EFFECT OF IRON-DEFICIENCY ON ELECTRON-TRANSPORT PROCESSES OF PLASMALEMMA-ENRICHED VESICLES ISOLATED FROM COTTON ROOTS

To investigate the accelerated transplasmalemma redox activity induced by iron starvation, cotton seedlings (Gossypium hirsutum L., cv Acala San Jose 2) are grown in nutrient solutions with or without Fe3+-ethylenediamine tetraacetic acid (FeEDTA). Vesicles enriched in plasmalemma are then isolated from the roots using a sucrose step gradient or by two-phase partitioning. Iron starvation stimulates by approximately two-fold the transport of electrons from NADH to FeEDTA, ferricyanide or cytochrome c when both donor and acceptor are added at the same face of the membrane. Under these conditions, NADH:duroquinone oxidoreductase activity is unaffected. It is concluded that not all of the redox systems at the plasmalemma are stimulated by iron starvation. Regardless of iron nutrition, Triton X-100 stimulates NADH oxidation of the vesicles when ferricyanide and duroquinone are the electron acceptors, but the detergent inhibits oxidation in the presence of cytochrome c. Transplasmalemma redox activity, which resembles more closely that occurring in vivo, is also detected in the vesicle system by measuring the pH gradient or the membrane potential generated as electrons are transported from ascorbate trapped within the vesicle to exogenous ferricyanide. The pH gradient observed while the membrane is not polarized, is steeper in vesicles from iron-starved roots; however, when the membrane potential is measured, Fe starvation results in its becoming less positive. These results are interpreted as being due to a stimulation by iron starvation of both transplasmalemma electron transport and of proton efflux. Based on the above observations and conclusions, we also suggest that plasmalemma-enriched vesicles provide a satisfactory model system to study aspects of the iron deficiency response.