Effect of a host-specific toxin (AM-toxin I) produced by Alternaria mali, an apple pathogen, on the ultrastructure of plasma membrane of cells in apple and Japanese pear leaves

Abstract

AM-toxin I (4 μg/ml), a host-specific toxin of Alternaria mali, caused permeability changes in susceptible apple and Nijisseiki pear leaves within 5 min of treatment. The first effect of the toxin appeared on plasma membranes of susceptible apple cells. One hour after toxin treatment, slight invaginations were evident in plasma membranes near plasmodesmata. Six hours after treatment, the spaces between cell wall and invaginated plasma membrane contained lomasome-like vesicles, membranous fragments, and desmotubules extending from plasmodesmata. The membranous materials appeared to originate from the plasma membrane. Thirty-one hours after treatment, necrotic cells of susceptible leaves showed the general disruptions of cellular membranes which might be caused by secondary effects of the toxin. The similar changes of plasma membrane and plasmodesmata also were found in Nijisseiki pear cells treated with AM-toxin I (4 μg/ml) for 1 h and 6 h. The invaginations of the plasma membrane at plasmodesmata were the first ultrastructural modifications in the toxin-treated Nijisseiki cells. AM-toxin I did not affect the ultrastructure of resistant apple and Chojuro pear cells. These results indicate that the initial sites for the toxin may be on the plasma membrane of the susceptible apple cells. Furthermore, the results suggest that the plasma membrane modifications are associated with permeability changes in the toxin-treated, susceptible plant cells except for original host cells treated with the toxin.