1990. Plasma membrane-bound NADH: Fe'+-EDTA reductase and iron deficiency in tomato {Lycopersicon esculentum). Is there a Turbo reductase? -Physiol. Plant. 79: 339^346.The properties of NADH-dependent Fe^*-EDTA reductase in plasma membranes (PM) from roots of iron-deficient and -sufficient tomato plants \Lycopersicon esculentum L. (Mill.) cv. Abunda] were examined. Iron deficiency resulted in a 3-fold increase of in vivo root iroB-chelate reductase activity with a K^ (Fe'+-EDTA) of 230 M.ln purified root PM, average specific activities of ferric chelate reductase of 410 and 254 nmol Fe (mg protein)"' min'' were obtained for iron-deficient and -sufficient plants, respectively. In both cases, the PM-bound activity showed a pH optimum at pH 6.8. Activity depended on NADH and not on NADPH and on the presence of detergent. The activity was inhibited 40-50% by superoxide dismutase (EC 1.15.1.1) and ca 30% by oxygen. Kinetic analysis of the membrane-boand enzyme revealed a K^ (Fe'^-EDIA) of ca 200 \iM for both iron-stressed and -sufficient plants. For NADH, Kjn values around 230 \iM were obtained. The ferric chelate reductase could be solubilised from salt-washed PM with Triton X-100 at a protein:detergent ratio of 1:2.8 (w/w). The Triton-soluble fraction revealed one enzyme-stained band in native polyacrylamide electrophoresis. Although the membranes showed no nitrate reductase (NR; EC 1,6.6.1) activity, anti-spinach NR immunoglobulin G (IgG) recognized a 54 kDa band both in the PM and the Triton-soluble fraction, but not in the enzymatically active material obtained from the native gel. No evidence could be found for the synthesis of a new, biochemically distinct PM-bound ferric chelate reductase under iron deficiency, which might be identified as the so-called Turbo reductase. It is concluded that iron deficiency in tomato induces increased expression of a ferric chelate reductase in root PM, which is already present in iron-sufficient plants and probably also in plants,, which do not contain the Turbo reductase, like the grasses. The iron reductase is not identical with the recently reported PM-associated nitrate reductase.