Effect of the host-specific toxin from Alternaria kikuchiana on the ultrastructure of plasma membranes of cells in leaves of Japanese pear

Abstract

Alternaria kikuchiana, causal fungus of the black spot disease of Japanese pear, produces a host-specific toxin (AK-toxin) which caused immediate changes in permeability of susceptible leaf tissues. The first obvious change in ultrastructure of susceptible cells in the vascular bundle sheath and in the mesophyll was an invagination of plasma membranes, evident by 1 h after exposure. There were still more invaginated plasma membranes by 3 and 6 h after exposure, when a high percentage of susceptible cells showed such changes. There was no obvious effect of toxin on cellular organelles of susceptible leaves after 1, 3 and 6 h of treatment; the organelles appeared to be normal even in cells with invagination of plasma membranes. After 10 h of toxin exposure, leaf cells were necrotic, but the cellular membrane systems still maintained much of their original structures. The degeneration that was evident may have been from secondary effects of toxin. The spaces between susceptible cell walls and invaginated membranes contained many lomasome-like vesicles, membrane fragments and darkly stained materials. Roland's staining method indicated that the vesicles were fragments from invaginated plasma membranes. However, ultrastructural changes in unit membranes of invaginated plasma membranes were not observed. Cells of resistant leaf tissues were not affected by toxin. These results are compatible with the hypothesis that the plasma membrane is the site of an initial effect of AK-toxin.