Scientists Pinpoint Two Dominant Genes That Shield Wheat From Devastating Spot Blotch

Spot blotch, a destructive fungal disease of wheat caused by the pathogen Bipolaris sorokiniana, has long been a thorn in the side of farmers across the warm and humid wheat-growing regions of the world. Now, a team of researchers in China has taken a major step toward disarming the disease at its genetic roots. In a study published in Theoretical and Applied Genetics, the group reports that resistance to spot blotch in a resistant wheat line is governed by two independent dominant genes, one of which, named SbWE35, is described here for the first time. The finding gives breeders a much sharper genetic toolkit for developing wheat varieties that can withstand a pathogen that thrives precisely in the environments where vulnerable smallholder farms are concentrated.

The research team, led by Beibei Li, Ping Lu and Zhiyong Liu, with colleagues at the Institute of Genetics and Developmental Biology of the Chinese Academy of Sciences, Xianghu Laboratory and partner institutions, focused on a wheat line called WE35, which had shown consistent resistance to spot blotch. To understand what lay behind that resistance, the researchers crossed WE35 with a susceptible line, S2199, and examined the disease responses of the resulting F2 and F2:3 generations. The segregation patterns in these populations told a clear story: rather than the diffuse, many-gene inheritance that often characterizes resistance to necrotrophic pathogens, spot blotch resistance in this cross behaved in a Mendelian fashion, controlled by two independent dominant genes acting together.

That discovery carried a double significance. One of the two genes was already familiar to the team: Sb3, a spot blotch resistance gene that had previously been identified and fine-mapped in earlier work. The second gene, however, was new to science. The researchers designated it SbWE35, after the resistant line in which it was found. Because each gene is dominant, a wheat plant needs only a single copy of either gene to mount resistance, and a plant carrying both gains a layered defense. For breeders, dominant genes are particularly attractive because they are easier to track and incorporate into elite cultivars through marker-assisted selection, and their effects are visible in the first generation after a cross.

Identifying the gene was only the first challenge; locating it on the vast and repetitive wheat genome was another. Wheat carries one of the largest genomes of any crop, roughly five times the size of the human genome, which makes precise genetic mapping a formidable technical task. The team turned to a method known as bulked segregant RNA-Seq, or BSR-Seq, which combines classical genetic segregation with high-throughput transcriptome sequencing. They crossed WE35 with a spot blotch susceptible line called 621-18-3, raised a segregating population, and then pooled the sequencing reads from the most resistant and most susceptible individuals. By comparing the allele frequencies between the two bulks, the researchers could pinpoint the genomic region where the resistance gene resided.

The BSR-Seq analysis delivered an unambiguous answer: SbWE35 maps to chromosome 4BL, the long arm of wheat chromosome 4B. With the rough location established, the team then embarked on fine mapping, the painstaking process of narrowing down the gene’s position using ever denser panels of molecular markers. Their efforts delimited SbWE35 to an extraordinarily tight interval of just 0.07 centimorgans, flanked by two markers named YK12918 and YK13104. In physical terms, that interval corresponds to approximately 1.08 megabases in the Chinese Spring reference genome, the standard assembly that wheat geneticists worldwide use as their coordinate system. An interval this small dramatically reduces the search space for the underlying causal gene and provides markers close enough to the resistance locus to be used directly in breeding programs.

The chromosome 4B location also rang a bell. A previously reported spot blotch resistance gene, Sb4, had been mapped to the same general region of chromosome 4BL in earlier work by members of the same research group. When the researchers performed comparative genetic mapping, aligning the SbWE35 interval with the Sb4 interval, they found substantial overlap between the two. This raises the tantalizing possibility that SbWE35 and Sb4 are not separate genes at all, but rather two views of the same genetic locus, identified independently in different resistant wheat lines. The authors are careful on this point: they note that the overlap suggests the two may represent the same locus, but formal allelism tests, which involve crossing the two resistant lines and examining whether the resistances segregate independently or together, will be required before the question can be settled definitively.

Why does this matter beyond the genetics laboratory? Spot blotch is a major constraint on wheat production in the warmer, more humid wheat regions of South Asia, Southeast Asia, Africa and Latin America, areas where hundreds of millions of people depend on wheat as a staple. The disease produces characteristic dark brown lesions on leaves, reducing the photosynthetic area of the plant and, in severe epidemics, cutting both yield and grain quality. Unlike some better-known wheat diseases that have attracted decades of international funding and breeding effort, spot blotch has historically received less attention, in part because its resistance is often quantitatively inherited and difficult to manipulate in breeding programs. Genes with large, clearly Mendelian effects, such as Sb3 and SbWE35, change that calculus entirely.

The study also fits into a broader and rapidly accelerating effort to decode wheat’s disease resistance arsenal at the molecular level. Around the world, researchers have recently cloned and characterized resistance genes for a range of wheat pathogens, including the receptor-kinase-like protein Stb6 that governs gene-for-gene resistance to Septoria tritici blotch, and genes involved in resistance to rusts and fusarium head blight. Each cloned or finely mapped resistance gene adds a molecular handle that breeders can grip with DNA markers, allowing them to stack multiple resistances into a single variety without having to wait for slow and weather-dependent field disease screenings at every generation. Stacking is especially valuable against a pathogen like Bipolaris sorokiniana, whose natural populations show considerable variability, meaning that a single resistance gene deployed alone could eventually be overcome.

For the immediate future, the tightly linked markers flanking SbWE35, YK12918 and YK13104, are the most practical takeaway from the study. Breeding programs in spot blotch-endemic regions can now screen seedlings for the presence of the WE35 resistance haplotype using these markers, long before any fungus appears in the field, and combine it with Sb3 to build durable, two-gene resistance into new cultivars. The work was supported by the National Key Research and Development Program of China, reflecting the strategic priority that wheat disease resistance holds for major wheat-producing nations facing shifting climates and evolving pathogen pressures.

As wheat continues to feed more than a third of the world’s population, and as warming temperatures push spot blotch-favorable conditions into new territories, the race to understand and deploy resistance genes has taken on fresh urgency. With SbWE35 now mapped, its relationship to Sb4 awaiting confirmation, and the genetic architecture of resistance in WE35 laid bare, breeders have two independent dominant shields to work with. The next chapters, formal allelism tests, candidate gene identification within the 1.08-megabase interval, and the deployment of marker-assisted selection in breeding pipelines, will determine how quickly this laboratory breakthrough translates into resistant varieties standing in farmers’ fields, quietly turning back one of wheat’s most persistent fungal adversaries.

Subject of Research: Genetic mapping of two dominant spot blotch resistance genes, Sb3 and SbWE35, in wheat

Article Title: Two independent dominant resistance genes confer resistance to spot blotch in wheat

Article References: Two independent dominant resistance genes confer resistance to spot blotch in wheat. (n.d.). https://doi.org/10.1007/s00122-026-05412-5

Image Credits: AI Generated

DOI: 10.1007/s00122-026-05412-5

Keywords: wheat, spot blotch, Bipolaris sorokiniana, disease resistance, SbWE35, Sb3, fine mapping, BSR-Seq, chromosome 4BL, plant genetics, marker-assisted selection, crop breeding