Volatile Products of the Lipoxygenase Pathway Evolved from Phaseolus vulgaris (L.) Leaves Inoculated with Pseudomonas syringae pv phaseolicola.

Activation of the "lipoxygenase pathway" in plants gives rise to a series of products derived from fatty acids. Analysis by gas chromatography-mass spectroscopy of volatile products produced by Phaseolus vulgaris (L.) cv Red Mexican leaves during a hypersensitive resistance response (HR) to the plant pathogenic bacterium Pseudomonas syringae pv phaseolicola showed evolution of several lipid-derived volatiles, including cis-3-hexenol and trans-2-hexenal, which arise from the 13-hydroperoxide of linolenic acid. These compounds were not produced in detectable amounts by buffer-inoculated leaves, nor did they evolve to such a high degree during comparable stages of the susceptible response. The absence of trans-2,cis-6-nonadienal, a product expected from 9-hydroperoxide of linolenic acid, suggests that lipid peroxidation during the HR proceeded primarily enzymically via bean lipoxygenase, which produces the 13-hydroperoxide, and not via autoxidative processes. The effects of trans-2-hexenal, cis-3-hexenol, and traumatic acid on P.s pv phaseolicola were investigaed. trans-2-Hexenal appeared to be highly bactericidal at low concentrations, whereas cis-3-hexenol was bactericidal only at much higher concentrations. Traumatic acid appeared to have no effect on P.s. pv. phaseolicola at the concentrations tested. These results demonstrate that during plant defense responses against microbial attack, several lipid-derived compounds are produced by the plant, some of which possess antimicrobial activity and conceivably are involved in plant disease resistance. The time of production of these substances, in amounts that would be expected to be antibacterial in vitro, correlated with a slowing down of the growth rate of bacteria in the leaves and was seen at a time before the accumulation of isoflavonoid phytoalexins in the host.