Don't shoot the (second) messenger: endomembrane transporters and binding proteins modulate cytosolic Ca2+ levels

Ammonium nutrition modifies cellular calcium distribution influencing ammonium-induced growth inhibition

Proper plant growth requires balanced nutrient levels. In this study, we analyzed the relationship between ammonium (NH4+) nutrition and calcium (Ca2+) homeostasis in the leaf tissues of wild-type and mutant Arabidopsis specimens provided with different nitrogen sources (NH4+ and nitrate, NO3-). Providing plants with NH4+ as the sole nitrogen source disrupts Ca2+ homeostasis, which is essential for activating signaling pathways and maintaining the cell wall structure. The results revealed that the lower Ca2+ content in Arabidopsis leaves under NH4+ stress might result from reduced transpiration pull, which could impair root-to-shoot Ca2+ transport. Moreover, NH4+ nutrition increased the expression of genes encoding proteins responsible for exporting Ca2+ from the cytosol of leaf cells. Furthermore, overexpression of the Ca2+/H+ antiporter 1 (CAX1) gene alleviates the effects of NH4+ syndrome, including stunted growth. The oeCAX1 plants, characterized by a lower apoplastic Ca2+ level, grew better under NH4+ stress than wild-type plants. Evaluation of the mechanical properties of the leaf blades, including stiffness, strength, toughness, and extensibility, showed that the wild-type and oeCAX1 plants responded differently to the nitrogen source, highlighting the role of cell wall metabolism in inhibiting the growth of NH4+-stressed plants.

Functional differentiation and genetic diversity of rice cation exchanger (CAX) genes and their potential use in rice improvement

Cation exchanger (CAX) genes play an important role in plant growth/development and response to biotic and abiotic stresses. Here, we tried to obtain important information on the functionalities and phenotypic effects of CAX gene family by systematic analyses of their expression patterns, genetic diversity (gene CDS haplotypes, structural variations, gene presence/absence variations) in 3010 rice genomes and nine parents of 496 Huanghuazhan introgression lines, the frequency shifts of the predominant gcHaps at these loci to artificial selection during modern breeding, and their association with tolerances to several abiotic stresses. Significant amounts of variation also exist in the cis-regulatory elements (CREs) of the OsCAX gene promoters in 50 high-quality rice genomes. The functional differentiation of OsCAX gene family were reflected primarily by their tissue and development specific expression patterns and in varied responses to different treatments, by unique sets of CREs in their promoters and their associations with specific agronomic traits/abiotic stress tolerances. Our results indicated that OsCAX1a and OsCAX2 as general signal transporters were in many processes of rice growth/development and responses to diverse environments, but they might be of less value in rice improvement. OsCAX1b, OsCAX1c, OsCAX3 and OsCAX4 was expected to be of potential value in rice improvement because of their associations with specific traits, responsiveness to specific abiotic stresses or phytohormones, and relatively high gcHap and CRE diversity. Our strategy was demonstrated to be highly efficient to obtain important genetic information on genes/alleles of specific gene family and can be used to systematically characterize the other rice gene families.

Ca2+ imaging and gene expression profiling of Lonicera Confusa in response to calcium-rich environment

As a medicinal plant widely planted in southwest karst of China, the study of adaptation mechanisms of Lonicera confusa, especially to karst calcium-rich environment, can provide important theoretical basis for repairing desertification by genetic engineering. In this study, the Ca²⁺ imaging in the leaves of L. confusa was explored by LSCM (Laser Scanning Confocal Microscopy) and TEM (Transmission Electron Microscopy), which revealed that the calcium could be transported to gland, epidermal hair and stoma in the leaves of L. confusa in high-Ca²⁺ environment. In addition, we simulated the growth environment of L. confusa and identified DEGs (Differentially Expressed Genes) under different Ca²⁺ concentrations by RNA sequencing. Further analysis showed that these DEGs were assigned with some important biological processes. Furthermore, a complex protein-protein interaction network among DEGs in L. Confusa was constructed and some important regulatory genes and transcription factors were identified. Taken together, this study displayed the Ca²⁺ transport and the accumulation of Ca²⁺ channels and pools in L. Confusa with high-Ca²⁺ treatment. Moreover, RNA sequencing provided a global picture of differential gene expression patterns in L. Confusa with high-Ca²⁺ treatment, which will help to reveal the molecular mechanism of the adaptation of L. confusa to high-Ca²⁺ environment in the future.

Heterodimerization of Arabidopsis calcium/proton exchangers contributes to regulation of guard cell dynamics and plant defense responses

Arabidopsis thaliana cation exchangers (CAX1 and CAX3) are closely related tonoplast-localized calcium/proton (Ca2+/H+) antiporters that contribute to cellular Ca2+ homeostasis. CAX1 and CAX3 were previously shown to interact in yeast; however, the function of this complex in plants has remained elusive. Here, we demonstrate that expression of CAX1 and CAX3 occurs in guard cells. Additionally, CAX1 and CAX3 are co-expressed in mesophyll tissue in response to wounding or flg22 treatment, due to the induction of CAX3 expression. Having shown that the transporters can be co-expressed in the same cells, we demonstrate that CAX1 and CAX3 can form homomeric and heteromeric complexes in plants. Consistent with the formation of a functional CAX1-CAX3 complex, CAX1 and CAX3 integrated into the yeast genome suppressed a Ca2+-hypersensitive phenotype of mutants defective in vacuolar Ca2+ transport, and demonstrated enzyme kinetics different from those of either CAX protein expressed by itself. We demonstrate that the interactions between CAX proteins contribute to the functioning of stomata, because stomata were more closed in cax1-1, cax3-1, and cax1-1/cax3-1 loss-of-function mutants due to an inability to buffer Ca2+ effectively. We hypothesize that the formation of CAX1-CAX3 complexes may occur in the mesophyll to affect intracellular Ca2+ signaling during defense responses.

Cadmium absorption and transportation pathways in plants

Controlling the uptake, transport, translocation, and accumulation of excessive amounts of cadmium from polluted environments is critical for plants and, consequently, humans with regard to food safety. Plants adopt various cellular and molecular mechanisms to minimize Cd toxicity. Upon exposure to Cd, plants initially implement avoidance strategies, such as production of organic acids, chelation, and sequestration, to prevent metal access to root cells. Nevertheless, Cd can be transported through the roots, stems, and leaves via apoplastic and symplastic pathways. These processes have been controlled by specific sites at the root surface and root cortex, in cells responsible for loading the root xylem, at the transition between the vascular systems of the root and the shoot, and in connecting tissues and cells at the stem. Although resistance to heavy metal cadmium can be achieved by either avoidance or tolerance, genetic basis to tolerance is therefore implied, in that these mechanisms are heritable attributes of tolerant mutants or genotypes.

Characterisation of Arabidopsis calnexin 1 and calnexin 2 in the endoplasmic reticulum and at plasmodesmata

Calnexin (CNX) is a highly conserved endoplasmic reticulum (ER) chaperone protein. Both calnexin and the homologous ER-lumenal protein, calreticulin, bind calcium ions and participate in protein folding. There are two calnexins in Arabidopsis thaliana, CNX1 and CNX2. GUS expression demonstrated that these are expressed in most Arabidopsis tissues throughout development. Calnexin transfer DNA (T-DNA) mutant lines exhibited increased transcript abundances of a number of other ER chaperones, including calreticulins, suggesting a degree of redundancy. CNX1 and CNX2 localised to the ER membrane including that within plasmodesmata, the intercellular channels connecting plant cells. This is comparable with the previous localisations of calreticulin in the ER lumen and at plasmodesmata. However, from green fluorescent protein (GFP) diffusion studies in single and double T-DNA insertion mutant lines, as well as overexpression lines, we found no evidence that CNX1 or CNX2 play a role in intercellular transport through plasmodesmata. In addition, calnexin T-DNA mutant lines showed no change in transcript abundance of a number of plasmodesmata-related proteins. CNX1 and CNX2 do not appear to have a specific localisation or function at plasmodesmata-rather the association of calnexin with the ER is simply maintained as the ER passes through plasmodesmata.

Functional characterization of a Glycine soja Ca(2+)ATPase in salt-alkaline stress responses

It is widely accepted that Ca(2+)ATPase family proteins play important roles in plant environmental stress responses. However, up to now, most researches are limited in the reference plants Arabidopsis and rice. The function of Ca(2+)ATPases from non-reference plants was rarely reported, especially its regulatory role in carbonate alkaline stress responses. Hence, in this study, we identified the P-type II Ca(2+)ATPase family genes in soybean genome, determined their chromosomal location and gene architecture, and analyzed their amino acid sequence and evolutionary relationship. Based on above results, we pointed out the existence of gene duplication for soybean Ca(2+)ATPases. Then, we investigated the expression profiles of the ACA subfamily genes in wild soybean (Glycine soja) under carbonate alkaline stress, and functionally characterized one representative gene GsACA1 by using transgenic alfalfa. Our results suggested that GsACA1 overexpression in alfalfa obviously increased plant tolerance to both carbonate alkaline and neutral salt stresses, as evidenced by lower levels of membrane permeability and MDA content, but higher levels of SOD activity, proline concentration and chlorophyll content under stress conditions. Taken together, for the first time, we reported a P-type II Ca(2+)ATPase from wild soybean, GsACA1, which could positively regulate plant tolerance to both carbonate alkaline and neutral salt stresses.

A molecular physiological review of vegetative desiccation tolerance in the resurrection plant Xerophyta viscosa (Baker)

MAIN CONCLUSION

Provides a first comprehensive review of integrated physiological and molecular aspects of desiccation tolerance Xerophyta viscosa. A synopsis of biotechnological studies being undertaken to improve drought tolerance in maize is given. Xerophyta viscosa (Baker) is a monocotyledonous resurrection plant from the family Vellociacea that occurs in summer-rainfall areas of South Africa, Lesotho and Swaziland. It inhabits rocky terrain in exposed grasslands and frequently experiences periods of water deficit. Being a resurrection plant it tolerates the loss of 95% of total cellular water, regaining full metabolic competency within 3 days of rehydration. In this paper, we review some of the molecular and physiological adaptations that occur during various stages of dehydration of X. viscosa, these being functionally grouped into early and late responses, which might be relevant to the attainment of desiccation tolerance. During early drying (to 55% RWC) photosynthesis is shut down, there is increased presence and activity of housekeeping antioxidants and a redirection of metabolism to the increased formation of sucrose and raffinose family oligosaccharides. Other metabolic shifts suggest water replacement in vacuoles proposed to facilitate mechanical stabilization. Some regulatory processes observed include increased presence of a linker histone H1 variant, a Type 2C protein phosphatase, a calmodulin- and an ERD15-like protein. During the late stages of drying (to 10% RWC) there was increased expression of several proteins involved in signal transduction, and retroelements speculated to be instrumental in gene silencing. There was induction of antioxidants not typically found in desiccation-sensitive systems, classical stress-associated proteins (HSP and LEAs), proteins involved in structural stabilization and those associated with changes in various metabolite pools during drying. Metabolites accumulated in this stage are proposed, inter alia, to facilitate subcellular stabilization by vitrification process which can include glass- and ionic liquid formation.

Transport pathways--proton motive force interrelationship in durum wheat mitochondria

In durum wheat mitochondria (DWM) the ATP-inhibited plant mitochondrial potassium channel (PmitoK(ATP)) and the plant uncoupling protein (PUCP) are able to strongly reduce the proton motive force (pmf) to control mitochondrial production of reactive oxygen species; under these conditions, mitochondrial carriers lack the driving force for transport and should be inactive. However, unexpectedly, DWM uncoupling by PmitoK(ATP) neither impairs the exchange of ADP for ATP nor blocks the inward transport of Pi and succinate. This uptake may occur via the plant inner membrane anion channel (PIMAC), which is physiologically inhibited by membrane potential, but unlocks its activity in de-energized mitochondria. Probably, cooperation between PIMAC and carriers may accomplish metabolite movement across the inner membrane under both energized and de-energized conditions. PIMAC may also cooperate with PmitoK(ATP) to transport ammonium salts in DWM. Interestingly, this finding may trouble classical interpretation of in vitro mitochondrial swelling; instead of free passage of ammonia through the inner membrane and proton symport with Pi, that trigger metabolite movements via carriers, transport of ammonium via PmitoK(ATP) and that of the counteranion via PIMAC may occur. Here, we review properties, modulation and function of the above reported DWM channels and carriers to shed new light on the control that they exert on pmf and vice-versa.

Analyses of Ca2+ accumulation and dynamics in the endoplasmic reticulum of Arabidopsis root cells using a genetically encoded Cameleon sensor

In planta, very limited information is available about how the endoplasmic reticulum (ER) contributes to cellular Ca(2+) dynamics and homeostasis. Here, we report the generation of an ER-targeted Cameleon reporter protein suitable for analysis of Ca(2+) accumulation and dynamics in the lumen of the ER in plant cells. Using stably transformed Arabidopsis (Arabidopsis thaliana) plants expressing this reporter protein, we observed a transiently enhanced accumulation of Ca(2+) in the ER in response to stimuli inducing cytosolic Ca(2+) rises in root tip cells. In all experimental conditions, ER Ca(2+) dynamics were substantially different from those monitored in the cytosol. A pharmacological approach enabled us to evaluate the contribution of the different ER-resident Ca(2+)-ATPase classes in the regulation of the ER Ca(2+) homeostasis. Taken together, our results do not provide evidence for a role of the ER as a major source that releases Ca(2+) for stimulus-induced increases in cytosolic Ca(2+) concentration. Instead, our results show that the luminal ER Ca(2+) elevations typically follow cytosolic ones, but with distinct dynamics. These findings suggest fundamental differences for the function of the ER in cellular Ca(2+) homeostasis in plants and animals.

Calcium oxalate crystals in eucalypt ectomycorrhizae: morphochemical characterization

Ectomycorrhizal fungi are ubiquitous in forest ecosystems, benefitting plants principally by increasing the uptake of water and nutrients such as calcium from the soil. Previous work has demonstrated accumulation of crystallites in eucalypt ectomycorrhizas, but detailed morphological and chemical characterization of these crystals has not been performed. In this work, cross sections of acetic acid-treated and cleared ectomycorrhizal fragments were visualized by polarized light microscopy to evaluate the location of crystals within cortical root cells. Ectomycorrhizal sections were also observed by scanning electron microscopy (SEM) coupled with energy dispersive x-ray (EDS) microprobe analysis. The predominant forms of crystals were crystal sand (granules) and concretions. Calcium, carbon and oxygen were detected by EDS as constituent elements and similar elemental profiles were observed between both crystal morphologies. All analyzed crystalline structures were characterized as calcium oxalate crystals. This is the first report of the stoichiometry and morphology of crystals occurring in eucalypt ectomycorrhizas in tropical soils. The data corroborates the role of ectomycorrhizae in the uptake and accumulation of calcium in the form of calcium oxalate crystals in hybrid eucalypt plants.

Ectopic expression of a maize calreticulin mitigates calcium deficiency-like disorders in sCAX1-expressing tobacco and tomato

Deregulated expression of an Arabidopsis H⁺/Ca²⁺ antiporter (sCAX1) in agricultural crops increases total calcium (Ca²⁺) but may result in yield losses due to Ca²⁺ deficiency-like symptoms. Here we demonstrate that co-expression of a maize calreticulin (CRT, a Ca²⁺ binding protein located at endoplasmic reticulum) in sCAX1-expressing tobacco and tomato plants mitigated these adverse effects while maintaining enhanced Ca²⁺ content. Co-expression of CRT and sCAX1 could alleviate the hypersensitivity to ion imbalance in tobacco plants. Furthermore, blossom-end rot (BER) in tomato may be linked to changes in CAX activity and enhanced CRT expression mitigated BER in sCAX1 expressing lines. These findings suggest that co-expressing Ca²⁺ transporters and binding proteins at different intracellular compartments can alter the content and distribution of Ca²⁺ within the plant matrix.

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 an Arabidopsis Ca2+/H+ antiporter CAX1 variant in petunia enhances cadmium tolerance and accumulation

Phytoremediation is a cost-effective and minimally invasive technology to cleanse soils contaminated with heavy metals. However, few plant species are suitable for phytoremediation of metals such as cadmium (Cd). Genetic engineering offers a powerful tool to generate plants that can hyperaccumulate Cd. An Arabidopsis CAX1 mutant (CAXcd), which confers enhanced Cd transport in yeast, was ectopically expressed in petunia to evaluate whether the CAXcd expression would enhance Cd tolerance and accumulation in planta. The CAXcd-expressing petunia plants showed significantly greater Cd tolerance and accumulation than the controls. After being treated with either 50 or 100μM CdCl(2) for 6 weeks, the CAXcd-expressing plants showed more vigorous growth compared with controls, and the transgenic plants accumulated significantly more Cd (up to 2.5-fold) than controls. Moreover, the accumulation of Cd did not affect the development and morphology of the CAXcd-expressing petunia plants until the flowering and ultimately the maturing of seeds. Therefore, petunia has the potential to serve as a model species for developing herbaceous, ornamental plants for phytoremediation.

Rapid cooling triggers forisome dispersion just before phloem transport stops

Phloem transport stops transiently within dicot stems that are cooled rapidly, but the cause remains unknown. Now it is known that (1) rapid cooling depolarizes cell membranes giving a transient increase in cytoplasmic Ca(2+), and (2) a rise of free calcium triggers dispersion of forisomes, which then occlude sieve elements (SEs) of fabacean plants. Therefore, we compared the effects of rapid chilling on SE electrophysiology, phloem transport and forisomes in Vicia faba. Forisomes dispersed after rapid cooling with a delay that was longer for slower cooling rates. Phloem transport stopped about 20 s after forisome dispersion, and then transport resumed and forisomes re-condensed within similar time frames. Transport interruption and forisome dispersion showed parallel behaviour--a cooling rate-dependent response, transience and desensitization. Chilling induced both a fast and a slow depolarization of SE membranes, the electrical signature suggesting strongly that the cause of forisome dispersion was the transient promotion of SE free calcium. This apparent block of SEs by dispersed forisomes may be assisted by other Ca(2+)-dependent sealing proteins that are present in all dicots.

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

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

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.

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

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

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

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

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

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

Removal of divalent cations induces structural transitions in red clover necrotic mosaic virus, revealing a potential mechanism for RNA release

The structure of Red clover necrotic mosaic virus (RCNMV), an icosahedral plant virus, was resolved to 8.5 A by cryoelectron microscopy. The virion capsid has prominent surface protrusions and subunits with a clearly defined shell and protruding domains. The structures of both the individual capsid protein (CP) subunits and the entire virion capsid are consistent with other species in the Tombusviridae family. Within the RCNMV capsid, there is a clearly defined inner cage formed by complexes of genomic RNA and the amino termini of CP subunits. An RCNMV virion has approximately 390 +/- 30 Ca2+ ions bound to the capsid and 420 +/- 25 Mg2+ ions thought to be in the interior of the capsid. Depletion of both Ca2+ and Mg2+ ions from RCNMV leads to significant structural changes, including (i) formation of 11- to 13-A-diameter channels that extend through the capsid and (ii) significant reorganization within the interior of the capsid. Genomic RNA within native capsids containing both Ca2+ and Mg2+ ions is extremely resistant to nucleases, but depletion of both of these cations results in nuclease sensitivity, as measured by a significant reduction in RCNMV infectivity. These results indicate that divalent cations play a central role in capsid dynamics and suggest a mechanism for the release of viral RNA in low-divalent-cation environments such as those found within the cytoplasm of a cell.

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.

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

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

Genetic manipulation for enhancing calcium content in potato tuber

Increased calcium (Ca) in potatoes may increase the production rate by enhancing tuber quality and storability. Additionally, increased Ca levels in important agricultural crops may help ameliorate the incidence of osteoporosis. However, the capacity to alter Ca levels in potato tubers through genetic manipulations has not been previously addressed. Here we demonstrate that potato tubers expressing the Arabidopsis H+/Ca2+ transporter sCAX1 (N-terminal autoinhibitory domain truncated version of CAtion eXchanger 1) contain up to 3-fold more Ca than wild-type tubers. The increased Ca appears to be distributed throughout the tuber. The sCAX1-expressing potatoes have normally undergone the tuber/plant/tuber cycle for three generations; the trait appeared stable through successive generations. The expression of sCAX1 does not appear to alter potato growth and development. Furthermore, increased Ca levels in sCAX1-expressing tubers do not appear to alter tuber morphology or yield. Given the preponderance of potato consumption worldwide, these transgenic plants may be a means of marginally increasing Ca intake levels in the population. To our knowledge, this study represents the first attempts to use biotechnology to increase the Ca content of potatoes.

Identification of a crucial histidine involved in metal transport activity in the Arabidopsis cation/H+ exchanger CAX1

In plants, yeast, and bacteria, cation/H+ exchangers (CAXs) have been shown to translocate Ca2+ and other metal ions utilizing the H+ gradient. The best characterized of these related transporters is the plant vacuolar localized CAX1. We have used site-directed mutagenesis to assess the impact of altering the seven histidine residues to alanine within Arabidopsis CAX1. The mutants were expressed in a Saccharomyces cerevisiae strain that is sensitive to Ca2+ and other metals. By utilizing a yeast growth assay, the H338A mutant was the only mutation that appeared to alter Ca2+ transport activity. The CAX1 His338 residue is conserved among various CAX transporters and may be located within a filter for cation selection. We proceeded to mutate His338 to every other amino acid residue and utilized yeast growth assays to estimate the transport properties of the 19 CAX mutants. Expression of 16 of these His338 mutants could not rescue any of the metal sensitivities. However, expression of H338N, H338Q, and H338K allowed for some growth on media containing Ca2+. Most interestingly, H338N exhibited increased tolerance to Cd2+ and Zn2+. Endomembrane fractions from yeast cells were used to measure directly the transport of H338N. Although the H338N mutant demonstrated 25% of the wild type Ca2+/H+ transport, it showed an increase in transport for both Cd2+ and Zn2+ reflected in a decrease in the Km for these substrates. This study provides insights into the CAX cation filter and novel mechanisms by which metals may be partitioned across membranes.

Regulation of the Slow Vacuolar Channel by Luminal Potassium: Role of Surface Charge

Voltage-dependent activation of slow vacuolar (SV) channels has been studied on isolated patches from red beet (Beta vulgaris L.) vacuoles. Isoosmotic variation of vacuolar K(+) from 10 to 400 mM in Ca(2+)-free solutions at the vacuolar side shifted the SV channel activation threshold to more positive voltages. The effect of K(+) could be mimicked by additions of choline or N-methyl D-glucamine and could be explained by unspecific screening of the negative surface charge. Fitting the dependence of voltage shift on K(+) concentration to the Gouy-Chapman model yields a surface charge density of 0.36 +/- 0.05 e(-)/nm(2). Negative surface potential also tended to increase the local concentration of permeable ions (K(+)), resulting in anomalously high single-channel conductance, approximately 200 pS in 10 mM KCl. An increase of ionic strength due to addition of impermeable cations greatly reduced the unitary conductance. Large positive shift of the SV channel voltage dependence, caused by physiological (0.5 mM) free vacuolar Ca(2+), was partly ameliorated by increasing luminal K(+). We interpreted these results as follows: K(+)induced a reduction of surface potential, hence i) causing a positive shift of the voltage dependence and ii) a dilution of Ca(2+) in the membrane vicinity, thus reducing the inhibitory effect of vacuolar Ca(2+) and causing a negative shift of the SV channel voltage dependence, with a sum of the two shifts being negative.

Evidence of differential pH regulation of the Arabidopsis vacuolar Ca2+/H+ antiporters CAX1 and CAX2

The Arabidopsis Ca(2+)/H(+) antiporters cation exchanger (CAX) 1 and 2 utilise an electrochemical gradient to transport Ca(2+) into the vacuole to help mediate Ca(2+) homeostasis. Previous whole plant studies indicate that activity of Ca(2+)/H(+) antiporters is regulated by pH. However, the pH regulation of individual Ca(2+)/H(+) antiporters has not been examined. To determine whether CAX1 and CAX2 activity is affected by pH, Ca(2+)/H(+) antiport activity was measured in vacuolar membrane vesicles isolated from yeast heterologously expressing either transporter. Ca(2+) transport by CAX1 and CAX2 was regulated by cytosolic pH and each transporter had a distinct cytosolic pH profile. Screening of CAX1/CAX2 chimeras identified an amino acid domain within CAX2 that altered the pH-dependent Ca(2+) transport profile so that it was almost identical to the pH profile of CAX1. Results from mutagenesis of a specific His residue within this domain suggests a role for this residue in pH regulation.

Functional and regulatory analysis of the Arabidopsis thaliana CAX2 cation transporter

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

Endoplasmic reticulum stress-induced programmed cell death in soybean cells

In animal cells, the endoplasmic reticulum may participate in programmed cell death by sensing and transducing apoptotic signals. In an attempt to analyze the role of the endoplasmic reticulum in plant programmed cell death we investigated the effect of cyclopiazonic acid, a specific blocker of plant endoplasmic reticulum-type IIA Ca2+-pumps, in soybean cells. Cyclopiazonic acid treatment elicited endoplasmic reticulum stress and a biphasic increase in cytosolic Ca2+ concentration, followed by the induction of a cell death program. Cyclopiazonic acid-induced programmed cell death occurred with accumulation of H2O2, cytochrome c release from mitochondria, caspase 9- and caspase 3-like protease activation, cytoplasmic shrinkage and chromatin condensation. Chelation of cytosolic Ca2+ with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (acetoxymethil ester) failed to inhibit cyclopiazonic acid-induced cell death. Taken together, our results provide evidence for a role of the endoplasmic reticulum and mitochondria in regulating cyclopiazonic acid-induced programmed cell death in soybean cells, probably via a cross-talk between the two organelles.

The generation of Ca(2+) signals in plants

The calcium ion is firmly established as a ubiquitous intracellular second messenger in plants. At their simplest, Ca(2+)-based signaling systems are composed of a receptor, a system for generating the increase in [Ca(2+)]cyt, downstream components that are capable of reacting to the increase in [Ca(2+)]cyt, and other cellular systems responsible for returning [Ca(2+)]cyt to its prestimulus level. Here we review the various mechanisms responsible for generating the stimulus-induced increases in [Ca(2+)]cyt known as Ca(2+) signals. We focus particularly on the mechanisms responsible for generating [Ca(2+)]cyt oscillations and transients and use Nod Factor signaling in legume root hairs and stimulus-response coupling in guard cells to assess the physiological significance of these classes of Ca(2+) signals.