Lipopeptide Phytotoxins Produced by Pseudomonas syringae pv. syringae: Comparison of the Biosurfactant and Ion Channel- Forming Activities of Syringopeptin and Syringomycin

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 poreforming
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.