Presenting and introducing a new approach and model for nitrogen fixation in cereal products

Nitrogen fixation is a chemical process by which molecular nitrogen (N۲), which has a strong triple covalent bond, is converted into ammonia (NH۳) or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. The nitrogen in air is molecular dinitrogen, a relatively nonreactive molecule that is metabolically useless to all but a few microorganisms. Biological nitrogen fixation or diazotrophy is an important microbe-mediated process that converts dinitrogen (N۲) gas to ammonia (NH۳) using the nitrogenase protein complex (Nif). Nitrogen fixation is essential to life because fixed inorganic nitrogen compounds are required for the biosynthesis of all nitrogen-containing organic compounds, such as amino acids and proteins, nucleoside triphosphates and nucleic acids. As part of the nitrogen cycle, it is essential for agriculture and the manufacture of fertilizer. It is also, indirectly, relevant to the manufacture of all nitrogen chemical compounds, which include some explosives, pharmaceuticals, and dyes. Nitrogen fixation is carried out naturally in soil by microorganisms termed diazotrophs that include bacteria, such as Azotobacter, and archaea. Some nitrogen-fixing bacteria have symbiotic relationships with plant groups, especially legumes. Looser non-symbiotic relationships between diazotrophs and plants are often referred to as associative, as seen in nitrogen fixation on rice roots. Nitrogen fixation occurs between some termites and fungi. It occurs naturally in the air by means of NOx production by lightning. Biological nitrogen fixation was discovered by Jean-Baptiste Boussingault in ۱۸۳۸. Later, in ۱۸۸۰, the process by which it happens was discovered by German agronomist Hermann Hellriegel and Hermann Wilfarth [de] and was fully described by Dutch microbiologist Martinus Beijerinck. "The protracted investigations of the relation of plants to the acquisition of nitrogen begun by Saussure, Ville, Lawes and Gilbert and others culminated in the discovery of symbiotic fixation by Hellriegel and Wilfarth in ۱۸۸۷." "Experiments by Bossingault in ۱۸۵۵ and Pugh, Gilbert & Lawes in ۱۸۸۷ had shown that nitrogen did not enter the plant directly. The discovery of the role of nitrogen fixing bacteria by Herman Hellriegel and Herman Wilfarth in ۱۸۸۶-۸ would open a new era of soil science." In ۱۹۰۱ Beijerinck showed that Azotobacter chroococcum was able to fix atmospheric nitrogen. This was the first species of the azotobacter genus, so-named by him. It is also the first known diazotroph, species that use diatomic nitrogen as a step in the complete nitrogen cycle. All biological reactions involving the process of nitrogen fixation are catalyzed by enzymes called nitrogenases. These enzymes contain iron, often with a second metal, usually molybdenum but sometimes vanadium. Nitrogen-fixing microbial associations with cereals have been of intense interest for more than a century (Roesch et al., Plant Soil ۲۰۰۸;۳۰۲:۹۱–۱۰۴; Triplett, Plant Soil ۱۹۹۶;۱۸۶:۲۹–۳۸; Mus et al., Appl. Environ. Microbiol. ۲۰۱۶;۸۲:۳۶۹۸–۳۷۱۰; Beatty and Good, Science ۲۰۱۱;۳۳۳:۴۱۶–۴۱۷). A recent report demonstrated that an indigenous Sierra Mixe maize landrace, characterized by an extensive development of aerial roots that secrete large amounts of mucilage, can acquire ۲۸–۸۲% of its nitrogen from atmospheric dinitrogen (Van Deynze et al., PLoS Biol. ۲۰۱۸;۱۶: e۲۰۰۶۳۵۲). Although the Sierra Mixe maize landrace is unique in the large quantity of mucilage produced, other cereal crops secrete mucilage from underground and aerial roots and we hypothesize that this may represent a general mechanism for cereals to support associations with microbial diazotrophs. We propose a model for the association of nitrogen-fixing microbes with maize mucilage and identify the four main functionalities for such a productive diazotrophic association.

Keywords:

Nitrogen-fixing , cereal crops , general mechanism , microbial