Molecular Mechanisms of Fungal Pathogenicity to Plants

Most of the approximately 100,000 known species of fungi are strictly sapro­ phytic and degrade dead organic materials for their source of nutrients. Only 8000 can cause disease on one or more plant species and only 100 are patho­ genic to humans or animals (1). What then distinguishes this small minority of pathogenic fungi from the majority of saprophytic fungi? Are there major differences between the genetic make up of these two groups? There are three possible answers: Saprophytic and pathogenic fungi both share the same set of genes, but some genes are differently regulated; pathogenic fungi possess unique genes that enable them to be pathogenic; or a combination of these two possibilities (71). How the fungus overcomes the plant's defense system, usually highly successful against microbial attack, is not known for most fungal diseases. Thus, the possible existence and the nature of fungal pathogenicity genes are inseparable from the plant's ability to recognize a foreign organism and set up defense mechanisms. Many fungi undergo a complex sequence of developmental and metabolic events to which plants, in tum, have evolved a series of different counter attacks. Some steps that may be crucial to the establishment of fungal pathogenicity: