Rhizobium genes open pathway for attachment in new bacteria

Researchers at Washington State University transferred a set of genes associated with biological nitrogen fixation from symbiotic rhizobia to bacterial strains previously lacking the ability to form nodules. The study demonstrated, in a proof-of-concept trial, the conversion of non-nodulating bacteria into endosymbionts capable of colonizing host plants. The research points to a route for understanding, and in the future exploring, microorganisms capable of reducing the dependence on nitrogen fertilizers in agricultural crops.

The team of scientists evaluated rhizobia of the genus Mesorhizobium, collected from natural environments in the western United States. These bacteria live in the soil. Some form nodules on the roots of legumes and fix nitrogen. Others do not perform this function.

Island of symbiosis

The researchers focused their analysis on a mobile genetic element called a symbiosis island. This segment carries genes necessary for nodulation and nitrogen fixation. The scientists marked symbiosis islands in donor strains and promoted bacterial crosses with recipient strains lacking the island. Then, they selected transformed bacteria and tested their symbiotic function in host plants.

The results indicated transmission equivalent to eighty percent of the tests. Transmission occurred in many genotypes. The donor island genotype explained forty-eight percent of the variation in the number of transformed bacteria. The recipient genotype explained twenty-five percent. The donor-recipient interaction explained twenty-six percent.

The acquisition of the symbiotic island allowed the formation of nodules in all the transformed strains evaluated. Before the transfer, these strains did not nodulate. In thirteen of the fifteen strains tested, the researchers recovered colony-forming units from the nodules. This result showed bacterial proliferation within the symbiotic structure.

Functional fixation

Not all nodulation resulted in functional nitrogen fixation. One-third of the transformed strains began acting as mutualists, increasing the nitrogen content in the plants. Two-thirds originated commensal relationships, without detectable cost to the host plant. The study found no evidence of measurable damage to the host in the new endosymbionts evaluated.

The genetic proximity between donors and recipients influenced performance. More closely related lineages generated transformants with greater nitrogen fixation, greater shoot mass, larger nodule diameter, and a greater number of nitrogen-fixing nodules. Donors and recipients originating from the same soil type, serpentine or non-serpentine, also produced transformants with better symbiotic performance.

The transfer of the genetic element did not show the same phylogenetic limit observed for its function. According to the study, the symbiotic island moved between different genomes. In some cases, it displaced other genomic elements at the insertion point associated with the phenylalanine transfer RNA gene. In a few transformants, partial transfer of the island occurred, with loss of genes linked to efficient nitrogen fixation.

Nitrogen fixation

Stephanie Porter, associate professor of biological sciences at WSU Vancouver, stated that there is interest in more natural ways to deliver nitrogen to crops. According to her, the team developed a way to move a large set of genes capable of enabling nitrogen fixation and plant colonization into bacteria that lack these functions.

The long-term goal involves transferring nitrogen-fixing capacity to microorganisms associated with fertilizer-dependent crops. Wheat and corn are examples of crops with high nitrogen demands. Porter also mentions microorganisms associated with corn or soybeans as possible targets for future transfer of this capacity.

Scientists still intend to identify genes and variants that contribute most to the success of the transfer. Agricultural application depends on this step. The study shows biotechnological potential, but remains in the proof-of-concept phase, with trials in rhizobia and host plants used as an experimental model.

Further information can be found at DOI 10.1016/j.cub.2026.04.071