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

Self-incompatibility is a reproductive strategy in which a plant’s own pollen fails to fertilize its own ovules, forcing out-crossing and thereby maintaining genetic diversity. In hybrid seed production, this trait is enormously valuable: if a female parent cannot self-pollinate, seed producers can generate hybrid seed without the laborious and expensive process of manual emasculation and hand pollination. In Indian mustard, breeders have been particularly interested in protogyny, a condition in which the female stigma matures and becomes receptive before the anthers release pollen, effectively preventing self-fertilization and promoting cross-pollination. Understanding the reproductive behavior of protogynous genotypes, the authors note, can help breeders select appropriate parental plants for hybrid-breeding programs and create novel mustard hybrids in a more systematic and logical manner.

The team’s first task was to work out how the protogyny-self-incompatibility trait, abbreviated PG-SI, is inherited. They crossed a protogynous, self-incompatible line with the popular variety Pusa Tarak and with ten normal, non-protogynous genotypes, generating first-generation hybrids and then F2 populations from selfed F1 plants. When the F2 generations were scored, the segregation pattern told a clear story: the PG-SI trait was controlled by duplicate dominant gene action at two loci, producing the classic 15:1 ratio expected when either of two dominant alleles is sufficient to confer the trait. This digenic-duplicate inheritance model means that both genes act redundantly, and a plant remains non-protogynous only when it lacks the dominant allele at both loci.

With the inheritance pattern established, the researchers turned to the molecular level. They developed protogynous and non-protogynous lines in three different genetic backgrounds, Pusa Tarak, Pusa Bold and Pusa Mahak, and subjected three PG-SI and three non-PG-SI genotypes to genotyping-by-sequencing, or GBS, on the Illumina platform. GBS is a reduced-representation sequencing approach that samples thousands of genomic sites across the genome, allowing researchers to discover and score single nucleotide polymorphisms, or SNPs, simultaneously across many individuals at relatively low cost. Because the PG-SI and non-PG-SI lines share most of their genetic background, SNPs that consistently differ between the two groups are strong candidates for being linked to the causal genes.

The sequencing effort was substantial. The team identified a total of 488,663 SNPs among the three PG-SI genotypes and 401,301 SNPs among the three non-PG-SI genotypes, reflecting the enormous natural sequence diversity present in the Brassica juncea genome. From this vast dataset, the researchers applied a stringent filter: they searched for SNP alleles that were consistently associated with all protogynous genotypes and consistently absent, in their alternate state, from all non-protogynous genotypes. Only four SNPs passed this test, and these markers co-segregated perfectly with the PG-SI phenotype across the three genetic backgrounds examined.

The four discriminating SNPs were mapped to specific genomic locations: A10:9559612 on chromosome A10, A09:694111 on chromosome A09, B05:2480898 on chromosome B05 and A03:11891971 on chromosome A03. Brassica juncea is an allotetraploid species carrying two distinct genomes, the A genome derived from Brassica rapa and the B genome from Brassica nigra, so the fact that candidate markers appear on both subgenomes is consistent with the digenic-duplicate mode of inheritance revealed by the segregation analysis. The presence of linked markers on chromosomes from both ancestral genomes suggests that the two duplicate genes controlling the trait may reside on different subgenomes, each contributing independently to the protogynous, self-incompatible phenotype.

Having localized the markers, the team then interrogated the genomic neighborhoods of these SNPs to identify the putative candidate genes responsible. Three genes emerged from this analysis: calcium-binding protein PBP1, a serine/threonine-protein kinase, and calcium-dependent protein kinase 5. The functional classes of these genes are strikingly coherent with what is known about self-incompatibility mechanisms in flowering plants. In the Brassicaceae, the self-incompatibility response is mediated by receptor kinases at the stigma surface, and calcium signaling acts as a rapid intracellular messenger that triggers the inhibition of pollen tube germination and growth. Calcium-dependent protein kinases decode calcium signals into phosphorylation events, while serine/threonine kinases are central components of the pollen-stigma recognition machinery that determines whether pollen is accepted or rejected.

The implications for hybrid breeding are considerable. Because the four SNPs co-segregate with the PG-SI trait across multiple genetic backgrounds, they can serve as molecular markers for marker-assisted selection, allowing breeders to track the trait in seedlings without having to wait for flowering and conduct time-consuming pollination tests. The authors emphasize that the associated SNPs and candidate genes predicted in the study could be used to transfer the protogyny-self-incompatibility trait into other elite genetic backgrounds, and subsequently exploited in hybrid breeding programs. This would enable the creation of novel mustard hybrids in a more systematic and logical manner, replacing labor-intensive emasculation with a genetically controlled selfing barrier.

The study also contributes to broader fundamental understanding of reproductive biology in crops. Self-incompatibility systems have been studied intensively in the Brassicaceae, where the stigma-pollen interface operates as a sophisticated stop-and-go signaling system, and recent work across plant families has revealed remarkable diversity in the molecular machinery underpinning this trait. By linking a classical phenotypic trait of agronomic importance to specific genomic regions and plausible signaling genes in an allotetraploid oilseed, the new research adds a valuable data point to this growing picture, and provides a concrete starting point for functional validation of the candidate genes.

For a crop that supplies a major share of edible oil across the Indian subcontinent and beyond, the ability to produce hybrid seed efficiently could translate into substantial yield gains, since hybrid vigor in mustard has been demonstrated repeatedly but has been difficult to commercialize at scale. The work of Kumar, Chakrabarty and colleagues, conducted at ICAR-Indian Agricultural Research Institute with support from the institute, shows how combining classical genetics with affordable high-throughput genotyping can convert a hard-to-manage reproductive trait into a breedable, trackable asset. As functional studies confirm the roles of the candidate genes and the markers are deployed in breeding pipelines, protogyny-based self-incompatibility may finally become a practical tool for delivering high-yielding mustard hybrids to farmers.

Subject of Research: Genetic basis of protogyny-associated self-incompatibility in Indian mustard

Article Title: Deciphering candidate genes governing Self-incompatibility reaction in Indian mustard [Brassica juncea (L.) Czern. and Coss.]

Article References: Kumar, S., Chakrabarty, S. K., Singh, S. K., Singh, S. K., Meena, V. K., Patel, M. K., Paul, D., & Basu, S. (2026). Deciphering candidate genes governing Self-incompatibility reaction in Indian mustard [Brassica juncea (L.) Czern. and Coss.]. Indian Journal of Genetics and Plant Breeding, 86(3), 345-353. https://doi.org/10.1007/s44489-026-00026-z

Image Credits: AI Generated

DOI: 10.1007/s44489-026-00026-z

Keywords: Brassica juncea, self-incompatibility, protogyny, genotyping-by-sequencing, SNP, candidate genes, hybrid breeding, marker-assisted selection, calcium-dependent protein kinase, Indian mustard, plant genetics, digenic inheritance