Effectiveness of nitrogen fixation in rhizobia

Abstract

Biological nitrogen fixation in rhizobia occurs primarily in root or stem nodules and is induced by the bacteria present in legume plants. This symbiotic process has fascinated researchers for over a century, and the positive effects of legumes on soils and their food and feed value have been recognized for thousands of years. Symbiotic nitrogen fixation uses solar energy to reduce the inert N₂ gas to ammonia at normal temperature and pressure, and is thus today, especially, important for sustainable food production. Increased productivity through improved effectiveness of the process is seen as a major research and development goal. The interaction between rhizobia and their legume hosts has thus been dissected at agronomic, plant physiological, microbiological and molecular levels to produce ample information about processes involved, but identification of major bottlenecks regarding efficiency of nitrogen fixation has proven to be complex. We review processes and results that contributed to the current understanding of this fascinating system, with focus on effectiveness of nitrogen fixation in rhizobia.

Figure 1

Summarized model for symbiotic nitrogen fixation in legumes by rhizobia.

Figure 2

Deduced energy metabolism in nodulating symbionts of Rhizobium freirei and hypothetical role of green complex. Reprinted from Degli Esposti and Martinez Romero (2016) with the permission of Oxford University Press.

Figure 3

Schematics of carbon and nitrogen metabolic pathways with key enzymes, metabolites and transporters in determinate nodules and indeterminate nodules. Reprinted from Liu et al. (2018) with the permission of Frontiers.

Figure 4

Comparison of NO signalling cascades in the symbiotic bacteria Bradyrhizobium japonicum, Sinorhizobium meliloti and Rhizobium etli. TU: transcription unit. Reprinted from Meilhoc et al. (2011) with the permission of Elsevier.