Involvement of an Inhibitory Compound in Induced Resistance of Maize toHelminthosporium carbonum

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Abstract
In maize [Zea mays], resistance to Helminthosporium carbonum race 1 is induced by prior inoculation with race 2. This induced resistance was consistently associated with the production of a compound(s), which reversibly inhibited conidial germination and germ tube elongation. It also prevented growth of phytopathogenic bacteria in a defined medium. The inhibitor was produced and diffused into liquid on the surface of the leaf in all maize lines tested and in response to inoculation with other fungi. When this inhibitory diffusate was added to the conidial inoculum, lesions did not develop on leaves of susceptible genotypes. The host-specific toxin (HC-toxin) produced only by race 1 prevented the synthesis or release of the inhibitor but did not affect its activity in germination bioassays or its ability to prevent lesion development. The results suggest that the inhibitor has a role in induced resistance.