Plasma membrane‐bound NADH: Fe3+‐EDTA reductase and iron deficiency in tomato (Lycopersicon esculentum). Is there a Turbo reductase?

The properties of NADH‐dependent Fe3+‐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 iron‐chelate reductase activity with a Km (Fe3+‐EDTA) of 230 μM. In purified root PM, average specific activities of ferric chelate reductase of 410 and 254 nmol Fe (mg protein)−1 min−1 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‐bound enzyme revealed a Km (Fe3+‐EDTA) of ca 200 μM for both iron‐stressed and ‐sufficient plants. For NADH, Km values around 230 μM 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.