Immunohistochemical studies of compatible and incompatible interactions of potato leaves with Phytophthora infestans and of the nonhost response to Phytophthora megasperma

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Abstract
Compatible and incompatible interactions with appropriate pathotypes of Phytophthora infestans (Mont.) de Bary (Pi, races Pi1 and Pi4, respectively) and the nonhost response to Phytophthora megasperma f.sp. glycinea kuan and Erwin (Pmg) were studied microscopically in leaves of one potato cultivar (Solanum tuberosium L. cv. Datura, carrying the Pi-resistance gene R1). Fungal growth was followed by immunohistochemical detection of hyphae in the plant tissue. The reaction of plant cells to the different pathotypes was monitored by staining with aniline blue (callose formation), autofluorescence (accumulation of phenolic compounds), and critical observation of the cell morphology, particularly the appearance of necrosis. Rapid responses occurred in epidermal cells in all types of interaction. Mesophyll cells responded rapidly only in incompatible interactions. Thus, all plant cells surrounding Pmg or Pi4 hyphae showed the hypersensitive reaction, which was rarely observed in the mesophyll of Pi1-infected leaves. This difference was more characteristic of the response of this system than either the timing of hyphal growth or the reaction of epidermal cells at the site of inoculation.