A short history of auxin-binding proteins

Recent progress in auxin biology

Like animals, plants have evolved into complex organisms. Developmental cohesion between tissues and cells is possible due to signaling molecules (messengers) like hormones. The first hormone discovered in plants was auxin. This phytohormone was first noticed because of its involvement in the response to directional light. Nowadays, auxin has been established as a central key player in the regulation of plant growth and development and in responses to environmental changes. At the cellular level, auxin controls division, elongation, and differentiation as well as the polarity of the cell. Auxin, to integrate so many different signals, needs to be regulated at many different levels. A tight regulation of auxin synthesis, activity, degradation as well as transport has been demonstrated. Another possibility to modulate auxin signaling is to modify the capacity of response of the cells by expressing differentially the signaling components. In this review, we provide an overview of the present knowledge in auxin biology, with emphasis on root development.

The AUX1 LAX family of auxin influx carriers is required for the establishment of embryonic root cell organization in Arabidopsis thaliana

BACKGROUND AND AIMS

The root meristem of the Arabidopsis thaliana mature embryo is a highly organized structure in which individual cell shape and size must be regulated in co-ordination with the surrounding cells. The objective of this study was to determine the role of the AUX1 LAX family of auxin import carriers during the establishment of the embryonic root cell pattern.

METHODS

The radicle apex of single and multiple aux1 lax mutant mature embryos was used to evaluate the effect of this gene family upon embryonic root organization and root cap size, cell number and cell size.

KEY RESULTS

It was demonstrated here that mutations within the AUX1 LAX family are associated with changes in cell pattern establishment in the embryonic quiescent centre and columella. aux1 lax mutants have a larger radicle root cap than the wild type and this is associated with a significant increase in the root-cap cell number, average cell size, or both. Extreme disorganization of the radicle apex was observed among quadruple aux1 lax1 lax2 lax3 mutant embryos, but not in single aux1 null or in lax1, lax2 and lax3 single mutants, indicating redundancy within the AUX1 LAX family.

CONCLUSIONS

It was determined that the AUX1 LAX family of auxin influx facilitators participates in the establishment of cell pattern within the apex of the embryonic root in a gene-redundant fashion. It was demonstrated that aux1 lax mutants are affected in cell proliferation and cell growth within the radicle tip. Thus AUX1 LAX auxin importers emerge as new players in morphogenetic processes involved in patterning during embryonic root formation.

ABP1: an auxin receptor for fast responses at the plasma membrane

Auxin-binding protein 1 (ABP1) is an auxin receptor for responses not primarily regulated by gene regulation. One fast response is protoplast swelling. By using immunological ABP1 tools we showed that the highly conserved box a is not alone important for auxin binding. Box c is another part of the auxin binding domain.1 Here we present a novel method to analyze auxin-induced, ABP1-mediated effects at the plasma membrane on single cell level in vivo. The fluorescence of FM4-64 in the plasma membrane is reduced by auxin and this response is mediated by ABP1. This method indicates a functional role of ABP1 at the plasma membrane.

Auxin-binding proteins without KDEL sequence in the moss Funaria hygrometrica

Whereas the important plant growth regulator auxin has multiple effects in flowering plants, it induces a specific cell differentiation step in the filamentous moss protonema. Here, we analyse the presence of classical auxin-binding protein (ABP1) homologues in the moss Funaria hygrometrica. Microsomal membranes isolated from protonemata of F. hygrometrica have specific indole acetic acid-binding sites, estimated to be about 3-5 pmol/mg protein with an apparent dissociation constant (K (d)) between 3 and 5 microM. Western analyses with anti-ABP1 antiserum detected the canonical endoplasmic reticulum (ER)-localised 22-24 kDa ABP1 in Zea mays, but not in F. hygrometrica. Instead, polypeptides of 31-33 and 46 kDa were labelled in the moss as well as in maize. In F. hygrometrica these proteins were found exclusively in microsomal membrane fractions and were confirmed as ABPs by photo-affinity labelling with 5-azido-[7-(3)H]-indole-3-acetic acid. Unlike the classical corn ABP1, these moss ABPs did not contain the KDEL ER retention sequence. Consistently, the fully sequenced genome of the moss Physcomitrella patens, a close relative of F. hygrometrica, encodes an ABP1-homologue without KDEL sequence. Our study suggests the presence of putative ABPs in F. hygrometrica that share immunological epitopes with ABP1 and bind auxin but are different from the classical corn ABP1.

The AUXIN BINDING PROTEIN 1 is required for differential auxin responses mediating root growth

Background

In plants, the phytohormone auxin is a crucial regulator sustaining growth and development. At the cellular level, auxin is interpreted differentially in a tissue- and dose-dependent manner. Mechanisms of auxin signalling are partially unknown and the contribution of the AUXIN BINDING PROTEIN 1 (ABP1) as an auxin receptor is still a matter of debate.

Methodology/Principal Findings

Here we took advantage of the present knowledge of the root biological system to demonstrate that ABP1 is required for auxin response. The use of conditional ABP1 defective plants reveals that the protein is essential for maintenance of the root meristem and acts at least on the D-type CYCLIN/RETINOBLASTOMA pathway to control entry into the cell cycle. ABP1 affects PLETHORA gradients and confers auxin sensitivity to root cells thus defining the competence of the cells to be maintained within the meristem or to elongate. ABP1 is also implicated in the regulation of gene expression in response to auxin.

Conclusions/Significance

Our data support that ABP1 is a key regulator for root growth and is required for auxin-mediated responses. Differential effects of ABP1 on various auxin responses support a model in which ABP1 is the major regulator for auxin action on the cell cycle and regulates auxin-mediated gene expression and cell elongation in addition to the already well known TIR1-mediated ubiquitination pathway.

Arabidopsis GLP4 is localized to the Golgi and binds auxin in vitro

Hormones are critical for cell differentiation, elongation, and division. The plant hormone auxin plays vital roles in plant growth and development and is essential for various physiologic processes. Previous studies showed that germin-like proteins (GLPs) are involved in multiple physiologic and developmental processes and that several GLP members could bind different auxin molecules. Here we showed that Arabidopsis thaliana GLP4 gene, which has a length of 660 bp and encodes a 219-aa polypeptide, contains the conserved auxin-binding region box A and binds indole-3-acetic acid and 2,4-dichlorophenoxyacetic acid (2,4-D) with low affinity, but not a-naphthaleneacetic acid, in vitro, by using assays equilibrium dialysis and nuclear magnetic resonance. This binding character is different from that of auxin-binding protein 1, which does not bind 2,4-D. GLP4 is highly transcribed in various tissues, but it shows low transcription in roots and during embryo development. In addition, transcription of GLP4 is stimulated by auxin treatment. Subcellular localization studies indicated that GLP4 protein is localized in the Golgi compartment and the N-terminus of GLP4 is crucial for its proper localization, which suggests that GLP4 may be involved in Golgi-dependent developmental processes.

Auxin-regulated cell polarity: an inside job?

Auxin is now known to be a key regulator of polar events in plant cells. The mechanism by which auxin conveys a polar signal to the cell is unknown, but one well-known hypothesis is that the auxin flux across the plasma membrane regulates vesicle trafficking. This hypothesis remains controversial because of its reliance on an as-yet-undiscovered membrane flux sensor. In this article I suggest instead that the polar signal is the auxin gradient within the cell cytoplasm. A computer model of vascular development is presented that demonstrates the plausibility of this scenario. The auxin-binding protein ABP1 might be the receptor for the auxin gradient.

Auxin signaling in algal lineages: fact or myth?

Auxin is of major importance throughout the life cycle of a plant, affecting several physiological and developmental processes, such as cell expansion and division. However, the evolutionary time point at which auxin became involved in such diverse processes is currently unclear. Despite some controversy, numerous reports demonstrate the presence of auxin in algal lineages and its effects on algal development, suggesting an early evolutionary origin of auxin-dependent mechanisms. Here, we review these reports and discuss in silico analyses of auxin signaling components. It seems that, at least in microalgae, the assumed major components of auxin signaling in land plants are absent. However, these microalgae might have alternative auxin signaling pathways that could account for their responses to auxin.

Conditional repression of AUXIN BINDING PROTEIN1 reveals that it coordinates cell division and cell expansion during postembryonic shoot development in Arabidopsis and tobacco

AUXIN BINDING PROTEIN1 (ABP1) has long been characterized as a potentially important mediator of auxin action in plants. Analysis of the functional requirement for ABP1 during development was hampered because of embryo lethality of the null mutant in Arabidopsis thaliana. Here, we used conditional repression of ABP1 to investigate its function during vegetative shoot development. Using an inducible cellular immunization approach and an inducible antisense construct, we showed that decreased ABP1 activity leads to a severe retardation of leaf growth involving an alteration in cell division frequency, an altered pattern of endocycle induction, a decrease in cell expansion, and a change in expression of early auxin responsive genes. In addition, local repression of ABP1 activity in the shoot apical meristem revealed an additional role for ABP1 in cell plate formation and cell shape. Moreover, cells at the site of presumptive leaf initiation were more sensitive to ABP1 repression than other regions of the meristem. This spatial context-dependent response of the meristem to ABP1 inactivation and the other data presented here are consistent with a model in which ABP1 acts as a coordinator of cell division and expansion, with local auxin levels influencing ABP1 effectiveness.

Does auxin binding protein 1 control both cell division and cell expansion?

The Auxin-Binding Protein 1 (ABP1) was identified over 30 years ago thanks to it's high affinity for active auxins. ABP1 plays an essential role in plant life yet to this day, its function remains 'enigmatic.' A recent study by our laboratory shows that ABP1 is critical for regulation of the cell cycle, acting both in G(1) and at the G(2)/M transition. We showed that ABP1 is likely to mediate the permissive auxin signal for entry into the cell cycle. These data were obtained by studying a conditional functional knock-out of ABP1 generated by cellular immunization in the model tobacco cell line, Bright Yellow 2.

Canalization without flux sensors: a traveling-wave hypothesis

In 1969, Tsvi Sachs published his seminal hypothesis of vascular development in plants: the canalization hypothesis. A positive feedback loop between the flux of the phytohormone auxin and the cells' auxin transport capacity would canalize auxin progressively into discrete channels, which would then differentiate into vascular tissues. Recent experimental studies confirm the central role of polar auxin flux in plant vasculogenesis, but it is unclear if and by which mechanism plant cells could respond to auxin flux. In this Opinion article, we review auxin perception mechanisms and argue that these respond more likely to auxin concentrations than to auxin flux. We propose an alternative mechanism for polar auxin channeling, which is more consistent with recent molecular observations.

Mechanism of auxin interaction with Auxin Binding Protein (ABP1): a molecular dynamics simulation study

Auxin Binding Protein 1 (ABP1) is ubiquitous in green plants. It binds the phytohormone auxin with high specificity and affinity, but its role in auxin-induced processes is unknown. To understand the proposed receptor function of ABP1 we carried out a detailed molecular modeling study. Molecular dynamics simulations showed that ABP1 can adopt two conformations differing primarily in the position of the C-terminus and that one of them is stabilized by auxin binding. This is in agreement with experimental evidence that auxin induces changes at the ABP1 C-terminus. Simulations of ligand egress from ABP1 revealed three main routes by which an auxin molecule can enter or leave the ABP1 binding site. Assuming the previously proposed orientation of ABP1 to plant cell membranes, one of the routes leads to the membrane and the other two to ABP1's aqueous surroundings. A network of hydrogen-bonded water molecules leading from the bulk water to the zinc-coordinated ligands in the ABP1 binding site was formed in all simulations. Water entrance into the zinc coordination sphere occurred simultaneously with auxin egress. These results suggest that the hydrogen-bonded water molecules may assist in protonation and deprotonation of auxin molecules and their egress from the ABP1 binding site.

Binding of ring-substituted indole-3-acetic acids to human serum albumin

The plant hormone, indole-3-acetic acid (IAA), and its ring-substituted derivatives have recently attracted attention as promising pro-drugs in cancer therapy. Here we present relative binding constants to human serum albumin for IAA and 34 of its derivatives, as obtained using the immobilized protein bound to a support suitable for high-performance liquid chromatography. We also report their octanol-water partition coefficients (logK(ow)) computed from retention data on a C(18) coated silica gel column. A four-parameter QSPR (quantitative structure-property relationships) model, based on physico-chemical properties, is put forward, which accounts for more than 96% of the variations in the binding affinities of these compounds. The model confirms the importance of lipophilicity as a global parameter governing interaction with serum albumin, but also assigns significant roles to parameters specifically related to the molecular topology of ring-substituted IAAs. Bulky substituents at ring-position 6 increase affinity, those at position 2 obstruct binding, while no steric effects were noted at other ring-positions. Electron-withdrawing substituents at position 5 enhance binding, but have no obvious effect at other ring positions.

Indole-3-acetic acid in microbial and microorganism-plant signaling

Diverse bacterial species possess the ability to produce the auxin phytohormone indole-3-acetic acid (IAA). Different biosynthesis pathways have been identified and redundancy for IAA biosynthesis is widespread among plant-associated bacteria. Interactions between IAA-producing bacteria and plants lead to diverse outcomes on the plant side, varying from pathogenesis to phyto-stimulation. Reviewing the role of bacterial IAA in different microorganism-plant interactions highlights the fact that bacteria use this phytohormone to interact with plants as part of their colonization strategy, including phyto-stimulation and circumvention of basal plant defense mechanisms. Moreover, several recent reports indicate that IAA can also be a signaling molecule in bacteria and therefore can have a direct effect on bacterial physiology. This review discusses past and recent data, and emerging views on IAA, a well-known phytohormone, as a microbial metabolic and signaling molecule.

The auxin-like activity of humic substances is related to membrane interactions in carrot cell cultures

A detailed characterization of two humic fractions was performed: One with low relative molecular mass (LMr<3,500 Da) and one with high relative molecular mass (HMr>3,500 Da). Distinct (1)H NMR spectroscopic patterns were observed for the two fractions. HMr showed an aromatic proton region, an intense and broad region (3.0-5.0 ppm) attributed to sugar-like and polyether components, and an intense doublet at 1.33 ppm (identified as protons of the beta-CH(3) in lactate). In contrast, LMr did not show resonances due to aromatic protons and was characterized by a broad unresolved region, assigned to sugar-like components. The (13)C NMR spectra showed that the LMr humic fraction was richer in carboxylic and aliphatic C groups compared to HMr fraction. These substances were fluorescein-labeled [fluorescein isothiocyanate (FITC)], and their interaction with carrot cells in culture was monitored for 10 d, and compared to FITC-indole-3-acetic acid (IAA) to clarify their mechanisms of biological activity. After different incubation times, fluorescein staining of carrot cells and decrease of fluorescein concentration in the culture medium were evaluated. Fluorescent membrane staining was only present in IAA and the LMr humic fraction treated cell cultures. A consequential decrease of fluorescein concentration in the culture media was also observed. Pretreatment of carrot cells with unconjugated IAA or LMr humic fraction markedly reduced fluorescein staining of both FITC-IAA and FITC-LMr humic fraction. Blocking tests gave indirect evidence of possible binding of the LMr humic fraction to IAA cell membrane receptors. These results indicate that the two humic fractions behave differently. Only LMr humic fraction, like IAA, interacts with cellular membranes in carrot cell cultures.

The auxin-binding protein 1 is essential for the control of cell cycle

The phytohormone auxin has been known for >50 years to be required for entry into the cell cycle. Despite the critical effects exerted by auxin on the control of cell division, the molecular mechanism by which auxin controls this pathway is poorly understood, and how auxin is perceived upstream of any change in the cell cycle is unknown. Auxin Binding Protein 1 (ABP1) is considered to be a candidate auxin receptor, triggering early modification of ion fluxes across the plasma membrane in response to auxin. ABP1 has also been proposed to mediate auxin-dependent cell expansion, and is essential for early embryonic development. We investigated whether ABP1 has a role in the cell cycle. Functional inactivation of ABP1 in the model plant cell system BY2 was achieved through cellular immunization via the conditional expression of a single-chain fragment variable (scFv). This scFv was derived from a well characterized anti-ABP1 monoclonal antibody previously shown to block the activity of the protein. We demonstrate that functional inactivation of ABP1 results in cell-cycle arrest, and provide evidence that ABP1 plays a critical role in regulation of the cell cycle by acting at both the G1/S and G2/M checkpoints. We conclude that ABP1 is essential for the auxin control of cell division and is likely to constitute the first step of the auxin-signalling pathway mediating auxin effects on the cell cycle.

Unrevealed structural requirements for auxin-like molecules by theoretical and experimental evidences

An computational-biostatistical approach, supported by ab initio optimizations of auxin-like molecules, was used to find biologically meaningful relationships between quantum chemical variables and fresh bioassay's data. It is proven that the auxin-like recognition requires different molecular assembling states. We suggest that the carboxyl group is not the determining factor in explaining the biological auxin-like conduct. The biological effects depends essentially on the chemical condition of the ring system. The aim to find active molecules (quantum objects) via statistical grouping-analysis of molecular quantum similarity measures was verified by bioactivity assays. Next, this approach led to the discovery of a non-carboxylated active auxin-like molecule (2,6-dibromo-phenol). This is the first publication on structure activity relationship of auxin-like molecules, which relies on highly standardized bioassays of different auxins screened in parallel as well as analysed by multi-dimensional scaling.

Auxin signaling

Auxin regulates a host of plant developmental and physiological processes, including embryogenesis, vascular differentiation, organogenesis, tropic growth, and root and shoot architecture. Genetic and biochemical studies carried out over the past decade have revealed that much of this regulation involves the SCF(TIR1/AFB)-mediated proteolysis of the Aux/IAA family of transcriptional regulators. With the recent finding that the TRANSPORT INHIBITOR RESPONSE1 (TIR1)/AUXIN SIGNALING F-BOX (AFB) proteins also function as auxin receptors, a potentially complete, and surprisingly simple, signaling pathway from perception to transcriptional response is now before us. However, understanding how this seemingly simple pathway controls the myriad of specific auxin responses remains a daunting challenge, and compelling evidence exists for SCF(TIR1/AFB)-independent auxin signaling pathways.

How ethylene works in the reproductive organs of higher plants: a signaling update from the third millennium

Ethylene (ET) is a notable signaling molecule in higher plants. In the year 1993 the ET receptor gene, ETR1, was identified; this ETR1 receptor protein being the first plant hormone receptor to be isolated. It is striking that there are six ET receptors in tomato instead of five in Arabidopsis, the two best-known signaling-model systems. Even though over the last few years great progress has been made in elucidating the genes and proteins involved in ET signaling, the complete pathway remains to be established. The present review examines the most representative successive advances that have taken place in this millennium in terms of the signaling pathway of ET, as well as the implications of the signaling in the reproductive organs of plants (i.e., flowers, fruits, seeds and pollen grains). A detailed comparative study is made on the advances in knowledge in the last decade, showing how the characterization of ET signaling provides clues for understanding how higher plants regulate their ET sensitivity. Also, it is indicated that ET signaling is at present sparking interest within phytohormonal molecular physiology and biology, and it is explained why several socio-economic aspects (flowering and fruit ripening) are undoubtedly involved in ET physiology.

Development of Erect Leaves in a Modern Maize Hybrid is Associated with Reduced Responsiveness to Auxin and Light of Young Seedlings In Vitro

Modern corn (Zea mays L.) varieties have been selected for their ability to maintain productivity in dense plantings. We have tested the possibility that the physiological consequence of the selection involves changes in responsiveness to light and auxin.Etiolated seedlings of two older corn hybrids 307 and 3306 elongated significantly more than seedlings of a modern corn hybrid 3394. The level of endogenous auxin and activity of PAT in 307 and 3394 were similar. Hybrid 3394 shows resistance to auxin- and light-induced responses at the seedling, cell and molecular levels. Intact 3394 plants exhibited less responsiveness to the inhibitory effect of R, FR and W, auxin, anti-auxin and inhibitors of PAT. In excised mesocotyl tissue 3394 seedlings also showed essentially low responsiveness to NAA. Cells of 3394 were insensitive to auxin- and light-induced hyperpolarization of the plasma membrane. Expression of ABP4 was much less in 3394 than in 307, and in contrast to 307, it was not upregulated by NAA, R and FR. Preliminary analysis of abp mutants suggests that ABPs may be involved in development of leaf angle in corn.Our results confirm the understanding that auxin interacts with light in the regulation of growth and development of young seedlings and suggest that in corn ABPs may be involved in growth of maize seedlings and development of leaf angle. We hypothesize that ABP4 plays an important role in the auxin- and/or light-induced growth responses. We also hypothesize that in the modern corn hybrid 3394, ABP4 is "mutated," which may result in the observed 3394 phenotypes, including upright leaves.

Higher extracellular pH suppresses tracheary element differentiation by affecting auxin uptake

In an optimized liquid medium containing auxin and cytokinin, mesophyll cells isolated from Zinnia elegans L. seedlings can be induced to differentiate into tracheary elements (TEs) at high frequency. However, it is known that buffering the medium at neutral pH severely suppresses TE differentiation. In the process of modifying the medium, we found that excessive administration of auxin restored the suppression. Based on this finding, we physiologically characterized auxin actions involved in TE differentiation by focusing on the influence of extracellular pH. First, dose/response relationships between auxin [1-naphthaleneacetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D)] concentrations and differentiated cell ratios were determined under various extracellular pH conditions. Secondly, intracellular concentrations of free forms and metabolites of auxin species were determined by analyzing extracts from cells cultured with radiolabeled NAA and 2,4-D under different extracellular pH conditions with liquid scintillation counting and thin-layer chromatography autoradiograms. Higher extracellular pH was found to reduce both the auxin potency for inducing TE differentiation and intracellular auxin accumulation. Reduction levels correlatively varied depending on the auxin species. These results suggest that the weakening in auxin potency at higher extracellular pH is ascribed to lower auxin uptake, which leads to decreased intracellular perception of the auxin signal. A model to predict auxin action that considers membrane transport, metabolism, and the perception of auxin is also presented.

Coulomb and Overlap Self-Similarities: A Comparative Selectivity Analysis of Structure−Function Relationships for Auxin−like Molecules

Auxins are defined mainly by a set of physiological actions, but the structure-effect relationship still is based on chemical intuition. Currently a well-defined auxin molecular structure is not available. The existence of different auxin binding proteins and mechanisms of auxin action, the wide diversity of the auxin molecules, and the pleiotropic effects of auxin imply a completely different mechanism as described for the animal hormone concept. Here, we present a computational approach dealing with semiempirical optimizations of the auxin molecules themselves, which represent a number of about 250 different chemical structures. Our approach uses molecular quantum similarity measures and additional quantum variables for the analysis of auxin-like molecules. The finding of similarities in molecules by focusing basically on their electron structure results in new insights in the relationship of the different auxin groups. Additional statistical analysis allows the identification of relationships between similarity groups and their biological activity, respectively. It is postulated that the auxin-like molecular recognition depends more on specific molecular assembling states than on a specific ring system or side chain.

Receptors for auxin: will it all end in TIRs?

The role of TIR1 in ubiquitination and regulated degradation of Aux/IAA transcription factors has been recognized for some years, but recent results have shown that TIR1 itself is also the binding site for auxin. The affinity and specificity of TIR1 match properties anticipated of a nuclear auxin receptor and we look at how they compare with the properties of ABP1. We also consider the mechanism of auxin action via TIR1 and the likelihood that the TIR1 family could account for all auxin responses. It seems likely that the TIR1 system can account for a large part of the repertoire of auxin-mediated responses, but maybe not all.

Identification of a glycosylphosphatidylinositol-anchored plasma membrane protein interacting with the C-terminus of auxin-binding protein 1: a photoaffinity crosslinking study

Synthetic peptides corresponding to the C-terminus of auxin-binding protein 1 (ABP1) have been shown to function as auxin agonists. To define a C-terminal receptor, photoaffinity crosslinking experiments were performed using an azido derivative of a C-terminal peptide and plasma membranes from maize (Zea mays L.). The crosslinking reaction was monitored by immunoblotting using anti-ABP1 antibodies. The crosslinked proteins were isolated by 2D gel electrophoresis and identified by mass spectrometric analysis. Further, the noncrosslinked forms of these proteins were also identified. Two proteins with apparent molecular masses of 73 kDa (termed C-terminal peptide-binding protein 1, CBP1) and 35 kDa (CBP2) were specifically linked with the C-terminal peptide. CBP2 is a cytoplasmic protein that consists of two conserved domains that are characteristic of a ricin-type lectin domain. CBP2 remained in the detergent-insoluble particles and was released from the particles by the addition of monosaccharides such as methyl-beta-D-galactopyranoside. CBP1 was released from the membranes by treatment with phosphatidylinositol-specific phospholipase C, indicating that CBP1 is a glycosylphosphatidylinositol (GPI)-anchored plasma membrane protein. CBP1 was found to be a copper-binding protein, and is highly homologous to Arabidopsis thaliana SKU5 that contributes to directional root growth processes. Further, it is similar to A. thaliana SKS6 that contributes to cotyledon vascular patterning and to Nicotiana tabacum NTP303 that contributes to pollen tube growth. The present results indicate that ABP1 may contribute to directional cell growth processes via the GPI-anchored plasma membrane protein SKU5 and its family members.

Exogenous auxin enhances the degradation of a light down-regulated and nuclear-localized OsiIAA1, an Aux/IAA protein from rice, via proteasome

Auxin regulates many aspects of plant growth and development by altering the expression of diverse genes. Among these, the early auxin-responsive genes of Aux/IAA class have been extensively studied in dicots but little information is available on monocots. Earlier, we reported the isolation of OsiIAA1 cDNA, first monocot member of Aux/IAA gene family from rice. Extending this work further, we have isolated the OsiIAA1 gene from rice localized on chromosome 3. The transcriptional start site was mapped to 158 bp upstream to the translational start site. The increased accumulation of OsiIAA1 transcript in auxin-treated rice coleoptiles even in the presence of a protein synthesis inhibitor, cycloheximide, suggested that OsiIAA1 is a primary auxin response gene; the expression of OsiIAA1 gene was also upregulated in the presence of cycloheximide alone. The OsiIAA1 transcript levels were down-regulated in etiolated rice coleoptiles irradiated with far-red, red and blue light, suggesting the existence of a cross-talk between auxin and light signaling. The antibodies raised against His6-OsiIAA1 recombinant protein could detect the OsiIAA1 protein in the plant extract only in the presence of a proteasome inhibitor, MG132, indicating that OsiIAA1 is rapidly degraded by proteasome complex. The degradation of the protein was enhanced by the application of exogenous auxin. Also, the proteasome inhibitor MG132 stabilized the purified His6-OsiIAA1 protein to some extent in the cell-free extracts of rice coleoptiles. The OsiIAA1 protein harbors two nuclear localization signals (NLSs), one bipartite and the other resembling SV40 type NLS. Although both the NLSs were able to target the protein to the nucleus, the bipartite NLS was more effective. These studies indicate that nuclear localization of OsiIAA1 could be a prerequisite for its role in auxin signal transduction.

Immunohistochemical localization of IAA and ABP1 in strawberry shoot apexes during floral induction

By using an anti-indole-acetic acid (anti-IAA) monoclonal antibody and an anti-auxin-binding protein 1 (anti-ABP1) polyclonal antibody, IAA and ABP1 were immunohistochemically localized in strawberry (Fragaria ananassa Duch.) shoot apexes during floral induction. The spatial distribution patterns of endogenous IAA and ABP1 and their dynamic changes during floral induction were investigated. In addition, the affect of 1-N-naphthylphtalamic acid (NPA) on IAA distribution during floral induction was also analyzed. The results showed that IAA was present in the shoot apexes throughout the floral induction process, gradually concentrating in the shoot apical meristem (SAM). The distribution of ABP1 and its dynamic changes were similar to those of IAA. In addition, the ABP1 immune signal in SAM gradually increased as floral induction developed. On a morphological level, these results indicate both the spatial distribution and dynamic changes in endogenous IAA and ABP1 during the floral induction process. The close correlation found between IAA and ABP1 indicates that a cooperation occurs during the regulation of floral induction. The results also suggest that IAA was the significant agent for floral induction, and that SAM might be the place of the main action. Treatment with NPA during floral induction prevented the accumulation of IAA in the SAM, delayed the process of floral differentiation and induced an abnormal flower development. It is likely that IAA in the shoot apex is produced in young leaves and transported through the vascular tissues to the SAM and other places of function. Finally, an appropriate amount of IAA in the SAM and normal polar auxin transport are essential for floral induction and differentiation in strawberries.

A new FronTIR in targeted protein degradation and plant development

Three papers, two in a recent issue of Nature and one in the July issue of Developmental Cell, identify a family of F box proteins as the long-sought receptors for the plant growth hormone auxin. The new studies reveal that auxin, a small molecule, regulates F box proteins, which are involved in ubiquitin-mediated protein degradation. This finding has profound implications for understanding plant physiology and development and for defining new modes of regulation of SCF ubiquitin ligase complexes.

The F-box protein TIR1 is an auxin receptor

The plant hormone auxin regulates diverse aspects of plant growth and development. Recent studies indicate that auxin acts by promoting the degradation of the Aux/IAA transcriptional repressors through the action of the ubiquitin protein ligase SCF(TIR1). The nature of the signalling cascade that leads to this effect is not known. However, recent studies indicate that the auxin receptor and other signalling components involved in this response are soluble factors. Using an in vitro pull-down assay, we demonstrate that the interaction between transport inhibitor response 1 (TIR1) and Aux/IAA proteins does not require stable modification of either protein. Instead auxin promotes the Aux/IAA-SCF(TIR1) interaction by binding directly to SCF(TIR1). We further show that the loss of TIR1 and three related F-box proteins eliminates saturable auxin binding in plant extracts. Finally, TIR1 synthesized in insect cells binds Aux/IAA proteins in an auxin-dependent manner. Together, these results indicate that TIR1 is an auxin receptor that mediates Aux/IAA degradation and auxin-regulated transcription.

The Arabidopsis F-box protein TIR1 is an auxin receptor

Despite 100 years of evidence showing a pivotal role for indole-3-acetic acid (IAA or auxin) in plant development, the mechanism of auxin perception has remained elusive. Central to auxin response are changes in gene expression, brought about by auxin-induced interaction between the Aux/IAA transcriptional repressor proteins and the ubiquitin-ligase complex SCF(TIR1), thus targeting for them proteolysis. Regulated SCF-mediated protein degradation is a widely occurring signal transduction mechanism. Target specificity is conferred by the F-box protein subunit of the SCF (TIR1 in the case of Aux/IAAs) and there are multiple F-box protein genes in all eukaryotic genomes examined so far. Although SCF-target interaction is usually regulated by signal-induced modification of the target, we have previously shown that auxin signalling involves the modification of SCF(TIR1). Here we show that this modification involves the direct binding of auxin to TIR1 and thus that TIR1 is an auxin receptor mediating transcriptional responses to auxin.

Absorption and fluorescence spectra of ring-substituted indole-3-acetic acids

The absorption and fluorescence spectra of indole-3-acetic acid (1), a plant growth regulator (auxin) and experimental cancer therapeutic, 29 ring-substituted derivatives and the 7-aza analogue (1H-pyrrolo[2,3b]pyridine-3-acetic acid) are compared. Two to four absorbance maxima in the 260-310-nm range are interpreted as overlapping vibronic lines of the 1La<--1A and 1Lb<--1A transitions. Two further maxima in the 200-230-nm region are assigned to the 1Ba<--1A and 1Bb<--1A transitions. 4- and 7-Fluoroindole-3-acetic acid exhibit blue shifts with respect to 1, most other derivatives show red shifts. All indole-3-acetic acids studied, with the exception of chloro-, bromo- and 4- or 7-fluoro-derivatives, fluoresce at 345-370 nm when excited at 275-280 nm. 7-Azaindole-3-acetic acid emits at 411 nm. The fluorescence quantum yield of 6-fluoroindole-3-acetic acid significantly exceeds that of 1 (0.3); the other derivatives have lower quantum yields. The plant-growth promoting activity of the ring-substituted indole-3-acetic acids studied correlates with the position of the 1Bb<--1A transition band.

Auxins

Auxin is a multifactorial phytohormone that is required for cell division. Fine gradients determine points of developmental change in time and space. It is associated intimately with the axiality of plant growth, and increasing doses lead to cell expansion or inhibition of cell expansion in different tissues. From embryonic patterning to fruit dehiscence every plant process has some involvement with auxin as a hormonal signal, including responses to wounding. Moreover, synthetic auxins have widespread uses as agrochemicals, particularly as selective herbicides. Despite the importance of auxin as a plant signal the pathways of its biosynthesis are still not clear. Much more is known about auxin perception and the mechanisms through which gene transcription is regulated. One receptor has been identified, and protein crystallography data has explained its auxin-binding capacity, but this is likely to control only a subset of auxin-mediated responses. Little is known of the signal transduction intermediates. A second receptor has been nominated and may be involved in controlling auxin-mediated gene transcription. A complex set of proteins comprising signalosome and proteasome contribute to the regulation of sets of transcription factors to confer regulation by derepression. A set of auxin transport proteins has been described with associated regulatory interactors, and these account for polar auxin flow and the control of auxin movements across cells, tissues, and around the plant. The gradients these transport systems build regulate the responses of growth and differentiation, including the plant's response to gravity. These areas are described and discussed by relating the physiology of the whole plant to the details of genetic and protein activities.

TIRs of joy: new receptors for auxin

Back-to-back papers have described the identification of a family of receptors for the plant hormone auxin. Most developmental processes in plants are dependent on auxin signalling making this discovery a landmark in the search for the mechanism of auxin action. The TIR1 gene translates into a protein with recognised motifs including an F-box domain and TIR1 forms part of an important ubiquitination complex that tags other proteins for degradation. Specific amongst the targets of TIR1 are a set of auxin-regulated transcription factors. The latest work has shown that TIR1 itself is also the binding site for auxin making it an auxin receptor with no requirement for a biochemical signalling cascade.

Auxin signals--turning genes on and turning cells around

The extremely wide spectrum of the plant processes that are influenced by auxin raises the question of how signals conveyed by a single molecule can trigger such a variety of responses. Although many aspects of auxin function remain elusive, others have become genetically tractable. The identification of crucial genes in auxin signal transduction and auxin transport in the past few years has led to molecularly testable concepts of how auxin signals regulate gene activities in individual cells, and how the polar transport of auxin could impact on patterning processes throughout the plant.

The indole alkaloids brucine, yohimbine, and hypaphorine are indole-3-acetic acid-specific competitors which do not alter auxin transport

The indole alkaloids brucine and yohimbine, just like hypaphorine, counteract indole-3-acetic acid (IAA) activity in seedling roots, root hairs and shoots, but do not appear to alter auxin transport in roots or in cultured cells. In roots, the interactions between IAA and these three alkaloids appear competitive and specific since these molecules interact with IAA but with neither 1-naphthaleneacetic acid (NAA) or 2,4-dichlorophenoxyacetic acid (2,4-D), two synthetic auxins. The data reported further support the hypothesis that hypaphorine brucine and yohimbine compete with IAA on some auxin-binding proteins likely to be auxin receptors and that 2,4-D and NAA are not always perceived by the same receptor as IAA or the same component of that receptor. At certain steps of plant development and in certain cells, endogenous indole alkaloids could be involved in IAA activity regulation together with other well-described mechanisms such as conjugation or degradation. Hypaphorine with other active indole alkaloids remaining to be identified, might be regarded as a new class of IAA antagonists.

Purification and characterization of a barley aleurone abscisic acid-binding protein

A protein designated ABAP1 and encoded by a novel gene (GenBank accession number AF127388) was purified and shown to specifically bind abscisic acid (ABA). ABAP1 protein is a 472-amino acid polypeptide containing a WW protein interaction domain and is induced by ABA in barley aleurone layers. Polyclonal antiidiotypic antibodies (AB2) cross-reacted with purified ABAP1 and with a corresponding 52-kDa protein associated with membrane fractions of ABA-treated barley aleurones. ABAP1 genes were detected in diverse monocot and dicot species, including wheat, tobacco, alfalfa, garden pea, and oilseed rape. The recombinant ABAP1 protein optimally bound (3)H-(+)-ABA at neutral pH. Denatured ABAP1 protein did not bind (3)H-(+)-ABA, nor did bovine serum albumin. The maximum specific binding as shown by Scatchard plot analysis was 0.8 mol of ABA mol(-1) protein with a linear function of r(2) = 0.94, an indication of one ABA-binding site with a dissociation constant (K(d)) of 28 x 10(-9) m. ABA binding in aleurone plasma membranes showed a maximum binding capacity of 330 nmol of ABA g(-1) protein with a K(d) of 26.5 x 10(-9) m. The similarities in the dissociation constants for ABA binding of the recombinant protein and that of the plasma membranes suggest that the protein within the plasma membrane fraction is the native form of ABAP1. The stereospecificity of ABAP1 was established by the incapability of ABA analogs and metabolites, including (-)-ABA, trans-ABA, phaseic acid, dihydrophaseic acid, and (+)-abscisic acid-glucose ester, to displace (3)H-(+)-ABA bound to ABAP1. However, two ABA precursors, (+)-ABA aldehyde and (+)-ABA alcohol, were able to displace (3)H-(+)-ABA, an indication that the structural requirement of ABAP1 at the C-1 position is not strict. Our data show that ABAP1 exerts high binding affinity for ABA. The interaction is reversible, follows saturation kinetics, and has stereospecificity, thus meeting the criteria for an ABA-binding protein.

Auxin action in a cell-free system

The plant hormone auxin regulates diverse aspects of plant growth and development. Despite its importance, the mechanisms of auxin action remain poorly understood. In particular, the identities of the auxin receptor and other signaling proteins are unknown. Recent studies have shown that auxin acts by promoting the degradation of a family of transcriptional regulators called the Aux/IAA proteins. These proteins interact with another large family of plant-specific transcription factors called Auxin Response Factors (ARF) and negatively regulate their activity. Auxin stimulates Aux/IAA degradation by promoting the interaction between a ubiquitin protein ligase (E3) called SCF(TIR1) and the Aux/IAA protein. In this report, we demonstrate that auxin promotes the interaction between the Aux/IAA proteins and SCF(TIR1) in a soluble extract free of membranes, indicating that this auxin response is mediated by a soluble receptor. In addition, we show that the response is not dependent on protein phosphorylation or dephosphorylation but rather is prevented by an inhibitor of peptidyl-prolyl isomerases.