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Open AccessArticle

Nitrogen Assimilation and Transport by Ex Planta Nitrogen-Fixing Bradyrhizobium diazoefficiens Bacteroids Is Modulated by Oxygen, Bacteroid Density and l-Malate

Department of Biochemistry, University of Missouri, Columbia, MO 65211, USA
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Author to whom correspondence should be addressed.
Int. J. Mol. Sci. 2020, 21(20), 7542; https://doi.org/10.3390/ijms21207542
Received: 1 August 2020 / Revised: 9 October 2020 / Accepted: 12 October 2020 / Published: 13 October 2020
(This article belongs to the Special Issue Amino Acids Transport and Metabolism 3.0)
Symbiotic nitrogen fixation requires the transfer of fixed organic nitrogen compounds from the symbiotic bacteria to a host plant, yet the chemical nature of the compounds is in question. Bradyrhizobium diazoefficiens bacteroids were isolated anaerobically from soybean nodules and assayed at varying densities, varying partial pressures of oxygen, and varying levels of l-malate. Ammonium was released at low bacteroid densities and high partial pressures of oxygen, but was apparently taken up at high bacteroid densities and low partial pressures of oxygen in the presence of l-malate; these later conditions were optimal for amino acid excretion. The ratio of partial pressure of oxygen/bacteroid density of apparent ammonium uptake and of alanine excretion displayed an inverse relationship. Ammonium uptake, alanine and branch chain amino acid release were all dependent on the concentration of l-malate displaying similar K0.5 values of 0.5 mM demonstrating concerted regulation. The hyperbolic kinetics of ammonium uptake and amino acid excretion suggests transport via a membrane carrier and also suggested that transport was rate limiting. Glutamate uptake displayed exponential kinetics implying transport via a channel. The chemical nature of the compounds released were dependent upon bacteroid density, partial pressure of oxygen and concentration of l-malate demonstrating an integrated metabolism. View Full-Text
Keywords: nitrogen fixation; Bradyrhizobium diazoefficiens; bacteroids; ammonia/ammonium; alanine nitrogen fixation; Bradyrhizobium diazoefficiens; bacteroids; ammonia/ammonium; alanine
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Waters, J.K.; Mawhinney, T.P.; Emerich, D.W. Nitrogen Assimilation and Transport by Ex Planta Nitrogen-Fixing Bradyrhizobium diazoefficiens Bacteroids Is Modulated by Oxygen, Bacteroid Density and l-Malate. Int. J. Mol. Sci. 2020, 21, 7542.

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