Because cutin and cellulose provide tough, protective barriers for the plant, cutinase and cellulase enzymes are necessary to the penetration of plant hosts by pathogenic fungi. They break down the cutin in the cuticle and the cellulose in the primary cell wall. Hydrolytic (digestive) enzymes also play important roles in pathogenesis. The organic food in the host is usually in the form of complex carbohydrates, fats, and proteins. To be absorbed by pathogens, they must be broken down to their simpler units: simple sugars, fatty acids, glycerol, and amino acids. Common digestive enzymes—amylases, cellulases, lipases, and proteases—produced by pathogens break down these complex foods.
The middle lamella, the area between cells in parenchyma tissue, has a high pectin content. For many diseases pathogenesis involves production of pectolytic enzymes that break down pectin. This causes dissolution and eventually death of the cells. Damping off, root rots, vascular wilts, and fruit and vegetable rots are caused by pathogens that produce large amounts of pec-tolytic enzymes.
Several toxins have been shown to be produced by plant pathogenic fungi and bacteria. Most of them are nonhost-specific toxins. They usually kill cells but may act on the permeability of the cytoplasmic membrane. Although they are involved in pathogenesis, in some cases strains of the pathogen that are unable to produce the toxin still can cause disease. In a few cases the toxin is host-specific and only affects that host at normal toxin concentrations.
Most plants are resistant to infection because of the presence of preexisting chemicals. However, there are many cases where chemicals that ward off infection are produced by the host only after the pathogen is present. These chemicals are called phytoalexins. This is a rather common phenomenon, with about three hundred chemicals from thirty different families of plants having been identified as phytoalexins. see also Agriculture, Modern; Chestnut Blight; Defenses, Chemical; Dutch Elm Disease; Eubacteria; Fungi; Herbicides; Hormones; Interaction, Plant-Fungal; Potato Blight.
Ira W. Deep
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