Oligandrin, the elicitin-like protein produced by the mycoparasite Pythium oligandrum, induces systemic resistance to Fusarium crown and root rot in tomato plants

Oligandrin, the elicitin-like protein produced by the mycoparasite Pythium oligandrum, crab shell chitosan and crude glucans, isolated from P. oligandrum cell walls were applied to decapitated tomato plants and evaluated for their potential to induce defence mechanisms in root tissues infected by Fusarium oxysporum f. sp. radicis-lycopersici. A significant decrease in disease incidence was monitored in oligandrin- and chitosan-treated plants as compared to water-treated plants whereas glucans from P. oligandrum cell walls failed to induce a resistance response. Ultrastructural investigations of the infected root tissues from water-treated (control) plants showed a rapid colonization of all tissues including the vascular stele. In root tissues from oligandrin-treated plants, restriction of fungal growth to the outer root tissues, decrease in pathogen viability and formation of aggregated deposits, which often accumulated at the surface of invading hyphae, were the most striking features of the reaction. In chitosan-treated plants, the main response was the formation of enlarged wall appositions at sites of attempted penetration, These wall appositions were found to vary greatly in their appearance from multi-textured to multi-layered structures and to contain large amounts of callose. The use of the WGA/ovomucoid-gold complex provided evidence that the wall-bound chitin component in Fusarium cells colonizing roots of oligandrin-treated tomato plants was not substantially altered even over cell walls of hyphae showing obvious signs of degradation. Evidence is provided in this study that oligandrin has the ability to induce systemic resistance in tomato. Exogenous, foliar applications of the fungal protein sensitize susceptible tomato plants to react more rapidly and more efficiently to F o. f. sp. radicis-lycopersici attack, mainly through the massive accumulation of fungitoxic compounds at sites of attempted pathogen penetration. Although cell wall modifications do not represent the central core of the oligandrin- mediated host response in tomato, they are part of the multicomponent defence system elaborated to fend off Fusarium invasion. (C) 2001 Editions scientifiques et medicales Elsevier SAS.