Molecular Genetics of Host-Specific Toxins in Plant Disease

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Many fungal genes responsible for the producion of host-specific toxins were cloned from Alternaria and Cochliobolus pathogens. Some of them were elucidated thier structures and function in the course of US/Japan Joint Research Project. A cyclic peptide synthetase gene (4MT) for AM-toxin of leaf blotch of apple pathogen was found to bel3kb in length with no introns and have four domains of amino acid activation specific for the amino acids constituting the peptide toxin. The gene cluster involved in AK-toxin biosynthesis in the black spot pathogen of Japanese pear were made structural and functional analyses AKT1 (fatty acid synthesis), AKT3(modification of fatty acid), AKTR(regulator), AKTS1 (similar to threonine dehydratase, its true role is unclear, but specific to the Japanese pear pathotype) and AKT2 (no similarity to known proteins in the databases). The strawberry pathotye had AKTl AKT3 and AKTR homologues, and the tangerine pathotype possessed AKTR homologue. The shared distribution of the genes were compatible with the molecular similarity of toxins. Also, structure and function of TOX1 and TOX2 in Cochijobolus were elucidated in depth. A gene for the sensitivity to ACR-toxin was cloned from mtDNA of host rough lemon, and a protein specific to the AM-toxin sensitivity was isolated from susceptible apple leaves by proteome analysis. Most fungal genes for host specific toxin biosynthesis would be horizontally transferred.