Department of Plant Pathology, Washington State University, Pullman 99164-6430, U.S.A.
The phytopathogenic bacterium Pseudomonas syringae pv. syringae produces two classes of necrosis-inducing lipodepsipeptide toxins commonly referred to as the syringomycins and syringopeptins. Members of the syringomycin class are pore-forming cytotoxins that act by promoting passive transmembrane ion flux. In this study, we test the hypothesis that syringopeptin forms SP22A and SP22B likewise function as pore-forming cytotoxins and are similar in activity to syringomycin in artificial and plant membranes. Correspondingly, syringopeptin increased the conductance of black-lipid membranes in a manner indicative of ion channel formation. In tobacco protoplast assays, syringopeptin forms SP22A and SP22B were equivalent in activity causing lysis of protoplasts and measurable 45Ca2+ influx at a threshold concentration of 50 ng/ml. A mixture of three forms of syringomycin did not show cytotoxic activity appreciably different from that of SP22A or SP22B in tobacco protoplast assays. Both forms of syringopeptin also displayed potent biosurfactant properties demonstrated by lowering of the interfacial tension of high-pressure liquid chromatography-grade water to 36 and 34.5 nm/m, respectively; the critical micellar concentration was 0.8 mg/ml for both forms of toxin. These results demonstrate that both classes of pore-forming lipodepsipeptides secreted by P. syringae pv. syringae are cytotoxic to plant cells at nanomolar concentrations and cause necrosis by forming ion channels that are freely permeable to divalent cations.