Location: Cereal Crops ResearchTitle: A ToxA-like protein from Cochliobolus heterostrophus induces light-dependent leaf necrosis and acts as a virulence factor with host selectivity on maize
|TURGEON, B - Cornell University - New York|
Submitted to: Fungal Genetics and Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/29/2015
Publication Date: 6/4/2015
Publication URL: https://handle.nal.usda.gov/10113/61007
Citation: Lu, S., Turgeon, B.G., Edwards, M.C. 2015. A ToxA-like protein from Cochliobolus heterostrophus induces light-dependent leaf necrosis and acts as a virulence factor with host selectivity on maize. Fungal Genetics and Biology. 81:12-24.
Interpretive Summary: Tan spot is a major diseases of wheat that is caused by the fungus Pyrenophora tritici-repentis (Ptr), which produces and relies on a small protein called ToxA to induce cell death on host plants. Southern Corn Leaf Blight is a major disease of maize that is caused by the fungus Cochliobolus heterostrophus (Ch), which is known to produce virulence-related toxins derived from secondary metabolites but not from proteins. In this manuscript, we describe, for the first time, the identification and functional characterization of a ToxA-like protein from C. heterostrophus that induces cell death on maize in a host-selective manner. We show that ChToxA and PtrToxA are encoded by genes with similar sequences; both proteins would adopt the same three-dimensional structures, and their toxicities on respective hosts all depend on light. We demonstrate further that the ChTOXA gene is up-regulated during fungal pathogenesis, and deletion of the ChTOXA gene from the fungal pathogen results in reduced virulence on specific ChToxA-sensitive maize lines. In addition, we describe potential ChToxA homologues identified in other 11 plant-pathogenic fungi. Our findings suggest that the production of host-selective proteinaceous toxins may be a common strategy employed by different fungal pathogens, and the sensitivity to structurally-related toxins may be controlled by similar pathways in related crop plants such as wheat and maize. Further studies on ChToxA and other ToxA-related proteins may help understand host-pathogen interactions and develop novel strategies for cereal crop protection.
Technical Abstract: ToxA, the first discovered fungal proteinaceous host-selective toxin (HST), was originally identified in 1989 from the tan spot fungus Pyrenophora tritici-repentis (Ptr). About 25 years later, a homolog was identified in the leaf/glume blotch fungus Stagonospora nodorum (Parastagonospora nodorum), also a pathogen of wheat. Here we report the identification and function of a ToxA-like protein from the maize pathogen Cochliobolus heterostrophus (Ch) that possesses necrosis-inducing activity specifically against maize. ChToxA is encoded by a 535-bp open reading frame featuring a ToxA-specific intron with unusual splicing sites (5’-ATAAGT…TAC-3’) at conserved positions relative to PtrToxA. The protein shows 64% similarity to PtrToxA and is predicted to adopt a similar three-dimensional structure, although lacking the arginyl-glycyl-aspartic acid (RGD) motif reported to be required for internalization into sensitive wheat mesophyll cells. Reverse-transcriptase PCR revealed that the ChTOXA gene expression is up-regulated in planta, relative to axenic culture. Plant assays indicated that the recombinant ChToxA protein induces light-dependent leaf necrosis in a host-selective manner on maize inbred lines. Gene deletion experiments confirmed that ChtoxA mutants are reduced in virulence on specific ChToxA-sensitive maize lines, relative to virulence caused by wild-type strains. Database searches identified potential ChToxA homologues in other plant-pathogenic ascomycetes. Sequence and phylogenetic analyses revealed that the corresponding ToxA-like proteins include one member recently shown to be associated with formation of penetration hypha. These results provide the first evidence that C. heterostrophus is capable of producing proteinaceous HSTs as virulence factors in addition to well-known secondary metabolite-type toxins produced biosynthetically by polyketide synthase megaenzymes. Further studies on ChToxA may provide new insights into effector evolution in host-pathogen interactions.