Monoclonal antibodies detect an auxin-induced conformational change in the maize auxin-binding protein

The monoclonal antibody MAC 256 precipitates specifically the auxin-binding protein (ABP) of maize membranes. Auxin-binding activity was recovered from the immunoprecipitate and MAC 256 can, therefore, bind undenatured, native ABP. A sandwich enzyme-linked immunosorbent assay was used to present native ABP to MAC 256 and under these conditions auxins inhibit antibody binding. Millimolar naphthalene-1-acetic acid completely blocks MAC 256 binding and the characteristics of monoclonal antibody MAC 259 are similar. The ability of a range of auxins and related compounds to displace MAC 256 correlates with the known structure-activity relationships of these compounds in vivo and in binding assays. The results are interpreted in terms of an auxin-induced conformational change in ABP, auxin binding leading to a change in, or concealment of, the epitope of the antibody. The epitope for MAC 256 and 259 lies close to the carboxy terminus of the protein, implying that the part of ABP containing the sequence of amino acids responsible for retention within the endoplasmic reticulum is conformationally active.Abbreviations ABP auxin-binding protein - ELISA enzyme-linked immunosorbent assay - IAA indole-3-acetic acid - Mab monoclonal antibody - NAA naphthalene-1-acetic acid - SDS-PAGE sodium dodecyl sulphate polyacrylamide gel electrophoresis - TIBA 2,3,5-triiodobenzoic acid - 2,4,5-T, 2,4,6-T 2,4,5-trichloro- and 2,4,6-trichlorophenoxyacetic acid, respectively We are grateful to Neville Huskisson and Pat Baker of the Microchemical Facility, AFRC IAPGR, Babraham, UK for the aminoacid sequencing and to the staff at the AFRC Monoclonal Antibody Centre, Babraham where the Mabs were produced. This work was partially funded by the Biotechnology Action Programme of the European Economic Community.To whom correspondence should be addressed.     

Plant regeneration in vitro from embryogenic cultures of spring- and winter-type barley (Hordeum vulgare L.) varieties

Summary Immature embryos of 41 lines of barley were screened in vitro for callus induction and somatic embryogenesis on different media to establish totipotent cultures. The use of modified MS and CC media, both supplemented with 1 g/l casein hydrolysate, and the substitution of agarose for agar resulted in the highest frequencies of somatic embryo induction. Embryogenic callus was induced and plants regenerated from 23 of the lines tested. The auxins 2,4-D, dicamba, picloram and 2,4,5-T were suitable for embryogenic callus induction. High frequencies of somatic embryo germination occurred on CC medium supplemented with 1 mg/l IAA and 0.05 mg/l zeatin. A strong genotypic effect on the capacity and frequency of embryogenic callus formation was found. Cultivar Golden Promise always gave the best results. Experiments with field grown material in 3 consecutive years showed that environmental factors also strongly influenced the induction of somatic embryogenesis and plant regeneration.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - 2,4,5-T 2,4,5-trichlorophenoxyacetic acid - dicamba 3,6-dichloro-o-anisic acid - picloram 4-amino-3,6,6-trichloropicolinic acid - NAA naphtaleneacetic acid - IAA indole-3-acetic acid - ABA abscisic acid - BAP 6-benzyl amino purine - 2iP 6-(3-methyl-2 butenyl 1-amino)purine - GA₃ gibberellic acid     

Comparison of mechanisms controlling uptake and accumulation of 2,4-dichlorophenoxy acetic acid,naphthalene-1-acetic acid,and indole-3-acetic acid in suspension-cultured tobacco cells

Accumulation of radiolabelled naphthalene-1-acetic acid (1-NAA), 2,4-dichlorophenoxyacetic acid (2,4-D), and indole-3-acetic acid (IAA) has been measured in suspension-cultured tobacco (Nicotiana tabacum) cells. In this paper is presented a simple methodology allowing activities of the auxin influx and efflux carriers to be monitored independently by measuring the cellular accumulation of [³H]NAA and [¹⁴C]2,4-D. We have shown that 1-NAA enters cells by passive diffusion and has its accumulation level controlled by the efflux carrier. By contrast, 2,4-D uptake is mostly ensured by the influx carrier and this auxin is not secreted by the efflux carrier. Both auxin carriers contribute to IAA accumulation. The kinetic parameters and specificity of each carrier have been determined and new information concerning interactions with naphthylphthalamic acid, pyrenoylbenzoic acid, and naphthalene-2-acetic acid are provided. The relative contributions of diffusion and carrier-mediated influx and efflux to the membrane transport of 2,4-D, 1-NAA, and IAA have been quantified, and the data indicate that plant cells are able to modulate over a large range their auxin content by modifying the activity of each carrier.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - 1-NAA naphthalene-1-acetic acid - 2-NAA naphthalene-2-acetic acid - NPA N-1-naphthylphthalamic acid - PBA 2-(1-pyrenoyl)benzoic acid - V_m maximum transport capacity of the carrier In honour of Professor Dieter Klämbt's 65th birthdayThe authors thank Drs. A.E. Geissler and G.F. Katekar (CSIRO, Canberra City, Australia) for providing auxin efflux carrier inhibitors CPD, CPP, and PBA, and Dr. H. Barbier-Brygoo (Institut des Sciences Végétales, CNRS, Gif-sur-Yvette, France) for helpful discussions. This work was supported by funds from the Centre National de la Recherche Scientifique (UPR0040).     

Auxin binding in roots: A comparison between maize roots and coleoptiles

Auxin binding onto membrane fractions of primary roots of maize seedlings has been demonstrated using naphth-1yl-acetic acid (NAA) and indol-3yl-acetic acid (IAA) as ligands. This binding is compared with the already well characterized interaction between auxins and coleoptile membranes. The results indicate that while kinetic parameters are of the same order for root and coleoptile binding, a number of differences occur with respect to location in cells and relative affinity. The possible significance of the existence of such binding sites in root cells is discussed in relation to auxin action.Abbreviations 4-Cl-PA 4-chlorophenoxyacetic acid - EDTA ethylene diamine tetracetic acid - IAA indol-3yl-acetic acid - MCPA 2-methyl-4-chlorophenoxyacetic acid - NAA naphth-1yl-acetic acid - 2-NAA naphth-2yl-acetic acid - Tris 2-amino-2-(hydroxymethyl) propane-1,3 diol - TIBA 2,3,5 triiodobenzoic acid - NPA naphthylphthalamic acid - PCIB 4-chlorophenoxyisobutyric acid - PCPP 4-chlorophenoxyisopropionic acid - 2,4-D 2,4-dichlorophenoxyacetic acid     

Initiation and growth of cell lines of Taxus brevifolia (Pacific yew)

Summary Conditions have been established for the induction and maintenance of callus cultures of Taxus brevifolia (Pacific yew) from bark, stem, and needle tissues. Cultures were established on a modified Gamborg's B5 medium, 1% sucrose, 0.2% casamino acids and 1 mg/L 2,4-D. There was no apparent inhibition of callus induction as a result of taxol concentration in the explant material. Cell lines derived from explants of individual trees were used to investigate growth characteristics. Although none of the cell lines contained taxol, some contained low levels of related taxanes. Variability was observed with each cell line in response to light, and auxin type and concentration. Growth index was most affected by cell line, followed by auxin type and concentration. These culturing methods may be useful for the goal of developing a highproducing cell line applicable for large-scale taxol production.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - NAA naphthaleneacetic acid - IAA indoleacetic acid - CA casamino acids - B5CA B5 with 0.2% casamino acids - IBA indolebutryric acid; Picloram (4-amino-3,5,6-tricnloro-2-pyridinecarboxylic acid) - 2,4,5-T 2,4,5-trichlorophenoxyacetic acid - BA 6-benzylaminopurine     

Somatic embryogenesis and organogenesis inGuizotia Abyssinica

Summary Induction of somatic embryogenesis, shoot organogenesis, and subsequent plant regeneration in niger seem to be dependent on genotype, choice of explant, and composition of media growth regulators. Two distinct regeneration protocols have been developed for somatic embryogenesis and shoot organogenesis. Somatic embryogenesis was induced from epicotyls and cotyledonary explants (9 to 35%) (but not from hypocotyls and roots) in presence of 2,4-dichlorophenoxyacetic acid, 2,4,5-trichlorophenoxyacetic acid, and 2,4,5-trichlorophenoxypropionic acid. These embryos matured in MS medium containing Kinetin plus naphthalene acetic acid (NAA), Kinetin plus Zeatin, and Kinetin plus abscisic acid (ABA). Matured embryos could be germinated on LS and MS basal media without hormones. Non-embryogenic callus initiated on Linsmaier and Skoog’s (LS) medium from cotyledons of six different genotypes produced shoots (9 to 32%) on Murashige and Skoog’s (MS) medium fortified with 6-benzylaminopurine (BAP, 0.5 mg · liter^−1), and BAP (1 mg · liter^−1) plus NAA (0.1 mg · liter^−1). These shoots were rooted with 100% frequency by using indole-3-acetic acid or NAA and transferred successfully to the soil.     

Auxin binding to corn coleoptile membranes: Kinetics and specificity

Summary Detailed examination of binding over the range 10^-7–10^-6 M suggests that membrane preparations from coleoptiles of Zea mays L., cv Kelvedon 33 contain at least two sets of high affinity binding sites for 1-naphthylacetic acid (NAA), with dissociation constants of 1.8×10^-7 M (site 1) and 14.5×10^-7 M (site 2). Similar studies with 3-indolylacetic acid (IAA) also indicate two sets of binding sites, whose concentrations are closely comparable to those deduced for NAA. A substantial proportion of the total binding activity is retained in a detergent-solubilized preparation. Using [¹⁴C]NAA the interactions of a range of analogues with each of the binding sites have been examined with the aid of double reciprocal plots. The specificity of site 2 is compatible with that expected for an auxin receptor, in that only active auxins, antiauxin transport inhibitors are able to compete with [¹⁴C]NAA for the binding sites. Site 1 on the other hand is less specific, since it appears to bind all compounds tested, including physiologically inactive analogues.Abbreviations NAA 1-naphthylacetic acid - IAA 3-indolylacetic acid - 2,4-D 2,4-dichlorophenoxyacetic acid - 2,6-D 2,6-dichlorophenoxyacetic acid - 2,4,5-T 2,4,5-trichlorophenoxyacetic acid - 2-CPIB -(2-chlorophenoxy)-isobutyric acid - 2,4-B 2,4-dichlorobenzoic acid - 2,6-B 2,6-dichlorobenzoic acid - TIBA 2,3,5-triiodobenzoic acid - NPA 1-N-naphthylphthalamic acid     

Influence of medium composition on callus induction and camptothecin(s) accumulation in Nothapodytes foetida

Camptothecin(s) production was examined in callus cultures derived from cotyledons of Nothapodytes foetida (Weigh) Sleumer. The calluses were grown on various combinations of Murashige and Skoog's basal media supplemented with auxins 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), a-napthalene acetic acid (NAA) and indole-3-acetic acid (IAA) with 6-benzyl aminopurine (BA)/kinetin in different concentrations. The presence of camptothecin (CPT) and 9-methoxycamptothecin (9-OMeCPT) were analyzed by HPLC in relation to the media composition. Hyper production of CPT(1.306% on dry wt. basis) was observed with a combination of 2,4-D with BA and 2,4,5-T and NAA in 1-month-old callus.     

The specificity of carrier-mediated auxin transport by suspension-cultured crown gall cells

1. The specificity of the auxin transport system of suspension-cultured crown gall cells from Parthenocissus tricuspidata Planch- is examined with regard to 2,4-Dichlorophenoxyacetic acid (2,4 D), l-Naphthylacetic acid (NAA) and Benzoic acid (BA) as well as for indole-3-acetic acid (IAA). — 2. All four weak acids can be accumulated by the cells from a medium more acidic than the cytoplasm. This is by virtue of non-specific passive diffusion of their lipid-soluble protonated forms down a concentration gradient. The corresponding anionic species are much less permeant. The extent of the accumulation is dependent on the pH difference that is maintained by the cells between their cytoplasm and the incubation medium. Studies of the concentration dependence of BA and NAA net uptake at a series of external pHs suggest that an acidification of the cytoplasm can be eventually brought about by the entry of weak acid into the cells. — 3. The uptake of 2,4 D, as well as that of IAA, has a saturable carrier-mediated component in addition to the passive diffusion of the undissociated acid. These saturable components probably represent anion uptake and appear to be mediated by a common carrier. The kinetic studies provided no evidence for the participation of carriers in the transport of BA or NAA. — 4. It was shown that the efflux of 2,4 D also has a carrier-mediated component and it is suggested that both the influx and efflux of IAA and 2,4 D occur on a common carrier. — 5. The inhibitor of polar auxin transport, 2,3,5-triiodobenzoic acid (TIBA), stimulates the net uptake of IAA by inhibiting carrier-mediated efflux of IAA from the cells. However, TIBA could not be demonstrated to have a significant effect on 2,4 D transport and any perturbation that occurs is very small in comparison with its effect on IAA movement. To account for this, the proposed common carrier could exhibit some difference in its internal binding characteristics betweend 2,4 D and IAA. An alternative explanation is that a second carrier is present, which mediates IAA efflux only, and which is inhibited by TIBA. — 6. TIBA has no significant effect on the transport of either BA or NAA, except to bring about an inhibition of net uptake, and a corresponding stimulation of efflux, when it is present at concentrations sufficient to acidify the cytoplasm. —7. The crown gall cells are compared to intact plant tissues capable of polar auxin transport with regard to the specificities exhibited for the transport of the auxins IAA, 2,4 D and NAA and the non-auxin BA.Abbreviations IAA indol-3-yl acetic acid - 2,4 D 2,4-Dichlorophenoxyacetic acid - NAA 1-Naphthylacetic acid - BA Benzoic acid - TIBA 2,3,5-triiodobenzoic acid     

Hormone-binding studies and the misuse of precipitation assays

Many methods are available for determining binding of small-molecular ligands to macromolecules. In studies on plant hormone binding, precipitation of putative hormone-protein complexes with ammonium sulphate is often used as the sole method of assay. Several such reports on auxin binding have been re-examined, using additional assay procedures. The results show that apparent binding activity measured by ammonium sulphate precipitation may be undetectable by independent, less ambiguous methods. It is concluded that hormone-binding studies that rely solely on the precipitation assay can be grossly misleading, and that this assay, if used at all, should always be validated against a procedure less likely to generate artefacts. An example of such validation for a plant protein is shown.Abbreviations BSA bovine serum albumin - IAA indole-3-acetic acid - NAA naphthalene-1-acetic acid - Tris 2-amino-2-(hydroxymethyl)-1,3-propanediol     

Somatic embryogenesis from mature embryo-derived leaflets of peanut (Arachis Hypogaea L.)

Summary Embryogenic masses were obtained from immature leaves of peanut (Arachis hypogaea L.) cultured on a medium containing 20 mg/l 2,4-D. Somatic embryos developed from these masses following transfer to a medium containing 3 mg/l 2,4-D. The embryo morphology was quite variable. Following transfer to hormone-free medium, these embryos germinated. Shoot elongation was obtained in 25% of the embryos following transfer to a medium supplemented with 0.5 mg/l each of BAP and Kn. The plants grown in vitro by this method survived in sand:soil mixture and were grown to maturity.Abbreviations ABA abscisic acid - BAP 6-benzyl amino purine - 2,4-D 2,4 dichlorophenoxyacetic acid - GA₃ gibberellic acid - Kn kinetin - NAA 1-naphthaleneacetic acid - 2,4,5-T 2,4,5-trichlorophenoxyacetic acid - Z zeatin     

Callus and embryoid formation from protoplasts of Helianthus annuus

Sunflower hypocotyl protoplasts have been isolated and cultured. Optimum plating density for cell division and colony formation was in the range of 5 to 7×10⁴ cells/mi in an agarose medium supplemented with BAP (1 mg/l) and NAA (1 mg/l). Plating efficiency was 60% after 21 days of culture. In the resultant culture a mixed population of calli and embryoids was observed. Thirty seven percent of the cell clusters exhibited a developmental pattern similar to an embryoid. Many stages of embryogenesis were observed in the same cultures.Abbreviations 2,4-D 2,4 dichlorophenoxyacetic acid - NAA 1-naphtaleneacetic acid - IAA Indole-3-acetic - BAP 6-benzylamino purine - GA₃ Gibberellic acid     

One step shoot tip multiplication and rooting of Digitalis thapsi L.

Shoot tips from seedlings of Digitalis thapsi L. were cultured on Murashige and Skoog's medium and the effect of various auxins (2,4-D, NAA and IAA) were analyzed alone or in combination with cytokinis (BA and kinetin). Shoot multiplication and direct rooting of the new shoots were obtained after four weeks of culture in MS medium without hormones, but callus formation and the appearance of abnormal phenotypes were frequent. The addition of auxins to the cultures prevented the formation of callus but not the appearance of variant phenotypes. Both drawbacks could be avoided by combination of NAA or IAA with BA or kinetin. The best results for shoot multiplication and direct rooting were obtained with 0.5 mg l^-1 NAA and 0.1 or 0.5 mg l^-1 kinetin.Abbreviations BA 6-benciladenine - 2,4-D 2,4-dichlorophenoxyacetic acid - IAA indole-3-acetic acid - Kin kinetin - NAA naphtalene acetic acid - MS Murashige and Skoog     

Somatic embryogenesis in cell cultures of Thapsia garganica

Cell cultures from different species of the genus Thapsia (Apiaceae) have been investigated. In one 4-yearold line of T. garganica L. spontaneous somatic embryogenesis up to the globular stage occurred in a suspension culture containing 1 mg l^–12,4-dichlorophenoxyacetic acid (2,4-D). Also callus cultures of this line, previously maintained on a medium containing 1 mg l^–1 2,4-D, when transferred to various media deprived of 2,4-D, produced somatic embryos that developed into plantlets. Cell culture, embryos and regenerated organs were analysed for their content of thapsigargins. The undifferentiated cell culture did not synthezise thapsigargins, but was found to produce a yet unidentified compound not present in planta. White embryos in the pre-cotyledonary stage did not synthezise thapsigargins either, but when the embryos developed to the cotyledonary stage and became green, the synthesis started. Regenerated roots and shoots also contained thapsigargins.Abbreviations BAP Benzylaminopurine - 2,4-D 2,4-Dichlorophenoxyacetic acid - EtOAc ethyl acetate - FDA fluorescein diacetate - IAA Indole-3-acetic acid - IBA indole-3-butyric acid - 2-iP 2-isopentenyladenine - NAA 1-Napthaleneacetic acid     

Somatic embryogenesis and plant regeneration of squash Cucurbita pepo L cv. YC 60

Plant regeneration from tissue cultures of summer squash, Cucurbita pepo L., cv. YC60, has been observed. Somatic embryos organized from shoot apex derived callus cultured on Murashige and Skoog (MS) medium supplemented with 1.2 mg/l 2,4,5-trichlorophenoxyacetic acid, 0.8 mg/l benzylaminopurine, and 0.1 mg/l kinetin. Embryos developed into plantlets by transfer of immature somatic embryos to MS medium with 0.05 mg/l NAA and 0.05 mg/l kinetin. Regenerated plants appeared morphologically normal and set fruits with seeds which could germinate normally.Abbreviation BAP 6-benzylaminopurine - 2,4-D 2, 4dichlorophenoxyacetic acid - IAA indole-3-acetic acid - KN kinetin - NAA -naphthyleneacetic acid - MS Murashige and Skoog - 2,4,5-T 2, 4,5-trichlorophenoxyacetic acid     

Somatic embryogenesis in Cucurbitaceae

Strategies based on the application of biotechnologies to crop improvement programmes generally require regeneration of whole plants from cells or tissues cultivated in vitro. In Cucurbitaceae, regeneration can occur either through a caulogenic or an embryogenic developmental pathway. Reports of somatic embryogenesis have dealt with the main cultivated crops, i.e. cucumber, melon, squash and watermelon. Somatic embryogenesis and plant recovery are obtained from numerous sources including protoplasts, but the best results are observed with explants coming from seedlings, especially cotyledons and hypocotyls. The genetic constitution of mother plants also seems to play a key role in the success of embryogenesis, but few systematic studies on genotype effect have been published. Somatic embryos can exhibit developmental abnormalities, particularly when they arise from protoplast-derived cultures. Generally, data concerning embryo yield, rate of germination and plant development and characteristics of regenerated plants and their progeny, has not been provided in previous reports. The potential use of somatic embryogenesis in cucurbit breeding programmes is stressed in this review.Abbreviations ABA abscisic acid - BAP 6-benzylaminopurine - B₅ Gamborg et al. (1968) - CH casein hydrolysate - CW coconut water - 2,4-d 2,4-dichlorophenoxy acetic acid - GA₃ gibberellin A₃ - GA₄ gibberellin A₄ - H Heller (1953) - IAA indole 3 acetic acid - IBA indole 3 butyric acid - 2ip 2 isopentenyladenine - KIN kinetin - MS Murashige & Skoog (1962) - N Nitsch (1951) - N6 Chu et al. (1975) - NAA 1 naphthalene acetic acid - TDZ thidiazuron - 2,4,5-T 2,4,5-trichlorophenoxyacetic acid     

Analogues of auxin modifying growth and development of some monocot and dicot plants

The dependence of morphogenetic processes in the formation of vegetative and generative organs in spring oilseed rape and barley on exogenously applied physiological analogues of auxin: 2,4-D (2,4-dichlorphenoxyacetic acid), NAA (naphthalene-1-acetic acid), TA-12 (1-[2-chloroethoxycarbonyl-methyl]-4-naphthalenesulfonic acid calcium salt) and TA-14 (1-[2-dimethylaminoethoxicarbonylmethyl]naphtalene chlormethylate) were investigated. The experiments were performed with hypocotyl tissue cultures of oilseed rape and barley microspores in vitro. The auxin analogues applied revealed differences of morphogenetic competence in dedifferentiation-redifferentiation processes that occurred in oilseed rape cultures. TA-12 and TA-14 applied together with NAA and BA (6-benzylaminopurine) caused more intensive callus growth in comparison with 2,4-D. Rhizogenesis was induced when 2,4-D was substituted by TA-12. Compound TA-14, unlike TA-12, facilitated the appearance and development of cotyledons in callus tissues. Hower the compounds TA-12 and TA-14 have no positive effect in monocot plant — barly anther culture for callogenesis and regeneration in comparison to indole-3-acetic acid (IAA). TA-14 and TA-12 showed similar but not identical auxin properties and demonstrated high efficiency as modifiers of rape-dicot plant growth and morphogenesis.     

Plant regeneration from cotyledons of niger

Callus initiation from seedling explants of niger (Guizotia abyssinica Cass) cv. Ootacamund was found to be better on LS medium containing kinetin (1.4 M) plus 2,4-dichlorophenoxyacetic acid (9 M) than its analogues. Embryoids were induced directly from cotyledons on LS medium supplemented with 2,4,5-trichlorophenoxyacetic acid and 2,4,5-trichlorophenoxypropionic acid. When cotyledon-derived callus was subcultured onto medium with 10.7 M naphthalene-acetic acid and 2.3 M kinetin, embryogenesis was observed. Multiple shoots were obtained from cotyledonary explants in presence of MS medium containing 4.4 M benzyladenine and 11.4 M indoleacetic acid. Regenerated plants that were transferred to pots and grown to maturity were morphologically normal and fertile.Abbreviations NAA naphthaleneacetic acid - IAA indoleacetic acid - BA benzyladenine - GA₃ gibberellic acid - 2,4-D 2,4-dichlorophenoxyacetic acid - 2,4,5-T 2,4,5-trichlorophenoxypropionic acid - 2,4,5-TP 2,4,5-trichlorophenoxypropionic acid - ABA abscisic acid     

Tissue culture and plant regeneration from immature embryo explants of Calotropis gigantea (Linn.) R. Br.

Callus cultures were established from immature embryos of Calotropis gigantea (Linn.) R. Br. on a modified basal medium of Murashige & Skoog supplemented with 1 mgl^-1 2,4-D. In addition to 0.1 mgl^-1 of NAA the optimal BAP concentration for promoting shoot bud formation and growth was 2 mgl^-1. Rooting was induced when shoots were transferred to auxin-supplemented Bonner's solution or half-strength MS basal salt solutions.Abbreviations NAA -naphthaleneacetic acid - 2,4-D 2,4-dichlorophenoxyacetic acid - IAA indole-3-acetic acid - IBA indole-butyric acid - BAP 6-benzylaminopurine - Kin kinetin     

In vitro binding affinities of 4-chloro-, 2-methyl-, 4-methyl-, and 4-ethylindoleacetic acid to auxin-binding protein 1 (ABP1) correlate with their growth-stimulating activities

4-Chlorindole-3-acetic acid (4-CI-IAA), an endogenous auxin in certain plant species of Fabaceae, has a higher efficiency in stimulating cell elongation of grass coleoptiles compared with indole-3-acetic acid (IAA), particularly at low concentrations. However, some investigations reported a 1,000-fold discrepancy between growth stimulation and binding affinity of 4-CI-IAA to auxin-binding protein 1 (ABP1) from maize. Here we report binding data of 4-CI-IAA and three alkylated IAA derivatives using purified ABP1 in equilibrium dialysis. There is a clear correlation between the growth-promoting effects and the binding affinity to ABP1 of the different IAA analogues measured by competition of [³H]naphthalene-1-acetic acid binding. Our data are consistent with the hypothesis that ABP1 mediates auxin-induced cell elongation.Abbreviations ABP1 auxin-binding protein 1 - 4-CI-IAA 4-chloroindole-3-acetic acid - NAA naphthalene-1-acetic acid - ER endoplasmic reticulum - IAA indole-3-acetic acid - 2-Me-IAA 2-methylindole-3-acetic acid - 4-Me-IAA 4-methylindole-3-acetic acid - 4-Et-IAA 4-ethylindole-3-acetic acid - MES 4-morpholineethanesulfonic acid - PAA phenylacetic acid