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1 Plasma Membrane Abiotic Stress Sensing

There is abundant information regarding plant abiotic stress-inducible signaling cascades and gene expression, but much less is known about plant abiotic stress sensors. Abiotic stress sensors in bacteria, cyanobacteria, and the yeast Saccharomyces cereviseae are primarily made up of receptor kinase two- or multicomponent systems consisting of a sensory histidine kinase linked directly or indirectly via a phosphorelay molecule to a response regulator (Murata and Los 2006; Lohrmann and Harter 2002). In some particular cases, the response regulator may also function directly as a transcription factor, thus eliminating the requirement for intermediate signaling cascades (Lohrmann and Harter 2002).

In Arabidopsis thaliana, accumulating evidence suggests that two members of the histidine kinase family are involved in osmosensing: AHK1, a protein that detects changes in water status and desiccation (Wohlbach et al. 2008), and CRE1/AHK4, which responds to hyperosmotic stress-related changes in turgor pressure in the presence of cytokinin. Both putative sensors complement a deletion mutant of the yeast osmosensing histidine kinase SLN1 (Reiser et al. 2003; Urao et al. 1999), and AHK1 can act as a positive regulator in osmotic stress signaling in Arabidopsis plants (Tran et al. 2007). While the assumption can be made that these sensors are PM-localized, it has not been confirmed experimentally, and neither protein has appeared in PM proteomic analyses. Moreover, AHK5, a related histidine kinase that responds to H2O2, has been shown to localize to both the cytoplasm and the PM in Arabidopsis (Desikan et al. 2008), and recently, the histidine kinase subfamily of ETR ethylene receptors were shown to be targeted to the endomembrane network and did not localize to the PM (Grefen et al. 2008). Two other related histidine kinase members have also been implicated in abiotic stress sensing; AHK2 was shown to be regulated by abscisic acid (ABA), osmotic stress, and salt stress, while AHK3 was shown to be induced by salt and cold stress (Wohlbach et al. 2008).

Other possible abiotic stress sensors at the plant PM are the receptor-like kinases, an unusually large family of proteins that consist of an extracellular receptor domain, a single transmembrane domain, and an intracellular serine/threonine kinase domain. While expression of the genes encoding these kinases has been shown to be regulated by different abiotic stress signals, there is no evidence of a direct role of these kinases in environmental sensing (Tamura et al. 2003). Rather, the evidence points to a more likely role in hormone signaling and host–pathogen recognition (Morris and Walker 2003).

Sensing of abiotic stress may not be limited to protein kinases. In yeast and mammals, PM proteins have been identified, which not only function as transporters but also act as sensors and signal relay proteins (Özcan et al. 1998; Diez-Sampedro et al. 2003). Interestingly, these proteins have unusually long C-terminal cytoplasmic domains, a trait shared by the Arabidopsis PM Na+/H+ exchanger SOS1/NHX7, which has been proposed to be an Na+ sensor (Zhu 2002). However, as of yet, a sensor function has not been clearly demonstrated for any of these proteins. Evidence that SOS1 is a downstream target of a CBL–CIPK calcium-sensing network (see below) suggests that SOS1 is not likely to function as a primary salt stress sensor.

Some insights into the role of the SOS1 C-terminus have come from a yeast two-hybrid screen which detected that the cytoplasmic tail portion binds RCD1/CEO1 (Katiyar-Agarwal et al. 2006), a salt and oxidative stress-responsive nuclear protein that interacts with transcription factors such as STO and DREB2A (Belles-Boix et al. 2000). Under stress conditions, RCD1/CEO1 localizes to both the nucleus and cytoplasm, thus suggesting it may relay a signal between the two compartments (Katiyar-Agarwal et al. 2006).

Cold, drought, and salt stress can induce production of the phytohormone/plant growth regulator ABA, which itself may be sensed at the PM. Candidate PM proteins for sensing and relaying the ABA signal are two proteins, AtGTG1 and AtGTG2 (Pandey et al. 2009). While these proteins show homology to a human Golgi-localized anion channel (Maeda et al. 2008), they share some properties of G protein-coupled receptors (GPCRs), as both proteins interacted with AtGPA1, a G-protein α subunit, itself localized to the PM by a lipid anchor (Temple and Jones 2007). As would be expected for loss of ABA sensing, mutants lacking both proteins were shown to be hyposensitive to ABA and impaired in upstream ABA signaling responses. It was reported that AtGTG1 and AtGTG2 bind ABA with high affinity (Pandey et al. 2009); however, a reevaluation of ABA-binding assays cautions the interpretation of these studies under conditions of suboptimal protein purification (Risk et al. 2009). The proteins are widely expressed in most tissues, although neither AtGTG1 nor AtGTG2 are transcriptionally regulated by abiotic stress (Pandey et al. 2009). ABA perception at the PM in turn regulates a wide variety of plant physiological and developmental processes that have been well-studied, especially as they pertain to guard cell physiology. These include the production of reactive oxygen species (ROS), increases in cytosolic free Ca2+ levels ([Ca2+]i), protein kinase cascades, activation and inhibition of PM channels, including anion channels, and K+ influx and K+ efflux channels; some of which are PM delimited events (Fan et al. 2004).

Perception of most abiotic stresses results in the generation of calcium signals within the plant cell, which, in turn, elicit distinct concentration-dependent responses that depend on the duration, oscillation, and location of the calcium signal (McAinsh and Pittman 2009). In plants, the calcineurin B-like protein (CBL) family represents a unique group of small calcium-binding proteins that act as calcium sensors. They play a key role in decoding calcium transients by interacting with and regulating members of a family of serine/threonine protein kinases known as CBL-interacting protein kinases (CIPKs) and generating a localized signaling cascade that results in phosphorylation of specific target proteins (Batistic and Kudla 2009; Luan 2009). Unique calcium signatures are obtained by the programmed mixing and matching of CBL proteins with CIPKs at specific locations throughout the cell, which include the PM (Kim et al. 2000). PM localization of particular CBL proteins occurs via lipid modifications, as has been demonstrated for both CBL4 and CBL1 (Ishitani et al. 2000; Batistic et al. 2008). Loss-of-function studies of PM CBLs highlight their role in plant abiotic stress tolerance. Disruption of CBL4/SOS3 in Arabidopsis results in plants that are hypersensitive to salt (Shi et al. 1999), disruption of CBL1 results in plants that are hypersensitive to drought, high salt, and hyperosmotic stress (Albrecht et al. 2003; Cheong et al. 2003), and disruption of CBL9 results in plants that are hypersensitive to ABA, salt, and osmotic stress (Pandey et al. 2004). Expression profiling also shows the importance of these sensors in abiotic stress response. In rice, expression analysis of ten CBL family members identified only two members, which showed no changes under any stress conditions, with most showing regulation by multiple stresses, and many containing stress-related cis-elements in their promotor (Gu et al. 2008).

2 PM Targets of Abiotic Stress Signals

Many targets of stress signaling mechanisms are localized to the PM. These PM delimited events mediate regulation of membrane-resident channels, transporters, pumps, and other target proteins. Recruitment of CIPK family members to the PM is thought to be orchestrated by specific CBL proteins, as the CIPK protein has no membrane spanning domains and no recognizable lipid modification sites (Batistic et al. 2008). Rather, a C-terminal regulatory domain of CIPK contains a 24-amino acid conserved NAF/FISL motif that mediates its interaction with the CBL protein by binding to a hydrophobic crevice (Akaboshi et al. 2008). Moreover, this domain exerts autoinhibitory effects on the kinase activity of the CIPK protein when in the unbound formation (Guo et al. 2001). CBL binding and subsequent targeting of CIPK’s to the PM allows for activation of the CIPK and subsequent phosphorylation of membrane-associated substrates. Many members of the CIPK family have been shown to be regulated at the level of transcription by one or more abiotic stress condition (Pandey 2008). In rice, of the 27 CIPK genes detected, 20 are regulated by salt, osmotic stress, low temperature, ABA, nutrient deficiency, or a combination of these (Xiang et al. 2007).

The physiological roles of several CBL–CIPK-target networks at the PM have been established in Arabidopsis, and these have been implicated in abiotic stress tolerance. The first elucidation of a CBL–CIPK-target interaction in plants was obtained from a genetic screen for salt overly sensitive mutants in Arabidopsis (Zhu et al. 1998) and resulted in the cloning of three genes (SOS1, SOS2, and SOS3), which are all required for salt tolerance. Analysis of these genes determined that the interaction of CBL4/SOS3 with CIPK24/SOS2 directed the kinase to the PM where it was shown to stimulate the activity of the Na+/H+ exchanger NHX7/SOS1 resulting in removal of excess cytoplasmic Na+ (Qiu et al. 2002). Reconstitution of the pathway in yeast confirmed that CIPK24 phosphorylated the NHX7/SOS1 protein in vitro (Quintero et al. 2002), and a phosphoproteomic study of the PM identified a putative phosphorylation site on the long hydrophilic C-terminal of the NHX7/SOS1 protein (Nühse et al. 2004).

The PM K+-uptake channel AKT1 has also been shown to be a target protein for phosphorylation by a CBL/CIPK signaling network involved in regulating the protein under K+-deficiency. Through combined genetic, biochemical, and electrophysiological studies, it was shown that two CBL proteins, CBL1 and CBL9, interacted independently with CIPK23 at the PM to phosphorylate and activate inward K+ currents through AKT1 (Xu et al. 2006). This multistep signaling cascade also includes the 2C-type protein phosphatase AIP1, which acts as a signaling terminator in conjunction with CIPK23 to dephosphorylate the AKT1 protein and thereby deactivate the K+-channel (Lee et al. 2007). Both cipk23 and cbl1/cbl9 double mutants are sensitive to low K+, while, conversely, overexpression of CIPK23, CBL1, or CBL9 resulted in plants that accumulated significantly more K+ than wild type and were hypertolerant to K+ deficiency (Xu et al. 2006). Moreover, CIPK23 expression increased under low K+ growth conditions, although this was not seen with either CBL1 or CBL9 (Xu et al. 2006; Cheong et al. 2007). Interestingly, cipk23 mutants are drought-tolerant and show reduced transpirational water loss and ABA hypersensitivity (Cheong et al. 2007). The complexity of this network was further demonstrated by Lee et al. (2007), who showed that AKT1 interacts with two additional CIPKs, CIPK6 and CIPK16, in addition to CIPK23, and that interaction with these CIPKs results in activation of inward K+ currents, albeit with reduced magnitude. No physiological function has been ascribed to the binding of CIPK6 or CIPK16 to the AKT1 to date.

A third, well characterized CBL–CIPK-target network localized to the PM is one resulting in the phosphorylation and subsequent negative regulation of an Arabidopsis P-type H+-ATPase, AHA2, which results in perturbations of apoplastic pH (Fuglsang et al. 2007). CBL2/SCaBP1 and CIPK11/PKS5 have been implicated as upstream mediators of this signaling cascade. Studies established that CIPK11/PKS5 phosphorylated AHA2 in vitro, with the putative phosphorylation site localized to a serine residue in the C-terminal. Yeast reconstitution of the CBL2–CIPK11–AHA2 pathway confirmed the phosphorylation and negative regulation of AHA2 and showed that phosphorylation-dependent changes in AHA2 structure destabilized the binding of a 14-3-3 protein (Fuglsang et al. 2007). However, phosphoproteomics studies have not detected an AHA2 phosphopeptide (Nühse et al. 2004), to corroborate these in vitro studies. Also, while differential regulation of CIPK11/PKS5 transcripts have been observed following treatment with NaCl, drought, ABA, and glucose, cipk11/pks5 mutant plants did not show any phenotypic defect under these conditions (Fuglsang et al. 2007), making it difficult to assign a physiological role in abiotic stress tolerance to the signaling pathway. Nevertheless, some insight can be gained from the observation that cipk11/pks5 mutants are more tolerant to high extracellular pH due to the ability of the deregulated AHA2 to acidify the extracellular space (Fuglsang et al. 2007).

PM transporters could function directly as relay mechanisms or effectors during the adaptive response of plant cells to stress conditions, mainly in cases where ions or metabolites must be transported across the membrane in order to maintain cell homeostasis and to remove toxic ions. One well-studied, but still controversial, mechanism implicated in adaptation to salinity stress is mediated by HKT transporters that have been variously identified as K+ and/or Na+ transport systems at the PM (Schachtman and Schroeder 1994; Rubio et al. 1995; Horie et al. 2001; Golldack et al. 2002; Rodríguez-Navarro and Rubio 2006). These transporters belong to a small family with only one member in Arabidopsis and nine members in rice (Garciadeblás et al. 2003). Initial characterization of HKT1;1 from Arabidopsis and HKT2;1 from rice demonstrated a large Na+-selective transport activity relative to K+ when expressed in yeast and Xenopus oocytes (Uozumi et al. 2000; Horie et al. 2001; Garciadeblás et al. 2003). However, Golldack et al. (2002) showed that HKT2;1 is competent in the transport of both K+ and Na+. More recent studies have demonstrated that HKT1;1 from Arabidopsis and its closest homolog SKC1 (HKT1;5) in rice function by removing Na+ from the xylem sap in the root, thus reducing both long distance Na+ transport and Na+ accumulation in the leaves (Ren et al. 2005; Sunarpi et al. 2005; Horie et al. 2007). SKC1 is linked with a QTL in rice that is associated with the maintenance of K+ content in the shoot under salt stress conditions (Ren et al. 2005). The SKC1/HKT1;5 locus encodes a functional Na+-selective transporter, whose activity was shown to be K+-independent. Contrary to expectations, the isoform encoded by the NSKC1 gene from the salt-resistant Nona Bokra rice variety was more active than KSKC1 from salt-sensitive Koshihikari variety (Ren et al. 2005). This difference in transport activity was attributed to four amino acid changes that exist in the sequence between the two rice varieties (Ren et al. 2005). GUS expression analyses demonstrated that SKC1 was mainly present in the xylem parenchyma cells surrounding the xylem vessels, where it could serve in the removal of Na+ from the xylem sap. Consistent with these results and supporting a direct role of HKT transporters in salt tolerance, Ren et al. (2005) reported lower levels of Na+ in the xylem and shoot tissues upon salt stress in salt-tolerant Nona Bokra as compared to the salt-sensitive Koshihikari. Similar observations have been made with the homologous HKT1;1 transporter from Arabidopsis (Sunarpi et al. 2005). HKT1;1 is expressed mainly in the cells surrounding the xylem vessels, with higher expression observed in the root than in the shoot. Analyses of three different mutants of HKT1, athkt1-3 in the Wassilewskija (Ws) background and athkt1-1, athkt1 (disruption allele FN1148; Gong et al. 2004) in the Columbia background, showed clear increases in the Na+ content of the xylem sap under control and Na+ stress conditions, indicating a role of AtHKT1;1 in lowering xylem Na+ content (Sunarpi et al. 2005). Associated with these changes, Sunarpi et al. (2005) also reported a decrease in Na+ levels in the phloem sap of the three different hkt1;1 mutants, thus confirming an earlier observation reported by Berthomieu et al. (2003) for a partial loss-of-function hkt1;1 allele, sas 2-1.

The decrease in Na+ phloem content observed in the different hkt1;1 mutants has been used to postulate that Arabidopsis HKT1;1 participates in the recirculation of Na+ from the shoot to the root via the phloem, thus decreasing the toxic effects of Na+ in shoot tissues. Further confirmation of the role of HKT1;1 in xylem Na+ removal was provided by its localization to the PM of leaf xylem parenchyma cells by immunoelectron microscopy. This confirmed previous work that showed PM localization of Mesembryanthemum crystallinum HKT1;1 (Su et al. 2003). However, as expression of Arabidopsis HKT1;1 was induced by 30mM NaCl or KCl and equimolar concentrations of mannitol or sorbitol, HKT1 transporters may also play an additional role in osmotic adaptation (Sunarpi et al. 2005).

3 Stomatal Guard Cells and Stress Responses at the PM

Studies of stomatal guard cell physiology have played a central role in efforts to identify stress-responsive adaptive mechanisms at the PM, as guard cells play a central role in the control of transpiration and water loss under conditions of drought, heat, salinity, and cold. In a mutant screen for ozone sensitivity, Vahisalu et al. (2008) identified the mutant slac1-1 that showed constitutively higher stomatal conductance than the wild type. Contemporaneously, Negi et al. (2008) isolated the allelic cdi3/slac1-2 mutant, which is impaired in CO2-dependent leaf temperature changes. SLAC1 encodes a membrane protein with ten predicted transmembrane helices and hydrophilic amino- and carboxy-terminals. SLAC1 is also predicted to possess a C4-dicarboxylate/malic acid transport protein domain, similar to that identified in the TehA and Mae1 proteins from Escherichia coli and Schizosaccharomyces pombe, respectively (Guzzo and Dubow 2000; Grobler et al. 1995). SLAC1 promoter GUS fusions showed the gene to be primarily expressed in guard cells, and GFP fusion studies demonstrated that it was localized to the PM (Vahisalu et al. 2008; Negi et al. 2008). Information on the participation of SLAC1 in stomatal control was gained from studies in slac1-1 plants, demonstrating alterations in stomatal conductance and stomatal closing upon transfer to the dark or upon exposure to low relative air humidity conditions, and a lack of response to large increases in atmospheric CO2 (Vahisalu et al. 2008). Similar results were also reported for the slac1-2 mutant (Negi et al. 2008). Moreover, slac1-1 guard cells were insensitive to ABA and downstream second messengers such as H2O2, NO, and oscillating cytoplasmic Ca2+.

Whole cell patch-clamp experiments demonstrated that slac1-1 guard cells lack the activity of the so-called S-type anion currents, which were clearly activated by μM levels of Ca2+ in wild type. From these results, it appears that SLAC1 plays a central role in stomatal closing under adverse environmental conditions (Vahisalu et al. 2008), although it is still not clear if this protein corresponds to the actual anion channel or to a regulatory subunit of the channel complex. In Arabidopsis, there are three other SLAC1-like genes (40–50% identity at the amino acid level) that encode proteins localized to the PM, but none of the genes are expressed in guard cells (Negi et al. 2008). However, two of the SLAC1 homologs, At1g62280 and At5g24030, complemented the CO2-insensitive phenotype of the slac1-2 mutant when expressed under the control of the SLAC1 guard cell-specific promoter. These findings indicate a conserved function despite tissue-specific expression. These studies highlight the importance of PM ion transport processes as direct adaptive mechanisms that mediate plant response to stress.

4 Membrane-Bound Transcription Factor and Response to Abiotic Stress

Another abiotic stress signaling target at the PM could be the recently described membrane-bound transcription factors (MBTF’s). Like other transcription factors, MBTFs are proteins that regulate the expression of downstream genes in response pathways and are latent until activation by internal or environmental signals. Signal-induced activation of transcription factors generally leads to a rapid transcriptional response. Until recently, it was assumed that transcription factors are present as cytosolic pools and that entry to the nucleus is mediated by protein–protein interactions, resulting in their phosphorylation (Leonard and O’Shea 1998) or release from an associated protein (e.g. NF-κB, IκB) (Ghosh et al. 1998). However, MBTFs are membrane-bound proteins that are inactive until undergoing a proteolytic step, which releases them from the resident membrane and allows their entry into the nucleus to modulate gene expression (Kim et al. 2006, 2007a, b; Chen et al. 2008; Yoon et al. 2008). Selective proteolysis of MBTFs to release the trans-acting DNA-binding domain from the membrane anchor adds another level of posttranslational regulation that can respond to abiotic stress at the PM.

As the role of MBTFs in Arabidopsis has been recently been reviewed (Chen et al. 2008), here we only describe the work that has demonstrated the association of stress-related MBTFs to the PM.

Structural analysis of NAC-type transcription factors was used to identify more than 13 members in Arabidopsis and 6 members in rice that possess strong α-helical domains suggestive of MBTF activity. These were designated NTLs for NTM1-like (Kim et al. 2007a), in reference to the well characterized intracellular membrane-associated NTM1 MTBF (Kim et al. 2006). Expression of most NTLs was detected in leaves, stems, and in roots, with apparently lower expression levels seen in flower and shoot apex tissues (Kim et al. 2006). Eleven NTLs were found to be differentially regulated by abiotic stress; NTL1 and NTL11 were found to be regulated by heat, and NTL4 and NTL7 by cold. NTL3, NTL6, and NTL8 were strongly upregulated by NaCl, NTL9 expression increased with osmotic stress, NTL5 was upregulated by drought and NaCl, and expression of NTL2 and NTL3 were influenced by cold, drought, and NaCl. Together, these responses indicate that individual NTLs play distinct roles in responses to abiotic stress conditions. However, NTL expression in response to hydrogen peroxide was low, suggesting that NTLs are not directly associated to responses to ROS (Kim et al. 2007a).

Evidence for the PM localization of NTL8 was demonstrated by GFP fusions to both full length NTL8 or to a truncated version of the protein (Kim et al. 2007a). The full length NTL8 GFP fusion was found to be associated with the PM, while the fusion lacking the NTL8 membrane anchor was detected in the nucleus, in agreement to the NTM1-like nature of NTL8 (Kim et al. 2007b). Overexpression of the truncated form of NTL8 caused a delay in flowering time, associated with a downregulation of specific flowering time genes including FLOWERING LOCUS T (FT), FRUITFUL (FUL), and CAULIFLOWER (CAL) (Kim et al. 2007b). Additionally, FT transcript levels were significantly reduced in plants exposed to NaCl, indicating that high salt delayed flowering by repressing FT expression via signaling mediated by NTL8. However, ntl8-1 mutants exhibited only slight differences from wild type in lateral root growth and flowering times (Kim et al. 2007b). Expression of NTL8 was also shown to be developmentally regulated, with maximal expression observed in germinating seeds (Kim et al. 2007b). A possible role of NTL8 in germination under salt stress was indicated by an observed decrease in the germination rate of seeds overexpressing the truncated form of NTL8, while germination rate increased in the ntl8 mutant (Kim et al. 2008).

Another well-studied MBTF of the NTL family, NTL9, is also localized to the PM and appears to be involved in abiotic stress gene regulation (Yoon et al. 2008). Plants overexpressing full length NTL9 did not show a clear phenotype, which contrasted to the dwarf phenotype with small and curled leaves often associated with abiotic stress that was observed in plants overexpressing NTL9 lacking the membrane anchor (Yoon et al. 2008). Increased expression of the stress-related genes SIAH-Interacting Protein (SIP) and Cold-Regulated 15a (COR15a) in plants overexpressing the truncated form of NTL9 further implicated NTL9 in stress responses (Yoon et al. 2008) as did evidence of posttranscriptional processing of NTL9 and increased abundance of both full length and truncated NTL9 with osmotic stress (Yoon et al. 2008).

5 Other Posttranslational Regulation of PM Proteins

Evidence exists for posttranslational regulation of other plant PM proteins in response to abiotic stress, although the associated upstream signaling is poorly characterized. The turnover of populations of integral PM proteins by trafficking to and from the membrane can be regulated by a variety of environmental stimuli. In the case of the KAT1 inward rectifying K+ channel, ABA induces the rapid and transient internalization of the channel to an endomembrane compartment from which it can be recycled to the PM upon removal of ABA (Sutter et al. 2007). The soluble N-ethylmaleimide-sensitive factor protein attachment protein receptor (SNARE) protein, SYP121, has been proposed to mediate the process (Grefen and Blatt 2009). It has been suggested that this trafficking is a posttranslational adaptive mechanism to allow rapid changes in osmotic solute flux required in response to abiotic stress.

Ubiquitination can also be induced by abiotic stress and alter the turnover and subcellular localization of PM target proteins such as the Arabidopsis aquaporin PIP2;1 (Lee et al. 2009). Overexpression of Arabidopsis RMA1, an ER-localized homolog of a RING membrane anchor E3 ubiquitin ligase, was shown to reduce PIP2;1 protein levels at the PM and inhibited trafficking of PIP2;1 from the ER in protoplasts. In addition, protoplasts from RNA interference (RNAi) plants with reduced expression of RMA1 showed increased levels of PIP2;1 (Lee et al. 2009). However, increased water permeability was not seen in these protoplasts, and compensatory expression of other PIP genes was not investigated. RMA1-mediated ubiquitination was further implicated in abiotic stress responses by the finding that Capsicum annuum plants exposed to cold, high salinity, and drought stress (but not ABA) showed rapid induction of transcripts for Rma1H1, and overexpression of the CaRMA1H1 gene in Arabidopsis resulted in plants that exhibited an increased tolerance to drought (Lee et al. 2009).

6 Abiotic Stress and Effects on PM Integrity

Exposure to abiotic stress can result in damage to PM lipids resulting in lesions and membrane leakiness, particularly when osmotic or cold stress factors are involved. Damaged PMs are thought to be repaired by fusion of exocytotic vesicles triggered by a rapid elevation in cytoplasmic Ca2+ resulting from entry through the damaged membrane and mediated by the calcium-sensing synaptotagmin protein functioning in parallel with SNARE proteins (Andrews 2005). In a proteomic study of PM proteins from Arabidopsis leaves induced by cold stress, a synaptotagmin protein was identified by its increased abundance (Kawamura and Uemura 2003). Subsequently, it was shown that loss-of-function mutations in Synaptotagmin1 (SYT1) exhibited reduced cellular viability as a consequence of compromised PM integrity and consequent increases in electrolyte leakage and hypersensitivity to NaCl, high osmotic stress, and freezing (Schapire et al. 2008). Further support for the role of this SYT1 in abiotic stress tolerance came from independent studies demonstrating that protoplasts or leaves from SYT1-RNA interference (RNAi) plants were freezing-sensitive even in the presence of calcium (Yamazaki et al. 2008).

7 Conclusion

It is evident that plants employ diverse mechanisms, many of which are localized to the PM; to contend with the adverse environmental conditions, they are continuously exposed. The PM not only functions as a protection barrier or interface involved in the maintenance of ionic and metabolite gradients, but it is directly involved in sensing the diversity of external signals, in the transduction of these signals, and in activating mechanisms that help maintain the cellular homeostasis that is disrupted by the imposition of multiple stress conditions.