Effects of Auxin Transport Inhibitors on Gibberellins in Pea

The effects of the auxin transport inhibitors 2,3,5-triiodobenzoic acid (TIBA), 9-hydroxyfluorene-9-carboxylic acid (HFCA), and 1-N-naphthylphthalamic acid (NPA) on gibberellins (GAs) in the garden pea (Pisum sativum L.) were studied. Application of these compounds to elongating internodes of intact wild type plants reduced markedly the endogenous level of the bioactive gibberellin A₁ (GA₁) below the application site. Indole-3-acetic acid (IAA) levels were also reduced, as was internode elongation. The auxin transport inhibitors did not affect the level of endogenous GA₁ above the application site markedly, nor that of GA₁ precursors above or below it. When plants were treated with [¹³C,³H]GA₂₀, TIBA reduced dramatically the level of [¹³C,³H]GA₁ recovered below the TIBA application site. The internodes treated with auxin transport inhibitors appeared to be still in the phase where endogenous GA₁ affects elongation, as indicated by the strong response to applied GA₁ by internodes of a GA₁-deficient line at the same stage of expansion. On the basis of the present results it is suggested that caution be exercised when attributing the developmental effects of auxin transport inhibitors to changes in IAA level alone. Received April 13, 1998; accepted April 14, 1998     

A Role for Auxin and Auxin Transport Inhibitors on the Ca Content of Artificially Induced Parthenocarpic Fruits

Artificially induced parthenocarpic fruits of apples, pears and tomatoes, as well as seeded fruits treated with 2,3,5-triiodobenzoic acid, frequently show symptoms of Ca deficiency and a low Ca content. It was concluded that auxins, probably produced by the seeds, play a significant role in Ca translocation into fruits. Exogenous indoleacetic acid but not 4-chlorophenoxyacetic acid applications could replace the effect of seeds in this respect. Auxin transport, rather than auxin accumulation, seems to be necessary for Ca transport, as can be concluded from the effect of auxin transport inhibitors.     

Combined Effects of Auxin Transport Inhibitors and Cytokinin: Alterations of Organ Development in Tobacco

We have examined the effects of the auxin transport inhibitors1-naphthylphthalamic acid (NPA) and 2,3,5-triiodobenzoic acid(TIBA) on leaf morphogenesis of transgenic Nicotiana tabacum(cv. Xanthi) plants expressing the Agrobacterium tumefacienscytokinin biosynthetic gene, ipt. We have observed the formationof saucer-shaped leaf-like organs at the shoot apex and at lateralbuds. The formation of apical saucer-shaped leaf-like organscan be duplicated by the application of exogenous NPA and cytokininto wild-type tobacco seedlings. We have also observed adventitiousleaf-like organs with altered petiole and blade morphology inthe transgenic plants treated with auxin transport inhibitors.These results suggest that the combination of diminished auxintransport and elevated cytokinin can lead to alterations inleaf development in tobacco. ⁴Present address: Genesis Research and Development Corporation,P.O. Box 50, Auckland, New Zealand     

Aryl-Substituted alpha-Aminooxycarboxylic Acids: A New Class of Auxin Transport Inhibitors

Certain herbicidal aminooxyisovalerate analogs were noted in whole plant phytotoxicity bioassays to cause disoriented roots. Since this symptom is often characteristic of interference with the transport of the plant hormone auxin, the ability of several of these compounds to compete for the N-1-naphthylphthalamic acid (NPA) binding site in corn (Zea mays L.) coleoptile membranes was measured. Significant NPA binding activity was found, expecially for the 2,4-dichlorophenyl analog. Application of structure-activity principles from traditional auxin transport inhibitors to this new class of molecules led to the synthesis of the naphthyl analogue. This molecule was extremely active in competing for NPA binding and in eliciting whole plant growth regulator effects. Possible relationships between these molecules and the mode of auxin transport are discussed.     

Auxin-Promoted Transport of Metabolites in Stems of Phaseolus vulgaris L.: Auxin Dose-Response Curves and Effects of Inhibitors of Polar Auxin Transport

Transport of ¹⁴C-photosynthate in decapitated stems of Phaseolusvulgaris explants was dependent on the concentration of indole-3-aceticacid (IAA) applied to the cut surfaces of the stem stumps. Thephysiological age of the stem influenced the nature of the transportresponse to IAA with stems that had ceased elongation exhibitinga more pronounced response with a distinct optimum. Increasednutrient status of the explants had little influence on theshape of the IAA dose-response curve but increased, by two ordersof magnitude, the IAA concentration that elicited the optimalresponse. Applications of the inhibitor of polar auxin transport,1-(2-carboxyphenyl)-3-phenylpropane-1, 3-dione (CPD), affectedIAA-promoted transport of ¹⁴C-photosynthates. At sub-optimalIAA concentrations, CPD inhibited transport, whereas at supra-optimalIAA concentrations, ¹⁴C-photosynthate transport was marginallystimulated by CPD. Treatment with CPD resulted in a significantreduction in stem levels of [¹⁴C]IAA below the site of inhibitorapplication, while above this point, levels of [¹⁴C]1AA remainedunaltered. The divergent responses of auxin-promoted transportto CPD treatment are most consistent with a remote action ofIAA on photosynthate transport in the decapitated stems. Key words: Auxin, photosynthate, transport     

Genetic Analysis of the Effects of Polar Auxin Transport Inhibitors on Root Growth in Arabidopsis thaliana

Polar auxin transport inhibitors, including N-1-naphthylphthalamicacid (NPA) and 2,3,5-triiodobenzoic acid (TIBA), have variouseffects on physiological and developmental events, such as theelongation and tropism of roots and stems, in higher plants.We isolated NPA-resistant mutants of Arabidopsis thaliana, withmutations designated pir1 and pir2, that were also resistantto TIBA. The mutations specifically affected the root-elongationprocess, and they were shown ultimately to be allelic to aux1and ein2, respectively, which are known as mutations that affectresponses to phytohormones. The mechanism of action of auxintransport inhibitors was investigated with these mutants, inrelation to the effects of ethylene, auxin, and the polar transportof auxin. With respect to the inhibition of root elongationin A. thaliana, we demonstrated that (1) the background levelof ethylene intensifies the effects of auxin transport inhibitors,(2) auxin transport inhibitors might act also via an inhibitorypathway that does not involve ethylene, auxin, or the polartransport of auxin, (3) the hypothesis that the inhibitory effectof NPA on root elongation is due to high-level accumulationof auxin as a result of blockage of auxin transport is not applicableto A. thaliana, and (4) in contrast to NPA, TIBA itself hasa weak auxin-like inhibitory effect. (Received April 12, 1996; Accepted September 2, 1996)     

Effect of Ethylene on Auxin Transport

Ethylene was found to have no influence on auxin transport in hypocotyls of Helianthus annuus and Phaseolus vulgaris; coleoptiles of Zea mays; petiole sections of Gossypium hirsutum, Phaseolus vulgaris, and Coleus blumei. In the experiments described here, the tissues were treated with ethylene only during the 3 hours of polar transport. This short treatment is in contrast to the methods of others who found an effect of ethylene on auxin transport when plants grown in ethylene are used as experimental tissues.     

Modeling Auxin Transport and Plant Development

The plant hormone auxin plays a critical role in plant development. Central to its function is its distribution in plant tissues, which is, in turn, largely shaped by intercellular polar transport processes. Auxin transport relies on diffusive uptake as well as carrier-mediated transport via influx and efflux carriers. Mathematical models have been used to both refine our theoretical understanding of these processes and to test new hypotheses regarding the localization of efflux carriers to understand auxin patterning at the tissue level. Here we review models for auxin transport and how they have been applied to patterning processes, including the elaboration of plant vasculature and primordium positioning. Second, we investigate the possible role of auxin influx carriers such as AUX1 in patterning auxin in the shoot meristem. We find that AUX1 and its relatives are likely to play a crucial role in maintaining high auxin levels in the meristem epidermis. We also show that auxin influx carriers may play an important role in stabilizing auxin distribution patterns generated by auxin-gradient type models for phyllotaxis.     

Plasmodesmata, Tropisms, and Auxin Transport

Attempts were made to disrupt the plasmodesmata between oatcoleoptile cells (Avena saliva L. cv. Victory) by severe plasmolysis.Coleoptiles, allowed to regain turgor after plasmolysis, wereable to execute geotropic and phototropic curvatures and segmentswould grow in response to applied auxin. In coleoptiles similarlytreated, studies with [¹⁴C]IAA have shown that longitudinal,basipetal transport of auxin still takes place and, as in controls,IAA is preferentially redistributed laterally within coleoptilesorientated horizontally. Physical continuity of the symplast of oat coleoptile cellsmay not always be disrupted by severe plasmolysis. Nevertheless,functional continuity appears to be interrupted. Despite this,all the processes involved in the execution of tropistic curvaturesremain intact, including transport of hormones. Plasmodesmatalcontinuity between oat coleoptile cells appears not to be anecessary requirement for auxin transport.     

Effect of Inhibitors of Auxin Transport and of Calmodulin on a Gravisensing-Dependent Current in Maize Roots

Some characteristics of the gravity sensing mechanism in maize root caps were investigated using a bioelectric current as an indicator of gravity sensing. This technique involves the measurement of a change in the current density which arises at the columella region coincidently with the presentation time. Two inhibitors of auxin transport, triiodobenzoic acid and naphthylphthalamic acid, blocked gravitropic curvature but not the change in current density. Two inhibitors of calmodulin activity, compound 48/80 and calmidazolium, blocked both curvature and gravity-induced current. The results suggest that auxin transport is not a component of gravity sensing in the root cap. By contrast, the results suggest that calmodulin plays an intrinsic role in gravity sensing.     

Auxin Transport Inhibitors: IV. EVIDENCE OF A COMMON MODE OF ACTION FOR A PROPOSED CLASS OF AUXIN TRANSPORT INHIBITORS: THE PHYTOTROPINS

The more active members of a proposed class of auxin transport inhibitors have been shown to have the ability to inhibit the active movement of auxin at concentrations where they have little effect on auxin action and no significant auxin activity. They have also been shown to give rise to characteristic biphasic dose-response curves on cress root growth. Based on these physiological similarities and other common physiological properties, it is concluded that they may achieve their effects by a common mode of action which differs from that of other known auxin transport inhibitors. It is suggested that the name "phytotropins" be given to the class of auxin transport inhibitors now defined by a similar mode of action and common chemical properties.     

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 Inhibitors and the Rooting of Hypocotyl Cuttings from Etiolated Mung-bean Vigna radiata (L.) Wilczek Seedlings

The auxin transport inhibitors 2, 3, 5-triiodobenzoic acid (TIBA)and naphthylphthalamic acid (NPA) inhibited adventitious rootformation (ARF) induced by indol-3-butyric acid (IBA) on cuttingsfrom etiolated mung-bean seedlings floated on solutions of thegrowth regulators. The concentrations of TIBA and NPA requiredfor a 25 per cent reduction in ARF with 10 µM IBA wereestimated by linear interpolation to be 11.3 µm and 0.42µM respectively. NPA is a particularly potent inhibitorof IBA-induced ARF. The inhibitory effect of either compoundwas reversible by higher concentrations of IBA. NPA had no effectwhen applied after the auxin treatment. The inhibitory effects of TIBA or NPA could not be explainedby effects on the uptake or metabolism of [2-^14C]IAA. Consideringthis and other evidence, it is suggested that NPA and possiblyTIBA are acting as specific antagonists of auxin in the inductionof ARF. Vigna radiata (L.), mung-bean, root induction, hypocotyl cuttings, auxin inhibitors, indol-3-butyric acid, 2,3,5-triiodobenzoic acid, naphthylphthalamic acid, auxin uptake, auxin metabolism, adventitious roots     

Effects of Inhibitors of Protein Synthesis on Transmembrane Auxin Transport in Cucurbita pepo L. Hypocotyl Segments

Pretreatment of 2?0 mm segments of etiolated zucchini (Cucurbitapepo L.) hypocotyl with cycloheximide (CH) or 2-(4-methyl-2,6-dinitroanilino)-N-methylpropionamide(MDMP) eliminated the stimulation by N-1-naphthylphthalamicacid (NPA) of net uptake of [1-¹⁴C]indol-3yl-acetic acid ([1-¹⁴C]IAA),but had relatively little effect on the net uptake of IAA inthe absence of NPA. The efflux of [1-¹⁴C]IAA from preloadedsegments was not substantially affected by inhibitor pretreatmentin the absence of NPA, but CH pretreatment significantly inhibitedthe reduction of efflux caused by NPA. Pretreatment with CHor MDMP did not affect net uptake by segments of the pH probe[2-¹⁴C]5,5-dimethyl-oxazolidine-2,4-dione ([2-¹⁴C]DMO), or thenet uptake of [¹⁴C]-labelled 3-O-methylglucose ([¹⁴C]3-0-MeGlu),suggesting that neither inhibitor affected intracellular pHor the general function of proton symporters in the plasma membrane.Both compounds reduced the incorporation of label from [³⁵S]methionineinto trichloroacetic acid (TCA)-insoluble fractions of zucchinitissue, confirming their inhibitory effect on protein synthesis. The steady-state association of [³H]IAA with microsomal vesiclesprepared from zucchini hypocotyl tissue was enhanced by theinclusion of NPA in the uptake medium. The stimulation by NPAof [³H]IAA association with microsomes was substantially reducedwhen the tissue was pretreated with CH. However, CH pretreatmentdid not affect the level of high affinity NPA binding to themembranes indicating that treatments did not result in lossof NPA receptors. It is suggested that the auxin transport site on the effluxcarrier system and the receptor site for NPA may reside on separateproteins linked by a third, rapidly turned-over, transducingprotein. Key words: Auxin carriers, auxin efflux, Cucurbita pepo, phytotropin receptors     

Metalloendoprotease Inhibitors Block Fast Axonal Transport

Metalloendoprotease activity that was sensitive to the metal chelator 1,10-phenanthroline and to synthetic dipeptide substrates of the enzyme was detected in homogenates of dorsal root ganglia (DRG) and spinal nerve from the bullfrog. Exposure of an intact in vitro preparation of DRG and spinal nerves to 1,10-phenanthroline led to a dose-dependent depression in the accumulation of fast-transported 3H-labeled protein proximal to a nerve ligature. In nonligated preparations, the chelator treatment reduced the amount of transported protein entering the nerve; no marked effect on the transport rate was observed. Exposure of a desheathed region of spinal nerve to 1,10-phenanthroline, while DRG were maintained in control medium, resulted in a slight depression of fast transport. This effect was not dose dependent over the range that produced a dose response when both DRG and spinal nerve were exposed to the drug. Treatment of DRG and spinal nerve with the metalloendoprotease substrate analogues carbobenzoxy (CBZ)-Ser-Leu-amide or CBZ-Gly-Leu-amide inhibited fast axonal transport, whereas treatment with CBZ-Gly-Gly-amide, which is not a substrate, had no detectable effect on transport. Selective exposure of desheathed nerve trunk to CBZ-Ser-Leu-amide inhibited fast transport, but the effect was less marked than when DRG and nerve trunk were treated. Although previous studies have focused on the role of metalloendoprotease activity in exocytosis, the present data suggest that the enzyme may also be involved in earlier stages of intracellular transport.     

Flavonoids and Auxin Transport Inhibitors Rescue Symbiotic Nodulation in the Medicago truncatula Cytokinin Perception Mutant cre1

Initiation of symbiotic nodules in legumes requires cytokinin signaling, but its mechanism of action is largely unknown. Here, we tested whether the failure to initiate nodules in the Medicago truncatula cytokinin perception mutant cre1 (cytokinin response1) is due to its altered ability to regulate auxin transport, auxin accumulation, and induction of flavonoids. We found that in the cre1 mutant, symbiotic rhizobia cannot locally alter acro- and basipetal auxin transport during nodule initiation and that these mutants show reduced auxin (indole-3-acetic acid) accumulation and auxin responses compared with the wild type. Quantification of flavonoids, which can act as endogenous auxin transport inhibitors, showed a deficiency in the induction of free naringenin, isoliquiritigenin, quercetin, and hesperetin in cre1 roots compared with wild-type roots 24 h after inoculation with rhizobia. Coinoculation of roots with rhizobia and the flavonoids naringenin, isoliquiritigenin, and kaempferol, or with the synthetic auxin transport inhibitor 2,3,5,-triiodobenzoic acid, rescued nodulation efficiency in cre1 mutants and allowed auxin transport control in response to rhizobia. Our results suggest that CRE1-dependent cytokinin signaling leads to nodule initiation through the regulation of flavonoid accumulation required for local alteration of polar auxin transport and subsequent auxin accumulation in cortical cells during the early stages of nodulation.     

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.     

Auxin Stimulates Its Own Transport by Shaping Actin Filaments

The directional transport of the plant hormone auxin has been identified as central element of axis formation and patterning in plants. This directionality of transport depends on gradients, across the cell, of auxin-efflux carriers that continuously cycle between plasma membrane and intracellular compartments. This cycling has been proposed to depend on actin filaments. However, the role of actin for the polarity of auxin transport has been disputed. The organization of actin, in turn, has been shown to be under control of auxin. By overexpression of the actin-binding protein talin, we have generated transgenic rice (Oryza sativa) lines, where actin filaments are bundled to variable extent and, in consequence, display a reduced dynamics. We show that this bundling of actin filaments correlates with impaired gravitropism and reduced longitudinal transport of auxin. We can restore a normal actin configuration by addition of exogenous auxins and restore gravitropism as well as polar auxin transport. This rescue is mediated by indole-3-acetic acid and 1-naphthyl acetic acid but not by 2,4-dichlorophenoxyacetic acid. We interpret these findings in the context of a self-referring regulatory circuit between polar auxin transport and actin organization. This circuit might contribute to the self-amplification of auxin transport that is a central element in current models of auxin-dependent patterning.In addition to its role as central regulator of growth, auxin is involved in pattern formation (Berleth and Sachs, 2001). Auxin-dependent patterning is linked to a directional flow of auxin, a cell-to-cell transport described by a modified chemiosmotic model (Lomax et al., 1995). The self-amplification of cell polarity by a polar auxin flow has been linked with directional intracellular traffic. Positive feedback of auxin on this traffic in combination with mutual competition of neighboring cells for free auxin are central for pattern formation (Merks et al., 2007). This so-called auxin canalization model has been originally deduced from an analysis of vascular bundles in regenerating stems (Sachs, 1969) but was successfully applied to venation in developing leaves (Sachs, 2000) and the patterning of leaf primordia (Reinhard et al., 2000). Thus, patterning would ultimately depend on the directionality of auxin transport.In the meantime, several plant-specific pin-formed (PIN) proteins have been identified as candidates for auxin-efflux carriers (for review, see Chen and Masson, 2006), and despite a long debate on the actual function of these proteins, the most recent results show that they are in fact rate-limiting for auxin efflux (Petrášek et al., 2006). PIN proteins undergo constitutive recycling between plasma membranes and endosomal compartments (Geldner et al., 2001; Paciorek et al., 2005). This recycling seems to be under control of small GTPases, the ADP-ribosylation factors (ARFs), and their associated guanine nucleotide exchange factors (Geldner et al., 2003). Mutation of one of these guanine nucleotide exchange factors is responsible for the phenotype of the Arabidopsis (Arabidopsis thaliana) mutant gnom causing a mislocalization of PIN1 that becomes trapped in intracellular compartments. This cellular mutant phenotype can be phenocopied by treatment of the wild type with brefeldin A, a fungal toxin that selectively blocks ARF-guanine nucleotide exchange factors (Geldner et al., 2001). This suggests that ARF-dependent vesicle trafficking is involved in the polar distribution of PIN proteins and, thus, in cell polarity.The internalization of PIN1 caused by brefeldin A is arrested by the actin inhibitor cytochalasin D (Geldner et al., 2001). Conversely, PIN3 is rapidly internalized upon treatment with cytochalasin (Friml et al., 2002). Moreover, the potent actin inhibitor latrunculin B (LatB) impaired the polar localization of PIN1 in protophloem cells, and with even higher sensitivity, of the auxin-efflux carrier AUX1 (Kleine-Vehn et al., 2006), and inhibition of myosin function with butane-2,3 monoxime inhibited basipetal auxin transport in flower stalks of Arabidopsis (Holweg, 2007). These findings suggest that actin participates in the cycling of some of the PIN proteins.The relation between actin and auxin seems to be bidirectional but complex; as early as 1937, Sweeney and Thimann (1937) demonstrated that auxin stimulates cytoplasmic streaming in oat (Avena sativa) coleoptiles. However, when streaming was inhibited by cytochalasin B, this delayed the onset of auxin transport but left the rate of auxin transport unaltered (Cande et al., 1973). The stimulation of coleoptile growth by auxin is accompanied by a debundling of actin bundles into finer strands (Waller et al., 2002; Holweg et al., 2004).Inhibition of auxin transport impaired the organization of actin in zygotes of the brown alga Fucus and inhibited signal-induced developmental polarity (Sun et al., 2004). Since the cycling of PIN proteins is regulated by auxin itself (Paciorek et al., 2005), there might be a feedback loop between actin and auxin. Consistent with this view, binding sites for 1-N-naphthylphthalamic acid (NPA), an inhibitor of polar auxin transport, have been found to cosediment with actin (Butler et al., 1998).However, models that link the polar localization of the PIN proteins to actin-dependent transport (Muday and Murphy, 2002; Blakeslee et al., 2005) are challenged by experiments where PIN proteins maintained their polar localization, although actin filaments had been eliminated (for instance, by cytochalasin D [Geldner et al., 2001], by low concentrations of LatB [Kleine-Vehn et al., 2006], or by the phytotropin NPA or artificial auxin 2,4-dichlorophenoxyacetic acid [2,4-D; Rahman et al., 2007]).On the other hand, a recent report (Dhonukshe et al., 2008) demonstrated that 2,3,5-triiodobenzoic acid (TIBA) and the phytotropin 2-(1-pyrenoyl) benzoic acid induced actin bundling not only in plants, but also in mammalian and yeast cells, i.e. in cells that are not to be expected to use auxin as signaling compound. This was interpreted as supportive evidence for a role of actin filaments in polar auxin transport. However, it was mentioned in the same work that NPA failed to cause actin bundling in nonplant cells, suggesting that its mode of action must be different. This is consistent with classical work demonstrating that different phytotropins act on different targets (for review, see Rubery, 1990). Summarizing, although actin seems to play a role for the polarity of auxin fluxes, this issue is, first, not simple and, second, far from being understood.The relationship between actin and auxin was studied in the context of patterned cell division using the tobacco (Nicotiana tabacum) cell line BY-2 (Maisch and Nick, 2007). In this cell line, cell division is partially synchronized within a cell file, leading to higher frequencies of files with even cell numbers compared with files with uneven cell numbers. This synchrony can be interrupted by low concentrations of NPA, an inhibitor of polar auxin flux. To address the role of actin in this synchrony, the actin-binding protein mouse talin was overexpressed in those cells, resulting in a bundled configuration of actin and a loss of synchrony similar to the effect of NPA (indicative for a reduced auxin transport). By addition of auxins that are transported in a polar fashion (but not auxin per se), both the normal organization of actin (with fine strands) and the synchrony of cell division could be restored. This demonstrated that debundled actin strands are necessary and sufficient for the synchrony of cell division. However, although being indicative for a functional auxin transport, this synchrony is not a direct measure of auxin transport.To measure auxin transport directly, it would be necessary to administer radioactively labeled auxin to one pole of the file and to quantify the radioactivity recovered in the opposite pole of the file. This is not possible in a tobacco cell culture that has to be cultivated as suspension in a liquid medium. We therefore have returned to the classical Graminean coleoptile system (for a classical review, see Goldsmith, 1977), where auxin has been discovered originally by its polar transport and where auxin transport can be easily measured by following the distribution of radioactively labeled indole-3-acetic acid (IAA) fed to the coleoptile apex. We generated transgenic rice (Oryza sativa) lines expressing the actin-binding protein talin to variable levels. In those lines, as a consequence of talin overexpression, actin filaments were bundled to variable extent. The bundling of actin filaments was accompanied by a reduced polar transport of auxin. We could restore a debundled configuration of actin by addition of exogenous auxin, and by this treatment we were able to restore auxin transport. This rescue was mediated by transportable auxin species, but not by the artificial auxin 2,4-D that lacks polar transport. Using this approach, we can now probe the causal relationship between actin configuration and polar auxin transport directly.