VIEW ARTICLE

Physiology and Biochemistry

The Relation Between Pisatin and the Development of Aphanomyces euteiches in Diseased Pisum sativum. Steven G. Pueppke, Former Graduate Research Assistant, Plant Pathology Department, Cornell University, Ithaca, NY 14853, Present address of the senior author: Department of Biology, University of Missouri-St. Louis, 8001 Natural Bridge Road, St. Louis 63121; Hans D. VanEtten, Assistant Professor, Plant Pathology Department, Cornell University, Ithaca, NY 14853. Phytopathology 66:1174-1185. Accepted for publication 5 April 1976. Copyright © 1976 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-66-1174.

When wounded Pisum sativum epicotyls were inoculated with the fungal pathogen. Aphanomyces euteiches, water-soaked lesions appeared within 1.5 days. In young lesions (1.5 to 2 days after inoculation) fungal growth was intercellular and restricted to macroscopically visible lesions. Aphanomyces euteiches was isolated from the site of inoculation and from margins of the lesions. The pathogen grew primarily in the cortex, infrequently in the stele, but never in epidermal cells. Although pisatin concentration in subepidermal tissues of young lesions was 20-40 times that which prevents mycelial growth in vitro (ED₁₀₀ ≥ 100 μg/ml), lesions continued to expand rapidly. Pisatin was induced in epicotyls by ultraviolet irradiation. When irradiated epicotyls containing approximately the ED₁₀₀ concentration of pisatin were inoculated, lesion development was not affected appreciably. Differential centrifugation of homogenates of irradiated tissues indicated that pisatin is not associated with particulate cell fractions. When protoplasts were prepared from irradiated leaflets by enzymatic degradation of the cell walls, pisatin was recovered from the enzyme solutions, but not from intact protoplasts. The ED₁₀₀ value of pisatin for A. euteiches was ≥ 100 μg/ml, whether pisatin was solubilized with ethanol or not. In four different liquid growth media, dry weight increase of A. euteiches was suppressed by 31 μg pisatin/ml medium. The daily addition of pisatin (31 μg/ml/day) to liquid cultures of A. euteiches did not result in adaptation of the organism to the accumulating phytoalexin. Dosage-response curves (radial growth versus pisatin concentration) of isolates recovered from A. euteiches-infected tissue were identical to that of the initial culture. Pisatin was not metabolically altered after incubation up to 4 days in liquid cultures of the pathogen. Low-molecular-weight components from healthy or infected epicotyls did not affect the sensitivity of A. euteiches to pisatin. Thus, although in vitro growth of A. euteiches always was prevented by pisatin at ≥ 100 μg/ml, much higher pisatin concentrations did not restrict the pathogen in vivo. The data appear to be inconsistent with concepts of pisatin as a primary resistance factor in pea.