Chapter Four Characterization of the Resistance Conditioned by the Rsv4 Gene for Resistance to SMV in Soybean

Resistance to Soybean mosaic virus (SMV; Genus Potyvirus; Family Potyviridae) in soybean (Glycine max [L.] Merr.) is controlled by single dominant genes at three distinct loci identified by genetic studies, molecular mapping, and responses to SMV pathotypes. The mechanism of resistance at the Rsv4 locus was investigated by tracking virus accumulation and movement over time. Soybean lines Peking, PI 88788, V94-5152, and Essex-Rsv4 isoline with the Rsv4 gene derived from V94-5152 were inoculated with SMV strains G1, G6 and G7 to monitor events during the infection process. Leaves were sampled 2, 4, 9, 14, 19 and 26 days post inculation and tested for SMV presence by leaf immuno-printing. The number of infection sites, rate and extent of viral movement, both cell-to-cell and long-distance, were all reduced in comparison to that in the susceptible cv. Essex or the resistant PI 96983 (Rsv1). Virus accumulation was reduced in the inoculated leaves and not detected in the upper, non-inoculated trifoliolate leaves. Vascular movement of the virus was also delayed and restricted. PI 88788 and Peking were more resistant to SMV than were Essex-Rsv4 and V94-5152, suggesting different alleles. It was concluded that Rsv4 resistance is non-strain-specific, non-necrotic to any strain, and does not function through a hypersensitive or necrotic response. Its mechanism is to reduce infection and accumulation, and limit and delay local and systemic invasion of plants by restricting both cell-to-cell and long distance movement of the virus.