Phosphorylation of the vacuolar anion exchanger AtCLCa is required for the stomatal response to abscisic acid

Eukaryotic anion/proton exchangers of the CLC (chloride channel) family mediate anion fluxes across intracellular membranes. The Arabidopsis thaliana anion/proton exchanger AtCLCa is involved in vacuolar accumulation of nitrate. We investigated the role of AtCLCa in leaf guard cells, a specialized plant epidermal cell that controls gas exchange and water loss through pores called stomata. We showed that AtCLCa not only fulfilled the expected role of accumulating anions in the vacuole during stomatal opening but also mediated anion release during stomatal closure in response to the stress hormone abscisic acid (ABA). We found that this dual role resulted from a phosphorylation-dependent change in the activity of AtCLCa. The protein kinase OST1 (also known as SnRK2.6) is a key signaling player and central regulator in guard cells in response to ABA. Phosphorylation of Thr(38) in the amino-terminal cytoplasmic domain of AtCLCa by OST1 increased the outward anion fluxes across the vacuolar membrane, which are essential for stomatal closure. We provide evidence that bidirectional activities of an intracellular CLC exchanger are physiologically relevant and that phosphorylation regulates the transport mode of this exchanger.

Voltage-dependent-anion-channels (VDACs) in Arabidopsis have a dual localization in the cell but show a distinct role in mitochondria

In mammals, the Voltage-dependent anion channels (VDACs) are predominant proteins of the outer mitochondrial membrane (OMM) where they contribute to the exchange of small metabolites essential for respiration. They were shown to be as well associated with the plasma membrane (PM) and act as redox enzyme or are involved in ATP release for example. In Arabidopsis, we show that four out of six genomic sequences encode AtVDAC proteins. All four AtVDACs are ubiquitously expressed in the plant but each of them displays a specific expression pattern in root cell types. Using two complementary approaches, we demonstrate conclusively that the four expressed AtVDACs are targeted to both mitochondria and plasma membrane but in differential abundance, AtVDAC3 being the most abundant in PM, and conversely, AtVDAC4 almost exclusively associated with mitochondria. These are the first plant proteins to be shown to reside in both these two membranes. To investigate a putative function of AtVDACs, we analyzed T-DNA insertion lines in each of the corresponding genes. Knock-out mutants for AtVDAC1, AtVDAC2 and AtVDAC4 present slow growth, reduced fertility and yellow spots in leaves when atvdac3 does not show any visible difference compared to wildtype plants. Analyses of atvdac1 and atvdac4 reveal that yellow areas correspond to necrosis and the mitochondria are swollen in these two mutants. All these results suggest that, in spite of a localization in plasma membrane for three of them, AtVDAC1, AtVDAC2 and AtVDAC4 have a main function in mitochondria.

Anion channels/transporters in plants: from molecular bases to regulatory networks

Anion channels/transporters are key to a wide spectrum of physiological functions in plants, such as osmoregulation, cell signaling, plant nutrition and compartmentalization of metabolites, and metal tolerance. The recent identification of gene families encoding some of these transport systems opened the way for gene expression studies, structure-function analyses of the corresponding proteins, and functional genomics approaches toward further understanding of their integrated roles in planta. This review, based on a few selected examples, illustrates that the members of a given gene family exhibit a diversity of substrate specificity, regulation, and intracellular localization, and are involved in a wide range of physiological functions. It also shows that post-translational modifications of transport proteins play a key role in the regulation of anion transport activity. Key questions arising from the increasing complexity of networks controlling anion transport in plant cells (the existence of redundancy, cross talk, and coordination between various pathways and compartments) are also addressed.

R type anion channel: a multifunctional channel seeking its molecular identity

Plant genomes code for channels involved in the transport of cations, anions and uncharged molecules through membranes. Although the molecular identity of channels for cations and uncharged molecules has progressed rapidly in the recent years, the molecular identity of anion channels has lagged behind. Electrophysiological studies have identified S-type (slow) and R-type (rapid) anion channels. In this brief review, we summarize the proposed functions of the R-type anion channels which, like the S-type, were first characterized by electrophysiology over 20 years ago, but unlike the S-type, have still yet to be cloned. We show that the R-type channel can play multiple roles.

The Arabidopsis vacuolar anion transporter, AtCLCc, is involved in the regulation of stomatal movements and contributes to salt tolerance

In plant cells, anion channels and transporters are essential for key functions such as nutrition, resistance to biotic or abiotic stresses, and ion homeostasis. In Arabidopsis, members of the chloride channel (CLC) family located in intracellular organelles have been shown to be required for nitrate homeostasis or pH adjustment, and previous results indicated that AtCLCc is involved in nitrate accumulation. We investigated new physiological functions of this CLC member in Arabidopsis. Here we report that AtCLCc is strongly expressed in guard cells and pollen and more weakly in roots. Use of an AtCLCc:GFP fusion revealed localization to the tonoplast. Disruption of the AtCLCc gene by a T-DNA insertion in four independent lines affected physiological responses that are directly related to the movement of chloride across the tonoplast membrane. Opening of clcc stomata was reduced in response to light, and ABA treatment failed to induce their closure, whereas application of KNO₃ but not KCl restored stomatal opening. clcc mutant plants were hypersensitive to NaCl treatment when grown on soil, and to NaCl and KCl in vitro, confirming the chloride dependence of the phenotype. These phenotypes were associated with modifications of chloride content in both guard cells and roots. These data demonstrate that AtCLCc is essential for stomatal movement and salt tolerance by regulating chloride homeostasis.

The proline 160 in the selectivity filter of the Arabidopsis NO(3)(-)/H(+) exchanger AtCLCa is essential for nitrate accumulation in planta

Nitrate, the major nitrogen source for plants, can be accumulated in the vacuole. Its transport across the vacuolar membrane is mediated by AtCLCa, an antiporter of the chloride channel (CLC) protein family. In contrast to other CLC family members, AtCLCa transports nitrate coupled to protons. Recently, the different behaviour towards nitrate of CLC proteins has been linked to the presence of a serine or proline in the selectivity filter motif GXGIP. By monitoring AtCLCa activity in its native environment, we show that if proline 160 in AtCLCa is changed to a serine (AtCLCa(P160S) ), the transporter loses its nitrate selectivity, but the anion proton exchange mechanism is unaffected. We also performed in vivo analyses in yeast and Arabidopsis. In contrast to native AtCLCa, expression of AtCLCa(P160S) does not complement either the ΔScCLC yeast mutant grown on nitrate or the nitrate under-accumulation phenotype of clca knockout plants. Our results confirm the significance of this amino acid in the conserved selectivity filter of CLC proteins and highlight the importance of the proline in AtCLCa for nitrate metabolism in Arabidopsis.

Functional evidence for an auxin receptor at the plasmalemma of tobacco mesophyll protoplasts

Tobacco mesophyll protoplasts were previously shown to respond to naphthaleneacetic acid by modifying their transmembrane potential difference. In the present work, evacuolated protoplasts were used to show that this response resides only at the plasmalemma. This electrical response was investigated by using polyclonal antibodies directed against plasma membrane antigens presumably involved in the reception and transduction of the auxin signal. An IgG fraction from an antiserum directed against the membrane auxin-binding protein from maize coleoptile completely inhibited the naphthaleneacetic acid-induced response of tobacco protoplasts. The suppression of the auxin-induced variation in the transmembrane potential difference by an IgG preparation directed against the plasmalemma ATPase from yeast demonstrated the involvement of the ATPase in the electrical response. Variation induced by fusicoccin in the transmembrane potential difference of tobacco protoplasts was unaffected by the anti-auxin-binding protein IgG fraction but was completely suppressed by the anti-ATPase IgG preparation. These results demonstrate the presence of a membrane receptor for auxin at the plasmalemma, the binding of the hormone to this receptor leading to the activation of the proton-pumping ATPase. They also show that at least the primary steps of activation by naphthaleneacetic acid are distinct from those of the fusicoccin-induced response.

Export of vacuolar manganese by AtNRAMP3 and AtNRAMP4 is required for optimal photosynthesis and growth under manganese deficiency

Manganese (Mn) is an essential element, acting as cofactor in numerous enzymes. In particular, a Mn cluster is indispensable for the function of the oxygen-evolving complex of photosystem II. Metal transporters of the Natural Resistance-Associated Macrophage Protein (NRAMP) family have the ability to transport both iron and Mn. AtNRAMP3 and AtNRAMP4 are required for iron mobilization in germinating seeds. The results reported here show that, in adult Arabidopsis (Arabidopsis thaliana) plants, AtNRAMP3 and AtNRAMP4 have an important role in Mn homeostasis. Vacuolar Mn accumulation in mesophyll cells of rosette leaves of adult nramp3nramp4 double mutant plants was dramatically increased when compared with the wild type. This suggests that a considerable proportion of the cellular Mn pool passes through the vacuole and is retrieved in an AtNRAMP3/AtNRAMP4-dependent manner. The impaired Mn release from mesophyll vacuoles of nramp3nramp4 double mutant plants is associated with reduced growth under Mn deficiency. However, leaf AtNRAMP3 and AtNRAMP4 protein levels are unaffected by Mn supply. Under Mn deficiency, nramp3nramp4 plants contain less functional photosystem II than the wild type. These data are consistent with a shortage of Mn to produce functional photosystem II, whereas mitochondrial Mn-dependent superoxide dismutase activity is maintained under Mn deficiency in both genotypes. The results presented here suggest an important role for AtNRAMP3/AtNRAMP4-dependent Mn transit through the vacuole prior to the import into chloroplasts of mesophyll cells.

R-type anion channel activation is an essential step for ROS-dependent innate immune response in Arabidopsis suspension cells

Plants are constantly exposed to environmental biotic and abiotic stresses. Plants cells perceive these factors and trigger early responses followed by delayed and complex adaptation processes. Using cell suspensions of Arabidopsis thaliana (L.) as a cellular model, we investigated the role of plasma membrane anion channels in Reactive Oxygen Species (ROS) production and in cell death which occurs during non-host pathogen infection. Protoplasts derived from Arabidopsis suspension cells display two anion currents with characteristics very similar to those of the slow nitrate-permeable (S-type) and rapid sulfate-permeable (R-type) channels previously characterised in hypocotyl cells and other cell types. Using seven inhibitors, we showed that the R-type channel and ROS formation in cell cultures present similar pharmacological profiles. The efficiency of anion channel blockers to inhibit ROS production was independent of the nature of the triggering signal (osmotic stress or general elicitors of plant defence), indicating that the R-type channel represents a crossroad in the signalling pathways leading to ROS production. In a second step, we show that treatment with R-type channel blockers accelerates cell death triggered by the non-specific plant pathogen Xanthomonas campestris. Finally, we discuss the hypothesis that the R-type channel is involved in innate immune response allowing cell defence via antibacterial ROS production.

ATP binding to the C terminus of the Arabidopsis thaliana nitrate/proton antiporter, AtCLCa, regulates nitrate transport into plant vacuoles

Nitrate, one of the major nitrogen sources for plants, is stored in the vacuole. Nitrate accumulation within the vacuole is primarily mediated by the NO(3)(-)/H(+) exchanger AtCLCa, which belongs to the chloride channel (CLC) family. Crystallography analysis of hCLC5 suggested that the C-terminal domain, composed by two cystathionine beta-synthetase motifs in all eukaryotic members of the CLC family is able to interact with ATP. However, interaction of nucleotides with a functional CLC protein has not been unambiguously demonstrated. Here we show that ATP reversibly inhibits AtCLCa by interacting with the C-terminal domain. Applying the patch clamp technique to isolated Arabidopsis thaliana vacuoles, we demonstrate that ATP reduces AtCLCa activity with a maximum inhibition of 60%. ATP inhibition of nitrate influx into the vacuole at cytosolic physiological nitrate concentrations suggests that ATP modulation is physiologically relevant. ADP and AMP do not decrease the AtCLCa transport activity; nonetheless, AMP (but not ADP) competes with ATP, preventing inhibition. A molecular model of the C terminus of AtCLCa was built by homology to hCLC5 C terminus. The model predicted the effects of mutations of the ATP binding site on the interaction energy between ATP and AtCLCa that were further confirmed by functional expression of site-directed mutated AtCLCa.

Review. CLC-mediated anion transport in plant cells

Plants need nitrate for growth and store the major part of it in the central vacuole of cells from root and shoot tissues. Based on few studies on the two model plants Arabidopsis thaliana and rice, members of the large ChLoride Channel (CLC) family have been proposed to encode anion channels/transporters involved in nitrate homeostasis. Proteins from the Arabidopsis CLC family (AtClC, comprising seven members) are present in various membrane compartments including the vacuolar membrane (AtClCa), Golgi vesicles (AtClCd and AtClCf) or chloroplast membranes (AtClCe). Through a combination of electrophysiological and genetic approaches, AtClCa was shown to function as a 2NO3-/1H+ exchanger that is able to accumulate specifically nitrate into the vacuole, in agreement with the main phenotypic trait of knockout mutant plants that accumulate 50 per cent less nitrate than their wild-type counterparts. The set-up of a functional complementation assay relying on transient expression of AtClCa cDNA in the mutant background opens the way for studies on structure-function relationships of the AtClCa nitrate transporter. Such studies will reveal whether important structural determinants identified in bacterial or mammalian CLCs are also crucial for AtClCa transport activity and regulation.

Two anion transporters AtClCa and AtClCe fulfil interconnecting but not redundant roles in nitrate assimilation pathways

* In plants, the knowledge of the molecular identity and functions of anion channels are still very limited, and are almost restricted to the large ChLoride Channel (CLC) family. In Arabidopsis thaliana, some genetic evidence has suggested a role for certain AtCLC protein members in the control of plant nitrate levels. In this context, AtClCa has been demonstrated to be involved in nitrate transport into the vacuole, thereby participating in cell nitrate homeostasis. * In this study, analyses of T-DNA insertion mutants within the AtClCa and AtClCe genes revealed common phenotypic traits: a lower endogenous nitrate content; a higher nitrite content; a reduced nitrate influx into the root; and a decreased expression of several genes encoding nitrate transporters. * This set of nitrate-related phenotypes, displayed by clca and clce mutant plants, showed interconnecting roles of AtClCa and AtClCe in nitrate homeostasis involving two different endocellular membranes. * In addition, it revealed cross-talk between two nitrate transporter families participating in nitrate assimilation pathways. The contribution to nitrate homeostasis at the cellular level of members of these different families is discussed.

Functional characterization of NRAMP3 and NRAMP4 from the metal hyperaccumulator Thlaspi caerulescens

The ability of metal hyperaccumulating plants to tolerate and accumulate heavy metals results from adaptations of metal homeostasis. NRAMP metal transporters were found to be highly expressed in some hyperaccumulating plant species. Here, we identified TcNRAMP3 and TcNRAMP4, the closest homologues to AtNRAMP3 and AtNRAMP4 in Thlaspi caerulescens and characterized them by expression analysis, confocal imaging and heterologous expression in yeast and Arabidopsis thaliana. TcNRAMP3 and TcNRAMP4 are expressed at higher levels than their A. thaliana homologues. When expressed in yeast TcNRAMP3 and TcNRAMP4 transport the same metals as their respective A. thaliana orthologues: iron (Fe), manganese (Mn) and cadmium (Cd) but not zinc (Zn) for NRAMP3; Fe, Mn, Cd and Zn for NRAMP4. They also localize at the vacuolar membrane in A. thaliana protoplasts. Inactivation of AtNRAMP3 and AtNRAMP4 in A. thaliana results in strong Cd and Zn hypersensitivity, which is fully rescued by TcNRAMP3 or TcNRAMP4 expression. However, metal tolerance conferred by TcNRAMP expression in nramp3nramp4 mutant does not exceed that of wild-type A. thaliana. Our data indicate that the difference between TcNRAMP3 and TcNRAMP4 and their A. thaliana orthologues does not lie in a different protein function, but probably resides in a different expression level or expression pattern.

AtMSL9 and AtMSL10: Sensors of plasma membrane tension in Arabidopsis roots

Plant cells, like those of animals and bacteria, are able to sense physical deformation of the plasma membrane. Mechanosensitive (MS) channels are proteins that transduce mechanical force into ion flux, providing a mechanism for the perception of mechanical stimuli such as sound, touch and osmotic pressure. We recently identified AtMSL9 and AtMSL10, two mechanosensitive channels in Arabidopsis thaliana, as molecular candidates for mechanosensing in higher plants.1 AtMSL9 and AtMSL10 are members of a family of proteins in Arabidopsis that are related to the bacterial MS channel MscS, termed MscS-Like (or MSL).2 MscS (Mechanosensitive channel of Small conductance) is one of the best-characterized MS channels, first identified as an electrophysiological activity in the plasma membrane (PM) of giant E. coli spheroplasts.3,4 Activation of MscS is voltage-independent, but responds directly to tension applied to the membrane and does not require other cellular proteins for this regulation.5,6 MscS family members are widely distributed throughout bacterial and archaeal genomes, are present in all plant genomes yet examined, and are found in selected fungal genomes.2,7,8 MscS homolgues have not yet been identified in animals.

Two MscS homologs provide mechanosensitive channel activities in the Arabidopsis root

In bacterial and animal systems, mechanosensitive (MS) ion channels are thought to mediate the perception of pressure, touch, and sound [1-3]. Although plants respond to a wide variety of mechanical stimuli, and although many mechanosensitive channel activities have been characterized in plant membranes by the patch-clamp method, the molecular nature of mechanoperception in plant systems has remained elusive [4]. Likely candidates are relatives of MscS (Mechanosensitive channel of small conductance), a well-characterized MS channel that serves to protect E. coli from osmotic shock [5]. Ten MscS-Like (MSL) proteins are found in the genome of the model flowering plant Arabidopsis thaliana[4, 6, 7]. MSL2 and MSL3, along with MSC1, a MscS family member from green algae, are implicated in the control of organelle morphology [8, 9]. Here, we characterize MSL9 and MSL10, two MSL proteins found in the plasma membrane of root cells. We use a combined genetic and electrophysiological approach to show that MSL9 and MSL10, along with three other members of the MSL family, are required for MS channel activities detected in protoplasts derived from root cells. This is the first molecular identification and characterization of MS channels in plant membranes.

A High Content in Lipid-modified Peripheral Proteins and Integral Receptor Kinases Features in the Arabidopsis Plasma Membrane Proteome

The proteomics of plasma membrane has brought to date only scarce and partial information on the actual protein repertoire. In this work, the plant plasma membrane proteome of Arabidopsis thaliana was investigated. A highly purified plasma membrane fraction was washed by NaCl and Na2CO3 salts, and the insoluble fractions were further analyzed by nano-LC-MS/MS. With 446 proteins identified, we hereby describe the largest plasma membrane proteome diversity reported so far. Half of the proteins were predicted to display transmembrane domains and/or to be anchored to the membrane, validating a posteriori the pertinence of the approach. A fine analysis highlighted two main specific and novel features. First, the main functional category is represented by a majority of as yet unreported signaling proteins, including 11% receptor-like kinases. Second, 16% of the identified proteins are predicted to be lipid-modified, specifically involving double lipid linkage through N-terminal myristoylation, S-palmitoylation, C-terminal prenylation, or glycosylphosphatidylinositol anchors. Thus, our approach led for the first time to the identification of a large number of peripheral proteins as part of the plasma membrane and allowed the functionality of the plasma membrane in the cell context to be reconsidered.

Anion channels and transporters in plant cell membranes

Anion channels/transporters appear as key players in signaling pathways leading to the adaptation of plant cells to abiotic and biotic environmental stresses, in the control of metabolism and in the maintenance of electrochemical gradients. Focusing on the most recent advances, this review aims at providing a description of the role of these channels in various physiological functions such as control of stomatal movements, plant-pathogen interaction, xylem loading, compartmentalization of metabolites and coupling with proton gradients. These functions have been demonstrated by a combination of electrophysiology, pharmacology and genetics approaches, the key issue being to identify the corresponding proteins and genes.

Different phosphorylation mechanisms are involved in the activation of sucrose non-fermenting 1 related protein kinases 2 by osmotic stresses and abscisic acid

In Arabidopsis cell suspension, hyperosmotic stresses (mannitol and NaCl) were previously shown to activate nine sucrose non-fermenting 1 related protein kinases 2 (SnRK2s) whereas only five of them were also activated by abscisic acid (ABA) treatment. Here, the possible activation by phosphorylation/ dephosphorylation of each kinase was investigated by studying their phosphorylation state after osmotic stress, using the Pro-Q Diamond, a specific dye for phosphoproteins. All the activated kinases were phosphorylated after osmotic stress but the induced phosphorylation changes were clearly different depending on the kinase. In addition, the increase of the global phosphorylation level induced by ABA application was lower, suggesting that different mechanisms may be involved in SnRK2 activation by hyperosmolarity and ABA. On the other hand, SnRK2 kinases remain activated by hyperosmotic stress in ABA-deficient and ABA-insensitive mutants, indicating that SnRK2 osmotic activation is independent of ABA. Moreover, using a mutant form of SnRK2s, a specific serine in the activation loop was shown to be phosphorylated after stress treatments and essential for activity and/or activation. Finally, SnRK2 activity was sensitive to staurosporine, whereas SnRK2 activation by hyperosmolarity or ABA was not, indicating that SnRK2 activation by phosphorylation is mediated by an upstream staurosporine-insensitive kinase, in both signalling pathways. All together, these results indicate that different phosphorylation mechanisms and at least three signalling pathways are involved in the activation of SnRK2 proteins in response to osmotic stress and ABA.

15N-Metabolic labeling for comparative plasma membrane proteomics in Arabidopsis cells

An important goal for proteomic studies is the global comparison of proteomes from different genotypes, tissues, or physiological conditions. This has so far been mostly achieved by densitometric comparison of spot intensities after protein separation by 2-DE. However, the physicochemical properties of membrane proteins preclude the use of 2-DE. Here, we describe the use of in vivo labeling by the stable isotope 15N as an alternative approach for comparative membrane proteomic studies in plant cells. We confirm that 15N-metabolic labeling of proteins is possible and efficient in Arabidopsis suspension cells. Quantification of 14N versus 15N MS signals reflects the relative abundance of 14N and 15N proteins in the sample analyzed. We describe the use of 15N-metabolic labeling to perform a partial comparative analysis of Arabidopsis cells following cadmium exposure. By focusing our attention on plasma membrane proteins, we were able to confidently identify proteins showing up to 5-fold regulation compared to unexposed cells. This study provides a proof of principle that 15N-metabolic labeling is a useful technique for comparative membrane proteome studies.

Two members of the Arabidopsis CLC (chloride channel) family, AtCLCe and AtCLCf, are associated with thylakoid and Golgi membranes, respectively

Though numerous pieces of evidence point to major physiological roles for anion channels in plants, progress in the understanding of their biological functions is limited by the small number of genes identified so far. Seven chloride channel (CLC) members could be identified in the Arabidopsis genome, amongst which AtCLCe and AtCLCf are both more closely related to bacterial CLCs than the other plant CLCs. It is shown here that AtCLCe is targeted to the thylakoid membranes in chloroplasts and, in agreement with this subcellular localization, that the clce mutants display a phenotype related to photosynthesis activity. The AtCLCf protein is localized in Golgi membranes and functionally complements the yeast gef1 mutant disrupted in the single CLC gene encoding a Golgi-associated protein.

Identification of features regulating OST1 kinase activity and OST1 function in guard cells

The phytohormone abscisic acid (ABA) mediates drought responses in plants and, in particular, triggers stomatal closure. Snf1-related kinase 2 (SnRK2) proteins from several plant species have been implicated in ABA-signaling pathways. In Arabidopsis (Arabidopsis thaliana) guard cells, OPEN STOMATA 1 (OST1)/SRK2E/SnRK2-6 is a critical positive regulator of ABA signal transduction. A better understanding of the mechanisms responsible for SnRK2 protein kinase activation is thus a major goal toward understanding ABA signal transduction. Here, we report successful purification of OST1 produced in Escherichia coli: The protein is active and autophosphorylates. Using mass spectrometry, we identified five target residues of autophosphorylation in recombinant OST1. Sequence analysis delineates two conserved boxes located in the carboxy-terminal moiety of OST1 after the catalytic domain: the SnRK2-specific box (glutamine-303 to proline-318) and the ABA-specific box (leucine-333 to methionine-362). Site-directed mutagenesis and serial deletions reveal that serine (Ser)-175 in the activation loop and the SnRK2-specific box are critical for the activity of recombinant OST1 kinase. Targeted expression of variants of OST1 kinase in guard cells uncovered additional features that are critical for OST1 function in ABA signaling, although not required for OST1 kinase activity: Ser-7, Ser-18, and Ser-29 and the ABA-specific box. Ser-7, Ser-18, Ser-29, and Ser-43 represent putative targets for regulatory phosphorylation and the ABA-specific box may be a target for the binding of signaling partners in guard cells.

The nitrate/proton antiporter AtCLCa mediates nitrate accumulation in plant vacuoles

Nitrate, the major nitrogen source for most plants, is widely used as a fertilizer and as a result has become a predominant freshwater pollutant. Plants need nitrate for growth and store most of it in the central vacuole. Some members of the chloride channel (CLC) protein family, such as the torpedo-fish ClC-0 and mammalian ClC-1, are anion channels, whereas the bacterial ClC-ec1 and mammalian ClC-4 and ClC-5 have recently been characterized as Cl-/H+ exchangers with unknown cellular functions. Plant members of the CLC family are proposed to be anion channels involved in nitrate homeostasis; however, direct evidence for anion transport mediated by a plant CLC is still lacking. Here we show that Arabidopsis thaliana CLCa (AtCLCa) is localized to an intracellular membrane, the tonoplast of the plant vacuole, which is amenable to electrophysiological studies, and we provide direct evidence for its anion transport ability. We demonstrate that AtCLCa is able to accumulate specifically nitrate in the vacuole and behaves as a NO3-/H+ exchanger. For the first time, to our knowledge, the transport activity of a plant CLC is revealed, the antiporter mechanism of a CLC protein is investigated in a native membrane system, and this property is directly connected with its physiological role.

Characterization of active oxygen-producing proteins in response to hypo-osmolarity in tobacco and Arabidopsis cell suspensions: identification of a cell wall peroxidase

The oxidative response induced by hypo-osmolarity is characterized in tobacco and Arabidopsis cells in order to identify the corresponding active oxygen-producing proteins. The pharmacological profiles of the oxidative responses were clearly different in the two plant materials, leading to the identification of distinct active oxygen producers in tobacco and Arabidopsis cells. In tobacco cells, a 100 kDa protein, localized in the plasma membrane, was demonstrated to produce active oxygen in the presence of NADPH. This production can be activated by fatty acids and is strongly depressed by diphenylene iodonium, as measured by an in vivo response. In Arabidopsis, 30 kDa and 34 kDa proteins localized in the cell wall were shown to be able to produce active oxygen in the presence of cofactors and the production is prevented by peroxidase inhibitors, as is the in vivo response. The two purified proteins were identified by mass spectrometry and both correspond to the peroxidase gene At5g64120.

Purification and fractionation of membranes for proteomic analyses

Proteomics is a very powerful approach to link the information contained in sequenced genomes, such as Arabidopsis, to the functional knowledge provided by studies of plant cell compartments. However, membrane proteomics remains a challenge. One way to bring into view the complex mixture of proteins present in a membrane is to develop proteomic analyses based on (1) the use of highly purified membrane fractions and (2) fractionation of membrane proteins to retrieve as many proteins as possible (from the most to the less hydrophobic ones). To illustrate such strategies, we choose two types of membranes, the plasma membrane and the chloroplast envelope membranes. Both types of membranes can be prepared in a reasonable degree of purity from different types of tissues: the plasma membrane from cultured cells and the chloroplast envelope membrane from whole plants. This article is restricted to the description of methods for the preparation of highly purified and characterized plant membrane fractions and the subsequent fractionation of these membrane proteins according to simple physicochemical criteria (i.e., chloroform/methanol extraction, alkaline or saline treatments) for further analyses using modern proteomic methodologies.

A versatile method for deciphering plant membrane proteomes

Proteomics is a very powerful approach to link the information contained in sequenced genomes, like that of Arabidopsis, to the functional knowledge provided by studies of plant cell compartments. This article summarizes the different steps of a versatile strategy that has been developed to decipher plant membrane proteomes. Initiated with envelope membranes from spinach chloroplasts, this strategy has been adapted to thylakoids, and further extended to a series of membranes from the model plant Arabidopsis: chloroplast envelope membranes, plasma membrane, and mitochondrial membranes. The first step is the preparation of highly purified membrane fractions from plant tissues. The second step in the strategy is the fractionation of membrane proteins on the basis of their physico-chemical properties. Chloroform/methanol extraction and washing of membranes with NaOH, NaCl or any other agent led to the simplification of the protein content of the fraction to be analysed. The next step is the genuine proteomic step, i.e. the separation of proteins by 1D-gel electrophoresis followed by in-gel proteolytic digestion of the polypeptides, analysis of the proteolytic peptides using mass spectrometry, and protein identification by searching through databases. The last step is the validation of the procedure by checking the subcellular location. The results obtained by using this strategy demonstrate that a combination of different proteomics approaches, together with bioinformatics, indeed provide a better understanding of the biochemical machinery of the different plant membranes at the molecular level.

Mobilization of vacuolar iron by AtNRAMP3 and AtNRAMP4 is essential for seed germination on low iron

Iron (Fe) is necessary for all living cells, but its bioavailability is often limited. Fe deficiency limits agriculture in many areas and affects over a billion human beings worldwide. In mammals, NRAMP2/DMT1/DCT1 was identified as a major pathway for Fe acquisition and recycling. In plants, AtNRAMP3 and AtNRAMP4 are induced under Fe deficiency. The similitude of AtNRAMP3 and AtNRAMP4 expression patterns and their common targeting to the vacuole, together with the lack of obvious phenotype in nramp3-1 and nramp4-1 single knockout mutants, suggested a functional redundancy. Indeed, the germination of nramp3 nramp4 double mutants is arrested under low Fe nutrition and fully rescued by high Fe supply. Mutant seeds have wild type Fe content, but fail to retrieve Fe from the vacuolar globoids. Our work thus identifies for the first time the vacuole as an essential compartment for Fe storage in seeds. Our data indicate that mobilization of vacuolar Fe stores by AtNRAMP3 and AtNRAMP4 is crucial to support Arabidopsis early development until efficient systems for Fe acquisition from the soil take over.

Distinct pH regulation of slow and rapid anion channels at the plasma membrane of Arabidopsis thaliana hypocotyl cells

Variations in both intracellular and extracellular pH are known to be involved in a wealth of physiological responses. Using the patch-clamp technique on Arabidopsis hypocotyl cells, it is shown that rapid-type and slow-type anion channels at the plasma membrane are both regulated by pH via distinct mechanisms. Modifications of pH modulate the voltage-dependent gating of the rapid channel. While intracellular alkalinization facilitates channel activation by shifting the voltage gate towards negative potentials, extracellular alkalinization shifts the activation threshold to more positive potentials, away from physiological resting membrane potentials. By contrast, pH modulates slow anion channel activity in a voltage-independent manner. Intracellular acidification and extracellular alkalinization increase slow anion channel currents. The possible role of these distinct modulations in physiological processes involving anion efflux and modulation of extracellular and/or intracellular pH, such as elicitor and ABA signalling, are discussed.

Focus on plant proteomics

Proteomics covers the systematic analysis of proteins expressed by a genome, from the identification of their primary amino-acid sequence to the determination of their relative amounts, their state of modification and association with other proteins or molecules of different types. Tremendous progress has been made in this field in the past few years, especially in plant biology, mostly due to major developments of mass spectrometry dedicated to protein analyses and advanced information technology. The aim of this special issue of Plant Physiology and Biochemistry devoted to Plant Proteomics is not to present a comprehensive coverage of this rapidly expanding field but to focus on the representation of some key aspects to illustrate the importance of proteomics in plant functional genomics.

Involvement of MPK4 in osmotic stress response pathways in cell suspensions and plantlets of Arabidopsis thaliana: activation by hypoosmolarity and negative role in hyperosmolarity tolerance

Three of the protein kinases activated by hypoosmotic stress in Arabidopsis thaliana cell suspensions were previously characterized [FEBS, 2002, 527, 43-50] as mitogen-activated protein (MAP) kinases and two of them corresponded to Arabidopsis mitogen-activated protein kinase 6 (MPK6) (44 kDa) and MPK3 (39 kDa). The third MAP kinase was identified here to MPK4, using a corresponding specific antibody. Like MPK6 and MPK3, MPK4 activity is clearly inhibited by apigenin and MPK4 activation by hypoosmolarity needs upstream phosphorylation events. Activation of the 3 MAP kinases, MPK3, 4 and 6, was confirmed in plantlets submitted to hypoosmotic stress. The action of a biotic signal, flagellin, was also demonstrated to induce the activations of the 3 MAP kinases. Using the mutant displaying MPK4 gene inactivation, the independence of the MPK3 and MPK6 activations towards the presence of MPK4 was demonstrated, both in hypoosmotic and flagellin signalling pathways. Although MPK4 was not activated by hyperosmolarity in cell suspensions nor in seedlings, a possible negative regulation of hyperosmolarity resistance by MPK4 is suggested, based both on phenotype and downstream gene expression studies.

Identification of nine sucrose nonfermenting 1-related protein kinases 2 activated by hyperosmotic and saline stresses in Arabidopsis thaliana

Several calcium-independent protein kinases were activated by hyperosmotic and saline stresses in Arabidopsis cell suspension. Similar activation profiles were also observed in seedlings exposed to hyperosmotic stress. One of them was identified to AtMPK6 but the others remained to be identified. They were assumed to belong to the SNF1 (sucrose nonfermenting 1)-related protein kinase 2 (SnRK2) family, which constitutes a plant-specific kinase group. The 10 Arabidopsis SnRK2 were expressed both in cells and seedlings, making the whole SnRK2 family a suitable candidate. Using a family-specific antibody raised against the 10 SnRK2, we demonstrated that these non-MAPK protein kinases activated by hyperosmolarity in cell suspension were SnRK2 proteins. Then, the molecular identification of the involved SnRK2 was investigated by transient expression assays. Nine of the 10 SnRK2 were activated by hyperosmolarity induced by mannitol, as well as NaCl, indicating an important role of the SnRK2 family in osmotic signaling. In contrast, none of the SnRK2 were activated by cold treatment, whereas abscisic acid only activated five of the nine SnRK2. The probable involvement of the different Arabidopsis SnRK2 in several abiotic transduction pathways is discussed.

Identification of New Intrinsic Proteins in Arabidopsis Plasma Membrane Proteome

Identification and characterization of anion channel genes in plants represent a goal for a better understanding of their central role in cell signaling, osmoregulation, nutrition, and metabolism. Though channel activities have been well characterized in plasma membrane by electrophysiology, the corresponding molecular entities are little documented. Indeed, the hydrophobic protein equipment of plant plasma membrane still remains largely unknown, though several proteomic approaches have been reported. To identify new putative transport systems, we developed a new proteomic strategy based on mass spectrometry analyses of a plasma membrane fraction enriched in hydrophobic proteins. We produced from Arabidopsis cell suspensions a highly purified plasma membrane fraction and characterized it in detail by immunological and enzymatic tests. Using complementary methods for the extraction of hydrophobic proteins and mass spectrometry analyses on mono-dimensional gels, about 100 proteins have been identified, 95% of which had never been found in previous proteomic studies. The inventory of the plasma membrane proteome generated by this approach contains numerous plasma membrane integral proteins, one-third displaying at least four transmembrane segments. The plasma membrane localization was confirmed for several proteins, therefore validating such proteomic strategy. An in silico analysis shows a correlation between the putative functions of the identified proteins and the expected roles for plasma membrane in transport, signaling, cellular traffic, and metabolism. This analysis also reveals 10 proteins that display structural properties compatible with transport functions and will constitute interesting targets for further functional studies.

AtNRAMP3, a multispecific vacuolar metal transporter involved in plant responses to iron deficiency

Metal homeostasis is critical for the survival of living organisms, and metal transporters play central roles in maintaining metal homeostasis in the living cells. We have investigated the function of a metal transporter of the NRAMP family, AtNRAMP3, in Arabidopsis thaliana. A previous study showed that AtNRAMP3 expression is upregulated by iron (Fe) starvation and that AtNRAMP3 protein can transport Fe. In the present study, we used AtNRAMP3 promoter beta-glucoronidase (GUS) fusions to show that AtNRAMP3 is expressed in the vascular bundles of roots, stems, and leaves under Fe-sufficient conditions. This suggests a function in long-distance metal transport within the plant. Under Fe-starvation conditions, the GUS activity driven by the AtNRAMP3 promoter is upregulated without any change in the expression pattern. We analyze the impact of AtNRAMP3 disruption and overexpression on metal accumulation in plants. Under Fe-sufficient conditions, AtNRAMP3 overexpression or disruption does not lead to any change in the plant metal content. Upon Fe starvation, AtNRAMP3 disruption leads to increased accumulation of manganese (Mn) and zinc (Zn) in the roots, whereas AtNRAMP3 overexpression downregulates Mn accumulation. In addition, overexpression of AtNRAMP3 downregulates the expression of the primary Fe uptake transporter IRT1 and of the root ferric chelate reductase FRO2. Expression of AtNRAMP3::GFP fusion protein in onion cells or Arabidopsis protoplasts shows that AtNRAMP3 protein localizes to the vacuolar membrane. To account for the results presented, we propose that AtNRAMP3 influences metal accumulation and IRT1 and FRO2 gene expression by mobilizing vacuolar metal pools to the cytosol.

Different protein kinase families are activated by osmotic stresses in Arabidopsis thaliana cell suspensions. Involvement of the MAP kinases AtMPK3 and AtMPK6

Five Ca(2+)-independent protein kinases were rapidly activated by hypoosmotic stress, moderate or high hyperosmolarity induced by several osmolytes, sucrose, mannitol or NaCl. Three of these kinases, transiently activated by hypoosmolarity, recognised by anti-phosphorylated mitogen-activated protein (MAP) kinase antibodies, sensitive to a MAP kinase inhibitor and inactivated by the action of a tyrosine phosphatase, corresponded to MAP kinases. Using specific antibodies, two of the MAP kinases were identified as AtMPK6 and AtMPK3. The two other protein kinases, durably activated by high hyperosmolarity, did not belong to the MAP kinase family. Activation of AtMPK6 and AtMPK3 by hypoosmolarity depended on upstream protein kinases sensitive to staurosporine and on calcium influx. In contrast, these two transduction steps were not involved in the activation of the two protein kinases activated by high hyperosmolarity.

Nitrate efflux is an essential component of the cryptogein signaling pathway leading to defense responses and hypersensitive cell death in tobacco

There is much interest in the transduction pathways by which avirulent pathogens or derived elicitors activate plant defense responses. However, little is known about anion channel functions in this process. The aim of this study was to reveal the contribution of anion channels in the defense response triggered in tobacco by the elicitor cryptogein. Cryptogein induced a fast nitrate (NO(3)(-)) efflux that was sensitive to anion channel blockers and regulated by phosphorylation events and Ca(2+) influx. Using a pharmacological approach, we provide evidence that NO(3)(-) efflux acts upstream of the cryptogein-induced oxidative burst and a 40-kD protein kinase whose activation seems to be controlled by the duration and intensity of anion efflux. Moreover, NO(3)(-) efflux inhibitors reduced and delayed the hypersensitive cell death triggered by cryptogein in tobacco plants. This was accompanied by a delay or a complete suppression of the induction of several defense-related genes, including hsr203J, a gene whose expression is correlated strongly with programmed cell death in plants. Our results indicate that anion channels are involved intimately in mediating defense responses and hypersensitive cell death.

Strategies to identify transport systems in plants

Since the first molecular structures of plant transporters were discovered over a decade ago, considerable advances have been made in the study of plant membrane transport, but we still do not understand transport regulation. The genes encoding the transport systems in the various cell membranes are still to be identified, as are the physiological roles of most transport systems. A wide variety of complementary strategies are now available to study transport systems in plants, including forward and reverse genetics, proteomics, and in silico exploitation of the huge amount of information contained in the completely known genomic sequence of Arabidopsis.

Nucleotides provide a voltage-sensitive gate for the rapid anion channel of arabidopsis hypocotyl cells

The rapid anion channel of Arabidopsis hypocotyl cells is highly voltage-dependent. At hyperpolarized potentials, the channel is closed, and membrane depolarization is required for channel activation. We have previously shown that channel gating is regulated by intracellular nucleotides. In the present study, we further analyze the channel gating, and we propose a mechanism to explain its regulation by voltage. In the absence of intracellular nucleotides, closure at hyperpolarized voltages is abolished. Structure-function studies of adenyl nucleotides show that the apparent gating charge of the current increases with the negative charge carried by nucleotides. We propose that the fast anion channel is gated by the voltage-dependent entry of free nucleotides into the pore, leading to a voltage-dependent block at hyperpolarized potentials. In agreement with this mechanism in which intracellular nucleotides need to be recruited to the channel pore, kinetic analyses of whole-cell and single-channel currents show that the rate of closure is faster when intracellular nucleotide concentration is increased, whereas the rate of channel activation is unchanged. Furthermore, decreasing the concentration of extracellular chloride enhances the intracellular nucleotide block. This result supports the hypothesis of a mechanism in which blocking nucleotides and permeant anions interact within the channel pore.

Protein kinases induced by osmotic stresses and elicitor molecules in tobacco cell suspensions: two crossroad MAP kinases and one osmoregulation-specific protein kinase

Two protein kinases displaying mitogen-activated protein kinase (MAPK) properties are activated both by an hypoosmotic stress and by oligogalacturonides in tobacco cell suspensions [Cazalé et al. (1999) Plant J. 19, 297-307]. Using specific antibodies, they were identified as the salicylic acid-induced protein kinase (SIPK) and wound-induced protein kinase (WIPK). The SIPK was also activated by an hyperosmotic stress, indicating that the same kinase may play a role both in hypo- and hyperosmotic signalling pathways, in addition to its involvement in the transduction of elicitor signals. Using immunoprecipitation followed by two-dimensional in-gel kinase assay, three molecular forms of the SIPK were observed, suggesting that additional modifications of the activated kinase may occur. In contrast to WIPK and SIPK, which are located at the crossroad of several transduction pathways initiated by elicitor or osmotic stimuli, a 44 kDa kinase, that would not belong to the MAPK family, appeared more specific to osmotic stress.

CLC-Nt1, a putative chloride channel protein of tobacco, co-localizes with mitochondrial membrane markers

The voltage-dependent chloride channel (CLC) family of membrane proteins has cognates in animals, yeast, bacteria and plants, and chloride-channel activity has been assigned to most of the animal homologues. Lack of evidence of CLC functions in plants prompted us to characterize the cellular localization of the tobacco CLC-Nt1 protein. Specific polyclonal antibodies were raised against an N-terminal polypeptide of CLC-Nt1. These antibodies were used to probe membrane proteins prepared by various cell-fractionation methods. These included aqueous two-phase partitioning (for plasma membranes), free-flow electrophoresis (for vacuolar and plasma membranes), intact vacuole isolation, Percoll-gradient centrifugation (for plastids and mitochondria) and stepped, linear, sucrose-density-gradient centrifugation (for mitochondria). Each purified membrane fraction was characterized with specific marker enzyme activities or antibodies. Our studies ruled out the possibility that the major cell localization of CLC-Nt1 was the vacuolar or plasma membranes, the endoplasmic reticulum, the Golgi apparatus or the plastids. In contrast, we showed that the tobacco CLC-Nt1 specifically co-localized with the markers of the mitochondrial inner membrane, cytochrome c oxidase and NAD9 protein. CLC-Nt1 may correspond to the inner membrane anion channel ('IMAC') described previously in animal and plant mitochondria.

Anion channels in higher plants: functional characterization, molecular structure and physiological role

Anion channels are well documented in various tissues, cell types and membranes of algae and higher plants, and current evidence supports their central role in cell signaling, osmoregulation, plant nutrition and metabolism. It is the aim of this review to illustrate through a few selected examples the variety of anion channels operating in plant cells and some of their regulation properties and unique physiological functions. In contrast, information on the molecular structure of plant anion channels has only recently started to emerge. Only a few genes coding for putative plant anion channels from the large chloride channel (CLC) family have been isolated, and current molecular data on these plant CLCs are presented and discussed. A major challenge remains to identify the genes encoding the various anion channels described so far in plant cells. Future prospects along this line are briefly outlined, as well as recent advances based on the use of knockout mutants in the model plant Arabidopsis thaliana to explore the physiological functions of anion channels in planta.

Characterization of a nitrate-permeable channel able to mediate sustained anion efflux in hypocotyl cells from Arabidopsis thaliana

We have characterized a new anionic current in Arabidopsis hypocotyl cells. This current, activated by membrane depolarization, has slow activation and deactivation kinetics in the 10 sec range. It presents many distinct properties from the rapid-type anion current already described on the same membrane. The slow-type channel is highly permeable to nitrate with a PNO3-/PCl- close to 20, but totally impermeable to sulphate. Activation of the channel requires cytosolic ATP and the slow current is partially inhibited by staurosporin, suggesting that channel regulation involves protein phosphorylation. The slow anion channel displays a unique pharmacological profile different from that of the rapid channel: the slow channel is inhibited by DIDS (4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid) with an IC50 of 26 microM. The slow and rapid anion channels are probably dedicated to specific functions: the first is able to mediate sustained anion efflux, while the second is a good candidate to be involved in fast electrical signalling.

Disruption of putative anion channel gene AtCLC-a in Arabidopsis suggests a role in the regulation of nitrate content

In animals and yeast, voltage-dependent chloride channels of the CLC family play a role in basic cellular functions such as epithelial transport, plasma membrane excitability, and control of pH and membrane potential in intracellular compartments. To assess the function of CLCs in plants, we searched for CLC insertion mutants in a library of Arabidopsis lines transformed by Agrobacterium tumefaciens transferred DNA (T-DNA). Using a polymerase chain reaction-based screening procedure, an Arabidopsis line that carries a T-DNA insertion within the C-terminus of the AtCLC-a coding sequence was identified. Progeny from this plant line, clca-1, showed dramatically altered transcription of the AtCLC-a gene. Plants homozygous for the clca-1 mutation exhibited normal development and a morphology indistinguishable from the wild-type. However, their capacity to accumulate nitrate under conditions of nitrate excess was reduced in roots and shoots, by approximately 50%, while chloride, sulphate and phosphate levels were similar to the wild-type. In addition, the herbicide chlorate, an analogue of nitrate, induced a faster and more pronounced chlorosis in mutant plants. Hypersensitivity to chlorate as well as decreased nitrate levels co-segregated with the T-DNA insertion. They were found at various time points of the clca-1 life cycle, supporting the idea that AtCLC-a has a general role in the control of the nitrate status in Arabidopsis. Concordant with such a function, AtCLC-a mRNA was found in roots and shoots, and its levels rapidly increased in both tissues upon addition of nitrate but not ammonium to the culture medium. The specificity of AtCLC-a function with respect to nitrate is further supported by a similar free amino acid content in wild-type and clca-1 plants. Although the cellular localization of AtCLC-a remains unclear, our results suggest that AtCLC-a plays a role in controlling the intracellular nitrate status.

A novel immunological approach establishes that the auxin-binding protein, Nt-abp1, is an element involved in auxin signaling at the plasma membrane

Interactions of a collection of monoclonal antibodies (mAbs) to the recombinant Nicotiana tabacum auxin-binding protein 1 (Nt-abp1) were extensively characterized using surface plasmon resonance. Dynamic interaction studies using combinations of Nt-abp1, synthetic peptides corresponding to conserved sequences within auxin-binding proteins, and the mAbs have shown that a number of the mAbs recognized discontinuous epitopes revealing the junction of distinct domains in the folded protein. In particular, the two putative auxin binding domains and the C terminus of the protein were shown to interact with each other in the folded protein. Using the auxin-induced electrical response of tobacco protoplasts as a functional assay, all the mAbs exhibited either auxin antagonist or hormonomimetic properties. These effects, measured for the first time in homologous conditions, confirm that Nt-abp1 is present at the plasma membrane and is involved in the activation of the auxin-dependent electrical response of tobacco protoplasts. Based on our surface plasmon resonance data, we propose that the key event leading to the activation of this auxin electrical response consists of a conformational change in Nt-abp1.

Sulfate is both a substrate and an activator of the voltage-dependent anion channel of Arabidopsis hypocotyl cells

On the basis of the anion content of in vitro-cultured Arabidopsis plantlets, we explored the selectivity of the voltage-dependent anion channel of the plasma membrane of hypocotyl cells. In the whole-cell configuration, substitution of cytosolic Cl(-) by different anions led to the following sequence of relative permeabilities: NO(3)(-) (2.6) >/= SO(4)(2-) (2.0) > Cl(-) (1.0) > HCO(3)(-) (0.8) >> malate(2-) (0.03). Large whole-cell currents were measured for NO(3)(-) and SO(4)(2-), about five to six times higher than the equivalent Cl(-) currents. Since SO(4)(2-) is usually considered to be a weakly permeant or non-permeant ion, the components of the large whole-cell current were explored in more detail. Aside from its permeation through the channel with a unitary conductance, about two-thirds that of Cl(-), SO(4)(2-) had a regulatory effect on channel activity by preventing the run-down of the anion current both in the whole-cell and the outside-out configuration, increasing markedly the whole-cell current. The fact that the voltage-dependent plasma membrane anion channel of hypocotyl cells can mediate large NO(3)(-) and SO(4)(2-) currents and is regulated by nucleotides favors the idea that this anion channel can contribute to the cellular homeostasis of important metabolized anions.

MAP kinase activation by hypoosmotic stress of tobacco cell suspensions: towards the oxidative burst response?

Hypoosmotic stress activates a phosphorylation-dependent oxidative burst. In-gel kinase assays were performed to characterize the protein kinases that could be implicated in osmoregulation and in the activation of the oxidative burst. Hypoosmotic stress activated several kinases among which 50 and 46 kDa proteins displayed mitogen-activated protein kinase (MAP kinase) properties. They phosphorylated myelin basic protein in the absence of calcium, were recognized by antibodies directed against human MAP kinases, and were phosphorylated on tyrosine. Immunoprecipitation with an antibody directed against the tobacco MAP kinase Ntf4 showed that at least one of the activated kinases would be Ntf4-like. Apigenin, a MAP kinase and cyclin-dependent kinase inhibitor which prevents the hypoosmotically induced oxidative burst (Cazale et al. 1998; Plant Physiol. 116, 659-669), inhibited these kinases in vitro suggesting that they may play a role in the activation of the oxidative burst. Like the oxidative response, activation of the kinases depended on extracellular calcium influx and protein kinases sensitive to staurosporine and 6-DMAP. However, kinase activation did not depend on effluxes through anion channels or on the oxidative burst. Two-dimensional in-gel kinase assays revealed the presence of three protein kinases with an apparent molecular mass of 50 kDa and one of 46 kDa, all four being activated by hypoosmotic stress. The same kinases were also activated by oligogalacturonides and salicylic acid, underlying the importance of these MAP kinases as common components of different signaling pathways triggered by different extracellular stimuli.

The sax1 dwarf mutant of Arabidopsis thaliana shows altered sensitivity of growth responses to abscisic acid, auxin, gibberellins and ethylene and is partially rescued by exogenous brassinosteroid

Genetic approaches using Arabidopsis thaliana aimed at the identification of mutations affecting events involved in auxin signalling have usually led to the isolation of auxin-resistant mutants. From a selection screen specifically developed to isolate auxin-hypersensitive mutants, one mutant line was selected for its increased sensitivity to auxin (x 2 to 3) for the root elongation response. The genetic analysis of sax1 (hypersensitive to abscisic acid and auxin) indicated that the mutant phenotype segregates as a single recessive Mendelian locus, mapping to the lower arm of chromosome 1. Sax1 seedlings grown in vitro showed a short curled primary root and small, round, dark-green cotyledons. In the greenhouse, adult sax1 plants were characterized by a dwarf phenotype, delayed development and reduced fertility. Further physiological characterization of sax1 seedlings revealed that the most striking trait was a large increase (x 40) in ABA-sensitivity of root elongation and, to a lesser extent, of ABA-induced stomatal closure; in other respects, hypocotyl elongation was resistant to gibberellins and ethylene. These alterations in hormone sensitivity in sax1 plants co-segregated with the dwarf phenotype suggesting that processes involved in cell elongation are modified. Treatment of mutant seedlings with an exogenous brassinosteroid partially rescued a wild-type size, suggesting that brassinosteroid biosynthesis might be affected in sax1 plants. Wild-type sensitivities to ABA, auxin and gibberellins were also restored in sax1 plants by exogenous application of brassinosteroid, illustrating the pivotal importance of the BR-related SAX1 gene.

The sax1 mutation defines a new locus involved in the brassinosteroid biosynthesis pathway in Arabidopsis thaliana

In this issue we described a dwarf mutant in Arabidopsis thaliana, sax1, which is affected in brassinosteroid biosynthesis. This primary defect is responsible for alterations in hormone sensitivity of sax1 plants characterized by the hypersensitivity of root elongation to abscisic acid and auxin and the insensitivity of hypocotyl growth to gibberellins and ethylene (Ephritikhine et al., 1999; Plant J. 18, 303-314). In this paper, we report the further characterization of the sax1 mutant aimed at identification of the mutated step in the brassinosteroid biosynthesis pathway. Rescue experiments with various intermediates of the pathway showed that the sax1 mutation alters a very early step catalyzing the oxidation and isomerization of 3 beta-hydroxyl, delta 5,6 precursors to 3-oxo, delta 4,5 steroids. The mapping of the mutation, the physiological properties of the mutant and the rescue experiments indicate that sax1 defines a new locus in the brassinosteroid biosynthesis pathway. The SAX1 protein is involved in brassinosteroid-dependent growth of seedlings in both light and dark conditions.

The auxin-binding protein Nt-ERabp1 alone activates an auxin-like transduction pathway

Hyperpolarization of tobacco protoplasts is amongst the earliest auxin responses described. It has been proposed that the auxin-binding protein, ABP1, or a related protein could be involved in the first step of auxin perception at the plasma membrane. Using for the first time homologous conditions for interaction between the protein Nt-ERabp1 or a synthetic peptide corresponding to the C-terminus and tobacco protoplasts, we have demonstrated that both can induce the hyperpolarization response. The results show that Nt-ERabp1 or the C-terminal peptide alone activates the auxin pathway from the outer face of the plasma membrane.

Anion-channel blockers interfere with auxin responses in dark-grown Arabidopsis hypocotyls

Anion channels are thought to participate in signal transduction and turgor regulation in higher plant cells. The regulation of hypocotyl cell elongation is a situation in which these channels could play important roles because it involves ionic fluxes that are implicated in turgor control and orchestrated by various signals. We have used a pharmacological approach to reveal the contribution of anion channels in the regulation of the development of hypocotyls by auxins. Auxins induce an inhibition of elongation, a disintegration of the cortical cell layers, and the formation of adventitious roots on Arabidopsis thaliana hypocotyls grown in the dark. Anion-channel blockers such as anthracene-9-carboxylic acid, 4,4'-diisothiocyanatostilbene-2-2'-disulfonic acid, 4-acetamido-4'-isothiocyanato-stilbene-2-2'-disulfonic acid, and R(+)-methylindazone; indanyloxyacteic acid-94, which produce little or no stimulation of hypocotyl elongation by themselves, are able to counteract the inhibition and the disintegration induced by auxins with various efficiencies. This interference appears to be specific for auxins and does not occur when hypocotyl elongation is inhibited by other growth regulators such as ethylene or cytokinins. The putative involvement of anion channels in auxin signal transduction is discussed.