Stress relaxation properties of the cell wall of tissue segments under different growth conditions

Stress-relaxation parameters were compared under different experimentalconditions using 5th internode segments of light-grown pea seedlingsand coleoptile segments of dark-grown Avena seedlings. The followingresults were obtained. 1. In a short incubation period at 25?C, IAA caused a decreasein the minimum relaxation time, To, of the epidermal cell wallof pea internodes when it induced elongation; the optimum concentrationof IAA for decreasing To was 10 mg/liter. 2. At all concentrations of IAA used, 0.1–1000 mg/liter,the relationship between the To value of the epidermal cellwall peeled from segments incubated for 2 hr and the subsequentelongation rate in 2–3 hr incubation was linear, indicatingthat the To value of the cell wall at a certain time regulatesthe rate of the following elongation. 3. When segments of pea epicotyls or Avena coleoptiles wereincubated in mannitol solution of various concentrations inthe presence and absence of IAA and then allowed to grow inthe absence of both mannitol and IAA, the segments extendeddifferently depending upon the mannitol concentration, whichwas less than 0.3 M, given during preincubation. 4. The T_o and b (relaxation rate, S/log t) values were smallerin the cell wall of segments which extended more, than in thosewhich extended less. In this case, 0.2 M mannitol solution wasmost effective, since it inhibited IAA-induced elongation duringpre-incubation and the segments thus incubated extended themost afterward. 5. Extensibility, mm/gr, seemed to parallel the elongation whichhad occurred during pre-incubation, indicating that this value,contrary to To, represented at least partly the result of elongation. From these results we concluded that the growth rate to followis regulated by the minimum stress relaxation time, To, andpossibly by the relaxation rate, b, of the cell wall beforeextension, and these parameters may represent certain biochemicalmodifications of the cell wall components needed for cell extension. (Received August 12, 1974; )     

Rhizobium-stimulated Callose Formation in Clover Root Hairs and its Relation to Infection

Callose was detected in the cell walls of the tips of growingroot hairs of Trifolium species and the non-legume Phleum pratenseusing u.v. fluorescence of fresh material stained with 0·005%aniline blue. Inoculation of the roots with Rhizobium trifolii,R. leguminosarum, R. meliloti, and R. japonicum, or additionof 10^–7 and 10^–8 M indole-3-acetic acid (IAA) increasedtip callose formation. Most tip callose was formed at 12 °C, and amounts declinedprogressively at 18, 24, and 30 °C, with very little formedat 36 °C. Tip calloso usually became less and disappearedin individual root hairs as they aged. Callose which appeared prominently in the host cell walls atthe points of initiation of infection threads did not usuallydisappear as the hairs matured. There was little or no extensionof callose along the infection thread and none in the threadtip or in the cell nucleus. Presumptive regions of callose hadsimilar structure and electron density as root hair wall materialand were sometimes related to arrays of vesicles in the hostcytoplasm. The external surface of the hair wall bore smallpegs or papillae (0·1–0·2 µm) continuouswith the outer layer of the wall and possibly associated withattachment of bacteria. Bacteria were usually umboriate at thepoint of attachment and their polyphosphate granules were muchlarger near the root hair than at the distal end.     

Auxin stimulates both deposition and breakdown of material in the pea outer epidermal cell wall,as measured interferometrically

The effect of auxin on the mass per area in the outer epidermal walls of third internodes of Pisum sativum L. cv. Alaska grown in dim red light was investigated using interference microscopy, and rates of net deposition of wall material were calculated. Examination of these net rates under different growth conditions showed that there is no simple relationship between the deposition of mass and growth. Net deposition can be proportional to growth when sufficient substrate for wall synthesis is available, as in intact plants, and in segments treated with indole-3-acetic acid (IAA) plus glucose. Net deposition can cause thickening of the walls when growth is small, as in the case of segments kept without IAA in the presence or absence of glucose, or segments whose growth is inhibited with mannitol. When substrate is limited and growth is large, however, wall expansion can occur with no net deposition, or an actual net loss of wall material can even take place. Auxin appears to induce a breakdown in the walls of segments treated in the absence of glucose, although it promotes synthesis when glucose is present. It is likely that IAA always induces a breakdown of wall material, but that the breakdown is masked when substrate is available for synthesis. Our results indicate that pea epidermal cells have two different auxin-stimulated mechanisms, wall synthesis and wall breakdown, potentially available to loosen their outer epidermal walls to bring about cell enlargement, alternatives which could be employed to different extents depending on substrate conditions.Abbreviation IAA indole-3-acetic acid M.S. Bret-Harte would like to thank Drs. Peter M. Ray, Stanford University, Winslow R. Briggs, Carnegie Institute of Washington, Stanford, Calif. USA, and Wendy K. Silk, of the University of California Davis USA, for helpful discussions, Dr. Briggs and the Carnegie Institute of Washington for the use of experimental facilities, and Dr. Ray for editorial assistance. This work was supported by a National Science Foundation Graduate Fellowship to M.S.B.-H., a National Science Foundation Postdoctoral Fellowship to T.I.B., and National Science Foundation grant DCB8801493 to P.B.G.     

Indole-3-Acetic Acid Control on Acidic Oat Cell Wall Peroxidases

Incubation of oat coleoptile segments with 40 μm indoleacetic acid (IAA) induced a decrease of 35–60% in peroxidase activity at the cell wall compartment. Treatment with IAA also produced a similar decrease in the oxidation of NADH and IAA at the cell wall. Isoelectric focusing of ionic, covalent, and intercellular wall peroxidase fractions showed that acidic isoforms (pI 4.0–5.5) were reduced preferentially by IAA treatment. Marked differences were found between acidic and basic wall isoperoxidases in relation to their efficacy in the oxidation of IAA. A peroxidase fraction containing acidic isoforms oxidized IAA with a V _max/s_0.5 value of 2.4 × 10⁻² min⁻¹· g fw⁻¹, 4.0 times higher than that obtained for basic peroxidase isoforms (0.6 × 10⁻² min⁻¹· g fw⁻¹). In contrast, basic isoforms were more efficient than acidic isoperoxidases in the oxidation of coniferyl alcohol or ferulic acid with H₂O₂ (5.6 and 2.1 times, respectively). The levels of diferulate and lignin in the walls of oat coleoptile segments were not altered by treatment with IAA. The decrease in cell wall peroxidase activity by IAA was related more to reduced oxidative degradation of the hormone than to covalent cell wall cross-linking. Received November 1, 1998; accepted December 14, 1998     

Cell Wall Acidification and its Role in Auxin-Stimulated Growth

The role of cell wall acidification in auxin-stimulated growthwas examined in abraded hypocotyl segments of etiolated Cucumis.sativus seedlings. Acidification of the medium by these segmentswas strongly inhibited by a pretreatment and the continued presenceof 1?0 mol m^–3 vanadate, widely used as an inhibitor ofplasma membrane ATPase activity. Elongation of segments in pH6?5 buffer was almost completely inhibited by such a treatmentwith vanadate, and the promotion of growth by indole-3-aceticacid (IAA) seen in the absence of vanadate was completely abolished.However, both inhibited and uninhibited segments showed a pronouncedelongation in response to pH 4?0) buffer. In pH 4?0 buffer,in contrast to the results obtained at pH 6?5, IAA significantlypromoted growth in both the presence and absence of vanadate.The results indicate that IAA can promote growth in the absenceof endogenous acidification, but that an acid wall is necessaryfor wall loosening to occur. Key words: Acidification, auxin-stimulated growth, Cucumis sativus, vanadate     

Calcium Antagonist TMB-8 Inhibits Cell Wall Formation and Growth in Pea

The effects on auxin-stimulated growth and cell wall formationof 8-(N, N-diethylamino)-octyl-3, 4, 5-trimethoxybenzoate.HCI(TMB-8), an intracellular Ca²⁺ antagonist, were investigatedin abraded stem segments from aetiolated seedlings of Pisumsativum L. cv. Alaska. Incubation of segments at pH 6.0 with200 mmol m^–3 TMB-8 resulted in a 50% inhibition of auxin-stimulatedgrowth. Added Ca²⁺ did not restore normal auxin-stimulated growth,presumably because of its well-known stiffening effect on thecell wall. In segments incubated at a pH (7–2) which preventedelongation, auxin promoted the incorporation of [³H]glucoseinto the cell wall relative to total uptake of label. TMB-8abolished about 60% of the total incorporation of label intocell walls in the presence of auxin, but was not effective inthe absence of auxin. Exogenous CaCl₂ reversed the inhibitoryeffect of TMB-8 on relative cell wall incorporation in a parabolicmanner, with a 50% reversal at about 100 mmol m^–3 andcomplete reversal at 1.0 mol m^–3 Ca²⁺. Other ions tested(Mg²⁺, Mn²⁺, Cu²⁺, Zn²⁺) were without substantial effect atconcentrations of 0.5 mol m^–3. Both apparent uptake ofCa²⁺ and consequent reversal of TMB-8 inhibition of cell wallincorporation were blocked by the Ca²⁺ channel blockers verapamiland La³⁺. The data provide further evidence that auxin-stimulatedgrowth is dependent upon continued cell wall incorporation,and suggest that a Ca²⁺ messenger system may be involved inthe promotory actions of auxin on cell wall synthesis and long-termgrowth. Key words: Auxin, calcium, cell wall synthesis     

Jasmonic Acid Inhibits the IAA-Induced Elongation of Oat Coleoptile Segments: a Possible Mechanism Involving the Metabolism of Cell Wall Polysaccharides

The effects of jasmonic acid (JA) on the IAA-induced elongationof segments of etiolated oat (Avena sativa L. cv. Victory) coleoptileswere studied. Exogenously applied JA substantially inhibitedIAA-induced elongation of oat coleoptile segments. The inhibitionof the growth of oat coleoptile segments due to JA appeared2 h after the application of JA with IAA. JA did not affectthe consumption of oxygen by the segments, the osmolarity ofthe cell sap or the IAA-induced loosening of cell walls, whichwas recognized as a decrease in the minimum stress-relaxationtime (T₀). JA was extremely effective in preventing increasesin the amount of the cell wall polysaccharides in both the non-cellulosicfraction and the cellulosic fraction during coleoptile growthin the presence and in the absence of IAA. Inhibition of thegrowth of oat coleoptile segments induced by JA was partiallyreversed by the simultaneous addition of sucrose to the testsolution. From these results, it appears that JA inhibits IAA-inducedelongation of oat coleoptile segments by interfering with someaspects of sugar metabolism that are related to the degradationand/or the synthesis of cell wall polysaccharides. (Received March 15, 1994; Accepted August 2, 1994)     

The Exodermal Casparian Band of Onion Roots Blocks the Apoplastic Movement of Sulphate Ions

Peterson, C. A. 1987. The exodermal Casparian band of onionroots blocks the apoplastic movement of sulphate ions.—J.exp. Bot. 38: 2068–2081. It has previously been established that the exodermal Casparianbands in the roots of onion and many other angiosperm speciesare impermeable to fluorescent, apoplastic dyes. The permeabilityof the onion root exodermal Casparian band to ions was testedby measuring sulphate free space in the walls using compartmentalanalysis by elution. Results of this analysis revealed fourcompartments for sulphate. The first was a surface film, between15 and 35 µm thick, which had a half-time of elution tooshort to be measured (i.e. less than 15 s). The second was awall compartment with an average half-time of elution of 1·0min. The third and fourth compartments were the cytoplasm andvacuole which had half-times of elution of 12·4 min and4·9 d, respectively. Experiments in which the temperatureand concentration of sulphate in the treatment solution werevaried indicated that the compartment identified as the wallwas a free space. Segments were excised from roots in regionswith a mature exodermal Casparian band. The sulphate free spacein the walls of ‘whole’ segments (i.e. segmentswith their cut ends sealed) was compared with that of ‘bisected’segments (i.e. segments which had been bisected and their stelesremoved). The free space in the walls of the whole segmentswas consistently smaller than the comparable free space in thebisected segments; the ratio whole/bisected ranged from 0·16to 0·26. This ratio compared favourably with predictedvalues obtained by taking the ratio of the wall volumes, i.e.walls external to the exodermal Casparian band/walls internaland external to the exodermal Casparian band. The ratios predictedfrom wall measurements varied from 0·25 to 0·27.Furthermore, the volume of the sulphate free space in the wholesegments was less than the volume of the walls external to theCasparian band. The results of the present study show that theexodermal Casparian band of onion is impermeable to sulphateions, which have a hydrated ionic radius of 0·378 nm. Key words: Allium cepa, exodermis, sulphate     

Xyloglucan Endotransglycosylase Activity, Microfibril Orientation and the Profiles of Cell Wall Properties Along Growing Regions of Maize Roots

A series of physical and chemical analyses were made on theexpanding zone of maize seedling roots grown in hydroponics.Comparison of longitudinal profiles of local relative elementalgrowth rate and turgor pressure indicated that cell walls becomelooser in the apical 5 mm and then tighten 5–10 mm fromthe root tip. Immersion of roots in 200 mol m^–3 mannitol(an osmotic stress of 0·48 MPa) rapidly and evenly reducedturgor pressure along the whole growing region. Growth was reducedto a greater extent in the region 5–10 mm from the roottip than in the apical region. This indicated rapid wall-looseningin the root tip, but not in the more basal regions. Following 24 h immersion in 400 mol m^–3 mannitol (an osmoticstress of 0·96 MPa) turgor had recovered to pre-stressedvalues. Under this stress treatment, growth was reduced in theregion 4–10 mm from the root tip, despite the recoveryof turgor, indicating a tightening of the wall. In the rootapex, local relative elemental growth rate was unchanged incomparison to control tissue, showing that wall properties herewere similar to the control values. Cellulose microfibrils on the inner face of cortical cell wallsbecame increasingly more parallel to the root axis along thegrowth profile of both unstressed and stressed roots. Orientationdid not correlate with the wall loosening in the apical regionof unstressed roots, or with the tightening in the region 5–10mm from the root tip following 24 h of osmotic stress. Longitudinal profiles of the possible wall-loosening enzymexyloglucan endotransglycosylase (XET) had good correspondencewith an increase in wall loosening during development. In thezone of wall tightening following osmotic stress, XET activitywas decreased per unit dry weight (compared with the unstressedcontrol), but not per unit fresh weight. Key words: Osmotic stress, turgor, growth, cell wall properties, microfibrils, XET     

The effect of ethylene and auxin on cell wall extensibility of the Semi-aquatic fern,Regnellidium diphyllum

Ethylene and auxin both enhance cell elongation growth in the rachis of the frond of Regnellidium diphyllum. Measurements of the stress relaxation modulus of the walls of methanol-killed rachis segments show that both auxin and ethylene cause an increase in cell wall extensibility, that the effects are additive, and that they occur in the presence of hypertonic solutions of mannitol that preclude cell elongation. The results are taken as evidence for the operation of two separate mechanisms for cell wall loosening.Abbreviation IAA indol-3yl-acetic acid     

Proton efflux from oat coleoptile cells and exchange with wall calcium after IAA or fusicoccin treatment

Elongation growth of plant cells occurs by stretching of cell walls under turgor pressure when intermolecular bonds in the walls are temporarily loosened. The acid-growth theory predicts that wall loosening is the result of wall acidification because treatments (including IAA and fusicoccin) that cause lowered wall pH cause elongation. However, conclusive evidence that IAA primarily reduces wall pH has been lacking. Calcium has been reported to stiffen the cell walls. We have used a microelectrode ion-flux measuring technique to observe directly, and non-invasively, the net fluxes of protons and calcium from split coleoptiles of oats (Avena sativa L.) in unbuffered solution. Normal net fluxes are 10 nmol · m⁻² · s⁻¹ proton efflux and zero calcium flux. The toxin fusicoccin (1 μM) causes immediate efflux from tissue not only of protons, but also of calcium, about 110 nmol · m⁻² · s⁻¹ in each case. The data fit the “weak acid Donnan Manning” model for ion exchange in the cell wall. Thus we associate the known “acid-growth” effect of fusicoccin with the displacement of calcium from the wall by exchange for protons extruded from the cytoplasm. Application of 10 μM IAA causes proton efflux to increase transiently by about 15 nmol · m⁻² · s⁻¹ with a lag of about 10 min. The calcium influx decreases immediately to an efflux of about 20 nmol · m⁻² · s⁻¹. It appears that auxin too causes an “acid-growth” effect, with extruded protons exchanging for calcium in the cell walls. I. Arif is currently recieving an AIDAB scholarship. This work was supported by an Australian Research Council grant to I.A. Newman.     

Turgor and Longitudinal Tissue 12Helianthus annuus

The quantitative relationship between turgor and the pressureexerted by the inner tissues (cortex, vascular tissue, and pith)on the peripheral cell walls (longitudinal tissue pressure)was investigated in hypocotyls of sunflower seedlings (Helianthusannuus L.) In etiolated hypocotyls cell turgor pressures, asmeasured with the pressure probe, were in the range 0·38to 0·55 MPa with an average of 0·48 MPa. In irradiatedhypocotyls turgor pressures varied from 0·40 to 0·57MPa with a, mean at 0·49 MPa. The pressure exerted bythe inner tissues on the outer walls was estimated by incubatingpeeled sections in a series of osmotic test solutions (polyethyleneglycol 8000). The length change was measured with a transducer.In both etiolated and irradiated hypocotyls an external osmoticpressure of 0·5 MPa was required to inhibit elongationof the inner tissues, i.e. the average cell turgor and the longitudinaltissue pressure are very similar quantities. The results indicatethat the turgor of the inner tissues is displaced to and borneby the thick, growth-limiting peripheral cell walls of the hypocotyl. Key words: Helianthus annuus, hypocotyl growth, tissue pressure, turgor pressure, wall stress     

Inhibiting Effect of Auxin on Glucosamine Incorporation into Cell Walls of Rice Coleoptiles

Auxin induced growth and decreased the hexosamine content ofthe cell walls of rice coleoptile sections. Indole-3-aceticacid (IAA) at 10^–5 M inhibited the incorporation of ¹⁴C-glucosamineinto the cell walls. IAA did not affect the ¹⁴C-incorporationinto the cytoplasm, while inhibitors of glycoprotein synthesis,unicamycin and monensin, suppressed the incorporation into boththe cytoplasm and the cell walls. The radioactivity due to labeledglucosamine in the cell walls increased during the chase, butthis increase was inhibited by IAA. Among the cell wall fractions,the increase in radioactivity and its inhibition by IAA wereconspicuous in the hemicellulose I fraction. The inhibitoryeffect of IAA on glucosamine incorporation into the cell wallswas observed even in the presence of 0.15 M mannitol solutionwhich completely suppressed the IAA-induced growth. These resultssuggest that auxin induces growth at least partly by inhibitingthe transport of asparagine-linked glycoproteins from the cytoplasmto the cell walls. ¹ Present address: Department of Biology, Faculty of Science,Osaka City University, Sumiyoshi-ku, Osaka 558, Japan (Received July 23, 1986; Accepted December 22, 1986)     

Relationships between Adsorption, Chemical State and Fluxes of Cadmium Applied as Cd(NO3)2 in Isolated Xylem Cell Walls of Tomato

Wolterbeek, H. Th. 1987. Relationships between adsorption, chemicalstate and fluxes of cadmium applied as Cd(NO₃)₂ in isolatedxylem cell walls of tomato.—J. exp. Bot. 38: 419–432. Isolated xylem cell wall pieces were applied as membranes inion diffusion experiments. The cell walls were isolated fromtomato internodes (Lycopersicon esculentum Mill, cv. Tiny Tim)and sealed in a two-compartment diffusion system. In flux andadsorption calculations, the cell wall was regarded as a leakymembrane with parallel fluxes through Donnan Free Space (DFS)and Water Free Space (WFS). During the experiments absorptioninto and diffusion across the walls was determined of Cd^{2 +}, applied as ¹¹⁵Cd(NO₃)₂. Flux experiments with ⁸²Br^–indicated that excluded volume effects and path tortuosity resultedin apparent WFS diffusion coefficients in the walls which were0·012 times as high as in water. The free proton concentration in the DFS was shown to be relatedto a complex formation between fixed charges and Cd^{2 +}. Thecell wall permeability for Cd^{2 +} and NO₃^– varied withapplied and absorbed concentrations, and the Cd^{2 +} flux curveshowed an inflexion point coinciding with a buffered degreeof dissociation of fixed charges in the DFS. The necessary couplingof fluxes of opposite charges resulted in relatively high NO₃^–and small Cd^{2 +} permeability of the DFS for strongly dilutedsolutions (P = 10^–4 m s^–1 and 10^–11 m s^–1for NO₃^– and Cd^{2 +} respectively). The results demonstratethe possible regulatory effects of the cell wall in processesof ion transfer from xylem vessels, or ion uptake in plant tissues. Key words: Cadmium, chemical state, DFS, WFS, ion flux, permeability, xylem cell walls, tomato, bromium, nitrate     

The Effect of Indoleacetic Acid on Cell Wall Extensibility in Avena Coleoptiles

The extensibility of cell walls in segments of living Avenacoleoptiles has been determined after treatment with IAA bythe use of a microextensometer. It has been demonstrated thatIAA has an effect both on the plastic and elastic extensibility,which is more marked if the segments are allowed to grow, exceptat temperatures nearing 0° C. The auxin-induced increasein extensibility is therefore not a direct effect on the walland metabolic factors must be involved. The results are interpreted,along cunent lines of thought, as indicating effects on thelinkage between protopectin or hemicellulose chains themselvesand between these and the microfibrillar network of cellulose.The bearing of these interpretations on the normal growth processin cell walls is briefly discussed.     

The Effect of pH on the Binding of Calcium to Pea Epicotyl Cell Walls and its Implications for the Control of Cell Extension

Cell walls were prepared from the epicotyls of dark-grown pea(Pisum sativum L.) seedlings. The walls were found to bind externally-added⁴⁵Ca²⁺, with a binding constant of 4 ? 10^–4 mol dm^–3and a maximum capacity of 1.5 ? 10^–8 g-ions of Ca²⁺ perg fresh weight of epicotyl. The binding capacity decreased asthe pH of the medium was decreased below 6.0, suggesting thatthe calcium was bound by an anionic group with an apparent pKof 4.7. More than half the calcium binding was due to polygalacturonicacid in the wall, since up to 60% of the calcium binding capacitywas removed by pre-incubation of the cell walls with polygalacturonase(E.C.3.2.1.15). Only small decreases in calcium binding wereseen following pre-incubation with protease, nucleases, phospholipaseand hemicellulase. These results indicate that calcium willbe displaced from the cell wall at hydrogen ion concentrationswhich are known to occur in the wall during wall extension.They are consistent with a mechanism by which calcium inhibitswall extension by forming ionic bridges between polygalacturonicacid molecules, and also with the hypothesis that calcium andhydrogen ions exert opposing influences on cell wall extensionby competing for the same binding sites on the polygalacturonicacid. Key words: Pea epicotyl, Cell wall, Calcium, pH     

Auxin-Induced Changes in the Molecular Weight Distribution of Cell Wall Xyloglucans in Avena Coleoptiles

The effect of auxin on the molecular weight (Mw) distributionof cell wall xyloglucans was investigated by gel permeationchromatography using coleoptile segments of Avena sativa L.cv. Victory, and the following results were obtained.

1. The water-insoluble hemicellulose (HC-A) mainly consisted ofxyloglucans. Iodine staining method revealed that relativelylarge amounts of xyloglucans were present in the water-solublehemicellulose (HC-B) and water-soluble polysaccharide (WS) fractions.
2. IAA did not cause remarkable changes in xyloglucan contentsin the hemicellulose, but significantly increased the xyloglucancontent in the WS fraction.
3. IAA substantially decreased theweight-average Mw of HC-A. Thiseffect became apparent within30 min of the incubation period,and was not affected by the0.15 M mannitol or 2% sucrose applied.Hydrogen ions also causeda decrease in the weight-average Mwof HC-A; its effect beingreversible.
4. Neither IAA nor hydrogen ions caused any remarkablechangesin the weightaverage Mw of water-soluble xyloglucansin theHC-B.

These results suggest that cell wall xyloglucans have an importantrole in auxininduced cell wall loosening in oat coleoptile cells. (Received May 10, 1984; Accepted August 20, 1984)     

Effect of {alpha}-Tomatine on the Response of Wheat Coleoptile Segments to Exogenous Indole-3-acetic Acid

A concentration of 10^–5 M tomatine had no effect on leakagefrom, or elongation of, wheat coleoptile segments, but consistentlyreduced IAA-enhanced extension growth by c. 50 per cent. Therewas no evidence of chemical interaction between the alkaloidand the auxin in solution, and IAA action was not affected bypre-treatment for up to 3 h with 10^–5 M tomatine. Studieswith [2-¹⁴C]IAA revealed that 10^–5 M tomatine did notinhibit uptake of auxin into segments. The effect of pre-treatingsegments for up to 3 h with IAA could be virtually nullifiedby 10^–5 M tomatine, as could also IAA-induced changesin properties of coleoptile cell walls. Results are discussedin relation to the ability of tomatine to disrupt membrane functionand to current hypotheses implicating membranes in the primaryaction of auxin.     

Auxin-dependent cell wall depositions in the epidermal periplasmic space of graviresponding nodes of Tradescantia fluminensis

Differential growth of the nodal regions of graviresponding Tradescantia fluminensis (Wandering Jew) was analysed with special respect to the extension-restricting epidermal cells of the opposite growing and growth-inhibited organ flanks. Gravicurvature of horizontally gravistimulated isolated nodes depends on auxin (indolyl-3-acetic acid, IAA) and shows a node-specific profile in which the third node below the tip showed the greatest response. Exogenously supplied gibberellic acid induced no gravitropic growth. Vertically oriented isolated nodes supplied with exogenous IAA showed, on an electron microscopical level, conspicuous membrane invaginations with adjacent wall depositions restricted to the outer tangential epidermal cell walls. Their number was more than doubled by exogenously supplied Ca2+, which inhibited IAA-induced growth. No such changes could be detected in water-incubated segments or inner tissues of IAA-supplied segments. Gravistimulated differential growth of nodes of intact shoots and of nodal segments was characterized by changes similar to the ones induced by exogenous IAA, with greatly increased numbers of wall depositions within the epidermal cells of the growth-inhibited upper organ flank. Similar to the gravistimulated wall depositions, an asymmetric distribution pattern of Ca2+ was detected in the epidermal cell walls employing x-ray energy spectrum analysis (EDX). The results indicate that growth of nodes of Tradescantia fluminensis is regulated via IAA-induced secretion and subsequent infiltration of wall components enabling wall extension. The data support the hypothesis that temporary differential growth during gravicurvature of Tradescantia fluminensis is mediated by the antagonistic effect of Ca(2+)-ions on the infiltration of IAA-induced wall-loosening components into the outer, extension-restricting epidermal walls thereby inhibiting growth.     

Auxin-induced changes in the cell wall structure: Changes in the sugar compositions, intrinsic viscosity and molecular weight distributions of matrix polysaccharides of the epicotyl cell wall of Vigna angularis

Rapid effects of indole-3-acetic acid (IAA) on the mechanical properties of cell wall, and sugar compositions, intrinsic viscosity and molecular weight distribution of cell wall polysaccharides were investigated with excised epicotyl segments of Vigna angularis Ohwi et Ohashi cv. Takara.

• 1 IAA caused cell wall loosening as studied by stress-relaxation analysis within 15 min after the IAA application.
• 2 IAA stimulated the decrease in the content of arabinose and galactose in the hemicellulose 1 h after its application. The amounts of other component sugars in the cell wall polysaccharides remained constant during the IAA-induced segment growth.
• 3 The intrinsic viscocity of the pectin increased as early as 30 min after the IAA application. This effect was not prevented when elongation growth of the segment was osmotically suppressed by 0.15 M mannitol.
• 4 Gel permeation chromatography of the pectin on a Sepharose 4 B column demonstrated that IAA caused increase in the mass-average molecular weight of the pectin. Analysis of the sugar compositions of the pectin eluted from the Sepharose 4 B column indicated that IAA increased the molecular weight of the polysaccharides composed of uronic acid, galactose, rhamnose and arabinose. This effect became apparent within 30 min after the IAA application. Furthermore, IAA increased the molecular weight of the pectin when elongation growth of the epicotyl segments was osmotically suppressed by 0.15 M mannitol.
• 5 Hemicellulose of the cell wall chromatographed on a Sepharose CL-4 B column. Analysis of the neutral sugar compositions and the iodine staining property (specific for xyloglucans) of the polysaccharide solution eluted from the column indicated that the hemicellulose consisted of xyloglucans, arabinogalactans and polysaccharides composed of xylose and/or mannose. IAA caused a decrease in the arabinogalactan content and depolymerization of xyloglucans. These IAA effects became apparent within 30 min after the IAA application. These changes occurred even when elongation growth of the epicotyl segments was osmotically suppressed by 0.15 M mannitol.

Polymerization of the pectin, degradation of arabinogalactans and depolymerization of xyloglucans appear to be involved in the mechanism by which IAA induces cell wall loosening and therefore extension growth of cells.