Abstract
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.
Highlights
Symbiotic nitrogen fixation is the process whereby rhizobia bacteria enter compatible host plant root cells and form a tumor-like expansion on the root referred to as a nodule [1,2]
A mixture of nitrogen compounds was found to be released from ex planta B. diazoefficiens bacteroids as the experimental conditions changed (Figures 6 and 10)
This demonstrates that ex planta bacteroids have the ability to adapt their amino acid metabolism and transport to the experimental conditions to maintain a continuous supply of amino acids in the assays
Summary
Symbiotic nitrogen fixation is the process whereby rhizobia bacteria enter compatible host plant root cells and form a tumor-like expansion on the root referred to as a nodule [1,2]. The symbiotic bacteria invade plant host cells encased in a plant-derived membrane called the symbiosome membrane, which separates the bacteria from the plant host cell cytoplasm. The bacteria differentiate into nitrogen-fixing forms called bacteroids. The transfer of nitrogen from the bacteroid to the plant is the most fundamental aspect of symbiotic nitrogen fixation, yet much remains to the learned about this process. Ammonium was shown to be the immediate product of nitrogenase, the enzyme complex that catalyzes the reduction of atmospheric dinitrogen: N2 + 8 H+ + 8 e− + 16 MgATP → 2 NH3 + H2 + 16 MgADP + 16 Pi [3]. In the absence of an ammonium assimilation enzyme, it was proposed that gaseous ammonia (NH3) diffused across the bacteroid membrane, into the symbiosome space surrounding the bacteroid, was protonated to ammonium (NH4+), and moved out of the symbiosome by an ammonium transporter in the symbiosome membrane [11,12]
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