A history of stress alters drought calcium signalling pathways in Arabidopsis

CIPK6, a CBL-interacting protein kinase is required for development and salt tolerance in plants

Calcineurin B-like proteins (CBL) and CBL-interacting protein kinases (CIPK) mediate plant responses to a variety of external stresses. Here we report that Arabidopsis CIPK6 is also required for the growth and development of plants. Phenotype of tobacco plants ectopically expressing a homologous gene (CaCIPK6) from the leguminous plant chickpea (Cicer arietinum) indicated its functional conservation. A lesion inAtCIPK6 significantly reduced shoot-to-root and root basipetal auxin transport, and the plants exhibited developmental defects such as fused cotyledons, swollen hypocotyls and compromised lateral root formation, in conjunction with reduced expression of a number of genes involved in auxin transport and abiotic stress response. The Arabidopsis mutant was more sensitive to salt stress compared to wild-type, while overexpression of a constitutively active mutant of CaCIPK6 promoted salt tolerance in transgenic tobacco. Furthermore, tobacco seedlings expressing the constitutively active mutant of CaCIPK6 showed a developed root system, increased basipetal auxin transport and hypersensitivity to auxin. Our results provide evidence for involvement of a CIPK in auxin transport and consequently in root development, as well as in the salt-stress response, by regulating the expression of genes.

Interaction of the WD40 domain of a myoinositol polyphosphate 5-phosphatase with SnRK1 links inositol, sugar, and stress signaling

In plants, myoinositol signaling pathways have been associated with several stress, developmental, and physiological processes, but the regulation of these pathways is largely unknown. In our efforts to better understand myoinositol signaling pathways in plants, we have found that the WD40 repeat region of a myoinositol polyphosphate 5-phosphatase (5PTase13; At1g05630) interacts with the sucrose nonfermenting-1-related kinase (SnRK1.1) in the yeast two-hybrid system and in vitro. Plant SnRK1 proteins (also known as AKIN10/11) have been described as central integrators of sugar, metabolic, stress, and developmental signals. Using mutants defective in 5PTase13, we show that 5PTase13 can act as a regulator of SnRK1 activity and that regulation differs with different nutrient availability. Specifically, we show that under low-nutrient or -sugar conditions, 5PTase13 acts as a positive regulator of SnRK1 activity. In contrast, under severe starvation conditions, 5PTase13 acts as a negative regulator of SnRK1 activity. To delineate the regulatory interaction that occurs between 5PTase13 and SnRK1.1, we used a cell-free degradation assay and found that 5PTase13 is required to reduce the amount of SnRK1.1 targeted for proteasomal destruction under low-nutrient conditions. This regulation most likely involves a 5PTase13-SnRK1.1 interaction within the nucleus, as a 5PTase13:green fluorescent protein was localized to the nucleus. We also show that a loss of function in 5PTase13 leads to nutrient level-dependent reduction of root growth, along with abscisic acid (ABA) and sugar insensitivity. 5ptase13 mutants accumulate less inositol 1,4,5-trisphosphate in response to sugar stress and have alterations in ABA-regulated gene expression, both of which are consistent with the known role of inositol 1,4,5-trisphosphate in ABA-mediated signaling. We propose that by forming a protein complex with SnRK1.1 protein, 5PTase13 plays a regulatory role linking inositol, sugar, and stress signaling.

Distinct roles of the pepper hypersensitive induced reaction protein gene CaHIR1 in disease and osmotic stress, as determined by comparative transcriptome and proteome analyses

A Capsicum annuum hypersensitive induced reaction protein1 (CaHIR1) was recently proposed as a positive regulator of hypersensitive cell death in plants. Overexpression of CaHIR1 in transgenic Arabidopsis plants conferred enhanced resistance against the hemi-biotrophic Pseudomonas syringae pv. tomato (Pst) and the biotrophic Hyaloperonospora parasitica. Infection by avirulent Pseudomonas strains carrying avrRpm1 or avrRpt2 caused enhanced resistance responses in transgenic plants, suggesting that CaHIR1 is involved in basal disease resistance in a race-nonspecific manner. H. parasitica exhibited low levels of asexual sporulation on CaHIR1 seedlings. In contrast, transgenic plants were susceptible not only to the necrotrophic fungal pathogen Botrytis cinerea but were also sensitive to osmotic stress caused by high salinity and drought. To identify proteins whose expression was altered by CaHIR1 overexpression in Arabidopsis leaves, a quantitative comparative proteome analysis using two-dimensional gel electrophoresis coupled with mass spectrometry was performed. Of about 400 soluble proteins, 11 proteins involved in several metabolic pathways were up- or down-regulated by CaHIR1 overexpression. Genes encoding glycine decarboxylase (At2g35370) and an unidentified protein (At2g03440), which were strongly upregulated in CaHIR1-overexpressing Arabidopsis, were also differentially induced at the transcriptional level by Pst infection. Arabidopsis carbonic anhydrase (At3g01500), highly similar to tobacco salicylic acid-binding protein 3, was up-regulated by CaHIR1 overexpression. The activity of an anti-oxidant enzyme, cooper/zinc superoxide dismutase (At2g28190), was also attenuated in transgenic Arabidopsis by CaHIR1 overexpression. Together, these results suggest that CaHIR1 overexpression in Arabidopsis mediates plant responses to biotrophic, hemi-biotrophic and necrotrophic pathogens, as well as to osmotic stress in different ways.

Pharmacological evidence for calcium involvement in the long-term processing of abiotic stimuli in plants

Information about abiotic conditions is stored for long periods in plants and, in flax seedlings, can lead to the production of meristems. To investigate the underlying mechanism, flax seedlings were given abiotic stimuli that included a mechanical stimulus (by manipulation), one or two cold shocks, a slow cold treatment and a drought stress and, if these seedlings were then subjected to a temporary (1 to 3 days) depletion of calcium, epidermal meristems were produced in the seedling hypocotyls. This production was inhibited by the addition to the nutrient media of EGTA, ruthenium red, lanthanum or gadolinium that affect calcium availability or calcium transport. Use of these agents revealed a period of vulnerability in information processing that was less than two min for mechanical stimuli and over five min for other abiotic stimuli, consistent with information about mechanical stimuli being stored particularly fast. We propose that external calcium is needed for the transduction/storage of the information for meristem production whilst a temporary depletion of external calcium is needed for the actual production of meristems. Such roles for calcium would be consistent with a mechanism based on ion condensation on charged polymers.

Effect of calcium and light on the germination of Urochondra setulosa under different salts

Urochondra setulosa (Trin.) C.E. Hubbard is a coastal halophytic grass thriving on the coastal dunes along the Pakistani seashore. This grass could be useful in coastal sand dune stabilization using seawater irrigation. The purpose of this investigation was to test the hypothesis that Ca(2+) (0.0, 2.5, 5.0, 10.0 and 50.0 mmol/L) alleviates the adverse effects of KCl, MgSO(4), NaCl and Na(2)SO(4) at 0, 200, 400, 600, 800 and 1000 mmol/L on the germination of Urochondra setulosa. Seed germination was inhibited with increase in salt concentration with few seeds germinated at and above 400 mmol/L concentration. No seed germinated in any of the KCl treatments. Inclusion of CaCl(2) substantially alleviated the inhibitory effects of all salts. Germination was higher under photoperiod in comparison to those seeds germinated under complete darkness. Among the CaCl(2) concentrations used, 10 mmol/L was most effective in alleviating salinity effects and allowing few seeds to germinate at 1000 mmol/L KCl, MgSO(4), NaCl and Na(2)SO(4) solution.

Priming: getting ready for battle

Infection of plants by necrotizing pathogens or colonization of plant roots with certain beneficial microbes causes the induction of a unique physiological state called "priming." The primed state can also be induced by treatment of plants with various natural and synthetic compounds. Primed plants display either faster, stronger, or both activation of the various cellular defense responses that are induced following attack by either pathogens or insects or in response to abiotic stress. Although the phenomenon has been known for decades, most progress in our understanding of priming has been made over the past few years. Here, we summarize the current knowledge of priming in various induced-resistance phenomena in plants.

Three grape CBF/DREB1 genes respond to low temperature, drought and abscisic acid

The C-repeat (CRT)-binding factor/dehydration-responsive element (DRE) binding protein 1 (CBF/ DREB1) transcription factors control an important pathway for increased freezing and drought tolerance in plants. Three CBF/DREB1-like genes, CBF 1-3, were isolated from both freezing-tolerant wild grape (Vitis riparia) and freezing-sensitive cultivated grape (Vitis vinifera). The deduced proteins in V. riparia are 63-70% identical to each other and 96-98% identical to the corresponding proteins in V. vinifera. All Vitis CBF proteins are 42-51% identical to AtCBF1 and contain CBF-specific amino acid motifs, supporting their identification as CBF proteins. Grape CBF sequences are unique in that they contain 20-29 additional amino acids and three serine stretches. Agro-infiltration experiments revealed that VrCBF1b localizes to the nucleus. VrCBF1a, VrCBF1b and VvCBF1 activated a green fluorescent protein (GFP) or glucuronidase (GUS) reporter gene behind CRT-containing promoters. Expression of the endogenous CBF genes was low at ambient temperature and enhanced upon low temperature (4 degrees C) treatment, first for CBF1, followed by CBF2, and about 2 d later by CBF3. No obvious significant difference was observed between V. riparia and V. vinifera genes. The expression levels of all three CBF genes were higher in young tissues than in older tissues. CBF1, 2 and 3 transcripts also accumulated in response to drought and exogenous abscisic acid (ABA) treatment, indicating that grape contains unique CBF genes.

Memory processes in the response of plants to environmental signals

Plants are sensitive to stimuli from the environment (e.g., wind, rain, contact, pricking, wounding). They usually respond to such stimuli by metabolic or morphogenetic changes. Sometimes the information corresponding to a stimulus may be "stored" in the plant where it remains inactive until a second stimulus "recalls" this information and finally allows it to take effect. Two experimental systems have proved especially useful in unravelling the main features of these memory-like processes.In the system based on Bidens seedlings, an asymmetrical treatment (e.g., pricking, or gently rubbing one of the seedling cotyledons) causes the cotyledonary buds to grow asymmetrically after release of apical dominance by decapitation of the seedlings. This information may be stored within the seedlings, without taking effect, for at least two weeks; then the information may be recalled by subjecting the seedlings to a second, appropriate, treatment that permits transduction of the signal into the final response (differential growth of the buds). Whilst storage is an irreversible, all-or-nothing process, recall is sensitive to a number of factors, including the intensity of these factors, and can readily be enabled or disabled. In consequence, it is possible to recall the stored message several times successively.In the system based on flax seedlings, stimulation such as manipulation stimulus, drought, wind, cold shock and radiation from a GSM telephone or from a 105 GHz Gunn oscillator, has no apparent effect. If, however, the seedlings are subjected at the same time to transient calcium depletion, numerous epidermal meristems form in their hypocotyls. When the calcium depletion treatment is applied a few days after the mechanical treatment, the time taken for the meristems to appear is increased by a number of days exactly equal to that between the application of the mechanical treatment and the beginning of the calcium depletion treatment. This means that a meristem-production information corresponding to the stimulation treatment has been stored in the plants, without any apparent effect, until the calcium depletion treatment recalls this information to allow it to take effect. Gel electrophoresis has shown that a few protein spots are changed (pI shift, appearance or disappearance of a spot) as a consequence of the application of the treatments that store or recall a meristem-production signal in flax seedlings. A SIMS investigation has revealed that the pI shift of one of these spots is probably due to protein phosphorylation. Modifications of the proteome have also been observed in Arabidopsis seedlings subjected to stimuli such as cold shock or radiation from a GSM telephone.

Complexity of the cold acclimation response in Drosophila melanogaster

Insects can increase their resistance to cold stress when they are exposed to non-lethal conditions prior to the stress; these plastic responses are normally described only in terms of immediate effects on mortality. Here we examine in Drosophila melanogaster the short- and longer-term effects of different conditions on several measures of cold resistance, but particularly chill coma recovery. Short-term exposure to sublethal temperature (cold hardening) did not decrease chill coma recovery times even though it decreased mortality. Exposure to 12 degrees C for 2 days (acclimation) decreased chill coma recovery times for a range of stressful temperatures when flies were cultured at 25 degrees C, but did not usually affect recovery times when flies were cultured at 19 degrees C. In contrast, 2-day exposure to 12 degrees C decreased mortality regardless of rearing temperature. Rearing at 19 degrees C decreased mortality and chill coma recovery time relative to rearing at 25 degrees C. Acclimation increased the eclosion rate of eggs from stressed females, but did not affect development time or size of the offspring. These results indicate that plastic responses to cold in D. melanogaster are complex when resistance is scored in different ways, and that effects can extend across generations.

Environmental stress alters genes expression and induces ovule abortion: reactive oxygen species appear as ovules commit to abort

Environmental stress dramatically reduces plant reproduction. Previous results showed that placing roots in 200 mM NaCl for 12 h caused 90% of the developing Arabidopsis ovules to abort (Sun et al. in Plant Physiol 135:2358-2367, 2004). To discover the molecular responses that occur during ovule abortion, gene expression was monitored using Affymetrix 24k genome arrays. Transcript levels were measured in pistils that were stressed for 6, 12, 18, and 24 h, then compared with the levels in healthy pistils. Over the course of this experiment, a total of 535 salt-responsive genes were identified. Cluster analysis showed that differentially expressed genes exhibited reproducible changes in expression. The expression of 65 transcription factors, some of which are known to be involved in stress responses, were modulated during ovule abortion. In flowers, salt stress led to a 30-fold increase in Na+ ions and modest, but significant, decreases in the accumulation of other ions. The expression of cation exchangers and ion transporters were induced, presumably to reestablish ion homeostasis following salt stress. Genes that encode enzymes that detoxify reactive oxygen species (ROS), including ascorbate peroxidase and peroxidase, were downregulated after ovules committed to abort. These changes in gene expression coincided with the synthesis of ROS in female gametophytes. One day after salt stress, ROS spread from the gametophytes to the maternal chalaza and integuments. In addition, genes encoding proteins that regulate ethylene responses, including ethylene biosynthesis, ethylene signal transduction and ethylene-responsive transcription factors, were upregulated after stress. Hypotheses are proposed on the basis of this expression analysis, which will be evaluated further in future experiments.

Enhancing Arabidopsis salt and drought stress tolerance by chemical priming for its abscisic acid responses

Drought and salt stress tolerance of Arabidopsis (Arabidopsis thaliana) plants increased following treatment with the nonprotein amino acid beta-aminobutyric acid (BABA), known as an inducer of resistance against infection of plants by numerous pathogens. BABA-pretreated plants showed earlier and higher expression of the salicylic acid-dependent PR-1 and PR-5 and the abscisic acid (ABA)-dependent RAB-18 and RD-29A genes following salt and drought stress. However, non-expressor of pathogenesis-related genes 1 and constitutive expressor of pathogenesis-related genes 1 mutants as well as transgenic NahG plants, all affected in the salicylic acid signal transduction pathway, still showed increased salt and drought tolerance after BABA treatment. On the contrary, the ABA deficient 1 and ABA insensitive 4 mutants, both impaired in the ABA-signaling pathway, could not be protected by BABA application. Our data demonstrate that BABA-induced water stress tolerance is based on enhanced ABA accumulation resulting in accelerated stress gene expression and stomatal closure. Here, we show a possibility to increase plant tolerance for these abiotic stresses through effective priming of the preexisting defense pathways without resorting to genetic alterations.

Calcium: just another regulator in the machinery of life?

*

BACKGROUND

Current hypotheses imply that stimulus-response systems in plants are networks of signal transduction pathways. It is usually assumed that these pathways connect receptors with effectors via chains of molecular events. Diverse intermediate signalling components (transducers) participate in these processes. However, many cellular transducers respond to several stimuli. Hence, there are no discrete chains but rather pathways that interconnect network-modules of different command structure. In particular, the cytosolic free Ca2+ concentration ([Ca2+](cyt)) is thought to perform many different tasks in a wide range of cellular events. However, this range of putative functions is so wide that it is often questioned how Ca2+ can comply with the definition of a second messenger. *THE Ca2+ SIGNATURE HYPOTHESIS: Some authors have suggested the concept of a specific signature of the ([Ca2+](cyt)) response. This implies that characteristics of the time course of changes in ([Ca2+](cyt)) and their localized sites of appearance in cells are used by the plant to identify the type and intensity of the stimulus. This hypothesis has triggered many investigations, which have yielded contradictory results. * THE CURRENT PICTURE: Much evidence suggests that the functions of calcium can be grouped into three classes: Ca2+ as a protective agent, Ca2+ as a chemical switch and Ca2+ as a 'digital' information carrier. Examples of the first two classes are presented here. The third is more controversial; while some investigations seem to support this idea, others call the Ca2+ signature hypothesis into question. Further investigations are needed to shed more light on Ca(2+)-driven signalling cascades.

Identification of pathogen-responsive regions in the promoter of a pepper lipid transfer protein gene (CALTPI) and the enhanced resistance of the CALTPI transgenic Arabidopsis against pathogen and environmental stresses

The 5' flanking region of the CALTPI gene, which encodes a basic lipid transfer protein, was isolated and characterized from the genomic DNA of Capsicum annuum. Four different regions of the promoter sequence of the CALTPI gene were fused to the beta-glucuronidase (GUS) coding region. In an Agrobacterium-mediated transient expression assay, the transcriptional activations of the promoter deletions were examined in tobacco leaves after infection with Pseudomonas syringae pv. tabaci, and treatment with ethylene and salicylic acid. The -808 bp region of the CALTPI gene promoter sequence exhibited full promoter activity. The W-box and ERE-box elements, which are essential for induction by all signals, were localized in the region between -555 bp and -391 bp upstream of the translation initiation site. A CALTPI transgene was then introduced under the control of the 35S promoter into the Arabidopsis ecotype Col-0. Transgenic Arabidopsis lines expressing the CALTPI gene developed rapidly compared to the wild-type plants, indicating that CALTPI may be involved in plant development. Overexpression of the CALTPI gene enhanced the resistance against infection by P. syringae pv. tomato and Botrytis cinerea. The transgenic plants expressing the CALTPI gene also showed high levels of tolerance to NaCl and drought stresses at various vegetative growth stages. No transcription of the PR-1, PR-2, PR-5, thionin, and RD29A genes was observed in untreated leaf tissues of the transgenic plants. The enhanced resistance to pathogen and environmental stresses in transgenic Arabidopsis correlated with the enhanced expression of the CALTPI gene.

Role of a Ca2+-ATPase induced by ABA and IAA in the generation of specific Ca2+ signals

The control of the Ca(2+)-ATPase gene (LCA1) that encodes two different membrane-located isoforms by two antagonic phytohormones, ABA and IAA, has been investigated. Strikingly both the growth regulators induce the LCA1 expression. By using a protoplast transient system, the cis-acting DNA elements responding to both, abiotic stress (ABA) and normal development (IAA), are dissected. ABA triggered a 4-fold increase in the GUS-activity. A single ACGT motif responsible for most of the LCA1 mRNA induction was localized at an unexpectedly large distance (1577 bp) upstream of the translational start. In the case of IAA, although there is a TGTCTC sequence that is known to be an important cis-acting element, two TGA motifs play a more critical role. It is proposed that the Ca(2+)-ATPase isoforms might intervene in the generation of specific Ca(2+) signals by restoring steady-state Ca(2+) levels, modulating both frequency and amplitude of Ca(2+) waves via wave interference.

ROS perception in Arabidopsis thaliana: the ozone-induced calcium response

Ozone is responsible for more crop losses than any other air pollutant. The changes in gene expression, which occur in plants in response to ozone, have been well characterized, yet little is known about how ozone is perceived or the signal transduction steps that follow. The earliest characterized response to ozone is an elevation in cytosolic-free calcium, which takes place within seconds of exposure. In this study, the calcium response to ozone was investigated in Arabidopsis thaliana seedlings using a variety of fumigation protocols. Ozone elicited distinct calcium responses in the aerial tissue and roots of seedlings. The calcium response in the cotyledons and leaves was biphasic and sensitive to the rate at which the ozone concentration increased. The response in the root was monophasic and insensitive to the rate of increase in ozone concentration. Experiments utilizing inhibitors of antioxidant metabolism demonstrated that the magnitude of the first peak in calcium in the aerial tissues was dependent upon the redox status of the plant. Seedlings were shown to be able to distinguish between ozone and hydrogen peroxide, producing a calcium signal in response to one of these reactive oxygen species (ROS) when they had become refractory to the other. Pre-treatment with ozone altered the calcium response to hydrogen peroxide and vice versa, indicating that the calcium response to a given ROS may reflect the stress history of the plant. These data suggest ROS signalling is more sophisticated than previously realized and raise questions over current models of ozone perception.

A logical (discrete) formulation for the storage and recall of environmental signals in plants

When subjected to an appropriate asymmetric stimulus, seedlings of Bidens pilosa L. "store" a symmetry-breaking instruction that will finally take effect (in the form of a differential growth of the cotyledonary buds) only if the plants are in a state in which they can "recall" this information. The ability of the plants to recall the stored symmetry-breaking instruction may be switched "on" or "off" by the application of a variety of stimuli. Although its detailed phenomenology is rather complicated, the overall behaviour of the plant storage/recall system can be modelled by use of an asynchronous, logical (discrete) description involving positive and negative feedback circuits, which are required for the existence of multi-stationarity and stability, respectively. The state tables, as used in this formalism, give a concise and easy-to-handle description of the evolution of the system and make it particularly easy to determine its stable states. This modelling approach may be extended to the formulation of many other experimental systems.

Long term transcript accumulation during the development of dehydration adaptation in Cicer arietinum

Cool season crops face intermittent drought. Exposure to drought and other abiotic stresses is known to increase tolerance of the plants against subsequent exposure to such stresses. Storage of environmental signals is also proposed. Preexposure to a dehydration shock improved adaptive response during subsequent dehydration treatment in a cool season crop chickpea (Cicer arietinum). We have identified 101 dehydration-inducible transcripts of chickpea by repetitive rounds of cDNA subtraction; differential DNA-array hybridization followed by northern-blot analysis and analyzed their responses to exogenous application of abscisic acid (ABA). Steady-state expression levels of the dehydration-induced transcripts were monitored during the recovery period between 2 consecutive dehydration stresses. Seven of them maintained more than 3-fold of expression after 24 h and more than 2-fold of expression level even at 72 h after the removal of stress. Noticeably, all of them were inducible by exogenous ABA treatment. When the seedlings were subjected to recover similarly after an exposure to exogenous ABA, the steady-state abundances of 6 of them followed totally different kinetics returning to basal level expression within 24 h. This observation indicated a correlation between the longer period of abundance of those transcripts in the recovery period and improved adaptation of the plants to subsequent dehydration stress and suggested that both ABA-dependent and -independent mechanisms are involved in the maintenance of the messages from the previous stress experience.

Abscisic acid induces CBF gene transcription and subsequent induction of cold-regulated genes via the CRT promoter element

Many cold-regulated genes of Arabidopsis are inducible by abscisic acid (ABA) as well as by cold. This has been thought to occur via two separate signaling pathways, with ABA acting via ABA-responsive promoter elements and low temperature activating the C-repeat element (CRT; dehydration-responsive) promoter element via CBF (DREB1) transcription factors. We show here that ABA is also capable of activating the CRT promoter element. Although the more recently discovered ABA-inducible CBF4 transcription factor might have accounted for this, we show here that CBF1-3 transcript levels also increase in response to elevated ABA levels. This increase in CBF1-3 transcript levels appears to be at least in part due to increased activity of the CBF promoters in response to ABA. A total of 125 bp of the CBF2 promoter, which has previously been shown to be sufficient for cold-, mechanical-, and cycloheximide-induced expression, was also sufficient for ABA-induced expression. However, the ABA-responsive promoter element-like motif within this region is not needed for ABA-induced expression. An observed increase in CBF protein levels after ABA treatment, together with previous data showing that increased CBF levels are sufficient for cold-regulated gene induction, suggests that ABA-induced increases in CBF1-3 transcript levels do have the potential to activate the CRT. Our data indicate therefore that activation of the CRT may also occur via a novel ABA-inducible signaling pathway using the normally cold-inducible CBFs.

Induction of defence gene expression by oligogalacturonic acid requires increases in both cytosolic calcium and hydrogen peroxide in Arabidopsis thaliana

Responses to oligogalacturonic acid (OGA) were determined in transgenic Arabidopsis thaliana seedlings expressing the calcium reporter protein aequorin. OGA stimulated a rapid, substantial and transient increase in the concentration of cytosolic calcium ([Ca2+]cyt) that peaked after ca. 15 s. This increase was dose-dependent, saturating at ca. 50 ug Gal equiv/ml of OGA. OGA also stimulated a rapid generation of H2O2. A small, rapid increase in H2O2 content was followed by a much larger oxidative burst, with H2O2 content peaking after ca. 60 min and declining thereafter. Induction of the oxidative burst by OGA was also dose-dependent, with a maximum response again being achieved at ca. 50 ug Gal equiv/mL. Inhibitors of calcium fluxes inhibited both increases in [Ca2+]cyt and [H2O2], whereas inhibitors of NADPH oxidase blocked only the oxidative burst. OGA increased strongly the expression of the defence-related genes CHS, GST, PAL and PR-1. This induction was suppressed by inhibitors of calcium flux or NADPH oxidase, indicating that increases in both cytosolic calcium and H2O2 are required for OGA-induced gene expression.

Low-temperature-induced transmembrane potential changes in mesophyll cells of Arabidopsis thaliana, Helianthus annuus and Vicia faba

Glass microelectrodes were inserted into mesophyll cells of intact leaves from higher plants: Arabidopsis thaliana, Helianthus annuus and Vicia faba var minor, and transient membrane potential changes were recorded in response to a sudden temperature drop. The cold-induced potential changes were unaffected by an anion channel inhibitor (anthracene-9-carboxylic acid) and potassium channel inhibitor (tetraethyl ammonium ion). Verapamil, a calcium channel inhibitor, caused significant suppression of the cold-induced potential changes. In the presence of calmoduline antagonists (trifluoperazine and N-6-aminohexyl-5-chloro-1-naphtalenesulphonamide), their amplitudes decreased and their durations were prolonged. Neomycin, which suppresses phospholipase C, also caused substantial inhibition of the amplitudes of the cold-induced potential changes. It is concluded that cold-evoked membrane potential changes are due to calcium influxes from both the apoplast and internal stores.

OXI1 kinase is necessary for oxidative burst-mediated signalling in Arabidopsis

Active oxygen species (AOS) generated in response to stimuli and during development can function as signalling molecules in eukaryotes, leading to specific downstream responses. In plants these include such diverse processes as coping with stress (for example pathogen attack, wounding and oxygen deprivation), abscisic-acid-induced guard-cell closure, and cellular development (for example root hair growth). Despite the importance of signalling via AOS in eukaryotes, little is known about the protein components operating downstream of AOS that mediate any of these processes. Here we show that expression of an Arabidopsis thaliana gene (OXI1) encoding a serine/threonine kinase is induced in response to a wide range of H2O2-generating stimuli. OXI1 kinase activity is itself also induced by H2O2 in vivo. OXI1 is required for full activation of the mitogen-activated protein kinases (MAPKs) MPK3 and MPK6 after treatment with AOS or elicitor and is necessary for at least two very different AOS-mediated processes: basal resistance to Peronospora parasitica infection, and root hair growth. Thus, OXI1 is an essential part of the signal transduction pathway linking oxidative burst signals to diverse downstream responses.

Ion binding properties of the dehydrin ERD14 are dependent upon phosphorylation

The ERD14 protein (early response to dehydration) is a member of the dehydrin family of proteins which accumulate in response to dehydration-related environmental stresses. Here we show the Arabidopsis dehydrin, ERD14, possesses ion binding properties. ERD14 is an in vitro substrate of casein kinase II; the phosphorylation resulting both in a shift in apparent molecular mass on SDS-PAGE gels and increased calcium binding activity. The phosphorylated protein bound significantly more calcium than the nonphosphorylated protein, with a dissociation constant of 120 microm and 2.86 mol of calcium bound per mol of protein. ERD14 is phosphorylated by extracts of cold-treated tissues, suggesting that the phosphorylation status of this protein might be modulated by cold-regulated kinases or phosphatases. Calcium binding properties of ERD14 purified from Arabidopsis extracts were comparable with phosphorylated Escherichia coli-expressed ERD14. Approximately 2 mol of phosphate were incorporated per mol of ERD14, indicating a minimum of two phosphorylation sites. Western blot analyses confirmed that threonine and serine are possible phosphorylation sites on ERD14. Utilizing matrix assisted laser desorption ionization-time of flight/mass spectrometry we identified five phosphorylated peptides that were present in both in vivo and in vitro phosphorylated ERD14. Our results suggest that the polyserine (S) domain is most likely the site of phosphorylation in ERD14 responsible for the activation of calcium binding.

Expression of rice chitinase gene triggered by the direct injection of Ca2+

In response to an elicitor, the Ca2+-dependent fluorescence (Fluo-3-Ca2+) increased transiently and then the expression of the chitinase gene (chi) followed. The gene expression was detected by Northern analysis. The deletion of Ca2+ from the medium or the addition of a Ca2+ channel blocker, verapamil, to the medium caused no gene expression, which supported the key role of Ca2+ in the signaling towards the chi expression. Then the Ca2+-injection experiment was done in order to investigate if it could trigger the chi expression. The plasmid pCHI-GFP (promoter: chi, reporter: green fluorescent protein gene (gfp)) was injected into the single-protoplasts, then after 1 day of incubation at 25 degrees C, 100 microM CaCl2 was injected into the same cells. After successive incubation for 1 day, 41 out of 85 cells showed the gene expression. The injection of 100 microM MgCl2, however, caused no gene expression. Therefore, Ca2+ could induce the chi of rice in the absence of the elicitor stimulus.

Stress memory in plants: a negative regulation of stomatal response and transient induction of rd22 gene to light in abscisic acid-entrained Arabidopsis plants

All organisms, including plants, perceive environmental stress, and they use this information to modify their behavior or development. Here, we demonstrate that Arabidopsis plants have memory functions related to repeated exposure to stressful concentrations of the phytohormone abscisic acid (ABA), which acts as a chemical signal. Repeated exposure of plants to ABA (40 micro m for 2 h) impaired light-induced stomatal opening or inhibited the response to a light stimulus after ABA-entrainment under both dark/light cycle and continuous light. Moreover, there were transient expressions of the rd22 gene during the same periods under both the growing conditions. Such acquired information in ABA-entrained plants produced a long-term sensitization. When the time of light application was changed, a transient induction of the rd22 gene in plants after ABA-entrainment indicated that these were light-associated responses. These transient effects were also observed in kin1, rab18, and rd29B. The transient expression of AtNCED3, causing the accumulation of endogenous ABA, indicated a possible regulation by ABA-dependent pathways in ABA-entrained plants. An ABA immunoassay supported this hypothesis: ABA-entrained plants showed a transient increase in endogenous ABA level from 220 to 250 pmol g-1 fresh mass at 1-2 h of the training period, whereas ABA-deficient (aba2) mutants did not. Taking into account these results, we propose that plants have the ability to memorize stressful environmental experiences, and discuss the molecular events in ABA-entrained plants.

Role of dynamics of intracellular calcium in aluminium-toxicity syndrome

This review is concentrating on the role of aluminium (Al)-calcium (Ca) interactions in Al toxicity syndrome in plants. Disruption of cytoplasmic Ca²⁺ homeostasis has been suggested as a primary trigger of Al toxicity. Aluminium causes an increase in cytosolic Ca²⁺ activity, potentially disrupting numerous biochemical and physiological processes, including those involved in the root growth. The source of Ca²⁺ for the increase in cytosolic Ca²⁺ activity under Al exposure is partly extracellular (likely to be due to the Al-resistant portion of the flux through depolarization-activated Ca²⁺ channels and fluxes through Ca²⁺ -permeable nonselective cation channels in the plasma membrane) as well as intracellular (increased cytosolic Ca²⁺ activity enhances the activity of Ca²⁺ release channels in the tonoplast and the endoplasmic reticulum membrane). The effect on increased cytosolic Ca²⁺ activity of possible Al-related inhibition of the plasma membrane and endo-membrane Ca²⁺ -ATPases and Ca²⁺ exchangers (CaX) that sequester Ca²⁺ out of the cytosol is insufficiently documented at present. The relationship between Al toxicity, cytoplasmic Ca²⁺ homeostasis and cytoplasmic pH needs to be elucidated. Technical improvements that would allow measurements of cytosolic Ca²⁺ activity within the short time after exposure to Al (seconds or shorter) are eagerly awaited. Contents I. Introduction 296 II. Symptoms of aluminium toxicity 296 III. Calcium - aluminium interactions 297 IV. The role of electrical properties of the plasma membrane in calcium-aluminium interactions 306 V. Oxidative stress 307 VI. Callose 308 VII. Cytoskeleton 308 VIII. Conclusions 309 Acknowledgements 309 References 309.

Low-temperature induced transmembrane potential changes in the liverwort Conocephalum conicum

Intracellular microelectrode measurements revealed that the liverwort Conocephalum conicum generates all-or-none action potentials (APs) in response to a sudden temperature drop. In plants with anion and potassium conductance blocked, dose-dependent voltage transients (VTs) were evoked by cold stimuli. These VTs did not propagate. When the external concentration of Ca(2+) was decreased or calcium channel inhibitors (La(3+), Gd(3+), verapamil, Mg(2+), Mn(2+)) were used, inhibition of VTs was observed. Amplitudes of both APs and VTs grew when Sr(2+) ions, known to release calcium from internal stores, were added to the medium. Neomycin, which suppresses phospholipase C and indirectly affects inositol triphosphate formation, caused substantial inhibition of both APs and VTs. It is concluded that a temperature drop elucidated membrane potential changes due to calcium influx both from external and internal stores.

Single-cell imaging of the Ca2+ influx into bovine endothelial cells occurring in response to an alternating electric stimulus

The electric control of cellular functions via Ca2+ was formerly suggested. From this viewpoint, the involvement of a Ca2+ channel was studied using bovine fetal arterial endothelial (BFAE) cells in which P2X4, an ATP-operated and fluid shear stress sensitive Ca2+ channel, exists predominantly. An electric stimulus (sine wave, 10 Hz, 10 VPP, 30 s) caused a marked influx of Ca2+ into BFAE cells from an extracellular solution. The magnitude of the [Ca2+]i change increased with a decrease in the frequency in the range from 100 Hz to 5 Hz. Regarding the pathway of this Ca2+ influx, single-cell imaging and an ATP depletion experiment strongly suggested the involvement of a pathway different from P2X4. This pathway was thought to be a non-specific one, because typical Ca2+ channel blockers, such as verapamil, Gd3+, and Co2+, could not inhibit the Ca2+ influx.

Temporal relationship between cytosolic free Ca(2+) and membrane potential during hypotonic turgor regulation in a brackish water Charophyte Lamprothamnium succinctum

Internodal cells of a brackish water charophyte, Lamprothamnium succinctum, regulate turgor pressure in response to changes in external osmotic pressure by modifying vacuolar concentrations of KCl. An increase in cytosolic concentration of free Ca(2+) ([Ca(2+)](c)) is necessary for the progress of turgor regulation induced by hypotonic treatment. Initial changes in membrane potential and [Ca(2+)](c) upon hypotonic treatment were measured to examine the temporal relationship between the two parameters. Fura-dextran (potassium salt, M(r) 10,000, anionic) that had been injected into the cytosol was used to measure [Ca(2+)](c). Membrane potential and membrane conductance under a current-clamp condition were also measured. Decrease in external osmotic pressure by 0.16 Osm induced a simultaneous increase in [Ca(2+)](c) with both depolarization of the membrane and increase in the membrane conductance. Decrease in external osmotic pressure by 0.05 Osm induced a simultaneous increase in [Ca(2+)](c) with membrane depolarization but the increase in membrane conductance started later than the other two processes. There was a close temporal relationship between the increase in [Ca(2+)](c) and membrane depolarization on the initial response of turgor regulation induced by hypotonic treatment.

Comprehensive analysis of gene expression in Nicotiana tabacum leaves acclimated to oxidative stress

The molecular mechanisms by which plants acclimate to oxidative stress are poorly understood. To identify the processes involved in acclimation, we performed a comprehensive analysis of gene expression in Nicotiana tabacum leaves acclimated to oxidative stress. Combining mRNA differential display and cDNA array analysis, we estimated that at least 95 genes alter their expression in tobacco leaves acclimated to oxidative stress, of which 83% are induced and 17% repressed. Sequence analysis of 53 sequence tags revealed that, in addition to antioxidant genes, genes implicated in abiotic and biotic stress defenses, cellular protection and detoxification, energy and carbohydrate metabolism, de novo protein synthesis, and signal transduction showed altered expression. Expression of most of the genes was enhanced, except for genes associated with photosynthesis and light-regulated processes that were repressed. During acclimation, two distinct groups of coregulated genes ("early-" and "late-response" gene regulons) were observed, indicating the presence of at least two different gene induction pathways. These two gene regulons also showed differential expression patterns on an oxidative stress challenge. Expression of "late-response" genes was augmented in the acclimated leaf tissues, whereas expression of "early-response" genes was not. Together, our data suggest that acclimation to oxidative stress is a highly complex process associated with broad gene expression adjustments. Moreover, our data indicate that in addition to defense gene induction, sensitization of plants for potentiated gene expression might be an important factor in oxidative stress acclimation.

Two-dimensional electrophoresis investigation of short-term response of flax seedlings to a cold shock

The flax, Linum usitatissimum L., is particularly suitable for studying the transduction and long-term signal storage of environmental signals. To investigate the underlying molecular mechanisms, we have focused on the initial changes in the proteome since these offer the possibility of reflecting the plant's history of exposure to stress. In principle, this 'proteome signature' might be revealed by two-dimensional electrophoresis (2-DE). We have therefore determined the potential of 2-DE to study the kinetics of changes to the proteome of flax induced by a 1 min cold shock. Protein identification is difficult with flax because of the lack of knowledge of gene sequences. Nevertheless, 2-DE analysis can be informative providing the significance of changes can be evaluated. We have developed a stringent threshold method to determine the significance of changes in gels obtained with proteins extracted from hypocotyls at different times after cold shock. This allowed us to reliably detect and characterize the kinetics of a set of seven spots that responded to cold shock and that constitute candidates for a proteome signature of long-term signal storage.

Hyperosmotic stress induces formation of tubulin macrotubules in root-tip cells of Triticum turgidum: their probable involvement in protoplast volume control

Treatment of root-tip cells of Triticum turgidum with 1 M mannitol solution for 30 min induces microtubule (Mt) disintegration in the plasmolyzed protoplasts. Interphase plasmolyzed cells possess many cortical, perinuclear and endoplasmic macrotubules, 35 nm in mean diameter, forming prominent arrays. In dividing cells macrotubules assemble into aberrant mitotic and cytokinetic apparatuses resulting in the disturbance of cell division. Putative tubulin paracrystals were occasionally observed in plasmolyzed cells. The quantity of polymeric tubulin in plasmolyzed cells exceeds that in control cells. Root-tip cells exposed for 2-8 h to plasmolyticum recover partially, although the volume of the plasmolyzed protoplast does not change detectably. Among other events, the macrotubules are replaced by Mts, chromatin assumes its typical appearance and the cells undergo typical cell divisions. Additionally, polysaccharidic material is found in the periplasmic space. Oryzalin and colchicine treatment induced macrotubule disintegration and a significant reduction of protoplast volume in every plasmolyzed cell type examined, whereas cytochalasin B had only minor effects restricted to differentiated cells. These results suggest that Mt destruction by hyperosmotic stress, and their replacement by tubulin macrotubules and putative tubulin paracrystals is a common feature among angiosperms and that macrotubules are involved in the mechanism of protoplast volume regulation.

Plants in a cold climate

Plants are able to survive prolonged exposure to sub-zero temperatures; this ability is enhanced by pre-exposure to low, but above-zero temperatures. This process, known as cold acclimation, is briefly reviewed from the perception of cold, through transduction of the low-temperature signal to functional analysis of cold-induced gene products. The stresses that freezing of apoplastic water imposes on plant cells is considered and what is understood about the mechanisms that plants use to combat those stresses discussed, with particular emphasis on the role of the extracellular matrix.

KCl activates phospholipase D at two different concentration ranges: distinguishing between hyperosmotic stress and membrane depolarization

Hyperosmotic stress induces the rapid formation of phosphatidic acid (PA) in Chlamydomonas moewusii via the activation of two signalling pathways: phospholipase D (PLD) and phospholipase C (PLC), the latter in combination with diacylglycerol kinase (DGK) (Munnik et al., 2000). A concomitant increase in cell Ca(2+) becomes manifest as deflagellation. When KCl was used as osmoticum we found that two concentration ranges activated deflagellation: one between 50 and 100 mm and another above 200 mm. Deflagellation in low KCl concentrations was complete within 30 sec whereas in high concentrations it took 5 min. PLC was not activated, as it was by high KCl concentrations that cause hyperosmotic stress. Moreover PLD was activated more strongly by low than by high KCl concentrations. Potassium was the most potent monovalent cation based on the induction of deflagellation and the formation of PA and PBut. During treatment, the external medium acidified, indicating an increase in H(+)-ATPase activity in order to re-establish the membrane potential. Activation of PLD and deflagellation at low KCl concentrations were abrogated by treatment with La(3+), Gd(3+) and EGTA, indicating the dependency on extracellular Ca(2+). This suggests that low concentrations of KCl depolarize the plasma membrane, resulting in the activation of H(+)-ATPases and opening voltage-dependent Ca(2+) +/- channels, observed as deflagellation and an increase in PLD activity.

A Ca2+/CaM-dependent kinase from pea is stress regulated and in vitro phosphorylates a protein that binds to AtCaM5 promoter

An immuno-homologue of maize Ca2+/calmodulin (CaM)-dependent protein kinase with a molecular mass of 72 kDa was identified in pea. The pea kinase (PsCCaMK) was upregulated in roots in response to low temperature and increased salinity. Exogenous Ca2+ application increased the kinase level and the response was faster than that obtained following stress application. Low temperature-mediated, but not salinity-mediated stress kinase increase was inhibited by the application of EGTA and W7, a CaM inhibitor. The purification of PsCCaMK using immuno-affinity chromatography resulted in coelution of the kinase with another polypeptide of molecular mass 40 kDa (p40). Western blot revealed the presence of PsCCaMK in nuclear protein extracts and was found to phosphorylate p40 in vitro. Gel mobility shift and South-Western analysis showed that p40 is a DNA-binding protein and it interacted specifically with one of the cis acting elements of the Arabidopsis CaM5 gene (AtCaM5) promoter. The binding of p40 to the specific elements in the AtCaM5 promoter was dependent of its dephosphorylated state. Our results suggest that p40 could be an upstream signal component of the stress responses.

A plant for all seasons: alterations in photosynthetic carbon metabolism during cold acclimation in Arabidopsis

Low temperatures lead to the inhibition of sucrose synthesis and photosynthesis. The biochemical and physiological adaptations of plants to low temperatures include the post-translational activation and increased expression of enzymes of the sucrose synthesis pathway, the changed expression of Calvin cycle enzymes, and changes in the leaf protein content. Recent progress has been made in understanding both the signals that trigger these processes and how the regulation of photosynthetic carbon metabolism interacts with other processes during cold acclimation.

Rapid low temperature-induced stomatal closure occurs in cold-tolerant Commelina communis leaves but not in cold-sensitive tobacco leaves, via a mechanism that involves apoplastic calcium but not abscisic acid

Commelina communis stomata closed within 1 h of transferring intact plants from 27 degrees C to 7 degrees C, whereas tobacco (Nicotiana rustica) stomata did not until the leaves wilted. Abscisic acid (ABA) did not mediate cold-induced C. communis stomatal closure: At low temperatures, bulk leaf ABA did not increase; ABA did not preferentially accumulate in the epidermis; its flux into detached leaves was lower; its release from isolated epidermis was not greater; and stomata in epidermal strips were less sensitive to exogenous ABA. Stomata of both species in epidermal strips on large volumes of cold KCl failed to close unless calcium was supplied. Therefore, the following cannot be triggers for cold-induced stomatal closure in C. communis: direct effects of temperature on guard or epidermal cells, long-distance signals, and effects of temperature on photosynthesis. Low temperature increased stomatal sensitivity to external CaCl(2) by 50% in C. communis but only by 20% in tobacco. C. communis stomata were 300- to 1,000-fold more sensitive to calcium at low temperature than tobacco stomata, but tobacco epidermis only released 13.6-fold more calcium into bathing solutions than C. communis. Stomata in C. communis epidermis incubated on ever-decreasing volumes of cold calcium-free KCl closed on the lowest volume (0.2 cm(3)) because the epidermal apoplast contained enough calcium to mediate closure if this was not over diluted. We propose that the basis of cold-induced stomatal closure exhibited by intact C. communis leaves is increased apoplastic calcium uptake by guard cells. Such responses do not occur in chill-sensitive tobacco leaves.

Unravelling response-specificity in Ca2+ signalling pathways in plant cells

Considerable advances have been made, both in the technologies available to study changes in intracellular cytosolic free Ca²⁺ ([Ca²⁺ ]_i ), and in our understanding of Ca²⁺ signalling cascades in plant cells, but how specificity can be generated from such a ubiquitous component as Ca²⁺ is questionable. Recently the concept of 'Ca²⁺ signatures' has been formulated; tight control of the temporal and spatial characteristics of alterations in [Ca²⁺ ]_i signals is thought to be responsible, at least in part, for the specificity of the response. However, the way in which Ca²⁺ signatures are decoded, which depends on the nature and location of the targets of the Ca²⁺ signals, has received little attention. In a few key systems, progress is being made on how diverse Ca²⁺ signatures might be transduced within cells in response to specific signals. Valuable pieces of the signal-specificity puzzle are being put together and this is illustrated here using some key examples; these emphasize the global importance of Ca²⁺ -mediated signal-transduction cascades in the responses of plants to a wide diversity of extracellular signals. However, the way in which signal specificity is encoded and transduced is still far from clear. Contents Summary 7 I. Introduction: Ca²⁺ as a signal transducer 8 II. Alterations in intracellular [Ca²⁺ ] 8 1. Measuring alterations in [Ca²⁺ ] 8 Imaging [Ca²⁺ ]_i using Ca²⁺ -sensitive dyes 8 Measuring [Ca²⁺ ]_i using aequorin 9 Imaging [Ca²⁺ ]_i using cameleon 10 2. The concept of the 'Ca²⁺ signature 10 3. How might specific Ca²⁺ signatures be generated? 11 Control of intracellular Ca²⁺ release 11 Control of influx of extracellular Ca²⁺ 12 4. Examples of Ca²⁺ signatures and cellular responses to increases in [Ca²⁺ ] 13 Ca²⁺ signatures in stomatal guard cells in response to abscisic acid signals 14 Ca²⁺ signals in response to abiotic stimuli1 8 Ca²⁺ signatures involved in plant-pathogen responses 19 Ca²⁺ signatures in control of plant reproduction 20 Ca²⁺ signatures in root hairs in response to nodulation signals 23 III. Decoding the [Ca²⁺ ]_i signatures 24 1. Coupling Ca²⁺ signals to responses through CaM 26 2. Coupling Ca²⁺ signals to responses through CDPK 27 3. Novel Ca²⁺ binding proteins as primary Ca²⁺ sensors 28 Conclusions and Perspective 28 References 29.

Osmotic stress activates distinct lipid and MAPK signalling pathways in plants

Plants are continuously exposed to all kinds of water stress such as drought and salinity. In order to survive and adapt, they have developed survival strategies that have been well studied, but little is known about the early mechanisms by which the osmotic stress is perceived and transduced into these responses. During the last few years, however, a variety of reports suggest that specific lipid and MAPK pathways are involved. This review briefly summarises them and presents a model showing that osmotic stress is transmitted by multiple signalling pathways.

Rapid accumulation of phosphatidylinositol 4,5-bisphosphate and inositol 1,4,5-trisphosphate correlates with calcium mobilization in salt-stressed arabidopsis

The phosphoinositide phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] is a key signaling molecule in animal cells. It can be hydrolyzed to release 1,2-diacyglycerol and inositol 1,4,5-trisphosphate (IP(3)), which in animal cells lead to protein kinase C activation and cellular calcium mobilization, respectively. In addition to its critical roles in constitutive and regulated secretion of proteins, PtdIns(4,5)P(2) binds to proteins that modify cytoskeletal architecture and phospholipid constituents. Herein, we report that Arabidopsis plants grown in liquid media rapidly increase PtdIns(4,5)P(2) synthesis in response to treatment with sodium chloride, potassium chloride, and sorbitol. These results demonstrate that when challenged with salinity and osmotic stress, terrestrial plants respond differently than algae, yeasts, and animal cells that accumulate different species of phosphoinositides. We also show data demonstrating that whole-plant IP(3) levels increase significantly within 1 min of stress initiation, and that IP(3) levels continue to increase for more than 30 min during stress application. Furthermore, using the calcium indicators Fura-2 and Fluo-3 we show that root intracellular calcium concentrations increase in response to stress treatments. Taken together, these results suggest that in response to salt and osmotic stress, Arabidopsis uses a signaling pathway in which a small but significant portion of PtdIns(4,5)P(2) is hydrolyzed to IP(3). The accumulation of IP(3) occurs during a time frame similar to that observed for stress-induced calcium mobilization. These data also suggest that the majority of the PtdIns(4,5)P(2) synthesized in response to salt and osmotic stress may be utilized for cellular signaling events distinct from the canonical IP(3) signaling pathway.

Abiotic stress signalling pathways: specificity and cross-talk

Plants exhibit a variety of responses to abiotic stresses that enable them to tolerate and survive adverse conditions. As we learn more about the signalling pathways leading to these responses, it is becoming clear that they constitute a network that is interconnected at many levels. In this article, we discuss the 'cross-talk' between different signalling pathways and question whether there are any truly specific abiotic stress signalling responses.

Hyperosmotic stress induces a rapid and transient increase in inositol 1,4,5-trisphosphate independent of abscisic acid in Arabidopsis cell culture

Phospholipid metabolism is involved in hyperosmotic-stress responses in plants. To investigate the role of phosphoinositide-specific phospholipase C (PI-PLC)-a key enzyme in phosphoinositide turnover-in hyperosmotic-stress signaling, we analyzed changes in inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) content in response to hyperosmotic shock or salinity in Arabidopsis thaliana T87 cultured cells. Within a few s, a hyperosmotic shock, caused by mannitol, NaCl, or dehydration, induced a rapid and transient increase in Ins(1,4,5)P3. However, no transient increase was detected in cells treated with ABA. Neomycin and U73122, inhibitors of PI-PLC, inhibited the increase in Ins(1,4,5)P3 caused by the hyperosmotic shock. A rapid increase in phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) in response to the hyperosmotic shock also occurred, but the rate of increase was much slower than that of Ins(1,4,5)P3. These findings indicate that the transient Ins(1,4,5)P3 production was due to the activation of PI-PLC in response to hyperosmotic stress. PI-PLC inhibitors also inhibited hyperosmotic stress-responsive expression of some dehydration-inducible genes, such as rd29A (lti78/cor78) and rd17 (cor47), that are controlled by the DRE/CRT cis-acting element but did not inhibit hyperosmotic stress-responsive expression of ABA-inducible genes, such as rd20. Taken together, these results suggest the involvement of PI-PLC and Ins(1,4,5)P3 in an ABA-independent hyperosmotic-stress signal transduction pathway in higher plants.

Altered patterns of tubulin polymerization in dividing leaf cells of Chlorophyton comosum after a hyperosmotic treatment

•  Microtubule organization and tubulin polymerization in meristematic leaf cells of Chlorophyton comosum treated with an aqueous solution of 1 M mannitol, inducing plasmolysis, were examined with immunofluorescence and transmission electron microscopy. •  Hyperosmotic treatment induced disintegration of the interphase microtubule systems. Free tubulin, either liberated from the depolymerized microtubules or pre-existing as a nonassembled pool, was incorporated into a network of paracrystals. In most of the dividing cells, mitotic and cytokinetic microtubule systems were replaced by atypical spindle-like structures displaying bipolarity and atypical phragmoplasts, respectively. These atypical mitotic and cytokinetic structures consisted of large densely packed bundles of macrotubules (32 nm diameter) or macrotubules and paracrystals. Tubulin paracrystals also occurred in ectopic positions in plasmolysed mitotic and cytokinetic cells. Dividing cells displaying paracrystals only did not form atypical mitotic and cytokinetic apparatuses. •  Short hyperosmotic stress causes disintegration of all microtubule arrays in dividing cells of C. comosum. Free tubulin is incorporated into macrotubules and tubulin paracrystals. The latter exhibit definite periodicity and characteristic fine structure.

Plant calcium signaling and monitoring: pros and cons and recent experimental approaches

This review focusses on Ca(2+)-mediated plant cell signaling and optical methods for in vivo [Ca2+] monitoring and imaging in plants. The cytosolic free calcium concentration has long been considered the central cellular key in plants. However, more and more data are turning up that critically question this view. Conflicting arguments show that there are still many open questions. One conclusion is that the cytosolic free Ca2+ concentration is just one of many cellular network parameters orchestrating complex cellular signaling. Novel experimental strategies which unveil interference of cellular parameters and communication of transduction pathways are required to understand this network. To date only optical methods are able to provide both kinetic and spatial information about cellular key parameters simultaneously. Focussing on calcium there are currently three classes of calcium indicators employed (i.e., chemical fluorescent dyes, luminescent indicators, and green-fluorescent-protein-based indicators). Properties and capabilities as well as advantages and disadvantages of these indicators when used in plant systems are discussed. Finally, general experimental strategies are mentioned which are able to answer open questions raised here.

Imaging spatial and cellular characteristics of low temperature calcium signature after cold acclimation in Arabidopsis

Cooling-induced 'calcium signatures' were imaged in aequorin-expressing Arabidopsis plants after cold acclimation or growth at ambient temperature. In all tissues, signatures were altered after acclimation. Characterization of the components generating this response indicates that cold acclimation increases cold-induced vacuolar Ca(2+) release, but does not affect the influx of extracellular calcium.

Expression of Drosophila homologue of senescence marker protein-30 during cold acclimation

Gene expression during cold acclimation at a moderately low temperature (15 degrees C) was studied in Drosophila melanogaster using a subtraction technique. A gene homologous to senescence marker protein-30 (SMP30), which has a Ca(2+)-binding function, was up-regulated at the transcription level after acclimation to 15 degrees C. This gene (henceforth referred to as Dca) was also expressed at a higher level in individuals reared at 15 degrees C from the egg stage than in those reared at 25 degrees C. Moreover, DCA mRNA increased at the senescent stage in Drosophila, although SMP30 is reported to decrease at senescent stages in mammals. In situ hybridization to polytene chromosomes revealed that the Dca gene was located at 88D on chromosome 3R. The 5' flanking region of this gene had AP-1 (a transcription factor of SMP30) binding sites, stress response element and some other transcription factor binding sites. The function of DCA was discussed in relation to the possible regulation of cytosolic Ca(2+) concentration.

An Arabidopsis gene encoding a Ca2+-binding protein is induced by abscisic acid during dehydration

An Arabidopsis thaliana RD20 cDNA, which was isolated as one of drought-inducible genes, encodes a putative protein with a conserved EF-hand Ca2+-binding domain. The recombinant RD20 protein was shown to bind Ca2+. The transcription of RD20 gene was induced not only by drought but also by ABA and high salinity.