Crops require nitrogen for growth and productivity because it is a major component of DNA, proteins, chlorophyll and energy-storage molecules such as ATP. Most crops depend on supplies of nitrate and ammonium from industrial synthetic fertilizers, but more than half of these inputs remain unassimilated, spilling over or leaching into rivers and lakes as a major source of pollution.
Leguminous crops such as peas and beans harbour bacteria that convert nitrogen gas directly into ammonia using an enzyme called nitrogenase. This process is known as biological nitrogen fixation. The introduction of nitrogenase genes into crop plants would provide the machinery needed to fix nitrogen independently. However, the process is extremely complex because many different individual proteins are required not only as the direct structural components of nitrogenase but also accessory proteins needed for its assembly and the provision of energy. The major protein components are also extremely oxygen sensitive. The researchers overcame this critical bottleneck by producing functional dinitrogenase reductase (Fe protein, NifH) and the nitrogenase cofactor maturase (NifB) in separate transgenic rice lines. This is a major bioengineering advance as it tears down two technical roadblocks and shows the path to make nitrogen-fixing cereals. The achievement removes one of the major constraints hindering biological nitrogen fixation in crops and sets the stage for the assembly of a complete and functional nitrogenase complex in plants. Further work to establish plants containing the full nitrogenase would have a lasting impact on global food security.
One of the major impacts of the work in the long term will be in low- and middle-income countries, which cannot afford expensive nitrogen fertilizers. The work is part of a research program funded by a grant from the Bill & Melinda Gates Foundation.
