Brown spot, caused by the seed-borne fungus Bipolaris oryzae, is a disease with a dark historical pedigree. It produces small circular lesions with brown centers and yellow halos on leaves, glumes, and sheaths, and under severe infection these lesions coalesce, devastating tissue quality and grain fill. Yield losses can climb as high as 45 percent, especially in nutrient-poor, stress-prone fields, and the disease was a major contributing factor to the Bengal Famine of 1943. The pathogen thrives in warm temperatures between 20 and 30 degrees Celsius and in soils deficient in silicon, nitrogen, potassium, manganese, and iron, conditions that climate change is making more common. Rising temperatures and erratic rainfall have been shown to intensify both disease severity and the virulence of the pathogen itself.

Despite this threat, brown spot has long lived in the shadow of rice blast and bacterial blight, the headline diseases of rice pathology. Its genetic basis remains poorly mapped. Earlier biparental studies identified only a handful of quantitative trait loci, such as BSq4.1 and BSq11.1 on chromosomes 4 and 11 in a doubled haploid population, and qBS2, qBS9, and qBS11 in a cross between resistant Tadukan and susceptible Hinohikari. Such mapping populations, however, suffer from low resolution and capture only the limited allelic diversity present in two parental lines. Genome-wide association studies, by contrast, can scan hundreds of diverse varieties simultaneously, detecting multiple resistance alleles at far finer resolution thanks to modern statistical models and high-throughput genotyping.

The research team, led by Parinda Barua and Sanjay Kumar Chetia at Assam Agricultural University, turned to a uniquely valuable resource: the Assam Rice Collection, a panel of 400 landraces spanning twelve ecotypes including Sali, Bora, Asra, Bao, Joha, Chakua, Khamti Lahi, and black rice. These traditional varieties, shaped by centuries of natural and farmer-mediated selection across varied agro-ecological niches, are widely regarded as reservoirs of stress-resilient alleles for climate-adaptive breeding. The same panel had previously been genotyped for blast resistance work, but its genetic architecture for brown spot resistance had never been examined.

Field screening took place over the 2022 and 2023 cropping seasons at the Assam Rice Research Institute in Titabar, Jorhat, in a permanent sick plot that has maintained a natural reservoir of Bipolaris oryzae inoculum across successive seasons. No artificial inoculation was used. Susceptible Mahsuri plants were sown as spreader rows after every twenty test entries and along the borders to guarantee uniform disease pressure, and moist jute sheets were laid over the young nursery to sustain the high humidity the fungus needs. Each accession was scored in three replications using the International Rice Research Institute’s standard 0-to-9 scale, where zero means no symptoms and nine means a highly susceptible plant.

The results revealed a striking gradient of susceptibility. Just 0.5 percent of accessions were highly resistant, 11.5 percent resistant, and 45.5 percent moderately resistant, while 42.5 percent were susceptible. Forty-eight accessions earned resistant scores, drawn mainly from the Sali, Chakua, and black rice groups, with two Sali landraces, Rupohi Sali and NCM-7-Solpona, standing out as highly resistant donors. Notably, nearly every black rice accession showed resistance, a pattern the authors link to the high phenolic and anthocyanin content of these pigmented grains, compounds known to carry antimicrobial activity and bolster plant defenses. Susceptibility, by contrast, was concentrated in the Asra, Bora, Joha, and Bao groups.

On the genotyping side, sequencing libraries prepared with the ApeKI restriction enzyme yielded 39,045 single nucleotide polymorphisms, of which 38,723 passed stringent quality filters requiring a minor allele frequency of at least 5 percent and no more than 10 percent missing data. The markers blanketed all twelve rice chromosomes, with chromosome 1 carrying the most variants and chromosome 12 the fewest. Transition mutations, the chemically conservative A-to-G and C-to-T swaps, outnumbered transversions, consistent with known mutation patterns in plant genomes. Population structure analysis resolved the panel into three genetic clusters, with aromatic, Joha, and Sali varieties grouping together, but a large share of accessions showed admixed ancestry, evidence of extensive gene flow through seed exchange and natural hybridization. Linkage disequilibrium across the genome was low, decaying rapidly on some chromosomes and slowly on others, a combination of weak structure and fast decay that makes the panel ideal for high-resolution association mapping.

The association scan itself, run through a compressed mixed linear model that corrects for both kinship and population structure, delivered ten significant SNPs at a strict threshold of P less than 0.0001: three clustered on chromosome 6 and seven on chromosome 10. Quantile-quantile plots confirmed the signals were genuine rather than statistical artifacts. Each SNP explained roughly 6 to 7 percent of phenotypic variance, modest effect sizes that fit the picture of brown spot resistance as a quantitative trait built from many loci of small cumulative effect. Importantly, the minor alleles were common in the panel, with frequencies between 0.28 and 0.45, meaning breeders could put them to work quickly through marker-assisted selection.

Digging into the 50-kilobase windows flanking each significant SNP, the team annotated sixteen candidate loci against the Nipponbare reference genome. Chromosome 10 emerged as the star of the show, hosting a battery of kinase genes: leucine-rich repeat receptor-like kinases, a putative XA21-like receptor protein, serine/threonine kinases, and cyclin-dependent kinases including OsCDKF2. Receptor-like kinases are the sentinels of plant innate immunity; the famous Xa21 gene, which confers resistance to bacterial blight, encodes exactly this class of protein, and related kinases have been tied to blast resistance as well. Calcium-dependent protein kinases act as molecular switches that convert calcium signals into phosphorylation cascades, and overexpressing the rice kinase OsCPK4 has been shown to block fungal penetration in blast infections. On chromosome 6, candidates included a ribonuclease H-like nucleic acid binding protein and an RDM1-like protein involved in small RNA-directed DNA methylation, hinting that epigenetic gene silencing, a pathway known to activate the defense gene OsWRKY45 against blast, may also shape brown spot responses. Perhaps most intriguing is Os10g0158400, a type III polyketide synthase called ARAS1, which belongs to an enzyme family that manufactures phytoalexins, antifungal compounds that accumulate rapidly at infection sites.

The authors are candid about the study’s limits. Phenotyping occurred at a single location across two seasons, and the candidate genes rest on positional inference and annotation rather than experimental proof; expression analysis, gene editing, and validation in independent populations remain future work. Still, the implications are considerable. No gene conferring direct resistance to brown spot has ever been identified, making chromosomes 6 and 10 the first well-supported genomic focal points for this neglected disease. The resistant landraces, especially Rupohi Sali, NCM-7-Solpona, and the black rice accessions, now stand as ready-made donor lines, and the ten SNPs offer breeders molecular handles to track resistance through crossing programs. As warming climates push brown spot into more fields, the old varieties of North-East India may hold the keys to keeping rice harvests, and the food security that depends on them, intact.

Subject of Research: Genome-wide association study of brown spot disease resistance in North-East Indian rice landraces

Article Title: A genome-wide association study in rice landraces of North-East India for brown spot resistance

Article References: Barua, P., Munda, S., Phukon, M., Kumar, A., Sruthi, R., Borah, J. L., Konwar, M. J., Bhattacharya, A., & Chetia, S. K. (2026). A genome-wide association study in rice landraces of North-East India for brown spot resistance. Heliyon, 12(15), Article e45469. https://doi.org/10.1016/j.heliyon.2026.e45469

Image Credits: AI Generated

DOI: Not provided

Keywords: rice, brown spot, Bipolaris oryzae, GWAS, landraces, North-East India, disease resistance, SNP markers, candidate genes, receptor-like kinases, marker-assisted selection, plant immunity