Rhizosphere Acidification by Iron Deficient Bean Plants: The Role of Trace Amounts of Divalent Metal Ions: A Study on Roots of Intact Plants with the Use of C- and P-NMR.

Rhizosphere acidification by Fe-deficient bean (Phaseolus vulgaris L.) plants was induced by trace amounts of divalent metal ions (Zn, Mn). The induction of this Fe-efficiency reaction was studied by (14)CO(2) and (11)CO(2) fixation experiments, and with (31)P-NMR on roots of whole plants. The starting and ending of an acidification cycle was closely coupled to parallel changes in CO(2) fixation, within the maximal resolution capacity of 20 min. (31)P-NMR experiments on intact root systems showed one peak which was ascribed to vacuolar free phosphate. At the onset of proton extrusion this peak shifted, indicating increase of pH in the cells. Proton extrusion was inhibited, with a lag period of 2 hours, by the protein synthesis inhibitors cycloheximide and hygromycin. It is assumed that Zn and Mn induce proton extrusion in Fe-deficient bean roots by activating the synthesis of a short-living polypeptide; the NMR data suggest a role for this peptide in the functioning of a proton pumping ATPase in the plasma membrane.

Proteins under the Control of the Gene for Fe Efficiency in Tomato.

Fe-deficient dicotyledons develop Fe-efficiency reactions, such as proton extrusion and ferric chelate reduction activity, which are located in the plasma membranes of the root epidermal cells. The fer mutant of tomato (Lycopersicon esculentum Mill.) cannot develop these reactions. Membranes were isolated from roots of wild-type (FER) and mutant (fer) tomato plants grown on nutrient solution with high and low Fe concentrations. Two proteins were identified which are synthesized under the control of the FER gene.

Characterization of Phloem iron and its possible role in the regulation of fe-efficiency reactions.

;Fe-efficiency reactions' are induced in the roots of dicotyledonous plants as a response to Fe deficiency. The role of phloem Fe in the regulation of these reactions was investigated. Iron travels in the phloem of Ricinus communis L. as a complex with an estimated molecular weight of 2400, as determined by gel exclusion chromatography. The complex is predominantly in the ferric form, but because of the presence of reducing compounds in the phloem sap, there must be a fast turnover in situ between ferric and ferrous (k approximately 1 min(-1)). Iron concentrations in R. communis phloem were determined colorimetrically or after addition of (59)Fe to the nutrient solution. The iron content of the phloem in Fe-deficient plants was lower (7 micromolar) than in Fe-sufficient plants (20 micromolar). Administration of Fe-EDTA to leaves of Phaseolus vulgaris L. increased the iron content of the roots within 2 days, and decreased proton extrusion and ferric chelate reduction. The increase in iron content of the roots was about the same as the difference between iron contents of roots grown on two iron levels with a concomitantly different expression of Fe-efficiency reactions. We conclude that the iron content of the leaves is reflected by the iron content of the phloem sap, and that the capacity of the phloem to carry iron to the roots is sufficient to influence the development of Fe-efficiency reactions. This does not preclude other ways for the shoot to influence these reactions.

Interrelationships between trans-Plasma Membrane Electron/Proton Transfer Stoichiometry, Organic Acid Metabolism, and Nitrate Reduction in Dwarf Bean (Phaseolus vulgaris).

Iron deficiency in dwarf bean (Phaseolus vulgaris L.) induces an increased activity of a system in the rhizodermal cells, which reduces extracellular ferric salts, and an active proton efflux from the roots, which is coupled to accumulation of citrate and malate in the roots and subsequent export of these compounds in the xylem. During reduction of extracellular ferricyanide by Fe-deficient plants, the stoichiometry of electron transport to proton efflux is 2e(-)/1H(+), and citrate and malate levels in the roots are strongly decreased. Reduction of ferricyanide by Fe-sufficient plants has no influence on root and shoot levels of citrate and malate, but in such plants the process is characterized by a e(-)/H(+) efflux stoichiometry close to unity. Apparently, organic acid metabolism and transport are closely associated with the e(-)/H(+) efflux ratio. To assess the significance of organic acid metabolism as one of the direct intracellular components of the induced unbalanced e(-)/H(+) efflux by roots, we studied NO(3) (-) reduction in shoots and roots of Fe-deficient and Fe-sufficient plants. Nitrate reductase activity in the roots was positively correlated with the level of citrate and malate, whereas the enzyme activity in the leaves responded positively to the import of these organic acid anions.

Involvement of superoxide radical in extracellular ferric reduction by iron-deficient bean roots.

The recent proposal of Tipton and Thowsen (Plant Physiol 79: 432-435) that iron-deficient plants reduce ferric chelates in cell walls by a system dependent on the leakage of malate from root cells was tested. Results are presented showing that this mechanism could not be responsible for the high rates of ferric reduction shown by roots of iron-deficient bean (Phaseolus vulgaris L. var Prélude) plants. The role of O(2) in the reduction of ferric chelates by roots of iron-deficient bean plants was also tested. The rate of Fe(III) reduction was the same in the presence and in the absence of O(2). However, in the presence of O(2) the reaction was partially inhibited by superoxide dismutase (SOD), which indicates a role for the superoxide radical, O(2) ([unk]), as a facultative intermediate electron carrier. The inhibition by SOD increased with substrate pH and with decrease in concentration of the ferrous scavenger bathophenanthroline-disulfonate. The results are consistent with a mechanism for transmembrane electron transport in which a flavin or quinone is the final electron carrier in the plasma membrane. The results are discussed in relation to the ecological importance that O(2) ([unk]) may have in the acquisition of ferric iron by dicotyledonous plants.

Control of the development of iron-efficiency reactions in potato as a response to iron deficiency is located in the roots.

Roots of potato plants (Solanum tuberosum cv Bintje) growing on low Fe nutrient solution developed the characteristic Fe efficiency reactions, such as high ferric reductase activity, proton extrusion and increased root hair formation. Roots from a tuber with sprout removed, when grown on Fe-free nutrient solution, also expressed these reactions; transfer to iron-containing medium resulted in their complete disappearance within 10 days. Roots growing on 2% sucrose in sterile Murashige-Skoog medium increased their ferric reductase activity upon withholding Fe and formed transfer cells. It is concluded that potato roots themselves control the development of Fe-efficiency reactions, and that the shoot may exert a modulating influence on their expression.

Rhizosphere acidification as a response to iron deficiency in bean plants.

Iron deficiency in higher plants causes accumulation of salts of organic acids in the roots, the most characteristic being citrate. We show that citrate and malate accumulate in beans (Phaseolus vulgaris L. var Prélude), not because of a lack of the iron-containing enzyme aconitase (EC 4.2.1.3), but in close coupling to the extrusion of protons during rhizosphere acidification, one of the ;Fe-efficiency' reactions of dicotyledonous plants. When proton excretion is induced in roots of control bean plants by addition of fusicoccin, only malate, not citrate, is accumulated. We propose that iron deficiency induces production of organic acids in the roots, which in beans leads to both proton excretion and an increased capacity to reduce ferric chelates via the induced electron transfer system in the root epidermis cells.

Free space iron pools in roots: generation and mobilization.

A rapid and simple method for the determination of a ferric iron pool in the free space of roots is described. Formation of this pool depended on the source of iron in the nutrient solution. During growth in water culture at pH 5 to 6 with Fe-ethylenediaminetetraacetate, a free space pool of 500 to 1000 nanomoles Fe per gram fresh weight was formed in the roots of bean (Phaseolus vulgaris L. var. Prélude), maize (Zea mays L. var. Capella), and chlorophytum (Chlorophytum comosum [Thunb.] Jacques). No significant pool (less than 100 nanomoles per gram fresh weight) was formed with ferrioxamine. Upon impending Fe deficiency, bean and chlorophytum were able to mobilize this pool. Fe-deficient bean plants mobilized iron from the free space iron pool of another plant in the same vessel.

Iron Deficiency Decreases Suberization in Bean Roots through a Decrease in Suberin-Specific Peroxidase Activity.

The suberin content of young root parts of iron-deficient and iron-sufficient Phaseolus vulgaris L. cv Prélude was determined. The aliphatic components that could be released from suberin-enriched fractions by LiAID(4) depolymerization were identified by gas chromatography-mass spectrometry. In the normal roots, the major aliphatic components were omega-hydroxy acids and dicarboxylic acids in which saturated C(16) and monounsaturated C(18) were the dominant homologues. Iron-deficient bean roots contained only 11% of the aliphatic components of suberin found in control roots although the relative composition of the constituents was not significantly affected by iron deficiency. Analysis of the aromatic components of the suberin polymer that could be released by alkaline nitrobenzene oxidation of bean root samples showed a 95% decrease in p-hydroxybenzaldehyde, vanillin, and syringaldehyde under iron-deficient conditions. The inhibition of suberin synthesis in bean roots was not due to a decrease in Fe-dependent omega-hydroxylase activity since normal omega-hydroxylation could be demonstrated, both in vitro with microsomal preparations and in situ by labeling of omega-hydroxy and dicarboxylic acids with [(14)C]acetate. The level of the isozyme of peroxidase that is specifically associated with suberization was suppressed by iron deficiency to 25% of that found in control roots. None of the other extracted isozymes of peroxidase was affected by the iron nutritional status. The activity of the suberin-associated peroxidase was restored within 3 to 4 days after application of iron to the growth medium. The results suggest that, in bean roots, iron deficiency causes inhibition of suberization by causing a decrease in the level of isoperoxidase activity which is required for polymerization of the aromatic domains of suberin, while the ability to synthesize the aliphatic components of the suberin polymer is not impaired.

Regulated redox processes at the plasmalemma of plant root cells and their function in iron uptake.

Plants take up iron as ferric chelates or, after reduction, as ferrous ions. Ferric reduction takes place at the plasma membrane of the root epidermis cells by a transmembrane redox system, which can be activated when iron is getting short. It is proposed that this inducible system, with NADPH as electron donor, is separate from a system, presumably present in all plant cells, which transports electrons from NADH or NADPH to ferricyanide, or, in vivo, oxygen.

Depolarization of Cell Membrane Potential during Trans-Plasma Membrane Electron Transfer to Extracellular Electron Acceptors in Iron-Deficient Roots of Phaseolus vulgaris L.

Transfer of electrons from the cytosol of bean (Phaseolus vulgaris L.) root cells to extracellular acceptors such as ferricyanide and Fe(III)EDTA causes a rapid depolarization of the membrane potential. This effect is most pronounced (30-40 millivolts) with root cells of Fe-deficient plants, which have an increased capacity to reduce extracellular ferric salts. Ferrocyanide has no effect. In the state of ferricyanide reduction, H(+) (1H(+)/2 electrons) and K(+) ions are excreted. The reduction of extracellular ferric salts by roots of Fe-deficient bean plants is driven by cellular NADPH (Sijmons, van den Briel, Bienfait 1984 Plant Physiol 75: 219-221). From this and from the membrane potential depolarization, we conclude that trans-plasma membrane electron transfer from NADPH is the primary process in the reduction of extracellular ferric salts.

Cytosolic NADPH is the electron donor for extracellular fe reduction in iron-deficient bean roots.

Pyridine nucleotides were determined in lateral roots of iron-deficient and iron-sufficient Phaseolus vulgaris L. cv Prelude. In iron-deficient plants, total NADP per gram fresh weight and the NADPH/NADP(+) ratio were twice the values found in iron-sufficient plants. The NADPH/NADP(+) ratio in iron-deficient plants was considerably lowered after a 2 minute incubation in 1 millimolar ferricyanide. Total NAD was not influenced by growth conditions and was mainly present in oxidized form.These results indicate that NADPH is the electron donor for the high Fe(III) reduction activity found in iron-deficient roots, a process that is part of the Fe-uptake mechanism.

Ferritin in bean leaves with constant and changing iron status.

Normal green leaves contain low levels of ferritin which stores 5 to 10% of the total iron (approximately 350 iron atoms/molecule). Chlorotic leaves do not have measurable amounts of ferritin, whereas iron-loaded leaves contain high levels of well-filled ferritin (1,500 to 2,500 iron atoms/molecule). The role of ferritin during a transient iron surplus in leaves was investigated. It is suggested that a short-term overdose of iron transported into the leaf is largely stored in or near the vessels in such a form that it can be quickly mobilized for export. Iron that reaches the mesophyll cells in an overdose situation is stored in ferritin and, when released, is most likely used for the leaf cells themselves and not for export.

The role of ferritin in developing primary bean leaves under various light conditions.

Ferritin and ferritin-iron in the primary leaves of Phaseolus vulgaris L. were determined during growth in the dark, in the light, and during de-etiolation. The ratio ferritin protein/total protein appeared to be rather constant. In dark-grown leaves maximally 50% of the total extractable iron was found to be present in ferritin. This percentage was lower in deetiolating and light-grown leaves. In ten-day-old green leaves no ferritin-iron could be measured. The translocation of iron from cotyledons to the developing plant appears to be related to the need for iron in the leaves. These results suggest that ferritin acts as a buffer molecule for iron in plants.

Rapid mobilization of ferritin iron by ascorbate in the presence of oxygen.

L-(-)-ascorbate mobilizes iron from horse-spleen ferritin in the presence of oxygen at pH 8.0. The reaction is strongly stimulated by Cu2+. Dehydroascorbate and other stable oxidation products of ascorbate are ineffective. We present evidence that monodehydroascorbate mobilizes ferritin iron by reduction.

Mitochondrial oxygen affinity as a function of redox and phosphate potentials.

1. The conditions under which mitochondria might catalyse a net reversal of oxidative phosphorylation are analysed. 2. Rat-liver mitochondria, incubated under such conditions, show a strongly diminished affinity for oxygen. 3. The velocity of respiration under these conditions is a hyperbolic function of the oxygen concentration. 4. The K-m for oxygen is less than 0.1 muM at low phosphate potential, irrespective of substrate, and 1-3 muM under reversal conditions. 5. The observed kinetics can be accounted for in a simple mechanism for cytochrome oxidase action.