Involvement of IAA in the interaction betweenAzospirillum brasilense andPanicum miliaceum roots

The possible involvement of IAA in the effect thatAzospirillum brasilense has on the elongation and morphology ofPanicum miliaceum roots was examined by comparing in a Petri dish system the effects of inoculation with a wild strain (Cd) with those of an IAA-overproducing mutant (FT-326). Both bacterial strains produced IAA in culture in the absence of tryptophan. At the stationary growth phase, production of IAA by FT-326 wasca. 12 times greater than that of Cd. When inoculation was made with bacterial concentrations higher than, 10⁶ colony forming units ml^–1 (CFU ml^–1), both strains inhibited root elongation to the same extent. At lower concentrations Cd enhanced elongation, by 15–20%, while FT-326 was ineffective. Both strains promoted root-hair development, and root-hairs were produced nearer the root tip the higher the bacterial concentration (e. g. root elongation region was reduced). Effects of FT-326 on root-hair development were greater than those of Cd. Acidified ether extracts of Cd and FT-326 cultures had inhibitory or promoting effects on root elongation depending on the dilution applied. At low dilutions, extracts from FT-326 were more inhibitory for elongation than those from Cd. At higher dilutions root elongation was promoted, but FT-326 extracts had to be more diluted than those from Cd. Dilutions that promoted root elongation contained supra-optimal concentrations of IAA, 1–3 orders of magnitude higher than those required for optimal enhancement by synthetic IAA. It is suggested that the bacteria produce in culture an IAA-antagonist or growth inhibitor that decreases the effectiveness of IAA action. The large variability reported for the effects ofAzospirillum on root elongation could be the result of the opposite effects on root elongation of IAA and other compounds, produced by the bacteria.     

Rapid Effects of IAA on Cell Surface Proteins from Intact Carrot Suspension Culture Cells

Suspension cultures of carrot (Daucus carrota L.) which had an absolute requirement for exogenously supplied auxin were grown in medium containing indoleacetic acid (IAA) as the sole auxin source. Putative cell surface proteins were extracted from the intact cells. Resupply of IAA to cultures partially depleted of auxin resulted in rapidly increased activities of three enzyme activities subsequently extracted. Two of the enzyme activities which increased, peroxidase and pectinesterase, have been implicated in the literature as important to cell wall development, structure, and growth. The other enzyme activity which was increased, IAA oxidase, may be involved in the degradation of IAA In vivo. Polypeptides in the extracts were found to increase equally as rapidly as the enzymes in response to IAA as determined with sodium dodecyl sulfate-polyacrylamide electrophoretic gels stained with silver. It is not known whether the changes in enzyme and polypeptide levels in the protein extracts were due to auxin effects on protein synthesis, transport, or extractability.     

Identification of Indole-3-Acetic Acid in the Basidiomycete Schizophyllum commune

Indole-3-acetic acid (IAA) was detected in the ether extracts of culture filtrates of indigotin-producing strains of the basidiomycete Schizophyllum commune. Several solvents, known to give distinctly different R_F values for IAA, and 3 location reagents gave identical results with synthetic IAA and IAA found in the extract. Confirmation was obtained by the Avena straight growth test, split pea test, and ultraviolet absorption spectrum.     

Growth Regulators in Picea abies

The occurrence of growth regulators active in the Avena coleoptile straight-growth test in sprouting buds and seedlings of Norway spruce (Picea abies Karst.) was investigated. The acid ether fraction contained a growth stimulator, the Rf of which in isopropanol: ammonia: water was 0.2–0.4. This substance behaved as indole-3-acetic acid (IAA) in elec-trophoresis, in chromatography in various solvent systems on paper and on a Sephadex column. It gave the colour typical of IAA when sprayed with Ehrlich reagent and its fluorescence characteristics corresponded to IAA. Acid ether-soluble inhibitors showed most activity at Rf 0.4–0.7, but due to tailing they interfered with the determination of the stimulator at the Rf of IAA in the bioassay. They also masked the activity of other stimulators. Colour reactions were obtained with Ehrlich reagent in the inhibiting chromatogram zone. When eluates from this zone were tested in high dilutions or after gel filtration growth stimulation was obtained. The acid fraction of seedling shoots also contained a stimulator with Rf 0.7–0.8. In the neutral-basic ether-soluble fraction growth stimulation was obtained at Rf 0.5–0.7. The extracts also contained stimulatory substances insoluble in ether but soluble in n-butanol and partly in ethyl acetate. When the butanol fraction was hydrolyzed in 1 M NaOH a substance behaving as IAA when chromatographed was released.     

Occurrence of Endogenous Indol-3yI-Aspartic Acid in Light and Dark-Grown Bean Seedlings (Phaseolus vulgaris)

An acid ether-soluble, strongly growth-stimulating substance revealed by the Avena coleoptile straight-growth test in methanol extracts from bean seedlings (Phaseolus vulgaris L.) was identified as indol-3yl-aspartic acid (IAAsp). Points of agreement between synthetic IAAsp and the investigated growth stimulator were indicated by chromatographic behavior, elution volume in gel filtration, mobility in paper electrophoresis, “colour reaction” with DMCA reagent, ability to form indol-3yl-acetic acid (IAA) and aspartic acid after hydrolysis and, finally, biological activity in the Avena test. Furthermore, some experiments demonstrated the occurrence of an inhibitor in extracts from light-grown tissue. This masked the stimulation of IAAsp in the Avena test when the extracts had been chromatographed in isopropanol: NH₃:H₂ O. A comparison of the levels of IAAsp between green and etiolated tissue did not reveal any distinct difference, demonstrating that the IAA conjugate IAAsp does not participate in the regulation of the photomorphogenesis.     

Auxin activity of 3-hydroxymethyl oxindole and 3-methylene oxindole in oat

Two photooxidation products of indol-3-ylacetic acid (IAA), 3-hydroxymethyl oxindole (HMO) and 3-methylene oxindole (Meox) were almost as effective as IAA in stimulation of growth of oat (Avena saliva cv. Kent) coleoptile sections. The IAA failed to give stimulation of growth after pretreatment of the coleoptiles with chemicals inhibiting oxidation of IAA, but the growth promoting effects of HMO or Meox remained unaffected by such pretreatment. After pretreatment with the chemicals which can form an adduct with Meox, the growth promoting activities of IAA, HMO or Meox were lost showing the involvement of the oxindole pathway of IAA metabolism in IAA action. Time-course experiments also showed that the stimulatory effects of HMO and Meox were the same as IAA.     

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.     

Biosynthesis of plant hormones during anaerobic digestion of instant coffee waste

A large amount of solid waste remains after the production of instant coffee. This waste has to be moved to dumps, where it poses a threat of environmental pollution. Treatment of this waste by anaerobic methanogenic thermophilic digestion produced, besides biogas, a digested slurry which was used as a growth medium for horticulture, and proved to be a suitable and economical substitute for peat moss. Biological tests with mung bean cuttings and Grevillea plantlets showed promotional effects on rooting of the slurry and its sieved fraction extract, washed with water (Capul). Green coffee beans, instant coffee waste, its anaerobically-digested slurry and Capul were extracted by various methods and the extracts were analyzed by TLC, HPLC and GC/MS. Examinations showed clearly the presence of IAA and IBA in free and bound forms in all the substrates. The values of free and bound IAA were calculated by use of an internal standard and GC/MS. The amount of conjugated IAA was found to be much higher than that of free IAA, in both the coffee beans and instant coffee waste (11.1 vs 2.7 nmol g^–1, respectively). In the digested slurry and Capul, however, most of the IAA was present as the free form and was approximately 23.5–33.0 nmol g^–1, which is almost ten times more than in the waste, and almost twice the total amount of IAA in coffee beans. It is postulated that the high levels of free IAA in the digested instant coffee waste are a result of catabolism of tryptophan by anaerobic bacteria.     

Growth stimulation by catecholamines in plant tissue/organ cultures

Addition of catecholamines at micromolar concentrations caused a dramatic stimulation of growth of tobacco (Nicotiana tabacum) thin cell layers (TCLs) and Acmella oppositifolia “hairy” root cultures. A threefold increase in the rate of ethylene evolution was observed in the catecholamine-treated explants. Aminooxyacetic acid and silver thiosulfate, inhibitors of ethylene biosynthesis and action, respectively, reduced the growth-promoting effect of dopamine. However, these compounds alone could also inhibit the growth of the TCL explants. When ethylene in the culture vessel was depleted by trapping with mercuric perchlorate, dopamine-stimulated growth was still obtained, suggesting that ethylene does not mediate the dopamine effect. Dopamine potentiated the growth of TCLs grown in Murashige and Skoog medium supplemented with indoleacetic acid (IAA) and kinetin. When IAA was replaced by 2,4-dichlorophenoxyacetic acid, dopamine addition showed no growth-promoting effect. Instead, 2,4-dichlorophenoxyacetic acid stimulated the growth of TCL explants to the same extent as that obtained with IAA plus dopamine. Because synthetic auxins do not appear to be substrates for IAA oxidizing enzymes, we hypothesized that catecholamines exert their effect by preventing IAA oxidation. Consistent with this explanation, dopamine (25 micromolar) inhibited IAA oxidase activity by 60 to 100% in crude enzyme extracts from tobacco roots and etiolated corn coleoptiles, but had no effect on peroxidase activity in the same extracts. Furthermore, addition of dopamine to TCL cultures resulted in a fourfold reduction in the oxidative degradation of [1-¹⁴C]IAA fed to the explants. Because the growth enhancement by catecholamines is observed in both IAA-requiring and IAA-independent cultures, we suggest that these aromatic amines may have a role in the regulation of IAA levels in vivo.     

Growth Regulator Interactions in the Growth of the Shoot System of Avena sativa Seedling: II. THE GROWTH OF THE FIRST LEAF AND THE COLEOPTILE1

1. Studies have been made of the growth in culture medium of thecomponent parts of compositesegments excised from 3 to 7-day-oldAvena sativa seedlings and comprising portions of coleoptileand first leaf bases and various lengths of first internodetissue.
2. The effects of various concentrations of gibberellicacid (GA)and indole-3- acetic acid (IAA) alone and in combinationhavebeen studied on the growth of these organs.
3. Both GA andIAA stimulate the growth of coleoptile base tissuebut in combinationtheir joint effects are less than additive.No synergism occurs.
4. The growth of the first-leaf base is greatly stimulated byGAbut is inhibited by IAA. In combination, the stimulatoryeffectof GA (up to 1 0 p.p.m.) may be virtually eliminatedby evenlow concentrations of IAA (0.01 p.p.m.).
5. The inclusionof first internode tissue in the segments considerablyincreasesthe growth of first leaf base tissue but has no consistenteffecton the growth of coleoptile base tissue. The presenceof firstinternode tissue also greatly increases the degreeof growthstimulation invoked by GA but does not influence thedegreeof IAA inhibition. It is postulated that the first internodetissue is the source of an unknown growth factor necessary forGA action in the first leaf and potentiating the action of endogenousgibberellin.
6. Kinetin, adenine sulphate, glutarnine, glutarnicacid, asparagine,glycine, arginine, histidine, lysine, aneurin,and pyridoxinewill not simulate the effects of this unknowngrowth factorin the growth of leaf tissue. Like IAA, kinetinvirtually eliminatesthe GA stimulation of leaf growth.
7. Astudy of extracts of internode tissue in various solvents,analysedby paper partition chromatography and assayed by thegrowthof the first leaf base, has indicated the presence ofgrowthinhibitors and gibberellin-like substances but has failedtoisolate the postulated endogenous GA-synergist.
8. The implicationsof these results for growth correlations andthe hormone controlof shoot growth in Avena sativa seedlingsis discussed.

     

Effect of coconut (<Emphasis Type="Italic">Cocos nucifera</Emphasis> L.) water extracts on adventitious root development in vegetative propagation of <Emphasis Type="Italic">Dracaena purplecompacta</Emphasis> L.

Propagation by softwood canes and cuttings is preferred as a practical system for vegetative reproduction of many ornamental plant species, despite the advances in tissue culture techniques. Dracaena purplecompacta L. is a species that has a high demand for exports. Conversely, coconut water (CW) is a rich supplement that naturally contains plant growth regulators such as indole acetic acid (IAA). The objective of this work was to evaluate the potential of CW extracts containing natural IAA, on adventitious root development in vegetative propagation of ornamental plant canes of D. purplecompacta L. Five different concentrations (28, 57, 143, 286, 571 μM of natural IAA) of CW extracts were tested. Another set of treatment was carried out with the same concentrations of authentic IAA hormone for comparison purpose. The 143-μM IAA CW extract recorded the best root induction and development. It was found that the root expression was faster (5 weeks) with the use of the novel method. In the conventional method, the canes are propagated by quick dip application of commercial product containing artificial hormone IAA and placing them on coir fiber dust beds. It takes up to 6 weeks for the canes to develop adventitious roots to the desired level. Steeping canes in 143-μM IAA CW extract improved rooting in D. purplecompacta L., and it was comparable to the application of 143-μM authentic IAA. The study indicates that adventitious root development, shoot development, and leaf emergence of D. purplecompacta L. is promoted by IAA CW extracts.     

Inhibitory action of auxin on root elongation not mediated by ethylene

The inhibitory effects of indole-3-acetic acid (IAA) and 1-aminocyclopropane-1-carboxylic acid (ACC) on elongation growth of pea (Pisum sativum L.) seedling roots were investigated in relation to the effects of these compounds on ethylene production by the root tips. When added to the growth solution both compounds caused a progressively increasing inhibition of growth within the concentration range of 0.01 to 1 micromolar. However, only ACC increased ethylene production in root tips excised from the treated seedlings after 24 hours. High auxin concentrations caused a transitory increase of ethylene production during a few hours in the beginning of the treatment period, but even in 1 micromolar IAA this increase was too low to have any appreciable effect on growth. ACC, but not IAA, caused growth curvatures, typical of ethylene treatment, in the root tips. IAA caused conspicuous swelling of the root tips while ACC did not. Cobalt and silver ions reversed the growth inhibitory effects induced by ACC but did not counteract the inhibition of elongation or swelling caused by IAA. The growth effects caused by the ACC treatments were obviously due to ethylene production. We found no evidence to indicate that the growth inhibition or swelling caused by IAA is mediated by ethylene. It is concluded that the inhibitory action of IAA on root growth is caused by this auxin per se.     

Biosynthesis of Indoleacetic Acid from Tryptophan-C in Cell-free Extracts of Pea Shoot Tips

A 2-step, 1-dimensional thin-layer chromatographic procedure for isolating indoleacetic acid (IAA) was developed and utilized in investigations of the biosynthesis of IAA from tryptophan-¹⁴C in cell-free extracts of pea (Pisum sativum L.) shoot tips. Identification of a ¹⁴C-product as IAA was by (a) co-chromatography of authentic IAA and ¹⁴C-product on thin-layer chromatography, and (b) gas-liquid and thin-layer chromatography of authentic and presumptive IAA methyl esters. Dialysis of enzyme extracts and addition of α-ketoglutaric acid and pyridoxal phosphate to reaction mixtures resulted in approximately 2- to 3-fold increases in net yields of IAA over yields in non-dialyzed reaction mixtures which did not contain additives essential to a transaminase reaction of tryptophan. Addition of thiamine pyrophosphate to reaction mixtures further enhanced net biosynthesis of IAA. It is concluded that the formation of indolepyruvic acid and its subsequent decarboxylation probably are sequential reactions in the major pathway of IAA biosynthesis from tryptophan in cell-free extracts of Pisum shoot tips. Comparison of maximum net IAA biosynthesis in extracts of shoot tips of etiolated and light-grown dwarf and tall pea seedlings revealed an order, on a unit protein N basis, of: light-grown tall > light-grown dwarf > etiolated tall etiolated dwarf. It is concluded that the different rates of stem elongation among etiolated and light-grown dwarf and tall pea seedlings are correlated, in general, with differences in net IAA biosynthesis and sensitivity of the tissues to IAA.     

Uptake and decarboxylation of indole-3-acetic acid during auxin-induced growth in lupin hypocotyl segments

The elongation growth of etiolated hypocotyl segments of lupin (Lupinus albus L.) was stimulated by acid pH (4.6 versus 6.5) and by IAA for periods of up to 4 h. After this time, the segments were unable to grow further. In the presence of an optimal IAA concentration (10 μM), acid pH increased the growth rate but had no effect on final growth. With suboptimal IAA (0.1 μM), however, acid pH increased growth in a more than additive way, suggesting a synergistic action between the two factors. This synergism may be explained by the increased IAA uptake and decarboxylation seen at an acid pH. These results reinforce the view that the effects of low pH and IAA on growth are not independent. Vanadate inhibited growth and also IAA uptake and decarboxylation. This inhibitor, therefore, probably inhibits growth not only by decreasing ATPase-mediated acidification but also by decreasing H⁺-dependent IAA uptake from the apoplasm. This dependence of IAA uptake on ATPase may be mediated by apoplasmic acidification. The amount of IAA decarboxylated increased when the assay conditions favored the growth of segments, indicating that IAA could be destroyed by decarboxylation during the auxin-induced growth.     

The Role of Boron in Plant Growth: IV. INTERRELATIONSHIPS BETWEEN BORON AND INDOL-3YL-ACETIC ACID IN THE METABOLISM OF BEAN RADICLES

The hypothesis that boron deficiency is equivalent to a stateof IAA toxicity was explored. Bioassays showed that extractsof substances similar to IAA taken from boron-deficient rootswere significantly more inhibitory to the growth of bean-rootsegments than those from normal roots. Supplied IAA and borondeficiency together restricted root growth to a greater extentthan either deficiency or IAA treatment separately. Roots werefound to recover more quickly from the inhibitory effects ofsupplied IAA if boron was present at high (0.5 ppm) rather thanlow (0.01 ppm) concentrations. Experiments with ¹⁴C-labelled IAA showed that deficient rootsabsorbed ¹⁴C more slowly than boron-fed roots and there wasalso a lower rate of decarboxylation in the deficient tissue.Consideration of the published evidence showed that many ofthe effects of boron deficiency could follow from an upset inIAA metabolism. It is suggested that boron-deficient tissuesuffers from excess auxin either because the element is necessaryfor some growth process, such as cell wall formation or nucleicacid synthesis, which, when impaired, results in the accumulationof auxin, or because the IAA-oxidation system is affected byphenolic inhibitors which boron normally inactivates by complexformation.     

Physiological significance of indoleacetic acid and factors determining its level in cotyledons of Lupinus albus during germination and growth

The results demonstrate the profile of the endogenous indole-3-acetic acid (IAA) in the cotyledons of Lupinus albus L. ( L. termis Forssk.) during germination and seedling growth. The auxin level increases markedly after seed hydration, especially during the time of radicle emergence 24 h after the onset of imbibition. This rise is accompanied by a minimal IAA-oxidase activity, formation of indoleacetylaspartic acid (IAAsp) and an increase in the endogenous tryptophan and tryptophan-carboxyl-¹⁴C degradation, though the latter cannot account for the high IAA level detected during early stages of germination. It is believed that cotyledons are a source of IAA to the developing embryonic axis. – The auxin level drops in the cotyledons during seedling growth, 2–18 days after sowing. This is true also for IAAsp and tryptophan-degrading activity of enzyme extracts. Conversely, endogenous tryptophan is increasingly liberated up to day 14, and IAA-oxidase activity climbs to a peak detected on day 12, prior to the appearance of senescence in the cotyledons. – The physiological significance of IAA and the factors regulating its level in the cotyledons during germination and growth are discussed.     

Promotion of stem elongation by indole-3-butyric acid in intact plants of Pisum sativum L.

While indole-3-butyric acid (IBA) has been confirmed to be an endogenous form of auxin in peas, and may occur in the shoot tip in a level higher than that of indole-3-acetic acid (IAA), the physiological significance of IBA in plants remains unclear. Recent evidence suggests that endogenous IAA may play an important role in controlling stem elongation in peas. To analyze the potential contribution of IBA to stem growth we determined the effectiveness of exogenous IBA in stimulating stem elongation in intact light-grown pea seedlings. Aqueous IBA, directly applied to the growing internodes via a cotton wick, was found to be nearly as effective as IAA in inducing stem elongation, even though the action of IBA appeared to be slower than that of IAA. Apically applied IBA was able to stimulate elongation of the subtending internodes, indicating that IBA is transported downwards in the stem tissue. The profiles of growth kinetics and distribution suggest that the basipetal transport of IBA in the intact plant stem is slower than that of IAA. Following withdrawal of an application, the residual effect of IBA in growth stimulation was markedly stronger than that of IAA, which may support the notion that IBA conjugates can be a better source of free auxin through hydrolysis than IAA conjugates. It is suggested that IBA may serve as a physiologically active form of auxin in contributing to stem elongation in intact plants.     

Indole-3-acetic acid metabolism in Lemna gibba undergoes dynamic changes in response to growth temperature

Auxin is the mobile signal controlling the rate of growth and specific aspects of the development of plants. It has been known for over a century that auxins act as the messenger linking plant development to specific environmental changes. An often overlooked aspect of how this is accomplished is the effect of the environment on metabolism of the major plant auxin, indole-3-acetic acid (IAA). We have studied the metabolism of IAA in relation to one environmental variable, growth temperature. The model system used was an inbred line of the aquatic monocot Lemna gibba G-3, 3F7-11 grown at temperatures ranging from 5 degrees C to 35 degrees C. IAA levels, the rate of IAA turnover, and the patterns of label incorporation from IAA precursors were measured using stable isotope-mass spectrometric techniques and were evaluated relative to growth at the experimental temperatures. IAA levels exhibited unusually high variability in plants grown at 15 degrees C and 20 degrees C. Turnover rates were quite rapid throughout the range of experimental temperatures except at 25 degrees C, where IAA turnover was notably slower. These results suggest that a transition occurred over these temperatures for some aspect of IAA metabolism. Analysis of [(15)N]anthranilate and [(2)H(5)]tryptophan (Trp) incorporation into IAA showed that Trp-dependent biosynthesis predominated at 15 degrees C; however, Trp-independent biosynthesis of IAA was the major route to IAA at 30 degrees C. The effects of growth temperature on auxin levels have been reported previously, but no prior studies correlated these effects with which pathway becomes the primary one for IAA production.     

Mutual interaction of auxin and cytokinins in regulating correlative dominance

Correlative dominance requires correlative signals from a dominant to a dominated organ. Auxins, particularly IAA, and cytokinins are obviously important components of this correlative system. Using a vegetative pea shoot and a generative apple and tomato fruit system it can be demonstrated that dominant organs always export more IAA and have a higher ³H-IAA transport capacity and velocity compared to dominated organs. In both systems the dominant organ can be replaced by the application of auxin, e.g. NAA, which maintains the differences in IAA export. This is an indication that similar regulatory mechanisms control dominance in both of these diverse systems. The possibility of replacing a dominant organ by auxin also makes it unlikely that growth of that organ or allocation of nutrients regulates the correlative inhibition of the dominated organ.It is suggested that differences in IAA export from, and transport capacities of, dominant and dominated shoots, may be explained by a mechanism of auxin transport autoinhibition (ATA), whereby the earlier and stronger export of IAA from the dominant shoot inhibits auxin export from the dominated shoot at the point where the two auxin streams converge. This hypothesis was tested with explants of pea, apple and tomato. It was shown that the basal application of cold IAA significantly reduced endogenous as well as exogenous IAA transport through these explants.Since the reduced IAA transport of dominated organs was not followed by an accumulation of IAA in the auxin producing subtending organ, it was concluded that IAA biosynthesis was possibly reduced and/or IAA conjugation stimulated. This could have been one of the determinants of their growth inhibition. ATA might also explain how the unidirectional IAA signal may affect the growth rate of organs even lateral or acropetal to its transport pathway and thus polar IAA-transport becomes a ``multidirectional' signal. From the experiments demonstrated it seems that ATA is a sufficient mechanism to impose growth inhibition in the dominated organ, without the need of other regulators.However, to release dominated organs from dominance cessation of ATA may not be sufficient and cytokinins are obviously a powerful antagonist to auxins. Their repeated exogenous application turns dominated lateral buds into strongly growing organs which ultimately may even dominate the previously dominant apex. These lateral shoots finally gain a strong IAA export capacity and inhibit, by ATA, IAA export from the hitherto dominant apex.In other experiments it was shown that interruption of polar IAA transport leads to a strong increase in root derived cytokinins. This can largely be prevented, in a concentration dependent manner, by the application of auxin, indicating that basipolar auxin may control cytokinin production in the roots and its possible delivery to lateral buds. In turn, the increased delivery of cytokinins to the lateral buds promotes a strong increase in IAA production and export. Thus there is a strong mutual interaction between auxin production in the shoots and cytokinin production in the roots, which may be important in regulating the balance between root and shoot growth.     

Chelation in Auxin Action: I. A STUDY OF THE INTERACTIONS OF 3-INDOLYLACETIC ACID AND SYNTHETIC CHELATING AGENTS AS AFFECTING THE GROWTH OF WHEAT ROOTS AND COLEOPTILE SECTIONS

Factorial experiments have been carried out on the effects,upon growth of roots of intact wheat seedlings and growth ofwheat coleoptile sections, of different concentrations of 3-indolylaceticacid (IAA) and various known chelating agents. These have demonstrateda similar mutual antagonism between pairs of agents whetherthese are IAA and a single known chelating agent or two knownchelating agents. This interaction takes the form that eitheragent alone in ‘high’ concentration severely inhibitsgrowth but this inhibitory effect is almost or entirely removedby the presence of one-millionth the concentration of the otheragent; when both agents are present in ‘high’ concentrationthe inhibition is again severe. The substitution of a non-chelatinganalogue for one of the agents either destroys the mutual characterof the antagonism or entirely prevents either agent at low concentrationfrom reducing measurably the inhibition caused by high concentrationof the other. The fact that IAA interacts with known chelatingagents, in controlling the growth both of roots and coleoptilesections, in the same unexpected and symmetrical way that theseinteract with each other, is held strongly to support the hypothesisthat it is here itself acting as a chelating or complexing agent;the absence of such interactions with a non-chelating analoguemakes this the more convincing. These results are concernedwith the removal of growth inhibition, due to supra-optimalconcentrations of one agent, by minute proportions of another;it cannot be regarded as proven that the promotion of growthby IAA in the absence of another agent is also due to chelationor complex formation. This seems probable, however, when thefindings here presented are taken in conjunction with the accumulatingevidence that IAA and other auxins can form complexes or chelateswith metals in vitro, and with the finding already publishedin detail that the eight chelating agents tried promoted growthin the wheat coleoptile test. The main criticisms to which this hypothesis has been subjectedhave been concerned with the relative magnitudes of effectsof IAA and chelating agents upon growth, with the low stabilityconstants of metal complexes with IAA and other auxins, withthe lack of parallelism between stability constants and growth-promotingactivity, and with the fact that one chelating agent (ethylenediamine-tetraaceticacid; EDTA) has been found inactive in certain growth tests.A series of factorial experiments comparing the authors' techniques(which are here described in detail), chemicals, and strainof wheat with those used by Fawcett et al. (1956) demonstratethat the discrepancies found, both as regards magnitudes ofeffects of IAA and EDTA and optimal concentrations, were partlydue to differences in strain but mainly to differences of technique.It is considered that ‘foreign’ molecules such asEDTA are likely to have side effects, which may well differin different strains or tests; competition with internal chelators(Burstrom and Tullin, 1957) is also likely to differ; differencesin rate of penetration and steric hindrance may also be involved.For these reasons effective chelating activity in vivo may bevery different from that in vitro and in the first instancethe magnitudes of growth-promoting effects of chelating agents(which may indeed be the net result of stimulatory and inhibitoryprocesses) seem less important than the fact that they are foundin so many instances. Possible ways in which IAA and other growth substances may regulategrowth by chelation or complex-formation are discussed.