Novel auxin transport inhibitors phenocopy the auxin influx carrier mutation aux1

3,4-Dichlorophenylacetic acid acts as an auxin analog and induces beneficial effects in various crops

Auxins and their analogs are widely used to promote root growth, flower and fruit development, and yield in crops. The action characteristics and application scope of various auxins are different. To overcome the limitations of existing auxins, expand the scope of applications, and reduce side effects, it is necessary to screen new auxin analogs. Here, we identified 3,4-dichlorophenylacetic acid (Dcaa) as having auxin-like activity and acting through the auxin signaling pathway in plants. At the physiological level, Dcaa promotes the elongation of oat coleoptile segments, the generation of adventitious roots, and the growth of crop roots. At the molecular level, Dcaa induces the expression of auxin-responsive genes and acts through auxin receptors. Molecular docking results showed that Dcaa can bind to auxin receptors, among which TIR1 has the highest binding activity. Application of Dcaa at the root tip of the DR5:GUS auxin-responsive reporter induces GUS expression in the root hair zone, which requires the PIN2 auxin efflux carrier. Dcaa also inhibits the endocytosis of PIN proteins like other auxins. These results provide a basis for the application of Dcaa in agricultural practices.

Unveiling Molecular Signatures in Light-Induced Seed Germination: Insights from PIN3, PIN7, and AUX1 in Arabidopsis thaliana

Light provides seeds with information that is essential for the adjustment of their germination to the conditions that are most favorable for the successful establishment of the future seedling. The promotion of germination depends mainly on environmental factors, like temperature and light, as well as internal factors associated with the hormonal balance between gibberellins (GA) and abscisic acid (ABA), although other hormones such as auxins may act secondarily. While transcriptomic studies of light-germinating Arabidopsis thaliana seeds suggest that auxins and auxin transporters are necessary, there are still no functional studies connecting the activity of the auxin transporters in light-induced seed germination. In this study, we investigated the roles of two auxin efflux carrier (PIN3 and PIN7) proteins and one auxin influx (AUX1) carrier protein during Arabidopsis thaliana seed germination. By using next-generation sequencing (RNAseq), gene expression analyses, hormonal sensitivity assays, and the quantification of indole-3-acetic acid (IAA) levels, we assessed the functional roles of PIN3, PIN7, and AUX1 during light-induced seed germination. We showed that auxin levels are increased 24 h after a red-pulse (Rp). Additionally, we evaluated the germination responses of pin3, pin7, and aux1 mutant seeds and showed that PIN3, PIN7, and AUX1 auxin carriers are important players in the regulation of seed germination. By using gene expression analysis in water, fluridone (F), and ABA+F treated seeds, we confirmed that Rp-induced seed germination is associated with auxin transport, and ABA controls the function of PIN3, PIN7, and AUX1 during this process. Overall, our results highlight the relevant and positive role of auxin transporters in germinating the seeds of Arabidopsis thaliana.

Design and synthesis of strong root gravitropism inhibitors with no concomitant growth inhibition

Herein, we describe a highly potent gravitropic bending inhibitor with no concomitant growth inhibition. Previously, we reported that (2Z,4E)-5-phenylpenta-2,4-dienoic acid (ku-76) selectively inhibits root gravitropic bending of lettuce radicles at 5 μM. Based on the structure-activity relationship study of ku-76 as a lead compound, we designed and synthesized various C4-substituted analogs of ku-76. Among the analogs, 4-phenylethynyl analog exhibited the highest potency for gravitropic bending inhibition, which was effective at only 0.01 μM. Remarkably, 4-phenylethynyl analog is much more potent than the known inhibitor, NPA. Substitution in the para position on the aromatic ring of 4-phenylethynyl group was tolerated without diminished activity. In addition, evaluation using Arabidopsis indicated that 4-phenylethynyl analog inhibits gravitropism by affecting auxin distribution in the root tips. Based on the effects on Arabidopsis phenotypes, 4-phenylethynyl analog may be a novel inhibitor that differs in action from the previously reported auxin transport inhibitors.

Endogenous auxin maintains embryonic cell identity and promotes somatic embryo development in Arabidopsis

Somatic embryogenesis (SE), or embryo development from in vitro cultured vegetative explants, can be induced in Arabidopsis by the synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) or by overexpression of specific transcription factors, such as AT-HOOK MOTIF NUCLEAR LOCALIZED 15 (AHL15). Here, we explored the role of endogenous auxin [indole-3-acetic acid (IAA)] during 2,4-D and AHL15-induced SE. Using the pWOX2:NLS-YFP reporter, we identified three distinct developmental stages for 2,4-D and AHL15-induced SE in Arabidopsis, with these being (i) acquisition of embryo identity; (ii) formation of pro-embryos; and (iii) somatic embryo patterning and development. The acquisition of embryo identity coincided with enhanced expression of the indole-3-pyruvic acid auxin biosynthesis YUCCA genes, resulting in an enhanced pDR5:GFP-reported auxin response in the embryo-forming tissues. Chemical inhibition of the indole-3-pyruvic acid pathway did not affect the acquisition of embryo identity, but significantly reduced or completely inhibited the formation of pro-embryos. Co-application of IAA with auxin biosynthesis inhibitors in the AHL15-induced SE system rescued differentiated somatic embryo formation, confirming that increased IAA levels are important during the last two stages of SE. Our analyses also showed that polar auxin transport, with AUXIN/LIKE-AUX influx and PIN-FORMED1 efflux carriers as important drivers, is required for the transition of embryonic cells to proembryos and, later, for correct cell fate specification and differentiation. Taken together, our results indicate that endogenous IAA biosynthesis and its polar transport are not required for the acquisition of embryo identity, but rather to maintain embryonic cell identity and for the formation of multicellular proembryos and their development into histodifferentiated embryos.

Screening of protonstatin-1 (PS-1) analogs for improved inhibitors of plant plasma membrane H+-ATPase activity

We previously identified protonstatin-1 (PS-1) as a selective inhibitor of plasma membrane H⁺-ATPase (PM H⁺-ATPase) activity and used it as a tool to validate the chemiosmotic model for polar auxin transport. Here, to obtain compounds with higher affinity than PS-1 for PM H⁺-ATPase, we synthesized 34 PS-1 analogs and examined their ability to inhibit PM H⁺-ATPase activity. The 34 analogs showed varying inhibitory effects on the activity of this enzyme. The strongest effect was observed for the small molecule PS-2, which was approximately five times stronger than PS-1. Compared to PS-1, PS-2 was also a stronger inhibitor of auxin uptake as well as acropetal and basipetal polar auxin transport in Arabidopsis thaliana seedlings. Because PS-2 is a more potent inhibitor of PM H⁺-ATPase than PS-1, we believe that this compound could be used as a tool to study the functions of this key plant enzyme.

New Wine in an Old Bottle: Utilizing Chemical Genetics to Dissect Apical Hook Development

The apical hook is formed by dicot seedlings to protect the tender shoot apical meristem during soil emergence. Regulated by many phytohormones, the apical hook has been taken as a model to study the crosstalk between individual signaling pathways. Over recent decades, the roles of different phytohormones and environmental signals in apical hook development have been illustrated. However, key regulators downstream of canonical hormone signaling have rarely been identified via classical genetics screening, possibly due to genetic redundancy and/or lethal mutation. Chemical genetics that utilize small molecules to perturb and elucidate biological processes could provide a complementary strategy to overcome the limitations in classical genetics. In this review, we summarize current progress in hormonal regulation of the apical hook, and previously reported chemical tools that could assist the understanding of this complex developmental process. We also provide insight into novel strategies for chemical screening and target identification, which could possibly lead to discoveries of new regulatory components in apical hook development, or unidentified signaling crosstalk that is overlooked by classical genetics screening.

Testing the polar auxin transport model with a selective plasma membrane H+ -ATPase inhibitor

Auxin is unique among plant hormones in that its function requires polarized transport across plant cells. A chemiosmotic model was proposed to explain how polar auxin transport is derived by the H⁺ gradient across the plasma membrane (PM) established by PM H⁺ -adenosine triphosphatases (ATPases). However, a classical genetic approach by mutations in PM H⁺ -ATPase members did not result in the ablation of polar auxin distribution, possibly due to functional redundancy in this gene family. To confirm the crucial role of PM H⁺ -ATPases in the polar auxin transport model, we employed a chemical genetic approach. Through a chemical screen, we identified protonstatin-1 (PS-1), a selective small-molecule inhibitor of PM H⁺ -ATPase activity that inhibits auxin transport. Assays with transgenic plants and yeast strains showed that the activity of PM H⁺ -ATPases affects auxin uptake as well as acropetal and basipetal polar auxin transport. We propose that PS-1 can be used as a tool to interrogate the function of PM H⁺ -ATPases. Our results support the chemiosmotic model in which PM H⁺ -ATPase itself plays a fundamental role in polar auxin transport.

Chemical Biology in Auxin Research

Molecular genetic and structural studies have revealed the mechanisms of fundamental components of key auxin regulatory pathways consisting of auxin biosynthesis, transport, and signaling. Chemical biology methods applied in auxin research have been greatly expanded through the understanding of auxin regulatory pathways. Many small-molecule modulators of auxin metabolism, transport, and signaling have been generated on the basis of the outcomes of genetic and structural studies on auxin regulatory pathways. These chemical modulators are now widely used as essential tools for dissecting auxin biology in diverse plants. This review covers the structures, primary targets, modes of action, and applications of chemical tools in auxin biosynthesis, transport, and signaling.

Cell kinetics of auxin transport and activity in Arabidopsis root growth and skewing

Auxin is a key regulator of plant growth and development. Local auxin biosynthesis and intercellular transport generates regional gradients in the root that are instructive for processes such as specification of developmental zones that maintain root growth and tropic responses. Here we present a toolbox to study auxin-mediated root development that features: (i) the ability to control auxin synthesis with high spatio-temporal resolution and (ii) single-cell nucleus tracking and morphokinetic analysis infrastructure. Integration of these two features enables cutting-edge analysis of root development at single-cell resolution based on morphokinetic parameters under normal growth conditions and during cell-type-specific induction of auxin biosynthesis. We show directional auxin flow in the root and refine the contributions of key players in this process. In addition, we determine the quantitative kinetics of Arabidopsis root meristem skewing, which depends on local auxin gradients but does not require PIN2 and AUX1 auxin transporter activities. Beyond the mechanistic insights into root development, the tools developed here will enable biologists to study kinetics and morphology of various critical processes at the single cell-level in whole organisms.

Indole-3-Acetic Acid Is Synthesized by the Endophyte Cyanodermella asteris via a Tryptophan-Dependent and -Independent Way and Mediates the Interaction with a Non-Host Plant

The plant hormone indole-3-acetic acid (IAA) is one of the main signals playing a role in the communication between host and endophytes. Endophytes can synthesize IAA de novo to influence the IAA homeostasis in plants. Although much is known about IAA biosynthesis in microorganisms, there is still less known about the pathway by which IAA is synthesized in fungal endophytes. The aim of this study is to examine a possible IAA biosynthesis pathway in Cyanodermella asteris. In vitro cultures of C. asteris were incubated with the IAA precursors tryptophan (Trp) and indole, as well as possible intermediates, and they were additionally treated with IAA biosynthesis inhibitors (2-mercaptobenzimidazole and yucasin DF) to elucidate possible IAA biosynthesis pathways. It was shown that (a) C. asteris synthesized IAA without adding precursors; (b) indole-3-acetonitrile (IAN), indole-3-acetamide (IAM), and indole-3-acetaldehyde (IAD) increased IAA biosynthesis; and (c) C. asteris synthesized IAA also by a Trp-independent pathway. Together with the genome information of C. asteris, the possible IAA biosynthesis pathways found can improve the understanding of IAA biosynthesis in fungal endophytes. The uptake of fungal IAA into Arabidopsis thaliana is necessary for the induction of lateral roots and other fungus-related growth phenotypes, since the application of the influx inhibitor 2-naphthoxyacetic acid (NOA) but not the efflux inhibitor N-1-naphtylphthalamic acid (NPA) were altering these parameters. In addition, the root phenotype of the mutation in an influx carrier, aux1, was partially rescued by C. asteris.

The TOR-Auxin Connection Upstream of Root Hair Growth

Plant growth and productivity are orchestrated by a network of signaling cascades involved in balancing responses to perceived environmental changes with resource availability. Vascular plants are divided into the shoot, an aboveground organ where sugar is synthesized, and the underground located root. Continuous growth requires the generation of energy in the form of carbohydrates in the leaves upon photosynthesis and uptake of nutrients and water through root hairs. Root hair outgrowth depends on the overall condition of the plant and its energy level must be high enough to maintain root growth. TARGET OF RAPAMYCIN (TOR)-mediated signaling cascades serve as a hub to evaluate which resources are needed to respond to external stimuli and which are available to maintain proper plant adaptation. Root hair growth further requires appropriate distribution of the phytohormone auxin, which primes root hair cell fate and triggers root hair elongation. Auxin is transported in an active, directed manner by a plasma membrane located carrier. The auxin efflux carrier PIN-FORMED 2 is necessary to transport auxin to root hair cells, followed by subcellular rearrangements involved in root hair outgrowth. This review presents an overview of events upstream and downstream of PIN2 action, which are involved in root hair growth control.

Auxin is required for the long coleoptile trait in japonica rice under submergence

Rice coleoptile elongation under submergence guarantees fast seedling establishment in the field. We investigated the role of auxin in influencing the capacity of rice to produce a long coleoptile under water. In order to explore the complexity of auxin's role in coleoptile elongation, we used gene expression analysis, confocal microscopy of an auxin-responsive fluorescent reporter, gas chromatography coupled to tandem mass spectrometry (GC-MS/MS), and T-DNA insertional mutants of an auxin transport protein. We show that a higher auxin availability in the coleoptile correlates with the final coleoptile length under submergence. We also identified the auxin influx carrier AUX1 as a component influencing this trait under submergence. The coleoptile tip is involved in the final length of rice varieties harbouring a long coleoptile. Our experimental results indicate that auxin biosynthesis and transport underlies the differential elongation between short and long coleoptile-harbouring japonica rice varieties.

Rice plants respond to ammonium stress by adopting a helical root growth pattern

High levels of ammonium nutrition reduce plant growth and different plant species have developed distinct strategies to maximize ammonium acquisition while alleviating ammonium toxicity through modulating root growth. To date, the mechanisms underlying plant tolerance or sensitivity towards ammonium remain unclear. Rice (Oryza sativa) uses ammonium as its main N source. Here we show that ammonium supply restricts rice root elongation and induces a helical growth pattern, which is attributed to root acidification resulting from ammonium uptake. Ammonium-induced low pH triggers the asymmetric distribution of auxin in rice root tips through changes in auxin signaling, thereby inducing a helical growth response. Blocking auxin signaling completely inhibited this root response. In contrast, this root response is not activated in ammonium-treated Arabidopsis. Acidification of Arabidopsis roots leads to the protonation of indole-3-acetic acid and dampening of the intracellular auxin signaling levels that are required for maintaining root growth. Our study suggests a different mode of action by ammonium on the root pattern and auxin response machinery in rice versus Arabidopsis, and the rice-specific helical root response towards ammonium is an expression of the ability of rice to moderate auxin signaling and root growth to utilize ammonium while confronting acidic stress.

An auxin transport network underlies xylem bridge formation between the hemi-parasitic plant Phtheirospermum japonicum and host Arabidopsis

Parasitic plants form vascular connections with host plants for efficient material transport. The haustorium is the responsible organ for host invasion and subsequent vascular connection. After invasion of host tissues, vascular meristem-like cells emerge in the central region of the haustorium, differentiate into tracheary elements and establish a connection, known as a xylem bridge, between parasite and host xylem systems. Despite the importance of this parasitic connection, the regulatory mechanisms of xylem bridge formation are unknown. Here, we show the role of auxin and auxin transporters during the process of xylem bridge formation using an Orobanchaceae hemiparasitic plant, Phtheirospermum japonicum The auxin response marker DR5 has a similar expression pattern to tracheary element differentiation genes in haustoria. Auxin transport inhibitors alter tracheary element differentiation in haustoria, but biosynthesis inhibitors do not, demonstrating the importance of auxin transport during xylem bridge formation. The expression patterns and subcellular localization of PIN family auxin efflux carriers and AUX1/LAX influx carriers correlate with DR5 expression patterns. The cooperative action of auxin transporters is therefore responsible for controlling xylem vessel connections between parasite and host.

Analysis of differentially expressed genes and pathways associated with male sterility lines in watermelon via bulked segregant RNA-seq

Genic male sterility (GMS) is a common and important trait, which is widely used for the production of hybrid seeds. However, the molecular mechanism of GMS in watermelon remains poorly understood. In this study, we comparatively analyzed the transcriptome profiles of sterile and fertile floral buds using the bulked segregant analysis (BSA) and transcriptome sequencing (RNA-seq). A total of 2507 differentially expressed genes (DEGs) including 593 up-regulated and 1914 down-regulated, were identified to be related to male sterility in watermelon line Se18. Gene ontology (GO) analysis showed that 57 GO terms were significantly enriched, while Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis revealed plant hormone signal transduction, glycolysis/gluconeogenesis, starch and sucrose metabolism, plant-pathogen interaction, phenylpropanoid biosynthesis pathways were obviously enriched. Furthermore, the efficiency of the RNA-seq analysis was validated by quantitative real-time PCR (qRT-PCR). Among the DEGs, some valuable candidate genes involved in pollen development were identified, such as gene Cla000029, a bHLH transcription factor and homologous to MS1 in Arabidopsis. Moreover, other DEGs including MYB gene Cla012590 (MYB26), Cla017100 (MYB21), etc., also provide useful information for further understanding the function of key genes involved in pollen development. This study provides new insights into the global network of male sterility in watermelon.

Essential structural features of (2Z,4E)-5-phenylpenta-2,4-dienoic acid for inhibition of root gravitropism

Previously, we found (2Z,4E)-5-phenylpenta-2,4-dienoic acid (ku-76) to be a selective inhibitor of root gravitropic bending of lettuce radicles at 5 μM, with no concomitant growth inhibition. Here, we describe a structure-activity relationship study of ku-76 to determine the essential structural features for the inhibitory activity. A series of ku-76 analogues was synthesized and the key features of ku-76 that are necessary for inhibition of lettuce root gravitropic bending were determined. The (2E,4E)-, (2Z,4Z)- (2E,4Z)- analogues were inactive, and 4,5-saturated and 4,5-alkynyl analogues also did not show inhibitory activity, demonstrating the importance of the (2Z,4E) diene unit. The aromatic ring was also crucial and could not be replaced with an alkyl chain. Derivatives in which the carboxylic acid was replaced with amides, alcohols, or esters were much less potent. These results suggest that the (2Z,4E)-diene, the carboxylic acid moiety, and the aromatic ring are essential for potent inhibitory activity against gravitropic bending.

New-Generation Chemical Tools for the Manipulation of Auxin Biosynthesis, Action, and Transport

Auxin is the master regulator for almost every aspect of plant growth and development. Small molecule inhibitors, fluorescently labeled molecule, and hormone analogs on auxin biosynthesis, transport, and signaling, so-called auxin chemical tools, have been widely utilized to dissect physiological functions of gene families in auxin biosynthesis, transport, and signaling. Auxin chemical tools can manipulate specific auxin-regulated events at any developmental stage. Chemical tools can modulate the function of orthologs of target proteins and also can overcome the redundant function of large family gene controlling auxin-regulated response. On the other hand, chemical tool might induce the off-target effects at high concentration, if the chemical tool shows insufficient specificity on target proteins. This chapter describes a brief overview and practical application of the auxin chemical tools.

Arabidopsis thaliana plants lacking the ARP2/3 complex show defects in cell wall assembly and auxin distribution

BACKGROUND AND AIM

The cytoskeleton plays an important role in the synthesis of plant cell walls. Both microtubules and actin cytoskeleton are known to be involved in the morphogenesis of plant cells through their role in cell wall building. The role of ARP2/3-nucleated actin cytoskeleton in the morphogenesis of cotyledon pavement cells has been described before. Seedlings of Arabidopsis mutants lacking a functional ARP2/3 complex display specific cell wall-associated defects.

METHODS

In three independent Arabidopsis mutant lines lacking subunits of the ARP2/3 complex, phenotypes associated with the loss of the complex were analysed throughout plant development. Organ size and anatomy, cell wall composition, and auxin distribution were investigated.

KEY RESULTS

ARP2/3-related phenotype is associated with changes in cell wall composition, and the phenotype is manifested especially in mature tissues. Cell walls of mature plants contain less cellulose and a higher amount of homogalacturonan, and display changes in cell wall lignification. Vascular bundles of mutant inflorescence stems show a changed pattern of AUX1-YFP expression. Plants lacking a functional ARP2/3 complex have decreased basipetal auxin transport.

CONCLUSIONS

The results suggest that the ARP2/3 complex has a morphogenetic function related to cell wall synthesis and auxin transport.

Advances in Understanding the Mechanism of Action of the Auxin Permease AUX1

In over 40 years of research on the cellular uptake of auxin it is somewhat chastening that we have elaborated so little on the original kinetic descriptions of auxin uptake by plant cells made by Rubery and Sheldrake in 1974. Every aspect of that seminal work has been investigated in detail, and the uptake activity they measured is now known to be attributed to the AUX1/LAX family of permeases. Recent pharmacological studies have defined the substrate specificity of AUX1, biochemical studies have evaluated its permeability to auxin in plant cell membranes, and rigourous kinetic studies have confirmed the affinity of AUX1 for IAA and synthetic auxins. Advances in genome sequencing have provided a rich resource for informatic analysis of the ancestry of AUX1 and the LAX proteins and, along with models of topology, suggest mechanistic links to families of eukaryotic proton co-transporters for which crystal structures have been presented. The insights gained from all the accumulated research reflect the brilliance of Rubery and Sheldrake’s early work, but recent biochemical analyses are starting to advance further our understanding of this vitally important family of auxin transport proteins.

Manipulation and Sensing of Auxin Metabolism, Transport and Signaling

The plant hormone auxin is involved in virtually every aspect of plant growth and development. A chemical genetic approach has greatly contributed to the identification of important genes in auxin biosynthesis, transport and signaling. Molecular genetic technologies and structural information for auxin regulatory components have accelerated the identification and characterization of many novel small molecule modulators in auxin biology. These modulators have been widely utilized to dissect auxin responses. Here we provide an overview of the structure, primary target, in planta activity and application of small molecule modulators in auxin biology.

Auxin molecular field maps define AUX1 selectivity: many auxin herbicides are not substrates

Developmental responses to auxin are regulated by facilitated uptake and efflux, but detailed molecular understanding of the carrier proteins is incomplete. We have used pharmacological tools to explore the chemical space that defines substrate preferences for the auxin uptake carrier AUX1. Total and partial loss-of-function aux1 mutants were assessed against wild-type for dose-dependent resistance to a range of auxins and analogues. We then developed an auxin accumulation assay with associated mathematical modelling to enumerate accurate IC50 values for a small library of auxin analogues. The structure activity relationship data were analysed using molecular field analyses to create a pharmacophoric atlas of AUX1 substrates. The uptake carrier exhibits a very high level of selectivity towards small substrates including the natural indole-3-acetic acid, and the synthetic auxin 2,4-dichlorophenoxyacetic acid. No AUX1 activity was observed for herbicides based on benzoic acid (dicamba), pyridinyloxyacetic acid (triclopyr) or the 6-arylpicolinates (halauxifen), and very low affinity was found for picolinic acid-based auxins (picloram) and quinolinecarboxylic acids (quinclorac). The atlas demonstrates why some widely used auxin herbicides are not, or are very poor substrates. We list molecular descriptors for AUX1 substrates and discuss our findings in terms of herbicide resistance management.

Enhanced Conjugation of Auxin by GH3 Enzymes Leads to Poor Adventitious Rooting in Carnation Stem Cuttings

Commercial carnation (Dianthus caryophyllus) cultivars are vegetatively propagated from axillary stem cuttings through adventitious rooting; a process which is affected by complex interactions between nutrient and hormone levels and is strongly genotype-dependent. To deepen our understanding of the regulatory events controlling this process, we performed a comparative study of adventitious root (AR) formation in two carnation cultivars with contrasting rooting performance, "2101-02 MFR" and "2003 R 8", as well as in the reference cultivar "Master". We provided molecular evidence that localized auxin response in the stem cutting base was required for efficient adventitious rooting in this species, which was dynamically established by polar auxin transport from the leaves. In turn, the bad-rooting behavior of the "2003 R 8" cultivar was correlated with enhanced synthesis of indole-3-acetic acid conjugated to aspartic acid by GH3 proteins in the stem cutting base. Treatment of stem cuttings with a competitive inhibitor of GH3 enzyme activity significantly improved rooting of "2003 R 8". Our results allowed us to propose a working model where endogenous auxin homeostasis regulated by GH3 proteins accounts for the cultivar dependency of AR formation in carnation stem cuttings.

Auxin transport in the evolution of branching forms

Contents 545 I. 545 II. 546 III. 546 IV. 548 V. 548 VI. 549 VII. 549 Acknowledgements 549 References 549 SUMMARY: Branching is one of the most striking aspects of land plant architecture, affecting resource acquisition and yield. Polar auxin transport by PIN proteins is a primary determinant of flowering plant branching patterns regulating both branch initiation and branch outgrowth. Several lines of experimental evidence suggest that PIN-mediated polar auxin transport is a conserved regulator of branching in vascular plant sporophytes. However, the mechanisms of branching and auxin transport and relationships between the two are not well known outside the flowering plants, and the paradigm for PIN-regulated branching in flowering plants does not fit bryophyte gametophytes. The evidence reviewed here suggests that divergent auxin transport routes contributed to the diversification of branching forms in distinct land plant lineages.

Auxin transport and response requirements for root hydrotropism differ between plant species

The direction of auxin transport changes in gravistimulated roots, causing auxin accumulation in the lower side of horizontally reoriented roots. This study found that auxin was similarly involved in hydrotropism and gravitropism in rice and pea roots, but hydrotropism in Lotus japonicus roots was independent of both auxin transport and response. Application of either auxin transport inhibitors or an auxin response inhibitor decreased both hydrotropism and gravitropism in rice roots, and reduced hydrotropism in pea roots. However, Lotus roots treated with these inhibitors showed reduced gravitropism but an unaltered or an enhanced hydrotropic response. Inhibiting auxin biosynthesis substantially reduced both tropisms in rice and Lotus roots. Removing the final 0.2 mm (including the root cap) from the root tip inhibited gravitropism but not hydrotropism in rice seedling roots. These results suggested that modes of auxin involvement in hydrotropism differed between plant species. In rice roots, although auxin transport and responses were required for both gravitropism and hydrotropism, the root cap was involved in the auxin regulation of gravitropism but not hydrotropism. Hydrotropism in Lotus roots, however, may be regulated by a novel mechanism that is independent of both auxin transport and the TIR1/AFBs auxin response pathway.

Regulation of seedling growth by ethylene and the ethylene-auxin crosstalk

This review highlights that the auxin gradient, established by local auxin biosynthesis and transport, can be controlled by ethylene, and steers seedling growth. A better understanding of the mechanisms in Arabidopsis will increase potential applications in crop species. In dark-grown Arabidopsis seedlings, exogenous ethylene treatment triggers an exaggeration of the apical hook, the inhibition of both hypocotyl and root elongation, and radial swelling of the hypocotyl. These features are predominantly based on the differential cell elongation in different cells/tissues mediated by an auxin gradient. Interestingly, the physiological responses regulated by ethylene and auxin crosstalk can be either additive or synergistic, as in primary root and root hair elongation, or antagonistic, as in hypocotyl elongation. This review focuses on the crosstalk of these two hormones at the seedling stage. Before illustrating the crosstalk, ethylene and auxin biosynthesis, metabolism, transport and signaling are briefly discussed.

Auxin Regulates Cotton Fiber Initiation via GhPIN-Mediated Auxin Transport

Cotton fibers are seed trichomes that make cotton unique compared with other plants. At anthesis, IAA, a major auxin in plants, accumulates in the fiber cell to promote cell initiation. However, many important aspects of this process are not clear. Here, auxin distribution patterns indicated by auxin-dependent DR5::GUS (β-glucuronidase) expression in cotton ovules were studied during fiber cell differentiation and cell initiation [-2 to 2 DPA (days post-anthesis)]. The nucellus and fiber cell were two major sites where auxin accumulates. The accumulation in the nucellus started from -1 DPA, and that in fiber cells from 0 DPA. Immunolocalization analysis further suggests that the IAA accumulation in fiber initials began before flower opening. Furthermore, we demonstrate that accumulated IAA in fiber initials was mainly from efflux transport and not from in situ synthesis. Eleven auxin efflux carrier (GhPIN) genes were identified, and their expression during ovule and fiber development was investigated. Ovule-specific suppression of multiple GhPIN genes in transgenic cotton inhibited both fiber initiation and elongation. In 0 DPA ovules, GhPIN3a, unlike other GhPIN genes, showed additional localization of the transcript in the outer integument. Collectively, these results demonstrate the important role of GhPIN-mediated auxin transport in fiber-specific auxin accumulation for fiber initiation.

Methodological Advances in Auxin and Cytokinin Biology

The history of auxin and cytokinin biology including the initial discoveries by father-son duo Charles Darwin and Francis Darwin (1880), and Gottlieb Haberlandt (1919) is a beautiful demonstration of unceasing continuity of research. Novel findings are integrated into existing hypotheses and models and deepen our understanding of biological principles. At the same time new questions are triggered and hand to hand with this new methodologies are developed to address these new challenges.

Inhibitors of plant hormone transport

Here we present an overview of what is known about endogenous plant compounds that act as inhibitors of hormonal transport processes in plants, about their identity and mechanism of action. We have also summarized commonly and less commonly used compounds of non-plant origin and synthetic drugs that show at least partial 'specificity' to transport or transporters of particular phytohormones. Our main attention is focused on the inhibitors of auxin transport. The urgent need to understand precisely the molecular mechanism of action of these inhibitors is highlighted.

Integrated mRNA and miRNA transcriptome reveal a cross-talk between developing response and hormone signaling for the seed kernels of Siberian apricot

Recently, our transcriptomic analysis has identified some functional genes responsible for oil biosynthesis in developing SASK, yet miRNA-mediated regulation for SASK development and oil accumulation is poorly understood. Here, 3 representative periods of 10, 30 and 60 DAF were selected for sRNA sequencing based on the dynamic patterns of growth tendency and oil content of developing SASK. By miRNA transcriptomic analysis, we characterized 296 known and 44 novel miRNAs in developing SASK, among which 36 known and 6 novel miRNAs respond specifically to developing SASK. Importantly, we performed an integrated analysis of mRNA and miRNA transcriptome as well as qRT-PCR detection to identify some key miRNAs and their targets (miR156-SPL, miR160-ARF18, miR164-NAC1, miR171h-SCL6, miR172-AP2, miR395-AUX22B, miR530-P2C37, miR393h-TIR1/AFB2 and psi-miRn5-SnRK2A) potentially involved in developing response and hormone signaling of SASK. Our results provide new insights into the important regulatory function of cross-talk between development response and hormone signaling for SASK oil accumulation.

Genome-wide survey and expression analysis of the amino acid transporter superfamily in potato (Solanum tuberosum L.)

Amino acid transporters (AATs) are integral membrane proteins responsible for the transmembrane transport of amino acids and play important roles in various physiological processes of plants. However, there has not yet been a genome-wide overview of the StAAT gene family to date and only StAAP1 has been previously studied in potato. In this paper, a total of 72 StAATs were identified using a series of bioinformatics searches and classified into 12 subfamilies based on their phylogenetic relationship with known Arabidopsis and rice AATs. Chromosomal localization revealed their distribution on all 12 chromosomes. Nearly one-third of StAAT genes (23 of 72) were derived from gene duplication, among which tandem duplication made the greatest contribution to the expansion of the StAAT family. Motif analysis showed that the same subfamily had similar conserved motifs in both numbers and varieties. Moreover, high-throughput sequencing data was used to analyze the expression patterns of StAAT genes and was verified by quantitative real-time RT-PCR. The expression of StAAT genes exhibited both abundant and tissue-specific expression patterns, which might be connected to their functional roles in long- and short-distance transport. This study provided a comprehensive survey of the StAAT gene family, and could serve as a theoretical foundation for the further functional identification and utilization of family members.

Inhibition of auxin transport and auxin signaling and treatment with far red light induces root coiling in the phospholipase-A mutant ppla-I-1. Significance for surface penetration?

When grown on a non-penetretable at a surface angle of 45°, Arabidopsis roots form wave-like structures and, in wild type rarely, but in certain mutants the tip root even may form circles. These circles are called coils. The formation of coils depends on the complex interaction of circumnutation, gravitropism and negative thigmotropism where - at least - gravitropism is intimately linked to auxin transport and signaling. The knockout mutant of patatin-related phospholipase-AI-1 (pplaI-1) is an auxin-signaling mutant which forms moderately increased numbers of coils on tilted agar plates. We tested the effects of the auxin efflux transport inhibitor NPA (1-naphthylphtalamic acid) and of the influx transport inhibitor 1-NOA (1-naphthoxyacetic acid) which both further increased root coil formation. The pPLAI-1 inhibitors HELSS (haloenol lactone suicide substrate=E-6-(bromomethylene)tetrahydro-3-(1-naphthalenyl)-2H-pyran-2-one) and ETYA (eicosatetraynoic acid) which are auxin signaling inhibitors also increased coil formation. In addition, far red light treatment increased coil formation. The results point out that a disturbance of auxin transport and signaling is one potential cause for root coils. As we show that the mutant pplaI-1 penetrates horizontal agar plates better than wild type plants root movements may help penetrating the soil.

Auxin and chloroplast movements

Auxin is involved in a wide spectrum of physiological processes in plants, including responses controlled by the blue light photoreceptors phototropins: phototropic bending and stomatal movement. However, the role of auxin in phototropin-mediated chloroplast movements has never been studied. To address this question we searched for potential interactions between auxin and the chloroplast movement signaling pathway using different experimental approaches and two model plants, Arabidopsis thaliana and Nicotiana tabacum. We observed that the disturbance of auxin homeostasis by shoot decapitation caused a decrease in chloroplast movement parameters, which could be rescued by exogenous auxin application. In several cases, the impairment of polar auxin transport, by chemical inhibitors or in auxin carrier mutants, had a similar negative effect on chloroplast movements. This inhibition was not correlated with changes in auxin levels. Chloroplast relocations were also affected by the antiauxin p-chlorophenoxyisobutyric acid and mutations in genes encoding some of the elements of the SCF(TIR1)-Aux/IAA auxin receptor complex. The observed changes in chloroplast movement parameters are not prominent, which points to a modulatory role of auxin in this process. Taken together, the obtained results suggest that auxin acts indirectly to regulate chloroplast movements, presumably by regulating gene expression via the SCF(TIR1)-Aux/IAA-ARF pathway. Auxin does not seem to be involved in controlling the expression of phototropins.

OsAUX1 controls lateral root initiation in rice (Oryza sativa L.)

Polar auxin transport, mediated by influx and efflux transporters, controls many aspects of plant growth and development. The auxin influx carriers in Arabidopsis have been shown to control lateral root development and gravitropism, but little is known about these proteins in rice. This paper reports on the functional characterization of OsAUX1. Three OsAUX1 T-DNA insertion mutants and RNAi knockdown transgenic plants reduced lateral root initiation compared with wild-type (WT) plants. OsAUX1 overexpression plants exhibited increased lateral root initiation and OsAUX1 was highly expressed in lateral roots and lateral root primordia. Similarly, the auxin reporter, DR5-GUS, was expressed at lower levels in osaux1 than in the WT plants, which indicated that the auxin levels in the mutant roots had decreased. Exogenous 1-naphthylacetic acid (NAA) treatment rescued the defective phenotype in osaux1-1 plants, whereas indole-3-acetic acid (IAA) and 2,4-D could not, which suggested that OsAUX1 was a putative auxin influx carrier. The transcript levels of several auxin signalling genes and cell cycle genes significantly declined in osaux1, hinting that the regulatory role of OsAUX1 may be mediated by auxin signalling and cell cycle genes. Overall, our results indicated that OsAUX1 was involved in polar auxin transport and functioned to control auxin-mediated lateral root initiation in rice.

Identification and Analysis of Medicago truncatula Auxin Transporter Gene Families Uncover their Roles in Responses to Sinorhizobium meliloti Infection

Auxin transport plays a pivotal role in the interaction between legume species and nitrogen-fixing bacteria to form symbioses. Auxin influx carriers auxin resistant 1/like aux 1 (AUX/LAX), efflux carriers pin-formed (PIN) and efflux/conditional P-glycoprotein (PGP/ABCB) are three major protein families participating in auxin polar transport. We used the latest Medicago truncatula genome sequence to characterize and analyze the M. truncatula LAX (MtLAX), M. truncatula PIN (MtPIN) and M. truncatula ABCB (MtABCB) families. Transient expression experiments indicated that three representative auxin transporters (MtLAX3, MtPIN7 and MtABCB1) showed cell plasma membrane localizations. The expression of most MtLAX, MtPIN and MtABCB genes was up-regulated in the roots and was down-regulated in the shoots by Sinorhizobium meliloti infection in the wild type (WT). However, the expression of these genes was down-regulated in both the roots and shoots of an infection-resistant mutant, dmi3. The different expression patterns between the WT and the mutant roots indicated that auxin relocation may be involved in rhizobial infection responses. Furthermore, IAA contents were significantly up-regulated in the shoots and down-regulated in the roots after Sinorhizobium meliloti infection in the WT. Inoculation of roots with rhizobia may reduce the auxin loading from shoots to roots by inhibiting the expression of most auxin transporter genes. However, the rate of change of gene expression and IAA contents in the dmi3 mutant were obviously lower than in the WT. The identification and expression analysis of auxin transporter genes helps us to understand the roles of auxin in the regulation of nodule formation in M. truncatula.

OsARF16, a transcription factor regulating auxin redistribution, is required for iron deficiency response in rice (Oryza sativa L.)

Plant response to iron deficiency is the most important feature for survival in Fe-limited soils. Several phytohormones, including auxin, are involved in iron uptake and homeostasis. However, the mechanisms behind how auxin participates in the iron deficiency response in rice are largely unknown. We found that OsARF16 was involved in the iron deficiency response and the induction of iron deficiency response genes. Most Fe-deficient symptoms could be partially restored in the osarf16 mutant, including dwarfism, photosynthesis decline, a reduction in iron content and root system architecture (RSA) regulation. OsARF16 expression was induced in the roots and shoots by Fe deprivation. Restoration of the phenotype could also be mimicked by 1-NOA, an auxin influx inhibitor. Furthermore, the qRT-PCR data indicated that the induction of Fe-deficiency response genes by iron deficiency was more compromised in the osarf16 mutant than in Nipponbare. In conclusion, osarf16, an auxin insensitive mutant, was involved in iron deficiency response in rice. Our results reveal a new biological function for OsARF16 and provide important information on how ARF-medicated auxin signaling affects iron signaling and the iron deficiency response. This work may help us to improve production or increased Fe nutrition of rice to iron deficiency by regulating auxin signaling.

Plant embryogenesis requires AUX/LAX-mediated auxin influx

The plant hormone auxin and its directional transport are known to play a crucial role in defining the embryonic axis and subsequent development of the body plan. Although the role of PIN auxin efflux transporters has been clearly assigned during embryonic shoot and root specification, the role of the auxin influx carriers AUX1 and LIKE-AUX1 (LAX) proteins is not well established. Here, we used chemical and genetic tools on Brassica napus microspore-derived embryos and Arabidopsis thaliana zygotic embryos, and demonstrate that AUX1, LAX1 and LAX2 are required for both shoot and root pole formation, in concert with PIN efflux carriers. Furthermore, we uncovered a positive-feedback loop between MONOPTEROS (ARF5)-dependent auxin signalling and auxin transport. This MONOPTEROS-dependent transcriptional regulation of auxin influx (AUX1, LAX1 and LAX2) and auxin efflux (PIN1 and PIN4) carriers by MONOPTEROS helps to maintain proper auxin transport to the root tip. These results indicate that auxin-dependent cell specification during embryo development requires balanced auxin transport involving both influx and efflux mechanisms, and that this transport is maintained by a positive transcriptional feedback on auxin signalling.

Genome-wide identification and expression analysis of auxin response factor gene family in Medicago truncatula

Auxin response factors (ARFs) bind specifically to auxin response elements (AuxREs) in the promoters of down-stream target genes and play roles in plant responses to diverse environmental factors. Using the latest updated Medicago truncatula reference genome sequence, a comprehensive characterization and analysis of 24 MtARF (M. truncatula ARF) genes were performed. To uncover the basic information and functions of MtARF genes during symbiosis, we analyzed the expression patterns of MtARF genes during the early phase of Sinorhizobium meliloti infection. The systematic analysis indicated that changes in MtARF gene expression occur during these early stages of infection, suggesting a functional role in symbiosis. Furthermore, the roles of MtARF-mediated auxin signaling in symbiosis were tested in the infection resistant mutant (dmi3). The expression responses of MtARFs to S. meliloti infection were attenuated in the mutant compared to wild-type A17. In summary, our results show that changes in MtARF gene expression occur during the response to S. meliloti infection, suggesting that members of this family may have important roles in the symbiotic interaction.

Gravitropism and Lateral Root Emergence are Dependent on the Trans-Golgi Network Protein TNO1

The trans-Golgi network (TGN) is a dynamic organelle that functions as a relay station for receiving endocytosed cargo, directing secretory cargo, and trafficking to the vacuole. TGN-localized SYP41-interacting protein (TNO1) is a large, TGN-localized, coiled-coil protein that associates with the membrane fusion protein SYP41, a target SNARE, and is required for efficient protein trafficking to the vacuole. Here, we show that a tno1 mutant has auxin transport-related defects. Mutant roots have delayed lateral root emergence, decreased gravitropic bending of plant organs and increased sensitivity to the auxin analog 2,4-dichlorophenoxyacetic acid and the natural auxin 3-indoleacetic acid. Auxin asymmetry at the tips of elongating stage II lateral roots was reduced in the tno1 mutant, suggesting a role for TNO1 in cellular auxin transport during lateral root emergence. During gravistimulation, tno1 roots exhibited delayed auxin transport from the columella to the basal epidermal cells. Endocytosis to the TGN was unaffected in the mutant, indicating that bulk endocytic defects are not responsible for the observed phenotypes. Together these studies demonstrate a role for TNO1 in mediating auxin responses during root development and gravistimulation, potentially through trafficking of auxin transport proteins.

Genome-wide identification and expression profiling analysis of the Aux/IAA gene family in Medicago truncatula during the early phase of Sinorhizobium meliloti infection

Background

Auxin/indoleacetic acid (Aux/IAA) genes, coding a family of short-lived nuclear proteins, play key roles in wide variety of plant developmental processes, including root system regulation and responses to environmental stimulus. However, how they function in auxin signaling pathway and symbiosis with rhizobial in Medicago truncatula are largely unknown. The present study aims at gaining deeper insight on distinctive expression and function features of Aux/IAA family genes in Medicago truncatula during nodule formation.

Principal Findings

Using the latest updated draft of the full Medicago truncatula genome, a comprehensive identification and analysis of IAA genes were performed. The data indicated that MtIAA family genes are distributed in all the M. truncatula chromosomes except chromosome 6. Most of MtIAA genes are responsive to exogenous auxin and express in tissues-specific manner. To understand the biological functions of MtIAA genes involved in nodule formation, quantitative real-time polymerase chain reaction (qRT-PCR) was used to test the expression profiling of MtIAA genes during the early phase of Sinorhizobium meliloti (S. meliloti) infection. The expression patterns of most MtIAA genes were down-regulated in roots and up-regulated in shoots by S. meliloti infection. The differences in expression responses between roots and shoots caused by S. meliloti infection were alleviated by 1-NOA application.

Conclusion

The genome-wide identification, evolution and expression pattern analysis of MtIAA genes were performed in this study. The data helps us to understand the roles of MtIAA-mediated auxin signaling in nodule formation during the early phase of S. meliloti infection.

The role of auxin transporters in monocots development

Auxin is a key regulator of plant growth and development, orchestrating cell division, elongation and differentiation, embryonic development, root and stem tropisms, apical dominance, and transition to flowering. Auxin levels are higher in undifferentiated cell populations and decrease following organ initiation and tissue differentiation. This differential auxin distribution is achieved by polar auxin transport (PAT) mediated by auxin transport proteins. There are four major families of auxin transporters in plants: PIN-FORMED (PIN), ATP-binding cassette family B (ABCB), AUXIN1/LIKE-AUX1s, and PIN-LIKES. These families include proteins located at the plasma membrane or at the endoplasmic reticulum (ER), which participate in auxin influx, efflux or both, from the apoplast into the cell or from the cytosol into the ER compartment. Auxin transporters have been largely studied in the dicotyledon model species Arabidopsis, but there is increasing evidence of their role in auxin regulated development in monocotyledon species. In monocots, families of auxin transporters are enlarged and often include duplicated genes and proteins with high sequence similarity. Some of these proteins underwent sub- and neo-functionalization with substantial modification to their structure and expression in organs such as adventitious roots, panicles, tassels, and ears. Most of the present information on monocot auxin transporters function derives from studies conducted in rice, maize, sorghum, and Brachypodium, using pharmacological applications (PAT inhibitors) or down-/up-regulation (over-expression and RNA interference) of candidate genes. Gene expression studies and comparison of predicted protein structures have also increased our knowledge of the role of PAT in monocots. However, knockout mutants and functional characterization of single genes are still scarce and the future availability of such resources will prove crucial to elucidate the role of auxin transporters in monocots development.

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 did 1-NOA. These results indicate that 7-B3 is a potential inhibitor of IAA influx that has almost no effect on IAA efflux or auxin signaling.

Gibberellins inhibit adventitious rooting in hybrid aspen and Arabidopsis by affecting auxin transport

Knowledge of processes involved in adventitious rooting is important to improve both fundamental understanding of plant physiology and the propagation of numerous plants. Hybrid aspen (Populus tremula × tremuloïdes) plants overexpressing a key gibberellin (GA) biosynthesis gene (AtGA20ox1) grow rapidly but have poor rooting efficiency, which restricts their clonal propagation. Therefore, we investigated the molecular basis of adventitious rooting in Populus and the model plant Arabidopsis. The production of adventitious roots (ARs) in tree cuttings is initiated from the basal stem region, and involves the interplay of several endogenous and exogenous factors. The roles of several hormones in this process have been characterized, but the effects of GAs have not been fully investigated. Here, we show that a GA treatment negatively affects the numbers of ARs produced by wild-type hybrid aspen cuttings. Furthermore, both hybrid aspen plants and intact Arabidopsis seedlings overexpressing AtGA20ox1, PttGID1.1 or PttGID1.3 genes (with a 35S promoter) produce few ARs, although ARs develop from the basal stem region of hybrid aspen and the hypocotyl of Arabidopsis. In Arabidopsis, auxin and strigolactones are known to affect AR formation. Our data show that the inhibitory effect of GA treatment on adventitious rooting is not mediated by perturbation of the auxin signalling pathway, or of the strigolactone biosynthetic and signalling pathways. Instead, GAs appear to act by perturbing polar auxin transport, in particular auxin efflux in hybrid aspen, and both efflux and influx in Arabidopsis.

Characterization of transmembrane auxin transport in Arabidopsis suspension-cultured cells

Polar auxin transport is a crucial process for control and coordination of plant development. Studies of auxin transport through plant tissues and organs showed that auxin is transported by a combination of phloem flow and the active, carrier-mediated cell-to-cell transport. Since plant organs and even tissues are too complex for determination of the kinetics of carrier-mediated auxin uptake and efflux on the cellular level, simplified models of cell suspension cultures are often used, and several tobacco cell lines have been established for auxin transport assays. However, there are very few data available on the specificity and kinetics of auxin transport across the plasma membrane for Arabidopsis thaliana suspension-cultured cells. In this report, the characteristics of carrier-mediated uptake (influx) and efflux for the native auxin indole-3-acetic acid and synthetic auxins, naphthalene-1-acetic and 2,4-dichlorophenoxyacetic acids (NAA and 2,4-D, respectively) in A. thaliana ecotype Landsberg erecta suspension-cultured cells (LE line) are provided. By auxin competition assays and inhibitor treatments, we show that, similarly to tobacco cells, uptake carriers have high affinity towards 2,4-D and that NAA is a good tool for studies of auxin efflux in LE cells. In contrast to tobacco cells, metabolic profiling showed that only a small proportion of NAA is metabolized in LE cells. These results show that the LE cell line is a useful experimental system for measurements of kinetics of auxin carriers on the cellular level that is complementary to tobacco cells.

Determination of auxin transport parameters on the cellular level

The accumulation of radioactively labelled compounds in cells is frequently used for the determination of activities of various transport systems located at the plasma membrane, including the system for carrier-mediated transport of plant hormone auxin. The measurements of auxin transport could be performed on the tissue level as well, but for more precise quantitative analysis of activity of individual auxin carriers the model of plant cell cultures represents an invaluable tool. Here, we describe the method for the determination of the activities of auxin influx and efflux carriers in plant cells grown in a suspension using radiolabelled synthetic auxins 2,4-dichlorophenoxyacetic acid (2,4-D) and naphthalene-1-acetic acid (NAA). By making use of specific inhibitors of active auxin influx and efflux, as well as cell lines overexpressing or silencing particular auxin carriers, this method allows the determination of kinetic parameters of auxin flow across the plasma membrane and the activity of those carriers.

Interaction between meristem tissue layers controls phyllotaxis

Phyllotaxis and vein formation are among the most conspicuous patterning processes in plants. The expression and polarization of the auxin efflux carrier PIN1 is the earliest marker for both processes, with mathematical models indicating that PIN1 can respond to auxin gradients and/or auxin flux. Here, we use cell-layer-specific PIN1 knockouts and partial complementation of auxin transport mutants to examine the interaction between phyllotactic patterning, which occurs primarily in the L1 surface layer of the meristem, and midvein specification in the inner tissues. We show that PIN1 expression in the L1 is sufficient for correct organ positioning, as long as the L1-specific influx carriers are present. Thus, differentiation of inner tissues can proceed without PIN1 or any of the known polar transporters. On theoretical grounds, we suggest that canalization of auxin flux between an auxin source and an auxin sink may involve facilitated diffusion rather than polar transport.

Defining the selectivity of processes along the auxin response chain: a study using auxin analogues

The mode of action of auxin is based on its non-uniform distribution within tissues and organs. Despite the wide use of several auxin analogues in research and agriculture, little is known about the specificity of different auxin-related transport and signalling processes towards these compounds. Using seedlings of Arabidopsis thaliana and suspension-cultured cells of Nicotiana tabacum (BY-2), the physiological activity of several auxin analogues was investigated, together with their capacity to induce auxin-dependent gene expression, to inhibit endocytosis and to be transported across the plasma membrane. This study shows that the specificity criteria for different auxin-related processes vary widely. Notably, the special behaviour of some synthetic auxin analogues suggests that they might be useful tools in investigations of the molecular mechanism of auxin action. Thus, due to their differential stimulatory effects on DR5 expression, indole-3-propionic (IPA) and 2,4,5-trichlorophenoxy acetic (2,4,5-T) acids can serve in studies of TRANSPORT INHIBITOR RESPONSE 1/AUXIN SIGNALLING F-BOX (TIR1/AFB)-mediated auxin signalling, and 5-fluoroindole-3-acetic acid (5-F-IAA) can help to discriminate between transcriptional and non-transcriptional pathways of auxin signalling. The results demonstrate that the major determinants for the auxin-like physiological potential of a particular compound are very complex and involve its chemical and metabolic stability, its ability to distribute in tissues in a polar manner and its activity towards auxin signalling machinery.

Ectopic localization of auxin and cytokinin in tobacco seedlings by the plant-oncogenic AK-6b gene of Agrobacterium tumefaciens AKE10

The oncogenic 6b gene of Agrobacterium tumefaciens induces a number of morphological and metabolic alterations in plants. Although molecular functions associated with the 6b genes have been proposed, including auxin transport, sugar transport, transcriptional regulation, and miRNA metabolism, so far an unequivocal conclusion has not been obtained. We investigated the association between auxin accumulation and tumor development of the tobacco seedlings expressing the AK-6b gene under the control of the dexamethasone-inducible promoter. Indole-3-acetic acid (IAA) localization was examined by immunochemical staining with monoclonal antibody against IAA and by histochemical analysis using the IAA-specific induced construct, DR5::GUS (β-glucuronidase). Both procedures indicated that IAA preferentially accumulated in the tumorous protrusions as well as in newly developing vascular bundles in the tumors. Furthermore, true leaves also showed abaxial IAA localization, leading to altered leaves in which the adaxial and abaxial identities were no longer evident. Co-localization of cytokinin and auxin in the abaxial tumors was verified by immunochemical staining with an antibody against cytokinin. Treatment of AK-6b-seedlings with N-1-naphthylphthalamic acid, an inhibitor of polar auxin transport, promoted the morphological severity of phenotypes, whereas 1-naphthoxyacetic acid, a specific auxin influx carrier inhibitor, induced tumor regression on cotyledons and new tumorous proliferations on hypocotyls. Prominent accumulation of both auxin and cytokinin was observed in both regressed and newly developing tumors. We suggest from these results that modulation of auxin/cytokinin localization as a result of AK-6b gene expression is responsible for the tumorous proliferation.

Root-derived auxin contributes to the phosphorus-deficiency-induced cluster-root formation in white lupin (Lupinus albus)

Formation of cluster roots is a typical morphological response to phosphorus (P) deficiency in white lupin (Lupinus albus), but its physiological and molecular mechanisms are still unclear. We investigated the role of auxin in the initiation of cluster roots by distinguishing the sources of auxin, measuring the longitudinal distribution patterns of free indole-3-acetic acid (IAA) along the root and the related gene expressions responsible for polar auxin transport (PAT) in different developmental stages of cluster roots. We found that removal of shoot apex or primary root apex and application of auxin-influx or -efflux transport inhibitors, 3-chloro-4-hydroxyphenylacetic acid, N-1-naphthylphthalamic acid and 2,3,5-triiodobenzoic acid, to the stem did not affect the number of cluster roots and the free-IAA concentration in the roots of P-deficient plants, but when these inhibitors were applied directly to the growth media, the cluster-root formation was greatly suppressed, suggesting the fundamental role of root-derived IAA in cluster-root formation. The concentration of free IAA in the roots was higher in P-deficient plants than in P-adequate ones, and the highest in the lateral-root apex and the lowest in the mature cluster roots. Meanwhile the expression patterns of LaAUX1, LaPIN1 and LaPIN3 transcripts related to PAT was consistent with concentrations of free IAA along the lateral root, indicating the contribution of IAA redistribution in the cluster-root development. We proposed that root-derived IAA plays a direct and important role in the P-deficiency-induced formation of cluster roots.