Ferricyanide-Mediated Transport of Chloride by Anaerobic Corn Roots

Active transport of salts by higher plant tissues has been shown to depend on aerobic respiration (13). Although active transport under anaerobic conditions has been reported for a number of animal tissues (19, 32) and for yeast (5, 6), transport in higher plants has been taken to be ?2 dependent. During aerobic respiration, electrons are transferred from respiratory intermediates to 02 via organized assemblies of carriers, the most important of which includes the cytochrome system. The mechanism linking electron transfer to active salt transport is the subject of various hypotheses. Mitchell (29) has suggested that metabolic energy participates in salt uptake by the formation and opening of covalent bonds between certain membrane constituents and the carried molecules. The hypotheses of Goldacre (9) and Bennet-Clark (2) propose the participation of high-energy phosphate compounds in ion transport. In these cases the connection with electron transfer is indirect. On the other hand Lundegardh (24, 25) has contended that ion transport, specifically anion transport, is effected directly by the cytochrome chain, the members of which in the oxidized state are positively charged and are held to bind anions electrostatically. In this view, electrons move from substrate to ?2 while avowedly causing anions to move in the opposite direction. Anion uptake by this mechanism is a property only of the complete cytochrome chain and begins with the oxidase (24); accumulation is explained by interposing a diffusion barrier somewhere along the chain. The foregoing scheme does not specifically explain how ions enter the vacuole, which Lundegardh (24) and many others regard as the final 'destination of the bulk of transported ions in plant cells. The evidence for the mitochondrial location of the cytochromes appears overwhelming, hence the electron ladder hypothesis at best refers to ion transport into the mitochondria. Moreover, even in this context the value of Lundegardh's hypothesis is unclear, since the spatial arrangement of the cytochromes with respect to the mitochondrial surface membranes remains undefined. The considerable evidence linking salt transport to oxidative phosphorylation, rather than to electron transport per se, has suggested an experimental approach to this question. Ferricyanide is representative of a class of compounds, including also the redox dyes, which accept electrons from intermediate carriers in the respiratory chain. Ferricyanide has been shown to serve this function in intact tissues (14, 16). Moreover, in animal mitochondria, electron transfer to ferricyanide through an abbreviated electron transfer chain is accompanied by oxidative phosphorylation (8). Electron transfer mediated by ferricyanide would thus be expected to support salt absorption if this process is dependent on oxidative phosphorylation but not if transport depends upon electron flow through the complete cytochrome system. A study of salt uptake under anaerobic conditions in which ferricyanide acts as sole external electron acceptor is described below.