The Relation Between Pisatin and the Development of Aphanomyces euteiches in Diseased Pisum sativum

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Abstract
When wounded P. sativum epicotyls were inoculated with the fungal pathogen, A. euteiches, water-soaked lesions appeared within 1.5 days. In young lesions (1.5-2 days after inoculation) fungal growth was intercellular and restricted to macroscopically visible lesions. A. 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 .ltoreq. 100 .mu.g/ml), lesions continued to expand rapidly. Pisatin was induced in epicotyls by UV 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 [lethal dose] value of pisatin for A. euteiches was .ltoreq. 100 .mu.g/ml. whether pisatin was solubilized with ethanol or not. In 4 different liquid growth media, dry weight increase of A. euteiches was suppressed by 31 .mu.g pisatin ml medium. The daily addition of pisatin (31 .mu.g 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 vs. 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. Although in vitro growth of A. euteiches always was prevented by pisatin at .ltoreq. 100 .mu.g/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.

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