| Soybean (Glycine max (L.) Merr.) sudden death syndrome (SDS), caused by Fusarium solani f. sp glycines (Fsg), reduces yields by 0.5–1% per year. The soybean cyst nematode, Heterodera glycines Ichinohe (Hg) reduces yields by 5–7% annually. Both SDS and SCN can be controlled with host resistance. However, host resistance is not easily achieved by conventional selection methods due to high costs and genotype by environment interactions. Molecular biotechnology could be used to increase host resistance via low cost and reproducible assays using DNA markers.; The region between the DNA markers BARC-Satt309 and SIUC-Sat122 markers on linkage group G contains a cluster of important loci for resistance to SDS (including Rfs 1, controlling leaf-scorch by restricting the spread of root infection by Fsg and Rfs2, controlling leaf scorch related to Fsg toxin) and a locus, Rhg1 for resistance to SCN Hg type 7 that causes plant cell necrosis by pathogen recognition at the feeding site. To locate the positions of these resistance genes, a fine map within the region between BARC-Satt309 and SIUC-Sat122 was constructed using 80 near isogenic lines (NILs) derived from the recombinant inbred lines ExF11 and ExF34. The fine map included nine DNA markers mapped within the interval spanning 5 cM at an average distance of 0.5 cM between markers. The marker order was consistent with published maps. Substitution maps were developed using recombinants within the interval identified from among 3500 NILs. Genotype groups were created based on the region containing the recombination events. Phenotype data were compared with genotype groups to determine the position of the genes.; Substitution mapping of the Rhg1 gene indicated that the gene may be located between ATG4-SCAR and TMD markers. The DNA marker TMD was found to be more closely linked to the Rhg1gene than other flanking markers. Substitution mapping of the Rfs1gene indicated that it may be located between OI03-P4 and SIUC-Sat122 markers. The Rfs2 gene is most likely located between TMD and SIUC-Sat1 markers. This study infers the existence of a new locus that was derived from a beneficial allele of Essex, Rfs7, located close to OI03-P4 in the interval between the DNA markers OI03-P4 and SIUC-Sat1. Gene interactions were evident. When Rfs1 was present in the absence of Rfs2, plants were more sensitive to leaf-scorch. There may be a toxin sensitivity gene interaction. Genes linked to Rfs1, such as Rfs7, may result in increased susceptibility to leaf-scorch in plants. DNA sequence annotation identified a laccase gene located downstream of the Rhg1 gene. Bioinformatics suggest this may be the Rfs2/Rft gene. Laccase has several potential physiological roles including plant wound healing, lignification, wood rotting, substrate degradation, and toxin detoxification.; It was difficult to study the effect of each gene for resistance to SDS due to their multigenic and complex nature. This study concluded that resistance to SDS may involve several genes whose effect cannot be measured without further fine mapping based on DNA sequence. Therefore, DNA sequences, gene maps, and gene interactions should be integrated to reveal the molecular basis of resistance to SDS. |
