Two action sites of AM-toxin I produced by apple pathotype of Alternaria alternata in host cells: an ultrastructural study

Abstract

The localization of primary action sites of AM-toxin I in host cells was examined by ultrastructural investigation and electron microscopic autoradiography. In susceptible apple leaves, the first effect of the toxin appeared 1 h after treatment in the plasma membranes and chloroplasts of mesophyll and vascular bundle sheath cells and in the plasma membranes of phloem and epidermal cells. Membranes and vesicles which were stained positively with a specific staining solution for grana lamellae were found in the matrix of the chloroplasts, showing that the membranous materials were derived from the disrupted grana. Cell wall lesions were formed around plasmodesmata where plasma membranes were invaginated. The invaginated sites were filled with amorphous materials from degraded cell walls, including membranes derived from plasma membranes and the desmotubules extending from plasmodesmata. The modified chloroplasts and plasma membranes were observed more often as the time after the toxin treatment was prolonged. Modified plastids were not found in the leaf cells. The other cellular membranes appeared normal even 10 h after the treatment. Resistant leaf cells were rarely affected by the toxin. Not all tissues from susceptible apples were sensitive as the toxin caused no necrosis or ultrastructural changes in petal cells. Resistant petal cells were also insensitive to the toxin, but the toxin causes necrosis and ultrastructural changes in moderately resistant petal cells in which the primary effect of the toxin appeared as plasma membrane modifications. Plastids were not affected by the toxin. These results indicate that the action sites of the toxin may be located on the plasma membrane – cell wall association in susceptible leaf cells and in moderately resistant petal cells and also on the chloroplasts of susceptible cells. The results of electron microscopic autoradiography also provided evidence that the action sites of the toxin were present on chloroplasts and the plasma membrane –cell wall association of susceptible leaf cells.