Regulation of Ginsenoside and Phytosterol Biosynthesis by RNA Interferences of Squalene Epoxidase Gene in Panax Ginseng

Squalene epoxidase catalyzes the first oxygenation step in phytosterol and triterpenoid saponin biosynthesis and is suggested to represent one of the rate-limiting enzymes in this pathway. Here, we investigated the roles of two squalene epoxidase genes (PgSQE1 and PgSQE2) in triterpene and phytosterol biosynthesis in Panax ginseng. PgSQE1 and PgSQE2 encoded deduced proteins of 537 and 545 amino acids, respectively. Amino acid sequences deduced from PgSQE1 and PgSQE2 share 83% homology, but the N-terminal regions (first 60 amino acids) are highly different. PgSQE1 mRNA abundantly accumulated in all organs. PgSQE2 was only weakly expressed and preferentially in petioles and flower buds. Methyl jasmonate (MeJA) treatment enhanced the accumulation of PgSQE1 mRNA in roots, but rather suppressed expression of PgSQE2. Precursor (squalene) treatment coordinately upregulated the expression of both PgSQE1 and PgSQE2. In situ hybridization analysis established that both PgSQE1 and PgSQE2 mRNAs accumulated preferentially in vascular bundle tissue and resin ducts of petioles. RNA interference of PgSQE1 in transgenic P. ginseng completely suppressed PgSQE1 transcription. Concomitantly, the interference of PgSQE1 resulted in reduction of ginsenoside production. Interestingly, silencing of PgSQE1 in RNAi roots strongly upregulated PgSQE2 and PNX (cycloartenol synthase) and resulted in enhanced phytosterol accumulation. These results indicate that expression of PgSQE1 and PgSQE2 were regulated in a different manner, and that PgSQE1 will regulate ginsenoside biosynthesis, but not that of phytosterols in P. ginseng.