RCN1-regulated phosphatase activity and EIN2 modulate hypocotyl gravitropism by a mechanism that does not require ethylene signaling

The roots curl in naphthylphthalamic acid1 (rcn1) mutant of Arabidopsis (Arabidopsis thaliana) has altered auxin transport, gravitropism, and ethylene response, providing an opportunity to analyze the interplay between ethylene and auxin in control of seedling growth. Roots of rcn1 seedlings were previously shown to have altered auxin transport, growth, and gravitropism, while rcn1 hypocotyl elongation exhibited enhanced ethylene response. We have characterized auxin transport and gravitropism phenotypes of rcn1 hypocotyls and have explored the roles of auxin and ethylene in controlling these phenotypes. As in roots, auxin transport is increased in etiolated rcn1 hypocotyls. Hypocotyl gravity response is accelerated, although overall elongation is reduced, in etiolated rcn1 hypocotyls. Etiolated, but not light grown, rcn1 seedlings also overproduce ethylene, and mutations conferring ethylene insensitivity restore normal hypocotyl elongation to rcn1. Auxin transport is unaffected by treatment with the ethylene precursor 1-aminocyclopropane carboxylic acid in etiolated hypocotyls of wild-type and rcn1 seedlings. Surprisingly, the ethylene insensitive2-1 (ein2-1) and ein2-5 mutations dramatically reduce gravitropic bending in hypocotyls. However, the ethylene resistant1-3 (etr1-3) mutation does not significantly affect hypocotyl gravity response. Furthermore, neither the etr1 nor the ein2 mutation abrogates the accelerated gravitropism observed in rcn1 hypocotyls, indicating that both wild-type gravity response and enhanced gravity response in rcn1 do not require an intact ethylene-signaling pathway. We therefore conclude that the RCN1 protein affects overall hypocotyl elongation via negative regulation of ethylene synthesis in etiolated seedlings, and that RCN1 and EIN2 modulate hypocotyl gravitropism and ethylene responses through independent pathways.     

Factors increasing ethylene production enhance the sensitivity of root growth to auxins

Light inhibits root elongation, increases ethylene production and enhances the inhibitory action of auxins on root elongation of pea ( Pisum sativum L. cv. Weibulls Marma) seedlings. To investigate the role of ethylene in the interaction between light and auxin, the level of ethylene production in darkness was increased to the level produced in light by supplying 1-aminocyclopropane-1-carboxylic acid (ACC) or benzylaminopurine (BAP). Ethylene production was measured in excised root tips after treatment of intact seedlings for 24 h, while root growth was measured after 48 h. Auxin, at a concentration causing a partial inhibition of root elongation, did not increase ethylene production significantly. A 4-fold increase in ethylene production, caused either by light, 0.1 μ M ACC or 0.1 μ M BAP, inhibited root elongation by 40–50%. The auxins 2,4-dichlorophenoxyacetic acid and indolebutyric acid applied at 0.1 μ M inhibited root elongation by 15–25% in darkness but by 50–60% in light. Supply of ACC or BAP in darkness enhanced the inhibitory effects of auxins to about the same extent as in light. The inhibition caused by the auxins as well as by the BAP was associated with swelling of the root tips. ACC and BAP treatment synergistically increased the swelling caused by auxins. We conclude that auxin and ethylene, when applied or produced in partially inhibitory concentrations, act synergistically to inhibit root elongation and increase root diameter. The effect of light on the response of the roots to auxins is mediated by a light-induced increase in ethylene production.     

The rib1 mutant of Arabidopsis has alterations in indole-3-butyric acid transport, hypocotyl elongation, and root architecture

Polar transport of the auxin indole-3-butyric acid (IBA) has recently been shown to occur in Arabidopsis (Arabidopis thaliana) seedlings, yet the physiological importance of this process has yet to be fully resolved. Here we describe the first demonstration of altered IBA transport in an Arabidopsis mutant, and show that the resistant to IBA (rib1) mutation results in alterations in growth, development, and response to exogenous auxin consistent with an important physiological role for IBA transport. Both hypocotyl and root IBA basipetal transport are decreased in rib1 and root acropetal IBA transport is increased. While indole-3-acetic acid (IAA) transport levels are not different in rib1 compared to wild type, root acropetal IAA transport is insensitive to the IAA efflux inhibitor naphthylphthalamic acid in rib1, as is the dependent physiological process of lateral root formation. These observed changes in IBA transport are accompanied by altered rib1 phenotypes. Previously, rib1 roots were shown to be less sensitive to growth inhibition by IBA, but to have a wild-type response to IAA in root elongation. rib1 is also less sensitive to IBA in stimulation of lateral root formation and in hypocotyl elongation under most, but not all, light and sucrose conditions. rib1 has wild-type responses to IAA, except under one set of conditions, low light and 1.5% sucrose, in which both hypocotyl elongation and lateral root formation show altered IAA response. Taken together, our results support a model in which endogenous IBA influences wild-type seedling morphology. Modifications in IBA distribution in seedlings affect hypocotyl and root elongation, as well as lateral root formation.     

Characterization of 1-aminocyclopropane-1-carboxylate (ACC) deaminase containing <Emphasis Type="Italic">Methylobacterium oryzae</Emphasis> and interactions with auxins and ACC regulation of ethylene in canola (<Emphasis Type="Italic">Brassica campestris</Emphasis>)

The possible interaction of the plant hormones auxin and ethylene and the role of 1-aminocyclopropane-1-carboxylate (ACC) deaminase containing bacteria on ethylene production in canola (Brassica campestris) in the presence of inhibitory concentrations of growth regulators were investigated. The effects of auxin (indole-3-acetic acid and 2,4-dichlorophenoxy acetic acid), auxin transport inhibitor 2-(p-chlorophenoxy)-2-methylpropionic acid, ethylene precursor 1-aminocyclopropane-1-carboxylate and ethylene synthesis inhibitor l-α-(2-aminoethoxyvinyl)glycine hydrochloride on root elongation were concentration dependent. Exogenous addition of growth regulators influences the enzyme activities of ethylene production and we have presented here evidences that support the hypothesis that inhibitory effects of auxin on root elongation are independent of ethylene. Additionally, we have proved that inoculation of ACC deaminase containing Methylobacterium oryzae sequester ACC exuded from roots and hydrolyze them lowering the concentration of ACC in root exudates. However, the inhibitory actions of exogenous additions of auxins could not be ameliorated by bacterial inoculation that reduces ethylene concentration in canola seedlings.     

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     

Genetic dissection of the role of ethylene in regulating auxin-dependent lateral and adventitious root formation in tomato

In this study we investigated the role of ethylene in the formation of lateral and adventitious roots in tomato ( Solanum lycopersicum ) using mutants isolated for altered ethylene signaling and fruit ripening. Mutations that block ethylene responses and delay ripening – Nr ( Never ripe ), gr ( green ripe ), nor ( non ripening ), and rin ( ripening inhibitor ) – have enhanced lateral root formation. In contrast, the epi ( epinastic ) mutant, which has elevated ethylene and constitutive ethylene signaling in some tissues, or treatment with the ethylene precursor 1-aminocyclopropane carboxylic acid (ACC), reduces lateral root formation. Treatment with ACC inhibits the initiation and elongation of lateral roots, except in the Nr genotype. Root basipetal and acropetal indole-3-acetic acid (IAA) transport increase with ACC treatments or in the epi mutant, while in the Nr mutant there is less auxin transport than in the wild type and transport is insensitive to ACC. In contrast, the process of adventitious root formation shows the opposite response to ethylene, with ACC treatment and the epi mutation increasing adventitious root formation and the Nr mutation reducing the number of adventitious roots. In hypocotyls, ACC treatment negatively regulated IAA transport while the Nr mutant showed increased IAA transport in hypocotyls. Ethylene significantly reduces free IAA content in roots, but only subtly changes free IAA content in tomato hypocotyls. These results indicate a negative role for ethylene in lateral root formation and a positive role in adventitious root formation with modulation of auxin transport as a central point of ethylene–auxin crosstalk.     

Auxin Transport Is Required for Hypocotyl Elongation in Light-Grown but Not Dark-Grown Arabidopsis

Many auxin responses are dependent on redistribution and/or polar transport of indoleacetic acid. Polar transport of auxin can be inhibited through the application of phytotropins such as 1-naphthylphthalamic acid (NPA). When Arabidopsis thaliana seedlings were grown in the light on medium containing 1.0 μm NPA, hypocotyl and root elongation and gravitropism were strongly inhibited. When grown in darkness, however, NPA disrupted the gravity response but did not affect elongation. The extent of inhibition of hypocotyl elongation by NPA increased in a fluence-rate-dependent manner to a maximum of about 75% inhibition at 50 μmol m⁻² s⁻¹ of white light. Plants grown under continuous blue or far-red light showed NPA-induced hypocotyl inhibition similar to that of white-light-grown plants. Plants grown under continuous red light showed less NPA-induced inhibition. Analysis of photoreceptor mutants indicates the involvement of phytochrome and cryptochrome in mediating this NPA response. Hypocotyls of some auxin-resistant mutants had decreased sensitivity to NPA in the light, but etiolated seedlings of these mutants were similar in length to the wild type. These results indicate that light has a significant effect on NPA-induced inhibition in Arabidopsis, and suggest that auxin has a more important role in elongation responses in light-grown than in dark-grown seedlings.     

Cytokinin-induced hypocotyl elongation in light-grown<Emphasis Type="Italic"> Arabidopsis</Emphasis> plants with inhibited ethylene action or indole-3-acetic acid transport

Cytokinins inhibit hypocotyl elongation in darkness but have no obvious effect on hypocotyl length in the light. However, we found that cytokinins do promote hypocotyl elongation in the light when ethylene action is blocked. A 50% increase in Arabidopsis thaliana (L.) Heynh. hypocotyl length was observed in response to N⁶-benzyladenine (BA) treatment in the presence of Ag⁺. The level of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid was strongly increased, indicating that ethylene biosynthesis was up-regulated by treatment with cytokinin. Furthermore, the effects of cytokinins on hypocotyl elongation were also tested using a series of mutants in the cascade of the ethylene-signal pathway. In the ethylene-insensitive mutants etr1-3 and ein2-1, cytokinin treatment resulted in hypocotyl lengths comparable to those of wild-type seedlings treated with both Ag⁺ and BA. A similar phenotypical response to cytokinin was observed when auxin transport was blocked by -naphthylphthalamic acid (NPA). Applied cytokinin largely restored cell elongation in the basal and middle parts of the hypocotyls of NPA-treated seedlings and at the same time abolished the NPA-induced decrease in indole-3-acetic acid levels. Our data support the hypothesis that, in the light, cytokinins interact with the ethylene-signalling pathway and conditionally up-regulate ethylene and auxin synthesis.     

The Arabidopsis mutant eer2 has enhanced ethylene responses in the light

By screening for ethylene response mutants in Arabidopsis, a novel mutant, eer2, was isolated which displays enhanced ethylene responses. On a low nutrient medium (LNM) light-grown eer2 seedlings showed a significant hypocotyl elongation in response to low levels of 1-amino-cyclopropane-1-carboxylate (ACC), the precursor of ethylene, compared with the wild type, indicating that eer2 is hypersensitive to ethylene. Treatment with 1-MCP (1-methylcyclopropene), a competitive inhibitor of ethylene signalling, suppressed this hypersensitive response, demonstrating that it is a bona fide ethylene effect. By contrast, roots of eer2 were less sensitive than the wild type to low concentrations of ACC. The ethylene levels in eer2 did not differ from the wild type, indicating that ethylene overproduction is not the primary cause of the eer2 phenotype. In addition to its enhanced ethylene response of hypocotyls, eer2 is also affected in the pattern of senescence and its phenotype depends on the nutritional status of the growth medium. Furthermore, linkage analysis of eer2 suggests that this mutant defines a new locus in ethylene signalling.     

The growth of tomato (Lycopersicon esculentum Mill.) hypocotyls in the light and in darkness differentially involves auxin.

Light and auxin antagonistically regulate hypocotyl elongation. We have investigated the physiological interactions of light and auxin in the control of tomato (Lycopersicon esculentum Mill.) hypocotyl elongation by studying the auxin-insensitive mutant diageotropica (dgt). The length of the hypocotyls of the dgt mutant is significantly reduced when compared to the wild type line Ailsa Craig (AC) in the dark and under red light, but not under the other light conditions tested, indicating that auxin sensitivity is involved in the elongation of hypocotyls only in these conditions. Similarly, the auxin transport inhibitor naphthylphthalamic [correction of naphtylphtalamic] acid (NPA) differentially affects elongation of dark- or light-grown hypocotyls of the MoneyMaker (MM) tomato wild type. Using different photomorphogenic mutants, we demonstrate that at least phytochrome A, phytochrome B1 and, to a much lesser extent [correction of extend], cryptochrome 1, are necessary for a switch from an auxin transport-dependent elongation of hypocotyls in the dark to an auxin transport-independent elongation in the light. Interestingly, the dgt mutant and NPA-treated seedlings exhibit a looped phenotype only under red light, indicating that the negative gravitropism of hypocotyls also differentially involves auxin in the various light conditions.     

Auxin and ethylene promote root hair elongation in Arabidopsis

Genetic and physiological studies implicate the phytohormones auxin and ethylene in root hair development. To learn more about the role of these compounds, we have examined the root hair phenotype of a number of auxin- and ethylene-related mutants. In a previous study, Masucci and Schiefelbein (1996) showed that neither the auxin response mutations aux1 and axr1 nor the ethylene response mutations etr1 and ein2 have a significant effect on root hair initiation. In this study, we found that mutants deficient in either auxin or ethylene response have a pronounced effect on root hair length. Treatment of wild-type, axr1 and etr1 seedlings with the synthetic auxin, 2,4-D, or the ethylene precursor ACC, led to the development of longer root hairs than untreated seedlings. Furthermore, axr1 seedlings grown in the presence of ACC produce ectopic root hairs and an unusual pattern of long root hairs followed by regions that completely lack root hairs. These studies indicate that both auxin and ethylene are required for normal root hair elongation.     

Ethylene regulates lateral root formation and auxin transport in Arabidopsis thaliana

Lateral root branching is a genetically defined and environmentally regulated process. Auxin is required for lateral root formation, and mutants that are altered in auxin synthesis, transport or signaling often have lateral root defects. Crosstalk between auxin and ethylene in root elongation has been demonstrated, but interactions between these hormones in the regulation of Arabidopsis lateral root formation are not well characterized. This study utilized Arabidopsis mutants altered in ethylene signaling and synthesis to explore the role of ethylene in lateral root formation. We find that enhanced ethylene synthesis or signaling, through the eto1-1 and ctr1-1 mutations, or through the application of 1-aminocyclopropane-1-carboxylic acid (ACC), negatively impacts lateral root formation, and is reversible by treatment with the ethylene antagonist, silver nitrate. In contrast, mutations that block ethylene responses, etr1-3 and ein2-5 , enhance root formation and render it insensitive to the effect of ACC, even though these mutants have reduced root elongation at high ACC doses. ACC treatments or the eto1-1 mutation significantly enhance radiolabeled indole-3-acetic acid (IAA) transport in both the acropetal and the basipetal directions. ein2-5 and etr1-3 have less acropetal IAA transport, and transport is no longer regulated by ACC. DR5-GUS reporter expression is also altered by ACC treatment, which is consistent with transport differences. The aux1-7 mutant, which has a defect in an IAA influx protein, is insensitive to the ethylene inhibition of root formation. aux1-7 also has ACC-insensitive acropetal and basipetal IAA transport, as well as altered DR5-GUS expression, which is consistent with ethylene altering AUX1-mediated IAA uptake, and thereby blocking lateral root formation.     

Light‐induced stabilization of ACS contributes to hypocotyl elongation during the dark‐to‐light transition in Arabidopsis seedlings

Hypocotyl growth during seedling emergence is a crucial developmental transition influenced by light and phytohormones such as ethylene. Ethylene and light antagonistically control hypocotyl growth in either continuous light or darkness. However, how ethylene and light regulate hypocotyl growth, including seedling emergence, during the dark‐to‐light transition remains elusive. Here, we show that ethylene and light cooperatively stimulate a transient increase in hypocotyl growth during the dark‐to‐light transition via the light‐mediated stabilization of 1‐aminocyclopropane‐1‐carboxylic acid (ACC) synthases (ACSs), the rate‐limiting enzymes in ethylene biosynthesis. We found that, in contrast to the known inhibitory role of light in hypocotyl growth, light treatment transiently increases hypocotyl growth in wild‐type etiolated seedlings. Moreover, ACC, the direct precursor of ethylene, accentuates the effects of light on hypocotyl elongation during the dark‐to‐light transition. We determined that light leads to the transient elongation of hypocotyls by stabilizing the ACS5 protein during the dark‐to‐light transition. Furthermore, biochemical analysis of an ACS5 mutant protein bearing an alteration in the C‐terminus indicated that light stabilizes ACS5 by inhibiting the degradation mechanism that acts through the C‐terminus of ACS5. Our study reveals that plants regulate hypocotyl elongation during seedling establishment by coordinating light‐induced ethylene biosynthesis at the post‐translational level. Moreover, the stimulatory role of light on hypocotyl growth during the dark‐to‐light transition provides additional insights into the known inhibitory role of light in hypocotyl development.     

The Ethylene-insensitive sickle mutant of Medicago truncatula shows altered auxin transport regulation during nodulation

We studied the ethylene-insensitive, hypernodulating mutant, sickle (skl), to investigate the interaction of ethylene with auxin transport during root nodulation in Medicago truncatula. Grafting experiments demonstrated that hypernodulation in skl is root controlled. Long distance transport of auxin from shoot to root was reduced by rhizobia after 24 h in wild type but not in skl. Similarly, the ethylene precursor 1-amino cyclopropane-1-carboxylic acid inhibited auxin transport in wild type but not in skl. Auxin transport at the nodule initiation zone was significantly reduced by rhizobia after 4 h in both wild type and skl. After 24 h, auxin transport significantly increased at the nodule initiation zone in skl compared to wild type, accompanied by an increase in the expression of the MtPIN1 and MtPIN2 (pin formed) auxin efflux transporters. Response assays to different auxins did not show any phenotype that would suggest a defect of auxin uptake in skl. The auxin transport inhibitor N-1-naphthylphtalamic acid inhibited nodulation in wild type but not skl, even though N-1-naphthylphtalamic acid still inhibited auxin transport in skl. Our results suggest that ethylene signaling modulates auxin transport regulation at certain stages of nodule development, partially through PIN gene expression, and that an increase in auxin transport relative to the wild type is correlated with higher nodule numbers. We also discuss the regulation of auxin transport in skl in comparison to previously published data on the autoregulation mutant, super numerary nodules (van Noorden et al., 2006).     

Ethylene modulates flavonoid accumulation and gravitropic responses in roots of Arabidopsis

Plant organs change their growth direction in response to reorientation relative to the gravity vector. We explored the role of ethylene in Arabidopsis (Arabidopsis thaliana) root gravitropism. Treatment of wild-type Columbia seedlings with the ethylene precursor 1-aminocyclopropane carboxylic acid (ACC) reduced root elongation and gravitropic curvature. The ethylene-insensitive mutants ein2-5 and etr1-3 had wild-type root gravity responses, but lacked the growth and gravity inhibition by ACC found in the wild type. We examined the effect of ACC on tt4(2YY6) seedlings, which have a null mutation in the gene encoding chalcone synthase, the first enzyme in flavonoid synthesis. The tt4(2YY6) mutant makes no flavonoids, has elevated indole-3-acetic acid transport, and exhibits a delayed gravity response. Roots of tt4(2YY6), the backcrossed line tt4-2, and two other tt4 alleles had wild-type sensitivity to growth inhibition by ACC, whereas the root gravitropic curvature of these tt4 alleles was much less inhibited by ACC than wild-type roots, suggesting that ACC may reduce gravitropic curvature by altering flavonoid synthesis. ACC treatment induced flavonoid accumulation in root tips, as judged by a dye that becomes fluorescent upon binding flavonoids in wild type, but not in ein2-5 and etr1-3. ACC also prevented a transient peak in flavonoid synthesis in response to gravity. Together, these experiments suggest that elevated ethylene levels negatively regulate root gravitropism, using EIN2- and ETR1-dependent pathways, and that ACC inhibition of gravity response occurs through altering flavonoid synthesis.     

Growth and stomata development of Arabidopsis hypocotyls are controlled by gibberellins and modulated by ethylene and auxins

The plant hormones gibberellin (GA), ethylene and auxin can promote hypocotyl elongation of Arabidopsis seedlings grown in the light on a low nutrient medium (LNM). In this study, we used hypocotyl elongation as a system to investigate interactions between GA and ethylene or auxin and analysed their influence on the development of stomata in the hypocotyl. When applied together, GA and ethylene or auxin exerted a synergistic effect on hypocotyl elongation. Stimulated cell elongation is the main cause of hypocotyl elongation. Furthermore, hypocotyls treated with GA plus either ethylene or auxin show an increased endoreduplication. In addition, a small but significant increase in cell number was observed in the cortical cell files of hypocotyls treated with ethylene and GA together. However, studies with transgenic seedlings expressing CycB1::uidA genes revealed that cell division in the hypocotyl occurs only in the epidermis and mainly to form stomata, a process strictly regulated by hormones. Stomata formation in the hypocotyl is induced by the treatment with either GA or ethylene. The effect of GA could be strongly enhanced by the simultaneous addition of ethylene or auxin to the growth medium. Gibberellin is the main signal inducing stomata formation in the hypocotyl. In addition, this signal regulates hypocotyl elongation and is modulated by ethylene and auxin. The implication of these three hormones in relation to cell division and stomata formation is discussed.     

An auxin-resistant mutant ofNicotiana plumbaginifolia Viv. is impaired in 1-naphthaleneacetic acid-induced hyperpolarization of hypocotyl cell membranes in intact seedlings

The electrical response to the synthetic auxin 1-naphthaleneacetic acid (1-NAA) ofNicotiana plumbaginifolia wild type and the monogenic, dominant auxinresistant mutant R25 was studied. Membrane potentials were continuously recorded in hypocotyl cells of light-grown, intact seedlings, and the time course of the response to 1-NAA addition was followed. Wild-type cells responded to ? 10^?5 M 1-NAA with a delayed, transient hyperpolarization. The R25 cells hyperpolarized significantly only in response to 1-NAA at 10^?3 M, and with maximal amplitudes lower than those recorded with the wild type. In contrast, the two genotypes reacted similarly in terms of kinetics and amplitude to 10^?5 M fusicoccin, which rapidly and strongly hyperpolarized the cells, and to 10^?3 M benzoic acid, which induced rapid and weak hyperpolarization. The resting membrane potentials of the wild type and R25 were also not significantly different. Unlike wild-type hypocotyls, those of R25 ceased elongating before the time chosen for the electrophysiological measurements, but control experiments performed at a time when the elongation of both genotypes had terminated indicated that the difference in electrical response to auxin is independent of hypocotyl growth. The inefficiency of 1-NAA in inducing hyperpolarization of R25 hypocotyl cells suggests a defect at an early step in auxin action.     

Transport of the two natural auxins, indole-3-butyric acid and indole-3-acetic acid, in Arabidopsis

Polar transport of the natural auxin indole-3-acetic acid (IAA) is important in a number of plant developmental processes. However, few studies have investigated the polar transport of other endogenous auxins, such as indole-3-butyric acid (IBA), in Arabidopsis. This study details the similarities and differences between IBA and IAA transport in several tissues of Arabidopsis. In the inflorescence axis, no significant IBA movement was detected, whereas IAA is transported in a basipetal direction from the meristem tip. In young seedlings, both IBA and IAA were transported only in a basipetal direction in the hypocotyl. In roots, both auxins moved in two distinct polarities and in specific tissues. The kinetics of IBA and IAA transport appear similar, with transport rates of 8 to 10 mm per hour. In addition, IBA transport, like IAA transport, is saturable at high concentrations of auxin, suggesting that IBA transport is protein mediated. Interestingly, IAA efflux inhibitors and mutations in genes encoding putative IAA transport proteins reduce IAA transport but do not alter IBA movement, suggesting that different auxin transport protein complexes are likely to mediate IBA and IAA transport. Finally, the physiological effects of IBA and IAA on hypocotyl elongation under several light conditions were examined and analyzed in the context of the differences in IBA and IAA transport. Together, these results present a detailed picture of IBA transport and provide the basis for a better understanding of the transport of these two endogenous auxins.