Molecular Aspects of Specificity in Interaction of Arabidopsis and Host-Specific Toxin Produced by Alternaria brassicicola

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Black leaf spot of Cruciferae caused by Alternaria brassicicola is a major disease in cultivated Brassica plants. Recently, we found that the pathogen produces host-specific toxin (HST), named AB-toxin. The toxin was released from the germinating spores only when spores were inoculated on host leaves, indicating that the production of AB-toxin by the pathogen is induced by substances in host leaves. Interestingly, the activity of AB-toxin was retained by membranes with a molecular weight cutoff of 5,000. In addition, the activity was heat unstable, and was lost by treatment with proteinase. Thus, unlike other HSTs, AB-toxin has a high molecular weight and is probably a protein (s). As a protein, AB-toxin may offer the advantage of being a direct gene product.Arabidopsis thariana which is known as a model plant for molecular biology, belongs to Cruciferae. Therefore, responses to the pathogen and AB-toxin of A.thariana were investigated. When the spores of the pathogen were inoculated on leaves of 7 ecotypes of A.thariana, black spot lesions appeared on 6 ecotypes except an ecotype RLD.AB-toxin was examined for biological activity on A.thariana. The toxin was toxic to 6 ecotypes, but was harmless to RLD.These results indicate that there are ecotypes susceptible and resistant to the pathogen and AB-toxin. To investigate hereditary background of the responses of A.thariana, susceptible and resistant ecotypes were crossed. The responses of the crossed plants indicated that the resistance is determined by a single dominant gene. On the other hand, AB-toxin induced electrolyte leakage from susceptible ecotype leaves within 1 hr after treatment, indicating that AB-toxin has a primary effect on the plasma membranes of host cells.