Soybean mosaic virus: A successful potyvirus with a wide distribution but restricted natural host range.

TaxonomySoybean mosaic virus (SMV) is a species within the genus Potyvirus, family Potyviridae, which includes almost one-quarter of all known plant RNA viruses affecting agriculturally important plants. The Potyvirus genus is the largest of all genera of plant RNA viruses with 160 species. ParticleThe filamentous particles of SMV, typical of potyviruses, are about 7500 angstrom long and 120 angstrom in diameter with a central hole of about 15 angstrom in diameter. Coat protein residues are arranged in helices of about 34 angstrom pitch having slightly less than nine subunits per turn. GenomeThe SMV genome consists of a single-stranded, positive-sense, polyadenylated RNA of approximately 9.6kb with a virus-encoded protein (VPg) linked at the 5 terminus. The genomic RNA contains a single large open reading frame (ORF). The polypeptide produced from the large ORF is processed proteolytically by three viral-encoded proteinases to yield about 10 functional proteins. A small ORF, partially overlapping the P3 cistron, pipo, is encoded as a fusion protein in the N-terminus of P3 (P3N+PIPO). Biological propertiesSMV's host range is restricted mostly to two plant species of a single genus: Glycine max (cultivated soybean) and G.soja (wild soybean). SMV is transmitted by aphids non-persistently and by seeds. The variability of SMV is recognized by reactions on cultivars with dominant resistance (R) genes. Recessive resistance genes are not known. Geographical distribution and economic importanceAs a consequence of its seed transmissibility, SMV is present in all soybean-growing areas of the world. SMV infections can reduce significantly seed quantity and quality (e.g. mottled seed coats, reduced seed size and viability, and altered chemical composition). ControlThe most effective means of managing losses from SMV are the planting of virus-free seeds and cultivars containing single or multiple R genes. Key attractionsThe interactions of SMV with soybean genotypes containing different dominant R genes and an understanding of the functional role(s) of SMV-encoded proteins in virulence, transmission and pathogenicity have been investigated intensively. The SMV-soybean pathosystem has become an excellent model for the examination of the genetics and genomics of a uniquely complex gene-for-gene resistance model in a crop of worldwide importance.