Indole-3-acetic Acid Transport Research Articles

Hormonal Control of Parthenocarpic Ovary Growth by the Apical Shoot in Pea

The role of the apical shoot as a source of inhibitors preventing fruit growth in the absence of a stimulus (e.g. pollination or application of gibberellic acid) has been investigated in pea (Pisum sativum L.). Plant decapitation stimulated parthenocarpic growth, even in derooted plants, and this effect was counteracted by the application of indole acetic acid (IAA) or abscisic acid (ABA) in agar blocks to the severed stump. The treatment of unpollinated ovaries with gibberellic acid blocked the effect of IAA or ABA applied to the stump. [3H]IAA and [3H]ABA applied to the stump were transported basipetally, and [3H]ABA but not [3H]IAA was also detected in unpollinated ovaries. The concentration of ABA in unpollinated ovaries increased significantly in the absence of a promotive stimulus. The application of IAA to the stump enhanced by 2- to 5-fold the concentration of ABA in the inhibited ovary, whereas the inhibition of IAA transport from the apical shoot by triiodobenzoic acid decreased the ovary content of ABA (to approximately one-half). Triiodobenzoic acid alone, however, was unable to stimulate ovary growth. Thus, in addition to removing IAA transport from the apical shoot, the accumulation of a promotive factor is also necessary to induce parthenocarpic growth in decapitated plants.

Auxin transport is required for hypocotyl elongation in light-grown but not dark-grown Arabidopsis.

Many auxin responses are dependent on redistribution and/or polar transport of indoleacetic acid. Polar transport of auxin can be inhibited through the application of phytotropins such as 1-naphthylphthalamic acid (NPA). When Arabidopsis thaliana seedlings were grown in the light on medium containing 1.0 microM NPA, hypocotyl and root elongation and gravitropism were strongly inhibited. When grown in darkness, however, NPA disrupted the gravity response but did not affect elongation. The extent of inhibition of hypocotyl elongation by NPA increased in a fluence-rate-dependent manner to a maximum of about 75% inhibition at 50 mumol m-2 s-1 of white light. Plants grown under continuous blue or far-red light showed NPA-induced hypocotyl inhibition similar to that of white-light-grown plants. Plants grown under continuous red light showed less NPA-induced inhibition. Analysis of photoreceptor mutants indicates the involvement of phytochrome and cryptochrome in mediating this NPA response. Hypocotyls of some auxin-resistant mutants had decreased sensitivity to NPA in the light, but etiolated seedlings of these mutants were similar in length to the wild type. These results indicate that light has a significant effect on NPA-induced inhibition in Arabidopsis, and suggest that auxin has a more important role in elongation responses in light-grown than in dark-grown seedlings.

Effect of changing the endogenous concentration of auxins and cytokinins and the production of ethylene in pea stem cuttings on adventitious root formation

Decapitation or treatment with naphthylphthalamic acid (NPA) ndash; an inhibitor of IAA transport ndash; or a synthetic cytokinin N-(2-chloro-4-pyridyl)-N-phenylurea (CPPU) caused a decrease in rooting of pea cuttings. However the mode of action of the above treatments was different. Decapitation resulted in a decrease of indole-3-acetic acid (IAA) and a simultaneous increase in cytokinin content at the base of the cuttings. NPA decreased IAA even more, but did not influence cytokinins. CPPU alone or in combination with amino-ethoxy-vinylglycine (AVG) ndash; an ethylene biosynthesis inhibitor ndash; increased IAA at the rooting zone, but CPPU was transported from apex to the base of the cuttings where it inhibited rooting. NAA, applied alone after decapitation, stimulated rooting, probably partly by being an auxin, and partly by inhibiting the accumulation of cytokinins at the base of the cuttings. Treatment with AVG neither influenced rooting nor auxin or cytokinin content. Therefore, ethylene production does not seem to be one of the main factors involved in the reduced rooting after the various treatments.

Brassinosteroid-induced rice lamina joint inclination and its relation to indole-3-acetic acid and ethylene

Brassinosteroid (BR)-induced rice (Oriza sativa L.) lamina joint (RLJ) inclination and its relationship to indole-3-acetic acid (IAA) and ethylene were investigated using BR isolated from beeswax. The effect of BR on RLJ inclination was time- and concentration-dependent. Etiolated lamina were more sensitive to BR than green lamina. The BR-induced inclination was accompanied by increased lamina fresh weight, total water content, free-water content, proton extrusion and ethylene production, and decreased bound-water content. Lamina dry weight was not changed. The inclination was due to greater expansion of the adaxial cells relative to the dorsal cells in the lamina joint. This response was caused by BR and/or BR-induced signal(s) that were transported from the leaf sheath to the leaf blade. Both BR-induced RLJ inclination and ethylene production were inhibited by cobalt chloride (CoCl2), an inhibitor of ACC oxidase. BR-induced inclination was much higher than that of IAA, and was inhibited by high concentration of 2,3,5-triiodobenzoic acid (TIBA), an inhibitor of IAA transport. A synergistic effect was observed between BR and IAA. These results suggest that the effects of BR on RLJ inclination and pulvinus cell expansion may be resulted from BR-increased water potential and proton extrusion in the lamina. The BR-induced RLJ inclination may involve the action of ethylene but may be independent of IAA.

Uptake and Transport of Indole-3-Butyric Acid in Arabidopsis thaliana: Comparison with Other Natural and Synthetic Auxins

Summary The uptake of3 H-indole-3-butyric acid (IBA) in Arabidopsis thalianaseedlings grown in liquid culture was studied and compared with other natural and synthetic auxins. It was shown that the uptake of IBA and also of indole-3-acetic acid (IAA) was linear during 1 h, but it was saturable after 5 and 12 h, respectively. The Km value for the saturable in vivo transport of IBA was 0.18 μmol/L, compared with 0.19 μmol/L for IAA, 3.1 μmol/L for naphthylacetic acid (NAA) and 1.2 μmol/L for phenylacetic acid (PAA). The uptake of 3 H-IBA was strongly inhibited by unlabelled IAA. The K; value for IAA was determined as 1.7 μmol/L. Long distance transport of IAA and IBA (acropetal and basipetal) has been investigated using 10 cm stem segments of Arabidopsis. In general, both hormones were transported better acropetally than basipetally. IBA was taken up better than IAA and was also transported more rapidly, although for both hormones a transport further than 6 cm after 4.5 h in the stem could not be detected. Our results indicate no prominent differences between IAA and IBA transport in Arabidopsis thaliana.

Effects of the Indole-3-Acetic Acid (IAA) Transport Inhibitors N-1-Naphthylphthalamic Acid and Morphactin on Endogenous IAA Dynamics in Relation to Compression Wood Formation in 1-Year-Old Pinus sylvestris (L.) Shoots.

Both N-1-naphthylphthalamic acid (NPA) and methyl-2-chloro-9-hydroxyfluorene-9-carboxylic acid (CF) inhibit the polar transport of indole-3-acetic acid (IAA) and, therefore, are attractive tools for investigating IAA's role in the regulation of plant growth. Ringing an intact conifer shoot with lanolin containing NPA or CF induces the formation of compression wood above the ring. This induction has been attributed to a postulated accumulation of IAA above the application site of the IAA transport inhibitor, but the validity of this postulation has never been confirmed. Using gas chromatography-selected ion monitoring-mass spectroscopy with [13C6]IAA as an internal standard, we measured the levels of endogenous free and conjugated IAA in 1-year-old Pinus sylvestris (L.) shoots ringed with NPA or CF. The level of free IAA was dramatically decreased below the ring, indicating that the polar transport of endogenous IAA was inhibited by the treatment. However, the free IAA level above the ring, where compression wood was formed, was also slightly lower than in control shoots. The lack of IAA accumulation above the site of the IAA transport inhibitor could not be explained by an increase in IAA conjugation. Furthermore, the turnover of [2-14C]IAA, measured using high-performance liquid chromatography with on-line radioactivity monitoring, was the same in NPA-treated and control shoots. The decrease in IAA level above a NPA or CF ring is attributed to these substances being transported acropetally and interfering with polar IAA transport along the shoot. It is concluded that compression wood formation above a NPA or CF ring is not associated with an overall increase in cambial region IAA level or increased IAA turnover. Instead, we suggest that acropetally transported NPA and CF induce compression wood formation by interacting with the NPA receptor in differentiating tracheids, thereby locally increasing IAA in these cells.

Interaction of Ethylene and Auxin in the Regulation of Hook Growth II. The Role for Ethylene in Different Growing Regions of the Hypocotyl Hook of Phaseolus vulgaris

In different growing regions of the hypocotyl hook of Phaseolus vulgaris synthesis and levels of endogenous ethylene are asymmetrically distributed. Significant difference exists between waterstress- and IAA-induced ethylene formation from isolated hook segments, which fits to the former results (Schwark and Schierle. J. Plant Physiol. 140, 562–570, 1992) that also endogenous auxin levels are asymmetrically distributed in the hook. 1-N-Naphthylphthalamic acid (NPA), a specific inhibitor of polar transport of indole-acetic-acid (IAA) equalizes the asymmetric distribution of aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene. This result indicates, that transport of endogenous IAA is causally linked with the asymmetric synthesis of ethylene in the hook.

Compartmentation of indole-3-acetic acid metabolism in protoplasts isolated from leaves of wild-type and IAA-overproducing transgenic tobacco plants

Cellular compartmentation of indole-3-acetamide (IAM), indole-3-acetic acid (IAA), and [15N1]IAA synthesised from [15N1]tryptophan was monitored in protoplasts isolated from sterile wild-type tobacco SRI plants, and in IAA-overproducing plants expressing the Agrobacterium tumefaciens T-DNA IAA genes iaaM and iaaH. Indole-3-acetamide was located exclusively in the cytosol of both iaaM and iaaM/iaaH protoplasts, being 75% lower than in iaaM protoplasts, presumably because of conversion into IAA by action of the iaaH-encoded hydrolase. The free-IAA level, however, was raised only 8% in iaaM/iaaH compared to iaaM protoplasts, whereas the level of IAA-conjugates was increased more than fivefold. For both genotypes, the location of IAA conjugates was restricted to the cytosol, while one-third of the free-IAA pool was present in chloroplasts. Transcription of the iaaM gene was increased by fusion to the strong cauliflower mosaic virus (CaMV) 35S promoter. Compared with the wildtype, this led to an 18-fold higher conversion of [15N1]tryptophan to [15N1]IAA, a three- to fourfold increase in free IAA, and a tenfold higher level of IAA conjugates in 35S-iaaM/iaaH protoplasts. Also in these genotypes, IAA conjugates were exclusively cytosolic. There was no major difference between transgenic and wildtype protoplasts in the proportion of chloroplastic to total cellular IAA, although the chloroplastic IAA and [15N1IAA pools in the transformant were threefold and eightfold higher, respectively. Since the IAM pool in transgenic plants is exclusively cytosolic, these findings suggest that the increased chloroplastic [15N1IAA pool in 35S-iaaM/iaaH protoplasts is synthesised in the cytosol but rapidly transported into the chloroplast. Furthermore, the presence of IAA in the chloroplast together with the exclusively cytosolic location of IAA conjugates, suggests the presence of two differentially subcellular pools of IAA. The first is located in the cytosol and mainly regulated by non-decarboxylative catabolism and conjugation (Sandberg et al. 1990, Planta 180, 562–568), whereas the second is located in the chloroplast and is seemingly not directly regulated inside the organelle by either of these two processes. The cytosolic IAA control mechanisms, however, also affect the chloroplastic pool size due to the rapid transport of IAA between the two compartments.

Effect of ethylene treatment on transport and metabolism of indole-3-butyric acid in citrus leaf midribs

Transport and metabolism of radiolabeled indole-3-butyric acid (IBA) were studied in midrib sections of Cleopatra mandarin (Citrus reticulata Blanco) and compared to that of indole-3-acetic acid (IAA). Exogenous IBA was metabolized by the midribs to a polar compound, probably an ester conjugate. Ethylene pretreatment of the midribs reduced their capacity to metabolize IBA by ca. 70% as compared to air pretreatment. IBA transport capacity in the leaf midribs was ca. two times greater in the basipetal direction than the acropetal. The basipetal transport capacity of 3H-IBA was lower than that of 14C-IAA (ca. 24% and 39% of the uptake, respectively). While ethylene treatment reduced basipetal transport of IAA by ca. 70% it did not affect the transport of IBA. Most of the transported label was found as free IBA, but the reduction of IBA conjugation by ethylene treatment did not affect the transport capacity.

The Decrease in Auxin Polar Transport Down the Lupin Hypocotyl Could Produce the Indole-3-Acetic Acid Distribution Responsible for the Elongation Growth Pattern

The variation of indole-3-acetic acid (IAA) transport along Lupinus albus L. hypocotyls was studied using decapitated seedlings and excised sections. To confirm that the mobile species was IAA and not IAA metabolites, dual isotope-labeled IAAs, [5-(3)H]IAA + [1-(14)C]IAA, were used. After apical application to decapitated seedlings, the longitudinal distribution of both isotopes at different transport periods showed that the velocity of IAA transport was higher in the apical, elongating region than in the basal, non-growing region. This variation in velocity was not a traumatic consequence of decapitation because after application of IAA to the basal region of decapitated seedlings, both the velocity and intensity of IAA transport were lower than in the apical treatment. The variation in IAA transport down the hypocotyl was confirmed when it was measured in excised sections located at different positions along the hypocotyl. The velocity and, to a greater extent, the intensity of IAA transport decreased from the apical to the basal sections. Consequently, if the amount of IAA reaching the apical zones of lupin hypocotyl were higher than the IAA transport capacity in the basal zones, accumulation of mobile IAA might be expected in zones located above the basal region. In fact, an IAA accumulation occurred in the elongating region during the first 4-h period of transport after apical treatment with IAA. It is proposed that the fall in IAA transport along the hypocotyl might be responsible for the IAA distribution and, consequently, for the growth distribution reported in this organ. An indirect proof of this was obtained from experiments that showed that the excision of the slowly transporting basal zones strongly reduced the growth in the remaining part of the organ, whereas excision of the root caused no significant modification in growth during a 20-h period.

Levels of endogenous indole‐3‐acetic acid and indole‐3‐acetylaspartic acid during adventitious root formation in pea cuttings

Levels of endogenous indole‐3‐acetic acid (IAA) and indole‐3‐acetylaspartic acid (IAAsp) were monitored in various parts of leafy cuttings of pea (Pisum sativum L. cv. Marma) during the course of adventitious root formation. IAA and IAAsp were identified by combined gas chromatography—mass spectrometry, and the quantitations were performed by means of high performance liquid chromatography with spectrofluorometric detection. IAA levels in the root forming tissue of the stem base, the upper part of the stem base (where no roots were formed), and the shoot apex remained constant during the period studied and were similar to levels occurring in the intact seedling. A reduction of the IAA level in the root regenerating zone, achieved by removing the shoot apex, resulted in almost complete inhibition of root formation. The IAAsp level in the shoot apex also remained constant, whereas in the stem base it increased 6‐fold during the first 3 days. These results show that root initiation may occur without increased IAA levels in the root regenerating zone. It is concluded that the steady‐state concentration is maintained by basipetal IAA transport from the shoot apex and by conjugation of excessive IAA with aspartic acid, thereby preventing accumulation of IAA in the tissue.

IAA synthesis and root induction with iaa genes under heat shock promoter control

We have devised a heat shock-inducible indole-3-acetic acid (IAA) synthesis system for plant cells, which is based on the iaa genes of the Agrobacterium tumefaciens T-DNA and the heat shock promoter hsp70 of Drosophila melanogaster. Two DNA constructs were tested: one contains the iaaM gene linked to the hsp70 promoter (hsp70-iaaM) and encodes the production of indoleacetamide (IAM), the other contains hsp70-iaaM and the wild-type iaaH gene which codes for the conversion of IAM into IAA (hsp70-iaaM/iaaH). Heat shock-controlled IAM and IAA synthesis was tested on two levels: biochemically by measuring IAM and IAA levels in Kalanchoe stem segments infected with the two constructs, and morphologically by IAA-dependent root formation on Kalanchoe plants, on carrot discs and on tobacco leaf fragments. At both levels the responses were found to be controlled by the heat shock promoter. IAM levels of segments infected with hsp70-iaaM increased 6-fold upon heat shock induction to 240 pmol IAM per stem segment. The accumulation of IAA in segments infected with hsp70-iaaM/iaaH and heat-shocked was found to be more variable, possibly due to IAA transport and metabolism. Heat shock treatment of Kalanchoe plants and tobacco leaf fragments infected with hsp70-iaaM/iaaH led to a strong increase in root formation. On carrot discs, heat shock-specific root induction was also demonstrated, but the responses differed between individual carrots.

Induction of acropetal 14C-photosynthate transport and radial growth by indole-3-acetic acid in Pinus sylvestris shoots

The relationship between radial growth and assimilate movement was determined in one-year-old Pinus sylvestris (L.) cuttings collected during the dormant period and reactivated for 1-27 days under environmental conditions favorable for growth. The cuttings were either left with buds intact, defoliated along the distal 8 cm, or debudded and treated apically with 0 or 1 mg g(-1) indole-3-acetic acid (IAA) in lanolin. Radial growth was measured as tracheid production and bark radial width. The distribution of (14)C-photosynthate 1 or 5 days after exposure to (14)CO(2) was used to indicate assimilate movement. Debudding inhibited tracheid production and decreased acropetal (14)C-photosynthate transport, whereas applying IAA to debudded cuttings promoted both these processes and, in addition, induced vigorous callus growth within the bark immediately below the application point. Distal defoliation markedly increased the amount of (14)C-photosynthate transported toward the apex without altering how debudding and exogenous IAA affected tracheid production and the distribution of (14)C-photosynthate. The production of tracheids induced by apically applied IAA in debudded and distally defoliated cuttings increased progressively in material collected on September 9, October 14 and November 24, whereas bark radial width and acropetal (14)C-photosynthate movement increased only between the first two collection dates. Ringing midway between the IAA source and the bottom of the distally defoliated region with a lanolin mixture of 10 mg g(-1)N-1-naphthylphthalamic acid (NPA), 1 mg g(-1) methyl-2-chloro-9-hydroxy-fluorene-9-carboxylic acid (CF) or 10 mg g(-1) phenylacetic acid (PAA) reduced radial growth and [1-(14)C]IAA transport below the ring and locally promoted radial growth and accumulated radioactivity above. (14)C-Photosynthate transport into the region above the ring either was not altered (CF, PAA) or was increased (NPA). The results indicate that apically applied IAA induces the acropetal movement of (14)C-photosynthate in debudded P. sylvestris shoots by locally promoting activity correlated with tracheid and callus production rather than by affecting radial growth or phloem transport processes, or both, at a distance below the IAA source.

In vivo metabolism of labelled indole-3-acetic acid during polar transport in etiolated hypocotyls of Lupinus albus: Relationship with growth

The in vivo metabolism of indole-3-acetic acid (IAA) in etiolated hypocotyls of lupin (Lupinus albus L., from Bari, Italy) was investigated by appliying IAA labelled with two radioisotopes ([1-14C]-IAA+[5-3H]-IAA) to the apical end of decapitated seedlings, followed by extraction of the radioactivity in the different regions along the hypocotyl. This method allowed detection of IAA decarboxylation in zones distant from the cut surface and, therefore, containing intact cells. When IAA was added directly in solution to the cut surface, decarboxylation was high especially in those hypocotyl regions where transient accumulations characteristic of the polar transport of IAA occurred. In 10-day-old seedlings such accumulations were observed both in the elongation zone (2nd, 3rd, and 4th cm) and in the non elongating basal zone (8th, 9th and 10th cm). When the IAA, instead, was applied with an agar block deposited on the cut surface, IAA metabolism (decarboxylation as well as conjugation) was increased but almost exclusively in tissues within 10 mm of the cut surface. In both kinds of experiment, the increase in IAA decarboxylation seemed to coincide with a decrease in the transport of IAA, since in the assay without agar the transient accumulations of radioactivity were probably due to a decrease in the transport velocity, while in the assay with agar the transport intensity was much lower than in the assay without agar. These results point to a competitive relationship between IAA metabolism and transport. Consequently, it is suggested that hypocotyl regions that probably use auxin for development processes (e.g., cell elongation and differentiation) may have a more intense IAA metabolism in parallel with their higher IAA concentrations.

Effect of Ethylene Treatment on Polar IAA Transport, Net IAA Uptake and Specific Binding of N-1-Naphthylphthalamic Acid in Tissues and Microsomes Isolated from Etiolated Pea Epicotyls

The effect of ethylene treatment on polar indole-3-acetic acid (IAA) transport, net IAA uptake in the presence and absence of N-1-naphthylphthalamic acid (NPA) and [(3)H]NPA binding characteristics was investigated in tissue segments or microsomes isolated from etiolated pea (Pisum sativum L. cv Alaska) epicotyls. Basipetal IAA transport in 5 millimeter segments isolated from ethylene-treated seedlings was inhibited by ethylene in a dose-dependent manner. Threshold, half-maximal and saturating concentrations of ethylene were 0.01, 0.55, 10.0 microliters per liter, respectively. This inhibition became apparent after 6 to 8 hours of ethylene treatment. Transport velocity in both control and ethylene-treated tissues was estimated to be 5 millimeters per hour. Net IAA uptake was stimulated in ethylene-treated tissues and the relative ability of the phytotropin NPA to enhance net IAA uptake was reduced in treated tissues. Specific binding of [(3)H]NPA to microsomes prepared from both control and ethylene-treated tissues was saturable and consistent with the existence of a single class of binding sites with an apparent affinity (K(d)) toward NPA of 8 to 9 nanomolar. The density of these binding sites (per milligram protein) was lower (36% of control) in ethylene-treated tissues. Direct application of ethylene to microsomal preparations isolated from untreated seedlings had no effect on the level of specific [(3)H]NPA binding.

Basipetal Auxin transport in lettuce and its possible involvement in acropetal calcium transport and incidence of tipburn

Abstract The relationship between polar basipetal indoleacetic acid (IAA) transport and acropetal calcium transport and the incidence of tipburn was studied in lettuce (Lactuca sativa). In lettuce plants treated with triiodo‐benzoic acid (TIBA), an auxin transport inhibitor, incidence of tipburn was increased. In young leaves, calcium concentrations were lowered by TIBA treatment, while the concentrations of potassium and magnesium were unaffected. In short‐term experiments with isolated shoot tips of lettuce it could be demonstrated in TIBA‐treated plants that both basipetal transport of IAA and acropetal transport of calcium ( Ca) to the growing tip were inhibited. The results support the view that a system for polar basipetal auxin transport exists in low transpiring vegetative tissue, which is associated with an acropetal calcium transport independent of transpiration.

Manipulating carnation petal senescence. I. The interaction and transport of benzyladenine and indoleacetic acid

Indoleacetic acid (IAA) promotes carnation petal senescence with the petals irreversibly wilting, whilst benzyladenine (BA) delays petal senescence so that they eventually “burn” or become necrotic at the petal perimeter. An anatomical study showed that with irreversible wilting the spongy mesophyll in the flag region becomes loosely packed. The rate and type of petal senescence was used as an indicator to measure the interaction of BA and IAA and, in relation to this parameter, each hormone was found to alter the physiological expression of senescence by the other. Although BA did alter IAA transport, it was concluded that the change in senescence pattern when these hormones were added together was not entirely due to differences in their transport.

Relationship between polar basipetal auxin transport and acropetal Ca2+ transport into tomato fruits

The dependence of acropetal Ca2+ transport on polar basipetal indoleacetic acid (IAA) transport was investigated in excised tomato fruits (Lycopersicon esculentum L. Mill.) using an in vitro fruit system. Auxin transport inhibitors like triiodobenzoic acid (TIBA), chlorofluorenolmethyl ester (CME) and naphthylphthalamic acid (NPA) were used in order to investigate the effect of restricted polar basipetal auxin transport on the acropetal transport of 45Ca2+, 86Rb+ and 98Sr2+ into the same fruits. TIBA and CME inhibited basipetal transport of IAA. particularly in 10‐ to 12‐day‐old tomato fruits, and simultaneously restricted the acropetal transport of 45Ca2+. The auxin transport inhibitors failed to significantly reduce the upward transport of 86Rb+ and the transport of 96Sr2+ was less inhibited than that of 45Ca2+. TIBA and CME did not significantly affect the acropetal transport of labelled water into the fruit, nor the cation‐exchange capacity or K+ and Mg2+ concentrations in the tomato fruit. These results support the view that a part of the Ca2+‐specific acropetal transport into tomato fruits is associated with the polar basipetal IAA transport. This Ca2+ transport is independent of the transpiration stream into the fruit and the cation exchange capacity of the fruit tissue.

Auxin carriers in Cucurbita vesicles

The association at 0° C of [(3)H]indole-3-acetic acid (IAA) with membrane vesicles prepared from Cucurbita pepo L. hypocotyls at pH 7.9 and resuspended at pH 6.0 was greatly diminished by osmotic shrinkage. Nonradioactive IAA inhibited a large proportion of this association thus revealing a saturable component, also decreased by osmotic shrinkage, which mainly represents operation of an auxin uptake carrier, with saturable binding having only a minor role. The contribution of this carrier to the steady state of IAA transport changed in the same direction as the proton motive force (transmembrane pH and electrical potential difference) which was manipulated using the K(+) ionophore valinomycin with differing K(+) concentration gradients over a range of assay pH values. We conclude that the uptake carrier is electroimpelled with IAA(-)/2H(+) (or IAAH/H(+)) symport as a possible mechanism. The same procedures were used to examine the behaviour of the IAA efflux carrier, whose inhibition by N-1-naphthylphthalamic acid (NPA) causes an increase in IAA accumulation by the vesicles. The extent of NPA-stimulation was linked to the magnitude of the electrical potential difference (negative inside) across the vesicle membranes. We conclude that IAA transport by the efflux carrier is electroimpelled, with IAA(-) a likely substrate.

The role of free space calcium in the basipetal transport of indole‐3‐acetic acid in sunflower hypocotyls1

Abstract Calcium has been reported to be involved in the basipetal transport of indole‐3‐acetic acid (IAA). It has been suggested that basipetal transport of IAA requires the influx of Ca2+ into the cytoplasm from the cell wall free space and that Ca2+ acts as a second messenger. Experiments were conducted to determine the interaction of free space Ca2+ with the basipetal transport of IAA. Four day old sunflower (Helianthus annus L. cv ‘Russian Mammoth') seedlings grown in 0.20 strength Hoagland's solution were either left intact or excised below the cotyledonary node to remove the meristem, the source of IAA in the shoot. After removal of the node, the plants were treated with or without 500 μM IAA in lanolin paste. After 24 h, 3 mm segments were cut from the hypocotyls and incubated in KCl. Solution Ca2+ concentration, representing free space Ca2+, increased when the source of IAA was removed. Plants which were provided a source of IAA, either endogenously or exogenously, had low levels of free space Ca...