Biological and sequence analysis of a novel European isolate of Barley mild mosaic virus that overcomes the barley rym5 resistance gene.

A Barley mild mosaic virus (BaMMV) isolate from France (BaMMV-Sil) capable of overcoming rym5-controlled resistance was inoculated to barley genotypes carrying various genes for resistance to the barley mosaic viruses. BaMMV-Sil was unable to infect genotypes carrying rym1, rym4, rym8, rym9, or rym11 but genotypes carrying rym3, rym5, rym6 or no known bymovirus resistance gene were susceptible. Plants carrying rym7 or rym10 showed partial resistance with delayed virus accumulation. The two genomic RNAs of BaMMV-Sil were sequenced and compared to published sequences and those of a further common strain isolate from the UK. Four amino acid differences were observed between BaMMV-Sil and European common strain isolates in the polypeptide encoded by RNA1, the RNA species which determines pathogenicity on the rym5 genotypes. Only two of these differences are likely to be functionally important (His rather than Gln at position1217 in the VPg cistron; His rather than Asp at position 1776 in the NIb cistron). Comparisons with related viruses in the genera Bymovirus and Potyvirus suggest that the change in the VPg, which occurs within a motif conserved amongst all viruses within the family Potyviridae, is the more likely cause of rym5 resistance-breaking.