Comparative Studies on Soybean Mosaic Virus Strains B and C: Nucleotide Sequences of the Capsid Protein Genes and Virulence in Soybean Cultivars.

Soybean mosaic virus (SMV), which belongs to the genus Potyvirus, causes the most widespread, economi cally important viral disease of soybean (Glycine max (L.) Merr.)3). It is transmitted by aphids or through seeds of infected soybean. Infection by SMV causes various symptoms, depending on the combination of soybean genotype and virus strain. Based on host range and virulence in several selective soybean lines, the 98 iso lates of SMV from seeds in the U.S. Department of Agriculture (USDA) soybean germplasm collections have been classified into seven virulence groups (Gl to G7)4). In a similar way, the SMV isolates occurring in Japan have been grouped into five strains and designated as A, B, C, D and E14) In a previous survey we showed the prevalence of SMV strains B and C in soybean fields in Niigata prefecture7). The soybean cv. Enrei, which is widely grown in this area, is resistant to SMV-B but susceptible to SMV-C. Consequently, most of the isolates affecting Enrei plants were identified as SMV-C, whereas SMV-B was predominantly identified in other soybean cultivars including the green soybean varieties. The five SMV strains in Japan were previously shown to serologically indistinguishable 13,16) As for the SMV strains in the United States, the capsid protein (CP) genes of three isolates (N, G2, G7) have been sequenced and shown to share at least 98% amino acid identity"). However, since strains GI to G7 and A to E were classified on the basis of different soybean cultivars, their mutual rela tionships remain obscure. In order to elucidate the inter viral relatedness and molecular variability among different SMV strains, we have cloned and sequenced the CP genes of the two SMV isolates which belong to the B and C strains. Brief reports describing parts of this study have been published elsewhere12,15). SMV-B87 was isolated from infected soybean seeds and propagated in soybean (cv. Tokachi nagaha) by mechanical inoculation. SMV-CN, an aphid non-trans missible isolate of SMV-C, was derived from naturally infected soybean plants (cv. Enrei) and propagated in soybean (cv. Enrei). The phenotypic reactions of various soybean lines to SMV-B87 and CN are shown in Table 1. All the inoculation experiments were done in a green house at various seasons and repeated at least six times. Virus infection (or non-infection) was verified serologi cally by dot immuno-binding assay (DIBA) as described