Molecular mapping of resistance to Pyrenophora tritici-repentis race 5 and sensitivity to Ptr ToxB in wheat

Tan spot, caused by Pyrenophora tritici-repentis (Ptr), is an economically important foliar disease in the major wheat growing areas of the world. Multiple races of the pathogen have been characterized based on their ability to cause necrosis and/or chlorosis in differential wheat lines. Isolates of race 5 cause chlorosis only, and they produce a host-selective toxin designated Ptr ToxB that induces chlorosis when infiltrated into sensitive genotypes. The international Triticeae mapping initiative (ITMI) mapping population was used to identify genomic regions harboring QTLs for resistance to fungal inoculations of Ptr race 5 and to determine the chromosomal location of the gene conditioning sensitivity to Ptr ToxB. The toxin-insensitivity gene, which we are designating tsc2, mapped to the distal tip of the short arm of chromosome 2B. This gene was responsible for the effects of a major QTL associated with resistance to the race 5 fungus and accounted for 69% of the phenotypic variation. Additional minor QTLs were identified on the short arm of 2A, the long arm of 4A, and on the long arm of chromosome 2B. Together, the major QTL on 2BS identified by tsc2 and the QTL on 4AL explained 73% of the total phenotypic variation for resistance to Ptr race 5. The results of this research indicate that Ptr ToxB is a major virulence factor, and the markers closely linked to tsc2 and the 4A QTL should be useful for introgression of resistance into adapted germplasm.