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On the road to making cereals that consume nitrogen from air


Some bacteria carry an enzyme called nitrogenase that enables them to grow incorporating nitrogen from air into biomass. Transferring nitrogenase to cereals would keep crop production high while reducing nitrogen fertilization.

Although nitrogen gas is the major component of the atmosphere, it is biologically inactive for plants, fungi and animals. Only some bacteria, called diazotrophs or nitrogen eaters, are able to convert (fix) nitrogen gas into ammonia, which is then used by themselves and other living organisms to grow biomass. Diazotrophs are therefore the primary producers of nitrogen in all ecosystems.

Agricultural systems in developed countries are largely based on cereal crops, which provide about 60% of the energy in our diet. Nitrogen and water availability are the most important factors limiting cereal crop productivity. Unlike legumes, cereals are unable to form symbioses with diazotrophs and their production is tied to the application of nitrogen fertilizers. During the last one hundred years, cereal crop yields have increased by addition of chemically synthesized nitrogen fertilizers at a cost exceeding 100,000 million US$ per year. Pervasive use of nitrogen fertilizers in developed countries have degraded the environment to a situation at times incompatible with animal life in marine ecosystems. In contrast, high cost and limited availability of chemical fertilizers prevent its use by poor farmers, causing poverty and hunger derived from low crop yields.

Credit: Pixabay

Diazotrophs use a two-component enzyme called nitrogenase to fix nitrogen. Because plants lack nitrogenase, they cannot “eat” nitrogen gas. The laboratory of Professor Luis Rubio is trying to solve the so-called “nitrogen problem” by transferring nitrogenase genes (nif) to plants. This would generate diazotrophic plants and thus crops much less dependent of chemical nitrogen fertilizers. This is considered as one of the most difficult challenges in plant biotechnology. Because nitrogenase is destroyed by oxygen gas, it has long been assumed that it would not work in cells from plants, animals or fungi, all of which require oxygen to obtain energy. Such incompatibility would be exacerbated in plant cells producing oxygen as result of photosynthesis.

However, a recent study from the Rubio laboratory demonstrated that the most oxygen sensitive component of nitrogenase functions in yeast (a fungi used as model to expedite outputs) as long as it is kept inside mitochondria, an intracellular organelle responsible for oxygenic respiration (López-Torrejón et al. 2016). The Rubio laboratory, together with the laboratory of Professor Christopher Voigt, a world-leading synthetic biologist at the Massachusetts Institute of Technology, has now taken a second step in assembling nitrogenase by transferring 9 nif genes to the genome of yeast and expressing them at controlled levels (Burén et al., 2017). This study proved correct formation of the other nitrogenase component in mitochondria, an essential step of nitrogenase assembly. The study also showed that nitrogenase was not fully functional, and that further bioengineering is needed to obtain active mitochondrial nitrogenase.

The research performed in the Rubio laboratory at the Centro de Biotecnología y Genómica de Plantas of Universidad Politécnica de Madrid is funded by the Bill & Melinda Gates Foundation and aims to help smallholder farmers in sub-Saharan Africa. Although much bioengineering remains to be done, research carried out at the Rubio laboratory will hopefully contribute to reduce nitrogen fertilization in developed countries and to increase cereal production for farmers in developing countries of Africa and Asia.

Buren, S.; Young, E.M.; Sweeny, E.A.; Lopez-Torrejon, G.; Veldhuizen, M.; Voigt, C.A.; Rubio, L.M.. Formation of Nitrogenase NifDK Tetramers in the Mitochondria of Saccharomyces cerevisiae ACS SYNTHETIC BIOLOGY, 6 (6):1043-1055; 10.1021/acssynbio.6b00371 JUN 2017

López-Torrejón, G; Jiménez-Vicente, E; Buesa, JM; Hernandez, JA; Verma, HK; Rubio, LM.  Expression of a functional oxygen-labile nitrogenase component in the mitochondrial matrix of aerobically grown yeast. NATURE COMMUNICATIONS. DOI: 10.1038/ncomms11426.

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