Proteomics of calcium-signaling components in plants

An Arabidopsis mutant impaired in intracellular calcium elevation is sensitive to biotic and abiotic stress

BACKGROUND

Ca2+, a versatile intracellular second messenger in various signaling pathways, initiates many responses involved in growth, defense and tolerance to biotic and abiotic stress. Endogenous and exogenous signals induce cytoplasmic Ca2+ ([Ca2+]cyt) elevation, which are responsible for the appropriate downstream responses.

RESULTS

Here we report on an ethyl-methane sulfonate-mediated Arabidopsis mutant that fails to induce [Ca2+]cyt elevation in response to exudate preparations from the pathogenic mibrobes Alternaria brassicae, Rhizoctonia solani, Phytophthora parasitica var. nicotianae and Agrobacterium tumefaciens. The cytoplasmic Ca2+elevation mutant1 (cycam1) is susceptible to infections by A. brassicae, its toxin preparation and sensitive to abiotic stress such as drought and salt. It accumulates high levels of reactive oxygen species and contains elevated salicylic acid, abscisic acid and bioactive jasmonic acid iso-leucine levels. Reactive oxygen species- and phytohormone-related genes are higher in A. brassicae-treated wild-type and mutant seedlings. Depending on the analysed response, the elevated levels of defense-related compounds are either caused by the cycam mutation and are promoted by the pathogen, or they are mainly due to the pathogen infection or application of pathogen-associated molecular patterns. Furthermore, cycam1 shows altered responses to abscisic acid treatments: the hormone inhibits germination and growth of the mutant.

CONCLUSIONS

We isolated an Arabidopsis mutant which fails to induce [Ca2+]cyt elevation in response to exudate preparations from various microbes. The higher susceptibility of the mutant to pathogen infections correlates with the higher accumulation of defense-related compounds, such as phytohormones, reactive oxygen-species, defense-related mRNA levels and secondary metabolites. Therefore, CYCAM1 couples [Ca2+]cyt elevation to biotic, abiotic and oxidative stress responses.

The transcriptional network of WRKY53 in cereals links oxidative responses to biotic and abiotic stress inputs

The transcription factor WRKY53 is expressed during biotic and abiotic stress responses in cereals, but little is currently known about its regulation, structure and downstream targets. We sequenced the wheat ortholog TaWRKY53 and its promoter region, which revealed extensive similarity in gene architecture and cis-acting regulatory elements to the rice ortholog OsWRKY53, including the presence of stress-responsive abscisic acid-responsive elements (ABRE) motifs and GCC-boxes. Four proteins interacted with the WRKY53 promoter in yeast one-hybrid assays, suggesting that this gene can receive inputs from diverse stress-related pathways such as calcium signalling and senescence, and environmental cues such as drought and ultraviolet radiation. The Ser/Thr receptor kinase ORK10/LRK10 and the apoplastic peroxidase POC1 are two downstream targets for regulation by the WRKY53 transcription factor, predicted based on the presence of W-box motifs in their promoters and coregulation with WRKY53, and verified by electrophoretic mobility shift assay (EMSA). Both ORK10/LRK10 and POC1 are upregulated during cereal responses to pathogens and aphids and important components of the oxidative burst during the hypersensitive response. Taken with our yeast two-hybrid assay which identified a strong protein-protein interaction between microsomal glutathione S-transferase 3 and WRKY53, this implies that the WRKY53 transcriptional network regulates oxidative responses to a wide array of stresses.

Whole genome transcriptome analysis of rice seedling reveals alterations in Ca(2+) ion signaling and homeostasis in response to Ca(2+) deficiency

Ca(2+) is an essential inorganic macronutrient, involved in regulating major physiological processes in plants. It has been well established as a second messenger and is predominantly stored in the cell wall, endoplasmic reticulum, mitochondria and vacuoles. In the cytosol, the concentration of this ion is maintained at nano-molar range. Upon requirement, Ca(2+) is released from intra-cellular as well as extracellular compartments such as organelles and cell wall. In this study, we report for the first time, a whole genome transcriptome response to short (5 D) and long (14 D) term Ca(2+) starvation and restoration in rice. Our results manifest that short and long term Ca(2+) starvation involves a very different response in gene expression with respect to both the number and function of genes involved. A larger number of genes were up- or down-regulated after 14 D (5588 genes) than after 5 D (798 genes) of Ca(2+) starvation. The functional classification of these genes indicated their connection with various metabolic pathways, ion transport, signal transduction, transcriptional regulation, and other processes related to growth and development. The results obtained here will enable to understand how changes in Ca(2+) concentration or availability are interpreted into adaptive responses in plants.

Overexpression of GmCaM4 in soybean enhances resistance to pathogens and tolerance to salt stress

Plant diseases inflict heavy losses on soybean yield, necessitating an understanding of the molecular mechanisms underlying biotic/abiotic stress responses. Ca(2) (+) is an important universal messenger, and protein sensors, prominently calmodulins (CaMs), recognize cellular changes in Ca(2) (+) in response to diverse signals. Because the development of stable transgenic soybeans is laborious and time consuming, we used the Bean pod mottle virus (BPMV)-based vector for rapid and efficient protein expression and gene silencing. The present study focuses on the functional roles of the gene encoding the soybean CaM isoform GmCaM4. Overexpression of GmCaM4 in soybean resulted in enhanced resistance to three plant pathogens and increased tolerance to high salt conditions. To gain an understanding of the underlying mechanisms, we examined the potential defence pathways involved. Our studies revealed activation/increased expression levels of pathogenesis-related (PR) genes in GmCaM4-overexpressing plants and the accumulation of jasmonic acid (JA). Silencing of GmCaM4, however, markedly repressed the expression of PR genes. We confirmed the in vivo interaction between GmCaM4 and the CaM binding transcription factor Myb2, which regulates the expression of salt-responsive genes, using the yeast two-hybrid (Y2H) system and bimolecular fluorescence complementation assays. GmCaM4 and Glycine max CaM binding receptor-like kinase (GmCBRLK) did not interact in the Y2H assays, but the interaction between GmCaM2 and GmCBRLK was confirmed. Thus, a GmCaM2-GmCBRLK-mediated salt tolerance mechanism, similar to that reported in Glycine soja, may also be functional in soybean. Confocal microscopy showed subcellular localization of the green fluorescent protein (GFP)-GmCaM4 fusion protein in the nucleus and cytoplasm.

Plant innate immunity: an updated insight into defense mechanism

Plants are invaded by an array of pathogens of which only a few succeed in causing disease. The attack by others is countered by a sophisticated immune system possessed by the plants. The plant immune system is broadly divided into two, viz. microbial-associated molecular-patterns-triggered immunity (MTI) and effector-triggered immunity (ETI). MTI confers basal resistance, while ETI confers durable resistance, often resulting in hypersensitive response. Plants also possess systemic acquired resistance (SAR), which provides long-term defense against a broad-spectrum of pathogens. Salicylic-acid-mediated systemic acquired immunity provokes the defense response throughout the plant system during pathogen infection at a particular site. Trans-generational immune priming allows the plant to heritably shield their progeny towards pathogens previously encountered. Plants circumvent the viral infection through RNA interference phenomena by utilizing small RNAs. This review summarizes the molecular mechanisms of plant immune system, and the latest breakthroughs reported in plant defense. We discuss the plant–pathogen interactions and integrated defense responses in the context of presenting an integral understanding in plant molecular immunity.

The Arabidopsis calmodulin-like protein, CML39, functions during early seedling establishment

During Ca(2+) signal transduction, Ca(2+)-binding proteins known as Ca(2+) sensors function to decode stimulus-specific Ca(2+) signals into downstream responses. Plants possess extended families of unique Ca(2+) sensors termed calmodulin-like proteins (CMLs) whose cellular roles are not well understood. CML39 encodes a predicted Ca(2+) sensor whose expression is strongly increased in response to diverse external stimuli. In the present study, we explored the biochemical properties of recombinant CML39, and used a reverse genetics approach to investigate its physiological role. Our data indicate that Ca(2+) binding by CML39 induces a conformational change in the protein that results in an increase in exposed-surface hydrophobicity, a property that is consistent with its predicted function as a Ca(2+) sensor. Loss-of-function cml39 mutants resemble wild-type plants under normal growth conditions but exhibit persistent arrest at the seedling stage if grown in the absence of sucrose or other metabolizable carbon sources. Under short-day conditions, cml39 mutants display increased sucrose-induced hypocotyl elongation. When grown in the dark, cml39 mutants show impaired hypocotyl elongation in the absence of sucrose. Promoter-reporter data indicate that CML39 expression is prominent in the apical hook in dark-grown seedlings. Collectively, our data suggest that CML39 functions in Arabidopsis as a Ca(2+) sensor that plays an important role in the transduction of light signals that promote seedling establishment.

Bioengineering for salinity tolerance in plants: state of the art

Genetic engineering of plants for abiotic stress tolerance is a challenging task because of its multifarious nature. Comprehensive studies for developing abiotic stress tolerance are in progress, involving genes from different pathways including osmolyte synthesis, ion homeostasis, antioxidative pathways, and regulatory genes. In the last decade, several attempts have been made to substantiate the role of "single-function" gene(s) as well as transcription factor(s) for abiotic stress tolerance. Since, the abiotic stress tolerance is multigenic in nature, therefore, the recent trend is shifting towards genetic transformation of multiple genes or transcription factors. A large number of crop plants are being engineered by abiotic stress tolerant genes and have shown the stress tolerance mostly at laboratory level. This review presents a mechanistic view of different pathways and emphasizes the function of different genes in conferring salt tolerance by genetic engineering approach. It also highlights the details of successes achieved in developing salt tolerance in plants thus far.

Individual and combined effects of CaCl₂ and UV-C on the biosynthesis of resveratrols in grape leaves and berry skins

The individual and combined effects of calcium chloride (CaCl2) and ultraviolet C (UV-C) light on the synthesis of resveratrol in grape leaves and berry skins were investigated. Results showed that all treatments could increase leaf resveratrol contents at least about 5 times, but the combination treatment was the most efficient. Moreover, compared with UV-C treatment, the combination treatment delayed the decline of resveratrol contents. The expression levels of phenylalanine ammonia lyase (PAL), cinnamate-4-hydroxylase (C4H), coumaroyl-CoA ligase (4CL), and stilbene synthase (STS) and 3-O-β-glycosyltransferases, which are related to the synthesis of resveratrol, increased in response to these treatments, paralleling the change in resveratrol content. All treatments also induced the biosynthesis of resveratrol in berry skins at room temperature. The berries of these treatments held at room temperature for 1 day were further stored under low temperature (-1 ± 0.5 °C, RH 95%) for 27 days, and the results showed that all treatments continuously increased berry skin resveratrol content, with the combination treatment being most efficient. During cold storage, resveratrol content remained at high levels and reached a maximum (about 247.7 μg/g FW) at 13 days, then showed a slight decline, though it remained high by the end of storage. Berry firmness and total soluble solids content showed slight changes during cold storage, but there were no differences among the treatments. Thus, the combination treatment of CaCl2 and UV-C could be an efficient method for increasing resveratrol content of table grapes during storage under low temperature. This would be potentially beneficial for producing functional fruits.

A cotton annexin protein AnxGb6 regulates fiber elongation through its interaction with actin 1

Annexins are assumed to be involved in regulating cotton fiber elongation, but direct evidence remains to be presented. Here we cloned six Annexin genes (AnxGb) abundantly expressed in fiber from sea-island cotton (G. barbadense). qRT-PCR results indicated that all six G. barbadense annexin genes were expressed in elongating cotton fibers, while only the expression of AnxGb6 was cotton fiber-specific. Yeast two hybridization and BiFC analysis revealed that AnxGb6 homodimer interacted with a cotton fiber specific actin GbAct1. Ectopic-expressed AnxGb6 in Arabidopsis enhanced its root elongation without increasing the root cell number. Ectopic AnxGb6 expression resulted in more F-actin accumulation in the basal part of the root cell elongation zone. Analysis of AnxGb6 expression in three cotton genotypes with different fiber length confirmed that AnxGb6 expression was correlated to cotton fiber length, especially fiber elongation rate. Our results demonstrated that AnxGb6 was important for fiber elongation by potentially providing a domain for F-actin organization.

The emerging function of IQD proteins as scaffolds in cellular signaling and trafficking

Calcium (Ca(2+)) signaling modules are essential for adjusting plant growth and performance to environmental constraints. Differential interactions between sensors of Ca(2+) dynamics and their molecular targets are at the center of the transduction process. Calmodulin (CaM) and CaM-like (CML) proteins are principal Ca(2+)-sensors in plants that govern the activities of numerous downstream proteins with regulatory properties. The families of IQ67-Domain (IQD) proteins are a large class of plant-specific CaM/CML-targets (e.g., 33 members in A. thaliana) which share a unique domain of multiple varied CaM retention motifs in tandem orientation. Genetic studies in Arabidopsis and tomato revealed first roles for IQD proteins related to basal defense response and plant development. Molecular, biochemical and histochemical analysis of Arabidopsis IQD1 demonstrated association with microtubules as well as targeting to the cell nucleus and nucleolus. In vivo binding to CaM and kinesin light chain-related protein-1 (KLCR1) suggests a Ca(2+)-regulated scaffolding function of IQD1 in kinesin motor-dependent transport of multiprotein complexes. Furthermore, because IQD1 interacts in vitro with single-stranded nucleic acids, the prospect arises that IQD1 and other IQD family members facilitate cellular RNA localization as one mechanism to control and fine-tune gene expression and protein sorting.

iTRAQ protein profile analysis of Citrus sinensis roots in response to long-term boron-deficiency

Seedlings of Citrus sinensis were fertilized with boron (B)-deficient (0μM H3BO3) or -sufficient (10μM H3BO3) nutrient solution for 15weeks. Thereafter, iTRAQ analysis was employed to compare the abundances of proteins from B-deficient and -sufficient roots. In B-deficient roots, 164 up-regulated and 225 down-regulated proteins were identified. These proteins were grouped into the following functional categories: protein metabolism, nucleic acid metabolism, stress responses, carbohydrate and energy metabolism, cell transport, cell wall and cytoskeleton metabolism, biological regulation and signal transduction, and lipid metabolism. The adaptive responses of roots to B-deficiency might include following several aspects: (a) decreasing root respiration; (b) improving the total ability to scavenge reactive oxygen species (ROS); and (c) enhancing cell transport. The differentially expressed proteins identified by iTRAQ are much larger than those detected using 2D gel electrophoresis, and many novel B-deficiency-responsive proteins involved in cell transport, biological regulation and signal transduction, stress responses and other metabolic processes were identified in this work. Our results indicate remarkable metabolic flexibility of citrus roots, which may contribute to the survival of B-deficient plants. This represents the most comprehensive analysis of protein profiles in response to B-deficiency.

BIOLOGICAL SIGNIFICANCE

In this study, we identified many new proteins involved in cell transport, biological regulation and signal transduction, stress responses and other metabolic processes that were not previously known to be associated with root B-deficiency responses. Therefore, our manuscript represents the most comprehensive analysis of protein profiles in response to B-deficiency and provides new information about the plant response to B-deficiency. This article is part of a Special Issue entitled: Translational Plant Proteomics.

Complementary biochemical approaches applied to the identification of plastidial calmodulin-binding proteins

Ca(2+)/Calmodulin (CaM)-dependent signaling pathways play a major role in the modulation of cell responses in eukaryotes. In the chloroplast, few proteins such as the NAD(+) kinase 2 have been previously shown to interact with CaM, but a general picture of the role of Ca(2+)/CaM signaling in this organelle is still lacking. Using CaM-affinity chromatography and mass spectrometry, we identified 210 candidate CaM-binding proteins from different Arabidopsis and spinach chloroplast sub-fractions. A subset of these proteins was validated by an optimized in vitro CaM-binding assay. In addition, we designed two fluorescence anisotropy assays to quantitatively characterize the binding parameters and applied those assays to NAD(+) kinase 2 and selected candidate proteins. On the basis of our results, there might be many more plastidial CaM-binding proteins than previously estimated. In addition, we showed that an array of complementary biochemical techniques is necessary in order to characterize the mode of interaction of candidate proteins with CaM.

CAMTA 1 regulates drought responses in Arabidopsis thaliana

BACKGROUND

Transcription factors (TF) play a crucial role in regulating gene expression and are fit to regulate diverse cellular processes by interacting with other proteins. A TF named calmodulin binding transcription activator (CAMTA) was identified in Arabidopsis thaliana (AtCAMTA1-6). To explore the role of CAMTA1 in drought response, the phenotypic differences and gene expression was studied between camta1 and Col-0 under drought condition.

RESULTS

In camta1, root development was abolished showing high-susceptibility to induced osmotic stress resulting in small wrinkled rosette leaves and stunted primary root. In camta1 under drought condition, we identified growth retardation, poor WUE, low photosystem II efficiency, decline in RWC and higher sensitivity to drought with reduced survivability. The microarray analysis of drought treated camta1 revealed that CAMTA1 regulates "drought recovery" as most indicative pathway along with other stress response, osmotic balance, apoptosis, DNA methylation and photosynthesis. Interestingly, majority of positively regulated genes were related to plasma membrane and chloroplast. Further, our analysis indicates that CAMTA1 regulates several stress responsive genes including RD26, ERD7, RAB18, LTPs, COR78, CBF1, HSPs etc. and promoter of these genes were enriched with CAMTA recognition cis-element. CAMTA1 probably regulate drought recovery by regulating expression of AP2-EREBP transcription factors and Abscisic acid response.

CONCLUSION

CAMTA1 rapidly changes broad spectrum of responsive genes of membrane integrity and photosynthetic machinery by generating ABA response for challenging drought stress. Our results demonstrate the important role of CAMTA1 in regulating drought response in Arabidopsis, thus could be genetically engineered for improving drought tolerance in crop.

Molecular characterization and analysis of a novel calcium-dependent protein kinase from Eimeria tenella

The calcium-dependent protein kinases (CDPKs) are unique enzymes found only in plants, green algae, ciliates and apicomplexan parasites. In this study, a novel CDPK gene of Eimeria tenella, designed EtCDPK3, was cloned using rapid amplification of cDNA ends (RACE) based on the expressed sequence tag (EST). The entire cDNA of EtCDPK3 contained 1637 nucleotides encoding 433 amino acids and the deduced EtCDPK3 protein had canonical characteristic domains identified in other CDPKs, including a well-conserved amino-terminal kinase domain and a carboxy-terminal calmodulin-like structure with 4 EF-hand motifs for calcium binding. The expression profiles of the EtCDPK3 gene in different development stages were investigated by real-time quantitative PCR. Messenger RNA levels from the EtCDPK3 gene were higher in sporozoites than in other stages (unsporulated oocysts, sporulated oocysts and merozoites). Western blot analysis showed that rabbit antiserum against recombinant EtCDPK3 could recognize a native 49 kDa protein band of parasite. Indirect immunofluorescent antibody labelling revealed dispersed localization of EtCDPK3 during the first schizogony and intense specific staining. EtCDPK3 was located at the apical end of the sporozoites after early infection of DF-1 cells and the protein was highly expressed. Inhibition of EtCDPK3 function using specific antibodies reduced the ability of E. tenella to invade host cells. These results suggested that EtCDPK3 may be involved in invasion and survival of the parasite intracellular stages of E. tenella. Because this kinase family is absent from hosts, it represents a valid target that could be exploited for chemotherapy against Eimeria spp.

Arabidopsis calmodulin-binding protein IQ67-domain 1 localizes to microtubules and interacts with kinesin light chain-related protein-1

Calcium (Ca(2+)) is a key second messenger in eukaryotes and regulates diverse cellular processes, most notably via calmodulin (CaM). In Arabidopsis thaliana, IQD1 (IQ67 domain 1) is the founding member of the IQD family of putative CaM targets. The 33 predicted IQD proteins share a conserved domain of 67 amino acids that is characterized by a unique arrangement of multiple CaM recruitment motifs, including so-called IQ motifs. Whereas IQD1 has been implicated in the regulation of defense metabolism, the biochemical functions of IQD proteins remain to be elucidated. In this study we show that IQD1 binds to multiple Arabidopsis CaM and CaM-like (CML) proteins in vitro and in yeast two-hybrid interaction assays. CaM overlay assays revealed moderate affinity of IQD1 to CaM2 (K(d) ∼ 0.6 μm). Deletion mapping of IQD1 demonstrated the importance of the IQ67 domain for CaM2 binding in vitro, which is corroborated by interaction of the shortest IQD member, IQD20, with Arabidopsis CaM/CMLs in yeast. A genetic screen of a cDNA library identified Arabidopsis kinesin light chain-related protein-1 (KLCR1) as an IQD1 interactor. The subcellular localization of GFP-tagged IQD1 proteins to microtubules and the cell nucleus in transiently and stably transformed plant tissues (tobacco leaves and Arabidopsis seedlings) suggests direct interaction of IQD1 and KLCR1 in planta that is supported by GFP∼IQD1-dependent recruitment of RFP∼KLCR1 and RFP∼CaM2 to microtubules. Collectively, the prospect arises that IQD1 and related proteins provide Ca(2+)/CaM-regulated scaffolds for facilitating cellular transport of specific cargo along microtubular tracks via kinesin motor proteins.

Evolutionary adaptation of plant annexins has diversified their molecular structures, interactions and functional roles

Annexins are an homologous, structurally related superfamily of proteins known to associate with membrane lipid and cytoskeletal components. Their involvement in membrane organization, vesicle trafficking and signaling is fundamental to cellular processes such as growth, differentiation, secretion and repair. Annexins exist in some prokaryotes and all eukaryotic phyla within which plant annexins represent a monophyletic clade of homologs descended from green algae. Genomic, proteomic and transcriptomic approaches have provided data on the diversity, cellular localization and expression patterns of different plant annexins. The availability of 35 complete plant genomes has enabled systematic comparative analysis to determine phylogenetic relationships, characterize structures and observe functional specificity between and within individual subfamilies. Short amino termini and selective erosion of the canonical type 2 calcium coordinating sites in domains 2 and 3 are typical of plant annexins. The convergent evolution of alternate functional motifs such as 'KGD', redox-sensitive Cys and hydrophobic Trp/Phe residues argues for their functional relevance and contribution to mechanistic diversity in plant annexins. This review examines recent findings and advances in plant annexin research with special focus on their structural diversity, cellular and molecular interactions and their potential integrated functions in the broader context of physiological responses.

The calcium-dependent protein kinase (PnCDPK1) is involved in Pharbitis nil flowering

Signaling pathways, and specifically the signaling pathway of calcium, have been widely implicated in the regulation of a variety of signals in plants. Calcium-dependent protein kinases (CDPKs) are essential sensor-transducers of calcium signaling pathways, the functional characterization of which is of great interest because they play important roles during growth and in response to a wide range of environmental and developmental stimuli. Here, we report the first evidence of transient and specific elevation of PnCDPK1 transcript level and enzyme activity following conversion of a leaf bud to a flower bud, as well as participation of PnCDPK1 in evocation and flower morphogenesis in Pharbitis nil. Fluorescence microscopy immunolocalization and biochemical analysis confirmed the presence of CDPK in shoot apexes. The protein level was low in leaves, vegetative apexes and increased significantly in apexes after a flowering long-induction night. In the vegetative apex, a very weak PnCDPK1 protein signal was accumulated prominently in the zone of the ground meristem and in external layers of tissues of the cortex. After the dark treatment, the signal in cells of the ground meristem was still present, but a significantly stronger signal appeared in epidermal cells, cortex tissue, and leaf primordium. At the onset of flower meristem development, the PnCDPK1 level diverged significantly. PnCDPK1 mRNA, protein level and enzyme activity were very low at the beginning of flower bud development and gradually increased in later stages, reaching the highest level in a fully open flower. Analysis of flower organs revealed that PnCDPK1 was accumulated mainly in petals and sepals rather than in pistils and stamens. Our results clearly indicate that PnCDPK1 is developmentally regulated and may be an important component in the signal transduction pathways for flower morphogenesis. Findings from this research are important for further dissecting mechanisms of flowering and functions of CDPKs in flowering plants.

The SbSOS1 gene from the extreme halophyte Salicornia brachiata enhances Na(+) loading in xylem and confers salt tolerance in transgenic tobacco

BACKGROUND

Soil salinity adversely affects plant growth and development and disturbs intracellular ion homeostasis resulting cellular toxicity. The Salt Overly Sensitive 1 (SOS1) gene encodes a plasma membrane Na(+)/H(+) antiporter that plays an important role in imparting salt stress tolerance to plants. Here, we report the cloning and characterisation of the SbSOS1 gene from Salicornia brachiata, an extreme halophyte.

RESULTS

The SbSOS1 gene is 3774 bp long and encodes a protein of 1159 amino acids. SbSOS1 exhibited a greater level of constitutive expression in roots than in shoots and was further increased by salt stress. Overexpressing the S. brachiata SbSOS1 gene in tobacco conferred high salt tolerance, promoted seed germination and increased root length, shoot length, leaf area, fresh weight, dry weight, relative water content (RWC), chlorophyll, K(+)/Na(+) ratio, membrane stability index, soluble sugar, proline and amino acid content relative to wild type (WT) plants. Transgenic plants exhibited reductions in electrolyte leakage, reactive oxygen species (ROS) and MDA content in response to salt stress, which probably occurred because of reduced cytosolic Na(+) content and oxidative damage. At higher salt stress, transgenic tobacco plants exhibited reduced Na(+) content in root and leaf and higher concentrations in stem and xylem sap relative to WT, which suggests a role of SbSOS1 in Na(+) loading to xylem from root and leaf tissues. Transgenic lines also showed increased K(+) and Ca(2+) content in root tissue compared to WT, which reflect that SbSOS1 indirectly affects the other transporters activity.

CONCLUSIONS

Overexpression of SbSOS1 in tobacco conferred a high degree of salt tolerance, enhanced plant growth and altered physiological and biochemical parameters in response to salt stress. In addition to Na(+) efflux outside the plasma membrane, SbSOS1 also helps to maintain variable Na(+) content in different organs and also affect the other transporters activity indirectly. These results broaden the role of SbSOS1 in planta and suggest that this gene could be used to develop salt-tolerant transgenic crops.

Abiotic stress responses promote Potato virus A infection in Nicotiana benthamiana

The effect of abiotic stress responses on Potato virus A (PVA; genus Potyvirus) infection was studied. Salt, osmotic and wounding stress all increased PVA gene expression in infected Nicotiana benthamiana leaves. According to the literature, an early response to these stresses is an elevation in cytosolic Ca(2+) concentration. The infiltration of 0.1 m CaCl(2) into the infected leaf area enhanced the translation of PVA RNA, and this Ca(2+) -induced effect was more profound than that induced solely by osmotic stress. The inhibition of voltage-gated Ca(2+) channels within the plasma membrane abolished the Ca(2+) effect, suggesting that Ca(2+) had to be transported into the cytosol to affect viral gene expression. This was also supported by a reduced wounding effect in the presence of the Ca(2+) -chelating agent ethylene glycol tetraacetic acid (EGTA). In the absence of viral replication, the intense synthesis of viral proteins in response to Ca(2+) was transient. However, a Ca(2+) pulse administered at the onset of wild-type PVA infection enhanced the progress of infection within the locally infected leaf, and the virus appeared earlier in the systemic leaves than in the control plants. This suggests that the cellular environment was thoroughly modified by the Ca(2+) pulse to support viral infection. One message of this study is that the sensing of abiotic stress, which leads to cellular responses, probably via Ca(2+) signalling, associated with enhanced virus infection, may lead to higher field crop losses. Therefore, the effect of abiotic stress on plant viral infection warrants further analysis.

Ca(2+) signals: the versatile decoders of environmental cues

Plants are often subjected to various environmental stresses that lead to deleterious effects on growth, production, sustainability, etc. The information of the incoming stress is read by the plants through the mechanism of signal transduction. The plant Ca(2+) serves as secondary messenger during adaptations to stressful conditions and developmental processes. A plethora of Ca(2+) sensors and decoders functions to bring about these changes. The cellular concentrations of Ca(2+), their subcellular localization, and the specific interaction affinities of Ca(2+) decoder proteins all work together to make this process a complex but synchronized signaling network. In this review, we focus on the versatility of these sensors and decoders in the model plant Arabidopsis as well as plants of economical importance. Here, we have also thrown light on the possible mechanism of action of these important components.

Characterization of StABF1, a stress-responsive bZIP transcription factor from Solanum tuberosum L. that is phosphorylated by StCDPK2 in vitro

ABF/AREB bZIP transcription factors mediate plant abiotic stress responses by regulating the expression of stress-related genes. These proteins bind to the abscisic acid (ABA)-responsive element (ABRE), which is the major cis-acting regulatory sequence in ABA-dependent gene expression. In an effort to understand the molecular mechanisms of abiotic stress resistance in cultivated potato (Solanum tuberosum L.), we have cloned and characterized an ABF/AREB-like transcription factor from potato, named StABF1. The predicted protein shares 45-57% identity with A. thaliana ABFs proteins and 96% identity with the S. lycopersicum SlAREB1 and presents all of the distinctive features of ABF/AREB transcription factors. Furthermore, StABF1 is able to bind to the ABRE in vitro. StABF1 gene is induced in response to ABA, drought, salt stress and cold, suggesting that it might be a key regulator of ABA-dependent stress signaling pathways in cultivated potato. StABF1 is phosphorylated in response to ABA and salt stress in a calcium-dependent manner, and we have identified a potato CDPK isoform (StCDPK2) that phosphorylates StABF1 in vitro. Interestingly, StABF1 expression is increased during tuber development and by tuber-inducing conditions (high sucrose/nitrogen ratio) in leaves. We also found that StABF1 calcium-dependent phosphorylation is stimulated by tuber-inducing conditions and inhibited by gibberellic acid, which inhibits tuberization.

Ca2+-dependent GTPase, extra-large G protein 2 (XLG2), promotes activation of DNA-binding protein related to vernalization 1 (RTV1), leading to activation of floral integrator genes and early flowering in Arabidopsis

Heterotrimeric G proteins, consisting of Gα, Gβ, and Gγ subunits, play important roles in plant development and cell signaling. In Arabidopsis, in addition to one prototypical G protein α subunit, GPA1, there are three extra-large G proteins, XLG1, XLG2, and XLG3, of largely unknown function. Each extra-large G (XLG) protein has a C-terminal Gα-like region and a ∼400 amino acid N-terminal extension. Here we show that the three XLG proteins specifically bind and hydrolyze GTP, despite the fact that these plant-specific proteins lack key conserved amino acid residues important for GTP binding and hydrolysis of GTP in mammalian Gα proteins. Moreover, unlike other known Gα proteins, these activities require Ca(2+) instead of Mg(2+) as a cofactor. Yeast two-hybrid library screening and in vitro protein pull-down assays revealed that XLG2 interacts with the nuclear protein RTV1 (related to vernalization 1). Electrophoretic mobility shift assays show that RTV1 binds to DNA in vitro in a non-sequence-specific manner and that GTP-bound XLG2 promotes the DNA binding activity of RTV1. Overexpression of RTV1 results in early flowering. Combined overexpression of XLG2 and RTV1 enhances this early flowering phenotype and elevates expression of the floral pathway integrator genes, FT and SOC1, but does not repress expression of the floral repressor, FLC. Chromatin immunoprecipitation assays show that XLG2 increases RTV1 binding to FT and SOC1 promoters. Thus, a Ca(2+)-dependent G protein, XLG2, promotes RTV1 DNA binding activity for a subset of floral integrator genes and contributes to floral transition.

Analysis of calcium signaling pathways in plants

BACKGROUND

Calcium serves as a versatile messenger in many adaptation and developmental processes in plants. Ca2+ signals are represented by stimulus-specific spatially and temporally defined Ca2+ signatures. These Ca2+ signatures are detected, decoded and transmitted to downstream responses by a complex toolkit of Ca2+ binding proteins that function as Ca2+ sensors.

SCOPE OF REVIEW

This review will reflect on advancements in monitoring Ca2+ dynamics in plants. Moreover, it will provide insights in the extensive and complex toolkit of plant Ca2+ sensor proteins that relay the information presented in the Ca2+ signatures into phosphorylation events, changes in protein-protein interaction or regulation of gene expression.

MAJOR CONCLUSIONS

Plants' response to signals is encoded by different Ca2+ signatures. The plant decoding Ca2+ toolkit encompasses different families of Ca2+ sensors like Calmodulins (CaM), Calmodulin-like proteins (CMLs), Ca2+-dependent protein kinases (CDPKs), Calcineurin B-like proteins (CBLs) and their interacting kinases (CIPKs). These Ca2+ sensors are encoded by complex gene families and form intricate signaling networks in plants that enable specific, robust and flexible information processing.

GENERAL SIGNIFICANCE

This review provides new insights about the biochemical regulation, physiological functions and of newly identified target proteins of the major plant Ca2+ sensor families. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signaling.

Phosphorylation of Thellungiella salsuginea dehydrins TsDHN-1 and TsDHN-2 facilitates cation-induced conformational changes and actin assembly

Group 2 late embryogenesis abundant (LEA) proteins, also known as dehydrins, are intrinsically disordered proteins that are expressed in plants experiencing extreme environmental conditions such as drought or low temperatures. These proteins are characterized by the presence of at least one conserved, lysine-rich K-segment and sometimes by one or more serine-rich S-segments that are phosphorylated. Dehydrins may stabilize proteins and membrane structures during environmental stress and can sequester and scavenge metal ions. Here, we investigate how the conformations of two dehydrins from Thellungiella salsuginea, denoted as TsDHN-1 (acidic) and TsDHN-2 (basic), are affected by pH, interactions with cations and membranes, and phosphorylation. Both TsDHN-1 and TsDHN-2 were expressed as SUMO fusion proteins for in vitro phosphorylation by casein kinase II (CKII), and structural analysis by circular dichroism and attenuated total reflection-Fourier transform infrared spectroscopy. We show that the polyproline II conformation can be induced in the dehydrins by their environmental conditions, including changes in the concentration of divalent cations such as Ca(2+). The assembly of actin by these dehydrins was assessed by sedimentation assays and viewed by transmission electron and atomic force microscopy. Phosphorylation allowed both dehydrins to polymerize actin filaments. These results support the hypothesis that dehydrins stabilize the cytoskeleton under stress conditions and further that phosphorylation may be an important feature of this stabilization.

cGMP regulates hydrogen peroxide accumulation in calcium-dependent salt resistance pathway in Arabidopsis thaliana roots

3',5'-cyclic guanosine monophosphate (cGMP) is an important second messenger in plants. In the present study, roles of cGMP in salt resistance in Arabidopsis roots were investigated. Arabidopsis roots were sensitive to 100 mM NaCl treatment, displaying a great increase in electrolyte leakage and Na(+)/K(+) ratio and a decrease in gene expression of the plasma membrane (PM) H(+)-ATPase. However, application of exogenous 8Br-cGMP (an analog of cGMP), H(2)O(2) or CaCl(2) alleviated the NaCl-induced injury by maintaining a lower Na(+)/K(+) ratio and increasing the PM H(+)-ATPase gene expression. In addition, the inhibition of root elongation and seed germination under salt stress was removed by 8Br-cGMP. Further study indicated that 8Br-cGMP-induced higher NADPH levels for PM NADPH oxidase to generate H(2)O(2) by regulating glucose-6-phosphate dehydrogenase (G6PDH) activity. The effect of 8Br-cGMP and H(2)O(2) on ionic homeostasis was abolished when Ca(2+) was eliminated by glycol-bis-(2-amino ethyl ether)-N,N,N',N'-tetraacetic acid (EGTA, a Ca(2+) chelator) in Arabidopsis roots under salt stress. Taken together, cGMP could regulate H(2)O(2) accumulation in salt stress, and Ca(2+) was necessary in the cGMP-mediated signaling pathway. H(2)O(2), as the downstream component of cGMP signaling pathway, stimulated PM H(+)-ATPase gene expression. Thus, ion homeostasis was modulated for salt tolerance.

Calmodulin and calmodulin-like proteins in plant calcium signaling

Calmodulin (CaM) is a primary calcium sensor in all eukaryotes. It binds calcium and regulates the activity of a wide range of effector proteins in response to calcium signals. The list of CaM targets includes plant-specific proteins whose functions are progressively being elucidated. Plants also possess numerous calmodulin-like proteins (CMLs) that appear to have evolved unique functions. Functional studies of CaM and CMLs in plants highlight the importance of this protein family in the regulation of plant development and stress responses by converting calcium signals into transcriptional responses, protein phosphorylation or metabolic changes. This review summarizes some of the significant progress made by biochemical and genetic studies in identifying the properties and physiological functions of plant CaMs and CMLs. We discuss emerging paradigms in the field and highlight the areas that need further investigation.

Experimental and computational approaches for the study of calmodulin interactions

Ca(2+), a universal messenger in eukaryotes, plays a major role in signaling pathways that control many growth and developmental processes in plants as well as their responses to various biotic and abiotic stresses. Cellular changes in Ca(2+) in response to diverse signals are recognized by protein sensors that either have their activity modulated or that interact with other proteins and modulate their activity. Calmodulins (CaMs) and CaM-like proteins (CMLs) are Ca(2+) sensors that have no enzymatic activity of their own but upon binding Ca(2+) interact and modulate the activity of other proteins involved in a large number of plant processes. Protein-protein interactions play a key role in Ca(2+)/CaM-mediated in signaling pathways. In this review, using CaM as an example, we discuss various experimental approaches and computational tools to identify protein-protein interactions. During the last two decades hundreds of CaM-binding proteins in plants have been identified using a variety of approaches ranging from simple screening of expression libraries with labeled CaM to high-throughput screens using protein chips. However, the high-throughput methods have not been applied to the entire proteome of any plant system. Nevertheless, the data provided by these screens allows the development of computational tools to predict CaM-interacting proteins. Using all known binding sites of CaM, we developed a computational method that predicted over 700 high confidence CaM interactors in the Arabidopsis proteome. Most (>600) of these are not known to bind calmodulin, suggesting that there are likely many more CaM targets than previously known. Functional analyses of some of the experimentally identified Ca(2+) sensor target proteins have uncovered their precise role in Ca(2+)-mediated processes. Further studies on identifying novel targets of CaM and CMLs and generating their interaction network - "calcium sensor interactome" - will help us in understanding how Ca(2+) regulates a myriad of cellular and physiological processes.

Coping with stresses: roles of calcium- and calcium/calmodulin-regulated gene expression

Abiotic and biotic stresses are major limiting factors of crop yields and cause billions of dollars of losses annually around the world. It is hoped that understanding at the molecular level how plants respond to adverse conditions and adapt to a changing environment will help in developing plants that can better cope with stresses. Acquisition of stress tolerance requires orchestration of a multitude of biochemical and physiological changes, and most of these depend on changes in gene expression. Research during the last two decades has established that different stresses cause signal-specific changes in cellular Ca(2+) level, which functions as a messenger in modulating diverse physiological processes that are important for stress adaptation. In recent years, many Ca(2+) and Ca(2+)/calmodulin (CaM) binding transcription factors (TFs) have been identified in plants. Functional analyses of some of these TFs indicate that they play key roles in stress signaling pathways. Here, we review recent progress in this area with emphasis on the roles of Ca(2+)- and Ca(2+)/CaM-regulated transcription in stress responses. We will discuss emerging paradigms in the field, highlight the areas that need further investigation, and present some promising novel high-throughput tools to address Ca(2+)-regulated transcriptional networks.

Structures of parasitic CDPK domains point to a common mechanism of activation

We recently determined the first structures of inactivated and calcium-activated calcium-dependent protein kinases (CDPKs) from Apicomplexa. Calcium binding triggered a large conformational change that constituted a new mechanism in calcium signaling and a novel EF-hand fold (CAD, for CDPK activation domain). Thus we set out to determine if this mechanism was universal to all CDPKs. We solved additional CDPK structures, including one from the species Plasmodium. We highlight the similarities in sequence and structure across apicomplexan and plant CDPKs, and strengthen our observations that this novel mechanism could be universal to canonical CDPKs. Our new structures demonstrate more detailed steps in the mechanism of calcium activation and possible key players in regulation. Residues involved in making the largest conformational change are the most conserved across Apicomplexa, leading us to propose that the mechanism is indeed conserved. CpCDPK3_CAD and PfCDPK_CAD were captured at a possible intermediate conformation, lending insight into the order of activation steps. PfCDPK3_CAD adopts an activated fold, despite having an inactive EF-hand sequence in the N-terminal lobe. We propose that for most apicomplexan CDPKs, the mode of activation will be similar to that seen in our structures, while specific regulation of the inactive and active forms will require further investigation.

Characterization of GmCaMK1, a member of a soybean calmodulin-binding receptor-like kinase family

Calmodulin(CaM)-regulated protein phosphorylation forms an important component of Ca(2+) signaling in animals but is less understood in plants. We have identified a CaM-binding receptor-like kinase from soybean nodules, GmCaMK1, a homolog of Arabidopsis CRLK1. We delineated the CaM-binding domain (CaMBD) of GmCaMK1 to a 24-residue region near the C-terminus, which overlaps with the kinase domain. We have demonstrated that GmCaMK1 binds CaM with high affinity in a Ca(2+)-dependent manner. We showed that GmCaMK1 is expressed broadly across tissues and is enriched in roots and developing nodules. Finally, we examined the CaMBDs of the five-member GmCaMK family in soybean, and orthologs present across taxa.

Cloning and characterization of a calcium binding EF-hand protein gene TaCab1 from wheat and its expression in response to Puccinia striiformis f. sp. tritici and abiotic stresses

Calcium is a ubiquitous and essential secondary messenger in eukaryotic signal transduction pathways. Calcium binding protein, as a component of pathways, plays various roles in response to biotic and abiotic stresses, as well as in developmental processes in plants. In this study, a calcium binding protein gene, designated as TaCab1 (Triticum aestivum calcium binding EF-hand protein 1), was isolated and characterized from wheat leaves (cv. Suwon 11) infected by Puccinia striiformis f. sp. tritici by in silico cloning and reverse transcription PCR (RT-PCR). TaCab1 did not have an intron and was predicted to encode a 216 amino acid protein which possesses an N-terminal region with a signal peptide, a transmembrane domain, an EF-hand motif and a caleosin domain. The results of transient assays with constructs of TaCab1 with green fluorescent protein (GFP) gene indicated that TaCab1 encodes a transmembrane protein. Quantitative real-time PCR (qRT-PCR) analyses revealed that TaCab1 was highly expressed in leaves than roots and stems. Although up-regulated expression profiles of TaCab1 were quite similar in both incompatible and compatible interactions, its transcript accumulation in the compatible interaction was much higher than in the incompatible interaction. The transcription of TaCab1 was also up-regulated at different degrees after treated by phytohormones [abscisic acid, benzyl adenine, ethylene, methyl jasmonate and salicylic acid (SA)] and stress stimuli [wounding, low temperature, polyethylene glycol and high salinity]. These results suggest that TaCab1 is involved in the plant-pathogen recognition, symptom development, and the basal tolerance to biotic and abiotic stresses through the SA signaling pathway.

Interaction of a plant pseudo-response regulator with a calmodulin-like protein

Calmodulin (CaM) plays a crucial role in the regulation of diverse cellular processes by modulating the activities of numerous target proteins. Plants possess an extended CaM family including numerous CaM-like proteins (CMLs), most of which appear to be unique to plants. We previously demonstrated a role for CML9 in abiotic stress tolerance and seed germination in Arabidopsis thaliana. We report here the isolation of PRR2, a pseudo-response regulator as a CML9 interacting protein by screening an expression library prepared from Arabidopsis seedlings with CML9 as bait in a yeast two-hybrid system. PRR2 is similar to the response regulators of the two-component system, but lacks the invariant residue required for phosphorylation by which response regulators switch their output response, suggesting the existence of alternative regulatory mechanisms. PRR2 was found to bind CML9 and closely related CMLs but not a canonical CaM. Mapping analyses indicate that an almost complete form of PRR2 is required for interaction with CML9, suggesting a recognition mode different from the classical CaM-target peptide complex. PRR2 contains several features that are typical of transcription factors, including a GARP DNA recognition domain, a Pro-rich region and a Golden C-terminal box. PRR2 and CML9 as fusion proteins with fluorescent tags co-localized in the nucleus of plant cells, and their interaction in the nuclear compartment was validated in planta by using a fluorophore-tagged protein interaction assay. These findings suggest that binding of PRR2 to CML9 may be an important mechanism to modulate the physiological role of this transcription factor in plants.

AtCPK6, a functionally redundant and positive regulator involved in salt/drought stress tolerance in Arabidopsis

The calcium-dependent protein kinase (CDPK) family is needed in plant signaling during various physiological pathways. The Arabidopsis AtCPK6 gene belongs to the subclass of stress-inducible CDPKs, which is stimulated by salt and osmotic stress. To elucidate the physiological function of AtCPK6, transgenic Arabidopsis plants under the control of double CaMV 35S promoter were obtained. AtCPK6 over-expressing plants showed enhanced tolerance to salt/drought stresses. The elevated tolerance of the AtCPK6 over-expressing plants was confirmed by the change of proline and malondialdehyde (MDA). Real-time PCR analyses revealed that the expression levels of several stress-regulated genes were altered in AtCPK6 over-expressing plants. However, cpk6 mutant displayed no obvious difference with control. These results are likely to indicate that AtCPK6 is functionally redundant and a positive regulator involved in the tolerance to salt/drought stress in Arabidopsis.

[Plant calcium sensors in osmotic signaling]

Calcium is an essential second messenger that mediates plant responses to developmental and environmental clues. Specific calcium signatures are sensed and decoded by diverse Ca(2+) sensors to induce appropriate downstream responses. Calmodulin is the most important and conserved Ca(2+) transducer in all eukaryotes. Additional plant-specific sensors are encoded by multigene families, i.e. calcineurin B-like and Ca(2+)-dependent protein kinases. Calcium binding induces structural conformational changes in Ca(2+) sensors, resulting in the modification of protein interaction or enzymatic activity. Activated Ca(2+) sensors subsequently regulate downstream targets which can be involved in signal transduction, like protein kinases and transcription factors, or in direct cell protection from stress damages, like ion transporters or detoxification enzymes. Ca(2+) plays an important role in osmotic signaling triggered by cold, drought and salinity. The multiplicity of plant calcium sensors associated with diverse cellular targets constitute a tightly regulated signaling network that induces specific stress responses to improve plant survival under unfavourable conditions.

Cytosolic calcium and pH signaling in plants under salinity stress

Calcium is one of the essential nutrients for growth and development of plants. It is an important component of various structures in cell wall and membranes. Besides some fundamental roles under normal condition, calcium functions as a major secondary-messenger molecule in plants under different developmental cues and various stress conditions including salinity stress. Also changes in cytosolic pH, pH(cyt), either individually, or in coordination with changes in cytosolic Ca(2+) concentration, [Ca(2+)](cyt), evoke a wide range of cellular functions in plants including signal transduction in plant-defense responses against stresses. It is believed that salinity stress, like other stresses, is perceived at cell membrane, either extra cellular or intracellular, which then triggers an intracellular-signaling cascade including the generation of secondary messenger molecules like Ca(2+) and protons. The variety and complexity of Ca(2+) and pH signaling result from the nature of the stresses as well as the tolerance level of the plant species against that specific stress. The nature of changes in [Ca(2+)](cyt) concentration, in terms of amplitude, frequency and duration, is likely very important for decoding the specific downstream responses for salinity stress tolerance in planta. It has been observed that the signatures of [Ca(2+)](cyt) and pH differ in various studies reported so far depending on the techniques used to measure them, and also depending on the plant organs where they are measured, such as root, shoot tissues or cells. This review describes the recent advances about the changes in [Ca(2+)](cyt) and pH(cyt) at both cellular and whole-plant levels under salinity stress condition, and in various salinity-tolerant and -sensitive plant species.

The language of calcium signaling

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

Breaking the code: Ca2+ sensors in plant signalling

Ca2+ ions play a vital role as second messengers in plant cells during various developmental processes and in response to environmental stimuli. Plants have evolved a diversity of unique proteins that bind Ca2+ using the evolutionarily conserved EF-hand motif. The currently held hypothesis is that these proteins function as Ca2+ sensors by undergoing conformational changes in response to Ca2+-binding that facilitate their regulation of target proteins and thereby co-ordinate various signalling pathways. The three main classes of these EF-hand Ca2+sensors in plants are CaMs [calmodulins; including CMLs (CaM-like proteins)], CDPKs (calcium-dependent protein kinases) and CBLs (calcineurin B-like proteins). In the plant species examined to date, each of these classes is represented by a large family of proteins, most of which have not been characterized biochemically and whose physiological roles remain unclear. In the present review, we discuss recent advances in research on CaMs and CMLs, CDPKs and CBLs, and we attempt to integrate the current knowledge on the different sensor classes into common physiological themes.

The calmodulin-related calcium sensor CML42 plays a role in trichome branching

Calcium (Ca(2+)) is a key second messenger in eukaryotes where it regulates a diverse array of cellular processes in response to external stimuli. An important Ca(2+) sensor in both animals and plants is calmodulin (CaM). In addition to evolutionarily conserved CaM, plants possess a unique family of CaM-like (CML) proteins. The majority of these CMLs have not yet been studied, and investigation into their physical properties and cellular functions will provide insight into Ca(2+) signal transduction in plants. Here we describe the characterization of CML42, a 191-amino acid Ca(2+)-binding protein from Arabidopsis. Ca(2+) binding to recombinant CML42 was assessed by fluorescence spectroscopy, NMR spectroscopy, microcalorimetry, and CD spectroscopy. CML42 displays significant alpha-helical secondary structure, binds three molecules of Ca(2+) with affinities ranging from 30 to 430 nm, and undergoes a Ca(2+)-induced conformational change that results in the exposure of one or more hydrophobic regions. Gene expression analysis revealed CML42 transcripts at various stages of development and in many cell types, including the support cells, which surround trichomes (leaf hairs) on the leaf surface. Using yeast two-hybrid screening we identified a putative CML42 interactor; kinesin-interacting Ca(2+)-binding protein (KIC). Because KIC is a protein known to function in trichome development, we examined transgenic CML42 knockout plants and found that they possess aberrant trichomes with increased branching. Collectively, our data support a role for CML42 as a Ca(2+) sensor that functions during cell branching in trichomes.

Identification of genes expressed during the self-incompatibility response in perennial ryegrass (Lolium perenne L.)

Self-incompatibility (SI) in Lolium perenne is controlled gametophytically by the S-Z two-locus system. S and Z loci mapped to L. perenne linkage groups 1 and 2, respectively, with their corresponding putative-syntenic regions on rice chromosome 5 (R5) and R4. None of the gene products of S and Z have yet been identified. SI cDNA libraries were developed to enrich for SI expressed genes in L. perenne. Transcripts were identified from the SI libraries that were orthologous to sequences on rice R4 and R5. These represent potential SI candidate genes. Altogether ten expressed SI candidate genes were identified. A rapid increase in gene expression within two minutes after pollen-stigma contact was revealed, reaching a maximum between 2 and 10 min. The potential involvement of these genes in the SI reactions is discussed.

Expression and interaction of the CBLs and CIPKs from immature seeds of kidney bean (Phaseolus vulgaris L.)

Protein phosphorylation plays a key regulatory role in a variety of cellular processes. To better understand the function of protein phosphorylation in seed maturation, a PCR-based cloning method was employed and five cDNA clones (pvcipk1-5) for protein kinases were isolated from a cDNA library prepared from immature seeds of kidney bean (Phaseolus vulgaris L.). The deduced amino acid sequences showed that the five protein kinases (PvCIPK1-5) are members of the sucrose non-fermenting 1-related protein kinase type 3 (SnRK3) family, which interacts with calcineurin B-like proteins (CBLs). Two cDNA clones (pvcbl1 and 2) for CBLs were further isolated from the cDNA library. The predicted primary sequences of the proteins (PvCBL1 and 2) displayed significant identity (more than 90%) with those of other plant CBLs. Semi-quantitative RT-PCR analysis showed that the isolated genes, except pvcbl1, are expressed in leaves and early maturing seeds, whereas pvcbl1 is constitutively expressed during seed development. Yeast two-hybrid assay indicated that among the five PvCIPKs, only PvCIPK1 interacts with both PvCBL1 and PvCBL2. These results suggest that calcium-dependent protein phosphorylation-signaling via CBL-CIPK complexes occurs during seed development.

Evidence that the BONZAI1/COPINE1 protein is a calcium- and pathogen-responsive defense suppressor

Copines are calcium-responsive, phospholipid-binding proteins involved in cellular signaling. The Arabidopsis BONZAI1/COPINE1 (BON1/CPN1) gene is a suppressor of defense responses controlled by the disease resistance (R) gene homolog SNC1. The BON1/CPN1 null mutant cpn1-1 has a recessive, temperature- and humidity-dependent, lesion mimic phenotype that includes activation of Pathogenesis-Related (PR) gene expression. Here, we demonstrated that the accumulation of BON1/CPN1 protein in wild-type plants was up-regulated by bacterial pathogen inoculation and by the activation of defense signaling responses controlled by two R genes, SNC1 and RPS2. Interestingly, however, over-accumulation of BON1/CPN1 in two BON1/CPN1 promoter T-DNA insertion mutants did not affect resistance to a bacterial pathogen. Promoter deletion analysis identified a 280 bp segment of the BON1/CPN1 promoter as being required for pathogen-induced gene expression; the same promoter region was also required for calcium ionophore-induced gene expression. Leaf infiltration with calcium ionophore triggered high-level PR gene expression specifically in cpn1-1 plants grown under permissive conditions, while co-infiltration of the calcium chelator EGTA attenuated this effect. These results explain the conditional nature of the cpn1-1 phenotype and are consistent with BON1/CPN1 being a calcium- and pathogen-responsive plant defense suppressor.

Cloning and characterization of a calcium dependent protein kinase gene associated with cotton fiber development

The gene GhCPK1 encoding a calcium dependent protein kinase was identified from cotton. Transcripts of GhCPK1 accumulated primarily in the elongating fiber, and Arabidopsis plants transformed with GhCPK1 promoter-GUS construct exhibited GUS activity mainly in the developing trichomes, roots, young leaves and sepals. In the bombarded onion epidermal cells, GhCPK1-GFP fusion proteins showed a subcellular distribution in the plasma membrane. In vitro assays indicated that GhCPK1 was a functional calcium-dependent kinase able to undergo autophosphorylation and phosphorylation of the known substrate histone III-S. Together, these results suggest that GhCPK1 may play a role in the calcium signaling events associated with fiber elongation.

Expression of TERF1 in rice regulates expression of stress-responsive genes and enhances tolerance to drought and high-salinity

Drought and high-salinity are the important constraints that severely affect plant development and crop yield worldwide. It has been established that ethylene response factor (ERF) proteins play important regulatory roles in plant response to abiotic and biotic stresses. Our previous researches have revealed that transgenic tobacco over-expressing TERF1 (encoding a tomato ERF protein) showed enhanced tolerance to abiotic stress. Here, we further investigate the function of TERF1 in transgenic rice. Compared with the wild-type plants, overexpression of TERF1 resulted in an increased tolerance to drought and high-salt in transgenic rice. And the enhanced tolerance may be associated with the accumulation of proline and the decrease of water loss. Furthermore, TERF1 can effectively regulate the expression of stress-related functional genes Lip5, Wcor413-l, OsPrx and OsABA2, as well as regulatory genes OsCDPK7, OsCDPK13 and OsCDPK19 under normal growth conditions. Our analyses of cis-acting elements show that there exist DRE/CRT and/or GCC-box existing in TERF1 targeted gene promoters. Our results revealed that ectopic expression of TERF1 in rice caused a series of molecular and physiological alterations and resulted in the transgenic rice with enhanced tolerance to abiotic stress, indicating that TERF1 might have similar regulatory roles in response to abiotic stress in tobacco and rice.

Plant calcineurin B-like proteins and their interacting protein kinases

Calcium serves as a critical messenger in many adaptation and developmental processes. Cellular calcium signals are detected and transmitted by sensor molecules such as calcium-binding proteins. In plants, the calcineurin B-like protein (CBL) family represents a unique group of calcium sensors and plays a key role in decoding calcium transients by specifically interacting with and regulating a family of protein kinases (CIPKs). Several CBL proteins appear to be targeted to the plasma membrane by means of dual lipid modification by myristoylation and S-acylation. In addition, CBL/CIPK complexes have been identified in other cellular localizations, suggesting that this network may confer spatial specificity in Ca2+ signaling. Molecular genetics analyses of loss-of function mutants have implicated several CBL proteins and CIPKs as important components of abiotic stress responses, hormone reactions and ion transport processes. The occurrence of CBL and CIPK proteins appears not to be restricted to the plant kingdom raising the question about the function of these Ca2+ decoding components in non-plant species.

Genetic approaches towards overcoming water deficit in plants - special emphasis on LEAs

Water deficit arises as a result of low temperature, salinity and dehydration, thereby affecting plant growth adversely and making it imperative for plants to surmount such situations by acclimatizing/adapting at various levels. Water deficit stress results in significant changes in gene expression, mediated by interconnected signal transduction pathways that may be triggered by calcium, and regulated via ABA dependent and/or independent pathways. Hence, adaptation of plants to such stresses involves maintaining cellular homeostasis, detoxification of harmful elements and also growth alterations. Stress in general cause excess production of reactive oxygen species (ROS) and the plants overcome the same by either preventing the accumulation of ROS or by eliminating the ROS formed. Ion homeostasis includes processes such as cellular uptake, sequestration and export in conjunction with long distance transport. Requisite amounts of osmolytes are hence synthesized under stress to maintain turgor along with maintaining the macromolecular structures and also for scavenging ROS. Another noteworthy response is the accumulation of novel proteins, including enzymes involved in the biosynthesis of osmoprotectants, heat-shock proteins (HSPs), late embryogenesis abundant (LEA) proteins, antifreeze proteins, chaperones, detoxification enzymes, transcription factors, kinases and phosphatases. The LEAs belong to a redundant protein family and are highly hydrophilic, boiling-soluble, non-globular and therefore have been defined and classified accordingly. The precise function of LEAs is still unknown, but substantial evidence indicates their involvement in dessication tolerance as the expression of LEAs confers increased resistance to stress in heterologous yeast system and also significantly improves water deficit tolerance in transgenic plants. Genetic manipulation of plants towards conferring abiotic stress tolerance is a daunting task, as the abiotic stress tolerance mechanism is highly complex and various strategies have been exploited to address and evaluate the stress tolerance mechanism, and the molecular responses to water deficit via complex signaling networks. Genomic technologies have recently been useful in integrating the multigenicity of the plant stress responses through, transcriptomics, proteomics and metabolite profilling and their interactions. This review deals with the recent developments on genetic approaches for water stress tolerance in plants, with special emphasis on LEAs.

The calmodulin-binding protein kinase 3 is part of heat-shock signal transduction in Arabidopsis thaliana

Based on our previous findings, we proposed a pathway for the participation of Ca(2+)/calmodulin (CaM) in heat-shock (HS) signal transduction. The specific mechanism by which CaM regulates activation of heat-shock transcription factors (HSFs) is not known. CaM-binding protein kinases (CBK) are the most poorly understood of the CaM target proteins in plants. In this study, using a yeast two-hybrid assay, we found that AtCBK3 interacts with AtHSFA1a. Fluorescence resonance energy transfer was used to confirm the interaction between AtCBK3-YFP and AtHSFA1a-CFP. Furthermore, we demonstrate that purified recombinant AtCBK3 phosphorylated recombinant AtHSFA1a in vitro. We also describe the results of both downregulation of AtCBK3 expression and ectopic overexpression in Arabidopsis thaliana. The T-DNA insertion AtCBK3 knockout lines had impaired basal thermotolerance, which could be complemented by transformation of plants with the native gene. Overexpression of AtCBK3 resulted in plants with increased basal thermotolerance. Results from real-time quantitative PCR and protein gel-blot analyses suggest that AtCBK3 regulates transcription of heat-shock protein (HSP) genes and synthesis of HSPs. The binding activity of HSF to the heat-shock element (HSE), the mRNA level of HSP genes and synthesis of HSPs were upregulated in AtCBK3-overexpressing lines after HS, but downregulated in AtCBK3 null lines. These results indicate that AtCBK3 controls the binding activity of HSFs to HSEs by phosphorylation of AtHSFA1a, and is an important component of the HS signal transduction pathway.