Expression of Tolerance to the Host-Specific Toxin of Alternaria alternata (AT Toxin) in Cultured Cells and Isolated Protoplasts of Tobacco

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Abstract
Crude AT toxin extracted from the culture filtrate of Alternaria alternata pathotype tobacco was applied to the suspension-cultured cells of tobacco cultivars that differed in susceptibility to the fungus, the causal organism for tobacco brown spot disease. Crude AT toxin at a concentration of 3% (v/v) inhibited cell growth in cell lines of all the tobacco cultivars, irrespective of the resistance of the cultivar to the fungus. However, cell mortality values, as judged by a vital staining method, were much lower for cell lines of resistant plants than for those of susceptible ones. These differences were also observed when leaf mesophyll protoplasts cultured for 4 days were treated with the toxin for 24 hr. Cell division and further colony formation occurred with protoplasts from resistant, but not from susceptible, plants. Thus, application of AT toxin to cultured cells or protoplasts might be an efficient alternative for evaluating the degree of resistance of tobacco lines to tobacco brown spot.