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 [1-¹⁴C]-or [5-³H]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 [1-¹⁴C]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 [1-¹⁴C]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 [1-¹⁴C]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 [1-¹⁴C]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.Abbreviations IAA indol-3yl-acetic acid - 2,4-D 2,4-dichlorophenoxyacetic acid - NAA 1-naphthaleneacetic acid - NPA N-1-naphthylphthalamic acid - TIBA 2,3,5-triiodobenzoic acid     

Transfer of exogenous auxin from the phloem to the polar auxin transport pathway in pea (Pisum sativum L.)

When [1-¹⁴C]indol-3yl-acetic acid ([1-¹⁴C]IAA) was applied to the upper surface of a mature foliage leaf of garden pea (Pisum sativum L. cv. Alderman), ¹⁴C effluxed basipetally but not acropetally from 30-mm-long internode segments excised 4 h after the application of [1-¹⁴C]IAA. This basipetal efflux was strongly inhibited by the inclusion of 3.10^–6 mol· dm³ N-1-naphthylphthalamic acid (NPA) in the efflux buffer. In contrast, when [¹⁴C] sucrose was applied to the leaf, the efflux of label from stem segments excised subsequently was neither polar nor sensitive to NPA. The [1-¹⁴C]IAA was initially exported from mature leaves in the phloem — transport was rapid and apolar; label was recovered from aphids feeding on the stem; and label was recovered in exudates collected from severed petioles in 20 mM ethylenediaminetetraacetic acid. No ¹⁴C was detected in aphids feeding on the stems of plants to which [1-¹⁴C]IAA had been applied apically, even though the internode on which they were feeding transported considerable quantities of label. Localised applications of NPA to the stem strongly inhibited the basipetal transport of apically applied [1-¹⁴C]IAA, but did not affect transport of [1-¹⁴C]IAA in the phloem. These results demonstrate for the first time that IAA exported from leaves in the phloem can be transferred into the extravascular polar auxin transport pathway but that reciprocal transfer probably does not occur. In intact plants, transfer of foliar-applied [1-¹⁴C]IAA from the phloem to the polar auxin transport pathway was confined to immature tissues at the shoot apex. In plants in which all tissues above the fed leaf were removed before labelling, a limited transfer of IAA occurred in more mature regions of the stem.Abbreviations IAA indol-3yl-acetic acid - EDTA ethylenediaminetetraacetic acid - NPA N-1-naphthylphthalamic acid We are grateful to the Nuffield Foundation for supporting this research under the NUF-URB95 scheme and for the provision of a bursary to A.J.C. We thank Professor Dennis A. Baker for constructive comments on a draft of this paper and Mrs. Rosemary Bell for her able technical assistance.     

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 M solution at pH 4.5 proceeded at a constant rate for at least 60 min and, up to approx. 100 M, 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 M) 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 M 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 M unlabelled PAA in the external medium greatly stimulated the uptake by pea stem segments of [1-¹⁴C]IAA (external concentration 0.2 M). 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 M, 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.Abbreviations IAA indol-3yl-acetic acid - DMO 5,5-dimethyloxazolidine-2,4-dione - NPA N-1-naphthylphthalamic acid - PAA phenylacetic acid - TIBA 2,3,5-triiodobenzoic acid     

Auxin transport and the regulation of petiole abscission in cotton (Gossypium hirsutum L.): A comparison of indol-3yl-acetic acid and phenylacetic acid

Distal applications of indol-3yl-acetic acid (IAA) to debladed cotyledonary petioles of cotton (Gossypium hirsutum L.) seedlings greatly delayed petiole abscission, but similar applications of phenylacetic acid (PAA) slightly accelerated abscission compared with untreated controls. Both compounds prevented abscission for at least 91 h when applied directly to the abscission zone at the base of the petiole. The contrasting effects of distal IAA and PAA on abscission were correlated with their polar transport behaviour-[1-¹⁴C]IAA underwent typical polar (basipetal) transport through isolated 30 mm petiole segments, but only a weak diffusive movement of [1-¹⁴C]PAA occurred.Removal of the shoot tip substantially delayed abscission of subtending debladed cotyledonary petioles. The promotive effect of the shoot tip on petiole abscission could be replaced in decapitated shoots by applications of either IAA or PAA to the cut surface of the stem. Following the application of [1-¹⁴C]IAA or [1-¹⁴C]PAA to the cut surface of decapitated shoots, only IAA was transported basipetally through the stem. Proximal applications of either compound stimulated the acropetal transport of [¹⁴C]sucrose applied to a subtending intact cotyledonary leaf and caused label to accumulate at the shoot tip. However, PAA was considerably less active than IAA in this response.It is concluded that whilst the inhibition of petiole abscission by distal auxin is mediated by effects of auxin in cells of the abscission zone itself, the promotion of abscission by the shoot tip (or by proximal exogenous auxin) is a remote effect which does not require basipetal auxin transport to the abscission zone. Possible mechanisms to explain this indirect effect of proximal auxin on abscission are discussed.     

The role of auxin efflux carriers in the reversible loss of polar auxin transport in the pea (Pisum sativum L.) stem

Correlatively inhibited pea shoots (Pisum sativum L.) did not transport apically applied ¹⁴C-labelled indol-3yl-acetic acid ([¹⁴C]IAA), and polar IAA transport did not occur in internodal segments cut from these shoots. Polar transport in shoots and segments recovered within 24 h of removing the dominant shoot apex. Decapitation of growing shoots also resulted in the loss of polar transport in segments from internodes subtending the apex. This loss was prevented by apical applications of unlabelled IAA, or by low temperatures (approx. 2° C) after decapitation. Rates of net uptake of [¹⁴C]IAA by 2-mm segments cut from subordinate or decapitated shoots were the same as those in segments cut from dominant or growing shoots. In both cases net uptake was stimulated to the same extent by competing unlabelled IAA and by N-1-naphthylphthalamic acid. Uptake of the pH probe [¹⁴C]-5,5-dimethyloxazolidine-2,4-dione from unbuffered solutions was the same in segments from both types of shoot. Patterns of [¹⁴C]IAA metabolism in shoots in which polar transport had ceased were the same as those in shoots capable of polar transport. The reversible loss of polar IAA transport in these systems, therefore, was not the result of loss or inactivation of specific IAA efflux carriers, loss of ability of cells to maintain transmembrane pH gradients, or the result of a change in IAA metabolism. Furthermore, in tissues incapable of polar transport, no evidence was found for the occurrence of inhibitors of IAA uptake or efflux. Evidence is cited to support the possibility that the reversible loss of polar auxin transport is the result of a gradual randomization of effluxcarrier distribution in the plasma membrane following withdrawal of an apical auxin supply and that the recovery of polar transport involves reestablishment of effluxcarrier asymmetry under the influence of vectorial gradients in auxin concentration.Abbreviations DMO 5,5-dimethyloxazolidine-2,4-dione - IAA indol-3yl-acetic acid - NPA N-1-naphthylphthalamic acid - TIBA 2,3,5-triiodobenzoic acid This work was supported by grant no. GR/D/08760 from the U.K. Science and Engineering Research Council. We thank Mrs. R.P. Bell for technical assistance.     

Effects of osmotic stress on polar auxin transport in Avena mesocotyl sections

Segments of mesocotyls of Avena sativa L. transported [1-¹⁴C]indol-3yl-acetic acid (IAA) with strictly basipetal polarity. Treatment of the segments with solutions of sorbitol caused a striking increase in basipetal auxin transport, which was greatest at concentrations around 0.5 M. Similar effects were observed with mannitol or quebrachitol as osmotica, but with glucose or sucrose the increases were smaller. Polar transport was still detectable in segments treated with 1.2 M sorbitol. The effects of osmotic stress on the polar transport of auxin were reversible, but treatment with sorbital solutions more concentrated than 0.5 M reduced the subsequent ability of mesocotyl segments to grow in response to IAA. The increased transport of auxin in the osmotically stressed segments could not be explained in terms of an increased uptake from donor blocks. The velocity of transport declined with higher concentrations of osmoticum. The reasons for the enhancement of auxin transport by osmotic stress are not known.     

Studies on the evolution of auxin carriers and phytotropin receptors: Transmembrane auxin transport in unicellular and multicellular Chlorophyta

The characteristics of transmembrane transport of ¹⁴C-labelled indol-3yl-acetic acid ([1-¹⁴C]IAA) were compared in Chlorella vulgaris Beij., a simple unicellular green alga, and in Chara vulgaris L., a branched, multicellular green alga exhibiting axial polarity and a high degree of cell and organ specialization. In Chara thallus cells, three distinguishable trans-plasmamembrane fluxes contributed to the net uptake of [1-¹⁴C]-IAA from an external solution, viz.: a non-mediated, pH-sensitive influx of undissociated IAA (IAAH); a saturable influx of IAA; and a saturable efflux of IAA. Both saturable fluxes were competitively inhibited by unlabelled IAA. Association of [³H]IAA with microsomal preparations from Chara thallus tissue was competitively inhibited by unlabelled IAA. Results indicated that up-take carriers occurred in the membranes at a much higher density than efflux carriers. The efflux component of IAA net uptake by Chara was not affected by several phytotropins (N-1-naphthylphthalmic acid, NPA; 2-(1-pyrenoyl)benzoic acid; and 5-(2-carboxyphenyl)-3-phenylpyrazole), which are potent non-competitive inhibitors of specific auxin-efflux carriers in more advanced plant groups, and no evidence was found for a specific association of [³H]NPA with Chara microsomal preparations. It was concluded that Chara lacked phytotropin receptors. Net uptake of [1-¹⁴C]IAA also was unaffected by 2,3,5-triiodobenzoic acid except at concentrations ( 10^–1 mol · m^–3) high enough to depress cytoplasmic pH (determined by uptake of 5,5-dimethyloxazolidine-2,4-dione). Chlorella cells accumulated [1-¹⁴C]IAA from an external solution by pH-sensitive diffusion of IAA across the plasma membrane and anion (IAA^–) trapping, but no evidence was found in Chlorella for the occurrence of IAA carriers. These results indicate that carrier systems capable of mediating the transmembrane transport of auxins appeared at a very early stage in the evolution of green plants, possibly in association with the origin of a differentiated, multicellular plant body. Phytotropin receptors evolved independently of the carriers.Abbreviations CPP 5-(2-carboxyphenyl)-3-phenylpyrazole - DMO 5,5-dimethyloxazolidine-2,4-dione - IAA indol-3yl-acetic acid - NPA N-1-naphthylphthalamic acid - PBA 2-(1-pyrenoyl)benzoic acid - TIBA 2,3,5-triiodobenzoic acid We thank the Nuffield Foundation for the award of an Undergraduate Research Bursary to J.E.D.-F., Dr. G.F. Katekar, C.S.I.R.O., Canberra, Australia for generous gifts of phytotropins, and Mrs. R.P. Bell for technical support.     

The influence of small direct electric currents on the transport of auxin in intact plants

When a d.c. potential of 9.0 V was applied to the stem of intact pea seedlings (Pisum sativum L. cv. Meteor and cv. Alderman) via 10 mM KCl-soaked filter paper electrodes placed ca. 50 mm apart the stem passed a steady current of 15–20 A (resistance ca. 100 k cm^-1). The basipetal transport of [1-¹⁴C]IAA applied to the apical bud was completely inhibited over the portion of the stem through which current flowed and ¹⁴C-labelled compounds accumulated in the vicinity of the upper electrode. The inhibition of transport was independent of the polarity of the applied potential. The basipetal transport of IAA in the stem above the electrode was not affected.Labelled auxin accumulated at the upper electrode both as unchanged IAA and as a compound tentatively identified as indol-3yl-acetyl aspartic acid (IAAsp). These compounds were only slowly remobilised when the current was interrupted. However, the ability of the transport system to move freshly-applied IAA was rapidly and fully restored when the potential was removed. No injury to the plant was detected after maintaining a current flow for up to 72 h. No leakage of ¹⁴C-labelled compounds into the KCl solution bathing the electrodes was detected.Abbreviations IAA indol-3yl-acetic acid - IAAsp indol-3yl-acetyl aspartic acid     

The effect of cycloheximide on IAA-stimulated transport of 14C-ABA and 14C-sucrose in long pea epicotyl segments

The effect of cycloheximide (CH) on the indol-3yl-acetic acid (IAA)-stimulated transport of ¹⁴C-labelled abscisic acid (ABA) and ¹⁴C-labelled sucrose was studied in 110 mm long pea epicotyl segments. IAA application resulted in elongation growth of the segments. This effect was decreased by CH treatment which also reduced [¹⁴C] ABA and [¹⁴C] sucrose accumulation in the growing apical part of the segments. A reduction in [¹⁴C] IAA uptake and in protein synthesis in this part of the segments was also observed. The simultaneous inhibition of protein synthesis and reduction of [¹⁴C] ABA and [¹⁴C] sucrose transport suggests that IAA can stimulate the transport of ABA and sucrose through a protein synthesis-based elongation growth.     

The effect of temperature on the velocity of exogenous auxin transport in intact chilling-sensitive and chilling-resistant plants

The velocity of exogenous indol-3yl-acetic acid ([1-¹⁴C]IAA) transport from the apical buds of intact pea, sunflower and cotton plants was determined from 0.5° C to 47° C. The minimum temperature at which transport occurred varied from 2° C (pea and sunflower) to 7° C (cotton). Above these temperatures the velocity of transport increased steadily to maxima near 44° C in all three species. Further increase in temperature resulted in a complete cessation of transport, suggesting a sudden high-temperature breakdown of the auxin transport system. Temperature coefficients (Q₁₀) for transport velocity calculated from Arrhenius plots were low (1.36 to 1.41 between 15° C and 30° C).Arrhenius plots for the chilling-sensitive cotton and sunflower plants exhibited abrupt discontinuities at 14.6° C and 8.7° C respectively. An Arrhenius plot for the chilling-resistant pea exhibited no such discontinuity over the whole temperature range at which transport occurred.Abbreviation IAA indol-3yl-acetic acid     

Components of auxin transport in stem segments of Pisum sativum L.

1. The uptake of indol-3-yl acetic acid ([1-¹⁴C]IAA, 0–2.0 M) into light-grown pea stem segments was measured under various conditions to investigate the extent to which mechanisms of auxin transport in crown gall suspension culture cells (Rubery and Sheldrake, Planta 118, 101–121, 1974) are also found in a tissue capable of polar auxin transport. — 2. IAA uptake increased as the external pH was lowered. IAA uptake was less than that of benzoic acid (BA), naphthylacetic acid (NAA) or 2,4 dichlorophenoxyacetic acid (2,4D) under equivalent conditions. TIBA enhanced net IAA uptake through inhibition of efflux, and to a lesser extent, also increased uptake of NAA and 2,4D while it had no effect on BA uptake. — 3. Both DNP and, at higher concentrations, BA, reduced IAA uptake probably because of a reduction of cytoplasmic pH. However, low concentrations of both BA and DNP caused a slight enhancement of IAA net uptake, possibly through a reduction of carrier-mediated IAA efflux. In the presence of TIBA, the inhibitory effects of DNP and BA were more severe and there was no enhancement of uptake at low concentrations. — 4. Non-radioactive IAA (10 M) reduced uptake of labelled IAA but further increases in concentration up to 1.0 mM produced first an inhibition (0–10 min) of labelled IAA uptake, followed by a stimulation at later times. Non-radioactive 2,4 D decreased, but was not observed to stimulate, uptake of labelled IAA. In the presence of TIBA labelled IAA uptake was inhibited by non-radioactive IAA regardless of its concentration. — 5. Sulphydryl reagents PCMB and PCMBS promoted or inhibited IAA uptake depending, respectively, on whether they penetrated or were excluded from the cells. The penetrant PCMB also reduced the promotion of labelled IAA uptake by TIBA or by high concentrations of added non-labelled IAA. — 6. Our findings are interpreted as being consistent with the diffusive entry of unionised IAA into cells together with some carrier-mediated uptake. Auxin efflux from the cells also appears to have a carrier-mediated contribution, at least part of which is inhibited by TIBA, and which has a capacity at least as great as that of the uptake carrier. The data indicate that pea stem segments contain cells whose mechanisms of trans-membrane auxin transport fit the model of polar auxin transport proposed from experiments with crown gall suspension cells, although differences, particularly of carrier specificity, are apparent between the two systems.Abbreviations IAA indol-3-yl acetic acid - BA benzoic acid - NAA 1-naphthylacetic acid - 2,4-D 2,4-dichlorophenoxyacetic acid - TIBA 2,3,5-triiodobenzoic acid - DNP 2,4-dinitrophenol - PCMB p-chloromercuribenzoic acid - PCMBS p-chloromercuribenzene sulphonic acid This work was performed in Cambridge during the tenure of a sabbatical leave by P.J.D. Supported by a grant for supplies from the American Philosophical Society to P.J.D.     

Targeting of auxin carriers to the plasma membrane: effects of monensin on transmembrane auxin transport in Cucurbita pepo L. tissue

Treatment of etiolated zucchini (Cucurbita pepo L.) hypocotyl tissue with sub-micromolar concentrations of the cationophore monensin rapidly (<20 min) inhibited the transport catalytic activity of the specific auxin-anion efflux carrier and reduced the inhibition of this carrier by the phytotropin N-1-naphthylphthalamic acid (NPA). Monensin inhibited the basipetal polar transport of indol-3yl-acetic acid (IAA) in long (30 mm) zucchini segments. At concentrations lower than 10^–5 mol·dm^–3 monensin did not affect uptake of the pH probe [2-¹⁴C]5,5-dimethyloxazolidine-2,4-dione (DMO) or that of the membrane-potential probe tetra[¹⁴C-phenyl]phosphonium bromide (TPP⁺), did not affect the response of IAA net uptake to external Ca²⁺ concentration and did not alter the metabolism of IAA. It was concluded that low concentrations of monensin inhibit transport through the Golgi apparatus of auxin efflux carrier protein and that the efflux carriers turn over very rapidly in the plasma membrane. Monensin pretreatment did not affect the saturable binding of [³H]NPA to microsomal membranes, indicating that the auxin-efflux catalytic sites and the NPA-binding sites are located on separate proteins. At higher concentrations (10^–5 mol·dm^–3) monensin inhibited both mediated uptake and mediated efflux components of IAA transport. This effect was at least in part attributable to perturbation by monensin of the driving forces for mediated uptake since high concentrations of monensin also reduced the uptake of DMO and TPP⁺.Abbreviations CH cycloheximide - DMO 5,5-dimethyloxazolidine-2,4-dione - MDMP 2-(4-methyl-2,6-dinitroanlilino)N-methyl-propionamide - NPA N-1-naphthylphthalamic acid - TPP⁺ tetraphenylphosphonium ion We thank Mrs. R.P. Bell for technical assistance and Drs. G.F. Katekar and M.A. Venis for generous gifts of NPA. S.W. was supported by the U.K. Science and Engineering Research Council.     

Targeting of auxin carriers to the plasma membrane: differential effects of brefeldin A on the traffic of auxin uptake and efflux carriers

Monensin and brefeldin A (BFA), inhibitors of Golgi-mediated protein secretion, rapidly perturb the transport catalytic activity of specific plasma membrane-associated efflux carriers for indole-3-acetic acid (IAA) and inhibit polar transport of IAA. To determine if these responses result solely from perturbation of the efflux carrier or whether specific auxin uptake carrier function is also affected, the influence of BFA on the cellular transport of a range of auxins with contrasting affinities for specific auxin uptake and efflux carriers was investigated in zucchini (Cucurbita pepo L.) hypocotyl tissue. In-flight addition of BFA (3 · 10⁻⁵ mol · dm⁻³) caused a rapid (lag < 10 min) and substantial (fourfold) increase in the rate of [1-¹⁴C]IAA net uptake by zucchini hypocotyl tissue. In the presence of the specific auxin efflux carrier inhibitor N-1-naphthylphthalamic acid (NPA; 3 · 10⁻⁶ mol · dm⁻³), BFA slightly reduced the rate of [1-¹⁴C]IAA net uptake. Stimulation of [1-¹⁴C]IAA net uptake by BFA was concentration-dependent. In the absence of BFA, net uptake of [1-¹⁴C]IAA exhibited the characteristic biphasic response to increasing concentrations of competing cold IAA but in the presence of BFA, [1-¹⁴C]IAA uptake decreased smoothly with increase in concentration of competing unlabelled IAA, indicating a loss of auxin efflux carrier activity but retention of functional uptake carriers. The half-time for mediated efflux of [1-¹⁴C]IAA from preloaded zucchini tissue was substantially increased by BFA (t_1/2 = 51 min, controls; 107 min, BFA-treated). Treatment with BFA and/or NPA did not significantly affect the net uptake by, or efflux from, zucchini tissue of [1-¹⁴C]2,4-dichlorophenoxyacetic acid ([1-¹⁴C]2,4-D), a substrate for the auxin uptake carrier but not the auxin efflux carrier. Uptake of [1-¹⁴C]2,4-D declined smoothly with increasing concentrations of competing unlabelled IAA whether or not BFA was included in the uptake medium, confirming the failure of BFA to perturb auxin uptake carrier function. Transport of 1-[4-³H]naphthaleneacetic acid (1-NAA) exhibited little response to BFA or NPA, confirming that it is only a weakly transported substrate for the efflux carrier in zucchini cells. Received: 12 November 1997 / Accepted: 27 January 1998     

Accumulation of 14C from exogenous labelled auxin in lateral root primordia of intact pea seedlings (Pisum sativum L.)

When [¹⁴C]indol-3yl-acetic acid was applied to the apical bud of 5-day old dwarf pea seedlings which possessed unbranched primary roots, a small amount of ¹⁴C was transported into the root system at a velocity of 11–14 mm h^-1. Most of the ¹⁴C which entered the primary root accumulated in the young lateral root primordia, including the smallest detectable (20–30 mm from the primary root tip). In older (8-d old) seedlings in which the primary root bore well-developed lateral roots, ¹⁴C also accumulated in the tertiary root primordia. In contrast, little ¹⁴C was detected in the apical region of the primary root or, in older plants, in the apices of the lateral roots.Abbreviations IAA indol-3yl-acetic acid     

Effects of temperature and sink activity on the transport of 14C-labelled indol-3yl-acetic acid in the intact pea plant (Pisum sativum L.)

The velocity and intensity of basipetal transport of ¹⁴C-labelled indol-3yl-acetic acid (IAA) applied to the apical bud of the intact pea plant were influenced by the temperature to which the stem was exposed and were not influenced by changes in the temperature of the root system when this was controlled independently between 5°C and 35°C. The velocity of transport increased steadily with temperature to a maximum in excess of 35°C and then fell sharply with further increase in temperature. The Q₁₀ for velocity, determined from Arrhenius plots, was low (ca. 1.3). Transport intensity increased to a maximum at about 25°C (Q₁₀=2.2) and then declined gradually with further increase in temperature. It is suggested that transport velocity and transport intensity are controlled independently.The characteristics of auxin transport through the stem were not affected by removal of the root system, or by the withdrawl of root aeration. Labelled IAA did not pass a region of the stem cooled to about 1.0°C, or through a narrow zone of stem tissue killed by heat treatment. In the latter case the heat treatment was shown not to interfere with the upward transport of water in the xylem. Labelled IAA continued to move into, and to accumulate in, the tissues immediately above a cooled or heat-killed region of the stem. It was concluded that the long-distance basipetal transport of auxin through the stem of the intact plant is driven by the transporting cells themselves and is independent of the activity of sinks for the transported auxin.The fronts of the observed tracer profiles in the stem were closely fitted by error function diffusion analogue curves. However, diffusion of IAA alone could not account for the observed characteristics of the transport and it is suggested that the curvilinear fronts of the profiles resulted from a diffusive mixing of exogenous IAA (or IAA-carrier complexes) with endogenous IAA already in the transport pathway.Abbreviations IAA indol-3yl-acetic acid - IAAsp indol-3yl-acetyl aspartic acid - CFM methyl 2-chloro-9-hydroxyfluorene-9-carboxylate (morphactin) - TIBA 2,3,5-triiodobenzoic acid - ABA abscisic acid     

Cell length,light and14C-labelled indol-3yl-acetic acid transport inPisum satisum L. andPhaseolus vulgaris L

The putative auxin-transporting cells of the intact herbaceous dicotyledon are the young, differentiating vascular elements. The length of these cells was found to be considerably greater in dwarf (Meteor) than in tall (Alderman) varieties ofPisum sativum L., and to be greater in etiolated than in light-grown plants ofP. sativum cv Meteor andPhaseolus vulgaris L. cv Mexican Black. Under given light conditions during transport these large differences in cell length did not influence the shapes of the transport profiles or the velocity of transport of¹⁴C-labelled indol-3yl-acetic acid (IAA) applied to the apical bud. However, in both etiolated and light-grown bean and dwarf pea plants the velocity of transport in darkness was ca. 25% lower than that in light. Under the same conditions of transport velocities in bean were about twice those observed in the dwarf pea. Exposure to light during transport increased the rate of export of¹⁴C from the labelled shoot apex in green dwarf pea plants but not in etiolated plants. The light conditions to which the plants were exposed during growth and transport had little effect on the rates of uptake of IAA from the applied solutions. The results indicate that the velocity of auxin transport is independent of the frequency of cell-to-cell interfaces along the transport pathway and it is suggested that in intact plants auxin transport is entirely symplastic.     

An assessment of auxin-promoted transport in decapitated stems and whole shoots of Phaseolus vulgaris L.

¹⁴C-photosynthate transfer in decapitated stems of P. vulgaris plants, treated with IAA (indol-3yl-acetic acid), appeared, as ascertained by microautoradiography, to be restricted to cells of sieve-element appearance. The IAA-induced promotion of photosynthate transport was found not to depend on any artifacts caused by the decapitation procedure. Rather, decapitation primarily served the purpose of removing photosynthate sources above the point of hormone application which otherwise suppressed the expression of the IAA effect on acropetal photosynthate transport. Furthermore, by manipulating stem levels of endogenous auxins with the inhibitor of polar auxin transport, 1-(2¹-carboxyphenyl)-3-phenylpropane-1,3-dione (ACP1.55), evidence was obtained indicating that photosynthate transfer to the shoot apex depended, at least in part, on endogenous levels of auxins at site(s) remote from the apical sink (i.e. shoot apex).Abbreviations ACP1.55 1-(2¹-carboxyphenyl)-3-phenylpropane-1,3-dione - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - IAA indol-3yl-acetic acid     

Transport of exogenous auxin in two-branched dwarf pea seedlings (Pisum sativum L.)

Dwarf pea plants bearing two cotyledonary shoots were obtained by removing the epicotyl shortly after germination, and the patterns of distribution of ¹⁴C in these plants was investigated following the application of [¹⁴C]IAA to the apex of one shoot. Basipetal transport to the root system occurred, but in none of the experiments was ¹⁴C ever detected in the unlabelled shoot even after transport periods of up to 48 h. This was true both of plants with two equal growing shoots and of plants in which one shoot had become correlatively inhibited by the other, and in the latter case applied whether the dominant or subordinate shoot was labelled. In contrast, when [¹⁴C]IAA was applied to a mature foliage leaf of one shoot transfer of ¹⁴C to the other shoot took place, although the amount transported was always low. Transport of ¹⁴C from the apex of a subordinate shoot on plants bearing one growing and one inhibited shoot was severely restricted compared with the transport from the dominant shoot apex, and in some individual plants no transport at all was detected. Removal of the dominant shoot apex rapidly restored the capacity of the subordinate shoot to transport apically-applied [¹⁴C]IAA, and at the same time led to rapid cambial development and secondary vascular differentiation in the previously inhibited shoot. Applications of 1% unlabelled IAA in lanolin to the decapitated dominant shoot maintained the inhibition of cambial development in the subordinate shoot and its reduced capacity for auxin transport. These results are discussed in relation to the polarity of auxin transport in intact plants and the mechanism of correlative inhibition.Abbreviations IAA Indol-3-yl-acetic acid - TIBA 2,3,5-triiodobenzoic acid - 2,4D 2,4-dichlorophenoxyacetic acid - IAAsp Indol-3-yl-acetyl aspartic acid     

Auxin gradient along the root of the maize seedling

[5-³H]Indol-3yl-acetic acid (IAA) applied to the shoot apices of intact 6-day-old maize (Zea mays L.) plants moved into the primary root and accumulated at the root apex. IAA from the shoot could partially satisfy the requirement of the primary root for IAA for growth.Abbreviation IAA indol-3yl-acetic acid     

Abscisic acid and apical dominance in Phaseolus coccineus L.

Abscisic acid (ABA) in lanolin, applied to the internode of decapitated runner bean plants enhances the outgrowth of lateral buds. The optimum concentration of the paste is 10^-5 M. The effect of ABA is counteracted by indoleacetic acid (IAA) but not by gibberellic acid (GA₃). There is no effect when ABA is applied to the apical bud or lateral buds of intact plants. However, 13.2 ng given to the lateral buds of decapitated plants stimulate their growth, whereas higher concentrations are inhibitory. Consequently, ABA enhances growth of lateral buds directly, but only when apical dominance is already weakened. The growth of the decapitated 2^nd internode was not affected by ABA. Radioactivity from [2-¹⁴C] ABA, applied to nonelongating 2^nd internode stumps of decapitated runner bean plants moves to the lateral buds, whereas [1-¹⁴C]IAA-and [³H]GA₁-translocation is much weaker. ABA transport is inhibited if IAA or [³H]GA₁ is applied simultaneously. In elongating internodes [¹⁴C]ABA is almost completely immobile. [¹⁴C]IAA-and [³H]GA₁-translocation is not affected by ABA. The amount of radioactivity from labelled ABA, translocated to the lateral buds, is highest during the early stages of bud outgrowth.Abbreviations ABA 2,4-cis, trans-(+)-abscisic acid - GA gibberellic acid - IAA indoleacetic acid - p.l. plain lanolin