Crosstalk During Fruit Ripening and Stress Response Among Abscisic Acid,Calcium-Dependent Protein Kinase and Phenylpropanoid

Phenylpropanoids are secondary metabolites produced by plants. They, by differential expression, are involved in responses to biotic and abiotic stresses and confer plant plasticity. In addition, they are synthesized under normal conditions during the fruit-ripening process. Therefore, the understanding of the mechanics involved in the accumulation of these compounds in plants is of extreme importance for the development of plants with greater resistance and tolerance to biotic and abiotic stresses, and plants with greater functional potential. There is evidence that one of the pathways of the induction of phenylpropanoids is dependent on abscisic acid (ABA) and it is generated by a signaling cascade involving calcium (Ca²⁺) and Ca²⁺-dependent protein kinases (CDPKs). Plants have several Ca²⁺ binding proteins that act as cellular sensors and represent the first points of signal transduction. CDPKs are mono-molecular Ca²⁺-sensor/kinase-effector proteins, which perceive Ca²⁺ signals and translate them into protein phosphorylation and thus represent an ideal tool for signal transduction. However, the mechanisms involved in the ABA–CDPK–phenylpropanoids crosstalk under stress conditions and during fruit ripening remains uncertain. Therefore, this review seeks to surface a new line of evidence as an attempt to understand the manner in which the induction of phenylpropanoids occurs in plants.     

Tetraploids Isatis indigotica are more responsive and adaptable to stresses than the diploid progenitor based on changes in expression patterns of a cold inducible Ii CPK1

In plants, Ca²⁺-dependent protein kinases (CDPKs) are characterized as important sensors of Ca²⁺ flux in response to varieties of biotic and abiotic stress. A comprehensive survey of global gene expression performed by using an Arabidopsis thaliana whole genome Affymetrix gene chip revealed that CDPK tends to be significantly higher in tetraploid Isatis indigotica than in diploid ones. To investigate different CDPK expression in response to polyploidy, a full-length cDNA clone (IiCPK1) encoding CDPK was isolated from the traditional Chinese medicinal herb I. indigotica cDNA library. IiCPK1 contains some basic features of CDPKs: a catalytic kinase domain including an ATP-binding domain and four EFhand calcium-binding motifs. Real-time PCR analysis indicated the expression of IiCPK1 from two kinds of I. indigotica (tetraploid and diploid). They both were induced in response to cold stress, but tetraploids I. indigotica which has good fertility, exhibited an enhanced resistance and higher yield, and presented to be more responsive and adaptable. Our results suggest that IiCPK1 gene plays a role in adapting to the environmental stress.     

植物体内钙信号及其在调节干旱胁迫中的作用

钙作为植物体内第二信使广泛参与了植物响应的各种非生物和生物胁迫的信号传导。胁迫信号通过激活位于细胞质膜上的钙离子通道,产生胞质内特异性的钙信号,传递至钙信号感受蛋白,如钙调素(calmodulin,CaM)、钙依赖蛋白激酶(Ca2+-dependent protein kinases,CDPK)和类钙调磷酸酶B蛋白(calcineurin B-like protein,CBL)等,进而引起胞内一系列生理生化变化,最终对胁迫做出响应。钙信号在植物响应干旱胁迫信号系统中起枢纽作用,主要通过调节气孔运动,水通道蛋白(aquaporin,AQP)和抗氧化酶活性来减少水分流失,提高水分利用率,最终降低干旱对植物细胞的伤害,并具有一定的生态学功能。该文对近年来国内外有关植物体内钙信号的研究进展以及在干旱逆境中的调节作用进行综述,并对今后的研究做了展望。     

OsCDPK13, a calcium-dependent protein kinase gene from rice,is induced in response to cold and gibberellin

Ca²⁺-dependent protein kinases (CDPKs) (EC 2.7.1.37) are the predominant Ca²⁺-regulated serine/threonine protein kinase in plants and their genes are encoded by a multigene family. CDPKs are important components in signal transduction, but the precise role of each individual CDPK is still largely unknown. A CDPK gene designated as OsCDPK13 was cloned from rice seedlings and it showed a high level of sequence similarities to rice and other plant CDPK genes. OsCDPK13 contains all conserved regions found in CDPKs. It was a single copy gene and was highly expressed in root and leaf sheath tissues of rice seedlings. OsCDPK13 expression was increased in leaf sheath segments treated with gibberellin or subjected to cold stress. The results in this investigation, together with our previous studies, suggest that OsCDPK13 may be an important signaling component in rice seedlings under cold stress condition and in response to gibberellin.     

CDPK-mediated abiotic stress signaling

Calcium-dependent protein kinases (CDPKs) constitute a large multigene family in various plant species. CDPKs have been shown to have important roles in various physiological processes, including plant growth and development and abiotic and biotic stress responses in plants. Functional analysis using gain-of-function and loss-of-function mutants has revealed the biological function of CDPKs in planta. Several CDPKs have been shown to be essential factors in abiotic stress tolerance, positively or negatively regulating stress tolerance by modulating ABA signaling and reducing the accumulation of reactive oxygen species (ROS). This review summarizes recent results describing the biological function of CDPKs that are involved in abiotic stress tolerance.     

Biotic and abiotic stress responses through calcium-dependent protein kinase (CDPK) signaling in wheat (Triticum aestivum L.)

Calcium-dependent protein kinases (CDPKs) sense the calcium concentration changes in plant cells and play important roles in signaling pathways for disease resistance and various stress responses as indicated by emerging evidences. Among the 20 wheat CDPK genes studied, 10 were found to respond to drought, salinity and ABA treatments. Consistent with previous observations, one CDPK gene was shown to respond to multiple abiotic stresses in wheat suggesting that CDPKs could be converging points for multiple signaling pathways. Among the 12 wheat CDPK genes that were responsive to Blumeria graminis tritici (Bgt) infection or the treatment of hydrogen peroxide (H₂O₂), eight also responded to abiotic stresses, suggesting a cross-talk between biotic and abiotic stress signaling pathways. Phylogenetic analysis indicated that some of these genes were closely related to CDPKs from other species, whose functions have been partially studied, suggesting similar functions wheat CDPK genes. Combining the up-to-date knowledge of CDPK functions and our observations, a model was developed to project the possible roles of wheat CDPK genes in the signaling of biotic and abiotic stress responses.Key words: CDPK, calcium, kinase, stress response, disease resistance, signal transduction, wheatSessile plants have developed sophisticated signaling pathways to deal with dramatic environmental changes that may affect their normal growth, such as pathogen attack, drought, and cold. Calcium is a universal secondary messenger that responds to these stimuli. The fluctuation in cytosolic Ca²⁺ levels can be sensed by calcium-dependent protein kinases (CDPKs), which will modify the phosphorylation status of substrate proteins.^1–3 Accumulating evidence indicate that CDPKs mediate biotic and abiotic stress signaling pathways.^4–7 For example, overexpression of the rice CDPK gene OsCDPK7 provides cold, salt, and drought tolerance for the transgenic rice plants, demonstrating the potential of CDPK engineering to generate stress tolerance enhanced crops.^8,9In wheat, 10 out of 14 CDPK genes appeared to respond to abiotic stresses including drought, NaCl, as well as ABA stimulus (Fig. 1A).¹⁰ Five CDPKs (TaCPK4, 6, 9, 10 and 18) were particularly interesting since they could respond to at least two of the three treatments, among which the expression level of TaCPK9 was enhanced under all three treatments suggesting that TaCPK9 is the point where multiple signaling pathways cross. In wheat, TaCPK4 responded to both ABA treatment and NaCl stress (Fig. 1A). Interestingly, its best Arabidopsis homologs AtCPK4 and AtCPK11, as suggested by a Neighbor-Joining phylogenetic analysis (Fig. 1B), have been postulated as two important positive regulators in CDPK/calcium-mediated ABA signaling pathways.¹¹ Such a correlation strongly supports the idea that TaCPK4 is a good candidate in wheat for ABA signaling. Figure 1A also shows that one wheat CDPK gene could respond to multiple abiotic stresses suggesting that CDPKs are converging points for multiple signaling pathways. On the other hand, multiple CDPKs were involved in single stress response. It is however not clear how these CDPKs are organized in one signaling pathway.Open in a separate windowFigure 1The roles of wheat CDPKs in abiotic and biotic stress responses. (A) One CDPK gene responded to multiple abiotic stresses and multiple CDPKs were required for single stress response. (B) Phylogenetic relationship of wheat CDPKs with functionally studied CDPKs from barley (HvCPKs), Arabidopsis (AtCPKs), and potato (StCDPKs) that are known to be involved in ABA signaling, oxidative burst regulation and defense to powdery mildew pathogenesis. (C) A model depicting CDPK-mediated signaling pathways under biotic and abiotic treatments in wheat (see text for details). Dotted lines with a question mark indicate unknown intermediate steps.Regarding the roles of CDPKs in defense reactions, 12 TaCPKs were found to be responsive to either Blumeria graminis tritici (Bgt) infection or H₂O₂ treatment. The response to H₂O₂ was investigated because cytosolic calcium influx and reactive oxygen species, such as H₂O₂ are known to be implicated in both plant innate immunity and abiotic stresses.^12–17 Among these CDPK genes, five responded to both treatments (Group II) whereas the ones that responded to Bgt infection (Group I) or H₂O₂ treatment (Group III) were four and three respectively. The differential expression patterns suggest different functional modes of these CDPK genes. Involvement of CDPK genes in plant defense response has been shown in multiple species.^5,7 Recently, two barley CDPK paralogs (HvCDPK3 and HvCDPK4) were found to play antagonistic roles during the early phase of powdery mildew pathogenesis.⁵ The close similarity between wheat CDPK genes (TaCPK2 and TaCPK5, Fig. 1B) with these two barley genes may suggest their potential roles in wheat powdery mildew resistance. Surprisingly, we did not detect the responsiveness of TaCPK5 to wheat Bgt infection, indicating the divergence of CDPK functions in these two members of Triticeae family. Recently, one potato (Solanum tuberosum) CDPK gene StCDPK5 has been shown to be directly involved in regulating oxidative burst via phosphorylation of the NADPH oxidase StRBOHB.¹⁸ In light of the close relationship of TaCPK2 with HvCDPK5 and StCDPK5 (Fig. 1B), we speculate that TaCPK2 could be associated with both biotic and abiotic stress response signaling pathways and therefore play multiple roles in wheat.A model was proposed in Figure 1C regarding the positions of wheat CDPK genes in signaling pathways for biotic and abiotic responses. The hypothesis depicted four different roles of wheat CDPK genes: (1) Group I genes that respond only to Bgt infection may, like potato StCDPK5, render defense response through an oxidase like NADPH oxidase that generates increased amount of H₂O₂;¹⁸ (2) At one aspect, Group II genes may participate in defense response in a manner similar to Group I genes; (3) On the other hand, since Group II genes also respond to H₂O₂ treatment directly, an auto-regulation circuit was proposed, which eventually joins the oxidase pathway; (4) Group III CDPK genes and some remaining CDPK genes are considered to be mainly involved in abiotic stress responses. The model positioned CDPKs both upstream and downstream of H₂O₂, presenting a complicated wiring of the signaling pathway network involving wheat CDPKs. Future biochemical, genetic, and transgenic analyses may help elucidate the genuineness of such a rather early model for the functions of wheat CDPK genes.     

Genome-wide identification, classification, and expression analysis of CDPK and its closely related gene families in poplar (Populus trichocarpa)

Calcium-dependent protein kinases (CDPKs) are Ca²⁺-binding proteins known to play crucial roles in Ca²⁺ signal transduction pathways which have been identified throughout plant kingdom and in certain types of protists. Genome-wide analysis of CDPKs have been carried out in Arabidopsis, rice and wheat, and quite a few of CDPKs were proved to play crucial roles in plant stress responsive signature pathways. In this study, a comprehensive analysis of Populus CDPK and its closely related gene families was performed, including phylogeny, chromosome locations, gene structures, and expression profiles. Thirty Populus CDPK genes and twenty closely related kinase genes were identified, which were phylogenetically clustered into eight distinct subfamilies and predominately distributed across fifteen linkage groups (LG). Genomic organization analyses indicated that purifying selection has played a pivotal role in the retention and maintenance of Populus CDPK gene family. Furthermore, microarray analysis showed that a number of Populus CDPK and its closely related genes differentially expressed across disparate tissues and under various stresses. The expression profiles of paralogous pairs were also investigated to reveal their evolution fates. In addition, quantitative real-time RT-PCR was performed on nine selected CDPK genes to confirm their responses to drought stress treatment. These observations may lay the foundation for future functional analysis of Populus CDPK and its closely related gene families to unravel their biological roles.     

钙调素结合蛋白参与调控植物逆境胁迫的研究进展

Ca²⁺是植物体内重要的第二信使,当植物受到各种环境刺激时,细胞内的Ca²⁺浓度瞬间产生变化,并被Ca²⁺信号效应器识别,通过与下游的靶蛋白结合并调节其活性,参与调控植物各种生理活动。钙调素结合蛋白以依赖Ca²⁺或不依赖Ca²⁺的方式结合钙调素。对目前已经鉴定的植物钙调素结合蛋白结构特点进行了综述,并着重介绍了钙调素结合蛋白是如何参与调节植物对生物胁迫和非生物胁迫的反应,为提高作物抗病抗逆能力研究提供理论基础。     

Cold stress-induced calcium-dependent protein kinase(s) in rice (Oryza sativa L.) seedling stem tissues

Ca²⁺-dependent protein kinases (CDPKs) play an important role in plant signal transduction. Protein kinase(s) activities induced by 5°C cold stress in rice (Oryza sativa L.) seedlings were investigated in both leaf and stem tissues in an early (up to 45 min) and late (up to 12 h) response study. The leaf had 37-, 47- and 55-kDa protein kinase activities, and the stem had 37-, 47- and 55-kDa protein kinase activities. A 16-kDa protein showed constitutive kinase activity in the rice seedling leaf and stem. It was further identified that the 47-kDa protein kinase activity induced by cold in both the cytosolic and membrane fractions of the stem was strictly Ca²⁺-dependent. This CDPK activitiy increased in the presence of the Ca²⁺ ionophore A23187 in stem segments, whereas it was decreased by the Ca²⁺ channel blocker, LaCl₃, and the Ca²⁺ chelator, EGTA. The general protein kinase inhibitor, staurosporine, completely inhibited this CDPK activity in vitro, and both W7, a calmodulin antagonist, and H7, a protein kinase C inhibitor, could only partially decrease this activity. The protein phosphatase inhibitor, okadaic acid, increased CDPK activity. This CDPK activity was also induced by salt, drought stress and the phytohormone abscicic acid. Among the 18 rice varieties tested, this cold-induced 47-kDa CDPK activity was stronger in the cold-tolerant varieties than in the sensitive ones. Received: 13 August 1999 / Accepted: 24 January 2000     

CDPK-mediated signalling pathways: specificity and cross-talk

Plants are constantly exposed to environmental changes and have to integrate a variety of biotic and abiotic stress stimuli. Calcium-dependent protein kinases (CDPKs) are implicated as important sensors of Ca2+ flux in plants in response to these stresses. CDPKs are encoded by multigene families, and expression levels of these genes are spatially and temporally controlled throughout development. In addition, a subset of CDPK genes responds to external stimuli. Biochemical evidence supports the idea that CDPKs are involved in signal transduction during stress conditions. Furthermore, loss-of-function and gain-of-function studies revealed that signalling pathways leading to cold, salt, drought or pathogen resistance are mediated by specific CDPK isoforms     

Chemical signaling under abiotic stress environment in plants

Many chemicals are critical for plant growth and development and play an important role in integrating various stress signals and controlling downstream stress responses by modulating gene expression machinery and regulating various transporters/pumps and biochemical reactions. These chemicals include calcium (Ca²⁺), cyclic nucleotides, polyphosphoinositides, nitric oxide (NO), sugars, abscisic acid (ABA), jasmonates (JA), salicylic acid (SA) and polyamines. Ca²⁺ is one of the very important ubiquitous second messengers in signal transduction pathways and usually its concentration increases in response to the stimuli including stress signals. Many Ca²⁺ sensors detect the Ca²⁺ signals and direct them to downstream signaling pathways by binding and activating diverse targets. cAMP or cGMP protects the cell with ion toxicity. Phosphoinositides are known to be involved both in transmission of signal across the plasma membrane and in intracellular signaling. NO activates various defense genes and acts as a developmental regulator in plants. Sugars affect the expression of many genes involved in photosynthesis, glycolysis, nitrogen metabolism, sucrose and starch metabolism, defense mechanisms and cell cycle regulation. ABA, JA, SA and polyamines are also involved in many stress responses. Cross-talk between these chemical signaling pathways is very common in plant responses to abiotic and bitotic factors. In this article we have described the role of these chemicals in initiating signaling under stress conditions mainly the abiotic stress.Key words: ABA, abiotic stress, Ca²⁺ binding proteins, calcium signaling, cyclic nucleotides, nitric oxide, phosphoinositides signaling, signal transduction, sugar signaling     

Calcium decoding mechanisms in plants

Ca²⁺ is a crucial second messenger that is involved in mediating responses to various biotic and abiotic environmental cues and in the regulation of many developmental processes in plants. Intracellular Ca²⁺ signals are realized by spatially and temporally defined changes in Ca²⁺ concentration that represent stimulus-specific Ca²⁺ signatures. These Ca²⁺ signatures are sensed, decoded and transmitted to downstream responses by a complex tool kit of Ca²⁺ binding proteins that function as Ca²⁺ sensors. Plants possess an extensive and complex array of such Ca²⁺ sensors that convey the information presented in the Ca²⁺ signatures into phosphorylation events, changes in protein-protein interactions or regulation of gene expression. Prominent Ca²⁺ sensors like, Calmodulins (CaM), Calmodulin-like proteins (CMLs), calcium dependent protein kinases (CDPKs), Calcineurin B-like proteins (CBLs) and their interacting kinases (CIPKs) exist in complex gene families and form intricate signaling networks in plants that are capable of robust and flexible information processing. In this review we reflect on the recently gained knowledge about the mechanistic principles of these Ca²⁺ sensors, their biochemical properties, physiological functions and newly identified targets proteins. These aspects will be discussed in the context of emerging functional principles that govern the information processing via these signaling modules.     

Stress responses mediated by the CBL calcium sensors in plants

Calcium ions (Ca²⁺) are involved as second messenger in plant responses to a broad array of environmental stimuli, including biotic and abiotic stresses. Therefore, understanding Ca²⁺-signaling mechanisms may lead to the development of transgenic crops with enhanced tolerance to adverse environmental conditions. In order to initiate the signaling cascades and give rise to relevant cellular and physiological responses, changes in the parameters of Ca²⁺ transients should be first detected by appropriate Ca²⁺ sensors in plant cells. In this regard, elucidations of plant Ca²⁺ sensors and their target molecules are critical steps for unraveling the Ca²⁺ signal transduction pathways. Recent studies have revealed that plants possess many Ca²⁺-binding proteins with different properties, which can serve as distinct Ca²⁺ sensors. This present review mainly focuses on a family of calcineurin B-like Ca²⁺ sensors which has been most recently identified from higher plants including Arabidopsis, rice, maize and pea.     

A phyloproteomic characterization of in vitro autophosphorylation in calcium-dependent protein kinases

Calcium-dependent protein kinases (CDPKs) are a novel class of signaling molecules that have been broadly implicated in relaying specific calcium-mediated responses to biotic and abiotic stress as well as developmental cues in both plants and protists. Calcium-dependent autophosphorylation has been observed in almost all CDPKs examined, but a physiological role for autophosphorylation has not been demonstrated. To date, only a handful of autophosphorylation sites have been mapped to specific residues within CDPK amino acid sequences. In an attempt to gain further insight into this phenomenon, we have mapped autophosphorylation sites and compared these phosphorylation patterns among multiple CDPK isoforms. From eight CDPKs and two CDPK-related kinases from Arabidopsis thaliana and Plasmodium falciparum, 31 new autophosphorylation sites were characterized, which in addition to the previously described sites, allowed the identification of five conserved loci. Of the 35 total sites analyzed approximately one-half were observed in the N-terminal variable domain. Homology models were generated for the protein kinase and calmodulin-like domains, each containing two of the five conserved sites, to allow intelligent speculation regarding subsequent lines of investigation.     

CPK12: A Ca2+-dependent protein kinase balancer in abscisic acid signaling

Ca²⁺ is believed to be a critical second messenger in ABA signal transduction. Ca²⁺-dependent protein kinases (CDPKs) are the best characterized Ca²⁺ sensors in plants. Recently, we identified an Arabidopsis CDPK member CPK12 as a negative regulator of ABA signaling in seed germination and post-germination growth, which reveals that different members of the CDPK family may constitute a regulation loop by functioning positively and negatively in ABA signal transduction. We observed that both RNA interference and overexpression of CPK12 gene resulted in ABA-hypersensitive phenotypes in seed germination and post-germination growth, suggesting a high complexity of the CPK12-mediated ABA signaling pathway. CPK12 stimulates a negative ABA-signaling regulator (ABI2) and phosphorylates two positive ABA-signaling regulators (ABF1 and ABF4), which may partly explain the ABA hypersensitivity induced by both downregulation and upregulation of CPK12 expression. Our data indicate that CPK12 appears to function as a balancer in ABA signal transduction in Arabidopsis.