Nitric oxide mediates elicitor-induced saponin synthesis in cell cultures of Panax ginseng

Abstract

The elicitor oligogalacturonic acid (OGA) stimulated nitric oxide (NO) accumulation in the growth medium of ginseng suspension cultures and induced increased nitric oxide synthase (NOS) activity in ginseng cells. OGA also stimulated accumulation of saponin, transcription of genes encoding squalene synthase (sqs) and squalene epoxidase (sqe), two early enzymes of saponin synthesis, and the accumulation of β-amyrin synthase protein (β-AS). Saponin accumulation, sqs and sqe gene expression, and increases in β-AS content were also induced by exposure to NO via the NO donor sodium nitroprusside (SNP). Inhibitors of mammalian nitric oxide synthase reduced both OGA-induced NO accumulation and NOS activity, suggesting that OGA-induced NO production occurs via a NOS-like enzyme. OGA-induced accumulation of β-AS and saponin, and transcription of sqs and sqe, were suppressed by treatments that removed NO or inhibited its production, indicating a role for NO in mediating OGA effects on these defence responses. NO accumulation and increased NOS activity were inhibited by calcium channel inhibitors and a protein kinase inhibitor, but not by a protein phosphatase inhibitor, indicating the requirement for calcium and protein phosphorylation during OGA-induced NO production. Saponin production and transcription, and accumulation of saponin biosynthetic genes and enzymes were also suppressed by these treatments, as well as by the protein phosphatase inhibitor okadaic acid.