| Research Abstract |
Fungal pathogenicity are predicted to release signaling molecules essential for path ogenicity during the process of infection. Although essential components for fungal pathogenesis are still unknown in many plant pathogenic fungi, certain fungal pathogens, especially the genera Alternaria and Cochliobolus, are known to produce host-specific toxins (HSTs) as compatibility agents. There are now ten or more Alternaria pathogena, mainly A. alternata pathotypes, known to produce HSTs, and the structures of most of the toxins have been elucidated. HSTs from A.alternata pathotypes have been classified into three groups based on their chemical structures, I.e.polyketides, cyclic peptides and decatrienoic acid esters. Recently a polyketied synthetase gene and a cyclic-peptide synthetase gene, involved in AAL-toxin and AM-toxin biosynthesis, respectively, have been cloned and characterized. In both cases, targeted gene disruption lead to elimination of toxin production and loss of pathogenisity indicating that HST biosynthetic genes are fungal pathogenicity determinants. These HST biosynthetic genes also reside on conditionally dispensable chromosomes found only in the pathogenic and HST-producing strains of A.alrernata and homologues of the genes were not detected in nonpathogenic strains of A.alrernata. This suggests that these genes may have been acquired by horizontal gene transfer and provides a possible mechanism whereby new pathotypes could arise in nature.
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