Candidate gene analysis of quantitative disease resistance in wheat

Knowledge of the biological significance underlying quantitative trait loci (QTLs) for disease resistance is generally limited. In recent years, advan ces in plant-microbe interactions and genome mapping have lead to an increa sed understanding of the genes involved in plant defense and quantitative d isease resistance. Here, we report on the application of the candidate-gene approach to the mapping of QTLs for disease resistance in a population of wheat recombinant inbreds. Over 50 loci, representing several classes of de fense response (DR) genes, were placed on an existing linkage map and the g enome was surveyed for QTLs associated with resistance to several diseases including tan spot, leaf rust, Karnal bunt, and stem rust. Analysis reveale d QTLs with large effects in regions of putative resistance (R) genes, as p reviously reported. Several candidate genes, including oxalate oxidase, per oxidase, superoxide dismutase, chitinase and thaumatin, mapped within previ ously identified resistance QTLs and explained a greater amount of the phen otypic variation. A cluster of closely linked DR genes on the long arm of c hromosome 7B, which included genes for catalase, chitinase, thaumatins and an ion channel regulator, had major effects for resistance to leaf rust of adult plants under conditions of natural infestation. The results of this s tudy indicate that many minor resistance QTLs may be from the action of DR genes, and that the candidate-gene approach can be an efficient method of Q TL identification.