MOLECULAR BASIS FOR SPECIALIZATION AND EVOLUTION OF PATHOGENICITY IN ALTERNARIA PATHOGENS THAT HOST-SPECIFIC TOXINS INVOLVE IN

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A new host-specific toxin (HST) was isolated from Australian strains of brown spot pathogen of tangerine, determined the structure, and named ACT-toxin IIIb. A rapid and sensitive procedure of AL-toxin determination was established. A key role of HSTs in host recognition by fungal pathogens was investigated in relation to plant defense reactins. For cloning gene (s) involved in biosynthesis of such HST,protoplasts of different pathotypes of Alternaria alternata were transformed utilizing ectopic integration of pAN7-1 plasmid carrying hph, gpd, and trpC genes in the presence of restriction enzymes. Several transformants were found lacked the ability to produce HSTs, and found they randomly have a single copy of pAN7-1 integrated at single site. Isolation of DNA fragments containing HST synthesis gene (s) by rescuing the integrated plasmid DNA with a flanking genomic sequence from HST-deficient transformants may possible. Electrophoretic karyotyping suggested that A.alternata has 8 to 9 chromosomes ranging from 0.6 Mb to 5.5 Mb, and its total genomesize probably is 22-24Mb. B-chromosomes might be correlated to specific pathogenicity of each pathotype. Some A.alternata have incompatibility in cell fusion.