Abstract
ricultural yields are often limited by nitrogen (N) availability, especially in countries of the developing world, whereas in industrialized nations the application of chemical N fertilizers has reached unsustainable levels that have resulted in severe environmental consequences. Finding alternatives to inorganic fertilizers is critical for sustainable and secure food production. Although gaseous nitrogen (N2) is abundant in the atmosphere, it cannot be assimilated by most living organisms. Only a selected group of microorganisms termed diazotrophs, have evolved the ability to reduce N2 to generate NH3 in a process known as biological nitrogen fixation (BNF) catalysed by nitrogenase, an oxygen-sensitive enzyme complex. This ability presents an opportunity to improve the nutrition of crop plants, through the introduction into cereal crops of either the N fixing bacteria or the nitrogenase enzyme responsible for N fixation. This review explores three potential approaches to obtain N-fixing cereals: (a) engineering the nitrogenase enzyme to function in plant cells; (b) engineering the legume symbiosis into cereals; and (c) engineering cereals with the capability to associate with N-fixing bacteria.
Highlights
This review explores three potential approaches to obtain N-fixing cereals: (a) engineering the nitrogenase enzyme to function in plant cells; (b) engineering the legume symbiosis into cereals; and (c) engineering cereals with the capability to associate with N-fixing bacteria
This review provides comprehensive and updated information on these different approaches to reducing the N fertilizers demand through improving biological nitrogen fixation (BNF): (1) direct transfer of bacterial nif genes for expressing heterologous nitrogenase in cereals; (2) engineering new symbioses between cereals and N2-fixing bacteria in a similar form to the legume–rhizobium symbiosis; and (3) improvement of N2-fixing bacterial endophytes naturally associated to cereals
The most important difficulties involved in the direct transfer of bacterial nif genes into the cereal are the sensitivity of nitrogenase to O2 and the complexity and fragility of nitrogenase biosynthesis
Summary
Garcia del Moral fuels in agriculture, with predictions that this process will consume around 2% of global energy by 2050 (Glendining et al, 2009) For this reason, considering the future menaces of a decline in petroleum reserves and the low efficiency with which cereal crops use chemical nitrogen fertilizers, the search for new alternative sources of nitrogen to reduce agricultural reliance on nitrogen fertilizers is an urgent need. Atmospheric nitrogen represents 78% of the air, due to the stability of the triple bond between the two nitrogen atoms, N2 is inaccessible to eukaryotes, since only a group of bacteria and archaea (called diazotrophos organisms) have developed the ability to fix N2 organic through the enzyme nitrogenase This biological nitrogen fixation (BNF) is the major contributor to the N economy of the biosphere, accounting for 30–50% of the total N in crop fields (Ormeño-Orrillo et al, 2013) and represents a promising substitute for chemical N fertilizers (Olivares et al, 2013; Dent & Cocking, 2017; Good, 2018). The process of N fixation through symbiosis is very complex, involving multiple events and their regulation in both the host and the rhizobia engineering a N-fixing symbiosis will requires adapting existing signaling and developmental mechanisms to provide a suitable environment for nitrogenase activity in the new cereal nodules
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