A stress-induced calcium-dependent protein kinase from Mesembryanthemum crystallinum phosphorylates a two-component pseudo-response regulator

Physiological and molecular responses of two contrasting drought resistance pistachio interspecific hybrid rootstocks

Pistachio (Pistacia vera L.) is a valuable nut crop that faces significant challenges due to drought stress, which can severely impact its growth, yield, and quality. Understanding the physiological and molecular mechanisms underlying drought tolerance is crucial for developing resilient pistachio rootstock. In this study, among nine-month-old saplings of seven clonal interspecies hybrids of Pistacia atlantica × Pistacia integerrima, two contrasting hybrids, 'C4-2' (sensitive) and 'C9-4' (resistant), were assessed for their morphological, physiological and molecular responses to 30 days of withholding irrigation. Water withholding induced alterations in root architecture in the resistant clone, accompanied by an increase in compatible solutes, including glycine betaine, proline, and total soluble carbohydrates. Enzyme activities of guaiacol peroxidase (GPX) and catalase (CAT) were elevated in the resistant clone under water stress. Both clones exhibited increased levels of malondialdehyde (MDA) and hydrogen peroxide (H2O2) during the stress period, with these changes being more pronounced in C4-2 compared to C9-4. In the resistant clone, both CDPK and ZEP genes were upregulated, suggesting their role in enhancing stress signaling and osmotic regulation under drought stress. The upregulation of CDPK indicates its involvement in calcium-mediated signaling pathways, which likely contribute to improved drought tolerance. Similarly, DHN expression was strongly influenced by CDPK activity, further emphasizing its role in maintaining cellular integrity during stress conditions. The findings provide valuable insights for developing more resilient pistachio rootstocks capable of thriving in water-limited environments. Specifically, C9-4 demonstrated significant drought tolerance in this study. Nonetheless, further research is necessary to validate the broader applicability of these findings and to evaluate its performance across various stress environments.

OsCPK12 phosphorylates OsCATA and OsCATC to regulate H2O2 homeostasis and improve oxidative stress tolerance in rice

Calcium-dependent protein kinases (CPKs), the best-characterized calcium sensors in plants, regulate many aspects of plant growth and development as well as plant adaptation to biotic and abiotic stresses. However, how CPKs regulate the antioxidant defense system remains largely unknown. We previously found that impaired function of OsCPK12 leads to oxidative stress in rice, with more H2O2, lower catalase (CAT) activity, and lower yield. Here, we explored the roles of OsCPK12 in oxidative stress tolerance in rice. Our results show that OsCPK12 interacts with and phosphorylates OsCATA and OsCATC at Ser11. Knockout of either OsCATA or OsCATC leads to an oxidative stress phenotype accompanied by higher accumulation of H2O2. Overexpression of the phosphomimetic proteins OsCATAS11D and OsCATCS11D in oscpk12-cr reduced the level of H2O2 accumulation. Moreover, OsCATAS11D and OsCATCS11D showed enhanced catalase activity in vivo and in vitro. OsCPK12-overexpressing plants exhibited higher CAT activity as well as higher tolerance to oxidative stress. Our findings demonstrate that OsCPK12 affects CAT enzyme activity by phosphorylating OsCATA and OsCATC at Ser11 to regulate H2O2 homeostasis, thereby mediating oxidative stress tolerance in rice.

TypiCal but DeliCate Ca++re: Dissecting the Essence of Calcium Signaling Network as a Robust Response Coordinator of Versatile Abiotic and Biotic Stimuli in Plants

Plant growth, development, and ultimately crop productivity are largely impacted by the interaction of plants with different abiotic and biotic factors throughout their life cycle. Perception of different abiotic stresses, such as salt, cold, drought, heat, and heavy metals, and interaction with beneficial and harmful biotic agents by plants lead to transient, sustained, or oscillatory changes of [calcium ion, Ca²⁺]_cyt within the cell. Significant progress has been made in the decoding of Ca²⁺ signatures into downstream responses to modulate differential developmental and physiological responses in the whole plant. Ca²⁺ sensor proteins, mainly calmodulins (CaMs), calmodulin-like proteins (CMLs), and others, such as Ca²⁺-dependent protein kinases (CDPKs), calcineurin B-like proteins (CBLs), and calmodulin-binding transcription activators (CAMTAs) have played critical roles in coupling the specific stress stimulus with an appropriate response. This review summarizes the current understanding of the Ca²⁺ influx and efflux system in plant cells and various Ca²⁺ binding protein-mediated signal transduction pathways that are delicately orchestrated to mitigate abiotic and biotic stresses. The probable interactions of different components of Ca²⁺ sensor relays and Ca²⁺ sensor responders in response to various external stimuli have been described diagrammatically focusing on established pathways and latest developments. Present comprehensive insight into key components of the Ca²⁺ signaling toolkit in plants can provide an innovative framework for biotechnological manipulations toward crop improvability in near future.

Salicylic acid: A key regulator of redox signalling and plant immunity

In plants, the reactive oxygen species (ROS) formed during normal conditions are essential in regulating several processes, like stomatal physiology, pathogen immunity and developmental signaling. However, biotic and abiotic stresses can cause ROS over-accumulation leading to oxidative stress. Therefore, a suitable equilibrium is vital for redox homeostasis in plants, and there have been major advances in this research arena. Salicylic acid (SA) is known as a chief regulator of ROS; however, the underlying mechanisms remain largely unexplored. SA plays an important role in establishing the hypersensitive response (HR) and systemic acquired resistance (SAR). This is underpinned by a robust and complex network of SA with Non-Expressor of Pathogenesis Related protein-1 (NPR1), ROS, calcium ions (Ca²⁺), nitric oxide (NO) and mitogen-activated protein kinase (MAPK) cascades. In this review, we summarize the recent advances in the regulation of ROS and antioxidant defense system signalling by SA at the physiological and molecular levels. Understanding the molecular mechanisms of how SA controls redox homeostasis would provide a fundamental framework to develop approaches that will improve plant growth and fitness, in order to meet the increasing global demand for food and bioenergy.

Response Mechanism of Plants to Drought Stress

With the global climate anomalies and the destruction of ecological balance, the water shortage has become a serious ecological problem facing all mankind, and drought has become a key factor restricting the development of agricultural production. Therefore, it is essential to study the drought tolerance of crops. Based on previous studies, we reviewed the effects of drought stress on plant morphology and physiology, including the changes of external morphology and internal structure of root, stem, and leaf, the effects of drought stress on osmotic regulation substances, drought-induced proteins, and active oxygen metabolism of plants. In this paper, the main drought stress signals and signal transduction pathways in plants are described, and the functional genes and regulatory genes related to drought stress are listed, respectively. We summarize the above aspects to provide valuable background knowledge and theoretical basis for future agriculture, forestry breeding, and cultivation.

Molecular cloning and expression of AmCDPK from mangrove Avicennia marina under elevated temperature

Considered as an essential calcium sensor, the calcium-dependent protein kinase (CDPK) family plays a critical part in terrestrial plants' responses to both biotic and abiotic stresses. In the study, Avicennia marina was proved to have better heat tolerance than other species. A CDPK gene was cloned from mangrove species A. marina using RACE-PCR and designated as AmCDPK. By predicting and analyzing its properties, structures and expression patterns, we found that the amino acid sequence, containing a kinase domain and four EF-hand Ca²⁺-binding sites, shared high identity with Handroanthus impetiginosus and Sesamum indicum. Quantitative real-time PCR data analysis suggested that AmCDPK demonstrated significant up-regulation under heat stress. It is likely that AmCDPK is a versatile gene involved in various stresses, including dehydration, cold, light, defense and ABA stress responses by analyzing cis-elements. It is the first time that CDPKs from mangroves have been cloned and our results brought evidence to the effect of AmCDPK on heat stress, which is particularly important under the background of global warming.

Calcium-dependent protein kinase CPK9 negatively functions in stomatal abscisic acid signaling by regulating ion channel activity in Arabidopsis

In this manuscript, we demonstrated the negative role of CPK9 in stomatal ABA signaling, and both CPK9 and CPK33 for accurate guard cell function was explored via cpk9/cpk33 double mutants' phenotype. Abscisic acid (ABA) can inhibit stomatal opening and promote stomatal closure by regulating ion channel activity in guard cell membranes. As an important second messenger, calcium (Ca²⁺) is essentially needed in ABA regulation of stomatal movement. Calcium-dependent protein kinases (CDPKs) have been proposed to contribute to central Ca²⁺ signal transduction in plants. Here, we report the functional characterization of CPK9 in Arabidopsis stomatal ABA signaling. CPK9 had high expression in guard cells and the protein was subcellularly located in the cell membrane. A loss-of-function mutant cpk9 showed a much more sensitive phenotype to ABA regulation of stomatal movement and ion channel activity, while CPK9 overexpression lines had opposite phonotypes. These findings demonstrated the negative role of CPK9 in stomatal ABA signaling. As the closest homolog of CPK33, we also proved that stomatal movement of the cpk9/cpk33 double mutants was more sensitive to ABA than either single mutants. These results revealed the role of CPK9 in guard cells, and the need of both CPK9 and CPK33 for accurate guard cell function.

Downstream components of the calmodulin signaling pathway in the rice salt stress response revealed by transcriptome profiling and target identification

BACKGROUND

Calmodulin (CaM) is an important calcium sensor protein that transduces Ca2+ signals in plant stress signaling pathways. A previous study has revealed that transgenic rice over-expressing the calmodulin gene OsCam1-1 (LOC_Os03g20370) is more tolerant to salt stress than wild type. To elucidate the role of OsCam1-1 in the salt stress response mechanism, downstream components of the OsCam1-1-mediated response were identified and investigated by transcriptome profiling and target identification.

RESULTS

Transcriptome profiling of transgenic 'Khao Dawk Mali 105' rice over-expressing OsCam1-1 and wild type rice showed that overexpression of OsCam1-1 widely affected the expression of genes involved in several cellular processes under salt stress, including signaling, hormone-mediated regulation, transcription, lipid metabolism, carbohydrate metabolism, secondary metabolism, photosynthesis, glycolysis, tricarboxylic acid (TCA) cycle and glyoxylate cycle. Under salt stress, the photosynthesis rate in the transgenic rice was slightly lower than in wild type, while sucrose and starch contents were higher, suggesting that energy and carbon metabolism were affected by OsCam1-1 overexpression. Additionally, four known and six novel CaM-interacting proteins were identified by cDNA expression library screening with the recombinant OsCaM1. GO terms enriched in their associated proteins that matched those of the differentially expressed genes affected by OsCam1-1 overexpression revealed various downstream cellular processes that could potentially be regulated by OsCaM1 through their actions.

CONCLUSIONS

The diverse cellular processes affected by OsCam1-1 overexpression and possessed by the identified CaM1-interacting proteins corroborate the notion that CaM signal transduction pathways compose a complex network of downstream components involved in several cellular processes. These findings suggest that under salt stress, CaM activity elevates metabolic enzymes involved in central energy pathways, which promote or at least maintain the production of energy under the limitation of photosynthesis.

An Overview of the Genetics of Plant Response to Salt Stress: Present Status and the Way Forward

Salinity is one of the major threats faced by the modern agriculture today. It causes multidimensional effects on plants. These effects depend upon the plant growth stage, intensity, and duration of the stress. All these lead to stunted growth and reduced yield, ultimately inducing economic loss to the farming community in particular and to the country in general. The soil conditions of agricultural land are deteriorating at an alarming rate. Plants assess the stress conditions, transmit the specific stress signals, and then initiate the response against that stress. A more complete understanding of plant response mechanisms and their practical incorporation in crop improvement is an essential step towards achieving the goal of sustainable agricultural development. Literature survey shows that investigations of plant stresses response mechanism are the focus area of research for plant scientists. Although these efforts lead to reveal different plant response mechanisms against salt stress, yet many questions still need to be answered to get a clear picture of plant strategy to cope with salt stress. Moreover, these studies have indicated the presence of a complicated network of different integrated pathways. In order to work in a progressive way, a review of current knowledge is critical. Therefore, this review aims to provide an overview of our understanding of plant response to salt stress and to indicate some important yet unexplored dynamics to improve our knowledge that could ultimately lead towards crop improvement.

The calcium-dependent kinase OsCPK24 functions in cold stress responses in rice

Calcium-dependent protein kinases (CPKs) are serine/threonine protein kinases that function in plant stress responses. Although CPKs are recognized as key messengers in signal transduction, the specific roles of CPKs and the molecular mechanisms underlying their activity remain largely unknown. Here, we characterized the function of OsCPK24, a cytosol-localized calcium-dependent protein kinase in rice. OsCPK24 was universally and highly expressed in rice plants and was induced by cold treatment. Whereas OsCPK24 knockdown plants exhibited increased sensitivity to cold compared to wild type (WT), OsCPK24-overexpressing plants exhibited increased cold tolerance. Plants overexpressing OsCPK24 exhibited increased accumulation of proline (an osmoprotectant) and glutathione (an antioxidant) and maintained a higher GSH/GSSG (reduced glutathione to oxidized glutathione) ratio during cold stress compared to WT. In addition to these effects in response to cold stress, we observed the kinase activity of OsCPK24 varied under different calcium concentrations. Further, OsCPK24 phosphorylated OsGrx10, a glutathione-dependent thioltransferase, at rates modulated by changes in calcium concentration. Together, our results support the hypothesis that OsCPK24 functions as a positive regulator of cold stress tolerance in rice, a process mediated by calcium signaling and involving phosphorylation and the inhibition of OsGrx10 to sustain higher glutathione levels.

The rice cold-responsive calcium-dependent protein kinase OsCPK17 is regulated by alternative splicing and post-translational modifications

Plant calcium-dependent protein kinases (CDPKs) are key proteins implicated in calcium-mediated signaling pathways of a wide range of biological events in the organism. The action of each particular CDPK is strictly regulated by many mechanisms in order to ensure an accurate signal translation and the activation of the adequate response processes. In this work, we investigated the regulation of a CDPK involved in rice cold stress response, OsCPK17, to better understand its mode of action. We identified two new alternative splicing (AS) mRNA forms of OsCPK17 encoding truncated versions of the protein, missing the CDPK activation domain. We analyzed the expression patterns of all AS variants in rice tissues and examined their subcellular localization in onion epidermal cells. The results indicate that the AS of OsCPK17 putatively originates truncated forms of the protein with distinct functions, and different subcellular and tissue distributions. Additionally, we addressed the regulation of OsCPK17 by post-translational modifications in several in vitro experiments. Our analysis indicated that OsCPK17 activity depends on its structural rearrangement induced by calcium binding, and that the protein can be autophosphorylated. The identified phosphorylation sites mostly populate the OsCPK17 N-terminal domain. Exceptions are phosphosites T107 and S136 in the kinase domain and S558 in the C-terminal domain. These phosphosites seem conserved in CDPKs and may reflect a common regulatory mechanism for this protein family.

Recent Advances in Substrate Identification of Protein Kinases in Plants and Their Role in Stress Management

Protein phosphorylation-dephosphorylation is a well-known regulatory mechanism in biological systems and has become one of the significant means of protein function regulation, modulating most of the biological processes. Protein kinases play vital role in numerous cellular processes. Kinases transduce external signal into responses such as growth, immunity and stress tolerance through phosphorylation of their target proteins. In order to understand these cellular processes at the molecular level, one needs to be aware of the different substrates targeted by protein kinases. Advancement in tools and techniques has bestowed practice of multiple approaches that enable target identification of kinases. However, so far none of the methodologies has been proved to be as good as a panacea for the substrate identification. In this review, the recent advances that have been made in the identifications of putative substrates and the implications of these kinases and their substrates in stress management are discussed.

Comprehensive Analysis of the CDPK-SnRK Superfamily Genes in Chinese Cabbage and Its Evolutionary Implications in Plants

The CDPK-SnRK (calcium-dependent protein kinase/Snf1-related protein kinase) gene superfamily plays important roles in signaling pathways for disease resistance and various stress responses, as indicated by emerging evidence. In this study, we constructed comparative analyses of gene structure, retention, expansion, whole-genome duplication (WGD) and expression patterns of CDPK-SnRK genes in Brassica rapa and their evolution in plants. A total of 49 BrCPKs, 14 BrCRKs, 3 BrPPCKs, 5 BrPEPRKs, and 56 BrSnRKs were identified in B. rapa. All BrCDPK-SnRK proteins had highly conserved kinase domains. By statistical analysis of the number of CDPK-SnRK genes in each species, we found that the expansion of the CDPK-SnRK gene family started from angiosperms. Segmental duplication played a predominant role in CDPK-SnRK gene expansion. The analysis showed that PEPRK was more preferentially retained than other subfamilies and that CPK was retained similarly to SnRK. Among the CPKs and SnRKs, CPKIII and SnRK1 genes were more preferentially retained than other groups. CRK was closest to CPK, which may share a common evolutionary origin. In addition, we identified 196 CPK genes and 252 SnRK genes in 6 species, and their different expansion and evolution types were discovered. Furthermore, the expression of BrCDPK-SnRK genes is dynamic in different tissues as well as in response to abiotic stresses, demonstrating their important roles in development in B. rapa. In summary, this study provides genome-wide insight into the evolutionary history and mechanisms of CDPK-SnRK genes following whole-genome triplication in B. rapa.

A Role for Barley Calcium-Dependent Protein Kinase CPK2a in the Response to Drought

Increasing the drought tolerance of crops is one of the most challenging goals in plant breeding. To improve crop productivity during periods of water deficit, it is essential to understand the complex regulatory pathways that adapt plant metabolism to environmental conditions. Among various plant hormones and second messengers, calcium ions are known to be involved in drought stress perception and signaling. Plants have developed specific calcium-dependent protein kinases that convert calcium signals into phosphorylation events. In this study we attempted to elucidate the role of a calcium-dependent protein kinase in the drought stress response of barley (Hordeum vulgare L.), one of the most economically important crops worldwide. The ongoing barley genome project has provided useful information about genes potentially involved in the drought stress response, but information on the role of calcium-dependent kinases is still limited. We found that the gene encoding the calcium-dependent protein kinase HvCPK2a was significantly upregulated in response to drought. To better understand the role of HvCPK2a in drought stress signaling, we generated transgenic Arabidopsis plants that overexpressed the corresponding coding sequence. Overexpressing lines displayed drought sensitivity, reduced nitrogen balance index (NBI), an increase in total chlorophyll content and decreased relative water content. In addition, in vitro kinase assay experiments combined with mass spectrometry allowed HvCPK2a autophosphorylation sites to be identified. Our results suggest that HvCPK2a is a dual-specificity calcium-dependent protein kinase that functions as a negative regulator of the drought stress response in barley.

A Glycine soja methionine sulfoxide reductase B5a interacts with the Ca(2+) /CAM-binding kinase GsCBRLK and activates ROS signaling under carbonate alkaline stress

Although research has extensively illustrated the molecular basis of plant responses to salt and high-pH stresses, knowledge on carbonate alkaline stress is poor and the specific responsive mechanism remains elusive. We have previously characterized a Glycine soja Ca(2+) /CAM-dependent kinase GsCBRLK that could increase salt tolerance. Here, we characterize a methionine sulfoxide reductase (MSR) B protein GsMSRB5a as a GsCBRLK interactor by using Y2H and BiFc assays. Further analyses showed that the N-terminal variable domain of GsCBRLK contributed to the GsMSRB5a interaction. Y2H assays also revealed the interaction specificity of GsCBRLK with the wild soybean MSRB subfamily proteins, and determined that the BoxI/BoxII-containing regions within GsMSRBs were responsible for their interaction. Furthermore, we also illustrated that the N-terminal basic regions in GsMSRBs functioned as transit peptides, which targeted themselves into chloroplasts and thereby prevented their interaction with GsCBRLK. Nevertheless, deletion of these regions allowed them to localize on the plasma membrane (PM) and interact with GsCBRLK. In addition, we also showed that GsMSRB5a and GsCBRLK displayed overlapping tissue expression specificity and coincident expression patterns under carbonate alkaline stress. Phenotypic experiments demonstrated that GsMSRB5a and GsCBRLK overexpression in Arabidopsis enhanced carbonate alkaline stress tolerance. Further investigations elucidated that GsMSRB5a and GsCBRLK inhibited reactive oxygen species (ROS) accumulation by modifying the expression of ROS signaling, biosynthesis and scavenging genes. Summarily, our results demonstrated that GsCBRLK and GsMSRB5a interacted with each other, and activated ROS signaling under carbonate alkaline stress.

The calcium-dependent protein kinase CPK7 acts on root hydraulic conductivity

The hydraulic conductivity of plant roots (Lp(r)) is determined in large part by the activity of aquaporins. Mechanisms occurring at the post-translational level, in particular phosphorylation of aquaporins of the plasma membrane intrinsic protein 2 (PIP2) subfamily, are thought to be of critical importance for regulating root water transport. However, knowledge of protein kinases and phosphatases acting on aquaporin function is still scarce. In the present work, we investigated the Lp(r) of knockout Arabidopsis plants for four Ca(2+)-dependent protein kinases. cpk7 plants showed a 30% increase in Lp(r) because of a higher aquaporin activity. A quantitative proteomic analysis of wild-type and cpk7 plants revealed that PIP gene expression and PIP protein quantity were not correlated and that CPK7 has no effect on PIP2 phosphorylation. In contrast, CPK7 exerts a negative control on the cellular abundance of PIP1s, which likely accounts for the higher Lp(r) of cpk7. In addition, this study revealed that the cellular amount of a few additional proteins including membrane transporters is controlled by CPK7. The overall work provides evidence for CPK7-dependent stability of specific membrane proteins.

Kinase-mediated orchestration of NADPH oxidase in plant immunity

Reactive oxygen species (ROS) are important signalling molecules, which participate in multiple physiological processes including immune response, development, cell elongation and hormonal signalling in plants. Plant NADPH oxidase, termed respiratory burst oxidase homologue (RBOH), is frequently studied as a main player for pathogen-responsive ROS burst. Our understanding of the activation mechanism of RBOH after pathogen recognition has increased in recent years. In this review, we focus on kinase-mediated regulatory mechanisms of RBOHs. Calcium-dependent protein kinases (CDPKs) are well known to activate RBOHs by direct phosphorylation. In addition to functions of CDPKs in plants, we also describe the involvement of receptor-like cytoplasmic kinases (RLCKs) and mitogen-activated protein kinases (MAPKs) in fine-tuning RBOH activity at the post-translational and transcriptional levels, respectively.

Identification of Differentially Expressed Genes by cDNA-AFLP Technique in Response to Drought Stress in Triticum durum

Drought is the single largest abiotic stress factor leading to reduced crop yields. The identification of differentially expressed genes and the understanding of their functions in environmentally stressful conditions are essential to improve drought tolerance. Transcriptomics is a powerful approach for the global analysis of molecular mechanisms under abiotic stress. To identify genes that are important for drought tolerance, we analyzed mRNA populations from untreated and drought-stressed leaves of Triticum durum by cDNA- -amplified fragment length polymorphism (cDNA-AFLP) technique. Overall, 76 transcript- -derived fragments corresponding to differentially induced transcripts were successfully sequenced. Most of the transcripts identified here, using basic local alignment search tool (BLAST) database, were genes belonging to different functional categories related to metabolism, energy, cellular biosynthesis, cell defense, signal transduction, transcription regulation, protein degradation and transport. The expression patterns of these genes were confirmed by quantitative reverse transcriptase real-time polymerase chain reaction (qRT- -PCR) based on ten selected genes representing different patterns. These results could facilitate the understanding of cellular mechanisms involving groups of genes that act in coordination in response to stimuli of water deficit. The identification of novel stress-responsive genes will provide useful data that could help develop breeding strategies aimed at improving durum wheat tolerance to field stress.

Functions of Calcium-Dependent Protein Kinases in Plant Innate Immunity

An increase of cytosolic Ca²⁺ is generated by diverse physiological stimuli and stresses, including pathogen attack. Plants have evolved two branches of the immune system to defend against pathogen infections. The primary innate immune response is triggered by the detection of evolutionarily conserved pathogen-associated molecular pattern (PAMP), which is called PAMP-triggered immunity (PTI). The second branch of plant innate immunity is triggered by the recognition of specific pathogen effector proteins and known as effector-triggered immunity (ETI). Calcium (Ca²⁺) signaling is essential in both plant PTI and ETI responses. Calcium-dependent protein kinases (CDPKs) have emerged as important Ca²⁺ sensor proteins in transducing differential Ca²⁺ signatures, triggered by PAMPs or effectors and activating complex downstream responses. CDPKs directly transmit calcium signals by calcium binding to the elongation factor (EF)-hand domain at the C-terminus and substrate phosphorylation by the catalytic kinase domain at the N-terminus. Emerging evidence suggests that specific and overlapping CDPKs phosphorylate distinct substrates in PTI and ETI to regulate diverse plant immune responses, including production of reactive oxygen species, transcriptional reprogramming of immune genes, and the hypersensitive response.

Two calcium-dependent protein kinases from Chlamydomonas reinhardtii are transcriptionally regulated by nutrient starvation

We report here, the transcriptional regulation of 2 Calcium Dependent Protein Kinases in response to nutrient starvation of Chlamydomonas reinhardtii vegetative cells. The CDPK proteins, CDPK1 and CDPK3; share 53% identity among themselves, a maximum of 57% and 52% to higher plants respectively and 42% to apicomplexan protozoans. We expressed a CDPK1-GFP fusion protein in the C. reinhardtii vegetative cells and showed its distribution both in the cell body and the membrane-matrix fraction of the flagella. The fusion protein exhibits mobility shift in the presence of Ca (2+), confirming its Ca (2+)-binding properties. To the best of our knowledge, this is the first report of transcriptional regulation of CDPKs from a unicellular chlorophyte in response to nutrient starvation namely acetate (A), phosphorus (P), and nitrogen (N).

Transcriptome analysis reveals crosstalk of responsive genes to multiple abiotic stresses in cotton (Gossypium hirsutum L.)

Abiotic stress is a major environmental factor that limits cotton growth and yield, moreover, this problem has become more and more serious recently, as multiple stresses often occur simultaneously due to the global climate change and environmental pollution. In this study, we sought to identify genes involved in diverse stresses including abscisic acid (ABA), cold, drought, salinity and alkalinity by comparative microarray analysis. Our result showed that 5790, 3067, 5608, 778 and 6148 transcripts, were differentially expressed in cotton seedlings under treatment of ABA (1 μM ABA), cold (4°C), drought (200 mM mannitol), salinity (200 mM NaCl) and alkalinity (pH=11) respectively. Among the induced or suppressed genes, 126 transcripts were shared by all of the five kinds of abiotic stresses, with 64 up-regulated and 62 down-regulated. These common members are grouped as stress signal transduction, transcription factors (TFs), stress response/defense proteins, metabolism, transport facilitation, as well as cell wall/structure, according to the function annotation. We also noticed that large proportion of significant differentially expressed genes specifically regulated in response to different stress. Nine of the common transcripts of multiple stresses were selected for further validation with quantitative real time RT-PCR (qRT-PCR). Furthermore, several well characterized TF families, for example, WRKY, MYB, NAC, AP2/ERF and zinc finger were shown to be involved in different stresses. As an original report using comparative microarray to analyze transcriptome of cotton under five abiotic stresses, valuable information about functional genes and related pathways of anti-stress, and/or stress tolerance in cotton seedlings was unveiled in our result. Besides this, some important common factors were focused for detailed identification and characterization. According to our analysis, it suggested that there was crosstalk of responsive genes or pathways to multiple abiotic or even biotic stresses, in cotton. These candidate genes will be worthy of functional study under diverse stresses.

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.

A maize calcium-dependent protein kinase gene, ZmCPK4, positively regulated abscisic acid signaling and enhanced drought stress tolerance in transgenic Arabidopsis

Calcium-dependent protein kinases (CDPKs) play essential roles in calcium-mediated signal transductions in plant response to abiotic stress. Several members have been identified to be regulators for plants response to abscisic acid (ABA) signaling. Here, we isolated a subgroup I CDPK gene, ZmCPK4, from maize. Quantitative real time PCR (qRT-PCR) analysis revealed that the ZmCPK4 transcripts were induced by various stresses and signal molecules. Transient and stable expression of the ZmCPK4-GFP fusion proteins revealed ZmCPK4 localized to the membrane. Moreover, overexpression of ZmCPK4 in the transgenic Arabidopsis enhanced ABA sensitivity in seed germination, seedling growth and stomatal movement. The transgenic plants also enhanced drought stress tolerance. Taken together, the results suggest that ZmCPK4 might be involved in ABA-mediated regulation of stomatal closure in response to drought stress.

Rice small C2-domain proteins are phosphorylated by calcium-dependent protein kinase

We previously reported that OsERG1 and OsERG3 encode rice small C2-domain proteins with different biochemical properties in Ca(2+)- and phospholipid-binding assays. Os-ERG1 exhibited Ca(2+)-dependent phospholipid binding, which was not observed with OsERG3. In the present study, we show that both OsERG1 and OsERG3 proteins exhibit oligomerization properties as determined by native polyacrylamide gel electrophoresis (PAGE) and glutaraldehyde cross-linking experiments. Furthermore, in vitro phosphorylation assays reveal the phosphorylation of OsERG1 and OsERG3 by a rice calcium-dependent protein kinase, OsCDPK5. Our mutation analysis on putative serine phosphorylation sites shows that the first serine (Ser) at position 41 of OsERG1 may be an essential residue for phosphorylation by OsCDPK5. Mutation of Ser41 to alanine (OsERG1S41A) and aspartate (OsERG1S41D) abolishes the ability of OsERG1 to bind phospholipids regardless of the presence or absence of Ca(2+) ions. In addition, unlike the OsERG1 wild-type form, the mutant OsERG1 (S41A)::smGFP construct lost the ability to translocate from the cytosol to the plasma membrane in response to calcium ions or fungal elicitor. These results indicate that Ser41 may be essential for the function of OsERG1.

PiSCP1 and PiCDPK2 Localize to Peroxisomes and Are Involved in Pollen Tube Growth in Petunia Inflata

Petunia inflata small CDPK-interacting protein 1 (PiSCP1) was identified as a pollen expressed PiCDPK1 interacting protein using the yeast two hybrid system and the interaction confirmed using pull-down and phosphorylation assays. PiSCP1 is pollen specific and shares amino acid homology with uncharacterized proteins from diverse species of higher plants, but no protein of known function. Expression of PiSCP1-GFP in vivo inhibited pollen tube growth and was shown to localize to peroxisomes in growing pollen tubes. As PiCDPK1 is plasma membrane localized, we investigated the localization of a second isoform, PiCDPK2, and show that it co-localizes to peroxisomes with PiSCP1 and that the two proteins interact in the yeast 2 hybrid interaction assay, suggesting that interaction with the latter CDPK isoform is likely the one of biological relevance. Both PiCDPK2 and PiSCP1 affect pollen tube growth, presumably by mediating peroxisome function, however how they do so is currently not clear.

Genome-wide identification of the maize calcium-dependent protein kinase gene family

In higher plants, calcium is a ubiquitous second messenger in eukaryotic signal transduction cascades. The plant-specific calcium-dependent protein kinases (CDPKs) play important roles regulating downstream components of calcium signaling. We conducted a genome-wide analysis of maize (Zea mays) CDPKs and identified 35 CDPK genes. Maize CDPKs were found to be similar to their counterparts in rice in gene structure, GC content and subgroup classification. Divergence time estimation suggested that maize-rice orthologs were largely consistent with the time when these two species diverged from the last common ancestor. Semiquantitative RT-PCR revealed that the 29 of total 35 maize CDPK genes were expressed in all tissues, including root, stem, leaf, tassel, ear, and kernel. Our genomic and bioinformatics analyses will provide an important foundation for further functional dissection of the maize CDPK gene family.

CDPKs in immune and stress signaling

Ca(2+) has long been recognized as a conserved second messenger and principal mediator in plant immune and stress responses. How Ca(2+) signals are sensed and relayed into diverse primary and global signaling events is still largely unknown. Comprehensive analyses of the plant-specific multigene family of Ca(2+)-dependent protein kinases (CDPKs) are unraveling the molecular, cellular and genetic mechanisms of Ca(2+) signaling. CDPKs, which exhibit overlapping and distinct expression patterns, sub-cellular localizations, substrate specificities and Ca(2+) sensitivities, play versatile roles in the activation and repression of enzymes, channels and transcription factors. Here, we review the recent advances on the multifaceted functions of CDPKs in the complex immune and stress signaling networks, including oxidative burst, stomatal movements, hormonal signaling and gene regulation.

Characterization of Arabidopsis calcium-dependent protein kinases: activated or not by calcium?

CDPKs (calcium-dependent protein kinases), which contain both calmodulin-like calcium binding and serine/threonine protein kinase domains, are only present in plants and some protozoans. Upon activation by a stimulus, they transduce the signal through phosphorylation cascades to induce downstream responses, including transcriptional regulation. To understand the functional specificities of CDPKs, 14 Arabidopsis CPKs (CDPKs in plants) representative of the three main subgroups were characterized at the biochemical level, using HA (haemagglutinin)-tagged CPKs expressed in planta. Most of them were partially or mainly associated with membranes, in agreement with acylation predictions. Importantly, CPKs displayed highly variable calcium-dependences for their kinase activities: seven CPKs from subgroups 1 and 2 were clearly sensitive to calcium with different intensities, whereas six CPKs from subgroup 3 exhibited low or no calcium sensitivity to two generic substrates. Interestingly, this apparent calcium-independence correlated with significant alterations in the predicted EF-hands of these kinases, although they all bound calcium. The noticeable exception, CPK25, was calcium-independent owing to the absence of functional EF-hands. Taken together, the results of the present study suggest that calcium binding differentially affects CDPK isoforms that may be activated by distinct molecular mechanisms.

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

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

Biochemical regulation of in vivo function of plant calcium-dependent protein kinases (CDPK)

Calcium (Ca(2+)) is a major second messenger in plant signal transduction mediating stress- and developmental processes. Plant Ca(2+)-dependent protein kinases (CDPKs) are mono-molecular Ca(2+)-sensor/protein kinase effector proteins, which perceive Ca(2+) signals and translate them into protein phosphorylation and thus represent an ideal tool for signal transduction. This review focuses on recent developments in CDPK structural analysis and CDPK in vivo phosphorylation substrate identification. We discuss mechanisms implicated in the in vivo regulation of CDPK activity including Ca(2+) binding to the CDPK EF-hands, Ca(2+)-triggered intra-molecular conformation changes, and CDPK (auto)-phosphorylation. Moreover, we address regulation and integration into signaling cascades of selected members of the plant CDPK family, for which in vivo function and phosphorylation in abiotic and biotic stress signaling have been demonstrated. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

The mechanism of substrate recognition of Ca2+-dependent protein kinases

Ca2+-dependent protein kinases (CDPKs) are encoded by a multigene family and are thought to play central roles in Ca2+ signaling in plants. Although the primary structures of CDPK isoforms are highly conserved, several studies suggested a distinct physiological function for each CDPK isoform in plants. Hence, there should be mechanisms by which individual CDPK specifically recognizes its substrate. Recently, the variable N-terminal domain of NtCDPK1 was shown to play an essential role in the specific recognition of the substrate. Because the variable N-terminal domain of other CDPKs may also be involved in the substrate recognition, the search for interacting proteins of the variable N-terminal domain would provide important clues to identify the physiological substrates of each CDPK. Additionally, manipulation of the variable N-terminal domain may enable us to engineer the substrate specificity of CDPK, leading a rational rewiring of cellular signaling pathways.

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

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

A novel interaction between CCaMK and a protein containing the Scythe_N ubiquitin-like domain in Lotus japonicus

In the Rhizobium-legume symbiosis, calcium/calmodulin-dependent protein kinase (CCaMK) is a key regulator for both rhizobial infection and nodule organogenesis. Deregulation of CCaMK by either a point mutation in the autophosphorylation site or the deletion of the carboxyl-terminal regulatory domain results in spontaneous nodule formation without rhizobia. However, the underlying biochemical mechanisms are poorly understood. Here, using the kinase domain of CCaMK as a bait in yeast two-hybrid screening, we identify a novel protein, CIP73 (for CCaMK-interacting protein of approximately 73 kD), that interacts with CCaMK. CIP73 contains a Scythe_N ubiquitin-like domain and belongs to the large ubiquitin superfamily. Deletion and mutagenesis analysis demonstrate that CIP73 could only interact with CCaMK when the calmodulin-binding domain and three EF-hand motifs are removed from the kinase domain. The amino-terminal 80 amino acid residues (80-160) of CCaMK are required for interacting with CIP73 in yeast cells. On the other hand, protein pull-down assay and bimolecular fluorescence complementation assay in Nicotiana benthamiana show that the full-length CCaMK could interact with CIP73 in vitro and in planta. Importantly, CCaMK phosphorylates the amino terminus of CIP73 in a Ca2+/calmodulin-dependent manner in vitro. CIP73 transcripts are preferentially expressed in roots, and very low expression is detected in leaves, stems, and nodules. The expression in roots is significantly decreased after inoculation of Mesorhizobium loti. RNA interference knockdown of CIP73 expression by hairy root transformation in Lotus japonicus led to decreased nodule formation, suggesting that CIP73 performed an essential role in nodulation.

Identification of a GCC transcription factor responding to fruit colour change events in citrus through the transcriptomic analyses of two mutants

BACKGROUND

External ripening in Citrus fruits is morphologically characterized by a colour shift from green to orange due to the degradation of chlorophylls and the accumulation of carotenoid pigments. Although numerous genes coding for enzymes involved in such biochemical pathways have been identified, the molecular control of this process has been scarcely studied. In this work we used the Citrus clementina mutants 39B3 and 39E7, showing delayed colour break, to isolate genes potentially related to the regulation of peel ripening and its physiological or biochemical effects.

RESULTS

Pigment analyses revealed different profiles of carotenoid and chlorophyll modification in 39B3 and 39E7 mutants. Flavedo from 39B3 fruits showed an overall delay in carotenoid accumulation and chlorophyll degradation, while the flavedo of 39E7 was devoid of the apocarotenoid β-citraurin among other carotenoid alterations. A Citrus microarray containing about 20,000 cDNA fragments was used to identify genes that were differentially expressed during colour change in the flavedo of 39B3 and 39E7 mutants respect to the parental variety. The results highlighted 73 and 90 genes that were respectively up- and down-regulated in both mutants. CcGCC1 gene, coding for a GCC type transcriptional factor, was found to be down-regulated. CcGCC1 expression was strongly induced at the onset of colour change in the flavedo of parental clementine fruit. Moreover, treatment of fruits with gibberellins, a retardant of external ripening, delayed both colour break and CcGCC1 overexpression.

CONCLUSIONS

In this work, the citrus fruit ripening mutants 39B3 and 39E7 have been characterized at the phenotypic, biochemical and transcriptomic level. A defective synthesis of the apocarotenoid β-citraurin has been proposed to cause the yellowish colour of fully ripe 39E7 flavedo. The analyses of the mutant transcriptomes revealed that colour change during peel ripening was strongly associated with a major mobilization of mineral elements and with other previously known metabolic and photosynthetic changes. The expression of CcGCC1 was associated with peel ripening since CcGCC1 down-regulation correlated with a delay in colour break induced by genetic, developmental and hormonal causes.

Roles of enzymatic and nonenzymatic antioxidants in plants during abiotic stress

Reactive oxygen species (ROS) are produced in plants as byproducts during many metabolic reactions, such as photosynthesis and respiration. Oxidative stress occurs when there is a serious imbalance between the production of ROS and antioxidant defense. Generation of ROS causes rapid cell damage by triggering a chain reaction. Cells have evolved an elaborate system of enzymatic and nonenzymatic antioxidants which help to scavenge these indigenously generated ROS. Various enzymes involved in ROS-scavenging have been manipulated, over expressed or downregulated to add to the present knowledge and understanding the role of the antioxidant systems. The present article reviews the manipulation of enzymatic and nonenzymatic antioxidants in plants to enhance the environmental stress tolerance and also throws light on ROS and redox signaling, calcium signaling, and ABA signaling.

Arabidopsis calcium-dependent protein kinase CPK10 functions in abscisic acid- and Ca2+-mediated stomatal regulation in response to drought stress

Plant calcium-dependent protein kinases (CDPKs) may function as calcium sensors and play important roles in the regulation of plant growth and development and in plant responses to biotic and abiotic stresses. The Arabidopsis (Arabidopsis thaliana) genome encodes 34 CDPKs, and most of them have not been functionally characterized. Here, we report the functional characterization of CPK10 in Arabidopsis response to drought stress. The cpk10 mutant, a T-DNA insertion mutant for the Arabidopsis CPK10 gene, showed a much more sensitive phenotype to drought stress compared with wild-type plants, while the CPK10 overexpression lines displayed enhanced tolerance to drought stress. Induction of stomatal closure and inhibition of stomatal opening by abscisic acid (ABA) and Ca(2+) were impaired in the cpk10 mutants. Using yeast two-hybrid methods, a heat shock protein, HSP1, was identified as a CPK10-interacting protein. The interaction between CPK10 and HSP1 was further confirmed by pull-down and bimolecular fluorescence complementation assays. The HSP1 knockout mutant (hsp1) plants showed a similar sensitive phenotype under drought stress as the cpk10 mutant plants and were similarly less sensitive to ABA and Ca(2+) in regulation of stomatal movements. Electrophysiological experiments showed that ABA and Ca(2+) inhibition of the inward K(+) currents in stomatal guard cells were impaired in the cpk10 and hsp1 mutants. All presented data demonstrate that CPK10, possibly by interacting with HSP1, plays important roles in ABA- and Ca(2+)-mediated regulation of stomatal movements.

[Plant calcium sensors in osmotic signaling]

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

Breaking the code: Ca2+ sensors in plant signalling

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

Transcript profiling of the salt-tolerant Festuca rubra ssp. litoralis reveals a regulatory network controlling salt acclimatization

We report an analysis of salt-stress responses in the monocotyledonous halophyte Festuca rubra ssp. litoralis. Salt-dependent expression of transcripts encoding a PIP2;1 aquaporin, V-ATPase subunit B, and the Na+/H+ antiporter NHX was characterized. Transcription of FrPIP2;1, FrVHA-B, and FrNHX1 was induced in root tissue of F. rubra ssp. litoralis by salt treatment, and during salt-stress F. rubra ssp. litoralis accumulated sodium in leaves and roots. Cell specificity of FrPIP2;1, FrVHA-B, and FrNHX1 transcription was analyzed by in situ PCR in roots of F. rubra ssp. litoralis. Expression of the genes was localized to the root epidermis, cortex cells, endodermis, and the vascular tissue. In plants treated with 500 mM NaCl, transcripts were repressed in the epidermis and the outer cortex cells, whereas endodermis and vasculature showed strong signals. These data demonstrate that transcriptional regulation of the aquaporin PIP2;1, V-ATPase, and the Na+/H+ antiporter NHX is correlated with salt tolerance in F. rubra ssp. litoralis and suggests coordinated control of ion homeostasis and water status at high salinity in plants. Salt-induced transcript accumulation in F. rubra ssp. litoralis was further monitored by cDNA-arrays with expressed sequence tags derived from a cDNA subtraction library. The salt-regulated transcripts included those involved in the control of gene expression and signal transduction elements such as a serine/threonine protein kinase, an SNF1-related protein kinase, and a WRKY-type transcription factor. Other ESTs with salt-dependent regulation included transcripts encoding proteins that function in metabolism, general stress responses, and defense and transport proteins.

Identification of proteins that interact with catalytically active calcium-dependent protein kinases from Arabidopsis

Calcium-dependent protein kinases (CDPKs) are essential sensor-transducers of calcium signaling pathways in plants. Functional characterization of CDPKs is of great interest because they play important roles during growth, development, and in response to a wide range of environmental stimuli. The Arabidopsis genome encodes 34 CDPKs, but very few substrates of these enzymes have been identified. In this study, we exploited the unique characteristics of CDPKs to develop an efficient approach for the discovery of CDPK-interacting proteins. High-throughput, semi-automated yeast two-hybrid interaction screens with two different cDNA libraries each containing 18 million prey clones were performed using catalytically impaired and constitutively active AtCPK4 and AtCPK11 variants as baits. The use of the constitutively active versions of the CPK baits improved the recovery of positive interacting proteins relative to the wild type kinase. Titration of interaction strength by growth under increasing concentrations of 3-aminotriazole (3-AT), a histidine analog and competitive inhibitor of the His3 gene product, confirmed these results. Possible mechanisms for this observed improvement are discussed. The reproducibility of this approach was assessed by the overlap of several interacting proteins of AtCPK4 and AtCPK11 and the recovery of several putative substrates and indicated that yeast two-hybrid screens using constitutively active and/or catalytically impaired forms of CDPK provides a useful tool to identify potential substrates of the CDPK family and potentially the entire protein kinase superfamily.

Calcium and calmodulin-mediated regulation of gene expression in plants

Sessile plants have developed a very delicate system to sense diverse kinds of endogenous developmental cues and exogenous environmental stimuli by using a simple Ca2+ ion. Calmodulin (CaM) is the predominant Ca2+ sensor and plays a crucial role in decoding the Ca2+ signatures into proper cellular responses in various cellular compartments in eukaryotes. A growing body of evidence points to the importance of Ca2+ and CaM in the regulation of the transcriptional process during plant responses to endogenous and exogenous stimuli. Here, we review recent progress in the identification of transcriptional regulators modulated by Ca2+ and CaM and in the assessment of their functional significance during plant signal transduction in response to biotic and abiotic stresses and developmental cues.

Evolutionary and functional study of the CDPK gene family in wheat (Triticum aestivum L.)

Calcium-dependent protein kinases (CDPKs) are crucial sensors of calcium concentration changes in plant cells under diverse endogenous and environmental stimuli. We identified 20 CDPK genes from bread wheat and performed a comprehensive study on their structural, functional and evolutionary characteristics. Full-length cDNA sequences of 14 CDPKs were obtained using various approaches. Wheat CDPKs were found to be similar to their counterparts in rice in genomic structure, GC content, subcellular localization, and subgroup classification. Divergence time estimation of wheat CDPK gene pairs and wheat-rice orthologs suggested that most duplicated genes already existed in the common ancestor of wheat and rice. The number of CDPKs in diploid wheat genome was estimated to be at least 26, a number close to that in rice, Arabidopsis, and poplar. However, polymorphism among EST sequences uncovered transcripts of all three homoeologous alleles for 13 out of 20 CDPKs. Thus, the hexaploid wheat should have 2-3 fold more CDPK genes expressing in their cells than the diploid species. Wheat CDPK genes were found to respond to various biotic and abiotic stimuli, including cold, hydrogen peroxide (H(2)O(2)), salt, drought, powdery mildew (Blumeria graminis tritici, Bgt), as well as phytohormones abscisic acid (ABA) and gibberellic acid (GA). Each CDPK gene often responded to multiple treatments, suggesting that wheat CDPKs are converging points for multiple signal transduction pathways. The current work represents the first comprehensive study of CDPK genes in bread wheat and provides a foundation for further functional study of this important gene family in Triticeae.

A chemical-genetic approach to elucidate protein kinase function in planta

The major objective in protein kinase research is the identification of the biological process, in which an individual enzyme is integrated. Protein kinase-mediated signalling is thereby often addressed by single knock-out mutation- or co-suppression-based reverse genetics approaches. If a protein kinase of interest is a member of a multi gene family, however, no obvious phenotypic alteration in the morphology or in biochemical parameters may become evident because mutant phenotypes may be compensated by functional redundancy or homeostasis. Here we establish a chemical-genetic screen combining ATP-analogue sensitive (as) kinase variants and molecular fingerprinting techniques to study members of the plant calcium-dependent protein kinase (CDPK) family in vivo. CDPKs have been implicated in fast signalling responses upon external abiotic and biotic stress stimuli. CDPKs carrying the as-mutation did not show altered phosphorylation kinetics with ATP as substrate, but were able to use ATP analogues as phosphate donors or as kinase inhibitors. For functional characterization in planta, we have substituted an Arabidopsis thaliana mutant line of AtCPK1 with the respective as-variant under the native CPK1 promoter. Seedlings of Arabidopsis wild type and AtCPK1 as-lines were treated with the ATP analogue inhibitor 1-NA-PP1 and exposed to cold stress conditions. Rapid cold-induced changes in the phosphoproteome were analysed by 2D-gel-electrophoresis and phosphoprotein staining. The comparison between wild type and AtCPK1 as-plants before and after inhibitor treatment revealed differential CPK1-dependent and cold-stress-induced phosphoprotein signals. In this study, we established the chemical-genetic approach as a tool, which allows the investigation of plant-specific classes of protein kinases in planta and which facilitates the identification of rapid changes of molecular biomarkers in kinase-mediated signalling networks.

Expression analysis of calcium-dependent protein kinase gene family during reproductive development and abiotic stress conditions in rice (Oryza sativa L. ssp. indica)

Calcium-dependent protein kinases (CDPKs) are important sensors of Ca(+2) flux in plants, which control plant development and responses by regulating downstream components of calcium signaling pathways. Availability of the whole genome sequence and microarray platform allows investigation of genome-wide organization and expression profile of CDPK genes in rice with a view to ultimately define their function in plant systems. Genome-wide analysis led to identification of 31 CDPK genes in rice after a thorough annotation exercise based upon HMM profiles. Twenty-nine already identified CDPK genes were verified and two new members were added to the CDPK gene family of rice. Relative expression of all these genes has been analyzed by using Affymetrix rice genome arraytrade mark during three vegetative stages, six stages of panicle (P1-P6) and five stages of seed (S1-S5) development along with three abiotic stress conditions, viz. cold, salt and desiccation, given to seedling. Thirty-one CDPK genes were found to express in at least one of the experimental stages studied. Of these, transcripts for twenty three genes accumulated differentially during reproductive developmental stages; nine of them were preferentially up-regulated only in panicle, five were up-regulated in stages of panicles as well as seed development, whereas, expression of one gene was found to be specific to the S1 stage of seed development. Eight genes were found to be down-regulated during the panicle and seed developmental stages. Six CDPK genes were found to be induced while the expression of one gene was down-regulated under stress conditions. The differential expression of CDPK genes during reproductive development and stress is suggestive of their involvement in the underlying signal transduction pathways. Furthermore, up-regulation of common genes both during reproductive development as well as stress responses is indicative of common element between reproduction and stress.

Isolation and characterization of a novel v-SNARE family protein that interacts with a calcium-dependent protein kinase from the common ice plant, Mesembryanthemum crystallinum

McCPK1 (Mesembryanthemum crystallinum calcium-dependent protein kinase 1) mRNA expression is transiently salinity- and dehydrationstress responsive. The enzyme also undergoes dynamic subcellular localization changes in response to these same stresses. Using the yeast-two hybrid system, we have isolated and characterized a M. crystallinum CPK1 Adaptor Protein 2 (McCAP2). We show that McCPK1 interacts with the C-terminal, coiled-coil containing region of McCAP2 in the yeast two-hybrid system. This interaction was confirmed in vitro between the purified recombinant forms of each of the proteins and in vivo by coimmunoprecipitation experiments from plant extracts. McCAP2, however, was not a substrate for McCPK1. Computational threading analysis suggested that McCAP2 is a member of a novel family of proteins with unknown function also found in rice and Arabidopsis. These proteins contain coiled-coil spectrin repeat domains present in the syntaxin super-family that participate in vesicular and protein trafficking. Consistent with the interaction data, subcellular localization and fractionation studies showed that McCAP2 colocalizes with McCPK1 to vesicular structures located on the actin cytoskeleton and within the endoplasmic reticulum in cells subjected to low humidity stress. McCAP2 also colocalizes with AtVTIl1a, an Arabidopsis v-SNARE [vesicle-soluble N-ethyl maleimide-sensitive factor (NSF) attachment protein (SNAP) receptor] present in the trans-Golgi network (TGN) and prevacuolar compartments (PVCs). Both interaction and subcellular localization studies suggest that McCAP2 may possibly serve as an adaptor protein responsible for vesicle-mediated trafficking of McCPK1 to or from the plasma membrane along actin microfilaments of the cytoskeleton.

A novel coiled-coil protein co-localizes and interacts with a calcium-dependent protein kinase in the common ice plant during low-humidity stress

McCPK1 (Mesembryanthemum crystallinum calcium-dependent protein kinase 1) mRNA expression is induced transiently by salinity and water deficit stress and also McCPK1 undergoes dynamic subcellular localization changes in response to these same stresses. Here we have confirmed that low humidity is capable of causing a drastic change in McCPK1's subcellular localization. We attempted to elucidate this phenomenon by isolating components likely to be involved in this process. McCAP1 (M. crystallinum CDPK adapter protein 1) was cloned in a yeast two-hybrid screen with a constitutively active McCPK1 as bait. We show that McCPK1 and McCAP1 can interact in the yeast two-hybrid system, in vitro, and in vivo as demonstrated by coimmunoprecipitation experiments from plant extracts. However, McCAP1 does not appear to be a substrate for McCPK1. DsRed-McCAP1 and EGFP-McCPK1 fusions colocalize in epidermal cells of ice plants exposed to low humidity. McCAP1 is homologous to a family of proteins in Arabidopsis with no known function. Computational threading analysis suggests that McCAP1 is likely to be an intermediate filament protein of the cytoskeleton.