In the present work we have studied the accumulation of
gentisic acid (2,5-
dihydroxybenzoic acid, a metabolic derivative of
salicylic acid, SA) in the plant-pathogen systems, Cucumis sativus and Gynura aurantiaca, infected with either prunus necrotic ringspot virus (PNRSV) or the exocortis viroid (CEVd), respectively. Both pathogens produced systemic
infections and accumulated large amounts of the intermediary signal molecule
gentisic acid as ascertained by electrospray ionization mass spectrometry (ESI-MS) coupled on line with high performance liquid chromatography (HPLC). The compound was found mostly in a conjugated (beta-
glucoside) form.
Gentisic acid has also been found to accumulate (although at lower levels) in cucumber inoculated with low doses of Pseudomonas syringae pv. tomato, producing a nonnecrotic reaction. In contrast, when cucumber was inoculated with high doses of this pathogen, a hypersensitive reaction occurred, but no
gentisic-acid signal was induced. This is consistent with our results supporting the idea that
gentisic-acid signaling may be restricted to nonnecrotizing reactions of the host plant (Bellés et al. in Mol Plant-Microbe Interact 12:227-235, 1999). In cucumber and Gynura plants, the activity of
gentisic acid as inducing signal was different to that of SA, thus confirming the data found for tomato. Exogenously supplied
gentisic acid was able to induce
peroxidase activity in both Gynura and cucumber plants in a similar way as SA or pathogens. However,
gentisic-acid treatments strongly induced
polyphenol oxidase activity in cucumber, whereas pathogen
infection or SA treatment resulted in a lower induction of this
enzyme. Nevertheless,
gentisic acid did not induce other defensive
proteins which are induced by SA in these plants. This indicates that
gentisic acid could act as an additional signal to SA for the activation of plant defenses in cucumber and Gynura plants.