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

Insights into Salinity Tolerance in Wheat

Salt stress has a detrimental impact on food crop production, with its severity escalating due to both natural and man-made factors. As one of the most important food crops, wheat is susceptible to salt stress, resulting in abnormal plant growth and reduced yields; therefore, damage from salt stress should be of great concern. Additionally, the utilization of land in coastal areas warrants increased attention, given diminishing supplies of fresh water and arable land, and the escalating demand for wheat. A comprehensive understanding of the physiological and molecular changes in wheat under salt stress can offer insights into mitigating the adverse effects of salt stress on wheat. In this review, we summarized the genes and molecular mechanisms involved in ion transport, signal transduction, and enzyme and hormone regulation, in response to salt stress based on the physiological processes in wheat. Then, we surveyed the latest progress in improving the salt tolerance of wheat through breeding, exogenous applications, and microbial pathways. Breeding efficiency can be improved through a combination of gene editing and multiple omics techniques, which is the fundamental strategy for dealing with salt stress. Possible challenges and prospects in this process were also discussed.

Natural immunity stimulation using ELICE16INDURES® plant conditioner in field culture of soybean

Recently, climate change has had an increasing impact on the world. Innate defense mechanisms operating in plants - such as PAMP-triggered Immunity (PTI) - help to reduce the adverse effects caused by various abiotic and biotic stressors. In this study, the effects of ELICE16INDURES® plant conditioner for organic farming, developed by the Research Institute for Medicinal Plants and Herbs Ltd. Budakalász Hungary, were studied in a soybean population in Northern Hungary. The active compounds and ingredients of this product were selected in such a way as to facilitate the triggering of general plant immunity without the presence and harmful effects of pathogens, thereby strengthening the healthy plant population and preparing it for possible stress effects. In practice, treatments of this agent were applied at two different time points and two concentrations. The conditioning effect was well demonstrated by using agro-drone and ENDVI determination in the soybean field. The genetic background of healthier plants was investigated by NGS sequencing, and by the expression levels of genes encoding enzymes involved in the catalysis of metabolic pathways regulating PTI. The genome-wide transcriptional profiling resulted in 13 contigs related to PAMP-triggered immunity and activated as a result of the treatments. Further analyses showed 16 additional PTI-related contigs whose gene expression changed positively as a result of the treatments. The gene expression values of genes encoded in these contigs were determined by in silico mRNA quantification and validated by RT-qPCR. Both - relatively low and high treatments - showed an increase in gene expression of key genes involving AOC, IFS, MAPK4, MEKK, and GST. Transcriptomic results indicated that the biosyntheses of jasmonic acid (JA), salicylic acid (SA), phenylpropanoid, flavonoid, phytoalexin, and cellular detoxification processes were triggered in the appropriate molecular steps and suggested that plant immune reactions may be activated also artificially, and innate immunity can be enhanced with proper plant biostimulants.

Transcriptome-based gene regulatory network analyses of differential cold tolerance of two tobacco cultivars

BACKGROUND

Cold is one of the main abiotic stresses that severely affect plant growth and development, and crop productivity as well. Transcriptional changes during cold stress have already been intensively studied in various plant species. However, the gene networks involved in the regulation of differential cold tolerance between tobacco varieties with contrasting cold resistance are quite limited.

RESULTS

Here, we conducted multiple time-point transcriptomic analyses using Tai tobacco (TT, cold susceptibility) and Yan tobacco (YT, cold resistance) with contrasting cold responses. We identified similar DEGs in both cultivars after comparing with the corresponding control (without cold treatment), which were mainly involved in response to abiotic stimuli, metabolic processes, kinase activities. Through comparison of the two cultivars at each time point, in contrast to TT, YT had higher expression levels of the genes responsible for environmental stresses. By applying Weighted Gene Co-Expression Network Analysis (WGCNA), we identified two main modules: the pink module was similar while the brown module was distinct between the two cultivars. Moreover, we obtained 100 hub genes, including 11 important transcription factors (TFs) potentially involved in cold stress, 3 key TFs in the brown module and 8 key TFs in the pink module. More importantly, according to the genetic regulatory networks (GRNs) between TFs and other genes or TFs by using GENIE3, we identified 3 TFs (ABI3/VP1, ARR-B and WRKY) mainly functioning in differential cold responses between two cultivars, and 3 key TFs (GRAS, AP2-EREBP and C2H2) primarily involved in cold responses.

CONCLUSION

Collectively, our study provides valuable resources for transcriptome- based gene network studies of cold responses in tobacco. It helps to reveal how key cold responsive TFs or other genes are regulated through network. It also helps to identify the potential key cold responsive genes for the genetic manipulation of tobacco cultivars with enhanced cold tolerance in the future.

Overexpression of PvSTK1 gene from Switchgrass (Panicum virgatum L.) affects flowering time and development of floral organ in transgenic Arabidopsis thaliana

Flowering means that the plant enters the reproductive growth stage, which is a crucial stage in the plant life cycle. Delaying flowering time to prolong vegetative growth is an important method to increase biomass yield and saccharification efficiency in switchgrass, It is of great significance to study the molecular mechanism of plant flowering and regulate the process of plant flowering in the process of biomass production. In this study, we identified 55 serine/threonine-protein kinase genes related to flower development from the switchgrass transcriptome database. Simultaneously, we cloned one of them, PvSTK1, whose expression level and differential fold were significantly higher than other members. PvSTK1 is located on chromosome 8N and its protein was in the cell membrane, cytoplasm, and nucleus. The spatio-temporal expression analysis of the PvSTK1 in switchgrass displayed that the PvSTK1 is crucial in vegetative period, however, not in the transition to reproductive period. Overexpression of PvSTK1 in Arabidopsis resulted in down-regulation of flower-promoting genes and up-regulation of flower-suppressing genes, thereby delaying flowering. In addition, PvSTK1 caused atrophy of the ovules of the florets at the base of the inflorescence, leading to sterility of the florets. The function of PvSTK1 is to inhibit the development of floral organs, and its overexpression can prolong its vegetative period. In the future, overexpression of the PvSTK1 gene in switchgrass will change the flowering time and increase yield and utilization efficiency of biomass.

Genome-Wide Analysis of the Protein Phosphatase 2C Genes in Tomato

The plant protein phosphatase 2C (PP2C) plays an irreplaceable role in phytohormone signaling, developmental processes, and manifold stresses. However, information about the PP2C gene family in tomato (Solanum lycopersicum) is relatively restricted. In this study, a genome-wide investigation of the SlPP2C gene family was performed. A total of 92 SlPP2C genes were identified, they were distributed on 11 chromosomes, and all the SlPP2C proteins have the type 2C phosphatase domains. Based on phylogenetic analysis of PP2C genes in Arabidopsis, rice, and tomato, SlPP2C genes were divided into eight groups, designated A–H, which is also supported by the analyses of gene structures and protein motifs. Gene duplication analysis revealed that the duplication of whole genome and chromosome segments was the main cause of SLPP2Cs expansion. A total of 26 cis-elements related to stress, hormones, and development were identified in the 3 kb upstream region of these SlPP2C genes. Expression profile analysis revealed that the SlPP2C genes display diverse expression patterns in various tomato tissues. Furthermore, we investigated the expression patterns of SlPP2C genes in response to Ralstonia solanacearum infection. RNA-seq and qRT-PCR data reveal that nine SlPP2Cs are correlated with R. solanacearum. The above evidence hinted that SlPP2C genes play multiple roles in tomato and may contribute to tomato resistance to bacterial wilt. This study obtained here will give an impetus to the understanding of the potential function of SlPP2Cs and lay a solid foundation for tomato breeding and transgenic resistance to plant pathogens.

Transcriptome profiling of Arabidopsis thaliana roots in response to allelopathic effects of Conyza canadensis

The molecular mechanisms underlying allelopathy and their role in the interactions between invasive weeds and native species remain unclear. In this study, we aimed to explore the physiological and molecular response of plant roots of a native species to allelopathy from an invasive weed. We examined the growth and development of roots of native Arabidopsis thaliana for a 2-week period after being treated with aqueous extracts at different concentrations from invasive Conyza canadensis. Extracts with higher concentration in the Murashige and Skoog (MS) media (i.e., 4 mg of extract/mL of MS) significantly affected the root growth of A. thaliana. Roots of A. thaliana displayed weakened root tip activity and an accumulation of reactive oxygen species (ROS) in response to extracts from C. canadensis. The transcriptome analysis of A. thaliana roots exposed to phytotoxicity revealed differentially expressed genes (DEGs) involved in cell wall formation, abiotic stress, transporter genes and signal transduction. We found that genes associated with nutrient transport, such as major facilitator superfamily (MFS) and amino acid permease (AAP3) transporters as well as genes involved in stress response, including leucine-rich repeat receptor-like protein kinases (LRR-RLKs) were down-regulated. In addition, we found that many transcription factors associated with plant stress (such as APETALA2/ethylene response factors) were up-regulated while others (e.g., zinc-finger proteins) were down-regulated. Allelochemicals from C. canadensis also induced the up-regulation of detoxification (DTX) genes, ROS related genes, calcineurin B-like interacting protein kinases (CIPKs) and calmodulin. Overall, our findings provided insights into allelopathy in C. canadensis at the molecular level, and contributes to our understanding of invasion mechanisms of alien plant species. CLINICAL TRIALS REGISTRATION: This study does not contain any studies with clinical trials performed by any of the authors.

Virus-induced gene silencing (VIGS) analysis shows involvement of the LsSTPK gene in lettuce (Lactuca sativaL.) in high temperature-induced bolting

To determine the effect of the serine/threonine protein kinase (STPK) gene on leaf lettuce bolting, we utilized virus-induced gene silencing (VIGS) using the TRV vector to silence the target gene. The 'GB30' leaf lettuce cultivar was the test material, and the methods included gene cloning, bioinformatics analysis, quantitative real-time PCR (qRT-PCR) and VIGS. LsSTPK, was cloned from the 'GB30' leaf lettuce cultivar via reverse transcription-polymerase chain reaction (RT-PCR). qRT-PCR analysis showed that the expression of LsSTPK in the stem of leaf lettuce was significantly greater than that in the roots and leaves, and after high-temperature treatment, the gene expression in the stems in the experimental group was markedly lower than that in the control groups. Following LsSTPK silencing via the VIGS method, the stem length in the treatment group was significantly greater than that in the blank and negative control groups, and the contents of auxin (IAA), GA3 and abscisic acid (ABA) in the treatment group were greater than those in the other two groups. Flower bud differentiation occurred in the treatment group but not in the control group. The above findings suggested that LsSTPK inhibits the bolting of leaf lettuce under high-temperature conditions.

A leucine-rich repeat receptor-like kinase, OsSTLK, modulates salt tolerance in rice

Leucine-rich repeat receptor-like kinases (LRR-RLKs) have been widely associated with plant abiotic stress responses. However, the functions of the majority of LRR-RLKs has not been well defined. Here, we identified a novel rice LRR-RLK member involved in salt tolerance and designated as OsSTLK (Oryza sativa L. Salt-Tolerance LRR-RLK). Transcript analysis showed that OsSTLK was significantly induced in response to salt stress in rice shoot and root in a time and dosage-dependent fashion. Phenotypic observations indicated that OsSTLK overexpression exhibited reduced salt sensitivity, and improved salt stress tolerance. Further physiological analysis showed that OsSTLK overexpression remarkably reduced electrolyte leakage, malondialdehyde (MDA) content, reactive oxygen species (ROS) accumulation under salt stress conditions by up-regulating ROS-scavenging activities and modifying stomatal patterning. Moreover, Na⁺/K⁺ ratio and MAPK phosphorylation level were also reduced in OsSTLK-overexpression transgenic rice plants compared with WT control. Taken together, our findings suggested that OsSTLK as an important positive regulator of salt stress tolerance perhaps through regulating ROS scavenging system, Na⁺/K⁺ ratio and MAPK signal pathway.

Molecular Characterization of the Transcription Factors in Susceptible Poplar Infected with Virulent Melampsora larici-populina

Transcription factors (TFs) have been shown to play important roles in determining poplar susceptibility. In this study, the transcript profiles of five resistance-related TF groups at different time points were investigated to study the roles of TFs in the compatible interaction between 'Robusta' (Populus nigra × P. deltoides) and the virulent E4 race of Melampsora larici-populina. The susceptibility test indicated that the parasitic process of E4 could be divided into two representative time periods: the infection phase and the production phase. Bioinformatics analysis showed that in these two phases, E4 infection induced a network of TFs in 'Robusta'. Although some TFs responded rapidly and positively, most TFs did not respond to E4, especially during the infection phase. The ethylene, jasmonic acid, and auxin pathways were downregulated, while a calcium-binding protein was upregulated. No other significantly changed phytohormone-related genes were found, which was consistent with the pathological process in the absence of an immune response, suggesting that the lack of response of most TFs during the infection phase of E4 is related to the susceptibility of 'Robusta'.

Control of plant growth and development by overexpressing MAP3K17, an ABA-inducible MAP3K, in Arabidopsis

Abscisic acid (ABA) plays an important role in plant growth, development, and stress responses. ABA regulates many aspects of plant growth and development, including seed maturation, dormancy, germination, the transition from vegetative to reproductive growth, leaf senescence and responses to environmental stresses, such as drought, high salinity and cold. It is also known that mitogen-activated protein kinase (MAPK) cascades function in ABA signaling. Recently, we and another group have identified the ABA-inducible MAP3Ks MAP3K17 and MAP3K18 as the upstream MAP3Ks of MKK3, implicating the MAP3K17/18-MKK3-MPK1/2/7/14 cascade in ABA signaling. It has also been reported that overexpression of MAP3K18 in Arabidopsis causes an early leaf senescence phenotype, ABA hypersensitive stomata closing, and drought tolerance. In this study, we generated transgenic plants overexpressing MAP3K17 (35S:MAP3K17) and its kinase-inactive form (35S:MAP3K17KN). The bolting of 35S:MAP3K17 was earlier than WT, and the fresh weights of the seedlings were smaller, whereas 35S:MAP3K17KN showed the opposite phenotype. These results indicate that the transition from vegetative to reproductive growth can be regulated by overexpression of MAP3K17 and its kinase-inactive form. Moreover, 35S:MAP3K17 showed lower sensitivity to ABA during post-germinated growth, whereas 35S:MAP3K17 KN showed the opposite phenotype, suggesting the negative roles of MAP3K17 in the response to ABA. Our work provides the possibility to regulate plant growth and development by the genetic manipulation of ABA-induced MAPK cascades, leading to improved crop growth and productivity.

Redox Regulation, Rather than Stress-Induced Phosphorylation, of a Hog1 Mitogen-Activated Protein Kinase Modulates Its Nitrosative-Stress-Specific Outputs

In all eukaryotic kingdoms, mitogen-activated protein kinases (MAPKs) play critical roles in cellular responses to environmental cues. These MAPKs are activated by phosphorylation at highly conserved threonine and tyrosine residues in response to specific inputs, leading to their accumulation in the nucleus and the activation of their downstream targets. A specific MAP kinase can regulate different downstream targets depending on the nature of the input signal, thereby raising a key question: what defines the stress-specific outputs of MAP kinases? We find that the Hog1 MAPK contributes to nitrosative-stress resistance in Candida albicans even though it displays minimal stress-induced phosphorylation under these conditions. We show that Hog1 becomes oxidized in response to nitrosative stress, accumulates in the nucleus, and regulates the nitrosative stress-induced transcriptome. Mutation of specific cysteine residues revealed that C156 and C161 function together to promote stress resistance, Hog1-mediated nitrosative-stress-induced gene expression, resistance to phagocytic killing, and C. albicans virulence. We propose that the oxidation of Hog1, rather than its phosphorylation, contributes to the nitrosative-stress-specific responses of this MAP kinase.

Mitogen-activated protein kinases play key roles in the responses of eukaryotic cells to extracellular signals and are critical for environmental-stress resistance. The widely accepted paradigm is that MAP kinases are activated by phosphorylation, which then triggers their nuclear accumulation and the activation of target proteins and genes that promote cellular adaptation. Our data suggest that alternative forms of posttranslational modification can modulate MAP kinase functionality in Candida albicans. We demonstrate that Hog1 is not significantly phosphorylated in response to nitrosative stress, yet it displays nuclear accumulation and contributes to the global transcriptional response to this stress, as well as promoting nitrosative-stress resistance. Instead, nitrosative stress triggers changes in the redox status of Hog1. We also show that specific Hog1 cysteine residues influence its activation of stress genes. Therefore, alternative posttranslational modifications appear to regulate the stress-specific outputs of MAP kinases.

Transcript Profiling and Gene Identification Involved in the Ethylene Signal Transduction Pathways of Creeping Bentgrass (Agrostis stolonifera) during ISR Response Induced by Butanediol

Creeping bentgrass (Agrostis stolonifera) is the preferred green lawn grass, with excellent turf characteristics but poor disease resistance. At present, the mechanisms of disease resistance in creeping bentgrass are poorly understood, especially the ethylene signal transduction pathway under the induced systemic resistance (ISR) response. In this study, butanediol (BDO), as a new type of disease-resistance compound, was applied to creeping bentgrass seedlings to induce the ISR response. Then, we measured ethylene production and related enzyme activities. Additionally, transcript profiling and gene identification were performed in association to ethylene signal transduction pathways. The changes of ethylene production and related enzyme 1-aminocyclopropane-1-carboxylic acid oxidase (ACO) and 1-aminocyclopropane-1-carboxylic acid synthases (ACS) activities showed significant difference at 24 h after Rhizoctonia solani inoculation among five treatments of various BDO concentrations. After 100 µmol L^-1 BDO treatment, ethylene production and related enzyme activities reached their peak levels. Additionally, 208,672 unigenes of creeping bentgrass were obtained by de novo assembly. In total, 15,903 annotated unigenes were grouped into 33 canonical pathways in the KEGG (Kyoto Encyclopedia of Genes and Genomes) analysis. Among those, 1803 unigenes were classified as 'signal transduction'. There were 6766 differentially expressed genes (DEGs) among B24 (inoculated-rhizobacteria in MS medium with 100 µmol L^-1 BDO for 24 h), NB24, B72 and NB24 (no rhizobacteria in MS medium with 100 µmol L^-1 BDO for 24 h) libraries, and 4,639 DEGs between B24 and B72 (inoculated-rhizobacteria in MS medium with 100 µmol L^-1 BDO for 72 h) libraries, with 4489 DEGs in all three libraries. As suggested by the RT-PCR assay, the expression levels of ethylene-responsive and defense-related genes were variable among treated samples during the BDO-induced ISR responses. The expression levels of EIN, ERF, NPR1, PR3 and PR4 genes increased and reached their peaks in the first 24 h after R. solani infection in the BDO-induced ISR reaction compared with NB24 treatments. This results is consistent with the changes of important ethylene biosynthetic enzymes and ethylene concentrations during the BDO-induced ISR responses. We further found the intermediate substances for the signaling pathway, and the relationships between the expression levels of BDO-induced ISR disease-resistance genes and those of the response genes for ethylene signal pathway. Our findings present a genetic basis for systemic resistance of creeping bentgrass through transcriptomic analysis and our study provides a theoretical and practical basis for the improvement of turfgrass disease resistance and quality.

Genome-wide identification and evolutionary analyses of the PP2C gene family with their expression profiling in response to multiple stresses in Brachypodium distachyon

Background

The type-2C protein phosphatases (PP2Cs), negatively regulating ABA responses and MAPK cascade pathways, play important roles in stress signal transduction in plants. Brachypodium distachyon is a new model plant for exploring the functional genomics of temperate grasses, cereals and biofuel crops. To date, genome-wide identification and analysis of the PP2C gene family in B. distachyon have not been investigated.

Results

In this study, 86 PP2C genes in B. distachyon were identified. Domain-based analyses of PP2C proteins showed that they all contained the phosphatase domains featured as 11 conserved signature motifs. Although not all phosphatase domains of BdPP2C members included all 11 motifs, tertiary structure analysis showed that four residues contributing to magnesium/manganese ions (Mg²⁺/Mn²⁺) coordination were conserved, except for two noncanonical members. The analysis of their chromosomal localizations showed that most of the BdPP2C genes were located within the low CpG density region. Phylogenetic tree and synteny blocks analyses among B. distachyon, Arabidopsis thaliana and Oryza sativa revealed that all PP2C members from the three species can be phylogenetically categorized into 13 subgroups (A–M) and BdPP2Cs were evolutionarily more closely related to OsPP2Cs than to AtPP2Cs. Segmental duplications contributed particularly to the expansion of the BdPP2C gene family and all duplicated BdPP2Cs evolved mainly from purifying selection. Real-time quantitative reverse transcription PCR (qRT-PCR) analysis showed that BdPP2Cs were broadly expressed in disparate tissues. We also found that almost all members displayed up-regulation in response to abiotic stresses such as cold, heat, PEG and NaCl treatments, but down-regulation to biotic stresses such as Ph14, Guy11 and F0968 infection.

Conclusions

In the present study, a comprehensive analysis of genome-wide identification and characterization of protein domains, phylogenetic relationship, gene and protein structure, chromosome location and expression pattern of the PP2C gene family was carried out for the first time in a new model monocot, i.e., B. distachyon. Our results provide a reference for genome-wide identification of the PP2C gene family of other species and also provide a foundation for future functional research on PP2C genes in B. distachyon.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2526-4) contains supplementary material, which is available to authorized users.

Mitogen-activated protein kinase kinase 5 (MKK5)-mediated signalling cascade regulates expression of iron superoxide dismutase gene in Arabidopsis under salinity stress

Superoxide dismutases (SODs) are involved in plant adaptive responses to biotic and abiotic stresses but the upstream signalling process that modulates their expression is not clear. Expression of two iron SODs, FSD2 and FSD3, was significantly increased in Arabidopsis in response to NaCl treatment but blocked in transgenic MKK5-RNAi plant, mkk5. Using an assay system for transient expression in protoplasts, it was found that mitogen-activated protein kinase kinase 5 (MKK5) was also activated in response to salt stress. Overexpression of MKK5 in wild-type plants enhanced their tolerance to salt treatments, while mkk5 mutant exhibited hypersensitivity to salt stress in germination on salt-containing media. Moreover, another kinase, MPK6, was also involved in the MKK5-mediated iron superoxide dismutase (FSD) signalling pathway in salt stress. The kinase activity of MPK6 was totally turned off in mkk5, whereas the activity of MPK3 was only partially blocked. MKK5 interacted with the MEKK1 protein that was also involved in the salt-induced FSD signalling pathway. These data suggest that salt-induced FSD2 and FSD3 expressions are influenced by MEKK1 via MKK5-MPK6-coupled signalling. This MAP kinase cascade (MEKK1, MKK5, and MPK6) mediates the salt-induced expression of iron superoxide dismutases.

Lead induced changes in phosphorylation of PSII proteins in low light grown pea plants

Light-intensity and redox-state induced thylakoid proteins phosphorylation involved in structural changes and in regulation of protein turnover. The presence of heavy metal ions triggers a wide range of cellular responses including changes in plant growth and photosynthesis. Plants have evolved a number of mechanisms to protect photosynthetic apparatus. We have characterized the effect of lead on PSII protein phosphorylation in pea (Pisum sativum L.) plants grown in low light conditions. Pb ions affected only slightly photochemical efficiency of PSII and had no effect on organization of thylakoid complexes. Lead activated strongly phosphorylation of PSII core D1 protein and dephosphorylation of this protein did not proceed in far red light. D1 protein was also not degraded in this conditions. However, phosphorylation of LHCII proteins was not affected by lead. These results indicate that Pb(2+) stimulate the phosphorylation of PSII core proteins and by disturbing the disassembly of supercomplexes play a role in PSII repair mechanism. LHCII phosphorylation could control the distribution of energy between the photosystems in low light conditions. This demonstrates that plants may respond to heavy metals by induction different pathways responsible for protein protection under stress conditions.

Analysis of global gene expression profile of rice in response to methylglyoxal indicates its possible role as a stress signal molecule

Methylglyoxal (MG) is a toxic metabolite produced primarily as a byproduct of glycolysis. Being a potent glycating agent, it can readily bind macromolecules like DNA, RNA, or proteins, modulating their expression and activity. In plants, despite the known inhibitory effects of MG on growth and development, still limited information is available about the molecular mechanisms and response pathways elicited upon elevation in MG levels. To gain insight into the molecular basis of MG response, we have investigated changes in global gene expression profiles in rice upon exposure to exogenous MG using GeneChip microarrays. Initially, growth of rice seedlings was monitored in response to increasing MG concentrations which could retard plant growth in a dose-dependent manner. Upon exposure to 10 mM concentration of MG, a total of 1685 probe sets were up- or down-regulated by more than 1.5-fold in shoot tissues within 16 h. These were classified into 10 functional categories. The genes involved in signal transduction such as, protein kinases and transcription factors, were significantly over-represented in the perturbed transcriptome, of which several are known to be involved in abiotic and biotic stress response indicating a cross-talk between MG-responsive and stress-responsive signal transduction pathways. Through in silico studies, we could predict 7-8 bp long conserved motif as a possible MG-responsive element (MGRE) in the 1 kb upstream region of genes that were more than 10-fold up- or down-regulated in the analysis. Since several perturbations were found in signaling cascades in response to MG, we hereby suggest that it plays an important role in signal transduction probably acting as a stress signal molecule.

Molecular cloning and expression analysis of mulberry MAPK gene family

Mitogen-activated protein kinase (MAPK) cascades play an important role in regulating various biotic and abiotic stresses in plants. Although MAPKs have been identified and characterized in a few model plants, there is little information available for mulberry Morus sp. L., one of the most ecologically and economically important perennial trees. This study identified 47 mulberry Morus notabilis MAPK (MnMAPK) family genes: 32 MnMAPKKK, five MnMAPKK and ten MnMAPK genes, and cloned ten MnMAPK cDNA genes based on a genome-wide analysis of the morus genome database. Comparative analysis with MAPK gene families from other plants suggested that MnMAPKs could be divided into five subfamilies (groups A, B, C, D and E) and they could have similar functions in response to abiotic and biotic stresses. MnMAPK gene expression analysis of different stresses (high/low temperature, salt and drought) and signal molecules (ABA, SA, H2O2 and methyl jasmonate (MeJA)) revealed that all ten MnMAPK genes responded to high/low temperature, salt and drought stresses, and that nine of the ten MnMAPKs (MnMAPK7 excepted) could be induced by ABA, SA, H2O2 and MeJA, which suggested that MnMAPKs may play pivotal roles in signal transduction pathways. Our results indicated that almost all of the MnMAPKs may be involved in environmental stress and defense responses, which provides the basis for further characterization of the physiological functions of MnMAPKs.

Modeling of the MAPK machinery activation in response to various abiotic and biotic stresses in plants by a system biology approach

Mitogen-Activated Protein Kinases (MAPKs) cascade plays an important role in regulating plant growth and development, generating cellular responses to the extracellular stimuli. MAPKs cascade mainly consist of three sub-families i.e. mitogen-activated protein kinase kinase kinase (MAPKKK), mitogen-activated protein kinase kinase (MAPKK) and mitogen activated protein kinase (MAPK), several cascades of which are activated by various abiotic and biotic stresses. In this work we have modeled the holistic molecular mechanisms essential to MAPKs activation in response to several abiotic and biotic stresses through a system biology approach and performed its simulation studies. As extent of abiotic and biotic stresses goes on increasing, the process of cell division, cell growth and cell differentiation slow down in time dependent manner. The models developed depict the combinatorial and multicomponent signaling triggered in response to several abiotic and biotic factors. These models can be used to predict behavior of cells in event of various stresses depending on their time and exposure through activation of complex signaling cascades.

MAPK machinery in plants: recognition and response to different stresses through multiple signal transduction pathways

The mitogen-activated protein kinase (MAPK) cascades play diverse roles in intra- and extra-cellular signaling in plants. MAP kinases are the component of kinase modules which transfer information from sensors to responses in eukaryotes including plants. They play a pivotal role in transduction of diverse extracellular stimuli such as biotic and abiotic stresses as well as a range of developmental responses including differentiation, proliferation and death. Several cascades are induced by different biotic and abiotic stress stimuli such as pathogen infections, heavy metal, wounding, high and low temperatures, high salinity, UV radiation, ozone, reactive oxygen species, drought and high or low osmolarity. MAPK signaling has been implicated in biotic stresses and has also been associated with hormonal responses. The cascade is regulated by various mechanisms, including not only transcriptional and translational regulation but through post-transcriptional regulation such as protein-protein interactions. Recent detailed analysis of certain specific MAP kinase pathways have revealed the specificity of the kinases in the cascade, signal transduction patterns, identity of pathway targets and the complexity of the cascade. The latest insights and finding are discussed in this paper in relation to the role of MAPK pathway modules in plant stress signaling.

Molecular characterization of the Salicornia brachiata SbMAPKK gene and its expression by abiotic stress

MAPK cascade is an important intracellular signaling module and function as a convergent point for crosstalk during abiotic stress signaling. In this study SbMAPKK gene has been isolated from Salicornia brachiata, a highly salt tolerant plant growing in costal marshes of Gujarat, India. The SbMAPKK gene is 1,023 bp long, encodes a 340 amino acid protein with an estimated molecular mass of 37.4 kDa. The SbMAPKK shows high sequence identity with NbMKK1 from N. benthamiana, LeMKK4 from Lycopersicon esculentum. SbMAPKK constitutes 11 conserved subdomains of protein kinase. Northern analysis revealed that SbMAPKK transcript expression is induced by different stresses like dehydration, cold and salt, however, maximum expression is observed during cold stress. The phylogenetic analysis and genomic organization confirms that it is an intron less gene belonging 'D' group in MAPKK family.

AtMKK1 and AtMPK6 are involved in abscisic acid and sugar signaling in Arabidopsis seed germination

Abscisic acid (ABA) and sugars have been well established to be crucial factors controlling seed germination of Arabidopsis. Here we demonstrate that AtMKK1 and AtMPK6 are both critical signals involved in ABA and sugar-regulated seed germination. Wild type plants depended on stratification and after-ripening for seed germination, whereas this dependence on either stratification or after-ripening was not required for mutants of mkk1 and mpk6 as well as their double mutant mkk1 mpk6. While seed germination of wild type plants was sensitively inhibited by ABA and glucose, mkk1, mpk6 and mkk1 mpk6 were all strongly resistant to ABA or glucose treatments, and in contrast, plants overexpressing MKK1 or MPK6 were super-sensitive to ABA and glucose. Glucose treatment significantly induced increases in MKK1 and MPK6 activities. These results clearly indicate that MKK1 and MPK6 are involved in the ABA and sugar signaling in the process of seed germination. Further experiments showed that glucose was capable of inducing ABA biosynthesis by up-regulating NCED3 and ABA2, and furthermore, this up-regulation of NCED3 and ABA2 was arrested in the mkk1 mpk6 double mutant, indicating that the inhibition of seed germination by glucose is potentially resulted from sugar-induced up-regulation of the ABA level.

AtMKK1 mediates ABA-induced CAT1 expression and H2O2 production via AtMPK6-coupled signaling in Arabidopsis

Catalase controls cellular H(2)O(2) and plays important roles in the adaptation of plants to various stresses, but little is known about the signaling events that lead to the expression of CAT1 and the production of H(2)O(2). Here we report the dependence of CAT1 expression and H(2)O(2) production on a mitogen-activated protein kinase (MAPK) cascade. CAT1 transcript was induced in an ABA-dependent way and the induction was abolished in the T-DNA insertion mutant mkk1 (SALK_015914), while AtMKK1 overexpression significantly enhanced the ABA-induced CAT1 expression and H(2)O(2) production. AtMPK6, another component in the MAPK cascade, was also involved: mpk6 mutant blocked and overexpressing AtMPK6 enhanced the ABA-dependent expression of CAT1 and H(2)O(2) production. The activity of AtMPK6 was increased by ABA in an AtMKK1-dependent manner. These data clearly suggest an ABA-dependent signaling pathway connecting CAT1 expression through a phosphorelay including AtMKK1 and AtMPK6. In further support of this view, mkk1 mutant reduced both the sensitivity to ABA during germination and the drought tolerance of seedlings, whereas the AtMKK1 overexpression line showed the opposite responses when compared with the wild type. The data suggest AtMKK1-AtMPK6 to be a key module in an ABA-dependent signaling cascade causing H(2)O(2) production and stress responses.

MAPK phosphorylation-induced stabilization of ACS6 protein is mediated by the non-catalytic C-terminal domain, which also contains the cis-determinant for rapid degradation by the 26S proteasome pathway

Ethylene is an important hormone in plant growth, development and responses to environmental stimuli. The ethylene-signaling pathway is initiated by the induction of ethylene biosynthesis, which is under tight regulation at both transcriptional and post-transcriptional levels by exogenous and endogenous cues. 1-Aminocyclopropane-1-carboxylic acid synthase (ACS) is the rate-limiting enzyme that catalyzes the committing step of ethylene biosynthesis. Recently, we found that ACS2 and ACS6, two isoforms of the Arabidopsis ACS family, are substrates of a stress-responsive mitogen-activated protein kinase (MAPK) cascade. Phosphorylation of ACS2/ACS6 by MPK6 leads to the accumulation of ACS proteins and the induction of ethylene. In this report, we demonstrate that unphosphorylated ACS6 protein is rapidly degraded by the 26S proteasome pathway. The degradation machinery targets the C-terminal non-catalytic domain of ACS6, which is sufficient to confer instability to green fluorescent protein and luciferase reporters. Phosphorylation of ACS6 introduces negative charges to the C-terminus of ACS6, which reduces the turnover of ACS6 by the degradation machinery. Consistent with this, other nearby conserved negatively charged amino acid residues are essential for ACS6 stability regulation. Protein degradation and phosphorylation are two important post-translational modifications of proteins. This research reveals an intricate interplay between these two important processes in controlling the levels of cellular ACS activity, and thus ethylene biosynthesis. The post-translational nature of both processes ensures a rapid response of ethylene induction, which is detectable within minutes after plants are exposed to stress.

Functional assignment of MAPK phosphatase domains

Mitogen-activated protein kinase (MAPK) pathways are well conserved in most organisms, from yeast to humans. The principal components of these pathways are MAP kinases whose activity is regulated by phosphorylation, implicating various MAPK protein effectors-in particular, protein phosphatases that inactivate MAPKs by dephosphorylation. The molecular basis of binding specificity of such regulatory phosphatases to MAPKs is poorly understood. To try to pinpoint potential functional regions within the sequences and to help identify new family members, we have applied a multimotif pattern-recognition approach to characterize two MAPK phosphatase subfamilies (tyrosine-specific and dual specificity) that are crucial in the regulation of MAPKs. We built "fingerprints" for these two subfamilies that are unique to, and highly discriminatory for, each group of proteins. The fingerprints were used in a genome-wide screen, identifying more than 80 MAPK phosphatase domains, several of which were in partial sequences or unclassified proteins. We confirmed experimentally that one predicted MAPK phosphatase orthologue in Xenopus binds to ERK1/2, suggesting a role in MAPK signaling and thus supporting our functional predictions. Further analysis, mapping the fingerprints on the three-dimensional structure of MAPK phosphatases, revealed that some of the fingerprint motifs reside in the N-terminal noncatalytic regions coinciding with reported MAPK binding sites, while others lie within the catalytic phosphatase domain. These results also suggest the presence of putative allosteric sites in the catalytic region for modulation of protein-protein interactions, and provide a framework for future experimental validation.

Chloroplast-generated reactive oxygen species are involved in hypersensitive response-like cell death mediated by a mitogen-activated protein kinase cascade

Plant defense against pathogens often includes rapid programmed cell death known as the hypersensitive response (HR). Recent genetic studies have demonstrated the involvement of a specific mitogen-activated protein kinase (MAPK) cascade consisting of three tobacco MAPKs, SIPK, Ntf4 and WIPK, and their common upstream MAPK kinase (MAPKK or MEK), NtMEK2. Potential upstream MAPKK kinases (MAPKKKs or MEKKs) in this cascade include the orthologs of Arabidopsis MEKK1 and tomato MAPKKKalpha. Activation of the SIPK/Ntf4/WIPK pathway induces cell death with phenotypes identical to pathogen-induced HR at macroscopic, microscopic and physiological levels, including loss of membrane potential, electrolyte leakage and rapid dehydration. Loss of membrane potential in NtMEK2(DD) plants is associated with the generation of reactive oxygen species (ROS), which is preceded by disruption of metabolic activities in chloroplasts and mitochondria. We observed rapid shutdown of carbon fixation in chloroplasts after SIPK/Ntf4/WIPK activation, which can lead to the generation of ROS in chloroplasts under illumination. Consistent with a role of chloroplast-generated ROS in MAPK-mediated cell death, plants kept in the dark do not accumulate H(2)O(2) in chloroplasts after MAPK activation, and cell death is significantly delayed. Similar light dependency was observed in HR cell death induced by tobacco mosaic virus, which is known to activate the same MAPK pathway in an N-gene-dependent manner. These results suggest that activation of the SIPK/Ntf4/WIPK cascade by pathogens actively promotes the generation of ROS in chloroplasts, which plays an important role in the signaling for and/or execution of HR cell death in plants.

Arabidopsis EIN2 modulates stress response through abscisic acid response pathway

The nuclear protein ETHYLENE INSENSITIVE2 (EIN2) is a central component of the ethylene signal transduction pathway in plants, and plays an important role in mediating cross-links between several hormone response pathways, including abscisic acid (ABA). ABA mediates stress responses in plants, but there is no report on the role of EIN2 on plant response to salt and osmotic stresses. Here, we show that EIN2 gene regulates plant response to osmotic and salt stress through an ABA-dependent pathway in Arabidopsis. The expression of the EIN2 gene is down-regulated by salt and osmotic stress. An Arabidopsis EIN2 null mutant was supersensitive to both salt and osmotic stress conditions. Disruption of EIN2 specifically altered the expression pattern of stress marker gene RD29B in response to the stresses, but not the stress- or ABA-responsive genes RD29A and RD22, suggesting EIN2 modulates plant stress responses through the RD29B branch of the ABA response. Furthermore, disruption of EIN2 caused substantial increase in ABA. Lastly, our data showed that mutations of other key genes in ethylene pathway also had altered sensitivity to abiotic stresses, indicating that the intact ethylene may involve in the stress response. Taken together, the results identified EIN2 as a cross-link node in ethylene, ABA and stress signaling pathways, and EIN2 is necessary to induce developmental arrest during seed germination, and seedling establishment, as well as subsequent vegetative growth, thereby allowing the survival and growth of plants under the adverse environmental conditions.

Computational identification and phylogenetic analysis of the MAPK gene family in Oryza sativa

The MAPK cascade plays greatly important roles in signal transduction pathways. The present study computationally identified 16 rice MAPKs (OsMPKs). The results of EST and cDNA hitting supported the reliability of OsMPKs in rice. Gene structure comparison showed large differences in exon numbers, from 2 to 12, among members of the rice MAPK family. Rice MAPKs were located on chromosomes 1, 2, 3, 5, 6, 10, and 11, particularly being distributed on the 1, 5 and 6 chromosomes. On a genome scale we revealed that the rice MAPK family should have evolved through segmental duplication produced by polyploidy, rather than through tandem amplification. Phylogenetic analysis divided the plant MAPK family members into four distinct groups (A-D), supported by highly significant bootstrap values. 11 out of the 16 OsMPKs belonged to group D, suggesting dramatic evolutionary expansion of this group and rapid gene losses in other groups. The divergence between and within plant MAPK groups should predate the monocot-dicot split. Notably, the divergences between plant, animal, fungi and parasite MAPKs were apparent, although MAP kinases might be conserved during long space of evolutionary time. Moreover, some orthologs and paralogs could be identified from the phylogenetic tree. It is suggested that members within each group might serve similar functions in different species. Thus, the annotation of published MAPKs would greatly facilitate the functional investigation of uncharacterized MAP kinases. However, group D genes were more complicated, and required extensive additional studies. Overall, the first description of the whole MAPK family will be expected to result in significant progress in investigating the regulation mechanism of MAPKs in response to extracellular stimuli.

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.

Activation of Ntf4, a tobacco mitogen-activated protein kinase, during plant defense response and its involvement in hypersensitive response-like cell death

Mitogen-activated protein kinase (MAPK) cascades are important signaling modules in eukaryotic cells. They function downstream of sensors/receptors and regulate cellular responses to external and endogenous stimuli. Recent studies demonstrated that SIPK and WIPK, two tobacco (Nicotiana spp.) MAPKs, are involved in signaling plant defense responses to various pathogens. Ntf4, another tobacco MAPK that shares 93.6% and 72.3% identity with SIPK and WIPK, respectively, was reported to be developmentally regulated and function in pollen germination. We found that Ntf4 is also expressed in leaves and suspension-cultured cells. Genomic analysis excluded the possibility that Ntf4 and SIPK are orthologs from the two parental lines of the amphidiploid common tobacco. In vitro and in vivo phosphorylation and activation assays revealed that Ntf4 shares the same upstream MAPK kinase, NtMEK2, with SIPK and WIPK. Similar to SIPK and WIPK, Ntf4 is also stress responsive and can be activated by cryptogein, a proteinaceous elicitin from oomycetic pathogen Phytophthora cryptogea. Tobacco recognition of cryptogein induces rapid hypersensitive response (HR) cell death in tobacco. Transgenic Ntf4 plants with elevated levels of Ntf4 protein showed accelerated HR cell death when treated with cryptogein. In addition, conditional overexpression of Ntf4, which results in high cellular Ntf4 activity, is sufficient to induce HR-like cell death. Based on these results, we concluded that Ntf4 is multifunctional. In addition to its role in pollen germination, Ntf4 is also a component downstream of NtMEK2 in the MAPK cascade that regulates pathogen-induced HR cell death in tobacco.

Molecular cloning and characterization of a novel MAP kinase gene in Chorispora bungeana

Chorispora bungeana Fisch. and C.A. Mey (Chorispora bungeana) is a rare alpine subnival plant species that is highly capable of resisting freezing environment. Since it is a stress-tolerant plant, we investigated the participation of mitogen-activated protein kinases (MAPKs) as possible mediators of abiotic stresses. We have isolated from Chorispora bungeana a new MAPK cDNA CbMAPK3 which encodes a 369 amino-acid protein with moderate to high nucleotide sequence similarity to previously reported plant MAPK genes. CbMAPK3 contains all 11 of the MAPK conserved subdomains and the phosphorylation motif TEY. The transcripts of CbMAPK3 were detected and no tissue-specific expression were observed in both roots and leaves, The transcripts of CbMAPK3 accumulated highly and rapidly when Chorispora bungeana treated with cold (4 and -4 degrees C), ABA and salinity stress. These results indicate that the CbMAPK3 may play an important role in response to environmental stresses.

A sphingolipid elicitor-inducible mitogen-activated protein kinase is regulated by the small GTPase OsRac1 and heterotrimeric G-protein in rice 1[w]

Mitogen-activated protein kinase (MAPK) cascades are activated in plants during responses to pathogens or to pathogen-derived elicitors and mediate intracellular stress responses. Here, we show that a rice (Oryza sativa) MAPK, OsMAPK6, was posttranslationally activated in a cell culture by a sphingolipid elicitor. Suppression of OsMAPK6 expression by RNA interference resulted in a strong reduction of pathogen-induced Phe ammonia-lyase mRNA, whereas the mRNA level of another rice MAPK, OsMAPK5a, was highly increased. Silencing of a small GTPase, OsRac1, by RNA interference or loss-of-function mutation (d1) of the heterotrimeric G-protein alpha-subunit gene resulted in a strong reduction of the OsMAPK6 protein levels and of kinase activation by a sphingolipid elicitor. Furthermore, coimmunoprecipitation experiments with OsRac1 and OsMAPK6 proteins showed that OsMAPK6 is closely associated with the active form of OsRac1, but not with inactive forms of OsRac1. These results indicate that these two G-proteins regulate an elicitor-inducible MAPK in rice at the protein level.

Exposure to toxic concentrations of aluminum activates a MAPK-like protein in cell suspension cultures of Coffea arabica

Addition of a toxic concentration of aluminum (Al) to cell suspension cultures of Coffea arabica L. induced the rapid and transient activation of a protein kinase that phosphorylates myelin basic protein (MBP), as revealed by in-gel kinase assays. This enzyme with an apparent molecular mass of 58 kDa was activated shortly after cells were exposed to 50 microM AlCl(3), a concentration previously shown to produce toxicity in plant cells in vitro. The activity of this kinase dropped to basal levels after 20 min of Al addition; this activity is specific for MBP as it could not be detected when casein or histone H1 were used as substrates. Analysis of the same cell extracts with antibodies that specifically recognize bis-phosphorylated (active) mitogen-activated protein kinases (MAP kinases), revealed the presence of a phosphoprotein with an apparent molecular mass of 58 kDa, which showed the same response to Al as the protein kinase revealed by the in-gel kinase assays. Furthermore, immunoprecipitation with an antibody directed against mammalian MAP kinases depleted both the enzymatic activity and the phosphoprotein from the cell extracts, suggesting that the 58 kDa kinase and the 58 kDa phosphoprotein from C. arabica cells are the same protein, and that it can be actually a member of the MAP kinase family of protein kinases. Since its activity is enhanced dramatically after addition of AlCl(3) to the medium, we can speculate that Al toxicity in plants could be perceived through the MAP kinase signal transduction pathway.

Phosphorylation of 1-aminocyclopropane-1-carboxylic acid synthase by MPK6, a stress-responsive mitogen-activated protein kinase, induces ethylene biosynthesis in Arabidopsis

Mitogen-activated protein kinases (MAPKs) are implicated in regulating plant growth, development, and response to the environment. However, the underlying mechanisms are unknown because of the lack of information about their substrates. Using a conditional gain-of-function transgenic system, we demonstrated that the activation of SIPK, a tobacco (Nicotiana tabacum) stress-responsive MAPK, induces the biosynthesis of ethylene. Here, we report that MPK6, the Arabidopsis thaliana ortholog of tobacco SIPK, is required for ethylene induction in this transgenic system. Furthermore, we found that selected isoforms of 1-aminocyclopropane-1-carboxylic acid synthase (ACS), the rate-limiting enzyme of ethylene biosynthesis, are substrates of MPK6. Phosphorylation of ACS2 and ACS6 by MPK6 leads to the accumulation of ACS protein and, thus, elevated levels of cellular ACS activity and ethylene production. Expression of ACS6(DDD), a gain-of-function ACS6 mutant that mimics the phosphorylated form of ACS6, confers constitutive ethylene production and ethylene-induced phenotypes. Increasing numbers of stress stimuli have been shown to activate Arabidopsis MPK6 or its orthologs in other plant species. The identification of the first plant MAPK substrate in this report reveals one mechanism by which MPK6/SIPK regulates plant stress responses. Equally important, this study uncovers a signaling pathway that modulates the biosynthesis of ethylene, an important plant hormone, in plants under stress.

Cadmium activates a mitogen-activated protein kinase gene and MBP kinases in rice

Mitogen-activated protein kinase (MAPK) pathways are modules involved in the transduction of extracellular signals to intracellular targets in all eukaryotes. In plants, it has been evidenced that MAPKs play a role in the signaling of biotic and abiotic stresses, plant hormones, and cell cycle cues. However, the effect of heavy metals on plant MAPKs has not been well examined. The Northern blot analysis of OsMAPK mRNA levels has shown that only OsMAPK2, but not OsMAPK3 and OsMAPK4, expressed in suspension-cultured cells in response to 100-400 microM Cd treatments. The OsMAPK2 transcripts increased within 12 h upon 400 microM Cd treatment. In addition, we found that 42- and 50-kDa MBP kinases were significantly activated by Cd treatment in rice suspension-cultured cells. And 40-, 42-, 50- and 64-kDa MBP kinases were activated in rice roots. Furthermore, GSH inhibits Cd-induced 40-kDa MBP kinase activation. By immunoblot analysis and immunoprecipitation followed by in-gel kinase assay, we confirmed that Cd-activated 42-kDa MBP kinase is a MAP kinase. Our results suggest that a MAP kinase cascade may function in the Cd-signalling pathway in rice.

MAP kinase-like activity in transformed Catharanthus roseus hairy roots varies with culture conditions such as temperature and hypo-osmotic shock

Mitogen activated protein (MAP) kinase-like activity was determined in extracts obtained from transformed Catharanthus roseus hairy roots by the ability to phosphorylate myelin basic protein (MBP). Both in solution and in gel kinase assays showed variation in activity, depending on root developmental stage. In gel kinase assays, using the extract soluble fraction, revealed a 56 kDa polypeptide with phosphorylation activity on MBP. In addition, another 75 kDa polypeptide was observed in the particulate fraction. Immunodetection with monoclonal antibodies against ERK-1, a mammalian MAP kinase, and with anti-phosphotyrosine antibodies cross-reacted with the 56 kDa polypeptide, named SMK56, from the soluble fraction, suggesting that this polypeptide could be related with members of the MAP kinase family. Antibodies against the dually phosphorylated threonine-tyrosine motif, characteristic of active forms of MAP kinases, also cross-reacted with this 56 kDa polypeptide. Changes in the levels of SMK56 were detected within the first 30 min of root exposure to low temperatures or hypo-osmotic shock, suggesting that this protein may be involved in the perception of environmental changes.

Plant sensitivity to low intensity 105 GHz electromagnetic radiation

Exposing seedlings of the flax, Linum usitatissimum L., to a variety of weak environmental stresses followed by a 2 day calcium deprivation, triggers the common response of production of epidermal meristems (actively dividing groups of cells) in the hypocotyl, which is the part of the stem between the root and the cotyledons (the pre-existing leaves in the embryo). This production reaches a plateau of 10-20 meristems after a month in the case of mechanical stimulation and cold shock. Recently, we have shown that radiation from a global system for mobile communication (GSM) telephone also triggers production of meristems with a plateau of around six meristems. Here, we show that a single 2 h exposure to radiation emitted at 105 GHz at non-thermal levels by a Gunn oscillator induces meristem production with kinetics similar to that induced by weak environmental stimuli and radiation from GSM telephone.

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.

Disease resistance and abiotic stress tolerance in rice are inversely modulated by an abscisic acid-inducible mitogen-activated protein kinase

Mitogen-activated protein kinase (MAPK) cascades play an important role in mediating stress responses in eukaryotic organisms. However, little is known about the role of MAPKs in modulating the interaction of defense pathways activated by biotic and abiotic factors. In this study, we have isolated and functionally characterized a stress-responsive MAPK gene (OsMAPK5) from rice. OsMAPK5 is a single-copy gene but can generate at least two differentially spliced transcripts. The OsMAPK5 gene, its protein, and kinase activity were inducible by abscisic acid as well as various biotic (pathogen infection) and abiotic (wounding, drought, salt, and cold) stresses. To determine its biological function, we generated and analyzed transgenic rice plants with overexpression (using the 35S promoter of Cauliflower mosaic virus) or suppression (using double-stranded RNA interference [dsRNAi]) of OsMAPK5. Interestingly, suppression of OsMAPK5 expression and its kinase activity resulted in the constitutive expression of pathogenesis-related (PR) genes such as PR1 and PR10 in the dsRNAi transgenic plants and significantly enhanced resistance to fungal (Magnaporthe grisea) and bacterial (Burkholderia glumae) pathogens. However, these same dsRNAi lines had significant reductions in drought, salt, and cold tolerance. By contrast, overexpression lines exhibited increased OsMAPK5 kinase activity and increased tolerance to drought, salt, and cold stresses. These results strongly suggest that OsMAPK5 can positively regulate drought, salt, and cold tolerance and negatively modulate PR gene expression and broad-spectrum disease resistance.

NDP kinase 2 interacts with two oxidative stress-activated MAPKs to regulate cellular redox state and enhances multiple stress tolerance in transgenic plants

NDP kinases (NDPKs) are multifunctional proteins that regulate a variety of eukaryotic cellular activities, including cell proliferation, development, and differentiation. However, much less is known about the functional significance of NDPKs in plants. We show here that NDPK is associated with H(2)O(2)-mediated mitogen-activated protein kinase signaling in plants. H(2)O(2) stress strongly induces the expression of the NDPK2 gene in Arabidopsis thaliana (AtNDPK2). Proteins from transgenic plants overexpressing AtNDPK2 showed high levels of autophosphorylation and NDPK activity, and they have lower levels of reactive oxygen species (ROS) than wild-type plants. Mutants lacking AtNDPK2 had higher levels of ROS than wild type. H(2)O(2) treatment induced the phosphorylation of two endogenous proteins whose molecular weights suggested they are AtMPK3 and AtMPK6, two H(2)O(2)-activated A. thaliana mitogen-activated protein kinases. In the absence of H(2)O(2) treatment, phosphorylation of these proteins was slightly elevated in plants overexpressing AtNDPK2 but markedly decreased in the AtNDPK2 deletion mutant. Yeast two-hybrid and in vitro protein pull-down assays revealed that AtNDPK2 specifically interacts with AtMPK3 and AtMPK6. Furthermore, AtNDPK2 also enhances the myelin basic protein phosphorylation activity of AtMPK3 in vitro. Finally, constitutive overexpression of AtNDPK2 in Arabidopsis plants conferred an enhanced tolerance to multiple environmental stresses that elicit ROS accumulation in situ. Thus, AtNDPK2 appears to play a previously uncharacterized regulatory role in H(2)O(2)-mediated MAPK signaling in plants.

Protein phosphatases in plants

Phosphorylation and dephosphorylation of a protein often serve as an "on-and-off" switch in the regulation of cellular activities. Recent studies demonstrate the involvement of protein phosphorylation in almost all signaling pathways in plants. A significant portion of the sequenced Arabidopsis genome encodes protein kinases and protein phosphatases that catalyze reversible phosphorylation. For optimal regulation, kinases and phosphatases must strike a balance in any given cell. Only a very small fraction of the thousands of protein kinases and phosphatases in plants has been studied experimentally. Nevertheless, the available results have demonstrated critical functions for these enzymes in plant growth and development. While serine/threonine phosphorylation is widely accepted as a predominant modification of plant proteins, the function of tyrosine phosphorylation, desptie its overwhelming importance in animal systems, had been largely neglected until recently when tyrosine phosphatases (PTPs) were characterized from plants. This review focuses on the structure, regulation, and function of protein phosphatases in higher plants.

Distinct regulation of salinity and genotoxic stress responses by Arabidopsis MAP kinase phosphatase 1

The Arabidopsis genome contains 20 genes encoding mitogen-activated protein kinases (MAPKs), which drastically outnumbers genes for their negative regulators, MAP kinase phosphatases (MKPs) (five at most). This contrasts sharply with genomes of other eukaryotes where the number of MAPKs and MKPs is approximately equal. MKPs may therefore play an important role in signal integration in plants, through concerted regulation of several MAPKs. Our previous studies identified Arabidopsis MKP1 and showed that its deficiency in the mkp1 mutant results in plant hypersensitivity to genotoxic stress. Here, we identify a set of MAPKs that interact with MKP1, and show that the activity level of one of these, MPK6, is regulated by MKP1 in vivo. Moreover, using expression profiling, we identified a specific group of genes that probably represent targets of MKP1 regulation. Surprisingly, the identity of these genes and interacting MAPKs suggested involvement of MKP1 in salt stress responses. Indeed, mkp1 plants have increased resistance to salinity. Thus MKP1 apparently plays a pivotal role in the integration and fine-tuning of plant responses to various environmental challenges.

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.

Hyperosmotic stress induces formation of tubulin macrotubules in root-tip cells of Triticum turgidum: their probable involvement in protoplast volume control

Treatment of root-tip cells of Triticum turgidum with 1 M mannitol solution for 30 min induces microtubule (Mt) disintegration in the plasmolyzed protoplasts. Interphase plasmolyzed cells possess many cortical, perinuclear and endoplasmic macrotubules, 35 nm in mean diameter, forming prominent arrays. In dividing cells macrotubules assemble into aberrant mitotic and cytokinetic apparatuses resulting in the disturbance of cell division. Putative tubulin paracrystals were occasionally observed in plasmolyzed cells. The quantity of polymeric tubulin in plasmolyzed cells exceeds that in control cells. Root-tip cells exposed for 2-8 h to plasmolyticum recover partially, although the volume of the plasmolyzed protoplast does not change detectably. Among other events, the macrotubules are replaced by Mts, chromatin assumes its typical appearance and the cells undergo typical cell divisions. Additionally, polysaccharidic material is found in the periplasmic space. Oryzalin and colchicine treatment induced macrotubule disintegration and a significant reduction of protoplast volume in every plasmolyzed cell type examined, whereas cytochalasin B had only minor effects restricted to differentiated cells. These results suggest that Mt destruction by hyperosmotic stress, and their replacement by tubulin macrotubules and putative tubulin paracrystals is a common feature among angiosperms and that macrotubules are involved in the mechanism of protoplast volume regulation.

Transcriptional regulation of a rice mitogen-activated protein kinase gene, OsMAPK4, in response to environmental stresses

Mitogen-activated protein kinase (MAPK) cascades play important roles in signal transduction of extracellular stimuli in eukaryotes. However, stimulatory signals for plant MAPKs have not been well elucidated. Here, a cDNA clone, termed Oryza sativa MAPK4 (OsMAPK4), from rice encoding a protein that showed homology with the eukaryotic MAPKs was isolated. According to the phylogenetic analysis, OsMAPK4 belongs to subgroup IV MAPK in plants. OsMAPK4 transcripts were expressed strongly in mature leaves and weakly in young leaves and panicles. The gene was also differentially expressed in roots at different developmental stages. In addition, the mRNA level of OsMAPK4 was up-regulated under sugar starvation, high salinity and cold treatments. These results suggest that this OsMAPK4 functions not only in developmental programs but also in stress-signaling pathways.