Involvement of Host Factors in the Production of a Protein Host-Specific Toxin Produced by Alternaria Brassicicola

Abstract

Alternaria brassicicola, the cause of black leaf spot of Brassica spp., produces a host-specific toxin named AB-toxin during spore germination at the infection site on host plants. Unlike other Alternaria host-specific toxins which are low-molecular-weight secondary metabolites, AB-toxin is a protein and estimated to be 35 kDa by SDS-polyacrylamide gel electrophoresis. AB-toxin was not detected in the culture filtrates of the pathogen. When the spores (5×105 – 106 spores/ml) suspended in water were dropped on host leaves, non-host leaves and plastic plates, the spores germinated only on host leaves and released AB-toxin in the spore germination fluid. Spores suspended in Czapek-Dox and PDB solutions germinated on plastic plates, but only trace of AB-toxin was detected. These results indicate that AB-toxin production by A. brassicicola is induced by host-related factors during early stages of host-parasite interactions.