REVERSIBLE EFFECTS OF INHIBITORY DIFFUSATES FROM MAIZE INOCULATED WITH COCHLIOBOLUS-CARBONUM

The localized resistance of maize to Cochliobolus carbonum race 1 induced by a prior inoculation with the weakly virulent race 2 is associated with the release of inhibitory compounds from affected leaf cells into the infection court after penetration. In this study, diffusates containing this inhibitory material harvested from the inoculated leaf surface were found to inhibit conidial germination and accompanying metabolic and biosynthetic activities-oxygen uptake and uridine and leucine incorporation. Addition of carboxyl-containing compounds (amino acids or salts of organic acids) to the germination medium prevented and reversed these inhibitory effects. Similarly, in the host-pathogen interaction, these compounds prevented the action of the diffusates, abolished the induced resistance, and promoted disease development in both resistant and susceptible isolines of maize by race 2 of C. carbonum. In this respect, sodium acetate mimicked the phenotypic effect of the host-specific toxin produced by race 1, which is essential for pathogenicity of C. carbonum. In contrast to the toxin, however, the effect of sodium acetate was not genotype-specific, suggesting that the inhibitory compounds produced by maize leaf cells are components of a general resistance response whose activity is suppressed by acidic compounds and whose synthesis by susceptible genotypes is suppressed by HC-toxin. © 1991.