High-resolution mapping of Ryd4^Hb, a major resistance gene to Barley yellow dwarf virus from Hordeum bulbosum

We mapped Ryd4^Hb in a 66.5 kbp interval in barley and dissociated it from a sublethality factor. These results will enable a targeted selection of the resistance in barley breeding. Virus diseases are causing high yield losses in crops worldwide. The Barley yellow dwarf virus (BYDV) complex is responsible for one of the most widespread and economically important viral diseases of cereals. While no gene conferring complete resistance (immunity) has been uncovered in the primary gene pool of barley, sources of resistance were searched and identified in the wild relative Hordeum bulbosum, representing the secondary gene pool of barley. One such locus, Ryd4^Hb, has been previously introgressed into barley, and was allocated to chromosome 3H, but is tightly linked to a sublethality factor that prevents the incorporation and utilization of Ryd4^Hb in barley varieties. To solve this problem, we fine-mapped Ryd4^Hb and separated it from this negative factor. We narrowed the Ryd4^Hb locus to a corresponding 66.5 kbp physical interval in the barley 'Morex' reference genome. The region comprises a gene from the nucleotide-binding and leucine-rich repeat immune receptor family, typical of dominant virus resistance genes. The closest homolog to this Ryd4^Hb candidate gene is the wheat Sr35 stem rust resistance gene. In addition to the fine mapping, we reduced the interval bearing the sublethality factor to 600 kbp in barley. Aphid feeding experiments demonstrated that Ryd4^Hb provides a resistance to BYDV rather than to its vector. The presented results, including the high-throughput molecular markers, will permit a more targeted selection of the resistance in breeding, enabling the use of Ryd4^Hb in barley varieties.