Transmembrane potential increase induced by auxin, benzyladenine and fusicoccin. Correlation with proton extrusion and cell enlargement

Congruence between PM H+-ATPase and NADPH oxidase during root growth: a necessary probability

Plasma membrane (PM) H⁺-ATPase and NADPH oxidase (NOX) are two key enzymes responsible for cell wall relaxation during elongation growth through apoplastic acidification and production of ˙OH radical via O₂˙^-, respectively. Our experiments revealed a putative feed-forward loop between these enzymes in growing roots of Vigna radiata (L.) Wilczek seedlings. Thus, NOX activity was found to be dependent on proton gradient generated across PM by H⁺-ATPase as evident from pharmacological experiments using carbonyl cyanide m-chlorophenylhydrazone (CCCP; protonophore) and sodium ortho-vanadate (PM H⁺-ATPase inhibitor). Conversely, H⁺-ATPase activity retarded in response to different ROS scavengers [CuCl₂, N, N' -dimethylthiourea (DMTU) and catalase] and NOX inhibitors [ZnCl₂ and diphenyleneiodonium (DPI)], while H₂O₂ promoted PM H⁺-ATPase activity at lower concentrations. Repressing effects of Ca⁺² antagonists (La⁺³ and EGTA) on the activity of both the enzymes indicate its possible mediation. Since, unlike animal NOX, the plant versions do not possess proton channel activity, harmonized functioning of PM H⁺-ATPase and NOX appears to be justified. Plasma membrane NADPH oxidase and H⁺-ATPase are functionally synchronized and they work cooperatively to maintain the membrane electrical balance while mediating plant cell growth through wall relaxation.

Production of reactive oxygen intermediates (O(2)(.-), H(2)O(2), and (.)OH) by maize roots and their role in wall loosening and elongation growth

Cell extension in the growing zone of plant roots typically takes place with a maximum local growth rate of 50% length increase per hour. The biochemical mechanism of this dramatic growth process is still poorly understood. Here we test the hypothesis that the wall-loosening reaction controlling root elongation is effected by the production of reactive oxygen intermediates, initiated by a NAD(P)H oxidase-catalyzed formation of superoxide radicals (O(2)(.-)) at the plasma membrane and culminating in the generation of polysaccharide-cleaving hydroxyl radicals ((.)OH) by cell wall peroxidase. The following results were obtained using primary roots of maize (Zea mays) seedlings as experimental material. (1) Production of O(2)(.-), H(2)O(2), and (.)OH can be demonstrated in the growing zone using specific histochemical assays and electron paramagnetic resonance spectroscopy. (2) Auxin-induced inhibition of growth is accompanied by a reduction of O(2)(.-) production. (3) Experimental generation of (.)OH in the cell walls with the Fenton reaction causes wall loosening (cell wall creep), specifically in the growing zone. Alternatively, wall loosening can be induced by (.)OH produced by endogenous cell wall peroxidase in the presence of NADH and H(2)O(2). (4) Inhibition of endogenous (.)OH formation by O(2)(.-) or (.)OH scavengers, or inhibitors of NAD(P)H oxidase or peroxidase activity, suppress elongation growth. These results show that juvenile root cells transiently express the ability to generate (.)OH, and to respond to (.)OH by wall loosening, in passing through the growing zone. Moreover, inhibitor studies indicate that (.)OH formation is essential for normal root growth.

The electrical response of maize to auxins

The electrical response of Zea mays coleoptiles and suspension cultured cells to several growth-promoting auxins (IAA, IBA, 2,4-D, 2,4,5-T, 1-NAA) and some of their structural analogues (2,3-D, 2-NAA) has been tested. In coleoptile two typical electrical responses to IAA are observed: an immediated rapid depolarization, and a hyperpolarization following 7-10 minutes after the first external addition of IAA. Of the other tested compounds only 1-NAA significantly depolarized the cells, whereas all auxins as well as the analogues evoked delayed hyperpolarizations. In contrast, the suspension cells were not hyperpolarized by any of the tested compounds, but were strongly depolarized by IAA, 1-NAA, and to a lesser extent by 2-NAA. In these cells IAA and 1-NAA induced inwardly directed currents of positive charge which both saturated around 12 mA/m2. The strong pH-dependence together with the half-maximal currents 0.49 microM IAA and 0.76 microM 1-NAA point to a symport of the anions with at least 2H+. The delayed plasma membrane hyperpolarization is a different response, and seems to be initiated by the protonated auxin species. In accordance with the current literature, it is interpreted as consequence of a stimulated proton extrusion. The finding that all tested compounds evoked a hyperpolarization, makes this response unspecific. It is concluded that a stimulation of proton extrusion is a necessary, but not sufficient step to induce elongation growth.

Structure and function of the interfaces in biotrophic symbioses as they relate to nutrient transport

In this review we compare the structure and function of the interfaces between symbionts in biotrophic associations. The emphasis is on biotrophic fungal parasites and on mycorrhizas, although necrotrophic parasitic associations and the Rhizobium/legume symbiosis are mentioned briefly. We take as a starting point the observations that in the parasitic associations nutrient transport is polarized towards the parasite, whereas in mutualistic associations it is bidirectional. The structure and function of the interfaces are then compared. An important common feature is that in nearly all cases the heterotrophic symbiont (whether mutualistic or parasitic) is located topologically outside the cytoplasm of the host cells, in an apoplastic compartment. This means that nutrient movements across the interface must involve transport into and out of this apoplastic region through membranes of both organisms. Basic principles of membrane transport in uninfected cells are briefly reviewed to set the scene for a discussion of transport mechanisms which may operate in parasitic and mycorrhizal symbioses. The presence and possible roles of ATPases associated with membranes at the interfaces are discussed. We conclude that cytochemical techniques (used to demonstrate the activity of these enzymes) need to he extended and complemented by biochemical and biophysical studies in order to confirm that the activity is due to transport ATPases. Nevertheless, the distribution of activity appears to he in accord with polarized transport mechanisms in some pathogens and with bidirectional transport in mycorrhizas. The absence of ATPases on many fungal membranes needs re-examination. We emphasize that transport mechanisms between mycorrhizal symbionts cannot be viewed simply as the exchange of carbon for phosphate. Additional features include provision for transport of carbon and nitrogen as amino acids or amides and for ions such as K⁺ and H⁺ involved in the maintenance of charge balance and pH regulation, processes which also occur in parasitic associations. Interplant transport of nutrients via mycorrhizal hyphae is discussed in the context of these complexities. Some suggestions for the directions of future work are made. CONTENTS Summary 1 I. Introduction 2 II. The availability of nutrients to the symbionts 3 III. Structure of interfaces between symbionts 4 IV. Identity of nutrients transferred: an overview 12 V. Membrane transport: basic principles 14 VI. Transport at the interface of biotrophic symbioses 15 VII. Regulation of pH in biotrophic symbioses 25 VIII. Conclusions: 26.

Kinetin-induced stimulation of electrogenic pumping in soybean suspension cultures is unrelated to signal transduction

Primary modes of action of cytokinins have been thought to involve stimulation of the electrogenic H(+) pump and-or opening of plasmamembrane Ca(2+) channels. In order to test these hypotheses, rapid changes in membrane transport in response to cytokinin application were studied in heterotrophic suspension-cultured callus of soybean (Glycine max (L.) Merr.) using electrophysiological techniques. Kinetin (N(6)-furfurylaminopurine; 2 μM) elicited membrane hyperpolarization of 13±1 mV. This effect occurred even at membrane poteintials more negative than the most negative ionic equilibrium potential, and therefore might have resulted either from stimulation of the electrogenic pump, or from closure of ionic channels. The former mechanism of action appears most likely because (i) kinetin-induced membrane hyperpolarization is not accompanied by a significant change in plasma-membrane resistivity and (ii) hyperpolarization is abolished by cyanide, which inhibits electrogenic pump activity by depletion of cellular ATP.Electrogenic pumping is also activated by two other cytokinins: N(6)-(benzyl)adenine and trans-zeatin. However, it is unlikely that the hormonal effect on electrogenesis is directly related to transduction of the cytokinin signal, for the following reasons: (i) hormonally inactive, but chemically related compounds (cis-zeatin, adenine) also elicited membrane hyperpolarization; (ii) hormonally active, N(9)-substituted cytokinins failed to stimulate electrogenesis; (iii) the chemically unrelated cytokinin N,N'-diphenylurea also failed to stimulate electrogenesis.The results imply that the kinetin effect on electrogenic pumping is related to adenine, or its metabolism, and not hormonal action. Adenine was absorbed by soybean cells, but not in sufficient quantities to have a significant effect on adeninenucleotide pools. It appears likely that the control of electrogenesis requires either the presence of a purine free base (i.e. no substituents at the N(9) position) or phosphoribosylation of the free base. No evidence was found for cytokinin-induced Ca(2+)-channel opening, though it is argued that such an event might be physiologically relevant, yet undetectable with the methods employed. It is essential that future studies on cytokinin signal transduction - especially as they relate to membrane transport - take into account the possibility that metabolic effects unrelated to hormone action are dominant.

The animal ether phospholipid platelet-activating factor stimulates acidification of the incubation medium by cultured soybean cells

Addition of the animal ether phospholipid platelet-activating factor, 1-0-alkyl-2-acetyl-sn-glycero-3-phosphocholine, (PAF) stimulates medium acidification in cultured soybean (Glycine max L.) cells. The pH of the medium after 8-10 hours is on the average one pH unit lower than in controls. With fusicoccin an average pH difference of 1.7 units is reached. Phospholipids, glycerol, 1-oleyl-2-acetyl-sn-glycerol, 1-0-hexadecyl-sn-glycerol, and triolein at the same concentrations as PAF had no stimulatory effect on medium acidification. The detergents CHAPS and deoxycholate lead to alkalinization of the medium whereas lysophosphatidylcholine (LPC), a detergent with structural similarity to PAF, shows no effect.

The proton pumps of the plasmalemma and the tonoplast of higher plants

Studies on intact cells, membrane vesicles, and reconstituted proteoliposomes have demonstrated in higher plants the existence of an ATP-driven electrogenic proton pump operating at the plasmalemma. There is also evidence of a second ATP-driven H+ pump localized at the tonoplast. The characteristics of both these ATP-driven pumps closely correspond to those of the plasmalemma and tonoplast proton pumps of Neurospora and yeasts.

Rapid response of the plasma-membrane potential in oat coleoptiles to auxin and other weak acids

We have compared the effects of the auxin, indole-3-acetic acid (IAA) with that of other weak acids on the plasma-membrane potential of oat (Avena sativa L.) coleoptile cells. Cells treated with 1 μM IAA at pH 6 depolarize 20-25 mV in 10-12 min, but they then repolarize, until by 20-25 min their potentials are about 25 mV more negative than the initial value. Similar concentrations of benzoic and butyric acids cause the initial depolarization, but not the subsequent hyperpolarization. The hyperpolarization is therefore specific to IAA. All the weak acids, including IAA, evoke a rapid hyperpolarization when their concentrations are raised to 10 mM. This result indicates that at high concentrations, the uptake of undissociated weak acids activates electrogenic proton pumping, most likely by lowering cytoplasmic pH. In contrast, the hyperpolarization observed with concentrations of IAA four orders of magnitude lower appears to be a specific hormonal effect. This specific, auxin-induced hyperpolarization occurs at the same time as the initiation of net proton secretion and supports the hypothesis that auxin initiates extension growth by increasing proton pumping.

Transmembrane electrical potentials in growing maize roots : Anti-auxin effects

The anti-auxin 4-chlorophenoxyisobutyric acid (PCIB) applied at a concentration of 10(-2) mol m(-3) to maize root segments was found to induce a transmembrane electrical potential of up to-130 mV (Δpd of 30 mV). The kinetics of this response were comparable to the time scale for PCIB-stimulated H(+)-extrusion. Both effects are eliminated by the addition of p-fluoromethoxycarbonyl cyanide phenylhydrazone (FCCP). Treatment with fusicoccin (FC) and PCIB together does not result in a hyperpolarization greater than with FC alone. Benzoic acid (10(-2) mol m(-3)) had no effect on the transmembrane electrical potentials. These results are discussed in relation to a possible electrogenic proton pump which may be regulated by perturbations in the cellular auxin content or activity.

Energetics of the response of maize coleoptile tissue to indoleacetic acid : Characterization by flow calorimetry as a function of time, IAA concentration, and pH

Indole-3-acetic acid (IAA) promotes an increase in steady-state heat production by corn (Zea mays L.) coleoptile tissue; this increase is associated with an elevation in aerobic respiration rates. A detailed time dependence of the exothermic response to IAA was obtained using flow calorimetry. The latent period and magnitude of response were evaluated as a function of IAA concentration and pH. The data indicate that more than one response may occur. The optimal change in heat production was produced by an IAA concentration of 3·10(-5) M. It was initiated within 5 min after the start of the IAA treatment, and reached a magnitude in excess of 25% of the tissue's basal heat production. Concentrations of IAA greater than 1·10(-4) M resulted in diminished response(s), but the effect was strongly pH dependent. Several possibilities for the increased heat production triggered by IAA are discussed.

The contribution of tonoplast and plasma membrane to the electrical properties of a higher-plant cell

The cytoplasm of subepidermal parenchyma cells of Avena sativa L. coleoptiles was collected at one end of the cell by centrifugation. The electrical properties of both plasmalemma and tonoplast were then examined with microelectrodes inserted into both cytoplasm and vacuole of the same cell. The input resistance of the cytoplasm measured with either electrode was 7.5±0.8 MΩ while that of the vacuole measured with the single vacuolar electrode and a bridge circuit was 29.2±3.1 MΩ. The latter value was not significantly different from that of control, uncentrifuged cells. The resistance of the tonoplast is therefore several times larger than the input resistance of the cytoplasm, but the specific resistance of the plasma membrane cannot be calculated without knowledge of the extent and pattern of intercellular coupling. Electrical coupling of the cytoplasms of adjacent cells was observed in only two out of eight experiments. The mean potential of the vacuoles,-77.8±6.4 mV, was not significantly different from that of the cytoplasm; however, all the available evidence indicates that variable tip potentials in impaled cells made absolute determination of the membrane potential uncertain. In fusicoccin, the cells hyperpolarized by 20 mV within 10 min. This reponse occurred entirely at the plasmalemma.

Short-term effects of plant hormones on membrane potential and membrane permeability of dwarf maize coleoptile cells (Zea mays L. d 1) in comparison with growth responses

The membrane potential difference of dwarf maize coleoptile cells is increased by both 10(-5)moll(-1) gibberellic acid (GA3) and indoleacetic acid (IAA) a few minutes after application. A final level is reached after 10-20 min. The membrane permeability ratio P Na:P K is altered by both hormones during the first 15 min after application, indicating a rapid effect on the membrane. Elongation growth of coleoptile segments, however, is only stimulated by IAA. The auxin-induced growth as well as the auxin effect on membrane permeability depends on the calcium ion concentration of the medium. It is concluded that IAA acts via a proton extrusion pump that is electrically balanced by a potassium ion uptake, driven by the electromotive force of the pump. The mode of action of GA3 on elongation growth is assumed to involve a process that depends on the physiologic state of the tissue and/or metabolic energy.

Fusicoccin-induced growth and hydrogen ion excretion of Avena coleoptiles: Relation to auxin responses

The fungal toxin fusicoccin (FC) induces both rapid cell elongation and H(+)-excretion in Avena coleoptiles. The rates for both responses are greater with FC than with optimal auxin, and in both cases the lag after addition of the hormone is less with FC. This provides additional support for the acid-growth theory. The FC responses resemble the auxin responses in that they are inhibited by a range of metabolic inhibitors, but the responses differ in three ways. First auxin, but not FC, requires continual protein synthesis for its action. The auxin-induced H(+)-excretion is inhibited by water stress or by low external pH, while the FC-induced H(+)-excretion is much less sensitive to either. It is concluded that auxin-induced and FC-induced H(+)-excretion may occur via different mechanisms.

Inhibition of ion uptake to barley roots by cycloheximide

Cycloheximide was shown to inhibit the transport of ions into the xylem of "salt-saturated" barley roots (Hordeum vulgare cv cape), and incorporation of L-[1-(14)C] leucine into protein within 40-60 min. Water flow across the roots of whole seedlings was not altered for at least 180 min. Uptake of ions into the cells of "salt-saturated" roots was not affected for 120 min, but was inhibited when treated with cycloheximide for more than 120 min. H(+) efflux from "lowsalt" roots was inhibited by cycloheximide at about the same time as ion uptake. Measurement of ATP levels and O2uptake indicated that cycloheximide was not acting as an uncoupler of oxidative phosphorylation. The present data are considered to support the view that secretion of ions into the xylem vessels involves a specific protein with a short effective half-life, and is a separate process from active uptake into the cortical cells.

Relation between permeability to potassium and sodium ions and fusicoccin-stimulated hydrogen-ion efflux in barley roots

Measurements are described of fusicoccin (FC)-stimulated H(+) efflux in barley (Hordeum vulgare L.) roots when K(+) and Na(+) concentrations were varied. In low-salt roots H(+) efflux was stimulated in both 5 mM KCl and NaCl. In salt-saturated roots H(+) efflux was stimulated more effectively in KCl than in NaCl solution. The stimulation of H(+) efflux thus is parallel with the selectivity of these different root preparations for K(+) and Na(+) and with estimates of permeability ratios (P Na/P K) determined from electrical measurements. It is suggested that the results support electrogenic coupling between FC-stimulated H(+) efflux and cation uptake.