Involvement of Membrane Lipid Peroxidation in the Development of a Bacterially Induced Hypersensitive Reaction

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Abstract
Membrane potential depolarization and increases in electrolyte leakage were observed during the development of a hypersensitive reaction in cucumber [Cucumis sativus] cotyledons infiltrated with the incompatible bacterial pathogen Pseudomonas syringae pv. pisi (P. s. pisi). These changes suggested an alteration of cellular membranes. Separation of the two membrane potential components suggested that both the passive component and the energy-dependent component were depolarized initially (after 2 and 3 hr of incubation) but that only the passive component showed continued depolarization throughout the 6-hr incubation period. We tested the possibility of lipid peroxidation as a mechanism by which the passive component of membrane potential was affected during a hypersensitive reaction. Lipid peroxidation and electrolyte leakage were determined and compared in cotyledons treated with the following live or heat-killed bacteria: P. s. pisi (incompatible pathogen), P. syringae pv. lachrymans (compatible pathogen), and P. fluorescens (a saprophyte). Lipid peroxidation and electrolyte leakage increased significantly only in the combination producing a hypersensitive reaction. In cotyledons inoculated with the incompatible pathogen, lipid peroxidation increased concomitantly with membrane depolarization and before electrolyte leakage. Therefore, we suggest that lipid peroxidation is a key step in the membrane alteration that produces hypersensitive reaction symptoms (electrolyte leakage and tissue collapse).