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

A dominant-negative Arabidopsis cation exchanger 1 (CAX1): N-terminal autoinhibition and membrane topology

Calcium (Ca2+) is essential for plant growth and cellular homeostasis, with cation exchangers (CAXs) regulating Ca2+ transport into plant vacuoles. In Arabidopsis, multiple CAXs feature a common structural arrangement, comprising an N-terminal autoinhibitory domain followed by two pseudosymmetrical modules. Mutations in CAX1 enhance stress tolerance, notably tolerance to anoxia (a condition marked by oxygen depletion), crucial for flood resilience. Here we engineered a dominant-negative CAX1 variant, named ½N-CAX1, incorporating the autoinhibitory domain and the N-terminal pseudosymmetrical module, which, when expressed in wild-type Arabidopsis plants, phenocopied the anoxia tolerance of cax1. Physiological evaluations, yeast assays, and calcium imaging demonstrated that wild-type plants expressing ½N-CAX1 have phenotypes consistent with inhibition of CAX1, which is likely through direct interaction of ½N-CAX1 with CAX1. Eliminating segments within the N-terminal pseudosymmetrical module, as well as incorporating modules from other plant CAXs and expressing these variants into wild-type plants, failed to produce anoxia tolerance. This underscores the requirement for both the CAX1 autoinhibitory domain and the intact pseudosymmetrical module to produce the dominant-negative phenotype. Our study elucidates the interaction of this ½N-CAX1 variant with CAX1 and its impact on anoxia tolerance, offering insights into further approaches for engineering plant stress tolerance.

CAX control: multiple roles of vacuolar cation/H+ exchangers in metal tolerance, mineral nutrition and environmental signalling

Plant vacuolar transporters, particularly CAX (Cation/H+ Exchangers) responsible for Ca2+/H+ exchange on the vacuole tonoplast, play a central role in governing cellular pH, ion balance, nutrient storage, metal accumulation, and stress responses. Furthermore, CAX variants have been employed to enhance the calcium content of crops, contributing to biofortification efforts. Recent research has uncovered the broader significance of these transporters in plant signal transduction and element partitioning. The use of genetically encoded Ca2+ sensors has begun to highlight the crucial role of CAX isoforms in generating cytosolic Ca2+ signals, underscoring their function as pivotal hubs in diverse environmental and developmental signalling networks. Interestingly, it has been observed that the loss of CAX function can be advantageous in specific stress conditions, both for biotic and abiotic stressors. Determining the optimal timing and approach for modulating the expression of CAX is a critical concern. In the future, strategically manipulating the temporal loss of CAX function in agriculturally important crops holds promise to bolster plant immunity, enhance cold tolerance, and fortify resilience against one of agriculture's most significant challenges, namely flooding.

CXIP4 depletion causes early lethality and pre-mRNA missplicing in Arabidopsis

Zinc knuckle (ZCCHC) motif-containing proteins are present in unicellular and multicellular eukaryotes and most ZCCHC proteins with known functions participate in the metabolism of various classes of RNA, such as mRNAs, ribosomal RNAs, and microRNAs. The Arabidopsis (Arabidopsis thaliana) genome encodes 69 ZCCHC-containing proteins, but the functions of most remain unclear. One of these proteins is CAX-INTERACTING PROTEIN 4 (CXIP4), which has been classified as a PTHR31437 family member, along with human SREK1-interacting protein 1 (SREK1IP1), which is thought to function in pre-mRNA splicing and RNA methylation. Metazoan SREK1IP1-like and plant CXIP4-like proteins only share a ZCCHC motif, and their functions remain almost entirely unknown. We studied two loss-of-function alleles of Arabidopsis CXIP4, the first mutations in PTHR31437 family genes described to date: cxip4-1 is likely null and shows early lethality, and cxip4-2 is hypomorphic and viable, with pleiotropic morphological defects. The cxip4-2 mutant exhibited deregulation of defense genes and upregulation of transcription factor encoding genes, some of which might explain its developmental defects. This mutant also exhibited increased intron retention events, and the specific functions of misspliced genes, such as those involved in "gene silencing by DNA methylation" and "mRNA polyadenylation factor" suggest that CXIP4 has additional functions. The CXIP4 protein localizes to the nucleus in a pattern resembling nuclear speckles, which are rich in splicing factors. Therefore, CXIP4 is required for plant survival and proper development, and mRNA maturation.

The cell biology of primary cell walls during salt stress

Salt stress simultaneously causes ionic toxicity, osmotic stress, and oxidative stress, which directly impact plant growth and development. Plants have developed numerous strategies to adapt to saline environments. Whereas some of these strategies have been investigated and exploited for crop improvement, much remains to be understood, including how salt stress is perceived by plants and how plants coordinate effective responses to the stress. It is, however, clear that the plant cell wall is the first contact point between external salt and the plant. In this context, significant advances in our understanding of halotropism, cell wall synthesis, and integrity surveillance, as well as salt-related cytoskeletal rearrangements, have been achieved. Indeed, molecular mechanisms underpinning some of these processes have recently been elucidated. In this review, we aim to provide insights into how plants respond and adapt to salt stress, with a special focus on primary cell wall biology in the model plant Arabidopsis thaliana.

TaHRC suppresses the calcium-mediated immune response and triggers wheat Fusarium head blight susceptibility

A wild-type allele of TaHRC suppresses the calcium-mediated immune response to Fusarium graminearum infection and facilitates the spread of Fusarium head blight disease symptoms within a wheat spike.

Long-distance mobile mRNA CAX3 modulates iron uptake and zinc compartmentalization

Iron deficiency in plants can lead to excessive absorption of zinc; however, important details of this mechanism have yet to be elucidated. Here, we report that MdCAX3 mRNA is transported from the leaf to the root, and that MdCAX3 is then activated by MdCXIP1. Suppression of MdCAX3 expression leads to an increase in the root apoplastic pH, which is associated with the iron deficiency response. Notably, overexpression of MdCAX3 does not affect the apoplastic pH in a MdCXIP1 loss-of-function Malus baccata (Mb) mutant that has a deletion in the MdCXIP1 promoter. This deletion in Mb weakens MdCXIP1 expression. Co-expression of MdCAX3 and MdCXIP1 in Mb causes a decrease in the root apoplastic pH. Furthermore, suppressing MdCAX3 in Malus significantly reduces zinc vacuole compartmentalization. We also show that MdCAX3 activated by MdCXIP1 is not only involved in iron uptake, but also in regulating zinc detoxification by compartmentalizing zinc in vacuoles to avoid iron starvation-induced zinc toxicity. Thus, mobile MdCAX3 mRNA is involved in the regulation of iron and zinc homeostasis in response to iron starvation.

Genome-wide analysis and expression profiling of Cation/H+ exchanger (CAX) family genes reveal likely functions in cadmium stress responses in poplar

Cadmium, a toxic heavy metal, seriously affects human health and ecological security. The cation/H⁺ exchanger (CAX) family is a unique metal transporter that plays a crucial role in Cd acquisition, transfer, and remission in plants. Although there are many studies related to the genome-wide analysis of Populus trichocarpa, little research has been done on the CAX family genes, especially concerning Cd stress. In this study, genome-wide analysis of the Populus CAX family identified seven stress-related CAX genes. The evolutionary tree indicated that the CaCA family genes were grouped into four clusters. Moreover, seven pairs of genes were derived by segmental duplication in poplars. Cis-acting element analysis identified numerous stress-related elements in the promoters of diverse PtrCAXs. Furthermore, some PtrCAXs were up-regulated by drought, beetle, and mechanical damage, indicating their possible function in regulating stress response. Under cadmium stress, all CAX genes in the roots were up-regulated. Our findings suggest that plants may regulate their response to Cd stress through the TF-CAXs module. Comprehensively investigating the CAX family provides a scientific basis for the phytoremediation of heavy metal pollution by Populus.

Advances in understanding the specificity function of transporters by machine learning

Understanding the underlying molecular mechanism of transporter activity is one of the major discussions in structural biology. A transporter can exclusively transport one ion (specific transporter) or multiple ions (general transporter). This study compared categorical and numerical features of general and specific calcium transporters using machine learning and attribute weighting models. To this end, 444 protein features, such as the frequency of dipeptides, organism, and subcellular location, were extracted for general (n = 103) and specific calcium transporters (n = 238). Aliphatic index, subcellular location, organism, Ile-Leu frequency, Glycine frequency, hydrophobic frequency, and specific dipeptides such as Ile-Leu, Phe-Val, and Tyr-Gln were the key features in differentiating general from specific calcium transporters. Calcium transporters in the cell outer membranes were specific, while the inner ones were general; additionally, when the hydrophobic frequency or Aliphatic index is increased, the calcium transporter act as a general transporter. Random Forest with accuracy criterion showed the highest accuracy (88.88% ±5.75%) and high AUC (0.964 ± 0.020), based on 5-fold cross-validation. Decision Tree with accuracy criterion was able to predict the specificity of calcium transporter irrespective of the organism and subcellular location. This study demonstrates the precise classification of transporter function based on sequence-derived physicochemical features.

Description of AtCAX4 in Response to Abiotic Stress in Arabidopsis

High-capacity tonoplast cation/H⁺ antiport in plants is partially mediated by a family of CAX transporters. Previous studies have reported that CAX activity is affected by an N-terminal autoinhibitory region. CAXs may be present as heterodimers in plant cells, and this phenomenon necessitates further study. In this study, we demonstrate that there is an interaction between CAX4 and CAX1 as determined by the use of a yeast two-hybrid system and a bimolecular fluorescence complementation assay. More specifically, the N-terminal of CAX4 interacts with CAX1. We further observed the over-expression and either a single or double mutant of CAX1 and CAX4 in response to abiotic stress in Arabidopsis. These results suggest that CAX1 and CAX4 can interact to form a heterodimer, and the N-terminal regions of CAX4 play important roles in vivo; this may provide a foundation for a deep study of CAX4 function in the future.

Genome-wide analysis of CCHC-type zinc finger (ZCCHC) proteins in yeast, Arabidopsis, and humans

The diverse eukaryotic proteins that contain zinc fingers participate in many aspects of nucleic acid metabolism, from DNA transcription to RNA degradation, post-transcriptional gene silencing, and small RNA biogenesis. These proteins can be classified into at least 30 types based on structure. In this review, we focus on the CCHC-type zinc fingers (ZCCHC), which contain an 18-residue domain with the CX2CX4HX4C sequence, where C is cysteine, H is histidine, and X is any amino acid. This motif, also named the "zinc knuckle", is characteristic of the retroviral Group Antigen protein and occurs alone or with other motifs. Many proteins containing zinc knuckles have been identified in eukaryotes, but only a few have been studied. Here, we review the available information on ZCCHC-containing factors from three evolutionarily distant eukaryotes-Saccharomyces cerevisiae, Arabidopsis thaliana, and Homo sapiens-representing fungi, plants, and metazoans, respectively. We performed systematic searches for proteins containing the CX2CX4HX4C sequence in organism-specific and generalist databases. Next, we analyzed the structural and functional information for all such proteins stored in UniProtKB. Excluding retrotransposon-encoded proteins and proteins harboring uncertain ZCCHC motifs, we found seven ZCCHC-containing proteins in yeast, 69 in Arabidopsis, and 34 in humans. ZCCHC-containing proteins mainly localize to the nucleus, but some are nuclear and cytoplasmic, or exclusively cytoplasmic, and one localizes to the chloroplast. Most of these factors participate in RNA metabolism, including transcriptional elongation, polyadenylation, translation, pre-messenger RNA splicing, RNA export, RNA degradation, microRNA and ribosomal RNA biogenesis, and post-transcriptional gene silencing. Several human ZCCHC-containing factors are derived from neofunctionalized retrotransposons and act as proto-oncogenes in diverse neoplastic processes. The conservation of ZCCHCs in orthologs of these three phylogenetically distant eukaryotes suggests that these domains have biologically relevant functions that are not well known at present.

Development of a rapid and efficient protoplast isolation and transfection method for chickpea (Cicer arietinum)

Chickpea (Cicer arietinum L.) is the second most important grain legume worldwide. Recent advances in the sequencing of the chickpea genome has provided a new and valuable resource to aid efforts in gene discovery and crop trait improvement. Technical difficulties in stable chickpea transgenics and the lack of a transient expression system for rapid analysis of gene expression and function; however, has limited the usefulness of this genomic resource. As a step toward alleviating this limitation, we report here the development of a simple and efficient transient gene expression protocol. Using leaves from chickpea seedlings, we have established a procedure that enables the generation of large quantities of vital chickpea protoplasts within only a few hours. In addition, we have optimized a PEG-calcium-mediated transfection method to efficiently deliver exogenous DNA into the chickpea protoplast. The current study is the first to present a detailed step-by-step procedures for protoplast isolation, evaluation, transfection, and application in chickpea. In addition, we optimize the transfection efficiency which has not been previously reported. Our protoplast transfection approach provides a platform that will allow rapid high-throughput screening and systematic characterization of gene expression and function. Knowledge gained through such studies will benefit current efforts to improve chickpea production and quality.•Modified enzymatic digestion solution for higher yield and viability.•Optimize transfection of chickpea protoplasts.

The grapevine CAX-interacting protein VvCXIP4 is exported from the nucleus to activate the tonoplast Ca2+/H+ exchanger VvCAX3

The nuclear-localized CAX-interacting protein VvCXIP4 is exported to the cytosol after a Ca²⁺ pulse, to activate the tonoplast-localized Ca²⁺/H⁺ exchanger VvCAX3. Vacuolar cation/H⁺ exchangers (CAXs) have long been recognized as 'housekeeping' components in cellular Ca²⁺ and trace metal homeostasis, being involved in a range of key cellular and physiological processes. However, the mechanisms that drive functional activation of the transporters are largely unknown. In the present study, we investigated the function of a putative grapevine CAX-interacting protein, VvCXIP4, by testing its ability to activate VvCAX3, previously characterized as a tonoplast-localized Ca²⁺/H⁺ exchanger. VvCAX3 contains an autoinhibitory domain that drives inactivation of the transporter and thus, is incapable of suppressing the Ca²⁺-hypersensitive phenotype of the S. cerevisiae mutant K667. In this study, the co-expression of VvCXIP4 and VvCAX3 in this strain efficiently rescued its growth defect at high Ca²⁺ levels. Flow cytometry experiments showed that yeast harboring both proteins effectively accumulated higher Ca²⁺ levels than cells expressing each of the proteins separately. Bimolecular fluorescence complementation (BiFC) assays allowed visualization of the direct interaction between the proteins in tobacco plants and in yeast, and also showed the self-interaction of VvCAX3 but not of VvCXIP4. Subcellular localization studies showed that, despite being primarily localized to the nucleus, VvCXIP4 is able to move to other cell compartments upon a Ca²⁺ stimulus, becoming prone to interaction with the tonoplast-localized VvCAX3. qPCR analysis showed that both genes are more expressed in grapevine stems and leaves, followed by the roots, and that the steady-state transcript levels were higher in the pulp than in the skin of grape berries. Also, both VvCXIP4 and VvCAX3 were upregulated by Ca²⁺ and Na⁺, indicating they share common regulatory mechanisms. However, VvCXIP4 was also upregulated by Li⁺, Cu²⁺ and Mn²⁺, and its expression increased steadily throughout grape berry development, contrary to VvCAX3, suggesting additional physiological roles for VvCXIP4, including the regulation of VvCAXs not yet functionally characterized. The main novelty of the present study was the demonstration of physical interaction between CXIP and CAX proteins from a woody plant model by BiFC assays, demonstrating the intracellular mobilization of CXIPs in response to Ca²⁺.

Mutation of a histidine-rich calcium-binding-protein gene in wheat confers resistance to Fusarium head blight

Head or ear blight, mainly caused by Fusarium species, can devastate almost all staple cereal crops (particularly wheat), resulting in great economic loss and imposing health threats on both human beings and livestock^1-3. However, achievement in breeding for highly resistant cultivars is still not satisfactory. Here, we isolated the major-effect wheat quantitative trait locus, Qfhs.njau-3B, which confers head blight resistance, and showed that it is the same as the previously designated Fhb1. Fhb1 results from a rare deletion involving the 3' exon of the histidine-rich calcium-binding-protein gene on chromosome 3BS. Both wheat and Arabidopsis transformed with the Fhb1 sequence showed enhanced resistance to Fusarium graminearum spread. The translation products of this gene's homologs among plants are well conserved and might be essential for plant growth and development. Fhb1 could be useful not only for curbing Fusarium head blight in grain crops but also for improving other plants vulnerable to Fusarium species.

Cloning and functional analysis of the VcCXIP4 and VcYSL6 genes as Cd-regulating genes in blueberry

Blueberries (Vaccinium ssp.) show relatively high resistance to pollution and have been reported to successfully colonize acid and heavy metal-contaminated soils. Blueberries were subjected to cadmium stress using a simulated pot-culture method. The intact CDS regions of VcCXIP4 and VcYSL6 were obtained, VcCXIP4 was located in the nucleus, while VcYSL6 was located in the chloroplast. Both genes were constructed into a modified plant expression vector pCambia1301 for tobacco transformation with agrobacterium infection methods. Results showed that VcCXIP4 did not function alone in regulating cadmium (Cd) transport. Cd content of Cd in the leaves of VcYSL6 transgenic tobacco by 15.57% under high Cd concentration. Both, VcCXIP4 and VcYSL6 genes were up-regulated under Cd stress. Blueberry primarily accumulated excess Cd in the root, but Cd content in the fruit was almost independent of Cd content in the soil. Further, the effect of soil Cd content on fruit Cd content was not significant. VcCXIP4 is likely to interact with other proteins to regulate excess Cd in blueberry, while VcYSL6 is a Cd transporter required for excess Cd detoxification in blueberry.

Rapid phosphoproteomic and transcriptomic changes in the rhizobia-legume symbiosis

Symbiotic associations between legumes and rhizobia usually commence with the perception of bacterial lipochitooligosaccharides, known as Nod factors (NF), which triggers rapid cellular and molecular responses in host plants. We report here deep untargeted tandem mass spectrometry-based measurements of rapid NF-induced changes in the phosphorylation status of 13,506 phosphosites in 7739 proteins from the model legume Medicago truncatula. To place these phosphorylation changes within a biological context, quantitative phosphoproteomic and RNA measurements in wild-type plants were compared with those observed in mutants, one defective in NF perception (nfp) and one defective in downstream signal transduction events (dmi3). Our study quantified the early phosphorylation and transcription dynamics that are specifically associated with NF-signaling, confirmed a dmi3-mediated feedback loop in the pathway, and suggested "cryptic" NF-signaling pathways, some of them being also involved in the response to symbiotic arbuscular mycorrhizal fungi.

Calcium efflux systems in stress signaling and adaptation in plants

Transient cytosolic calcium ([Ca(2+)](cyt)) elevation is an ubiquitous denominator of the signaling network when plants are exposed to literally every known abiotic and biotic stress. These stress-induced [Ca(2+)](cyt) elevations vary in magnitude, frequency, and shape, depending on the severity of the stress as well the type of stress experienced. This creates a unique stress-specific calcium "signature" that is then decoded by signal transduction networks. While most published papers have been focused predominantly on the role of Ca(2+) influx mechanisms to shaping [Ca(2+)](cyt) signatures, restoration of the basal [Ca(2+)](cyt) levels is impossible without both cytosolic Ca(2+) buffering and efficient Ca(2+) efflux mechanisms removing excess Ca(2+) from cytosol, to reload Ca(2+) stores and to terminate Ca(2+) signaling. This is the topic of the current review. The molecular identity of two major types of Ca(2+) efflux systems, Ca(2+)-ATPase pumps and Ca(2+)/H(+) exchangers, is described, and their regulatory modes are analyzed in detail. The spatial and temporal organization of calcium signaling networks is described, and the importance of existence of intracellular calcium microdomains is discussed. Experimental evidence for the role of Ca(2+) efflux systems in plant responses to a range of abiotic and biotic factors is summarized. Contribution of Ca(2+)-ATPase pumps and Ca(2+)/H(+) exchangers in shaping [Ca(2+)](cyt) signatures is then modeled by using a four-component model (plasma- and endo-membrane-based Ca(2+)-permeable channels and efflux systems) taking into account the cytosolic Ca(2+) buffering. It is concluded that physiologically relevant variations in the activity of Ca(2+)-ATPase pumps and Ca(2+)/H(+) exchangers are sufficient to fully describe all the reported experimental evidence and determine the shape of [Ca(2+)](cyt) signatures in response to environmental stimuli, emphasizing the crucial role these active efflux systems play in plant adaptive responses to environment.

Expression of biomineralization-related ion transport genes in Emiliania huxleyi

Biomineralization in the marine phytoplankton Emiliania huxleyi is a stringently controlled intracellular process. The molecular basis of coccolith production is still relatively unknown although its importance in global biogeochemical cycles and varying sensitivity to increased pCO₂ levels has been well documented. This study looks into the role of several candidate Ca²⁺, H⁺ and inorganic carbon transport genes in E. huxleyi, using quantitative reverse transcriptase PCR. Differential gene expression analysis was investigated in two isogenic pairs of calcifying and non-calcifying strains of E. huxleyi and cultures grown at various Ca²⁺ concentrations to alter calcite production. We show that calcification correlated to the consistent upregulation of a putative HCO₃⁻ transporter belonging to the solute carrier 4 (SLC4) family, a Ca²⁺/H⁺ exchanger belonging to the CAX family of exchangers and a vacuolar H⁺-ATPase. We also show that the coccolith-associated protein, GPA is downregulated in calcifying cells. The data provide strong evidence that these genes play key roles in E. huxleyi biomineralization. Based on the gene expression data and the current literature a working model for biomineralization-related ion transport in coccolithophores is presented.

Characterization of Arabidopsis Ca2+/H+ exchanger CAX3

Plant calcium (Ca(2+)) gradients, millimolar levels in the vacuole and micromolar levels in the cytoplasm, are regulated in part by high-capacity vacuolar cation/H(+) exchangers (CAXs). Several CAX transporters, including CAX1, appear to contain an approximately 40-amino acid N-terminal regulatory region (NRR) that modulates transport through N-terminal autoinhibition. Deletion of the NRR from several CAXs (sCAX) enhances function in plant and yeast expression assays; however, to date, there are no functional assays for CAX3 (or sCAX3), which is 77% identical and 91% similar in sequence to CAX1. In this report, we create a series of truncations in the CAX3 NRR and demonstrate activation of CAX3 in both yeast and plants by truncating a large portion (up to 90 amino acids) of the NRR. Experiments with endomembrane-enriched vesicles isolated from yeast expressing activated CAX3 demonstrate that the gene encodes Ca(2+)/H(+) exchange with properties distinct from those of CAX1. The phenotypes produced by activated CAX3-expressing in transgenic tobacco lines are also distinct from those produced by sCAX1-expressing plants. These studies demonstrate shared and unique aspects of CAX1 and CAX3 transport and regulation.

Plant cation/H+ exchangers (CAXs): biological functions and genetic manipulations

Inorganic cations play decisive roles in many cellular and physiological processes and are essential components of plant nutrition. Therefore, the uptake of cations and their redistribution must be precisely controlled. Vacuolar antiporters are important elements in mediating the intracellular sequestration of these cations. These antiporters are energized by the proton gradient across the vacuolar membrane and allow the rapid transport of cations into the vacuole. CAXs (for CAtion eXchanger) are members of a multigene family and appear to predominately reside on vacuoles. Defining CAX regulation and substrate specificity have been aided by utilising yeast as an experimental tool. Studies in plants suggest CAXs regulate apoplastic Ca(2+) levels in order to optimise cell wall expansion, photosynthesis, transpiration and plant productivity. CAX studies provide the basis for making designer transporters that have been used to develop nutrient enhanced crops and plants for remediating toxic soils.

A mammalian monothiol glutaredoxin, Grx3, is critical for cell cycle progression during embryogenesis

Glutaredoxins (Grxs) have been shown to be critical in maintaining redox homeostasis in living cells. Recently, an emerging subgroup of Grxs with one cysteine residue in the putative active motif (monothiol Grxs) has been identified. However, the biological and physiological functions of this group of proteins have not been well characterized. Here, we characterize a mammalian monothiol Grx (Grx3, also termed TXNL2/PICOT) with high similarity to yeast ScGrx3/ScGrx4. In yeast expression assays, mammalian Grx3s were localized to the nuclei and able to rescue growth defects of grx3grx4 cells. Furthermore, Grx3 inhibited iron accumulation in yeast grx3gxr4 cells and suppressed the sensitivity of mutant cells to exogenous oxidants. In mice, Grx3 mRNA was ubiquitously expressed in developing embryos, adult tissues and organs, and was induced during oxidative stress. Mouse embryos absent of Grx3 grew smaller with morphological defects and eventually died at 12.5 days of gestation. Analysis in mouse embryonic fibroblasts revealed that Grx3(-/-) cells had impaired growth and cell cycle progression at the G(2) /M phase, whereas the DNA replication during the S phase was not affected by Grx3 deletion. Furthermore, Grx3-knockdown HeLa cells displayed a significant delay in mitotic exit and had a higher percentage of binucleated cells. Therefore, our findings suggest that the mammalian Grx3 has conserved functions in protecting cells against oxidative stress and deletion of Grx3 in mice causes early embryonic lethality which could be due to defective cell cycle progression during late mitosis.

Vacuolar Ca(2+) uptake

Calcium transporters that mediate the removal of Ca(2+) from the cytosol and into internal stores provide a critical role in regulating Ca(2+) signals following stimulus induction and in preventing calcium toxicity. The vacuole is a major calcium store in many organisms, particularly plants and fungi. Two main pathways facilitate the accumulation of Ca(2+) into vacuoles, Ca(2+)-ATPases and Ca(2+)/H(+) exchangers. Here I review the biochemical and regulatory features of these transporters that have been characterised in yeast and plants. These Ca(2+) transport mechanisms are compared with those being identified from other vacuolated organisms including algae and protozoa. Studies suggest that Ca(2+) uptake into vacuoles and other related acidic Ca(2+) stores occurs by conserved mechanisms which developed early in evolution.

The language of calcium signaling

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

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

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

Shaping the calcium signature

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

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

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

Relationship between calcium decoding elements and plant abiotic-stress resistance

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

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

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

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

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

Simultaneous flux and current measurement from single plant protoplasts reveals a strong link between K+ fluxes and current, but no link between Ca2+ fluxes and current

We present a thorough calibration and verification of a combined non-invasive self-referencing microelectrode-based ion-flux measurement and whole-cell patch clamp system as a novel and powerful tool for the study of ion transport. The system is shown to be capable of revealing the movement of multiple ions across the plasma membrane of a single protoplast at multiple voltages and in complex physiologically relevant solutions. Wheat root protoplasts are patch clamped in the whole-cell configuration and current-voltage relations obtained whilst monitoring net K+ and Ca2+ flux adjacent to the membrane with ion-selective electrodes. At each voltage, net ion flux (nmol m(-2) sec(-1)) is converted to an equivalent current density (mA m(-2)) taking into account geometry and electrode efficiency, and compared with the net current density measured with the patch clamp system. Using this technique, it is demonstrated that the K+-permeable outwardly rectifying conductance (KORC) is responsible for net outward K+ movement across the plasma membrane [1:1 flux-to-current ratio (1.21 +/- 0.14 SEM, n = 15)]. Variation in the K+ flux-to-current ratio among single protoplasts suggests a heterogeneous distribution of KORC channels on the membrane surface. As a demonstration of the power of the technique we show that despite a significant Ca2+ permeability being associated with KORC (analysis of tail current reversal potentials), there is no correlation between Ca2+ flux and KORC activity. A very significant observation is that large Ca2+ fluxes are electrically silent and probably tightly coupled to compensatory charge movements. This analysis demonstrates that it is mandatory to measure flux and currents simultaneously to investigate properly Ca2+ transport mechanisms and selectivity of ion channels in general.

Heavy metal stress and leaf senescence induce the barley gene HvC2d1 encoding a calcium-dependent novel C2 domain-like protein

By comparing cDNA populations derived from chromium-stressed primary leaves of barley (Hordeum vulgare L.) with controls, differentially expressed cDNA fragments could be identified. The deduced amino acid sequence of one of these cDNAs [named 'C2 domain 1' (HvC2d1)] exhibits a motif that is similar to the known C2 domain and a nuclear localization signal (NLS). Expression of this member of a novel class of plant C2 domain-like proteins was studied using real-time PCR, and subcellular localization was investigated using green fluorescent protein (GFP) fusion constructs. Calcium binding was analysed using a (45)Ca(2+) overlay assay. HvC2d1 was transiently induced after exposure to different heavy metals and its mRNA accumulated during the phase of leaf senescence. HvC2d1 expression responded to changes in calcium levels caused by the calcium ionophore A23187 and to treatment with methylviologen resulting in the production of reactive oxygen species (ROS). Using overexpressed and purified HvC2d1, the binding of calcium could be confirmed. Chimeric HvC2d1-GFP protein was localized in onion epidermal cells at the plasma membrane, cytoplasm and the nucleus. After addition of calcium ionophore A23187 green fluorescence was only visible in the nucleus. The data suggest a calcium-dependent translocation of HvC2d1 to the nucleus. A possible role of HvC2d1 in stress- and development-dependent signalling in the nucleus is discussed.

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

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