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Physiology and Biochemistry

Localization of Peroxidase in Rhizoctonia solani-Infected Cotton Seedlings. Joseph A. Veech, Research Plant Physiologist, U.S. Department of Agriculture, Agricultural Research Service, National Cotton Pathology Research Laboratory, P.O. Drawer JF, College Station, Texas 77840; Phytopathology 66:1072-1076. Accepted for publication 24 March 1976. Copyright © 1976 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-66-1072.

Cotton (Gossypium hirsutum) seedlings become less susceptible to soreshin pathogen (Rhizoctonia solani) with age. In noninoculated seedlings peroxidase activity was prominent in the epidermis, hypodermis, endodermis, and some vascular parenchyma cells of 5- and 11-day-old hypocotyls. Generally, activity was minor in the cell walls of the cortex. Occasionally, 11-day-old hypocotyls had peroxidase activity in the cortical cells that apparently had been crushed by secondary growth. Peroxidase activity increased in pre-existing sites of localization in response to infection by R. solani. New sites of increased activity were induced, first in the cell walls and later in the cytoplasm, as the disease developed; these developed near the advancing margin of the necrotic lesion and prior to tissue browning. Since phenolase activity was not detected, it was concluded that peroxidase was responsible for the oxidation of polyphenols, the major component of the brown pigment. Infection-induced sites of peroxidase activity were distributed similarly in seedlings inoculated at 5 or 11 days of age. Thus, the sites of constitutive or infection-induced peroxidase activity apparently are not related to the increased resistance of older seedlings to R. solani.

Additional keywords: histochemistry, phenoloxidation, necrosis.