Can cereals be engineered to form nitrogen-fixing root nodules?

Legumes host bacteria in root nodules that turn nitrogen from the air into a form the plant can use; cereals such as wheat, rice and maize cannot, and depend on fertiliser. Transferring the nodule symbiosis to cereals is a long-standing goal of plant science that has not yet been reached.

Question

Can cereals such as rice, wheat, maize and barley be engineered to form root nodules that host nitrogen-fixing bacteria and supply the plant with fixed nitrogen, as legumes do?

Background

The panellists added this question themselves, as a topic missing from the submissions; it closes the category "Molecular Approaches to Fundamental Plant Biology" of the 2023 list of 100 important questions facing plant science [1]. Making nitrogen fertiliser consumes large amounts of energy, its overuse causes greenhouse gas emissions and eutrophication, and where it is scarce yields stay low [2].

What is known

• A century of research has revealed the cellular, molecular and genetic control of nodulation, including genes that evolved specifically for nodule formation and bacterial infection, yet nitrogen-fixing nodulation has not been engineered into a non-legume crop [3].
• In the legume Lotus japonicus, the receptors NFR1 and NFR5 form the core complex that starts both nodule organogenesis and infection; barley receptors that activate nodule organogenesis were also identified [4].
• Two amino acid residues in the kinase domain decide whether such receptors signal for immunity or for symbiosis: changing them converted an immune receptor of Lotus and the barley receptor RLK4 to enable symbiotic signalling in Lotus [5].
• A maize landrace from nitrogen-poor soils in Oaxaca, Mexico, obtains 29–82% of its nitrogen from fixation by bacteria living in the mucilage of its aerial roots, a route that needs no nodules [6].

What would count as progress

No single result settles a question like this; contributions like these would move it forward:

• A cereal line that forms nodule-like organs in response to rhizobia, with confirmed bacterial infection and nitrogen fixation measured by ¹⁵N labelling, reported with the full genetic constructs so others can repeat it.
• Milestones demonstrated and quantified in a cereal: symbiotic signalling, infection, organ formation, oxygen control inside the organ, and transfer of fixed nitrogen to the plant.
• Greenhouse and field measurements of how much nitrogen engineered or associative systems supply, and at what cost to yield.

References

1. Armstrong, E. M. et al. (2023). One hundred important questions facing plant science: an international perspective. New Phytologist 238(2), 470–481.
2. Rogers, C. & Oldroyd, G. E. D. (2014). Synthetic biology approaches to engineering the nitrogen symbiosis in cereals. Journal of Experimental Botany 65(8), 1939–1946.
3. Huisman, R. & Geurts, R. (2020). A roadmap toward engineered nitrogen-fixing nodule symbiosis. Plant Communications 1(1), 100019.
4. Rübsam, H. et al. (2023). Nanobody-driven signaling reveals the core receptor complex in root nodule symbiosis. Science 379(6629), 272–277.
5. Tsitsikli, M. et al. (2025). Two residues reprogram immunity receptors for nitrogen-fixing symbiosis. Nature 648(8093), 443–450.
6. Van Deynze, A. et al. (2018). Nitrogen fixation in a landrace of maize is supported by a mucilage-associated diazotrophic microbiota. PLOS Biology 16(8), e2006352.

Source

Imported from One hundred important questions facing plant science, question 100, curated by Armstrong et al. Listed as open in the source (2023) and still an active research question as of 2026-10-20. The statement above is written in our own words; see the source for the original wording, history and discussion.