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.     

Decarboxylation and transport of auxin in segments of sunflower and cabbage roots

Summary The movement of ¹⁴C from indole-3-acetic acid (IAA) ¹⁴C has been examined in 5 mm root segments of dark-grown seedlings of Helianthus annuus and Brassica oleracea. Contaminants from distilled water, phosphate buffer and the razor-blade cutter increase the decarboxylation of IAA-¹⁴C, and cutting of root segments results in an activation of IAA-destroying enzymes at the cut surfaces. When these sources of errors were eliminated the following was shown: a) Both in sunflower and cabbage there is a slight acropetal flux of ¹⁴C through the root segments into the agar receiver blocks. The amount of ¹⁴C found in the receiver blocks increases with the lenght of the transport period. b) When the root segments, after the transport period, are cut in two equal parts and these assayed separately, the amounts of ¹⁴C in the two parts indicate a greater acropetal than basipetal transport. c) The total radioactivity of the receiver blocks is in part due to IAA-¹⁴C and in part to ¹⁴CO₂, the latter being a result of enzymatic destruction of auxin. d) Addition of ferulic acid, an inhibitor of IAA oxidases, to the receiver blocks markedly inhibits the decarboxylation of IAA-¹⁴C and thus increases the amount transported. This effect is more pronounced after a 20 hr than after a 6 hr transport period.     

Disruption of the Polar Auxin Transport System in Cotton Seedlings following Treatment with the Defoliant Thidiazuron

The effect of the defoliant thidiazuron (TDZ) on basipetal auxin transport in petiole segments isolated from cotton (Gossypium hirsutum L. cv LG102) seedlings was examined using the donor/receiver agar block technique. Treatment of intact seedlings with TDZ at concentrations of 1 micromolar or greater resulted in a dose-dependent inhibition of ¹⁴C-IAA transport in petiole segments isolated 1 or 2 days after treatment. Using 100 micromolar TDZ, the inhibition was detectable 19 hours after treatment and was complete by 27 hours. Both leaves and petiole segments exhibited a marked increase in ethylene production following treatment with TDZ at concentrations of 0.1 micromolar or greater. The involvement of ethylene in this TDZ response was evaluated by examining the effects of two inhibitors of ethylene action: silver thiosulfate, 2,5-norbornadiene. One day after treatment, both inhibitors effectively antagonized the TDZ-induced inhibition of auxin transport. Two days after TDZ treatment both inhibitors were ineffective. The decrease in IAA transport in TDZ treated tissues was associated with increased metabolism of IAA. The transport of ¹⁴C-2,4-dichlorophenoxyacetic acid was also inhibited by TDZ treatment. This inhibition was not accompanied by increased metabolism. Incorporation of TDZ into the receiver blocks had no effect on auxin transport. The ability of the phytotropin N-1-naphthylphthalamic acid to stimulate IAA uptake from a bathing medium was reduced in TDZ-treated tissues. This reduction is thought to reflect a decline in the auxin efflux system following TDZ treatment.     

Nature of cell-to-cell transfer of auxin in polar transport

Summary By application of agar blocks (side blocks) against the inner and outer epidermis of maize (Zea mays L.) coleoptiles whose cuticle has been abraded it is found that radioactive auxin in the polar transport stream exchanges rapidly with the tissue's free space and therefore does not move confined within the symplast. Polar transport of IAA is demonstrable in Avena coleoptile segments plasmolyzed in 0.5 and 0.7 M mannitol, in which most of the plasmodesmatal connections between successive cells in the polar transport pathway appear to have been broken. We conclude that during polar transport IAA probably moves from cell to cell by crossing the plasmalemmas and the free space between successive cells, rather than via plasmodesmata. Auxin in the polar transport stream exchanges rapidly with side blocks by a cyanide-and azide-insensitive, presumably passive, process. A similarly passive uptake takes place into the cells from an external donor. NPA almost completely inhibits efflux from the polar transport stream even though it does not inhibit uptake; its inhibition of efflux is completely reversed by azide or cyanide. These findings are compatible either with the traditional model of polar transport as passive uptake combined with an active basal efflux pump for IAA, or with the model of purely passive polar transport driven by pH and/or potential differences across the plasma membrane, provided certain ad hoc assumptions are made about the characteristics of the IAA anion carrier that would be operating in either model.Abbreviations IAA indoleacetic acid - NPA N-1-naphthylphthalamic acid - TIBA 2,3,5-triiodobenzoic acid     

Influence of 2,3,5-Triiodobenzoic Acid and 1-N-Naphthylphthalamic Acid on Indoleacetic Acid Transport in Carnation Cuttings: Relationship with Rooting

³H-IAA transport in excised sections of carnation cuttings was studied by using two receiver systems for recovery of transported radioactivity: agar blocks (A) and wells containing a buffer solution (B). When receivers were periodically renewed, transport continued for up to 8 h and ceased before 24 h. If receivers were not renewed, IAA transport decreased drastically due to immobilization in the base of the sections. TIBA was as effective as NPA in inhibiting the basipetal transport irrespective of the application site (the basal or the apical side of sections). The polarity of IAA transport was determined by measuring the polar ratio (basipetal/acropetal) and the inhibition caused by TIBA or NPA. The polar ratio varied with receiver, whereas the inhibition by TIBA or NPA was similar. Distribution of immobilized radioactivity along the sections after a transport period of 24 h showed that the application of TIBA to the apical side or NPA to the basal side of sections, increased the radioactivity in zones further from the application site, which agrees with a basipetal and acropetal movement of TIBA and NPA, respectively. The existence of a slow acropetal movement of the inhibitor was confirmed by using ³H-NPA. From the results obtained, a methodological approach is proposed to measure the variations in polar auxin transport. This method was used to investigate whether the variations in rooting observed during the cold storage of cuttings might be related to changes in polar auxin transport. As the storage period increased, a decrease in intensity and polarity of auxin transport occurred, which was accompanied by a delay in the formation and growth of adventitious roots, confirming the involvement of polar auxin transport in supplying the auxin for rooting. Received April 19, 1999; accepted December 2, 1999     

Morphogenesis in selaginella: auxin transport in the stem

Selaginella willdenovii Baker is a prostrate vascular cryptogam with a dorsiventral stem. At each major branching of the stem apex a dorsal and a ventral angle meristem is formed. The ventral meristem becomes determined as a root, and the dorsal meristem as a shoot. The present investigation examined the distribution and transport of ¹⁴C-indoleacetic acid through stem tissues as a basis for the pattern of meristem determination. Externally applied indoleacetic acid is transported into receiver blocks with a velocity of 12 millimeters per hour. Much of the auxin becomes immobilized in the tissue and is not transported. The polar ratio of auxin transport is approximately 2. Auxin is transported equally on the dorsal and the ventral sides of the stem axis, and the auxin flux in vascular tissue is twice that in the cortex. In the branch junctions twice as much auxin is transported on the dorsal side as on the ventral side, and this is held to be the consequence of the lateral branch vascular tissue connecting with the dorsal and median, but not with the ventral vascular strand of the stem axis.     

Auxin transport in roots

Summary The reasons underlying the initial increase and subsequent decrease in the amount of radioactivity in the receiver block at the apical end of a Zea root segment supplied with a basal donor block containing labelled IAA have been investigated.The phenomenon was observed in segments supplied with IAA-1-¹⁴C, IAA-2-¹⁴C and IAA-5-³H. An acropetal polarity in the movement of radioactivity into the receiver blocks was observed using donor blocks containing IAA-5-³H at concentrations as low as 10^-10M.The decrease in the amount of radioactivity in the receiver block begins after 6–8 h of transport at 25° C, and is unaffected by renewal of the donor block every 2 h, or the presence of 2% sucrose in the donor and receiver blocks.The net export of radioactivity into the receiver block at the apical end of the segment virtually ceases after 6–8 h of transport at 25° C, and is not prolonged by the presence of 2% sucrose in the donor and receiver blocks. At 10° C, net export of radioactivity continues for at least the first 50 h of transport, and the amount of radioactivity in a continuously applied receiver block continues to increase over this period.Receiver blocks removed from the apical end of segments after 8 h of transport and placed on planchettes show little or no decrease in the amount of radioactivity they contain as a function of time, in marked contrast to those left in contact with the segment.There is a marked, and metabolically dependent, resorption of radioactivity from the receiver block at the apical end of the segment after about 8 h of transport at 25° C; most of the resorbed radioactivity remains in the apical 2–4 mm of the segment.There is a loss of radioactive CO₂ from segments supplied with a basal donor block containing 10^-6M IAA-1-¹⁴C at 25° C, the emission beginning after 6–8 h of transport. Segments similarly supplied with 10^-6M IAA-2-¹⁴C did not begin to lose radioactive CO₂ until after about 10–12 h of transport.The ability of the segments to transport radioactivity in a polar manner declines with time after they are excised from the root, regardless of whether their cut ends are kept in the intervening period in contact with plain agar blocks, or ones containing unlabelled IAA at 10^-6M. By the 6th h after excision at 25° C no transport of radioactivity through the segments and into the receiver blocks could be detected in either the aropetal or basipetal direction.The decrease in radioactivity in the receiver block after transport periods of 6–8 h at 25° C is therefore due to (1) a cessation of net export of radioactivity into the block, and (2) the onset of a metabolically-dependent, net resorption of radioactivity. At this time substantial amounts of radioactive CO₂ begin to be evolved from segments supplied with IAA-1-¹⁴C, whereas with IAA-2-¹⁴C radioactive CO₂ is not evolved for a further 4–6 h.     

The movement of 2,4-dichlorophenoxy acetic acid in root segments of Pisum sativum L.

Summary An acropetal polarisation of the movement of 2,4-dichlorophenoxy acetic acid (2,4-D) through subapical segments of Pisum seedling primary roots has been monitored throughout a 60 h transport period in darkness at 25° C using [1-¹⁴C]2,4-D and [2-¹⁴C]2,4-D. Uptake of 2,4-D does not proceed at a constant rate; periods in which the amount of ¹⁴C in the root segments and receiver blocks increases rapidly are followed by periods in which the amount of radioactivity remains relatively constant or declines slightly. These oscillations do not appear to be related to the time of day at which the experiments are begun or ended. Immobilisation and degradation of 2,4-D during transport in the segments seems to be low. Replacement of [1-¹⁴C]2,4-D donor blocks after 25 h by blocks containing unlabelled 2,4-D results in continued transport of the compound into receiver blocks, with only small amounts of ¹⁴C remaining in the root tissues. Radioactivity is also exported from the segments into the blocks used to replace the donor blocks, with larger amounts being exported into the blocks applied to the apical ends than into those applied to the basal ends of the segments. This radioactivity may be taken-up again by the segments but more ¹⁴C is exported into these blocks towards the end of the experiments. The possibility of regular oscillations in uptake and movement of 2,4-D in Pisum root segments is discussed.     

Polar Transport of Auxin across Gravistimulated Roots of Maize and Its Enhancement by Calcium

The effect of Ca on the polar movement of [³H]indoleacetic acid ([³H] IAA) in gravistimulated roots was examined using 3-day-old seedlings of maize (Zea mays L.). Transport of label was measured by placing an agar donor block containing [³H]IAA on one side of the elongation zone and measuring movement of label across the root into an agar receiver block on the opposite side. In vertically oriented roots, movement of label across the elongation zone into the receiver was slight and was not enhanced by incorporating 10 millimolar CaCl₂ into the receiver block. In horizontally oriented roots, movement of label across the root was readily detectable and movement to a receiver on the bottom was about 3-fold greater than movement in the opposite direction. This polarity was abolished in roots from which the caps were removed prior to gravistimulation. When CaCl₂ was incorporated into the receivers, movement of label across horizontally oriented intact roots was increased about 3-fold in both the downward and upward direction. The ability of Ca to enhance the movement of label from [³H]IAA increased with increasing Ca concentration in the receiver up to 5 to 10 millimolar CaCl₂. With the inclusion of CaCl₂ in the receiver blocks, gravity-induced polar movement of label into receiver blocks from applied [³H]IAA was detectable within 30 minutes, and asymmetric distribution of label within the tissue was detectable within 20 minutes. The results indicate that gravistimulation induces a physiological asymmetry in the auxin transport system of maize roots and that Ca increases the total transport of auxin across the root.     

Transport and metabolism of indole-3-acetic Acid in coleus petiole segments of increasing age

Transport and metabolism of IAA-1-¹⁴C in Coleus blumei Benth. was studied by means of a combination of liquid scintillation counting, autoradiography and thin-layer chromatography. Transport of IAA in petiole segments of increasing age (No. 2-8) was strictly polar in a basipetal direction. No acropetal movement occurred in either young or old tissues. The greatest amount, expressed as a percentage of the radioactivity lost from the donor block, was found in basal receivers on petiole number 2. There was gradually less transport in older segments. The recovery as a percentage of the radioactivity not accounted for by donor and receiver blocks, measured by counting the radioactivity in an acetonitrile-extract of petiole segments, was low: 25 to 50%. In this acetonitrile-soluble fraction evidence for different radioactive compounds was found, depending on the age of the tissue. A possible relationship between the amounts of auxin transported in the tissue and its corresponding metabolism is discussed.     

Cell wall pH and auxin transport velocity

According to the chemiosmotic polar diffusion hypothesis, auxin pulse velocity and basal secretion should increase with decreasing cell wall pH. Experiments were designed to test this prediction. Avena coleoptile sections were preincubated in either fusicoccin (FC), cycloheximide, pH 4.0, or pH 8.0 buffer and subsequently their polar transport capacities were determined. Relative to controls, FC enhanced auxin (IAA) uptake while CHI and pH 8.0 buffer reduced IAA uptake. Nevertheless, FC reduced IAA pulse velocity while cycloheximide increased velocity. Additional experiments showed that delivery of auxin to receivers is enhanced by increased receiver pH. This phenomenon was overcome by a pretreatment of the tissue with IAA. Our data suggest that while acidic wall pH values facilitate cellular IAA uptake, they do not enhance pulse velocity or basal secretion. These findings are inconsistent with the chemiosmotic hypothesis for auxin transport.     

Effects of ethylene on auxin transport

The effect of ethylene on the uptake, distribution and polar transport of C¹⁴ from indole-3-acetic acid-2-C¹⁴ and naphthalene acetic acid-1-C¹⁴ in tissue sections was studied. Test species were cotton (Gossypium hirsutum, L.) and cowpea (Vigna sinensis, Endl.). Generally, incubation of tissue or intact plants with ethylene reduced the degree of polar auxin transport. Ethylene inhibited the movement of both auxins in stem tissue and IAA in petiole tissue of cotton. The effect of ethylene on auxin movement in cow-peas was more complex. Ethylene apparently inhibited transport in younger petiole and stem tissue, but stimulated the process to a small but significant degree in basal petiole segments.

Ethylene, in some experiments, reduced C¹⁴ (auxin) uptake. This reduction was consistently smaller than the inhibition of transport. Effects upon transport were observed when uptake was not different. Differences in uptake declined as the period of incubation with auxin was lengthened, but transport was inhibited for up to 23 hours.

It is proposed that ethylene may, through its effect on transport, cause localized shortages and surpluses of auxin which in turn contribute to symptoms now associated with the response of sensitive species to ethylene.

     

A mechanism of respiration-dependent water uptake enhanced by auxin

Summary There are many contradictory observations on the mechanohydraulic relation of growing higher plant cells and tissues. Graphical analysis of the simultaneous equations which govern irreversible wall yielding and water absorption has made more comprehensive the understanding of this relation when relative growth rate is plotted against turgor pressure. It suggests that some respiration-dependent and auxin sensitive process might regulate the difference of osmotic potential between cells and water source. Based on anatomical and electrophysiological knowledge of the pea stem xylem, we propose the wall canal system as the mechanism of respiration-dependent water uptake which is sensitive to auxin. This system consists of the xylem apoplastic walls, the xylem proton pumps, active solute uptake system and cell membranes. In the simplest case, third-order simultaneous differential equations are involved. Numerical analysis showed that net uptake of solutes enables water to be taken up against an opposing gradient of water potential. The behaviour of this wall canal system describes well the mechano-hydraulic relation of enlarging plant cells and tissues. Recent typical, but incompatible, interpretations of this relation are critically discussed based on our model.Abbreviations V the volume of enlarging symplast - the average extensibility of the wall - Pⁱ turgor pressure - Y the yield threshold of the wall - L the relative hydraulic conductance - the solute reflection coefficient of the plasmamembrane - Cⁱ the osmotic concentration of the symplast cells - C^x the osmotic concentration of the xylem vessels - P^x hydrostatic pressure in the xylem vessels - R the gas constant - T absolute temperature - ^o water potential of xylem fluid - ⁱ water potential of symplast cells     

Abscisic Acid Action on Basipetal Auxin Transport

Several experiments have been performed to analyse the ABA effects on the basipetal transport of IAA-2-¹⁴C, using sections of epicotyls prepared from etiolated Lens seedlings. The sections were incubated in an ABA solution or ABA was applied in the donor blocks containing IAA. For each type of assay, the uptake (analyses of the donor blocks) and the movement of IAA-C¹⁴ (analyses of the receiver blocks) were inhibited by ABA. The distribution of continuous decrease of the radioactivity, along the sections' axis, showed a ¹⁴C level from the apical towards the basal segments. ABA caused a decrease in the ¹⁴C concentration for the total sections, but a relative increase for the basal segment. When ABA was applied simultaneously with IAA in the donor blocks, the transport velocity of IAA, through the sections, was not changed significantly, while an ABA pretreatment caused a significant decrease.     

Auxin Transport in Tendril Segments of Passiflora caerulea

The movement of auxin through tendril segments of Passiflora caerulca L. has been investigated using IAA-2-¹⁴C. It has been shown that (1) flux of IAA through the segments is strongly polarized basipetally: (2) the amount of ¹⁴C recovered in the basal receiver blocks increases linearly within a transport period of 6 h; (3) velocity of basipetal transport is 14.5 mm h^?1; (4) at least 70% of the radioactivity in the receiver blocks is confined to the IAA molecule: approximately 55% of ¹⁴C from methanolic extracts of the segments is IAA: (5) at low temperatures (2–4°C) the basipetal transport is abolished; (6) white light promotes basipetal transport, and this effect is abolished in a CO₂-free atmosphere; (7) no difference could be detected in ¹⁴C content between dorsal and ventral halves of tendril segments nor among individual dorsal and ventral receiver blocks.     

The transportable auxin pool

Evidences from experiments with stem sections of sunflower seedlings suggest that the transport of auxin may be limited by a restricted pool size of transportable auxin and restrictions in the availability of transport sites. A steady state of transport is observed over a range of lengths of stem sections, and over a wide range of auxin contents. The capacity of the sections to transport a pulse of auxin declines with aging after cutting, 50% decline occurring at about 10⁺ hours; the transportability of a pulse of auxin declines rapidly after the completion of uptake, 50% decline occurring at about 1 hour. A chase treatment with unlabeled auxin does not alter transport, but a pretreatment with auxin depressed subsequent transport for about 1 hour. In depleted tissues such pretreatment is not inhibitory but rather is promotive of transport. The interpretation offered is that transport is limited by the pool size and transport sites, and roles for these factors are suggested in relation to the auxin transport gradient and the tropistic responses.     

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     

Quantitative predictions for the chemiosmotic uptake of auxin

1. The predictions of a general kinetic model for the chemiosmotic uptake of auxin and other weak acids are compared with experimental results for the auxin indoleacetic acid. The proposed mechanism involves diffusional flux of undissociated acid, a saturable, voltage-sensitive flux of anion (A^-), and a carrier-mediated symport of H⁺ and A^-, all operating in parallel. During much of uptake, the electrochemical gradients are such that the net symport and the net anion flux are in opposition: the symport contributes more to influx; the anion path, to efflux. The voltage-sensitive flux of A^- therefore constitutes a leak. 2. The presence of a symport, whose carrier can distribute across the membrane in response to the internal and external concentrations of auxin, can speed the rate of uptake, but does not by itself alter the accumulation of auxin at equilibrium. 3. The accumulation ratio at equilibrium is less at low concentrations of auxin than at higher concentrations, indicating the presence of a saturable anion path. The concentration dependence of the transition depends on several factors, and is not a reliable indicator of the A^--carrier binding constant. 4. Observed uptake near neutral pH appears larger than is consistent with a voltage-sensitive anion flux being the only carrier-mediated path across the membrane. This observation provides indirect evidence for the presence of an auxin-proton symport in addition to a saturable A^- carrier. 5. The change in kinetics of uptake of [³H]indole-3-acetic acid (IAA), observed as the total concentration of IAA is raised from 0.1 to 100 M, is consistent with either (i) a symport that saturates at low concentrations, or (ii) activation of an A^- efflux by intermediate concentrations of auxin. 6. The data on the concentration dependence of uptake of auxin are not consistent with a multi-proton symport.Abbreviations A^- auxin anion - HA weak acid, particularly IAA - HXA carrier in electroneutral complex with a proton and the auxin anion - H₂XA carrier in electroneutral complex with two protons and the auxin anion - IAA indole-3-acetic acid - X auxin carrier - XA carrier-auxin anion complex     

Transport and Metabolism of Indol-3yl-(Acetic Acid-2-14C) in Pea Roots

¹⁴C from indol-3-yl-(acetic acid-2-¹⁴C) (IAA-¹⁴C) was transportedin a weak but definitely polar manner through segments of youngand matured regions of pea roots. Greater quantities of ¹⁴C-labelledmaterial moved acropetally than basipetally. Up to 70 per centof radioactivity originally present in donor agar blocks wastaken up by the root segments, but only approximately 2 to 3per cent of this emerged into the receiver agar blocks. Anydifferences in uptake, transport, or binding of auxin were veryslight in the three regions of root studied. The IAA-¹⁴C wasmetabolized during passage through the root segments, yieldingtwo principal radioactive products. The identities of thesewere not determined, but they appeared to have auxin activityand may be formed spontaneously, but more slowly, in solutionsof IAA-¹⁴C. IAA-¹⁴C was transported into receiver blocks morereadily than its radioactive derivatives.