Abstract
BackgroundHC-toxin, a cyclic tetrapeptide, is a virulence determinant for the plant pathogenic fungus Cochliobolus carbonum. It was recently discovered that another fungus, Alternaria jesenskae, also produces HC-toxin.ResultsThe major genes (collectively known as AjTOX2) involved in the biosynthesis of HC-toxin were identified from A. jesenskae by genomic sequencing. The encoded orthologous proteins share 75-85% amino acid identity, and the genes for HC-toxin biosynthesis are duplicated in both fungi. The genomic organization of the genes in the two fungi show a similar but not identical partial clustering arrangement. A set of representative housekeeping proteins show a similar high level of amino acid identity between C. carbonum and A. jesenskae, which is consistent with the close relatedness of these two genera within the family Pleosporaceae (Dothideomycetes).ConclusionsThis is the first report that the plant virulence factor HC-toxin is made by an organism other than C. carbonum. The genes may have moved by horizontal transfer between the two species, but it cannot be excluded that they were present in a common ancestor and lost from other species of Alternaria and Cochliobolus.
Highlights
HC-toxin, a cyclic tetrapeptide, is a virulence determinant for the plant pathogenic fungus Cochliobolus carbonum
Alternaria jesenskae produces HC-toxin An isolate of A. jesenskae was obtained and its taxonomic identity confirmed by sequencing of the ITS regions [15]
Alternaria jesenskae has unmistakable orthologs of the TOX2 genes The genome of A. jesenskae was determined to ~10× coverage by pyrosequencing followed by assembly
Summary
HC-toxin, a cyclic tetrapeptide, is a virulence determinant for the plant pathogenic fungus Cochliobolus carbonum. Many secondary metabolites play important ecological roles in the interactions between microbes and other organisms. Some, such as the host-selective toxins, are virulence factors for plant pathogenic fungi [1]. HC-toxin is a host-selective toxin that endows the pathogenic fungus Cochliobolus carbonum with exceptional virulence on maize varieties that lack a functional copy of HM1 and/ or HM2, both of which encode a carbonyl reductase that detoxifies HC-toxin [5].
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