Two Resistance Alleles, One Locus: Study Reveals Hidden Diversity in Soybean Stem Canker Defense

Southern stem canker is one of the most destructive diseases of soybean, capable of stripping away between 20 and 80 percent of yield depending on how susceptible a cultivar is and how favorable the environment is for the pathogen. Over the past two decades alone, the disease has caused an estimated 226 million dollars in economic losses across the southern United States soybean production region. The culprit is Diaporthe aspalathi, an ascomycete fungus that invades stem tissue and produces cankers that girdle plants and cut off water and nutrient transport. For growers, the most effective and sustainable line of defense has long been genetic resistance, and the single most important source of that resistance is a locus on chromosome 14 known as Rdm3. A new study published in Theoretical and Applied Genetics now shows that this celebrated resistance hub is more genetically diverse than anyone had confirmed before, a finding with direct consequences for how breeders deploy it.

Researchers at the University of Georgia, led by M. Habib Widyawan together with James W. Buck and Zenglu Li, set out to answer a deceptively simple question: do two well-known resistant soybean lines, the cultivar Crockett and the plant introduction PI 398469, carry the same resistance allele at Rdm3 or two different ones? Both lines had been mapped to the same locus, and both resist a broad range of D. aspalathi isolates collected from the United States and South America. Yet indirect evidence, including differences in how the two lines respond to Brazilian isolates and differences in their haplotype structure, hinted that they might not be identical. Resolving that question matters because breeders who unknowingly combine the same allele twice gain nothing, while combining genuinely different alleles can broaden and prolong resistance in the field.

To settle the matter, the team crossed Crockett with PI 398469 and developed a population of 290 F5-derived recombinant inbred lines, advancing generations through field seasons in Georgia and winters in Puerto Rico. They then used kompetitive allele-specific PCR markers that distinguish the Rdm3 haplotypes of each parent to sort the lines into four classes: those carrying the Crockett haplotype, those carrying the PI 398469 haplotype, and two classes of recombinants carrying mixed marker combinations from both parents. Each class was challenged in the greenhouse with a virulent D. aspalathi isolate from Georgia using a toothpick inoculation assay, in which colonized toothpicks are inserted into wounds made between the cotyledon and unifoliate nodes of V3-stage plants.

The results were striking. Lines carrying either parental haplotype, or a recombinant combination that retained both resistance haplotypes, showed mean disease ratings of roughly 1.2 to 1.3 on a five-point scale, essentially as resistant as the parents themselves. But the second recombinant class, which carried alternate haplotype combinations, averaged a rating of 2.0 and displayed a much wider spread of individual responses, ranging from fully resistant to moderately diseased. In other words, recombination within the Rdm3 interval had broken up favorable allele combinations and produced progeny that were weaker and more variable than either parent. This kind of transgressive phenotypic disruption is a classic signature of allelic differences at a resistance locus, echoing earlier findings at the flax L locus, where chimeric resistance alleles generated susceptibility despite resistant parents.

Sequence analysis strengthened the case. The team compiled eighteen candidate genes within the Rdm3 interval from previous mapping work and used gene ontology enrichment to prioritize two of them. The first, Glyma.14G023500, encodes a calcium-dependent protein kinase, a class of enzyme central to protein phosphorylation and immune signal transduction in plants. A marker that defines the Crockett haplotype sits inside this gene and corresponds to a threonine-to-alanine amino acid substitution specific to Crockett, while whole-genome resequencing revealed a second substitution, glycine to arginine, that separates Crockett, PI 398469, and the susceptible line G81-2057. Notably, susceptible recombinant inbred lines carried two additional amino acid substitutions within the protein kinase domain, a region where altered function could impair calcium-mediated defense signaling. The second candidate, Glyma.14G022600, encodes a bromodomain-containing protein and showed dramatic divergence between the two resistant lines, with only about 41 percent amino acid similarity between Crockett and PI 398469 in the aligned regions.

Expression profiling added a dynamic layer to the story. Using calcofluor white staining and fluorescence microscopy, the researchers tracked fungal colonization over time and found that hyphae grew freely in the susceptible line as early as nine hours after inoculation, whereas in both resistant lines fungal expansion remained restricted even though colonization was detectable by 72 hours. Guided by these observations, the team measured candidate gene expression at 9 hours, 24 hours, 72 hours, and 7 days post inoculation. Glyma.14G023500 behaved differently depending on genotype: Crockett showed strong repression of the gene at 72 hours, coinciding with active fungal presence, while PI 398469 instead showed significant upregulation at 7 days after inoculation, and the susceptible line stayed near baseline throughout. Because loss of calcium-dependent protein kinase function is known to increase susceptibility in other plant-pathogen systems, these contrasting temporal patterns suggest that the two alleles regulate calcium-dependent immune signaling in distinct ways.

The second candidate gene told a similar tale of allele-specific regulation. Glyma.14G022600 was suppressed in Crockett but upregulated in the susceptible line at 9 hours post inoculation, remained stable in PI 398469, and by 72 hours showed strong repression in the susceptible genotype while resistant lines sustained expression before all lines converged back to baseline by 7 days. Recombinant inbred lines from separate mapping populations mirrored these parental patterns, with resistant and susceptible progeny from Crockett-derived and PI 398469-derived crosses each following the expression trajectory of the parent that donated their resistance allele. Together, the sequence and expression data paint a picture of two resistance alleles that, while sitting at the same locus, operate through measurably different regulatory dynamics.

Perhaps the most unexpected discovery concerned how the Crockett allele is inherited and where it came from. Across multiple independent populations, including crosses of Crockett with PI 398469, Tracy-M, and the susceptible line G81-2057, the Crockett resistance allele was consistently underrepresented, a statistical distortion of Mendelian segregation that was absent from populations derived from Tracy-M or PI 398469. The allele is also rare in the wider gene pool, appearing at only about 0.8 percent of accessions in the USDA Soybean Germplasm Collection and at low frequency in Brazilian lines and elite southern breeding material. This pattern suggests the Crockett allele may depend on additional genetic factors for stable transmission, a phenomenon paralleled at the soybean cyst nematode resistance locus rhg1, where incompatible allele combinations cause segregation distortion and reduced viability.

Pedigree analysis then delivered a genuine surprise. Neither of Crockett’s documented parents, Hampton 266A and PI 171451, nor its recorded grandparents carried the Crockett-type Rdm3 haplotype. PI 171451, the resistant parent, carries a haplotype matching that of Tracy-M, while Hampton 266A and its ancestor Lee carry a non-resistance haplotype. Whole-genome resequencing showed that 28 percent of Crockett’s SNP alleles on chromosome 14 were inconsistent with both parents, with the mismatches concentrated in a roughly 7.5 megabase region encompassing Rdm3 and enriched within the two candidate genes. Structural variant analysis revealed dense clusters of breakend signals in this interval, and Glyma.14G023500 shares 93 percent sequence identity with a paralogous calcium-dependent protein kinase gene on chromosome 2. The authors propose that non-allelic homologous recombination between these paralogous sequences may have generated the novel Crockett allele de novo during breeding, a rare event that can nonetheless produce alleles with adaptive advantages such as broader pathogen resistance.

The practical implications reach well beyond soybean genetics. Because Crockett, PI 398469, and Tracy-M each appear to harbor distinct resistance alleles at Rdm3, breeders can now stack genuinely different sources of resistance rather than unknowingly redeploying the same one, a strategy central to durable disease control. The allele-specific KASP markers developed in this work make it possible to track each variant through breeding populations and to avoid the transmission problems associated with the Crockett haplotype. The authors caution that long-read sequencing, copy-number analysis, and functional validation will be needed to fully resolve the structural architecture of the Rdm3 interval and the molecular mechanism of each allele. But the core message is already clear: a single resistance locus can hide a small library of functionally distinct defenses, and recognizing that diversity is the first step toward using it.

Subject of Research: Allelic variation at the Rdm3 locus conferring resistance to soybean southern stem canker

Article Title: Allele variation at the Rdm3 locus conferring resistance to soybean southern stem canker

Article References: Widyawan, M. H., Buck, J. W., & Li, Z. (2026). Allele variation at the Rdm3 locus conferring resistance to soybean southern stem canker. Theoretical and Applied Genetics, 139(10), Article 282. https://doi.org/10.1007/s00122-026-05381-9

Image Credits: AI Generated

DOI: 10.1007/s00122-026-05381-9

Keywords: soybean, southern stem canker, Diaporthe aspalathi, Rdm3 locus, disease resistance, allelic variation, recombinant inbred lines, calcium-dependent protein kinase, segregation distortion, structural variation, plant breeding, marker-assisted selection