The initiation of membrane lipid peroxidation during bacteria-induced hypersensitive reaction

Abstract

The initiation of lipid peroxidation reported previously may be initiated by free radicals or lipoxygenase. Levels of the free radicals can be affected by activity of protective enzymes (such as catalase and Superoxide dismutase), levels of antioxidants and rate of free radical production. We followed and compared the activity of lipoxygenase, catalase and superoxide dismutase in cucumber cotyledons infiltrated with: (1) Pseudomonas syringae pv. pisi (incompatible pathogen); (2) Pseudomonas syringae pv. lachrymans (compatible pathogen); and (3) Pseudomonas fluorescens (saprophyte). The only difference in enzyme activity was an increase in lipoxygenase activity in cotyledons infiltrated with Pseudomonas syringae pv. pisi. To assess the possibility of free radical initiation of lipid peroxidation we infiltrated cotyledons with P. syringae pv. pisi plus free radical scavengers (superoxide dismutase, catalase, a-tocopherol-acetate and dipyridamole) and followed membrane alteration (assessed by electrolyte leakage and fluorescein accumulation). Also, these results were compared with the results of experiments in which the free radical scavengers and paraquat (which is proposed to initiate lipid peroxidation via the production of superoxide radicals) were infiltrated together. We observed a significant reduction of membrane alteration induced by P. syringae pv. pisi or paraquat by addition of superoxide dismutase. We suggest that lipid peroxidation mediated membrane alteration induced by P. syringae pv. pisi is initiated by free radicals and the superoxide radical is a likely candidate. We propose that the increase in lipoxygenase activity may be a response to fatty acids released by free radical attack of membrane lipids during lipid peroxidation.