Production of a host-specific toxin by germinating spores of Alternaria brassicicola

Spore suspensions of Alternaria brassicicola pathogenic to Brassica spp. were incubated on the surface of Brassica leaves. The spore germination fluid was collected and examined for biological activity. In a detached leaf assay, the fluid induced water-soaking symptoms followed by brown necrotic lesions on Brassica leaves susceptible to the pathogen but did not cause visible symptoms on nonhost leaves. When spores of the non-pathogenic species Altenaria alternata were combined with the fluid and inoculated on the leaves, the spores could invade Brassica leaves, just as the pathogen did. These results indicate that A. brassicicola produces a host-specific toxin(s) and releases it during spore germination on the leaves. Toxin activity of the fluid was retained by an ultrafiltration membrane with a 5 kDa cut off. The toxin was purified by ion exchange chromatography and gel filtration HPLC. The molecular weight of the toxin was estimated to be 35 kDa by SDS-polyacrylamide gel electrophoresis. The activity of the toxin was heat labile and was also lost after treatment with proteinase K. Thus, the toxin named AB-toxin is probably a protein, the first described among the many host-specific toxins produced by Alternaria species. (C) 1998 Academic Press.