The beneficial role of different auxins and polyamines at successive stages of somatic embryo formation and development of Panax ginseng in vitro

The production of viable plantlets via somatic embryogenesis in Panax ginseng requires different culture media corresponding to successive developmental stages. The effects of several auxins and polyamines have been tested at various steps. Multiplication of the embryogenic root-derived callus has been optimized on half-strength MS medium supplemented with 3-(benzo[b]selenyl)acetic acid (BSAA, a synthetic auxin) and kinetin; exogenously applied polyamines were deleterious at this stage, causing browning of the callus, diminished capacity of embryo initiation, and an increased tendency to hyperhydricity. BSAA again appeared to be the most favourable auxin at the initiation stage, but here its action was reinforced by the presence of polyamines, spermidine being the most efficient. Among the auxin needed at the next step, i.e., for the regeneration of embryos, the two seleniated auxins BSAA and seleniated 2,4-dichlorophenoxyacetic acid (2,4-D-Se) were the more efficient. For the harmonious development of plantlets, i.e., the simultaneous outgrowth of shoots and roots, the polyamines were favourable, with a greater efficiency for spermine.     

Effects of plant growth regulators on acetylene-reducing associations between Azospirillum brasilense and wheat

Treatment of wheat seedlings with the synthetic auxin, 2,4-dichlorophenoxyacetic acid (2,4-d), induced nodule-like structures or tumours (termed para-nodules) where lateral roots would normally emerge. The formation of these structures promoted increased rates of acetylene reduction at reduced oxygen pressure (0.02–0.04 atm) in seedling inoculated with Azospirillum brasilense, compared to seedlings inoculated without auxin treatment. Fluorescent microscopy, laser scanning confocal microscopy and direct bacterial counts all showed that the 2,4-d treatment stimulated internal colonization of the root system with azospirilla, particularly in the basal region of the nodular structures. Both colonization with azospirilla and acetylene-reducing activity were further stimulated by simultaneous treatment with another synthetic auxin, naphthaleneacetic acid (NAA) and, less reliably, with indoleacetic acid (IAA) and indolebutyric acid (IBA). These auxins produced shortening of many initiated lateral roots, although 20 times the concentration of NAA was required to achieve rounded structures similar to those obtained with 2,4-d. Treatment with NAA, IAA or IBA alone also stimulated colonization with azospirilla and acetylene reduction rates at 0.02 atm oxygen, but less effectively than by treatment with 2,4-d. Such exogenous treatments of wheat seedlings with synthetic growth regulators provide an effective laboratory model for studies on the development of a N₂-fixing system in cereals.     

Auxin structure and activity on tomato morphogenesis in vitro and pea stem elongation

In order to understand better the relationship between auxin structure and activity on morphogenesis and cell elongation, six different auxins were tested on the regeneration of tomato (Lycopersicon esculentum Miller var. Alice) from cotyledons and on pea (Pisum sativum L. var. Alaska) stem elongation. The auxins were: indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), 1, 2-benzisoxazole-3-acetic acid (BOA), 1,2-benzisothiazole-3-acetic acid (BIA), 1-naphthalenacetic acid (NAA), 2,4-dichlorophenoxyacetic acid (2,4-D). All these compounds obey the minimum requirement rules for auxin activity and all were effective on cell elongation. At the dose of 10 M and in the absence of cytokinin, they all, except 2,4-D, induced roots, while in the presence of cytokinin they induced shoots, roots, hairy root-like filaments (HRLF) or callus depending on their concentration. The morphogenetic pattern did not change by varying cytokinin concentration. We conclude that auxin structure plays a minor role in morphogenesis or cell elongation, because it is only responsible for variations in the level of auxin activity.     

Analysis of mycorrhizal associations formed by Cistus incanus transformed root clones with Terfezia boudieri isolates

One clone (M-2), out of several Agrobacterium rhizogenes transformed root clones of Cistus incanus, formed ecto- or endomycorrhiza in vitro with two isolates of Terfezia boudieri collected in Israel. All other clone-fungal isolate combinations formed ectomycorrhiza. The endomycorrhiza-forming isolate secreted smaller amounts of auxin than an ectomycorrhiza-forming isolate. Addition of 2,4-dichlorophenoxyacetic acid (2,4-D) led to ectomycorrhiza formation by the M-2 clone on low P medium. Endomycorrhizas were formed by both M-2 and a control clone with the same T. boudieri isolates on high P medium with 2,4-D. The M-2 clone of C. incanus exhibited greater sensitivity to exogenous auxins (IAA and 2,4-D) than other clones, and clonal sensitivity to auxin was increased tenfold under low P conditions. Results are discussed in relation to phosphate and auxin influence on T. boudieri–C. incanus interaction.     

Incubation of corn coleoptiles with auxin enhances in-vitro fusicoccin binding

Binding of fusicoccin (FC) to microsomal preparations of corn (Zea mays L.) coleoptiles is enhanced after incubation of the tissue with indole-3-acetic acid (IAA). Treatment of the kinetic data according to Scatchard shows that the enhancement is a consequence of an increase in the number of high-affinity FC-binding sites without changes of their K_D. The minimal effective concentration of IAA is 10^-7 M; above 10^-5 M the effect declines. The stimulation is insensitive to protein-synthesis inhibitors (cycloheximide and puromycin). The same effect is observed with the synthetic auxins 2,4-dichlorophenoxyacetic acid and naphtalene-1-acetic acid while it is abolished by the auxin antagonists naphtalene-2-acetic acid and p-chlorophenoxyisobutyric acid. Since the above effect is only observed with intact tissue and not after incubation of IAA with microsomal preparations, a direct interaction of IAA with the FC-binding sites is ruled out and an alternative mechanism must be sought.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - FC fusicoccin - [³H]FC ³H-labeled dihydrofusicoccin - IAA indole-3-acetic acid - 1-NAA naphtalene-1-acetic acid - 2-NAA naphtalene-2-acetic acid - PCIB p-chlorophenoxyisobutyric acid     

Characterization of competence during induction of somatic embryogenesis in alfalfa tissue culture

Systematic manipulations of the culture protocol leading to somatic embryogenesis in alfalfa petiole-derived callus were performed to study the competence phase during somatic embryogenesis in alfalfa. These demonstrated a requirement for the acquisition of competence prior to the induction of the embryogenesis pathway. Induction was triggered by a number of synthetic auxins including 2,4-dichlorophenoxyacetic acid (2,4-d). Competence could be acquired in the presence of these auxins as well as phenylacetic acid (PAA), an auxin that did not induce embryogenesis. Different degrees of competence were apparently acquired by exposure to 2,4-d and PAA. Some degree of competence was acquired in the absence of auxin treatment. The current understanding of the concept of competence is discussed.PRC contribution no. 1431     

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     

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     

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     

Auxin binding to subcellular fractions from Cucurbita hypocotyls: In vitro evidence for an auxin transport carrier

An auxin binding sive, with characteristics different from the previously described auxin binding sites I and II in maize coleoptiles, is reported in homogenates of zucchini (Cucurbita pepo L. cv. Black Beauty) hypocotyls. Evidence from differential centrifugation and sucrose and metrizamide density gradients indicates that the site is localized on the plasma membrane. The site has a K_D of 1–2×10^–6 M for indole acetic acid and has a pH optimum of 5.0. Binding specificity measured with several auxins, weak auxins, and anti-auxins generally parallels the activities of the same compounds as inhibitors of auxin transport. 1-N-naphthylphthalamic acid and 2,3,5-triiodobenzoic acid (2,3,5-TIBA), both auxin transport inhibitors in vivo, increase specific auxin binding to this site. 3,4,5-TIBA, which can partially reverse 2,3,5-TIBA's transport inhibition when the two substances are added together in vivo, partially reverses 2,3,5-TIBA's increase in specific auxin binding to the plasma membrane site when added with 2,3,5-TIBA in vitro. Preliminary investigations indicate that a similar plasma membrane site exists in maize (Zea mays L.) coleoptiles. It is suggested that different conformations of this site may function during active auxin transport.Abbreviations IAA indole-3-acetic acid - NPA 1-N-naphthylphthalamie acid - 2,3,5-TIBA 2,3,5-triiodobenzoic acid - 3,4,5-TIBA 3,4,5-triiodobenzoic acid - 1-NAA 1-naphthaleneacetic acid - 2-NAA 2-naphthaleneacetic acid - 2,4-D 2,4-dichlorophenoxyacetic acid - DTE dithioerythritol - MOPS N-morpholino-3-propansulfonic acid - CCO cytochrome c oxidase - CCR NADH: cytochrome c reductase - glu I glucan synthetase I - ER endoplasmic reticulum     

Increases in jasmonic acid caused by indole-3-acetic acid and auxin herbicides in cleavers (Galium aparine)

The effects of indole-3-acetic acid and auxin herbicides on endogenous jasmonic acid (JA) concentrations were studied in relation to changes in ethylene and abscisic acid (ABA) levels in cleavers (Galium aparine). When plants were root-treated with increasing concentrations of indole-3-acetic acid (IAA), ethylene biosynthesis was stimulated in response to the accumulation of endogenous IAA in the shoot tissue. Within 25h of treatment, stimulated ethylene formation was accompanied by increases in immunoreactive concentrations of JA and ABA, which reached maxima of 4.5-fold and 26-fold of the control, respectively, at 100 microM of applied IAA. Corresponding effects were obtained using synthetic auxins and when the ethylene-releasing compound ethephon was applied exogenously. This represents the first report, to our knowledge, of an auxin-mediated increase in JA levels. The increase in JA may be triggered by ethylene.     

Different Behaviour of Indole-3-Acetic Acid and Indole-3-Butyric Acid in Stimulating Lateral Root Development in Rice (Oryza sativa L.)

The plant hormone auxin has been shown to be involved in lateral root development and application of auxins, indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA), increases the number of lateral roots in several plants. We found that the effects of two auxins on lateral root development in the indica rice (Oryza sativa L. cv. IR8) were totally different from each other depending on the application method. When the roots were incubated with an auxin solution, IAA inhibited lateral root development, while IBA was stimulatory. In contrast, when auxin was applied to the shoot, IAA promoted lateral root formation, while IBA did not. The transport of [³H]IAA from shoot to root occurred efficiently (% transported compared to supplied) but that of [³H]IBA did not, which is consistent with the stimulatory effect of IAA on lateral root production when applied to the shoot. The auxin action of IBA has been suggested to be due to its conversion to IAA. However, in rice IAA competitively inhibited the stimulatory effect of IBA on lateral root formation when they were applied to the incubation solution, suggesting that the stimulatory effect of IBA on lateral root development is not through its conversion to IAA.     

Properties of auxin binding sites in different subcellular fractions from maize coleoptiles

In-vitro binding of labeled auxins to sedimentable particles was tested in subcellular fractions from homogenates of maize (Zea mays L.) coleoptiles. The material was fractionated by differential centrifugation or on sucrose density gradients. It was confirmed that the major saturable binding activity (site I) for 1-naphthyl[1-¹⁴C]acetic acid is associated with vesicles derived from the endoplasmatic reticulum. A second type of specific auxin binding (site II) could be distinguished by several criteria, e.g. by the low affinity towards phenylacetic acid. The particles carrying site II could be clearly separated from markers of the endoplasmatic reticulum, the plasmalemma, the mitochondria and the nuclei, while their density as well as sedimentation velocity correlated with particle-bound acid phosphatase, indicating a localization at the tonoplast. In contrast to site I, binding at site II was hardly affected by a supernatant factor and by sulfhydryl groups. However, the specificity pattern of site II towards auxins and auxin analogs was very similar to that of site I tested in the presence of supernatant factor. The existence of a third auxin receptor localized in plasma membrane-rich gradient fractions was indicated by a preferential in-vitro binding of 2,4-dichlorophenoxyacetic acid.Abbreviations 1-NAA 1-naphthyl acetic acid - 2-NAA 2-naphthyl acetic acid - IAA 3-indolyl acetic acid - PAA phenyl acetic acid - 2,4-D 2,4-D-dichlorophenoxy acetic acid - D-2,4-DP dichlorophenoxy isopropionic acid - NPA 1-N-naphthyl phthalamic acid - ER endoplasmatic reticulum - SF supernatant factor     

Arabidopsis iba response5 suppressors separate responses to various hormones

Auxin controls numerous plant growth processes by directing cell division and expansion. Auxin-response mutants, including iba response5 (ibr5), exhibit a long root and decreased lateral root production in response to exogenous auxins. ibr5 also displays resistance to the phytohormone abscisic acid (ABA). We found that the sar3 suppressor of auxin resistant1 (axr1) mutant does not suppress ibr5 auxin-response defects, suggesting that screening for ibr5 suppressors might reveal new components important for phytohormone responsiveness. We identified two classes of Arabidopsis thaliana mutants that suppressed ibr5 resistance to indole-3-butyric acid (IBA): those with restored responses to both the auxin precursor IBA and the active auxin indole-3-acetic acid (IAA) and those with restored response to IBA but not IAA. Restored IAA sensitivity was accompanied by restored ABA responsiveness, whereas suppressors that remained IAA resistant also remained ABA resistant. Some suppressors restored sensitivity to both natural and synthetic auxins; others restored responsiveness only to auxin precursors. We used positional information to determine that one ibr5 suppressor carried a mutation in PLEIOTROPIC DRUG RESISTANCE9 (PDR9/ABCG37/At3g53480), which encodes an ATP-binding cassette transporter previously implicated in cellular efflux of the synthetic auxin 2,4-dichlorophenoxyacetic acid.     

Mutations in an auxin receptor homolog AFB5 and in SGT1b confer resistance to synthetic picolinate auxins and not to 2,4-dichlorophenoxyacetic acid or indole-3-acetic acid in Arabidopsis

Although a wide range of structurally diverse small molecules can act as auxins, it is unclear whether all of these compounds act via the same mechanisms that have been characterized for 2,4-dichlorophenoxyacetic acid (2,4-D) and indole-3-acetic acid (IAA). To address this question, we used a novel member of the picolinate class of synthetic auxins that is structurally distinct from 2,4-D to screen for Arabidopsis (Arabidopsis thaliana) mutants that show chemically selective auxin resistance. We identified seven alleles at two distinct genetic loci that conferred significant resistance to picolinate auxins such as picloram, yet had minimal cross-resistance to 2,4-D or IAA. Double mutants had the same level and selectivity of resistance as single mutants. The sites of the mutations were identified by positional mapping as At4g11260 and At5g49980. At5g49980 is previously uncharacterized and encodes auxin signaling F-box protein 5, one of five homologs of TIR1 in the Arabidopsis genome. TIR1 is the recognition component of the Skp1-cullin-F-box complex associated with the ubiquitin-proteasome pathway involved in auxin signaling and has recently been shown to be a receptor for IAA and 2,4-D. At4g11260 encodes the tetratricopeptide protein SGT1b that has also been associated with Skp1-cullin-F-box-mediated ubiquitination in auxin signaling and other pathways. Complementation of mutant lines with their corresponding wild-type genes restored picolinate auxin sensitivity. These results show that chemical specificity in auxin signaling can be conferred by upstream components of the auxin response pathway. They also demonstrate the utility of genetic screens using structurally diverse chemistries to uncover novel pathway components.     

Auxin carriers in Cucurbita vesicles

Carrier-mediated uptake of indole-3-acetic acid (IAA) by microsomal vesicles from Cucurbita pepo L. hypocotyls was strongly inhibited by 2,4-dichlorophenoxyacetic acid (2,4-D; i ₅₀= 0.3 M) but only weakly by 1-naphthylacetic acid (NAA). The fully ionised auxin indol-3-yl methanesulphonic acid also inhibited (i ₅₀=3 M). The same affinity ranking of these auxins for the uptake carrier, an electroimpelled auxin anion-H⁺ symport, is demonstrable in hypocotyl segments. The specificity of the auxin-anion eflux carrier was tested by the ability of different nonradioactive auxins to compete with [³H]IAA and reduce the stimulation of net radioactive uptake by N-1-naphthylphthalamic acid (NPA), a noncompetitive inhibitor of this carrier. By this criterion, NAA and IAA had comparable affinities, with 2,4-D interaction more weakly. Stimulation of [³H]IAA uptake by NAA, as a result of competition for the efflux carrier, could also be demonstrated when a suitable concentration of 2,4-D was used selectively to inhibit the uptake carrier. However, when [³H]NAA was used, no stimulation of its association with vesicles by NPA, 2,3,5-triiodobenzoic acid, or nonradioactive NAA was found. In hypocotyl segments, [³H]NAA net uptake was much less sensitive to NPA stimulation than was [¹⁴C]IAA uptake. The apparent contradictions concerning NAA could be explained by carrier-mediated auxin efflux making a smaller relative contribution to the overall transport of NAA than of IAA. The relationship between carrier specificity as manifested in vitro and the specificity of polar auxin transport is discussed.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - IAA indole-3-acetic acid - ION3 mixture of 4 M carbonylcyanide m-chlorophenylhydrazone, nigericin and valinomycin - IMS indol-3-yl methanesulphonic acid - NAA 1-naphthylacetic aci - NPA N-1-naphthylphthalamic acid     

The Effects of Exogenous Auxins on Endogenous Indole-3-Acetic Acid Metabolism (The Implications for Carrot Somatic Embryogenesis)

The effect of auxin application on auxin metabolism was investigated in excised hypocotyl cultures of carrot (Daucus carota). Concentrations of both free and conjugated indole-3-acetic acid (IAA), [2H4]IAA, 2,4-dichlorophenoxyacetic acid, and naphthaleneacetic acid (NAA) were measured by mass spectroscopy using stable-isotope-labeled internal standards. [13C1]NAA was synthesized for this purpose, thus extending the range of auxins that can be assayed by stable-isotope techniques. 2,4-Dichlorophenoxyacetic acid promoted callus proliferation of the excised hypocotyls, accumulated as the free form in large quantities, and had minor effects on endogenous IAA concentrations. NAA promoted callus proliferation and the resulting callus became organogenic, producing both roots and shoots. NAA was found mostly in the conjugated form and had minor effects on endogenous IAA concentrations. [2H4]IAA had no visible effect on the growth pattern of cultured hypocotyls, possibly because it was rapidly metabolized to form inactive conjugates or possibly because it mediated a decrease in endogenous IAA concentrations by an apparent feedback mechanism. The presence of exogenous auxins did not affect tryptophan labeling of either the endogenous tryptophan or IAA pools. This suggested that exogenous auxins did not alter the IAA biosynthetic pathway, but that synthetic auxins did appear to be necessary to induce callus proliferation, which was essential for excised hypocotyls to gain the competence to form somatic embryos.     

The Effects of Synthetic Growth-regulator Treatments on the Levels of Free Endogenous Growth-substances in Plants

The possible effects of synthetic auxins and anti-auxins onthe metabolism of indole-3-acetic acid (IAA) in plant tissueshave not been properly studied in the past. For this reasonseedlings of peas, beans, and sunflower have been treated withthe synthetic auxin, 2,4-dichlorophenoxyacetic acid (2,4-D)and two supposed anti-auxins, 2,3,5-tri-iodobenzoic acid (TIBA)and maleic hydrazide (MH), at non-toxic levels sufficient tocause well-marked growth responses. Estimates of the contentof alcohol-extractable growth-substances have subsequently beendetermined, after separation by paper partition chromatography.Although at least six active natural compounds have been indicatedin such extracts, only the effects of treatment on IAA levelshave been followed in detail. 2,4-D treatment of both leaves and roots has no detectable effecton the levels of free endogenous IAA, and it is thereby concludedthat 2,4-D is an auxin in its own right and does not act ongrowth via a disturbance of IAA metabolism. There are indicationsthat considerable amounts of the absorbed 2,4-D are convertedin plant tissues to a neutral detoxication product which iseasily decomposed to liberate 2,4-D during chromatographic analysis. TIBA treatment of pea roots dramatically reduces their freeendogenous IAA content, in some cases to 1/10,000 the normallevel. The implications of these findings are discussed in termsof the physiological and morphological responses of plants toTIBA treatment. There are indications that MH may put up slightly the levelof free endogenous auxin in pea roots but further confirmatorywork is required.     

The indole alkaloids brucine, yohimbine, and hypaphorine are indole-3-acetic acid-specific competitors which do not alter auxin transport

The indole alkaloids brucine and yohimbine, just like hypaphorine, counteract indole-3-acetic acid (IAA) activity in seedling roots, root hairs and shoots, but do not appear to alter auxin transport in roots or in cultured cells. In roots, the interactions between IAA and these three alkaloids appear competitive and specific since these molecules interact with IAA but with neither 1-naphthaleneacetic acid (NAA) or 2,4-dichlorophenoxyacetic acid (2,4-D), two synthetic auxins. The data reported further support the hypothesis that hypaphorine brucine and yohimbine compete with IAA on some auxin-binding proteins likely to be auxin receptors and that 2,4-D and NAA are not always perceived by the same receptor as IAA or the same component of that receptor. At certain steps of plant development and in certain cells, endogenous indole alkaloids could be involved in IAA activity regulation together with other well-described mechanisms such as conjugation or degradation. Hypaphorine with other active indole alkaloids remaining to be identified, might be regarded as a new class of IAA antagonists.     

Early time course and specificity of auxin effects on turgor movement of the bean pulvinus

Auxin application to the upper side of the pulvinus of primary leaves of Phaseolus vulgaris L. promoted bending away from the place of application. The effect had a latency of less than 20 min and was specifically induced by substances known as active auxins in growth tests (indoleacetic and 1-naphthaleneacetic acid) but not by inactive auxin analogs (2-naphthaleneacetic, 3-indolepropionic and benzoic acid); 2,4-dichlorophenoxyacetic acid, and L-(-)-2,4-dichlorophenoxyisopropionic acid were of intermediate activity. Auxin-promoted bending was reversible and presumably caused by turgor increase in the treated cells.Abbreviations IAA 3-indolacetic acid - NAA naphthylacetic acid