The tobacco N and Arabidopsis RPS2 genes, among several recently cloned disease-resistance genes, share a highly conserved structure, a nucleotide-binding site (NBS). Using degenerate oligonucleotide primers for the NBS region of N and RPS2, we have amplified and cloned the NBS sequences from soybean. Each of these PCR-derived NBS clones detected low-or moderate-copy soybean DNA sequences and belongs to 1 of 11 different classes. Sequence analysis showed that all PCR clones encode three motifs (P-loop, kinase-2, and kinase-3a) of NBS nearly identical to those in N and RPS2. The intervening region between P-loop and kinase-3a of the 11 classes has high (26% average) amino acid sequence similarity to the N gene although not as high (19% average) to RPS2. These 11 classes represent a superfamily of NBS-containing soybean genes that are homologous to N and RPS2. Each class or subfamily was assessed for its positional association with known soybean disease-resistance genes through near-isogenic line assays, followed by linkage analysis in F2 populations using restriction fragment length polymorphisms. Five of the 11 subfamilies have thus far been mapped to the vicinity of known soybean genes for resistance to potyviruses (Rsvl and Rpv), Phytophthora root rot (Rpsl, Rps2, and Rps3), and powdery mildew (rmd). The conserved N-or RPS2-homologous NBS sequences and their positional associations with mapped soybean-resistance genes suggest that a number of the soybean disease-resistance genes may belong to this superfamily. The candidate subfamilies of NBScontaining genes identified by genetic mapping should greatly facilitate the molecular cloning of disease-resistance genes.Over the past few years, we have witnessed a breakthrough in the molecular cloning of disease-resistance genes (for review, see ref.

A quantitative, reproducible, and efficient phytic acid assay procedure is needed to screen breeding populations and support genetic studies in soybeans [Glycine max (L.) Merr.]. The objective of this study was to modify the colorimetric Wade reagent method and compare the accuracy and applicability of this new method in determining seed phytic acid content in soybean with three well‐established phytic acid assay methods: anion exchange column (AEC), high‐performance liquid chromatography (HPLC), and 31P nuclear magnetic resonance (NMR). The CV for repeated measurements of a low phytic acid soybean mutant, CX1834‐1‐6, ranged from 1.8 to 4.2% (n = 5), indicating the results were precise and reproducible. Phytic acid content of 42 soybean genotypes as determined by this method showed a correlation of 93.7 to 96.6% with the measurements by AEC, HPLC, and NMR. According to analysis of covariance, using inorganic P content as a predictor, phytic acid P content in a given sample analyzed by the four assay methods can be estimated with four linear regression models at the α = 0.01 level. Compared with HPLC, AEC, and 31P NMR, this modified colorimetric method is simpler and less expensive for assaying a large number of samples, allowing its effective application in breeding and genetic studies of low phytic acid soybean.

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.

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