The Mechanism of Transmembrane Auxin Transport and Its Relation to the Chemiosmotic Hypothesis of the Polar Transport of Auxin. In: Skoog F, editor. Plant Growth Substances 1979. Berlin: Springer Berlin Heidelberg; 1980. pp. 50-60. [DOI: 10.1007/978-3-642-67720-5_4]

Higher extracellular pH suppresses tracheary element differentiation by affecting auxin uptake

In an optimized liquid medium containing auxin and cytokinin, mesophyll cells isolated from Zinnia elegans L. seedlings can be induced to differentiate into tracheary elements (TEs) at high frequency. However, it is known that buffering the medium at neutral pH severely suppresses TE differentiation. In the process of modifying the medium, we found that excessive administration of auxin restored the suppression. Based on this finding, we physiologically characterized auxin actions involved in TE differentiation by focusing on the influence of extracellular pH. First, dose/response relationships between auxin [1-naphthaleneacetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D)] concentrations and differentiated cell ratios were determined under various extracellular pH conditions. Secondly, intracellular concentrations of free forms and metabolites of auxin species were determined by analyzing extracts from cells cultured with radiolabeled NAA and 2,4-D under different extracellular pH conditions with liquid scintillation counting and thin-layer chromatography autoradiograms. Higher extracellular pH was found to reduce both the auxin potency for inducing TE differentiation and intracellular auxin accumulation. Reduction levels correlatively varied depending on the auxin species. These results suggest that the weakening in auxin potency at higher extracellular pH is ascribed to lower auxin uptake, which leads to decreased intracellular perception of the auxin signal. A model to predict auxin action that considers membrane transport, metabolism, and the perception of auxin is also presented.

Oxygen influences benzyladenine and 2,4-dichlorophenoxyacetic acid levels in cultured embryogenic tissue of Norway spruce

It is known that reducing the partial pressure of O₂ influences the induction of somatic embryogenesis. We tested the hypothesis that O₂ causes changes in the endogenous levels of exogenously supplied benzyladenine (BA) or 2,4-dichlorophenoxyacetic acid (2,4-D). Embryogenic tissue of Picea abies was incubated under reduced (2.5, 5 kPa) and ambient (21 kPa) levels of O₂ for 1, 3, 7 and 11 days and the endogenous concentrations of BA and 2,4-D were measured. For all treatments the concentration of BA in the tissue increased until the third day. At day 3, the ratio of BA in the tissue relative to the initial concentration in the medium, was 3.9, 2.8 and 1.9 for tissue incubated under 2.5, 5 and 21 kPa O₂ , respectively. The BA concentration then declined gradually. Uptake of 2,4-D was inhibited at low O₂ levels. However, 2,4-D gradually accumulated in tissue grown under hypoxia, so that high levels were reached by day 11. These shifts in the BA and 2,4-D levels also caused a transient increase in the BA to 2,4-D ratio in tissue incubated under hypoxia. Although relevant for the previously reported effects of oxygen on induction of embryogenic tissue, it is unlikely that oxygen-induced alterations in BA and 2,4-D levels alone suffice to explain these findings.

On polar auxin transport in plant cells

We present here explicit mathematical formulas for calculating the concentration, mass, and velocity of movement of the center of mass of the plant growth regulator auxin during its polar movement through a linear file of cells. The results of numerical computations for two cases, (a) the conservative, in which the mass in the system remains constant and (b) the non-conservative, in which the system acquires mass at one end and loses it at the other, are graphically presented. Our approach differs from that of Mitchison's (Mitchison 1980) in considering both initial effects of loading and end effects of substance leaving the file of cells. We find the velocity varies greatly as mass is entering or leaving the file of cells but remains constant as long as most of the mass is within the cells. This is also the time for which Mitchison's formula for the velocity, which neglects end effects, reflects the true velocity of auxin movement. Finally, the predictions of the model are compared with two sets of experimental data. Movement of a pulse of auxin through corn coleoptiles is well described by the theory. Movement of auxin through zucchini shoots, however, shows the need to take into account immobilization of auxin by this tissue during the course of transport.

Applicability of the chemiosmotic polar diffusion theory to the transport of indol-3yl-acetic acid in the intact pea (Pisum sativum L.)

The transport of exogenous indol-3yl-acetic acid (IAA) from the apical tissues of intact, light-grown pea (Pisum sativum L. cv. Alderman) shoots exhibited properties identical to those associated with polar transport in isolated shoot segments. Transport in the stem of apically applied [1-(14)C]-or [5-(3)H]IAA occurred at velocities (approx. 8-15 mm·h(-1)) characteristic of polar transport. Following pulse-labelling, IAA drained from distal tissues after passage of a pulse and the rate characteristics of a pulse were not affected by chases of unlabelled IAA. However, transport of [1-(14)C]IAA was inhibited through a localised region of the stem pretreated with a high concentration of unlabelled IAA or with the synthetic auxins 1-napthaleneacetic acid and 2,4-dichlorophenoxyacetic acid, and label accumulated in more distal tissues. Transport of [1-(14)C]IAA was also completely prevented through regions of the intact stem treated with N-1-naphthylphthalamic acid (NPA) and 2,3,5-triiodobenzoic acid.Export of IAA from the apical bud into the stem increased with total concentration of IAA applied (labelled+unlabelled) but approached saturation at high concentrations (834 mmol·m(-3)). Transport velocity increased with concentration up to 83 mmol·m(-3) IAA but fell again with further increase in concentration.Stem segments (2 mm) cut from intact plants transporting apically applied [1-(14)C]IAA effluxed 93% of their initial radioactivity into buffer (pH 7.0) in 90 min. The half-time for efflux increased from 32.5 to 103.9 min when 3 mmol·m(-3) NPA was included in the efflux medium. Long (30 mm) stem sections cut from immediately below an apical bud 3.0 h after the apical application of [1-(14)C]IAA effluxed IAA when their basal ends, but not their apical ends, were immersed in buffer (pH 7.0). Addition of 3 mmol·m(-3) NPA to the external medium completely prevented this basal efflux.These results support the view that the slow long-distance transport of IAA from the intact shoot apex occurs by polar cell-to-cell transport and that it is mediated by the components of IAA transmembrane transport predicted by the chemiosmotic polar diffusion theory.

Regulation of auxin transport in pea (Pisum sativum L.) by phenylacetic acid: inhibition of polar auxin transport in intact plants and stem segments

The transport of [(14)C]phenylacetic acid (PAA) in intact plants and stem segments of light-grown pea (Pisum sativum L. cv. Alderman) plants was investigated and compared with the transport of [(14)C]indiol-3yl-acetic acid (IAA). Although PAA was readily taken up by apical tissues, unlike IAA it did not undergo long-distance transport in the stem. The absence of PAA export from the apex was shown not to be the consequence of its failure to be taken up or of its metabolism. Only a weak diffusive movement of PAA was observed in isolated stem segments which readily transported IAA. When [1-(14)C]PAA was applied to a mature foliage leaf in light, only 5.4% of the (14)C recovered in ethanol extracts (89.6% of applied (14)C) had been exported from the leaf after 6.0 h. When applied to the corresponding leaf, [(14)C]sucrose was readily exported (46.4% of the total recovered ethanol-soluble (14)C after 6.0 h). [1-(14)C]phenylacetic acid applied to the root system was readily taken up but, after 5.0 h, 99.3% of the recovered (14)C was still in the root system.When applied to the stem of intact plants (either in lanolin at 10 mg·g(-1), or as a 10(-4) M solution), unlabelled PAA blocked the transport through the stem of [1-(14)C]IAA applied to the apical bud, and caused IAA to accumulate in the PAA-treated region of the stem. Applications of PAA to the stem also inhibited the basipetal polar transport of [1-(14)C]IAA in isolated stem segments. These results are consistent with recent observations (C.F. Johnson and D.A. Morris, 1987, Planta 172, 400-407) that no carriers for PAA occur in the plasma membrane of the light-grown pea stem, but that PAA can inhibit the carrier-mediated efflux of IAA from cells. The possible functions of endogenous PAA are discussed and its is suggested that an important role of the compound may be to modulate the polar transport and-or accumulation by cells of IAA.

Regulation of auxin transport in pea (Pisum sativum L.) by phenylacetic acid: effects on the components of transmembrane transport of indol-3yl-acetic acid

Phenylacetic acid (PAA), a naturally-occurring acidic plant growth substance, was readily taken up by pea (Pisum sativum L. cv. Alderman) stem segments from buffered external solutions by a pH-dependent, non-mediated diffusion. Net uptake from a 0.2 μM solution at pH 4.5 proceeded at a constant rate for at least 60 min and, up to approx. 100 μM, the rate of uptake was directly proportional to the external concentration of the compound. The net rate of uptake of PAA was not affected by the inclusion of indol-3yl-acetic acid (IAA) in the uptake medium (up to approx. 30 μM) and, unlike the net uptake of IAA, was not stimulated by N-1-naphthylphthalamic acid (NPA) or 2,3,5-triiodobenzoic acid. At an external concentration of 0.2 μM and pH 4.5, the net rate of uptake of PAA was about twice that of IAA. It was concluded that the uptake of PAA did not involve the participation of carriers and that PAA was not a transported substrate for the carriers involved in the uptake and polar transport of IAA. Nevertheless, the inclusion of 3-100 μM unlabelled PAA in the external medium greatly stimulated the uptake by pea stem segments of [1-(14)C]IAA (external concentration 0.2 μM). It was concluded that whilst PAA was not a transported substrate for the NPA-sensitive IAA efflux carrier, it interacted with this carrier to inhibit IAA efflux from cells. Over the concentration range 3-100 μM, PAA progressively reduced the stimulatory effect of NPA on IAA uptake, indicating that PAA also inhibited carrier-mediated uptake of IAA. The consequences of these observations for the regulation of polar auxin transport are discussed.

Auxin carriers in Cucurbita vesicles : I. Imposed perturbations of transmembrane pH and electrical potential gradients characterised by radioactive probes

The behaviour of radioactive probes of transmembrane pH gradients ([(14)C]butyric acid) and electrical potential gradients ((86)Rb(+) and S(14)CN(-)) has been characterised in membrane vesicles from Cucurbita pepo L. hypocotyls subjected to changing gradients and ionophore-mediated changes in the conductivity of K(+) and H(+) ions. The vesicles are sealed in the presence of 25 mM KCl. The K(+) ionophore valinomycin changes the distribution of (86)Rb(+) and of S(14)CN(-) in opposite directions, which depend on the external K(+) concentration. The accumulation of [(14)C]butyric acid by the vesicles and its association with the membranous phase are enhanced by increasing acidity, as expected for this lipophilic weak acid (pKa=4.8). Uptake of S(14)CN(-) increases and valinomycin-stimulated (86)Rb(+) uptake decreases when the external pH is lowered, indicating a strongly pH-dependent H(+) diffusion potential.

Auxin carriers in Cucurbita vesicles : II. Evidence that carrier-mediated routes of both indole-3-acetic acid influx and efflux are electroimpelled

The association at 0° C of [(3)H]indole-3-acetic acid (IAA) with membrane vesicles prepared from Cucurbita pepo L. hypocotyls at pH 7.9 and resuspended at pH 6.0 was greatly diminished by osmotic shrinkage. Nonradioactive IAA inhibited a large proportion of this association thus revealing a saturable component, also decreased by osmotic shrinkage, which mainly represents operation of an auxin uptake carrier, with saturable binding having only a minor role. The contribution of this carrier to the steady state of IAA transport changed in the same direction as the proton motive force (transmembrane pH and electrical potential difference) which was manipulated using the K(+) ionophore valinomycin with differing K(+) concentration gradients over a range of assay pH values. We conclude that the uptake carrier is electroimpelled with IAA(-)/2H(+) (or IAAH/H(+)) symport as a possible mechanism. The same procedures were used to examine the behaviour of the IAA efflux carrier, whose inhibition by N-1-naphthylphthalamic acid (NPA) causes an increase in IAA accumulation by the vesicles. The extent of NPA-stimulation was linked to the magnitude of the electrical potential difference (negative inside) across the vesicle membranes. We conclude that IAA transport by the efflux carrier is electroimpelled, with IAA(-) a likely substrate.

Auxin carriers in membranes of lupin hypocotyls

The pH-driven accumulation of [(3)H]indolyl-3-acetic acid (IAA) has been found to occur in membrane vesicles of lupin (Lupinus albus L.) hypocotyls. Most of this association of auxin with membranes is very sensitive to osmotic shock, high concentrations of permeable weak acids, incubation at 20° C for 20 min and to some ionophores. Long incubation times also depress the ability to accumulate radioactive IAA but this ability can be partially restored by a treatment that presumably reconstitutes the pH gradient across the membranes. Two specific inhibitors of auxin transport, N-1-naphtylphthalamic acid and 2,3,5-triiodobenzoic acid, stimulate net IAA uptake with an optimum at about 10(-6) M (pH 5.0). At least two auxin carriers appear to be present in the lupin membrane vesicles. An uptake carrier seems to be saturated at 10(-7) M IAA in the presence of N-1-naphtylphthalamic acid, but higher IAA concentrations are needed to saturate an efflux carrier. The uptake carrier also shows a high affinity for IAA and 2,4-dichlorophenoxyacetic acid and a low affinity for 1-naphthylacetic acid.

Electrical properties of the vertically growing root tip of Lepidium sativum L

Electrical transmembrane potential differences and resistances in different tissues of intact root tips of Lepidium sativum L. were investigated in a humid atmosphere by conventional glass-microelectrode techniques with the reference electrode at the surface (apoplast) of the root. The resting potential (inside negative) in cells of the root cap rose from-80 mV in external cell layers (secretion cells) to approx.-140 mV in central cells (statocytes). Measurements of the electric input resistance within the apoplast of the root tip (calyptra, meristem and elongation zone) yielded a preference for longitudinal contact (resistance per length of tissue approx. 3.4 GOhm m(-1)) compared with transversal contact (approx. 14 GOhm m(-1)). Similarly, the symplastic coupling expressed as the characteristic length (L) where a signal is reduced to 1/c compared with the origin yielded L y =390 μm in the longitudinal (y) direction and L x =140 μm in the transversal (x) direction. Cable analytical treatment of the symplastic input resistances (approx. 10 MOhm) resulted in low membrane resistances in the y-direction at the ends of cells compared with the membrane resistances in the x-direction (approx. 0.2 Ohm m(2)) of the lateral membranes in the approximately cylindrical cells. This anisotropy is discussed in terms of model calculations. The resistivity of the symplast was calculated to be about 2.5 Ohm m. The input current-voltage relationship displayed a slight curvature with increasing slope for the more negative membrane potential typical of membranes with electrogenic pumps. Even after massive electrical stimulation in the range from-50 to-150mV carried out to trace current-voltage curves, electrical excitations (action potentials) were not detected in the cells investigated.

The uptake of gibberellin A1 by suspension-cultured Spinacia oleracea cells has a carrier-mediated component

The kinetics of the uptake of [(3)H]gibberellin A1 (GA1) by light- and dark-grown suspension-cultured cells of Spinacia oleracea (spinach) have been studied. Use of nonradioactive GA1 and gibberellic acid (GA3) show that the uptake has a saturable and a nonsaturable component. The nonsaturable component increases as the pH is lowered at a fixed concentration of [(3)H]GA1 and is probably caused by non-mediated diffusion of the uncharged protonated species of GA1. The saturable component is not the result of metabolic transformation or to GA1 binding to the cell wall and is suggested to represent the operation of a transport carrier for which GA1 and GA3 are substrates. Auxin, abscisic acid and a cytokinin did not alter the GA1 uptake. The Km is approx. 0.3 μmol dm(-3) at pH 4.4 in light- and dark-grown cells. The Vmax of the carrier is higher in the light-grown cells. The optimum pH for the carrier at a physiological GA1 concentration (3 nmol dm(-3)) was pH 4.0, with no activity detectable at pH 7.0. Both saturable and nonsaturable components were decreased by protonophores indicating that the pH gradient between the cells and the medium may be a component of the driving forces for both types of transport. Both the permeability coefficient for the undissociated GA1 and the ratio V max/K m for the carrier are lower than the corresponding values for the indole-3-acetic acid and abscisic acid carriers studied in other species.

Carriers for abscisic acid and indole-3-acetic acid in primary roots: their regional localisation and thermodynamic driving forces

A carrier for the uptake of abscisic acid (ABA) is present in the tips and elongating zones of primary roots of both leguminous (runner bean, French bean, pea) and non-leguminous (sunflower, maize) seedlings. No ABA carrier was present in more mature root regions. For indole-3-acetic acid both carrier-mediated uptake and a 2,3,5-triiodobenzoate-sensitive efflux component are present in growing and in non-elongating runner-bean root tissues. Both ABA and indole-3-acetic acid carriers were inactivated by protein-modifying reagents. The driving forces for the carrier systems were studied using reagents, (KCl, fusicoccin, vanadate, dicyclohexylcarbodiimide, proton ionophores and azide) known to modify transmembrane pH (ΔpH) and electricla gradients (ΔE) and whose effects were independently monitored using radiolabelled, lipophilic, weak acids as probes. For abscisic acid the carrier-mediated uptake depend on ΔpH and the nonsaturable component of uptake, due to diffusion of undissociated ABA. The maximum velocity of the carrier is greater at pH 4 than at pH 5, although the Michaelis constants are similar. Modification of ΔE did not alter ABA net uptake but effects on the indole-3-acetic acid system consistent with perturbation of an electrogenic 2,3,5-triiodobenzoate-sensitive component were observed. It is suggested that the ABA carrier is an ABA anion/hydrogen ion symport or, less likely, represents facilitated diffusion of undissociated ABA.