Mechanisms of survival of necrotrophic fungal plant pathogens in hosts expressing the hypersensitive response.

The hypersensitive response (HR), elicited when resistant hosts are infected by incompatible races of biotrophic fungi, has been researched extensively. New studies on host responses to necrotrophic fungi are beginning to show that when the HR occurs in hosts colonized by necrotrophs, fungal growth is accelerated rather than retarded. We review current knowledge about how necrotrophs survive in host plants in which the HR is expressed. We discuss how necrotrophs cope with the environmental factors formed as a result of the HR. Necrotrophs contain an array of enzymes, which can help in exploiting the hostile environment in order to colonize the host and to remove or inactivate active oxygen species (AOS). Among this array of enzymes are superoxide dismutase (SOD), peroxidases, catalase, and perhaps laccases and polyphenol oxidases. Of these, only SOD and catalase have been studied in any detail. The precise significance of SOD and catalase in host invasion and fungal resistance is still not adequately known. The importance of different peroxidases is also still far from clear. We speculate that AOS species may trigger the response of necrotrophs to the host environment.

The possible function of the glucan sheath of Botrytis cinerea: effects on the distribution of enzyme activities.

The effect of a beta-1,3-glucanase (Glucanex) on cultures of Botrytis cinerea was examined. The enzyme released reducing sugars from the mycelium and from the glucan secreted into the culture medium. The morphology of the mycelium was changed in the presence of Glucanex. The measured activity of guaiacol peroxidase, laccase, and catalase was increased when the mycelium was treated with Glucanex. Culture of the mycelium in the presence of Glucanex resulted in an increase in catalase activity. We suggest that the glucan plays a role in protecting the fungus from host response and may assist in the initial stages of host infection.

Enzymes of Botrytis cinerea capable of breaking down hydrogen peroxide.

The amounts of intra- and extracellular guaiacol peroxidase, ascorbic peroxidase, glutathione peroxidase, superoxide dismutase, laccase, and catalase present in Botrytis cinerea, cultured in three different media: Kovac synthetic medium, Sabouraud fluid medium, and a medium containing malt extract, were determined. The activity of two enzymes, ascorbic peroxidase and glutathione peroxidase, has not been previously described in B. cinerea. The detected amount of the enzymes showed considerable variability in the three different culture media. The presence of an array of enzymes capable of metabolizing hydrogen peroxide, whose levels are determined by the conditions under which the fungus grows, shows that B. cinerea is well equipped to contend with the occurrence of host-produced active oxygen species.