Stress Signaling II: Calcium Sensing And Signaling
Chapter 4Stress Signaling II: Calcium Sensing and SignalingMarie Boudsocq and Jen Sheen*Department of Molecular Biology, Massachusetts General Hospital & Departmentof Genetics, Harvard Medical School, Boston, MA 02114, USASummary.I Introduction.II Calcium Signals.A Calcium Signatures.B Role of Calcium Signatures.C Calcium Channels, Pumps and Transporters.III Calcium Sensing and Signaling.A Sensor Relays.1 Calmodulin and Calmodulin-Like Sensors.2 Calcineurin B-Like Sensors.B Sensor Protein Kinases.1 Calcium-Dependent Protein Kinases.2 Calcium and Calmodulin-Dependent Protein Kinases.3 Other Calcium-Binding Proteins.IV 787879798183838585868686SummaryCalcium is an essential second messenger in plant signaling networks. Many environmentaland developmental stimuli induce an increase in cytosolic calcium to trigger different physiologicalresponses. The specificity of Ca2 signaling is achieved by a combination of distinct calcium signaturesthat are generated by specific calcium channels, pumps and transporters, and diverse calcium sensorsthat differ by their expression pattern, sub-cellular localization, substrate specificities and calcium sensitivities. Calcium binding modifies the structural conformation or enzymatic activity of the calcium sensors,which subsequently regulate downstream targets. Calmodulin is the most important Ca2 transducer ineukaryotes and regulates numerous proteins with diverse cellular functions, including protein kinases.Plants also possess specific multigene families of protein kinases that play crucial roles in mediatingcalcium signaling. The multiplicity and diversity of plant calcium sensors, as well as the interconnections between various signal transduction pathways, constitute a tightly regulated signaling network thatinduces specific stress responses to improve plant survival.Keywords Calcineurin B-like calcium calcium-dependent protein kinase calcium sensing calciumsignatures calmodulin stress signaling* Author for Correspondence, e-mail: sheen@molbio.mgh.harvard.eduA. Pareek, S.K. Sopory, H.J. Bohnert and Govindjee (eds.), Abiotic Stress Adaptation in Plants: Physiological, Molecularand Genomic Foundation, pp.75–90.DOI 10.1007/978-90-481-3112-9 4, Springer Science Business Media B.V. 2010.75
76M. Boudsocq and J. SheenI IntroductionCalcium is an essential plant nutrient that playsstructural roles in the cell wall and membranes,and regulates plant growth and development(Hepler 2005). However, to avoid toxicity, calcium is maintained at low levels in the cytosolthrough the activation of calcium pumps andstorage in multiple intracellular compartmentsas well as extracellular spaces (Fig. 1) (Sanderset al. 2002). While the role of calcium seems to belimited in prokaryotes (Dominguez 2004), it hasevolved to be a ubiquitous second messenger inplants that mediates complex responses to developmental and environmental cues. Many external and internal signals can strongly, rapidly andtransiently increase cytosolic calcium [Ca2 ]cyt,through the regulation of diverse calcium transport systems (Fig. 1). The abundance of bufferingcalcium binding proteins in the cytosol can reducecalcium mobility and facilitate the localized andspatially distinct elevations in calcium concentrations (White and Broadley 2003). These calciumsignals can be decoded by protein sensors whichdisplay an altered conformation and/or activityupon calcium binding. Understanding the specificity of calcium signaling has been a major challenge in plant biology for decades, since manydiverse stimuli generate Ca2 signals to triggertotally different responses. This signaling specificity can be achieved by different features ofcalcium signatures, distinct calcium sensitivities,expression and localization of calcium sensorsand their downstream relay partners, as well asinteractions with other signaling cascades. Thisreview provides an overview of plant calcium signaling in response to abiotic stresses.Abbreviations ABA – abscisic acid; ACA – auto-inhibited Ca2 ATPase; cADPR – cyclic ADP Ribose; CaM – calmodulin;CaMBP – calmodulin-binding protein; CAMTA – calmodulinbinding transcription activator; CBK – calmodulin-bindingprotein kinase; CBL – calcineurin B-like; CCaMK – calciumand calmodulin-dependent protein kinase; CDPK – calciumdependent protein kinase; CIPK – CBL-interacting proteinkinase; CML – calmodulin-like; CNGC – cyclic-nucleotidegated channel; cNMP – cyclic nucleotide monophosphate;CRK – CDPK-related protein kinase; DGK – diacylglycerolkinase; GABA – g-aminobutyric acid; GAD – glutamate decarboxylase; IP3 – inositol triphosphate; MAPK – mitogen-activatedprotein kinase; PA – phosphatidic acid; PI-PLC – phosphoinositide-specific phospholipase C; PLD – phospholipase D;SOS – salt-overly sensitiveFig. 1. Schematic representation of Ca2 -permeable channels,pumps and transporters that are proposed to be involvedin calcium signaling in response to abiotic stresses. Ca2 permeable channels (cylinders) can be regulated by voltage, either hyperpolarization (HAC) or depolarization(DAC) or ligands. The ligand-gated channels include IP3receptors (IP3-R), cADPR receptors (cADPR-R), glutamate receptors (GLR) and cyclic nucleotide-gated channels (CNGC). Genes encoding HAC, DAC, IP3-R andcADPR-R have not been identified in plants. Ca2 -pumpsand transporters (ovals) comprise ACA and ECA Ca2 ATPases, and the CAX Ca2 /H -antiporters. Biochemicaland electrophysiological evidence indicate the presenceof Ca2 transport systems involved in stress responses inthe mitochondria (MT) and the nucleus, but their molecular identity is not clear yet. Currently, there is no evidencefor the involvement of plastids (PL) in regulating abioticstress Ca2 signals. The estimated calcium concentrationis indicated for each cellular compartment (Pauly et al.2001; Reddy and Reddy 2004) (Adapted from Reddy andReddy 2004).II Calcium SignalsA Calcium SignaturesValuable tools have been developed to monitor[Ca2 ]cyt. Fluorescent dyes, like fluo-4, fura-2 andindo-1, allow single-cell calcium imaging, whereasthe calcium-sensitive luminescent protein aequorin
477Calcium Signaling in Abiotic Stress Responsescan be expressed in different cellular compartments(Knight et al. 1991; Reddy and Reddy 2004). Thecameleon probe, which is based on green fluorescent protein, has been adapted for plant systemsto provide non-invasive features and high calciumsensitivity (Allen et al. 1999). Using these tools,increase in [Ca2 ]cyt has been monitored in responseto many abiotic stresses in plants (Scrase-Field andKnight 2003; White and Broadley 2003). Calciumsignals are defined by kinetic parameters (amplitude, duration, frequency, lag time) and spatialfeatures (calcium origin and localization), and aparticular combination of these factors appears tobe specific to each stimulus (Table 1). The calcium response also depends on the strength of thestimulus, allowing a tight regulation of subsequentresponses (Pauly et al. 2001). The use of calciumchelators or inhibitors of calcium channels indicates that different calcium sources are involved,depending on the stimuli (White and Broadley2003). For example, similar calcium kineticsinduced by cold and touch result from differentcalcium sources and locations (Knight et al. 1991;Wood et al. 2000), which eventually contributesto response specificity. Furthermore, refractoryperiods, during which seedlings can still respondto other stimuli, have been described (Price et al.1994), further demonstrating that distinct signalsmobilize calcium from different stores. In additionto the cytosol, abiotic stresses also induce calciumelevation in other cellular compartments, including the nucleus and mitochondria (Subbaiah et al.1998; van der Luit et al. 1999; Pauly et al. 2001).Interestingly, the Ca2 signatures of organelles areindependent of the cytosolic Ca2 signals (Paulyet al. 2000; Logan and Knight 2003). Calciumsignatures are also cell type and organ-specific inresponse to various abiotic stresses (Kiegle et al.2000; White and Broadley 2003).B Role of Calcium SignaturesBecause calcium changes have been associated with various downstream physiologicalresponses to abiotic stresses (Reddy and Reddy2004), calcium signatures may be relevant forencoding specific information for proper adaptation to distinct conditions. For example, impairing calcium signals with chelators or channelinhibitors reduces plant tolerance to freezing(Monroy et al. 1993) and heat shock (Gong et al.1998), whereas calcium treatment increases plantsurvival. Although calcium has been proposedto act simply as a chemical switch (Scrase-Fieldand Knight 2003), several lines of evidence suggest that calcium signals can also carry specificinformation that distinguishes the various abioticstresses. For example, in tobacco seedlings, windand cold induce the expression of NpCaM-1 ina Ca2 -dependent manner. Although both stressesincrease Ca2 level in cytosol and nucleus,cytosolic calcium triggers NpCaM-1 induction bycold, whereas nuclear calcium is responsible forNpCaM-1 induction by wind (van der Luit et al.1999). Thus, calcium elevation in the same cellular compartment may display different functions,depending on the stimulus. Recently, artificialcytosolic calcium transients have been shown toinduce rapid transcriptome changes resemblingabscisic acid (ABA) responses in Arabidopsisseedlings, further demonstrating that a particularcalcium signal can induce specific gene expressionpatterns (Kaplan et al. 2006). Studies on stomatalregulation in guard cells also support a specificTable. 1. Calcium signatures in response to abiotic stresses.StimulusFeatures of the cytosolic calcium signalCold shockSlow coolingRapid and transient Ca peak (seconds)Bimodal Ca2 elevation (minutes)Hyperosmoticand salt stressHypoosmotic stressMechanical stressOxidative stressAnoxiaHeat shockSingle or biphasic Ca2 elevation (20–60 s)2 Rapid and bimodal Ca2 elevation (minutes)Rapid and transient Ca2 peak (seconds)Single Ca2 peak (minutes)Rapid and sustained Ca2 elevation (hours)Sustained Ca2 increase (15–30 min)References: See review Scrase-Field and Knight (2003), White and Broadley (2003).Calcium storesMainly externalExternal and internal (vacuole,IP3-dependent)External and internal (vacuole,IP3-dependent)External and internal (ER)InternalExternal and internalInternal (mitochondria)External and internal
78role of calcium signatures. In the det3 mutant,the altered calcium signal, induced by oxidativestress, fails to trigger stomatal closure, while calcium responses to cold and ABA are maintained.Artificially imposing the calcium oscillations,observed in wild-type plants, restores stomatalclosure in det3, indicating that the calcium signalitself carries the information that induces specificresponses (Allen et al. 2000). In addition, pretreatment of seedlings with a stimulus modifiescalcium signals induced by other stresses, suggesting that calcium may act as a memory signalto help adjust better to subsequent unfavorableconditions (White and Broadley 2003).C Calcium Channels, Pumpsand TransportersIncrease in [Ca2 ]cyt results from a combination ofcalcium influx into the cytosol via Ca2 -permeablechannels, according to the electrochemicalpotential, and calcium efflux out of the cytosolthrough energy-dependent calcium ATPasesand transporters (Fig. 1) (Sanders et al. 2002).Ca2 -permeable channels, which can be activated by hyper-polarization, depolarization orligand binding, such as glutamate, inositol triphosphate (IP3), cyclic ADP ribose (cADPR)and cyclic nucleotide monophosphate (cNMPs),have been found in many different plant membranes (White and Broadley 2003; Hetheringtonand Brownlee 2004). Although the molecularidentity of these channels is mostly unknown,their activities in response to abiotic stressesand the ability of the ligands to elicit calciumsignals have been well documented (White andBroadley 2003; Reddy and Reddy 2004; Peiteret al. 2005; Carpaneto et al. 2007). For example, IP3 and cADPR can induce calcium releasefrom the vacuole and trigger the induction ofstress-responsive genes such as RD29A (Wuet al. 1997; Xiong et al. 2002). The recent annotation and cloning of genes encoding putativecalcium channels provides important tools tostudy their involvement in generating calciumsignals (Sanders et al. 2002). The glutamatereceptor GLR3.3 mediates calcium entry intothe cytosol (Qi et al. 2006) and over-expressionof AtGluR2/GLR3.2 confers hypersensitivityto Na and K ions, but not to mannitol (Kimet al. 2001). Thus, AtGluR2/GLR3.2 may playM. Boudsocq and J. Sheena specific role in Ca2 -mediated adaptation toionic stresses. Recently, the Ca2 -sensing receptor CAS has been shown to control the Ca2 resting level and to regulate IP3 concentrations inArabidopsis (Tang et al. 2007). Cyclic-nucleotidegated channels (CNGCs), that are activatedby cNMPs, can conduct several types of cations, including calcium (Sanders et al. 2002;Lemtiri-Chlieh and Berkowitz 2004). However,the functional role of CAS and CNGCs in mediating abiotic stress signaling requires furtherinvestigation.Calcium efflux from the cytosol allows replenishment of internal and external stores (Fig. 1),and a return to resting calcium levels, which maycontribute to shaping the specific and distinct calcium signatures. Ca2 pumps, whose expressionis induced by salt stress, include the endoplasmicreticulum (ER)-type Ca2 -ATPases (ECA or typeIIA) and the auto-inhibited Ca2 -ATPases (ACA ortype IIB) (Fig. 1) (Geisler et al. 2000; Sze et al.2000). Interestingly, the Arabidopsis vacuolarACA4 restores growth on NaCl and mannitol ina mutant yeast strain, suggesting a positive roleof ACA4 in plant stress tolerance (Geisler et al.2000). Among the transporters, the vacuolarCa2 /H antiporter CAX1, which is induced bycold, has been shown to negatively regulate thecold-acclimation response in Arabidopsis byrepressing the expression of CBF/DREB1 genesand their downstream targets (Hirschi 1999;Catala et al. 2003).III Calcium Sensing and SignalingAny modification in the concentration of calciummust subsequently be decoded in the targetedcells and organs to induce appropriate responsesdepending on the stimulus. Calcium sensors havebeen divided into two groups: the sensor relays,including calmodulin (CaMs) and calcineurinB-like (CBLs) proteins, and the sensor proteinkinases, such as calcium-dependent protein kinases(CDPKs) as well as calcium and calmodulindependent protein kinases (CCaMKs). CaMsand CBLs do not possess any intrinsic activityand have to transmit the calcium-induced modification to target proteins, whereas CDPKs andCCaMKs are directly activated upon calciumbinding (Fig. 2).
4Calcium Signaling in Abiotic Stress ResponsesFig. 2. Domain structure of plant calcium sensors. (a) Sensorrelays and protein kinase partners. CaM and CBL are sensor relays that bind calcium through EF-hand motifs. CaMsubsequently regulates many different target proteins including protein kinases (CRKs), whereas CBLs mainly activateCIPKs by interacting with the FISL/NAF domain (CBL binding) to release auto-inhibition; (b) Sensor protein kinases: Incontrast to CRKs and CIPKs, the kinases CDPK and CCaMKcan directly bind calcium through their EF-hand motifs. As aresult, CDPKs function independently of other Ca2 sensorswhereas CCaMK activity can be further modulated by CaM(Adapted from Harper et al. 2004).A Sensor Relays1 Calmodulin and Calmodulin-LikeSensors1.1 Biochemical Functions and Regulationof CalmodulinCalmodulin is a small protein composed of twopairs of Ca2 binding sites named EF-hands (Luanet al. 2002). Calcium binding modifies the CaMglobular structure into an open conformationthat allows interaction with proteins (Yamniukand Vogel 2005). This interaction subsequentlyactivates (Lee et al. 2000) or inhibits (Choi et al.2005b; Yoo et al. 2005) CaM targets, translatinga calcium signal into a biochemical response.The Arabidopsis genome contains seven CaMgenes encoding four isoforms that differ by onlyone to four amino acids. In addition, Arabidopsiscontains 50 genes encoding CaM-like (CMLs)proteins with more divergent sequences andsometimes extra-domains that confer additionalproperties (McCormack and Braam 2003).Specificity of CaM-mediated responses results79from different expression patterns, specific targets,calcium affinities, sub-cellular localization andmethylation (Luan et al. 2002; McCormack andBraam 2003). CaM isoforms differ in their abilityto regulate target proteins (Lee et al., 2000; Yooet al. 2005), possibly due to different structuralinteractions of the targets with CaM (Yamniukand Vogel 2005). A recent protein array studyhas identified 173 protein targets of seven CaMs/CMLs in Arabidopsis. Among them, about 25%interact with all CaMs/CMLs tested, 50% with atleast two of them, and 25% are specific to oneCaM/CML (Popescu et al. 2007). CaMs sharingthe same targets can compete for binding (Leeet al. 1999), indicating that target proteins aretightly regulated depending on the amount of eachCaM isoform. Interestingly, a mutation convertingthree amino acids of rice OsCaM1 into those ofOsCaM61, confers an ability to activate OsCBKalmost as efficiently as OsCaM61 (Li et al. 2006).Thus, CaMs exhibit outstanding target specificities despite high levels of sequence identity. Different Ca2 sensitivities were observed dependingon CaM and target proteins, adding another layerof regulation (Lee et al. 2000; Luoni et al. 2006).CaMs also display multiple sub-cellular localizations (Yang and Poovaiah 2003). Interestingly,the petunia CaM53 and rice OsCaM61 are targeted to membranes or the nucleus dependingon their prenylation status (Luan et al. 2002).Finally, CaM methylation may be a specific regulatory mechanism for a s
Stress Signaling II: Calcium Sensing and Signaling Marie Boudsocq and Jen Sheen* Department of Molecular Biology, Massachusetts General Hospital & Department of Genetics, Harvard Medical School, Boston, MA 02114, USA Summary Calcium is an essential second
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