ROLE OF BIOSURFACTANT AND ION CHANNEL-FORMING ACTIVITIES OF SYRINGOMYCIN IN TRANSMEMBRANE ION FLUX - A MODEL FOR THE MECHANISM OF ACTION INTHE PLANT-PATHOGEN INTERACTION

Syringomycin is a necrosis-inducing lipopeptide toxin synthesized and secreted by the phytopathogen, Pseudomonas syringae pv, syringae, Alth ough small quantities of syringomycin are known to activate a cascade of physiological events in plasma membranes, the mechanism of action o f 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 c ations. Accordingly, direct measurement of ion fluxes were performed u sing artificial bilayers. The hemolytic properties and surface activit y of HPLC-purified syringomycin were quantified by use of an erythrocy te lysis assay and by the drop weight method, Assays were performed us ing syringomycin form SRE alone or a mixture containing all forms of t he phytotoxin, At a threshold concentration of 500 ng/ml, syringomycin induced hemolysis by forming ion channels in membranes, Osmotic prote ction studies indicated a channel radius of between 0.6 and 1 nm, The ion channel-forming activity was insensitive and permeable to both mon ovalent and divalent cations, suggesting that syringomycin causes lysi s 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/m l and the gamma(CMC) was 33 mN/m. A model is presented depicting the m echanism-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.