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Bipolaris maydis race C strain 523 (C523) induces severer leaf blight on cytoplasmic male sterility (CMS)-C maize than on normal (N) maize.Previously,a pathotoxin isolated from C523 (C-toxin) was shown to be responsible for the disease.To understand the basis of the differential responses between CMS-C and N maizes to this fungus,protein synthesis in vitro by mitochondria from N and CMS-C cytoplasms was monitored after their incubation in a solution containing the toxin (0.3%).Similar protein products were detected between the two alloplasmic lines,indicating that the toxin does not directly act on the mitochondrial membrane,nor inhibits the expression of mitochondrial genes.To further locate the action site of the toxin,intact leaves from both N and several subtypes of CMS-C lines were treated by 0.3% toxin.Analysis of electrolyte leakage of leaf cells showed that the leakage rates were similar to one another among the alloplasmic maize lines.In contrast,at a lower concentration of the toxin (0.05%),the leaf cells from CMS-C line were more susceptible to the toxin than those of the other lines.All these results indicate that the target of the toxin action appears to be the cellular-membrane rather than mitochondria,suggesting that the variable susceptibilities to B.maydis between the alloplasmic maize lines might be related to a difference in their cellular-membranes.