Adenylate control of respiration in plants: the contribution of rotenone‐insensitive electron transport to ADP‐limited oxygen consumption by soybean mitochondria

Abstract

The aim was to test the hypothesis that rotenone‐insensive electron transport (bypass of complex I) may underlie rapid state 4 (ADP‐limited) mitochondrial respiration. A comparison of mitochondria from soybean (Glycine max L. cv. Bragg) cotyledons and nodules showed that ADP‐sufficient (state 3) malate plus pyruvate oxidation by mitochondria from 7‐day‐old cotyledons was inhibited 50% by rotenone and state 4 rates were rapid, whereas nodule mitochondria were 80% inhibited by rotenone and had slower state 4 rates of malate plus pyruvate oxidation. Respiration of malate alone (pH 7.6) by cotyledon mitochondria was slow, especially in the absence of ADP; subsequent addition of pyruvate dramatically increased state 4 oxygen uptake concomitant with a rapid rise in mitochondrial NADH (determined by fluorimetry). Rotenone had no effect on this increased rate of state 4 respiration. The rate of malate oxidation by nodule mitochondria was relatively rapid compared with cotyledon mitochondria. The addition of pyruvate in state 4 caused a slow increase in matrix NADH and only a slight stimulation of oxygen uptake. Rotenone inhibited state 4 malate plus pyruvate oxidation by 50% in these mitochondria. From a large number of cotyledon and nodule mitochondrial preparations, a close correlation was found between the rate of state 4 oxygen uptake and rotenone‐resistance. During cotyledon development increased rotenone‐resistance was associated with an increase in the alternative oxidase. Addition of pyruvate to cotyledon mitochondria, during state 4 oxidation of malate in the presence of antimycin A, significantly stimulated O2 uptake and also almost eliminated respiratory control. Such combined operation of the rotenone‐insensitive bypass and the alternative oxidase in vivo will significantly affect the extent to which adenylates control the rate of electron transport.