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Metabolic Changes Associated with Systemic Induced 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 61:293-300. Accepted for publication 28 September 1970. DOI: 10.1094/Phyto-61-293.

The induction of systemic resistance in upper leaves of Samsun NN tobacco by inoculation of lower leaves with tobacco mosaic virus (TMV) was not accompanied by permanent change in the rate of oxygen uptake or in the activities of phosphohexoisomerase, aconitase, cytochrome oxidase, polyphenol oxidase, ascorbic acid oxidase, or glucose-6-phosphate dehydrogenase. Peroxidase and catalase activities increased parallel with the development of resistance and remained at high levels. Following inoculation with high-titer TMV inocula, oxygen uptake and the activities of all enzymes increased markedly at about the time of lesion formation, reached a maximum shortly afterwards, then declined sharply (except catalase and peroxidase); increases were detected sooner and the maxima reached were earlier and higher in resistant than in nonresistant leaves. Lesions appeared sooner and in greater numbers in resistant than in nonresistant leaves. We conclude that inoculation with a lesion-inducing virus elicits a nonspecific host response characterized by a general increase in enzyme synthesis and oxidative metabolism in infected and surrounding cells. Resistance is considered a consequence of high peroxidase activity, which leads to early killing of infected cells, which in turn leads to rapid and enhanced changes in advance of infection, including early formation of a barrier to virus spread.