Transport of [3H]IAA label in gravistimulated primary roots of maize

The curvature of roots in response to gravity is attributed to the development of a differential concentration gradient of IAA in the top and bottom of the elongation region of roots. The development of the IAA gradient has been attributed to the redistribution of IAA from the stele to cortical tissues in the elongation region. The gravistimulated redistribution of IAA was investigated by applying [³H]IAA to the cut surface of 5 mm apical primary root segments. The movement of label from the stele-associated [³H]IAA into the root, tip, root cap, and cortical tissues on the top and bottom of the elongation region was determined in vertically growing roots and gravistimulated roots. Label from the stele moved into the region of cell differentiation (root tip) prior to accumulating in the elongation region. Little label was observed in the root cap. Gravistimulation did not increase the amount of label moving from the stele; but gravistimulation did increase the amount of label accumulating in cortical tissues on the lower side of the elongation region, and decreased the amount of label accumulating in cortical tissues on the upper side of the elongation region. Removal of the cap prior to or immediately following gravity stimulation rendered the roots partially insensitive to gravity and also prevented gravity-induced asymmetric redistribution of label. However, removal of the root cap following 30 min of gravistimulation did not alter root curvature or the establishment of an IAA asymmetry across the region of root elongation. These results suggest that a signal originating in the root cap directs auxin redistribution in tissues behind the root cap, leading to the development of an asymmetry of IAA concentration in the elongation region that in turn causes the differential growth rate in the elongation region of a graviresponding root.     

Transport of [3H]IAA label in gravistimulated primary roots of maize

The curvature of roots in response to gravity is attributed to the development of a differential concentration gradient of IAA in the top and bottom of the elongation region of roots. The development of the IAA gradient has been attributed to the redistribution of IAA from the stele to cortical tissues in the elongation region. The gravistimulated redistribution of IAA was investigated by applying [³H]IAA to the cut surface of 5 mm apical primary root segments. The movement of label from the stele-associated [³H]IAA into the root, tip, root cap, and cortical tissues on the top and bottom of the elongation region was determined in vertically growing roots and gravistimulated roots. Label from the stele moved into the region of cell differentiation (root tip) prior to accumulating in the elongation region. Little label was observed in the root cap. Gravistimulation did not increase the amount of label moving from the stele; but gravistimulation did increase the amount of label accumulating in cortical tissues on the lower side of the elongation region, and decreased the amount of label accumulating in cortical tissues on the upper side of the elongation region. Removal of the cap prior to or immediately following gravity stimulation rendered the roots partially insensitive to gravity and also prevented gravity-induced asymmetric redistribution of label. However, removal of the root cap following 30 min of gravistimulation did not alter root curvature or the establishment of an IAA asymmetry across the region of root elongation. These results suggest that a signal originating in the root cap directs auxin redistribution in tissues behind the root cap, leading to the development of an asymmetry of IAA concentration in the elongation region that in turn causes the differential growth rate in the elongation region of a graviresponding root.     

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.     

Maize root growth and localized indol-3yl-acetic Acid treatment: a new methodological approach

Resin beads loaded with indol-3yl-acetic acid (IAA) were used as asymmetrical donors along the elongation zone of intact primary Zea mays L. roots. A strong curvature, towards and above the bead, occurred when IAA was applied at a mean distance of 2.20 mm from the tip. No curvature was detected after applications at 3.89 and 5.71 mm from the tip. Correspondence analysis, a new methodological approach in plant hormone studies, permitted the evaluation of the relative influence of several factors on the curvature observed for each root. The parameters considered were the initial growth rate, the exact location of the bead (1.64-2.73 millimeters from tip) and the quantity of IAA absorbed. Roots which grew rapidly bent earlier than slowly growing ones and the more basal the treatment was, the less curvature occurred. Surprisingly, the amount of IAA taken up (between 1.2 and 2.2 times the endogenous IAA content) was found to have no influence on either the time-course or the magnitude of this growth inhibition (curvature). The usefulness of this multivariate analysis is also discussed.     

Patterns of auxin and abscisic acid movement in the tips of gravistimulated primary roots of maize

Because both abscisic acid (ABA) and auxin (IAA) have been suggested as possible chemical mediators of differential growth during root gravitropism, we compared with redistribution of label from applied ³H-IAA and ³H-ABA during maize root gravitropism and examined the relative basipetal movement of ³H-IAA and ³H-ABA applied to the caps of vertical roots. Lateral movement of ³H-ABA across the tips of vertical roots was non-polar and about 2-fold greater than lateral movement of ³H-IAA (also non-polar). The greater movement of ABA was not due to enhanced uptake since the uptake of ³H-IAA was greater than that of ³H-ABA. Basipetal movement of label from ³H-IAA or ³H-ABA applied to the root cap was determined by measuring radioactivity in successive 1 mm sections behind the tip 90 minutes after application. ABA remained largely in the first mm (point of application) whereas IAA was concentrated in the region 2–4 mm from the tip with substantial levels found 7–8 mm from the tip. Pretreatment with inhibitors of polar auxin transport decreased both gravicurvature and the basipetal movement of IAA. When roots were placed horizontally, the movement of ³H-IAA from top to bottom across the cap was enhanced relative to movement from bottom to top whereas the pattern of movement of label from ³H-ABA was unaffected. These results are consistent with the hypothesis that IAA plays a role in root gravitropism but contrary to the idea that gravi-induced asymmetric distribution of ABA contributes to the response.     

Transport and Degradation of Auxin in Relation to Geotropism in Roots of Phaseolus vulgaris

The movement of auxin in Phaseolus vulgaris roots has been examined after injection of IAA^?3H into the basal root/hypocotyl region of intact, dark-grown seedlings. Only a portion of the applied IAA^?3H was transported unchanged to the root tip. The major part of the chromatographed, labelled compounds translocated to the roots was indole-3-acetylaspartic acid (IAAsp) and an unidentified compound running near the front in isopropanol, ammonia, water. The velocity of the auxin transport (7.2 mm per hour) was calculated from scintillation countings of methanol extracts from serial sections of the root. An accumulation of radioactive compounds in the extreme root tip, was observed 5 h after the injection of IAA. The influence of exogenous IAA on the geotropical behaviour of the bean seedling roots was examined. Pretreated roots were stimulated for 5 min in the horizontal position and then rotated parallel to the horizontal axis of the klinostat for 60 or 90 min. The resulting geotropic curvature of IAA-injected and control roots showed significantly different patterns of development. When the stimulation was started 5 h after application of the auxin, the geotropic curvature became larger in roots of the injected plants than in the controls. If, however, the translocation period was extended to 20 h the geotropic curvature was significantly smaller in the roots of the injected plants. The auxin injection did not significally affect the rate of root elongation. The change in geotropical behaviour of the roots is interpreted as a result of the influence of the conversion products of the applied IAA on the geotropical responsiveness.     

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.     

Effect of calmodulin antagonists on the growth and graviresponsiveness of primary roots of maize

We examined the effect of calmodulin (CaM) antagonists applied at the root tip on root growth, gravity-induced root curvature, and the movement of calcium across the root tip and auxin (IAA) across the elongation zone of gravistimulated roots. All of the CaM antagonists used in these studies delayed gravity-induced curvature at a concentration (1 M) that did not affect root growth. Calmodulin antagonists ( 1M) inhibited downward transport of label from ⁴⁵Ca²⁺ across the caps of gravistimulated roots relative to the downward transport of ⁴⁵Ca²⁺ in gravistimulated roots which were not treated with CaM antagonists. Application of CaM antagonists at the root tip ( 1M) also decreased the relative downward movement of label from ³H-IAA applied to the upper side of the elongation zone of gravistimulated roots. In general, tip application of antagonists inhibited neither the upward transport of ⁴⁵Ca²⁺ in the root tip nor the upward movement of label from ³H-IAA in the elongation zone of gravistimulated roots. Thus, roots treated with CaM antagonists ( 1 M) become less graviresponsive and exhibit reduced or even a reversal of downward polarity of calcium transport across the root tip and IAA transport across the elongation zone. The results indicate that calmodulin-regulated events play a role in root gravitropism.     

Local application of indole-3-acetic acid,by resin beads to intact growing maize roots

Five types of anion-exchanger resin beads which had adsorbed indole-3-acetic acid (IAA) were tested as IAA donors. The rate of IAA-uptake by beads was a function of time and pH. The release was relatively steady during 6 h application on vertical maize roots. No IAA degradation occurred in the beads (Amberlite IRA 400 type) but 45.8% was metabolised in the roots during treatment. Beads loaded with IAA and placed on one side of the root (at 2.20±0.03 mm from the tip) induced a curvature towards and above the bead (23.3±1.1 degrees after 5.25 h application). In contrast, control beads (without IAA) did not change the axial growth rate. Applied IAA seemed to move differently from endogenous IAA. The use of resin beads loaded with IAA offers a technique to study the effects of local IAA application on intact growing roots.Abbreviations 3,3-DGA 3,3 dimethyl-glutaric acid - HPLC high-performance liquid chromatography - IAA indole-3-acetic acid - Ox-IAA oxindole-3-acetic acid     

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; )     

Mediation of tropisms by lateral translocation of endogenous indole-3-acetic acid in maize coleoptiles

Abstract. The hypothesis that tropic responses result from lateral auxin gradients was examined in coleoptiles of red-light-grown maize ( Zea mays L.) by measuring endogenous IAA (indole-3-acetic acid) using a physicochemical method. Phototropic stimulation (unilateral blue light; 8s at 0.33 μmol m⁻²s⁻¹) was found to induce a lateral gradient of solvent-extractable IAA in a subapical zone (2-7mm from the tip). The gradient occurred in advance of the bending response, with a decrease of IAA in the irradiated half and a compensatory increase in the shaded half. The maximal gradient measured was about 1:2 (irradiated: shaded). Diffusible IAA, obtained from the cut end of an excised coleoptile tip (3mm long, with its base split by 1mm), was similarly redistributed between the two sides, indicating that IAA is laterally translocated in the tip and that the resulting IAA gradient migrates to the subapical zone. A smaller gradient was induced in a basal zone (12-17mm from the tip). This gradient was initiated about 20 min later than that at the subapical zone, in agreement with a similar delay of bending observed in this zone. Gravitropic stimulation (60° from the vertical) also resulted in a lateral gradient of extractable IAA in the subapical zone, the gradient preceding the bending response. It is concluded that the tropisms of maize coleoptiles are mediated by IAA gradients, which are most likely caused by lateral IAA transport as the Cholodny-Went theory of tropisms describes. From IAA measurement data, the mean velocity of basipetally-polar transport of endogenous IAA was estimated to be 12 mm h⁻¹.     

Changes in IAA responsiveness in the elongation region of graviresponding mung bean roots

IAA responsiveness of sections of root tissue taken from the top and bottom of mung bean roots was assessed prior to and at varying times following gravistimulation. Prior to gravistimulation, root tissue sections from the sides of the elongation zone responded similarly to IAA. After gravistimulation (within 5 min), root sections from the bottom of the elongation zone became more responsive to IAA than sections collected from the upper side of the elongation zone. The change in IAA responsiveness of these tissue sections was transient with root sections from both the top and bottom of the elongation zone again exhibiting similar responsiveness to IAA following 15 minutes of gravistimulation.These studies also examined if the root tip is required for the gravity-induced shift in IAA responsiveness in the tissues of the elongation zone. The IAA responsiveness of top and bottom sections of the elongation zone from decapped mung bean roots was assessed at varying times following gravistimulation. The responsiveness to IAA of top and bottom sections changed rapidly in decapped roots, just as had been previously found for intact roots. Although the alteration in responsiveness was transient in decapped roots (just as intact roots), the time it took for the sections to recover previous responsiveness to IAA was extended.These results suggest that the initial growth response of graviresponding roots may be due to a change in the IAA responsiveness of tissues in the elongation zone and not an asymmetric accumulation of IAA on the lower side of the elongation zone. The results also indicate that the gravity-induced shift in IAA responsiveness in the elongation zone occurs independently of the root cap, suggesting that the cells in the elongation region can perceive and respond to gravity independently of the root cap during the intial phases of the gravity response.     

MECHANICAL PROPERTIES WITHIN THE GROWTH ZONE OF CORN ROOTS INVESTIGATED BY BENDING EXPERIMENTS II. DISTRIBUTIONS OF MODULUS AND COMPLIANCE IN BENDING

A bending technique was used to infer the spatial distributions of rheological properties within the growth zone of the root of corn, Zea mays. “Bending modulus” (ratio of stress to strain, calculated from engineering theory of bending) falls from 20 MPa near the root tip (3 mm from the tip) to 6 MPa at the location 6 mm from the tip and then remains uniform through the basal region of the growth zone. Where growth stops, at 11–12 mm, there is a sharp rise in bending modulus. The profile of bending moduli is not changed by root incubation temperature during the growth period prior to bending, but it is shifted to the left in roots growing more slowly than the average at either of two temperatures (19 and 29 C). The spatial distribution of “compliance” (reciprocal of bending modulus and a measure of tissue extensibility) resembles the distribution of swelling in response to osmotic perturbation. The distribution of compliance does not parallel that of growth rate. Attempts to explain the discrepancy between compliance and growth rate lead us to examine the theoretical basis for the calculations and to suggest that the dependence of compliance on rate of stretching is physiologically important.     

Auxin signalling is involved in cadmium-induced glutathione-S-transferase activity in barley root

Transient exposure of barley roots to Cd, IAA or H₂O₂ for 30 min resulted in a significant root growth inhibition. Cd significantly increased the GST activity of roots 6 h after the end of short-term treatment. This increase was more relevant in root segment containing differentiation zone than in root segment just immediately behind the root apex. In contrast to Cd treatment, the short-term exposure of barley roots to IAA resulted in a significant increase of GST activity along the whole root tip and this increase was detectable already 3 h after the treatment with 10 μM IAA. Similarly to IAA, exogenously applied 10 mM H₂O₂ for 30 min caused significant increase of GST activity along the whole root tip 6 h after the treatment. This increase was already detectable 3 h after the exposure, but only in the differentiation zone of root tip. Auxin influx or signalling inhibitor considerable decreased the Cd- or IAA-induced GST activity in barley root tips. The strong activation of GST even after a brief exposure of barley roots to Cd support the crucial role of GST in the Cd-induced stress response in which presumably IAA and H₂O₂ play an important signalling role including the activation of GST.     

Differential growth and hormone redistribution in gravireacting maize roots

When growing roots are placed in a horizontal position gravity induces a positive curvature. It is classically considered to be the consequence of a faster elongation rate by the upper side compared to the lower side. A critical examination indicates that the gravireaction is caused by differential cell extension depending on several processes. Some of the endogenous regulators which may control the growth and gravitropism of elongating roots are briefly presented. The growth inhibitors produced or released from the root cap move preferentially in a basipetal direction and accumulate in the lower side of the elongation zone of horizontally maintained roots. The identity of these compounds is far from clear, but one of these inhibitors could be abscisic acid (ABA). However, indol-3y1 acetic acid (IAA) is also important for root growth and gravitropism. ABA may interact with IAA. Two other aspects of root cell extension have also to be carefully considered. An elongation gradient measured from the tip to the base of the root was found to be important for the growth of both vertical and horizontal gravireactive roots. It was changed significantly during the gravipresentation and can be considered as the origin of the differential elongation. Sephadex beads have been used as both growth markers and as monitors of surface pH changes when they contain some pH indicator. This technique has shown that the distribution of cell extension along the main root axis is related to a pH gradient, the proton efflux being larger for faster growing parts of roots. A lateral movement of calcium is obtained when Ca2+ is applied across the tips of horizontally placed roots with a preferential transport towards the lower side. Endogenous calcium, which may accumulate inside the endoplasmic reticulum of some cap cells, may also act in the gravireception. These observations and several others strongly suggest that calcium may play an essential role in controlling root growth and several steps of the root gravireaction.     

The spatially variable inhibition by water deficit of maize root growth correlates with altered profiles of proton flux and cell wall pH

Growth of elongating primary roots of maize (Zea mays) seedlings was approximately 50% inhibited after 48 h in aerated nutrient solution under water deficit induced by polyethylene glycol 6000 at -0.5 MPa water potential. Proton flux along the root elongation zone was assayed by high resolution analyses of images of acid diffusion around roots contacted for 5 min with pH indicator gel. Profiles of root segmental elongation correlated qualitatively and quantitatively (r(2) = 0.74) with proton flux along the surface of the elongation zone from water-deficit and control treatments. Proton flux and segmental elongation in roots under water deficit were remarkably well maintained in the region 0 to 3 mm behind the root tip and were inhibited from 3 to 10 mm behind the tip. Associated changes in apoplastic pH inside epidermal cell walls were measured in three defined regions along the root elongation zone by confocal laser scanning microscopy using a ratiometric method. Finally, external acidification of roots was shown to specifically induce a partial reversal of growth inhibition by water deficit in the central region of the elongation zone. These new findings, plus evidence in the literature concerning increases induced by acid pH in wall-extensibility parameters, lead us to propose that the apparently adaptive maintenance of growth 0 to 3 mm behind the tip in maize primary roots under water deficit and the associated inhibition of growth further behind the tip are related to spatially variable changes in proton pumping into expanding cell walls.     

The response to auxin of rapeseed (Brassica napus L.) roots displaying reduced gravitropism due to transformation by Agrobacterium rhizogenes

It has recently been documented that, compared to untransformed controls, the roots of oilseed rape (Brassica napus L. CV CrGC5) seedlings transformed by Agrobacterium rhizogenes A4 show a reduced gravitropic reaction (Legué et al. 1994, Physiol Plant 91: 559–566). After stimulation at 90°C or 135°, the transformed root tips curve, but never reach a vertical orientation. In the present study, we investigated the causes of reduced gravitropic bending observed in stimulated transformed root tips. First, we localized the gravitropic curvature in normal and in transformed roots after 1.5 h of stimulation. The cells involved in root curvature (target cells) corresponded at the cellular level to the apical part of the zone of increasing cell length. In transformed roots grown in the vertical position, these cells showed a reduction in cell length compared to controls. Because auxin is considered to be the gravitropic mediator, the response of normal and transformed roots to exogenous auxin was studied. Indole-3-acetic acid (IAA) was applied along the first 3 mm using resin beads loaded with the hormone. In comparison to normal roots, transformed roots showed reduced bending toward the bead at all points of bead application. Moreover, the cells which responded to IAA corresponded to the target cells involved in the gravitropic reaction. The level of endogenous IAA was lower in transformed roots. Thus, it was concluded that the modified behavior of transformed roots during gravitropic stimulation could be due to differences either in IAA levels or in reactivity of the target cells to the message from the cap.Abbreviations DEZ distal elongation zone - ELISA enzymelinked immunosorbent assay - T-DNA DNA transferred from Agrobacterium rhizogenes to the plant genome This work was supported by the Centre National d'Etudes Spatiales.     

Sites and regulation of auxin biosynthesis in Arabidopsis roots

Auxin has been shown to be important for many aspects of root development, including initiation and emergence of lateral roots, patterning of the root apical meristem, gravitropism, and root elongation. Auxin biosynthesis occurs in both aerial portions of the plant and in roots; thus, the auxin required for root development could come from either source, or both. To monitor putative internal sites of auxin synthesis in the root, a method for measuring indole-3-acetic acid (IAA) biosynthesis with tissue resolution was developed. We monitored IAA synthesis in 0.5- to 2-mm sections of Arabidopsis thaliana roots and were able to identify an important auxin source in the meristematic region of the primary root tip as well as in the tips of emerged lateral roots. Lower but significant synthesis capacity was observed in tissues upward from the tip, showing that the root contains multiple auxin sources. Root-localized IAA synthesis was diminished in a cyp79B2 cyp79B3 double knockout, suggesting an important role for Trp-dependent IAA synthesis pathways in the root. We present a model for how the primary root is supplied with auxin during early seedling development.     

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.