Structure-Guided Identification of Critical Residues in the Vacuolar Cation/Proton Antiporter NHX1 from Arabidopsis thaliana

Cation/Proton Antiporters (CPA) acting in all biological membranes regulate the volume and pH of cells and of intracellular organelles. A key issue with these proteins is their structure-function relationships since they present intrinsic regulatory features that rely on structural determinants, including pH sensitivity and the stoichiometry of ion exchange. Crystal structures are only available for prokaryotic CPA, whereas the eukaryotic ones have been modeled using the former as templates. Here, we present an updated and improved structural model of the tonoplast-localized K+, Na+/H+ antiporter NHX1 of Arabidopsis as a representative of the vacuolar NHX family that is essential for the accumulation of K+ into plant vacuoles. Conserved residues that were judged as functionally important were mutated, and the resulting protein variants were tested for activity in the yeast Saccharomyces cerevisiae. The results indicate that residue N184 in the ND-motif characteristic of CPA1 could be replaced by the DD-motif of CPA2 family members with minimal consequences for their activity. Attempts to alter the electroneutrality of AtNHX1 by different combinations of amino acid replacements at N184, R353 and R390 residues resulted in inactive or partly active proteins with a differential ability to control the vacuolar pH of the yeast.

A salt stress-activated GSO1-SOS2-SOS1 module protects the Arabidopsis root stem cell niche by enhancing sodium ion extrusion

Soil salinity impairs plant growth reducing crop productivity. Toxic accumulation of sodium ions is counteracted by the Salt Overly Sensitive (SOS) pathway for Na⁺ extrusion, comprising the Na⁺ transporter SOS1, the kinase SOS2, and SOS3 as one of several Calcineurin-B-like (CBL) Ca^{2 +} sensors. Here, we report that the receptor-like kinase GSO1/SGN3 activates SOS2, independently of SOS3 binding, by physical interaction and phosphorylation at Thr16. Loss of GSO1 function renders plants salt sensitive and GSO1 is both sufficient and required for activating the SOS2-SOS1 module in yeast and in planta. Salt stress causes the accumulation of GSO1 in two specific and spatially defined areas of the root tip: in the endodermis section undergoing Casparian strip (CS) formation, where it reinforces the CIF-GSO1-SGN1 axis for CS barrier formation; and in the meristem, where it creates the GSO1-SOS2-SOS1 axis for Na⁺ detoxification. Thus, GSO1 simultaneously prevents Na⁺ both from diffusing into the vasculature, and from poisoning unprotected stem cells in the meristem. By protecting the meristem, receptor-like kinase-conferred activation of the SOS2-SOS1 module allows root growth to be maintained in adverse environments.

The phosphoinositide PI(3,5)P2 inhibits the activity of plant NHX proton/potassium antiporters: Advantages of a novel electrophysiological approach

In the present work, we discuss the way in which the parallel application of the patch-clamp technique and the 2′,7′-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) fluorescence detection for recording luminal proton changes allows the functional characterization of nonelectrogenic potassium/proton vacuolar antiporters of the NHX (Na+/H+ exchanger) family. Moreover, we review the functional role of the tonoplast-specific phosphoinositide PI(3,5)P2, able to simultaneously inhibit the activity of NHXs and CLC-a transporters, whose coordinated action can play an important role in the water balance of plant cells.

The Arabidopsis protein NPF6.2/NRT1.4 is a plasma membrane nitrate transporter and a target of protein kinase CIPK23

Nitrate and potassium nutrition is tightly coordinated in vascular plants. Physiological and molecular genetics studies have demonstrated that several NPF/NRT1 nitrate transporters have a significant impact on both uptake and the root-shoot partition of these nutrients. However, how these traits are biochemically connected remain controversial since some NPF proteins, e.g. NPF7.3/NRT1.5, have been suggested to mediate K⁺/H⁺ exchange instead of nitrate fluxes. Here we show that NPF6.2/NRT1.4, a protein that gates nitrate accumulation at the leaf petiole of Arabidopsis thaliana, also affects the root/shoot distribution of potassium. We demonstrate that NPF6.2/NRT1.4 is a plasma membrane nitrate transporter phosphorylated at threonine-98 by the CIPK23 protein kinase that is a regulatory hub for nitrogen and potassium nutrition. Heterologous expression of NPF6.2/NRT1.4 and NPF7.3/NRT1.5 in yeast mutants with altered potassium uptake and efflux systems showed no evidence of nitrate-dependent potassium transport by these proteins.

Beyond the patch-clamp resolution: functional activity of nonelectrogenic vacuolar NHX proton/potassium antiporters and inhibition by phosphoinositides

We combined the patch-clamp technique with ratiometric fluorescence imaging using the proton-responsive dye BCECF as a luminal probe. Upon application of a steep cytosol-directed potassium ion (K⁺ ) gradient in Arabidopsis mesophyll vacuoles, a strong and reversible acidification of the vacuolar lumen was detected, whereas no associated electrical currents were observed, in agreement with electroneutral cation/H⁺ exchange. Our data show that this acidification was generated by NHX antiport activity, because: it did not distinguish between K⁺ and sodium (Na⁺ ) ions; it was sensitive to the NHX inhibitor benzamil; and it was completely absent in vacuoles from nhx1 nhx2 double knockout plants. Our data further show that NHX activity could be reversed, was voltage-independent and specifically impaired by the low-abundance signaling lipid PI(3,5)P₂ , which may regulate salt accumulation in plants by acting as a common messenger to coordinately shut down secondary active carriers responsible for cation and anion uptake inside the vacuole. Finally, we developed a theory based on thermodynamics, which supports the data obtained by our novel experimental approach. This work, therefore, represents a proof-of-principle that can be applied to the study of proton-dependent exchangers from plants and animals, which are barely detectable using conventional techniques.

The role of PQL genes in response to salinity tolerance in Arabidopsis and barley

While soil salinity is a global problem, how salt enters plant root cells from the soil solution remains underexplored. Non-selective cation channels (NSCCs) are suggested to be the major pathway for the entry of sodium ions (Na+), yet their genetic constituents remain unknown. Yeast PQ loop (PQL) proteins were previously proposed to encode NSCCs, but the role of PQLs in plants is unknown. The hypothesis tested in this research is that PQL proteins constitute NSCCs mediating some of the Na+ influx into the root, contributing to ion accumulation and the inhibition of growth in saline conditions. We identified plant PQL homologues, and studied the role of one clade of PQL genes in Arabidopsis and barley. Using heterologous expression of AtPQL1a and HvPQL1 in HEK293 cells allowed us to resolve sizable inwardly directed currents permeable to monovalent cations such as Na+, K+, or Li+ upon membrane hyperpolarization. We observed that GFP-tagged PQL proteins localized to intracellular membrane structures, both when transiently over-expressed in tobacco leaf epidermis and in stable Arabidopsis transformants. Expression of AtPQL1a, AtPQL1b, and AtPQL1c was increased by salt stress in the shoot tissue compared to non-stressed plants. Mutant lines with altered expression of AtPQL1a, AtPQL1b, and AtPQL1c developed larger rosettes in saline conditions, while altered levels of AtPQL1a severely reduced development of lateral roots in all conditions. This study provides the first step toward understanding the function of PQL proteins in plants and the role of NSCC in salinity tolerance.

Distinct Roles of N-Terminal Fatty Acid Acylation of the Salinity-Sensor Protein SOS3

The Salt-Overly-Sensitive (SOS) pathway controls the net uptake of sodium by roots and the xylematic transfer to shoots in vascular plants. SOS3/CBL4 is a core component of the SOS pathway that senses calcium signaling of salinity stress to activate and recruit the protein kinase SOS2/CIPK24 to the plasma membrane to trigger sodium efflux by the Na/H exchanger SOS1/NHX7. However, despite the well-established function of SOS3 at the plasma membrane, SOS3 displays a nucleo-cytoplasmic distribution whose physiological meaning is not understood. Here, we show that the N-terminal part of SOS3 encodes structural information for dual acylation with myristic and palmitic fatty acids, each of which commands a different location and function of SOS3. N-myristoylation at glycine-2 is essential for plasma membrane association and recruiting SOS2 to activate SOS1, whereas S-acylation at cysteine-3 redirects SOS3 toward the nucleus. Moreover, a poly-lysine track in positions 7-11 that is unique to SOS3 among other Arabidopsis CBLs appears to be essential for the correct positioning of the SOS2-SOS3 complex at the plasma membrane for the activation of SOS1. The nuclear-localized SOS3 protein had limited bearing on the salt tolerance of Arabidopsis. These results are evidence of a novel S-acylation dependent nuclear trafficking mechanism that contrasts with alternative subcellular targeting of other CBLs by S-acylation.

Recognition and Activation of the Plant AKT1 Potassium Channel by the Kinase CIPK23

Plant growth largely depends on the maintenance of adequate intracellular levels of potassium (K⁺). The families of 10 Calcineurin B-Like (CBL) calcium sensors and 26 CBL-Interacting Protein Kinases (CIPKs) of Arabidopsis (Arabidopsis thaliana) decode the calcium signals elicited by environmental inputs to regulate different ion channels and transporters involved in the control of K⁺ fluxes by phosphorylation-dependent and -independent events. However, the detailed molecular mechanisms governing target specificity require investigation. Here, we show that the physical interaction between CIPK23 and the noncanonical ankyrin domain in the cytosolic side of the inward-rectifier K⁺ channel AKT1 regulates kinase docking and channel activation. Point mutations on this domain specifically alter binding to CIPK23, enhancing or impairing the ability of CIPK23 to regulate channel activity. Our data demonstrate the relevance of this protein-protein interaction that contributes to the formation of a complex between CIPK23/CBL1 and AKT1 in the membrane for the proper regulation of K⁺ transport.

Coordinated Transport of Nitrate, Potassium, and Sodium

Potassium (K+) and nitrogen (N) are essential nutrients, and their absorption and distribution within the plant must be coordinated for optimal growth and development. Potassium is involved in charge balance of inorganic and organic anions and macromolecules, control of membrane electrical potential, pH homeostasis and the regulation of cell osmotic pressure, whereas nitrogen is an essential component of amino acids, proteins, and nucleic acids. Nitrate (NO3 -) is often the primary nitrogen source, but it also serves as a signaling molecule to the plant. Nitrate regulates root architecture, stimulates shoot growth, delays flowering, regulates abscisic acid-independent stomata opening, and relieves seed dormancy. Plants can sense K+/NO3 - levels in soils and adjust accordingly the uptake and root-to-shoot transport to balance the distribution of these ions between organs. On the other hand, in small amounts sodium (Na+) is categorized as a "beneficial element" for plants, mainly as a "cheap" osmolyte. However, at high concentrations in the soil, Na+ can inhibit various physiological processes impairing plant growth. Hence, plants have developed specific mechanisms to transport, sense, and respond to a variety of Na+ conditions. Sodium is taken up by many K+ transporters, and a large proportion of Na+ ions accumulated in shoots appear to be loaded into the xylem by systems that show nitrate dependence. Thus, an adequate supply of mineral nutrients is paramount to reduce the noxious effects of salts and to sustain crop productivity under salt stress. In this review, we will focus on recent research unraveling the mechanisms that coordinate the K+-NO3 -; Na+-NO3 -, and K+-Na+ transports, and the regulators controlling their uptake and allocation.

A Critical Role of Sodium Flux via the Plasma Membrane Na+/H+ Exchanger SOS1 in the Salt Tolerance of Rice

Rice (Oryza sativa) stands among the world's most important crop species. Rice is salt sensitive, and the undue accumulation of sodium ions (Na⁺) in shoots has the strongest negative correlation with rice productivity under long-term salinity. The plasma membrane Na⁺/H⁺ exchanger protein Salt Overly Sensitive 1 (SOS1) is the sole Na⁺ efflux transporter that has been genetically characterized to date. Here, the importance of SOS1-facilitated Na⁺ flux in the salt tolerance of rice was analyzed in a reverse-genetics approach. A sos1 loss-of-function mutant displayed exceptional salt sensitivity that was correlated with excessive Na⁺ intake and impaired Na⁺ loading into the xylem, thus indicating that SOS1 controls net root Na⁺ uptake and long-distance Na⁺ transport to shoots. The acute Na⁺ sensitivity of sos1 plants at low NaCl concentrations allowed analysis of the transcriptional response to sodicity stress without effects of the osmotic stress intrinsic to high-salinity treatments. In contrast with that in the wild type, sos1 mutant roots displayed preferential down-regulation of stress-related genes in response to salt treatment, despite the greater intensity of stress experienced by the mutant. These results suggest there is impaired stress detection or an inability to mount a comprehensive response to salinity in sos1 In summary, the plasma membrane Na⁺/H⁺ exchanger SOS1 plays a major role in the salt tolerance of rice by controlling Na⁺ homeostasis and possibly contributing to the sensing of sodicity stress.

Regulation of K+ Nutrition in Plants

Modern agriculture relies on mineral fertilization. Unlike other major macronutrients, potassium (K+) is not incorporated into organic matter but remains as soluble ion in the cell sap contributing up to 10% of the dry organic matter. Consequently, K+ constitutes a chief osmoticum to drive cellular expansion and organ movements, such as stomata aperture. Moreover, K+ transport is critical for the control of cytoplasmic and luminal pH in endosomes, regulation of membrane potential, and enzyme activity. Not surprisingly, plants have evolved a large ensemble of K+ transporters with defined functions in nutrient uptake by roots, storage in vacuoles, and ion translocation between tissues and organs. This review describes critical transport proteins governing K+ nutrition, their regulation, and coordinated activity, and summarizes our current understanding of signaling pathways activated by K+ starvation.

Regulation of K+ Nutrition in Plants

Modern agriculture relies on mineral fertilization. Unlike other major macronutrients, potassium (K⁺) is not incorporated into organic matter but remains as soluble ion in the cell sap contributing up to 10% of the dry organic matter. Consequently, K⁺ constitutes a chief osmoticum to drive cellular expansion and organ movements, such as stomata aperture. Moreover, K⁺ transport is critical for the control of cytoplasmic and luminal pH in endosomes, regulation of membrane potential, and enzyme activity. Not surprisingly, plants have evolved a large ensemble of K⁺ transporters with defined functions in nutrient uptake by roots, storage in vacuoles, and ion translocation between tissues and organs. This review describes critical transport proteins governing K⁺ nutrition, their regulation, and coordinated activity, and summarizes our current understanding of signaling pathways activated by K⁺ starvation.

Pleiotropic effects of enhancing vacuolar K/H exchange in tomato

Cation antiporters of the NHX family are widely regarded as determinants of salt tolerance due to their capacity to drive sodium (Na) and sequester it into vacuoles. Recent work shows, however, that NHX transporters are primarily involved in vacuolar potassium (K) storage. Over-expression of the K/H antiporter AtNHX1 in tomato increases K accumulation into vacuoles and plant sensitivity to K deprivation. Here we show that the appearance of early leaf symptoms of K deficiency was associated with higher concentration of polyamines. Transgenic roots exhibited a greater sensitivity than shoots to K deprivation with changes in the composition of the free amino acids pool, total sugars and organic acids. Concentrations of amides (glutamine), amino acids (arginine) and sugars significantly increased in root, together with a reduction in malate and succinate concentrations. The concentration of pyruvate and the activity of pyruvate kinase were greater in the transgenic roots before K withdrawal although both parameters were depressed by K deprivation and approached wild-type levels. In the longer term, the over-expression of the NHX1 antiporter affected root growth and biomass partitioning (shoot/root ratio). Greater ethylene release produced longer stem internodes and leaf curling in the transgenic line. Our data show that enhanced sequestration of K by the NHX antiporter in the vacuoles altered cellular K homeostasis and had deeper physiological consequences than expected. Early metabolic changes lead later on to profound morphological and physiological adjustments resulting eventually in the loss of nutrient use efficiency.

Feature extraction for robust physical activity recognition

This paper presents the development of a Human Activity Recognition (HAR) system that uses a network of nine inertial measurement units situated in different body parts. Every unit provides 3D (3-dimension) acceleration, 3D angular velocity, 3D magnetic field orientation, and 4D quaternions. This system identifies 33 different physical activities (walking, running, cycling, lateral elevation of arms, etc.). The system is composed of two main modules: a feature extractor for obtaining the most relevant characteristics from the inertial signals every second, and a machine learning algorithm for classifying between the different activities. This paper focuses on the feature extractor module, evaluating several types of features and proposing different normalization approaches. This paper also analyses the performance of every sensor included in the inertial measurement units. The main experiments have been done using a public available dataset named REALDISP Activity Recognition dataset. This dataset includes recordings from 17 subjects performing 33 different activities in three different scenarios. Final results demonstrate that the proposed HAR system significantly improves the classification accuracy compared to previous works on this dataset. For the best configuration, the system accuracy is 99.1%. This system has been also evaluated with the OPPORTUNITY dataset obtaining competitive results.

Control of vacuolar dynamics and regulation of stomatal aperture by tonoplast potassium uptake

Stomatal movements rely on alterations in guard cell turgor. This requires massive K(+) bidirectional fluxes across the plasma and tonoplast membranes. Surprisingly, given their physiological importance, the transporters mediating the energetically uphill transport of K(+) into the vacuole remain to be identified. Here, we report that, in Arabidopsis guard cells, the tonoplast-localized K(+)/H(+) exchangers NHX1 and NHX2 are pivotal in the vacuolar accumulation of K(+) and that nhx1 nhx2 mutant lines are dysfunctional in stomatal regulation. Hypomorphic and complete-loss-of-function double mutants exhibited significantly impaired stomatal opening and closure responses. Disruption of K(+) accumulation in guard cells correlated with more acidic vacuoles and the disappearance of the highly dynamic remodelling of vacuolar structure associated with stomatal movements. Our results show that guard cell vacuolar accumulation of K(+) is a requirement for stomatal opening and a critical component in the overall K(+) homeostasis essential for stomatal closure, and suggest that vacuolar K(+) fluxes are also of decisive importance in the regulation of vacuolar dynamics and luminal pH that underlie stomatal movements.

The Salt Overly Sensitive (SOS) pathway: established and emerging roles

Soil salinity is a growing problem around the world with special relevance in farmlands. The ability to sense and respond to environmental stimuli is among the most fundamental processes that enable plants to survive. At the cellular level, the Salt Overly Sensitive (SOS) signaling pathway that comprises SOS3, SOS2, and SOS1 has been proposed to mediate cellular signaling under salt stress, to maintain ion homeostasis. Less well known is how cellularly heterogenous organs couple the salt signals to homeostasis maintenance of different types of cells and to appropriate growth of the entire organ and plant. Recent evidence strongly indicates that different regulatory mechanisms are adopted by roots and shoots in response to salt stress. Several reports have stated that, in roots, the SOS proteins may have novel roles in addition to their functions in sodium homeostasis. SOS3 plays a critical role in plastic development of lateral roots through modulation of auxin gradients and maxima in roots under mild salt conditions. The SOS proteins also play a role in the dynamics of cytoskeleton under stress. These results imply a high complexity of the regulatory networks involved in plant response to salinity. This review focuses on the emerging complexity of the SOS signaling and SOS protein functions, and highlights recent understanding on how the SOS proteins contribute to different responses to salt stress besides ion homeostasis.

Ion exchangers NHX1 and NHX2 mediate active potassium uptake into vacuoles to regulate cell turgor and stomatal function in Arabidopsis

Intracellular NHX proteins are Na(+),K(+)/H(+) antiporters involved in K(+) homeostasis, endosomal pH regulation, and salt tolerance. Proteins NHX1 and NHX2 are the two major tonoplast-localized NHX isoforms. Here, we show that NHX1 and NHX2 have similar expression patterns and identical biochemical activity, and together they account for a significant amount of the Na(+),K(+)/H(+) antiport activity in tonoplast vesicles. Reverse genetics showed functional redundancy of NHX1 and NHX2 genes. Growth of the double mutant nhx1 nhx2 was severely impaired, and plants were extremely sensitive to external K(+). By contrast, nhx1 nhx2 mutants showed similar sensitivity to salinity stress and even greater rates of Na(+) sequestration than the wild type. Double mutants had reduced ability to create the vacuolar K(+) pool, which in turn provoked greater K(+) retention in the cytosol, impaired osmoregulation, and compromised turgor generation for cell expansion. Genes NHX1 and NHX2 were highly expressed in guard cells, and stomatal function was defective in mutant plants, further compromising their ability to regulate water relations. Together, these results show that tonoplast-localized NHX proteins are essential for active K(+) uptake at the tonoplast, for turgor regulation, and for stomatal function.

The AtNHX1 exchanger mediates potassium compartmentation in vacuoles of transgenic tomato

NHX-type antiporters in the tonoplast have been reported to increase the salt tolerance of various plants species, and are thought to mediate the compartmentation of Na(+) in vacuoles. However, all isoforms characterized so far catalyze both Na(+)/H(+) and K(+)/H(+) exchange. Here, we show that AtNHX1 has a critical involvement in the subcellular partitioning of K(+), which in turn affects plant K(+) nutrition and Na(+) tolerance. Transgenic tomato plants overexpressing AtNHX1 had larger K(+) vacuolar pools in all growth conditions tested, but no consistent enhancement of Na(+) accumulation was observed under salt stress. Plants overexpressing AtNHX1 have a greater capacity to retain intracellular K(+) and to withstand salt-shock. Under K(+)-limiting conditions, greater K(+) compartmentation in the vacuole occurred at the expense of the cytosolic K(+) pool, which was lower in transgenic plants. This caused the early activation of the high-affinity K(+) uptake system, enhanced K(+) uptake by roots, and increased the K(+) content in plant tissues and the xylem sap of transformed plants. Our results strongly suggest that NHX proteins are likely candidates for the H(+)-linked K(+) transport that is thought to facilitate active K(+) uptake at the tonoplast, and the partitioning of K(+) between vacuole and cytosol.

Loss of halophytism by interference with SOS1 expression

The contribution of SOS1 (for Salt Overly Sensitive 1), encoding a sodium/proton antiporter, to plant salinity tolerance was analyzed in wild-type and RNA interference (RNAi) lines of the halophytic Arabidopsis (Arabidopsis thaliana)-relative Thellungiella salsuginea. Under all conditions, SOS1 mRNA abundance was higher in Thellungiella than in Arabidopsis. Ectopic expression of the Thellungiella homolog ThSOS1 suppressed the salt-sensitive phenotype of a Saccharomyces cerevisiae strain lacking sodium ion (Na(+)) efflux transporters and increased salt tolerance of wild-type Arabidopsis. thsos1-RNAi lines of Thellungiella were highly salt sensitive. A representative line, thsos1-4, showed faster Na(+) accumulation, more severe water loss in shoots under salt stress, and slower removal of Na(+) from the root after removal of stress compared with the wild type. thsos1-4 showed drastically higher sodium-specific fluorescence visualized by CoroNa-Green, a sodium-specific fluorophore, than the wild type, inhibition of endocytosis in root tip cells, and cell death in the adjacent elongation zone. After prolonged stress, Na(+) accumulated inside the pericycle in thsos1-4, while sodium was confined in vacuoles of epidermis and cortex cells in the wild type. RNAi-based interference of SOS1 caused cell death in the root elongation zone, accompanied by fragmentation of vacuoles, inhibition of endocytosis, and apoplastic sodium influx into the stele and hence the shoot. Reduction in SOS1 expression changed Thellungiella that normally can grow in seawater-strength sodium chloride solutions into a plant as sensitive to Na(+) as Arabidopsis.

Mutants of the Arabidopsis thaliana cation/H+ antiporter AtNHX1 conferring increased salt tolerance in yeast: the endosome/prevacuolar compartment is a target for salt toxicity

Mutants of the plant cation/H(+) antiporter AtNHX1 that confer greater halotolerance were generated by random mutagenesis and selected in yeast by phenotypic complementation. The amino acid substitutions that were selected were conservative and occurred in the second half of the membrane-associated N terminus. AtNHX1 complemented the lack of endogenous ScNHX1 in endosomal protein trafficking assays. Growth enhancement on hygromycin B and vanadate media agreed with a generally improved endosomal/prevacuolar function of the mutated proteins. In vivo measurements by (31)P NMR revealed that wild-type and mutant AtNHX1 transporters did not affect cytosolic or vacuolar pH. Surprisingly, when yeast cells were challenged with lithium, a tracer for sodium, the main effect of the mutations in AtNHX1 was a reduction in the amount of compartmentalized lithium. When purified and reconstituted into proteoliposomes or assayed in intact vacuoles isolated from yeast cells, a representative mutant transporter (V318I) showed a greater cation discrimination favoring potassium transport over that of sodium or lithium. Together, our data suggest that the endosome/prevacuolar compartment is a target for salt toxicity. Poisoning by toxic cations in the endosome/prevacuolar compartment is detrimental for cell functions, but it can be alleviated by improving the discrimination of transported alkali cations by the resident cation/H(+) antiporter.

The phosphate transporter PHT4;6 is a determinant of salt tolerance that is localized to the Golgi apparatus of Arabidopsis

Insertion mutations that disrupt the function of PHT4;6 (At5g44370) cause NaCl hypersensitivity of Arabidopsis seedlings that is characterized by reduced growth of the primary root, enhanced lateral branching, and swelling of root tips. Mutant phenotypes were exacerbated by sucrose, but not by equiosmolar concentrations of mannitol, and attenuated by low inorganic phosphate in the medium. Protein PHT4;6 belongs to the Major Facilitator Superfamily of permeases that shares significant sequence similarity to mammalian type-I Pi transporters and vesicular glutamate transporters, and is a member of the PHT4 family of putative intracellular phosphate transporters of plants. PHT4;6 localizes to the Golgi membrane and transport studies indicate that PHT4;6 facilitates the selective transport of Pi but not of chloride or inorganic anions. Phenotypic similarities with other mutants displaying root swelling suggest that PHT4;6 likely functions in protein N-glycosylation and cell wall biosynthesis, which are essential for salt tolerance. Together, our results indicate that PHT4;6 transports Pi out of the Golgi lumenal space for the re-cycling of the Pi released from glycosylation processes.

Phosphorylation of SOS3-LIKE CALCIUM BINDING PROTEIN8 by SOS2 protein kinase stabilizes their protein complex and regulates salt tolerance in Arabidopsis

The Salt Overly Sensitive (SOS) pathway plays an important role in the regulation of Na+/K+ ion homeostasis and salt tolerance in Arabidopsis thaliana. Previously, we reported that the calcium binding proteins SOS3 and SOS3-LIKE CALCIUM BINDING PROTEIN8 (SCaBP8) nonredundantly activate the protein kinase SOS2. Here, we show that SOS2 phosphorylates SCaBP8 at its C terminus but does not phosphorylate SOS3. In vitro, SOS2 phosphorylation of SCaBP8 was enhanced by the bimolecular interaction of SOS2 and SCaBP8 and did not require calcium ions. In vivo, this phosphorylation was induced by salt stress, occurred at the membrane, stabilized the SCaBP8-SOS2 interaction, and enhanced plasma membrane Na+/H+ exchange activity. When a Ser at position 237 in the SCaBP8 protein (the SOS2 phosphorylation target) was mutated to Ala, SCaBP8 was no longer phosphorylated by SOS2 and the mutant protein could not fully rescue the salt-sensitive phenotype of the scabp8 mutant. By contrast, when Ser-237 was mutated to Asp to mimic the charge of a phosphorylated Ser residue, the mutant protein rescued the scabp8 salt sensitivity. These data demonstrate that calcium sensor phosphorylation is a critical component of SOS pathway regulation of salt tolerance in Arabidopsis.

SCABP8/CBL10, a putative calcium sensor, interacts with the protein kinase SOS2 to protect Arabidopsis shoots from salt stress

The SOS (for Salt Overly Sensitive) pathway plays essential roles in conferring salt tolerance in Arabidopsis thaliana. Under salt stress, the calcium sensor SOS3 activates the kinase SOS2 that positively regulates SOS1, a plasma membrane sodium/proton antiporter. We show that SOS3 acts primarily in roots under salt stress. By contrast, the SOS3 homolog SOS3-LIKE CALCIUM BINDING PROTEIN8 (SCABP8)/CALCINEURIN B-LIKE10 functions mainly in the shoot response to salt toxicity. While root growth is reduced in sos3 mutants in the presence of NaCl, the salt sensitivity of scabp8 is more prominent in shoot tissues. SCABP8 is further shown to bind calcium, interact with SOS2 both in vitro and in vivo, recruit SOS2 to the plasma membrane, enhance SOS2 activity in a calcium-dependent manner, and activate SOS1 in yeast. In addition, sos3 scabp8 and sos2 scabp8 display a phenotype similar to sos2, which is more sensitive to salt than either sos3 or scabp8 alone. Overexpression of SCABP8 in sos3 partially rescues the sos3 salt-sensitive phenotype. However, overexpression of SOS3 fails to complement scabp8. These results suggest that SCABP8 and SOS3 are only partially redundant in their function, and each plays additional and unique roles in the plant salt stress response.

Conservation of the salt overly sensitive pathway in rice

The salt tolerance of rice (Oryza sativa) correlates with the ability to exclude Na+ from the shoot and to maintain a low cellular Na+/K+ ratio. We have identified a rice plasma membrane Na+/H+ exchanger that, on the basis of genetic and biochemical criteria, is the functional homolog of the Arabidopsis (Arabidopsis thaliana) salt overly sensitive 1 (SOS1) protein. The rice transporter, denoted by OsSOS1, demonstrated a capacity for Na+/H+ exchange in plasma membrane vesicles of yeast (Saccharomyces cerevisiae) cells and reduced their net cellular Na+ content. The Arabidopsis protein kinase complex SOS2/SOS3, which positively controls the activity of AtSOS1, phosphorylated OsSOS1 and stimulated its activity in vivo and in vitro. Moreover, OsSOS1 suppressed the salt sensitivity of a sos1-1 mutant of Arabidopsis. These results represent the first molecular and biochemical characterization of a Na+ efflux protein from monocots. Putative rice homologs of the Arabidopsis protein kinase SOS2 and its Ca2+-dependent activator SOS3 were identified also. OsCIPK24 and OsCBL4 acted coordinately to activate OsSOS1 in yeast cells and they could be exchanged with their Arabidopsis counterpart to form heterologous protein kinase modules that activated both OsSOS1 and AtSOS1 and suppressed the salt sensitivity of sos2 and sos3 mutants of Arabidopsis. These results demonstrate that the SOS salt tolerance pathway operates in cereals and evidences a high degree of structural conservation among the SOS proteins from dicots and monocots.

Alkali cation exchangers: roles in cellular homeostasis and stress tolerance

Uptake and translocation of cations play essential roles in plant nutrition, signal transduction, growth, and development. Among them, potassium (K+) and sodium (Na+) have been the focus of numerous physiological studies because K+ is an essential macronutrient and the most abundant inorganic cation in plant cells, whereas Na+ toxicity is a principal component of the deleterious effects associated with salinity stress. Although the homeostasis of these two ions was long surmised to be fine tuned and under complex regulation, the myriad of candidate membrane transporters mediating their uptake, intracellular distribution, and long-distance transport is nevertheless perplexing. Recent advances have shown that, in addition to their function in vacuolar accumulation of Na+, proteins of the NHX family are endosomal transporters that also play critical roles in K+ homeostasis, luminal pH control, and vesicle trafficking. The plasma membrane SOS1 protein from Arabidopsis thaliana, a highly specific Na+/H+ exchanger that catalyses Na+ efflux and that regulates its root/shoot distribution, has also revealed surprising interactions with K+ uptake mechanisms by roots. Finally, the function of individual members of the large CHX family remains largely unknown but two CHX isoforms, AtCHX17 and AtCH23, have been shown to affect K+ homeostasis and the control of chloroplast pH, respectively. Recent advances on the understanding of the physiological processes that are governed by these three families of cation exchangers are reviewed and discussed.

Osmotin Is a Homolog of Mammalian Adiponectin and Controls Apoptosis in Yeast through a Homolog of Mammalian Adiponectin Receptor

The antifungal activity of the PR-5 family of plant defense proteins has been suspected to involve specific plasma membrane component(s) of the fungal target. Osmotin is a tobacco PR-5 family protein that induces apoptosis in the yeast Saccharomyces cerevisiae. We show here that the protein encoded by ORE20/PHO36 (YOL002c), a seven transmembrane domain receptor-like polypeptide that regulates lipid and phosphate metabolism, is an osmotin binding plasma membrane protein that is required for full sensitivity to osmotin. PHO36 functions upstream of RAS2 in the osmotin-induced apoptotic pathway. The mammalian homolog of PHO36 is a receptor for the hormone adiponectin and regulates cellular lipid and sugar metabolism. Osmotin and adiponectin, the corresponding "receptor" binding proteins, do not share sequence similarity. However, the beta barrel domain of both proteins can be overlapped, and osmotin, like adiponectin, activates AMP kinase in C2C12 myocytes via adiponectin receptors.

Impact of the human immunodeficiency virus on the epidemiology of tuberculosis in area 15 of the Valencian community in Spain

SETTING

Area 15 in Valencia.

OBJECTIVES

To describe the epidemiology (1987-2001) of tuberculosis (TB) in human immunodeficiency virus (HIV) patients.

METHODS

Study of annual incidence, age distribution, excess cases attributed to HIV, etiological risk fraction (ERF), population attributable fraction (PAF) and f factor.

RESULTS

Of 476 cases diagnosed, 459 were TB, 16 environmental and one mixed; 76% of environmental cases were HIV-positive (P < 0.001). There was a mean annual TB incidence of 24.6/100000, with an annual reduction of 4%. Seventy-three patients were HIV coinfected (16%) (mean incidence 3834/100 000 seropositives). The principal risk factor was drug use (59%) for HIV+ and contact with TB for HIV-. We found no difference in pulmonary or extra-pulmonary location between groups, contrary to mixed cases (P < 0.001). In HIV+ there was a lower frequency of infiltrates (P < 0.001) and cavitation (P < 0.01), and a higher frequency of adenitis (P < 0.001), miliary or nodular pattern and normal X-ray (P < 0.001). Seropositives had a 174 times higher probability of developing TB. The mean ERF attributed to HIV was 99%, the PAF was 16% and the f factor was 1.19. Highly active antiretroviral therapy (HAART) reduced the risk of TB in HIV+ by 80%.

CONCLUSIONS

TB has continued its decline, although HIV generated an excess of cases in the 1990s. HAART has reduced the TB risk in HIV+ and possibly the overall rate of TB.

Inorganic cations mediate plant PR5 protein antifungal activity through fungal Mnn1- and Mnn4-regulated cell surface glycans

Antimicrobial activities of many defense proteins are profoundly altered by inorganic cations, thereby controlling disease pathologies in a number of mammalian systems, such as cystic fibrosis in humans. Protein-based active defense systems in plants also are influenced by cations; however, little is known of how these cation effects are mediated. Cytotoxicity of the pathogenesis-related protein osmotin against the model fungus Saccharomyces cerevisiae was progressively abolished by K+. By the use of S. cerevisiae mannosylation mutants, this effect was shown to require mannosephosphate residues in the cell wall. However, osmotin activity was not suppressed by even high concentrations of Ca2+. Rather, submillimolar levels of Ca2+ specifically facilitated osmotin's activity, as well as its binding to the cell surface. This effect also was dependent on mannosephosphate groups on the cell surface, and appeared to require negative charge on a portion of the osmotin protein. Results suggest that Ca2+ modulates osmotin action by facilitating its binding to the fungal cell surface, but that K+ blocks this interaction by competing for binding to mannosephosphate groups. Therefore, we have identified glycan interaction as a mechanism for antimicrobial protein activity modulation by cations, a pattern that may apply to diverse innate defense responses.

Transgenic evaluation of activated mutant alleles of SOS2 reveals a critical requirement for its kinase activity and C-terminal regulatory domain for salt tolerance in Arabidopsis thaliana

In Arabidopsis thaliana, the calcium binding protein Salt Overly Sensitive3 (SOS3) interacts with and activates the protein kinase SOS2, which in turn activates the plasma membrane Na(+)/H(+) antiporter SOS1 to bring about sodium ion homeostasis and salt tolerance. Constitutively active alleles of SOS2 can be constructed in vitro by changing Thr(168) to Asp in the activation loop of the kinase catalytic domain and/or by removing the autoinhibitory FISL motif from the C-terminal regulatory domain. We expressed various activated forms of SOS2 in Saccharomyces cerevisiae (yeast) and in A. thaliana and evaluated the salt tolerance of the transgenic organisms. Experiments in which the activated SOS2 alleles were coexpressed with SOS1 in S. cerevisiae showed that the kinase activity of SOS2 is partially sufficient for SOS1 activation in vivo, and higher kinase activity leads to greater SOS1 activation. Coexpression of SOS3 with SOS2 forms that retained the FISL motif resulted in more dramatic increases in salt tolerance. In planta assays showed that the Thr(168)-to-Asp-activated mutant SOS2 partially rescued the salt hypersensitivity in sos2 and sos3 mutant plants. By contrast, SOS2 lacking only the FISL domain suppressed the sos2 but not the sos3 mutation, whereas truncated forms in which the C terminus had been removed could not restore the growth of either sos2 or sos3 plants. Expression of some of the activated SOS2 proteins in wild-type A. thaliana conferred increased salt tolerance. These studies demonstrate that the protein kinase activity of SOS2 is partially sufficient for activation of SOS1 and for salt tolerance in vivo and in planta and that the kinase activity of SOS2 is limiting for plant salt tolerance. The results also reveal an essential in planta role for the SOS2 C-terminal regulatory domain in salt tolerance.

Regulation of Vacuolar Na+/H+ Exchange in Arabidopsis thaliana by the Salt-Overly-Sensitive (SOS) Pathway

For plants growing in highly saline environments, accumulation of sodium in the cell cytoplasm leads to disruption of metabolic processes and reduced growth. Maintaining low levels of cytoplasmic sodium requires the coordinate regulation of transport proteins on numerous cellular membranes. Our previous studies have linked components of the Salt-Overly-Sensitive pathway (SOS1-3) to salt tolerance in Arabidopsis thaliana and demonstrated that the activity of the plasma membrane Na+/H+ exchanger (SOS1) is regulated by SOS2 (a protein kinase) and SOS3 (a calcium-binding protein). Current studies were undertaken to determine if the Na+/H+ exchanger in the vacuolar membrane (tonoplast) of Arabidopsis is also a target for the SOS regulatory pathway. Characterization of tonoplast Na+/H+ exchange demonstrated that it represents activity originating from the AtNHX proteins since it could be inhibited by 5-(N-methyl-N-isobutyl)amiloride and by anti-NHX1 antibodies. Transport activity was selective for sodium (apparent Km=31 mm) and electroneutral (one sodium ion for each proton). When compared with tonoplast Na+/H+-exchange activity in wild type, activity was significantly higher, greatly reduced, and unchanged in sos1, sos2, and sos3, respectively. Activated SOS2 protein added in vitro increased tonoplast Na+/H+-exchange activity in vesicles isolated from sos2 but did not have any effect on activity in vesicles isolated from wild type, sos1, or sos3. These results demonstrate that (i) the tonoplast Na+/H+ exchanger in Arabidopsis is a target of the SOS regulatory pathway, (ii) there are branches to the SOS pathway, and (iii) there may be coordinate regulation of the exchangers in the tonoplast and plasma membrane.

The STT3a subunit isoform of the Arabidopsis oligosaccharyltransferase controls adaptive responses to salt/osmotic stress

Arabidopsis stt3a-1 and stt3a-2 mutations cause NaCl/osmotic sensitivity that is characterized by reduced cell division in the root meristem. Sequence comparison of the STT3a gene identified a yeast ortholog, STT3, which encodes an essential subunit of the oligosaccharyltransferase complex that is involved in protein N-glycosylation. NaCl induces the unfolded protein response in the endoplasmic reticulum (ER) and cell cycle arrest in root tip cells of stt3a seedlings, as determined by expression profiling of ER stress-responsive chaperone (BiP-GUS) and cell division (CycB1;1-GUS) genes, respectively. Together, these results indicate that plant salt stress adaptation involves ER stress signal regulation of cell cycle progression. Interestingly, a mutation (stt3b-1) in another Arabidopsis STT3 isogene (STT3b) does not cause NaCl sensitivity. However, the stt3a-1 stt3b-1 double mutation is gametophytic lethal. Apparently, STT3a and STT3b have overlapping and essential functions in plant growth and developmental processes, but the pivotal and specific protein glycosylation that is a necessary for recovery from the unfolded protein response and for cell cycle progression during salt/osmotic stress recovery is associated uniquely with the function of the STT3a isoform.

An osmotically induced cytosolic Ca2+ transient activates calcineurin signaling to mediate ion homeostasis and salt tolerance of Saccharomyces cerevisiae

Hyperosmotic stress caused by NaCl, LiCl, or sorbitol induces an immediate and short duration ( approximately 1 min) transient cytosolic Ca(2+) ([Ca(2+)](cyt)) increase (Ca(2+)-dependent aequorin luminescence) in Saccharomyces cerevisiae cells. The amplitude of the osmotically induced [Ca(2+)](cyt) transient was attenuated by the addition of chelating agents EGTA or BAPTA, cation channel pore blockers, competitive inhibitors of Ca(2+) transport, or mutations (cch1Delta or mid1Delta) that reduce Ca(2+) influx, indicating that Ca(ext)(2+) is a source for the transient. An osmotic pretreatment (30 min) administered by inoculating cells into media supplemented with either NaCl (0.4 or 0.5 m) or sorbitol (0.8 or 1.0 m) enhanced the subsequent growth of these cells in media containing 1 m NaCl or 2 m sorbitol. Inclusion of EGTA in the osmotic pretreatment media or the cch1Delta mutation reduced cellular capacity for NaCl but not hyperosmotic adaptation. The stress-adaptive effect of hyperosmotic pretreatment was mimicked by exposing cells briefly to 20 mm CaCl(2). Thus, NaCl- or sorbitol-induced hyperosmotic shock causes a [Ca(2+)](cyt) transient that is facilitated by Ca(2+) influx, which enhances ionic but not osmotic stress adaptation. NaCl-induced ENA1 expression was inhibited by EGTA, cch1Delta mutation, and FK506, indicating that the [Ca(2+)](cyt) transient activates calcineurin signaling to mediate ion homeostasis and salt tolerance.

Comparative study of the thermodynamic behaviour of the binary mixtures dimethyl carbonate + (benzene, n-heptane, cyclohexane, or toluene)

Isobaric molar heat capacities at 288.15, 298.15, and 308.15 K and densities and speeds of sound at 288.15, 293.15, 298.15, and 308.15 K were determined for the dimethyl carbonate + benzene system over the whole composition range and at atmospheric pressure. In addition, speeds of sound at the same temperatures for the dimethyl carbonate + n-heptane system are reported. These experimental data were used to obtain molar volumes, isobaric thermal expansivities, isentropic and isothermal compressibilities, and isochoric molar heat capacities. Excess quantities of the above-mentioned properties were calculated using the Benson and Kiyohara criterion. The thermodynamic behaviour of the dimethyl carbonate + (n-heptane, cyclohexane, benzene, or toluene) systems is comparatively analyzed.Key words: binary mixtures, excess properties, dimethyl carbonate, benzene, hydrocarbon.

A plant defense response effector induces microbial apoptosis

Osmotin is a tobacco PR-5 protein that has antifungal activity and is implicated in host-plant defense. We show here that osmotin induces apoptosis in Saccharomyces cerevisiae. Induction of apoptosis was correlated with intracellular accumulation of reactive oxygen species and was mediated by RAS2, but not RAS1. Osmotin treatment resulted in suppression of transcription of stress-responsive genes via the RAS2/cAMP pathway. It was therefore concluded that osmotin induced proapoptotic signaling in yeast. The results indicate that the ability of antimicrobial proteins to induce microbial apoptosis could be an important factor in determining a pathogen's virulence and could therefore be targeted for the design of new antifungal drugs.

Tobacco and Arabidiopsis SLT1 mediate salt tolerance of yeast

A tobacco cDNA (NtSLT1, for Nicotiana tabacum sodium- and lithium-tolerant) was isolated by functional complementation of the salt-sensitive phenotype of a calcineurin (CaN)-deficient yeast mutant (cnb delta, regulatory subunit null). CaN is a Ca2+/calmodulin-dependent type 2B protein phosphatase that regulates Na+ homeostasis in yeast. This phosphatase modulates plasma membrane K+/Na+ selectivity through the activation of high-affinity K+ transport, and increaseses extracellular Na+ efflux by activation and transcriptional induction of the Na+/Li+ translocating P-type ATPase encoded by ENA1. Expression of N-terminally truncated NtSLT1 (Met-304), but not full-length protein, suppressed salt sensitivity of cnb1. Truncated NtSLT1 also increased salt tolerance of wild-type yeast, indicating functional sufficiency. NtSLT1 encodes a protein of yet unknown function but experimentation in yeast confirms it as a salt tolerance determinant. The Arabidopsis thaliana orthologue, AtSLT1, also suppressed salt sensitivity of cnb delta but only when expressed without the N-terminus (Met-301), suggesting that this region of the proteins from these evolutionarily diverse plant species contains an autoinhibitory domain. NtSLT1 enhanced transcription of the CaN-dependent ENA1 gene promoter and compensated the salt sensitivity of a mutant deficient in TCN1--a transcription factor that is activated by CaN and then induces ENA1 expression. NtSLT1 partially suppressed the salt sensitivity of ena1-4 indicating that NtSLT1 has both ENA-dependent and independent functions. NtSLT1 suppressed spk1 hal4 (SPK1/HAL4 which encodes a serine-threonine kinase that regulates TRK1-2 transporters to have high K+/Na+ selectivity) but not ena1-4 trk1-2 implicating the ENA-independent function to be through TRK1-2. Together, these results implicate SLT1 as a signal regulatory molecule that mediates salt tolerance by modulating Na+ homeostasis.

Resistance to the plant PR-5 protein osmotin in the model fungus Saccharomyces cerevisiae is mediated by the regulatory effects of SSD1 on cell wall composition

The capacity of plants to counter the challenge of pathogenic fungal attack depends in part on the ability of plant defense proteins to overcome fungal resistance by being able to recognize and eradicate the invading fungi. Fungal genes that control resistance to plant defense proteins are therefore important determinants that define the range of fungi from which an induced defense protein can protect the plant. Resistance of the model fungus Saccharomyces cerevisiae to osmotin, a plant defense PR-5 protein, is strongly dependent on the natural polymorphism of the SSD1 gene. Expression of the SSD1-v allele afforded resistance to the antifungal protein. Conversely, yeast strains carrying the SSD1-d allele or a null ssd1Delta mutation displayed high sensitivity to osmotin. The SSD1-v protein mediates osmotin resistance in a cell wall-dependent manner. Deletion of SSD1-v or SSD1-d impeded sorting of the PIR proteins (osmotin-resistance factors) to the cell wall without affecting mRNA levels, indicating that SSD1 functions in post-transcriptional regulation of gene expression. The sensitivity of ssd1Delta cells to osmotin was only partially suppressed by over-accumulation of PIR proteins in the cell wall, suggesting an additional function for SSD1 in cell wall-mediated resistance. Accordingly, cells carrying a null ssd1 mutation also displayed aberrant cell-wall morphology and lower levels of alkali-insoluble cell-wall glucans. Therefore SSD1 is an important regulator of fungal cell-wall biogenesis and composition, including the deposition of PIR proteins which block the action of plant antifungal PR-5 proteins.

Fungal cell wall phosphomannans facilitate the toxic activity of a plant PR-5 protein

Osmotin is a plant PR-5 protein. It has a broad spectrum of antifungal activity, yet also exhibits specificity for certain fungal targets. The structural bases for this specificity remain unknown. We show here that full sensitivity of Saccharomyces cerevisiae cells to the PR-5 protein osmotin is dependent on the function of MNN2, MNN4 and MNN6. MNN2 is an alpha-1, 2-mannosyltransferase catalyzing the addition of the first mannose to the branches on the poly l,6-mannose backbone of the outer chain of cell wall N-linked mannans. MNN4 and MNN6 are required for the transfer of mannosylphosphate to cell wall mannans. Null mnn2, mnn4 or mnn6 mutants lack phosphomannans and are defective in binding osmotin to the fungal cell wall. Both antimannoprotein antibody and the cationic dye alcian blue protect cells against osmotin cytotoxicity. MNN1 is an alpha-1,3-mannosyltransferase that adds the terminal mannose to the outer chain branches of N-linked mannan, masking mannosylphosphate. Null mnn1 cells exhibit enhanced osmotin binding and sensitivity. Several cell wall mannoproteins can bind to immobilized osmotin, suggesting that their polysaccharide constituent determines osmotin binding. Our results demonstrating a causal relationship between cell surface phosphomannan and the susceptibility of a yeast strain to osmotin suggest that cell surface polysaccharides of invading pathogens control target specificity of plant PR-5 proteins.

Functional conservation between yeast and plant endosomal Na(+)/H(+) antiporters

Vacuolar compartmentation of Na(+) is an essential mechanism for salinity tolerance since it lowers cytosolic Na(+) levels while contributing to osmotic adjustment for cell turgor and expansion. The AtNHX1 protein of Arabidopsis thaliana substituted functionally for ScNHX1, the endosomal Na(+)/H(+) antiporter of yeast. Ion tolerance conferred by AtNHX1 and ScNHX1 correlated with ion uptake into an intracellular pool that was energetically dependent on the vacuolar (H(+))ATPase. AtNHX1 localized to vacuolar membrane fractions of yeast. Hence, both transporters share an evolutionarily conserved function in Na(+) compartmentation. AtNHX1 mRNA levels were upregulated by ABA and NaCl treatment in leaf but not in root tissue.

Stress signaling through Ca2+/calmodulin-dependent protein phosphatase calcineurin mediates salt adaptation in plants

Calcineurin (CaN) is a Ca2+- and calmodulin-dependent protein phosphatase (PP2B) that, in yeast, is an integral intermediate of a salt-stress signal transduction pathway that effects NaCl tolerance through the regulation of Na+ influx and efflux. A truncated form of the catalytic subunit and the regulatory subunit of yeast CaN were coexpressed in transgenic tobacco plants to reconstitute a constitutively activated phosphatase in vivo. Several different transgenic lines that expressed activated CaN also exhibited substantial NaCl tolerance, and this trait was linked to the genetic inheritance of the CaN transgenes. Enhanced capacity of plants expressing CaN to survive NaCl shock was similar when evaluation was conducted on seedlings in tissue culture raft vessels or plants in hydroponic culture that were transpiring actively. Root growth was less perturbed than shoot growth by NaCl in plants expressing CaN. Also, NaCl stress survival of control shoots was enhanced substantially when grafted onto roots of plants expressing CaN, further implicating a significant function of the phosphatase in the preservation of root integrity during salt shock. Together, these results indicate that in plants, like in yeast, a Ca2+- and calmodulin-dependent CaN signal pathway regulates determinants of salt tolerance required for stress adaptation. Furthermore, modulation of this pathway by expression of an activated regulatory intermediate substantially enhanced salt tolerance.

Osmotin, a plant antifungal protein, subverts signal transduction to enhance fungal cell susceptibility

The plant pathogenesis-related protein osmotin is an antifungal cytotoxic agent that causes rapid cell death in the yeast S. cerevisiae. We show here that osmotin uses a signal transduction pathway to weaken defensive cell wall barriers and increase its cytotoxic efficacy. The pathway activated by osmotin includes the regulatory elements of the mating pheromone response STE4, STE18, STE20, STE5, STE11, STE7, FUS3, KSS1, and STE12. Neither the pheromone receptor nor its associated G protein alpha subunit GPA1 are required for osmotin action. However, mutation of SST2, a negative regulator of G alpha proteins, resulted in supersensitivity to osmotin. Phosphorylation of STE7 was rapidly stimulated by osmotin preceding any changes in cell vitality or morphology. These results demonstrate that osmotin subverts target cell signal transduction as part of its mechanism of action.

Stress proteins on the yeast cell surface determine resistance to osmotin, a plant antifungal protein

Strains of the yeast Saccharomyces cerevisiae differ in their sensitivities to tobacco osmotin, an antifungal protein of the PR-5 family. However, cells sensitive to tobacco osmotin showed resistance to osmotin-like proteins purified from the plant Atriplex nummularia, indicating a strict specificity between the antifungal protein and its target cell. A member of a gene family encoding stress proteins induced by heat and nitrogen limitation, collectively called Pir proteins, was isolated among the genes that conveyed resistance to tobacco osmotin to a susceptible strain. We show that overexpression of Pir proteins increased resistance to osmotin, whereas simultaneous deletion of all PIR genes in a tolerant strain resulted in sensitivity. Pir proteins have been immunolocalized to the cell wall. The enzymatic digestion of the cell wall of sensitive and resistant cells rendered spheroplasts equally susceptible to the cytotoxic action of tobacco osmotin but not to other osmotin-like proteins, indicating that the cell membrane interacts specifically with osmotin and facilitates its action. Our results demonstrate that fungal cell wall proteins are determinants of resistance to antifungal PR-5 proteins.

Activated calcineurin confers high tolerance to ion stress and alters the budding pattern and cell morphology of yeast cells

The PP2B protein phosphatase, also known as calcineurin, is a regulator of ion homeostasis in yeast cells. We have investigated the physiological consequences of constitutive expression of a recombinant form of calcineurin in which the Ca2+/calmodulin-binding and autoinhibitory domains of the catalytic subunit were deleted. The concomitant expression of the regulatory subunit along with the truncated catalytic subunit resulted in high tolerance to toxic levels of Na+ and Li+. This activated form of calcineurin substituted for the Na+ stress signal to promote the expression of the ENA1 gene, encoding a P-ATPase pump, and to induce the transition of the K+ uptake system to the high affinity mode that restricts influx of Na+ and Li+. In addition, the transcriptional responsiveness of ENA1 to Na+ stress was enhanced. These results demonstrate that calcineurin has a pivotal role in a signaling cascade activated by ion stress in yeast. Moreover, we found that changes in the level of calcineurin activity affected budding pattern and cell morphology. Cells expressing the truncated calcineurin were elongated and budded in an unipolar pattern, whereas calcineurin-deficient mutants budded randomly. These results suggest that calcineurin may also act in the establishment of cell polarity.

Phosphate transporters from the higher plant Arabidopsis thaliana

Two cDNAs (AtPT1 and AtPT2) encoding plant phosphate transporters have been isolated from a library prepared with mRNA extracted from phosphate-starved Arabidopsis thaliana roots, The encoded polypeptides are 78% identical to each other and show high degree of amino acid sequence similarity with high-affinity phosphate transporters of Saccharomyces cerevisiae, Neurospora crassa, and the mycorrhizal fungus Glomus versiforme. The AtPT1 and AtPT2 polypeptides are integral membrane proteins predicted to contain 12 membrane-spanning domains separated into two groups of six by a large charged hydrophilic region. Upon expression, both AtPT1 and AtPT2 were able to complement the pho84 mutant phenotype of yeast strain NS219 lacking the high-affinity phosphate transport activity. AtPT1 and AtPT2 are representatives of two distinct, small gene families in A. thaliana. The transcripts of both genes are expressed in roots and are not detectable in leaves. The steady-state level of their mRNAs increases in response to phosphate starvation.