Alteration of Oxidative Enzymes in Potato Tuber Tissue by Infection with Phytophthora infestans

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Abstract
Studies on the distribution of polyphenol oxidase and peroxidase in the potato tuber tissue neighboring the cells infected with P. infestans were carried out. When the tuber was cut, or was cut and infected with a compatible strain of P. infestans to which the tissue was susceptible, peak of polyphenol-oxidase activity appeared in a tissue zone 0.5-1.0 mm below the cut surface 2 days after cutting. On the outer side of this peak the activity dropped sharply, and in the inner side it decreased to normal levels within about 2.5-5 mm from the cut surface. When the cut surface was infected with an incompatible race of P. infestans to which the tissue was resistant or was treated with pectinase, an increased polyphenol-oxidase activity was found in the inner side of this peak which extended much deeper into the underlying tissue, to about 5 mm from the cut surface. No increase in polyphenol-oxidase or peroxidase activities was induced in the tissues infected by the compatible race to which the tissue was susceptible. In contrast, the peroxidase activity was highest in the superficial tissue in all cases and decreased gradually toward the inner tissue without showing any peak. The increase of peroxidase caused by cutting, infection, or treatment with pectinase was limited to about 1.5 mm from the cut surface. The O2 uptake of the tissues was more sensitive to DIECA in the zone with the marked increase of polyphenol oxidase following treatment of the cut surface with pectinase or infection by the parasite than in that from the cut untreated tissues. A correlation was found between the increase in O2 uptake and the increase in the activity of polyphenol oxidase. The supernatant proteins extracted from potato tubers were fractionated by means of DEAE cellulose chromatography and vertical starch gel electrophoresis. Some components of this peroxidase isozyme system appeared to increase more than others following cutting, infection, or pectinase treatment of the tubers. It is suggested that enzymes such as pectinase may play a role in inducing the increase in respiration and polyphenol-oxidase activity of plant tissue adjacent to a site of infection or injury.