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

Abstract

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.

Iron limitation results in induction of ferricyanide reductase and ferric chelate reductase activities in Chlamydomonas reinhardtii

“…Contrary to some reports from higher plants (Cakmak et al 1987;BruÈ ggemann et al 1990), addition of exogenous superoxide dismutase did not decrease the rate of Fe(III) reduction, nor did the addition of exogenous NADH stimulate iron reduction (Table 1). It has been suggested that NADH-dependent peroxidase activity is able to catalyze reduction of extracellular Fe(III) to Fe(II), in a process that also results in a stimulation of O 2 consumption and is sensitive to inhibition by superoxide dismutase (MacrõÁ et al 1992).…”

Ferric reduction bygeum urbanum: A kinetic study

“…In contrast to the reductive capacity for iron chelates which is expressed in iron-stressed roots of dicots and most monocots (Turbo-System), the reduction of ferricyanide is a general feature of all plants and tissues (Standard-System). However, recent investigations on purified plasma membrane vesicles showed that no biochemically distinct enzyme is expressed when plants were starved of iron (7,8,17). The substrate dependence of the reduction of ferricyanide by iron-deficient roots is shown in Fig.…”

“…This hypothesis was supported by findings of Bienfait et al (5) and Romheld and Marschner (6), based on results obtained with intact plants. Initial attempts to demonstrate a direct involvement of PM-bound reductase(s) in the reduction of extracellular Felll using both intact tissue and isolated PM-vesicles have been carried out recently (7,8,9).…”

Iron reductase systems on the plant plasma membrane—A review

“…Investigations on ferric reduction of purified PM from iron-deficient and ironsufficient plants were compared with in vivo data and brought the original model into question. In their study, Brtiggemann et al (1990) could not confirm the synthesis of a biochemically distinct enzyme formed under iron stress, but proposed that the in vivo response to iron starvation relies on the enhanced synthesis or activation of a constitutively existing redox system. However, till the time of this work (July 1993), no iron chelate reductase system has been isolated from PM and purified to homogeneity so far.…”