Five soybean [Glycine max (L.) Merr.] cultivars that have been used to differentiate strain groups of soybean mosaic virus (SMV) were studied to determine the allelic relationships among genes conditioning reaction to SMV. The cultivars PI 96983, ‘Ogden’, ‘York’, ‘Marshall’, and ‘Kwanggyo’ (PI 406710) are each known to have single dominant gene conditioning resistance to SMV. These parents were crossed in all possible combinations with each other and with a susceptible cultivar. The crosses were evaluated in the F2 and with F2‐derived F3 lines in the field and in the greenhouse for reaction to inoculation with the G1 strain of SMV (SMV‐G1). Results confirm that each of the resistant cultivars has a single dominant gene for resistance to SMV‐G1, but that dominance is sometimes incomplete. The lack of segregation for susceptibility in F2 and F3 progenies from the resistant ✕ resistant crosses indicates a high probability that the resistance genes in these cultivars are alleles at a common locus. Gene symbols Rsvy1, Rsvm1 and Rsvk1 are proposed for the resistance alleles in York, Marshall, and Kwanggyo, respectively. The systemic necrosis reaction that sometimes occurs following SMV‐G1 inoculation of segregating populations is highly associated with plants heterozygous for the resistance gene, but may be influenced by environment and genetic background.
The soybean Rsv1 gene for resistance to soybean mosaic virus (SMV; Potyvirus) has previously been described as a single-locus multi-allelic gene mapping to molecular linkage group (MLG) F. Various Rsv1 alleles condition different responses to the seven (G1-G7) described strains of SMV, including extreme resistance, localized and systemic necrosis, and mosaic symptoms. We describe the cloning of a cluster of NBS-LRR resistance gene candidates from MLG F of the virus-resistant soybean line PI96983 and demonstrate that multiple genes within this cluster interact to condition unique responses to SMV strains. In addition to cloning 3gG2, a strong candidate for the major Rsv1 resistance gene from PI96983, we describe various unique resistant and necrotic reactions coincident with the presence or absence of other members of this gene cluster. Responses of recombinant lines from a high-resolution mapping population of PI96983 (resistant) ϫ Lee 68 (susceptible) demonstrate that more than one gene in this region of the PI96983 chromosome conditions resistance and/or necrosis to SMV. In addition, the soybean cultivars Marshall and Ogden, which carry other previously described Rsv1 alleles, are shown to possess the 3gG2 gene in a NBS-LRR gene cluster background distinct from PI96983. These observations suggest that two or more related non-TIR-NBS-LRR gene products are likely involved in the allelic response of several Rsv1-containing lines to SMV.
Resistance to Soybean mosaic virus (SMV) was identified in PI 88788 soybean, a germ plasm accession from China that is used widely as a source of resistance to soybean cyst nematode. Strains SMV-G1 through -G7 infected the inoculated leaves of PI 88788 but were not detected in upper, noninoculated trifoliolate leaves. Inheritance of resistance was determined by inoculating progenies of crosses of PI 88788 with susceptible cvs. Essex and Lee 68 with SMV strains G1 and G7. Allelomorphic relationships with known genes for resistance to SMV were tested in crosses with the resistant genotypes PI 96983, L29, and V94-5152, possessing Rsv1, Rsv3, and Rsv4 genes, respectively. Data analyses showed that resistance in PI 88788 to SMV-G1 is controlled by a single, partially dominant gene; however, to SMV-G7, the same gene was completely dominant. The PI 88788 gene was independent of the Rsv1 and Rsv3 loci, but allelic to Rsv4 in V94-5152. Expression of the Rsv4 gene in PI 88788 resulted in a reduced number of infection sites and restricted short- and long-distance movement of virus, rather than hypersensitivity. A unique late susceptible phenotype was strongly associated with heterozygosity. This gene has potential value for use in gene pyramiding to achieve resistance to several SMV strains, as well as for rate-reducing resistance.
Soybean [Glycine max (L.) Merr.] PI486355 is resistant to all the identified strains of soybean mosaic virus (SMV) and possesses two independently inherited resistance genes. To characterize the two genes, PI486355 was crossed with the susceptible cultivars 'Lee 68' and 'Essex' and with cultivars 'Ogden' and 'Marshall', which are resistant to SMV-G1 but systemically necrotic to SMV-G7. The F2 populations and F2∶3 progenies from these crosses were inoculated with SMV-G7 in the greenhouse. The two resistance genes were separated in two F3∶4 lines, 'LR1' and 'LR2', derived from Essex x PI486355. F1 individuals from the crosses of LR1 and LR2 with Lee 68, Ogden, and 'York' were tested with SMV-G7 in the greenhouse; the F2 populations were tested with SMV-G1 and G7. The results revealed that expression of the gene in LR1 is gene-dosage dependent, with the homozygotes conferring resistance but the heterozygotes showing systemic necrosis to SMV-G7. This gene was shown to be an allele of the Rsv1 locus and was designated as Rsv1-s. It is the only allele identified so far at the Rsv1 locus which confers resistance to SMV-G7. Rsv1-s also confers resistance to SMV-G1 through G4, but results in systemic necrosis with SMV-G5 and G6. The gene in LR2 confers resistance to strains SMV-G1 through G7 and exhibits complete dominance. It appears to be epistatic to genes at the Rsv1 locus, inhibiting the expression of the systemic necrosis conditioned by the Rsv1 alleles. SMV-G7 induced a pin-point necrotic reaction on the inoculated primary leaves in LR1 but not in LR2. The unique genetic features of the two resistance genes from PI486355 will facilitate their proper use and identification in breeding and contribute to a better understanding of the interaction of SMV strains with soybean resistance genes.
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