Effect of Cyclanilide on Auxin Activity

Cyclanilide is a plant growth regulator that is registered for use in cotton at different stages of growth, to either suppress vegetative growth (in combination with mepiquat chloride) or accelerate senescence (enhance defoliation and boll opening, used in combination with ethephon). This research was conducted to study the mechanism of action of cyclanilide: its potential interaction with auxin (IAA) transport and signaling in plants. The activity of cyclanilide was compared with the activity of the auxin transport inhibitors NPA and TIBA. Movement of [³H]IAA was inhibited in etiolated corn coleoptiles by 10 μM cyclanilide, NPA, and TIBA, which demonstrated that cyclanilide affected polar auxin transport. Although NPA inhibited [³H]IAA efflux from cells in etiolated zucchini hypocotyls, cyclanilide had no effect. NPA did not inhibit the influx of IAA into cells in etiolated zucchini hypocotyls, whereas cyclanilide inhibited uptake 25 and 31% at 10 and 100 μM, respectively. Also, NPA inhibited the gravitropic response in tomato roots (85% at 1 μM) more than cyclanilide (30% at 1 μM). Although NPA inhibited tomato root growth (30% at 1 μM), cyclanilide stimulated root growth (165% of control at 5 μM). To further characterize cyclanilide action, plasma membrane fractions from etiolated zucchini hypocotyls were obtained and the binding of NPA, IAA, and cyclanilide studied. Cyclanilide inhibited the binding of [³H]NPA and [³H]IAA with an IC₅₀ of 50 μM for both. NPA did not affect the binding of IAA, nor did IAA affect the binding of NPA. Kinetic analysis indicated that cyclanilide is a noncompetitive inhibitor of both NPA and IAA binding, with inhibition constants (K _i) of 40 and 2.3 μM, respectively. These data demonstrated that cyclanilide interacts with auxin-regulated processes via a mechanism that is distinct from other auxin transport inhibitors. This research identifies a possible mechanism of action for cyclanilide when used as a plant growth regulator.     

Cooperation of Ethylene and Auxin in the Growth Regulation of Rice Coleoptile Segments

Elongation of coleoptile segments, having or not having a tip,excised from rice (Oryza sativa L. cv. Sasanishiki) seedlingswas promoted by exogenous ethylene above 0.3 µl l^–1as well as by IAA above 0.1 µM. Ethylene production ofdecapitated segments was stimulated by IAA above 1.0µM,and this was strongly inhibited by 1.0 µM AVG. AVG inhibitedthe IAA-stimulated elongation of the decapitated segment witha 4 h lag period, and this was completely recovered by ethyleneapplied at the concentration of 0.03 µl l^–1, whichhad no effect on elongation without exogenous IAA. The effectsof IAA and ethylene on elongation were additive. These factsshow that ethylene produced in response to IAA promotes ricecoleoptile elongation in concert with IAA, probably by prolongingthe possible duration of the IAA-stimulated elongation, butthat they act independently of each other. Moreover, AVG stronglyinhibited the endogenous growth of coleoptile segments withtips and this effect was nullified by the exogenous applicationof 0.03 µl l^–1 ethylene. These data imply that theelongation of intact rice coleoptiles may be regulated cooperativelyby endogenous ethylene and auxin in the same manner as foundin the IAA-stimulated elongation of the decapitated coleoptilesegments. Key words: oryza sativa, Ethylene, Auxin, Coleoptile growth     

Ethylene Interacts with Auxin in Regulating Developmental Attenuation of Gravitropism in Flax Root

Previous research shows that gravity-sensing in flax (Linum usitatissimum) root is initiated during seed imbibition and precedes root emergence. In this study we investigated the developmental attenuation of flax root gravitropism post-germination and the involvement of ethylene. Gravity response deteriorated significantly from 3 to 11?h after root emergence, which occurred at around 19?h after imbibition (that is, from “age” 22 to 30?h). Although the root elongation rate increased from 22 to 30?h, the gravitropic curving rate declined steadily. Older roots were able to tolerate higher levels of exogenous IAA before inhibition of elongation and gravitropism occurred. The age-dependent effect of IAA on root growth and gravitropism suggests that young roots are more sensitive to auxin and respond to a smaller vertical auxin gradient than older roots upon horizontal gravistimulation. The ethylene synthesis inhibitor AVG (2-aminoethoxyvinyl glycine, 10?μM) or ethylene action inhibitor Ag⁺ (10?μM) stimulated gravitropic curvature of 30?h roots by 24 and 32%, respectively, but had no effect on 22?h roots, suggesting that as roots age, ethylene begins to play a role in root gravitropism. The auxin transport inhibitor NPA (N-naphthylphthalamic acid, 50?μM) reduced gravitropic curvature of 30?h roots by 24% but had no effect on 22?h roots. On the other hand, treating roots simultaneously with the auxin transport inhibitor and ethylene synthesis or action inhibitor stimulated gravitropic curvature of 30?h roots but not 22?h roots. Taken together, these data indicate that as roots develop, their weakened gravity response is due to decreased auxin sensitivity and possibly auxin transport regulated by ethylene.     

Mechanism by which<Emphasis Type="Italic">Bacillus</Emphasis>-Derived 2-Aminobenzoic acid inhibits the growth of<Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> Roots

To analyze the growth inhibitory mechanism of a 2-aminobenzoic acid (2-AA) derived fromBacillus cereus EJ-121, we treatedArabidopsis thaliana plants with 2-AA, 2-AA analogs, auxin (NAA), a known auxin transport inhibitor [2,3,5-triiodobenzoic acid (TIBA)], and an ethylene action inhibitor [silver thiosulfate (Ag)]. Root development was significantly inhibited by 50 μM 2-AA, whereas the growth of bacteria and yeast was undeterred. The application of two 2-AA analogs - 3-aminobenzoic acid (3-AA) and 4-aminobenzoic acid (4-AA) - did not impairArabidopsis root growth at concentrations below 100 μM. These results suggest that the effect of 2-AA is not due to its chemical structure, but because of its conversion to another metabolite, IAA. To confirm this, we supplemented TIBA in the growth medium, and found that the degree of inhibition was significantly reduced. Similarly, when plants were co-treated with 100 μM Ag, the negative effect of 50 μM 2-AA was greatly diminished. All of these observations support the proposal that this inhibition results from the conversion of 2-AA to IAA. Furthermore, the increased auxin level leads to a rise in ethylene synthesis, which then blocks root growth and, ultimately, retards overall plant development.     

Ethylene promotes the induction by auxin of the cortical microtubule randomization required for low-pH-induced root hair initiation in lettuce (Lactuca sativa L.) seedlings

Transverse cortical microtubule (CMT) arrays in lettuce root epidermal cells randomize soon after a shift from pH 6.0 to pH 4.0, and this randomization is essential for root hair initiation. We investigated the hormonal regulation of CMT randomization. At pH 4.0, 1 micro M of the auxin competitive inhibitor 2-(p-chlorophenoxy)-2-methylpropionic acid (PCIB), 0.1 micro M of the ethylene biosynthesis inhibitor aminoethoxyvinylglycine (AVG) or 0.1 micro M of the ethylene action inhibitor Ag(+) suppressed CMT randomization and root hair initiation. At pH 6.0, addition of 0.1 micro M indole-3-acetic acid (IAA) or 1 micro M of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) induced CMT randomization and root hair initiation. Culturing with 0.1 micro M IAA plus 0.1 micro M AVG, or 1 micro M ACC plus 1 micro M PCIB also induced these phenomena. ACC (1 micro M) plus 100 micro M PCIB inhibited CMT randomization and root hair initiation, but 1 micro M AVG with 0.1 micro M Ag(+) and 0.1 micro M IAA induced them. These results suggest that auxin is essential for CMT randomization. As a higher concentration of PCIB was required to suppress CMT randomization when ACC was added, the greater amount of ethylene produced at pH 4.0 may promote the induction by auxin of CMT randomization in hair-forming cells.     

Cytokinin and Ethylene Affect Auxin Transport-Dependent Rhizogenesis in Hypocotyls of Common Ice Plant (<Emphasis Type="Italic">Mesembryanthemum crystallinum</Emphasis> L<Emphasis Type="Italic">.</Emphasis>)

Hypocotyl explants of Mesembryanthemum crystallinum regenerated roots when cultured vertically with either the apical end (AE) or basal end (BE) in media containing indole-3-acetic acid (IAA). IAA alone induced roots regularly from the basal end of the explants, either from the cut surface immersed in the medium or from the opposite side. The inhibitors of auxin efflux carriers, α-naphthylphthalamic acid (NPA) and 2,3,5-triiodobenzoic acid (TIBA), inhibited rhizogenesis only from AE-cultured explants, indicating the role of polar auxin transport in root regeneration in this system. Cytokinin (zeatin, kinetin, BAP) added to auxin-containing medium reduced rhizogenesis from the explants maintained with BE and AE and additionally changed the IAA-induced pattern of rooting in AE-cultured explants by favoring rooting from the apical end and middle part of the hypocotyl with its concomitant reduction from the basal end. The addition of kinetin did not influence the content of IAA in the explants maintained with AE, suggesting that the cytokinin effect on root patterning was not dependent on auxin biosynthesis. Kinetin, however, strongly enhanced ethylene production. The importance of ethylene in regulating PAT-dependent rhizogenesis was tested by using an ethylene antagonist AgNO₃, an inhibitor of ethylene synthesis aminoethoxyvinylglycine (AVG), and a precursor of ethylene, 1-aminocyclopropane-1-carboxylic acid (ACC). AgNO₃ applied together with IAA or with IAA and kinetin strongly reduced the production of ethylene, inhibited rhizogenesis, and induced nonregenerative callus from BE, suggesting the need for ethylene signaling to elicit the rhizogenic action of auxin. A reduction of rhizogenesis and decrease of ethylene biosynthesis was also caused by AVG. In addition, AVG at 10 μM reversed the effect of cytokinin on root patterning, resulting in roots emerging only from BE on the medium with IAA and kinetin. Conversely, ACC at 200 μM markedly enhanced the production of ethylene and partly mimicked the effect of cytokinin when applied with IAA alone, thus confirming that in cultured hypocotyls of ice plant, cytokinin affects IAA-induced rhizogenesis through an ethylene-dependent pathway.     

Nitric oxide mediates humic acids-induced root development and plasma membrane H+-ATPase activation

It is widely reported that some humic substances behave as exogenous auxins influencing root growth by mechanisms that are not yet completely understood. This study explores the hypothesis that the humic acids’ effects on root development involve a nitric oxide signaling. Maize seedlings were treated with HA 20 mg C L⁻¹, IAA 0.1 nM, and NO donors (SNP or GSNO), in combination with either the auxin-signaling inhibitor PCIB, the auxin efflux inhibitor TIBA, or the NO scavenger PTIO. H⁺-transport-competent plasma membrane vesicles were isolated from roots to investigate a possible link between NO-induced H⁺-pump and HA bioactivity. Plants treated with either HA or SNP stimulated similarly the lateral roots emergence even in the presence of the auxin inhibitors, whereas NO scavenger diminished this effect. These treatments induced H⁺-ATPase stimulation by threefold, which was abolished by PTIO and decreased by auxin inhibitors. HA-induced NO synthesis was also detected in the sites of lateral roots emergence. These data depict a new scenario where the root development stimulation and the H⁺-ATPase activation elicited by either HA or exogenous IAA depend essentially on mechanisms that use NO as a messenger induced site-specifically in the early stages of lateral root development.     

A Dominant Mutation in ARL2 Causes Impaired Adventitious Root Development in Rice

Adventitious roots are vital for water and nutrient assimilation by cereal crops because they comprise the bulk of the fibrous root system. We isolated and analyzed a rice mutant, adventitious rootless 2 (arl2), which failed to initiate adventitious root primordia during early development. Its seminal root produced fewer lateral roots than from the wild type. This mutant also exhibited pleiotropic phenotypes of longer and thicker seminal roots, a different morphology for the first leaf, delayed heading, and a greater tiller angle. Physiological experiments showed that exogenous auxin and ethylene could rescue adventitious root growth, a response opposite that for two previously reported mutants, arl1 and gnom1. Activity in the auxin signal pathway and the polar auxin transport system was normal for arl2. Compared with the wild type, arl2 plants showed enhanced sensitivity to ethephon but decreased sensitivity to AgNO₃, an inhibitor of ethylene. Genetics analysis demonstrated that this mutant is controlled by a single dominant gene; ARL2 was mapped within a 100-kb interval on the short arm of chromosome 2.     

Auxin and Ethylene-stimulated Adventitious Rooting in Relation to Tissue 9

We have previously shown that both endogenous auxin and ethylenepromote adventitious root formation in the hypocotyls of derootedsunflower (Helianthus annuus) seedlings. Experiments here showedthat promotive effects on rooting of the ethylene precursor,1-aminocyclopropane-l-carboxylic acid (ACC) and the ethylene-releasingcompound, ethephon (2-chloro-ethylphosphonic acid), dependedon the existence of cotyledons and apical bud (major sourcesof auxin) or the presence of exogenously applied indole-3-aceticacid (IAA). Ethephon, ACC, aminoethoxyvinylglycine (an inhibitorof ethylene biosynthesis), and silver thiosulphate (STS, aninhibitor of ethylene action), applied for a length of timethat significantly influenced adventitious rooting, showed noinhibitory effect on the basipetal transport of [³H]IAA. Theseregulators also had no effect on the metabolism of [³H]IAA andendogenous IAA levels measured by gas chromatography-mass spectrometry.ACC enhanced the rooting response of hypocotyls to exogenousIAA and decreased the inhibition of rooting by IAA transportinhibitor, N-1-naphthylphthalamic acid (NPA). STS reduced therooting response of hypocotyls to exogenous IAA and increasedthe inhibition of rooting by NPA. Exogenous auxins promotedethylene production in the rooting zone of the hypocotyls. Decapitationof the cuttings or application of NPA to the hypocotyl belowthe cotyledons did not alter ethylene production in the rootingzone, but greatly reduced the number of root primordia. We concludethat auxin is a primary controller of adventitious root formationin sunflower hypocotyls, while the effect of ethylene is mediatedby auxin. Key words: Auxin, ethylene, adventitious rooting, sunflower     

Ethephon and auxin induce mycorrhiza-like changes in the morphology of root organ cultures of Mugo pine

The effects of ethylene and auxin on the morphology and anatomy of root organ cultures of Pinus mugo Turra var. mugo were investigated to test the hypothesis that changes in root morphology associated with formation of ectomycorrhizae may be related to ethylene produced by ectomycorrhizal fungi or by host plant roots in response to fungus-produced auxin. Morphological changes characteristic of mycorrhizal infection include dichotomous branching of lateral roots, inhibition of root hair formation and enlargement of cortical cells. Lateral roots on non-mycorrhizal root organ cultures, grown in a defined medium, underwent dichtotomous branching while root hair formation was inhibited in response to the ethylene released by 50 and 100 μ M ethephon (2-chloroethylphosphonic acid), but no effect on cortical cell dimensions was observed. The auxin, naphthaleneacetic acid (1 and 10 μ M ) also stimulated dichotomous branching and inhibited root hair formation, but to a lesser extent and with a greater lag time than ethephon. Auxin-stimulated ethylene production by root organ cultures was demonstrated. This appeared to be responsible, at least in part, for the auxin-induced dichotomous branching since the ethylene action inhibitor, silver thiosulfate (0.1 m M ) inhibited the response to auxin by 35%.     

Hormonal Effects on Growth and Morphology of Normal and Hairy Roots of Hyoscyamus muticus

Treatment of normal and Agrobacterium rhizogenes-transformed root cultures of Hyoscyamus muticus with three different auxins, indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), and naphthaleneacetic acid (NAA), revealed that the response varied considerably among auxins, between transformed and normal roots, and depending on the parameter. In normal roots all three auxins provoked abundant branching, with IBA and NAA being the most effective at 2.5 and 0.5 μm, respectively, whereas IAA was most effective at low concentrations (0.05 and 0.1 μm). In transformed roots exogenously supplied auxins were generally inhibitory or, at best, without effect on growth and branching. Only 0.01 μm IAA significantly enhanced lateral root number, whereas at the higher concentrations IBA, although inhibitory, was the least effective auxin. In both root types IBA had little effect on primary root growth, but normal roots were more sensitive to IAA and NAA. These results suggest a different sensitivity to auxins of normal and transformed roots since there was no significant difference in endogenous free and conjugated IAA content nor in IAA uptake capacity. Ethylene production and biosynthesis were approximately threefold higher in hairy roots, but production could be stimulated up to tenfold that of control levels in normal roots by supplying NAA or 1-aminocyclopropane-1-carboxylic acid (ACC). Treatment with 2.5 μm NAA, but not IAA or IBA, also enhanced ethylene biosynthesis in normal roots but not in transformed ones. ACC and malonyl-1-aminocyclopropane-1-carboxylic acid accumulated to detectable levels only after treatment with an auxin (NAA). Received March 3, 1997; accepted May 28, 1997     

A Comparative Study of the Changes in Root Growth, Induced by Coumarin, Auxin, Ethylene, Kinetin and Gibberellic Acid

The effect of coumarin, IAA, ethylene, kinetin and gibberellic acid on roots of maize and wheat was investigated. Sterile attached and detached roots and isolated elongation zones were used. In some experiments a semi-sterile procedure was followed. The effects of the different regulators separately or in various combinations together with coumarin were studied on the root growth with regard to division, elongation and swelling of the cells. The ethylene production in isolated elongation zones was measured after treatment with coumarin, IAA, PCIB, kinetin, colchicine and dinitrophenol. The results show the following: 1) Each substance produces a specific morphologic pattern. 2) Changes in polarity were demonstrated for auxin-induced swelling in cell divisions and cell expansion and for coumarin-induced swelling in cell divisions. Other cell expansion in swollen parts was due to cylindric cells increasing in width while retaining their cylindric form. 3) Coumarin-induced inhibition could not be counteracted by IAA, PCIB, carbon dioxide, kinetin, gibberellic acid or Cycocel. 4) The ethylene production in isolated elongation zones increases noticeably after kinetin treatment, less strongly after auxin treatment and least after coumarin treatment. The production of ethylene does not seem to be correlated with the morphogenetic effect of the different substances. 5) The isolated elongation zones reacted to a) IAA and kinetin with an increase in length in some cases and b) gibberellic acid with a reduction of their width. 6) The inhibitory effect of coumarin on the growth in length of the elongation zones was diminished by kinetin. The swelling produced by coumarin in these zones was reduced by gibberellic acid. The effects just mentioned of kinetin and gibberellic acid were considered as indirect ones. - From the present findings it was concluded that concomitant effects of auxin, ethylene, cytokinins and gibberellins are not obligatory for coumarin to exert its morphogenetic effects on root growth. In discussing the results some pitfalls in studies of growth reactions after application of hormones to roots containing meristem were emphasized.     

Evidence for a Relationship between H Excretion and Auxin in Shoot Gravitropism

The role of auxin and protons in the gravitropic response of the sunflower (Helianthus annuus L. cv Sungold) hypocotyl has been investigated. No physiological asymmetry in acid-growth capacity could be detected between the upper and lower surfaces of gravistimulated hypocotyls. These data imply that neutral buffers inhibit shoot gravitropism by preventing the establishment of a lateral proton gradient along gravitropically stimulated hypocotyls. Indirect evidence that auxin is involved in the establishment and/or maintenance of such a gradient derives from the quantitative assessment of the effects of exogenous auxin, anti-auxins, and vanadate on gravicurvature. At low concentrations, exogenous auxin accelerated curvature; at high concentrations, curvature was prevented. Vanadate, an inhibitor of auxin-enhanced H⁺ secretion, α-(p-chlorophenoxy)isobutyric acid (PCIB), an anti-auxin, and 2,3,5-triiodobenzoic acid (TIBA), an auxin-transport inhibitor, prevented observable asymmetric proton excretion using a brom cresol purple agar technique and also inhibited gravicurvature. Vanadate, PCIB, and TIBA inhibition of gravicurvature could be reversed with acid treatment to the lower surface of a gravistimulated hypocotyl. Auxin treatment to the lower surface of a gravistimulated hypocotyl did not reverse vanadate-induced inhibition, but it did partially reverse PCIB- and TIBA-induced inhibition. These results indicate a close relationship between the acid-growth theory and the differential growth responses of the sunflower hypocotyl during gravitropism.     

Regulation of root growth by auxin and ethylene is influenced by phosphate supply in white clover (<Emphasis Type="Italic">Trifolium repens</Emphasis> L.)

The influence of P-supply on root system architecture (primary root length, number and total length of lateral roots) through the effects of ethylene (ACC) and auxin [1-naphthylacetic acid (NAA)] has been examined in the legume white clover (Trifolium repens L.). Higher concentrations (1 and 10 μM) of ACC and NAA (100 nM) inhibited growth, while lower concentrations (100 nM ACC, 5 nM NAA) either had no effect or stimulated growth in P-sufficient (1 mM Pi) roots. In response to low (10 μM) P, a stimulation of primary root growth, number of lateral roots and mean length of lateral roots was observed, while a super-stimulation of these growth parameters occurred in response to subsequent 100 nM ACC treatment suggesting that the low P treatment increased the sensitivity of the roots to ethylene. Examination of the primary roots of DR5p::GUS transformants suggests that this change in sensitivity induced by low P occurs through the promotion of auxin signalling/transport to the root apex. These results are discussed in terms of the role of ethylene and the significance of changes in sensitivity to the hormone in modulating root system architecture in response to low P-supply.     

Effects of low nitrogen medium on endogenous changes in ethylene, auxins, and cytokinins in in vitro shoot formation from rhizomes of Cymbidium kanran

Summary Shoot formation from rhizome explants of Cymbidium kanran was promoted on Murashige and Skoog (MS) medium: (1) with 1 mgl⁻¹ (4.4μM) 6-benzyladenine (BA) and 0.1 mgl⁻¹ (0.54μM) α-naphthaleneacetic acid (NAA); (2) with ethylene inhibitor (silver nitrate, AgNO₃); or (3) by reducing ammonium nitrate (NH₄NO₃) and potassium nitrate (KNO₃) to 25 and 50%, respectively, of their original concentrations. Shoot formation by BA and NAA was strongly inhibited with the application of ethephon, an ethylene releaser. The ethylene production from the rhizome explants was reduced 30–55% on low nitrogen medium after 1–3 mo. of culture and 52% on BA and NAA medium after 1 mo. of culture compared with explants on standard MS medium. No difference in endogenous auxin (indole-3-acetic acid, IAA) and cytokinin (isopentenyl adenosine, iPA) contents in the rhizomes was found between the treatments. Low ethylene levels were correlated with higher frequency of shoot formation from the rhizomes.     

Activity of 1-aminocyclopropane carboxylic acid synthase and growth of mustard (<Emphasis Type="Italic">Brassica juncea</Emphasis>) following defoliation

To examine the possible relationship between the activity of 1-aminocyclopropane carboxylic acid synthase (ACS; EC 4.4.1.14) and growth of mustard (Brassica juncea L.), ACS activity, ethylene and plant growth were studied in the presence of ACS activity modulators in no-defoliation and defoliated plants. Growth of plants was greatest when subjected to defoliation of 50% lower leaves in the plant axis compared to defoliation of 25% lower leaves or no-defoliation. The activity of ACS in no-defoliation and defoliated plants was correlative with growth of plants. ACS activity and ethylene evolution in no-defoliation plants treated with 10 μM indole-3-acetic acid (IAA) and defoliated plants treated with water were equal and resulted in maximum plant growth. On the contrary, the application of 10 μM IAA on defoliated plants resulted in the increase in ACS activity and ethylene evolution to an extent that inhibited the growth. The application of 100 μM IAA on no-defoliation and defoliated plants increased ACS activity and ethylene evolution maximally and proved inhibitory for the plant growth. The association of ACS activity, ethylene evolution and growth of plants was further substantiated with the use of 50 μM aminoethoxyvinyl glycine (AVG) applied alone or in combination with 10 or 100 μM IAA. The application of AVG resulted in the inhibition of ACS activity and the growth of no-defoliation or defoliated plants. The results indicate that there exists a correlation between ACS activity, ethylene and the growth of mustard plants.     

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.     

Auxin is a positive regulator for ethylene-mediated response in the growth of Arabidopsis roots

The requirement of auxin for the ethylene-mediated growth response in the root of Arabidopsis thaliana seedlings was investigated using two ethylene-resistant mutants, aux1-7 and eir1-1, whose roots have been shown to have a defect in the auxin influx and efflux carriers, respectively. A 50% inhibition of growth (I(50)) was achieved with 0.84 microl liter(-1) ethylene in wild-type roots, but 71.3 microl liter( -1) ethylene was required to induce I(50) in eir1-1 roots. In aux1-7 roots, I(50) was not obtained even at 1,000 microl liter(-1) ethylene. By contrast, in the presence of 10 nM 1-naphthaleneacetic acid (NAA), the concentrations of ethylene required to induce I(50) in eir1-1 and aux1-7 roots were greatly reduced nearly to the level required in wild-type roots. Since the action of NAA to restore the ethylene response in aux1-7 roots was not replaced by IAA, an increase in the intracellular level of auxin is likely to be the cause for the restoration of ethylene response. NAA at 10 nM did not inhibit root growth when applied solely, but it was the optimum concentration to recover the ethylene response in the mutant roots. These results suggest that auxin is a positive regulator for ethylene-induced inhibition in root elongation.     

Lateral root formation in rice (Oryza Sativa L.): differential effects of indole-3-acetic acid and indole-3-butyric acid

Comparative effects of indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) on lateral root (LR) formation were studied using 2-day-old seedlings of IR8 rice (Oryza sativa L.). Results showed that IBA at all concentrations (0.8–500 nmol/L) increased the number of LRs in the seminal root. However exogenous IAA, failed to increase the number of LRs. On the other hand, both IBA and IAA caused inhibition of seminal root elongation and promotion of LR elongation, but IAA can only reach to the same degree of that of IBA at a more than 20-fold concentration. Exogenous IBA had no effect on endogenous IAA content. We conclude from the results that IBA could act directly as a distinct auxin, promoting LR formation in rice, and that the signal transduction pathway for IBA is at least partially different from that for IAA.