Effects of Host-Selective SV-Toxin from Stemphylium vesicarium, the Cause of Brown Spot of European Pear Plants, on Ultrastructure of Leaf Cells

Brown spot of European pear "Pyrus communis L[# caused by Stemphylium vesicarium "Wallr[# E[ Simmons is a disease of economic importance in fruit!growing areas of southern Europe[ The pathogen produces two host! speci_c SV!toxins "SV!toxin I and II# in culture _ltrateswhich selectively induce veinal necrosis only on sus! ceptible cultivars[ Ultrastructural changes in leaf cells treated with SV!toxin were observed by transmission electron microscopy[ Plasma membrane modi_cation speci_cally occurred at the plasmodesmata of susceptible leaf cells[ The plasmalemmal invaginations appeared at both ends of the plasmodesmata 2 h after toxin treatment[ Many Golgi vesicles were found in the cytoplasm close to or fused to the invaginated plasma membranes[ Poly! saccharide!like materials were released from the fused vesicles to extracellular spaces between cell walls and invaginated plasma membranes[ Membrane fragments from modi_ed plasma membranes and extended desmo! tubules from plasmodesmata were also observed at invaginated sites[ No changes in ultrastructure were detected in other organelles[ These results suggest that the target site of SV!toxin action may be on the plasma membranes of susceptible cells