Auxin-gibberellin relationships in their effects on hypocotyl elongation of light-grown cucumber seedlings. Responses of sections to auxin, gibberellin and sucrose

The sensitivity of light-grown cucumber hypocotyl sections toIAA and GA₃ depends on the degree of aging of the tissue. Agreater response to GA₃ was obtained with young tissue, whilethat to IAA was obtained with relatively old tissue. The responseto IAA reached a maximum at about 15 hr of incubation; the youngerthe tissue the earlier the time of maximum response. The responseto GA₃ continued for more than 70 hr with a constant growthrate. Very young tissue started to respond to GA₃ without lagtime; the older the tissue the later the start of the response. Sucrose (2%) inhibited IAA-induced elongation, while there wasa distinct synergism between GA₃ and sucrose. The promotiveeffect of sucrose on GA₃-induced elongation was also obtainedwhen sections were pretreated with sucrose, then transferredto GA₃. Mannitol (1%) strongly inhibited IAA-induced elongation,but not GA₃-induced elongation. (Received December 6, 1972; )     

Effect of Xyloglucan Nonasaccharide on Cell Elongation Induced by 2,4-Dichlorophenoxyacetic Acid and Indole-3-acetic Acid

Xyloglucan nonasaccharide (XG9) is recognized as an inhibitorof 2,4-D-induced long-term growth of segments of pea stems.In the presence of 10^–5 M 2,4-D, inhibition by 10^–9M XG9 of elongation of third internode segments of pea seedlingswas detected within 2 h after the start of incubation, in someexperiments. Analysis by double-reciprocal (Lineweaver-Burk)plots of elongation in the presence of various concentrationsof 2,4-D, with or without XG9, gave parallel lines, indicatingthat XG9 inhibited 2,4-D-induced elongation in an uncompetitivemanner. XG9 did not influence the 2,4-D-induced cell wall loosening.Thus, XG9 does not fulfill the proposed definition of an "antiauxin". XG9 at 10^–11 to 10^–6 M did not influence IAA-inducedelongation of segments from pea third internodes, azuki beanepicotyls, cucumber hypocotyls, or oat coleoptiles. Inhibitionof IAA-induced elongation by XG9 was not observed even whenthe segments from pea or azuki bean were abraded. Furthermore,fucosyl-lactose at 10^–11 to 10^–4 M did not affectthe IAA-induced elongation of segments of pea internodes orof azuki bean epicotyls. XG9 may be incapable of inhibitingthe IAA-induced cell elongation (especially in oat) or, alternatively,the endogenous levels of XG9 may be so high that exogenouslyapplied XG9 has no inhibitory effect on IAA-induced elongation. (Received February 28, 1991; Accepted May 25, 1991)     

Actinomycin D Inhibits the GA3-Induced Elongation of Azuki Bean Epicotyls and the Reorientation of Cortical Microtubules

Actinomycin D inhibited the elongation of epicotyl segmentsfrom azuki bean seedlings that was induced by simultaneous treatmentwith IAA and GA₃. The drug also inhibited the elongation ofthe segments that was caused by IAA alone when it was appliedtogether with IAA. When the segments were pretreated with GA₃and then incubated with IAA, GA₃ promoted the elongation causedby IAA and brought about a predominance of transverse corticalmicrotubules (MTs) in the epidermal cells of the segments. Thechange in the arrangement of MTs caused by pretreatment withGA₃ was evident 1 h after the start of subsequent incubationwith IAA when the effect of pretreatment with GA₃ on the elongationhad not yet become apparent. Pretreatment with GA₃ did not causeany change in the arrangement of MTs when GA₃-pretreated segmentswere not incubated subsequently with IAA. Although actinomycinD applied before treatment with IAA did not inhibit the IAA-inducedelongation, the drug diminished the promotion of the elongationcaused by pretreatment with GA₃ and prevented GA₃ from bringingabout a predominance of transverse MTs when the drug was appliedduring the pretreatment with GA₃. GA₃-induced synthesis of mRNA seems to be involved in the promotionby GA₃ of IAA-induced elongation and in the GA₃-induced rearrangementof cortical MTs. (Received June 15, 1993; Accepted August 16, 1993)     

Auxin-gibberellin relationships in their effects on hypocotyl elongation of light-grown cucumber seedlings III. Gibberellin specificity in its enhancing effect on IAA-induced elongation

Pretreatment effects of different gibberellins, helminthosporicacid, cyclic AMP and Kinetin on subsequent IAA-induced elongationwere tested in cucumber hypocotyl sections. Gibberellin A₇ wasmore active than GA₃, while gibberellin A₃ was almost inactive.Both helminthosporic acid and cyclic AMP mimicked GA₃-action,though the degree of their activity was less. Kinetin pretreatmentresulted in marked inhibition of IAA-induced elongation. Thepretreatment effect of GA₃ was also reflected in a greater responceof the sections to synthetic auxins. (Received October 6, 1973; )     

UDP-Glucose level as a limiting factor for IAA-induced cell elongation in Avena coleoptile segments

The effects of galactose on IAA-induced elongation and endogenous level of UDP-glucose (UDPG) in oat ( Avena sativa L. cv. Victory) coleoptile segments were examined under various growth conditions to see if there was a correlation between the level of UDPG and auxin-induced growth. The following results were obtained:

• (1)

  Galactose (10 m M ) inhibited the auxin-induced cell elongation of oat coleoptile segments after a lag of ca 2 h. Determinations of cell wall polysaccharides and UDP-sugars indicated that galactose, when inhibiting the cell wall polysaccharide synthesis, decreased the level of UDPG but caused an increase in the levels of Gal-1-P and UDP-Gal.
• (2)

  When coleoptile segments treated with IAA and galactose were transferred to galactose-free IAA-solution, the segment elongation was restored and the amounts of cell wall polysaccharides increased. During this period, the amount of UDPG increased and the levels of Gal-1-P and UDP-Gal slightly decreased or leveled off. The UDP-pentoses changed similarly as UDPG did.
• (3)

  Addition of sucrose (30 m M ) enhanced IAA-induced cell elongation and removed growth inhibition by 1 m M galactose. Sucrose increased the amounts of the cell wall polysaccharides and the level of UDPG in the presence or absence of IAA and also counteracted the decrease in UDPG caused by galactose.

These results indicate that the level of UDPG is an important limiting factor for cell wall biosynthesis and, thus, for auxin-induced elongation.     

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     

Hydroxyl radical-induced cell-wall loosening in vitro and in vivo: implications for the control of elongation growth

Hydroxyl radicals (OH) are capable of unspecifically cleaving cell-wall polysaccharides in a site-specific reaction. I investigated the hypothesis that cell-wall loosening underlying the elongation growth of plant organs is controlled by apoplastically produced OH attacking load-bearing cell-wall matrix polymers. Isolated cell walls (operationally, frozen/thawed, abraded segments from coleoptiles or hypocotyls, respectively) from maize, cucumber, soybean, sunflower or Scots pine seedlings were pre-loaded with catalytic Cu or Fe ions and then incubated in a mixture of ascorbate + H2O2 for generating OH in the walls. This treatment induced irreversible wall extension (creep) in walls stretched in an extensiometer. The reaction could be promoted by acid pH and inhibited by several OH scavengers. Generation of OH by the same reaction in living coleoptile or hypocotyl segments caused elongation growth. Auxin-induced elongation growth of maize coleoptiles could be inhibited by OH scavengers. Auxin promoted the production of superoxide radicals (O2(-)), an OH precursor, in the growth-controlling outer epidermis of maize coleoptiles. It is concluded that OH fulfils basic criteria for a wall-loosening factor acting in auxin-mediated elongation growth of plant species with widely differing cell-wall polysaccharide compositions.     

Auxin-gibberellin relationships in their effects on hypocotyl elongation of light-grown cucumber seedlings II. Effect of GA3-pretreatment on IAA-induced elongation

IAA-induced growth of light-grown cucumber hypocotyl sectionsis markedly enhanced by GA₃-pretreatment of the sections; thereis a distinct synergism between IAA and GA₃. Water pretreatmentalso enhances IAA-induced growth. On the other hand, IAA-pretreatedsections showed practically no further growth in response topost treatment with GA₃. The enhancing effect of GA₃ is obtainedwith only 30 min pretreatment, the maximum effect occuring with2 hr pretreatment. Pretreatment longer than 8 hr is less effective.This enhancing effect of GA₃ can be observed soon after posttreatment with IAA. The response of GA₃-pretreated sectionsto IAA is greater in pretreatment with higher concentrationsof GA₃, and higher degrees of synergism between IAA and GA₃are obtained at IAA concentrations less than 10^-4 M. This synergisticinteraction between GA₃ and IAA is more marked in aged hypocotylsections than in young sections. From these results we concludedthat gibberellin sensitizes hypocotyl cells to the subsequenteffect of auxin on cell elongation. (Received October 6, 1973; )     

Involvement of wall microtubules in gibberllin promotion and kinetin inhibition of stem elongation

The elongation of light-grown azuki bean (Azukia angularis =Vigna angularis) epicotyl segments was promoted by indoleaceticacid (IAA) and this IAA-induced elongation was inhibited byboth kinetin and benzimidazole (BIA). Increased stem thickeningwas observed with kinetin- or BIA-treated segments, but thiswas not accompanied by incresed cell number in the transversedirection, suggesting that both kinetin and BIA promoted lateralcell expansion. Colchicine at a concentration with no effecton IAA-induced elongation reversed both the kinetic- and BIA-inducedinhibition. Electron-microscopic examination revealed that wall microtubulesin cells treated with kinetin together with IAA ran parallelto the cell axis, while wall microtubules in cells treated withonly IAA were randomly oriented. In the cell treated with gibberellintogether with IAA, wall microtubules ran tranverse to the cellaxis. (Received July 13, 1973; )     

Inhibition of stem elongation in cucumis seedlings by blue light requires calcium

The effects of blue light and calcium on elongation of hypocotyl segments of Cucumber (Cucumis sativa L. cv Burpee's Pickler) were studied. Cucumber seedlings grown in dim red light showed a rapid decline in the rate of hypocotyl elongation when irradiated with high intensity (100 micromoles per square meter per second) blue light. In intact, 4-day-old seedlings the inhibition began within 2 minutes after the onset of blue-light irradiation and reached a maximum of approximately 55% within 4 minutes. Hypocotyl segments cut from 4-day-old seedlings also showed an inhibition of elongation in response to blue light when segments were floated on aqueous buffer and exposed to blue light for 3 hours. In the presence of 2 micromolar indole-3-acetic acid, blue light caused a 50% inhibition of elongation. Buffering free calcium in the incubation medium with 0.1 millimolar ethylene glycol bis(-aminoethyl ether)- N,N,N′,N′-tetraacetic acid eliminated the blue-light inhibition of segment elongation. Several experiments confirmed a specific requirement for calcium for the blue-light-induced inhibition of segment elongation. Treating segments with 0.2 micromolar fusicoccin abolished the inhibition of elongation by blue light as did buffering the medium at pH 4. Adding 1 millimolar ascorbate to incubation medium also eliminated the inhibition of segment elongation caused by blue light. Several compounds implicated in cell-wall redox reactions alter the magnitude of the blue-light-induced inhibition. The activity of peroxidase isolated from the cell-wall free space of cucumber hypocotyls was inhibited by ascorbate and low pH. The results are consistent with the hypothesis that blue light inhibits elongation by inducing an increase in cell-wall peroxidase activity and implicate calcium ions in the response to blue light.     

Relationship between IAA-induced elongation growth and plasma membrane NADH oxidase in soybean (Glycine max Merr.) hypocotyls

A relationship between the activity of NADH oxidase of the plasma membrane and the IAA-induced elongation growth of hypocotyl segments in etiolated soybean (Glycine max Merr.) seedlings was investigated. The plasma membrane NADH oxidase activity increased in parallel to IAA effect on elongation growth in hypocotyl segments. Actually, NADH oxidase activity was stimulated 3-fold by 1 u,M IAA, and the elongation rate of segments was stimulated 10-fold by 10 iM IAA. The short-term elongation growth kinetics, however, showed that the IAA-induced elongation of hypocotyl segments was completely inhibited by plasma membrane redox inhibitors such as actinomycin D and adriamycin, at 80 μM and 50 μM respectively. In addition, 1 mM actinomycin D inhibited the IAA-stimulated NADH oxidase activity by about 80%. However, adriamycin had no effect on NADH oxidase activity of plasma membrane vesicles. Based on these results, the plasma membrane redox reactions seemed to be involved in IAA-induced elongation growth of hypocotyls, and the redox component responding to IAA was suggested to be NADH oxidase.     

Galactose prevents proton excretion but not IAA-induced growth in segments of azuki bean epicotyls

We investigated the effect of galactose on IAA-induced elongation and proton excretion in azuki bean (Vigna angularis Ohwi et Ohashi) segments in order to confirm whether or not protons were involved in auxin-induced growth. Galactose inhibited the IAA-induced decrease in the solution pH but had no inhibitory effect on IAA-induced growth in segments of azuki bean epicotyls. On the other hand, galactose inhibited both IAA-induced growth and proton excretion in oat (Avena sativa L.) coleoptile segments. From these results it is unlikely that IAA-induced growth is mediated by proton excretion at least in azuki bean epicotyls.Abbreviations IAA indole-3-acetic acid - FC fusicoccin     

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)     

Update on the possible mode of action of the jasmonates: Focus on the metabolism of cell wall polysaccharides in relation to growth and development

Jasmonic acid (JA) and its related compounds (jasmonates) applied to plant tissues exert either inhibitory or promotive effects in growth and developmental processes, which in some ways are similar to abscisic acid. However, little is known about the mode of action of the jamonates at the tissue or organ levels. Here, we review partial evidence for the physiological action of the jasmonates on cell elongation and abscission.
Jasmonates inhibit the IAA-induced cell elongation of oat coleoptile segments not by affecting energy production, osmoregulation and cell wall loosening, but by inhibiting the synthesis of cell wall polysaccharides. The inhibition is partially reversed by simultaneous application of sucrose. Inhibition of IAA-induced elongation by JA is only observed in monocotyledons, not in dicotyledons. These effects suggest that jasmonates exert their inhibitory effect on cell elongation by affecting the metabolism of the cell wall polysaccharides in monocotyledons.
Jasmonates promote the abscission of bean petiole explants without enhancing ethylene production. Cells in the petiole adjacent to the abscission zone expand during abscission. In the abscission zone, jasmonates decrease the amount of cellulosic but not that of noncellulosic polysaccharides. Jasmonates increase the activities of cellulase and decrease the levels of UDP-sugars, which are important intermediates for the synthesis of cell wall polysaccharides in the abscission zone, probably resulting in the decreased level of cellulose and the mechanical weakness of cell walls.
Thus, it is suggested that jasmonates exert their multiple physiological effects by affecting the metabolic processes of cell wall polysaccharides.     

Interaction of Gibberellin A3 and Ancymidol in the Growth and Cell-Wall Extensibility of Dwarf Pea Roots

Effects of ancymidol (Anc) and gibberellin A₃ (GA₃) on rootgrowth, osmotic concentration and cell-wall extensibility ofthe root were investigated in the gibberellin-sensitive cultivarof dwarf pea, Little Marvel. Anc strongly suppressed elongationof both shoots and roots in darkness. Although the elongationof shoots of this dwarf cultivar was severely retarded in thelight, it was repressed still further by Anc. GA₃ promoted elongationof shoots both in the presence and in the absence of Anc, whereasit reversed suppression of root elongation by Anc. The concentrationof GA₃ required for the recovery of root elongation was lowerthan that required for the promotion of shoot elongation. Treatmentwith Anc led to increased thickening of roots with increasednumbers of cells per cross section and lateral expansion ofcells in the cortex. GA₃ had little effect on the osmotic concentration of cell sapobtained from root segments. Anc-treated roots did not respondto acid solutions by elongation, whereas GA₃-treated roots respondednormally to such solutions. Anc suppressed but GA₃ enhancedthe cell-wall extensibility of roots as measured in vivo andin vitro. These results indicate that a low concentration of gibberellinplays a role in normal elongation of roots by maintaining theextensibility of the cell wall in this gibberellin-sensitivedwarf pea. (Received January 17, 1994; Accepted July 15, 1994)     

Effect of inhibition of protein glycosylation on auxin-induced growth and the occurrence of osmiophilic particles in maize (Zea mays L.) coleoptiles

The dependence of auxin (IAA)-induced elongation growth on proteinglycosylation was investigated in abraded maize (Zea mays L.)coleoptile segments, employing 2-deoxy-D-glucose (DOG) and tunicamycin(TUM) as inhibitors of protein glycosylation. TUM had no detectableeffect on growth at 100µg ml^–1. DOG impaired growthat concentrations larger than 1 mM. Total inhibition of growthoccurred at a concentration of 20 mM. Similar effects were observedwith mannose and glucosamine. The effect on wall-synthetic processesin the growth-limiting epidermis was analysed by tracer incorporationstudies. Within 30 min hemicellulose and cellulose synthesis,measured as ³H-glucose incorporation, was not affected by DOG,indicating that inhibition of growth is not causally relatedto synthesis of both wall components. In contrast, protein synthesisand secretion into the walls, measured as incorporation of ³H-leucineinto the TCA-precipitable protoplasmic and wall-bound protein,was rapidly inhibited by DOG. Concomitant with the effect ongrowth, DOG as well as mannose inhibited the occurrence of osmiophilicparticles (OPs) which specifically occur at the growth-limitingepidermis during IAA-induced growth. The results provide evidencethat IAA-induced wall loosening underlying elongation growthis dependent on O-glycosylation of proteins and their subsequentsecretion into the epidermal walls. It appears that interferencewith these processes is responsible for inhibition of IAA inducedgrowth by hexoses acting as anti-glucose antimetabolites. Key words: Auxin-induced growth, cell-wall synthesis, 2-deoxy-D-glucose, mannose, osmiophilic particles, tunicamycin     

The effect of the seed coat, embryonic axis and aeration conditions on starch degradation hi cotyledons of Lens culinaris

The effect of exogenously applied galactose on the cell wall polysaccharide synthesis and UDP-sugar levels in oat ( Avena sativa L. cv. Victory I) coleoptile segments was studied to clarify the mechanism of inhibition of IAA-induced cell elongation by galactose, and the following results were obtained: (1) The inhibition of IAA-induced cell elongation by galactose became apparent after a 2 h-lag, while the lag was shortened to 1 h when galactose was added to the segments after more than 1 h of IAA application. (2) Galactose inhibited the [¹⁴C]-glucose incorporation into cellulosic and non-cellulosic fractions of the cell wall and the increase in net polysaccharide content in the fractions during long-term incubation. (3) The dominant sugar nucleotide in oat coleoptiles was UDP-glucose (2.1 nmol segment⁻¹). Galactose application caused a remarkable decrease in the UDP-glucose level, accompanying a strong accumulation of galactose-1-phosphate and UDP-galactose. (4) Galactose-1-phosphate competitively inhibited the UTP: a- d -glucose-1-phosphate uridylyltransferase (EC 2.7.7.9) activity of the crude enzyme preparation from oat coleoptiles. From these results we conclude that galactose inhibits the IAA-induced cell elongation by inhibiting the formation of UDP-glucose, which is a key intermediate of cell wall polysaccharide synthesis.     

Auxin-gibberellin relationships in their effects on hypocotyl elongation of light-grown cucumber seedlings VI. Promotive and inhibitory effects of kinetin

The interaction of kinetin with IAA and GA₃ on the elongationof hypocotyl sections of Cucumis sativus L. cv. National Picklingwas studied. Kinetin in the concentration range of 10^–7M to 10^–4 M markedly inhibited IAA-induced elongation,while in a lower range from 10^–10 M to 10^–8 M, itsynergistically enhanced IAA-induced elongation. Kinetin alonein this range had no effect. A 5-to 15-min pulse treatment seemsenough to induce the maximum effect for both inhibition andpromotion. Since the magnitude of the maximum inhibition dependedon the concentration and not on the duration of treatment, thereaction in the cell caused by kinetin seemed to be completedwithin a short period. Washing of the sections with distilledwater after kinetin treatment (30 min) did not significantlyeliminate the kinetin effect. This probably indicates that thebinding of kinetin molecules to a supposed acceptor is not reversible.Interaction of kinetin with GA₃ in their pretreatment effectson IAA-induced elongation shows that in the inhibitory concentrationrange, the kinetin effect was partly overcome by GA₃, and thatin the promotive range, the magnitude of the enhancement wasdetermined by kinetin regardless of the presence of GA₃. Theeffect of kinetin seems to dominate over that of GA₃ indicatingthat the modes of their pretreatment effects differ from oneanother. (Received June 24, 1977; )     

ffects of Some Inhibitors on Potassium-and IAA-Induced Adventitious Root Formation of Excised Cucumber Cotyledon

The effects of some inhibitors on potassium- and IAA-induced rooting were studied adopting the root-formation bioassay in the excised cucumber ( Cucumis sativus L. ) cotyledon. 5-fluomuracil at 7 Ï 10-4 – 7 Ï10-1 mmol/L and cycloheximide at 3.5 Ï 10-4 – 1.05 Ï10-2 mmol/L significantly inhibited potassium- and IAA-induced adventitious root formation of the excised cucumber cotyledons, respectively. Na3VO4 at 0.1 – 1.0 mmol/L obviously inhibited potassium and IAA-induced adventitious rooting of the excised cucumber cotyledons, and similar inhibitory effect was found with 2,3,5-triiodobenzoic acid (TIBA) at 2 Ï 10-4 – 2 Ï 10-l mmol/L.There is a close relationship between potassium and IAA-induced adventitious rooting and the promotive effect of potassium on rooting is possibly brought about via affecting the endogenous level of IAA.     

Auxin-gibberellin relationships in. their effects on hypocotyl elongation of light-grown cucumber seedlings IV. Inhibition of IAA-induced elongation by N,N'-dicyclohexylcarbodiimide and its reversal by gibberellin A3

IAA-induced elongation and control growth of light-grown cucumberhypocotyl sections were markedly inhibited by DCCD, an inhibitorof membrane-bound ATPases. The concentration effective for inducingmarked inhibition was more than 10^–5 M. At 10^–5M DCCD, there was an apparent antagonism between IAA and DCCD.At 5 x 10^–5 M DCCD, the inhibition was partially recoveredby 10^–4 M of IAA. The results might indicate a close associationof the auxin action with membrane-bound ATPases. The DCCD inhibitionwas so strong that treatment with 10^–4 M DCCD for about5 min significantly suppressed further growth and longer incubationkilled the sections. In contrast, DCCD had not inhibitory effecton both control growth and IAA-induced elongation if GA₃ waspresent simultaneously. DCCD treatment followed by GA₃ treatmentstill resulted in the inhibition, suggesting that the inhibitionwas not reversible. In order to obtain reversal of DCCD inhibitionby GA₃ both compounds must be present at the same time. TheGA₃ effect is discussed in connection with the mechanism ofDCCD action on membrane-bound ATPases. (Received October 6, 1975; )