A primary question in plant pathology has been what makes a pathogen a pathogen. This research will characterize a cluster of fungal genes with properties similar to what have been called "pathogenicity islands" in bacteria. This cluster was identified in a strain of the fungus Nectria haematococca which is pathogenic on pea and is called the Pea Pathogenicity or PEP gene cluster because it contains genes for pathogenicity on pea. The PEP cluster is located on a 1.6 Mb "conditionally dispensable" (CD) chromosome. CD chromosomes are dispensable for normal growth in culture but contain unique genes that appear to allow strains containing the chromosomes to live in specific habitats; in this case pea tissue. The PEP gene cluster contains six genes that are expressed during infection of pea tissue but the biochemical function of only one of the genes is known with certainty. This gene, PDA1, encodes a specific cytochrome P450 that confers resistance to pisatin, an antibiotic produced by pea plants. Three of the PEP genes, in addition to PDA1, can independently confer some level of virulence to an isolate lacking the CD chromosome; functions for two of these three genes are hypothesized based on predicted amino acid sequences. The deduced amino acid sequence of another transcribed portion of the PEP cluster, as well as three apparently non-transcribed open reading frames in this 25 kb region, have a high degree of similarity to known fungal transposases. Differences between the G+C content and codon usage of the six genes in the PEP cluster and chromosomal DNA suggest that the PEP cluster and possibly the CD chromosome might have been acquired horizontally. Several of the features of the PEP cluster - a cluster of pathogenicity genes, the presence of transposable elements and the indication of an exogenous origin - are shared by pathogenicity islands in animal and plant pathogenic bacteria. The objectives of this research are 1) localization of the physical borders of the PEP gene cluster, 2) evaluation of the relative contributions of the individual PEP genes to the lesion-causing ability of N. haematococca, 3) characterization of the distribution of PEP homologues, 4) analysis of the expression of PEP genes and 5) determination of their biochemical function(s). The results of the proposed research may define new mechanisms of pathogenicity as well as contribute to a better understanding of host specialization, evolution and variation in plant pathogenic fungi.
Most diseases of plants, including economically important crop plants, are caused by fungi. The strains of fungi that cause plant disease are often very closely related to harmless, free-living fungi. This research has identified a cluster of genes found in a plant pathogenic fungus that is missing or altered in related but nonpathogenic strains. This research into the function of these genes and their inheritance can lead to a greater understanding of how pathogenic microbes develop from their harmless nearest relatives.
