ANALYSIS OF RESPIRATORY-CHAIN REGULATION IN ROOTS OF SOYBEAN SEEDLINGS

Changes in the respiratory rate and the contribution of the cytochrome (Cyt) c oxidase and alternative oxidase (COX and AOX, respectively) w ere investigated in soybean (Glycine max I.. cv Stevens) root seedling s using the O-18-discrimination method, In 4-d-old roots respiration p roceeded almost entirely via COX, but by d 17 more than 50% of the flu x occurred via AOX. During this period the capacity of COX, the theore tical yield of ATP synthesis, and the root relative growth rate all de creased substantially. In extracts from whole roots of different ages, the ubiquinone pool was maintained at 50% to 60% reduction, whereas p yruvate content fluctuated without a consistent trend. In whole-root i mmunoblots, AOX protein was largely in the reduced, active form at 7 a nd 17 d but was partially oxidized at 4 d. In isolated mitochondria, C yt pathway and succinate dehydrogenase capacities and COX I protein ab undance decreased with root age, whereas both AOX capacity and protein abundance remained unchanged. The amount of mitochondrial protein on a dry-mass basis did not vary significantly with root age. It is concl uded that decreases in whole-root respiration during growth of soybean seedlings can be largely explained by decreases in maximal rates of e lectron transport via COX. Flux via AOX is increased so that the ubiqu inone pool is maintained in a moderately reduced state.