Chapter Five Mechanisms of the Soybean Rsv3 Gene for Resistance to Soybean mosaic virus

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, Rsv1, Rsv3 and Rsv4. The mechanisms of resistance at the Rsv3 locus were investigated in Harosoy and the Williams isoline L29 by tracking virus replication and movement. Two other cultivars with two genes for resistance to SMV were also tested: Columbia with Rsv3 and Rsv4, and Tousan 140 with Rsv1 and Rsv3. Greenhouse-grown plants of each line were inoculated with SMV strains G1, G6 and G7 to monitor events during the infection process. Leaves were sampled over time and tested for the presence of SMV by immuno-printing. Number and size of infection sites and proportion of leaf area infected were determined. Results confirmed that the SMV resistance conditioned by Rsv3 in Harosoy is strain specific and its mechanisms involve restriction of virus accumulation and movement, both cell-to-cell and long distance with G6 and G7. Interactions of L29 and SMV confirmed that Rsv3 in L29 conditions an extreme resistance (ER) with no detectable virus to both G6 and G7. The Rvs3 alleles in Harosoy and L29 are different, therefore we designated them at Rsv3 and Rsv3. Cultivars carrying both Rsv1 and Rsv3 or Rsv3 and Rsv4 genes were resistant to all strains tested. These results support the use of gene pyramiding to insure a wider and more durable resistance to SMV.