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 vesic arium (Wallr.) E. Simmons is a disease of economic importance in fruit-grow ing areas of southern Europe. The pathogen produces two host-specific SV-to xins (SV-toxin I and II) in culture filtrates, which selectively induce vei nal necrosis only on susceptible cultivars. Ultrastructural changes in leaf cells treated with SV-toxin were observed by transmission electron microsc opy. Plasma membrane modification specifically occurred at the plasmodesmat a of susceptible leaf cells. The plasmalemmal invaginations appeared at bot h ends of the plasmodesmata 3 h after toxin treatment. Many Golgi vesicles were found in the cytoplasm close to or fused to the invaginated plasma mem branes. Polysaccharide-like materials were released from the fused vesicles to extracellular spaces between cell walls and invaginated plasma membrane s. Membrane fragments from modified plasma membranes and extended desmotubu les 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 s usceptible cells.