Submitted to: Cellular Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/20/2008
Publication Date: 4/22/2008
Citation: Friesen, T.L., Faris, J.D., Solomon, P.S., Oliver, R.P. 2008. Host specific toxins; effectors of necrotrophic pathogenicity. Cellular Microbiology. 10(7) 1421-1428
Interpretive Summary: Host-specific toxins (HSTs) are important virulence factors for several pathogens found in the Dothideomycete class of fungi. HSTs are used by these pathogens to incite disease, causing substantial yield losses worldwide. Fungal genera in this class include Cochliobolus, Alternaria, Stagonospora, and Pyrenophora. These genera contain pathogens that cause diseases such as tan spot of wheat, net blotch of barley, spot blotch of barley, Southern and Northern corn leaf blight, Stagonospora leaf and glume blotch of wheat, Victoria blight of oat, as well as many others. Stagonospora nodorum has developed into a model system for studying the Dothidiomycetes due to its production of multiple proteinaceous HSTs which interact directly or indirectly with the products of dominant host susceptibility genes resulting in disease development. This review evaluates this system and also speculates on the potential for lateral gene transfer of these virulence genes between pathogens in this class.
Technical Abstract: Host-specific toxins are defined as pathogen effectors that induce toxicity and promote disease only in the host species and only in cultivars of that host, and with few exceptions, expressing a specific dominant susceptibility gene. They are a feature of a small but well studied group of fungal plant pathogens. Classical HST pathogens include species of Cochliobolus, Alternaria and Pyrenophora. Recent studies have shown that Stagonospora nodorum produces at least four separate host-specific toxins that interact with some of the many quantitative resistance loci found in the host wheat. Rationalisation of fungal phylogenetics has placed these pathogens in the Pleosporales order of the Class Dothideomycetes. It is possible that all HST pathogens lie in this order. Strong evidence of the recent lateral gene transfer of the ToxA gene from Stagonospora nodorum to Pyrenophora tritici-repentis has been obtained. Hallmarks of lateral gene transfer are present for all the studied HST genes although final proof is lacking. We therefore suggest that the Pleosporales pathogens may have a conserved propensity to acquire HST genes by lateral transfer.