Partial characterization and aspects of the mode of action of a hypersensitivity-inhibiting factor (HIF) isolated from Phytophthora infestans

Abstract

Factors which inhibit the hypersensitive reaction of potato tuber tissue (Kennebec, R 1) to Phytophthora infestans were isolated from mycelia and zoospores of race 1234 (compatible) and race 4 (incompatible) of the fungus. They were partially characterized as glucans containing β-1→3 and β-1→6 linkages and 17 to 23 glucose units. The glucans from both mycelia and zoospores included a non-anionic glucan and an anionic glucan; one or two residues of the latter were esterified with a phosphoryl monoester. Death of host cells, browning and the accumulation of rishitin (hypersensitive reaction) in tuber slices inoculated with race 4 or treated with an elicitor from the fungus were suppressed by pretreatment of slices with the glucans. The glucans from the compatible race were more active in suppressing the hypersensitive reaction than those from the incompatible race. The anionic glucan was more active than the non-anionic glucan. Crude elicitors from races 4 and 1234 lost terpenoid-eliciting activity when mixed with a microsomal fraction prepared from potato tuber tissue. The glucans from the compatible race, but not the incompatible race, markedly reduced the loss resulting from the reaction between the crude elicitor and the microsomal fraction. The data suggest that the compatible interaction between potato tissue and P. infestans may be caused by a suppression of the hypersensitive response of the host tissue by water-soluble glucans from the fungus.