Abstract
In agroecosystems, nitrogen is one of the major nutrients limiting plant growth. To meet the increased nitrogen demand in agriculture, synthetic fertilizers have been used extensively in the latter part of the twentieth century, which have led to environmental challenges such as nitrate pollution. Biological nitrogen fixation (BNF) in plants is an essential mechanism for sustainable agricultural production and healthy ecosystem functioning. BNF by legumes and associative, endosymbiotic, and endophytic nitrogen fixation in non-legumes play major roles in reducing the use of synthetic nitrogen fertilizer in agriculture, increased plant nutrient content, and soil health reclamation. This review discusses the process of nitrogen-fixation in plants, nodule formation, the genes involved in plant-rhizobia interaction, and nitrogen-fixing legume and non-legume plants. This review also elaborates on current research efforts involved in transferring nitrogen-fixing mechanisms from legumes to non-legumes, especially to economically important crops such as rice, maize, and wheat at the molecular level and relevant other techniques involving the manipulation of soil microbiome for plant benefits in the non-legume root environment.
Highlights
A healthy, functioning soil ensures nutrient cycling for optimum plant growth for agricultural production [1]
Symbiotic Nitrogen Fixation in Legume Nodules. In both natural and cultivated ecosystems, legumes supply a significant amount of nitrogen [84,85,86], and the nitrogen fixed by perennial forage legumes can be as high as the amounts of nitrogen fertilizers used in conventional farming practices [87,88]
Symbiotic bacteria such as rhizobia and Frankia are located in nodules, whereas in rhizosphere-associative systems, the diazotrophic bacteria are essential in the free-living state and fix nitrogen using the supply of carbohydrates from the environment [104,105] in accord with the excretion of carbohydrates from the roots and the degradation of soil organic matter
Summary
A healthy, functioning soil ensures nutrient cycling for optimum plant growth for agricultural production [1]. Soil nitrogen input for plant nutrition and crop productivity largely depends on organic matter degradation, synthetic fertilizer applications, and biological nitrogen fixation (BNF) via nitrogenase enzyme activity [4,5]. This limited bio-availability of N and the escalating reliance of crop growth on N have created a colossal N-based fertilizer industry worldwide [6,7]. Despite the phylogenic and ecological diversity among diazotrophic bacteria and their hosts, a synchronized interaction is always a prerequisite between the microbial entities and the host plant to achieve a successful nitrogen fixation system The importance of this process is enormous as it reduces the dependence on nitrogen fertilizers for plants and for agriculture overall. Obligate aerobes, such as Azotobacter vinelandii can shield nitrogenase from oxygen and perform nitrogen fixation by consuming oxygen via cytochrome oxidases [31,32]
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