Fungal elicitor induces singlet oxygen generation, ethylene release and saponin synthesis in cultured cells of Panax ginseng C. A. Meyer

Singlet oxygen is a high-energy molecular oxygen species. As one of the most active intermediates involved in chemical and biochemical reactions, singlet oxygen plays essential roles in plant responses to UV and strong light. Here, we report that Cle, an elicitor derived from fungal cell walls, induces the generation of singlet oxygen in cell cultures of ginseng, Panax ginseng. Cle treatment also triggers the activation of plasma membrane NADPH oxidase and 1-aminocyclopropane-1-carboxylic acid oxidase (ACO), subsequently leading to ethylene release and increased saponin synthesis, as shown by increased mRNA expression of squalene synthase (SQS) and squalene epoxidase (SQE), and accumulation of beta-amyrin synthase (beta-AS). Suppression of Cle-induced singlet oxygen generation or inhibition of ethylene production blocks saponin synthesis, whereas treatment of ginseng cells with ethylene or singlet oxygen induces the synthesis of saponin. Together, these results indicate that Cle-induced production of both singlet oxygen and ethylene is required for saponin synthesis, and that singlet oxygen may function upstream of ethylene during Cle-induced saponin synthesis.