Effects of ethylene on auxin transport

The effect of ethylene on the uptake, distribution and polar transport of C¹⁴ from indole-3-acetic acid-2-C¹⁴ and naphthalene acetic acid-1-C¹⁴ in tissue sections was studied. Test species were cotton (Gossypium hirsutum, L.) and cowpea (Vigna sinensis, Endl.). Generally, incubation of tissue or intact plants with ethylene reduced the degree of polar auxin transport. Ethylene inhibited the movement of both auxins in stem tissue and IAA in petiole tissue of cotton. The effect of ethylene on auxin movement in cow-peas was more complex. Ethylene apparently inhibited transport in younger petiole and stem tissue, but stimulated the process to a small but significant degree in basal petiole segments.

Ethylene, in some experiments, reduced C¹⁴ (auxin) uptake. This reduction was consistently smaller than the inhibition of transport. Effects upon transport were observed when uptake was not different. Differences in uptake declined as the period of incubation with auxin was lengthened, but transport was inhibited for up to 23 hours.

It is proposed that ethylene may, through its effect on transport, cause localized shortages and surpluses of auxin which in turn contribute to symptoms now associated with the response of sensitive species to ethylene.

     

Insensitivity of the diageotropica tomato mutant to auxin

The sensitivity of excised hypocotyl segments to indoleacetic acid (IAA) in two assays, ethylene production and elongation, was determined in the ethylene-requiring tomato (Lycopersicon esculentum Mill.) mutant, diageotropica (dgt), and its isogenic parent, cv VFN8. Endogenous (uninduced) ethylene synthesis rates were slightly lower in dgt hypocotyls than in VFN8 hypocotyls. Ethylene production was essentially unaffected by IAA in dgt, but was stimulated up to 10-fold by 10 micromolar IAA in VFN8. Elongation of dgt hypocotyls was also insensitive to concentrations of IAA as high as 100 micromolar, as compared to significant elongation of VFN8 hypocotyls in response to 0.1 micromolar IAA. A range of IAA analogs active in VFN8 was also ineffective in stimulating elongation of dgt hypocotyls, suggesting that the differences were not due to rapid metabolism of IAA by dgt tissues. Auxin-induced elongation of VFN8 hypocotyls was unaffected by 2,3,5-triiodobenzoic acid and naphthylphthalamic acid, indicating that polar auxin transport was not a factor in these experiments. Exogenous and auxin-induced ethylene had no effect on the elongation respone of either genotype, nor did exogenous ethylene restore the sensitivity of dgt hypocotyls to IAA. Despite their apparent insensitivity to auxin, dgt hypocotyls elongated dramatically and synthesized ethylene rapidly in response to 1.2 micromolar fusicoccin. These results suggest that the primary effect of the dgt mutation is to reduce the sensitivity of the tissue to auxin. As altered regulation of ethylene synthesis is only one symptom of this fundamental deficiency, dgt should more properly be considered to be the auxin-insensitive tomato mutant.     

Auxin transport and the regulation of petiole abscission in cotton (Gossypium hirsutum L.): A comparison of indol-3yl-acetic acid and phenylacetic acid

Distal applications of indol-3yl-acetic acid (IAA) to debladed cotyledonary petioles of cotton (Gossypium hirsutum L.) seedlings greatly delayed petiole abscission, but similar applications of phenylacetic acid (PAA) slightly accelerated abscission compared with untreated controls. Both compounds prevented abscission for at least 91 h when applied directly to the abscission zone at the base of the petiole. The contrasting effects of distal IAA and PAA on abscission were correlated with their polar transport behaviour-[1-¹⁴C]IAA underwent typical polar (basipetal) transport through isolated 30 mm petiole segments, but only a weak diffusive movement of [1-¹⁴C]PAA occurred.Removal of the shoot tip substantially delayed abscission of subtending debladed cotyledonary petioles. The promotive effect of the shoot tip on petiole abscission could be replaced in decapitated shoots by applications of either IAA or PAA to the cut surface of the stem. Following the application of [1-¹⁴C]IAA or [1-¹⁴C]PAA to the cut surface of decapitated shoots, only IAA was transported basipetally through the stem. Proximal applications of either compound stimulated the acropetal transport of [¹⁴C]sucrose applied to a subtending intact cotyledonary leaf and caused label to accumulate at the shoot tip. However, PAA was considerably less active than IAA in this response.It is concluded that whilst the inhibition of petiole abscission by distal auxin is mediated by effects of auxin in cells of the abscission zone itself, the promotion of abscission by the shoot tip (or by proximal exogenous auxin) is a remote effect which does not require basipetal auxin transport to the abscission zone. Possible mechanisms to explain this indirect effect of proximal auxin on abscission are discussed.     

Transport and metabolism of indole-3-acetic Acid in coleus petiole segments of increasing age

Transport and metabolism of IAA-1-¹⁴C in Coleus blumei Benth. was studied by means of a combination of liquid scintillation counting, autoradiography and thin-layer chromatography. Transport of IAA in petiole segments of increasing age (No. 2-8) was strictly polar in a basipetal direction. No acropetal movement occurred in either young or old tissues. The greatest amount, expressed as a percentage of the radioactivity lost from the donor block, was found in basal receivers on petiole number 2. There was gradually less transport in older segments. The recovery as a percentage of the radioactivity not accounted for by donor and receiver blocks, measured by counting the radioactivity in an acetonitrile-extract of petiole segments, was low: 25 to 50%. In this acetonitrile-soluble fraction evidence for different radioactive compounds was found, depending on the age of the tissue. A possible relationship between the amounts of auxin transported in the tissue and its corresponding metabolism is discussed.     

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.     

Ethylene-induced leaf abscission in cotton seedlings : the physiological bases for age-dependent differences in sensitivity

The speed of ethylene-induced leaf abscission in cotton (Gossypium hirsutum L. cv LG-102) seedlings is dependent on leaf position (i.e. physiological age). Fumigation of intact seedlings for 18 hours with 10 microliters per liter of ethylene resulted in 40% abscission of the still-expanding third true (3°) leaves but had no effect on the fully expanded first true (1°) leaves. After 42 hours of fumigation with 50 microliters per liter of ethylene, total abscission of the 3° leaves occurred while <50% abscission of the 1° leaves was observed. On a leaf basis, endogenous levels of free IAA in 1° leaves were approximately twice those of 3° leaves. Free IAA levels were reduced equally (approximately 55%) in both leaf types after 18 hours of ethylene (10 microliters per liter) treatment. Ethylene treatment of intact seedlings inhibited the basipetal movement of [¹⁴C]IAA in petiole segments isolated from both leaf types in a dose-dependent manner. The auxin transport inhibitor N-1-naphthylphthalamic acid increased the rate and extent of ethylene-induced leaf abscission at both leaf positions but did not alter the relative pattern of abscission. Abscission-zone explants prepared from 3° leaves abscised faster than 1° leaf explants when exposed to ethylene. Ethyleneinduced abscission of 3° explants was not appreciably inhibited by exogenous IAA while 1° explants exhibited a pronounced and protracted inhibition. The synthetic auxins 2,4-D and 1-naphthaleneacetic acid completely inhibited ethylene-induced abscission of both 1° and 3° explants for 40 hours. It is proposed that the differential abscission response of cotton seedling leaves is primarily a result of the limited abscission-inhibiting effects of IAA in the abscission zone of the younger leaves.     

Effect of Ethylene Treatment on Polar IAA Transport, Net IAA Uptake and Specific Binding of N-1-Naphthylphthalamic Acid in Tissues and Microsomes Isolated from Etiolated Pea Epicotyls

The effect of ethylene treatment on polar indole-3-acetic acid (IAA) transport, net IAA uptake in the presence and absence of N-1-naphthylphthalamic acid (NPA) and [³H]NPA binding characteristics was investigated in tissue segments or microsomes isolated from etiolated pea (Pisum sativum L. cv Alaska) epicotyls. Basipetal IAA transport in 5 millimeter segments isolated from ethylene-treated seedlings was inhibited by ethylene in a dose-dependent manner. Threshold, half-maximal and saturating concentrations of ethylene were 0.01, 0.55, 10.0 microliters per liter, respectively. This inhibition became apparent after 6 to 8 hours of ethylene treatment. Transport velocity in both control and ethylene-treated tissues was estimated to be 5 millimeters per hour. Net IAA uptake was stimulated in ethylene-treated tissues and the relative ability of the phytotropin NPA to enhance net IAA uptake was reduced in treated tissues. Specific binding of [³H]NPA to microsomes prepared from both control and ethylene-treated tissues was saturable and consistent with the existence of a single class of binding sites with an apparent affinity (K_d) toward NPA of 8 to 9 nanomolar. The density of these binding sites (per milligram protein) was lower (36% of control) in ethylene-treated tissues. Direct application of ethylene to microsomal preparations isolated from untreated seedlings had no effect on the level of specific [³H]NPA binding.     

Auxin and Ethylene Regulation of Petiole Epinasty in Two Developmental Mutants of Tomato, diageotropica and Epinastic

The epinastic growth responses of petioles to auxin and ethylene were quantified in two developmental mutants of tomato (Lycopersicon esculentum Mill.). In the wild type parent line, cultivar VFN8, the epinastic response of excised petiole sections was approximately log-linear between 0.1 and 100 micromolar indole-3-acetic acid (IAA) and 2,4-dichlorophenoxyacetic acid (2,4-D) concentrations, with a greater response to 2,4-D at any concentration. When ethylene synthesis was inhibited by aminoethoxyvinylglycine (AVG), epinasty was no longer induced by auxin, but could be restored by the addition of ethylene gas. In the auxin-insensitive mutant, diageotropica (dgt), no epinastic response to IAA was observed at IAA concentrations that effectively induced epinasty in VFN8. In the absence of added IAA, epinastic growth of dgt petioles in 1.3 microliters per liter exogenous ethylene gas was more than double that of VFN8 petioles. IAA had little additional effect in dgt, but promoted epinasty in VFN8. These results confirm that tomato petiole cells respond directly to ethylene and make it unlikely that the differential growth responsible for epinasty results from lateral auxin redistribution. The second mutant, Epinastic (Epi), exhibits constitutively epinasty, cortical swelling, and root branching symptomatic of possible alternation in auxin or ethylene regulation of growth. Only minor quantitative differences were observed between the epinastic responses to auxin and ethylene of VFN8 and Epi. However, in contrast to VFN8, when ethylene synthesis or action was inhibited in Epi, auxin still induced 40 to 50% of the epinastic response observed in the absence of inhibitors. This indicates that the target cells for epinastic growth in Epi are qualitatively different from those of VFN8, having gained the ability to grow differentially in response to auxin alone. The dgt and Epi mutants provide useful systems in which to study the genetic determination of target cell specificity for hormone action.     

On ethylene and stem elongation in green pea seedlings

Maximum elongation of excised internodal stem sections of light-grown pea (Pisum sativum L.) seedlings occurred at 10⁻⁵ molar indoleacetic acid (IAA), with submaximal responses occurring at 10⁻⁴ and 10⁻³ molar. Accompanying elongation at concentrations of IAA of 10⁻⁶ to 10⁻³ molar was production of ethylene, with the amount increasing up to 10⁻⁴ molar IAA and then becoming nearly constant. Elongation of light-grown sections was not inhibited by exogenous ethylene up to 10,000 ppm in the presence of 10⁻⁵ molar IAA. Marked (up to 50%) inhibition of elongation of internodal segments in situ was observed after treating whole light-grown seedlings with exogenous ethylene for 20 hours. It is concluded that ethylene is not responsible for the submaximal elongation responses of green pea stem sections at high auxin concentrations, but that IAA per se is accountable.     

Characterization of the growth and auxin physiology of roots of the tomato mutant,diageotropica

Roots of the tomato (Lycopersicon esculentum, Mill.) mutant diageotropica (dgt) exhibit an altered phenotype. These roots are agravitropic and lack lateral roots. Relative to wild-type (VFN8) roots, dgt roots are less sensitive to growth inhibition by exogenously applied IAA and auxin transport inhibitors (phytotropins), and the roots exhibit a reduction in maximal growth inhibition in response to ethylene. However, IAA transport through roots, binding of the phytotropin, tritiated naphthylphthalamic acid ([³H]NPA), to root microsomal membranes, NPA-sensitive IAA uptake by root segments, and uptake of [³H]NPA into root segments are all similar in mutant and wild-type roots. We speculate that the reduced sensitivity of dgt root growth to auxin-transport inhibitors and ethylene is an indirect result of the reduction in sensitivity to auxin in this single gene, recessive mutant. We conclude that dgt roots, like dgt shoots, exhibit abnormalities indicating they have a defect associated with or affecting a primary site of auxin perception or action.Abbreviations BCA bicinchoninic acid - IAA indole 3-acetic acid - dgt diageotropica - IC₅₀ concentration for 50% inhibition of growth - NPA N-1-naphthylphthalamic acid - SCB-1 semicarbazone 1 This research was supported by grants from Sandoz Agro, Inc. (G.K.M), the National Aeronautics and Space Administration (NASA) and the National Science Foundation (T.L.L), and NASA (D.L.R.).     

Effect of Transverse Gravity Stimulation, Gibberellin and Indoleacetie Acid upon Polar Transport of IAAC14 in the Stem of Helianthus annuus

Using IAA^C14, polar transport has been studied in Helianthus annuus shoots in which stem elongation was inhibited by a transverse gravity stimulus induced by horizontal orientation of the plant with daily rotation. Marked inhibition of polar transport of IAA ^C14 occurred in the treated plants. A similar degree of inhibition occurred in the upper and lower halves of non-rotated horizontally trained shoots. Horizontal orientation of stem segments during the transport test had no consistent effect upon IAA transport. Pretreatment of erect plants with gibberellin greatly enhanced IAA^C14 transport and also reduced the inhibitory effect of horizontal orientation. Pretreatment of erect plants with non-radioactive IAA or ethylene inhibited transport of IAA ^C14 and induced the same symptoms in the shoot as the transverse gravity stimulus. The similarities between the response of the auxin transport system to gravity stimulation, IAA and ethylene are discussed.     

Role of calcium in the polar secretion of indoleacetic Acid

The rate of auxin transport in sunflower hypocotyls (Helianthus annuus L. cv `Russian mammoth') or corn coleoptiles (Zea mays L. cv `WF9 × 38') was less in seedlings grown in Ca-deficient medium than in controls. The rate of IAA transport depended on the concentration of Ca in the root medium up to 1 millimolar. Further increases in auxin transport were observed when the isolated segments were incubated in medium containing up to 30 millimolar Ca. We suggest that the rate of auxin transport in plant tissue is dependent on the pool of ionic Ca in the extracellular space.

Segments from Ca-deficient seedlings exhibited a high specific requirement for Ca²⁺ in auxin transport. Magnesium, strontium, and several other divalent cations tested for their ability to replace Ca²⁺ in restoring auxin transport showed no effect; partial replacement by lanthanum was observed.

Auxin transport, or auxin flux through the segment, which is the result of IAA secretion by individual cells, was reduced in the low Ca²⁺ segments due both to lowered velocity and to reduced capacity of transport. The requirement for Ca²⁺ in the secretion of auxin is believed to be equivalent to the phenomenon observed in animal cell secretion, where the influx of Ca²⁺ serves as a link between an external stimulus and the secretion response.

     

The physiological significance of phenylacetic Acid in abscising cotton cotyledons

The physiological role of phenylacetic acid (PAA) as an endogenous regulator of cotyledon abscission was examined using cotton (Gossypium hirsutum L. cv LG 102) seedlings. Application of 100 micromolar or more PAA to leafless cotyledon abscission-zone explants resulted in the retardation of petiole abscission and a decrease in the rise of ethylene evolution that normally accompanies aging of these explants in vitro. The partial inhibition of ethylene evolution in these explants by PAA was indirect since application of this compound stimulated short-term (<24 hours) ethylene production. PAA treatment partially suppressed the stimulation of petiole abscission elicited by either ethylene or abscisic acid. Both free and an acid-labile, bound form of PAA were identified in extracts prepared from cotyledons. No discernible pattern of changes in free or bound PAA was found during the course of ethylene-induced cotyledon abscission. Unlike indole-3-acetic acid, transport of PAA in isolated petiole segments was limited and exhibited little polarity. On the whole, these results are not consistent with the direct participation of PAA in the endogenous regulation of cotyledon abscission.     

Auxin and Ethylene Control of Growth in Seedlings of Zea mays L. and Avena sativa L

The effects of applied ethylene on the growth of coleoptilesand mesocotyls of etiolated monocot seedlings (oat and maize)have been compared with those on the epicotyl of a dicot seedling(the etiolated pea). Significant inhibition of elongation by ethylene (10 µll^–1for 24 h) was found in intact seedlings of all three species,but lateral expansion growth was observed only in the pea internodeand oat mesocotyl tissue. The sensitivity of the growth of seedlingparts to ethylene is in the decreasing order pea internode,oat coleoptile and oat mesocotyl, with maize exhibiting theleast growth response. Although excised segments of mesocotyland coleoptile or pea internode all exhibit enhanced elongationgrowth in IAA solutions (10^–6–2 ? 10^–5 moll^–1), no consistent effects were found in ethylene. Ethyleneproduction in segments was significantly enhanced by applicationof auxin (IAA, 10^–5 mol l^–6 or less) in all tissuesexcept those of the eat mesocotyl. Segments of maize show a slow rate of metabolism of applied[2-¹⁴C]IAA (30 per cent converted to other metabolites within9 h) and a high capacity for polar auxin transport. Ethylene(10 µl l^–1 for 24 h) has little effect on eitherof these processes. The oat has a smaller capacity for polartransport than maize and the rate ef metabolism of auxin isas fast as in the pea (90 per cent metabolized in 6 h). Althoughethylene pretreatment does not change the rate of auxin metabolismin oat, there is a marked reduction in auxin transport. It is proposed that the insensitivity of maize seedlings toethylene is related to the supply and persistence of auxin whichcould protect the seedling against the effects of applied orendogenously produced ethylene. Although the mesocotyl of oatis sensitive to applied ethylene it may be in part protectedagainst ethylene in vivo by the absence of an auxin-enhancedethylene production system. The results are discussed in relationto a model for the auxin and ethylene control of cell growthin the pea.     

The role of auxin efflux carriers in the reversible loss of polar auxin transport in the pea (Pisum sativum L.) stem

Correlatively inhibited pea shoots (Pisum sativum L.) did not transport apically applied ¹⁴C-labelled indol-3yl-acetic acid ([¹⁴C]IAA), and polar IAA transport did not occur in internodal segments cut from these shoots. Polar transport in shoots and segments recovered within 24 h of removing the dominant shoot apex. Decapitation of growing shoots also resulted in the loss of polar transport in segments from internodes subtending the apex. This loss was prevented by apical applications of unlabelled IAA, or by low temperatures (approx. 2° C) after decapitation. Rates of net uptake of [¹⁴C]IAA by 2-mm segments cut from subordinate or decapitated shoots were the same as those in segments cut from dominant or growing shoots. In both cases net uptake was stimulated to the same extent by competing unlabelled IAA and by N-1-naphthylphthalamic acid. Uptake of the pH probe [¹⁴C]-5,5-dimethyloxazolidine-2,4-dione from unbuffered solutions was the same in segments from both types of shoot. Patterns of [¹⁴C]IAA metabolism in shoots in which polar transport had ceased were the same as those in shoots capable of polar transport. The reversible loss of polar IAA transport in these systems, therefore, was not the result of loss or inactivation of specific IAA efflux carriers, loss of ability of cells to maintain transmembrane pH gradients, or the result of a change in IAA metabolism. Furthermore, in tissues incapable of polar transport, no evidence was found for the occurrence of inhibitors of IAA uptake or efflux. Evidence is cited to support the possibility that the reversible loss of polar auxin transport is the result of a gradual randomization of effluxcarrier distribution in the plasma membrane following withdrawal of an apical auxin supply and that the recovery of polar transport involves reestablishment of effluxcarrier asymmetry under the influence of vectorial gradients in auxin concentration.Abbreviations DMO 5,5-dimethyloxazolidine-2,4-dione - IAA indol-3yl-acetic acid - NPA N-1-naphthylphthalamic acid - TIBA 2,3,5-triiodobenzoic acid This work was supported by grant no. GR/D/08760 from the U.K. Science and Engineering Research Council. We thank Mrs. R.P. Bell for technical assistance.     

Cobalt and plant development: interactions with ethylene in hypocotyl growth

Co²⁺ promoted elongation of hypocotyl segments of light-grown cucumber (Cucumis sativus) seedlings. Time course and dose response data are presented and interactions with IAA, gibberellin, cyclohexanol, and cotyledons described. Segments without cotyledons responded to Co²⁺ only if grown in gas-tight vessels with IAA added. When bases of cotyledons were ringed with an inhibitor of auxin transport, Co²⁺ caused no growth promotion in the hypocotyl. Co²⁺ prevented lateral swelling of hypocotyls treated with supraoptimal IAA. Removal of ethylene from the atmosphere reduced the Co²⁺ response, but Co²⁺ did not counteract the inhibitory effect of increased ethylene levels. These results are consistent with the hypothesis that Co²⁺ promotes hypocotyl elongation by inhibiting ethylene production. The hypothesis was confirmed by a direct demonstration that Co²⁺, at growth-promoting concentrations, powerfully inhibited ethylene production in the cucumber hypocotyl.     

Abscission: the initial effect of ethylene is in the leaf blade

The leaf blade of cotton (Gossypium hirsutum L. cv. Stoneville 213) was investigated as the initial site of ethylene action in abscission. Ethylene applied at 14 μl/l to intact 3-week-old plants caused abscission of the third true leaf within 3 days. However, keeping only the leaf blade of this leaf in air during ethylene treatment of the rest of the plant completely prevented its abscission for up to 7 days. This inhibition of abscission was apparently the result of continued auxin production in the blade since (a) the application of an auxin transport inhibitor to the petiole of the air-treated leaf blade restored ethylene sensitivity to the leaf in terms of abscission; (b) repeated applications of naphthaleneacetic acid to the leaf blade of the third true leaf, when the entire plant was exposed to ethylene, had the same preventive effect on abscission of this leaf as keeping its leaf blade in air; and (c) the inhibitory effect of ethylene on auxin transport in the petiole, which is reduced by auxin treatment, was also reduced by placing the leaf blade in air.     

Decarboxylation and transport of auxin in segments of sunflower and cabbage roots

Summary The movement of ¹⁴C from indole-3-acetic acid (IAA) ¹⁴C has been examined in 5 mm root segments of dark-grown seedlings of Helianthus annuus and Brassica oleracea. Contaminants from distilled water, phosphate buffer and the razor-blade cutter increase the decarboxylation of IAA-¹⁴C, and cutting of root segments results in an activation of IAA-destroying enzymes at the cut surfaces. When these sources of errors were eliminated the following was shown: a) Both in sunflower and cabbage there is a slight acropetal flux of ¹⁴C through the root segments into the agar receiver blocks. The amount of ¹⁴C found in the receiver blocks increases with the lenght of the transport period. b) When the root segments, after the transport period, are cut in two equal parts and these assayed separately, the amounts of ¹⁴C in the two parts indicate a greater acropetal than basipetal transport. c) The total radioactivity of the receiver blocks is in part due to IAA-¹⁴C and in part to ¹⁴CO₂, the latter being a result of enzymatic destruction of auxin. d) Addition of ferulic acid, an inhibitor of IAA oxidases, to the receiver blocks markedly inhibits the decarboxylation of IAA-¹⁴C and thus increases the amount transported. This effect is more pronounced after a 20 hr than after a 6 hr transport period.     

Polar transport of kinetin in tissues of radish

Polar transport of kinetin-8-¹⁴C occurred in segments of petioles, hypocotyls, and roots of radish (Raphanus sativus L.). The polarity was basipetal in petioles and hypocotyls and acropetal in roots. In segments excised from seedlings with fully expanded cotyledons, indole-3-acetic acid was required for polarity to develop. In hypocotyl segments isolated at this stage, basipetal and acropetal movements were equal during the first 12 hours of auxin treatment after which time acropetal movement declined. Pretreatment with auxin eliminated this delay in the appearance of polarity. In hypocotyl segments excised from seedlings with expanding cotyledons, exogenous auxin was unnecessary for polarity. Potassium cyanide abolished polarity at both stages of growth by allowing increased acropetal movement. The rate of accumulation of kinetin in receiver blocks was greater than the in vivo increase in cytokinin content of developing radish roots.     

Enhancement of auxin sensitivity in Ranunculus sceleratus by ethylene: A mechanism to escape from hypoxia under temporary submergence

We analyzed auxin-induced and ethylene-enhanced elongation of petiole segments in Ranunculus sceleratus L. The early time course of elongation in petiolar segments was monitored with a computer-based video digitizer system. The application of ethylene-releasing ethrel slightly increased the elongation rate in the absence of IAA. When IAA alone was applied, elongation increased after a latent period of approximately 30 min. Maximal elongation rate was attained immediately after the latent period, and then the stabilized steady rate was recorded. During this phase, addition of ethrel strongly increased the elongation rate after a period of approximately 18 min. Although ethrel could acidify the growth medium, only a small part of the enhanced elongation was due to an acid-growth effect. Most of the growth stimulation was auxin-dependent and must be ascribed to the presence of ethylene. In the presence of ethrel, the log-concentration-response curve of IAA appeared to be shifted to the left. This kinetic analysis indicates an increase, due to ethylene, in the sensitivity of the R. sceleratus petiole to auxin, which results in inducing rapid growth to escape from hypoxia under temporary submergence.