MICROAEROBIC RESPIRATION AND OXIDATIVE-PHOSPHORYLATION BY SOYBEAN NODULE MITOCHONDRIA - IMPLICATIONS FOR NITROGEN-FIXATION

The infected cells of soybean (Glycine max) root nodules require ATP p roduction for ammonia assimilation and purine synthesis under microaer obic conditions. It is likely that the bulk of this demand is supplied through mitochondrial oxidative phosphorylation, Mitochondria purifie d from root nodules respired and synthesized ATP in sub-micromolar oxy gen concentrations as measured by leghaemoglobin spectroscopy and luci ferase luminescence, Both oxygen uptake and the apparent ATP/O ratio d eclined significantly as the oxygen concentration fell below 100 mu mo l m(-3). Cytochrome-pathway respiration by root nodule mitochondria ha d a higher apparent affinity for oxygen (K-m 50 mu mol m(-3)) than did mitochondria isolated from roots (K-m 125 mu mol m(-3)), Electron mic rographs showed that mitochondria predominated at the periphery of inf ected cells adjacent to gas-filled intercellular spaces, where the oxy gen concentration is predicted to be highest. Calculations of oxygen c oncentration and nitrogen fixation rates on an infected cell basis sug gest that the measured rates of ATP production by isolated mitochondri a are sufficient for the quantifiable in vivo requirements of ammonia assimilation and purine synthesis. The possible roles of mitochondrial respiration in the control of infected cell metabolism are also discu ssed.