The Arabidopsis cax1 mutant exhibits impaired ion homeostasis, development, and hormonal responses and reveals interplay among vacuolar transporters

Comparative physiology of elemental distributions in plants

BACKGROUND

Plants contain relatively few cell types, each contributing a specialized role in shaping plant function. With respect to plant nutrition, different cell types accumulate certain elements in varying amounts within their storage vacuole. The role and mechanisms underlying cell-specific distribution of elements in plants is poorly understood.

SCOPE

The phenomenon of cell-specific elemental accumulation has been briefly reviewed previously, but recent technological advances with the potential to probe mechanisms underlying elemental compartmentation have warranted an updated evaluation. We have taken this opportunity to catalogue many of the studies, and techniques used for, recording cell-specific compartmentation of particular elements. More importantly, we use three case-study elements (Ca, Cd and Na) to highlight the basis of such phenomena in terms of their physiological implications and underpinning mechanisms; we also link such distributions to the expression of known ion or solute transporters.

CONCLUSIONS

Element accumulation patterns are clearly defined by expression of key ion or solute transporters. Although the location of element accumulation is fairly robust, alterations in expression of certain solute transporters, through genetic modifications or by growth under stress, result in perturbations to these patterns. However, redundancy or induced pleiotropic expression effects may complicate attempts to characterize the pathways that lead to cell-specific elemental distribution. Accumulation of one element often has consequences on the accumulation of others, which seems to be driven largely to maintain vacuolar and cytoplasmic osmolarity and charge balance, and also serves as a detoxification mechanism. Altered cell-specific transcriptomics can be shown, in part, to explain some of this compensation.

Calcium signals: the lead currency of plant information processing

Ca(2+) signals are core transducers and regulators in many adaptation and developmental processes of plants. Ca(2+) signals are represented by stimulus-specific signatures that result from the concerted action of channels, pumps, and carriers that shape temporally and spatially defined Ca(2+) elevations. Cellular Ca(2+) signals are decoded and transmitted by a toolkit of Ca(2+) binding proteins that relay this information into downstream responses. Major transduction routes of Ca(2+) signaling involve Ca(2+)-regulated kinases mediating phosphorylation events that orchestrate downstream responses or comprise regulation of gene expression via Ca(2+)-regulated transcription factors and Ca(2+)-responsive promoter elements. Here, we review some of the remarkable progress that has been made in recent years, especially in identifying critical components functioning in Ca(2+) signal transduction, both at the single-cell and multicellular level. Despite impressive progress in our understanding of the processing of Ca(2+) signals during the past years, the elucidation of the exact mechanistic principles that underlie the specific recognition and conversion of the cellular Ca(2+) currency into defined changes in protein-protein interaction, protein phosphorylation, and gene expression and thereby establish the specificity in stimulus response coupling remain to be explored.

Arabidopsis V-ATPase activity at the tonoplast is required for efficient nutrient storage but not for sodium accumulation

The productivity of higher plants as a major source of food and energy is linked to their ability to buffer changes in the concentrations of essential and toxic ions. Transport across the tonoplast is energized by two proton pumps, the vacuolar H(+)-ATPase (V-ATPase) and the vacuolar H(+)-pyrophosphatase (V-PPase); however, their functional relation and relative contributions to ion storage and detoxification are unclear. We have identified an Arabidopsis mutant in which energization of vacuolar transport solely relies on the activity of the V-PPase. The vha-a2 vha-a3 double mutant, which lacks the two tonoplast-localized isoforms of the membrane-integral V-ATPase subunit VHA-a, is viable but shows day-length-dependent growth retardation. Nitrate content is reduced whereas nitrate assimilation is increased in the vha-a2 vha-a3 mutant, indicating that vacuolar nitrate storage represents a major growth-limiting factor. Zinc is an essential micronutrient that is toxic at excess concentrations and is detoxified via a vacuolar Zn(2+)/H(+)-antiport system. Accordingly, the double mutant shows reduced zinc tolerance. In the same way the vacuolar Na(+)/H(+)-antiport system is assumed to be an important component of the system that removes sodium from the cytosol. Unexpectedly, salt tolerance and accumulation are not affected in the vha-a2 vha-a3 double mutant. In contrast, reduction of V-ATPase activity in the trans-Golgi network/early endosome (TGN/EE) leads to increased salt sensitivity. Taken together, our results show that during gametophyte and embryo development V-PPase activity at the tonoplast is sufficient whereas tonoplast V-ATPase activity is limiting for nutrient storage but not for sodium tolerance during vegetative and reproductive growth.

The language of calcium signaling

Ca(2+) signals are a core regulator of plant cell physiology and cellular responses to the environment. The channels, pumps, and carriers that underlie Ca(2+) homeostasis provide the mechanistic basis for generation of Ca(2+) signals by regulating movement of Ca(2+) ions between subcellular compartments and between the cell and its extracellular environment. The information encoded within the Ca(2+) transients is decoded and transmitted by a toolkit of Ca(2+)-binding proteins that regulate transcription via Ca(2+)-responsive promoter elements and that regulate protein phosphorylation. Ca(2+)-signaling networks have architectural structures comparable to scale-free networks and bow tie networks in computing, and these similarities help explain such properties of Ca(2+)-signaling networks as robustness, evolvability, and the ability to process multiple signals simultaneously.

Characterization of a PutCAX1 gene from Puccinellia tenuiflora that confers Ca2+ and Ba2+ tolerance in yeast

The gene for a novel cation/H+ antiporter from Puccinellia tenuiflora, PutCAX1, was cloned from a cDNA library. The PutCAX protein was localized in the vacuolar membrane using a GFP marker. Several yeast transformants were created using full-length and truncated form of PutCAX1 and their growths in the presence of various cations (Mg2+, Ca2+, Mn2+, Ni2+, Cu2+, Zn2+, Se2+, and Ba2+) were analyzed. PutCAX1 expression was found to affect the response to Ca2+ and Ba2+ in yeast. The PutCAX1 and C-terminally truncated PutCAX1 (DeltaCPutCAX1) transformants grew in the presence of 70 mM Ca2+ as well as in the presence of 8 mM Ba2+. However, the DeltaCPutCAX1 transformant was able to grow in the presence of 20 mM Ba2+ while the PutCAX1 transformant could not. On the other hand, expression of the N-terminally truncated form and the N- and C-terminally truncated form failed to suppress the Ca2+ or Ba2+ sensitivity of yeast. These results suggest that PutCAX1 can complement the active Ca2+ transporters at some level and confer yeast Ba2+ tolerance, and that the N- and C-terminal regions of PutCAX1 play important roles in increasing the Ca2+ or Ba2+ tolerance of yeast.

Comparative analysis of CAX2-like cation transporters indicates functional and regulatory diversity

Internal compartmentalization of metals is an important metal tolerance mechanism in many organisms. In plants and fungi, sequestration into the vacuole is a major detoxification mechanism for metals. Cation transport into the vacuole can be mediated by CAX (cation exchanger) transporters. The Arabidopsis thaliana AtCAX2 transporter was shown previously to transport Ca(2+), Cd(2+) and Mn(2+). To assess the conservation of the functional and regulatory characteristics of CAX2-like transporters in higher plants, we have characterized AtCAX2 orthologues from Arabidopsis (AtCAX5), tomato (LeCAX2) and barley (HvCAX2). Substrate specificity and regulatory activity were assessed using a yeast heterologous-expression assay. Each CAX could transport Ca(2+) and Mn(2+) into the yeast vacuole, but they each had different cation transport kinetics. Most notably, there was variation in the regulation of the transporters. As found with AtCAX2 previously, only expression of an N-terminally truncated form of AtCAX5 in yeast was able to mediate Ca(2+) and Mn(2+) transport, indicating that activity may be controlled by an autoregulatory region at the N-terminus. In contrast, either full-length or truncated LeCAX2 could efficiently transport Ca(2+), although Mn(2+) transport was controlled by the N-terminus. HvCAX2 did not appear to possess an N-terminal regulatory domain. Expression of AtCAX2 was not significantly modulated by metal stress; however, AtCAX5 and HvCAX2 were transcriptionally up-regulated by high Mn(2+) treatment, and by Ca(2+) and Na(+) stress respectively. It is therefore apparent that, despite the high sequence identity between plant CAX2 orthologues, there is significant diversity in their functional characteristics, particularly with regard to regulatory mechanisms.

Molecular regulators of phosphate homeostasis in plants

An appropriate cellular phosphate (Pi) concentration is indispensable for essential physiological and biochemical processes. To maintain cellular Pi homeostasis, plants have developed a series of adaptive responses to facilitate external Pi acquisition and to limit Pi consumption and to adjust Pi recycling internally when the Pi supply is inadequate. Over the past decade, significant progress has been made toward understanding such regulation at the molecular level. In this review, the focus is on the molecular regulators that mediate cellular Pi concentrations. The regulators are introduced and organized according to their original identification procedures, by the forward genetic approach of mutant screening or by reverse genetic analysis. These genes are involved in Pi uptake, allocation or remobilization or are upstream regulators, such as transcriptional factors or signalling molecules. In the future, integration of current knowledge and exploration of new technology is expected to offer new insights into molecular mechanisms that maintain Pi homeostasis.

Root development under metal stress in Arabidopsis thaliana requires the H+/cation antiporter CAX4

* The Arabidopsis vacuolar CAtion eXchangers (CAXs) play a key role in mediating cation influx into the vacuole. In Arabidopsis, there are six CAX genes. However, some members are yet to be characterized fully. * In this study, we show that CAX4 is expressed in the root apex and lateral root primordia, and that expression is increased when Ni(2+) or Mn(2+) levels are elevated or Ca(2+) is depleted. * Transgenic plants expressing increased levels of CAX4 display symptoms consistent with increased sequestration of Ca(2+) and Cd(2+) into the vacuole. When CAX4 is highly expressed in an Arabidopsis cax1 mutant line with weak vacuolar Ca(2+)/H(+) antiport activity, a 29% increase in Ca(2+)/H(+) antiport is measured. A cax4 loss-of-function mutant and CAX4 RNA interference lines display altered root growth in response to Cd(2+), Mn(2+) and auxin. The DR5::GUS auxin reporter detected reduces auxin responses in the cax4 lines. * These results indicate that CAX4 is a cation/H(+) antiporter that plays an important function in root growth under heavy metal stress conditions.

Shaping the calcium signature

In numerous plant signal transduction pathways, Ca2+ is a versatile second messenger which controls the activation of many downstream actions in response to various stimuli. There is strong evidence to indicate that information encoded within these stimulus-induced Ca2+ oscillations can provide signalling specificity. Such Ca2+ signals, or 'Ca2+ signatures', are generated in the cytosol, and in noncytosolic locations including the nucleus and chloroplast, through the coordinated action of Ca2+ influx and efflux pathways. An increased understanding of the functions and regulation of these various Ca2+ transporters has improved our appreciation of the role these transporters play in specifically shaping the Ca2+ signatures. Here we review the evidence which indicates that Ca2+ channel, Ca2+-ATPase and Ca2+ exchanger isoforms can indeed modulate specific Ca2+ signatures in response to an individual signal.

A cation/proton-exchanging protein is a candidate for the barley NecS1 gene controlling necrosis and enhanced defense response to stem rust

We characterized three lesion mimic necS1 (necrotic Steptoe) mutants, induced by fast neutron (FN) treatment of barley cultivar Steptoe. The three mutants are recessive and allelic. When infected with Puccinia graminis f. sp. tritici pathotypes MCC and QCC and P. graminis f. sp. secalis isolate 92-MN-90, all three mutants exhibited enhanced resistance compared to parent cultivar Steptoe. These results suggested that the lesion mimic mutants carry broad-spectrum resistance to stem rust. In order to identify the mutated gene responsible for the phenotype, transcript-based cloning was used. Two genes, represented by three Barley1 probesets (Contig4211_at and Contig4212_s_at, representing the same gene, and Contig10850_s_at), were deleted in all three mutants. Genetic analysis suggested that the lesion mimic phenotype was due to a mutation in one or both of these genes, named NecS1. Consistent with the increased disease resistance, all three mutants constitutively accumulated elevated transcript levels of pathogenesis-related (PR) genes. Barley stripe mosaic virus (BSMV) has been developed as a virus-induced gene-silencing (VIGS) vector for monocots. We utilized BSMV-VIGS to demonstrate that silencing of the gene represented by Contig4211_at, but not Contig10850_s_at caused the necrotic lesion mimic phenotype on barley seedling leaves. Therefore, Contig4211_at is a strong candidate for the NecS1 gene, which encodes a cation/proton exchanging protein (HvCAX1).

Interaction between Arabidopsis Ca2+/H+ exchangers CAX1 and CAX3

In plants, high capacity tonoplast cation/H(+) antiport is mediated in part by a family of CAX (cation exchanger) transporters. Functional association between CAX1 and CAX3 has previously been inferred; however, the nature of this interaction has not been established. Here we analyze the formation of "hetero-CAX" complexes and their transport properties. Co-expressing both CAX1 and CAX3 mediated lithium and salt tolerance in yeast, and these phenotypes could not be recapitulated by expression of deregulated versions of either transporter. Coincident expression of Arabidopsis CAX1 and CAX3 occurs during particular stress responses, flowering, and seedling growth. Analysis of cax1, cax3, and cax1/3 seedlings demonstrated similar stress sensitivities. When plants expressed high levels of both CAXs, alterations in transport properties were evident that could not be recapitulated by high level expression of either transporter individually. In planta coimmunoprecipitation suggested that a protein-protein interaction occurred between CAX1 and CAX3. In vivo interaction between the CAX proteins was shown using a split ubiquitin yeast two-hybrid system and gel shift assays. These findings demonstrate cation exchange plasticity through hetero-CAX interactions.

AtCCX3 is an Arabidopsis endomembrane H+ -dependent K+ transporter

The Arabidopsis (Arabidopsis thaliana) cation calcium exchangers (CCXs) were recently identified as a subfamily of cation transporters; however, no plant CCXs have been functionally characterized. Here, we show that Arabidopsis AtCCX3 (At3g14070) and AtCCX4 (At1g54115) can suppress yeast mutants defective in Na(+), K(+), and Mn(2+) transport. We also report high-capacity uptake of (86)Rb(+) in tonoplast-enriched vesicles from yeast expressing AtCCX3. Cation competition studies showed inhibition of (86)Rb(+) uptake in AtCCX3 cells by excess Na(+), K(+), and Mn(2+). Functional epitope-tagged AtCCX3 fusion proteins were localized to endomembranes in plants and yeast. In Arabidopsis, AtCCX3 is primarily expressed in flowers, while AtCCX4 is expressed throughout the plant. Quantitative polymerase chain reaction showed that expression of AtCCX3 increased in plants treated with NaCl, KCl, and MnCl(2). Insertional mutant lines of AtCCX3 and AtCCX4 displayed no apparent growth defects; however, overexpression of AtCCX3 caused increased Na(+) accumulation and increased (86)Rb(+) transport. Uptake of (86)Rb(+) increased in tonoplast-enriched membranes isolated from Arabidopsis lines expressing CCX3 driven by the cauliflower mosaic virus 35S promoter. Overexpression of AtCCX3 in tobacco (Nicotiana tabacum) produced lesions in the leaves, stunted growth, and resulted in the accumulation of higher levels of numerous cations. In summary, these findings suggest that AtCCX3 is an endomembrane-localized H(+)-dependent K(+) transporter with apparent Na(+) and Mn(2+) transport properties distinct from those of previously characterized plant transporters.

Overexpression of the tomato K+/H+ antiporter LeNHX2 confers salt tolerance by improving potassium compartmentalization

Here, the function of the tomato (Solanum lycopersicon) K+/H+ antiporter LeNHX2 was studied using 35S-driven gene overexpression of a histagged LeNHX2 protein in Arabidopsis thaliana and LeNHX2 gene silencing in tomato. Transgenic A. thaliana plants expressed the histagged LeNHX2 both in shoots and in roots, as assayed by western blotting. Transitory expression of a green fluorescent protein (GFP) tagged protein showed that the antiporter is present in small vesicles. Internal membrane vesicles from transgenic plants displayed enhanced K+/H+ exchange activity, confirming the K+/H+ antiporter function of this enzyme. Transgenic A. thaliana plants overexpressing the histagged tomato antiporter LeNHX2 exhibited inhibited growth in the absence of K+ in the growth medium, but were more tolerant to high concentrations of Na+ than untransformed controls. When grown in the presence of NaCl, transgenic plants contained lower concentrations of intracellular Na+, but more K+, as compared with untransformed controls. Silencing of LeNHX2 in S. lycopersicon plants produced significant inhibition of plant growth and fruit and seed production as well as increased sensitivity to NaCl. The data indicate that regulation of K+ homeostasis by LeNHX2 is essential for normal plant growth and development, and plays an important role in the response to salt stress by improving K+ accumulation.

A cytoplasmic Ca2+ functional assay for identifying and purifying endogenous cell signaling peptides in Arabidopsis seedlings: identification of AtRALF1 peptide

Transient increases in the cytoplasmic Ca(2+) concentration are key events that initiate many cellular signaling pathways in response to developmental and environmental cues in plants; however, only a few extracellular mediators regulating cytoplasmic Ca(2+) singling are known to date. To identify endogenous cell signaling peptides regulating cytoplasmic Ca(2+) signaling, Arabidopsis seedlings expressing aequorin were used for an in vivo luminescence assay for Ca(2+) changes. These seedlings were challenged with fractions derived from plant extracts. Multiple heat-stable, protease-sensitive peaks of calcium elevating activity were observed after fractionation of these extracts by high-performance liquid chromatography. Tandem mass spectrometry identified the predominant active molecule isolated by a series of such chromatographic separations as a 49-amino acid polypeptide, AtRALF1 (the rapid alkalinization factor protein family). Within 40 s of treatment with nanomolar concentrations of the natural or synthetic version of the peptides, the cytoplasmic Ca(2+) level increased and reached its maximum. Prior treatment with a Ca(2+) chelator or inhibitor of IP 3-dependent signaling partially suppressed the AtRALF1-induced Ca(2+) concentration increase, indicating the likely involvement of Ca(2+) influx across the plasma membrane as well as release of Ca(2+) from intracellular reserves. Ca(2+) imaging using seedlings expressing the FRET-based Ca(2+) sensor yellow cameleon (YC) 3.6 showed that AtRALF1 could induce an elevation in Ca(2+) concentration in the surface cells of the root consistent with the very rapid effects of addition of AtRALF1 on Ca(2+) levels as reported by aequorin. Our data support a model in which the RALF peptide mediates Ca(2+)-dependent signaling events through a cell surface receptor, where it may play a role in eliciting events linked to stress responses or the modulation of growth.

Exchangers man the pumps: Functional interplay between proton pumps and proton-coupled Ca exchangers

Tonoplast-localised proton-coupled Ca(2+) transporters encoded by cation/H(+)exchanger (CAX) genes play a critical role in sequestering Ca(2+) into the vacuole. These transporters may function in coordination with Ca(2+) release channels, to shape stimulus-induced cytosolic Ca(2+) elevations. Recent analysis of Arabidopsis CAX knockout mutants, particularly cax1 and cax3, identified a variety of phenotypes including sensitivity to abiotic stresses, which indicated that these transporters might play a role in mediating the plant's stress response. A common feature of these mutants was the perturbation of H(+)-ATPase activity at both the tonoplast and the plasma membrane, suggesting a tight interplay between the Ca(2+)/H(+) exchangers and H(+) pumps. We speculate that indirect regulation of proton flux by the exchangers may be as important as the direct regulation of Ca(2+) flux. These results suggest cautious interpretation of mutant Ca(2+)/H(+) exchanger phenotypes that may be due to either perturbed Ca(2+) or H(+) transport.

Relationship between calcium decoding elements and plant abiotic-stress resistance

Serving as an important second messenger, calcium ion has unique properties and universal ability to transmit diverse signals that trigger primary physiological actions in cells in response to hormones, pathogens, light, gravity, and stress factors. Being a second messenger of paramount significance, calcium is required at almost all stages of plant growth and development, playing a fundamental role in regulating polar growth of cells and tissues and participating in plant adaptation to various stress factors. Many researches showed that calcium signals decoding elements are involved in ABA-induced stomatal closure and plant adaptation to drought, cold, salt and other abiotic stresses. Calcium channel proteins like AtTPC1 and TaTPC1 can regulate stomatal closure. Recently some new studies show that Ca(2+) is dissolved in water in the apoplast and transported primarily from root to shoot through the transpiration stream. The oscillating amplitudes of [Ca(2+)](o) and [Ca(2+)](i) are controlled by soil Ca(2+) concentrations and transpiration rates. Because leaf water use efficiency (WUE) is determined by stomatal closure and transpiration rate, so there may be a close relationship between Ca(2+) transporters and stomatal closure as well as WUE, which needs to be studied. The selection of varieties with better drought resistance and high WUE plays an increasing role in bio-watersaving in arid and semi-arid areas on the globe. The current paper reviews the relationship between calcium signals decoding elements and plant drought resistance as well as other abiotic stresses for further study.

Reduced V-ATPase activity in the trans-Golgi network causes oxylipin-dependent hypocotyl growth Inhibition in Arabidopsis

Regulated cell expansion allows plants to adapt their morphogenesis to prevailing environmental conditions. Cell expansion is driven by turgor pressure created by osmotic water uptake and is restricted by the extensibility of the cell wall, which in turn is regulated by the synthesis, incorporation, and cross-linking of new cell wall components. The vacuolar H(+)-ATPase (V-ATPase) could provide a way to coordinately regulate turgor pressure and cell wall synthesis, as it energizes the secondary active transport of solutes across the tonoplast and also has an important function in the trans-Golgi network (TGN), which affects synthesis and trafficking of cell wall components. We have previously shown that det3, a mutant with reduced V-ATPase activity, has a severe defect in cell expansion. However, it was not clear if this is caused by a defect in turgor pressure or in cell wall synthesis. Here, we show that inhibition of the tonoplast-localized V-ATPase subunit isoform VHA-a3 does not impair cell expansion. By contrast, inhibition of the TGN-localized isoform VHA-a1 is sufficient to restrict cell expansion. Furthermore, we provide evidence that the reduced hypocotyl cell expansion in det3 is conditional and due to active, hormone-mediated growth inhibition caused by a cell wall defect.

The Arabidopsis cax3 mutants display altered salt tolerance, pH sensitivity and reduced plasma membrane H+-ATPase activity

Perturbing CAX1, an Arabidopsis vacuolar H+/Ca2+ antiporter, and the related vacuolar transporter CAX3, has been previously shown to cause severe growth defects; however, the specific function of CAX3 has remained elusive. Here, we describe plant phenotypes that are shared among cax1 and cax3 including an increased sensitivity to both abscisic acid (ABA) and sugar during germination, and an increased tolerance to ethylene during early seedling development. We have also identified phenotypes unique to cax3, namely salt, lithium and low pH sensitivity. We used biochemical measurements to ascribe these cax3 sensitivities to a reduction in vacuolar H+/Ca2+ transport during salt stress and decreased plasma membrane H+-ATPase activity. These findings catalog an array of CAX phenotypes and assign a specific role for CAX3 in response to salt tolerance.

SOS2 promotes salt tolerance in part by interacting with the vacuolar H+-ATPase and upregulating its transport activity

The salt overly sensitive (SOS) pathway is critical for plant salt stress tolerance and has a key role in regulating ion transport under salt stress. To further investigate salt tolerance factors regulated by the SOS pathway, we expressed an N-terminal fusion of the improved tandem affinity purification tag to SOS2 (NTAP-SOS2) in sos2-2 mutant plants. Expression of NTAP-SOS2 rescued the salt tolerance defect of sos2-2 plants, indicating that the fusion protein was functional in vivo. Tandem affinity purification of NTAP-SOS2-containing protein complexes and subsequent liquid chromatography-tandem mass spectrometry analysis indicated that subunits A, B, C, E, and G of the peripheral cytoplasmic domain of the vacuolar H+-ATPase (V-ATPase) were present in a SOS2-containing protein complex. Parallel purification of samples from control and salt-stressed NTAP-SOS2/sos2-2 plants demonstrated that each of these V-ATPase subunits was more abundant in NTAP-SOS2 complexes isolated from salt-stressed plants, suggesting that the interaction may be enhanced by salt stress. Yeast two-hybrid analysis showed that SOS2 interacted directly with V-ATPase regulatory subunits B1 and B2. The importance of the SOS2 interaction with the V-ATPase was shown at the cellular level by reduced H+ transport activity of tonoplast vesicles isolated from sos2-2 cells relative to vesicles from wild-type cells. In addition, seedlings of the det3 mutant, which has reduced V-ATPase activity, were found to be severely salt sensitive. Our results suggest that regulation of V-ATPase activity is an additional key function of SOS2 in coordinating changes in ion transport during salt stress and in promoting salt tolerance.

Novel tonoplast transporters identified using a proteomic approach with vacuoles isolated from cauliflower buds

Young meristematic plant cells contain a large number of small vacuoles, while the largest part of the vacuome in mature cells is composed by a large central vacuole, occupying 80% to 90% of the cell volume. Thus far, only a limited number of vacuolar membrane proteins have been identified and characterized. The proteomic approach is a powerful tool to identify new vacuolar membrane proteins. To analyze vacuoles from growing tissues we isolated vacuoles from cauliflower (Brassica oleracea) buds, which are constituted by a large amount of small cells but also contain cells in expansion as well as fully expanded cells. Here we show that using purified cauliflower vacuoles and different extraction procedures such as saline, NaOH, acetone, and chloroform/methanol and analyzing the data against the Arabidopsis (Arabidopsis thaliana) database 102 cauliflower integral proteins and 214 peripheral proteins could be identified. The vacuolar pyrophosphatase was the most prominent protein. From the 102 identified proteins 45 proteins were already described. Nine of these, corresponding to 46% of peptides detected, are known vacuolar proteins. We identified 57 proteins (55.9%) containing at least one membrane spanning domain with unknown subcellular localization. A comparison of the newly identified proteins with expression profiles from in silico data revealed that most of them are highly expressed in young, developing tissues. To verify whether the newly identified proteins were indeed localized in the vacuole we constructed and expressed green fluorescence protein fusion proteins for five putative vacuolar membrane proteins exhibiting three to 11 transmembrane domains. Four of them, a putative organic cation transporter, a nodulin N21 family protein, a membrane protein of unknown function, and a senescence related membrane protein were localized in the vacuolar membrane, while a white-brown ATP-binding cassette transporter homolog was shown to reside in the plasma membrane. These results demonstrate that proteomic analysis of highly purified vacuoles from specific tissues allows the identification of new vacuolar proteins and provides an additional view of tonoplastic proteins.

Plant proton pumps

Chemiosmotic circuits of plant cells are driven by proton (H(+)) gradients that mediate secondary active transport of compounds across plasma and endosomal membranes. Furthermore, regulation of endosomal acidification is critical for endocytic and secretory pathways. For plants to react to their constantly changing environments and at the same time maintain optimal metabolic conditions, the expression, activity and interplay of the pumps generating these H(+) gradients have to be tightly regulated. In this review, we will highlight results on the regulation, localization and physiological roles of these H(+)- pumps, namely the plasma membrane H(+)-ATPase, the vacuolar H(+)-ATPase and the vacuolar H(+)-PPase.

Regulation of telomerase in Arabidopsis by BT2, an apparent target of TELOMERASE ACTIVATOR1

Telomerase, an enzyme essential for the synthesis and maintenance of telomeric DNA and the long-term stability of the genome, is developmentally regulated in plants. Telomerase activity is abundant in reproductive organs but low or undetectable in vegetative organs. Treatment with exogenous auxin, however, overrides this developmental control and induces telomerase in mature leaves. The Arabidopsis thaliana transcription factor TELOMERASE ACTIVATOR1 (TAC1) potentiates some responses to auxin, including the induction of telomerase activity in leaves. Here, we report that BT2, a protein with BTB, TAZ, and calmodulin binding domains, is an essential component of the TAC1-mediated telomerase activation pathway. Steady state concentration of BT2 mRNA increases in response to TAC1 expression, and TAC1 specifically binds the BT2 promoter both in vitro and in yeast one-hybrid assays. Constitutive expression of BT2 induces telomerase activity in leaves, whereas a null mutation of BT2 blocks TAC1-mediated telomerase induction, indicating that BT2 acts downstream of TAC1 to regulate telomerase activity in mature vegetative organs.

Identification of microspore-active promoters that allow targeted manipulation of gene expression at early stages of microgametogenesis in Arabidopsis

BACKGROUND

The effective functional analysis of male gametophyte development requires new tools enabling the spatially and temporally controlled expression of both marker genes and modified genes of interest. In particular, promoters driving expression at earlier developmental stages including microspores are required.

RESULTS

Transcriptomic datasets covering four progressive stages of male gametophyte development in Arabidopsis were used to select candidate genes showing early expression profiles that were male gametophyte-specific. Promoter-GUS reporter analysis of candidate genes identified three promoters (MSP1, MSP2, and MSP3) that are active in microspores and are otherwise specific to the male gametophyte and tapetum. The MSP1 and MSP2 promoters were used to successfully complement and restore the male transmission of the gametophytic two-in-one (tio) mutant that is cytokinesis-defective at first microspore division.

CONCLUSION

We demonstrate the effective application of MSP promoters as tools that can be used to elucidate gametophytic gene functions in microspores in a male-specific manner.

A proteomics dissection of Arabidopsis thaliana vacuoles isolated from cell culture

To better understand the mechanisms governing cellular traffic, storage of various metabolites, and their ultimate degradation, Arabidopsis thaliana vacuole proteomes were established. To this aim, a procedure was developed to prepare highly purified vacuoles from protoplasts isolated from Arabidopsis cell cultures using Ficoll density gradients. Based on the specific activity of the vacuolar marker alpha-mannosidase, the enrichment factor of the vacuoles was estimated at approximately 42-fold with an average yield of 2.1%. Absence of significant contamination by other cellular compartments was validated by Western blot using antibodies raised against specific markers of chloroplasts, mitochondria, plasma membrane, and endoplasmic reticulum. Based on these results, vacuole preparations showed the necessary degree of purity for proteomics study. Therefore, a proteomics approach was developed to identify the protein components present in both the membrane and soluble fractions of the Arabidopsis cell vacuoles. This approach includes the following: (i) a mild oxidation step leading to the transformation of cysteine residues into cysteic acid and methionine to methionine sulfoxide, (ii) an in-solution proteolytic digestion of very hydrophobic proteins, and (iii) a prefractionation of proteins by short migration by SDS-PAGE followed by analysis by liquid chromatography coupled to tandem mass spectrometry. This procedure allowed the identification of more than 650 proteins, two-thirds of which copurify with the membrane hydrophobic fraction and one-third of which copurifies with the soluble fraction. Among the 416 proteins identified from the membrane fraction, 195 were considered integral membrane proteins based on the presence of one or more predicted transmembrane domains, and 110 transporters and related proteins were identified (91 putative transporters and 19 proteins related to the V-ATPase pump). With regard to function, about 20% of the proteins identified were known previously to be associated with vacuolar activities. The proteins identified are involved in ion and metabolite transport (26%), stress response (9%), signal transduction (7%), and metabolism (6%) or have been described to be involved in typical vacuolar activities, such as protein and sugar hydrolysis. The subcellular localization of several putative vacuolar proteins was confirmed by transient expression of green fluorescent protein fusion constructs.

Endomembrane proton pumps: connecting membrane and vesicle transport

pH-homeostasis in the endomembrane system requires the activity of proton-pumps. In animals, the progressive acidification of compartments along the endocytic and secretory pathways is critical for protein sorting and vesicle trafficking, and is achieved by the activity of the vacuolar H(+)-ATPase (V-ATPase). Plants have an additional endomembrane pump, the vacuolar H(+)-pyrophosphatase (V-PPase), and previous research was largely focused on the respective functions of the two pumps in secondary active transport across the tonoplast. Recent approaches, including reverse genetics, have not only provided evidence that both enzymes play unique and essential roles but have also highlighted the important functions of the two proton pumps in endocytic and secretory trafficking.

Identification of three distinct phylogenetic groups of CAX cation/proton antiporters

Ca(2+)/cation antiporter (CaCA) proteins are integral membrane proteins that transport Ca(2+) or other cations using the H(+) or Na(+) gradient generated by primary transporters. The CAX (for CAtion eXchanger) family is one of the five families that make up the CaCA superfamily. CAX genes have been found in bacteria, Dictyostelium, fungi, plants, and lower vertebrates, but only a small number of CAXs have been functionally characterized. In this study, we explored the diversity of CAXs and their phylogenetic relationships. The results demonstrate that there are three major types of CAXs: type I (CAXs similar to Arabidopsis thaliana CAX1, found in plants, fungi, and bacteria), type II (CAXs with a long N-terminus hydrophilic region, found in fungi, Dictyostelium, and lower vertebrates), and type III (CAXs similar to Escherichia coli ChaA, found in bacteria). Some CAXs were found to have secondary structures that are different from the canonical six transmembrane (TM) domains-acidic motif-five TM domain structure. Our phylogenetic tree indicated no evidence to support the cyanobacterial origin of plant CAXs or the classification of Arabidopsis exchangers CAX7 to CAX11. For the first time, these results clearly define the CAX exchanger family and its subtypes in phylogenetic terms. The surprising diversity of CAXs demonstrates their potential range of biochemical properties and physiologic relevance.

Expression of an Arabidopsis CAX2 variant in potato tubers increases calcium levels with no accumulation of manganese

Previously, we made a chimeric Arabidopsis thaliana vacuolar transporter CAX2B [a variant of N-terminus truncated form of CAX2 (sCAX2) containing the "B" domain from CAX1] that has enhanced calcium (Ca(2+)) substrate specificity and lost the manganese (Mn(2+)) transport capability of sCAX2. Here, we demonstrate that potato (Solanum tuberosum L.) tubers expressing the CAX2B contain 50-65% more calcium (Ca(2+)) than wild-type tubers. Moreover, expression of CAX2B in potatoes did not show any significant increase of the four metals tested, particularly manganese (Mn(2+)). The CAX2B-expressing potatoes have normally undergone the tuber/plant/tuber cycle for three generations; the trait appeared stable through the successive generations and showed no deleterious alternations on plant growth and development. These results demonstrate the enhanced substrate specificity of CAX2B in potato. Therefore, CAX2B can be a valuable tool for Ca(2+) nutrient enrichment of potatoes with reduced accumulation of undesirable metals.

AtGRXcp, an Arabidopsis chloroplastic glutaredoxin, is critical for protection against protein oxidative damage

Glutaredoxins (Grxs) are ubiquitous small heat-stable disulfide oxidoreductases and members of the thioredoxin (Trx) fold protein family. In bacterial, yeast, and mammalian cells, Grxs appear to be involved in maintaining cellular redox homeostasis. However, in plants, the physiological roles of Grxs have not been fully characterized. Recently, an emerging subgroup of Grxs with one cysteine residue in the putative active motif (monothiol Grxs) has been identified but not well characterized. Here we demonstrate that a plant protein, AtGRXcp, is a chloroplast-localized monothiol Grx with high similarity to yeast Grx5. In yeast expression assays, AtGRXcp localized to the mitochondria and suppressed the sensitivity of yeast grx5 cells to H2O2 and protein oxidation. AtGRXcp expression can also suppress iron accumulation and partially rescue the lysine auxotrophy of yeast grx5 cells. Analysis of the conserved monothiol motif suggests that the cysteine residue affects AtGRXcp expression and stability. In planta, AtGRXcp expression was elevated in young cotyledons, green tissues, and vascular bundles. Analysis of atgrxcp plants demonstrated defects in early seedling growth under oxidative stresses. In addition, atgrxcp lines displayed increased protein carbonylation within chloroplasts. Thus, this work describes the initial functional characterization of a plant monothiol Grx and suggests a conserved biological function in protecting cells against protein oxidative damage.

Diverse functions and molecular properties emerging for CAX cation/H+ exchangers in plants

Steep concentration gradients of many ions are actively maintained, with lower concentrations typically located in the cytosol, and higher concentrations in organelles and outside the cell. The vacuole is an important storage organelle for many ions. The concentration gradient of cations is established across the plant tonoplast, in part, by high-capacity cation/H+ (CAX) exchange activity. While plants may not be green yeast, analysis of CAX regulation and substrate specificity has been greatly aided by utilizing yeast as an experimental tool. The basic CAX biology in ARABIDOPSIS has immediate relevance toward understanding the functional interplay between diverse transport processes. The long-range applied goals are to identify novel transporters and express them in crop plants in order to "mine" nutrients out of the soil and into plants. In doing so, this could boost the levels of essential nutrients in plants.

Proteome analysis of rice root proteins regulated by gibberellin

To gain an enhanced understanding of the mechanism by which gibberellins (GAs) regulate the growth and development of plants, it is necessary to identify proteins regulated by GA. Proteome analysis techniques have been applied as a direct, effective, and reliable tool in differential protein expressions. In previous studies, sixteen proteins showed differences in accumulation levels as a result of treatment with GA₃, uniconazole, or abscisic acid (ABA), and/or the differences between the GA-deficient semi-dwarf mutant, Tan-ginbozu, and normal cultivars. Among these proteins, aldolase increased in roots treated with GA₃, was present at low levels in Tan-ginbozu roots, and decreased in roots treated with uniconazole or ABA. In a root elongation assay, the growth of aldolase-antisense transgenic rice was half of that of vector control transgenic rice. These results indicate that increases in aldolase activity stimulate the glycolytic pathway and may play an important role in the GA-induced growth of roots. In this review, we discuss the relationship among GA, aldolase, and root growth.

Expression of the vacuolar Ca2+/H+ exchanger, OsCAX1a, in rice: cell and age specificity of expression, and enhancement by Ca2+

Calcium is an essential macronutrient for plants and functions in signal transduction. Regulation of the cytosolic calcium concentration is required for normal cell growth. In calcium homeostasis in plant cells, Ca(2+)/H(+) exchangers are involved in Ca(2+) compartmentalization into intracellular compartments. Here, we examine the intracellular localization of a rice Ca(2+)/H(+) exchanger, OsCAX1a, fused to a green fluorescent protein and transiently expressed in onion epidermis and rice protoplasts. Green fluorescence was observed in the vacuolar membrane. After sucrose gradient centrifugation of the homogenate of rice plants, OsCAX1a was detected in the same fraction as the vacuolar membrane aquaporin gamma-TIP. We then quantified the mRNA and protein of OsCAX1a in plants grown with metal ions. OsCAX1a mRNA was induced in roots by high concentrations of Ca(2+). The protein level in shoots was also increased in the presence of high concentrations of Ca(2+). Furthermore, transgenic rice plants transformed with the OsCAX1a promoter fused to beta-glucuronidase showed reporter expression in vascular bundles, stomata, trichomes, steles, flowers, embryos and aleurone layers. In the case of stomata and trichomes, transcription of OsCAX1a was particularly high in aged organs. These results suggest that OsCAX1a transports Ca(2+) into vacuoles and is involved in Ca(2+) homeostasis in cells that suffer from high concentrations of Ca(2+).

The role of monovalent cation transporters in plant responses to salinity

Exposure to high ambient levels of NaCl affects plant water relations and creates ionic stress in the form of the cellular accumulation of Cl- and, in particular, Na+ ions. However, salt stress also impacts heavily on the homeostasis of other ions such as Ca2+, K+, and NO(3)(-) and therefore requires insights into how transport and compartmentation of these nutrients is altered during salinity stress. A genomics approach can greatly help with the identification of genes, and therefore potentially gene products, that are involved in plant salinity. Both the literature and public databases contain the results of many genomics studies and, in this report, those data are collated in the context of cation membrane transport and salinity. The efficacy of genomics approaches in isolation is low due to large inherent variability and the exclusion of gene products that are predominantly regulated post-transcriptionally. In conjunction with complementary approaches, however, transcriptomics can help identify important transcripts and relevant associations between physiological processes. This analysis identified (i) vascular K+ circulation, (ii) root shoot translocation of Ca2+, and (iii) transition metal homeostasis as potentially important aspects of the plant response to salt stress.

Up-regulation of a H+-pyrophosphatase (H+-PPase) as a strategy to engineer drought-resistant crop plants

Engineering drought -resistant crop plants is a critically important objective. Overexpression of the vacuolar H(+)-pyrophosphatase (H(+)-PPase) AVP1 in the model plant Arabidopsis thaliana results in enhanced performance under soil water deficits. Recent work demonstrates that AVP1 plays an important role in root development through the facilitation of auxin fluxes. With the objective of improving crop performance, we expressed AVP1 in a commercial cultivar of tomato. This approach resulted in (i) greater pyrophosphate-driven cation transport into root vacuolar fractions, (ii) increased root biomass, and (iii) enhanced recovery of plants from an episode of soil water deficit stress. More robust root systems allowed transgenic tomato plants to take up greater amounts of water during the imposed water deficit stress, resulting in a more favorable plant water status and less injury. This study documents a general strategy for improving drought resistance of crops.

A putative plasma membrane cation/proton antiporter from soybean confers salt tolerance in Arabidopsis

Cation transport is thought to be an important process for ion homeostasis in plant cells. Here, we report that a soybean putative cation/proton antiporter GmCAX1 may be a mediator of this process. GmCAX1 is expressed in all tissues of the soybean plants but at a lower level in roots. Its expression was induced by PEG, ABA, Ca(2+), Na(+) and Li(+) treatments. The GmCAX1-GFP fusion protein was mainly localized in plasma membrane of the transgenic Arabidopsis plant cells and onion epidermal cells. Transgenic Arabidopsis plants overexpressing GmCAX1 accumulated less Na(+), K(+), and Li(+), and were more tolerant to elevated Li(+) and Na(+) levels during germination when compared with the controls. These results suggest that GmCAX1 may function as an antiporter for Na(+), K(+) and Li(+). Modulation of this antiporter may be beneficial for regulation of ion homeostasis and thus plant salt tolerance.

Expression profile of the genes for rice cation/H+ exchanger family and functional analysis in yeast

We identified five cation/H(+) exchangers (CAX) from rice, and phylogenetically divided them into two clusters. Gene expression and absolute amounts of mRNA in different organs were analyzed by real-time PCR. OsCAX1a showed high expression in almost all organs. OsCAX1b and OsCAX1c were detected in a limited number of organs and their expression levels were very low. The mRNA levels of OsCAX2 and OsCAX3 varied with the organ. OsCAXs were heterologously expressed in yeast to characterize the ion transport activity. All exchangers, except for OsCAX2, conferred tolerance to calcium. OsCAX1a and OsCAX3 conferred tolerance to manganese. The diversity of expression sites and substrates suggest the broad range function of CAX.

CML24, regulated in expression by diverse stimuli, encodes a potential Ca2+ sensor that functions in responses to abscisic acid, daylength, and ion stress

Changes in intracellular calcium (Ca(2+)) levels serve to signal responses to diverse stimuli. Ca(2+) signals are likely perceived through proteins that bind Ca(2+), undergo conformation changes following Ca(2+) binding, and interact with target proteins. The 50-member calmodulin-like (CML) Arabidopsis (Arabidopsis thaliana) family encodes proteins containing the predicted Ca(2+)-binding EF-hand motif. The functions of virtually all these proteins are unknown. CML24, also known as TCH2, shares over 40% amino acid sequence identity with calmodulin, has four EF hands, and undergoes Ca(2+)-dependent changes in hydrophobic interaction chromatography and migration rate through denaturing gel electrophoresis, indicating that CML24 binds Ca(2+) and, as a consequence, undergoes conformational changes. CML24 expression occurs in all major organs, and transcript levels are increased from 2- to 15-fold in plants subjected to touch, darkness, heat, cold, hydrogen peroxide, abscisic acid (ABA), and indole-3-acetic acid. However, CML24 protein accumulation changes were not detectable. The putative CML24 regulatory region confers reporter expression at sites of predicted mechanical stress; in regions undergoing growth; in vascular tissues and various floral organs; and in stomata, trichomes, and hydathodes. CML24-underexpressing transgenics are resistant to ABA inhibition of germination and seedling growth, are defective in long-day induction of flowering, and have enhanced tolerance to CoCl(2), molybdic acid, ZnSO(4), and MgCl(2). MgCl(2) tolerance is not due to reduced uptake or to elevated Ca(2+) accumulation. Together, these data present evidence that CML24, a gene expressed in diverse organs and responsive to diverse stimuli, encodes a potential Ca(2+) sensor that may function to enable responses to ABA, daylength, and presence of various salts.

Plants, symbiosis and parasites: a calcium signalling connection

A unique family of protein kinases has evolved with regulatory domains containing sequences that are related to Ca(2+)-binding EF-hands. In this family, the archetypal Ca(2+)-dependent protein kinases (CDPKs) have been found in plants and some protists, including the malarial parasite, Plasmodium falciparum. Recent genetic evidence has revealed isoform-specific functions for a CDPK that is essential for Plasmodium berghei gametogenesis, and for a related chimeric Ca(2+) and calmodulin-dependent protein kinase (CCaMK) that is essential to the formation of symbiotic nitrogen-fixing nodules in plants. In Arabidopsis thaliana, the analysis of 42 isoforms of CDPK and related kinases is expected to delineate Ca(2+) signalling pathways in all aspects of plant biology.

Functional association of Arabidopsis CAX1 and CAX3 is required for normal growth and ion homeostasis

Cation levels within the cytosol are coordinated by a network of transporters. Here, we examine the functional roles of calcium exchanger 1 (CAX1), a vacuolar H+/Ca2+ transporter, and the closely related transporter CAX3. We demonstrate that like CAX1, CAX3 is also localized to the tonoplast. We show that CAX1 is predominately expressed in leaves, while CAX3 is highly expressed in roots. Previously, using a yeast assay, we demonstrated that an N-terminal truncation of CAX1 functions as an H+/Ca2+ transporter. Here, we use the same yeast assay to show that full-length CAX1 and full-length CAX3 can partially, but not fully, suppress the Ca2+ hypersensitive yeast phenotype and coexpression of full-length CAX1 and CAX3 conferred phenotypes not produced when either transporter was expressed individually. In planta, CAX3 null alleles were modestly sensitive to exogenous Ca2+ and also displayed a 22% reduction in vacuolar H+-ATPase activity. cax1/cax3 double mutants displayed a severe reduction in growth, including leaf tip and flower necrosis and pronounced sensitivity to exogenous Ca2+ and other ions. These growth defects were partially suppressed by addition of exogenous Mg2+. The double mutant displayed a 42% decrease in vacuolar H+/Ca2+ transport, and a 47% decrease in H+-ATPase activity. While the ionome of cax1 and cax3 lines were modestly perturbed, the cax1/cax3 lines displayed increased PO4(3-), Mn2+, and Zn2+ and decreased Ca2+ and Mg2+ in shoot tissue. These findings suggest synergistic function of CAX1 and CAX3 in plant growth and nutrient acquisition.

Mutations in CAX1 produce phenotypes characteristic of plants tolerant to serpentine soils

Plant tolerance of serpentine soils is potentially an excellent model for studying the genetics of adaptive variation in natural populations. A large-scale viability screen of Arabidopsis thaliana mutants on a defined nutrient solution with a low Ca(2+) : Mg(2+) ratio (1 : 24 mol : mol), typical of serpentine soils, yielded survivors with null alleles of the tonoplast calcium-proton antiporter CAX1. cax1 mutants have most of the phenotypes associated with tolerance to serpentine soils, including survival in solutions with a low Ca(2+) : Mg(2+) ratio; requirement for a high concentration of Mg(2+) for maximum growth; reduced leaf tissue concentration of Mg(2+); and poor growth performance on 'normal' levels of Ca(2+) and Mg(2+). A physiological model is proposed to explain how loss-of-function cax1 mutations could produce all these phenotypes characteristic of plants adapted to serpentine soils, why 'normal' plants are unable to survive on serpentine soil, and why serpentine-adapted plants are unable to compete on 'normal' soils.

Development of transgenic rice plants overexpressing the Arabidopsis H+/Ca2+ antiporter CAX1 gene

The gene of the Arabidopsis thaliana H+/Ca2+ transporter, CAX1 (cation exchanger 1) was introduced into Japonica cultivars of rice (Ilpumbyeo) by Agrobacterium-mediated transformation, and a large number of transgenic plants were produced. The neomycin phosphotransferase II (NPTII) gene was used as a selectable marker. The activity of neomycin phosphotransferase could be successfully detected in transgenic rice callus. The introduction of the CAX1 gene was also proven by PCR using CAX1-specific oligonucleotide primers in regenerated plants. Stable integration and expression of the CAX1 gene in T0 plants and T1 progeny were confirmed by DNA hybridization, Northern blot analysis, and luminescent analysis.

Two isoforms of the A subunit of the vacuolar H(+)-ATPase in Lycopersicon esculentum: highly similar proteins but divergent patterns of tissue localization

The plant vacuolar H(+)-translocating ATPase (V-ATPase, EC 3.6.1.34) generates a H+ electro-chemical gradient across the tonoplast membrane. We isolated two full-length cDNA clones (VHA-A1 and VHA-A2) from tomato (Lycopersicon esculentum Mill. cv. Large Cherry Red) coding for two isoforms of the V-ATPase catalytic subunit (V-ATPases A1 and A2). The cDNA clones encoding the two isoforms share 90% identity at the nucleotide level and 96% identity at the amino acid level. The 5'- and 3'-untranslated regions, however, are highly diverse. Both V-ATPase A1 and A2 isoforms encode polypeptides of 623 amino acids, with calculated molecular masses of 68,570 and 68,715, respectively. The expression of VHA-A1 and accumulation of V-ATPase A1 polypeptide were ubiquitous in all tissues examined. In response to salinity, the abundances of both transcript (VHA-A1) and protein (V-ATPase A1) of the A1 isoform in leaves were nearly doubled. In contrast to the A1 isoform, VHA-A2 transcript and V-ATPase A2 polypeptide were only detected in abundance in roots, and in minor quantities in mature fruit. In roots, accumulation of transcripts and polypeptides did not change in response to salinity for either isoform. Subcellular localization indicated that the highest levels of both V-ATPase A1 and A2 isoforms were in the tonoplast. However, significant quantities of both isoforms were detected in membranes associated with endoplasmic reticulum and/or Golgi. Immunoprecipitation of dissociated V1 domains using isoform-specific antibodies showed that V1 domains consist of either V-ATPase A1 or A2 catalytic subunit isoforms.

Characterization of fructose-bisphosphate aldolase regulated by gibberellin in roots of rice seedling

Fructose-bisphosphate aldolase is a glycolytic enzyme whose activity increases in rice roots treated with gibberellin (GA). To investigate the relationship between aldolase and root growth, GA-induced root aldolase was characterized. GA3 promoted an increase in aldolase accumulation when 0.1 microM GA3 was added exogenously to rice roots. Aldolase accumulated abundantly in roots, especially in the apical region. To examine the effect of aldolase function on root growth, transgenic rice plants expressing antisense aldolase were constructed. Root growth of aldolase-antisense transgenic rice was repressed compared with that of the vector control transgenic rice. Although aldolase activity increased by 25% in vector control rice roots treated with 0.1 microM GA3, FBPA activity increased very little by 0.1 microM GA3 treatment in the root of aldolase-antisense transgenic rice. Furthermore, aldolase co-immunoprecipitated with antibodies against vacuolar H+ -ATPase in rice roots. In the root of OsCDPK13-antisense transgenic rice, aldolase did not accumulate even after treatment with GA3. These results suggest that the activation of glycolytic pathway function accelerates root growth and that GA3-induced root aldolase may be modulated through OsCDPK13. Aldolase physically associates with vacuolar H-ATPase in roots and may regulate the vacuolar H-ATPase mediated control of cell elongation that determines root length.

Functional and regulatory analysis of the Arabidopsis thaliana CAX2 cation transporter

The vacuolar sequestration of metals is an important metal tolerance mechanism in plants. The Arabidopsis thaliana vacuolar transporters CAX1 and CAX2 were originally identified in a Saccharomyces cerevisiae suppression screen as Ca2+/H+ antiporters. CAX2 has a low affinity for Ca2+ but can transport other metals including Mn2+ and Cd2+. Here we demonstrate that unlike cax1 mutants, CAX2 insertional mutants caused no discernable morphological phenotypes or alterations in Ca2+/H+ antiport activity. However, cax2 lines exhibited a reduction in vacuolar Mn2+/H+ antiport and, like cax1 mutants, reduced V-type H+ -ATPase (V-ATPase) activity. Analysis of a CAX2 promoter beta-glucoronidase (GUS) reporter gene fusion confirmed that CAX2 was expressed throughout the plant and strongly expressed in flower tissue, vascular tissue and in the apical meristem of young plants. Heterologous expression in yeast identified an N-terminal regulatory region in CAX2, suggesting that Arabidopsis contains multiple cation/H+ antiporters with shared regulatory features. Furthermore, despite significant variations in morphological and biochemical phenotypes, cax1 and cax2 lines both significantly alter V-ATPase activity, hinting at coordinate regulation among transporters driven by H+ gradients and the V-ATPase.

Expression patterns of a novel AtCHX gene family highlight potential roles in osmotic adjustment and K+ homeostasis in pollen development

A combined bioinformatic and experimental approach is being used to uncover the functions of a novel family of cation/H(+) exchanger (CHX) genes in plants using Arabidopsis as a model. The predicted protein (85-95 kD) of 28 AtCHX genes after revision consists of an amino-terminal domain with 10 to 12 transmembrane spans (approximately 440 residues) and a hydrophilic domain of approximately 360 residues at the carboxyl end, which is proposed to have regulatory roles. The hydrophobic, but not the hydrophilic, domain of plant CHX is remarkably similar to monovalent cation/proton antiporter-2 (CPA2) proteins, especially yeast (Saccharomyces cerevisiae) KHA1 and Synechocystis NhaS4. Reports of characterized fungal and prokaryotic CPA2 indicate that they have various transport modes, including K(+)/H(+) (KHA1), Na(+)/H(+)-K(+) (GerN) antiport, and ligand-gated ion channel (KefC). The expression pattern of AtCHX genes was determined by reverse transcription PCR, promoter-driven beta-glucuronidase expression in transgenic plants, and Affymetrix ATH1 genome arrays. Results show that 18 genes are specifically or preferentially expressed in the male gametophyte, and six genes are highly expressed in sporophytic tissues. Microarray data revealed that several AtCHX genes were developmentally regulated during microgametogenesis. An exciting idea is that CHX proteins allow osmotic adjustment and K(+) homeostasis as mature pollen desiccates and then rehydrates at germination. The multiplicity of CHX-like genes is conserved in higher plants but is not found in animals. Only 17 genes, OsCHX01 to OsCHX17, were identified in rice (Oryza sativa) subsp. japonica, suggesting diversification of CHX in Arabidopsis. These results reveal a novel CHX gene family in flowering plants with potential functions in pollen development, germination, and tube growth.

New insight into the structure and regulation of the plant vacuolar H+-ATPase

Plant cells are characterized by a highly active secretory system that includes the large central vacuole found in most differentiated tissues. The plant vacuolar H+-ATPase plays an essential role in maintaining the ionic and metabolic gradients across endomembranes, in activating transport processes and vesicle dynamics, and, hence, is indispensable for plant growth, development, and adaptation to changing environmental conditions. The review summarizes recent advances in elucidating the structure, subunit composition, localization, and regulation of plant V-ATPase. Emerging knowledge on subunit isogenes from Arabidopsis and rice genomic sequences as well as from Mesembryanthemum illustrates another level of complexity, the regulation of isogene expression and function of subunit isoforms. To this end, the review attempts to define directions of future research on plant V-ATPase.

Proteomics of calcium-signaling components in plants

Calcium functions as a versatile messenger in mediating responses to hormones, biotic/abiotic stress signals and a variety of developmental cues in plants. The Ca(2+)-signaling circuit consists of three major "nodes"--generation of a Ca(2+)-signature in response to a signal, recognition of the signature by Ca2+ sensors and transduction of the signature message to targets that participate in producing signal-specific responses. Molecular genetic and protein-protein interaction approaches together with bioinformatic analysis of the Arabidopsis genome have resulted in identification of a large number of proteins at each "node"--approximately 80 at Ca2+ signature, approximately 400 sensors and approximately 200 targets--that form a myriad of Ca2+ signaling networks in a "mix and match" fashion. In parallel, biochemical, cell biological, genetic and transgenic approaches have unraveled functions and regulatory mechanisms of a few of these components. The emerging paradigm from these studies is that plants have many unique Ca2+ signaling proteins. The presence of a large number of proteins, including several families, at each "node" and potential interaction of several targets by a sensor or vice versa are likely to generate highly complex networks that regulate Ca(2+)-mediated processes. Therefore, there is a great demand for high-throughput technologies for identification of signaling networks in the "Ca(2+)-signaling-grid" and their roles in cellular processes. Here we discuss the current status of Ca2+ signaling components, their known functions and potential of emerging high-throughput genomic and proteomic technologies in unraveling complex Ca2+ circuitry.

Characterization of CXIP4, a novel Arabidopsis protein that activates the H+/Ca2+ antiporter, CAX1

Precise regulation of calcium transporters is essential for modulating the Ca2+ signaling network that is involved in the growth and adaptation of all organisms. The Arabidopsis H+/Ca2+ antiporter, CAX1, is a high capacity and low affinity Ca2+ transporter and several CAX1-like transporters are found in Arabidopsis. When heterologously expressed in yeast, CAX1 is unable to suppress the Ca2+ hypersensitivity of yeast vacuolar Ca2+ transporter mutants due to an N-terminal autoinhibition mechanism that prevents Ca2+ transport. Using a yeast screen, we have identified CAX nteracting Protein 4 (CXIP4) that activated full-length CAX1, but not full-length CAX2, CAX3 or CAX4. CXIP4 encodes a novel plant protein with no bacterial, fungal, animal, or mammalian homologs. Expression of a GFP-CXIP4 fusion in yeast and plant cells suggests that CXIP4 is targeted predominantly to the nucleus. Using a yeast growth assay, CXIP4 activated a chimeric CAX construct that contained specific portions of the N-terminus of CAX1. Together with other recent studies, these results suggest that CAX1 is regulated by several signaling molecules that converge on the N-terminus of CAX1 to regulate H+/Ca2+ antiport.

Cell expansion in roots

Cell expansion in roots is crucial for the exploration and exploitation of the soil substrate and the plethora of activities that roots engage in. Expansion requires the coordinated activities of many cell processes. Central to this is the control of ion transport during vacuolar growth, which mediates the increase in cell size and the concomitant production of new wall and membrane at the surface of growing cells. The cytoskeleton plays an important role in growth and the control of growth direction. Evidence is accumulating to show that plant hormones also coordinate cell expansion throughout the plant by controlling the activities of growth-regulating DELLA proteins.

Involvement of the R2R3-MYB, AtMYB61, in the ectopic lignification and dark-photomorphogenic components of the det3 mutant phenotype

Overexpression of a pine MYB, PtMYB4, in Arabidopsis caused ectopic lignin deposition and allowed the plants to undergo photomorphogenesis even when they were grown in the dark. The phenotype caused by PtMYB4 overexpression was reminiscent of the previously characterised dark-photomorphogenic mutant, de-etiolated 3 (det3); consequently, we tested the hypothesis that MYB misexpression may explain aspects of the det3 phenotype. We show here that AtMYB61, a member of the Arabidopsis R2R3-MYB family, is misexpressed in the det3 mutant. Semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) experiments suggested that AtMYB61 was misexpressed in a det3 background relative to wild-type plants. Examination of AtMYB61 promoter activity in a det3 background showed that the spatial control of AtMYB61 expression was lost. In order to determine if such misexpression could explain the mutant phenotype, AtMYB61 was overexpressed in wild-type Arabidopsis plants. Transgenic plants that overexpressed AtMYB61 had the same ectopic lignification and dark-photomorphogenic phenotype as that of the det3 mutant. In order to test if AtMYB61 was necessary for these aspects of the det3 phenotype, AtMYB61 expression was downregulated in det3 plants in both antisense and sense suppression experiments. Suppression of AtMYB61 in a det3 mutant background restored all mutant phenotypes of the det3 mutant associated with development in the dark. Taken together, these results suggest that AtMYB61 misexpression was both sufficient and necessary to explain the ectopic lignification and dark-photomorphogenic phenotypes of the det3 mutant.