Ice plant root plasma membrane aquaporins are regulated by clathrin-coated vesicles in response to salt stress

The regulation of root Plasma membrane (PM) Intrinsic Protein (PIP)-type aquaporins (AQPs) is potentially important for salinity tolerance. However, the molecular and cellular details underlying this process in halophytes remain unclear. Using free-flow electrophoresis and label-free proteomics, we report that the increased abundance of PIPs at the PM of the halophyte ice plant (Mesembryanthemum crystallinum L.) roots under salinity conditions is regulated by clathrin-coated vesicles (CCV). To understand this regulation, we analyzed several components of the M. crystallinum CCV complexes: clathrin light chain (McCLC) and subunits μ1 and μ2 of the adaptor protein (AP) complex (McAP1μ and McAP2μ). Co-localization analyses revealed the association between McPIP1;4 and McAP2μ and between McPIP2;1 and McAP1μ, observations corroborated by mbSUS assays, suggesting that AQP abundance at the PM is under the control of CCV. The ability of McPIP1;4 and McPIP2;1 to form homo- and hetero-oligomers was tested and confirmed, as well as their activity as water channels. Also, we found increased phosphorylation of McPIP2;1 only at the PM in response to salt stress. Our results indicate root PIPs from halophytes might be regulated through CCV trafficking and phosphorylation, impacting their localization, transport activity, and abundance under salinity conditions.

Plasma and vacuolar membrane sphingolipidomes: composition and insights on the role of main molecular species

Lipid structures affect membrane biophysical properties such as thickness, stability, permeability, curvature, fluidity, asymmetry, and interdigitation, contributing to membrane function. Sphingolipids are abundant in plant endomembranes and plasma membranes (PMs) and comprise four classes: ceramides, hydroxyceramides, glucosylceramides, and glycosylinositolphosphoceramides (GIPCs). They constitute an array of chemical structures whose distribution in plant membranes is unknown. With the aim of describing the hydrophobic portion of sphingolipids, 18 preparations from microsomal (MIC), vacuolar (VM), PM, and detergent-resistant membranes (DRM) were isolated from Arabidopsis (Arabidopsis thaliana) leaves. Sphingolipid species, encompassing pairing of long-chain bases and fatty acids, were identified and quantified in these membranes. Sphingolipid concentrations were compared using univariate and multivariate analysis to assess sphingolipid diversity, abundance, and predominance across membranes. The four sphingolipid classes were present at different levels in each membrane: VM was enriched in glucosylceramides, hydroxyceramides, and GIPCs; PM in GIPCs, in agreement with their key role in signal recognition and sensing; and DRM in GIPCs, as reported by their function in nanodomain formation. While a total of 84 sphingolipid species was identified in MIC, VM, PM, and DRM, only 34 were selectively distributed in the four membrane types. Conversely, every membrane contained a different number of predominant species (11 in VM, 6 in PM, and 17 in DRM). This study reveals that MIC, VM, PM, and DRM contain the same set of sphingolipid species but every membrane source contains its own specific assortment based on the proportion of sphingolipid classes and on the predominance of individual species.

CD43 (sialophorin) is involved in the induction of extracellular matrix remodeling and angiogenesis by lung cancer cells

Aberrant expression of CD43 in malignant tumors of nonhematopoietic origin such as those from lung, cervix, colon, and breast has been shown to correlate with poor prognosis, providing tumor cells with enhanced motility, anchorage-independent growth, and in vivo tumor size, while protecting the cells of NK lysis and apoptosis. To further characterize the role of CD43 in cell transformation, we tested whether interfering its expression modified the capacity of the A549 non-small cell lung cancer cells to secrete molecules contributing to malignancy. The proteomic analysis of the secretome of serum-starved A549 cells revealed that cells expressing normal levels of CD43 released significantly high levels of molecules involved in extracellular matrix organization, angiogenesis, platelet degranulation, collagen degradation, and inflammation, as compared to CD43 RNAi cells. This data reveals a novel and unexpected role for CD43 in lung cancer development, mainly in remodeling the tumor microenvironment.

Integrative study of microbial community dynamics and water quality along The Apatlaco River

The increasing demand for clean water resources for human consumption, is raising concerning about the sustainable worldwide provisioning. In Mexico, rivers near to high-density urbanizations are subject to irrational exploitation where polluted water is a risk for human health. Therefore, the aims of this study are to analyze water quality parameters and bacterial community dynamics to understand the relation between them, in the Apatlaco river, which presents a clear environmental perturbance. Parameters such as total coliforms, chemical oxygen demand, harness, ammonium, nitrite, nitrate, total Kjeldahl nitrogen, dissolved oxygen, total phosphorus, total dissolved solids, and temperature were analyzed in 17 sampling points along the river. The high pollution level was registered in the sampling point 10 with 480 mg/L chemical oxygen demand, 7 mg/L nitrite, 34 mg/L nitrate, 2 mg/L dissolved oxygen, and 299 mg/L of total dissolved solids. From these sites, we selected four samples for DNA extraction and performed a metagenomic analysis using a whole metagenome shotgun approach, to compare the microbial communities between polluted and non-polluted sites. In general, Proteobacteria was the most representative phylum in all sites. However, the clean water reference point was enriched with microorganism from the Limnohabitans genus, a planktonic bacterium widespread in freshwater ecosystems. Nevertheless, in the polluted sampled sites, we found a high abundance of potential opportunistic pathogen genera such as Acinetobacter, Arcobacter, and Myroides, among others. This suggests that in addition to water contamination, an imminent human health risk due to pathogenic bacteria can potentially affect a population of ∼1.6 million people dwelling nearby. These results will contribute to the knowledge regarding anthropogenic pollution on the microbial population dynamic and how they affect human health and life quality.

Transcriptomic and Proteomic Analysis of the Tentacles and Mucus of Anthopleura dowii Verrill, 1869

Sea anemone venom contains a complex and diverse arsenal of peptides and proteins of pharmacological and biotechnological interest, however, only venom from a few species has been explored from a global perspective to date. In the present study, we identified the polypeptides present in the venom of the sea anemone Anthopleura dowii Verrill, 1869 through a transcriptomic and proteomic analysis of the tentacles and the proteomic profile of the secreted mucus. In our transcriptomic results, we identified 261 polypeptides related to or predicted to be secreted in the venom, including proteases, neurotoxins that could act as either potassium (K⁺) or sodium (Na⁺) channels inhibitors, protease inhibitors, phospholipases A2, and other polypeptides. Our proteomic data allowed the identification of 156 polypeptides—48 exclusively identified in the mucus, 20 in the tentacles, and 88 in both protein samples. Only 23 polypeptides identified by tandem mass spectrometry (MS/MS) were related to the venom and 21 exclusively identified in the mucus, most corresponding to neurotoxins and hydrolases. Our data contribute to the knowledge of evolutionary and venomic analyses of cnidarians, particularly of sea anemones.

Functional analysis of the Chloroplast GrpE (CGE) proteins from Arabidopsis thaliana

The function of proteins depends on specific partners that regulate protein folding, degradation and protein-protein interactions, such partners are the chaperones and cochaperones. In chloroplasts, proteins belonging to several families of chaperones have been identified: chaperonins (Cpn60s), Hsp90s (Hsp90-5/Hsp90C), Hsp100s (Hsp93/ClpC) and Hsp70s (cpHsc70s). Several lines of evidence have demonstrated that cpHsc70 chaperones are involved in molecular processes like protein import, protein folding and oligomer formation that impact important physiological aspects in plants such as thermotolerance and thylakoid biogenesis. Despite the vast amount of data existing around the function of cpHcp70s chaperones, very little attention has been paid to the roles of DnaJ and GrpE cochaperones in the chloroplast. In this study, we performed a phylogenetic analysis of the chloroplastic GrpE (CGE) proteins from 71 species. Based on their phylogenetic relationships and on a motif enrichment analysis, we propose a classification system for land plants' CGEs, which include two independent groups with specific primary structure traits. Furthermore, using in vivo assays we determined that the two CGEs from A. thaliana (AtCGEs) complement the mutant phenotype displayed by a knockout E. coli strain defective in the bacterial grpE gene. Moreover, we determined in planta that the two AtCGEs are bona fide chloroplastic proteins, which form the essential homodimers needed to establish direct physical interactions with the cpHsc70-1 chaperone. Finally, we found evidence suggesting that AtCGE1 is involved in specific physiological phenomena in A. thaliana, such as the chloroplastic response to heat stress, and the correct oligomerization of the photosynthesis-related LHCII complex.

Cadmium and zinc activate adaptive mechanisms in Nicotiana tabacum similar to those observed in metal tolerant plants

MAIN CONCLUSION

Tobacco germinated and grew in the presence of high concentrations of cadmium and zinc without toxic symptoms. Evidence suggests that these ions are sequestered into the vacuole by heavy metal/H + exchanger mechanisms. Heavy metal hyperaccumulation and hypertolerance are traits shared by a small set of plants which show specialized physiological and molecular adaptations allowing them to accumulate and sequester toxic metal ions. Nicotiana tabacum was used to test its potential as a metal-accumulator in a glass house experiment. Seed germination was not affected in the presence of increasing concentrations of zinc and cadmium. Juvenile and adult plants could concentrate CdCl2 and ZnSO4 to levels exceeding those in the hydroponic growth medium and maintained or increased their leaf dry weight when treated with 0.5- or 1-mM CdCl2 or 1-mM ZnSO4 for 5 days. Accumulation of heavy metals did not affect the chlorophyll and carotenoid levels, while variable effects were observed in cell sap osmolarity. Heavy metal-dependent H+ transport across the vacuole membrane was monitored using quinacrine fluorescence quenching. Cadmium- or zinc-dependent fluorescence recovery revealed that increasing concentrations of heavy metals stimulated the activities of the tonoplast Cd2+ or Zn2+/H+ exchangers. Immunodetection of the V-ATPase subunits showed that the increased proton transport by zinc was not due to changes in protein amount. MTP1 and MTP4 immunodetection and semiquantitative RT-PCR of NtMTP1, NtNRAMP1, and NtZIP1 helped to identify the genes that are likely involved in sequestration of cadmium and zinc in the leaf and root tissue. Finally, we demonstrated that cadmium and zinc treatments induced an accumulation of zinc in leaf tissues. This study shows that N. tabacum possesses a hyperaccumulation response, and thus could be used for phytoremediation purposes.

Membrane Proteomic Insights into the Physiology and Taxonomy of an Oleaginous Green Microalga

Ettlia oleoabundans is a nonsequenced oleaginous green microalga. Despite the significant biotechnological interest in producing value-added compounds from the acyl lipids of this microalga, a basic understanding of the physiology and biochemistry of oleaginous microalgae is lacking, especially under nitrogen deprivation conditions known to trigger lipid accumulation. Using an RNA sequencing-based proteomics approach together with manual annotation, we are able to provide, to our knowledge, the first membrane proteome of an oleaginous microalga. This approach allowed the identification of novel proteins in E. oleoabundans, including two photoprotection-related proteins, Photosystem II Subunit S and Maintenance of Photosystem II under High Light1, which were considered exclusive to higher photosynthetic organisms, as well as Retinitis Pigmentosa Type 2-Clathrin Light Chain, a membrane protein with a novel domain architecture. Free-flow zonal electrophoresis of microalgal membranes coupled to liquid chromatography-tandem mass spectrometry proved to be a useful technique for determining the intracellular location of proteins of interest. Carbon-flow compartmentalization in E. oleoabundans was modeled using this information. Molecular phylogenetic analyses of protein markers and 18S ribosomal DNA support the reclassification of E. oleoabundans within the trebouxiophycean microalgae, rather than with the Chlorophyceae class, in which it is currently classified, indicating that it may not be closely related to the model green alga Chlamydomonas reinhardtii A detailed survey of biological processes taking place in the membranes of nitrogen-deprived E. oleoabundans, including lipid metabolism, provides insights into the basic biology of this nonmodel organism.

An alternative mode of CD43 signal transduction activates pro-survival pathways of T lymphocytes

CD43 is one of the most abundant co-stimulatory molecules on a T-cell surface; it transduces activation signals through its cytoplasmic domain, contributing to modulation of the outcome of T-cell responses. The aim of this study was to uncover new signalling pathways regulated by this sialomucin. Analysis of changes in protein abundance allowed us to identify pyruvate kinase isozyme M2 (PKM2), an enzyme of the glycolytic pathway, as an element potentially participating in the signalling cascade resulting from the engagement of CD43 and the T-cell receptor (TCR). We found that the glycolytic activity of this enzyme was not significantly increased in response to TCR+CD43 co-stimulation, but that PKM2 was tyrosine phosphorylated, suggesting that it was performing moonlight functions. We report that phosphorylation of both Y¹⁰⁵ of PKM2 and of Y⁷⁰⁵ of signal transducer and activator of transcription 3 was induced in response to TCR+CD43 co-stimulation, resulting in activation of the mitogen-activated protein kinase kinase 5/extracellular signal-regulated kinase 5 (MEK5/ERK5) pathway. ERK5 and the cAMP response element binding protein (CREB) were activated, and c-Myc and nuclear factor-κB (p65) nuclear localization, as well as Bad phosphorylation, were augmented. Consistent with this, expression of human CD43 in a murine T-cell hybridoma favoured cell survival. Altogether, our data highlight novel signalling pathways for the CD43 molecule in T lymphocytes, and underscore a role for CD43 in promoting cell survival through non-glycolytic functions of metabolic enzymes.

Single-cell-type quantitative proteomic and ionomic analysis of epidermal bladder cells from the halophyte model plant Mesembryanthemum crystallinum to identify salt-responsive proteins

BACKGROUND

Epidermal bladder cells (EBC) are large single-celled, specialized, and modified trichomes found on the aerial parts of the halophyte Mesembryanthemum crystallinum. Recent development of a simple but high throughput technique to extract the contents from these cells has provided an opportunity to conduct detailed single-cell-type analyses of their molecular characteristics at high resolution to gain insight into the role of these cells in the salt tolerance of the plant.

RESULTS

In this study, we carry out large-scale complementary quantitative proteomic studies using both a label (DIGE) and label-free (GeLC-MS) approach to identify salt-responsive proteins in the EBC extract. Additionally we perform an ionomics analysis (ICP-MS) to follow changes in the amounts of 27 different elements. Using these methods, we were able to identify 54 proteins and nine elements that showed statistically significant changes in the EBC from salt-treated plants. GO enrichment analysis identified a large number of transport proteins but also proteins involved in photosynthesis, primary metabolism and Crassulacean acid metabolism (CAM). Validation of results by western blot, confocal microscopy and enzyme analysis helped to strengthen findings and further our understanding into the role of these specialized cells. As expected EBC accumulated large quantities of sodium, however, the most abundant element was chloride suggesting the sequestration of this ion into the EBC vacuole is just as important for salt tolerance.

CONCLUSIONS

This single-cell type omics approach shows that epidermal bladder cells of M. crystallinum are metabolically active modified trichomes, with primary metabolism supporting cell growth, ion accumulation, compatible solute synthesis and CAM. Data are available via ProteomeXchange with identifier PXD004045.

Cell type-specific responses to salinity - the epidermal bladder cell transcriptome of Mesembryanthemum crystallinum

Mesembryanthemum crystallinum (ice plant) exhibits extreme tolerance to salt. Epidermal bladder cells (EBCs), developing on the surface of aerial tissues and specialized in sodium sequestration and other protective functions, are critical for the plant's stress adaptation. We present the first transcriptome analysis of EBCs isolated from intact plants, to investigate cell type-specific responses during plant salt adaptation. We developed a de novo assembled, nonredundant EBC reference transcriptome. Using RNAseq, we compared the expression patterns of the EBC-specific transcriptome between control and salt-treated plants. The EBC reference transcriptome consists of 37 341 transcript-contigs, of which 7% showed significantly different expression between salt-treated and control samples. We identified significant changes in ion transport, metabolism related to energy generation and osmolyte accumulation, stress signalling, and organelle functions, as well as a number of lineage-specific genes of unknown function, in response to salt treatment. The salinity-induced EBC transcriptome includes active transcript clusters, refuting the view of EBCs as passive storage compartments in the whole-plant stress response. EBC transcriptomes, differing from those of whole plants or leaf tissue, exemplify the importance of cell type-specific resolution in understanding stress adaptive mechanisms.

Single cell-type comparative metabolomics of epidermal bladder cells from the halophyte Mesembryanthemum crystallinum

One of the remarkable adaptive features of the halophyte Mesembryanthemum crystallinum are the specialized modified trichomes called epidermal bladder cells (EBC) which cover the leaves, stems, and peduncle of the plant. They are present from an early developmental stage but upon salt stress rapidly expand due to the accumulation of water and sodium. This particular plant feature makes it an attractive system for single cell type studies, with recent proteomics and transcriptomics studies of the EBC establishing that these cells are metabolically active and have roles other than sodium sequestration. To continue our investigation into the function of these unusual cells we carried out a comprehensive global analysis of the metabolites present in the EBC extract by gas chromatography Time-of-Flight mass spectrometry (GC-TOF) and identified 194 known and 722 total molecular features. Statistical analysis of the metabolic changes between control and salt-treated samples identified 352 significantly differing metabolites (268 after correction for FDR). Principal components analysis provided an unbiased evaluation of the data variance structure. Biochemical pathway enrichment analysis suggested significant perturbations in 13 biochemical pathways as defined in KEGG. More than 50% of the metabolites that show significant changes in the EBC, can be classified as compatible solutes and include sugars, sugar alcohols, protein and non-protein amino acids, and organic acids, highlighting the need to maintain osmotic homeostasis to balance the accumulation of Na(+) and Cl(-) ions. Overall, the comparison of metabolic changes in salt treated relative to control samples suggests large alterations in M. crystallinum epidermal bladder cells.

Identification of rice cornichon as a possible cargo receptor for the Golgi-localized sodium transporter OsHKT1;3

Membrane proteins are synthesized and folded in the endoplasmic reticulum (ER), and continue their path to their site of residence along the secretory pathway. The COPII system has been identified as a key player for selecting and directing the fate of membrane and secretory cargo proteins. Selection of cargo proteins within the COPII vesicles is achieved by cargo receptors. The cornichon cargo receptor belongs to a conserved protein family found in eukaryotes that has been demonstrated to participate in the selection of integral membrane proteins as cargo for their correct targeting. Here it is demonstrated at the cellular level that rice cornichon OsCNIH1 interacts with OsHKT1;3 and, in yeast cells, enables the expression of the sodium transporter to the Golgi apparatus. Physical and functional HKT-cornichon interactions are confirmed by the mating-based split ubiquitin system, bimolecular fluorescence complementation, and Xenopus oocyte and yeast expression systems. The interaction between the two proteins occurs in the ER of plant cells and their co-expression in oocytes leads to the sequestration of the transporter in the ER. In the yeast cornichon mutant erv14, OsHKT1;3 is mistargeted, preventing the toxic effects of sodium transport in the cell observed in wild-type cells or in the erv14 mutant that co-expressed OsHKT1;3 with either OsCNIH1 or Erv14p. Identification and characterization of rice cornichon as a possible cargo receptor opens up the opportunity to improve our knowledge on membrane protein targeting in plant cells.

Comparative 2D-DIGE analysis of salinity responsive microsomal proteins from leaves of salt-sensitive Arabidopsis thaliana and salt-tolerant Thellungiella salsuginea

Halophytes have evolved unique molecular strategies to overcome high soil salinity but we still know very little about the main mechanisms that these plants use to complete their lifecycle under salinity stress. One useful approach to further our understanding in this area is to directly compare the response to salinity of two closely related species which show diverse levels of salt tolerance. Here we present a comparative proteomic study using DIGE of leaf microsomal proteins to identify salt-responsive membrane associated proteins in Arabidopsis thaliana (a glycophyte) and Thellungiella salsuginea (a halophyte). While a small number of distinct protein abundance changes were observed upon salt stress in both species, the most notable differences were observed between species and specifically, in untreated plants with a total of 36 proteins displaying significant abundance changes. Gene ontology (GO) term enrichment analysis showed that the majority of these proteins were distributed into two functional categories; transport (31%) and carbohydrate metabolism (17%). Results identify several novel salt responsive proteins in this system and support the theory that T. salsuginea shows a high degree of salt-tolerance because molecular mechanisms are primed to deal with the stress. This intrinsic ability to anticipate salinity stress distinguishes it from the glycophyte A. thaliana.

BIOLOGICAL SIGNIFICANCE

There is significant interest in understanding the molecular mechanisms that plants use to tolerate salinity as soil salinization is becoming an increasing concern for agriculture with high soil Na(+) levels leading to reduced yields and economic loss. Much of our knowledge on the molecular mechanisms employed by plants to combat salinity stress has come from work on salt-sensitive plants, but studies on naturally occurring highly salt-resistant plants, halophytes, and direct comparisons between closely related glycophytes and halophytes, could help to further our understanding of salinity tolerance mechanisms. In this study, employing two closely related species which differ markedly in their salt-tolerance, we carried out a quantitative proteomic approach using 2D-DIGE to identify salt-responsive proteins and compare and contrast the differences between the two plant species. Our work complements a previous study using iTRAQ technology (34) and highlights the benefits of using alternative technologies and approaches to gain a broader representation of the salt-responsive proteome in these species.

Growing Arabidopsis in vitro: cell suspensions, in vitro culture, and regeneration

An understanding of basic methods in Arabidopsis tissue culture is beneficial for any laboratory working on this model plant. Tissue culture refers to the aseptic growth of cells, organs, or plants in a controlled environment, in which physical, nutrient, and hormonal conditions can all be easily manipulated and monitored. The methodology facilitates the production of a large number of plants that are genetically identical over a relatively short growth period. Techniques, including callus production, cell suspension cultures, and plant regeneration, are all indispensable tools for the study of cellular biochemical and molecular processes. Plant regeneration is a key technology for successful stable plant transformation, while cell suspension cultures can be exploited for metabolite profiling and mining. In this chapter we report methods for the successful and highly efficient in vitro regeneration of plants and production of stable cell suspension lines from leaf explants of both Arabidopsis thaliana and Arabidopsis halleri.

Progress and challenges for abiotic stress proteomics of crop plants

Plants are continually challenged to recognize and respond to adverse changes in their environment to avoid detrimental effects on growth and development. Understanding the mechanisms that crop plants employ to resist and tolerate abiotic stress is of considerable interest for designing agriculture breeding strategies to ensure sustainable productivity. The application of proteomics technologies to advance our knowledge in crop plant abiotic stress tolerance has increased dramatically in the past few years as evidenced by the large amount of publications in this area. This is attributed to advances in various technology platforms associated with MS-based techniques as well as the accessibility of proteomics units to a wider plant research community. This review summarizes the work which has been reported for major crop plants and evaluates the findings in context of the approaches that are widely employed with the aim to encourage broadening the strategies used to increase coverage of the proteome.

Quantitative proteomics of heavy metal exposure in Arabidopsis thaliana reveals alterations in one-carbon metabolism enzymes upon exposure to zinc

Plant zinc (Zn) homeostasis must be tightly regulated as the requirement for this micronutrient necessitates its uptake. However, excessive Zn can lead to toxicity and the plant must respond rapidly and effectively within its capacity to minimize damage. To detect mechanisms that may be important for coping with excess Zn we carried out a quantitative proteomics approach using 2D-DIGE to identify Zn-responsive proteins in microsomal fractions from leaves of 4day, 200μM Zn-treated, Arabidopsis thaliana plants. Of the eight proteins which showed significant changes in abundance in the Zn-treated samples and which met all of the selection criteria following statistical analysis, six were successfully identified by LC-MS/MS with 2 or more unique peptides. Three of the proteins were found to be involved in the one-carbon metabolism pathway; including glycine decarboxylase P protein, serine hydroxymethyltransferase (SHMT) and methionine synthase, all of which showed reduced abundance in the Zn-treated samples. Western blot analysis confirmed the decrease in SHMT, while changes in metal tolerance protein indicated plants were most likely actively sequestering Zn. Interestingly, excess Zn led to increased petiole length, a phenotype which may reflect the reduced levels of methionine, a key product of the one-carbon metabolism pathway.

BIOLOGICAL SIGNIFICANCE

Metal contamination is becoming an increasingly common environmental problem. High levels of zinc can be found in certain soils naturally or as a result of long-term anthropogenic activity which leads to its accumulation; i.e. use of fertilizers or industrial waste. The study of metal tolerant plants, particularly those classified as hyperaccumulators has been driven by the potential use of these plants for bioremediation purposes. However, the effects of heavy metal exposure on sensitive plants and the different cellular processes that are affected have received significantly less attention. We are interested in identifying proteins in A. thaliana that are induced as a result of exposure to subtoxic levels of heavy metals with the aim of discovering novel participants in heavy metal stress and adaptation.

CD43 activates moonlight functions of pyruvate kinase (P1113)

CD43 is the most abundant costimulatory molecule on the T cell surface; it transduces activation signals through its cytoplasmic domain, modulating the outcome of T cell responses. CD43 KO mice revealed a positive role for CD43 in T cell migration and in the regulation of cell number and homeostasis. The aim of this study was to identify new signaling pathways modulated by CD43. Protein expression profiles of normal human CD4+ T cells stimulated through TCR and CD43 identified changes in 189 proteins. Bioinformatic analysis revealed that glucose metabolism was significantly represented. We evaluated pyruvate kinase (PK) and lactate dehydrogenase (LDH) activities, as well as glucose uptake. TCR+CD43 stimulated cells showed a 1.5 fold increase in PK activity, but only a moderate increase in LDH activity and glucose uptake. Interestingly, tyrosine phosphorylation of PK triggers its ability to phosphorylate Stat3. Phosphorylation of Y105 of PK and of Y705 of Stat3 was induced in response to TCR+CD43 and TCR+CD28. Since phosphorylation of Y705 of Stat3 by PK induces MEK5 transcription, we evaluated the MEK5/ERK5 pathway activation by measuring the levels of pCREB at S133, an ERK5 target. CREB pS133 was increased in TCR+CD43 and TCR+CD28 stimulated T cells. Altogether, our results show that CD43 regulates monosaccharide metabolism, as well as moonlight functions of metabolic enzymes.

Protein profiling of epidermal bladder cells from the halophyte Mesembryanthemum crystallinum

Plant epidermal trichomes are as varied in morphology as they are in function. In the halophyte Mesembryanthemum crystallinum, specialized trichomes called epidermal bladder cells (EBC) line the surface of leaves and stems, and increase dramatically in size and volume upon plant salt-treatment. These cells have been proposed to have roles in plant defense and UV protection, but primarily in sodium sequestration and as water reservoirs. To gain further understanding into the roles of EBC, a cell-type-specific proteomics approach was taken in which precision single-cell sampling of cell sap from individual EBC was combined with shotgun peptide sequencing (LC-MS/MS). Identified proteins showed diverse biological functions and cellular locations, with a high representation of proteins involved in H(+)-transport, carbohydrate metabolism, and photosynthesis. The proteome of EBC provides insight into the roles of these cells in ion and water homeostasis and raises the possibility that they are photosynthetically active and functioning in Crassulacean acid metabolism.

Day/night regulation of aquaporins during the CAM cycle in Mesembryanthemum crystallinum

Mesembryanthemum crystallinum exhibits induction of Crassulacean acid metabolism (CAM) after a threshold stage of development, by exposure to long days with high light intensities or by water and salt stress. During the CAM cycle, fluctuations in carbon partitioning within the cell lead to transient drops in osmotic potential, which are likely stabilized/balanced by passive movement of water via aquaporins (AQPs). Protoplast swelling assays were used to detect changes in water permeability during the day/night cycle of CAM. To assess the role of AQPs during the same period, we followed transcript accumulation and protein abundance of four plasma membrane intrinsic proteins (PIPs) and one tonoplast intrinsic protein (TIP). CAM plants showed a persistent rhythm of specific AQP protein abundance changes throughout the day/night cycle, including changes in amount of McPIP2;1, McTIP1;2, McPIP1;4 and McPIP1;5, while the abundance of McPIP1;2 was unchanged. These protein changes did not appear to be coordinated with transcript levels for any of the AQPs analysed; however, they did occur in parrallel to alterations in water permeability, as well as variations in cell osmolarity, pinitol, glucose, fructose and phosphoenolpyruvate carboxylase (PEPc) levels measured throughout the day/night CAM cycle. Results suggest a role for AQPs in maintaining water balance during CAM and highlight the complexity of protein expression during the CAM cycle.

Ser123 is essential for the water channel activity of McPIP2;1 from Mesembryanthemum crystallinum

The increased expression of McPIP2;1 (MipC), a root-specific aquaporin (AQP) from Mesembryanthemum crystallinum, under salt stress has suggested a role for this AQP in the salt tolerance of the plant. However, whether McPIP2;1 transports water or another solute and how its activity is regulated are so far unknown. Therefore, wild type (wt) or mutated McPIP2;1 protein was expressed in Xenopus laevis oocytes. Then, the osmotic water permeability (P(f)) of the oocytes membrane was assessed by hypotonic challenges. Selectivity of McPIP2;1 to water was determined by radiolabeled glycerol or urea uptake assays. Moreover, swelling and in vitro phosphorylation assays revealed that both water permeation and phosphorylation status of McPIP2;1 were significantly increased by the phosphorylation agonists okadaic acid (OA), phorbol myristate acetate (PMA), and 8-Br-cAMP, and markedly decreased by the inhibitory peptides PKI 14-22 and PKC 20-28, inhibitors of protein kinases A (PKA) and C (PKC), respectively. Substitution of Ser(123) or both, Ser(123) and Ser(282), abolished the water channel activity of McPIP2;1 while substitution of Ser(282) only partially inhibited it (51.9% inhibition). Despite lacking Ser(123) and/or Ser(282), the McPIP2;1 mutant forms were still phosphorylated in vitro, which suggests that phosphorylation may have a dual role on this AQP. Our results indicate that McPIP2;1 water permeability depends completely on Ser(123) and is positively regulated by PKA- and PKC-mediated phosphorylation. Regulation of the phosphorylation status of McPIP2;1 may contribute to control water transport through root cells when the plant is subjected to high salinity conditions.

Quantitative proteomics of the tonoplast reveals a role for glycolytic enzymes in salt tolerance

To examine the role of the tonoplast in plant salt tolerance and identify proteins involved in the regulation of transporters for vacuolar Na(+) sequestration, we exploited a targeted quantitative proteomics approach. Two-dimensional differential in-gel electrophoresis analysis of free flow zonal electrophoresis separated tonoplast fractions from control, and salt-treated Mesembryanthemum crystallinum plants revealed the membrane association of glycolytic enzymes aldolase and enolase, along with subunits of the vacuolar H(+)-ATPase V-ATPase. Protein blot analysis confirmed coordinated salt regulation of these proteins, and chaotrope treatment indicated a strong tonoplast association. Reciprocal coimmunoprecipitation studies revealed that the glycolytic enzymes interacted with the V-ATPase subunit B VHA-B, and aldolase was shown to stimulate V-ATPase activity in vitro by increasing the affinity for ATP. To investigate a physiological role for this association, the Arabidopsis thaliana cytoplasmic enolase mutant, los2, was characterized. These plants were salt sensitive, and there was a specific reduction in enolase abundance in the tonoplast from salt-treated plants. Moreover, tonoplast isolated from mutant plants showed an impaired ability for aldolase stimulation of V-ATPase hydrolytic activity. The association of glycolytic proteins with the tonoplast may not only channel ATP to the V-ATPase, but also directly upregulate H(+)-pump activity.

Zinc tolerance and accumulation in stable cell suspension cultures and in vitro regenerated plants of the emerging model plant Arabidopsis halleri (Brassicaceae)

Arabidopsis halleri is increasingly employed as a model plant for studying heavy metal hyperaccumulation. With the aim of providing valuable tools for studies on cellular physiology and molecular biology of metal tolerance and transport, this study reports the development of successful and highly efficient methods for the in vitro regeneration of A. halleri plants and production of stable cell suspension lines. Plants were regenerated from leaf explants of A. halleri via a three-step procedure: callus induction, somatic embryogenesis and shoot development. Efficiency of callus proliferation and regeneration depended on the initial callus induction media and was optimal in the presence of 1 mg L(-1) 2,4-dichlorophenoxyacetic acid, and 0.05 mg L(-1) benzylaminopurine. Subsequent shoot and root regeneration from callus initiated under these conditions reached levels of 100% efficiency. High friability of the callus supported the development of cell suspension cultures with minimal cellular aggregates. Characterization of regenerated plants and cell cultures determined that they maintained not only the zinc tolerance and requirement of the whole plant but also the ability to accumulate zinc; with plants accumulating up to 50.0 micromoles zinc g(-1) FW, and cell suspension cultures 30.9 micromoles zinc g(-1) DW. Together this work will provide the experimental basis for furthering our knowledge of A. halleri as a model heavy metal hyperaccumulating plant.

Enhanced separation of membranes during free flow zonal electrophoresis in plants

Free flow zonal electrophoresis (FFZE) is a versatile technique that allows for the separation of cells, organelles, membranes, and proteins based on net surface charge during laminar flow through a thin aqueous layer. We have been optimizing the FFZE technique to enhance separation of plant vacuolar membranes (tonoplast) from other endomembranes to pursue a directed proteomics approach to identify novel tonoplast transporters. Addition of ATP to a mixture of endomembranes selectively enhanced electrophoretic mobility of acidic vesicular compartments during FFZE toward the positive electrode. This has been attributed to activation of the V-ATPase generating a more negative membrane potential outside the vesicles, resulting in enhanced migration of acidic vesicles, including tonoplast, to the anode (Morré, D. J.; Lawrence, J.; Safranski, K.; Hammond, T.; Morré, D. M. J. Chromatogr., A 1994, 668, 201-213). We confirm that ATP does induce a redistribution of membranes during FFZE of microsomal membranes isolated from several plant species, including Arabidopsis thaliana, Thellungiella halophila, Mesembryanthemum crystallinum, and Ananas comosus. However, we demonstrate, using V-ATPase-specific inhibitors, nonhydrolyzable ATP analogs, and ionophores to dissipate membrane potential, that the ATP-dependent migrational shift of membranes under FFZE is not due to activation of the V-ATPase. Addition of EDTA to chelate Mg2+, leading to the production of the tetravalent anionic form of ATP, resulted in a further enhancement of membrane migration toward the anode, and manipulation of cell surface charge by addition of polycations also influenced the ATP-dependent migration of membranes. We propose that ATP enhances the mobility of endomembranes by screening positive surface charges on the membrane surface.

Salt stress in Thellungiella halophila activates Na+ transport mechanisms required for salinity tolerance

Salinity is considered one of the major limiting factors for plant growth and agricultural productivity. We are using salt cress (Thellungiella halophila) to identify biochemical mechanisms that enable plants to grow in saline conditions. Under salt stress, the major site of Na+ accumulation occurred in old leaves, followed by young leaves and taproots, with the least accumulation occurring in lateral roots. Salt treatment increased both the H+ transport and hydrolytic activity of salt cress tonoplast (TP) and plasma membrane (PM) H(+)-ATPases from leaves and roots. TP Na(+)/H+ exchange was greatly stimulated by growth of the plants in NaCl, both in leaves and roots. Expression of the PM H(+)-ATPase isoform AHA3, the Na+ transporter HKT1, and the Na(+)/H+ exchanger SOS1 were examined in PMs isolated from control and salt-treated salt cress roots and leaves. An increased expression of SOS1, but no changes in levels of AHA3 and HKT1, was observed. NHX1 was only detected in PM fractions of roots, and a salt-induced increase in protein expression was observed. Analysis of the levels of expression of vacuolar H(+)-translocating ATPase subunits showed no major changes in protein expression of subunits VHA-A or VHA-B with salt treatment; however, VHA-E showed an increased expression in leaf tissue, but not in roots, when the plants were treated with NaCl. Salt cress plants were able to distribute and store Na+ by a very strict control of ion movement across both the TP and PM.

Novel regulation of aquaporins during osmotic stress

Aquaporin protein regulation and redistribution in response to osmotic stress was investigated. Ice plant (Mesembryanthemum crystallinum) McTIP1;2 (McMIPF) mediated water flux when expressed in Xenopus leavis oocytes. Mannitol-induced water imbalance resulted in increased protein amounts in tonoplast fractions and a shift in protein distribution to other membrane fractions, suggesting aquaporin relocalization. Indirect immunofluorescence labeling also supports a change in membrane distribution for McTIP1;2 and the appearance of a unique compartment where McTIP1;2 is expressed. Mannitol-induced redistribution of McTIP1;2 was arrested by pretreatment with brefeldin A, wortmannin, and cytochalasin D, inhibitors of vesicle trafficking-related processes. Evidence suggests a role for glycosylation and involvement of a cAMP-dependent signaling pathway in McTIP1;2 redistribution. McTIP1;2 redistribution to endosomal compartments may be part of a homeostatic process to restore and maintain cellular osmolarity under osmotic-stress conditions.

Expression of the cation transporter McHKT1 in a halophyte

From the ice plant, Mesembryanthemum crystallinum, McHKT1 was isolated encoding a protein 41-61% identical to other plant HKT1-like sequences previously described as potassium or sodium/potassium transporters. McHKT1 acts as a potassium transporter in yeast with specificity similar to that of wheat HKT1. In Xenopus oocytes it transports cations with a specificity Rb+ > Cs+ > [K+ = Na+ = Li+]. McHKT1 is exclusively localized to the plasma membrane. The isoform isolated is most highly expressed in leaves and is present in stems, flowers and seed pods but absent from the root where, according to immunological data, a second isoform exists which does not cross-hybridize with the leaf form in RNA blots at high stringency. McHKT1 transcript amounts increase during the first 6-10 h of stress and then decline to pre-stress levels with kinetics reminiscent of the initial influx of sodium into this halophyte. Immunocytological localization showed strong signals in the leaf vasculature and surrounding mesophyll cells but low-intensity signals are also detected in other cell types. In roots, McHKT is mainly confined to endodermis and stele. Possible functions of McHKT1 in ion homeostasis in the halophytic ice plant are discussed.

Na+/H+ exchange activity in the plasma membrane of Arabidopsis

In plants, Na+/H+ exchangers in the plasma membrane are critical for growth in high levels of salt, removing toxic Na+ from the cytoplasm by transport out of the cell. The molecular identity of a plasma membrane Na+/H+ exchanger in Arabidopsis (SOS1) has recently been determined. In this study, immunological analysis provided evidence that SOS1 localizes to the plasma membrane of leaves and roots. To characterize the transport activity of this protein, purified plasma membrane vesicles were isolated from leaves of Arabidopsis. Na+/H+ exchange activity, monitored as the ability of Na to dissipate an established pH gradient, was absent in plants grown without salt. However, exchange activity was induced when plants were grown in 250 mm NaCl and increased with prolonged salt exposure up to 8 d. H+-coupled exchange was specific for Na, because chloride salts of other monovalent cations did not dissipate the pH gradient. Na+/H+ exchange activity was dependent on Na (substrate) concentration, and kinetic analysis indicated that the affinity (apparent Km) of the transporter for Na+ is 22.8 mm. Data from two experimental approaches supports electroneutral exchange (one Na+ exchanged for one proton): (a) no change in membrane potential was measured during the exchange reaction, and (b) Na+/H+ exchange was unaffected by the presence or absence of a membrane potential. Results from this research provide a framework for future studies into the regulation of the plant plasma membrane Na+/H+ exchanger and its relative contribution to the maintenance of cellular Na+ homeostasis during plant growth in salt.

Na+/H+ exchange in the halophyte Mesembryanthemum crystallinum is associated with cellular sites of Na+ storage

The tonoplast Na⁺/H⁺ exchanger is involved in sequestering Na⁺ in plant vacuoles, providing solutes for osmotic adjustment while avoiding cytoplasmic Na⁺ toxicity. As such it is assumed to be one of the key mechanisms involved in salt-tolerance in plants. In this study, we measured tonoplast Na⁺/H⁺ exchange in roots and different leaf tissues of adult Mesembryanthemum crystallinum L. plants to determine if activity of the exchanger follows the gradient from roots to leaves previously observed for Na⁺ and pinitol accumulation. Na⁺/H⁺ exchange was absent from roots of control and NaCl-treated plants. In contrast, leaves showed constitutive Na⁺/H⁺ exchange that was enhanced by growth of the plants in NaCl. Highest activity was measured in the epidermal bladder cells in agreement with the highest concentrations of Na⁺ found in this tissue. Tonoplast H⁺-translocating ATPase activity was also greatest in this tissue, as were protein levels for myo-inositol-O-methyltransferase, a key enzyme in the pinitol biosynthesis pathway. The strong correlation between Na⁺/H⁺ exchange and Na⁺ accumulation confirms the role of this transporter in vacuolar sequestration of Na⁺ and plant salt tolerance.

Expression of water channel proteins in Mesembryanthemum crystallinum

We have characterized transcripts for nine major intrinsic proteins (MIPs), some of which function as water channels (aquaporins), from the ice plant Mesembryanthemum crystallinum. To determine the cellular distribution and expression of these MIPs, oligopeptide-based antibodies were generated against MIP-A, MIP-B, MIP-C, or MIP-F, which, according to sequence and functional characteristics, are located in the plasma membrane (PM) and tonoplast, respectively. MIPs were most abundant in cells involved in bulk water flow and solute flux. The tonoplast MIP-F was found in all cells, while signature cell types identified different PM-MIPs: MIP-A predominantly in phloem-associated cells, MIP-B in xylem parenchyma, and MIP-C in the epidermis and endodermis of immature roots. Membrane protein analysis confirmed MIP-F as tonoplast located. MIP-A and MIP-B were found in tonoplast fractions and also in fractions distinct from either the tonoplast or PM. MIP-C was most abundant but not exclusive to PM fractions, where it is expected based on its sequence signature. We suggest that within the cell, MIPs are mobile, which is similar to aquaporins cycling through animal endosomes. MIP cycling and the differential regulation of these proteins observed under conditions of salt stress may be fundamental for the control of tissue water flux.

Towards the production of salt-tolerant crops

Crop production is affected by numerous environmental factors, with soil salinity and drought having the most detrimental effects. Attempts to improve yield under stress conditions by plant breeding have been unsuccessful, primarily due to the multigenic origin of the adaptive responses. The transfer of genes through genetic engineering of crop plants appears more feasible. Important adaptive mechanisms targeted for potential gene transfer would be the tonoplast Na+/H+ antiport, compatible solute synthesis and, regulation of water channel activity and expression, mechanisms involved in cellular osmoregulation. In this review we discuss recent advances in our understanding of these adaptive mechanisms.

Salt stress in Mesembryanthemum crystallinum L. cell suspensions activates adaptive mechanisms similar to those observed in the whole plant

A salt-tolerant stable cell-suspension culture from the halophyte Mesembryanthemum crystallinum L. has been established from calli generated from leaves of 6-week-old well-watered plants. Optimal cell growth was observed in the presence of 200 mM NaCl, and within 7 d cells were able to concentrate Na+ to levels exceeding those in the growth medium. Accumulation of Na+ was paralled by increases in the compatible solute pinitol and myo-inositol methyl transferase (IMT), a key enzyme in pinitol biosynthesis. Increasing concentrations of NaCl stimulated the activities of tonoplast and plasma-membrane H(+)-ATPases. Immunodetection of the ATPases showed that the increased activity was not due to changes in protein amount that could be attributed to treatment conditions. A specific role for these mechanisms in salt-adaptation is supported by the inability of mannitol-induced water stress to elicit the same responses, and the absence of enzyme activity and protein expression associated with Crassulacean acid metabolism in the cells. Results demonstrate that these M. crystallinum cell suspensions show a halophytic growth response, comparable to that of the whole plant, and thus provide a valuable tool for studying signaling and biochemical pathways involved in salt recognition and response.

Plant Defense Response to Fungal Pathogens (II. G-Protein-Mediated Changes in Host Plasma Membrane Redox Reactions)

Elicitor preparations containing the avr5 gene products from races 4 and 2.3 of Cladosporium fulvum, and tomato (Lycopersicon esculentum L.) cells containing the resistance gene Cf5 were used to investigate the involvement of redox processes in the production of active oxygen species associated with the plant response to the fungal elicitors. Here we demonstrate that certain race-specific elicitors of C. fulvum induced an increase in ferricyanide reduction in enriched plasma membrane fractions of tomato cells. The addition of elicitors to plasma membranes also induced increases in NADH oxidase and NADH-dependent cytochrome c reductase activities, whereas ascorbate peroxidase activity was decreased. These results suggest that changes in the host plasma membrane redox processes, transferring electrons from reducing agents to oxygen, could be involved in the increased production of active oxygen species by the race-specific elicitors. Our results also show that the dephosphorylation of enzymes involved in redox reactions is responsible for the race-specific induced redox activity. The effects of guanidine nucleotide analogs and mastoparan on the activation of plasma membrane redox reactions support the role of GTP-binding proteins in the transduction of signals leading to the activation of the defense response mechanisms of tomato against fungal pathogens.

Plant Defense Response to Fungal Pathogens (Activation of Host-Plasma Membrane H+-ATPase by Elicitor-Induced Enzyme Dephosphorylation)

Elicitor preparations containing the avr5 gene products from race 4 of Cladosporium fulvum and tomato (Lycopersicon esculentum L.) cells near isogenic for the resistance gene Cf5 were used to investigate events following the treatment of host plasma membranes with elicitor. A 4-fold increase in H+-ATPase activity, coincident with the acidification of the extracellular medium, was detected immediately after elicitor treatment. The elicitor-induced stimulation of the plasma membrane H+-ATPase was inhibited by okadaic acid but not by staurosporine, suggesting that protein dephosphorylation was required for increased H+-ATPase activity. This observation was confirmed by [gamma]-32P labeling and immunodetection of the plasma membrane H+-ATPase. Effects of guanidine nucleotide analogs and mastoparan on the ATPase activity suggested the role of GTP-binding proteins in mediating the putative elicitor-receptor binding, resulting in activation of a phosphatase(s), which in turn stimulates the plasma membrane H+-ATPase by dephosphorylation.

Effect of Specific Elicitors of Cladosporium fulvum on Tomato Suspension Cells : Evidence for the Involvement of Active Oxygen Species

Intercellular fluid (IF) obtained from tomato (Lycopersicon esculentum L.) leaflets colonized by Cladosporium fulvum Cooke contains specific elicitors that induce necrosis in tomato cultivars resistant to the race of C. fulvum used to produce the IF. The responses of cell-suspension cultures produced from tomato lines near-isogenic for resistance genes Cf 4 and Cf 5 to IF produced from leaves infected by races 4 (virulent on Cf 4 but not Cf 5 plants), 2.4.5, and 2.4.5.9 (both virulent on Cf 4 and Cf 5 plants) were used to investigate the possibility that active oxygen (AO) species were involved in the initial host reaction to these elicitors. Concurrently, the same assays were used to determine if the cell lines retained the elicitor specificity of the original plants. An IF/cell combination that gives an incompatible reaction in leaves (race 4 IF and Cf 5 cells) showed reduced oxygen uptake and increases in malonaldehyde (a product of lipid peroxidation); cytochrome c reducing activity, which was inhibited by superoxide dismutase (SOD) (an assay for superoxide); luminol-dependent chemiluminescence (an assay for several AO species); activity of extracellular peroxidases; and extracellular phenolic compounds. In contrast, compatible combinations (IF from races 2.4.5 or 2.4.5.9 and Cf 4 or Cf 5 cells; race 4 IF and Cf 4 cells) did not exhibit any of these changes. The addition of catalase, SOD, ascorbate (a scavenger of superoxide), mannitol (a scavenger of the hydroxyl radical), KCN, or salicyl hydroxamic acid (both inhibitors of peroxidases) prior to IF treatment reduced the IF-induced increases in malonaldehyde and extracellular phenolics. Catalase was an effective inhibitor of the IF-induced changes in oxygen uptake and cytochrome c reducing activity. These results demonstrate the specificity of the IF-induced cell responses and confirm that AO species are involved in the initial cell response.