Protective Effect of Homologues of Host-Specific AF-Toxin I Produced by Alternaria alternata Strawberry Pathotype on Strawberry Cells

The protective effect of homologues of host-specific AF-toxin I from Alternaria alternata strawberry pathotype on susceptible strawberry cells was quantitatively examined by using the cultured cells. Among three AF-toxin molecules, AF-toxin II did not exhibit toxicity to strawberry cells, although AF-toxin I and III were highly toxic to the cells. AF-toxin II protected strawberry cells from AF-toxin I action. The protection was remarkable when the cultured cells were exposed to excess amounts of AF-toxin II both prior to and simultaneously with AF-toxin I addition. The simultaneous treatment, however, was most effective at preventing AF-toxin I action: it gave complete protection at a 50 : 1 (toxin II : toxin I) ratio on a molar basis. The epoxy-decatrienoic acid moiety of the AF-toxins also prevented the cell death caused by AF-toxin I, but was less effective than AF-toxin II. Protection was apparent when the cultured cells were exposed to AF-toxin II for 3 h, rinsed to remove free AF-toxin II and then exposed to AF-toxin I. These results suggest that the toxoids of AF-toxin I, such as AF-toxin II and the decatrienoic acid, act as competitive inhibitors of AF-toxin I.