Mitochondria of Mycorrhizal Roots of Fagus Sylvatica

SUMMARY Mitochondria were isolated from beech mycorrhizas, by disruption of the tissue in a medium containing PVP and reducing agents, to counteract the enzyme denaturing actions of endogenous phenols, tannins and the products of phenoloxidase activity. Electron microscopic examination of the isolated mitochondria showed that they were structurally intact, and spectral analysis revealed that their cytochrome composition was marked by a particularly high content of cytochrome b 557. The mitochondria actively respired with the Krebs cycle intermediates α‐ketoglutarate, malate + pyruvate and succinate, and with exogenous NADH. In each case the oxidation was tightly coupled to phosphorylation, and ADP:O ratios were obtained which indicated that three sites of oxidative phosphorylation were associated with the respiration of the NAD‐linked substrates α‐ketoglutarate and malate + pyruvate, and two sites with the respiration of succinate and exogenous NADH. The inhibitors rotenone, antimycin A and cyanide blocked electron flow at the known sites of action in the electron transport chain. Rotenone completely inhibited the oxidation of both α‐ketoglutarate and malate + pyruvate, but had no effect on the oxidation of succinate or NADH. Oxidation of the four substrates was found to be partially sensitive to antimycin and to cyanide which caused a maximal inhibition of 50–60% in the rate of respiration. The antimycin‐ and cyanide‐insensitive respiration was abolished by mCLAM. It was concluded that there was at least one fraction of the mitochondrial population which had a cyanide‐insensitive pathway of respiration, and that the respiratory system of the mitochondria in this fraction could best be explained by a model in which the respiratory chain is branched from substrate to oxygen, one branch via cyanide‐sensitive cytochrome oxidase and the other via a cyanide‐insensitive alternate oxidase. No oxidative phosphorylation was found to be associated with the alternate pathway of oxidation with succinate as substrate. Evidence is presented to show that the cyanide‐sensitive pathway was the favoured route of oxidation under phosphorylating conditions, whereas the alternate pathway only operated when electron flow along the cyanide sensitive pathway was restricted either by inhibitors or by the imposition of respiratory control. Some implications of these findings in relation to the physiology of the mycorrhizal system are discussed.