Correlation of cultivation time of Panax ginseng with metabolic profiles of nine ginsenosides and mRNA expression of genes encoding major biosynthetic enzymes

Ginseng (Panax ginseng C. A. Meyer) is one of the most important Chinese medicinal herbs. This medicinal plant produces numerous bioactive substances, including triterpene saponins of the ginsenoside-type. The endogenous levels of all types of ginsenosides largely determine the therapeutic quality of P. ginseng. Here, a field investigation was conducted to compare the changes in ginsenoside composition and mRNA expression levels of some key genes in the biosynthetic pathway of P. ginseng seedlings cultivated for different times (1, 2, 3 or 4 years). After establishing a high performance liquid chromatography method to simultaneously determine nine different ginsenosides, we measured the variations of ginsenoside contents in P. ginseng seedlings collected from our field base in Jilin province, China. The levels of total ginsenosides obviously increased with longer cultivation times and reached peak values in the 4-year seedlings. In light of the changes of ginsenosides composition, protopanaxatriol (PPT)-type ginsenosides displayed a synchronized change with total ones, while the peak point of protopanaxadiol (PPD)-type ginsenoside contents appeared in the 3-year plants. Transcript analyses of squalene synthase (SS), squalene epoxidase (SQE1) and dammarenediol synthase (DS), which are the rate-limiting enzymes in the ginsenoside biosynthetic pathway, showed that the gene expression levels of PgSS and PgSQE1 were highest in the 2-year seedlings, and could indirectly control saponins. While the PgDS gene was highly expressed in 4-year plants, and another important gene pleiotropic drug resistance (PgPDR) has a consistent trend. The further correlation analysis between gene expressions and metabolic changes showed that PgDS transcript level significantly correlated with PPT-type ginsenosides, Rg1, Re, and Rf. PgPDR expression levels positively correlated with total ginsenoside, Rg1, Re, and Rb1 (p < 0.01) and Rg3 negatively correlated with PgDS and PgPDR genes expression (p < 0.01). However, the PgSS gene expression was negatively correlated with Rb1 (p < 0.05). In conclusion, we propose that PPT-type ginsenoside and PPD or dammarenediol-II compounds have a close relationship. Particularly, we highlight recent reports on functional characterization of key genes dedicated to the production of ginsenosides in P. ginseng seedling with cultivated time.