life. Cultivation of P. ginseng is difficult because of its long cultivation period of more than four years, its sun shading, absence of recurring cultivation and various diseases. Conventional breeding of P. ginseng is also difficult and impractical since the procedure takes more than 50 years. In view of these facts, biotechnological applications have been considered as an alternative approach for ginseng improvement and propagation and the production of raw materials for medicinal use.
The representative secondary compound accumulated in roots of ginseng species is ginsenoside, a triterpenoid saponin. Ginsenosides are considered to be the main bioactive compounds derived from the roots and rhizomes of different Panax species (Araliaceae). The enzyme squalene synthase catalyzes the first step from the central isoprenoid pathway towards sterol and triterpenoid biosynthesis. Both phytosterols and triterpenes in plants are synthesized from the product of cyclization of 2,3-oxidosqualene. The step in ginsenoside synthesis involves cyclization of 2,3-oxidosqualene to oleanane and a dammarene-type triterpene skeleton. Enzymes at the later step of ginsenoside biosynthesis are cytochrome P450s and glycosyltransferases.
The genes for biochemical pathways involved in saponin biosynthesis are of considerable interest in the area of ginseng biotechnology. We investigated the roles of squalene synthase (PgSS1) protein on the biosynthesis of phytosterols and triterpenoids. Over-expression of the PgSS1 gene in adventitious roots of transgenic P. ginseng resulted in the up-regulation of the downstream genes, such as squalene epoxidase, beta-amyrin synthase and cycloartenol synthase. Transgenic P. ginseng also exhibited a remarkable increase in the production of phytosterol (beta-sitosterol, stigmasterol, campesterol) and triterpene saponins (ginsenosides).
The first committed step in ginsenoside synthesis is the cyclization of 2,3-oxidosqualene to dammarenediol II by the oxidosqualene cyclase (dammarenediol synthase). The gene encoding dammarenediol synthase was characterized by our research group. We reported that ectopic expression of dammarenediol synthase gene (DDS) in yeast mutant (erg7) lacking lanosterol synthase resulted in the production of dammarenediol and hydroxydammarenone which were confirmed by LC/APCIMS. RNA interference (RNAi) of DDS in transgenic P. ginseng resulted in silencing of DDS expression which leads reduction of ginsenosides production to 84.5% in roots. Now we are focused on the characterization of cytochrome P450 for biosynthesis of protopanaxadiol and protopanaxatriol used as backbones for ginsenoside

• Yong Eui Choi(College of Forest and Environmental Sciences, Kangwon National University)