In 2012, the first domestic commercial edamame processing plant was established in Arkansas and edamame production was contracted out to local growers. Although the state is a major soybean producer, studies of nematode effects on edamame are limited. A survey of nematode genera and density in 64 contracted edamame production fields was conducted in 2013 and 2014. In both years, Meloidogyne and Heterodera were present in less than half of the surveyed fields while Pratylenchus was the most prevalent in 2013 and Helicotylenchus in 2014. A microplot study was conducted in 2014 in two locations to evaluate the effects of root-knot nematode (Meloidogyne incognita, race 3) and soybean cyst nematode (Heterodera glycines, HG type 2.5.7) on plant growth, yield and food quality components of edamame. Yield was the most consistent factor influenced by nematode pressure with increasing nematode population densities resulting in suppressed pod and seed weight. Additionally, seed protein content was reduced in the highest tested population density of H. glycines. In greenhouse studies, 22 advanced edamame breeding lines from the University of Arkansas soybean breeding program were compared with two susceptible commercial cultivars for suitability as hosts for both M. incognita and H. glycines independently. Four lines showed consistent reductions in M. incognita reproduction relative to the commercial cultivars and could represent sources of moderate resistance for development of future root-knot nematode resistant edamame cultivars.
Reniform nematode (Rotylenchulus reniformis) is a yield-limiting pathogen of soybean (Glycine max) in the southeastern region of the United States. A population of 250 recombinant inbred lines (RIL) (F2:8) developed from a cross between reniform nematode resistant soybean cultivar Forrest and susceptible cultivar Williams 82 was utilized to identify regions associated with host suitability. A genetic linkage map was constructed using single-nucleotide polymorphism markers generated by genotyping-by-sequencing. The phenotype was measured in the RIL population and resistance was characterized using normalized and transformed nematode reproduction indices in an optimal univariate cluster analysis. Quantitative trait loci (QTL) analysis using normalized phenotype scores identified two QTLs on each arm of chromosome 18 (rrn-1 and rrn-2). The same QTL analysis performed with log10(x) transformed phenotype data also identified two QTLs: one on chromosome 18 overlapping the same region in the other analysis (rrn-1), and one on chromosome 11 (rrn-3). While rrn-1 and rrn-3 have been reported associated with reduced reproduction of reniform nematode, this is the first report of the rrn-2 region associated with host suitability to reniform nematode. The resistant parent allele at rrn-2 showed an inverse relationship with the resistance phenotype, correlating with an increase in nematode reproduction or host suitability. Several candidate genes within these regions corresponded with host plant defense systems. Interestingly, a characteristic pathogen resistance gene with a leucine-rich repeat was discovered within rrn-2. These genetic markers can be used by soybean breeders in marker-assisted selection to develop lines with resistance to reniform nematode.
Reniform nematode (Rotylenchulus reniformis, Linford and Oliveira) is a sedentary, semi-endoparasite that infects a wide range of plant hosts and is one of the top three nematode pathogens affecting soybean in the southeastern United States. Previous studies have linked resistance to reniform nematode in soybean to two quantitative trait loci on chromosomes 11 and 18. A Kompetitive Allele-Specific PCR (KASP) assay was designed using SNP markers within these two regions to distinguish reniform nematode-resistant soybean based on genotype. A collection of 44 soybean plant introductions with resistant phenotype to reniform nematode and 40 susceptible soybean lines were genotyped at the two target loci to validate the KASP assay design. Of the 44 observed resistant lines, two carried the susceptible genotype; PI 438489B at the locus on chromosome 18 and PI 495017C on chromosome 11. Of the 40 observed susceptible soybean lines, only 25 had the expected susceptible genotype at the loci on chromosome 18 and 13 on chromosome 11. Our KASP assay was 68% accurate in predicting the phenotype of 84 soybean accessions based on their genotype at the SNP marker on chromosome 18 and 83% accurate at chromosome 11. These results indicate a moderate correlation of soybean SNP markers GlyREN18_46 and GlyREN11_190 with reniform nematode resistance. Further research is required to improve the accuracy of KASP assays to predict soybean response to reniform nematode, particularly host susceptibility.
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