Nitrogen-fixing cereals

A cereal that supplied its own nitrogen would be the largest single change available to agriculture. It has not been achieved, it is not close, and the reasons are chemical rather than organisational. This page is about why.

The enzyme is the problem

Biological nitrogen fixation is done by one enzyme family. Nitrogenase consists of an iron protein and a molybdenum–iron protein carrying the FeMo-cofactor, and it reduces dinitrogen at that metal cluster using low-potential electrons delivered one at a time, each transfer driven by ATP hydrolysis. The stoichiometry is unforgiving: even in the ideal case the reaction consumes sixteen ATP per molecule of nitrogen fixed and obligately evolves hydrogen alongside the ammonia, so part of the energy is lost by design.

Worse, the metal clusters are irreversibly damaged by oxygen — the iron protein within seconds of exposure. Nitrogenase and aerobic metabolism are therefore mutually exclusive in the same compartment at the same time, and every organism that fixes nitrogen has an answer to this: a heterocyst with a thickened wall, temporal separation between day and night, life in an anoxic niche, or a nodule.

What a nodule is, and what it costs

The legume answer is an organ. Rhizobia inside root nodule cells are enclosed in a plant-derived membrane, and the nodule cortex forms a variable oxygen diffusion barrier. Inside, leghaemoglobin — a plant globin, present at high concentration — binds oxygen with high affinity, holding the free concentration in the nanomolar range while still ferrying enough flux to the bacteroid to run the respiration that pays for nitrogenase. It is a solution to two opposing requirements at once: almost no free oxygen, plenty of oxygen delivery.

The plant pays for this in carbon. Nodulation, nodule maintenance and the ATP burden of fixation together consume a substantial share of photosynthate, which is why a nodulated legume yields less than it would with the same nitrogen supplied from soil, and why nodulation is switched off when nitrate is abundant. Free nitrogen is not free.

Why transferring it has not worked

The genetic burden is the second obstacle. Fixation in a model diazotroph requires around twenty nif genes — structural subunits, cofactor biosynthesis, electron transfer, maturation — with a defined stoichiometry between them. Transferring that into a plant means expressing the set in a compartment that can supply ATP, low-potential reductant and iron–sulfur cluster assembly while excluding oxygen; mitochondria and plastids are the candidate compartments for that reason. Individual Nif proteins have been expressed and some shown active in isolation in yeast and plant organelles, but no functional nitrogenase has been assembled in a plant. The alternative programme — teaching a cereal to nodulate, exploiting the signalling components it shares with the older mycorrhizal symbiosis — is at the same stage.

What is actually deployable

The tractable route is bacterial and lives outside the plant cell. Associative and endophytic diazotrophs — Azospirillum, Herbaspirillum, Gluconacetobacter and relatives — colonise cereal roots and interior tissues without forming a nodule, and can be engineered to keep nif expression on in the presence of ammonium and to excrete ammonium rather than assimilate it. Without a nodule there is no diffusion barrier, so what these organisms fix is limited by their own oxygen protection and their carbon supply. The quantities are small relative to a cereal’s demand, they are hard to measure unambiguously in the field, and published estimates for associative fixation have been revised downwards more than once. Treat this as a partial fertiliser offset, not a replacement.