Plasma membrane-bound H+-ATPase and reductase activities in Fe-deficient cucumber roots

Physiological and biochemical modifications induced by Fe-deficiency have been studied in cucumber (Cucumis sativus L. cv. Marketer) roots, a Strategy I plant that initiates a rapid acidification of the medium and an increase in the electric potential difference when grown under Fe-deficiency. Using the aqueous two-phase partitioning method, a membrane fraction which has the plasmalemma characteristics was purified from roots of plants grown in the absence and in the presence of iron. The plasma membrane vesicles prepared from Fe-deficient plants showed an H+-ATPase activity (EC 3.6.1.35) that is twice that of the non-deficient control. Furthermore, membranes from Fe-deficient plants showed a higher capacity to reduce Fe3+-chelates. The difference observed in the reductase activity was small with ferricyanide (only 30%) but was much greater with Fe3-EDTA and Fe3-citrate (210 and 250%, respectively). NADH was the preferred electron donor for the reduction of Fe3+ compounds. Fe3+ reduction in plasma membrane from cucumber roots seems to occur with utilisation of superoxide anion, since addition of superoxide dismutase (SOD; EC 1.15.1.1) “in vitro” decreased Fe3+ reduction by 60%. The response and the difference induced by iron starvation on these two plasma membrane activities together with a possible involvement of O2 in controlling the Fe3+/Fe2+ ratio in the rhizosphere are discussed.