Expression of three members of the calcium-dependent protein kinase gene family in Arabidopsis thaliana

Transcriptome sequencing and expression analysis in peanut reveal the potential mechanism response to Ralstonia solanacearum infection

BACKGROUND

Bacterial wilt caused by Ralstonia solanacearum severely affects peanut (Arachis hypogaea L.) yields. The breeding of resistant cultivars is an efficient means of controlling plant diseases. Therefore, identification of resistance genes effective against bacterial wilt is a matter of urgency. The lack of a reference genome for a resistant genotype severely hinders the process of identification of resistance genes in peanut. In addition, limited information is available on disease resistance-related pathways in peanut.

RESULTS

Full-length transcriptome data were used to generate wilt-resistant and -susceptible transcript pools. In total, 253,869 transcripts were retained to form a reference transcriptome for RNA-sequencing data analysis. Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis of differentially expressed genes revealed the plant-pathogen interaction pathway to be the main resistance-related pathway for peanut to prevent bacterial invasion and calcium plays an important role in this pathway. Glutathione metabolism was enriched in wilt-susceptible genotypes, which would promote glutathione synthesis in the early stages of pathogen invasion. Based on our previous quantitative trait locus (QTL) mapping results, the genes arahy.V6I7WA and arahy.MXY2PU, which encode nucleotide-binding site-leucine-rich repeat receptor proteins, were indicated to be associated with resistance to bacterial wilt.

CONCLUSIONS

This study identified several pathways associated with resistance to bacterial wilt and identified candidate genes for bacterial wilt resistance in a major QTL region. These findings lay a foundation for investigation of the mechanism of resistance to bacterial wilt in peanut.

The Roles of CDPKs as a Convergence Point of Different Signaling Pathways in Maize Adaptation to Abiotic Stress

The calcium ion (Ca²⁺), as a well-known second messenger, plays an important role in multiple processes of growth, development, and stress adaptation in plants. As central Ca²⁺ sensor proteins and a multifunctional kinase family, calcium-dependent protein kinases (CDPKs) are widely present in plants. In maize, the signal transduction processes involved in ZmCDPKs' responses to abiotic stresses have also been well elucidated. In addition to Ca²⁺ signaling, maize ZmCDPKs are also regulated by a variety of abiotic stresses, and they transmit signals to downstream target molecules, such as transport proteins, transcription factors, molecular chaperones, and other protein kinases, through protein interaction or phosphorylation, etc., thus changing their activity, triggering a series of cascade reactions, and being involved in hormone and reactive oxygen signaling regulation. As such, ZmCDPKs play an indispensable role in regulating maize growth, development, and stress responses. In this review, we summarize the roles of ZmCDPKs as a convergence point of different signaling pathways in regulating maize response to abiotic stress, which will promote an understanding of the molecular mechanisms of ZmCDPKs in maize tolerance to abiotic stress and open new opportunities for agricultural applications.

Constitutive Active CPK30 Interferes With Root Growth and Endomembrane Trafficking in Arabidopsis thaliana

Calcium-dependent protein kinases (CPK) are key components of a wide array of signaling pathways, translating stress and nutrient signaling into the modulation of cellular processes such as ion transport and transcription. However, not much is known about CPKs in endomembrane trafficking. Here, we screened for CPKs that impact on root growth and gravitropism, by overexpressing constitutively active forms of CPKs under the control of an inducible promoter in Arabidopsis thaliana. We found that inducible overexpression of an constitutive active CPK30 (CA-CPK30) resulted in a loss of root gravitropism and ectopic auxin accumulation in the root tip. Immunolocalization revealed that CA-CPK30 roots have reduced PIN protein levels, PIN1 polarity defects and impaired Brefeldin A (BFA)-sensitive trafficking. Moreover, FM4-64 uptake was reduced, indicative of a defect in endocytosis. The effects on BFA-sensitive trafficking were not specific to PINs, as BFA could not induce aggregation of ARF1- and CHC-labeled endosomes in CA-CPK30. Interestingly, the interference with BFA-body formation, could be reverted by increasing the extracellular pH, indicating a pH-dependence of this CA-CPK30 effect. Altogether, our data reveal an important role for CPK30 in root growth regulation and endomembrane trafficking in Arabidopsis thaliana.

Identification of CDPK Gene Family in Solanum habrochaites and Its Function Analysis under Stress

Tomato is an important vegetable crop. In the process of tomato production, it will encounter abiotic stress, such as low temperature, drought, and high salt, and biotic stress, such as pathogen infection, which will seriously affect the yield of tomato. Calcium-dependent protein kinase (CDPK) is a class of major calcium signal receptor which has an important regulatory effect on the perception and decoding of calcium signals. CDPK plays a key role in many aspects of plant growth, such as the elongation of pollen tubes, plant growth, and response to biotic and abiotic stress. While some studies have concentrated on Arabidopsis and pepper, Solanum habrochaites is a wild species relative of cultivated tomato and there is no report on CDPK in Solanum habrochaites to date. Using tomato genomic data, this study identified 33 members of the CDPK gene family. Evolutionary analysis divides family members into four Asian groups, of which the CDPK family members have 11 gene replication pairs. Subcellular location analysis showed that most proteins were predicted to be located in the cytoplasm, and less protein existed on the cell membrane. Not all CDPK family members have a transmembrane domain. Cis regulatory elements relating to light, hormones, and drought stress are overrepresented in the promoter region of the CDPK genes in Solanum habrochaites. The expression levels of each gene under biotic stress and abiotic stress were quantified by qRT-PCR. The results showed that members of the CDPK family in Solanum habrochaites respond to different biotic and abiotic stresses. Among them, the expression of ShCDPK6 and ShCDPK26 genes change significantly. ShCDPK6 and ShCDPK26 genes were silenced using VIGS (virus-induced gene silencing), and the silenced plants illustrated reduced stress resistance to Botrytis cinerea, cold, and drought stress. The results of this study will provide a basis for the in-depth study of the CDPK gene family in Solanum habrochaites, laying the foundation for further analysis of the function of the gene family.

Genome-wide survey and expression analysis of calcium-dependent protein kinase (CDPK) in grass Brachypodium distachyon

BACKGROUND

Ca2+ played as a ubiquitous secondary messenger involved in plant growth, development, and responses to various environmental stimuli. Calcium-dependent protein kinases (CDPK) were important Ca2+ sensors, which could directly translate Ca2+ signals into downstream phosphorylation signals. Considering the importance of CDPKs as Ca2+ effectors for regulation of plant stress tolerance and few studies on Brachypodium distachyon were available, it was of interest for us to isolate CDPKs from B. distachyon.

RESULTS

A systemic analysis of 30 CDPK family genes in B. distachyon was performed. Results showed that all BdCDPK family members contained conserved catalytic Ser/Thr protein kinase domain, autoinhibitory domain, and EF-hand domain, and a variable N-terminal domain, could be divided into four subgroup (I-IV), based upon sequence homology. Most BdCDPKs had four EF-hands, in which EF2 and EF4 revealed high variability and strong divergence from EF-hand in AtCDPKs. Synteny results indicated that large number of syntenic relationship events existed between rice and B. distachyon, implying their high conservation. Expression profiles indicated that most of BdCDPK genes were involved in phytohormones signal transduction pathways and regulated physiological process in responding to multiple environmental stresses. Moreover, the co-expression network implied that BdCDPKs might be both the activator and the repressor involved in WRKY transcription factors or MAPK cascade genes mediated stress response processes, base on their complex regulatory network.

CONCLUSIONS

BdCDPKs might play multiple function in WRKY or MAPK mediated abiotic stresses response and phytohormone signaling transduction in B. distachyon. Our genomics analysis of BdCDPKs could provide fundamental information for further investigation the functions of CDPKs in integrating Ca2+ signalling pathways in response to environments stresses in B. distachyon.

Expression, Subcellular Localization, and Interactions of CPK Family Genes in Maize

Calcium-dependent protein kinase (CPKs) is a key player in the calcium signaling pathway to decode calcium signals into various physiological responses. cDNA sequences of 9 ZmCPK genes were successfully cloned from all four phylogenetic groups in maize. qRT-PCR analysis showed the expression variation of these selected genes under abscisic acid (ABA) and calcium chloride (CaCl2) treatment. Due to the presence of N-myristoylation/palmitoylation sites, the selected ZmCPK members were localized in a plasma membrane. To clarify whether ZmCPK, a key player in calcium signaling, interacts with key players of ABA, protein phosphatase 2Cs (PP2Cs) and the SNF1-related protein kinase 2s (SnRK2s) and mitogen-activated protein kinase (MAPK) signaling pathways in maize, we examined the interaction between 9 CPKs, 8 PP2Cs, 5 SnRKs, and 20 members of the MPK family in maize by using yeast two-hybrid assay. Our results showed that three ZmCPKs interact with three different members of ZmSnRKs while four ZmCPK members had a positive interaction with 13 members of ZmMPKs in different combinations. These four ZmCPK proteins are from three different groups in maize. These findings of physical interactions between ZmCPKs, ZmSnRKs, and ZmMPKs suggested that these signaling pathways do not only have indirect influence but also have direct crosstalk that may involve the defense mechanism in maize. The present study may improve the understanding of signal transduction in plants.

CircRNA Expression Pattern and ceRNA and miRNA-mRNA Networks Involved in Anther Development in the CMS Line of Brassica campestris

Male-sterile plants provide an important breeding tool for the heterosis of hybrid crops, such as Brassicaceae. In the last decade, circular RNAs (circRNAs), as a novel class of covalently closed and single-stranded endogenous non-coding RNAs (ncRNAs), have received much attention because of their functions as “microRNA (miRNA) sponges” and “competing endogenous RNAs” (ceRNAs). However, the information about circRNAs in the regulation of male-sterility and anther development is limited. In this study, we established the Polima cytoplasm male sterility (CMS) line “Bcpol97-05A”, and the fertile line, “Bcajh97-01B”, in Brassicacampestris L. ssp. chinensis Makino, syn. B. rapa ssp. chinensis, and performed RNA expression profiling comparisons between the flower buds of the sterile line and fertile line by whole-transcriptome sequencing. A total of 31 differentially expressed (DE) circRNAs, 47 DE miRNAs, and 4779 DE mRNAs were identified. By using Cytoscape, the miRNA-mediated regulatory network and ceRNA network were constructed, and the circRNA A02:23507399|23531438 was hypothesized to be an important circRNA regulating anther development at the post-transcriptional level. The gene ontology (GO) analysis demonstrated that miRNAs and circRNAs could regulate the orderly secretion and deposition of cellulose, sporopollenin, pectin, and tryphine; the timely degradation of lipids; and the programmed cell death (PCD) of tapetum cells, which play key roles in anther development. Our study revealed a new circRNA–miRNA–mRNA network, which is involved in the anther development of B. campestris, which enriched the understanding of CMS in flowering plants, and laid a foundation for further study on the functions of circRNAs and miRNAs during anther development.

The Arabidopsis Calcium-Dependent Protein Kinases (CDPKs) and Their Roles in Plant Growth Regulation and Abiotic Stress Responses

As a ubiquitous secondary messenger in plant signaling systems, calcium ions (Ca2+) play essential roles in plant growth and development. Within the cellular signaling network, the accurate decoding of diverse Ca2+ signal is a fundamental molecular event. Calcium-dependent protein kinases (CDPKs), identified commonly in plants, are a kind of vital regulatory protein deciphering calcium signals triggered by various developmental and environmental stimuli. This review chiefly introduces Ca2+ distribution in plant cells, the classification of Arabidopsis thaliana CDPKs (AtCDPKs), the identification of the Ca2+-AtCDPK signal transduction mechanism and AtCDPKs’ functions involved in plant growth regulation and abiotic stress responses. The review presents a comprehensive overview of AtCDPKs and may contribute to the research of CDPKs in other plants.

Genome-wide survey and expression analysis of calcium-dependent protein kinase in Gossypium raimondii

Calcium-dependent protein kinases (CDPKs) are one of the largest protein kinases in plants and participate in different physiological processes through regulating downstream components of calcium signaling pathways. In this study, 41 CDPK genes, from GrCPK1 to GrCPK41, were identified in the genome of the diploid cotton, Gossypium raimondii. The phylogenetic analysis indicated that all these genes were divided into four subgroups and members within the same subgroup shared conserved exon-intron structures. The expansion of GrCPKs family in G. raimondii was due to the segmental duplication events, and the analysis of Ka/Ks ratios implied that the duplicated GrCPKs had mainly undergone strong purifying selection pressure with limited functional divergence. The cold-responsive elements in promoter regions were detected in the majority of GrCPKs. The expression analysis of 11 selected genes showed that GrCPKs exhibited tissue-specific expression patterns and the expression of GrCPKs were induced or repressed by cold treatment. These observations would lay an important foundation for functional and evolutionary analysis of CDPK gene family in Gossypium species.

A mathematical model on water redistribution mechanism of the seismonastic movement of Mimosa pudica

A theoretical model based on the water redistribution mechanism is proposed to predict the volumetric strain of motor cells in Mimosa pudica during the seismonastic movement. The model describes the water and ion movements following the opening of ion channels triggered by stimulation. The cellular strain is related to the angular velocity of the plant movement, and both their predictions are in good agreement with experimental data, thus validating the water redistribution mechanism. The results reveal that an increase in ion diffusivity across the cell membrane of <15-fold is sufficient to produce the observed seismonastic movement.

Arabidopsis CDPK6 phosphorylates ADF1 at N-terminal serine 6 predominantly

KEY MESSAGE

We found that Arabidopsis AtADF1 was phosphorylated by AtCDPK6 at serine 6 predominantly and the phosphoregulation plays a key role in the regulation of ADF1-mediated depolymerization of actin filaments.

ABSTRACT

Since actin-depolymerizing factor (ADF) is highly conserved among eukaryotes, it is one of the key modulators for actin organization. In plants, ADF is directly involved in the depolymerization of actin filaments, and therefore important for F-actin-dependent cellular activities. The activity of ADF is tightly controlled through a number of molecular mechanisms, including phosphorylation-mediated inactivation of ADF. To investigate Arabidopsis ADF1 phosphoregulation, we generated AtADF1 phosphorylation site-specific mutants. Using transient expression and stable transgenic approaches, we analyzed the ADF1 phosphorylation mutants in the regulation of actin filament organizations in plant cells. By in vitro phosphorylation assay, we showed that AtADF1 is phosphorylated by AtCDPK6 at serine 6 predominantly. Chemically induced expression of AtCDPK6 can negatively regulate the wild-type AtADF1 in depolymerizing actin filaments, but not those of the mutants AtADF1(S6A) and AtADF1(S6D). These results demonstrate a regulatory function of Arabidopsis CDPK6 in the N-terminal phosphorylation of AtADF1.

14-3-3-regulated Ca(2+)-dependent protein kinase CPK3 is required for sphingolipid-induced cell death in Arabidopsis

In eukaryotic cells, sphingoid long chain bases (LCBs) such as sphingosine or phytosphingosine (PHS) behave as second messengers involved in various processes including programmed cell death (PCD). In plants, induction of PCD by LCBs has now been described, but the signalling pathway is still enigmatic. Using Arabidopsis, we identify new key steps in this pathway. We demonstrate that PHS induces activation of the calcium-dependent kinase CPK3, which phosphorylates its binding partners, the 14-3-3 proteins. This phosphorylation leads to the disruption of the complex and to CPK3 degradation. Using cpk3 knockout lines, we demonstrate that CPK3 is a positive regulator of LCB-mediated PCD. These findings establish 14-3-3-regulated CPK3 as a key component of the LCB pathway leading to PCD in plants.

The Ca(2+) -dependent protein kinase CPK3 is required for MAPK-independent salt-stress acclimation in Arabidopsis

Plants use different signalling pathways to respond to external stimuli. Intracellular signalling via calcium-dependent protein kinases (CDPKs) or mitogen-activated protein kinases (MAPKs) present two major pathways that are widely used to react to a changing environment. Both CDPK and MAPK pathways are known to be involved in the signalling of abiotic and biotic stresses in animal, yeast and plant cells. Here, we show the essential function of the CDPK CPK3 (At4g23650) for salt stress acclimation in Arabidopsis thaliana, and test crosstalk between CPK3 and the major salt-stress activated MAPKs MPK4 and MPK6 in the salt stress response. CPK3 kinase activity was induced by salt and other stresses after transient overexpression in Arabidopsis protoplasts, but endogenous CPK3 appeared to be constitutively active in roots and leaves in a strictly Ca(2+) -dependent manner. cpk3 mutants show a salt-sensitive phenotype comparable with mutants in MAPK pathways. In contrast to animal cells, where crosstalk between Ca(2+) and MAPK signalling is well established, CPK3 seems to act independently of those pathways. Salt-induced transcriptional induction of known salt stress-regulated and MAPK-dependent marker genes was not altered, whereas post-translational protein phosphorylation patterns from roots of wild type and cpk3 plants revealed clear differences. A significant portion of CPK3 was found to be associated with the plasma membrane and the vacuole, both depending on its N-terminal myristoylation. An initial proteomic study led to the identification of 28 potential CPK3 targets, predominantly membrane-associated proteins.

Expression and localization of calcium-dependent protein kinase isoforms in chickpea

Calcium-dependent protein kinases (CPKs) play important roles in multiple signal transduction pathways but the precise role of individual CPK is largely unknown. We isolated two cDNAs encoding two CPK isoforms (Cicer arietinum CPKs-CaCPK1 and CaCPK2) of chickpea. Their expression in various organs and in response to various phytohormones, and dehydration, high salt stress and fungal spore in excised leaves as well as localization in leaf and stem tissues were analyzed in this study. CaCPK1 protein and its activity were ubiquitous in all tissues examined. In contrast, CaCPK2 transcript, CaCPK2 protein and its activity were almost undetectable in flowers and fruits. Both CaCPK1 and CaCPK2 transcripts and proteins were abundant in roots but in minor quantities in leaves and stems. Of the three phytohormones tested, viz. indole-3-acetic acid (IAA), gibberellin (GA(3)) and benzyladenine (BA), only BA increased both CaCPK1 and CaCPK2 transcripts, proteins and their activities. GA(3) induced accumulation of CaCPK2 transcript and protein but CaCPK1 remained unaffected. The expression of CaCPK1 and CaCPK2 in leaves was enhanced in response to high salt stress. Treatments with Aspergillus sp. spores increased expression of CaCPK1 in chickpea leaf tissue but had no effect on CaCPK2. Excised leaves subjected to dehydration showed increase in CaCPK2 expression but not in CaCPK1. Both isoforms were located in the plasma membrane (PM) and chloroplast membrane of leaf mesophyll cells as well as in the PM of stem xylem parenchyma cells. These results suggest specific roles for CaCPK isoforms in phytohormone/defense/stress signaling pathways.

VfCPK1, a gene encoding calcium-dependent protein kinase from Vicia faba, is induced by drought and abscisic acid

Calcium, one of the most ubiquitous second messengers, has been shown to be involved in a wide variety of responses in plants. Calcium-dependent protein kinases (CDPKs) (EC 2.7.1.37) are the predominant Ca(2+)-regulated serine/threonine protein kinase in plants and play an important role in plant calcium signal transduction. CDPKs are encoded by a large multigene family in many plants, which has been showed so far; however, the precise role of each specific CDPK is still largely unknown. A novel CDPK gene designated as VfCPK1 was cloned from epidermal peels of broad bean (Vicia faba L.) leaves using the rapid amplification of cDNA ends (RACE)-PCR technique and its expression was studied in detail. The VfCPK1 cDNA is 1783 bp long and contains an open reading frame of 1482 bp encoding 493 amino acids. VfCPK1 contains all conserved regions found in CDPKs and shows a high level of sequence similarity to many other plant CDPKs. VfCPK1 was highly expressed in leaves, especially in leaf epidermal peels of broad bean in mRNA and protein levels. Expressions of VfCPK1 at both the mRNA and protein levels were increased in leaves treated with abscisic acid or subjected to drought stress. Potential roles of VfCPK1 in epidermal peels are discussed. The nucleotide sequence data reported here were deposited in the GenBank database under accession number AY753552.

Molecular Cloning and Characterization of a Novel Calcium-dependent Protein Kinase Gene IiCPK2 Responsive to Polyploidy from Tetraploid Isatis indigotica

A novel calcium-dependent protein kinase gene (designated as IiCPK2) was cloned from tetraploid Isatis indigotica. The full-length cDNA of IiCPK2 was 2585 bp long with an open reading frame (ORF) of 1878 bp encoding a polypeptide of 625 amino acid residues. The predicted IiCPK2 polypeptide included three domains: a kinase domain, a junction domain (or autoinhibitory region), and a C-terminal calmodulin-like domain (or calcium-binding domain), which presented a typical structure of plant CDPKs. Further analysis of IiCPK2 genomic DNA revealed that it contained 7 exons, 6 introns and the length of most exons was highly conserved. Semi-quantitative RTPCR revealed that the expression of IiCPK2 in root, stem and leaf were much higher in tetraploid sample than that in diploid progenitor. Further expression analysis revealed that gibberellin (GA3), NaCl and cold treatments could upregulate the IiCPK2 transcription. All our findings suggest that IiCPK2 might participate in the cold, high salinity and GA3 responsive pathways.

Purification and characterization of two protein kinases acting on the aquaporin SoPIP2;1

Aquaporins are water channel proteins that facilitate the movement of water and other small solutes across biological membranes. Plants usually have large aquaporin families, providing them with many ways to regulate the water transport. Some aquaporins are regulated post-translationally by phosphorylation. We have previously shown that the water channel activity of SoPIP2;1, an aquaporin in the plasma membrane of spinach leaves, was enhanced by phosphorylation at Ser115 and Ser274. These two serine residues are highly conserved in all plasma membrane aquaporins of the PIP2 subgroup. In this study we have purified and characterized two protein kinases phosphorylating Ser115 and Ser274 in SoPIP2;1. By anion exchange chromatography, the Ser115 kinase was purified from the soluble protein fraction isolated from spinach leaves. The Ca2+-dependent Ser274 kinase was purified by peptide affinity chromatography using plasma membranes isolated from spinach leaves. When characterized, the Ser115 kinase was Mg2+-dependent, Ca2+-independent and had a pH-optimum at 6.5. In accordance with previous studies using the oocyte expression system, site-directed mutagenesis and kinase and phosphatase inhibitors, the phosphorylation of Ser274, but not of Ser115, was increased in the presence of phosphatase inhibitors while kinase inhibitors decreased the phosphorylation of both Ser274 and Ser115. The molecular weight of the Ser274 kinase was approximately 50 kDa. The identification and characterization of these two protein kinases is an important step towards elucidating the signal transduction pathway for gating of the aquaporin SoPIP2;1.

Cloning and biochemical properties of CDPK gene OsCDPK14 from rice

A rice CDPK gene, OsCDPK14 (AY144497), was cloned from developing caryopses of rice (Oryza sativa cv. Zhonghua 15). Its cDNA sequence (1922 bp) contains an ORF encoding a 514 amino acids protein (56.7kD, pl 5.18). OsCDPK14 shows the typical structural features of the CDPK family, including a conserved catalytic Ser/Thr kinase domain, an autoinhibitory domain and a CaM-like domain with four putative Ca2+-binding EF hands. Subcellular targeting indicated that OsCDPK14 was located in the cytoplasm, probably due to the absence of myristoylation and palmitoylation motifs. OsCDPK14 was expressed in Escherichia coli and purified from bacterial extracts. The recombinant protein was shown to be a functional protein kinase using Syntide-2, a synthetic peptide. Kinase activity was shown to be Ca2+-dependent, and this activation was strongly enhanced by Mn2+ and inhibited by W7 in vitro. These results provide significant insights into the regulation and biochemical properties of OsCDPK14, suggesting OsCDPK14 may be a signal factor of cytoplasm in rice plant.

Purification and characterization of a PMA-stimulated kinase and identification of PMA-induced phosphorylation of a polypeptide that is dephosphorylated by low temperature in Brassica juncea

Heterologous classical protein kinase C (cPKC) rat polyclonal antibodies showed presence of PKC homolog in Brassica juncea seedlings. It was purified to homogeneity by ammonium sulfate precipitation, diethyl amino ethyl (DEAE)-Sephacel, gel-filtration chromatography, and preparative gel electrophoresis. PKC-like kinase activity was fractionated into three distinct peaks after DEAE-Sephacel chromatography. The kinase activity was associated with a 55 kDa polypeptide. It was calcium dependent and lipids (phosphatidylserine, PS and oleyl acetylglycerol, OAG) stimulated it further, suggesting it to be a classical type protein kinase C. This was further confirmed by the stimulation of the kinase activity by phorbol 12-myristate 13-acetate (PMA) (a diacylglycerol, DAG analog) and its inhibition by H-7 (a general kinase inhibitor) and staurosporine (a PKC specific inhibitor). Histone was the preferred substrate over casein and BSA. Phosphoamino acid analysis showed it to be a serine/threonine kinase. Western blotting with the purified polypeptide showed an immunopositive 55 kDa polypeptide. In search of the substrate for the kinase in vitro phosphorylation was done in presence of kinase inhibitors. PKC-dependent phosphorylation was observed which was inhibited by PKC inhibitors (H-7 and staurosporine) and enhanced by PKC activator (PMA). Low temperature induced dephosphorylation of the same polypeptide. Direct involvement of PKC-dependent phosphorylation in early LT signaling is indicated for the first time.

Variations in the level of enzyme activity and immunolocalization of calcium-dependent protein kinases in the phloem of different cucumber organs

Calcium-dependent protein kinases (CDPKs) constitute a unique family of enzymes in plants that are characterized by a C-terminal calmodulin (CaM)-like domain. Through protein kinase assays, we have examined the levels of cucumber calcium-dependent kinase (CsCDPK) activity in various organs of cucumber seedlings and plants. The activity of CsCDPK was highest in cucumber plant leaves followed by seedling roots and hypocotyls; however, cucumber plant flowers, seedling cotyledons, and hooks had levels that were barely detectable. The CsCDPKs were immunolocalized using polyclonal antibodies that are highly specific against a part of the kinase domain of a calcium-dependent protein kinase (CsCDPKS) in the phloem sieve elements (SEs) in various organs of cucumber. In addition, this study indicates the presence of CsCDPKs in organelle-like bodies associated with the plasma membrane of sieve elements in mature stems and roots as well as in the storage bodies of immature seeds. These findings are discussed in terms of the likely roles played by CDPKs in the signal transduction pathways for Ca2+-regulated phloem transport of assimilates from leaves to various organs during growth and development of cucumber seedlings and plants.

Membrane-associated protein kinase activities in the developing mesocarp of grape berry

Fruit development is a process involving various signals and gene expression. Protein phosphorylation catalyzed by protein kinases is known to play a key role in eukaryotic cell signalling and so may be involved in the regulation of fruit development. Using the method of exogenous substrate phosphorylation, we characterised the calcium-dependent and calmodulin-independent protein kinase (CDPK) activity and the myelin basic protein (MBP)-phosphoralating activity that could be due to a mitogen-activated protein kinase (MAPK)-like activity in the developing mesocarp of grape berry. The CDPK activity was shown to be predominantly localised in the plasma membrane, while the MAPK-like activity was predominantly associated with endomembranes. The assays of bivalent cation requirement showed that Mn2+ could to a certain extent replace Mg2+ in the incubation system for the protein kinase activities. Both CDPK and MAPK-like activities were resistant to heat treatment. The activities of the two enzymes were fruit developmental stage-specific with the highest activities of both enzymes in the lag growth phase before the ripening stage, suggesting strongly the important roles of the detected CDPK and MAPK-like activities in the fruit development.

Organization and evolution of multifunctional Ca2+/CaM-dependent protein kinase genes

The "multi-functional" Ca(2+) and calmodulin-dependent protein kinase, type II (CaMK-II) is an evolutionarily conserved protein. It has been found as a single gene in the horseshoe crab, marine sponge, sea urchin, nematode, and fruit fly, whereas most vertebrates possess four genes (alpha, beta, gamma, and delta). Species from fruit flies to humans encode alternative splice variants which are differentially targeted to phosphorylate diverse downstream targets of Ca(2+) signaling. By comparing known CaMK-II protein and nucleotide sequences, we have now provided evidence for the evolutionary relatedness of CaMK-IIs. Parsimony analyses unambiguously indicate that the four vertebrate CaMK-II genes arose via repeated duplications. Nucleotide phylogenies show consistent but moderate support for the placement of the vertebrate delta CaMK-II as the earliest diverging vertebrate gene. delta CaMK-II is the only gene with both central and C-terminal variable domains and has three to four times more intronic sequence than the other three genes. beta and gamma CaMK-II genes show strong sequence similarity and have comparable exon and intron organization and utilization. alpha CaMK-II is absent from amphibians (Xenopus laevis) and has the most restricted tissue specificity in mammals, whereas beta, gamma, and delta CaMK-IIs are expressed in most tissues. All 38 known mammalian CaMK-II splice variants were compiled with their tissue specificity and exon usage. Some of these variants use alternative 5' and 3' donors within a single exon as well as alternative promoters. These findings serve as an important benchmark for future phylogenetic, developmental, or biochemical studies on this important, conserved, and highly regulated gene family.

The Arabidopsis CDPK-SnRK superfamily of protein kinases

The CDPK-SnRK superfamily consists of seven types of serine-threonine protein kinases: calcium-dependent protein kinase (CDPKs), CDPK-related kinases (CRKs), phosphoenolpyruvate carboxylase kinases (PPCKs), PEP carboxylase kinase-related kinases (PEPRKs), calmodulin-dependent protein kinases (CaMKs), calcium and calmodulin-dependent protein kinases (CCaMKs), and SnRKs. Within this superfamily, individual isoforms and subfamilies contain distinct regulatory domains, subcellular targeting information, and substrate specificities. Our analysis of the Arabidopsis genome identified 34 CDPKs, eight CRKs, two PPCKs, two PEPRKs, and 38 SnRKs. No definitive examples were found for a CCaMK similar to those previously identified in lily (Lilium longiflorum) and tobacco (Nicotiana tabacum) or for a CaMK similar to those in animals or yeast. CDPKs are present in plants and a specific subgroup of protists, but CRKs, PPCKs, PEPRKs, and two of the SnRK subgroups have been found only in plants. CDPKs and at least one SnRK have been implicated in decoding calcium signals in Arabidopsis. Analysis of intron placements supports the hypothesis that CDPKs, CRKs, PPCKs and PEPRKs have a common evolutionary origin; however there are no conserved intron positions between these kinases and the SnRK subgroup. CDPKs and SnRKs are found on all five Arabidopsis chromosomes. The presence of closely related kinases in regions of the genome known to have arisen by genome duplication indicates that these kinases probably arose by divergence from common ancestors. The PlantsP database provides a resource of continuously updated information on protein kinases from Arabidopsis and other plants.

Membrane-associated protein kinase activities in developing apple fruit

Fruit development is a process involving various signals and gene expression. Protein phosphorylation catalysed by protein kinases is known to play a key role in eukaryotic cell signalling and so may be involved in the regulation of fruit development. Using the method of exogenous substrate phosphorylation, the activity of calcium-dependent and calmodulin-independent protein kinase (CDPK) that was stimulated by phosphatidylserine, and the myelin basic protein (MBP)-phosphorylating activity that could be due to a calcium-independent mitogen-activated protein kinase-like (MAPK-like) activity in the developing apple fruits were identified. The CDPK activity was shown to be predominantly localized in the plasma membrane, whereas in the presence of phosphatidylserine, the high activity of CDPK was detected in both plasma membrane and endomembranes. The MAPK-like activity was predominantly associated with endomembranes. The assays of bivalent cation requirement showed that Mn2+ could replace Mg2+ in the incubation system for the protein kinase activities and stimulate CDPK activity more than Mg2+. Heat treatment abolished CDPK but stimulated MAPK-like activity. The activities of the phosphatidylserine-stimulated CDPK and of the MAPK-like were fruit developmental stage-specific with higher activities of both enzymes in the early and middle developmental stages in comparison with the late developmental stage. These data suggest that the detected protein kinases may play an important role in the fruit development.

Phytochrome signalling modulates the SA-perceptive pathway in Arabidopsis

The interaction of phytochrome signalling with the SA signal transduction pathway has been investigated in Arabidopsis using single and multiple mutants affected in light perception (phyA and phyB deficient) and light-signal processing (psi2, phytochrome signalling). The induction of PR1 by SA and functional analogues has been found to strictly correlate with the activity of the signalling pathway controlled by both phyA and phyB photoreceptors. In darkness as well as dim light, and independently of a carbohydrate source, SA-induced PR gene expression as well as the hypersensitive response to pathogens (HR) are strongly reduced. Moreover, the initiation of HR also exhibits a strict dependence upon both the presence and the amplitude of a phytochrome-elicited signal. The growth of an incompatible strain of bacterial a pathogen (Pseudomonas syringae pv. tomato) was enhanced in phyA-phyB and decreased in psi2 mutants. While functional chloroplasts were found necessary for the development of an HR, the induction of PRs was strictly dependent on light, but independent of functional chloroplasts. Taken together, these data demonstrate that the light-induced signalling pathway interacts with the pathogen/SA-mediated signal transduction route. These results are summarized in a formalism that allows qualitative computer simulation.

Calcium signaling through protein kinases. The Arabidopsis calcium-dependent protein kinase gene family

In plants, numerous Ca(2+)-stimulated protein kinase activities occur through calcium-dependent protein kinases (CDPKs). These novel calcium sensors are likely to be crucial mediators of responses to diverse endogenous and environmental cues. However, the precise biological function(s) of most CDPKs remains elusive. The Arabidopsis genome is predicted to encode 34 different CDPKs. In this Update, we analyze the Arabidopsis CDPK gene family and review the expression, regulation, and possible functions of plant CDPKs. By combining emerging cellular and genomic technologies with genetic and biochemical approaches, the characterization of Arabidopsis CDPKs provides a valuable opportunity to understand the plant calcium-signaling network.

Analysis of EF-hand-containing proteins in Arabidopsis

BACKGROUND

In plants, calcium (Ca2+) has emerged as an important messenger mediating the action of many hormonal and environmental signals, including biotic and abiotic stresses. Many different signals raise cytosolic calcium concentration ([Ca2+]cyt), which in turn is thought to regulate cellular and developmental processes via Ca2+-binding proteins. Three out of the four classes of Ca2+-binding proteins in plants contain Ca2+-binding EF-hand motif(s). This motif is a conserved helix-loop-helix structure that can bind a single Ca2+ ion. To identify all EF-hand-containing proteins in Arabidopsis, we analyzed its completed genome sequence for genes encoding EF-hand-containing proteins.

RESULTS

A maximum of 250 proteins possibly having EF-hands were identified. Diverse proteins, including enzymes, proteins involved in transcription and translation, protein- and nucleic-acid-binding proteins and a large number of unknown proteins, have one or more putative EF-hands. Phylogenetic analysis identified six major groups that contain some families of proteins.

CONCLUSIONS

The presence of EF-hand motif(s) in a diversity of proteins is consistent with the involvement of Ca2+ in regulating many cellular and developmental processes. Thus far, only 47 of the possible 250 EF-hand proteins have been reported in the literature. Various domains that we identified in many of the uncharacterized EF-hand-containing proteins should help in elucidating their cellular role(s). Our analyses suggest that the Ca2+ messenger system is widely used in plants and that EF-hand-containing proteins are likely to be the key transducers mediating Ca2+ action.

A Ca(2+)-dependent protein kinase that endows rice plants with cold- and salt-stress tolerance functions in vascular bundles

A rice Ca(2+)-dependent protein kinase, OsCDPK7, is a positive regulator commonly involved in the tolerance to cold and salt/drought. We carried out in situ detection of the transcript and immunolocalization of the protein. In the wild-type rice plants under both stress conditions, OsCDPK7 was expressed predominantly in vascular tissues of crowns and roots, vascular bundles and central cylinder, respectively, where water stress occurs most severely. This enzyme was also expressed in the peripheral cylinder of crown vascular bundles and root sclerenchyma. Similar localization patterns with stronger signals were observed in stress-tolerant OsCDPK7 over-expressing transformants with the cauliflower mosaic virus 35S promoter. The transcript of a putative target gene of the OsCDPK7 signaling pathway, rab16A, was also detected essentially in the same tissues upon salt stress, suggesting that the OsCDPK7 pathway operates predominantly in these regions. We propose that the use of the 35S promoter fortuitously strengthened the localized expression of OsCDPK7, resulting in enhancement of the stress signaling in the inherently operating regions leading to improved stress tolerance.

Characterization of a Ca2+-dependent protein kinase from rice root: differential response to cold and regulation by abscisic acid

The Ca2+-dependent protein kinases (CDPKs) and abscisic acid (ABA) are known to be involved in low-temperature stress response. The focus of this study was to characterize the 45 kDa protein kinase identified in the crude extract of rice (Oryza sativa L.) seedling roots in response to cold (5 degrees C) stress. The activity of the 45 kDa protein kinase decreased at low temperature as evident by an in-gel kinase assay using histone III-S as a substrate. Also, the Ca2+-dependent activity of this protein kinase was suppressed by cold in the membrane fractions of the root. A general protein kinase inhibitor and Ca2+ chelator inhibited the activity of the 45 kDa protein kinase, suggesting that it was a plant CDPK. The 45 kDa CDPK identified was found to be independent of photosynthetic tissues such as the leaf and leaf sheath of rice seedlings, supporting a direct sensing mechanism in the roots of rice seedlings to cold stress. The suppressed activity of the 45 kDa CDPK was reverted by supplementing with 5 microM ABA under cold stress. The 45 kDa CDPK activity was stronger in the cold-tolerant variety of the 4 types tested than it was in the cold-sensitive one. These results suggest the involvement of endogenous ABA in regulating the activity of the 45 kDa CDPK in response to cold stress.

Auxin and heat shock activation of a novel member of the calmodulin like domain protein kinase gene family in cultured alfalfa cells

A calmodulin like domain protein kinase (CPK) homologue was identified in alfalfa and termed MsCPK3. The full-length sequence of cDNA encoded a 535 amino acid polypeptide with a molecular weight of 60.2 kDa. The deduced amino acid sequence showed all the conserved motifs that define other members of this kinase family, such as serine-threonine kinase domain, a junction region and four potential Ca2+ -binding EF sites. The recombinant MsCPK3 protein purified from E. coli was activated by Ca2+ and inhibited by calmodulin antagonist (W-7) in in vitro phosphorylation assays. The expression of MsCPK3 gene increased in the early phase of the 2,4-D induced alfalfa somatic embryogenesis. Heat shock also activated this gene while kinetin, ABA and NaCl treatment did not result in MsCPK3 mRNA accumulation. The data presented suggest that the new alfalfa CPK differs in stress responses from the previously described homologues and in its potential involvement in hormone and stress-activated reprogramming of developmental pathways during somatic embryogenesis.

Overexpression of the calcium-dependent protein kinase OsCDPK2 in transgenic rice is repressed by light in leaves and disrupts seed development

Independent transgenic rice lines overexpressing the rice CDPK isoform OsCDPK2 were generated by particle bombardment. High levels of OsCDPK2 were detected in leaves removed from etiolated plants, as well as in stems and flowers. However, there was no overexpression in green leaves that had been exposed to light, confirming that OsCDPK2 protein stability was subject to light regulation. The morphological phenotype of transgenic plants producing high levels of recombinant OsCDPK2 was normal until the onset of seed development. Flowers developed normally, producing well-shaped ovaries and stigmas, and mature anthers filled with pollen grains. However, seed formation in these plants was strongly inhibited, with only 3-7% of the flowers producing seeds. Seed development was arrested at an early stage. We discuss these data with respect to the possible requirement for specific CDPK isoforms during rice seed development.

SUGAR-INDUCED SIGNAL TRANSDUCTION IN PLANTS

Sugars have important signaling functions throughout all stages of the plant's life cycle. This review presents our current understanding of the different mechanisms of sugar sensing and sugar-induced signal transduction, including the experimental approaches used. In plants separate sensing systems are present for hexose and sucrose. Hexokinase-dependent and -independent hexose sensing systems can further be distinguished. There has been progress in understanding the signal transduction cascade by analyzing the function of the SNF1 kinase complex and the regulatory PRL1 protein. The role of sugar signaling in seed development and in seed germination is discussed, especially with respect to the various mechanisms by which sugar signaling controls gene expression. Finally, recent literature on interacting signal transduction cascades is discussed, with particular emphasis on the ethylene and ABA signal transduction pathways.

Comparative modeling studies of the calmodulin-like domain of calcium-dependent protein kinase from soybean

Calmodulin-like domain protein kinases (CDPKs) represent a new class of calcium-dependent protein-phosphorylating enzymes that are not activated by calmodulin or phospholipid compounds. They have been found exclusively in plant and protozoal tissues. CDPKs are typified by four distinct domains: an N-terminal leader sequence, a protein kinase (PK) domain, a calmodulin-like domain (CLD), and a junction domain (JD) between the PK domain and CLD. Structural characterization of the CLD of CDPKalpha from soybean was undertaken based on the amino acid sequence homology of CLD to the structurally well-characterized calmodulin (CaM) family of structures. Tertiary models of apo-CLD, Ca(2+)-CLD complex, and intermolecularly bound Ca(2+)-CLD-JD complexes were obtained via automated and non-automated homology building methods. The resulting structures were compared and validated based on energy differences, phi-psi angle distribution, solvent accessibility, and hydrophobic potential. Circular dichroism, one-dimensional, and two-dimensional nuclear magnetic resonance spectroscopy studies of the CLD and peptides encompassing the JD provide experimental support to the models. The results suggest that there is a possible interaction between the CLD and JD domain similar to that of the CaM/calmodulin-dependent protein kinase II system. At low Ca(2+) levels, the JD may act as an autoinhibitory domain for kinase activity, and during calcium activation an intramolecular CLD-JD complex may form, relieving inhibition of the PK domain. Interactions between the JD and the C terminus of the CLD appear to be particularly important. The outcome of this study supports an intramolecular binding model for calcium activation of CDPK, although not exclusively.

Spatio-temporal accumulation and activity of calcium-dependent protein kinases during embryogenesis, seed development, and germination in sandalwood

Western-blot analysis and protein kinase assays identified two Ca(2+)-dependent protein kinases (CDPKs) of 55 to 60 kD in soluble protein extracts of embryogenic cultures of sandalwood (Santalum album L.). However, these sandalwood CDPKs (swCDPKs) were absent in plantlets regenerated from somatic embryos. swCDPKs exhibited differential expression (monitored at the level of the protein) and activity in different developmental stages. Zygotic embryos, seedlings, and endosperm showed high accumulation of swCDPK, but the enzyme was not detected in the soluble proteins of shoots and flowers. swCDPK exhibited a temporal pattern of expression in endosperm, showing high accumulation and activity in mature fruit and germinating stages; the enzyme was localized strongly in the storage bodies of the endosperm cells. The study also reports for the first time to our knowledge a post-translational inhibition/inactivation of swCDPK in zygotic embryos during seed dormancy and early stages of germination. The temporal expression of swCDPK during somatic/zygotic embryogenesis, seed maturation, and germination suggests involvement of the enzyme in these developmental processes.

Molecular cloning, genomic organization, and biochemical characterization of myristoyl-CoA:protein N-myristoyltransferase from Arabidopsis thaliana

Myristoyl-CoA:protein N-myristoyltransferase (NMT, EC 2.3.1.97) catalyzes the co-translational addition of myristic acid to the amino-terminal glycine residue of a number of important proteins of diverse functions. We have isolated a full-length Arabidopsis thaliana cDNA encoding NMT (AtNMT1), the first described from a higher plant. This AtNMT1 cDNA clone has an open reading frame of 434 amino acids and a predicted molecular mass of 48,706 Da. The primary structure is 50% identical to the mammalian NMTs. Analyses of Southern blots, genomic clones, and database sequences suggested that the A. thaliana genome contains two copies of NMT gene, which are present on different chromosomes and have distinct genomic organizations. The recombinant AtNMT1 expressed in Escherichia coli exhibited a high catalytic efficiency for the peptides derived from putative plant myristoylated proteins AtCDPK6 and Fen kinase. The AtNMT was similar to the mammalian NMTs with respect to a relative specificity for myristoyl CoA among the acyl CoA donors and also inhibition by the bovine brain NMT inhibitor NIP(71). The AtNMT1 expression profile indicated ubiquity in roots, stem, leaves, flowers, and siliques (approximately 1.7 kb transcript and approximately 50 kDa immunoreactive polypeptide) but a greater level in the younger tissue, which are developmentally very active. NMT activity was also evident in all these tissues. Subcellular distribution studies indicated that, in leaf extracts, approximately 60% of AtNMT activity was associated with the ribosomal fractions, whereas approximately 30% of the activity was observed in the cytosolic fractions. The NMT is biologically important to plants, as noted from the stunted development when the AtNMT1 was down-regulated in transgenic Arabidopsis under the control of an enhanced CaMV 35S promoter. The results presented in this study provide the first direct molecular evidence for plant protein N-myristoylation and a mechanistic basis for understanding the role of this protein modification in plants.

Lipid-linked proteins of plants

Increasing numbers of plant proteins are being shown to have posttranslationally-attached lipids. The modifications include N-myristoylation, S-palmitoylation, prenylation by farnesyl or geranylgeranyl moieties, or attachment of glycosylphosphatidylinositol anchors. This report summarizes recent findings regarding the structure, metabolism and physiological functions of these important protein-linked lipids.

Signaling events leading to crassulacean acid metabolism induction in the common ice plant

A rapid, semiquantitative reverse transcriptase-polymerase chain reaction assay was developed to investigate signal transduction events involved in the induction of Crassulacean acid metabolism (CAM) in detached common ice plant (Mesembryanthemum crystallinum) leaves. Transcript abundance of Ppc1, a gene encoding the CAM-specific isoform of phosphoenolpyruvate carboxylase, increased rapidly in response to osmotic stress (dehydration and mannitol), ionic stress (NaCl), and exogenous abscisic acid treatment, but failed to accumulate in response to exogenous cytokinin or methyl jasmonate. Stress-induced accumulation of Ppc1, GapC1, and Mdh1 transcripts was inhibited by pretreating leaves with the calcium chelator ethyleneglycol-bis(aminoethyl ether)-N,N'-tetraacetic acid, suggesting that extracellular calcium participates in signaling events leading to CAM induction. Treatment of unstressed detached leaves with ionomycin, a Ca(2+) ionophore, and thapsigargin, a Ca(2+)-ATPase inhibitor, enhanced Ppc1 transcript accumulation, indicating that elevations in cytosolic [Ca(2+)] are likely to participate in signaling CAM induction. Inhibitors of Ca(2+)- or calmodulin-dependent protein kinases (N-[6-aminohexyl]-5-chloro-1-napthalenesulfonamide, Lavendustin C) and protein phosphatase 1 and 2A (okadaic acid) activity suppressed Ppc1 transcript accumulation in response to ionic and osmotic stresses, as well as abscisic acid treatment. These results suggest that both protein phosphorylation and dephosphorylation events participate in signaling during CAM induction. In contrast, pretreatment with cyclosporin A or ascomycin, inhibitors of protein phosphatase 2B activity, stimulated Ppc1 gene expression either directly or indirectly through promoting water loss.

PLANT PROTEIN SERINE/THREONINE KINASES: Classification and Functions

The first plant protein kinase sequences were reported as recently as 1989, but by mid-1998 there were more than 500, including 175 in Arabidopsis thaliana alone. Despite this impressive pace of discovery, progress in understanding the detailed functions of protein kinases in plants has been slower. Protein serine/threonine kinases from A. thaliana can be divided into around a dozen major groups based on their sequence relationships. For each of these groups, studies on animal and fungal homologs are briefly reviewed, and direct studies of their physiological functions in plants are then discussed in more detail. The network of protein-serine/threonine kinases in plant cells appears to act as a "central processor unit" (cpu), accepting input information from receptors that sense environmental conditions, phytohormones, and other external factors, and converting it into appropriate outputs such as changes in metabolism, gene expression, and cell growth and division.

Isolation of a gene encoding Arabidopsis membrane-associated acyl-CoA binding protein and immunolocalization of its gene product

Until recently, only cytosolic acyl-CoA binding proteins (ACBPs) have been characterized. The isolation of an Arabidopsis thaliana cDNA encoding a novel membrane-associated ACBP that accumulates in developing seeds, designated ACBP1, has provided evidence for the existence of membrane-associated forms of ACBPs (Chye, 1998, Plant Mol. Biol. 38, 827-838). We now report on the isolation of its corresponding gene from an A. thaliana Columbia genomic library using the ACBP1 cDNA as a hybridization probe. Nucleotide sequence analysis of Arabidopsis ACBP1 showed that its promoter lacks a TATA box, resembling the promoters of rat, Drosophila and human genes encoding cytosolic ACBP and suggesting that it is a housekeeping gene. We show by Western blot analysis that ACBP1 expression in developing seeds coincides with lipid deposition and that homologues of membrane-associated ACBP1 exist in other plants. Using light microscopy, we show that ACBP1 is strongly expressed in the embryo at the cotyledons, hypocotyl, procambium of the axis and in most peripheral cells of the cotyledons and hypocotyl. Immunogold labelling localized ACBP1 to vesicles, to the plasma membrane especially at epidermal cells of heart, torpedo and cotyledonary stage embryos, and to the cell wall of the outer integument cells at the seed coat. Our results suggest that ACBP1 is involved in intermembrane lipid transport from the ER via vesicles to the plasma membrane where it could maintain a membrane-associated acyl pool; its immunolocalization to the cell wall of outer integument cells at the seed coat suggests a role in cuticle and cutin formation.

Potato protein kinase StCPK1: a putative evolutionary link between CDPKs and CRKs

Calcium-dependent protein kinases (CDPKs) in plants are characterized by a four-domain structure including conserved sequences in the catalytic domain, and in the C-terminal calmodulin-like domain. Based on this conservation we have PCR-amplified and isolated a potato cDNA clone (StCPK1) from a library representing an early stage of tuber development. DNA sequence analysis revealed that in the catalytic domain, StCPK1 shares more homology with CDPK-related kinases than with CDPKs; however, like CDPKs, it possesses canonical EF-hands at the calmodulin-like 3' end. StCPK1 exists in a few copies in the potato genome and is abundantly expressed in the sepals of mature flowers. Floral expression of genes homologous to StCPK1 appears to be widespread in the family Solanaceae.

Kinetic and calcium-binding properties of three calcium-dependent protein kinase isoenzymes from soybean

Calmodulin-like domain protein kinases (CDPKs) are a family of calcium- but not calmodulin-dependent protein kinases found in a wide variety of plants and in protists. CDPKs are encoded by large multigene families, and to assess whether family members play distinct or redundant roles in vivo, we characterized soybean CDPK isoforms alpha, beta, and gamma, which share 60-80% identity in amino acid sequence. RNA blot analysis showed that the three CDPKs were expressed in most plant tissues examined and in suspension-cultured soybean cells. Recombinant CDPKalpha, -beta, and -gamma phosphorylated peptide substrates containing the four-residue motif R/K-X-X-S/T, but CDPKalpha was the most selective for residues outside of the motif. The CDPKs were inhibited by the general protein kinase inhibitors K252a and staurosporine and by calphostin C, which is an inhibitor of protein kinase C. The calcium-binding properties of each CDPK were distinct. The Kd's for Ca2+ determined by flow dialysis in the absence of substrates were 51, 1.4, and 1.6 micro M for CDPKalpha, -beta, and -gamma, respectively. In the presence of the peptide substrate syntide-2 the Kd of CDPKalpha decreased to 0.6 microM. Also, the sensitivity of this isoenzyme's activity to calcium varied with protein substrate. The concentrations of Ca2+ required for half-maximal activity (K0.5) for each CDPK with syntide-2 as substrate were 0.06, 0.4, and 1 micro M, respectively. These results show that members of the CDPK family differ in biochemical properties and support the hypothesis that each isoform may have a distinct role in calcium signal transduction.

[Protein phosphatases and protein kinases in higher plants]

The recent gain in knowledge concerning enzymes involved in signal transduction pathways is a direct consequence of the considerable advances made in molecular biology. Protein kinases and protein phosphatases, the two major enzymes implicated in post-translational modifications, have been studied in particular. The number of characterized plant genes and/or cDNAs encoding these enzymes is increasing everyday. Since 1991, 26 genes and cDNAs coding for plant protein phosphatases have been isolated and characterized. The huge number of protein kinases (estimated at several thousands) makes it impossible to give an exhaustive list of the genes already identified, but a classification of these enzymes, based on phylogenetic criteria, allows us to appreciate the range of functions this protein family may play in plants.