Glutamate as a Carbon Source for N2-Fixing Bacteroids Prepared from Soybean Root Nodules

Abstract

The possibility that glutamate might serve as a source of reducing power, supporting fixation of N2 to NH3 by bacteroids in soybean root nodules, was investigated. Suspensions of bacteroids were retained within a reaction chamber supplied with a flow of solution containing dissolved air, buffer, oxyleghaemoglobin and either disodium glutamate (1 or 10mM) or no added substrate (2 or 20 mM-NaC1). The solution passing out of the chamber was analysed for dissolved NH3 and proportional oxygenation of leghaemoglobin was determined spectrophotometrically. Rates of production of NH3 and of consumption of O2 and the concentrations of free, dissolved O2 prevailing during steady states were calculated. At 1 mM, glutamate stimulated O2 demand of the bacteroids and enhanced NH, production. However, increased production of NH3 was shown to arise entirely from deamination of [lSN]glutamate and not from N2 fixation. At 10 mM-glutamate, the proportion of NH3 arising from [I SN]glutamate increased with increased steady state concentrations of free, dissolved O2 in the chamber, but there was also significant and efficient stimulation of N2 fixation when the concentration of free, dissolved O2 was between 10 and 70nM. The kinetics of O2 consumption by bacteroids using endogenous substrates or 10 mmglutamate were similar.

Metabolism of glutamate and aspartate in bacteroids isolated from soybean root nodules

“…However, when the PBUs were supplied with [ 14C]glutamate at a concentration of 2mM, the respiratory utilization of glutamate by the enclosed bacteroids was about 45% of that by free bacteroids, whereas malate utilization was also inhibited to about 60% by the presence of the PBM. In soybean nodules estimates of the in vivo concentration of glutamate in the plant cell cytosol range from 1 to 10 mM (Kouchi & Yoneyama, 1986;Streeter, 1987;Bergersen & Turner, 1988). Therefore, the PBM may not work as a potential barrier to the supply of glutamate to the bacteroids from plant cytosol within the range of concentrations expected in vivo in the cytosol in nodules.…”

“…We have also studied the metabolism of glutamate and aspartate by bacteroids of soybean nodules because : (1) bacteroids possess an active uptake system for glutamate (Salminen & Streeter, 1987;Udvardi et af., 1988); ( 2 ) exogenous glutamate enhances O2 consumption and N fixation by isolated bacteroids (Bergersen & Turner, 1988); (3) the site of natural I5N enrichment in soybean nodules is predominantly in the bacteroids (Reinero et al, 1983;Yoneyama et a/., 1987), suggesting that H . Kouchi, K .…”

Factors contributing to the accumulation of glutamate in Bradyrhizobium japonicum bacteroids under microaerobic conditions

“…japonicum bacteroids can absorb and respire glutamate (Bergersen & Turner, 1988;Salminen & Streeter, 1987), and some workers have even suggested that glutamate may be an important carbon source for bacteroids (Kahn et af., 1985). However, glutamate transport through the peribacteroid membrane may be restricted (Udvardi et al, 1988), so it would appear that glutamate accumulation in bacteroids may be unrelated to glutamate imported from the host cytoplasm.…”

Labeling of Carbon Pools in Bradyrhizobium japonicum and Rhizobium leguminosarum bv viciae Bacteroids following Incubation of Intact Nodules with 14CO2

“…Evidence from '5N2 feeding to intact nodules also indicates glutamate labeling in bacteroids (18). Glutamate has also been implicated as a possible carbon source for B. japonicum bacteroids (2), although most of the available data strongly point to dicarboxylic acids as the principal carbon source for bacteroids (3,17,31 Conclusions from studies of carbon uptake and metabolism by isolated bacteroids need to be confirmed with bacteroids in their natural environment, i.e. in the intact nodules.…”