Potato lectin lyses zoospores of Phytophthora infestans and precipitates elicitors of terpenoid accumulation produced by the fungus

Abstract

Lectins were isolated and partially purified from 4 potato cullivars with different R-genes for resistance to Phytophthora infestans. The 4 lectins lysed zoospores from 5 races of the fungus, and the ability to lyse was not related to the resistance of the cultivar. Solutions containing 25 μg glucose equivalent of lectin ml−1 caused 80 to 100% lysis of zoospores within 1 h. Highly purified lectins prepared from 2 potato cultivars exhibited a similar lytic effect on zoospores. Lysis was prevented when solutions containing lectin were mixed with solutions containing trimers and tetramers of N-acetyl- D -glucosamine prior to mixing with the zoospores. Sucrose, dextran, soluble starch, inulin, bovine serum albumin, laminarin and collagen at 300 μg ml−1 did not lyse zoospores. Agglutination or lysis was not observed when cystospores, sporangia or mycelia were treated with potato lectins, Concanavalin A, and lectins from Ricinus communis and Dolichos biflorns were tested for their effect on zoospores, and only concanavalin A was as active as the potato lectins in causing lysis. Like potato lectin, concanavalin A did not agglutinate, lyse or affect the germination or growth of cystospores or sporangia. Potato lectin (100 μg glucose equivalent ml−1) precipitated elicitors of terpenoid accumulation extracted from the fungus. The lectin-elicitor complexes nonetheless retained most of the elicitor activity when applied to potato discs. In contrast to potato lectin, concanavalin A in the absence of salts did not precipitate the elicitor. In the presence of Mn2+ and Ga2+, concanavalin A at 100 and 200 μg ml−1 precipitated some but not all the elicitor activity. Though evidence is lacking that potato lectin has a direct role in determining specificity in the interaction between potato and P. infestans, the results from studies of lysis, agglutination and the precipitation of elicitor by lectins suggest that the elicitor may not be exposed on the surface of cystospores, sporangia and mycelium.