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Changes in Phenol Metabolism Associated with Induced Systemic Resistance to Tobacco Mosaic Virus in Samsun NN Tobacco. Thomas J. Simons, Department of Plant Pathology, Cornell University, Ithaca, New York 14850, Present address of senior author: Botany Department, University College, Cork, Ireland; A. Frank Ross, Department of Plant Pathology, Cornell University, Ithaca, New York 14850. Phytopathology 61:1261-1265. Accepted for publication 27 May 1971. DOI: 10.1094/Phyto-61-1261.

Induction of systemic resistance in upper leaves of Samsun NN tobacco following inoculation of the lower leaves with tobacco mosaic virus was not accompanied by changes in activity of phenylalanine ammonium lyase or in concentration of total phenols, orthodihydroxyphenols, or chlorogenic acid. Challenge inoculation of these resistant upper leaves, however, effected an earlier and greater decrease in the concentration of these phenolic compounds during and subsequent to lesion formation than that occurring when nonresistant leaves were similarly inoculated. Also, the abrupt increase in phenylalanine ammonium lyase activity occurring at about the time of initial lesion formation was greater in resistant than in nonresistant leaves. A few hours later, activity of this enzyme decreased abruptly in both types of leaves. These results support the hypothesis that accelerated necrotic and metabolic responses of resistant leaves after challenge inoculation are due to rapid peroxidase-mediated quinone accumulation at points of infection. We suggest that the mechanism responsible for cessation of lesion expansion is the eventual collapse, largely due to quinone accumulation, of a ring of cells just in advance of virus infection or shortly thereafter.