Ultrastructure of Cells ofUlmus americanaCultured in Vitro and Exposed to the Culture Filtrate ofCeratocystis ulmi

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Abstract
Calli of American elm susceptible and resistant to Dutch elm disease were exposed to a culture filtrate of a pathogenic isolate of Ceratocystis ulmi. Cells from untreated tissue exhibited typical internal composition associated with healthy, actively growing cells. All cells exposed to culture filtrate showed appreciable ultrastructural changes. Cells from susceptible American elm displayed mitochondrial and cytoplasmic disruption. Damage to membranes was evident by the appearance of incomplete cell walls and distinctive plasmolysis. Cells of the resistant American elm differed dramatically with the presence and accumulation of phenoliclike deposits. The response seen with the resistant elm could be associated with a defensive mechanism against toxic metabolites of C. ulmi.