Calcium-independent activation of salicylic acid-induced protein kinase and a 40-kilodalton protein kinase by hyperosmotic stress

Spermine signalling in tobacco: activation of mitogen-activated protein kinases by spermine is mediated through mitochondrial dysfunction

Polyamines (PAs) play important roles in cell proliferation, growth and environmental stress responses of all living organisms. In this study, we examine whether these compounds act as signal mediators. Spermine (Spm) specifically activated protein kinases of tobacco leaves, which were identified as salicylic acid (SA)-induced protein kinase (SIPK) and wound-induced protein kinase (WIPK), using specific antibodies. Upon Spm treatment, upregulation of WIPK, but not SIPK, was observed. Spm-induced mitogen-activated protein kinases (MAPKs) activation and WIPK upregulation were prevented upon pre-treatment with antioxidants and Ca2+ channel blockers. Additionally, Spm specifically stimulated expression of the alternative oxidase (AOX) gene, which was disrupted by these antioxidants and Ca2+ channel blockers. Bongkrekic acid (BK), an inhibitor of the opening of mitochondrial permeability transition (PT) pores, suppressed MAPKs activation and accumulation of WIPK and AOX mRNA. Our data collectively suggest that Spm causes mitochondrial dysfunction via a signalling pathway in which reactive oxygen species and Ca2+ influx are involved. As a result, the phosphorylation activities of the two MAPK enzymes SIPK and WIPK are stimulated.

Pleiotropic effect of the insertion of the Agrobacterium rhizogenes rolD gene in tomato ( Lycopersicon esculentum Mill.)

The Agrobacterium rhizogenes rolD gene, coding for an ornithine cyclodeaminase involved in the biosynthesis of proline from ornithine, has been inserted in Lycopersicon esculentum cv Tondino with the aim of studying its effects on plant morphological characters including pathogen defense response. The analysis of plants transgenic for rolD did not show major morphological modifications. First generation transgenic plants however were found to flower earlier, and showed an increased number of inflorescences and higher fruit yield. Transformed plants were also analysed for parameters linked to pathogen defense response, i.e. ion leakage in the presence of the toxin produced by the fungus Fusarium oxysporum f. sp. lycopersici, and expression of the pathogenesis-related PR-1 gene. All the plants harbouring the rolD gene were shown to be more tolerant to the toxin in ion leakage experiments, with respect to the untransformed regenerated controls and the cv Tondino. PR-1 gene expression was quantitated by means of real-time PCR both at the basal level and after treatment with salicylic acid, an inducer of Systemic Acquired Resistance. In both cases the amount of PR-1 mRNA was higher in the transgenic plants. It seems therefore that the transformation of tomato plants with rolD could lead to an increased competence for defense response, as shown by toxin tolerance and increased expression of the Systemic Acquired Resistance marker gene PR-1. The results are finally discussed in view of their possible economic relevance.

Interaction between two mitogen-activated protein kinases during tobacco defense signaling

Plant mitogen-activated protein kinases (MAPKs) represented by tobacco wounding-induced protein kinase (WIPK) have unique regulation at the level of transcription in response to stresses. By using transcriptional and translational inhibitors, it has been shown previously that WIPK gene expression and de novo protein synthesis are required for the high-level activity of WIPK in cells treated with elicitins from Phytophthora spp. However, regulation of WIPK expression and the role(s) of WIPK in plant disease resistance are unknown. In this report, we demonstrate that WIPK gene transcription is regulated by phosphorylation and de-phosphorylation events. Interestingly, salicylic acid-induced protein kinase (SIPK) was identified as the kinase involved in regulating WIPK gene expression based on both gain-of-function and loss-of-function analyses. This finding revealed an additional level of interaction between SIPK and WIPK, which share an upstream MAPKK, NtMEK2. Depending on whether WIPK shares its downstream targets with SIPK, it could either function as a positive feed-forward regulator of SIPK or initiate a new pathway. Consistent with the first scenario, co-expression of WIPK with the active mutant of NtMEK2 leads to accelerated hypersensitive response (HR)-like cell death in which SIPK also plays a role. Mutagenesis analysis revealed that the conserved common docking domain in WIPK is required for its function. Together with prior reports that (i) WIPK is activated in NN tobacco infected with tobacco mosaic virus, and (ii) PVX virus-induced gene silencing of WIPK attenuated N gene-mediated resistance, we concluded that WIPK plays a positive role in plant disease resistance, possibly through accelerating the pathogen-induced HR cell death.

Ankyrin protein kinases: a novel type of plant kinase gene whose expression is induced by osmotic stress in alfalfa

Interaction between Medicago spp. and Sinorhizobium meliloti leads to the development of a novel organ, the root nodule. A gene, Msapk1, encoding a novel type of plant protein kinase containing a N-terminal region with an ankyrin domain, was identified and shown to be expressed both in S. meliloti-infected and spontaneous nodules in alfalfa. This gene is not exclusively associated to nodulation since its expression was detected in other plant organs. Several genes coding for ankyrin protein kinases (APKs) were detected in various plants and animals. Three closest A. thaliana homologues of Msapk1 were identified in databases and two of them were shown to express differentially in various organs using gene-specific RT-PCR. In contrast, Southern analysis suggests that a single-copy gene exists in diploid M. truncatula. By screening a M. truncatula BAC library the Mtapk1 genomic region was isolated and sequenced. Two neighbouring genes showing homologies to previously identified sequences in data banks were detected in the vicinity of the Mtapk1 gene and compared to similar regions of the three Atapk genes. The distribution of exons/introns was the same for all expressed genes of both species although Mtapk1 contained larger introns. Upon osmotic stress Msapk1 expression was induced in roots of alfalfa starting from three hours up to two days of treatment. These data suggest that Msapk1, involved in alfalfa osmotic stress responses, belongs to a novel class of plant protein kinases.

Changes in phosphorylation of 50 and 53 kDa soluble proteins in graviresponding oat (Avena sativa) shoots

The present work indicates that phosphorylation of a 50 kDa soluble protein is involved in the gravitropic response in graviresponsive pulvini of oat (Avena sativa) stems. This 50 kDa protein shows a differential pattern of phosphorylation between lower and upper halves of pulvini both in vivo and in vitro. The differential phosphorylation of this protein is detected only when stem segments are gravistimulated for short and long time periods. The differential phosphorylation of the 50 kDa protein occurs as early as 5 min after the initiation of gravistimulation. This corresponds closely to the presentation time of 5.2 min. This differential phosphorylation pattern was changed by treatments with cycloheximide, implying that a newly-synthesized protein is involved in the differential phosphorylation during the gravitropic response. An autophosphorylation experiment shows that the 50 kDa protein has kinase activity. The phosphorylation patterns of a 53 kDa protein were similar to those of the 50 kDa protein, but were only expressed in vitro. These findings indicate that the differential phosphorylation of the 50 (and 53 kDa) soluble proteins in graviresponding oat shoots may be an important component of the gravity signal transduction pathway.

Mitogen-activated protein kinases play an essential role in oxidative burst-independent expression of pathogenesis-related genes in parsley

Plants are continuously exposed to attack by potential phytopathogens. Disease prevention requires pathogen recognition and the induction of a multifaceted defense response. We are studying the non-host disease resistance response of parsley to the oomycete, Phytophthora sojae using a cell culture-based system. Receptor-mediated recognition of P. sojae may be achieved through a thirteen amino acid peptide sequence (Pep-13) present within an abundant cell wall transglutaminase. Following recognition of this elicitor molecule, parsley cells mount a defense response, which includes the generation of reactive oxygen species (ROS) and transcriptional activation of genes encoding pathogenesis-related (PR) proteins or enzymes involved in the synthesis of antimicrobial phytoalexins. Treatment of parsley cells with the NADPH oxidase inhibitor, diphenylene iodonium (DPI), blocked both Pep-13-induced phytoalexin production and the accumulation of transcripts encoding enzymes involved in their synthesis. In contrast, DPI treatment had no effect upon Pep-13-induced PR gene expression, suggesting the existence of an oxidative burst-independent mechanism for the transcriptional activation of PR genes. The use of specific antibodies enabled the identification of three parsley mitogen-activated protein kinases (MAPKs) that are activated within the signal transduction pathway(s) triggered following recognition of Pep-13. Other environmental challenges failed to activate these kinases in parsley cells, suggesting that their activation plays a key role in defense signal transduction. Moreover, by making use of a protoplast co-transfection system overexpressing wild-type and loss-of-function MAPK mutants, we show an essential role for post-translational phosphorylation and activation of MAPKs for oxidative burst-independent PR promoter activation.

Different protein kinase families are activated by osmotic stresses in Arabidopsis thaliana cell suspensions. Involvement of the MAP kinases AtMPK3 and AtMPK6

Five Ca(2+)-independent protein kinases were rapidly activated by hypoosmotic stress, moderate or high hyperosmolarity induced by several osmolytes, sucrose, mannitol or NaCl. Three of these kinases, transiently activated by hypoosmolarity, recognised by anti-phosphorylated mitogen-activated protein (MAP) kinase antibodies, sensitive to a MAP kinase inhibitor and inactivated by the action of a tyrosine phosphatase, corresponded to MAP kinases. Using specific antibodies, two of the MAP kinases were identified as AtMPK6 and AtMPK3. The two other protein kinases, durably activated by high hyperosmolarity, did not belong to the MAP kinase family. Activation of AtMPK6 and AtMPK3 by hypoosmolarity depended on upstream protein kinases sensitive to staurosporine and on calcium influx. In contrast, these two transduction steps were not involved in the activation of the two protein kinases activated by high hyperosmolarity.

Opposite changes in membrane fluidity mimic cold and heat stress activation of distinct plant MAP kinase pathways

Mitogen-activated protein kinases (MAPKs) appear to be ubiquitously involved in signal transduction during eukaryotic responses to extracellular stimuli. In plants, no heat shock-activated MAPK has so far been reported. Also, whereas cold activates specific plant MAPKs such as alfalfa SAMK, mechanisms of such activation are unknown. Here, we report a heat shock-activated MAPK (HAMK) immunologically related to ERK (Extracellular signal-Regulated Kinase) superfamily of protein kinases. Molecular mechanisms of heat-activation of HAMK and cold-activation of SAMK were investigated. We show that cold-activation of SAMK requires membrane rigidification, whereas heat-activation of HAMK occurs through membrane fluidization. The temperature stress- and membrane structure-dependent activation of both SAMK and HAMK is mimicked at 25 degrees C by destabilizers of microfilaments and microtubules, latrunculin B and oryzalin, respectively; but is blocked by jasplakinolide, a stabilizer of actin microfilaments. Activation of SAMK or HAMK by temperature, chemically modulated membrane fluidity, or by cytoskeleton destabilizers is inhibited by blocking the influx of extracellular calcium. Activation of SAMK or HAMK is also prevented by an antagonist of calcium-dependent protein kinases (CDPKs). In summary, our data indicate that cold and heat are sensed by structural changes in the plasma membrane that translates the signal via cytoskeleton, Ca2+ fluxes and CDPKs into the activation of distinct MAPK cascades.

Nitrate efflux is an essential component of the cryptogein signaling pathway leading to defense responses and hypersensitive cell death in tobacco

There is much interest in the transduction pathways by which avirulent pathogens or derived elicitors activate plant defense responses. However, little is known about anion channel functions in this process. The aim of this study was to reveal the contribution of anion channels in the defense response triggered in tobacco by the elicitor cryptogein. Cryptogein induced a fast nitrate (NO(3)(-)) efflux that was sensitive to anion channel blockers and regulated by phosphorylation events and Ca(2+) influx. Using a pharmacological approach, we provide evidence that NO(3)(-) efflux acts upstream of the cryptogein-induced oxidative burst and a 40-kD protein kinase whose activation seems to be controlled by the duration and intensity of anion efflux. Moreover, NO(3)(-) efflux inhibitors reduced and delayed the hypersensitive cell death triggered by cryptogein in tobacco plants. This was accompanied by a delay or a complete suppression of the induction of several defense-related genes, including hsr203J, a gene whose expression is correlated strongly with programmed cell death in plants. Our results indicate that anion channels are involved intimately in mediating defense responses and hypersensitive cell death.

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.

Salt and drought stress signal transduction in plants

Salt and drought stress signal transduction consists of ionic and osmotic homeostasis signaling pathways, detoxification (i.e., damage control and repair) response pathways, and pathways for growth regulation. The ionic aspect of salt stress is signaled via the SOS pathway where a calcium-responsive SOS3-SOS2 protein kinase complex controls the expression and activity of ion transporters such as SOS1. Osmotic stress activates several protein kinases including mitogen-activated kinases, which may mediate osmotic homeostasis and/or detoxification responses. A number of phospholipid systems are activated by osmotic stress, generating a diverse array of messenger molecules, some of which may function upstream of the osmotic stress-activated protein kinases. Abscisic acid biosynthesis is regulated by osmotic stress at multiple steps. Both ABA-dependent and -independent osmotic stress signaling first modify constitutively expressed transcription factors, leading to the expression of early response transcriptional activators, which then activate downstream stress tolerance effector genes.

Salt and drought stress signal transduction in plants

Salt and drought stress signal transduction consists of ionic and osmotic homeostasis signaling pathways, detoxification (i.e., damage control and repair) response pathways, and pathways for growth regulation. The ionic aspect of salt stress is signaled via the SOS pathway where a calcium-responsive SOS3-SOS2 protein kinase complex controls the expression and activity of ion transporters such as SOS1. Osmotic stress activates several protein kinases including mitogen-activated kinases, which may mediate osmotic homeostasis and/or detoxification responses. A number of phospholipid systems are activated by osmotic stress, generating a diverse array of messenger molecules, some of which may function upstream of the osmotic stress-activated protein kinases. Abscisic acid biosynthesis is regulated by osmotic stress at multiple steps. Both ABA-dependent and -independent osmotic stress signaling first modify constitutively expressed transcription factors, leading to the expression of early response transcriptional activators, which then activate downstream stress tolerance effector genes.

Inducers of glycinebetaine synthesis in barley

Glycinebetaine is an osmoprotectant accumulated by barley (Hordeum vulgare) plants in response to high levels of NaCl, drought, and cold stress. Using barley seedlings in hydroponic culture, we characterized additional inducers of glycinebetaine accumulation. These included other inorganic salts (KCl, MgCl(2), LiCl, and Na(2)SO(4)), oxidants (H(2)O(2) and cumene hydroperoxide), and organic compounds (abscisic acid, polymixin B, n-butanol, salicylic acid, and aspirin). Stress symptoms brought on by high NaCl and other inducers, and not necessarily correlated with glycinebetaine accumulation, include wilting, loss of chlorophyll, and increase in thiobarbituric acid reacting substances. For NaCl, Ca(2+) ions at 10 to 20 mM decrease these stress symptoms without diminishing, or even increasing, glycinebetaine induction. Abscisic acid induces glycinebetaine accumulation without causing any of the stress symptoms. NaCl, KCl, and H(2)O(2) (but not other inducers) induce glycinebetaine at concentrations below those needed for the other stress symptoms. Mg(2+) at 10 to 20 mM induces both stress symptoms and glycinebetaine, but only at low (0.2 mM) Ca(2+). Although illumination is needed for optimal induction, a significant increase in the leaf glycinebetaine level is found in complete darkness, also.

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.

Cell signaling under salt, water and cold stresses

Forward genetics and biochemical approaches to studying plant responses to salt, water and cold stresses began to bear fruit recently. Analysis of salt overly sensitive (sos) Arabidopsis mutants revealed a novel calcium-regulated protein kinase pathway for response to the ionic aspect of salt stress. In-gel kinase assays identified several SOS-independent protein kinases that are either activated specifically by osmotic stress or by multiple abiotic and biotic stresses. Molecular analysis revealed a transcriptional cascade in cold-regulated gene expression.

Oxidative stress activates ATMPK6, an Arabidopsis homologue of MAP kinase

Mitogen-activated protein kinase (MAPK) cascades function in biotic and abiotic stress responses in plants. We analysed effect of oxidative stress on the activation of ATMPK6, an Arabidopsis thaliana MAPK, in Arabidopsis T87 cultured cells and rosette leaves using anti-ATMPK6 specific antibody. ATMPK6 in T87 cells was strongly activated by reactive oxygen species (ROS) such as H(2)O(2) and KO(2). In leaves, ATMPK6 was activated by paraquat and 3-amino-1,2,4-triazole (a catalase inhibitor). These results indicate that ATMPK6 is one of the candidates for signal mediators in response to abiotic or biotic sources for ROS in Arabidopsis.

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.

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.

Osmotic stress activates distinct lipid and MAPK signalling pathways in plants

Plants are continuously exposed to all kinds of water stress such as drought and salinity. In order to survive and adapt, they have developed survival strategies that have been well studied, but little is known about the early mechanisms by which the osmotic stress is perceived and transduced into these responses. During the last few years, however, a variety of reports suggest that specific lipid and MAPK pathways are involved. This review briefly summarises them and presents a model showing that osmotic stress is transmitted by multiple signalling pathways.

Abiotic stress signal transduction in plants: Molecular and genetic perspectives

Low temperature, drought and salinity are major adverse environmental factors that limit plant productivity. Understanding the mechanisms by which plants perceive and transduce these stress signals to initiate adaptive responses is essential for engineering stress-tolerant crop plants. Molecular and biochemical studies suggest that abiotic stress signaling in plants involves receptor-coupled phosphorelay, phosphoinositol-induced Ca2+ changes, mitogen-activated protein kinase cascades and transcriptional activation of stress-responsive genes. In addition, protein posttranslational modifications and adapter or scaffold-mediated protein-protein interactions are also important in abiotic stress signal transduction. Most of these signaling modules, however, have not been genetically established to function in plant abiotic stress signal transduction. To overcome the scarcity of abiotic stress-specific phenotypes for conventional genetic screens, molecular genetic analysis using stress-responsive promoter-driven reporter is suggested as an alternative approach to genetically dissect abiotic stress signaling networks in plants.

A harpin binding site in tobacco plasma membranes mediates activation of the pathogenesis-related gene HIN1 independent of extracellular calcium but dependent on mitogen-activated protein kinase activity

Harpin from the bean halo-blight pathogen Pseudomonas syringae pv phaseolicola (harpin(Psph)) elicits the hypersensitive response and the accumulation of pathogenesis-related gene transcripts in the nonhost plant tobacco. Here, we report the characterization of a nonproteinaceous binding site for harpin(Psph) in tobacco plasma membranes, which is assumed to mediate the activation of plant defense responses in a receptor-like manner. Binding of 125I-harpin(Psph) to tobacco microsomal membranes (dissociation constant = 425 nM) and protoplasts (dissociation constant = 380 nM) was specific, reversible, and saturable. A close correlation was found between the abilities of harpin(Psph) fragments to elicit the transcript accumulation of the pathogenesis-related tobacco gene HIN1 and to compete for binding of 125I-harpin(Psph) to its binding site. Another elicitor of the hypersensitive response and HIN1 induction in tobacco, the Phytophthora megasperma-derived beta-elicitin beta-megaspermin, failed to bind to the putative harpin(Psph) receptor. In contrast to activation by beta-megaspermin, harpin(Psph)-induced activation of the 48-kD salicylic acid-responsive mitogen-activated protein kinase (MAPK) and HIN1 transcript accumulation were independent of extracellular calcium. Moreover, use of the MAPK kinase inhibitor U0126 revealed that MAPK activity was essential for pathogenesis-related gene expression in harpin(Psph)-treated tobacco cells. Thus, a receptor-mediated MAPK-dependent signaling pathway may mediate the activation of plant defense responses induced by harpin(Psph).

Hyperosmotic stress induces the rapid phosphorylation of a soybean phosphatidylinositol transfer protein homolog through activation of the protein kinases SPK1 and SPK2

Although phosphatidylinositol transfer proteins (PITPs) are known to serve critical functions in regulating a varied array of signal transduction processes in animals and yeast, the discovery of a similar class of proteins in plants occurred only recently. Here, we report the participation of Ssh1p, a soybean PITP-like protein, in the early events of osmosensory signal transduction in plants, a function not attributed previously to animal or yeast PITPs. Exposure of plant tissues to hyperosmotic stress led to the rapid phosphorylation of Ssh1p, a modification that decreased its ability to associate with membranes. An osmotic stress-activated Ssh1p kinase activity was detected in several plant species by presenting recombinant Ssh1p as a substrate in in-gel kinase assays. Elements of a similar osmosensory signaling pathway also were conserved in yeast, an observation that facilitated the identification of soybean protein kinases SPK1 and SPK2 as stress-activated Ssh1p kinases. This study reveals the activation of SPK1 and/or SPK2 and the subsequent phosphorylation of Ssh1p as two early successive events in a hyperosmotic stress-induced signaling cascade in plants. Furthermore, Ssh1p is shown to enhance the activities of a plant phosphatidylinositol 3-kinase and phosphatidylinositol 4-kinase, an observation that suggests that the ultimate function of Ssh1p in cellular signaling is to alter the plant's capacity to synthesize phosphoinositides during periods of hyperosmotic stress.

Hyperosmotic stress induces the rapid phosphorylation of a soybean phosphatidylinositol transfer protein homolog through activation of the protein kinases SPK1 and SPK2

Although phosphatidylinositol transfer proteins (PITPs) are known to serve critical functions in regulating a varied array of signal transduction processes in animals and yeast, the discovery of a similar class of proteins in plants occurred only recently. Here, we report the participation of Ssh1p, a soybean PITP-like protein, in the early events of osmosensory signal transduction in plants, a function not attributed previously to animal or yeast PITPs. Exposure of plant tissues to hyperosmotic stress led to the rapid phosphorylation of Ssh1p, a modification that decreased its ability to associate with membranes. An osmotic stress-activated Ssh1p kinase activity was detected in several plant species by presenting recombinant Ssh1p as a substrate in in-gel kinase assays. Elements of a similar osmosensory signaling pathway also were conserved in yeast, an observation that facilitated the identification of soybean protein kinases SPK1 and SPK2 as stress-activated Ssh1p kinases. This study reveals the activation of SPK1 and/or SPK2 and the subsequent phosphorylation of Ssh1p as two early successive events in a hyperosmotic stress-induced signaling cascade in plants. Furthermore, Ssh1p is shown to enhance the activities of a plant phosphatidylinositol 3-kinase and phosphatidylinositol 4-kinase, an observation that suggests that the ultimate function of Ssh1p in cellular signaling is to alter the plant's capacity to synthesize phosphoinositides during periods of hyperosmotic stress.

Arabidopsis map kinase 4 negatively regulates systemic acquired resistance

Transposon inactivation of Arabidopsis MAP kinase 4 produced the mpk4 mutant exhibiting constitutive systemic acquired resistance (SAR) including elevated salicylic acid (SA) levels, increased resistance to virulent pathogens, and constitutive pathogenesis-related gene expression shown by Northern and microarray hybridizations. MPK4 kinase activity is required to repress SAR, as an inactive MPK4 form failed to complement mpk4. Analysis of mpk4 expressing the SA hydroxylase NahG and of mpk4/npr1 double mutants indicated that SAR expression in mpk4 is dependent upon elevated SA levels but is independent of NPR1. PDF1.2 and THI2.1 gene induction by jasmonate was blocked in mpk4 expressing NahG, suggesting that MPK4 is required for jasmonic acid-responsive gene expression.

Various abiotic stresses rapidly activate Arabidopsis MAP kinases ATMPK4 and ATMPK6

Mitogen-activated protein kinase (MAP kinase, MAPK) cascades play pivotal roles in signal transduction of extracellular stimuli, such as environmental stresses and growth regulators, in various organisms. Arabidopsis thaliana MAP kinases constitute a gene family, but stimulatory signals for each MAP kinase have not been elucidated. Here we show that environmental stresses such as low temperature, low humidity, hyper-osmolarity, touch and wounding induce rapid and transient activation of the Arabidopsis MAP kinases ATMPK4 and ATMPK6. Activation of ATMPK4 and ATMPK6 was associated with tyrosine phosphorylation but not with the amounts of mRNA or protein. Kinetics during activation differ between these two MAP kinases. These results suggest that ATMPK4 and ATMPK6 are involved in distinct signal transduction pathways responding to these environmental stresses.

Multiple levels of tobacco WIPK activation during the induction of cell death by fungal elicitins

Three protein kinases of 48, 44 and 40 kDa are activated at different stages in tobacco cells treated with fungal elicitins. Previously we demonstrated that the rapidly activated 48 kDa protein kinase is encoded by SIPK. Here we report that the elicitin-activated 44 kDa kinase is encoded by WIPK. Activation of this kinase occurred 2-4 h after elicitin treatment and was preceded by dramatic increases in WIPK mRNA and protein levels. Studies using actinomycin D and cycloheximide demonstrated that de novo transcription and translation were required for this activation of the kinase activity. Strikingly, the kinetics of WIPK activation following elicitin treatment correlated with the onset of hypersensitive response (HR)-like cell death. Moreover, staurosporine and K-252a, two Ser/Thr protein kinase inhibitors that blocked WIPK activation, suppressed cell death. The timing for elicitin-treated cells to commit to a death program correlated with the appearance of high levels of WIPK activity. These correlative data suggest that WIPK may play a role during HR development in tobacco. Interestingly, a fungal cell-wall elicitor that does not cause cell death induced WIPK mRNA and protein to similar levels as those observed with the elicitins. However, no corresponding increase in WIPK activity was detected. Thus WIPK appears to be controlled at multiple levels.