Cereal crops that obtain most of their nitrogen from the air, through their own nitrogenase, an engineered symbiosis or a partnered microbiome, so that high yields do not depend on synthetic nitrogen fertilizer.
Scope
In: maize, wheat, rice and other cereals that fix nitrogen in the plant or in a stable association with it. Out: fertilizer-applied microbial inoculants that supplement but do not replace fertilizer (they appear here only as an approach), and plant gene-editing tools, which are biotech/plant-genome-engineering.
Nitrogen derived from the atmosphere (%Ndfa) headline18 points to go
Share of a plant's nitrogen that comes from biological fixation of atmospheric N2, with the measurement method (15N isotope dilution, 15N natural abundance, ureide or N difference) stated in the conditions. Higher is better.
Current (2018-01-01)82%
Target100%
Limit–
Conditions. Share of the crop's nitrogen from biological fixation of atmospheric nitrogen, measured with 15N methods.
Why this target. A crop that fully meets its nitrogen from the air is the 'N-self-fertilizing' crop described in the literature as capable of autonomous fixation, avoiding the need for chemical fertilizers. The target applies to elite cultivars at full yield, which is what the current value does not show.
Note. Upper end of a 29-82% range, in an indigenous landrace grown in nitrogen-depleted soil. Not a modern high-yield cultivar: no elite cereal has shown this.
Fixation shown in a landrace, not in high-yield cultivars
criticalscientific unknownlayer: systemactive
The high fixation share was measured in a landrace with aerial roots that secrete mucilage, grown in nitrogen-depleted soil. Moving the trait into elite cultivars at full yield, and under fertilization, has not been shown. Engineered root-associated bacteria that keep fixing in fertilized fields have been commercialized, but their abstract reports a yield gain over fertilizer alone, not a share of nitrogen from fixation.
Approaches
Engineered root-associated diazotroph that fixes nitrogen despite fertilizer wen2021enabling
Expressing the nitrogenase components in plant mitochondria is the route to a crop that fixes its own nitrogen. Sixteen nitrogenase proteins have each been expressed and targeted to the mitochondrial matrix of a model plant, but the NifD component is the least abundant and a full working complex in a plant has not been shown.
Root-nodule symbiosis not yet engineered into non-legumes
highscientific unknownlayer: principleopen
Legumes host nitrogen-fixing rhizobia in root nodules. The objective of engineering nodulation in non-leguminous crops has not been achieved; the open questions are the signalling, infection and nodule-organogenesis programs.