An hypothesis for the role of malic enzyme in symbiotic nitrogen fixation in soybean nodules

Abstract

Recent measurements of respiration and nitrogen fixation by isolated soybean bacteroids, incubated with oxyleghaemoglobin in a flow chamber under steady-state conditions, have shown that while addition of malate stimulated respiration, the effect on N2 fixation depended on the concentration of malate and oxygen. At low malate concentrations, N2 fixation was stimulated, but at higher malate concentrations (more than 0.5 mM at less than 60 nM O2) N2 fixation was inhibited and carbon diverted to poly-β-hydroxybutyrate formation. These results are interpreted in terms of the redox poise of pyridine nucleotides and the relative rates of poly-β-hydroxybutyrate synthesis and tricarboxylic acid cycle operation. Soybean bacteroids contain both NAD- and NADP-linked malic enzymes which have very different affinities for malate, and thereby have the capacity to alter the NAD(P)H/NAD(P) ratios in the bacteroid in response to varying malate concentrations. It is suggested that in vivo the rate of delivery of malate to the bacteroid must be carefully regulated to optimise N2 fixation.