Activation of a mitogen-activated protein kinase pathway is involved in disease resistance in tobacco

Double jeopardy: both overexpression and suppression of a redox-activated plant mitogen-activated protein kinase render tobacco plants ozone sensitive

In plants, the role of mitogen-activated protein kinase (MAPK) in reactive oxygen species (ROS)-based signal transduction processes is elusive. Despite the fact that ROS can induce MAPK activation, no direct genetic evidence has linked ROS-induced MAPK activation with the hypersensitive response, a form of programmed cell death. In tobacco, the major ROS-induced MAPK is salicylate-induced protein kinase (SIPK). We found through gain-of-function and loss-of-function approaches that both overexpression and RNA interference-based suppression of SIPK render the plant sensitive to ROS stress. Transgenic lines overexpressing a nonphosphorylatable version of SIPK were not ROS sensitive. Analysis of the MAPK activation profiles in ROS-stressed transgenic and wild-type plants revealed a striking interplay between SIPK and another MAPK (wound-induced protein kinase [WIPK]) in the different kinotypes. During continuous ozone exposure, abnormally prolonged activation of SIPK was seen in the SIPK-overexpression genotype, without WIPK activation, whereas strong and stable activation of WIPK was observed in the SIPK-suppressed lines. Thus, one role of activated SIPK in tobacco cells upon ROS stimulation appears to be control of the inactivation of WIPK.

NPK1, an MEKK1-like mitogen-activated protein kinase kinase kinase, regulates innate immunity and development in plants

Mitogen-activated protein kinase (MAPK) cascades are rapidly activated upon plant recognition of invading pathogens. Here, we describe the use of virus-induced gene silencing (VIGS) to study the role of candidate plant MAP kinase kinase kinase (MAPKKK) homologs of human MEKK1 in pathogen-resistance pathways. We demonstrate that silencing expression of a tobacco MAPKKK, Nicotiana Protein Kinase 1 (NPK1), interferes with the function of the disease-resistance genes N, Bs2, and Rx, but does not affect Pto- and Cf4-mediated resistance. Further, NPK1-silenced plants also exhibit reduced cell size, defective cytokinesis, and an overall dwarf phenotype. Our results provide evidence that NPK1 functions in the regulation of N-, Bs2-, and Rx-mediated resistance responses and may play a role in one or more MAPK cascades, regulating multiple cellular processes.

Innate immunity in plants and animals: emerging parallels between the recognition of general elicitors and pathogen-associated molecular patterns

Recent findings have highlighted remarkable similarities in the innate pathogen defense systems of plants, animals and insects. Pathogen-associated molecular patterns (PAMP) that are similar to those activating innate immune responses in animals have been shown to mediate the activation of plant defense. Moreover, recognition complexes that are structurally related to animal PAMP receptors are now being discovered in plants, suggesting a common evolutionary origin of pathogen defense systems in higher eukaryotes.

Mitogen-activated protein kinase cascades in plants: a new nomenclature

Mitogen-activated protein kinase (MAPK) cascades are universal signal transduction modules in eukaryotes, including yeasts, animals and plants. These protein phosphorylation cascades link extracellular stimuli to a wide range of cellular responses. In plants, MAPK cascades are involved in responses to various biotic and abiotic stresses, hormones, cell division and developmental processes. Completion of the Arabidopsis genome-sequencing project has revealed the existence of 20 MAPKs, 10 MAPK kinases and 60 MAPK kinase kinases. Here, we propose a simplified nomenclature for Arabidopsis MAPKs and MAPK kinases that might also serve as a basis for standard annotation of these gene families in all plants.

Control of plant cytokinesis by an NPK1-mediated mitogen-activated protein kinase cascade

Cytokinesis is the last essential step in the distribution of genetic information to daughter cells and partition of the cytoplasm. In plant cells, various proteins have been found in the phragmoplast, which corresponds to the cytokinetic apparatus, and in the cell plate, which corresponds to a new cross wall, but our understanding of the functions of these proteins in cytokinesis remains incomplete. Reverse genetic analysis of NPK1 MAPKKK (nucleus- and phragmoplast-localized protein kinase 1 mitogen-activated protein kinase kinase kinase) and investigations of factors that might be functionally related to NPK1 have helped to clarify new aspects of the mechanisms of cytokinesis in plant cells. In this review, we summarize the evidence for the involvement of NPK1 in cytokinesis. We also describe the characteristics of a kinesin-like protein and the homologue of a mitogen-activated protein kinase that we identified recently, and we discuss possible relationships among these proteins in cytokinesis.

The tobacco mosaic virus resistance gene, N

Summary In this mini review we discuss recent advances in the understanding of the N gene-mediated resistance to tobacco mosaic virus (TMV). The tobacco N gene belongs to toll-interleukin-1 receptor homology/nucleotide binding/leucine rich repeat (TIR-NB-LRR) class of resistance genes. It encodes two transcripts, N(S) and N(L), by alternative splicing, both of which are required to confer resistance to TMV. The structure-function analysis of the N gene indicates that the TIR, NB and LRR domains are indispensable for its function. The N gene response is elicited by the C-terminal helicase domain of the 126 kDa TMV replicase protein. Tobacco N gene can also confer resistance to TMV in heterologous plants like tomato and Nicotiana benthamiana. Recent studies on N-mediated signalling suggest that EDS1, Rar1 and NPR1 genes play an important role in TMV resistance. Finally, we discuss current status of the N-mediated signal transduction and speculate directions for future work to understand N-TMV interaction.

Phosphorylation of cucumber mosaic virus RNA polymerase 2a protein inhibits formation of replicase complex

The 2a (polymerase) protein of cucumber mosaic virus (CMV) was shown to be phosphorylated both in vivo and in vitro. In vitro assays using 2a protein mutants and tobacco protein kinases showed that the 2a protein has at least three phosphorylation sites, one of which is located within the N-terminal 126 amino acid region. This region is essential and sufficient for interaction with the CMV 1a protein. When phosphorylated in vitro, the 2a protein N-terminal region failed to interact with the 1a protein. Since the 1a-2a interaction is essential for the replication of CMV, this suggests that phosphorylation of the N-terminal region of the 2a protein negatively modulates the interaction in vivo, and may have a regulatory role acting directly in viral infection.

Towards genomic and proteomic studies of protein phosphorylation in plant-pathogen interactions

Phosphorylation is an effective method of post-translational protein modification but understanding its significance is hindered by its biological complexity. Many protein kinases and phosphatases have been identified that connect signal perception mechanisms to plant defence responses. Recent studies of mitogen-activated protein kinases, calcium-dependent protein kinases and other kinases and phosphatases have revealed some important mechanisms, but have also raised new questions. The regulation of any phosphorylation pathway is complex and dynamic. There are many protein kinases and phosphatases in the plant genome, which makes it hard to delineate the phosphorylation machinery fully. Genomics and proteomics have already identified new components and will continue to influence the study of phosphorylation profoundly in plant-pathogen interactions.

The need for a standard nomenclature for gene classification (a Nucleotide Function Code) and an automated data-based tool to assist in understanding the molecular associations in cell signalling in plant-pathogen interactions

summary Despite the adoption of Arabidopsis thaliana as a model plant system and the plethora of molecular information being obtained from its use, it is disappointing that the scientific community has not devised a cell signalling model integrating and visualizing these data. Lack of common systems of nomenclature and the sheer size and complexity of the task inhibit any individual from bringing together the knowledge into a unified structure. There are clearly many aspects of cell biology that are similar, even between plants and animals, that could facilitate development of a generic model. A gene-coding or nucleotide classification system which is 'user-friendly' would be beneficial to building such a model and enable rapid identification of orthologues of genes from different organisms. Whilst some international projects seek to address the problem of assigning unique numbers to genes, none suggest a nucleotide classification system that provides biological information that is transparent within the code. This paper discusses these issues and identifies the need for a more formal, semi-automated approach to modelling signal transduction utilizing the strengths of the proposed classification approach. By way of illustration, an example of a possible nucleotide function code is suggested, to demonstrate more clearly the benefits of such a system. Further discussion of this topic will be encouraged on websites (<www.bspp.org.uk> and <www.drastic.org.uk>).

Activation of AtMEK1, an Arabidopsis mitogen-activated protein kinase kinase, in vitro and in vivo: analysis of active mutants expressed in E. coli and generation of the active form in stress response in seedlings

The mitogen-activated protein kinase (MAPK) cascade, consisting of MAPK, MAPK kinase (MAPKK) and MAPK kinase kinase (MAPKKK), is the signaling system that relays various external signals, including mitogens and stresses in eukaryotes. MAPKK is activated by phosphorylation in the consensus motif, SXXXS/T, in animals, but the regulation mechanism for the plant MAPKK by phosphorylation, having the putative phosphorylation motif of S/TXXXXXS/T, is not yet fully clarified. Here we constructed a series of mutants of AtMEK1, an Arabidopsis MAPKK, having the sequence T218-X-S220-X-X-X-S224 that fits both of the plant- and animal-type motifs. We show that the two double-mutant proteins replacing Thr-218/Ser-224 and Ser-220/Ser-224 by Glu expressed in Escherichia coli show a constitutive activity to phosphorylate the Thr and Tyr residues of the kinase-negative mutant of an Arabidopsis MAPK, named ATMPK4, in vitro. The mutation analysis of AtMEK1 replacing Thr-218 and Ser-220 to Ala suggested that Thr-218 is autophosphorylated by the enzyme. The wild-type ATMPK4 was also phosphorylated by the active mutants of AtMEK1 and showed a high protein kinase activity toward myelin basic proteins. In contrast, ATMPK3, another Arabidopsis MAPK, was a poor substrate of this plant MAPKK, indicating that AtMEK1 has a substrate specificity preferring ATMPK4 to ATMPK3, at least in vitro. Furthermore, AtMEK1 immunoprecipitated from Arabidopsis seedlings stimulated with wounding, cold, drought, and high salt showed an elevated protein kinase activity toward the kinase-negative ATMPK4, while the amounts of the AtMEK1 protein did not change significantly. These data indicate that the AtMEK1 becomes an active form through phosphorylation and activates its downstream target ATMPK4 in stress response in Arabidopsis.

MAP kinase signalling cascade in Arabidopsis innate immunity

There is remarkable conservation in the recognition of pathogen-associated molecular patterns (PAMPs) by innate immune responses of plants, insects and mammals. We developed an Arabidopsis thaliana leaf cell system based on the induction of early-defence gene transcription by flagellin, a highly conserved component of bacterial flagella that functions as a PAMP in plants and mammals. Here we identify a complete plant MAP kinase cascade (MEKK1, MKK4/MKK5 and MPK3/MPK6) and WRKY22/WRKY29 transcription factors that function downstream of the flagellin receptor FLS2, a leucine-rich-repeat (LRR) receptor kinase. Activation of this MAPK cascade confers resistance to both bacterial and fungal pathogens, suggesting that signalling events initiated by diverse pathogens converge into a conserved MAPK cascade.

Cell death mediated by MAPK is associated with hydrogen peroxide production in Arabidopsis

Rapid and localized programmed cell death, known as the hypersensitive response (HR) is frequently associated with plant disease resistance. In contrast to our knowledge about the regulation and execution of apoptosis in animal system, information about plant HR is limited. Recent studies implicated the mitogen-activated protein kinase (MAPK) cascade in regulating plant HR cell death as well as several other defense responses during incompatible interactions between plants and pathogens. Here, we report the generation of transgenic Arabidopsis plants that express the active mutants of AtMEK4 and AtMEK5, two closely related MAPK kinases under the control of a steroid-inducible promoter. Induction of the transgene expression by the application of dexamethasone, a steroid, leads to HR-like cell death, which is preceded by the activation of endogenous MAPKs and the generation of hydrogen peroxide. Both prolonged MAPK activation and reactive oxygen species generation have been implicated in the regulation of HR cell death induced by incompatible pathogens. As a result, we speculate that the prolonged activation of the MAPK pathway in cells could disrupt the redox balance, which leads to the generation of reactive oxygen species and eventually HR cell death.

MAPK cascades in plant defense signaling

The Arabidopsis genome encodes approximately 20 different mitogen-activated protein kinases (MAPKs) that are likely to be involved in growth, development and responses to endogenous and environmental cues. Several plant MAPKs are activated by a variety of stress stimuli, including pathogen infection, wounding, temperature, drought, salinity, osmolarity, UV irradiation, ozone and reactive oxygen species. Recent gain-of-function studies show that two tobacco MAPKs induce the expression of defense genes and cause cell death. By contrast, loss-of-function studies of other MAPK pathways revealed negative regulation of disease resistance. This 'push-and-pull' regulation by different MAPK pathways might provide a more precise control of plant defense responses.

Ectopic expression of an Arabidopsis calmodulin-like domain protein kinase-enhanced NADPH oxidase activity and oxidative burst in tomato protoplasts

Among plant defense responses to pathogen attack, the release of active oxygen species (AOS), termed the oxidative burst, may affect the attacking pathogen and the host plant cells at the infection site, thereby limiting the spread of the pathogen. Plasma membrane-associated NADPH oxidase represents a key enzyme in mediating the oxidative burst. The mechanisms of NADPH oxidase activation, however, remains unclear. Ectopic expression of AK1-6H, an Arabidopsis calmodulin-like domain protein kinase (CDPK) in tomato protoplasts enhanced plasma membrane-associated NADPH oxidase activity. Arabidopsis protein phosphatase 2A abolished this enhancement, whereas Arabidopsis dual-specificity protein tyrosine phosphatase 1 or maize protein phosphatase 1 had no effect tMEK2MUT, a constitutively activated, mitogen-activated protein kinase kinase from tomato, did not enhance NADPH oxidase activity when overexpressed. In a cell-free system, AK1-6H moderately stimulated the NADPH oxidase activity on plasma membrane. AK1-6H, but not tMEK2MUT, also enhanced production of AOS in intact protoplasts. Our results show that ectopic expression of a heterologous CDPK can enhance NADPH oxidase activity and stimulate an oxidative burst in tomato protoplasts.

Plant mitogen-activated protein kinase signaling cascades

Mitogen-activated protein kinase (MAPK) cascades have emerged as a universal signal transduction mechanism that connects diverse receptors/sensors to cellular and nuclear responses in eukaryotes. Recent studies in plants indicate that MAPK cascades are vital to fundamental physiological functions involved in hormonal responses, cell cycle regulation, abiotic stress signaling, and defense mechanisms. New findings have revealed the complexity and redundancy of the signaling components, the antagonistic nature of distinct pathways, and the use of both positive and negative regulatory mechanisms.

Protein kinases in the plant defence response

Protein kinases play a central role in signalling during pathogen recognition and the subsequent activation of plant defence mechanisms. Members of different kinase subfamilies, such as calcium-dependent protein kinases and MAP kinases, are involved. Nevertheless, often, only a single component of a signalling cascade in an experimental plant system has been characterised. The future challenge is to understand how these kinases work, which cellular responses they mediate, and how they fit into the bigger picture of defence signalling. This challenge has become increasingly feasible with the recent introduction of new techniques: these techniques include reverse genetics, which will allow the allocation of biological function to kinase isoforms, (phospho) proteomics combined with mass spectrometry, and transient expression of kinases in a (constitutively) active form, mimicking the induction of defence responses in a biological system.

Mapping out the roles of MAP kinases in plant defense

Arabidopsis contains 20 MAP kinase genes, but their roles in plant physiology have remained largely unknown because of a lack of mutants. Recent papers from two groups have shed new light on the function of two different MAP kinases. The Arabidopsis MPK4 gene appears to negatively regulate salicylic acid-mediated defense responses and positively regulate jasmonic acid-induced responses. The tobacco SIPK gene (orthologous to Arabidopsis MPK6) appears to positively regulate programmed cell death.

Signal transmission in the plant immune response

Genetic and biochemical dissection of signaling pathways regulating plant pathogen defense has revealed remarkable similarities with the innate immune system of mammals and Drosophila. Numerous plant proteins resembling eukaryotic receptors have been implicated in the perception of pathogen-derived signal molecules. Receptor-mediated changes in levels of free calcium in the cytoplasm and production of reactive oxygen species and nitric oxide constitute early events generally observed in plant-pathogen interactions. Positive and negative regulation of plant pathogen defense responses has been attributed to mitogen-activated protein kinase cascades. In addition, salicylic acid, jasmonic acid and ethylene are components of signaling networks that provide the molecular basis for specificity of plant defense responses. This article reviews recent advances in our understanding of early signaling events involved in the establishment of plant disease resistance.

Harpin induces activation of the Arabidopsis mitogen-activated protein kinases AtMPK4 and AtMPK6

Mitogen-activated protein kinases (MAPKs) are key enzymes that mediate adaptive responses to various abiotic and biotic stresses, including pathogen challenge. The proteinaceous bacterial elicitor harpin (secreted by Pseudomonas syringae pv syringae) activates two MAPKs in suspension cultures of Arabidopsis var. Landsberg erecta. In this study, we show that harpin and exogenous hydrogen peroxide (H(2)O(2)) activate myelin basic protein kinases in Arabidopsis leaves. Using anti-AtMPK4 and anti-AtMPK6 antibodies, we identify the harpin-activated MAPKs in both leaves and suspension cultures as AtMPK4 and AtMPK6, and show that H(2)O(2), generated by Arabidopsis cells in response to challenge with harpin, activates only AtMPK6. However, treatments with catalase, which removes H(2)O(2), or diphenylene iodonium, which inhibits superoxide and H(2)O(2) production, do not inhibit harpin-induced activation of AtMPK4 or AtMPK6. In addition, activation of AtMPK4 but not AtMPK6 is inhibited by the MAPK kinase inhibitor PD98059. Neither harpin nor H(2)O(2) has any effect on AtMPK4 or AtMPK6 gene expression. In addition, the expression of AtMEKK1, AtMEK1, or AtMKK2, previously shown to be potential functional partners of AtMPK4, were not affected by either harpin or H(2)O(2) treatments. These data suggest that harpin activates several signaling pathways, one leading to stimulation of the oxidative burst and others leading to the activation of AtMPK4 or AtMPK6.

Recent advances in plant MAP kinase signalling

Mitogen activated protein kinases (MAPK) are important mediators in signal transmission, connecting the perception of external stimuli to cellular responses. MAPK cascades are involved in signalling various biotic and abiotic stresses, like wounding and pathogen infection, temperature stress or drought, but are also involved in mediating the action of some plant hormones, such as ethylene and auxin. Moreover, MAPKs have been implicated in cell cycle and developmental processes. In Arabidopsis mutant screens and in vivo assays several components of plant MAPK cascades have been identified. This review gives an update of recent advances in plant MAPK signalling and discusses the emerging mechanisms of some selected MAPK pathways.

Genetic dissection of systemic acquired resistance

Significant progress has been made in the past year in understanding the mechanism of systemic acquired resistance. Mitogen-activated protein kinase cascades have been implicated as negative regulators of salicyclic acid accumulation and the induction of resistance. The salicylic acid signal is transduced through NPR1, a nuclear-localized protein that interacts with transcription factors that are involved in regulating salicylic-acid-mediated gene expression. Both promoter analyses and genetic studies have shown that gene expression in systemic acquired resistance requires not only the activation of a transcriptional activator(s) but also inhibition of a transcriptional repressor(s). Microarray experiments have been performed to search for those genes whose expression is transcriptionally regulated during systemic acquired resistance and to identify common promoter elements that control these genes.

Activation of salicylic acid-induced protein kinase, a mitogen-activated protein kinase, induces multiple defense responses in tobacco

The activation of mitogen-activated protein kinases (MAPKs) is one of the earliest responses in plants challenged by avirulent pathogens or cells treated with pathogen-derived elicitors. Expression of a constitutively active MAPK kinase, NtMEK2(DD), in tobacco induces the expression of defense genes and hypersensitive response-like cell death, which are preceded by the activation of two endogenous MAPKs, salicylic acid-induced protein kinase (SIPK) and wounding-induced protein kinase (WIPK). However, the roles that SIPK and WIPK each play in the process are unknown. Here we report that SIPK alone is sufficient to activate these defense responses. In tobacco leaves transiently transformed with SIPK under the control of a steroid-inducible promoter, the induction of SIPK expression after the application of dexamethasone, a steroid, leads to an increase of SIPK activity. The increase of SIPK activity is dependent on the phosphorylation of newly synthesized SIPK by its endogenous upstream kinase. In contrast, the expression of WIPK under the same conditions fails to increase its activity, even though the protein accumulates to a similar level. Studies using chimeras of SIPK and WIPK demonstrated that the C terminus of SIPK contains the molecular determinant for its activation, which is rather surprising because the N termini of SIPK and WIPK are more divergent. SIPK has been implicated previously in the regulation of both plant defense gene activation and hypersensitive response-like cell death based on evidence from pharmacological studies using kinase inhibitors. This gain-of-function study provided more direct evidence for its role in the signaling of multiple defense responses in tobacco.