Nitrogen-fixing cereals

In plain language

Plants need nitrogen to build proteins and DNA, and most of the nitrogen used in modern farming comes from synthetic fertiliser, which takes a lot of energy to make. Legumes such as beans and peas partly solve this problem with bacteria that live in nodules on their roots and turn nitrogen gas from the air into a form the plant can use. Cereals such as maize, wheat and rice do not form these partnerships, so researchers are asking whether they can be made to fix nitrogen, either directly or through bacteria that live on their roots.

There are two broad routes. The first is to add microbes that fix nitrogen near the roots, for example as a seed or soil treatment. A Mexican maize landrace already does something similar: its aerial roots produce a sugary slime, called mucilage, that hosts nitrogen-fixing bacteria. Companies have gene-edited some bacteria to fix more nitrogen. Their results are promising in some field trials but inconsistent in others. The second route is to engineer the cereal plant, or its cell compartments, to carry nitrogenase, the enzyme that does the fixing. That enzyme is hard to build in plants and is damaged by oxygen, and in the laboratory the work has only reached early steps.

Technical explanation

Biological nitrogen fixation is carried out by nitrogenase, a multi-protein enzyme complex found in some bacteria. Its activity is sensitive to oxygen, which is one reason it is hard to move into plant cells. In Van Deynze and colleagues (2018), a traditional maize landrace from the Sierra Mixe region of Oaxaca, Mexico, showed nitrogenase activity in its aerial root mucilage by the acetylene reduction assay. No activity was detected in underground roots, leaves, stems or aerial roots before mucilage production. Using 15N-enrichment and nitrogen-difference methods over field work spanning five years, the authors reported that atmospheric nitrogen fixation contributed 29% to 82% of the plant's nitrogen nutrition. The 15N-enrichment method alone gave 31% to 55%. The authors say the genetic basis of the trait and the origin of the bacteria remain unresolved, and that all the techniques available are prone to artefacts.

In the 2024 Scientific Reports study, gene-edited non-transgenic strains of Klebsiella variicola and Kosakonia sacchari were used in the commercial product PROVEN 40. Controlled-environment 15N2 labelling showed elevated 15N in maize chlorophyll (p < 0.001), but not with nif-knockout control strains. Field estimates were small and uncertain. The 2022 inoculation-rate trials estimated 21.2 kg N/ha by the VT-R1 stage, with a 95% confidence interval of -0.16 to 42.6 kg N/ha, so the interval includes zero. A 2021 on-farm comparison on 58 fields found no yield difference when 39 to 45 kg N/ha of fertiliser was replaced by PROVEN 40 (-0.0135 Mg/ha, p = 0.871).

Engineering nitrogenase into cereals is at an earlier stage. Jiang and colleagues (2021, Communications Biology) tested 32 NifH sequences expressed in tobacco (Nicotiana benthamiana), and only two were consistently detected in mitochondria. Protein purified from tobacco leaves was inactive until its iron-sulphur cluster was reconstituted in vitro, and the authors identify plant mitochondrial iron-sulphur cluster availability as a bottleneck. Rogers and Oldroyd (2014) set out an initial target of 25 to 50 kg fixed N/ha, noted that none of the current approaches is likely to reach fertiliser-equivalent levels in the short term, and observed that one generation of maize takes approximately a year, which slows crop testing.

What the evidence says

Each statement is typed by the strength of its evidence and linked to the source that supports it.

Limitations and open problems

• Measured nitrogen fixation in field trials is small and highly uncertain. The 2022 confidence interval includes zero, and leaf-tip sampling, used to estimate fixation, has not been validated as representative of the whole plant (Sci Rep, 2024).
• Yield responses to commercial nitrogen-fixing products are inconsistent between sites and years. A 2025 University of Kentucky review of trials reported no nitrogen credit values, and a North Central region study found a yield benefit at only 2 of 61 site-years (Kentucky review, 2025). In an Illinois study summarised by the same review, PROVEN 40 raised yield by 1.8 bu/acre on average across four site-years, but only one site-year showed a significant response.
• The 2022 season was described as abnormally dry. The authors could not determine how drought affects the strains, expected populations to decline later in the season, and reported little evidence of fixation after flowering (Sci Rep, 2024).
• No active nitrogenase has been shown working in plant cells. Oxygen sensitivity and the availability of iron-sulphur clusters in plant mitochondria are described as bottlenecks (Jiang and colleagues, 2021; Rogers and Oldroyd, 2014).
• Most authors of the 2024 field study are affiliated with the company that sells the product, which holds patents on the strains. The commercial trials were non-replicated side-by-side comparisons, so independent replication is needed before wider conclusions are drawn (Sci Rep, 2024).
• The 2024 authors say effects on nitrous oxide emissions and nitrate leaching require further research, so the climate benefit of reduced fertiliser use is not yet established.

Possible future uses

Timeline

1. 31 Mar 2014Rogers and Oldroyd publish a review in the Journal of Experimental Botany that proposes an initial target of 25 to 50 kg fixed N/ha for engineered cereals, and state that current approaches are unlikely to reach fertiliser-equivalent levels in the short term. Sources: [4]
2. 7 Aug 2018Van Deynze and colleagues report that a Sierra Mixe maize landrace obtains 29% to 82% of its nitrogen nutrition from atmospheric fixation, with nitrogenase activity detected in aerial root mucilage. Sources: [1]
3. 4 Jan 2021Jiang and colleagues report that only two of 32 NifH variants were consistently detected in plant mitochondria, and identify iron-sulphur cluster availability as a bottleneck. No nitrogen fixation in whole plants was reported. Sources: [5]
4. 12 Nov 2024Scientific Reports publishes field and isotope evidence for gene-edited Klebsiella and Kosakonia strains in maize, including small and variable fixation estimates. Sources: [2]
5. 24 Mar 2025University of Kentucky Grain Crops review finds inconsistent corn yield responses across biological nitrogen fixation trials. Sources: [3]

Futures this could help bring about

How far away?

Farming without synthetic fertiliser

Pilots · 2 of 6 steps done

Glossary

Nitrogen fixation

Conversion of nitrogen gas from the air into ammonia or related compounds that living things can use.

Nitrogenase

The enzyme complex that carries out nitrogen fixation in certain bacteria. It is damaged by oxygen.

Mucilage

A sticky, carbohydrate-rich slime that some plant roots release, which can support bacteria.

Acetylene reduction assay

A laboratory test that measures nitrogenase activity by how much acetylene the sample converts to ethylene.

Sources

1. peer reviewed paper
   Nitrogen fixation in a landrace of maize is supported by a mucilage-associated diazotrophic microbiota
   Allen Van Deynze, Pablo Zamora, Pierre-Marc Delaux et al.. PLoS Biology, 7 Aug 2018. DOI: 10.1371/journal.pbio.2006352. Accessed 21 Oct 2026.
2. peer reviewed paper
   Genetic remodeling of soil diazotrophs enables partial replacement of synthetic nitrogen fertilizer with biological nitrogen fixation in maize
   Rafael Martinez-Feria, Maegen B. Simmonds, Bilge Ozaydin et al.. Scientific Reports, 12 Nov 2024. DOI: 10.1038/s41598-024-78243-3. Accessed 21 Oct 2026.
3. other
   Biological N fixation products for corn: update
   John Grove, Chad Lee. University of Kentucky Martin-Gatton College of Agriculture, Food and Environment, Grain and Forage Center of Excellence, 24 Mar 2025. Accessed 21 Oct 2026.
4. review article
   Synthetic biology approaches to engineering the nitrogen symbiosis in cereals
   Christian Rogers, Giles E. D. Oldroyd. Journal of Experimental Botany, 31 Mar 2014. DOI: 10.1093/jxb/eru098. Accessed 21 Oct 2026.
5. peer reviewed paper
   Exploiting genetic diversity and gene synthesis to identify superior nitrogenase NifH protein variants to engineer N2-fixation in plants
   Xi Jiang, Lucía Payá-Tormo, Diana Coroian et al.. Communications Biology, 4 Jan 2021. DOI: 10.1038/s42003-020-01536-6. Accessed 21 Oct 2026.

Revision history

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