Gene regulation by MAP kinase cascades

The Arabidopsis thaliana mitogen-activated protein kinases MPK3 and MPK6 target a subclass of 'VQ-motif'-containing proteins to regulate immune responses

Mitogen-activated protein kinase (MAPK) cascades play key roles in plant immune signalling, and elucidating their regulatory functions requires the identification of the pathway-specific substrates. We used yeast two-hybrid interaction screens, in vitro kinase assays and mass spectrometry-based phosphosite mapping to study a family of MAPK substrates. Site-directed mutagenesis and promoter-reporter fusion studies were performed to evaluate the impact of substrate phosphorylation on downstream signalling. A subset of the Arabidopsis thaliana VQ-motif-containing proteins (VQPs) were phosphorylated by the MAPKs MPK3 and MPK6, and renamed MPK3/6-targeted VQPs (MVQs). When plant protoplasts (expressing these MVQs) were treated with the flagellin-derived peptide flg22, several MVQs were destabilized in vivo. The MVQs interact with specific WRKY transcription factors. Detailed analysis of a representative member of the MVQ subset, MVQ1, indicated a negative role in WRKY-mediated defence gene expression - with mutation of the VQ-motif abrogating WRKY binding and causing mis-regulation of defence gene expression. We postulate the existence of a variety of WRKY-VQP-containing transcriptional regulatory protein complexes that depend on spatio-temporal VQP and WRKY expression patterns. Defence gene transcription can be modulated by changing the composition of these complexes - in part - through MAPK-mediated VQP degradation.

Genome-wide identification and expression analysis of MAPK and MAPKK gene family in Malus domestica

MAPK signal transduction modules play crucial roles in regulating many biological processes in plants, which are composed of three classes of hierarchically organized protein kinases, namely MAPKKKs, MAPKKs, and MAPKs. Although genome-wide analysis of this family has been carried out in some species, little is known about MAPK and MAPKK genes in apple (Malus domestica). In this study, a total of 26 putative apple MAPK genes (MdMPKs) and 9 putative apple MAPKK genes (MdMKKs) have been identified and located within the apple genome. Phylogenetic analysis revealed that MdMAPKs and MdMAPKKs could be divided into 4 subfamilies (groups A, B, C and D), respectively. The predicted MdMAPKs and MdMAPKKs were distributed across 13 out of 17 chromosomes with different densities. In addition, analysis of exon-intron junctions and of intron phase inside the predicted coding region of each candidate gene has revealed high levels of conservation within and between phylogenetic groups. According to the microarray and expressed sequence tag (EST) analysis, the different expression patterns indicate that they may play different roles during fruit development and rootstock-scion interaction process. Moreover, MAPK and MAPKK genes were performed expression profile analyses in different tissues (root, stem, leaf, flower and fruit), and all of the selected genes were expressed in at least one of the tissues tested, indicating that the MAPKs and MAPKKs are involved in various aspects of physiological and developmental processes of apple. To our knowledge, this is the first report of a genome-wide analysis of the apple MAPK and MAPKK gene family. This study provides valuable information for understanding the classification and putative functions of the MAPK signal in apple.

From models to ornamentals: how is flower senescence regulated?

Floral senescence involves an ordered set of events coordinated at the plant, flower, organ and cellular level. This review assesses our current understanding of the input signals, signal transduction and cellular processes that regulate petal senescence and cell death. In many species a visible sign of petal senescence is wilting. This is accompanied by remobilization of nutrients from the flower to the developing ovary or to other parts of the plant. In other species, petals abscise while still turgid. Coordinating signals for floral senescence also vary across species. In some species ethylene acts as a central regulator, in others floral senescence is ethylene insensitive and other growth regulators are implicated. Due to the variability in this coordination and sequence of events across species, identifying suitable models to study petal senescence has been challenging, and the best candidates are reviewed. Transcriptomic studies provide an overview of the MAP kinases and transcription factors that are activated during petal senescence in several species including Arabidopsis. Our understanding of downstream regulators such as autophagy genes and proteases is also improving. This gives us insights into possible signalling cascades that regulate initiation of senescence and coordination of cell death processes. It also identifies the gaps in our knowledge such as the role of microRNAs. Finally future prospects for using all this information from model to non-model species to extend vase life in ornamental species is reviewed.

The rice MAPKK-MAPK interactome: the biological significance of MAPK components in hormone signal transduction

Mitogen-activated protein kinase (MAPK) signaling cascades are evolutionarily conserved fundamental signal transduction pathways. A MAPK cascade consists of many distinct MAPKKK-MAPKK-MAPK modules linked to various upstream receptors and downstream targets through sequential phosphorylation and activation of the cascade components. These cascades collaborate in transmitting a variety of extracellular signals and in controlling cellular responses and processes such as growth, differentiation, cell death, hormonal signaling, and stress responses. Although MAPK proteins play central roles in signal transduction pathways, our knowledge of MAPK signaling in hormonal responses in rice has been limited to a small subset of specific upstream and downstream interacting targets. However, recent studies revealing direct MAPK and MAPKK interactions have provided the basis for elucidating interaction specificities, functional divergence, and functional modulation during hormonal responses. In this review, we highlight current insights into MAPKK-MAPK interaction patterns in rice, with emphasis on the biological significance of these interacting pairs in SA (salicylic acid), JA (jasmonic acid), ET (ethylene), and ABA (abscisic acid) responses, and discuss the challenges in understanding functional signal transduction networks mediated by these hormones.

Genome-wide identification and analysis of expression profiles of maize mitogen-activated protein kinase kinase kinase

Mitogen-activated protein kinase (MAPK) cascades are highly conserved signal transduction model in animals, yeast and plants. Plant MAPK cascades have been implicated in development and stress responses. Although MAPKKKs have been investigated in several plant species including Arabidopsis and rice, no systematic analysis has been conducted in maize. In this study, we performed a bioinformatics analysis of the entire maize genome and identified 74 MAPKKK genes. Phylogenetic analyses of MAPKKKs from maize, rice and Arabidopsis have classified them into three subgroups, which included Raf, ZIK and MEKK. Evolutionary relationships within subfamilies were also supported by exon-intron organizations and the conserved protein motifs. Further expression analysis of the MAPKKKs in microarray databases revealed that MAPKKKs were involved in important signaling pathways in maize different organs and developmental stages. Our genomics analysis of maize MAPKKK genes provides important information for evolutionary and functional characterization of this family in maize.

OsWRKY30 is activated by MAP kinases to confer drought tolerance in rice

Both the WRKY transcription factor (TF) and MAP kinases have been shown to regulate gene expression in response to biotic and abiotic stresses in plants. Several reports have shown that WRKY TFs may function downstream of MAPK cascades. Here, we have shown that OsWRKY30 interacted with OsMPK3, OsMPK4, OsMPK7, OsMPK14, OsMPK20-4, and OsMPK20-5, and could be phosphorylated by OsMPK3, OsMPK7, and OsMPK14. Overexpression of OsWRKY30 in rice dramatically increased drought tolerance. Overexpression of OsWRKY30AA, in which all SP (serine residue followed by proline residue) sites were replaced by AP (A, alanine), resulted in no improvement in drought tolerance. In addition, the function of transcriptional activation of OsWRKY30 was impaired after SP was replaced by AP. These results proved that the phosphorylation of OsWRKY30 by MAPKs was crucial in order for OsWRKY30 to perform its biological function.

Rice mitogen-activated protein kinase interactome analysis using the yeast two-hybrid system

Mitogen-activated protein kinase (MAPK) cascades support the flow of extracellular signals to intracellular target molecules and ultimately drive a diverse array of physiological functions in cells, tissues, and organisms by interacting with other proteins. Yet, our knowledge of the global physical MAPK interactome in plants remains largely fragmented. Here, we utilized the yeast two-hybrid system and coimmunoprecipitation, pull-down, bimolecular fluorescence complementation, subcellular localization, and kinase assay experiments in the model crop rice (Oryza sativa) to systematically map what is to our knowledge the first plant MAPK-interacting proteins. We identified 80 nonredundant interacting protein pairs (74 nonredundant interactors) for rice MAPKs and elucidated the novel proteome-wide network of MAPK interactors. The established interactome contains four membrane-associated proteins, seven MAP2Ks (for MAPK kinase), four MAPKs, and 59 putative substrates, including 18 transcription factors. Several interactors were also validated by experimental approaches (in vivo and in vitro) and literature survey. Our results highlight the importance of OsMPK1, an ortholog of tobacco (Nicotiana benthamiana) salicyclic acid-induced protein kinase and Arabidopsis (Arabidopsis thaliana) AtMPK6, among the rice MAPKs, as it alone interacts with 41 unique proteins (51.2% of the mapped MAPK interaction network). Additionally, Gene Ontology classification of interacting proteins into 34 functional categories suggested MAPK participation in diverse physiological functions. Together, the results obtained essentially enhance our knowledge of the MAPK-interacting protein network and provide a valuable research resource for developing a nearly complete map of the rice MAPK interactome.

CrMPK3, a mitogen activated protein kinase from Catharanthus roseus and its possible role in stress induced biosynthesis of monoterpenoid indole alkaloids

BACKGROUND

Mitogen activated protein kinase (MAPK) cascade is an important signaling cascade that operates in stress signal transduction in plants. The biologically active monoterpenoid indole alkaloids (MIA) produced in Catharanthus roseus are known to be induced under several abiotic stress conditions such as wounding, UV-B etc. However involvement of any signaling component in the accumulation of MIAs remains poorly investigated so far. Here we report isolation of a novel abiotic stress inducible Catharanthus roseus MAPK, CrMPK3 that may have role in accumulation of MIAs in response to abiotic stress.

RESULTS

CrMPK3 expressed in bacterial system is an active kinase as it showed auto-phosphorylation and phosphorylation of Myelin Basic Protein. CrMPK3 though localized in cytoplasm, moves to nucleus upon wounding. Wounding, UV treatment and MeJA application on C. roseus leaves resulted in the transcript accumulation of CrMPK3 as well as activation of MAPK in C. roseus leaves. Immuno-precipitation followed by immunoblot analysis revealed that wounding, UV treatment and methyl jasmonate (MeJA) activate CrMPK3. Transient over-expression of CrMPK3 in C. roseus leaf tissue showed enhanced expression of key MIA biosynthesis pathway genes and also accumulation of specific MIAs.

CONCLUSION

Results from our study suggest a possible involvement of CrMPK3 in abiotic stress signal transduction towards regulation of transcripts of key MIA biosynthetic pathway genes, regulators and accumulation of major MIAs.

Abiotic stresses activate a MAPkinase in the model grass species Lolium temulentum

Forage and turf grasses are utilized in diverse environments that expose them to a variety of abiotic stresses, however very little is known concerning the perception or molecular responses to these various stresses. In the model grass species Lolium temulentum, a 46kDa mitogen-activated protein kinase (MAPK) was activated in the leaf within 10min of exposing the roots to salt stress. When plants were subjected cold stress, no significant activation of the MAPK was observed. However, the 46kDa MAPK was rapidly activated in the leaves of plants within 3min of exposure to heat stress. Previously, mechanical wounding has been shown to rapidly activate a 46kDa and a 44kDa MAPK in L. temulentum. The wound activation of the MAPKs was delayed and diminished in plants undergoing cold treatment. In plants subjected simultaneously to 40°C and wounding, the activation of the 46kDa MAPK was enhanced. However if plants were subjected to heat and cold stress for more than 2h or exposed to 300mM NaCl for 24h prior to wounding, the wound activation of the 46kDa and a 44kDa MAPKs were significantly inhibited. These results suggest that the 46kDa MAPK plays a role in the response to various environmental stimuli.

Phosphorylation by AtMPK6 is required for the biological function of AtMYB41 in Arabidopsis

Mitogen-activated protein kinases (MPKs) are involved in a number of signaling pathways that control plant development and stress tolerance via the phosphorylation of target molecules. However, so far only a limited number of target molecules have been identified. Here, we provide evidence that MYB41 represents a new target of MPK6. MYB41 interacts with MPK6 not only in vitro but also in planta. MYB41 was phosphorylated by recombinant MPK6 as well as by plant MPK6. Ser(251) in MYB41 was identified as the site phosphorylated by MPK6. The phosphorylation of MYB41 by MPK6 enhanced its DNA binding to the promoter of a LTP gene. Interestingly, transgenic plants over-expressing MYB41(WT) showed enhanced salt tolerance, whereas transgenic plants over-expressing MYB41(S251A) showed decreased salt tolerance during seed germination and initial root growth. These results indicate that the phosphorylation of MYB41 by MPK6 is required for the biological function of MYB41 in salt tolerance.

The bHLH Rac Immunity1 (RAI1) Is Activated by OsRac1 via OsMAPK3 and OsMAPK6 in Rice Immunity

The Rac/Rop GTPase OsRac1 plays an essential role in rice immunity. However, the regulatory genes acting downstream of OsRac1 are largely unknown. We focused on the RAI1 gene, which is up-regulated in suspension cells expressing a constitutively active form of OsRac1. RAI1 encodes a putative basic helix-loop-helix transcription factor. A microarray analysis of cells transformed with an inducible RAI1 construct showed increased expression of PAL1 and OsWRKY19 genes after induction, suggesting that these genes are regulated by RAI1. This was confirmed using RAI1 T-DNA activation-tagged and RNA interference lines. The PAL1 and OsWRKY19 genes were also up-regulated by sphingolipid and chitin elicitors, and the RAI1 activation-tagged plants had increased resistance to a rice blast fungus. These results indicated that RAI1 is involved in defense responses in rice. RAI1 interacted with OsMAPK3 and OsMAPK6 proteins in vivo and in vitro. Also, RAI1 was phosphorylated by OsMAPK3/6 and OsMKK4-dd in vitro. Overexpression of OsMAPK6 and/or OsMAPK3 together with OsMKK4-dd increased PAL1 and OsWRKY19 expression in rice protoplasts. Therefore, the regulation of PAL1 and OsWRKY19 expression by RAI1 could be controlled via an OsMKK4-OsMAPK3/6 cascade. Co-immunoprecipitation assays indicated that OsMAPK3 and OsRac1 occur in the same complex as OsMAPK6. Taken together, our results indicate that RAI1 could be regulated by OsRac1 through an OsMAPK3/6 cascade. In this study, we have identified RAI1 as the first transcription factor acting downstream of OsRac1. This work will help us to understand the immune system regulated by OsRac1 in rice and its orthologs in other plant species.

cDNA-AFLP analysis of salt-inducible genes expression in Chrysanthemum lavandulifolium under salt treatment

Chrysanthemum lavandulifolium (Fisch. ex Trautv.) Makino is a halophyte species that belongs to the Asteraceae family, and the genus Chrysanthemum. It is one of the ancestors of C.×morifolium Ramatella. Understanding the tolerance mechanism associated with salt stress in C. lavandulifolium could provide important information for explaining the salt tolerance of higher plants and could also help enhancing breeding programs of cultivated Chrysanthemum. In this study, cDNA amplified fragment length polymorphism (cDNA-AFLP) was used to detect differential gene expression in leaves of C. lavandulifolium in response to NaCl treatment. The determination of membrane permeablility, peroxidase activity (POD), malon-dialdehyde (MDA), as well as proline and leaf chlorophyll contents under different NaCl concentrations showed that a 200 mM NaCl treatment was an optimal condition for the cDNA-AFLP experiment. Using this concentration during different times (0, 3 h, 12 h, 24 h and 48 h), we obtained 1930 cDNA fragments using 64 primers. After sequencing 234 randomly chosen cDNA clones and BLASTx analyzing, we got 129 expressed sequence tags (ESTs) which had no significant homology with other sequences, 85 ESTs were homologous to genes with known functions, whereas the rest of ESTs showed homology to unclassified or putative proteins. 25 ESTs that were similar to known functional genes involved in several abiotic and biotic stresses were confirmed by semi-quantitative RT-PCR and qRT-PCR. The expression patterns of these salt-responsive genes not only responded to salt stress but also to plant hormones, such as abscisic acid (ABA), and to other abiotic stresses such as drought and cold. These results indicate an extensive cross-talk among several stresses. Our results provide interesting information for further understanding the molecular mechanisms of salt tolerance in C. lavandulifolium.

Roles of mitogen-activated protein kinase cascades in ABA signaling

Abscisic acid (ABA) is a universal hormone in higher plants and plays a major role in various aspects of plant stress, growth, and development. Mitogen-activated protein kinase (MAPK) cascades are key signaling modules for responding to various extracellular stimuli in plants. The available data suggest that MAPK cascades are involved in some ABA responses, including antioxidant defense, guard cell signaling, and seed germination. Some MAPK phosphatases have also been demonstrated to be implicated in ABA responses. The goal of this review is to piece together the findings concerning MAPK cascades in ABA signaling. Questions and further perspectives of the roles played by MAPK cascades in ABA signaling are also furnished.

Stress-responsive mitogen-activated protein kinases interact with the EAR motif of a poplar zinc finger protein and mediate its degradation through the 26S proteasome

Mitogen-activated protein kinases (MAPKs) contribute to the establishment of plant disease resistance by regulating downstream signaling components, including transcription factors. In this study, we identified MAPK-interacting proteins, and among the newly discovered candidates was a Cys-2/His-2-type zinc finger protein named PtiZFP1. This putative transcription factor belongs to a family of transcriptional repressors that rely on an ERF-associated amphiphilic repression (EAR) motif for their repression activity. Amino acids located within this repression motif were also found to be essential for MAPK binding. Close examination of the primary protein sequence revealed a functional bipartite MAPK docking site that partially overlaps with the EAR motif. Transient expression assays in Arabidopsis (Arabidopsis thaliana) protoplasts suggest that MAPKs promote PtiZFP1 degradation through the 26S proteasome. Since features of the MAPK docking site are conserved among other EAR repressors, our study suggests a novel mode of defense mechanism regulation involving stress-responsive MAPKs and EAR repressors.

ZmMKK4 regulates osmotic stress through reactive oxygen species scavenging in transgenic tobacco

Mitogen-activated protein kinase kinase (MAPKKs) are important components of MAPK cascades, which are universal signal transduction modules and play important role in regulating both plant development and biotic or abiotic stress responses. In this study, we identified the group C MAPKK gene, ZmMKK4, in maize (Zea mays L.). Overexpression of ZmMKK4 in tobacco enhanced tolerance to osmotic stress by increased proline content and antioxidant enzyme (POD) activities compared with wild-type plants. RT-PCR revealed that one peroxidase (POX) gene, NtPOX1, was higher in ZmMKK4-overexpressing plants than in the wild-type plants. In addition, the accumulation of reactive oxygen species (ROS) in ZmMKK4-overexpressing plants is much less than that of wild-type plants. These results suggest that ZmMKK4 may be involved in ROS signaling. Taken together, these results indicate that ZmMKK4 is a positive regulator of osmotic stress by regulating scavenging of ROS in plants.

Cloning and molecular characterization of a mitogen-activated protein kinase gene from Poncirus trifoliata whose ectopic expression confers dehydration/drought tolerance in transgenic tobacco

The mitogen-activated protein kinase (MAPK) cascade plays pivotal roles in diverse signalling pathways related to plant development and stress responses. In this study, the cloning and functional characterization of a group-I MAPK gene, PtrMAPK, in Poncirus trifoliata (L.) Raf are reported. PtrMAPK contains 11 highly conserved kinase domains and a phosphorylation motif (TEY), and is localized in the nucleus of transformed onion epidermal cells. The PtrMAPK transcript level was increased by dehydration and cold, but was unaffected by salt. Transgenic overexpression of PtrMAPK in tobacco confers dehydration and drought tolerance. The transgenic plants exhibited better water status, less reactive oxygen species (ROS) generation, and higher levels of antioxidant enzyme activity and metabolites than the wild type. Interestingly, the stress tolerance capacity of the transgenic plants was compromised by inhibitors of antioxidant enzymes. In addition, overexpression of PtrMAPK enhanced the expression of ROS-related and stress-responsive genes under normal or drought conditions. Taken together, these data demonstrate that PtrMAPK acts as a positive regulator in dehydration/drought stress responses by either regulating ROS homeostasis through activation of the cellular antioxidant systems or modulating transcriptional levels of a variety of stress-associated genes.

Protein kinase signaling networks in plant innate immunity

In plants and animals, innate immunity is triggered through pattern recognition receptors (PRRs) in response to microbe-associated molecular patterns (MAMPs) to provide the first line of inducible defense. Plant receptor protein kinases (RPKs) represent the main plasma membrane PRRs perceiving diverse MAMPs. RPKs also recognize secondary danger-inducible plant peptides and cell-wall signals. Both types of RPKs trigger rapid and convergent downstream signaling networks controlled by calcium-activated PKs and mitogen-activated PK (MAPK) cascades. These PK signaling networks serve specific and overlapping roles in controlling the activities and synthesis of a plethora of transcription factors (TFs), enzymes, hormones, peptides and antimicrobial chemicals, contributing to resistance against bacteria, oomycetes and fungi.

Erk1 positively regulates osteoclast differentiation and bone resorptive activity

The extracellular signal-regulated kinases (ERK1 and 2) are widely-expressed and they modulate proliferation, survival, differentiation, and protein synthesis in multiple cell lineages. Altered ERK1/2 signaling is found in several genetic diseases with skeletal phenotypes, including Noonan syndrome, Neurofibromatosis type 1, and Cardio-facio-cutaneous syndrome, suggesting that MEK-ERK signals regulate human skeletal development. Here, we examine the consequence of Erk1 and Erk2 disruption in multiple functions of osteoclasts, specialized macrophage/monocyte lineage-derived cells that resorb bone. We demonstrate that Erk1 positively regulates osteoclast development and bone resorptive activity, as genetic disruption of Erk1 reduced osteoclast progenitor cell numbers, compromised pit formation, and diminished M-CSF-mediated adhesion and migration. Moreover, WT mice reconstituted long-term with Erk1^−/− bone marrow mononuclear cells (BMMNCs) demonstrated increased bone mineral density as compared to recipients transplanted with WT and Erk2^−/− BMMNCs, implicating marrow autonomous, Erk1-dependent osteoclast function. These data demonstrate Erk1 plays an important role in osteoclast functions while providing rationale for the development of Erk1-specific inhibitors for experimental investigation and/or therapeutic modulation of aberrant osteoclast function.

A cotton group C MAP kinase gene, GhMPK2, positively regulates salt and drought tolerance in tobacco

Mitogen-activated protein kinase (MAPK) cascades play important roles in mediating biotic and abiotic stress responses. In plants, MAPKs are classified into four major groups (A-D) according to their sequence homology and conserved phosphorylation motifs. Compared with well-studied MAPKs in groups A and B, little is known about group C. In this study, we functionally characterised a stress-responsive group C MAPK gene (GhMPK2) from cotton (Gossypium hirsutum). Northern blot analysis indicated that GhMPK2 was induced by abscisic acid (ABA) and abiotic stresses, such as NaCl, PEG, and dehydration. Subcellular localization analysis suggested that GhMPK2 may activate its specific targets in the nucleus. Constitutive overexpression of GhMPK2 in tobacco (Nicotiana tabacum) conferred reduced sensitivity to ABA during both seed germination and vegetative growth. Interestingly, transgenic plants had a decreased rate of water loss and exhibited enhanced drought and salt tolerance. Additionally, transgenic plants showed improved osmotic adjustment capacity, elevated proline accumulation and up-regulated expression of several stress-related genes, including DIN1, Osmotin and NtLEA5. β-glucuronidase (GUS) expression driven by the GhMPK2 promoter was clearly enhanced by treatment with NaCl, PEG, and ABA. These results strongly suggest that GhMPK2 positively regulates salt and drought tolerance in transgenic plants.

ZmMKK4, a novel group C mitogen-activated protein kinase kinase in maize (Zea mays), confers salt and cold tolerance in transgenic Arabidopsis

Mitogen-activated protein kinase (MAPK) cascades are signalling modules that transduce extracellular signalling to a range of cellular responses. Plant MAPK cascades have been implicated in development and stress response. In this study, we isolated a novel group C MAPKK gene, ZmMKK4, from maize. Northern blotting analysis revealed that the ZmMKK4 transcript expression was up-regulated by cold, high salt and exogenous H(2)O(2,) but down-regulated by exogenous abscisic acid (ABA). Over-expression of ZmMKK4 in Arabidopsis conferred tolerance to cold and salt stresses by increased germination rate, lateral root numbers, plant survival rate, chlorophyll, proline and soluble sugar contents, and antioxidant enzyme [peroxidase (POD), catalase (CAT)] activities compared with control plants. Furthermore, ZmMKK4 enhanced a 37 kDa kinase activity after cold and salt stresses. RT-PCR analysis revealed that the transcript levels of stress-responsive transcription factors and functional genes were higher in ZmMKK4-over-expressing plants than in control plants. In addition, ZmMKK4 protein is localized in the nucleus. Taken together, these results indicate that ZmMKK4 is a positive regulator of salt and cold tolerance in plants.

Capsicum annuum WRKYb transcription factor that binds to the CaPR-10 promoter functions as a positive regulator in innate immunity upon TMV infection

In plant, some WRKY transcription factors are known to play an important role in the transcriptional reprogramming associated with the immune response. By using WRKY-domain-specific differential display procedure, we isolated CaWRKYb gene, which is rapidly induced during an incompatible interaction between hot pepper and Tobacco mosaic virus (TMV) pathotype P(0) infection. The recombinant CaWRKYb bound to the W box-containing CaPR-10 promoter probes efficiently and the specificity of binding was confirmed by mutant study and competition with cold oligonucleotides. Also, in GUS reporter activity assay using Arabidopsis protoplasts with the CaPR-10 promoter, GUS activity was increased in the presence of CaWRKYb. And CaWRKYb-knockdown plant showed reduced number of hypersensitive response local lesions upon TMV-P(0) infection. Furthermore, CaWRKYb-knockdown plant exhibited compromised resistance to TMV-P(0) by accumulating more TMV, apparently through decreased expression of CaPR-10, CaPR-1, and CaPR-5. These results suggest that CaWRKYb is involved as a positive transcription factor in defense-related signal transduction pathways in hot pepper.

Phosphorylation-dependent differential regulation of plant growth, cell death, and innate immunity by the regulatory receptor-like kinase BAK1

Plants rely heavily on receptor-like kinases (RLKs) for perception and integration of external and internal stimuli. The Arabidopsis regulatory leucine-rich repeat RLK (LRR-RLK) BAK1 is involved in steroid hormone responses, innate immunity, and cell death control. Here, we describe the differential regulation of three different BAK1-dependent signaling pathways by a novel allele of BAK1, bak1-5. Innate immune signaling mediated by the BAK1-dependent RKs FLS2 and EFR is severely compromised in bak1-5 mutant plants. However, bak1-5 mutants are not impaired in BR signaling or cell death control. We also show that, in contrast to the RD kinase BRI1, the non-RD kinases FLS2 and EFR have very low kinase activity, and we show that neither was able to trans-phosphorylate BAK1 in vitro. Furthermore, kinase activity for all partners is completely dispensable for the ligand-induced heteromerization of FLS2 or EFR with BAK1 in planta, revealing another pathway specific mechanistic difference. The specific suppression of FLS2- and EFR-dependent signaling in bak1-5 is not due to a differential interaction of BAK1-5 with the respective ligand-binding RK but requires BAK1-5 kinase activity. Overall our results demonstrate a phosphorylation-dependent differential control of plant growth, innate immunity, and cell death by the regulatory RLK BAK1, which may reveal key differences in the molecular mechanisms underlying the regulation of ligand-binding RD and non-RD RKs.

Plants need to adapt to their ever-changing environment for survival. Transmembrane receptor kinases are essential to translate extracellular stimuli into intracellular responses. A key question is how plants maintain signaling specificity in response to multiple stresses and endogenous hormones. Growth responses induced by steroid hormones and innate immunity triggered by recognition of conserved microbial molecules depend on the common regulatory receptor-like kinase BAK1, which is also involved in cell death control. It is still unclear if BAK1 provides signaling specificity or if it is a mere signaling enhancer. Here, we describe the novel protein variant BAK1-5 that specifically blocks innate immune responses without affecting steroid responses or cell death. This unambiguously demonstrates that the role of BAK1 in plant signaling can be mechanistically separated. Importantly, the impairment of immune signaling is not caused by a loss of interaction of BAK1-5 with immune receptors but is due to an altered kinase activity. Thus, BAK1-dependent signaling pathways are under a differential phosphorylation-dependent regulation. The examination of this novel mutant version of BAK1 will enable detailed studies into the mechanistic role of BAK1 in plant innate immunity, but also more generally will provide invaluable insights into transmembrane receptor signaling specificity in plants.

Cotton GhMPK2 is involved in multiple signaling pathways and mediates defense responses to pathogen infection and oxidative stress

Mitogen-activated protein kinase (MAPK) cascades play important roles in mediating pathogen responses and reactive oxygen species signaling. In plants, MAPKs are classified into four major groups (A-D). Previous studies have mainly focused on groups A and B, but little is known about group C. In this study, we functionally characterized a stress-responsive group C MAPK gene (GhMPK2) from cotton. Northern blot analysis indicated that GhMPK2 was induced not only by signaling molecules, such as ethylene and methyl jasmonate, but also by methyl viologen-mediated oxidative stress. Transgenic tobacco (Nicotiana tabacum) plants that overexpress GhMPK2 displayed enhanced resistance to fungal and viral pathogens, and the expression of the pathogenesis-related (PR) genes, including PR1, PR2, PR4, and PR5, was significantly increased. Interestingly, the transcription of 1-aminocyclopropane-1-carboxylic acid synthase (ACS) and 1-aminocyclopropane-1-carboxylic acid oxidase (ACO) was significantly upregulated in transgenic plants, suggesting that GhMPK2 positively regulates ethylene synthesis. Moreover, overexpression of GhMPK2 elevated the expression of several antioxidant enzymes, conferring on transgenic plants enhanced reactive oxygen species scavenging capability and oxidative stress tolerance. These results increased our understanding of the role of the group C GhMPK2 gene in multiple defense-signaling pathways, including those that are involved in responses to pathogen infection and oxidative stress.

The age of protein kinases

Major progress has been made in unravelling of regulatory mechanisms in eukaryotic cells. Modification of target protein properties by reversible phosphorylation events has been found to be one of the most prominent cellular control processes in all organisms. The phospho-status of a protein is dynamically controlled by protein kinases and counteracting phosphatases. Therefore, monitoring of kinase and phosphatase activities, identification of specific phosphorylation sites, and assessment of their functional significance are of crucial importance to understand development and homeostasis. Recent advances in the area of molecular biology and biochemistry, for instance, mass spectrometry-based phosphoproteomics or fluorescence spectroscopical methods, open new possibilities to reach an unprecidented depth and a proteome-wide understanding of phosphorylation processes in plants and other species. In addition, the growing number of model species allows now deepening evolutionary insights into signal transduction cascades and the use of kinase/phosphatase systems. Thus, this is the age where we move from an understanding of the structure and function of individual protein modules to insights how these proteins are organized into pathways and networks. In this introductory chapter, we briefly review general definitions, methodology, and current concepts of the molecular mechanisms of protein kinase function as a foundation for this methods book. We briefly review biochemistry and structural biology of kinases and provide selected examples for the role of kinases in biological systems.

Expression analysis of five maize MAP kinase genes in response to various abiotic stresses and signal molecules

Mitogen-activated protein kinase (MAPK) cascades are universal signal transduction modules in eukaryotes. Plant MAPK cascades are complicated networks and play vital roles in signal transduction induced by biotic and abiotic stresses. In this paper, expression patterns of MAPKs in maize roots treated with low-temperature, osmotic stresses, wounding, plant hormones and UV-C irradiation were investigated. Semi-quantitative RT-PCR reveals that the expression of MAPKs in maize roots which treated with low-temperature in light or in low light are inducible. The expression patterns of MAPKs in maize roots with treatments of CaCl2, SA, GA and wounding are approximately the same. A detailed time course experiment shows that the expression patterns of ZmSIMK are different with treatments of PEG and NaCl, respectively. These results suggest that the expression patterns of MAPKs are complicated and the signal pathways are interlaced into a network in maize roots.

Hubs and bottlenecks in plant molecular signalling networks

Conditional control of plant cell function and development relies on appropriate signal perception, signal integration and processing. The development of high throughput technologies such as proteomics and interactomics has enabled the identification of protein interaction networks that mediate signal processing from inputs to appropriate outputs. Such networks can be depicted in graphical representations using nodes and edges allowing for the immediate visualization and analysis of the network's topology. Hubs are network elements characterized by many edges (often degree grade k ≥ 5) which confer a degree of topological importance to them. The review introduces the concept of networks, hubs and bottlenecks and describes four examples from plant science in more detail, namely hubs in the redox regulatory network of the chloroplast with ferredoxin, thioredoxin and peroxiredoxin, in mitogen activated protein (MAP) kinase signal processing, in photomorphogenesis with the COP9 signalosome, COP1 and CDD, and monomeric GTPase function. Some guidance is provided to appropriate internet resources, web repositories, databases and their use. Plant networks can be generated from existing public databases and this type of analysis is valuable in support of existing hypotheses, or to allow for the generation of new concepts or ideas. However, intensive manual curating of in silico networks is still always necessary.

Stomatal development and movement: the roles of MAPK signaling

Stomata are epidermal pores on plant surface used for gas exchange with the atmosphere. Stomatal development and movement are regulated by environmental and internal signals. Mitogen-activated protein kinase (MAPK) cascades are universal transducers of extracellular signals among all eukaryotes. In plant, MAPK cascades regulate diverse cellular processes occurring during the whole ontogenetic plant life and ranging from normal cell proliferation to stress-inducing plant-to-environment interactions. Recent reports reveal that MAPK signaling is involved in both stomatal development and movement. This mini-review summarizes the roles of MAPK signaling in stomatal development and movement. How MAPK specificity is maintained in stomatal development and movement is also discussed.

Chitooligosaccharide sensing and downstream signaling: contrasted outcomes in pathogenic and beneficial plant-microbe interactions

In plants, short chitin oligosaccharides and chitosan fragments (collectively referred to as chitooligosaccharides) are well-known elicitors that trigger defense gene expression, synthesis of antimicrobial compounds, and cell wall strengthening. Recent findings have shed new light on chitin-sensing mechanisms and downstream activation of intracellular signaling networks that mediate plant defense responses. Interestingly, chitin receptors possess several lysin motif domains that are also found in several legume Nod factor receptors. Nod factors are chitin-related molecules produced by nitrogen-fixing rhizobia to induce root nodulation. The fact that chitin and Nod factor receptors share structural similarity suggests an evolutionary conserved relationship between mechanisms enabling recognition of both deleterious and beneficial microorganisms. Here, we will present an update on molecular events involved in chitooligosaccharide sensing and downstream signaling pathways in plants and will discuss how structurally related signals may lead to such contrasted outcomes during plant-microbe interactions.

Convergence and specificity in the Arabidopsis MAPK nexus

Although mitogen-activated protein kinase (MAPK) signal transduction cascades are known regulators of various aspects of plant biology, our knowledge of these systems has been largely restricted to a small subset of the MAPKs. However, global analyses are now revealing that many more of these kinases are probably engaged in modulating developmental and fitness adaptation processes in the plant kingdom. In this review, we show how these new findings are beginning to define the overall architecture of plant MAPK signaling, with a particular focus on the interplay between the terminal MPKs and their activators, inactivators and cellular targets.

Mitogen-activated protein kinase signaling in plants

Eukaryotic mitogen-activated protein kinase (MAPK) cascades have evolved to transduce environmental and developmental signals into adaptive and programmed responses. MAPK cascades relay and amplify signals via three types of reversibly phosphorylated kinases leading to the phosphorylation of substrate proteins, whose altered activities mediate a wide array of responses, including changes in gene expression. Cascades may share kinase components, but their signaling specificity is maintained by spaciotemporal constraints and dynamic protein-protein interactions and by mechanisms that include crossinhibition, feedback control, and scaffolding. Plant MAPK cascades regulate numerous processes, including stress and hormonal responses, innate immunity, and developmental programs. Genetic analyses have uncovered several predominant MAPK components shared by several of these processes including the Arabidopsis thaliana MAPKs MPK3, 4, and 6 and MAP2Ks MKK1, 2, 4, and 5. Future work needs to focus on identifying substrates of MAPKs, and on understanding how specificity is achieved among MAPK signaling pathways.