Pyridine nucleotide oxidation by a plasma membrane fraction from red beet (Beta vulgaris L.) storage tissue

Enhanced proton translocating pyrophosphatase activity improves nitrogen use efficiency in Romaine lettuce

Plant nitrate (NO3(-)) acquisition depends on the combined activities of root high- and low-affinity NO3(-) transporters and the proton gradient generated by the plasma membrane H(+)-ATPase. These processes are coordinated with photosynthesis and the carbon status of the plant. Here, we present the characterization of romaine lettuce (Lactuca sativa 'Conquistador') plants engineered to overexpress an intragenic gain-of-function allele of the type I proton translocating pyrophosphatase (H(+)-PPase) of Arabidopsis (Arabidopsis thaliana). The proton-pumping and inorganic pyrophosphate hydrolytic activities of these plants are augmented compared with control plants. Immunohistochemical data show a conspicuous increase in H(+)-PPase protein abundance at the vasculature of the transgenic plants. Transgenic plants displayed an enhanced rhizosphere acidification capacity consistent with the augmented plasma membrane H(+)-ATPase proton transport values, and ATP hydrolytic capacities evaluated in vitro. These transgenic lines outperform control plants when challenged with NO3(-) limitations in laboratory, greenhouse, and field scenarios. Furthermore, we report the characterization of a lettuce LsNRT2.1 gene that is constitutive up-regulated in the transgenic plants. Of note, the expression of the LsNRT2.1 gene in control plants is regulated by NO3(-) and sugars. Enhanced accumulation of (15)N-labeled fertilizer by transgenic lettuce compared with control plants was observed in greenhouse experiments. A negative correlation between the level of root soluble sugars and biomass is consistent with the strong root growth that characterizes these transgenic plants.

Arabidopsis sodium dependent and independent phenotypes triggered by H⁺-PPase up-regulation are SOS1 dependent

Coordinate regulation of transporters at both the plasma membrane and vacuole contribute to plant cell's ability to adapt to a changing environment and play a key role in the maintenance of the chemiosmotic circuits required for cellular growth. The plasma membrane (PM) Na⁺/H⁺ antiporter (SOS1) is involved in salt tolerance, presumably in sodium extrusion; the vacuolar type I H⁺-PPase AVP1 is involved in vacuolar sodium sequestration, but its overexpression has also been shown to alter the abundance and activity of the PM H⁺-ATPase. Here we investigate the relationship between these transporters utilizing loss-of-function mutants of SOS1 (sos1) and increased expression of AVP1 (AVP1OX). Heightened expression of AVP1 enhances pyrophosphate-dependent proton pump activity, salt tolerance, ion vacuolar sequestration, K⁺ uptake capacity, root hair development, osmotic responses, and PM ATPase hydrolytic and proton pumping activities. In sos1 lines overexpressing AVP1, these phenotypes are negatively affected demonstrating that sos1 is epistatic to AVP1. Enhanced AVP1 protein levels require SOS1 and this regulation appears to be post-translational.

Transmembrane electron transport in sealed and NAD(P)H-loaded right-side-out plasma membrane vesicles isolated from maize (Zea mays L.) roots

Electron transport across plasma membranes has been observed in vivo in several plant species and tissues after the application of ferricyanide (hexacyanoferrate III, HCF III). In the present work, a transmembrane electron flow was demonstrated in sealed and NAD(P)H-loaded right-side-out (apoplastic-side-out) plasma membrane vesicles isolated from maize (Zea mays L.) roots. HCF III was reduced at a rate of up to 126 nmol min(-1) mg(-1) protein by NADPH-loaded vesicles, while reduction rates with NADH-loaded vesicles were several-fold lower. Coincident with the reduction of HCF III, NAD(P)H oxidation was observed inside the vesicles. The dependence of reduction on K+ indicated an electrogenic transmembrane electron flow. Application of 100 microM calcium decreased HCF III reduction up to 66%, while pre-incubation with 200 microM warfarin or diphenylene iodonium inhibited transmembrane electron transport only weakly. Fe(3+)-EDTA was not reduced significantly by NADPH-loaded plasma membrane vesicles, whereas XTT was reduced at a rate of 765 pmol min(-1) mg(-1) protein. The results suggested a major function for NADPH in transmembrane electron flow and were discussed in conjunction with in vivo experiments.

Oxidoreductases in plant plasma membranes

Electron transporting oxidoreductases at biological membranes mediate several physiological processes. While such activities are well known and widely accepted as physiologically significant for other biological membranes, oxidoreductase activities found at the plasma membrane of plants are still being neglected. The ubiquity of the oxidoreductases in the plasma membrane suggests that the activity observed is of major importance in fact up to now no plant without redox activity at the plasmalemma is known. Involvement in proton pumping, membrane energization, ion channel regulation, iron reduction, nutrient uptake, signal transduction, and growth regulation has been proposed. However, positive proof for one of the numerous theories about the physiological function of the system is still missing. Evidence for an involvement in signalling and regulation of growth and transport activities at the plasma membrane is strong, but the high activity of the system displayed in some experiments also suggests function in defense against pathogens.

Molecular components and biochemistry of electron transport in plant plasma membranes (review)

It is worthwhile emphasizing the importance of electron transport across lipid membranes. Mitochondrial and electron transport in chloroplasts were elucidated in great detail many years ago. Plasma membrane-bound electron transfer may be involved in several processes such as membrane energization, signalling, regulation of transport and/or growth, and generation or scavenging of free radicals. We here give an overview of plasma membrane-bound electron transfer, of possible compounds of the electron transporting systems isolated from plasma membranes, and of their biochemical characteristics. Both the progress made in purification of redox enzymes and compounds and data from biochemical characterization of the activities found, support the discussion concerning models of the molecular structure of the electron transport systems of plant plasma membranes.

Electron and proton transport across the plasma membrane

Transplasm membrane electron transport in both plant and animal cells activates proton release. The nature and components of the electron transport system and the mechanism by which proton release is activated remains to be discovered. Reduced pyridine nucleotides are substrates for the plasma membrane dehydrogenases. Both plant and animal membranes have unusual cyanide-insensitive oxidases so oxygen can be the natural electron acceptor. Natural ferric chelates or ferric transferrin can also act as electron acceptors. Artificial, impermeable oxidants such as ferricyanide are used to probe the activity. Since plasma membranes contain b cytochromes, flavin, iron, and quinones, components for electron transport are present but their participation, except for quinone, has not been demonstrated. Stimulation of electron transport with impermeable oxidants and hormones activates proton release from cells. In plants the electron transport and proton release is stimulated by red or blue light. Inhibitors of electron transport, such as certain antitumor drugs, inhibit proton release. With animal cells the high ratio of protons released to electrons transferred, stimulation of proton release by sodium ions, and inhibition by amilorides indicates that electron transport activates the Na+/H+ antiport. In plants part of the proton release can be achieved by activation of the H+ ATPase. A contribution to proton transfer by protonated electron carriers in the membrane has not been eliminated. In some cells transmembrane electron transport has been shown to cause cytoplasmic pH changes or to stimulate protein kinases which may be the basis for activation of proton channels in the membrane. The redox-induced proton release causes internal and external pH changes which can be related to stimulation of animal and plant cell growth by external, impermeable oxidants or by oxygen.

Transplasma membrane electron transport in plants

The presence of transplasma membrane electron transport in a variety of plant cells and tissues is reported. It is now agreed that this property of eukaryotic cells is of ubiquitous nature. Studies with highly purified plasma membranes have established the presence of electron transport enzymes. Two types of activities have been identified. One, termed "Standard" reductase, is of general occurrence. The other, inducible under iron deficiency and relatively more active, is "Turbo" reductase. However, the true nature of components participating in electron transport and their organization in the plasma membrane is not known. The electron transport is associated with proton release and uses intracellular NAD(P)H as substrate. The electron flow leads to changes in intracellular redox status, pH, and metabolic energy. The responsiveness of this system to growth hormones is also observed. These findings suggest a role for electron flow across the plasma membrane in cell growth and regulation of ion transport. Involvement of this system in many other cellular functions is also argued.

The possible role of redox-associated protons in growth of plant cells

The protons excreted by plant cells may arise by two different mechanisms: (1) by the action of the plasma membrane H(+)-ATPase and (2) by plasma membrane redox reactions. The exact proportion from each source is not known, but the plasma membrane H(+)-ATPase is, by far, the major contributor to proton efflux. There is still some questions of whether the redox-associated protons produced by NADH oxidation on the inner side of the plasma membrane traverse the membrane in a 1:1 relationship with electrons generated in the redox reactions. Membrane depolarization observed in the presence of ferricyanide reduction by plasma membranes of whole cells or tissues or the lag period between ferricyanide reduction and medium acidification argue that only scalar protons may be involved. The other major argument against tight coupling between protons and electrons involves the concept of strong charge compensation. When ferricyanide is reduced to ferrocyanide on the outside of cells or tissues, an extra negative charge arises, which is compensated for by the release of H+ or K+, so that the total ratio of increased H+ plus K+ equals the electrons transferred by transmembrane electron transport. These are strong arguments against a tight coupling between electrons and protons excreted by the plasma membrane. On the other hand, there is no question that inhibitor studies provide evidence for two mechanisms of proton generation by plasma membranes. When the H(+)-ATPase activity is totally inhibited, the addition of ferricyanide induces a burst of extra proton excretion, or vice versa, when plasma membrane redox reactions are inhibited, the H(+)-ATPase can function normally. Since plasma membrane redox reactions and associated H+ excretion are related to growth it is possible that in plants the ATPase-generated protons have a different function from redox-associated protons. The H(+)-ATPase-generated protons have been considered for many years to be necessary for cell wall expansion, allowing elongation to take place. A special function of the redox-generated protons may be in initiating proliferative cell growth, based on the presence of a hormone-stimulated NADH oxidase in membranes of soybean hypocotyls and stimulation of root growth by low concentrations of oxidants. Here we propose that this NADH oxidase and the redox protons released by its action control growth. The mechanism for this may be the evolution of protons into a special membrane domain, from which a signal to initiate cell proliferation may originate, independent of the action of the H(+)-ATPase-generated protons.(ABSTRACT TRUNCATED AT 400 WORDS)

Generation of superoxide anion and hydrogen peroxide at the surface of plant cells

In addition to well-known cell wall peroxidases, there is now evidence for the presence of this enzyme at the plasma membrane of the plant cells (surface peroxidase). Both are able to catalyze, through a chain of reactions involving the superoxide anion, the oxidation of NADH to generate hydrogen peroxide. The latter is oxidized by other wall-bound peroxidases to convert cinnamoyl alcohols into radical forms, which, then polymerize to generate lignin. However, there are other enzymes at the surface of plasma membranes capable of generating hydrogen peroxide (cell wall polyamine oxidase), superoxide anion (plasma membrane Turbo reductase), or both (plasma membrane flavoprotein?). These enzymes utilize NAD(P)H as a substrate. The Turbo reductase and the flavoprotein catalyze the univalent reduction of Fe3+ and then of O2 to produce Fe2+ and O2-., respectively. The superoxide anion, in the acidic environment of the cell wall, may then dismutate to H2O2. These superoxide anion- and hydrogen peroxide-generating systems are discussed in relation to their possible involvement in physiological and pathological processes in the apoplast of plant cells.

Hydrogen peroxide formation and iron ion oxidoreduction linked to NADH oxidation in radish plasmalemma vesicles

Previously, we showed the presence in radish (Raphanus sativus L.) plasmalemma vesicles of an NAD(P)H oxidase, active at pH 4.5-5.0, which elicits the formation of anion superoxide (Vianello and Macrí (1989) Biochim. Biophys. Acta 980, 202-208). In this work, we studied the role of hydrogen peroxide and iron ions upon this oxidase activity. NADH oxidation was stimulated by ferrous ions and, to a lesser extent, by ferric ions. Salicylate and benzoate, two known hydroxyl radical scavengers, inhibited both basal and iron-stimulated NADH oxidase activity. The iron chelators EDTA (ethylenediaminetetraacetic acid) and DFA (deferoxamine melysate) at high concentrations (2 mM) inhibited the NADH oxidation, whereas they were ineffective at lower concentrations (80 microM); the subsequent addition of ferrous ions caused a rapid and limited increase of oxygen consumption which later ceased. Hydrogen peroxide was not detected during NADH oxidation but, in the presence of salicylate, its formation was found in significant amounts. NADH oxidase activity was also associated to a Fe2+ oxidation which was only partially inhibited by salicylate. Ferrous ion oxidation was partially inhibited by catalase and prevented by superoxide dismutase, while ferric ion reduction was abolished by catalase and unaffected by superoxide dismutase. These results show that during NADH oxidation iron ions undergo oxidoreduction and that hydrogen peroxide is produced and rapidly consumed. As previously suggested, this oxidation appears linked to the univalent oxidoreduction of iron ions by a reduced flavoprotein of radish plasmalemma which is then converted to a radical form. The latter, reacting with oxygen generates the superoxide anion which dismutases to H2O2. Hydrogen peroxide, through a Fenton's reaction, may react with Fe2+ to produce hydroxyl radicals, or with Fe3+ to generate the superoxide anion.