Indole-3-acetic Acid Transport Research Articles

Potassium deficiency inhibits lateral root development in tobacco seedlings by changing auxin distribution

Potassium deficiency (LK) is a common abiotic stress that affects plant root development. We aimed to identify the role of auxin in LK effects on root growth. We investigated the effects of LK on the formation and elongation of first- and second-order lateral roots (LRs), auxin concentration, DR5::GUS expression, [3H]IAA transport and the expressions of six PIN genes in roots of tobacco plants. We also examined the effects of exogenous auxin (NAA) and a transport inhibitor (NPA) on LR growth and DR5::GUS expression. Potassium deficiency reduced root growth, mostly by impairing the formation and elongation of first-order LRs. Indole acetic acid (IAA) concentration and DR5::GUS expression levels decreased in leaves and roots subjected to LK, indicating that LK affected auxin distribution. [3H]IAA transport and the expression levels of PIN genes were reduced by LK, showing that auxin polar transport was inhibited by LK. Application of NAA to LK-treated plants increased first-order LR formation and elongation to levels similar to the control, an outcome that was consistent with the similarity in DR5::GUS expression levels between treatments. NPA acted in a converse manner. Potassium deficiency inhibited LR formation and elongation in tobacco plants by shifting auxin distribution.

Abscisic Acid Is a General Negative Regulator of Arabidopsis Axillary Bud Growth.

Branching is an important process controlled by intrinsic programs and by environmental signals transduced by a variety of plant hormones. Abscisic acid (ABA) was previously shown to mediate Arabidopsis (Arabidopsis thaliana) branching responses to the ratio of red light (R) to far-red light (FR; an indicator of competition) by suppressing bud outgrowth from lower rosette positions under low R:FR. However, the role of ABA in regulating branching more generally was not investigated. This study shows that ABA restricts lower bud outgrowth and promotes correlative inhibition under both high and low R:FR. ABA was elevated in buds exhibiting delayed outgrowth resulting from bud position and low R:FR and decreased in elongating buds. ABA was reduced in lower buds of hyperbranching mutants deficient in auxin signaling (AUXIN RESISTANT1), MORE AXILLARY BRANCHING (MAX) signaling (MAX2), and BRANCHED1 (BRC1) function, and partial suppression of branch elongation in these mutants by exogenous ABA suggested that ABA may act downstream of these components. Bud BRC1 expression was not altered by exogenous ABA, consistent with a downstream function for ABA. However, the expression of genes encoding the indole-3-acetic acid (IAA) biosynthesis enzyme TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS1, the auxin transporter PIN-FORMED1, and the cell cycle genes CYCLIN A2;1 and PROLIFERATING CELL NUCLEAR ANTIGEN1 in buds was suppressed by ABA, suggesting that it may inhibit bud growth in part by suppressing elements of the cell cycle machinery and bud-autonomous IAA biosynthesis and transport. ABA was found to suppress bud IAA accumulation, thus confirming this aspect of its action.

Abscission research: what we know and what we still need to study

Purpose of review: Abscission is a programmed developmental process initiated by auxin depletion. This review summarizes the mechanisms leading to auxin depletion in the abscission zone (AZ), evaluates the methods for estimation of the spatio-temporal auxin levels, demonstrates how auxin depletion occurs during natural, stress-induced, and artificially-induced organ abscission, and presents new evidence for early and late events resulting from auxin depletion which lead to organ abscission.Findings: Auxin depletion occurs during natural developmental processes which end in organ abscission (leaf and flower senescence, fruit ripening, and self-pruning) and stress-induced abscission, and following artificial organ removal in the tomato model system. Stress-induced auxin depletion is mediated by increased ethylene and reactive oxygen species (ROS) production and carbohydrate starvation. Similar changes in auxin-related genes occurred in both flower AZ (FAZ) and leaf AZ (LAZ) following flower or leaf removal, respectively, suggesting a similar regulation of the abscission process of these organs. Auxin depletion resulted from decreased indole-3-acetic acid (IAA) biosynthesis and transport, as well as from enhanced IAA transport autoinhibition (ATA), conjugation and oxidative IAA catabolism. Functional analyses of several target genes delaying abscission, such as Knotted-Like Homeobox Protein1 (KD1), Tomato Proline Rich Protein (TPRP), Ethylene Responsive Factor52 (ERF52), and Ribonuclease LX (LX), shed light on various events operating in response to auxin depletion in tomato FAZ and/or LAZ. The information gained allows a better understanding of the abscission process driven by auxin depletion, and might lead to development of improved methods for abscission control in horticultural crops.Direction for future research: A better understanding of abscission regulation as it pertains to auxin depletion will require advanced molecular tools such as microarrays, new generation sequencing (NGS), transcriptomic, functional, and proteomic analyses of target genes and proteins found to operate in the abscission process.

Sugars take a central position in plant growth, development and, stress responses. A focus on apical dominance.

Sugars take a central position in plant growth, development and, stress responses. A focus on apical dominance.

A 2,4-dichlorophenoxyacetic acid analog screened using a maize coleoptile system potentially inhibits indole-3-acetic acid influx in Arabidopsis thaliana

Studies using inhibitors of indole-3-acetic acid (IAA) transport, not only for efflux but influx carriers, provide many aspects of auxin physiology in plants. 1-Naphtoxyacetic acid (1-NOA), an analog of the synthetic auxin 1-N-naphtalene acetic acid (NAA), inhibits the IAA influx carrier AUX1. However, 1-NOA also shows auxin activity because of its structural similarity to NAA. In this study, we have identified another candidate inhibitor of the IAA influx carrier. The compound, “7-B3; ethyl 2-[(2-chloro-4-nitrophenyl)thio]acetate,” is a 2,4-dichlorophenoxyacetic acid (2,4-D) analog. At high concentrations (> 300 µM), 7-B3 slightly reduced IAA transport and tropic curvature of maize coleoptiles, whereas lower concentrations had almost no effect. We have analyzed the effects of 7-B3 on Arabidopsis thaliana seedlings. 7-B3 rescued the 2,4-D-inhibited root elongation, but not the NAA-inhibited root elongation. The effect of 7-B3 was weaker than that of 1-NOA. Both 1-NOA and 7-B3 inhibited DR5::GUS expression induced by IAA and 2,4-D, but not that induced by NAA. At high concentrations, 1-NOA exhibited auxin activity, but 7-B3 did not. Furthermore, 7-B3 inhibited apical hook formation in etiolated seedlings more effectively than 1-NOA did. These results indicate that 7-B3 is a potential inhibitor of IAA influx that has almost no effect on IAA efflux or auxin signaling.

Phytotoxicity of aluminum on root growth and indole-3-acetic acid accumulation and transport in alfalfa roots

Aluminum (Al) toxicity in acid soils is a major constraint on crop production. The objective of this study of alfalfa (Medicago sativa L.) was to determine whether Al-induced inhibition of root growth in alfalfa (M. sativa L.) is related to Al distribution in different root tissues, changes in endogenous level of indole-3-acetic acid (IAA) in root tips, and the expression of key genes in IAA metabolism and translocation. Roots of alfalfa were exposed to 100μM Al3+ in half-strength Hoagland's nutrient solution. The inhibitory effects of Al on root elongation was more pronounced than on root and shoot biomass accumulation. Lumogallion, an Al specific stain, was used to monitor tissue locations of Al in the root. Lumogallion-Al was mainly detected in the root cap, epidermis, and stele of Al-treated roots. In the meristematic region of the root tip, Al accumulated mainly in the cell wall, intracellular membrane system and center of the nucleus. The similar distribution of Al ions in the root with that of auxin revealed that Al in roots may affect IAA levels. High performance liquid chromatography analysis demonstrated that IAA levels increased in the base of the root and decreased in the root tips treated with 100μM Al3+ for 3d compared to that of the control (without Al). Reverse transcription and quantitative PCR showed that the expressions of three genes, auxin transporter-like protein, auxin efflux carrier component, and cationic peroxidase, were significantly higher in Al-stressed alfalfa roots than in the control while the expression of auxin conjugate hydrolase was significantly lower. These results suggested that Al-inhibition of root elongation could be associated with Al accumulation in apoplast and membrane system, and alteration of IAA transport in the root.

The choice of auxin analogue for in vitro root induction influences post-induction root development in Eucalyptus grandis

Previous studies on in vitro rooting for improved micropropagation of eucalypts indicated that root graviperception and post-acclimatisation architecture are determined by the relative exogenous auxin analogue and its stability, supplied during the pre-rooting culture stages. The specific roles of indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) in the rooting medium on the in vitro root morphological processes were explored using a good-rooting clone. In vitro rooting percentage was significantly reduced when either of the auxin inhibitors 2,3,5-triiodobenzoic acid (TIBA) and ρ-chlorophenoxyisobutyric acid (PCIB) or the auxin antagonist kinetin was supplied at rooting, with or without exogenous auxin. For all treatments, at the time of root induction, shoots did not possess a vascular cambium, only procambial tissue, from where adventitious roots formed. However, when the inhibitors or the antagonist were supplied to the roots 3 days after root induction, they affected root growth and graviperception. Kinetin and PCIB significantly reduced the mean root diameter from 552.8 µm (control) to 129.2 µm and 278.6 µm, respectively, over 3 weeks. While the PCIB treatment resulted in a significant increase in delta root length over this period, the TIBA treatment significantly decreased delta root length and increased mean root diameter to 833.4 µm. Restricting IAA transport with TIBA further altered root vascular patterning and, as with PCIB, resulted in the collapse of the columella region. Nevertheless, only a disruption in IAA transport and subsequent auxin distribution by TIBA treatment resulted in altered root graviperception. The results suggest the necessary inclusion of IAA in eucalypt micropropagation protocols to ensure good quality roots.

Auxins: Biosynthesis, metabolism, and transport

Indole-3-acetic acid (IAA) is the key plant hormone of the auxin class, which controls all processes of plant growth and development. Information on IAA biosynthesis, metabolism, and transport is of great importance for understanding nearly all morphogenetic processes in plant development. Nevertheless, our knowledge about the organization of this auxin machine, which is of significance to all plants, is by no means complete. At present, six independent pathways of IAA biosynthesis are known, each of which has its own regulation and genetic base. Auxin metabolism and active transport are no less intricate systems with internal and external regulation. A lot of genes that determine the behavior of the auxin machine are still unknown, but those revealed can be employed in genetic engineering to obtain new plant forms with improved properties. The review presents modern concepts of the biosynthesis, metabolism, and active transport of auxin with regard to the genes underlying these processes.

Localized Induction of the ATP-Binding Cassette B19 Auxin Transporter Enhances Adventitious Root Formation in Arabidopsis

Adventitious roots emerge from aerial plant tissues, and the induction of these roots is essential for clonal propagation of agriculturally important plant species. This process has received extensive study in horticultural species but much less focus in genetically tractable model species. We have explored the role of auxin transport in this process in Arabidopsis (Arabidopsis thaliana) seedlings in which adventitious root initiation was induced by excising roots from low-light-grown hypocotyls. Inhibition of auxin transport from the shoot apex abolishes adventitious root formation under these conditions. Root excision was accompanied by a rapid increase in radioactive indole-3-acetic acid (IAA) transport and its accumulation in the hypocotyl above the point of excision where adventitious roots emerge. Local increases in auxin-responsive gene expression were also observed above the site of excision using three auxin-responsive reporters. These changes in auxin accumulation preceded cell division events, monitored by a cyclin B1 reporter (pCYCB1;1:GUS), and adventitious root initiation. We examined excision-induced adventitious root formation in auxin influx and efflux mutants, including auxin insensitive1, pin-formed1 (pin1), pin2, pin3, and pin7, with the most profound reductions observed in ATP-binding cassette B19 (ABCB19). An ABCB19 overexpression line forms more adventitious roots than the wild type in intact seedlings. Examination of transcriptional and translational fusions between ABCB19 and green fluorescent protein indicates that excision locally induced the accumulation of ABCB19 transcript and protein that is temporally and spatially linked to local IAA accumulation leading to adventitious root formation. These experiments are consistent with localized synthesis of ABCB19 protein after hypocotyl excision leads to enhanced IAA transport and local IAA accumulation driving adventitious root formation.

Correlative polar auxin transport to explain the thinning mode of action of benzyladenine on apple

Chemical thinning of young apple (Malus domestica, Borkh.) fruit has become standard agricultural practice worldwide. The general understanding about the mode of action of thinning substances is, however, still very limited. Benzyladenine (BA), one of the most promising newer thinning chemicals has been chosen as an example to study its “mode of action”. The following aspects were investigated: (1) the site of action, (2) effects on seed number and development, (3) fruit and vegetative growth, (4) photosynthesis and (5) polar indoleacetic acid transport (IAAPAT). When BA was applied at 100mgL−1 on three apple cultivars on whole trees or leaves-only, thinning effects were comparable to or even stronger than hand thinning. BA applied only on fruit showed insignificant thinning. At June drop the weight of abscising fruit was comparable or even higher when treated with BA. Growth of remaining fruit was not affected until harvest when only fruit were treated. However, when leaves-only or whole trees were treated remaining fruit showed a decreased fruit growth initially but surpassed control fruit thereafter. No direct correlations were found between thinning efficacy and seed number, shoot growth, photosynthesis and by indirect conclusion, assimilate allocation. Thus, these assessments provided no clear explanation for a possible mode of action of BA. Remarkably, a decrease in IAAPAT of lateral fruit was opposed to an increase in IAAPAT of bourse shoot tips after BA application. This was only observed when leaves/shoot tips but not fruit only were treated. Assuming a correlative IAA transport autoinhibition (ATA) of fruit by the IAAPAT of shoots, particularly of bourse shoot tips, may explain fruit abscission and the mode of action of BA applications in apple. The latter aspects are discussed in more detail.

Exogenous indole-3-acetic acid could reduce the accumulation of aluminum in root apex of wheat (Triticum aestivum L.) under Al stress

Indole-3-acetic acid (IAA) is hormones in higher plants and participates in plant growth regulation and stress resistance including Al stress. The reduction of root apex aluminum (Al) content by exogenous IAA treatment was hypothesized to be effective in alleviating the adverse effect of high Al concentration in wheat root growth. To investigate the role of IAA in lowering root apex Al content, Al-tolerant wheat (ET8) was studied in Al solution (50 µM), co-treated with IAA (25 µM) and anion channel inhibitors (5 µM NIF or A9C) or IAA transport inhibitors (5 µM TIBA or NPA) under acidic condition for 24 h. Treatments were as fellows: control (0.5 µM CaCl 2 ), Al (50 µM), Al (50 µM) + IAA (25 µM), Al (50 µM) + IAA (25 µM) + NIF (or A9C, 5 mM), Al (50 µM) + NPA (or TIBA, 5 µM). Al content in root apex, rhizosphere pH and PM H + -ATPase activity were studied. The results showed that co-treatment with IAA reduced root apex Al content by 42% compared to Al treatment. Anion channel inhibitors enhanced the accumulation of Al in root apex by 27% (or 32%) (NIF or A9C), while addition of IAA neutralized its promoting effect. The co-treatment with IAA increased rhizosphere pH by alleviating the decrease of plasma membrane H + -ATPase activity, while IAA transport inhibitors (NPA or TIBA) suppressed the elevation of rhizosphere pH. The current results suggested that IAA could be effective in alleviating Al toxicity through reducing Al accumulation in wheat root apex.

Vacuolar CAX1 and CAX3 Influence Auxin Transport in Guard Cells via Regulation of Apoplastic pH

CATION EXCHANGERs CAX1 and CAX3 are vacuolar ion transporters involved in ion homeostasis in plants. Widely expressed in the plant, they mediate calcium transport from the cytosol to the vacuole lumen using the proton gradient across the tonoplast. Here, we report an unexpected role of CAX1 and CAX3 in regulating apoplastic pH and describe how they contribute to auxin transport using the guard cell's response as readout of hormone signaling and cross talk. We show that indole-3-acetic acid (IAA) inhibition of abscisic acid (ABA)-induced stomatal closure is impaired in cax1, cax3, and cax1/cax3. These mutants exhibited constitutive hypopolarization of the plasma membrane, and time-course analyses of membrane potential revealed that IAA-induced hyperpolarization of the plasma membrane is also altered in these mutants. Both ethylene and 1-naphthalene acetic acid inhibited ABA-triggered stomatal closure in cax1, cax3, and cax1/cax3, suggesting that auxin signaling cascades were functional and that a defect in IAA transport caused the phenotype of the cax mutants. Consistent with this finding, chemical inhibition of AUX1 in wild-type plants phenocopied the cax mutants. We also found that cax1/cax3 mutants have a higher apoplastic pH than the wild type, further supporting the hypothesis that there is a defect in IAA import in the cax mutants. Accordingly, we were able to fully restore IAA inhibition of ABA-induced stomatal closure in cax1, cax3, and cax1/cax3 when stomatal movement assays were carried out at a lower extracellular pH. Our results suggest a network linking the vacuolar cation exchangers to apoplastic pH maintenance that plays a crucial role in cellular processes.

Transport of Indole-3-Butyric Acid and Indole-3-Acetic Acid in Arabidopsis Hypocotyls Using Stable Isotope Labeling

The polar transport of the natural auxins indole-3-butyric acid (IBA) and indole-3-acetic acid (IAA) has been described in Arabidopsis (Arabidopsis thaliana) hypocotyls using radioactive tracers. Because radioactive assays alone cannot distinguish IBA from its metabolites, the detected transport from applied [3H]IBA may have resulted from the transport of IBA metabolites, including IAA. To test this hypothesis, we used a mass spectrometry-based method to quantify the transport of IBA in Arabidopsis hypocotyls by following the movement of [13C1]IBA and the [13C1]IAA derived from [13C1]IBA. We also assayed [13C6]IAA transport in a parallel control experiment. We found that the amount of transported [13C1]IBA was dramatically lower than [13C6]IAA, and the IBA transport was not reduced by the auxin transport inhibitor N-1-naphthylphthalamic acid. Significant amounts of the applied [13C1]IBA were converted to [13C1]IAA during transport, but [13C1]IBA transport was independent of IBA-to-IAA conversion. We also found that most of the [13C1]IBA was converted to ester-linked [13C1]IBA at the apical end of hypocotyls, and ester-linked [13C1]IBA was also found in the basal end at a level higher than free [13C1]IBA. In contrast, most of the [13C6]IAA was converted to amide-linked [13C6]IAA at the apical end of hypocotyls, but very little conjugated [13C6]IAA was found in the basal end. Our results demonstrate that the polar transport of IBA is much lower than IAA in Arabidopsis hypocotyls, and the transport mechanism is distinct from IAA transport. These experiments also establish a method for quantifying the movement of small molecules in plants using stable isotope labeling.

Comparative Genomics and Molecular Characterization of the Maize PIN Family Proteins

Comparative Genomics and Molecular Characterization of the Maize PIN Family Proteins

The Arabidopsis concentration‐dependent influx/efflux transporter ABCB4 regulates cellular auxin levels in the root epidermis

Arabidopsis ATP-binding cassette B4 (ABCB4) is a root-localised auxin efflux transporter with reported auxin uptake activity in low auxin concentrations. Results reported here demonstrate that ABCB4 is a substrate-activated regulator of cellular auxin levels. The contribution of ABCB4 to shootward auxin movement at the root apex increases with auxin concentration, but in root hair elongation assays ABCB4-mediated uptake is evident at low concentrations as well. Uptake kinetics of ABCB4 heterologously expressed in Schizosaccharomyces pombe differed from the saturation kinetics of AUX1 as uptake converted to efflux at threshold indole-3-acetic acid (IAA) concentrations. The concentration dependence of ABCB4 appears to be a direct effect on transporter activity, as ABCB4 expression and ABCB4 plasma membrane (PM) localisation at the root apex are relatively insensitive to changes in auxin concentration. However, PM localization of ABCB4 decreases with 1-naphthylphthalamic acid (NPA) treatment. Unlike other plant ABCBs studied to date, and consistent with decreased detergent solubility, ABCB4(pro) :ABCB4-GFP is partially internalised in all cell types by 0.05% DMSO, but not 0.1% ethanol. In trichoblasts, ABCB4(pro) :ABCB4-GFP PM signals are reduced by >200 nm IAA and 2,4-dichlorophenoxyacetic acid (2,4-D). In heterologous systems and in planta, ABCB4 transports benzoic acid with weak affinity, but not the oxidative catabolism products 2-oxindole-3-acetic-acid and 2-oxindole-3-acetyl-β-D-glucose. ABCB4 mediates uptake, but not efflux, of the synthetic auxin 2,4-D in cells lacking AUX1 activity. Results presented here suggest that 2,4-D is a non-competitive inhibitor of IAA transport by ABCB4 and indicate that ABCB4 is a target of 2,4-D herbicidal activity.

Low-Fluence Red Light Increases the Transport and Biosynthesis of Auxin

In plants, light is an important environmental signal that induces photomorphogenesis and interacts with endogenous signals, including hormones. We found that light increased polar auxin transport in dark-grown Arabidopsis (Arabidopsis thaliana) and tomato (Solanum lycopersicum) hypocotyls. In tomato, this increase was induced by low-fluence red or blue light followed by 1 d of darkness. It was reduced in phyA, phyB1, and phyB2 tomato mutants and was reversed by far-red light applied immediately after the red or blue light exposure, suggesting that phytochrome is involved in this response. We further found that the free indole-3-acetic acid (IAA) level in hypocotyl regions below the hook was increased by red light, while the level of conjugated IAA was unchanged. Analysis of IAA synthesized from [¹³C]indole or [¹³C]tryptophan (Trp) revealed that both Trp-dependent and Trp-independent IAA biosynthesis were increased by low-fluence red light in the top section (meristem, cotyledons, and hook), and the Trp-independent pathway appears to become the primary route for IAA biosynthesis after red light exposure. IAA biosynthesis in tissues below the top section was not affected by red light, suggesting that the increase of free IAA in this region was due to increased transport of IAA from above. Our study provides a comprehensive view of light effects on the transport and biosynthesis of IAA, showing that red light increases both IAA biosynthesis in the top section and polar auxin transport in hypocotyls, leading to unchanged free IAA levels in the top section and increased free IAA levels in the lower hypocotyl regions.

Ethylene inhibits lateral root development, increases IAA transport and expression of PIN3 and PIN7 auxin efflux carriers

We used genetic and molecular approaches to identify mechanisms by which the gaseous plant hormone ethylene reduces lateral root formation and enhances polar transport of the hormone auxin. Arabidopsis thaliana mutants, aux1, lax3, pin3 and pin7, which are defective in auxin influx and efflux proteins, were less sensitive to the inhibition of lateral root formation and stimulation of auxin transport following treatment with the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC). By contrast, pin2 and abcb19 mutants exhibited wild-type ACC responses. ACC and indole-3-acetic acid (IAA) increased the abundance of transcripts encoding auxin transport proteins in an ETR1 and EIN2 (ethylene signaling)-dependent and TIR1 (auxin receptor)-dependent fashion, respectively. The effects of ACC on these transcripts and on lateral root development were still present in the tir1 mutant, suggesting independent signaling networks. ACC increased auxin-induced gene expression in the root apex, but decreased expression in regions where lateral roots form and reduced free IAA in whole roots. The ethylene synthesis inhibitor aminoethoxyvinylglycine (AVG) had opposite effects on auxin-dependent gene expression. These results suggest that ACC affects root development by altering auxin distribution. PIN3- and PIN7-GFP fluorescence was increased or decreased after ACC or AVG treatment, respectively, consistent with the role of PIN3 and PIN7 in ACC-elevated transport. ACC treatment abolished a localized depletion of fluorescence of PIN3- and PIN7-GFP, normally found below the site of primordia formation. These results suggest that ACC treatment increased PIN3 and PIN7 expression, resulting in elevated auxin transport, which prevented the localized accumulation of auxin needed to drive lateral root formation.

Effect of indole-3-acetic acid on aluminum-induced efflux of malic acid from wheat (Triticum aestivum L.)

Indole-3-acetic acid (IAA) has been found to be involved in plant resistance to various types of environmental stress. Aluminum (Al) toxicity, as one of the most important environmental stress in acid soils, is coped by most plants through the efflux of organic acids via anion channel. This study aims to evaluate the effect of IAA on efflux of malic acid from wheat (Triticum aestivum L.) under Al stress. Hydroponic experiments were performed by wheat ET8 (Al-tolerant). The efflux of malic acid was investigated under different treatments. Results showed that Al treatments increased the accumulation of endogenous IAA, but decreased the activity of IAA oxidase in a dose-dependent manner. A good correlation between all the data of malic acid efflux rate and endogenous IAA content was obtained (R2 = 0.9859**). IAA treatment alone had no effect on the efflux of malic acid. But compared to Al (50 μM) treatment, the efflux of malic acid increased significantly under the co-treatment of IAA (50 μM) and Al (50 μM). In split-root experiments, the root with half of it being treated with Al (CK/Al), the other part (CK) showed significantly higher malic acid efflux rate and endogenous IAA content in root apexes, compared with the root without such treatment (CK/CK). The Al-induced malic acid efflux decreased under the treatments of IAA transport inhibitor N-1-napthyl-phtalamic acid (NPA) (or 2,3,5-triiodobenzoic acid, TIBA). These above results suggested the possible involvement of IAA in the stimulation of malic acid efflux under Al stress. In addition, anion channel inhibitor treatment experiment showed that IAA (50 μM) relieved the inhibiting effect of 5 μM anthracene-9-carboxylic acid (A9C) (or niflumic acid, NIF) on malic acid efflux induced by Al (50 μM), compared to the co-treatment of Al (50 μM) and 5 μM anion channel inhibitor A9C (or NIF) it is thus speculated that the anion channel might have been activated when IAA was involved in malic acid efflux. This study showed that IAA was involved in aluminum-induced efflux of malic acid from wheat.

NPH3- and PGP-like genes are exclusively expressed in the apical tip region essential for blue-light perception and lateral auxin transport in maize coleoptiles

Phototropic curvature results from differential growth on two sides of the elongating shoot, which is explained by asymmetrical indole-3-acetic acid (IAA) distribution. Using 2 cm maize coleoptile segments, 1st positive phototropic curvature was confirmed here after 8 s irradiation with unilateral blue light (0.33 μmol m−2 s−1). IAA was redistributed asymmetrically by approximately 20 min after photo-stimulation. This asymmetric distribution was initiated in the top 0–3 mm region and was then transmitted to lower regions. Application of the IAA transport inhibitor, 1-N-naphthylphthalamic acid (NPA), to the top 2 mm region completely inhibited phototropic curvature, even when auxin was simultaneously applied below the NPA-treated zone. Thus, lateral IAA movement occurred only within the top 0–3 mm region after photo-stimulation. Localized irradiation experiments indicated that the photo-stimulus was perceived in the apical 2 mm region. The results suggest that this region harbours key components responsible for photo-sensing and lateral IAA transport. In the present study, it was found that the NPH3- and PGP-like genes were exclusively expressed in the 0–2 mm region of the tip, whereas PHOT1 and ZmPIN1a, b, and c were expressed relatively evenly along the coleoptile, and ZmAUX1, ZMK1, and ZmSAURE2 were strongly expressed in the elongation zone. These results suggest that the NPH3-like and PGP-like gene products have a key role in photo-signal transduction and regulation of the direction of auxin transport after blue light perception by phot1 at the very tip region of maize coleoptiles.

Detailed quantitative analysis of architectural traits of basal roots of young seedlings of bean in response to auxin and ethylene.

Vertical placement of roots within the soil determines their efficiency of acquisition of heterogeneous belowground resources. This study quantifies the architectural traits of seedling basal roots of bean (Phaseolus vulgaris), and shows that the distribution of root tips at different depths results from a combined effect of both basal root growth angle (BRGA) and root length. Based on emergence locations, the basal roots are classified in three zones, upper, middle, and lower, with each zone having distinct architectural traits. The genotypes characterized as shallow on BRGA alone produced basal roots with higher BRGA, greater length, and more vertically distributed roots than deep genotypes, thereby establishing root depth as a robust measure of root architecture. Although endogenous indole-3-acetic acid (IAA) levels were similar in all genotypes, IAA and 1-N-naphthylphthalamic acid treatments showed different root growth responses to auxin because shallow and deep genotypes tended to have optimal and supraoptimal auxin levels, respectively, for root growth in controls. While IAA increased ethylene production, ethylene also increased IAA content. Although differences in acropetal IAA transport to roots of different zones can account for some of the differences in auxin responsiveness among roots of different emergence positions, this study shows that mutually dependent ethylene-auxin interplay regulates BRGA and root growth differently in different genotypes. Root length inhibition by auxin was reversed by an ethylene synthesis inhibitor. However, IAA caused smaller BRGA in deep genotypes, but not in shallow genotypes, which only responded to IAA in the presence of an ethylene inhibitor.