Auxin Transport in Roots: V. EFFECTS OF TEMPERATURE ON THE MOVEMENT OF IAA IN ZEA ROOTS

The effects of temperature on the polar movement of IAA through6-mm and 12-mm segments of Zea mays roots have been investigatedover the range from 1 to 50°C. At all temperatures an acropetal polar movement of IAA predominated,although at low temperatures and at 50°C the 6-mm segmentsshowed a transient basipetal polarity, before the persistentacropetal polarity developed. At 1°C the differences betweenacropetal and basipetal movement of IAA were less distinct thanat the other temperatures. There is, however, a marked metabolically-dependentacropetal movement of IAA through the tissues at 1°C, becausewhen the segments were deprived of oxygen the acropetal movementwas severely reduced while the basipetal movement was reducedto a smaller extent. At 1°C and at 5°C there was alwaysa persistent basipetal polarity of IAA movement through 6-mmand 12-mm segments under anaerobic conditions. The velocity of acropetal movement (mm h^–1) was the samethrough the 6-mm and the 12-mm segments and was markedly affectedby temperature. It increased from 1°C to a maximum valueof 8 mm h^–1 at 31°C and then decreased again at 40and 50°C. The velocity of basipetal movement could be assessedonly at 1 and 5°C at which temperatures it was greater thanthe velocity of acropetal movement, and virtually independentof segment length. The acropetal flux of IAA (cpm h^–1) was much less through12-mm segments than through 6-mm segments. For both lengthsof segment, however, the flux showed a complex relationshipwith ambient temperature, increasing from 1°C to a maximumat 10–15°C, declining to a minimum value at 31°Cand then rising again at 40 and 50°C. The basipetal fluxof IAA could be astimated only at 1 and 5°C at which itwas very much smaller than the acropetal flux. The amount of IAA in the receiver blocks increased linearlywith time at the lower temperatures. At temperatures withinthe range 15°C to about 31°C, however, the amount ofIAA in the receiver blocks began to decline if the transportperiods exceeded a certain length. The time at which this declinein the IAA in the receiver block began was related to the ambienttemperature. Chromatographic analysis indicated one radioactive substancein receiver blocks at the apical end of segments supplied withIAA-1-¹⁴C at the basal end after transport periods of 6 h at25°C, and 72 h at 5°C. The Rf of this substance wasclosely similar to that of the radioactive IAA supplied in thedonor blocks.     

Auxin transport in roots

Summary The movement of IAA has been investigated in roots of dark-grown seedlings of Zea mays using IAA-I-¹⁴C.With 6-mm segments excised 1 mm below the apex of the root it has been shown that: (a) There is a strictly acropetal flux of IAA through the tissues, the amount of IAA found in an apical receiving block increasing almost linearly with increasing transport period up to about 6–7 hours, but thereafter declining for at least a further 18 hours. The onset of this decline appears to be dependent upon the concentration of IAA in the donor block. (b) The amount of IAA recovered in the apical receiving block increases with increasing concentration of IAA in the donor block over the range from 0.1–10 M, with transport periods of both 4 and 9 hours. (c) The radioactivity in the receiving block is confined to the IAA molecule. (d) The orientation of the segment with respect to gravity did not significantly affect the acropetal polar flux of IAA in the tissue.With non-decapitated 7-mm root apices it has been found that the presence of the apex has no effect on the strictly acropetal flux of IAA in the tissues, but that it entirely prevented the emergence of IAA into an apical receiving block.     

Effect of auxin on acropetal auxin transport in roots of corn

Acropetal [¹⁴C]indoleacetic acid (IAA) transport was investigated in roots of corn. At least 40 to 50% of this movement is dependent on activities in the root apex. Selective excision of various populations of cells comprising the root apex, e.g. the root cap, quiescent center, or proximal meristem show that the proximal meristem is the critical region in the apex with regard to influencing IAA movement. The quiescent center has no influence and the root cap has only a minor effect. Excision and replacement of the proximal meristem with an exogenous supply of 10⁻⁸ to 10⁻⁹ molar IAA prevents the reduction in acropetal IAA transport which would normally occur in the absence of this meristem. Substituting 10⁻⁹ molar IAA for the excised root cap brings about a significant increase in the amount of IAA moved acropetally, as compared to intact roots with the root cap still in place. From this and previous work, it is concluded that IAA synthesis occurring in the proximal meristem stimulates the movement of IAA from the basal to apical end of the root.     

Transport of 14C-lableled indoleacetic acid in Vicia root segments

IAA transport in Vicia root segments was investigated for comparisonwith that in intact roots. Lanolin paste (1-mm-wide ring) oragar blocks (3?3?1.5mm), both containing IAA-2-¹⁴C were appliedto the surface or a cut end of the root segments, respectively;transported ¹⁴C was collected in receiver agar blocks placedon the cut end of the segments. When lanolin paste was appliedto 5-mm segments, basipetal transport of IAA predominated overacropetal transport. When agar blocks were applied to 1- and2-mm segments, the same was true; in longer segments (3 and5 mm long), however, basipetal movement occurred predominantlyat first but was surpassed by acropetal movement after 2–3hr. Among the segments tested (regions 2–4, 4–6and 8–10 mm from the tip), the most apical one showedthe distinctest predominancy of basipetal movement. The velocitiesof the acropetal and basipetal movement of the ¹⁴C were estimatedat 3–3.8 and 8–12 mm/hr, respectively. Autoradiographicstudy and the experiment in which wire was inserted longitudinallythrough the central part of the segments showed that basipetalmovement occurred mainly through the outer part of the rootsand acropetal movement mainly through the central cylinder.The present results were compatible with those obtained previouslywith intact roots. Some properties of polar movement, such asits specificity, inhibition by TIBA, and dependency on terneprature are described. (Received March 22, 1978; )     

Transport and Metabolism of Kinetin-8-14C in the Mesocotyl and Coleoptile of Zea mays L.

The movement and metabolism of kinetin-8-¹⁴C in the mesocotyland coleoptile of Zea mays have been investigated. The segments (10 mm) of coleoptile with and without an apexwere used. The presence of apex decreases the quantity of countswhich move basipetally in the segments. The quantity transported acropetally in the segments (apex incontact with receiver blocks) was about three times higher thanthat transported basipetally (apex in contact with donor blocks). Using 10 mm coleoptile segments excised I mm below the apex,there was no appreciable difference between acropetal and basipetalmovement. The transport of kinetin in the mesocotyl was studied with andwithout vascular tissue. The quantity transported in segmentswithout vascular tissue was always lower than in the segmentswith vascular tissue. The presence of the node between the coleoptile and mesocotyldoes not disturb the movement of kinetin in the basipetal position(node in contact with donor blocks). Thin layer chromatography of plant extracts after a 48-hourtransport period revealed several radioactive substances whichappeared to be adenine with R_f 0.5 and other components appearedat R_f 0.6.     

Auxin transport in roots

Summary Light promotes the net acropetal movement of ¹⁴C through 6-mm subapical segments of dark-grown roots of Zea mays supplied at their basal ends with 1 M IAA-1-¹⁴C in agar blocks. This promotion occurs only when the segments are irradiated during the transport period, and both red and blue light appear to be as effective as white light at the radiant flux densities used in this investigation. The promotion is not found if the segments are pretreated with light and then returned to darkness before the trasport of IAA-1-¹⁴C is determined. The very slight basipetal movement of ¹⁴C through the segments supplied with an apical source of IAA-1-¹⁴C is unaffected by light.Only one radioactive substance is found in the apical receiver blocks. This substance has an R_f virtually identical to those of the stock solution of IAA incorporated into the donor block and of unlabelled IAA. The movement of radioactivity into the receiver blocks through, the illuminated segments therefore appears to reflect the movement of IAA. Light thus increases the acropetal movement of IAA through the Zea root segment.The primary roots of Zea mays var. Giant Horse Tooth seedlings grown in total darkness do not exhibit a positive geotropic response. When the seed is orientated with the embryo uppermost the radicle grows out horizontally. On exposure to light, however, the roots bend down. This reaction appears about 3–9 hours after the onset of illumination, and white, red and blue light appear to be equally effective at the flux densities employed in this study. Green light in the spectral band between 510–530 nm did not appear to induce this positive geotropic responsiveness.     

Basipetally polarised transport of [3H]gibberellin A1 and [14C]gibberellin A3, and acropetal polarity of [14C]indole-3-acetic acid transport,in stelar tissues of Phaseolus coccineus roots

Summary Movement of both [³H]GA₁ and [¹⁴C]GA₃ through root segments from P. coccineus seedlings was basipetally polarised. The basipetal/acropetal ratio of radioactivity from [³H]GA₁ in agar receiver blocks was 9.2 for apical, elongating segments, and 4.0 for more basal, non-elongating segments. Polarity of gibberellin transport was restricted to the stele, and absent from cortical tissues. Transport of [¹⁴C]IAA through root segments to agar receivers was preferentially acropetal, particularly so in the stele. Despite the existence of basipetal polarity of gibberellin transport in the root, [³H]GA₁ injected into cotyledons moved into and acropetally along the seedling root.     

Auxin transport in roots

Summary The net uptake and movement of radioactivity by 12-mm root segments of Zea mays have been studied as a function of time at 5, 15 and 25° C. Segments were supplied with an agar donor block containing 1 M IAA-1-¹⁴C or IAA-2-¹⁴C continuously or for a limited period of time (pulse-labelling). In the latter case the original donor block was replaced either by a plain agar block or by one containing 1 M unlabelled IAA. Receiver blocks were placed at the other end of the segments.The net uptake of radioactivity from the donor block at 15° C was greater at the basal end than at the apical end of the segment. At 5 and 15° C, the net uptake from a basal donor was virtually linear with time but at 25° C the rate of net accumulation decreased after about 10 h. Decarboxylation of IAA undoubtedly occurred at 15 and 25° C when the concentration in the tissue attained a high value.An acropetally polarised movement of radioactivity into the receiver blocks occurred regardless of whether the results were based on the actual amounts of radioactivity in the receiver block, or on the amounts in the receiver block expressed as a percentage of the net total radioactivity accumulated from the donor block. Only one radioactive substance was present in the receiver block and it ran to the same R_f as IAA in the isopropanol: ammonium: water solvent system.The amounts of radioactivity moving into that part of the root segment at least 6 mm distant from the end in contact with either an apical or a basal donor block were assessed. An acropetal polarity in the movement of radioactivity was observed on the basis of the actual amounts of radioactivity in these distal parts of the segments, but no such polarity was evident when the amounts of radioactivity were expressed as a percentage of the net total accumulated from the donor block. At least 3 radioactive substances were present in the tissue in addition to the substance running to the same R_f as IAA. The distribution of radioactivity in the segment cannot therefore be used to assess the distribution of IAA.Acropetal movement of radioactivity into an apical receiver block is not dependent upon the continued uptake of IAA at the basal end of the segment. No distinct pulses of radioactivity were detected moving through the root segments.Only a small part of the radioactivity in the root segment appears to be located in the polar transport system, while the bulk is not. The polarity found in the movement of the bulk radioactivity within the segment seems to be related to the polarity in IAA uptake from the donor blocks.     

Maintenance of polar auxin transport in Zea coleoptiles by anaerobic metabolism

Summary The basipetal movement of IAA in 5-mm Zea coleoptile segments is drastically reduced under anaerobic conditions, but it remains greater than acropetal movement which is closely similar in the presence and absence of oxygen. The polarity of IAA movement has thus been confirmed in Zea coleoptile segments which have been deprived of oxygen. This net polar flux is dependent upon anaerobic metabolism since it is abolished in the presence of the metabolic inhibitiors sodium fluoride and iodoacetic acid.Acropetal movement of IAA is unaffected by the presence of sodium fluoride in air or anaerobic conditions. Uptake of IAA from a basal donor is not affected by sodium fluoride in air, but under anaerobic conditions the inhibitor decreased uptake by approximately 13%.Under anaerobic conditions both inhibitors reduce basipetal movement of IAA to the level of acropetal movement, and both decrease the total uptake of IAA from an apical donor by up to 30–45%. Under aerobic conditions sodium fluoride has no marked effect upon either the uptake of IAA from an apical donor or the basipetal movement of IAA by the segments. On the other hand, iodoacetic acid greatly decreased the uptake of IAA by the segments in air, but the same fraction of the total IAA taken up was recovered in the receiving block in the presence and absence of the inhibitor.This research was supported by Grant Number 83/6 to Professor M. B. Wilkins from the U. K. Agricultural Research Council.     

MOVEMENT OF IAA IN SECTIONS FROM SPIDER FLOWER (CLEOME HASSLERIANA) STAMEN FILAMENTS

Movement of IAA in spider flower (Cleome hassleriana Chod.) stamen filaments was studied by placing 2-mm sections horizontally between donor agar blocks containing ¹⁴C-labeled IAA and plain agar receiver blocks and measuring radioactivity in the donor and receiver blocks and filament sections by scintillation counting after the desired transport time. Movement was strictly polar and basipetal at all stages of floral development, except in open flowers just before stamen abscission when the amounts moving acropetally and basipetally were equal. The amount of IAA moved depended upon the stage of development. As buds aged more IAA was moved, until the buds opened and the stamen filaments reached maximum elongation; then the amount of IAA moving basipetally dropped drastically. There was an insignificant amount of acropetal IAA movement except just before stamen abscission. This change in IAA movement is not due to a change in filament diameter. In time-course studies the amount of IAA moved basipetally increased with time up to 5 hr and then declined slightly. The amount of radioactivity retained by sections increased until 8 hr. The amount of IAA moved in tip sections was less than that in mid or base sections; however, this can be partially explained by differences in uptake area of these sections. The relationship of these results to the hypothesis that changes in IAA movement are important in the control of stamen filament elongation and abscission is discussed.     

Auxin, transport and growth in intact roots of Vicia faba

Transport of IAA applied to the intact root of Vicia seedlingswas investigated in relation to root growth. The root was treatedat 3–4, 4–5 or 7–8 mm from the tip with anarrow ring of lanolin paste containing IAA or IAA-2-¹⁴C ingrowth or transport experiments, respectively. The growth processalong the root axis was examined in every 1-mm part from thetip at 30 min, 1 or 4 hr intervals. The elongation zone of thecontrol root was 1–9 mm from the tip. IAA treatment broughtabout no significant change in the growth of the region apicalto the treated site, whereas distinct inhibition of growth occurredin the region basal to the treated site within 1 hr. The transportof radioactivity was observed in both acropetal and basipetaldirections within 1 hr, but the latter predominated for 8 hror more; the nearer to the tip the treatment site, the longerthe predominance lasted. The velocities of acropetal and basipetaltransport were estimated at about 4 and 8 mm/hr, respectively.Autoradiographs of transverse section of roots showed that basipetaltransport occurred mainly through the outer part of the root,whereas acropetal transport occurred mainly through the innerpart, the central cylinder. It may be concluded that the basipetallytransported IAA which passed through the outer part of the rootinhibited the elongation of the intact root. (Received November 25, 1975; )     

Evidence for Three Different Systems of Movement of Indoleacetic Acid in Intact Roots of Phaseolus coccineus

Indoleacetic acid (IAA)-5-³H (2 × 10⁻⁹M) was applied to intact roots of Phaseolus coccineus seedlings, at the apex or 2 cm above the apex, at various pHs and in the presence of Cu²⁺ and NaCl. The transport of label in the roots was then examined after 6 h by cutting the roots into 1 mm sections above and below the zone of treatment. Basipetal movement from 2 cm above the apex was unafected by pH, Cu²⁺ or NaCl. Acropetal movement from the same area decreased with increasing pH from 5.4 to 8.0, probably due to an effect of pH on the entry of IAA into the cells. pH had no effect on sucrose transport. Cu²⁺ also inhibited acropetal movement but NaCl had no effect. Basipetal movement of label from the apex was reduced by Cu²⁺ and increasing pH, but not as much as with acropetal movement, and increased by the presence of NaCl. These facts are interpreted as showing 3 different systems of IAA movement in intact roots: basipetal from 2 cm up the root in some extracellular physical system; acropetal from 2 cm up the root, and basipetal from the apex, in a metabolically dependent intracellular system, but in different tissues of the root. It is proposed that endogenous IAA not only moves into the root from the stem but is also synthesized in the root apex, and moves basipetally for a short distance to the root growing zone in a separate system from the IAA descending from the stem.     

Effects of Cations on Hormone Transport in Primary Roots of Zea mays

We examined the influence of aluminum and calcium (and certain other cations) on hormone transport in corn roots. When aluminum was applied unilaterally to the caps of 15 mm apical root sections the roots curved strongly away from the aluminum. When aluminum was applied unilaterally to the cap and ³H-indole-3-acetic acid was applied to the basal cut surface twice as much radioactivity (assumed to be IAA) accumulated on the concave side of the curved root as on the convex side. Auxin transport in the apical region of intact roots was preferentially basipetal, with a polarity (basipetal transport divided by acropetal transport) of 6.3. In decapped 5 mm apical root segments, auxin transport was acropetally polar (polarity = 0.63). Application of aluminum to the root cap strongly promoted acropetal transport of auxin reducing polarity from 6.3 to 2.1. Application of calcium to the root cap enhanced basipetal movement of auxin, increasing polarity from 6.3 to 7.6. Application of the calcium chelator, ethylene-glycol-bis-(β-aminoethylether)-N,N,N′, N′-tetraacetic acid, greatly decreased basipetal auxin movement, reducing polarity from 6.3 to 3.7. Transport of label after application of tritiated abscisic acid showed no polarity and was not affected by calcium or aluminum. The results indicate that the root cap is particularly important in maintaining basipetal polarity of auxin transport in primary roots of corn. The induction of root curvature by unilateral application of aluminum or calcium to root caps is likely to result from localized effects of these ions on auxin transport. The findings are discussed relative to the possible role of calcium redistribution in the gravitropic curvature of roots and the possibility of calmodulin involvement in the action of calcium and aluminum on auxin transport.     

Movement of Indoleacetic Acid in Coleoptiles of Avena sativa L. II. Suspension of Polarity by Total Inhibition of the Basipetal Transport

Acropetal and basipetal movement of indole-3-acetic acid through coleoptiles of Avena sativa L. was studied. Sections 10-mm long were supplied with either apical or basal sources containing C(14) carboxyl-labeled indoleacetic acid (10(-5)m). Anaerobic conditions inhibit metabolically dependent movement (transport) thus reducing basipetal but not acropetal movement. Total inhibition of basipetal transport abolishes the polarity of auxin uptake and movement. The nonpolar movement that remains in anaerobic sections is free diffusion with an average diffusion coefficient of approximately 1 x 10(-4) mm(2) per second. During an 8-hour diffusion, at least the first millimeter of the section comes to equilibrium at approximately the same concentration as the donor.Acropetal movement is probably by diffusion and is accompanied by an aerobic immobilization of indoleacetic acid that increases more than proportionally to concentration. Anaerobic conditions totally prevent this immobilization and reduce acropetal uptake but not the amount of indoleacetic acid moving into the upper parts of the section; there is, therefore, no evidence for acropetal transport.Polarity of auxin movement in aerobic coleoptile sections is achieved by strict basipetal transport of auxin. The basipetal transport may intensify the polarity by recycling auxin that is moving acropetally.     

Decarboxylation and Transport of Auxin in Segments of Sunflower and Cabbage Roots. II. A Chromatographic Study Using IAA-1-14C and IAA-5-3H

The polar movement of IAA has been examined in 5-mm root segments of Brassica oleracea and Helianthus annum. The movement was studied partly with IAA-1-¹⁴C and partly with IAA-5-³H. In both plants a slight acropetal flux of ¹⁴C and IAA-³H was found through the segments. The recovered radioactivity in the agar receiver blocks and in the receiver end of the segments increased as a function of time. A large portion of the applied IAA was converted on the cut surfaces and in the tissues of the segments. Chromatographic analysis indicated different destruction products when estimated by scintillation counting and by spraying with in-dole reagent (DMCA). Chromatograms run in isopropanol: ammonia: water, 8:1:1, yielded three different substances, one spot near the starting line and one near the front, neither of which has been identified. Finally there was a spot with R_f 0.4–0.6, probably representing IAA.     

Endogenous and exogenous auxin in the control of root growth

The endogenous indol-3yl-acetic acid (IAA) of detipped apical segments from roots of maize (cv ORLA) was greatly reduced by an exodiffusion technique which depended upon the preferential acropetal transport of the phytohormone into buffered agar. When IAA was applied to the basal cut ends of freshly prepared root segments only growth inhibitions were demonstrable but after the endogenous auxin concentration had been reduced by the exodiffusion technique it became possible to stimulate growth by IAA application. The implications of the interaction between exogenous and endogenous IAA in the control of root segment growth are discussed with special reference to the role of endogenous IAA in the regulation of root growth and geotropism.Abbreviations IAA indol-3yl-acetic acid - GC-MS gas chromatography-mass spectrometry     

Transport of shoot- and cotyledon-applied indole-3-acetic acid to Vicia faba root

To clarify the participation of indole-3-acetic acid (IAA) originatingfrom the shoot in root growth regulation and the mechanism ofIAA translocation from shoot to root, the movement of ¹⁴C-IAAwhich was applied to the epicotyl or the cotyledon of Viciafaba seedlings was investigated. The radioactivity of IAA appliedto the cotyledon moved faster to the root tip than that appliedto the epicotyl. On the basis of the effect of 2,3,5-triiodobenzoic acid on IAAmovement, a comparison with ¹⁴C-glucose movement and autoradiographicexamination, the nature of IAA movement was concluded to bepolar transport from the epicotyl to the basal part of the roots,while IAA movement from the epicotyl to the cotyledon, fromthe basal part of roots to the apical part, and from the cotyledonto the epicotyl and to the root took place in the phloem. Theradioactivity from ¹⁴C-IAA applied to the cotyledon accumulatedin lateral root primordia and vascular bundles. These factssuggest that IAA produced in cotyledons may participate in theregulation of Vicia root development. (Received December 21, 1979; )     

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.     

ONTOGENY OF THE INFLORESCENCE OF SAURURUS CERNUUS (SAURURACEAE)

The spicate inflorescence of Saururus cernuus L. (Saururaceae) results from the activity of an inflorescence apical meristem which produces 200–300 primordia in acropetal succession. The inflorescence apex arises by conversion of the terminal vegetative apex. During transition the apical meristem increases greatly in height and width and changes its cellular configuration from one of tunica-corpus to one of mantle (with two tunica layers) and core. Primordia are initiated by periclinal divisions in the subsurface layer. These are “common” primordia, each of which subsequently divides to produce a floral apex above and a bract primordium below. The bract later elongates so that the flower appears borne on the bract. All common primordia are formed by the time the inflorescence is about 4.4 mm long; the apical meristem ceases activity at this stage. As cessation approaches, cell divisions become rare in the apical meristem, and height and width of the meristem above the primordia diminish, as primordia continue to be initiated on the flanks. Cell differentiation proceeds acropetally into the apical meristem and reaches the summital tunica layers last of all. Solitary bracts are initiated just before apical cessation, but no imperfect or ebracteate flowers are produced in Saururus. The final event of meristem activity is hair formation by individual cells of the tunica at the summit, a feature not previously reported for apical meristems.     

Basipetal auxin transport is required for gravitropism in roots of Arabidopsis

Auxin transport has been reported to occur in two distinct polarities, acropetally and basipetally, in two different root tissues. The goals of this study were to determine whether both polarities of indole-3-acetic acid (IAA) transport occur in roots of Arabidopsis and to determine which polarity controls the gravity response. Global application of the auxin transport inhibitor naphthylphthalamic acid (NPA) to roots blocked the gravity response, root waving, and root elongation. Immediately after the application of NPA, the root gravity response was completely blocked, as measured by an automated video digitizer. Basipetal [(3)H]IAA transport in Arabidopsis roots was inhibited by NPA, whereas the movement of [(14)C]benzoic acid was not affected. Inhibition of basipetal IAA transport by local application of NPA blocked the gravity response. Inhibition of acropetal IAA transport by application of NPA at the root-shoot junction only partially reduced the gravity response at high NPA concentrations. Excised root tips, which do not receive auxin from the shoot, exhibited a normal response to gravity. The Arabidopsis mutant eir1, which has agravitropic roots, exhibited reduced basipetal IAA transport but wild-type levels of acropetal IAA transport. These results support the hypothesis that basipetally transported IAA controls root gravitropism in Arabidopsis.