Transcriptionally and post-transcriptionally regulated response of 13 calmodulin genes to tobacco mosaic virus-induced cell death and wounding in tobacco plant

Expression of calmodulin and lipid transfer protein genes in Prunus incisa x serrula under different stress conditions

Calcium-binding proteins and pathogenesis-related (PR) proteins are crucial components of the inducible repertoire of plant stress and defence. Considering the important role played by calmodulin (CaM) and lipid transfer protein (LTP) in mediating plant signal transduction, the present study investigated the expression of Ltp and CaM genes in Prunus incisa x serrula (PIS) under various abiotic stress conditions. The aim of this study is to find out whether expression of these proteins is regulated in parallel or independently and to compare the expression profiles of CaM and allergenic proteins like Ltp under different stress conditions. Southern blot analyses indicated that Ltp and CaM are encoded by at least two to four genes, which might be indicative for the expected variability and presence of isoforms. Transcription levels of both genes were analysed in leaves and roots of micropropagated plantlets under low and high temperatures, salicylic acid and wounding stress, harvested after 0, 0.5, 1, 2, 4, 10, 24 and 72 h. Real Time qPCR data showed that both genes respond differently to various stresses. Furthermore, a high variation in transcription levels of both genes was observed in leaf tissues, while in roots both genes were expressed at a lower extent and down-regulated. Western blot analyses indicated that after 24 h the amount of CaM protein is higher, while the amount of LTP is lower in various stresses. Results obtained suggest that CaM and LTP are differentially regulated in response to different stresses in PIS plants, and additionally show tissue-specific expression, hinting at a potential role of different isoforms.

Cloning and characterization of Sdga gene encoding alpha-subunit of heterotrimeric guanosine 5'-triphosphate-binding protein complex in Scoparia dulcis

A homology-based cloning strategy yielded Sdga, a cDNA clone presumably encoding alpha-subunit of heterotrimeric guanosine 5'-triphosphate-binding protein complex, from leaf tissues of Scoparia dulcis. Phylogenetic tree analysis of G-protein alpha-subunits from various biological sources suggested that, unlike in animal cells, classification of Galpha-proteins into specific subfamilies could not be applicable to the proteins from higher plants. Restriction digests of genomic DNA of S. dulcis showed a single hybridized signal in Southern blot analysis, suggesting that Sdga is a sole gene encoding Galpha-subunit in this plant. The expression level of Sdga appeared to be maintained at almost constant level after exposure of the leaves to methyl jasmonate as analyzed by reverse-transcription polymerase chain reaction. These results suggest that Sdga plays roles in methyl jasmonate-induced responses of S. dulcis without a notable change in the transcriptional level.

A simple way to identify non-viable cells within living plant tissue using confocal microscopy

BACKGROUND

Plant cell death is a normal process during plant development. Mutant plants may exhibit misregulation of this process, which can lead to severe growth defects. Simple ways of visualising cell death in living plant tissues can aid the study of plant development and physiology.

RESULTS

Spectral variants of the fluorescent SYTOX dyes were tested for their usefulness for the detection of non-viable cells within plant embryos and roots using confocal laser-scanning microscopy. The dyes were selective for non-viable cells and showed very little background staining in living cells. Simultaneous detection of SYTOX dye and fluorescent protein (e.g. GFP) fluorescence was possible.

CONCLUSION

The fluorescent SYTOX dyes are useful for an easy and quick first assay of plant cell viability in living plant samples using fluorescence and confocal laser-scanning microscopy.

Disturbance of the Ca(2+)/calmodulin-dependent signalling pathway is responsible for the resistance of Arabidopsis dnd1 against Pectobacterium carotovorum infection

SUMMARY Arabidopsis thaliana wild-type Col-0 and its mutant, 'defence, no death' (dnd) 1-1, were infected with biotrophic Pseudomonas syringae pv. tomato strain DC3000 and necrotrophic Pectobacterium carotovorum strain KACC 10228, and cellular and molecular responses among them were then analysed. Col-0 wild-type was susceptible to both pathogens. By contrast, neither DC3000 nor KACC 10228 infected dnd1-1 (Yu et al., 1998. Proc. Natl. Acad. Sci. USA 95: 7819-7824). Neither of the pathogens triggered cell death or accumulation of active oxygen species in dnd1-1. KACC 10228 induced accelerated transcriptions of PDF1.2 and AtEBP genes in wild-type Col-0, while DC3000-induced transcriptions of them were relatively retarded. Neither of the pathogens modified the constitutive transcription of PR1 in dnd1-1. PDF1.2 and AtEBP transcriptions were not induced by the same treatments. Hydrogen peroxide scavengers, catalase and ascorbic acid, and LaCl(3), an inhibitor of Ca(2+) influx, diminished cell death and protected the wild-type plant from KACC 10228 infection, while EGTA inhibited cell death and pathogen growth. Exogenous Ca(2+) nullified resistance against KACC 10228 challenge in dnd1-1. W-7 and chloropromazine, two calmodulin antagonists, also triggered cell death in dnd1-1 and abolished resistance against KACC 10228. In summary, cell death is correlated with KACC 10228 infection and disease development. Furthermore, the resistance of dnd1-1 against P. carotovorum is dependent on calmodulin and inhibition of cytosolic Ca(2+) increment.

Cloning and expression of calmodulin gene in Scoparia dulcis

A homology-based cloning strategy yielded a cDNA clone, designated Sd-cam, encoding calmodulin protein from Scoparia dulcis. The restriction digests of genomic DNA of S. dulcis showed a single hybridized signal when probed with the fragment of this gene in Southern blot analyses, suggesting that Sd-cam occurs as a sole gene encoding calmodulin in the plant. The reverse-transcription polymerase chain reaction analysis revealed that Sd-cam was appreciably expressed in leaf, root and stem tissues. It appeared that transcription of this gene increased transiently when the leaf cultures of S. dulcis were treated with methyl jasmonate and calcium ionophore A23187. These results suggest that transcriptional activation of Sd-cam is one of the early cellular events of the methyl jasmonate-induced responses of S. dulcis.

Pathogen-induced binding of the soybean zinc finger homeodomain proteins GmZF-HD1 and GmZF-HD2 to two repeats of ATTA homeodomain binding site in the calmodulin isoform 4 (GmCaM4) promoter

Calmodulin (CaM) is involved in defense responses in plants. In soybean (Glycine max), transcription of calmodulin isoform 4 (GmCaM4) is rapidly induced within 30 min after pathogen stimulation, but regulation of the GmCaM4 gene in response to pathogen is poorly understood. Here, we used the yeast one-hybrid system to isolate two cDNA clones encoding proteins that bind to a 30-nt A/T-rich sequence in the GmCaM4 promoter, a region that contains two repeats of a conserved homeodomain binding site, ATTA. The two proteins, GmZF-HD1 and GmZF-HD2, belong to the zinc finger homeodomain (ZF-HD) transcription factor family. Domain deletion analysis showed that a homeodomain motif can bind to the 30-nt GmCaM4 promoter sequence, whereas the two zinc finger domains cannot. Critically, the formation of super-shifted complexes by an anti-GmZF-HD1 antibody incubated with nuclear extracts from pathogen-treated cells suggests that the interaction between GmZF-HD1 and two homeodomain binding site repeats is regulated by pathogen stimulation. Finally, a transient expression assay with Arabidopsis protoplasts confirmed that GmZF-HD1 can activate the expression of GmCaM4 by specifically interacting with the two repeats. These results suggest that the GmZF-HD1 and -2 proteins function as ZF-HD transcription factors to activate GmCaM4 gene expression in response to pathogen.

Pathogen-induced calmodulin isoforms in basal resistance against bacterial and fungal pathogens in tobacco

Thirteen tobacco calmodulin (CaM) genes fall into three distinct amino acid homology types. Wound-inducible type I isoforms NtCaM1 and 2 were moderately induced by tobacco mosaic virus (TMV)-mediated hypersensitive reaction, and the type III isoform NtCaM13 was highly induced, while the type II isoforms NtCaM3-NtCaM12 showed little response. Type I and III knockdown tobacco lines were generated using inverted repeat sequences from NtCaM1 and 13, respectively, to evaluate the contribution of pathogen-induced calmodulins (CaMs) to disease resistance. After specific reduction of type I and III CaM gene expression was confirmed in both transgenic lines, we analyzed the response to TMV infection, and found that TMV susceptibility was slightly enhanced in type III CaM knockdown lines compared with the control line. Resistance to a compatible strain of the bacterial pathogen Ralstonia solanacearum, and fungal pathogens Rhizoctonia solani and Pythium aphanidermatum was significantly lower in type III but not in type I CaM knockdown plants. Expression of jasmonic acid (JA)- and/or ethylene-inducible basic PR genes was not affected in these lines, suggesting that type III CaM isoforms are probably involved in basal defense against necrotrophic pathogens in a manner that is independent of JA and ethylene signaling.

Genome-wide identification and analyses of the rice calmodulin and related potential calcium sensor proteins

BACKGROUND

A wide range of stimuli evoke rapid and transient increases in [Ca2+]cyt in plant cells which are transmitted by protein sensors that contain EF-hand motifs. Here, a group of Oryza sativa L. genes encoding calmodulin (CaM) and CaM-like (CML) proteins that do not possess functional domains other than the Ca2+-binding EF-hand motifs was analyzed.

RESULTS

By functional analyses and BLAST searches of the TIGR rice database, a maximum number of 243 proteins that possibly have EF-hand motifs were identified in the rice genome. Using a neighbor-joining tree based on amino acid sequence similarity, five loci were defined as Cam genes and thirty two additional CML genes were identified. Extensive analyses of the gene structures, the chromosome locations, the EF-hand motif organization, expression characteristics including analysis by RT-PCR and a comparative analysis of Cam and CML genes in rice and Arabidopsis are presented.

CONCLUSION

Although many proteins have unknown functions, the complexity of this gene family indicates the importance of Ca2+-signals in regulating cellular responses to stimuli and this family of proteins likely plays a critical role as their transducers.

Transcriptional Induction of Two Genes for CCaPs, Novel Cytosolic Proteins, in Arabidopsis thaliana in the Dark

Ca2+-signaling in downstream effectors is supported by many kinds of Ca2+-binding proteins, which function as a signal mediator and a Ca2+-buffering protein. We found in Arabidopsis thaliana a new type of Ca2+-binding protein, CCaP1, which consists of 152 amino acid residues, and binds (45)Ca2+ even in the presence of a high concentration of Mg2+. We found two other proteins with similar motifs, CCaP2 and CCaP3. These three proteins had no organelle localization signal and their green fluorescent protein (GFP) fusions were detected in the cytosol. Real-time PCR and histochemical analysis of promoter-beta-glucuronidase fusions revealed that CCaP1 was predominantly expressed in petioles while CCaP2 was expressed in roots. CCaP3 was hardly expressed. Expression of CCaP1 and CCaP2 was enhanced in darkness and became maximal after 24 h. Immunoblotting revealed petiole-specific accumulation of CCaP1. Expression of CCaP1 and CCaP2 was suppressed by a high concentration of Ca2+ and other metal ions. Deletion of sucrose from the medium markedly increased the mRNA levels of CCaP1 and CCaP2 within 2 h. Gibberellic acid enhanced the expression of CCaP1 and CCaP2 by 5- and 2.5-fold, respectively, after 6 h. CCaP1 and CCaP2 were suppressed in the petiole and the root, respectively, by light and the product of photosynthesis (sucrose) or both. These results suggest that CCaP1 functions as a mediator in response to continuous dark or gibberellic acid.

Early signaling events induced by elicitors of plant defenses

Plant pathogen attacks are perceived through pathogen-issued compounds or plant-derived molecules that elicit defense reactions. Despite the large variety of elicitors, general schemes for cellular elicitor signaling leading to plant resistance can be drawn. In this article, we review early signaling events that happen after elicitor perception, including reversible protein phosphorylations, changes in the activities of plasma membrane proteins, variations in free calcium concentrations in cytosol and nucleus, and production of nitric oxide and active oxygen species. These events occur within the first minutes to a few hours after elicitor perception. One specific elicitor transduction pathway can use a combination or a partial combination of such events which can differ in kinetics and intensity depending on the stimulus. The links between the signaling events allow amplification of the signal transduction and ensure specificity to get appropriate plant defense reactions. This review first describes the early events induced by cryptogein, an elicitor of tobacco defense reactions, in order to give a general scheme for signal transduction that will be use as a thread to review signaling events monitored in different elicitor or plant models.

Calcium in plant defence-signalling pathways

In plant cells, the calcium ion is a ubiquitous intracellular second messenger involved in numerous signalling pathways. Variations in the cytosolic concentration of Ca2+ ([Ca2+]cyt) couple a large array of signals and responses. Here we concentrate on calcium signalling in plant defence responses, particularly on the generation of the calcium signal and downstream calcium-dependent events participating in the establishment of defence responses with special reference to calcium-binding proteins.

Catalytic activation of the plant MAPK phosphatase NtMKP1 by its physiological substrate salicylic acid-induced protein kinase but not by calmodulins

MAPK phosphatases (MKPs) are negative regulators of MAPKs. Previously, we identified NtMKP1 as a novel calmodulin (CaM)-binding protein (Yamakawa, H., Katou, S., Seo, S., Mitsuhara, I., Kamada, H., and Ohashi, Y. (2004) J. Biol. Chem. 279, 928-936). In this study, we characterized the interaction of NtMKP1 with substrate MAPKs and CaM. NtMKP1 (produced by in vitro transcription/translation) inactivated salicylic acid-induced protein kinase (SIPK) through dephosphorylation of the TEY motif of SIPK. CaM bound but unexpectedly did not activate the phosphatase activity of NtMKP1. NtMKP1 has four characteristic domains, viz. a dual-specificity phosphatase catalytic domain, a gelsolin homology domain, a CaM-binding domain, and C-terminal domain. Deletion analysis revealed that the N-terminal non-catalytic region of NtMKP1 bound SIPK and was essential for inactivating SIPK, whereas the CaM-binding and C-terminal domains were dispensable. Moreover, the phosphatase activity of NtMKP1 was increased strongly by the binding of SIPK, but weakly by another MAPK, wound-induced protein kinase. Swapping and site-directed mutagenesis of SIPK and wound-induced protein kinase revealed that the strong activation of NtMKP1 phosphatase activity by SIPK partially depended on the putative common docking domain of SIPK. On the other hand, conversion of Lys(41) and Arg(43) of NtMKP1 to Ala (K41A/R43A) abolished the interaction with SIPK. Expression of constitutively active MAPK kinase in Nicotiana benthamiana induced activation of SIPK and cell death. Simultaneous expression of either NtMKP1 or NtMKP1 L443R, which was unable to bind CaM, compromised the constitutively active MAPK kinase-induced responses, whereas that of NtMKP1 K41A/R43A did not. These results indicate that the regulation of NtMKP1 activity by SIPK binding, but not by CaM binding, is important for the function of NtMKP1.

Calmodulin-like proteins from Arabidopsis and tomato are involved in host defense against Pseudomonas syringae pv. tomato

Complex signal transduction pathways underlie the myriad plant responses to attack by pathogens. Ca(2+) is a universal second messenger in eukaryotes that modulates various signal transduction pathways through stimulus-specific changes in its intracellular concentration. Ca(2+)-binding proteins such as calmodulin (CaM) detect Ca(2+) signals and regulate downstream targets as part of a coordinated cellular response to a given stimulus. Here we report the characterization of a tomato gene (APR134) encoding a CaM-related protein that is induced in disease-resistant leaves in response to attack by Pseudomonas syringae pv. tomato. We show that suppression of APR134 gene expression in tomato (Solanum lycopersicum), using virus-induced gene silencing (VIGS), compromises the plant's immune response. We isolated APR134-like genes from Arabidopsis, termed CML42 and CML43, to investigate whether they serve a functionally similar role. Gene expression analysis revealed that CML43 is rapidly induced in disease-resistant Arabidopsis leaves following inoculation with Pseudomonas syringae pv. tomato. Overexpression of CML43 in Arabidopsis accelerated the hypersensitive response. Recombinant APR134, CML42, and CML43 proteins all bind Ca(2+ )in vitro. Collectively, our data support a role for CML43, and APR134 as important mediators of Ca(2+)-dependent signals during the plant immune response to bacterial pathogens.

Cloning and expression of calmodulin genes regulating phytoalexin production in carrot cells

A homology-based cloning strategy yielded four cDNA clones encoding the open reading frame of carrot calmodulin, designated cam-4 and cam-8 from an oligogalacturonide elicitor-treated cell culture and cam-11 and cam-16 from cells exposed to ethylene, respectively. Reverse-transcription polymerase chain reaction analyses revealed that the expression of the cam-4 gene increased transiently when carrot seedlings were treated with oligogalacturonides, although, the cells incubated with fungal mycelial walls or ethylene did not show a significant change in the expression level. In contrast, marked and transient expression was observed for either cam-11 or cam-16 only when carrot cells were treated with ethylene. These results suggest that two classes of stimuli which are capable of triggering phytoalexin production in carrot cells, oligogalacturonides and ethylene, evoke the activation of the Ca2+ cascade in the cells by expressing distinct calmodulin genes to initiate the biosynthesis of the compound.

Molecular cloning and characterization of calmodulin genes in young oak seedlings (Quercus petraea L.) during early flooding stress

As part of an integrated study on the molecular response of woody plants to flooding, three CaM genes were isolated from oak seedlings (Quercus petraea Liebl.) and characterized. QpCaM-1 was almost exclusively expressed in roots, whereas QpCaM-2 and -3 were more evenly distributed throughout the plant. The present paper documents the differential expression of these genes during hypoxia.

Multiple Forms of Calmodulin Genes in Carrot Treated with Fungal Mycelial Walls

Eleven cDNA clones encoding the open-reading frame of calmodulin and calmodulin-like proteins were isolated from carrot seedlings treated with fungal mycelial walls. These calmodulin genes were mainly expressed in the stems and leaves of carrot, although the transcriptional level was almost negligible in the seeds and root tissues. Based on nucleotide homology, these genes were divided into two classes, class I and class II, and reverse transcription-polymerase chain reaction analyses revealed that the expression level of class II genes transiently increased when carrot seedlings were treated with the mycelial walls. In contrast, the level of class I genes apparently did not show any significant change even after treatment with the fungal components. These results suggest that the defense responses induced in the fungi-infected carrot accompany the increase in the transcriptional levels of a certain calmodulin gene(s).

Plant-specific calmodulin-binding proteins

Calmodulin CaM is the most prominent Ca2+ transducer in eukaryotic cells, regulating the activity of numerous proteins with diverse cellular functions. Many features of CaM and its downstream targets are similar in plants and other eukaryotes. However, plants possess a unique set of CaM-related proteins, and several unique CaM target proteins. This review discusses recent progress in identifying plant-specific CaM-binding proteins and their roles in response to biotic and abiotic stresses and development. The review also addresses aspects emerging from recent structural studies of CaM interactions with target proteins relevant to plants.

Three types of tobacco calmodulins characteristically activate plant NAD kinase at different Ca2+ concentrations and pHs

We previously reported that three types of tobacco calmodulin (CaM) isoforms originated from 13 genes are differently regulated at the transcript and protein levels in response to wounding and tobacco mosaic virus-induced hypersensitive reaction (HR); wound-inducible type I and HR-inducible type III levels increased after wounding and HR, respectively, while type II, whose expression is constitutive and wound responsible, remained unchanged. Here, we show that these CaMs differentially activate target enzymes; rat NO synthase was activated most effectively by type III, moderately by type I and weakly by type II, and plant NAD kinase (NADK) was activated in the inverse order. Furthermore, we found that a suitable Ca2+ concentration differs by type; type II activated NADK at lower Ca2+ of around 0.1 microM, which is the cytosolic concentration in unstimulated cells, type I did so at 1-5 microM, which is the increased Ca2+ concentration in stimulated cells, while type III did not at any Ca2+ level. NADK activation was highest over a pH range of 7.1-6.8 for which the cytosolic pH reportedly changed from 7.5 after being stimulated. Thus, tobacco CaMs, especially type I, effectively activate NADK in stimuli-induced conditions.

A tobacco calcium/calmodulin-binding protein kinase functions as a negative regulator of flowering

A tobacco calcium/calmodulin-binding protein kinase (NtCBK1) was isolated and identified. The predicted NtCBK1 protein has 599 amino acids, an N-terminal kinase domain, and shares high homology with other calmodulin (CaM)-related kinases. Whereas NtCBK1 phosphorylates itself and substrates such as histone IIIS and syntide-2 in the absence of CaM, its kinase activity can be stimulated by tobacco CaMs. However, unlike another tobacco protein kinase designated NtCBK2, NtCBK1 was not differentially regulated by the different CaM isoforms tested. The CaM-binding domain of NtCBK1 was located between amino acids 436 and 455, and this domain was shown to be necessary for CaM modulation of kinase activity. RNA in situ hybridization showed that NtCBK1 was highly regulated in the transition to flowering. Whereas NtCBK1 mRNA was accumulated in the shoot apical meristem during vegetative growth, its expression was dramatically decreased in the shoot apical meristem after floral determination, and in young flower primordia. The expression of NtCBK1 was up-regulated to high levels in floral organ primordia. Fluctuations in NtCBK1 expression were verified by analysis of tobacco plants expressing green fluorescent protein under the control of the NtCBK1 promoter, suggesting a role of NtCBK1 in the transition to flowering. This conclusion was confirmed by overexpressing NtCBK1 in transgenic tobacco plants, where maintenance of high levels of NtCBK1 in the shoot apical meristem delayed the switch to flowering and extended the vegetative phase of growth. Further work indicated that overexpression of NtCBK1 in transgenic tobacco did not affect the expression of NFL, a tobacco homologue of the LFY gene that controls meristem initiation and floral structure in tobacco. In addition, the promotion of tobacco flowering time by DNA demethylation cannot be blocked by the overexpression of NtCBK1.

Characterization of a novel calcium/calmodulin-dependent protein kinase from tobacco

A cDNA encoding a calcium (Ca2+)/calmodulin (CaM)-dependent protein kinase (CaMK) from tobacco (Nicotiana tabacum), NtCaMK1, was isolated by protein-protein interaction-based screening of a cDNA expression library using 35S-labeled CaM as a probe. The genomic sequence is about 24.6 kb, with 21 exons, and the full-length cDNA is 4.8 kb, with an open reading frame for NtCaMK1 consisting of 1,415 amino acid residues. NtCaMK1 has all 11 subdomains of a kinase catalytic domain, lacks EF hands for Ca2+-binding, and is structurally similar to other CaMKs in mammal systems. Biochemical analyses have identified NtCaMK1 as a Ca2+/CaMK since NtCaMK1 phosphorylated itself and histone IIIs as substrate only in the presence of Ca2+/CaM with a Km of 44.5 microm and a Vmax of 416.2 nm min(-1) mg(-1). Kinetic analysis showed that the kinase not previously autophosphorylated had a Km for the synthetic peptide syntide-2 of 22.1 microm and a Vmax of 644.1 nm min(-1) mg(-1) when assayed in the presence of Ca2+/CaM. Once the autophosphorylation of NtCaMK1 was initiated, the phosphorylated form displayed Ca2+/CaM-independent behavior, as many other CaMKs do. Analysis of the CaM-binding domain (CaMBD) in NtCaMK1 with truncated and site-directed mutated forms defined a stretch of 20 amino acid residues at positions 913 to 932 as the CaMBD with high CaM affinity (Kd = 5 nm). This CaMBD was classified as a 1-8-14 motif. The activation of NtCaMK1 was differentially regulated by three tobacco CaM isoforms (NtCaM1, NtCaM3, and NtCaM13). While NtCaM1 and NtCaM13 activated NtCaMK1 effectively, NtCaM3 did not activate the kinase.

Proteomics of calcium-signaling components in plants

Calcium functions as a versatile messenger in mediating responses to hormones, biotic/abiotic stress signals and a variety of developmental cues in plants. The Ca(2+)-signaling circuit consists of three major "nodes"--generation of a Ca(2+)-signature in response to a signal, recognition of the signature by Ca2+ sensors and transduction of the signature message to targets that participate in producing signal-specific responses. Molecular genetic and protein-protein interaction approaches together with bioinformatic analysis of the Arabidopsis genome have resulted in identification of a large number of proteins at each "node"--approximately 80 at Ca2+ signature, approximately 400 sensors and approximately 200 targets--that form a myriad of Ca2+ signaling networks in a "mix and match" fashion. In parallel, biochemical, cell biological, genetic and transgenic approaches have unraveled functions and regulatory mechanisms of a few of these components. The emerging paradigm from these studies is that plants have many unique Ca2+ signaling proteins. The presence of a large number of proteins, including several families, at each "node" and potential interaction of several targets by a sensor or vice versa are likely to generate highly complex networks that regulate Ca(2+)-mediated processes. Therefore, there is a great demand for high-throughput technologies for identification of signaling networks in the "Ca(2+)-signaling-grid" and their roles in cellular processes. Here we discuss the current status of Ca2+ signaling components, their known functions and potential of emerging high-throughput genomic and proteomic technologies in unraveling complex Ca2+ circuitry.

Plant MAPK phosphatase interacts with calmodulins

A mitogen-activated protein kinase (MAPK) phosphatase gene, designated NtMKP1, was isolated as a candidate gene for a calmodulin (CaM)-binding protein from tobacco. NtMKP1 protein has four characteristic domains conserved among plant MAPK phosphatases reported so far, namely a dual specificity protein phosphatase catalytic domain, gelsolin-like domain, putative CaM-binding domain (CaMBD), and serine-rich region, indicating that NtMKP1 is the ortholog of Arabidopsis MKP1. The bacterially expressed NtMKP1 protein physically interacted with three plant-specific types of CaM in an overlay assay with labeled CaMs, showing high affinity to NtCaM1 and NtCaM3 but lower affinity to NtCaM13. The peptide for the putative CaMBD bound both NtCaM1 and NtCaM3 significantly but bound NtCaM13 only slightly. Moreover, CaM overlay assays with mutated CaMBDs revealed that Trp440 and Leu443 in the middle of the basic amphiphilic alpha-helix motif (amino acids 436-453) are critical for binding CaM. In comparison with the transient accumulation of a wound-induced MAPK, WIPK transcript, a prolonged activation of NtMKP1 expression was found in response to wounding and tobacco mosaic virus-induced hypersensitive reaction. In transgenic tobacco plants overexpressing NtMKP1, wound-induced activation of SIPK, salicylic acid-induced MAPK, and WIPK was inhibited. These results suggest that plant CaMs are involved in these stress-activated MAPK cascades via NtMKP1.

A tobacco (Nicotiana tabaccum) calmodulin-binding protein kinase, NtCBK2, is regulated differentially by calmodulin isoforms

A calcium (Ca2+)/calmodulin (CaM)-binding protein kinase (CBK) from tobacco (Nicotiana tabaccum ), NtCBK2, has been characterized molecularly and biochemically. NtCBK2 has all 11 conserved subdomains of the kinase-catalytic domain and a CaM-binding site as shown by other kinases, including Ca2+-dependent protein kinase and chimaeric Ca2+/CaM-dependent protein kinases. However, this kinase does not contain an EF-hand motif for Ca2+ binding, and its activity was not regulated by Ca2+. Whereas NtCBK2 phosphorylated both itself and other substrates, such as histone IIIS and syntide-2, in a Ca2+/CaM-independent manner, as also shown by OsCBK, a CaM-binding protein kinase from rice (Oryza sativa ), the kinase activity of NtCBK2 was greatly stimulated by Ca2+/CaM, whereas that of OsCBK was not. By molecular dissection analyses, the CaM-binding domain of NtCBK2 has been localized in a stretch of 30 amino acid residues at residue positions 431-460 as a 1-5-10 protein motif. Three tobacco CaM isoforms (NtCaM1, NtCaM3 and NtCaM13) used in the present study have been shown to bind to NtCBK2, but with different dissociation constants ( K(d)s), as follows: NtCaM1, 55.7 nM; NtCaM3, 25.4 nM; and NtCaM13, 19.8 nM, indicating that NtCBK2 has a higher affinity for NtCaM3 and NtCaM13 than for NtCaM1. The enzymic activity of NtCBK2 was also modulated differently by various CaM isoforms. Whereas the phosphorylation activity of NtCBK2 was shown by assay to be enhanced only approximately 2-3-fold by the presence of NtCaM1, the activity could be amplified up to 8-9-fold by NtCaM3 or 10-11-fold by NtCaM13, suggesting that NtCaM3 and NtCaM13 are better activators than NtCaM1 for NtCBK2.

Differential expression and functional analysis of three calmodulin isoforms in germinating pea (Pisum sativum L.) seeds

Implication of the ubiquitous, highly conserved, Ca2+ sensor calmodulin (CaM) in pea seed germination has been investigated. Mass spectrometry analysis of purified CaM revealed the coexistence in seeds of three protein isoforms, diverging from each other by single amino acid substitution in the N-terminal alpha-helix. CaM was shown to be encoded by a small multigenic family, and full-length cDNAs of the three isoforms (PsCaM1, 2 and 3) were isolated to allow the design of specific primers in more divergent 5' and 3' untranslated regions. Expression studies, performed by semiquantitative RT-PCR, demonstrated differential expression patterns of the three transcripts during germination. PsCaM1 and 2 were detected at different levels in dry axes and cotyledons, and they accumulated during imbibition and prior to radicle protrusion. In contrast, PsCaM3 appeared only upon radicle protrusion, then gradually increased in both tissues. To characterise the biochemical properties of the CaM isoforms, functional analyses were conducted in vitro using recombinant Strep-tagged proteins (CaM1-ST, CaM2-ST and CaM3-ST) expressed in Escherichia coli. Gel mobility shift assays revealed that CaM1-ST exhibited a stoichiometric binding of a synthetic amphiphilic CaM kinase II peptide while CaM2-ST and CaM3-ST affinities for the same peptide were reduced. Affinity differences were also observed for CaM isoform binding to Trp-3, an idealised helical CaM-binding peptide. However, the three proteins activated in the same way the CaM-dependent pea NAD kinase. Finally, the significance of the single substitutions upon CaM interaction with its targets is discussed in a structural context.

Induced expression of a temperature-sensitive leucine-rich repeat receptor-like protein kinase gene by hypersensitive cell death and wounding in tobacco plant carrying the N resistance gene

A gene encoding a receptor-like protein kinase was isolated as the gene induced in the early period of N gene-dependent hypersensitive cell death in tobacco leaves. The kinase domain expressed as a glutathione S-transferase fusion protein was capable of autophosphorylation, indicating that this gene encodes an active protein kinase. A high level of the transcript accumulated before necrotic lesion formation in tobacco mosaic virus (TMV)-inoculated tobacco leaves carrying the N gene but it was low in a tobacco cultivar lacking the N gene. A small but reproducible increase in the transcript was found 1-2 h after a temperature shift from 30 degrees C to 20 degrees C even in healthy leaves, suggesting the gene expression is temperature sensitive. The gene was named WRK for wound-induced receptor-like protein kinase, because the transcript increased to a maximum within 15-30 min of wounding. In suspension cultured tobacco cells, an increase in the transcript was found 15 min after transfer to a new medium, but it was suppressed under high osmotic pressures. The wound-induced WRK accumulation was enhanced by cycloheximide treatment, but not by known defense signal compounds (salicylic acid, jasmonic acid, 1-aminocyclopropan-1-carboxylic acid and abscisic acid) and some plant hormones. Thus, WRK is a wound-inducible and temperature-sensitive protein kinase gene induced before hypersensitive cell death probably through unknown signaling pathways.