Rph22: A Gene Conferring Partial Resistance to Barley Leaf Rust Derived from the Non-Host Hordeum bulbosum

Plant breeders are always searching for effective resistance to problem pathogens such as barley leaf rust (Puccinia hordei), which causes significant losses of yield in barley crops. Hordeum bulbosum is an undomesticated relative of cultivated barley that is a non-host to P. hordei and as such is a potential source of effective resistance. The previous development of introgression lines between barley and H. bulbosum has resulted in the identification of resistance loci to barley leaf rust on seven different chromosome arms. These different resistance loci include examples of both hypersensitive and non-hypersensitive resistance mechanisms and are likely to include some of the components of the H. bulbosum non-host resistance system to P. hordei. One locus, assigned Rph22, confers a very high level of non-hypersensitive resistance to P. hordei when transferred from H. bulbosum into the susceptible barley cultivar ‘Golden Promise’. We have fine mapped this locus to the distal end of chromosome 2HL between the flanking markers H35_15816 and H35_759 (HarvEST database assembly 35). Marker assisted selection has also allowed us to reduce the size of the introgression surrounding Rph22 and thus minimize the linkage drag of undomesticated alleles from H. bulbosum. This same marker interval is also known to harbor a locus (Rphq2) conferring partial resistance to barley leaf rust from the barley cultivar ‘Vada’. Further investigation of these two loci (Rph22 and Rphq2) may reveal more information on the nature of host/non-host resistance mechanisms and may lead to future barley cultivars with greater protection from disease threats.