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Salt is a severe abiotic stress that reduces soybean [Glycine max (L.) Merr.] yield by >20% in saline soils and irrigated fields. Marker-assisted selection (MAS) is an efficient method to identify salt-tolerant soybean lines. 'Osage' is a salt-tolerant and high-yielding cultivar in the Mid-South of the United States and is a distant progeny tracing five generations back to 'S-100'. The objective of this study was to determine the inheritance of salt tolerance in Osage soybean by identifying the quantitative trait loci (QTL) controlling the trait. A chloride (Cl-) includer, 'RA-452', was crossed with Cl- excluder, Osage, to develop an F4:6 mapping population. The F4:6 lines were genotyped by 5403 single nucleotide polymorphism (SNP) markers spanning 20 chromosomes (Chr.), of which, 1269 were polymorphic. The salt stress response of F4:6 lines, along with parental genotypes, was evaluated in the greenhouse in April and June 2013. Three different treatments (120 mM NaCl, 120 mM KCl, and water) were initiated at V1 stage and continued for ~18 to 21 d. The leaf Cl- contents were quantified by inductively coupled plasma optical emission spectroscopy. Composite interval mapping analysis indicated that a major Cl--tolerant QTL was confirmed and narrowed down on Chr. 3 in both NaCl and KCl treatments, a novel Cl--tolerant QTL on Chr. 15 was identified in NaCl treatment, and a novel Cl--tolerant QTL on Chr. 13 was identified in the KCl treatment. Two annotated genes, Glyma.13G161800 and Glyma.15G091600, were proposed to be the candidate gene conferring the Cl- tolerance within QTL region on Chr. 13 and 15, respectively. A total of 27 SNP markers were significantly associated with Cl- tolerance, which could be used for MAS in breeding salt-tolerant soybean lines. [ABSTRACT FROM AUTHOR]

Soybean mosaic virus (SMV), a member of the Potyviridae family, is the most common virus negatively affecting yield and seed quality in soybean [Glycine max (L.) Merr.]. Seven SMV strains, G1 through G7, and three independent SMV resistance genes (R-genes), Rsv1, Rsv3, and Rsv4, have been previously identified. The Rsv1 locus contains at least 10 alleles conferring differential plant reactions to SMV strains, and it was mapped at very complex resistancegene-rich region. In this study, two alleles of the Rsv1 locus were analyzed by crossing PI 96983 (Rsv1) and cultivar York (Rsv1-y) soybean to evaluate whether Rsv1 and Rsv1-y belong to the same or different but closely linked loci. To break possible linkage, 3000 F2:3 lines were developed and investigated using infections of the SMVG1 strain in a greenhouse. The occurrence of segregating and susceptible lines indicated tight linkage between two genes. The recombination frequency (RF) was estimated using the maximum likelihood (ML) formula concluding that Rsv1 and Rsv1-y are two tightly linked loci located apart with genetic distance of 2.2 cM on chromosome 13. A new symbol of Rsv5 has been assigned for the resistance gene in York to replace the original Rsv1-y allele assignment. This research provided the first evidence for existence of two R-genes on chromosome 13, conferring resistance to different SMV strains. Both loci, Rsv1 and Rsv5, can be easily transferred into susceptible cultivars in a breeding program to provide broad and durable protection against SMV strains. [ABSTRACT FROM AUTHOR]