Effect of indoleacetic acid on the metabolic activity oftaraxacum root segments

Inoubation of thin slioes ofTaraxacum root segments in 3-indoleaoetic acid solutions enhanced the level of total proteins, RNA, DNA and a number of enzymes reaching an optimum at 0.01 mg 1^-1. It has been shown that the auxin promotes the synthesis of DNA, again with an optimum promotion at 0.01 mg 1^-1 IAA. This work was carried out at the Department of Botany, University of Sheffield, Sheffield, England. The author wishes to thank Dr. A. Booth for taking a keen interest in this work.     

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; )     

Transport of indoleacetic acid in intact roots of Phaseolus coccineus

Summary Indoleacetic acid (IAA)-5-³H (2×10^-9) was applied to intact roots of Phaseolus coccineus seedlings at the apex or 2 cm above the apex, and the movement of IAA-³H and its metabolites traced by sectioning and chromatography. Basipetal movement of label occurred for 2 cm or less, declining exponentially, and the amount increased with time. Acropetal transport from above the apex showed quantitatively less movement of radioactivity. After a 6h treatment period a decline of label occurred in the first 0.5cm, below which there was a long distance movement of small amounts of label, mainly in IAA, towards the apex where the label concentrated by a factor of approximately 2. Short-distance basipetal movement consisted of about equal amounts of IAA and metabolites, and only metabolites were found in areas more basipetal than 2cm. Label from solutions of sucrose-¹⁴C and ³H₂O followed the same general pattern of movement as label from IAA-³H, except that acropetal movement of water showed a steady decrease in the amount of label as the distance from the area of application increased. The short distance basipetal transport of label with the breakdown of IAA-³H indicates that the extent of basipetal movement was limited by catabolic processes. The acropetal pattern of IAA-³H movement with the concentration of the transported material close to the apex, is possibly the result of transport in the phloem.     

Transport of auxin (indoleacetic acid) through lipid bilayer membranes

Summary Diffusion of auxin (indole-3-acetic acid) through planar lipid bilayer membranes was studied as a function of pH and auxin concentration. Membranes were made of egg or soybean lecithin or phosphatidyl serine inn-decane (25–35 mg/ml). Tracer and electrical techniques were used to estimate the permeabilities to nonionized (HA) and ionized (A^–) auxin. The auxin tracer flux is unstirred layer limited at low pH and membrane limited at high pH, i.e., when [A^–][HA]. The tracer flux is not affected by the transmembrane voltage and is much higher than the flux predicted from the membrane conductance. Thus, only nonionized auxin crosses the membrane at a significant rate. Auxin transport shows saturation kinetics, but this is due entirely to unstirred layer effects rather than to the existence of an auxin carrier in the membrane. A rapid interconversion of A^– and HA at the membrane surface allows A^– to facilitate the auxin flux through the unstirred layer. Thus, the total flux is higher than that expected for the simple diffusion of HA alone. The relation between flux (J _A), concentrations and permeabilities is: 1/J _A=1/P ^UL([A^–]+[HA])+1/P _HA ^M [HA]. By fitting this equation to our data we find thatP ^UL=6.9×10^–4 cm/sec andP _HA ^M =3.3×10^–3 cm/sec for egg lecithin-decane bilayers. Similar membrane permeabilities were observed with phosphatidyl serine or soybean lipids. Thus, auxin permeability is not affected by a net surface charge on the membrane. Our model describing diffusion and reaction in the unstirred layers can explain the anomolous relationship between pH and weak acid (or weak base) uptake observed in many plant cells.     

Content of indoleacetic acid and its metabolism in root nodules ofMelilotus alba

The root nodules ofMelilotus alba, a leguminous fodder herb, contain a high amount of indoleacetic acid (IAA). The tryptophan pool present in the nodule might serve as a source for the IAA production. Metabolism of IAA in the nodules was evidenced by the presence of IAA-metabolizing enzymes, IAA oxidase and peroxidase. A high amount of IAA was produced by the symbiont isolated from the nodules in culture, when supplemented with tryptophan. For IAA production, the bacteria preferred thel-isomer over thedl- ord-isomer of tryptophan. The possible role of nodular IAA production on the legume-Rhizobium symbiosis is discussed.     

Initial phases of indoleacetic acid induced growth in coleoptile segments of Avena sativa L.

The intial phases of auxin-induced growth in coleoptile segments of Avena sativa L. were investigated using a high resolution growth recording technique, based on an angular position sensing transducer. The first response to the hormone is a slight, transient reduction of the growth rate lasting about 5 min. After this phase growth rate increases to a maximum. The duration of the increase and the maximum clearly depend on the concentration of the hormone. With increasing auxin concentration the duration of the growth rate increase is reduced from about 80 min in 10^-9 M indoleacetic acid (IAA) to about 14 min in 10^-4 M IAA. After the maximum the growth rate declines. Looking at the maximum of the growth rate, we obtained a dose-response curve with a sharp increase between 10^-9 M and 10^-6 M IAA and a slight decline between 10^-6 M and 10^-4 M IAA. This result is confirmed by growth rates measured one and two hours after the application of the hormone.Abbreviations IAA indoleacetic acid     

Abscisic acid distribution in horizontal maize root segments

Using gas chromatography-mass spectrometry (GC/MS) techniques of analyses, it has been found that endogenous abscisic acid (ABA) becomes asymmetrically distributed in the elongation zone of horizontal Zea mays (cv. LG 11) roots which are showing a positive gravitropic response. There is a relative increase in the ABA content of the lower half and a concomitant decrease for the upper half in such roots. Asymmetric distribution of ABA is also detected in the elongation zone of half-decapped roots.Abbreviations IAA indoleacetic acid - ABA abscisic acid - GC/MS gas chromatography-mass spectrometry     

The transport and metabolism of 14C-labelled indoleacetic acid in intact pea seedlings

Summary Part of the IAA-I- or IAA-2-¹⁴C applied at low concentrations to the apices of intact, light-grown dwarf pea seedling was transported unchanged to the root system The calculated velocity of transport in the stem was 11 mm per hour. In the root the label accumulated in the developing lateral root primordia.A large proportion of the applied IAA was converted by tissues of the apical bud, stem and root to indole-3-acetyl-aspartic acid (IAAsp). This compound was not transported. In addition evidence was obtained for the formation of IAA-protein complexes in the apex and roots, but not in the fully-expanded internodes.Large quantities of a decarboxylation product of IAA, tentatively indentified as indole-3-aldehyde (IAld), and several minor metabolites of IAA, were detected in extracts of the roots and first internodes, but not in the above-ground organs exposed to light. These compounds were readily transported through stem and root tissues. Together, the decarboxylation of IAA and the formation of IAAsp operated to maintain a relatively constant level of free IAA-¹⁴C in the root system.     

Role of Pseudomonas putida indoleacetic acid in development of the host plant root system

Many plant-associated bacteria synthesize the phytohormone indoleacetic acid (IAA). While IAA produced by phytopathogenic bacteria, mainly by the indoleacetamide pathway, has been implicated in the induction of plant tumors, it is not clear whether IAA synthesized by beneficial bacteria, usually via the indolepyruvic acid pathway, is involved in plant growth promotion. To determine whether bacterial IAA enhances root development in host plants, the ipdc gene that encodes indolepyruvate decarboxylase, a key enzyme in the indolepyruvic acid pathway, was isolated from the plant growth-promoting bacterium Pseudomonas putida GR12-2 and an IAA-deficient mutant constructed by insertional mutagenesis. The canola seedling primary roots from seeds treated with wild-type P. putida GR12-2 were on average 35 to 50% longer than the roots from seeds treated with the IAA-deficient mutant and the roots from uninoculated seeds. In addition, exposing mung bean cuttings to high levels of IAA by soaking them in a suspension of the wild-type strain stimulated the formation of many, very small, adventitious roots. Formation of fewer roots was stimulated by treatment with the IAA-deficient mutant. These results suggest that bacterial IAA plays a major role in the development of the host plant root system.     

Transport of indoleacetic Acid in intact corn coleoptiles

We have characterized the transport of [³H]indoleacetic acid (IAA) in intact corn (Zea mays L.) coleoptiles. We have used a wide range of concentrations of added IAA (28 femtomoles to 100 picomoles taken up over 60 minutes). The shape of the transport curve varies with the concentration of added IAA, although the rate of movement of the observed front of tracer is invariant with concentration. At the lowest concentration of tracer used, the labeled IAA in the transport stream is not detectably metabolized or immobilized, curvature does not develop as a result of tracer application, and normal phototropic and gravitropic responsiveness are not affected. Therefore we believe we are observing the transport of true tracer quantities of labeled auxin at this lowest concentration.     

The transport of radiolabeled indoleacetic acid and its conjugates in nodal stem segments of Phaseolus vulgaris L.

The transport of radiolabeled indoleacetic acid (IAA), and some of its conjugates, was investigated in nodal stem segments of Phaseolus vulgaris L. Donor agar blocks containing either [2-acetyl-¹⁴C]-IAA; [2-acetyl-¹⁴C]-indole-3-acetyl-L-aspartate (IAAsp); [2-acetyl-¹⁴C]-indole-3-acetyl-L-glycine (IAGly); or [2-acetyl-¹⁴C]-indole-3-acetyl-L-alanine (IAAla) were placed on either the apical or basal cut surface of stem segments each bearing an axillary bud at the midline. In some experiments, a receiver block was placed on the end opposite to the donor. After transport was terminated, the segments were divided into five equal sections plus the bud, and the radioactivity of donors, receivers and each part of the stem segment was counted.For all four substances tested, the amount of ¹⁴C transported to the axillary bud from the base was the same or greater than that from the apical end. After basipetal transport, the distribution of ¹⁴C in the segment declined sharply from apex to base. The inverse was true for acropetal transport. Transport for the three IAA conjugates did not differ substantially from each other.The IAA transport inhibitor, N-1-naphthylphthalamic acid (NPA), inhibited basipetal ¹⁴C-IAA transport to the base of the stem segment but did not alter substantially the amount of ¹⁴C-IAA recovered from the bud. Transport of ¹⁴C-IAA from the apical end to all parts of the stem segment declined when the base of the section was treated with nonradioactive IAA. Taken together with data presented in the accompanying article [Tamas et al. (1989) Plant Growth Regul 8: 165–183], these results suggest that the transport of IAA plays a role in axillary bud growth regulation, but its effect does not depend on the accumulation of IAA in the axillary bud itself.     

Fluorescence and photoinactivation of indoleacetic acid

These results indicate quite clearly that the induction of the photoinactivation of indoleacetic acid (IAA) is by no means a peculiarity of riboflavin but is a property common to many fluorescent substances. It is not essential that the compounds be colored. Colorless materials are also able to bring about IAA photoinactivation provided that they absorb ultraviolet light.After exposure to light, the reaction mixture becomes turbid and develops a light-pink coloration. Experiments are being performed to elucidate the nature of the reaction product.It is suggested that besides riboflavin other naturally occurring fluorescent substances in plants may play a role in light-induced growth reactions.     

The effect of kinetin on the indoleacetic acid level and indoleacetic acid oxidase activity in roots of young plants

Kinetin treatment increased the extractable IAA content in roots of young plants of Phaseolus vulgaris L., Zea mays L. and Avena sativa L. The highest increase was obtained with roots of beans and the lowest with oat roots. Maize was intermediate between these two species. Kinetin treatment decreased the activity of IAA-oxidase but the correlation between the decrease of the activity of this enzyme and the increase in the level of IAA was not good. The decrease of the oxidase activity was greatest in oat roots where kinetin had very little effect on the IAA level, and was rather small in bean roots, where kinetin treatment significantly increased the IAA level.     

Effect of malformin on adventitious root formation and metabolism of indoleacetic acid-2-14C by Phaseolus vulgaris

Malformin inhibited rooting on cuttings of Phaseolus vulgaris.IAA antagonized malformin-induced inhibition of rooting, butmalformin inhibited IAA-induced swelling on the base of thecuttings. It was suggested that IAA-induced swelling was mediatedby ethylene. Malformin did not inhibit transport of root-promotingsubstances from upper portions of the cuttings or polar transportof IAA-2-¹⁴C, nor did it alter the melting point of DNA or thebinding of DNA to histone. Although malformin appeared to alterthe metabolism of IAA-2-¹⁴C, the effect may have been the resultof a marked and selective stimulation of efflux of IAA-2-¹⁴Cmetabolites by malformin. Efflux of IAA or its metabolites maycontribute toward inhibition of rooting by malformin. ¹ Journal Paper No. 4688 of the Purdue Agricultural ExperimentStation. Supported in part by grant GB-7158 from the NationalScience Foundation. ² Present address: Botanisches Institut der Technischen UniversitätBraunschweig, 3300 Braunschweig, Humboldtstraße 1. (Received March 9, 1972; )     

Effect of indoleacetic acid,abscisic acid,root tips and coleoptile tips on growth and curvature of maize roots

The effect of externally applied indoleacetic acid (IAA) and abscisic acid (ABA) on the growth of roots of Zea mays L. was measured. Donor blocks of agar with IAA or ABA were placed laterally on the roots and root curvature was measured. When IAA was applied to vertical roots, a curvature directed toward the donor block was observed. This curvature corresponded to a growth inhibition at the side of the root where the donor was applied. When IAA was applied to horizontal roots from the upper side, normal geotropic downward bending was delayed or totally inhibited. The extent of retardation and the inhibition of curvature were found to depend on the concentration of IAA in the donor block. ABA neither induced curvature in vertical roots nor inhibited geotropic curvature in horizontal roots; thus the growth of roots was not inhibited by ABA. However, when, instead of donor blocks, root tips or coleoptile tips were placed onto vertical roots, a curvature of the roots was observed.Abbreviations ABA abscisic acid - IAA 3-indoleacetic acid     

Transport de l'auxine-14C en provenance de jeunes gousses de Vicia faba L.

Summary After the injection of [¹⁴C]indole acetic acid (IAA) into very young pods of broad-bean (Vicia faba L.) the movement of the ¹⁴C in the peduncle and stem was followed by autoradiography. In samples with only one young pod the basipetal transport was always clearly dominant. Most of the radioactivity was found in the bundles, particularly in the outer region of the bundle and also in the inner region (protoxylem parenchyma). The progression of the tracer was relatively complex. The rate of movement of the radioactive «front» could be as much as 2 cm·h^-1 but most of the ¹⁴C moved towards the base at rates clearly less than that of the «front». Chromatograms with several solvent systems showed that IAA was the main or the only mobile radioactive substance. During transport, a part of IAA was converted into indole-3-aldehyde (IAld) and indole-3-acetyl-aspartic acid (IAAsp). IAAsp and possibly also IAld, which were found mainly near the donor pod, seemed immobile. This work is part of a study on the interchange of phytohormones between fruit and plant.

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Abréviations AIA acide indolyl-acétique - AIAsp acide indolyl-acetyl-aspartique - IAld indolyl-aldéhyde     

Distribution of 14C-labelled sucrose in seedlings of Pisum sativum L. Treated with indoleacetic acid and kinetin

Summary The influence of decapitation and treatment with IAA and/or kinetin on the pattern of distribution of ¹⁴C-labelled sucrose applied to the third leaf of 14-day old dwarf pea seedlings was investigated. Decapitation resulted in a diversion of the labelled metabolites to the lateral buds, and greatly increased the radioactivity present in the root system indicating that in these seedlings the roots and apex actively competed for translocates from the third leaf. Application of IAA to the decapitated internode prevented the growth of the lateral buds for the duration of the experiment and restored the pattern of distribution of labelled metabolites found in the intact plant. Application of kinetin alone resulted in a marked accumulation of labelled materials in the lateral buds, but when kinetin was applied with IAA metabolites were once again diverted from the lateral buds to the treated internode. Neither of these treatments had any influence on the proportion of the translocated materials which accumulated in the root system when compared with intact plants. The results are discussed in relation to current concepts of hormone-directed transport of nutrients in plants.