Abstract
SummaryBiological 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.
Highlights
Nod and Nif proteins are encoded by accessory genes of bacteria that are placed in transmissible genetic elements like plasmids, symbiotic islands and chromids
Microbial Biotechnology published by John Wiley & Sons Ltd and Society for Applied Microbiology., Microbial Biotechnology, 13, 1314–1335
Not advisable to use the method for quantification of amounts of N2 fixed over time, but for activity comparisons between rhizobial strains and activity measurements in diverse field conditions it is excellent
Summary
Biological nitrogen fixation in rhizobia occurs primarily in root or stem nodules and is induced by the bacteria present in legume plants. The words efficiency, efficacy and effectiveness are used in rhizobium literature almost without distinction Since they do mean different things, a distinction might be useful when discussing symbiotic nitrogen fixation (SNF) in associations between rhizobia and legumes. One line of research aims at applying synthetic biology and biotechnology to engineer a biocatalyst for fertilizer production Another main direction is to take on the challenge of engineering nonlegumes to either harbour nitrogenase without rhizobial infection or to become nodulated by rhizobia. Studies with these aims result in a great deal of basic research on biological mechanisms of SNF, some of which could be applied when looking for efficiency and trying to understand effectiveness in rhizobia. This minireview mainly deals with the latter, though it is not wise to draw sharp borderlines between concepts defined to help us keep track on complex biological processes
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