The study, led by Sara Rodriguez-Mena of the Institute for Sustainable Agriculture in Córdoba, Spain, together with Mario González, Diego Rubiales and collaborators in Portugal and Iran, set out with a simple but urgent question. Grass pea cultivation declined for decades in many regions where Aphanomyces is endemic, which may explain why the pathogen was never reported as a problem in this crop. But renewed interest in grass pea as a model crop for sustainable agriculture, driven by its resilience and its ability to fix atmospheric nitrogen, means that expanded cultivation could bring the crop back into contact with infested soils. As legume acreage grows across Europe and beyond, so too does the risk that Aphanomyces spreads with it. Understanding whether grass pea can withstand this pathogen before farmers commit to the crop is therefore a matter of practical urgency.

To answer it, the researchers assembled a globally diverse panel of 169 grass pea accessions, drawn from the Mediterranean basin, South Asia, Eastern Europe, Sub-Saharan Africa, North Asia and the Americas. The collection spanned differences in seed colour and seed size that would later prove biologically meaningful. Each accession was inoculated under controlled conditions with the RB84 isolate of A. euteiches, a pea-derived strain known for its ability to cross host boundaries and infect multiple legume species. Ten-day-old seedlings received a zoospore suspension adjusted to 1,000 zoospores per millilitre, applied directly at the base of each stem. Twenty days later, the team scored both foliar symptoms on a scale of 0 to 5 and root rot symptoms on a scale of 0 to 9, with the highly susceptible pea cultivar Messire serving as a positive control to confirm that the inoculation had worked.

The results revealed an extraordinary spectrum of responses. Average foliar symptom scores ranged from 0.08 to 4.50, and root rot scores from 1.64 to 8.73, meaning the collection captured everything from near-complete susceptibility to something approaching immunity. Twelve accessions qualified as resistant, with low scores on both scales, and two of them, PI283570_A and PI283580, displayed complete resistance to the tested isolate, a level of protection that has never been documented in pea, where breeders have managed only to identify partial resistance. Ten further accessions showed partial resistance, including STUDENICA, a commercial variety with strong agronomic credentials that could be recommended for cultivation in infested soils right away. Notably, several of the resistant accessions, including PI283580, BGE17185 and PTLS1006, had previously shown low symptom levels against another soil-borne pathogen, Fusarium oxysporum f. sp. pisi, making them doubly valuable given how frequently Aphanomyces and Fusarium co-infect legume fields.

Genotype was the dominant driver of symptom variation, with high broad-sense heritability for both foliar and root scores, exactly the property breeders need for genetic improvement. But seed characteristics left a visible fingerprint too. Accessions with large, light-coloured seeds, which are typical of Mediterranean origins, showed significantly lower disease symptoms than small, dark-seeded accessions of South Asian provenance. This echoes earlier findings in the same collection for Fusarium resistance, hinting that shared defence mechanisms may underpin grass pea tolerance to both soil-borne pathogens. The correlation between foliar and root symptoms was moderate but significant, confirming that what happens underground reliably manifests above ground, as it does in pea.

The real breakthrough came from combining these phenotypes with an upgraded genomic data set. Building on a previously published grass pea reference genome, the team re-called variants from genotyping-by-sequencing data and mapped DArTseq markers onto the seven chromosomes of the L. sativus assembly. The original data set of 26,876 SNPs, of which only 5,651 passed earlier quality filters, was transformed into a far denser resource: 44,383 SNPs detected in total, with 12,974 high-quality markers passing rigorous filtering for minor allele frequency and missing data. This nearly tripled the usable marker count and provided comprehensive genome-wide coverage, giving the association study the resolution it needed.

Using four different genome-wide association models, the team identified 20 SNPs significantly associated with resistance, distributed across five of the seven grass pea chromosomes, with chromosomes 2 and 3 entirely free of associations. Thirteen markers tracked foliar symptoms, eight tracked root rot, and one, SNP007517 on chromosome 6, was associated with both. Two markers, SNP032149 and SNP007517, were independently detected by two different statistical models, reinforcing confidence in their relevance. The proportion of phenotypic variance explained by individual markers ranged from a modest 0.13 percent to 11.62 percent, a pattern that screams polygenic inheritance: no single locus dominates, and resistance emerges from the combined action of many genes scattered across the genome. Clusters of closely linked markers on chromosomes 5 and 6 point to candidate quantitative trait loci that future studies can target with finer mapping.

In silico annotation of the genomic regions surrounding the significant SNPs identified seven putative candidate genes with plausible roles in defence. A wax ester synthase gene on chromosome 1 and a fatty-acid metabolism gene on chromosome 5 connect to evidence that fatty-acid accumulation mediates reactive oxygen species production during Aphanomyces infection in pea. A component of the exocyst complex on chromosome 4, involved in vesicle trafficking, has been implicated in plant immunity, though its specific role against this oomycete remains poorly understood. Chromosome 6 contributed two protein kinase genes and an ethylene receptor gene, and here the parallels with other legumes are striking: serine/threonine protein kinases and ethylene signalling have both been repeatedly linked to Aphanomyces defence in pea and barrel medic, with the ethylene pathway promoting lignin deposition that forms a protective ring around the root’s central cylinder, physically blocking pathogen entry. A callose synthase gene on chromosome 7 rounds out the list, suggesting reinforcement of cell walls at infection sites to limit hyphal spread.

What makes these findings particularly interesting from an evolutionary and agronomic standpoint is that the genomic distribution of resistance markers in grass pea does not overlap with the conserved syntenic QTL regions identified in pea, lentil, faba bean and barrel medic. Grass pea appears to have evolved its own, independently assembled resistance toolkit. This lack of a conserved pattern could complicate cross-species marker transfer, but it also opens exciting possibilities: if breeders can overcome crossability barriers between grass pea and its close relatives, these novel resistance loci could diversify the genetic defences available across legume agriculture, reducing the selective pressure that intensively cultivated, genetically similar crops place on pathogen populations. Because Aphanomyces populations are genetically diverse and reproduce both sexually and asexually, quantitative, polygenic resistance stacked across multiple loci is widely regarded as the most durable long-term strategy, far more robust than reliance on any single resistance gene.

The authors are careful to note the caveats. The RB84 isolate is a pea pathogen, so the response characterised here is a non-host-specific infection; grass pea-specific isolates, if they exist, could be more aggressive and might overcome even the complete resistance observed in PI283570_A and PI283580. Field validation is essential, as is testing against a broader panel of isolates to determine whether the identified regions confer broad-spectrum or pathotype-specific resistance, and whether the two fully resistant accessions owe their protection to monogenic or polygenic mechanisms. Functional validation of the seven candidate genes in planta will clarify their actual roles and guide precision breeding. Still, the significance of this work is hard to overstate. For the first time, breeders have identified sources of resistance to Aphanomyces root rot in grass pea, together with associated loci and candidate genes, transforming an uncharted vulnerability into a well-mapped opportunity. As climate change pushes agriculture toward resilient crops, grass pea may be entering hostile territory, but it is arriving armed with a genetic arsenal that science is only beginning to decode.

Subject of Research: Genetic resistance of grass pea (Lathyrus sativus) to the oomycete root rot pathogen Aphanomyces euteiches

Article Title: Aphanomyces euteiches causes disease in Lathyrus sativus with a globally diverse, polygenic resistance landscape

Article References: Aphanomyces euteiches causes disease in Lathyrus sativus with a globally diverse, polygenic resistance landscape. (n.d.). https://doi.org/10.1007/s00122-026-05372-w

Image Credits: AI Generated

DOI: 10.1007/s00122-026-05372-w

Keywords: grass pea, Lathyrus sativus, Aphanomyces euteiches, root rot, genome-wide association study, polygenic resistance, candidate genes, legume breeding, plant pathology, SNP markers, quantitative resistance, sustainable agriculture