RESUMEN
Twelve Rsv resistance-breaking (RB) isolates of Soybean mosaic virus (SMV) were obtained from field-grown soybean plants showing mosaic symptoms and subsequently examined biologically and molecularly. All of these RB isolates were identified as SMV based on serological and infectivity assays, and the amplification of P1 gene products by reverse transcription-polymerase chain reaction (RT-PCR). Differential soybean cultivars, lines or accessions Lee 68 (rsv), PI 96983, York, Marshall, Ogden, Kwanggyo, Suweon 97 (Rsv1 alleles), L29 (Rsv3), and V94-5152 (Rsv4), following inoculation with each RB isolate, showed similar systemic symptoms suggesting that these RB isolates can overcome Rsv resistance at three loci. To differentiate the 12 RB isolates molecularly, the P1 coding region for each isolate was amplified, cloned, sequenced and compared to known SMV strains. The P1 region from the RB isolates shared 86-90% and 90-99% similarities in amino acid (aa) and nucleotide sequence, respectively, with known SMV strains. Comparison of aa sequences indicated that these RB isolates are newly emerging isolates capable of breaking Rsv resistance. Phylogenetic analysis further suggested that the RB isolates can be classified as three major types. However, recombination was not observed within the coding region of P1 protein among the types. This is the first report on the emergence of SMV isolates capable of overcoming all of the known resistance alleles at the Rsv1 locus, as well as distinct resistance genes at Rsv3 and Rsv4.