Virus diseases are widespread threats for crop production, which can, in many cases, be controlled efficiently by exploiting naturally occurring resistance. Barley, an important cereal species of the Triticeae, carries two genes, rym4 and rym5, which are located in the telomeric region of chromosome 3HL and confer recessive resistance to various strains of the Barley yellow mosaic virus complex. The barley 'eukaryotic translation initiation factor 4E' (Hv-eIF4E) was identified as a candidate for resistance gene function by physical mapping on a 650 kb contig. It is located in a chromosomal region characterized by suppressed recombination, in a position collinear to its homologue on rice chromosome 1L. Sequence diversity in the coding region of Hv-eIF4E, as calculated from a collection of unrelated barley accessions, revealed non-silent single nucleotide polymorphisms (SNPs) in four of its five exons. Stable transformation of a resistant barley genotype with a genomic fragment or a full-length cDNA of Hv-eIF4E derived from susceptible cultivars induced susceptibility to Barley mild mosaic virus. Moreover, the identification of SNPs diagnostic for rym4 and rym5 provides evidence that these are two alleles, which confer different resistance specificities. These findings demonstrate that variants of Hv-eIF4E confer multiallelic recessive virus resistance in a monocot species. The identification of eIF4E as the causal host factor for bymovirus resistance illustrates that mutations in this basic component of the eukaryotic translation complex form a seminal mechanism for recessive virus resistance in both dicot and monocot plants.
