A tobacco protein kinase, NPK2, has a domain homologous to a domain found in activators of mitogen-activated protein kinases (MAPKKs)

Exome sequencing of bulked segregants identified a novel TaMKK3-A allele linked to the wheat ERA8 ABA-hypersensitive germination phenotype

Using bulked segregant analysis of exome sequence, we fine-mapped the ABA-hypersensitive mutant ERA8 in a wheat backcross population to the TaMKK3-A locus of chromosome 4A. Preharvest sprouting (PHS) is the germination of mature grain on the mother plant when it rains before harvest. The ENHANCED RESPONSE TO ABA8 (ERA8) mutant increases seed dormancy and, consequently, PHS tolerance in soft white wheat 'Zak.' ERA8 was mapped to chromosome 4A in a Zak/'ZakERA8' backcross population using bulked segregant analysis of exome sequenced DNA (BSA-exome-seq). ERA8 was fine-mapped relative to mutagen-induced SNPs to a 4.6 Mb region containing 70 genes. In the backcross population, the ERA8 ABA-hypersensitive phenotype was strongly linked to a missense mutation in TaMKK3-A-G1093A (LOD 16.5), a gene associated with natural PHS tolerance in barley and wheat. The map position of ERA8 was confirmed in an 'Otis'/ZakERA8 but not in a 'Louise'/ZakERA8 mapping population. This is likely because Otis carries the same natural PHS susceptible MKK3-A-A660^S allele as Zak, whereas Louise carries the PHS-tolerant MKK3-A-C660^R allele. Thus, the variation for grain dormancy and PHS tolerance in the Louise/ZakERA8 population likely resulted from segregation of other loci rather than segregation for PHS tolerance at the MKK3 locus. This inadvertent complementation test suggests that the MKK3-A-G1093A mutation causes the ERA8 phenotype. Moreover, MKK3 was a known ABA signaling gene in the 70-gene 4.6 Mb ERA8 interval. None of these 70 genes showed the differential regulation in wild-type Zak versus ERA8 expected of a promoter mutation. Thus, the working model is that the ERA8 phenotype results from the MKK3-A-G1093A mutation.

Cellular Complexity in MAPK Signaling in Plants: Questions and Emerging Tools to Answer Them

Mitogen activated protein kinase (MAPK) cascades play an important role in many aspects of plant growth, development, and environmental response. Because of their central role in many important processes, MAPKs have been extensively studied using biochemical and genetic approaches. This work has allowed for the identification of the MAPK genes and proteins involved in a number of different signaling pathways. Less well developed, however, is our understanding of how MAPK cascades and their corresponding signaling pathways are organized at subcellular levels. In this review, we will provide an overview of plant MAPK signaling, including a discussion of what is known about cellular mechanisms for achieving signaling specificity. Then we will explore what is currently known about the subcellular localization of MAPK proteins in resting conditions and after pathway activation. Finally, we will discuss a number of new experimental methods that have not been widely deployed in plants that have the potential to provide a deeper understanding of the spatial and temporal dynamics of MAPK signaling.

Convergence of Multiple MAP3Ks on MKK3 Identifies a Set of Novel Stress MAPK Modules

Since its first description in 1995 and functional characterization 12 years later, plant MKK3-type MAP2Ks have emerged as important integrators in plant signaling. Although they have received less attention than the canonical stress-activated mitogen-activated protein kinases (MAPKs), several recent publications shed light on their important roles in plant adaptation to environmental conditions. Nevertheless, the MKK3-related literature is complicated. This review summarizes the current knowledge and discrepancies on MKK3 MAPK modules in plants and highlights the singular role of MKK3 in green plants. In the light of the latest data, we hypothesize a general model that all clade-III MAP3Ks converge on MKK3 and C-group MAPKs, thereby defining a set of novel MAPK modules which are activated by stresses and internal signals through the transcriptional regulation of MAP3K genes.

The mitogen-activated protein kinase cascade MKK3-MPK6 is an important part of the jasmonate signal transduction pathway in Arabidopsis

The plant hormone jasmonic acid (JA) plays a key role in the environmental stress responses and developmental processes of plants. Although ATMYC2/JASMONATE-INSENSITIVE1 (JIN1) is a major positive regulator of JA-inducible gene expression and essential for JA-dependent developmental processes in Arabidopsis thaliana, molecular mechanisms underlying the control of ATMYC2/JIN1 expression remain largely unknown. Here, we identify a mitogen-activated protein kinase (MAPK) cascade, MAPK KINASE 3 (MKK3)-MAPK 6 (MPK6), which is activated by JA in Arabidopsis. We also show that JA negatively controls ATMYC2/JIN1 expression, based on quantitative RT-PCR and genetic analyses using gain-of-function and loss-of-function mutants of the MKK3-MPK6 cascade. These results indicate that this kinase unit plays a key role in JA-dependent negative regulation of ATMYC2/JIN1 expression. Both positive and negative regulation by JA may be used to fine-tune ATMYC2/JIN1 expression to control JA signaling. Moreover, JA-regulated root growth inhibition is affected by mutations in the MKK3-MPK6 cascade, which indicates important roles in JA signaling. We provide a model explaining how MPK6 can convert three distinct signals - JA, pathogen, and cold/salt stress - into three different sets of responses in Arabidopsis.

Signaling through MAP kinase networks in plants

Protein phosphorylation is the most important mechanism for controlling many fundamental cellular processes in all living organisms including plants. A specific class of serine/threonine protein kinases, the mitogen-activated protein kinases (MAP kinases) play a central role in the transduction of various extra- and intracellular signals and are conserved throughout eukaryotes. These generally function via a cascade of networks, where MAP kinase (MAPK) is phosphorylated and activated by MAPK kinase (MAPKK), which itself is activated by MAPKK kinase (MAPKKK). Signaling through MAP kinase cascade can lead to cellular responses including cell division, differentiation as well as response to various stresses. In plants, MAP kinases are represented by multigene families and are organized into a complex network for efficient transmission of specific stimuli. Putative plant MAP kinase cascades have been postulated based on experimental analysis of in vitro interactions between specific MAP kinase components. These cascades have been tested in planta following expression of epitope-tagged kinases in protoplasts. It is known that signaling for cell division and stress responses in plants are mediated through MAP kinases and even auxin, ABA and possibly ethylene and cytokinin also utilize a MAP kinase pathway. Most of the biotic (pathogens and pathogen-derived elicitors) including wounding and abiotic stresses (salinity, cold, drought, and oxidative) can induce defense responses in plants through MAP kinase pathways. In this article we have covered the historical background, biochemical assay, activation/inactivation, and targets of MAP kinases with emphasis on plant MAP kinases and the responses regulated by them. The cross-talk between plant MAP kinases is also discussed to bring out the complexity within this three-component module.

The transcript-level-independent activation of ornithine decarboxylase in suspension-cultured BY2 cells entering the cell cycle

The regulation of ornithine decarboxylase (ODC) expression was studied in suspension-cultured tobacco (Nicotiana tabacum L.) BY2 cells. ODC activity increased rapidly 3 h after cells re-entered the cell cycle from the stationary phase, corresponding to the G1 phase, and continued to increase in the subsequent S phase, while the ODC transcript level increased only transiently. ODC activity was suppressed by sucrose-deficiency, while the ODC transcript level was not affected. U0126, a specific inhibitor of mammalian MAPK kinases (MEKs), significantly reduced ODC enzyme activity, but not the ODC transcript level. These results suggest that ODC activity is regulated independently of its transcript level in BY2 cells, and that sucrose and a U0126-sensitive protein kinase are required for the transcript-level-independent activation of ODC.

Two novel mitogen-activated protein signaling components, OsMEK1 and OsMAP1, are involved in a moderate low-temperature signaling pathway in rice

Rice (Oryza sativa) anther development is easily damaged by moderately low temperatures above 12 degrees C. Subtractive screening of cDNA that accumulated in 12 degrees C-treated anthers identified a cDNA clone, OsMEK1, encoding a protein with features characteristic of a mitogen-activated protein (MAP) kinase kinase. The putative OsMEK1 protein shows 92% identity to the maize (Zea mays) MEK homolog, ZmMEK1. OsMEK1 transcript levels were induced in rice anthers by 12 degrees C treatment for 48 h. Similar OsMEK1 induction was observed in shoots and roots of seedlings that were treated at 12 degrees C for up to 24 h. It is interesting that no induction of OsMEK1 transcripts was observed in 4 degrees C-treated seedlings. In contrast, rice lip19, encoding a bZIP protein possibly involved in low temperature signal transduction, was not induced by 12 degrees C treatment but was induced by 4 degrees C treatment. Among the three MAP kinase homologs cloned, only OsMAP1 displayed similar 12 degrees C-specific induction pattern as OsMEK1. A yeast two-hybrid system revealed that OsMEK1 interacts with OsMAP1, but not with OsMAP2 and OsMAP3, suggesting that OsMEK1 and OsMAP1 probably function in the same signaling pathway. An in-gel assay of protein kinase activity revealed that a protein kinase (approximately 43 kD), which preferentially uses myelin basic protein as a substrate, was activated by 12 degrees C treatment but not by 4 degrees C treatment. Taken together, these results lead us to conclude that at least two signaling pathways for low temperature stress exist in rice, and that a MAP kinase pathway with OsMEK1 and OsMAP1 components is possibly involved in the signaling for the higher range low-temperature stress.

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 signal transduction pathways in plants

The mitogen-activated protein kinase (MAP kinase) signal transduction cascades are routes through which eukaryotic cells deliver extracellular messages to the cytosol and nucleus. These signalling pathways direct cell division, cellular differentiation, metabolism, and both biotic and abiotic stress responses. In plants, MAP kinases and the upstream components of the cascades are represented by multigene families, organized into different pathways which are stimulated and interact in complex ways. Experimental strategies for the analysis of MAP kinase cascades include the yeast two-hybrid system; using this approach in vitro interactions between specific MAP kinase cascade components have been analysed and putative plant cascades postulated. Transient transformation of protoplasts with epitope-tagged kinases has allowed cascades to be tested in planta. There is clear evidence for the involvement of MAP kinases in plant cell division and in the regulation of auxin signalling. Biotic (pathogens and pathogen-derived elicitors from fungi, bacteria and viruses) and abiotic stresses including wounding, mechanical stimulation, cold, drought and ozone can elicit defence responses in plants through MAP kinase pathways. There are data suggesting that ABA signalling utilizes a MAP kinase pathway, and probably ethylene and perhaps cytokinins do so also. The objective of this paper is to review this rapidly advancing field. Contents Summary 67 I. Introduction 68 II. Background 68 III. MAP kinase targets and targeting specificity 69 IV. Assays and inhibitors 70 V. Two well characterized MAP kinase pathways, Hog1 and Sevenless 71 VI. MAP kinases in plants 73 VII. MAP kinases and cell division 76 VIII. MAP kinases and plant hormones 76 IX. MAP kinase and abiotic stress 78 X. MAP kinase and biotic stress 80 XI. Future perspectives for MAP kinase research in plants 83 Acknowledgements 84 References 84.

Mitogen-activated protein [MAP] kinase pathways in plants: versatile signaling tools

Mitogen-activated protein kinases (MAPKs) are important signaling tools in all eukaryotes, and function in mediating an enormous variety of external signals to appropriate cellular responses. MAPK pathways have been studied extensively in yeast and mammalian cells, and a large body of knowledge on their functioning has accumulated, which is summarized briefly. Plant MAPK pathways have attracted increasing interest, resulting in the isolation of a large number of different components of MAPK cascades. Studies on the functions of these components have revealed that MAPKs play important roles in the response to a broad variety of stresses, as well as in the signaling of most plant hormones and in developmental processes. Finally, the involvement of various plant phosphatases in the inactivation of MAPKs is discussed.

ATMPK4, an Arabidopsis homolog of mitogen-activated protein kinase, is activated in vitro by AtMEK1 through threonine phosphorylation

The modulation of mitogen-activated protein kinase (MAPK) activity regulates many intracellular signaling processes. In animal and yeast cells, MAP kinases are activated via phosphorylation by the dual-specificity kinase MEK (MAP kinase kinase). Several plant homologs of MEK and MAPK have been identified, but the biochemical events underlying the activation of plant MAPKs remain unknown. We describe the in vitro activation of an Arabidopsis homolog of MAP kinase, ATMPK4. ATMPK4 was phosphorylated in vitro by an Arabidopsis MEK homolog, AtMEK1. This phosphorylation occurred principally on threonine (Thr) residues and resulted in elevated ATMPK4 kinase activity. A second Arabidopsis MEK isoform, ATMAP2Kalpha, failed to phosphorylate ATMPK4 in vitro. Tyr dephosphorylation by the Arabidopsis Tyr-specific phosphatase AtPTP1 resulted in an almost complete loss of ATMPK4 activity. Immunoprecipitates of Arabidopsis extracts with anti-ATMPK4 antibodies displayed myelin basic protein kinase activity that was sensitive to treatment with AtPTP1. These results demonstrate that a plant MEK can phosphorylate and activate MAPK, and that Tyr phosphorylation is critical for the catalytic activity of MAPK in plants. Surprisingly, in contrast to the animal enzymes, AtMEK1 may not be a dual-specificity kinase but, rather, the required Tyr phosphorylation on ATMPK4 may result from autophosphorylation.

Molecular cloning and characterization of a tobacco MAP kinase kinase that interacts with SIPK

A tobacco MAP kinase termed SIPK (Salicylic acid-Induced Protein Kinase) is activated in response to a variety of stress signals, including pathogen attack and wounding (S. Zhang and D.F. Klessig, Proc. Natl. Acad. Sci. USA 95:7225-7230, 1998; S. Zhang and D.F. Klessig, Proc. Natl. Acad. Sci. USA 95:7433-7438, 1998). Using the yeast two-hybrid system, we have identified a gene encoding a protein that interacts with SIPK but not the wounding induced protein kinase (WIPK), which is another tobacco MAP kinase. Sequence analysis indicated that this SIPK-interacting protein is a member of the MAP kinase kinase family; thus, it was named SIPK kinase (SIPKK). Co-immunoprecipitation experiments demonstrated that SIPKK and SIPK interact in vitro. Consistent with its putative function as a kinase, SIPKK phosphorylated myelin basic protein in vitro. Interestingly, SIPKK was induced at the mRNA level after Tobacco mosaic virus (TMV) infection or wounding, albeit with kinetics that are too slow to account for the activation of SIPK following these stimuli.

MAP kinase cascades in Arabidopsis: their roles in stress and hormone responses

Mitogen-activated protein kinase (MAPK) cascades have essential roles in diverse intracellular signaling processes in plants, animals and yeasts. In plants, MAPK and MAPK-like kinase activities are transiently activated in response to environmental stresses and plant hormones. In addition, transcription of genes encoding protein kinases involved in MAPK cascades in upregulated by environmental stresses. A possible MAPK cascade of Arabidopsis was identified based on both the yeast two-hybrid analysis and functional complementation analysis of yeast mutants. This MAPK cascade may have important roles in stress signal transduction pathways in Arabidopsis.

PLANT PROTEIN SERINE/THREONINE KINASES: Classification and Functions

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

Molecular characterisation of plant cDNAs BnMAP4Kalpha1 and BnMAP4Kalpha2 belonging to the GCK/SPS1 subfamily of MAP kinase kinase kinase kinase

Several yeast and mammal MAP kinase modules require, upstream of their MAP kinase kinase kinase (MAP3K), a MAP3K kinase (MAP4K). An Arabidopsis thaliana EST clone, sharing identity to MAP4Ks from yeast and mammals, has been used to isolate cDNA clones from a Brassica napus microspore-derived embryo cDNA library. The BnMAP4Kalpha1 and BnMAP4K-alpha2 clones encode putative proteins possessing the 12 subdomains of the serine/threonine protein kinase catalytic domain. A detailed analysis showed that they belong to the GCK/SPS1 subfamily of MAP4K proteins which possess an amino terminal catalytic domain and a long carboxy terminal tail. A Southern blot analysis suggested that the two proteins are encoded by a small multigene family. Expression studies revealed the presence of BnMAP4Kalpha1 and -alpha2 transcripts in all the tissues examined; however, they are most abundant in roots, siliques and flower buds. The expression of BnMAP4Kalpha1 and -alpha2 at the three main developmental stages of microspore-derived embryos (i.e., globular/heart, torpedo and cotyledonary) was confirmed by northern blot and RT-PCR analysis. An expression analysis of the above genes using synchronised Arabidopsis thaliana cell suspensions showed that the homologues genes are cell cycle regulated.

Mitogen-activated protein kinase: conservation of a three-kinase module from yeast to human

Mitogen-activated protein kinases (MAPK) are serine-threonine protein kinases that are activated by diverse stimuli ranging from cytokines, growth factors, neurotransmitters, hormones, cellular stress, and cell adherence. Mitogen-activated protein kinases are expressed in all eukaryotic cells. The basic assembly of MAPK pathways is a three-component module conserved from yeast to humans. The MAPK module includes three kinases that establish a sequential activation pathway comprising a MAPK kinase kinase (MKKK), MAPK kinase (MKK), and MAPK. Currently, there have been 14 MKKK, 7 MKK, and 12 MAPK identified in mammalian cells. The mammalian MAPK can be subdivided into five families: MAPKerk1/2, MAPKp38, MAPKjnk, MAPKerk3/4, and MAPKerk5. Each MAPK family has distinct biological functions. In Saccharomyces cerevisiae, there are five MAPK pathways involved in mating, cell wall remodelling, nutrient deprivation, and responses to stress stimuli such as osmolarity changes. Component members of the yeast pathways have conserved counterparts in mammalian cells. The number of different MKKK in MAPK modules allows for the diversity of inputs capable of activating MAPK pathways. In this review, we define all known MAPK module kinases from yeast to humans, what is known about their regulation, defined MAPK substrates, and the function of MAPK in cell physiology.

Isolation of ATMEKK1 (a MAP kinase kinase kinase)-interacting proteins and analysis of a MAP kinase cascade in Arabidopsis

In plants, a number of MAP kinase (MAPK), MAPK kinase (MAPKK), and MAPKK kinase (MAPKKK) homologues have been reported. However, there have been no reports of protein-protein interactions between these kinases or molecular analysis of MAPK cascades in higher plants. To analyze a possible MAPK cascade in Arabidopsis thaliana, we took two molecular approaches. One is the two-hybrid screening of ATMEKK1 (a MAPKKK)-interacting proteins; the other is an analysis of physical and functional interactions among isolated MAPK, MAPKK, and MAPKKK homologues from Arabidopsis. In two-hybrid screening using ATMEKK1 as bait, we isolated a novel MAPKK homologue, ATMKK2, a MAPK homologue, ATMPK4, and an unknown protein. ATMKK2 has high sequence similarity with MEK1 (a MAPKK) in Arabidopsis. Based on yeast two-hybrid analysis, we detected protein-protein interactions between ATMEKK1 and ATMKK2/MEK1 (MAPKKs), between ATMKK2/MEK1 and ATMPK4 (a MAPK), and between ATMPK4 and ATMEKK1. ATMPK4 and ATMKK2/MEK1 interacted with two distinct regions of ATMEKK1, the N-terminal regulatory domain and the C-terminal kinase domain, respectively. Coexpression of ATMEKK1 increased the ability of two closely related MAPKKs, ATMKK2 and MEK1, to complement a growth defect of the yeast pbs2 mutant. Coexpression of ATMPK4 and MEK1 complemented a growth defect of the yeast mpk1 and bck1 mutants. By contrast, other combinations of MAPKs and MAPKKs did not suppress these yeast mutations. These results suggest that ATMEKK1, ATMKK2/MEK1, and ATMPK4 may constitute a MAP kinase cascade.

Cytosolic acidification but not auxin at physiological concentration is an activator of MAP kinases in tobacco cells

In higher plants, MAP kinase cascades are involved in the transduction of numerous stress-related signals but much less is known about the effect of mitogenic signals. We have analysed MAP kinase activation in tobacco cells after treatment by auxin, a growth factor required at physiological concentrations for mitosis in plant cell cultures. From in-gel assay of myelin basic protein kinase and from immunochemical detection of ERK related kinases, we show that the mitogenic effect of auxin, which was confirmed by the specific increase of several mRNAs species, did not rely on MAP kinase activation within the first 2 hours. These data contest previous results which could be due to the activation of MAP kinase by a signal other than auxin. In the second part of this study, we show that the treatment of the cells with high concentrations of various weak lipophilic acids such as auxin, in a nonphysiological concentration range, butyric or acetic acid is sufficient to activate transiently a MAP kinase. The data show that MAP kinase activation is the consequence of cytosolic acidification. Moreover, it is not sensitive to the protein kinase inhibitor staurosporine. These results suggest a functional role for cytosolic acidification as a second messenger mediating MAP kinase activation in the response of plant cells to various stresses.

[Protein phosphatases and protein kinases in higher plants]

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

Environmental stress response in plants: the role of mitogen-activated protein kinases

Mitogen-activated protein kinase (MAPK) cascades have essential roles in diverse intracellular signaling processes in plants, animals and yeasts. In plants, transcription of genes encoding protein kinases involved in MAPK cascades is upregulated by environmental stresses and plant hormones; in addition, MAPK-like kinase activities are transiently activated in response to environmental stresses. Consequently, MAPK cascades are now thought to have important roles in stress signal transduction pathways in higher plants.

Stress signaling in plants: a mitogen-activated protein kinase pathway is activated by cold and drought

Yeast and animals use mitogen-activated protein (MAP) kinase cascades to mediate stress and extracellular signals. We have tested whether MAP kinases are involved in mediating environmental stress responses in plants. Using specific peptide antibodies that were raised against different alfalfa MAP kinases, we found exclusive activation of p44MMK4 kinase in drought- and cold-treated plants. p44MMK4 kinase was transiently activated by these treatments and was correlated with a shift in the electrophoretic mobility of the p44MMK4 protein. Although transcript levels of the MMK4 gene accumulated after drought and cold treatment, no changes in p44MMK4 steady state protein levels were observed, indicating a posttranslational activation mechanism. Extreme temperatures, drought, and salt stress are considered to be different forms of osmotic stress. However, high salt concentrations or heat shock did not induce activation of p44MMK4, indicating the existence of distinct mechanisms to mediate different stresses in alfalfa. Stress adaptation in plants is mediated by abscisic acid (ABA)-dependent and ABA-independent processes. Although ABA rapidly induced the transcription of an ABA-inducible marker gene, MMK4 transcript levels did not increase and p44MMK4 kinase was not activated. These data indicate that the MMK4 kinase pathway mediates drought and cold signaling independently of ABA.

A gene encoding a mitogen-activated protein kinase kinase kinase is induced simultaneously with genes for a mitogen-activated protein kinase and an S6 ribosomal protein kinase by touch, cold, and water stress in Arabidopsis thaliana

We describe here the cloning and characterization of a cDNA encoding a protein kinase that has high sequence homology to members of the mitogen-activated protein kinase (MAPK) kinase kinase (MAPKKK or MEKK) family; this cDNA is named cATMEKKI (Arabidopsis thaliana MAP kinase or ERK kinase kinase 1). The catalytic domain of the putative ATMEKK1 protein shows approximately 40% identity with the amino acid sequences of the catalytic domains of MAPKKKs (such as Byr2 from Schizosaccharomyces pombe, Ste11 from Saccharomyces cerevisiae, Bck1 from S. cerevisiae, MEKK from mouse, and NPK1 from tobacco). In yeast cells that overexpress ATMEKK1, the protein kinase replaces Ste11 in responding to mating pheromone. In this study, the expression of three protein kinases was examined by Northern blot analyses: ATMEKK1 (structurally related to MAPKKK), ATMPK3 (structurally related to MAPK), and ATPK19 (structurally related to ribosomal S6 kinase). The mRNA levels of these three protein kinases increased markedly and simultaneously in response to touch, cold, and salinity stress. These results suggest that MAP kinase cascades, which are thought to respond to a variety of extracellular signals, are regulated not only at the posttranslational level but also at the transcriptional level in plants and that MAP kinase cascades in plants may function in transducing signals in the presence of environmental stress.

MMK2, a novel alfalfa MAP kinase, specifically complements the yeast MPK1 function

Mitogen-activated protein (MAP) kinases are serine/threonine protein kinases that are activated in response to a variety of stimuli. Here we report the isolation of an alfalfa cDNA encoding a functional MAP kinase, termed MMK2. The predicted amino acid sequence of MMK2 shares 65% identity with a previously identified alfalfa MAP kinase, termed MMK1. Both alfalfa cDNA clones encode functional kinases when expressed in bacteria, undergoing autophosphorylation and activation to phosphorylate myelin basic protein in vitro. However, only MMK2 was able to phosphorylate a 39 kDa protein from the detergent-resistant cytoskeleton of carrot cells. The distinctiveness of MMK2 was further shown by complementation analysis of three different MAP kinase-dependent yeast pathways; this revealed a highly specific replacement of the yeast MPK1(SLT2) kinase by MMK2, which was found to be dependent on activation by the upstream regulators of the pathway. These results establish the existence of MAP kinases with different characteristics in higher plants, suggesting the possibility that they could mediate different cellular responses.

Cutting activates a 46-kilodalton protein kinase in plants

Using SDS/polyacrylamide gels that contained myelin basic protein, we identified a 46-kDa protein kinase in tobacco that is transiently activated by cutting. Although the activity of the kinase was rarely detectable in mature leaves, marked activity became apparent within several minutes after isolation of leaf discs and subsided within 30 min. In the presence of cycloheximide (CHX), the kinase activity did not diminish after the isolation over the course of 2 hr, suggesting that protein synthesis was not required for the activation of the kinase. A second cutting of leaf discs between 30 min and 60 min after the isolation failed to activate the kinase, whereas a second cutting given 3 hr after isolation apparently activated the kinase. These results suggest that the 46-kDa protein kinase is desensitized immediately after the first activation, which can be blocked by CHX, but the response ability recovers with time. When protein extracts containing the active kinase were treated with serine/threonine-specific or tyrosine-specific protein phosphatase, the kinase activity was abolished. After immunoprecipitation with antibody against phosphotyrosine, activity of the kinase was recovered in the immunoprecipitate. These results suggest that the active form of the kinase is phosphorylated at both serine/threonine and tyrosine residues. It seems likely that the 46-kDa protein kinase can be activated by dual phosphorylation. The activity of a 46-kDa protein kinase was also detected in leaves of a wide variety of plant species including dicotyledonous and monocotyledonous plants. We propose the name PMSAP (plant multisignal-activated protein) kinase for this kinase because the kinase was also activated by various signals other than cutting.