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

Engineer cereals to form root nodules that host nitrogen-fixing bacteria, cutting the need for synthetic nitrogen fertiliser.

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	1	+# Can cereals be engineered to form nitrogen-fixing root nodules?
	2	+
	3	+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.
	4	+
	5	+## Question
	6	+
	7	+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?
	8	+
	9	+## Background
	10	+
	11	+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].
	12	+
	13	+## What is known
	14	+
	15	+- 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].
	16	+- 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].
	17	+- 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].
	18	+- 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].
	19	+
	20	+## What would count as progress
	21	+
	22	+No single result settles a question like this; contributions like these would move it forward:
	23	+
	24	+- 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.
	25	+- 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.
	26	+- Greenhouse and field measurements of how much nitrogen engineered or associative systems supply, and at what cost to yield.
	27	+
	28	+## References
	29	+
	30	+1. Armstrong, E. M. et al. (2023). [One hundred important questions facing plant science: an international perspective](https://doi.org/10.1111/nph.18771). New Phytologist 238(2), 470–481.
	31	+2. Rogers, C. & Oldroyd, G. E. D. (2014). [Synthetic biology approaches to engineering the nitrogen symbiosis in cereals](https://doi.org/10.1093/jxb/eru098). Journal of Experimental Botany 65(8), 1939–1946.
	32	+3. Huisman, R. & Geurts, R. (2020). [A roadmap toward engineered nitrogen-fixing nodule symbiosis](https://doi.org/10.1016/j.xplc.2019.100019). Plant Communications 1(1), 100019.
	33	+4. Rübsam, H. et al. (2023). [Nanobody-driven signaling reveals the core receptor complex in root nodule symbiosis](https://doi.org/10.1126/science.ade9204). Science 379(6629), 272–277.
	34	+5. Tsitsikli, M. et al. (2025). [Two residues reprogram immunity receptors for nitrogen-fixing symbiosis](https://doi.org/10.1038/s41586-025-09696-3). Nature 648(8093), 443–450.
	35	+6. Van Deynze, A. et al. (2018). [Nitrogen fixation in a landrace of maize is supported by a mucilage-associated diazotrophic microbiota](https://doi.org/10.1371/journal.pbio.2006352). PLOS Biology 16(8), e2006352.
	36	+
	37	+## Source
	38	+
	39	+Imported from [One hundred important questions facing plant science, question 100](https://doi.org/10.1111/nph.18771), 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.