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.