Host-specific alterations in viral RNA accumulation and infection spread in a brome mosaic virus isolate with an expanded host range.

To facilitate studies of virus-host interaction and the determinants of viral host range, we constructed full-length cDNA clones to all three genomic RNAs of an unusual brome mosaic virus (BMV) isolate with an expanded host range. While other BMV strains, including the previously cloned M1 strain, systemically infect barley and other grasses but not legumes, the expanded-host-range isolate and the set of transcripts from its cDNA clones, designated the M2 strain of BMV, systemically infect both barley and cowpea line TVu-612, a legume. All reassorted combinations of M1 and M2 genomic RNAs were equally competent for replication in barley protoplasts and systemic infection of barley plants but showed widely varying levels of viral RNA accumulation in cowpea protoplasts and systemic infection in TVu-612 cowpea plants. Systemic infection levels were influenced by all three genomic RNAs. M2 RNA2 and M2 RNA3 made independent and additive contributions to the frequency with which reassortants infected TVu-612 systemically. The greater individual effect segregated with M2 RNA3, which encodes functions required for infection spread (the 3a movement protein and coat protein). M2 RNA3 also directed accelerated expansion of BMV lesions in inoculated TVu-612 leaves. If the inoculum contained M2 RNA3, the frequency with which reassortants infected TVu-612 systemically could be further enhanced by the presence of M2 RNA1 rather than M1 RNA1. RNA1 encodes the 1a RNA replication protein, and despite similar accumulation in barley protoplasts, in cowpea protoplasts all reassortants bearing M2 RNA1 accumulated positive- and negative-strand RNAs to levels at least six- to eightfold higher than reassortants bearing M1 RNA1. Overall, the results indicate that changes in several distinct virus functions contribute to adapting BMV-M2 to systemically infect TVu-612 cowpea.