Pathogenic specialization in phytopathogenic fungi producing host-specific toxins

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Host-specific toxins are critical determinants of host-specific pathogenicity or virulence in several plant pathogenic fungi. Seven variants of Alternaria alternata produce host-specific toxins and cause severe diseases on different plants. These host-specific forms have been designated as pathotypes of A.alternata. To understand the molecular basis of pathogenic specialization in A.alternata, this research project has focused on cloning the genes required for toxin biosynthesis of the pathogens. We have studied mainly three pathotypes, Japanese pear, strawberry and tangerine, which produce AK-, AF- and ACT-toxins, respectively. These toxins have a common structural moiety, 9,10-epoxy-8-hydroxy-9-methyl-decatrienoic acid. Summary of results obtained in this research is as follows.(1) We isolated the gene cluster required for AK-toxin biosynthesis of the Japanese pear pathotype and found 11 putative open reading frames (ORFs) within an about 80-kb region. Structural analysis also found … More several transposon-like sequences within the region. Functional analysis of the ORFs identified eight genes (AKT genes) involved in AK-toxin biosynthesis.(2) We observed distribution of the AKT gene homologues in seven pathotypes and nonpathogenic stains of A.alternata by DNA gel blot analysis and found homologues of six genes in the strawberry and tangerine pathotypes.(3) We constructed cosmid genomic libraries of the strawberry and tangerine pathotypes and isolated clones containing the AKT homologues.(4) We isolated five genes (AFT genes) homologous to AKT genes and newly found three ORFs. One of the ORFs was identified as a gene specific for AF-toxin biosynthesis. We also found that the genes are located on a dispensable 1.1-Mb chromosome.(5) We isolated three genes (ACT genes) homologous to AKT genes and newly found three ORFs. We produced ACT-toxin-deficient mutants by transformation-mediated targeting of an ACT gene (ACTR). Toxin production of the mutant could be restored by introduction of the homologous gene (AKTR) of the Japanese pear pathotype, showing the functional identity of ACTR and AKTR. Less