Host-pathogen interactions IV. Studies on the polysaccharide-degrading enzymes secreted by Fusarium oxysporum f. sp. lycopersici

Abstract

When grown in liquid culture containing 1% isolated cell walls from tomato ( Lycopersicon esculentum) stems as the sole carbon source, Fusarium oxysporum secretes a variety of polysaccharide-degrading enzymes. These include a polygalacturonase, a cellulase, α- and β-galactosidases, an α- l-arabinofuranosidase, and a β-xylosidase. These enzyme activities appear in the culture fluid in a definite order: the polygalacturonase in first to reach maximal activity, followed by the glycosidases, with the cellulase reaching a peak after the other enzyme activities have diminished. A small fraction of the tomato stem cell wall, which contains a polysaccharide rich in galacturonic acid, has been shown to be a better inducer of the Fusarium oxysporum polygalacturonase than walls from which the inducer has been extracted. The ability of the Fusarium oxysporum culture filtrate enzymes to degrade cell walls isolated from tomato stems has also been demonstrated. Approximately 80% of the galacturonic acid present in tomato stem cell walls is removed in 6 h of enzymolysis with dialyzed Fusarium oxysporum culture filtrates. A group of proteins, isolated from the cell walls of tomato stems, bean hypocotyls and suspension-cultured sycamore ( Acer pseudoplatanus) cells, has been shown to inhibit the action of the Fusarium oxysporum polygalacturonase and, in doing so, to prevent cell wall degradation by the mixture of fungal enzymes present in dialyzed culture filtrates.