The strain’s remarkable stress tolerance was immediately apparent in laboratory assays. IRAMC:0087 retained 77 percent of its growth under continuous exposure to 40°C and maintained over 90 percent of its growth after a short heat shock at 47°C. Even more striking was its resilience to salinity: the bacterium kept growing at sodium chloride concentrations of up to 510 millimolar, maintaining 62 percent of its control growth rate. Under osmotic stress simulated by polyethylene glycol, the strain showed moderate sensitivity, with 20 percent PEG 6000 reducing growth by nearly half. These phenotypes are not surprising given the strain’s origin in the Nafta region, where sandy soils with high electrical conductivity impose relentless selective pressure on microbial life.

Beyond surviving extreme conditions, IRAMC:0087 displayed a suite of plant growth-promoting traits. It produced indole-3-acetic acid at measurable concentrations, solubilized inorganic phosphate, and secreted siderophores that chelate iron from the environment. Genome annotation confirmed the molecular machinery behind these phenotypes, including the complete tryptophan-dependent IAA biosynthesis pathway, phosphate mobilization genes such as phoD and the phnFGIJKLM operon, and an eight-gene rhizobactin biosynthesis cluster located on an accessory plasmid. Interestingly, although ACC deaminase activity was not detected under free-living conditions, the genome contains an acdS gene on pSymA, suggesting that its expression may require specific plant-derived signals or symbiotic contexts not reproduced in vitro.

The symbiotic behavior of IRAMC:0087 proved to be its most surprising feature. When tested against six legume species under controlled growth chamber conditions, the strain formed effective nitrogen-fixing nodules only on Vachellia tortilis, the umbrella thorn acacia well adapted to arid environments. Pink nodules appeared after four weeks, and inoculated plants showed significantly increased shoot dry weight compared to uninoculated controls, confirming active biological nitrogen fixation. Meanwhile, the classical hosts of S. meliloti—Medicago sativa, Medicago truncatula, and Medicago arborea—were not nodulated at all. Yet the strain still promoted growth in some of these non-host plants: Medicago truncatula seedlings showed a 153 percent increase in root dry weight and a 68 percent increase in shoot dry weight following inoculation, indicating that the bacterium can assist plants through non-symbiotic mechanisms even when nodulation fails.

Confocal microscopy using a red fluorescent protein-tagged version of the strain revealed the details of these host-dependent interactions. On Vachellia tortilis roots, IRAMC:0087 initially colonized the primary root surface and then accumulated near sites of secondary root emergence, eventually achieving dense colonization as lateral roots developed. On Medicago sativa, the bacteria adhered to the main root near the base of root hairs but showed no evidence of penetration. In Trifolium subterraneum, the strain colonized both root hairs and intercellular spaces, suggesting the initiation of an endophytic interaction rather than a nodulating symbiosis. These observations confirm that IRAMC:0087’s infection behavior is strictly host-dependent, with the molecular dialogue between bacterium and plant determining whether the interaction progresses to nodule formation or stalls at the root surface.

To understand the genetic basis of these traits, the researchers assembled a complete circular genome of 7.37 megabases using combined Oxford Nanopore long-read and Illumina short-read sequencing. The genome comprises five replicons: a 3.65-megabase chromosome, two megaplasmids corresponding to pSymA (0.59 megabases) and pSymB (1.67 megabases), and two accessory plasmids of 1.25 and 0.20 megabases. The overall GC content is 62 percent, and annotation identified 6,950 coding sequences, including 6,881 protein-coding genes, 326 pseudogenes, and a full complement of rRNA, tRNA, and non-coding RNA genes. Phylogenomic analysis, average nucleotide identity of 98.96 percent, and digital DNA–DNA hybridization of 91.60 percent against the type strain all confirmed the assignment to S. meliloti.

But the real story emerged when the researchers compared IRAMC:0087’s genome architecture to that of the model strain S. meliloti 1,021. While the chromosome and pSymB replicons showed high conservation in gene order and content, the pSymA plasmid of IRAMC:0087 was approximately 2.5 times smaller than its counterpart in the model strain. More remarkably, phylogenomic analysis of pSymA sequences revealed that IRAMC:0087’s symbiotic plasmid is most closely related to those of S. garamanticum and S. numidicum, two species originally isolated from nodules of Argyrolobium uniflorum in Tunisia. The repABC replication operon, which controls plasmid maintenance and partitioning, clustered IRAMC:0087 firmly with these non-meliloti species in a strongly supported clade, clearly separated from the canonical S. meliloti 1,021 lineage.

This chimeric genome architecture has direct consequences for host specificity. Phylogenetic analysis of the nodC gene, a key determinant of Nod factor structure and host recognition, placed IRAMC:0087 within symbiovar tunisiaense, a recently described symbiotic lineage associated with wild legumes of North Africa. The strain shared 98.82 percent sequence similarity in nodA and 97.11 percent in nodC with S. garamanticum, reinforcing the conclusion that its symbiotic identity is governed by plasmid-borne genes acquired through horizontal transfer rather than by the taxonomic identity of the bacterial chromosome. Synteny analysis of the nodulation and nitrogen fixation gene clusters further showed that IRAMC:0087 shares a conserved multi-block gene arrangement with S. garamanticum and S. numidicum, in contrast to the single contiguous cluster found in S. meliloti 1,021.

Pan-genome analysis of 74 Sinorhizobium genomes revealed the extraordinary openness of this species’ gene pool: only 1.3 percent of orthologous gene clusters were strictly core, while 81.6 percent were present in fewer than 11 genomes, indicating massive strain-specific accessory content. Within this landscape, IRAMC:0087 carried 4,520 plant growth-promoting trait gene families, about 121 more than the ingroup mean. Functional profiling showed a distinctive reorganization of its gene repertoire, with significant expansions in quorum-sensing and biofilm formation genes, bacterial secretion systems, exopolysaccharide production, and surface attachment functions—all determinants of rhizosphere competitiveness. Conversely, the strain showed a notable depletion in nitrogen acquisition genes compared to other S. meliloti strains, a pattern consistent with the idea that host-range specificity in rhizobia is governed by discrete sets of horizontally transferable symbiotic genes rather than by species boundaries.

The insertion sequence landscape of the accessory plasmid p_A1 provided clues about how this genomic reshuffling occurred. Alignment between p_A1 and pSymA of the model strain revealed extensive block-level rearrangement, with breakpoint junctions concentrated in discrete hotspots along the plasmid, most notably around the 0.5-megabase position. Several insertion sequence families showed markedly different copy numbers between the two replicons, and some were exclusive to one or the other, consistent with IS-mediated recombination as a driver of the extensive structural divergence. The researchers also identified 39 gene clusters unique to IRAMC:0087, most encoding hypothetical proteins, suggesting that the accessory genome harbors strain-specific adaptive functions that remain to be functionally characterized.

The complete genome also revealed a comprehensive stress-tolerance toolkit distributed across all replicons. Heat shock proteins of the HSP20, HSP33, and HSP70 families, the major chaperone systems DnaK/DnaJ and GroEL-GroES, and the sigma factor RpoH were present in multiple copies, providing a genomic basis for the thermotolerant phenotype. Osmoprotection is mediated by genes for the biosynthesis and uptake of glycine betaine, choline, and trehalose, with all three trehalose biosynthetic pathways represented. The exopolysaccharide biosynthesis operon on pSymB and four copies of the regulatory gene mucR distributed across different replicons further support biofilm formation and cell envelope protection under drought and salinity. Antioxidant defenses including catalase, peroxidase, superoxide dismutase, and methionine sulfoxide reductases, together with the redox-sensitive regulators OxyR and SoxR, complete the picture of a bacterium equipped for life in one of Earth’s most challenging environments.

These findings position IRAMC:0087 as a compelling model for understanding how rhizobial genomes evolve under extreme environmental pressure. The strain demonstrates that species identity alone is a poor predictor of symbiotic capability in Sinorhizobium, and that symbiovar affiliation—the specific combination of nodulation and nitrogen fixation genes carried on the symbiotic plasmid—is the more meaningful determinant of host range. For sustainable agriculture in arid and climate-vulnerable regions, desert-adapted strains like IRAMC:0087 represent an underexplored reservoir of stress-tolerant, competitively fit bioinoculants that could improve biological nitrogen fixation, reduce dependence on synthetic fertilizers, and support the resilience of legume-based cropping systems under the combined pressures of heat, salinity, and water scarcity that define the Saharan landscape.

Subject of Research: Genomic architecture and symbiotic plasmid evolution of a stress-tolerant Sinorhizobium meliloti strain isolated from Saharan legume nodules

Article Title: Genomic plasticity, stress tolerance, and a distinctive pSymA in the symbiotic bacterium Sinorhizobium meliloti IRAMC:0087

Article References: Ben Gaied, R., Brígido, C., Kaur, S., DiCenzo, G. C., & Sbissi, I. (2026). Genomic plasticity, stress tolerance, and a distinctive pSymA in the symbiotic bacterium Sinorhizobium meliloti IRAMC:0087. Frontiers in Sustainable Food Systems, 10, Article 1917758. https://doi.org/10.3389/fsufs.2026.1917758

Image Credits: AI Generated

DOI: 10.3389/fsufs.2026.1917758

Keywords: Sinorhizobium meliloti, pSymA, symbiovar tunisiaense, biological nitrogen fixation, arid agriculture, genomic plasticity, horizontal gene transfer, stress tolerance, rhizobia, Vachellia tortilis, plant growth-promoting traits, Sahara

The motivation behind the study lies in a stubborn agricultural problem. Continuous monoculture—planting the same crop season after season on the same land—degrades soil structure, depletes available nutrients, and encourages the buildup of soil-borne pathogens. Tobacco production, in particular, suffers from low fertilizer use efficiency and the chemical fixation of phosphorus and potassium, elements that are often abundant in soil minerals but locked in forms that plant roots cannot absorb. Phosphorus, for example, is frequently bound to calcium, iron, or aluminum in ways that render it inaccessible, while potassium can be trapped within the lattice of soil minerals. The conventional remedy has been to apply ever-larger doses of chemical fertilizer, an approach that inflates costs, pollutes waterways, and degrades soil biology over time. Plant growth-promoting rhizobacteria, or PGPR, offer an alternative: microbes that colonize the root zone and mobilize nutrients through their own metabolism.

To find a candidate strain worth sequencing, the team screened bacteria from tobacco rhizosphere soil and put EL9 through a battery of functional assays. In colorimetric tests, the strain produced IAA at a level equivalent to 55.47 milligrams per liter, a substantial output for a single isolate. IAA is the principal auxin hormone in plants; it stimulates cell elongation, root initiation, and overall vegetative development, so a root-dwelling bacterium that secretes IAA effectively hands its host plant a growth stimulus from the outside. In parallel assays, EL9 solubilized phosphate at 427.60 milligrams per liter and mobilized potassium at 172.29 milligrams per liter, confirming in the laboratory what the genome later explained in molecular detail: this organism is a triple-threat nutrient mobilizer.

The centerpiece of the study is the whole-genome sequence itself. EL9 carries a genome of approximately 5.10 megabases—a moderately sized bacterial genome typical of the Bacillaceae family, to which Priestia megaterium (formerly classified in the genus Bacillus) belongs. Within those five-plus million base pairs, the researchers identified a tryptophan biosynthesis gene cluster along with the amiE gene, genetic features that they link to the bacterium’s IAA-producing capacity. The connection is biochemically logical: the most common microbial route to IAA runs through tryptophan, an amino acid precursor that bacteria convert to auxin via several enzymatic pathways. A strain that can manufacture its own tryptophan and process it has an internal supply chain for hormone production. The amiE gene, encoding amidase activity, has been associated in prior literature with the conversion of indole-3-acetamide into active IAA, providing a plausible enzymatic step in that pathway.

Beyond auxin, the genome revealed genes involved in phosphorus transport, sulfate assimilation, and core carbon and nitrogen metabolism. Phosphorus-solubilizing bacteria typically accomplish their work by secreting organic acids that chelate the metal cations binding phosphate, or by releasing phosphatases that cleave phosphate from organic molecules; the transport genes allow the freed phosphate to be imported into the cell, creating a sink that keeps the dissolution reaction moving forward. Sulfate assimilation genes point to the bacterium’s ability to take up inorganic sulfur and convert it into the sulfur-containing amino acids and cofactors it needs—an indicator of metabolic self-sufficiency in the nutrient-poor rhizosphere. Together, these gene families sketch the picture of a generalist capable of thriving in marginal soils while actively reworking the nutrient chemistry around plant roots.

Genomic sequences alone, however convincing, do not prove that a strain will perform in a living field. The team therefore moved from in silico analysis to pot experiments, testing EL9 on three crop species: tobacco, Chinese cabbage, and wheat. Across all three, inoculation with EL9 significantly increased the levels of IAA, available phosphorus, and available potassium in the rhizosphere soil, and these chemical changes were mirrored by measurable improvements in plant growth and root development. Root architecture matters enormously in agriculture—deeper, denser root systems capture more water and nutrients and confer drought resilience—so the observation that EL9-treated plants developed enhanced roots is among the most practically significant findings of the study.

The researchers then scaled up to field trials with tobacco, the crop from which the strain originally came. The results confirmed improvements in agronomic traits and, critically, in the quality of cured leaves, the end product on which tobacco farmers’ income depends. Field performance is where many laboratory-promising biofertilizer candidates falter, because real soils present competition from resident microbiota, fluctuating moisture and temperature, and heterogeneous nutrient distributions. That EL9 maintained its effects under field conditions strengthens the case that its genome-encoded traits translate into genuine agronomic value rather than remaining a petri-dish curiosity.

Safety is a non-negotiable concern for any organism intended for large-scale environmental release, and the team addressed it directly with a genomic risk assessment. In silico analyses of the EL9 genome revealed no complete or obvious pathogenicity determinants—no integrated arsenal of toxin genes, virulence factors, or antibiotic resistance cassettes of the kind that would raise red flags for regulators. This matters because the genus historically placed in Bacillus includes Bacillus anthracis, the anthrax agent, and any agricultural relative must be shown to lack the genetic machinery for harming animals or humans. Additionally, plate assays suggested preliminary antagonistic activity against Fusarium oxysporum, a notorious soil-borne fungus that causes vascular wilt diseases in a wide range of crops. If EL9’s antifungal capacity holds up in further testing, the strain could offer disease suppression as a fourth benefit stacked on top of hormone production and phosphorus and potassium mobilization.

The significance of the work extends beyond one bacterium. Biofertilizer development has long suffered from a disconnect between genomic potential and field performance: strains are identified, their genes catalogued, and then the products underperform in real soils, or they work for one crop but not others. EL9’s combination of a well-characterized genetic repertoire, demonstrated efficacy across three botanically distinct crops—tobacco is a solanaceous broadleaf, Chinese cabbage a brassica, and wheat a cereal grass—and confirmed field results makes it an unusually well-documented candidate. The multi-crop success also hints that the strain’s benefits derive from general mechanisms of nutrient mobilization and hormone provision rather than from a narrow, host-specific interaction.

There are still hurdles between the current results and commercial deployment. The authors describe the antifungal activity as preliminary, based on plate assays, and field-scale disease suppression has not yet been demonstrated. Formulation science—how to deliver live bacteria to fields in a stable, shelf-stable product—remains a separate engineering challenge, as does registration under agricultural regulations, which vary by country. The researchers note that the article is being shared early as a citable, peer-reviewed accepted manuscript, with a final version of record to follow. Funding for the work came from the China National Tobacco Corporation’s Science and Technology Key Program and the Natural Science Foundation of Henan Province.

Nevertheless, the study offers a template for how modern genomics can accelerate the search for sustainable agricultural inputs. Rather than relying solely on trial and error, researchers can now sequence a promising isolate, read its functional genes like a parts list, verify safety computationally before any environmental exposure, and only then invest in greenhouse and field validation. In an era when agriculture must produce more with fewer chemical inputs and less environmental damage, a single microorganism that can feed plants, stimulate their roots, and potentially shield them from fungal attackers is exactly the kind of multifunctional tool the field has been looking for. EL9 may prove to be one of the clearer examples of a microbe whose genome tells the whole story—a story that ends in healthier soil and crops grown with a lighter chemical footprint.