The molecular genetics of disease resistance in barley

The molecular genetics of disease resistance of barley and its wild relatives is reviewed, and the implications of recent findings for resistance breeding and the potential for disease control using gene technologies are discussed. As a resource for barley researchers and breeders, a chromosome map and list of mapped resistance genes, their source, and associated molecular markers are presented, updated to ultimo 2002. Genetic mapping of major genes and quantitative trait loci for many major diseases is revealing a heterogeneous distribution of resistance loci on chromosomes, with more than half of mapped loci occurring in clusters. Relatively few resistance loci have been identified in the cultivated barley germplasm. Studies have shown that wild Hordeum species contain resistance genes for the major diseases, although their allelic relationship to previously mapped genes is unknown. The structure of genes involved in race- specific and race- non- specific barley powdery mildew resistance has been determined. Isolation of resistance genes for other major diseases is essential and may be accelerated via genomics techniques such as EST sequencing, subtractive hybridisation, or expression profiling. Current strategies for molecular manipulation of barley disease resistance are based on the over- expression of defence- related or disease- signalling genes from other species.