Evidence for the acid-growth theory of fusicoccin action

Three predictions of the acid-growth theory of fusicoccin (FC) action in inducing cell elongation were reinvestigated using abraded segments of maize (Zea mays L.) coleoptiles. i) Quantitative comparison of segment elongation and medium-acidification kinetics measured in the same sample of tissue shows that these FC-induced processes are strictly correlated in time and respond coordinately to cations present in the medium. ii) Fusicoccin (1 mol l^-1) induces a rapid acidification of the cell-wall solution, reaching a final level of pH 3.8–4.0. Exogenous protons are able to substitute quantitatively for FC in causing segment elongation at pH 3.8–4.0. At pH 4, FC has no additional effect on cell elongation. iii) Neutral buffers (pH 7) completely abolish the FC-mediated growth response. iv) Cycloheximide (10 mg l^-1) inhibits both FC-induced and acid-buffer(pH 4)-induced elongation after a lag of 40–45 min, and FC-induced H⁺ excretion after a lag of 2 h. Under the same conditions, indole-3-acetic acid-induced elongation and H⁺ excretion are inhibited without detectable lag. It is concluded that these results are fully compatible with the acid-growth theory of FC action.Abbreviations IAA indole-3-acetic acid - CHI cycloheximide - FC fusicoccin     

Auxin-induced H+-pump stimulation does not depend on the presence of epidermal cells in corn coleoptiles

Cell-wall acidification and electrical reactions (depolarization and hyperpolarization) are typical auxin responses in maize (Zea mays L.) coleoptiles. In an attempt to test the role of the outer epidermis in these responses, they have been measured and compared in intact and peeled coleoptile fragments. To exclude interactions between parenchymal and epidermal cells, the coleoptile pieces were completely stripped of their outer epidermis. This preparation was monitored by means of a scanning electron microscope. When externally applied indole-3-acetic acid was tested, we found that neither cell-wall acidification nor the electrical membrane responses depended on the presence of intact epidermal cells.Abbreviations IAA Indole-3-acetic acid - MES 2-[N morpholino-ethane-sulfonic acid - TRIS 2-Amino-2-hydroxymethyl-1,3-propanediol We thank Kuki Kaethner for her excellent technical assistance. This work was supported by the Hessische Graduiertenförderung and the Deutsche Forschungsgemeinschaft.     

In-vivo measurement of cell-wall extensibility in maize coleoptiles: Effects of auxin and abscisic acid

Plastic and elastic in-vivo extensibilities (E_pl and E_el, respectively) of cell walls of growing maize (Zea mays L.) coleoptile segments were measured by stretching living tissue at constant force (creep test) in an extensiometer. The linear displacement transducer used as a measuring device permits the determination of load-induced extensions in the range of 0–1% of the segment's length, leading to a minimal disturbance of the hydraulic parameters of the tissue and allowing the measurement of unidirectional cell-wall creep at virtually unchanged turgor and metabolic activity. A rein-vestigation of the time-course of indole-3-acetic acid-promoted and abscisic acid-inhibited wall loo-sening revealed that the in-vivo creep test yields results very similar to those obtained previously with the in-vitro creep test [Kutschera and Schopfer, 1986, Planta 167, 527–535]. The hormones affect elongation rate and E_pl in a closely correlated manner both in step-up as well as step-down growth changes whereas E_el remains unaltered. It is argued that both hormones influence growth by modifying E_pl of the outer epidermis and that this effect can be quantitatively measured, in relative units, by either the in-vivo or the in-vitro creep test.Abbreviations ABA ±abscisic acid - E_el, E_pl elastic and plastic in-vivo cell-wall extensibility, respectively - E_tot E_el+E_pl - IAA indole-3-acetic acid; m, cell-wall yielding coefficient     

Auxin causes oscillations of cytosolic free calcium and pH inZea mays coleoptiles

In epidermal cells of maize (Zea mays L.) coleoptiles, cytosolic pH (pH_c), cytosolic free calcium, membrane potential and changes thereof were monitored continuously and simultaneously (pH_c/, _m, Ca²⁺/ _m) using double-barrelled ion-sensitive microelectrodes. In the resting cells the cytosolic pH was 7.3–7.5 and the concentration of free calcium was 119±24 nM. One-micromolar indole-3-acetic acid (IAA), added to the external medium at pH 6.0 triggered oscillations in _m, pH_c and free calcium with a period of 20 to 30 min. Acidification of the cytosolic pH increased the cytosolic free calcium. The _m oscillations are attributed to changes in activity of the H⁺-extrusion pump at the plasmalemma, triggered off by pH and controlled by pH regulation (pH oscillation). The origin of the pH_c and Ca²⁺ changes remains unclear, but is possibly caused by auxin-receptor-induced lipid breakdown and subsequent second-messenger formation. It is suggested that the observed cytosolic pH and Ca²⁺ changes are intrinsically interrelated, and it is concluded that this onset of regulatory processes through the phytohormone IAA is indicative of calcium and protons mediating early auxin action in maize coleoptiles. It is further concluded that the double-barrelled ion-sensitive microelectrode is an invaluable tool for investigating in-vivo hormone action in plant tissues.Abbreviations and symbols FC fusicoccin - IAA indole-3-acetic acid - Mes 2-(N-morpholino)ethanesulfonic acid - pH_c cytosolic pH - Tris 2-amino-2-(hydroxymethyl)-1,3-propanediol - _m membrane potential difference (mV)     

Tip-localised H+-fluxes and the applicability of the acid-growth hypothesis to tip-growing cells: Control of chloronemal extension in Funaria hygrometrica by auxin and light

The role of tip-localised H⁺ secretion in regulating chloronemal tip growth in the moss Funaria hygrometrica Hedw. was investigated. pH was monitored with pH microelectrodes placed close to the cell surface while the rate of extension growth was manipulated by illumination and by the application of indole-3-acetic acid. Growth stimulations were accompanied by acidification of the external solution; this acidification was most pronounced at the growing tip. The timing and extent of acification external to the tip correlated well with the magnitude and time course of growth stimulations. The maintenance of both growth and H⁺ efflux under CO₂-free conditions indicated that neither photosynthetic nor respiratory CO₂ metabolism were involved. Artificially acidifying the nutrient solution rapidly but transiently stimulated elongation in both white light and darkness. Furthermore, the stimulation of elongation caused by white light was inhibited if the nutrient solution was buffered strongly near neutrality. We conclude that the acid growth hypothesis is applicable to tip growth in Funaria and that light and exogenous indole-3-acetic acid act at least in part by stimulating localised H⁺-ion efflux.Abbreviations D darkness - IAA indole-3-acetic acid - WL white light     

Role of cell-wall biogenesis in the initiation of auxin-mediated growth in coleoptiles of Zea mays L.

The involvement of cell-wall polymer synthesis in auxin-mediated elongation of coleoptile segments from Zea mays L. was investigated with particular regard to the growth-limiting outer epidermis. There was no effect of indole acetic acid (IAA) on the incorporation of labeled glucose into the major polysaccharide wall fractions (cellulose, hemicellulose) within the first 2 h of IAA-induced growth. 2,6-Dichlorobenzonitrile inhibited cellulose synthesis strongly but had no effect on IAA-induced segment elongation even after a pretreatment period of 24 h, indicating that the growth response is independent of the apposition of new cellulose microfibrils at the epidermal cell wall. The incorporation of labeled leucine into total and cell-wall protein of the epidermis was promoted by IAA during the first 30 min of IAA-induced growth. Inhibition of IAA-induced growth by protein and RNA-synthesis inhibitors (cycloheximide, cordycepin) was accompanied by an inhibition of leucine incorporation into the epidermal cell wall during the first 30 min of induced growth but had no effect on the concomitant incorporation of monosaccharide precursors into the cellulose or hemicellulose fractions of this wall. It is concluded that at least one of the epidermal cell-wall proteins fulfills the criteria for a growth-limiting protein induced by IAA at the onset of the growth response. In contrast, the synthesis of the polysaccharide wall fractions cellulose and hemicellulose, as well as their transport and integration into the growing epidermal wall, appears to be independent of growth-limiting protein and these processes are therefore no part of the mechanism of growth control by IAA.Abbreviations CHI cycloheximide - COR cordycepin - DCB 2,6-dichlorobenzonitrile - GLP growth-limiting protein(s) - IAA indole-3-acetic acid     

Cooperation of epidermis and inner tissues in auxin-mediated growth of maize coleoptiles

The function of the epidermis in auxinmediated elongation growth of maize (Zea mays L.) coleoptile segments was investigated. The following results were obtained: i) In the intact organ, there is a strong tissue tension produced by the expanding force of the inner tissues which is balanced by the contracting force of the outer epidermal wall. The compression imposed by the stretched outer epidermal wall upon the inner tissues gives rise to a wall-pressure difference which can be transformed into a water-potential difference between inner tissues and external medium (water) by removal of the outer epidermal wall. ii) Peeled segments fail to respond to auxin with normal growth. The plastic extensibility of the inner-tissue cell walls (measured with a constant-load extensiometer using living segments) is not influenced by auxin (or abscisic acid) in peeled or nonpeeled segments. It is concluded that auxin induces (and abscisic acid inhibits) elongation of the intact segment by increasing (decreasing) the extensibility specifically in the outer epidermal wall. In addition, tissue tension (and therewith the pressure acting on the outer epidermal wall) is maintained at a constant level over several hours of auxin-mediated growth, indicating that the inner cells also contribute actively to organ elongation. However, this contribution does not involve an increase of cell-wall extensibility, but a continuous shifting of the potential extension threshold (i.e., the length to which the inner tissues would extend by water uptake after peeling) ahead of the actual segment length. Thus, steady growth involves the coordinated action of wall loosening in the epidermis and regeneration of tissue tension by the inner tissues. iii) Electron micrographs show the accumulation of striking osmiophilic material (particles of approx. 0.3 m diameter) specifically at the plasma membrane/cell-wall interface of the outer epidermal wall of auxin-treated segments. iv) Peeled segments fail to respond to auxin with proton excretion. This is in contrast to fusicoccin-induced proton excretion and growth which can also be readily demonstrated in the absence of the epidermis. However, peeled and nonpeeled segments show the same sensitivity to protons with regard to the induction of acid-mediated in-vivo elongation and cell-wall extensibility. The observed threshold at pH 4.5–5.0 is too low to be compatible with a second messenger function of protons also in the growth response of the inner tissues. Organ growth is described in terms of a physical model which takes into account tissue tension and extensibility of the outer epidermal wall as the decisive growth parameters. This model states that the wall pressure increment, produced by tissue tension in the outer epidermal wall, rather than the pressure acting on the inner-tissue walls, is the driving force of growth.Abbreviations and symbols E _el, E _pl elastic and plastic in-vitro cell-wall extensibility, respectively - E _tot E _el+E _pl - FC fusicoccin - IAA indole-3-acetic acid - IT inner tissue - ITW inner-tissue walls - OEW outer epidermal wall - osmotic pressure - P wall pressure - water potential     

Auxin action on proton influx in corn roots and its correlation with growth

At concentrations inhibitory to the elongation of corn (Zea mays L.) roots, the auxins, indole-3-acetic acid (IAA) and α-naphthaleneacetic acid (α-NAA), cause an increase in the pH of the bathing medium; this increase occurs with an average latent period shorter than the latent period for the inhibitory effect of these auxins on elongation. Indole-2-carboxylic acid, an inactive structural analogue of IAA, and β-naphthaleneacetic acid, an inactive analogue of α-NAA, affect neither growth nor the pH of the medium. Since acid pH is known to promote and basic pH to inhibit root elongation, the data are consistent with the hypothesis that hormone-induced modification of cell-wall pH plays a role in the control of elongation of roots, as has been proposed for elongation of stems and coleoptiles.     

Evidence against the acid-growth theory of auxin action

Four experimental predictions of the acid-growth theory of auxin (indole-3-acetic acid, IAA) action in inducing cell elongation were reinvestigated using abraded segments of maize (Zea mays L.) coleoptiles. i) Quantitative comparison of segment elongation and medium-acidification kinetics measured in the same sample of tissue reveals that these IAA-induced processes are neither correlated in time nor responding coordinately to cations present in the medium. ii) Exogenous protons are not able to substitute for IAA in causing segment elongation at the predicted pH of 4.5–5.0. Instead, external buffers induce significant segment elongation only below pH 4.5, reaching a maximal response at pH 1.75–2.5. Acid and IAA coact additively, and therefore independently, in the whole range of feasible pH values. iii) Neutral or alkaline buffers (pH 6–10) are unable to abolish the IAA-mediated growth response and have no effect on its lag-phase. iv) Fusicoccin, at a concentration producing the same H⁺ excretion as high concentrations of IAA, is ineffective in inducing segment elongation. Moreover, sucrose and other sugars can quantiatively substritute for IAA in inducing H⁺ excretion but are likewise ineffective in inducing elongation. It is concluded that these results are incompatible with the acid-growth theory of auxin action.Abbreviations IAA indole-3-acetic acid - FC fusicoccin     

In-vitro auxin transport in membrane vesicles from maize coleoptiles

When membrane vesicles from maize (Zea mays L.) coleoptiles are extracted at high buffer strength, a pH-driven, saturable association of [¹⁴C] indole-3-acetic acid is found, similar to the in-vitro auxin-transport system previously described for Cucurbita hypocotyls. The phytotropins naphthylphthalamic acid and pyrenoylbenzoic acid increase net uptake, pressumably by inhibiting the auxin-efflux carrier.Abbreviations IAA indole-3-acetic acid - ION3 ionophore mixture of carbonylcyanide-3-chlorophenylhydrazone, nigericin and valinomycin - 1-NAA, 2-NAA 1-, 2-naphthaleneacetic acid - NPA 1-N-naphthylphthalamic acid - PBA 2-(1-pyrenoyl)benzoic acid     

Inhibitory action of red light on the growth of the maize mesocotyl: evaluation of the auxin hypothesis

Brief irradiation of 3-d-old maize (Zea mays L.) seedlings with red light (R; 180 J m^-2) inhibits elongation of the mesocotyl (70–80% inhibition in 8 h) and reduces its indole-3-acetic acid (IAA) content. The reduction in IAA content, apparent within a few hours, is the result of a reduction in the supply of IAA from the coleoptile unit (which includes the shoot apex and primary leaves). The fluence-response relationship for the inhibition of mesocotyl growth by R and far-red light closely resemble those for the reduction of the IAA supply from the coleoptile. The relationship between the concentration of IAA (1–10 M) supplied to the cut surface of the mesocotyl of seedlings with their coleoptile removed and the growth increment of the mesocotyl, measured after 4 h, is linear. The hypothesis that R inhibits mesocotyl growth mainly by reducing the IAA supply from the coleoptile is supported. However, mesocotyl growth in seedlings from which the coleoptiles have been removed is also inhibited by R (about 25% inhibition in 8 h). This inhibition is not related to changes in the IAA level, and not relieved by applied IAA. In intact seedlings, this effect may also participate in the inhibition of mesocotyl growth by R. Inhibition of cell division by R, whose mechanism is not known, will also result in reduced mesocotyl elongation especially in the long term (e.g. 24 h).Abbreviations FR far-red light - IAA indole-3-acetic acid - P_fr phytochrome in the far-red-absorbing form - P_r phytochrome in the red-absorbing form - R red light     

Indole-3-methanol is the main product of the oxidation of indole-3-acetic acid catalyzed by two cytosolic basic isoperoxidases from Lupinus

The nature of the products of the auxin catabolism mediated by both basic and acidic isoperoxidases has been studied. While indole-3-methanol is only a minor product of the oxidation of indole-3-acetic acid catalyzed by extracellular acidic isoperoxidases, it is the only product of the oxidation of indole-3-acetic acid catalyzed by two cytosolic basic isoperoxidases (EC 1.11.1.7) from lupin (Lupinus albus L.) hypocotyls. The putative indole-3-methanol formed by these latter isoperoxidases was isolated and then characterized by mass spectrometry and ¹H-nuclear magnetic resonance spectrometry. These results are discussed with respect to the diversity and compartmentation of the catabolism of indole-3-acetic acid in plant tissues.Abbreviations DCP 2,4-dichlorophenol - IAA indole-3-acetic acid - IM indole-3-methanol     

Heterogeneity of auxin-accumulating membrane vesicles from Cucurbita and Zea: a possible reflection of cell polarity

When microsomes from hypocotyls of Cucurbita pepo L. or coleoptiles of Zea mays L. were centrifuged on dextran-sucrose gradients a heterogeneity of auxin-accumulating vesicles was observed. Vesicles from the top part of the gradient showed saturable, specific accumulation of indole-3-acetic acid with only a small stimulation by phytotropins, and with very few binding sites for 1-N-naphthylphthalamic acid. In the vesicles from the lower part of the gradient, net accumulation of indole-3-acetic acid could be strongly increased by addition of phytotropins; binding of 1-N-naphthylphthalamic acid was high in this region. After two-phase partitioning, both kinds of vesicles were found in the upper-phase membrane fraction considered to be purified plasma membrane. The hypothesis is discussed that vesicles can be separated from the apical and basal parts of the cell's plasmalemma.Abbreviations CCO cytochrome-c oxidase - CCR KCN-insensitive NADH-dependent cytochrome-c reductase - 2,4-D 2,4-dichlorophenoxyacetic acid - IAA indole-3-acetic acid - IDPase inosine 5-diphosphatase - ION3 ionophore mixture of carbonylcyanide-3-chlorophenylhydrazone, nigericin and valinomycin - 1-NAA 1-naphthaleneacetic acid - NPA 1-N-naphthylphthalamic acid - PBA 2-(1-pyrenoyl)benzoic acid - UDPG uridine diphosphoglucose     

Role of protein and RNA synthesis in the initiation of auxin-mediated growth in coleoptiles of Zea mays L.

The kinetics of inhibition by protein- and RNA-synthesis inhibitors (cycloheximide and cordycepin, respectively) of indole-3-acetic acid (IAA)-induced elongation growth were investigated using abraded coleoptile segments of Zea mays L. Removal of the cuticle — a diffusion barrier for solutes — by mechanical abrasion of the outer epidermal cell wall increased the effectiveness of inhibitors tremendously. In an attempt to elucidate the role of growth-limiting protein(s) (GLP) in the growth mechanism the following results were obtained. The elongation induced by IAA was completely inhibited when cycloheximide (10 mol·l^-1) was applied to abraded coleoptile segments as shortly as 10 min before the onset of the growth response (=5 min after administration of IAA). However, when cycloheximide was applied after 60 min of IAA treatment (when a steady-state growth rate is reached), the time required for complete cessation of growth was much longer (about 40 min). Cycloheximide inhibited the incorporation of [³H]leucine into protein within about 5 min. Cordycepin (400 mol·l^-1) prevented IAA-induced growth when applied as shortly as 25 min before the onset of the growth response (=10 min before administration of IAA) but required more than 60 min for a full inhibition of steady-state growth. The incorporation of [³H]adenosine into RNA was inhibited by cordycepin within 10 min. It is concluded that, contrary to previous investigations with nonabraded organ segments, the initiation of growth by IAA depends directly on the synthesis of GLP. Moreover, the apparent lifetime of GLP is at least four times longer than the time required by cycloheximide to inhibit the initiation of growth by IAA. This is interpreted to mean that GLP is not present before IAA starts to act but is synthesized as a consequence of IAA action starting a few minutes before the initiation of growth. Interpreting the kinetics of growth inhibition by cordycepin in a similar way, we further conclude that GLP synthesis is mediated by IAA-induced synthesis of the corresponding mRNA which starts about 10 min before the onset of GLP synthesis. Inhibition by cycloheximide and cordycepin of IAA-induced growth cannot be alleviated by acidifying the cell wall to pH 4-5, indicating that these inhibitors do not act on growth via an inhibition of auxin-mediated proton excretion.Abbreviations CHI cycloheximide - COR cordycepin - GLP growth-dimiting protein(s) - IAA indole-3-acetic acid - mRNA_GLP mRNA coding for GLP     

Acetic acid esters and permeable weak acids induce active proton extrusion and extension growth of coleoptile segments by lowering the cytoplasmic pH

In Avena coleoptile segments a decrease of cytoplasmic pH activates energy-dependent H⁺ extrusion into the apoplast, thereby triggering extension growth. This sequence of events cannot be inhibited by cycloheximide and is induced by the following conditions and compounds. (i) A short anaerobic treatment of coleoptile segments results in the formation of lactic acid and an intracellular decrease of pH. For a period of 20 min after transfer to normal air, the growth rate is up to six times higher than the rate before anaerobiosis. (ii) Similarly, incubation of segments with CN^– (0.1 mM) in the presence of oxygen causes and accumulation of lactic acid and a fall in cell-sap pH. After removing CN^– a growth burst occurs. (iii) Higher concentrations of permeable acids (10 mM in buffer pH 5.8) induce extension growth. This growth is O₂-dependent and therefore differs from the acid growth, which can be triggered under anaerobic conditions by acid buffers of pH5 via the direct increase of cell-wall plasticity. (iv) A short application of CO₂-saturated buffer (pH 5.8) causes CO₂-induced elongation growth; after a 3-min pulse the growth rate is enhanced for about 15 min. (v) Lipophilic esters of acetic acid or propionic acid, such as naphthylacetate, naphthylpropionate, phenylacetate, benzylacetate induce elongation growth. These compounds, when taken up into the cell, are hydrolized by esterases; the acids released lower the cytoplasmic pH (shown by the pH indicator, fluorescein). The highest esterase activity was found in a microsomal membrane fraction of coleoptiles. While the carboxyester-induced extension growth is completely inhibited under anoxia, the initial acidification of the bathing solution can still be observed. This decrease in external pH is obviously the result of ester hydrolysis, caused by damaged cells, and is not the result of pH changes within the cell-wall compartment. It is suggested that a fast uptake of carboxyesters and the shift in equilibrium caused by their internal hydrolysis leads to a continuous formation of acids which lowers the cytoplasmic pH and activates the ATP-dependent H⁺ extrusion. In most experiments fusicoccin (a diacetic acid ester) acts similarly to naphthylacetate and the other carboxyesters, although quantitative differences exist. Therefore, it is possible that fusicoccin is effective partly on the basis of its ester characteristic. The effects observed are discussed with regard to the very narrow pH optimum of plasma-membrane H⁺-ATPases exhibiting their highest levels of activity at pH 6.5 (Hager and Biber 1984, Z. Naturforsch. C 39, 927–937).Abbreviations CHM cycloheximide - DMO dimethadione (5.5-dimethyl-2,4-oxazolidinedione) - FC fusicoccin - IAA indole-3-acetic acid - Mes 2-(N-morpholino)ethanesulfonic acid - NA (or )-naphthylacetate (acetic acid-1(or-2-)naphthylester) - NAA (or )-naphthaleneacetic acid - PA phenylacetate (acetic acid phenylester)     

Gibberellic acid induces cytoplasmic acidification in maize coleoptiles

Indole-3-acetic acid (IAA), fusicoccin and weak acids all lower the cytoplasmic pH (pH_i) and induce elongation growth of maize (Zea mays L.) coleoptiles. Gibberellic acid (GA₃) also induces elongation growth and we have used confocal laser scanning microscopy to study the effects of GA₃ on pH_i employing the pH-indicator dyes, 2,7-bis(2-carboxyethyl)-5-(and-6) carboxyfluorescein and carboxy-semi-naphthorhodafluor-1. We confirm that GA₃ induces growth significantly in light-grown but only slightly or not at all in dark-grown coleoptiles. The growth induced by IAA treatment was similar in light- and dark-grown coleoptiles. The pH_i decreased by up to 0.6 units during the first 7 min of GA₃ or IAA treatment of both light- and dark-grown coleoptiles. Gibberellic acid inhibited IAA-induced growth of dark-grown coleoptiles. Hence, in dark-grown coleoptiles GA₃ may activate either directly or indirectly reactions that interfere with the signalling pathway leading to elongation growth. The possible role of pH_i in growth is discussed.Abbreviations ABA abscisic acid - AM acetoxymethyl ester - BCECF 2,7-bis(2-carboxyethyl)-5-(and-6) carboxyfluorescein - [Ca²⁺]_i cytoplasmic free calcium - GA_(n) gibberellin A_(n) - GA₃ gibberellic acid - IAA indole-3-acetic acid - PGR plant growth regulator - pH_i cytoplasmic pH - Pipes piperazine-N,N-bis[2-ethanesulfonic acid] - Snarf-1 carboxy-semi-naphthorhodafluor-1 We thank Dr R. King (CSIRO, Canberra) for providing the GA₁ and T. Phillips for processing the photographic material. H.R. Irving was supported by an Australian Research Council Research Fellowship and the work was supported by an Australian Research Council grant.     

Deuterium-labelled indole-3-acetic acid neo-synthesis in plantlets and excised roots of maize

Synthesis of indole-3-acetic acid (IAA), using stable-isotope incorporation, was investigated in Zea mays L. Incorporation of ²H from ²H₂O into IAA molecules was shown to occur in intact plantlets and excised primary roots cultured in vitro. This demonstrates the de-novo formation of IAA, a process which is quantitatively well defined and is initiated early in germination.Abbreviations IAA indole-3-acetic acid     

Hexacyanoferrate (III) stimulation of elongation in coleoptile segments fromZea mays L.

Summary The influence of exogenous potassium hexacyanoferrate (III) (HCF III) on elongation of maize (Zea mays L.) coleoptile segments was investigated. Addition of HCF III led to a strong stimulation of growth both in the presence and absence of indole-3-acetic acid (IAA). The magnitude of growth stimulation was dependent on the presence of IAA, HCF III concentration, incubation time, and phase growth. The reduced form, potassium hexacyanoferrate (II), was without effect on growth. In the presence of HCF III, elongation was suppressed when coleoptile segments were treated with N,N-dicyclohexylcarbodiimide, cycloheximide or atebrine (quinacrine). The addition of HCF III stimulated the IAA-induced proton extrusion, and the e^–/H⁺ ratio decreased with incubation time. HCF III also strongly stimulated elongation ofAvena saliva L. coleoptile segments andGlycine max L. hypocotyl segments. These results suggested that a plasma membrane redox system (NADH oxidase type I) may be involved in the regulation of growth through the activity of the plasma membrane-bound ATPase.Abbreviations CH cycloheximide - DCCD N,N-dicyclohexylcarbodiimide - HCF III potassium hexacyanoferrate (III) (potassium ferricyanide) - HCF II potassium hexacyanoferrate (II) (potassium ferrocyanide) - IAA indole-3-acetic acid     

Effect of cadmium on growth, proton extrusion and membrane potential in maize coleoptile segments

Cd accumulation, its effects on elongation growth of maize coleoptile segments, pH changes of their incubation medium and the membrane potential of parenchymal cells were studied. The Cd content increased significantly with exposure to increasing cadmium concentrations. Coleoptile segments accumulated the metal more efficiently in the range 10–100 μM Cd, than in the range 100–1000 μM Cd. Cd at concentrations higher than 1.0 μM produced a significant inhibition of both growth and proton extrusion. 100 μM Cd caused depolarization of the plasma membrane (PM) potential in parenchymal cells. The simultaneous treatment of maize coleoptile segments by indole-3-acetic acid (IAA) and Cd, counteracted the toxic effect of Cd on growth. Moreover, our data also showed that 100 μM Cd suppressed the characteristic IAA-induced hyperpolarization of the membrane potential, causing membrane depolarization. These results indicate that the toxic effect of Cd on growth of maize coleoptile segments might be, at least in part, caused via reduced PM H⁺-ATPase activity.