Tan spot of wheat: A different view of gene-for-gene

Pyrenophora tritici-repentis is the causal agent of tan spot of wheat, a serious disease of worldwide, economic importance. Our work and the work of others has established that pathogenicity/virulence of this pathogen is dependent on the production of host-selective toxins (HSTs). The fungus produces a number of HSTs and the complex race structure of the pathogen appears to arise as a result of the differential expression of one or a combination of these HSTs. This disease interaction appears to be a true gene-for-gene interaction except that in this case, virulence is the dominant feature of the pathogen and disease susceptibility is the dominant feature of the host. This is the inverse of "classical" gene-for-gene interactions where avirulence and resistance are dominant. Consequently, just as the molecular evaluation of "classical" gene-for-gene interactions has contributed so much to our understanding of avirulence and disease resistance (incompatibility), we feel that a molecular evaluation of tan spot of wheat holds tremendous promise for understanding virulence and disease susceptibility (compatibility). Thus, tan spot of wheat presents not only a disease interaction of significant economic impact where characterization of virulence determinants could be exploited for the control of this wide-ranging and serious disease, but also a disease interaction with the potential to become a "model" gene-for-gene system for understanding pathogen virulence and host disease susceptibility.