Helminthosporium oryzae toxin suppresses phenol metabolism in rice plants and aids pathogen colonization

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Abstract

Infection by Helminthosporium oryzae, the nice brown spot pathogen, caused a decrease in phenolics content, and in the activities of peroxidase and phenylalanine-ammonia lyase in rice leaves at advanced stages. At later stages of infection, the pathogen toxin accumulated in infected tissues. The partially purified toxin, isolated from culture filtrates of the fungus, also caused a decrease in phenolics content and phenylalanine-ammonia lyase activities in rice leaves. Catechol, quinic acid, phenylalanine, Ethrel, and WL 28325 were fed to rice plants to increase their phenol metabolism. These treatments, which increased the total soluble phenolics content in leaves, delayed the symptoms appreciably. When toxin was applied to the leaves of the chemically treated plants the leaves' phenolic content decreased rapidly. It appears that the major role of the toxin in pathogenesis is the suppression of defence mechanisms of rice plants.

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