Modulation of K+ channels in Vicia stomatal guard cells by peptide homologs to the auxin-binding protein C terminus

A charged existence: A century of transmembrane ion transport in plants

If the past century marked the birth of membrane transport as a focus for research in plants, the past 50 years has seen the field mature from arcane interest to a central pillar of plant physiology. Ion transport across plant membranes accounts for roughly 30% of the metabolic energy consumed by a plant cell, and it underpins virtually every aspect of plant biology, from mineral nutrition, cell expansion, and development to auxin polarity, fertilization, plant pathogen defense, and senescence. The means to quantify ion flux through individual transporters, even single channel proteins, became widely available as voltage clamp methods expanded from giant algal cells to the fungus Neurospora crassa in the 1970s and the cells of angiosperms in the 1980s. Here, I touch briefly on some key aspects of the development of modern electrophysiology with a focus on the guard cells of stomata, now without dispute the premier plant cell model for ion transport and its regulation. Guard cells have proven to be a crucible for many technical and conceptual developments that have since emerged into the mainstream of plant science. Their study continues to provide fundamental insights and carries much importance for the global challenges that face us today.

BnKAT2 Positively Regulates the Main Inflorescence Length and Silique Number in Brassica napus by Regulating the Auxin and Cytokinin Signaling Pathways

Brassica napus is the dominant oil crop cultivated in China for its high quality and high yield. The length of the main inflorescence and the number of siliques produced are important traits contributing to rapeseed yield. Therefore, studying genes related to main inflorescence and silique number is beneficial to increase rapeseed yield. Herein, we focused on the effects of BnKAT2 on the main inflorescence length and silique number in B. napus. We explored the mechanism of BnKAT2 increasing the effective length of main inflorescence and the number of siliques through bioinformatics analysis, transgenic technology, and transcriptome sequencing analysis. The full BnKAT2(BnaA01g09060D) sequence is 3674 bp, while its open reading frame is 2055 bp, and the encoded protein comprises 684 amino acids. BnKAT2 is predicted to possess two structural domains, namely KHA and _CNMP-binding domains. The overexpression of BnKAT2 effectively increased the length of the main inflorescence and the number of siliques in B. napus, as well as in transgenic Arabidopsis thaliana. The type-A Arabidopsis response regulator (A-ARR), negative regulators of the cytokinin, are downregulated in the BnKAT2-overexpressing lines. The Aux/IAA, key genes in auxin signaling pathways, are downregulated in the BnKAT2-overexpressing lines. These results indicate that BnKAT2 might regulate the effective length of the main inflorescence and the number of siliques through the auxin and cytokinin signaling pathways. Our study provides a new potential function gene responsible for improvement of main inflorescence length and silique number, as well as a candidate gene for developing markers used in MAS (marker-assisted selection) breeding to improve rapeseed yield.

The Story of Auxin-Binding Protein 1 (ABP1)

The auxin-binding protein 1 (ABP1) has endured a history of undulating prominence as a candidate receptor for this important phytohormone. Its capacity for binding auxin has not been in doubt, a feature adequately explained by its crystal structure, but any relevance of this to auxin signaling and plant development has been far more demanding to define. Over its research lifetime, it has been associated with many auxin-induced activities, including ion fluxes across the plasma membrane, rearrangement of the cytoskeleton and cell shape, and the abundance of PIN proteins at the plasma membrane via control of endocytosis, all of which required its presence in the apoplast. Yet, ABP1 has a KDEL sequence that targets it to the endoplasmic reticulum, where most of it remains. This mismatch has been more than adequately compensated for by the need for an auxin receptor to account for responses far too rapid to be executed through transcription and translation and the TIR1/AuxIAA coreceptor system. However, discoveries showing that abp1-null mutants are not compromised for auxin signaling or development, that TIR1 or AFB1 are necessarily involved with very rapid responses at the plasma membrane, and that these rapid responses are mediated with intracellular auxin all suggest that ABP1's auxin-binding capacity is not physiologically relevant. Nevertheless, ABP1 is ubiquitous in higher plants and throughout plant tissues. We need to complete its history by defining its function inside plant cells.

Membrane voltage as a dynamic platform for spatiotemporal signaling, physiological, and developmental regulation

Membrane voltage arises from the transport of ions through ion-translocating ATPases, ion-coupled transport of solutes, and ion channels, and is an integral part of the bioenergetic "currency" of the membrane. The dynamics of membrane voltage-so-called action, systemic, and variation potentials-have also led to a recognition of their contributions to signal transduction, both within cells and across tissues. Here, we review the origins of our understanding of membrane voltage and its place as a central element in regulating transport and signal transmission. We stress the importance of understanding voltage as a common intermediate that acts both as a driving force for transport-an electrical "substrate"-and as a product of charge flux across the membrane, thereby interconnecting all charge-carrying transport across the membrane. The voltage interconnection is vital to signaling via second messengers that rely on ion flux, including cytosolic free Ca2+, H+, and the synthesis of reactive oxygen species generated by integral membrane, respiratory burst oxidases. These characteristics inform on the ways in which long-distance voltage signals and voltage oscillations give rise to unique gene expression patterns and influence physiological, developmental, and adaptive responses such as systemic acquired resistance to pathogens and to insect herbivory.

Do Plasmodesmata Play a Prominent Role in Regulation of Auxin-Dependent Genes at Early Stages of Embryogenesis?

Plasmodesmata form intercellular channels which ensure the transport of various molecules during embryogenesis and postembryonic growth. However, high permeability of plasmodesmata may interfere with the establishment of auxin maxima, which are required for cellular patterning and the development of distinct tissues. Therefore, diffusion through plasmodesmata is not always desirable and the symplastic continuum must be broken up to induce or accomplish some developmental processes. Many data show the role of auxin maxima in the regulation of auxin-responsive genes and the establishment of various cellular patterns. However, still little is known whether and how these maxima are formed in the embryo proper before 16-cell stage, that is, when there is still a nonpolar distribution of auxin efflux carriers. In this work, we focused on auxin-dependent regulation of plasmodesmata function, which may provide rapid and transient changes of their permeability, and thus take part in the regulation of gene expression.

Salinity Effects on Guard Cell Proteome in Chenopodium quinoa

Epidermal fragments enriched in guard cells (GCs) were isolated from the halophyte quinoa (Chenopodium quinoa Wild.) species, and the response at the proteome level was studied after salinity treatment of 300 mM NaCl for 3 weeks. In total, 2147 proteins were identified, of which 36% were differentially expressed in response to salinity stress in GCs. Up and downregulated proteins included signaling molecules, enzyme modulators, transcription factors and oxidoreductases. The most abundant proteins induced by salt treatment were desiccation-responsive protein 29B (50-fold), osmotin-like protein OSML13 (13-fold), polycystin-1, lipoxygenase, alpha-toxin, and triacylglycerol lipase (PLAT) domain-containing protein 3-like (eight-fold), and dehydrin early responsive to dehydration (ERD14) (eight-fold). Ten proteins related to the gene ontology term “response to ABA” were upregulated in quinoa GC; this included aspartic protease, phospholipase D and plastid-lipid-associated protein. Additionally, seven proteins in the sucrose–starch pathway were upregulated in the GC in response to salinity stress, and accumulation of tryptophan synthase and L-methionine synthase (enzymes involved in the amino acid biosynthesis) was observed. Exogenous application of sucrose and tryptophan, L-methionine resulted in reduction in stomatal aperture and conductance, which could be advantageous for plants under salt stress. Eight aspartic proteinase proteins were highly upregulated in GCs of quinoa, and exogenous application of pepstatin A (an inhibitor of aspartic proteinase) was accompanied by higher oxidative stress and extremely low stomatal aperture and conductance, suggesting a possible role of aspartic proteinase in mitigating oxidative stress induced by saline conditions.

Unusual Roles of Secretory SNARE SYP132 in Plasma Membrane H+-ATPase Traffic and Vegetative Plant Growth

The plasma membrane proton (H⁺)-ATPases of plants generate steep electrochemical gradients and activate osmotic solute uptake. H⁺-ATPase-mediated proton pumping orchestrates cellular homeostasis and is a prerequisite for plastic cell expansion and plant growth. All evidence suggests that the population of H⁺-ATPase proteins at the plasma membrane reflects a balance of their roles in exocytosis, endocytosis, and recycling. Auxin governs both traffic and activation of the plasma membrane H⁺-ATPase proteins already present at the membrane. As in other eukaryotes, in plants, SNARE-mediated membrane traffic influences the density of several proteins at the plasma membrane. Even so, H⁺-ATPase traffic, its relationship with SNAREs, and its regulation by auxin have remained enigmatic. Here, we identify the Arabidopsis (Arabidopsis thaliana) Qa-SNARE SYP132 (Syntaxin of Plants132) as a key factor in H⁺-ATPase traffic and demonstrate its association with endocytosis. SYP132 is a low-abundant, secretory SNARE that primarily localizes to the plasma membrane. We find that SYP132 expression is tightly regulated by auxin and that augmented SYP132 expression reduces the amount of H⁺-ATPase proteins at the plasma membrane. The physiological consequences of SYP132 overexpression include reduced apoplast acidification and suppressed vegetative growth. Thus, SYP132 plays unexpected and vital roles in auxin-regulated H⁺-ATPase traffic and associated functions at the plasma membrane.

Protoplast Swelling and Hypocotyl Growth Depend on Different Auxin Signaling Pathways

Members of the TRANSPORT INHIBITOR RESPONSE1/AUXIN SIGNALING F-BOX PROTEIN (TIR1/AFB) family are known auxin receptors. To analyze the possible receptor function of AUXIN BINDING PROTEIN1 (ABP1), an auxin receptor currently under debate, we performed different approaches. We performed a pharmacological approach using α-(2,4-dimethylphenylethyl-2-oxo)-indole-3-acetic acid (auxinole), α-(phenylethyl-2-oxo)-indole-3-acetic acid (PEO-IAA), and 5-fluoroindole-3-acetic acid (5-F-IAA) to discriminate between ABP1- and TIR1/AFB-mediated processes in Arabidopsis (Arabidopsis thaliana). We used a peptide of the carboxyl-terminal region of AtABP1 as a tool. We performed mutant analysis with the null alleles of ABP1, abp1-c1 and abp1-TD1, and the TILLING mutant abp1-5 We employed Coimbra, an accession that exhibits an amino acid exchange in the auxin-binding domain of ABP1. We measured either volume changes of single hypocotyl protoplasts or hypocotyl growth, both at high temporal resolution. 5-F-IAA selectively activated the TIR1/AFB pathway but did not induce protoplast swelling; instead, it showed auxin activity in the hypocotyl growth test. In contrast, PEO-IAA induced an auxin-like swelling response but no hypocotyl growth. The carboxyl-terminal peptide of AtABP1 induced an auxin-like swelling response. In the ABP1-related mutants and Coimbra, no auxin-induced protoplast swelling occurred. ABP1 seems to be involved in mediating rapid auxin-induced protoplast swelling, but it is not involved in the control of rapid auxin-induced growth.

Leaf expansion in Phaseolus: transient auxin-induced growth increase

Control of leaf expansion by auxin is not well understood. Evidence from short term exogenous applications and from treatment of excised tissues suggests auxin positively influences growth. Manipulations of endogenous leaf auxin content, however, suggests that, long-term, auxin suppresses leaf expansion. This study attempts to clarify the growth effects of auxin on unifoliate (primary) leaves of the common bean (Phaseolus vulgaris) by reexamining the response to auxin treatment of both excised leaf strips and attached leaves. Leaf strips, incubated in culture conditions that promoted steady elongation for up to 48 h, treated with 10 μM NAA responded with an initial surge of elongation growth complete within 10 hours followed by insensitivity. A range of NAA concentrations from 0.1 μM to 300 μM induced increased strip elongation after 24 hours and 48 hours. Increased elongation and epinastic curvature of leaf strips was found specific to active auxins. Expanding attached unifoliates treated once with aqueous auxin α-naphthalene acetic acid (NAA) at 1.0 mM showed both an initial surge in growth lasting 4-6 hours followed by growth inhibition sustained at least as long as 24 hours post treatment. Auxin-induced inhibition of leaf expansion was associated with smaller epidermal cell area. Together the results suggest increasing leaf auxin first increases growth then slows growth through inhibition of cell expansion. Excised leaf strips, retain only the initial increased growth response to auxin and not the subsequent growth inhibition, either as a consequence of wounding or of isolation from the plant.

Auxin Signaling in Regulation of Plant Translation Reinitiation

The mRNA translation machinery directs protein production, and thus cell growth, according to prevailing cellular and environmental conditions. The target of rapamycin (TOR) signaling pathway-a major growth-related pathway-plays a pivotal role in optimizing protein synthesis in mammals, while its deregulation triggers uncontrolled cell proliferation and the development of severe diseases. In plants, several signaling pathways sensitive to environmental changes, hormones, and pathogens have been implicated in post-transcriptional control, and thus far phytohormones have attracted most attention as TOR upstream regulators in plants. Recent data have suggested that the coordinated actions of the phytohormone auxin, Rho-like small GTPases (ROPs) from plants, and TOR signaling contribute to translation regulation of mRNAs that harbor upstream open reading frames (uORFs) within their 5'-untranslated regions (5'-UTRs). This review will summarize recent advances in translational regulation of a specific set of uORF-containing mRNAs that encode regulatory proteins-transcription factors, protein kinases and other cellular controllers-and how their control can impact plant growth and development.

The Membrane Transport System of the Guard Cell and Its Integration for Stomatal Dynamics

Stomatal guard cells are widely recognized as the premier plant cell model for membrane transport, signaling, and homeostasis. This recognition is rooted in half a century of research into ion transport across the plasma and vacuolar membranes of guard cells that drive stomatal movements and the signaling mechanisms that regulate them. Stomatal guard cells surround pores in the epidermis of plant leaves, controlling the aperture of the pore to balance CO₂ entry into the leaf for photosynthesis with water loss via transpiration. The position of guard cells in the epidermis is ideally suited for cellular and subcellular research, and their sensitivity to endogenous signals and environmental stimuli makes them a primary target for physiological studies. Stomata underpin the challenges of water availability and crop production that are expected to unfold over the next 20 to 30 years. A quantitative understanding of how ion transport is integrated and controlled is key to meeting these challenges and to engineering guard cells for improved water use efficiency and agricultural yields.

Auxin regulation of cell polarity in plants

Auxin is well known to control pattern formation and directional growth at the organ/tissue levels via the nuclear TIR1/AFB receptor-mediated transcriptional responses. Recent studies have expanded the arena of auxin actions as a trigger or key regulator of cell polarization and morphogenesis. These actions require non-transcriptional responses such as changes in the cytoskeleton and vesicular trafficking, which are commonly regulated by ROP/Rac GTPase-dependent pathways. These findings beg for the question about the nature of auxin receptors that regulate these responses and renew the interest in ABP1 as a cell surface auxin receptor, including the work showing auxin-binding protein 1 (ABP1) interacts with the extracellular domain of the transmembrane kinase (TMK) receptor-like kinases in an auxin-dependent manner, as well as the debate on this auxin binding protein discovered about 40 years ago. This review highlights recent work on the non-transcriptional auxin signaling mechanisms underscoring cell polarity and shape formation in plants.

Spatial Regulation of Root Growth: Placing the Plant TOR Pathway in a Developmental Perspective

Plant cells contain specialized structures, such as a cell wall and a large vacuole, which play a major role in cell growth. Roots follow an organized pattern of development, making them the organs of choice for studying the spatio-temporal regulation of cell proliferation and growth in plants. During root growth, cells originate from the initials surrounding the quiescent center, proliferate in the division zone of the meristem, and then increase in length in the elongation zone, reaching their final size and differentiation stage in the mature zone. Phytohormones, especially auxins and cytokinins, control the dynamic balance between cell division and differentiation and therefore organ size. Plant growth is also regulated by metabolites and nutrients, such as the sugars produced by photosynthesis or nitrate assimilated from the soil. Recent literature has shown that the conserved eukaryotic TOR (target of rapamycin) kinase pathway plays an important role in orchestrating plant growth. We will summarize how the regulation of cell proliferation and cell expansion by phytohormones are at the heart of root growth and then discuss recent data indicating that the TOR pathway integrates hormonal and nutritive signals to orchestrate root growth.

Complementation of the embryo-lethal T-DNA insertion mutant of AUXIN-BINDING-PROTEIN 1 (ABP1) with abp1 point mutated versions reveals crosstalk of ABP1 and phytochromes

The function of the extracytoplasmic AUXIN-BINDING-PROTEIN1 (ABP1) is largely enigmatic. We complemented a homozygous T-DNA insertion null mutant of ABP1 in Arabidopsis thaliana Wassilewskia with three mutated and one wild-type (wt) ABP1 cDNA, all tagged C-terminally with a strepII-FLAG tag upstream the KDEL signal. Based on in silico modelling, the abp1 mutants were predicted to have altered geometries of the auxin binding pocket and calculated auxin binding energies lower than the wt. Phenotypes linked to auxin transport were compromised in these three complemented abp1 mutants. Red light effects, such as elongation of hypocotyls in constant red (R) and far-red (FR) light, in white light supplemented by FR light simulating shade, and inhibition of gravitropism by R or FR, were all compromised in the complemented lines. Using auxin- or light-induced expression of marker genes, we showed that auxin-induced expression was delayed already after 10 min, and light-induced expression within 60 min, even though TIR1/AFB or phyB are thought to act as receptors relevant for gene expression regulation. The expression of marker genes in seedlings responding to both auxin and shade showed that for both stimuli regulation of marker gene expression was altered after 10-20 min in the wild type and phyB mutant. The rapidity of expression responses provides a framework for the mechanics of functional interaction of ABP1 and phyB to trigger interwoven signalling pathways.

Molecular dynamics simulations of the auxin-binding protein 1 in complex with indole-3-acetic acid and naphthalen-1-acetic acid

Auxin-binding protein 1 (ABP1) is suggested to be an auxin receptor which plays an important role in several processes in green plants. Maize ABP1 was simulated with the natural auxin indole-3-acetic acid (IAA) and the synthetic analog naphthalen-1-acetic acid (NAA), to elucidate the role of the KDEL sequence and the helix at the C-terminus. The KDEL sequence weakens the intermolecular interactions between the monomers but stabilizes the C-terminal helix. Conformational changes at the C-terminus occur within the KDEL sequence and are influenced by the binding of the simulated ligands. This observation helps to explain experimental findings on ABP1 interactions with antibodies that are modulated by the presence of auxin, and supports the hypothesis that ABP1 acts as an auxin receptor. Stable hydrogen bonds between the monomers are formed between Glu40 and Glu62, Arg10 and Thr97, Lys39, and Glu62 in all simulations. The amino acids Ile22, Leu25, Trp44, Pro55, Ile130, and Phe149 are located in the binding pocket and are involved in hydrophobic interactions with the ring system of the ligand. Trp151 is stably involved in a face to end interaction with the ligand. The calculated free energy of binding using the linear interaction energy approach showed a higher binding affinity for NAA as compared to IAA. Our simulations confirm the asymmetric behavior of the two monomers, the stronger interaction of NAA than IAA and offers insight into the possible mechanism of ABP1 as an auxin receptor.

Clustering of the K+ channel GORK of Arabidopsis parallels its gating by extracellular K+

GORK is the only outward-rectifying Kv-like K(+) channel expressed in guard cells. Its activity is tightly regulated to facilitate K(+) efflux for stomatal closure and is elevated in ABA in parallel with suppression of the activity of the inward-rectifying K(+) channel KAT1. Whereas the population of KAT1 is subject to regulated traffic to and from the plasma membrane, nothing is known about GORK, its distribution and traffic in vivo. We have used transformations with fluorescently-tagged GORK to explore its characteristics in tobacco epidermis and Arabidopsis guard cells. These studies showed that GORK assembles in puncta that reversibly dissociated as a function of the external K(+) concentration. Puncta dissociation parallelled the gating dependence of GORK, the speed of response consistent with the rapidity of channel gating response to changes in the external ionic conditions. Dissociation was also suppressed by the K(+) channel blocker Ba(2+) . By contrast, confocal and protein biochemical analysis failed to uncover substantial exo- and endocytotic traffic of the channel. Gating of GORK is displaced to more positive voltages with external K(+) , a characteristic that ensures the channel facilitates only K(+) efflux regardless of the external cation concentration. GORK conductance is also enhanced by external K(+) above 1 mm. We suggest that GORK clustering in puncta is related to its gating and conductance, and reflects associated conformational changes and (de)stabilisation of the channel protein, possibly as a platform for transmission and coordination of channel gating in response to external K(+) .

Ca(2+)-Transport through Plasma Membrane as a Test of Auxin Sensitivity

Auxin is one of the crucial regulators of plant growth and development. The discovered auxin cytosolic receptor (TIR1) is not involved in the perception of the hormone signal at the plasma membrane. Instead, another receptor, related to the ABP1, auxin binding protein1, is supposed to be responsible for the perception at the plasma membrane. One of the fast and sensitive auxin-induced reactions is an increase of Ca(2+) cytosolic concentration, which is suggested to be dependent on the activation of Ca(2+) influx through the plasma membrane. This investigation was carried out with a plasmalemma enriched vesicle fraction, obtained from etiolated maize coleoptiles. The magnitude of Ca(2+) efflux through the membrane vesicles was estimated according to the shift of potential dependent fluorescent dye diS-C₃-(5). The obtained results showed that during coleoptiles ageing (3rd, 4th and 5th days of seedling etiolated growth) the magnitude of Ca(2+) efflux from inside-out vesicles was decreased. Addition of ABP1 led to a recovery of Ca(2+) efflux to the level of the youngest and most sensitive cells. Moreover, the efflux was more sensitive, responding from 10(-8) to 10(-6) M 1-NAA, in vesicles containing ABP1, whereas native vesicles showed the highest efflux at 10(-6) M 1-NAA. We suggest that auxin increases plasma membrane permeability to Ca(2+) and that ABP1 is involved in modulation of this reaction.

AUXIN BINDING PROTEIN1 links cell wall remodeling, auxin signaling, and cell expansion in arabidopsis

Cell expansion is an increase in cell size and thus plays an essential role in plant growth and development. Phytohormones and the primary plant cell wall play major roles in the complex process of cell expansion. In shoot tissues, cell expansion requires the auxin receptor AUXIN BINDING PROTEIN1 (ABP1), but the mechanism by which ABP1 affects expansion remains unknown. We analyzed the effect of functional inactivation of ABP1 on transcriptomic changes in dark-grown hypocotyls and investigated the consequences of gene expression on cell wall composition and cell expansion. Molecular and genetic evidence indicates that ABP1 affects the expression of a broad range of cell wall-related genes, especially cell wall remodeling genes, mainly via an SCF(TIR/AFB)-dependent pathway. ABP1 also functions in the modulation of hemicellulose xyloglucan structure. Furthermore, fucosidase-mediated defucosylation of xyloglucan, but not biosynthesis of nonfucosylated xyloglucan, rescued dark-grown hypocotyl lengthening of ABP1 knockdown seedlings. In muro remodeling of xyloglucan side chains via an ABP1-dependent pathway appears to be of critical importance for temporal and spatial control of cell expansion.

Extracellular signals and receptor-like kinases regulating ROP GTPases in plants

Rho-like GTPase from plants (ROPs) function as signaling switches that control a wide variety of cellular functions and behaviors including cell morphogenesis, cell division and cell differentiation. The Arabidopsis thaliana genome encodes 11 ROPs that form a distinct single subfamily contrarily to animal or fungal counterparts where multiple subfamilies of Rho GTPases exist. Since Rho proteins bind to their downstream effector proteins only in their GTP-bound "active" state, the activation of ROPs by upstream factor(s) is a critical step in the regulation of ROP signaling. Therefore, it is critical to examine the input signals that lead to the activation of ROPs. Recent findings showed that the plant hormone auxin is an important signal for the activation of ROPs during pavement cell morphogenesis as well as for other developmental processes. In contrast to auxin, another plant hormone, abscisic acid, negatively regulates ROP signaling. Calcium is another emerging signal in the regulation of ROP signaling. Several lines of evidence indicate that plasma membrane localized-receptor like kinases play a critical role in the transmission of the extracellular signals to intracellular ROP signaling pathways. This review focuses on how these signals impinge upon various direct regulators of ROPs to modulate various plant processes.

Calcium: The Missing Link in Auxin Action

Due to their sessile lifestyles, plants need to deal with the limitations and stresses imposed by the changing environment. Plants cope with these by a remarkable developmental flexibility, which is embedded in their strategy to survive. Plants can adjust their size, shape and number of organs, bend according to gravity and light, and regenerate tissues that were damaged, utilizing a coordinating, intercellular signal, the plant hormone, auxin. Another versatile signal is the cation, Ca(2+), which is a crucial second messenger for many rapid cellular processes during responses to a wide range of endogenous and environmental signals, such as hormones, light, drought stress and others. Auxin is a good candidate for one of these Ca(2+)-activating signals. However, the role of auxin-induced Ca(2+) signaling is poorly understood. Here, we will provide an overview of possible developmental and physiological roles, as well as mechanisms underlying the interconnection of Ca(2+) and auxin signaling.

Phototropism: translating light into directional growth

Phototropism allows plants to align their photosynthetic tissues with incoming light. The direction of incident light is sensed by the phototropin family of blue light photoreceptors (phot1 and phot2 in Arabidopsis), which are light-activated protein kinases. The kinase activity of phototropins and phosphorylation of residues in the activation loop of their kinase domains are essential for the phototropic response. These initial steps trigger the formation of the auxin gradient across the hypocotyl that leads to asymmetric growth. The molecular events between photoreceptor activation and the growth response are only starting to be elucidated. In this review, we discuss the major steps leading from light perception to directional growth concentrating on Arabidopsis. In addition, we highlight links that connect these different steps enabling the phototropic response.

OnGuard, a computational platform for quantitative kinetic modeling of guard cell physiology

Stomatal guard cells play a key role in gas exchange for photosynthesis while minimizing transpirational water loss from plants by opening and closing the stomatal pore. Foliar gas exchange has long been incorporated into mathematical models, several of which are robust enough to recapitulate transpirational characteristics at the whole-plant and community levels. Few models of stomata have been developed from the bottom up, however, and none are sufficiently generalized to be widely applicable in predicting stomatal behavior at a cellular level. We describe here the construction of computational models for the guard cell, building on the wealth of biophysical and kinetic knowledge available for guard cell transport, signaling, and homeostasis. The OnGuard software was constructed with the HoTSig library to incorporate explicitly all of the fundamental properties for transporters at the plasma membrane and tonoplast, the salient features of osmolite metabolism, and the major controls of cytosolic-free Ca²⁺ concentration and pH. The library engenders a structured approach to tier and interrelate computational elements, and the OnGuard software allows ready access to parameters and equations 'on the fly' while enabling the network of components within each model to interact computationally. We show that an OnGuard model readily achieves stability in a set of physiologically sensible baseline or Reference States; we also show the robustness of these Reference States in adjusting to changes in environmental parameters and the activities of major groups of transporters both at the tonoplast and plasma membrane. The following article addresses the predictive power of the OnGuard model to generate unexpected and counterintuitive outputs.

Systems dynamic modeling of the stomatal guard cell predicts emergent behaviors in transport, signaling, and volume control

The dynamics of stomatal movements and their consequences for photosynthesis and transpirational water loss have long been incorporated into mathematical models, but none have been developed from the bottom up that are widely applicable in predicting stomatal behavior at a cellular level. We previously established a systems dynamic model incorporating explicitly the wealth of biophysical and kinetic knowledge available for guard cell transport, signaling, and homeostasis. Here we describe the behavior of the model in response to experimentally documented changes in primary pump activities and malate (Mal) synthesis imposed over a diurnal cycle. We show that the model successfully recapitulates the cyclic variations in H⁺, K⁺, Cl⁻, and Mal concentrations in the cytosol and vacuole known for guard cells. It also yields a number of unexpected and counterintuitive outputs. Among these, we report a diurnal elevation in cytosolic-free Ca²⁺ concentration and an exchange of vacuolar Cl⁻ with Mal, both of which find substantiation in the literature but had previously been suggested to require additional and complex levels of regulation. These findings highlight the true predictive power of the OnGuard model in providing a framework for systems analysis of stomatal guard cells, and they demonstrate the utility of the OnGuard software and HoTSig library in exploring fundamental problems in cellular physiology and homeostasis.

The interaction and integration of auxin signaling components

IAA, a naturally occurring auxin, is a simple signaling molecule that regulates many diverse steps of plant development. Auxin essentially coordinates plant development through transcriptional regulation. Auxin binds to TIR1/AFB nuclear receptors, which are F-box subunits of the SCF ubiquitin ligase complex. The auxin signal is then modulated by the quantitative and qualitative responses of the Aux/IAA repressors and the auxin response factor (ARF) transcription factors. The specificity of the auxin-regulated gene expression profile is defined by several factors, such as the expression of these regulatory proteins, their post-transcriptional regulation, their stability and the affinity between these regulatory proteins. Auxin-binding protein 1 (ABP1) is a candidate protein for an auxin receptor that is implicated in non-transcriptional auxin signaling. ABP1 also affects TIR1/AFB-mediated auxin-responsive gene expression, implying that both the ABP1 and TIR1/AFB signaling machineries coordinately control auxin-mediated physiological events. Systematic approaches using the comprehensive mapping of the expression and interaction of signaling modules and computational modeling would be valuable for integrating our knowledge of auxin signals and responses.

Uniform auxin triggers the Rho GTPase-dependent formation of interdigitation patterns in pavement cells

The investigation of Rho-family GTPases has uncovered mechanisms for spatiotemporal control of cellular processes such as cell polarization, movement, morphogenesis and cell division. Now Rho GTPase plays another leading role in the discovery of a new signaling mechanism for auxin, a multifunctional hormone that regulates pattern formation in plants. Arabidopsis leaf epidermal pavement cells (PCs) develop the puzzle-piece cell shape with interlocking lobes and indentations via interdigitated cellular growth.1 Through the ABP1 (Auxin Binding Protein 1) cell surface receptor, auxin coordinately activates 2 mutually exclusive Rho GTPase signaling pathways that are activated in the complementary lobing and indenting sides of adjacent cells: the ROP2 pathway for lobe formation and the ROP6 pathway for promoting indentation. This new signaling mechanism also involves ROP2-dependent polar accumulation of PIN1 in the plasma membrane, a member of the PIN auxin efflux carrier family that is critical for the formation of various developmental patterns including the PC interdigitation pattern. This Rho-dependent auxin signaling mechanism explains how interdigitated cellular growth is coordinated. In this extra view, we propose that the same mechanism can also explain how a uniform auxin signal initiates the formation of the interdigitated pattern.

Is ABP1 an auxin receptor yet?

AUXIN BINDING PROTEIN 1 (ABP1) has long been proposed as an auxin receptor to regulate cell expansion. The embryo lethality of ABP1-null mutants demonstrates its fundamental role in plant development, but also hinders investigation of its involvement in post-embryonic processes and its mode of action. By taking advantage of weak alleles and inducible systems, several recent studies have revealed a role for ABP1 in organ development, cell polarization, and shape formation. In addition to its role in the regulation of auxin-induced gene expression, ABP1 has now been shown to modulate non-transcriptional auxin responses. ABP1 is required for activating two antagonizing ROP GTPase signaling pathways involved in cytoskeletal reorganization and cell shape formation, and participates in the regulation of clathrin-mediated endocytosis to subsequently affect PIN protein distribution. These exciting discoveries provide indisputable evidence for the auxin-induced signaling pathways that are downstream of ABP1 function, and suggest intriguing mechanisms for ABP1-mediated polar cell expansion and spatial coordination in response to auxin.

AUXIN BINDING PROTEIN1: the outsider

AUXIN BINDING PROTEIN1 (ABP1) is one of the first characterized proteins that bind auxin and has been implied as a receptor for a number of auxin responses. Early studies characterized its auxin binding properties and focused on rapid electrophysiological and cell expansion responses, while subsequent work indicated a role in cell cycle and cell division control. Very recently, ABP1 has been ascribed a role in modulating endocytic events at the plasma membrane and RHO OF PLANTS-mediated cytoskeletal rearrangements during asymmetric cell expansion. The exact molecular function of ABP1 is still unresolved, but its main activity apparently lies in influencing events at the plasma membrane. This review aims to connect the novel findings with the more classical literature on ABP1 and to point out the many open questions that still separate us from a comprehensive model of ABP1 action, almost 40 years after the first reports of its existence.

AUXIN-BINDING-PROTEIN1, the second auxin receptor: what is the significance of a two-receptor concept in plant signal transduction?

Since we are living in the 'age of transcription', awareness of aspects other than transcription in auxin signal transduction seems to have faded. One purpose of this review is to recall these other aspects. The focus will also be on the time scales of auxin responses and their potential or known dependence on either AUXIN BINDING PROTEIN 1 (ABP1) or on TRANSPORT-INHIBITOR-RESISTANT1 (TIR1) as a receptor. Furthermore, both direct and indirect evidence for the function of ABP1 as a receptor will be reviewed. Finally, the potential functions of a two-receptor system for auxin and similarities to other two-receptor signalling systems in plants will be discussed. It is suggested that such a functional arrangement is a property of plants which strengthens tissue autonomy and overcomes the lack of nerves or blood circulation which are responsible for rapid signal transport in animals.

The heterozygous abp1/ABP1 insertional mutant has defects in functions requiring polar auxin transport and in regulation of early auxin-regulated genes

AUXIN-BINDING PROTEIN 1 (ABP1) is not easily accessible for molecular studies because the homozygous T-DNA insertion mutant is embryo-lethal. We found that the heterozygous abp1/ABP1 insertion mutant has defects in auxin physiology-related responses: higher root slanting angles, longer hypocotyls, agravitropic roots and hypocotyls, aphototropic hypocotyls, and decreased apical dominance. Heterozygous plants flowered earlier than wild-type plants under short-day conditions. The length of the main root, the lateral root density and the hypocotyl length were little altered in the mutant in response to auxin. Compared to wild-type plants, transcription of early auxin-regulated genes (IAA2, IAA11, IAA13, IAA14, IAA19, IAA20, SAUR9, SAUR15, SAUR23, GH3.5 and ABP1) was less strongly up-regulated in the mutant by 0.1, 1 and 10 μm IAA. Surprisingly, ABP1 was itself an early auxin-up-regulated gene. IAA uptake into the mutant seedlings during auxin treatments was indistinguishable from wild-type. Basipetal auxin transport in young roots was slower in the mutant, indicating a PIN2/EIR1 defect, while acropetal transport was indistinguishable from wild-type. In the eir1 background, three of the early auxin-regulated genes tested (IAA2, IAA13 and ABP1) were more strongly induced by 1 μm IAA in comparison to wild-type, but eight of them were less up-regulated in comparison to wild-type. Similar but not identical disturbances in regulation of early auxin-regulated genes indicate tight functional linkage of ABP1 and auxin transport regulation. We hypothesize that ABP1 is involved in the regulation of polar auxin transport, and thus affects local auxin concentration and early auxin gene regulation. In turn, ABP1 itself is under the transcriptional control of auxin.

Indolyl-3-butyric acid-induced Arabidopsis stomatal opening mediated by 3',5'-cyclic guanosine-monophosphate

It has been pharmacologically suggested that 3',5'-cyclic guanosine-monophosphate (cGMP) mediates indolyl-3-butyric acid (IBA)-induced stomatal opening. In Arabidopsis thaliana (L.) Heynh., such investigations compared the wild type (Columbia and Ws ecotypes) to mutants knockout for either GTP-binding protein (G protein) α subunit 1 (gpa1-4), putative G protein-coupled receptor 1 (gcr1-5), calcineurin B-like isoform 1 (cbl1) or 9 (cbl9), or the NADPH oxidases AtrbohD and AtrbohF (atrbohD/F). Stomatal opening to IBA or the permeant cGMP analogue, 8-bromo-cGMP (8-Br-cGMP) was abolished in the atrbohD/F mutant. The IBA response was fully or partially suppressed, respectively, in the gcr1-5 mutant, or the gpa1-4 and cbl1 mutants. In the cbl9 mutant, the response to IBA or 8-Br-cGMP, respectively, was partially or fully suppressed. Phenylarsine oxide (PAO) affected the IBA response, which the cbl1 mutant overlapped or the gpa1-4 and cbl9 mutants increased up to 100% inhibition. 6-anilino-5,8-quinolinedione, mas17, the (Rp)-diastereomer of 8-bromo-3',5'-cyclic guanosine monophosphorothioate (Rp-8-Br-cGMPS), nicotinamide, ruthenium red (RRed), 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), cyclosporine A (CsA) and FK506 converged to affect the IBA response, which the gpa1-4 and cbl9 mutants overlapped or the cbl1 mutant and PAO increased up to 100% inhibition. Rp-8-Br-cGMPS, nicotinamide, RRed, BAPTA, CsA or FK506 paralled the cbl9 and atrbohD/F mutants to abolish the 8-Br-cGMP response. Based on so far revealed features of these mutants and pharmacological compounds, these results confirmed cGMP as a Ca(2+)-mobilizing second messenger for apoplastic auxin whose perception and transduction would implicate a seven-transmembrane receptor - G protein - guanylyl cyclase unit at the guard cell plasma membrane.

AUXIN BINDING PROTEIN 1: functional and evolutionary aspects

In this review, we examine the role of AUXIN BINDING PROTEIN 1 (ABP1) in mediating growth and developmental responses. ABP1 is involved in a broad range of cellular responses to auxin, acting either as the main regulator of the response, such as seen for entry into cell division or, as a fine-tuning device as for the regulation of expression of early auxin response genes. Phylogenetic analysis has revealed that ABP1 is an ancient protein that was already present in various algae and has acquired a motif of retention in the endoplasmic reticulum only recently. An evaluation of the evidence for ABP1 function according to its cellular localization supports the plasma membrane as a starting point for ABP1-mediated auxin signaling.

Cellular responses to auxin: division versus expansion

The phytohormone auxin is a major regulator of plant growth and development. Many aspects of these processes depend on the multiple controls exerted by auxin on cell division and cell expansion. The detailed mechanisms by which auxin controls these essential cellular responses are still poorly understood, despite recent progress in the identification of auxin receptors and components of auxin signaling pathways. The purpose of this review is to provide an overview of the present knowledge of the molecular mechanisms involved in the auxin control of cell division and cell expansion. In both cases, the involvement of at least two signaling pathways and of multiple targets of auxin action reflects the complexity of the subtle regulation of auxin-mediated cellular responses. In addition, it offers the necessary flexibility for generating differential responses within a given cell depending on its developmental context.

A new perspective on auxin perception

An important question in modern plant biology concerns the mechanisms of auxin perception. Despite the recently discovered soluble receptor, the F-box protein TIR1, there is no doubt that another type of signal perception exists, and is linked to the plasma membrane. Two models for the receptor have been suggested: either the receptor includes a protein kinase, or it is coupled with a G-protein. We propose a third model, acting through Ca(2+)-channels in the plasma membrane. The model is based on the revealed rapid auxin-induced reactions, including changes in the membrane potential, shifts in cytosol concentration of Ca(2+) and H(+) and modulation of cell sensitivity to hormones by the external Ca(2+) concentration. Detailed inhibitor analysis with both living cells and isolated plasma membranes show that auxin might directly stimulate Ca(2+) transport through the plasma membrane. A hypothetical scheme of auxin perception at the plasma membrane is suggested together with further transduction events. In addition, comparative analyses of auxin and serotonin perceptions are provided.

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.

Threonine at position 306 of the KAT1 potassium channel is essential for channel activity and is a target site for ABA-activated SnRK2/OST1/SnRK2.6 protein kinase

The Arabidopsis thaliana K+ channel KAT1 has been suggested to have a key role in mediating the aperture of stomata pores on the surface of plant leaves. Although the activity of KAT1 is thought to be regulated by phosphorylation, the endogenous pathway and the primary target site for this modification remained unknown. In the present study, we have demonstrated that the C-terminal region of KAT1 acts as a phosphorylation target for the Arabidopsis calcium-independent ABA (abscisic acid)-activated protein kinase SnRK2.6 (Snf1-related protein kinase 2.6). This was confirmed by LC-MS/MS (liquid chromatography tandem MS) analysis, which showed that Thr306 and Thr308 of KAT1 were modified by phosphorylation. The role of these specific residues was examined by single point mutations and measurement of KAT1 channel activities in Xenopus oocyte and yeast systems. Modification of Thr308 had minimal effect on KAT1 activity. On the other hand, modification of Thr306 reduced the K+ transport uptake activity of KAT1 in both systems, indicating that Thr306 is responsible for the functional regulation of KAT1. These results suggest that negative regulation of KAT1 activity, required for stomatal closure, probably occurs by phosphorylation of KAT1 Thr306 by the stress-activated endogenous SnRK2.6 protein kinase.

Hormone interactions in stomatal function

Research in recent years on the biology of guard cells has shown that these specialized cells integrate both extra- and intra-cellular signals in the control of stomatal apertures. Among the phytohormones, abscisic acid (ABA) is one of the key players regulating stomatal function. In addition, auxin, cytokinin, ethylene, brassinosteroids, jasmonates, and salicylic acid also contribute to stomatal aperture regulation. The interaction of multiple hormones can serve to determine the size of stomatal apertures in a condition-specific manner. Here, we discuss the roles of different phytohormones and the effects of their interactions on guard cell physiology and function.

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.

Characterization of a tobacco TPK-type K+ channel as a novel tonoplast K+ channel using yeast tonoplasts

The tonoplast K(+) membrane transport system plays a crucial role in maintaining K(+) homeostasis in plant cells. Here, we isolated cDNAs encoding a two-pore K(+) channel (NtTPK1) from Nicotiana tabacum cv. SR1 and cultured BY-2 tobacco cells. Two of the four variants of NtTPK1 contained VHG and GHG instead of the GYG signature sequence in the second pore region. All four products were functional when expressed in the Escherichia coli cell membrane, and NtTPK1 was targeted to the tonoplast in tobacco cells. Two of the three promoter sequences isolated from N. tabacum cv. SR1 were active, and expression from these was increased approximately 2-fold by salt stress or high osmotic shock. To determine the properties of NtTPK1, we enlarged mutant yeast cells with inactivated endogenous tonoplast channels and prepared tonoplasts suitable for patch clamp recording allowing the NtTPK1-related channel conductance to be distinguished from the small endogenous currents. NtTPK1 exhibited strong selectivity for K(+) over Na(+). NtTPK1 activity was sensitive to spermidine and spermine, which were shown to be present in tobacco cells. NtTPK1 was active in the absence of Ca(2+), but a cytosolic concentration of 45 microM Ca(2+) resulted in a 2-fold increase in the amplitude of the K(+) current. Acidification of the cytosol to pH 5.5 also markedly increased NtTPK1-mediated K(+) currents. These results show that NtTPK1 is a novel tonoplast K(+) channel belonging to a different group from the previously characterized vacuolar channels SV, FV, and VK.

Mitochondrial sequestration of BCECF after ester loading in the giant alga Chara australis

Ratiometric fluorescent dyes are often used to monitor free ion concentrations in vivo, especially in cells that are recalcitrant to transformation with genetically encoded fluorescent markers. Although intracellular dye distributions are often found to be cytosolic, dye localisation has often not been examined in detail. We began exploring the use of BCECF (2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein) to monitor pH in the giant alga Chara australis and discovered that younger leaf cells could be loaded using the acetoxymethyl ester of BCECF. However, we were puzzled to find in microphotometric measurements that the fluorescence ratio appeared insensitive to manipulations affecting cytosolic pH. Confocal imaging of C. australis cells loaded with BCECF showed an accumulation of the dye in two locations: (1) on the outside of the chloroplasts in irregularly shaped stationary bodies; (2) within 1-1.5 mum structures that moved rapidly with the pericellular cytoplasmic streaming. Together with the streaming cytoplasm, these organelles were rendered stationary with 50 muM cytochalasin D. Rhodamine 123, a mitochondrionspecific dye, highlighted organelles outside of the chloroplasts, similar to those shown by BCECF in location 1. We conclude that in the cytoplasmic compartment, BCECF was sequestered within cytoplasmic mitochondria in immature and fast-growing cells and within the cortical mitochondrial system in older and slowly growing cells. Thus, BCECF-AM is unsuitable for reporting changes in cytosolic pH in C. australis but might be employed in future to study pH changes in the mitochondria.

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.

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.

Auxin-dependent cell division and cell elongation. 1-Naphthaleneacetic acid and 2,4-dichlorophenoxyacetic acid activate different pathways

During exponential phase, the tobacco (Nicotiana tabacum) cell line cv Virginia Bright Italia-0 divides axially to produce linear cell files of distinct polarity. This axial division is controlled by exogenous auxin. We used exponential tobacco cv Virginia Bright Italia-0 cells to dissect early auxin signaling, with cell division and cell elongation as physiological markers. Experiments with 1-naphthaleneacetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D) demonstrated that these 2 auxin species affect cell division and cell elongation differentially; NAA stimulates cell elongation at concentrations that are much lower than those required to stimulate cell division. In contrast, 2,4-D promotes cell division but not cell elongation. Pertussis toxin, a blocker of heterotrimeric G-proteins, inhibits the stimulation of cell division by 2,4-D but does not affect cell elongation. Aluminum tetrafluoride, an activator of the G-proteins, can induce cell division at NAA concentrations that are not permissive for division and even in the absence of any exogenous auxin. The data are discussed in a model where the two different auxins activate two different pathways for the control of cell division and cell elongation.

Role of the plasma membrane H+-ATPase in auxin-induced elongation growth: historical and new aspects

This article will cover historical and recent aspects of reactions and mechanisms involved in the auxin-induced signalling cascade that terminates in the dramatic elongation growth of cells and plant organs. Massive evidence has accumulated that the final target of auxin action is the plasma membrane H(+)-ATPase, which excretes H(+) ions into the cell wall compartment and, in an antiport, takes up K(+) ions through an inwardly rectifying K(+) channel. The auxin-enhanced H(+) pumping lowers the cell wall pH, activates pH-sensitive enzymes and proteins within the wall, and initiates cell-wall loosening and extension growth. These processes, induced by auxin or by the "super-auxin" fusicoccin, can be blocked instantly and specifically by a voltage inhibition of the H(+)-ATPase due to removal of K(+) ions or the addition of K(+)-channel blockers. Vice versa, H(+) pumping and growth are immediately switched on by addition of K(+) ions. Furthermore, the treatment of segments either with auxin or with fusicoccin (which activates the H(+)-ATPase irreversibly) or with acid buffers (from outside) causes an identical transformation and degradation pattern of cell wall constituents during cell-wall loosening and growth. These and other results described below are in agreement with the acid-growth theory of elongation growth. However, objections to this theory are also discussed.

A developmental gradient in the mechanism of K+ uptake during light-stimulated leaf growth in Nicotiana tabacum L

Light causes growth of dicotyledonous leaves by stimulating proton efflux, cell wall acidification and loosening, and solute accumulation for turgor maintenance. For cells still undergoing cell division at the base of expanding tobacco ( Nicotiana tabacum L. cv. Xanthi) leaves, light-stimulated growth depends on K+ uptake, and is inhibited by the potassium channel blocker tetraethylammonium (TEA). The generality of this mechanism has been tested by comparing the effect of light on the growth-associated physiology of dividing and expanding cells in the base with cells at the tip growing by cell expansion only. The magnitude of the light-induced growth response of excised leaf discs is greatest at the leaf base and declines as cells mature. Basal tissue is more sensitive to exogenous potassium, which enhances light-stimulated growth at <1 mM, whereas tip tissue requires higher levels (>10 mM). Growth is inhibited by TEA similarly in tip and base. However, light-stimulated K+ uptake and proton efflux respond differently to TEA in tip and basal tissue. In basal tissue, TEA reduces light-stimulated K+ uptake by 60% and inhibits light-stimulated proton efflux. These results agree with those presented by M. Claussen et al. (1997, Planta 201:227-234) showing that auxin-stimulated H+ pump activity and growth in coleoptiles require K+ uptake as an electrical counterbalance to H+ efflux. In contrast, in tip tissue, TEA inhibits light-stimulated K+ uptake by only 17% and does not inhibit proton efflux. Our results suggest that the basipetal gradient in the effect of TEA on light-regulated growth physiology can be explained by TEA effects on K+ uptake: TEA inhibits light-stimulated H+ pump activity, wall acidification and membrane hyperpolarization only in cells dependent on TEA-sensitive channels for light-stimulated K+ uptake. Further, our data suggest that younger, basal tissue is dependent on TEA-sensitive, sucrose-stimulatable channels for light-stimulated K+ uptake whereas older, tip tissue is able to use an additional, TEA-insensitive K+ transporter to mediate light-stimulated K+ uptake.

The beta-subunit of the Arabidopsis G protein negatively regulates auxin-induced cell division and affects multiple developmental processes

Plant cells respond to low concentrations of auxin by cell expansion, and at a slightly higher concentration, these cells divide. Previous work revealed that null mutants of the alpha-subunit of a putative heterotrimeric G protein (GPA1) have reduced cell division. Here, we show that this prototypical G protein complex acts mechanistically by controlling auxin sensitivity toward cell division. Loss-of-function G protein mutants have altered auxin-mediated cell division throughout development, especially during the auxin-induced formation of lateral and adventitious root primordia. Ectopic expression of the wild-type Galpha-subunit phenocopies the Gbeta mutants (auxin hypersensitivity), probably by sequestering the Gbetagamma-subunits, whereas overexpression of Gbeta reduces auxin sensitivity and a constitutively active (Q222L) mutant Galpha behaves like the wild type. These data are consistent with a model in which Gbetagamma acts as a negative regulator of auxin-induced cell division. Accordingly, basal repression of approximately one-third of the identified auxin-regulated genes (47 of 150 upregulated genes among 8300 quantitated) is lost in the Gbeta transcript-null mutant. Included among these are genes that encode proteins proposed to control cell division in root primordia formation as well as several novel genes. These results suggest that although auxin-regulated cell division is not coupled directly by a G protein, the Gbeta-subunit attenuates this auxin pathway upstream of the control of mRNA steady state levels.

Crystal structure of auxin-binding protein 1 in complex with auxin

The structure of auxin-binding protein 1 (ABP1) from maize has been determined at 1.9 A resolution, revealing its auxin-binding site. The structure confirms that ABP1 belongs to the ancient and functionally diverse germin/seed storage 7S protein superfamily. The binding pocket of ABP1 is predominantly hydrophobic with a metal ion deep inside the pocket coordinated by three histidines and a glutamate. Auxin binds within this pocket, with its carboxylate binding the zinc and its aromatic ring binding hydrophobic residues including Trp151. There is a single disulfide between Cys2 and Cys155. No conformational rearrangement of ABP1 was observed when auxin bound to the protein in the crystal, but examination of the structure reveals a possible mechanism of signal transduction.

Channelling auxin action: modulation of ion transport by indole-3-acetic acid

The growth hormone auxin is a key regulator of plant cell division and elongation. Since plants lack muscles, processes involved in growth and movements rely on turgor formation, and thus on the transport of solutes and water. Modern electrophysiological techniques and molecular genetics have shed new light on the regulation of plant ion transporters in response to auxin. Guard cells, hypocotyls and coleoptiles have advanced to major model systems in studying auxin action. This review will therefore focus on the molecular mechanism by which auxin modulates ion transport and cell expansion in these model cell types.