Abstract
The signalling pathways mediating defence responses induced in suspension cultures of ginseng,Panax ginsengC. A. Meyer, by the plant cell wall‐derived elicitor oligogalacturonic acid (OGA), were investigated. OGA induced the rapid generation and release of hydrogen peroxide, H2O2, potentially via an NADPH oxidase‐like enzyme. In addition, OGA elicited the accumulation of jasmonic acid (JA) and the defence compound saponin, and expression of genes encoding squalene synthase (SQS; EC 2.5.1.21) and squalene epoxidase (SQE; EC 1.14.99.7), key enzymes of saponin biosynthesis. H2O2and JA also induced saponin accumulation and transcription ofsqsandsqe. The accumulation of JA, but not that of H2O2, was inhibited by an inhibitor of JA biosynthesis, whereas the accumulation of both H2O2and JA were inhibited by treatments that removed H2O2or inhibited its production. Moreover, H2O2stimulated JA accumulation, but JA treatment had no effect on H2O2generation. These data place H2O2and JA as early signalling intermediaries in the cellular responses mediating saponin biosynthesis in response to OGA, and position H2O2upstream of JA. Inhibitors of calcium fluxes and protein phosphorylation inhibited all responses to some extent, indicating the requirements for a protein kinase and calcium in at least one step in the signal cascade leading to saponin accumulation.
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