Syringomycin is a
necrosis-inducing
lipopeptide toxin synthesized and secreted by the phytopathogen, Pseudomonas syringae pv. syringae. Although small quantities of
syringomycin are known to activate a cascade of physiological events in plasma membranes, the mechanism of action of the phytotoxin has never been fully characterized. The objective of this study was to test the hypothesis that the primary mode of action of
syringomycin is to form transmembrane pores that are permeable to
cations. Accordingly, direct measurement of ion fluxes were performed using artificial bilayers. The hemolytic properties and surface activity of HPLC-purified
syringomycin were quantified by use of an erythrocyte lysis assay and by the drop weight method. Assays were performed using
syringomycin form SRE alone or a mixture containing all forms of the phytotoxin. At a threshold concentration of 500 ng/ml,
syringomycin induced
hemolysis by forming
ion channels in membranes. Osmotic protection studies indicated a channel radius of between 0.6 and 1 nm. The
ion channel-forming activity was insensitive and permeable to both monovalent and
divalent cations, suggesting that
syringomycin causes lysis of erythrocytes by
colloid osmotic lysis. In addition,
syringomycin, like other
lipopeptide antibiotics, is a potent biosurfactant capable of lowering the interfacial tension of water to 31 mN/m. The critical micellar concentration of
syringomycin was calculated to be 1.25 mg/ml and the gamma CMC was 33 mN/m. A model is presented depicting the mechanism of action of
syringomycin in the plant-pathogen interaction. The model integrates known effects of the toxin on ion flux in plasma membranes with formation of
ion channels and the consequential cascade of effects associated with cellular signalling.