The phytotoxic lipodepsipeptide syringopeptin 25A from Pseudomonas syringae pv syringae forms ion channels in sugar beet vacuoles

Syringopeptin 25A (SP25A) belongs to a family of cyclic lipodepsipeptides (LDPs) produced by the gram-negative bacterium Pseudomonas syringae, a phytopathogenic organism that affects several plants of agronomic interest. LDPs increase the permeability of plasma and, possibly, intracellular membranes in plant cells. Consistently, SP25A forms ion channels in planar lipid bilayers and other model membranes. Here we used sugar beet tonoplasts as a new biological model system to study toxin action. When applied to the vacuoles by a fast perfusion procedure, SP25A increases membrane permeability by forming discrete ion channels even at low applied potentials. The SP25A channel displays anion selectivity (with a Cl-/K+ permeability ratio of 6.7 ± 1.3) and has intrinsic rectification properties that derive from a different channel conductance at negative and positive voltages, presumably owing to an asymmetric distribution of fixed charges on the pore. Substitution of chloride with different anions reveals the following selectivity sequence NO3- ≈ Cl- >F- >gluconate-, suggesting that the permeation pore is filled with water. The properties of the SP25A channels in vacuolar membranes are similar to those observed in planar lipid membranes prepared with asolectin. This work provides a direct demonstration of toxin effects on a native plant membrane, extending to a biological system previous results obtained on artificial planar lipid membranes