Nitrogen-Fixing Bacteria: The Friends in Need

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Nitrogen is one of the most abundantly available elements in the atmosphere, but it is not available in a biologically utilizable form. In nature, lightning storms and biological nitrogen fixation (BNF) by prokaryotes are responsible for converting the inorganic atmospheric nitrogen into forms that can be utilized by biological systems. The process of BNF occurs only in prokaryotes expressing the enzyme nitrogenase. Some plants, like legumes and a few non-legumes, can form symbiotic relationships with specific nitrogen-fixing bacteria, forming a specialized organ called nodules. Low oxygen content inside the nodules and easy access to sugars from the host plant help the bacteria in expressing nitrogenase. The host plant in turn benefits by utilizing the fixed nitrogen. Most of the staple food crops do not have the capacity to form nodules for harbouring nitrogen-fixing microbes; further, the increase in population has compounded the intensity of cultivation across agroecosystems. Hence, nitrogen becomes limiting for crop plants and it becomes imperative to encourage sustainable methods like BNF for providing N. Synthetic fertilizers used presently by the farmers, are a major cause of pollution. In such a scenario, the associative, endophytic, and free-living nitrogen fixers or biotechnological interventions hold the key to providing pollution-free nitrogen to these crop plants. In this chapter, we will discuss the diversity of nitrogen-fixing systems and the methods of assessing and utilizing these microbes for crop benefit.