ABA activation of an MBP kinase in Pisum sativum epidermal peels correlates with stomatal responses to ABA

Integrative study of transcriptome and microbiome to reveal the response of Rhododendron decorum to cadmium stress

The anomalies of cadmium (Cd) in karst region pose a severe threat to plant growth and development. In this study, the responses of Rhododendron decorum to Cd stress were investigated at physiological, molecular, and endophytic microbial levels, and the potential correlation among these responses was assessed. The Cd stress impeded R. decorum growth and led to an increase in malondialdehyde (MDA) and hydrogen peroxide (H2O2) levels, as well as enhanced superoxide dismutase (SOD) and catalase (CAT) activities. Meanwhile, Cd stress increased the Cd (up to 80 times compared to the control), sodium (Na), aluminum (Al), and zinc (Zn) contents, while decreased the magnesium (Mg) and manganese (Mn) contents in R. decorum leaves. Transcriptome suggested that Cd significantly regulated the pathways including "protein repair", "hormone-mediated signaling pathway", and "ATP-binding cassette (ABC) transporters". Additionally, q-PCR analysis showed that Cd stress significantly up-regulated the expressions of ABCB19-like and pleiotropic drug resistance, while down-regulated the expressions of indole-3-acetic acid-amido synthetase and cytokinin dehydrogenase. The Cd stress influenced the composition of endophytic microbial communities in R. decorum leaves and enhanced the interspecific bacterial associations. Furthermore, the bacterial genera Achromobacter, Aureimonas and fungal genus Vishniacozyma exhibited a high degree of connectivity with other nodes in networks constructed by the metal element contents, differentially expressed genes (DEGs), and microbial communities, respectively. These findings provide a comprehensive insight into the response of R. decorum to Cd-induced stress, which might facilitate the breeding of the Cd-tolerant R. decorum.

Transcriptome profiling of Bergenia purpurascens under cold stress

Bergenia purpurascens is an important medicinal, edible and ornamental plant. It generally grows in high-altitude areas with complex climates. There have been no reports about how B. purpurascens survives under cold stress. Here, the B. purpurascens under low temperature were subjected to transcriptomics analysis to explore the candidate genes and pathways that involved in the cold tolerance of B. purpurascens. Compared with the control treatment, we found 9,600 up-regulated differentially expressed genes (DEGs) and 7,055 down-regulated DEGs. A significant number of DEGs were involved in the Ca2+ signaling pathway, mitogen-activated protein kinase (MAPK) cascade, plant hormone signaling pathway, and lipid metabolism. A total of 400 transcription factors were found to respond to cold stress, most of which belonged to the MYB and AP2/ERF families. Five novel genes were found to be potential candidate genes involved in the cold tolerance of B. purpurascens. The study provide insights into further investigation of the molecular mechanism of how B. purpurascens survives under cold stress.

Transcriptome and proteome depth analysis indicate ABA, MAPK cascade and Ca2+ signaling co-regulate cold tolerance in Rhododendron chrysanthum Pall

INTRODUCTION

Cold stress is a global common problem that significantly limits plant development and geographical distribution. Plants respond to low temperature stress by evolving interrelated regulatory pathways to respond and adapt to their environment in a timely manner. Rhodoendron chrysanthum Pall. (R. chrysanthum) is a perennial evergreen dwarf shrub used for adornment and medicine that thrives in the Changbai Mountains at high elevations and subfreezing conditions.

METHODS

In this study, a comprehensive investigation of cold tolerance (4°C, 12h) in R. chrysanthum leaves under cold using physiological combined with transcriptomic and proteomic approaches.

RESULTS

There were 12,261 differentially expressed genes (DEGs) and 360 differentially expressed proteins (DEPs) in the low temperature (LT) and normal treatment (Control). Integrated transcriptomic and proteomic analyses showed that MAPK cascade, ABA biosynthesis and signaling, plant-pathogen interaction, linoleic acid metabolism and glycerophospholipid metabolism were significantly enriched in response to cold stress of R. chrysanthum leaves.

DISCUSSION

We analyzed the involvement of ABA biosynthesis and signaling, MAPK cascade, and Ca²⁺ signaling, that may jointly respond to stomatal closure, chlorophyll degradation, and ROS homeostasis under low temperature stress. These results propose an integrated regulatory network of ABA, MAPK cascade and Ca²⁺ signaling comodulating the cold stress in R. chrysanthum, which will provide some insights to elucidate the molecular mechanisms of cold tolerance in plants.

Arbuscular mycorrhizal fungi enhance photosynthesis and drought tolerance by regulating MAPK genes expressions of Populus simonii × P. nigra

Arbuscular mycorrhizal fungi (AMF) promote plants to absorb more water and nutrients and improve their stress resistance. As the main signal transducer, the mitogen-activated protein kinase (MAPK) cascade plays a vital role in drought stress. However, how the MAPK family genes of mycorrhizal plants respond to stress is still not clear. Our study analyzed physiological indexes and expression profiles of MAPK family genes of Populus simonii × P. nigra under two inoculation treatments (inoculated with or without Rhizophagus irregularis) and two water conditions (well-watered or drought stress). The results showed that the stronger photosynthesis of mycorrhizal plants may be mediated by MAPK genes induced by AMF. Mycorrhizal plants showed lower oxidative damage and drought sensitivity. Mycorrhiza downregulated the expression of PsnMAPK7-2, PsnMAPK16-1, PsnMAPK19-2, and PsnMAPK20-2 which negatively regulate drought tolerance and induced specific PsnMAPKs in roots which activate transcription factors to regulate downstream gene expressions, enhancing drought tolerance. This is the first time to identify part of the MAPK gene family of P. simonii × P. nigra at the genome level and study MAPK genes in mycorrhizal forest trees. This is helpful to understand the function of the MAPK gene family in response to drought of mycorrhizal plants and lays a foundation for afforestation by using mycorrhizal saplings.

Mitogen-Activated Protein Kinase Is Involved in Salt Stress Response in Tomato (Solanum lycopersicum) Seedlings

Salt stress impairs plant growth and development, thereby causing low yield and inferior quality of crops. In this study, tomato (Solanum lycopersicum L. ‘Micro-Tom’) seedlings treated with different concentrations of sodium chloride (NaCl) were investigated in terms of decreased plant height, stem diameter, dry weight, fresh weight, leaves relative water content and root activity. To reveal the response mechanism of tomato seedlings to salt stress, the transcriptome of tomato leaves was conducted. A total of 6589 differentially expressed genes (DEGs) were identified and classified into different metabolic pathways, especially photosynthesis, carbon metabolism, biosynthesis of amino acids and mitogen-activated protein kinase (MAPK) signaling pathway. Of these, approximately 42 DEGs were enriched in the MAPK signaling pathway, most of which mainly included plant hormone, hydrogen peroxide (H₂O₂), wounding and pathogen infection signaling pathways. To further explore the roles of MAPK under salt stress, MAPK phosphorylation inhibitor SB203580 (SB) was applied. We found that SB further decreased endogenous jasmonic acid, abscisic acid and ethylene levels under salt stress condition. Additionally, in comparison with NaCl treatment alone, SB + NaCl treatment reduced the content of O²⁻ and H₂O₂ and the activities of antioxidant enzyme and downregulated the expression levels of genes related to pathogen infection. Together, the results revealed that MAPK might be involved in the salinity response of tomato seedlings by regulating hormone balance, ROS metabolism, antioxidant capacity and plant immunity.

Research on the Molecular Interaction Mechanism between Plants and Pathogenic Fungi

Plant diseases caused by fungi are one of the major threats to global food security and understanding the interactions between fungi and plants is of great significance for plant disease control. The interaction between pathogenic fungi and plants is a complex process. From the perspective of pathogenic fungi, pathogenic fungi are involved in the regulation of pathogenicity by surface signal recognition proteins, MAPK signaling pathways, transcription factors, and pathogenic factors in the process of infecting plants. From the perspective of plant immunity, the signal pathway of immune response, the signal transduction pathway that induces plant immunity, and the function of plant cytoskeleton are the keys to studying plant resistance. In this review, we summarize the current research progress of fungi–plant interactions from multiple aspects and discuss the prospects and challenges of phytopathogenic fungi and their host interactions.

The phytotoxicity of exposure to two polybrominated diphenyl ethers (BDE47 and BDE209) on photosynthesis and the response of the hormone signaling and ROS scavenging system in tobacco leaves

To reveal the response and adaptative mechanism of plants to the organic pollutants PBDEs, physiological and transcriptomic techniques were used to study the effects of exposure to BDE47 and BDE209 on tobacco (Nicotiana tabacum L.) plant growth, physiological function and response of key genes. Exposure to both BDE47 and BDE209 inhibited the growth of tobacco plants. The number of down-regulated DEGs following exposure to BDE47 was significantly higher than that following exposure to BDE209. Enrichment analysis using the KEGG showed that BDE47 and BDE209 primarily affected tobacco leaf photosynthesis-antenna proteins, photosynthesis, plant hormone signal transduction and α-linolenic acid metabolism. BDE47 primarily inhibits the synthesis of Chl a, and BDE209 has a more significant impact on Chl b. Most photosynthesis-related DEGs were concentrated in PSII and PSI; the number of down-regulated DEGs in PSI was significantly higher than that in PSII, and the range in which the PSI activity was reduced was also higher than that of PSII, i.e., PSII and PSI (particularly PSI) were sensitive to the effects of exposure to BDE47 and BDE209 on photosynthesis. The increase of the ratio of regulatory energy dissipation played an important protective role in alleviating the photoinhibition of PSII. Exposure to BDE47 and BDE209 can lead to the accumulation of ROS in tobacco leaves, but correspondingly, the activities of antioxidant enzymes SOD, POD, CAT, APX and GPX and the up-regulated expression of their coding genes play an important role in preventing excessive oxidative damage. Exposure to BDE47 and BDE209 promoted the up-regulation of gene expression related to Pro synthesis. In particular, the Pro synthetic process of the Orn pathway was promoted. Exposure to BDE47 and BDE209 induced the up-regulated expression of genes related to the synthesis of ABA and JA, promoted the synthesis of ABA and JA, and activated ABA and JA signal transduction pathways. In conclusion, both BDE47 and BDE209 inhibit the synthesis of chlorophyll and hinder the process of light energy capture and electron transfer in tobacco leaves. BDE47 was more toxic than BDE209. However, tobacco leaves can also adapt to BDE47 and BDE209 by regulating the antioxidant system, accumulating Pro and initiating the hormone signal transduction process. The results of this study provide a theoretical basis for the phytotoxicity mechanism of PBDEs.

6-Benzyladenine increasing subsequent waterlogging-induced waterlogging tolerance of summer maize by increasing hormone signal transduction

Summer maize is frequently subjected to waterlogging damage because of increased and variable rainfall during the growing season. The application of 6-benzyladenine (6-BA) can effectively mitigate the waterlogging effects on plant growth and increase the grain yield of waterlogged summer maize. However, the mechanisms underlying this process and the involvement of 6-BA in relevant signal transduction pathways remain unclear. In this study, we explored the effects of 6-BA on waterlogged summer maize using a phosphoproteomic technique to better understand the mechanism by which summer maize growth improves following waterlogging. Application of 6-BA inhibited the waterlogging-induced increase in abscisic acid (ABA) content and increased the phosphorylation levels of proteins involved in ABA signaling; accordingly, stomatal responsiveness to exogenous ABA increased. In addition, the application of 6-BA had a long-term effect on signal transduction pathways and contributed to rapid responses to subsequent stresses. Plants primed with 6-BA accumulated more ethylene and jasmonic acid in response to subsequent waterlogging; accordingly, leaf SPAD, antioxidase activity, and root traits improved by 6-BA priming. These results suggest that the effects of 6-BA on hormone signal transduction pathways are anamnestic, which enables plants to show faster or stronger defense responses to stress.

Adaptive Responses of Citrus grandis Leaves to Copper Toxicity Revealed by RNA-Seq and Physiology

Copper (Cu)-toxic effects on Citrus grandis growth and Cu uptake, as well as gene expression and physiological parameters in leaves were investigated. Using RNA-Seq, 715 upregulated and 573 downregulated genes were identified in leaves of C. grandis seedlings exposed to Cu-toxicity (LCGSEC). Cu-toxicity altered the expression of 52 genes related to cell wall metabolism, thus impairing cell wall metabolism and lowering leaf growth. Cu-toxicity downregulated the expression of photosynthetic electron transport-related genes, thus reducing CO₂ assimilation. Some genes involved in thermal energy dissipation, photorespiration, reactive oxygen species scavenging and cell redox homeostasis and some antioxidants (reduced glutathione, phytochelatins, metallothioneins, l-tryptophan and total phenolics) were upregulated in LCGSEC, but they could not protect LCGSEC from oxidative damage. Several adaptive responses might occur in LCGSEC. LCGSEC displayed both enhanced capacities to maintain homeostasis of Cu via reducing Cu uptake by leaves and preventing release of vacuolar Cu into the cytoplasm, and to improve internal detoxification of Cu by accumulating Cu chelators (lignin, reduced glutathione, phytochelatins, metallothioneins, l-tryptophan and total phenolics). The capacities to maintain both energy homeostasis and Ca homeostasis might be upregulated in LCGSEC. Cu-toxicity increased abscisates (auxins) level, thus stimulating stomatal closure and lowering water loss (enhancing water use efficiency and photosynthesis).

A Method for Assaying of Protein Kinase Activity In Vivo and Its Use in Studies of Signal Transduction in Strawberry Fruit Ripening

Strawberry (Fragaria × ananassa) fruit ripening is regulated by a complex of cellular signal transduction networks, in which protein kinases are key components. Here, we report a relatively simple method for assaying protein kinase activity in vivo and specifically its application to study the kinase, FaMPK6, signaling in strawberry fruit. Green fluorescent protein (GFP)-tagged FaMPK6 was transiently expressed in strawberry fruit and after stimuli were applied to the fruit it was precipitated using an anti-GFP antibody. The precipitated kinase activity was measured in vitro using ³²P-ATP and myelin basic protein (MBP) as substrates. We also report that FaMPK6 is not involved in the abscisic acid (ABA) signaling cascade, which is closely associated with FaMPK6 signaling in other plant species. However, methyl jasmonate (MeJA), low temperature, and high salt treatments were all found to activate FaMPK6. Transient manipulation of FaMPK6 expression was observed to cause significant changes in the expression patterns of 2749 genes, of which 264 were associated with MeJA signaling. The data also suggest a role for FaMPK6 in modulating cell wall metabolism during fruit ripening. Taken together, the presented method is powerful and its use will contribute to a profound exploration to the signaling mechanism of strawberry fruit ripening.

Decoding ABA and osmostress signalling in plants from an evolutionary point of view

The plant hormone abscisic acid (ABA) is fundamental for land plant adaptation to water-limited conditions. Osmostress, such as drought, induces ABA accumulation in angiosperms, triggering physiological responses such as stomata closure. The core components of angiosperm ABA signalling are soluble ABA receptors, group A protein phosphatase type 2C and SNF1-related protein kinase2 (SnRK2). ABA also has various functions in non-angiosperms, however, suggesting that its role in adaptation to land may not have been angiosperm-specific. Indeed, among land plants, the core ABA signalling components are evolutionarily conserved, implying their presence in a common ancestor. Results of ongoing functional genomics studies of ABA signalling components in bryophytes and algae have expanded our understanding of the evolutionary role of ABA signalling, with genome sequencing uncovering the ABA core module even in algae. In this review, we describe recent discoveries involving the ABA core module in non-angiosperms, tracing the footprints of how ABA evolved as a phytohormone. We also cover the latest findings on Raf-like kinases as upstream regulators of the core ABA module component SnRK2. Finally, we discuss the origin of ABA signalling from an evolutionary perspective.

Chemical Manipulation of Abscisic Acid Signaling: A New Approach to Abiotic and Biotic Stress Management in Agriculture

The phytohormone abscisic acid (ABA) is the best-known stress signaling molecule in plants. ABA protects sessile land plants from biotic and abiotic stresses. The conserved pyrabactin resistance/pyrabactin resistance-like/regulatory component of ABA receptors (PYR/PYL/RCAR) perceives ABA and triggers a cascade of signaling events. A thorough knowledge of the sequential steps of ABA signaling will be necessary for the development of chemicals that control plant stress responses. The core components of the ABA signaling pathway have been identified with adequate characterization. The information available concerning ABA biosynthesis, transport, perception, and metabolism has enabled detailed functional studies on how the protective ability of ABA in plants might be modified to increase plant resistance to stress. Some of the significant contributions to chemical manipulation include ABA biosynthesis inhibitors, and ABA receptor agonists and antagonists. Chemical manipulation of key control points in ABA signaling is important for abiotic and biotic stress management in agriculture. However, a comprehensive review of the current knowledge of chemical manipulation of ABA signaling is lacking. Here, a thorough analysis of recent reports on small-molecule modulation of ABA signaling is provided. The challenges and prospects in the chemical manipulation of ABA signaling for the development of ABA-based agrochemicals are also discussed.

AtDPG1 is involved in the salt stress response of Arabidopsis seedling through ABI4

Although the genes controlling chloroplast development play important roles in plant responses to environmental stresses, the molecular mechanisms remain largely unclear. In this study, an Arabidopsis mutant dpg1 (delayed pale-greening1) with a chloroplast development defect was studied. By using quantitative RT-PCR and histochemical GUS assays, we demonstrated that AtDPG1 was mainly expressed in the green tissues of Arabidopsis seedlings and could be induced by salt stress. Phenotypic analysis showed that mutation in AtDPG1 lead to an enhanced sensitivity to salt stress in Arabidopsis seedlings. Further studies demonstrated that disruption of the AtDPG1 in Arabidopsis increases its sensitivity to salt stress in an ABA-dependent manner. Moreover, expression levels of various stress-responsive and ABA signal-related genes were remarkably altered in the dpg1 plants under NaCl treatment. Notably, the transcript levels of ABI4 in dpg1 mutant increased more significantly than that in wild type plants under salt conditions. The seedlings of dpg1/abi4 double mutant exhibited stronger resistance to salt stress after salt treatment compared with the dpg1 single mutant, suggesting that the salt-hypersensitive phenotype of dpg1 seedlings could be rescued via loss of ABI4 function. These results reveal that AtDPG1 is involved in the salt stress response of Arabidopsis seedling through ABI4.

GSHR, a Web-Based Platform Provides Gene Set-Level Analyses of Hormone Responses in Arabidopsis

Phytohormones regulate diverse aspects of plant growth and environmental responses. Recent high-throughput technologies have promoted a more comprehensive profiling of genes regulated by different hormones. However, these omics data generally result in large gene lists that make it challenging to interpret the data and extract insights into biological significance. With the rapid accumulation of theses large-scale experiments, especially the transcriptomic data available in public databases, a means of using this information to explore the transcriptional networks is needed. Different platforms have different architectures and designs, and even similar studies using the same platform may obtain data with large variances because of the highly dynamic and flexible effects of plant hormones; this makes it difficult to make comparisons across different studies and platforms. Here, we present a web server providing gene set-level analyses of Arabidopsis thaliana hormone responses. GSHR collected 333 RNA-seq and 1,205 microarray datasets from the Gene Expression Omnibus, characterizing transcriptomic changes in Arabidopsis in response to phytohormones including abscisic acid, auxin, brassinosteroids, cytokinins, ethylene, gibberellins, jasmonic acid, salicylic acid, and strigolactones. These data were further processed and organized into 1,368 gene sets regulated by different hormones or hormone-related factors. By comparing input gene lists to these gene sets, GSHR helped to identify gene sets from the input gene list regulated by different phytohormones or related factors. Together, GSHR links prior information regarding transcriptomic changes induced by hormones and related factors to newly generated data and facilities cross-study and cross-platform comparisons; this helps facilitate the mining of biologically significant information from large-scale datasets. The GSHR is freely available at http://bioinfo.sibs.ac.cn/GSHR/.

Mitogen-Activated Protein Kinase Cascades in Plant Hormone Signaling

Mitogen-activated protein kinase (MAPK) modules play key roles in the transduction of environmental and developmental signals through phosphorylation of downstream signaling targets, including other kinases, enzymes, cytoskeletal proteins or transcription factors, in all eukaryotic cells. A typical MAPK cascade consists of at least three sequentially acting serine/threonine kinases, a MAP kinase kinase kinase (MAPKKK), a MAP kinase kinase (MAPKK) and finally, the MAP kinase (MAPK) itself, with each phosphorylating, and hence activating, the next kinase in the cascade. Recent advances in our understanding of hormone signaling pathways have led to the discovery of new regulatory systems. In particular, this research has revealed the emerging role of crosstalk between the protein components of various signaling pathways and the involvement of this crosstalk in multiple cellular processes. Here we provide an overview of current models and mechanisms of hormone signaling with a special emphasis on the role of MAPKs in cell signaling networks. One-sentence summary: In this review we highlight the mechanisms of crosstalk between MAPK cascades and plant hormone signaling pathways and summarize recent findings on MAPK regulation and function in various cellular processes.

Overexpression of Populus trichocarpa Mitogen-Activated Protein Kinase Kinase4 Enhances Salt Tolerance in Tobacco

Mitogen-activated protein kinase (MAPK) is one of the factors of cascade reactions affecting responses to signal pathway of environmental stimuli. Throughout the life of plants, MAPK family members participate in signal transduction pathways and regulate various intracellular physiological and metabolic reactions. To gain insights into regulatory function of MAPK kinase (MAPKK) in Populus trichocarpa under salt stress, we obtained full-length cDNA of PtMAPKK4 and analyzed different expression levels of PtMAPKK4 gene in leaves, stems, and root organs. The relationship between PtMAPKK4 and salt stress was studied by detecting expression characteristics of mRNA under 150 mM NaCl stress using real-time quantitative polymerase chain reaction. The results showed that expression of PtMAPKK4 increased under salt (NaCl) stress in leaves but initially reduced and then increased in roots. Thus, salt stress failed to induce PtMAPKK4 expression in stems. PtMAPKK4 possibly participates in regulation of plant growth and metabolism, thereby improving its salt tolerance. We used Saccharomyces cerevisiae strain INVScI to verify subcellular localization of PtMAPKK4 kinase. The yeast strains containing pYES2-PtMAPKK4-GFP plasmid expressed GFP fusion proteins under the induction of d-galactose, and the products were located in nucleus. These results were consistent with network prediction and confirmed location of PtMAPKK4 enzyme in the nucleus. We tested NaCl tolerance in transgenic tobacco lines overexpressing PtMAPKK4 under the control of 35S promoter at germination stage to detect salt tolerance function of PtMAPKK4. Compared withK326 (a wild-type tobacco), lines overexpressing PtMAPKK4 showed a certain degree of improvement in tolerance, germination, and growth. NaCl inhibited growth of overexpressed line and K326 at the seedling stage. However, statistical analysis showed longer root length, higher fresh weight, and lower MDA content in transgenic lines in comparison with that in K326.

Role and interrelationship of MEK1-MPK6 cascade, hydrogen peroxide and nitric oxide in darkness-induced stomatal closure

Pharmacological data have suggested the involvement of mitogen-activated protein kinase (MPK) cascades in dark-induced stomatal closure, but which specific MPK cascade participates in the darkness guard cell signaling and its relationship with hydrogen peroxide (H₂O₂) and nitric oxide (NO) remain unclear. In this paper, we observed that darkness induced activation of MPK6 in leaves of wild-type Arabidopsis (Arabidopsis thaliana) and mutants for nitrate reductase 1 (NIA1), but this effect was inhibited in mutants for MPK Kinase 1 (MEK1) and ATRBOHD/F. Mutants for MEK1, MPK6 and NIA1 showed defect of dark-induced NO production in guard cells and stomatal closure, but were normal in the dark-induced H₂O₂ generation, while stomata of mutant AtrbohD/F showed defect of dark-induced H₂O₂ and NO production and subsequent closure. Moreover, exogenous NO rescued the defect of dark-induced stomatal closure in mutants of AtrbohD/F, mek1 and mpk6, while exogenous H₂O₂ could not rescue the defect of dark-induced stomatal closure in mutants of mek1, mpk6 and nia1. These genetic and biochemical evidences not only show that MEK1-MPK6 cascade, AtRBOHD/F-dependent H₂O₂ and NIA1-dependent NO are all involved in dark-induced stomatal closure in Arabidopsis, also indicate that MEK1-MPK6 cascade functions via working downstream of H₂O₂ and upstream of NO.

Interactions between plant hormones and heavy metals responses

Heavy metals are natural non-biodegradable constituents of the Earth's crust that accumulate and persist indefinitely in the ecosystem as a result of human activities. Since the industrial revolution, the concentration of cadmium, arsenic, lead, mercury and zinc, amongst others, have increasingly contaminated soil and water resources, leading to significant yield losses in plants. These issues have become an important concern of scientific interest. Understanding the molecular and physiological responses of plants to heavy metal stress is critical in order to maximize their productivity. Recent research has extended our view of how plant hormones can regulate and integrate growth responses to various environmental cues in order to sustain life. In the present review we discuss current knowledge about the role of the plant growth hormones abscisic acid, auxin, brassinosteroid and ethylene in signaling pathways, defense mechanisms and alleviation of heavy metal toxicity.

The Role of MAPK Modules and ABA during Abiotic Stress Signaling

To respond to abiotic stresses, plants have developed specific mechanisms that allow them to rapidly perceive and respond to environmental changes. The phytohormone abscisic acid (ABA) was shown to be a pivotal regulator of abiotic stress responses in plants, triggering major changes in plant physiology. The ABA core signaling pathway largely relies on the activation of SnRK2 kinases to mediate several rapid responses, including gene regulation, stomatal closure, and plant growth modulation. Mitogen-activated protein kinases (MAPKs) have also been implicated in ABA signaling, but an entire ABA-activated MAPK module was uncovered only recently. In this review, we discuss the evidence for a role of MAPK modules in the context of different plant ABA signaling pathways.

Abscisic Acid and Abiotic Stress Tolerance in Crop Plants

Abiotic stress is a primary threat to fulfill the demand of agricultural production to feed the world in coming decades. Plants reduce growth and development process during stress conditions, which ultimately affect the yield. In stress conditions, plants develop various stress mechanism to face the magnitude of stress challenges, although that is not enough to protect them. Therefore, many strategies have been used to produce abiotic stress tolerance crop plants, among them, abscisic acid (ABA) phytohormone engineering could be one of the methods of choice. ABA is an isoprenoid phytohormone, which regulates various physiological processes ranging from stomatal opening to protein storage and provides adaptation to many stresses like drought, salt, and cold stresses. ABA is also called an important messenger that acts as the signaling mediator for regulating the adaptive response of plants to different environmental stress conditions. In this review, we will discuss the role of ABA in response to abiotic stress at the molecular level and ABA signaling. The review also deals with the effect of ABA in respect to gene expression.

Abscisic Acid and Abiotic Stress Tolerance in Crop Plants

Abiotic stress is a primary threat to fulfill the demand of agricultural production to feed the world in coming decades. Plants reduce growth and development process during stress conditions, which ultimately affect the yield. In stress conditions, plants develop various stress mechanism to face the magnitude of stress challenges, although that is not enough to protect them. Therefore, many strategies have been used to produce abiotic stress tolerance crop plants, among them, abscisic acid (ABA) phytohormone engineering could be one of the methods of choice. ABA is an isoprenoid phytohormone, which regulates various physiological processes ranging from stomatal opening to protein storage and provides adaptation to many stresses like drought, salt, and cold stresses. ABA is also called an important messenger that acts as the signaling mediator for regulating the adaptive response of plants to different environmental stress conditions. In this review, we will discuss the role of ABA in response to abiotic stress at the molecular level and ABA signaling. The review also deals with the effect of ABA in respect to gene expression.

Identification of candidate genes for drought tolerance in coffee by high-throughput sequencing in the shoot apex of different Coffea arabica cultivars

BACKGROUND

Drought is a widespread limiting factor in coffee plants. It affects plant development, fruit production, bean development and consequently beverage quality. Genetic diversity for drought tolerance exists within the coffee genus. However, the molecular mechanisms underlying the adaptation of coffee plants to drought are largely unknown. In this study, we compared the molecular responses to drought in two commercial cultivars (IAPAR59, drought-tolerant and Rubi, drought-susceptible) of Coffea arabica grown in the field under control (irrigation) and drought conditions using the pyrosequencing of RNA extracted from shoot apices and analysing the expression of 38 candidate genes.

RESULTS

Pyrosequencing from shoot apices generated a total of 34.7 Mbp and 535,544 reads enabling the identification of 43,087 clusters (41,512 contigs and 1,575 singletons). These data included 17,719 clusters (16,238 contigs and 1,575 singletons) exclusively from 454 sequencing reads, along with 25,368 hybrid clusters assembled with 454 sequences. The comparison of DNA libraries identified new candidate genes (n = 20) presenting differential expression between IAPAR59 and Rubi and/or drought conditions. Their expression was monitored in plagiotropic buds, together with those of other (n = 18) candidates genes. Under drought conditions, up-regulated expression was observed in IAPAR59 but not in Rubi for CaSTK1 (protein kinase), CaSAMT1 (SAM-dependent methyltransferase), CaSLP1 (plant development) and CaMAS1 (ABA biosynthesis). Interestingly, the expression of lipid-transfer protein (nsLTP) genes was also highly up-regulated under drought conditions in IAPAR59. This may have been related to the thicker cuticle observed on the abaxial leaf surface in IAPAR59 compared to Rubi.

CONCLUSIONS

The full transcriptome assembly of C. arabica, followed by functional annotation, enabled us to identify differentially expressed genes related to drought conditions. Using these data, candidate genes were selected and their differential expression profiles were confirmed by qPCR experiments in plagiotropic buds of IAPAR59 and Rubi under drought conditions. As regards the genes up-regulated under drought conditions, specifically in the drought-tolerant IAPAR59, several corresponded to orphan genes but also to genes coding proteins involved in signal transduction pathways, as well as ABA and lipid metabolism, for example. The identification of these genes should help advance our understanding of the genetic determinism of drought tolerance in coffee.

MAPK Cascades in Guard Cell Signal Transduction

Guard cells form stomata on the epidermis and continuously respond to endogenous and environmental stimuli to fine-tune the gas exchange and transpirational water loss, processes which involve mitogen-activated protein kinase (MAPK) cascades. MAPKs form three-tiered kinase cascades with MAPK kinases and MAPK kinase kinases, by which signals are transduced to the target proteins. MAPK cascade genes are highly conserved in all eukaryotes, and they play crucial roles in myriad developmental and physiological processes. MAPK cascades function during biotic and abiotic stress responses by linking extracellular signals received by receptors to cytosolic events and gene expression. In this review, we highlight recent findings and insights into MAPK-mediated guard cell signaling, including the specificity of MAPK cascades and the remaining questions.

Carbon monoxide regulates the expression of the wound-inducible gene ipomoelin through antioxidation and MAPK phosphorylation in sweet potato

Carbon monoxide (CO), one of the haem oxygenase (HO) products, plays important roles in plant development and stress adaptation. However, the function of CO involved in wounding responses is seldom studied. A wound-inducible gene, ipomoelin (IPO), of sweet potato (Ipomoea batatas cv. Tainung 57) was used as a target to study the regulation of CO in wounding responses. After wounding for 1h, the endogenous CO content and IbHO expression level were significantly reduced in leaves. IPO expression upon wounding was prohibited by the HO activator hemin, whereas the HO inhibitor zinc protoporphyrin IX elevated IPO expression. The IPO expression induced by wounding, H2O2, or methyl jasmonate was inhibited by CO. CO also affected the activities of ascorbate peroxidase, catalase, and peroxidase, and largely decreased H2O2 content in leaves. CO inhibited the extracellular signal-regulated kinase (ERK) phosphorylation induced by wounding. IbMAPK, the ERK of sweet potato, was identified by immunoblotting, and the interaction with its upstream activator, IbMEK1, was further confirmed by bimolecular fluorescence complementation and co-immunoprecipitation. Conclusively, wounding in leaves repressed IbHO expression and CO production, induced H2O2 generation and ERK phosphorylation, and then stimulated IPO expression.

Copper amine oxidase and phospholipase D act independently in abscisic acid (ABA)-induced stomatal closure in Vicia faba and Arabidopsis

Recent evidence has demonstrated that both copper amine oxidase (CuAO; EC 1.4.3.6) and phospholipase D (PLD; EC 3.1.4.4) are involved in abscisic acid (ABA)-induced stomatal closure. In this study, we investigated the interaction between CuAO and PLD in the ABA response. Pretreatment with either CuAO or PLD inhibitors alone or that with both additively led to impairment of ABA-induced H2O2 production and stomatal closure in Vicia faba. ABA-stimulated PLD activation could not be inhibited by the CuAO inhibitor, and CuAO activity was not affected by the PLD inhibitor. These data suggest that CuAO and PLD act independently in the ABA response. To further examine PLD and CuAO activities in ABA responses, we used the Arabidopsis mutants cuaoζ and pldα1. Ablation of guard cell-expressed CuAOζ or PLDα1 gene retarded ABA-induced H2O2 generation and stomatal closure. As a product of PLD, phosphatidic acid (PA) substantially enhanced H2O2 production and stomatal closure in wide type, pldα1, and cuaoζ. Moreover, putrescine (Put), a substrate of CuAO as well as an activator of PLD, induced H2O2 production and stomatal closure in WT but not in both mutants. These results suggest that CuAO and PLD act independently in ABA-induced stomatal closure.

Protein phosphorylation in stomatal movement

As research progresses on how guard cells perceive and transduce environmental cues to regulate stomatal movement, plant biologists are discovering key roles of protein phosphorylation. Early research efforts focused on characterization of ion channels and transporters in guard cell hormonal signaling. Subsequent genetic studies identified mutants of kinases and phosphatases that are defective in regulating guard cell ion channel activities, and recently proteins regulated by phosphorylation have been identified. Here we review the essential role of protein phosphorylation in ABA-induced stomatal closure and in blue light-induced stomatal opening. We also highlight evidence for the cross-talk between different pathways, which is mediated by protein phosphorylation.

Phosphorylation networks in the abscisic Acid signaling pathway

Abscisic acid (ABA) is one of the major phytohormones and regulates various processes in the plant life cycle, for example, seed development and abiotic/biotic stress responses. Recent studies have made significant progress in elucidating ABA signaling and established a simple ABA signaling model consisting of three core components: PYR/PYL/RCAR receptors, 2C-type protein phosphatases, and SnRK2 protein kinases. This model highlights the importance of protein phosphorylation mediated by SnRK2, but the downstream substrates of SnRK2 remain to be determined to complete the model. Previous studies have identified several SnRK2 substrates involving transcription factors and ion channels. Recently, SnRK2 substrates have been further surveyed by a phosphoproteomic approach, giving new insights on the SnRK2 downstream pathway. Other protein kinases, e.g., Ca(2+)-dependent protein kinase (CDPK) and mitogen-activated protein kinase (MAPK), have been identified as ABA signaling factors. Some evidence suggests that the SnRK2 pathway partially interacts with CDPK or MAPK pathways. In this chapter, recent advances in ABA signaling study are summarized, primarily focusing on two major protein kinases, SnRK2 and MAPK. Challenges for further study of the ABA-dependent protein phosphorylation network are also discussed.

The role of ABA and MAPK signaling pathways in plant abiotic stress responses

As sessile organisms, plants have developed specific mechanisms that allow them to rapidly perceive and respond to stresses in the environment. Among the evolutionarily conserved pathways, the ABA (abscisic acid) signaling pathway has been identified as a central regulator of abiotic stress response in plants, triggering major changes in gene expression and adaptive physiological responses. ABA induces protein kinases of the SnRK family to mediate a number of its responses. Recently, MAPK (mitogen activated protein kinase) cascades have also been shown to be implicated in ABA signaling. Therefore, besides discussing the role of ABA in abiotic stress signaling, we will also summarize the evidence for a role of MAPKs in the context of abiotic stress and ABA signaling.

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

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

Two guard cell-preferential MAPKs, MPK9 and MPK12, regulate YEL signalling in Arabidopsis guard cells

We report that two mitogen-activated protein kinases (MAPKs), MPK9 and MPK12, positively regulate abscisic acid (ABA)-induced stomatal closure in Arabidopsis thaliana. Yeast elicitor (YEL) induced stomatal closure accompanied by intracellular reactive oxygen species (ROS) accumulation and cytosolic free calcium concentration ([Ca(2+) ]cyt ) oscillation. In this study, we examined whether these two MAP kinases are involved in YEL-induced stomatal closure using MAPKK inhibitors, PD98059 and U0126, and MAPK mutants, mpk9, mpk12 and mpk9 mpk12. Both PD98059 and U0126 inhibited YEL-induced stomatal closure. YEL induced stomatal closure in the mpk9 and mpk12 mutants but not in the mpk9 mpk12 mutant, suggesting that a MAPK cascade involving MPK9 and MPK12 functions in guard cell YEL signalling. However, YEL induced extracellular ROS production, intracellular ROS accumulation and cytosolic alkalisation in the mpk9, mpk12 and mpk9 mpk12 mutants. YEL induced [Ca(2+) ]cyt oscillations in both wild type and mpk9 mpk12 mutant. These results suggest that MPK9 and MPK12 function redundantly downstream of extracellular ROS production, intracellular ROS accumulation, cytosolic alkalisation and [Ca(2+) ]cyt oscillation in YEL-induced stomatal closure in Arabidopsis guard cells and are shared with ABA signalling.

MAP-kinase activity in etiolated Cucumis sativus cotyledons: the effect of red and far-red light irradiation

Phytochrome (phy) signalling in plants may be transduced through protein phosphorylation. Mitogen-activated protein kinase (MAP-kinase, MAPK) activity and the effect of R (red) and FR (far-red) light irradiation on MAPK activity were studied in etiolated Cucumis sativus L. cotyledons. By in vitro protein phosphorylation and in-gel assays with myelin basic protein (MBP), a protein band (between 48 and 45 kDa) with MAPK-like activity was detected. The addition to the phosphorylation buffer of specific protein phosphatase (PTP) inhibitors (Na(3)VO(4) and NaF) and genistein, apigenin or PD98059 as MAPK inhibitors allowed us to confirm the MAPK activity of the protein band. Irradiation of etiolated cotyledons with FR light for 5, 10 or 60 min rapidly and transiently stimulated the MAPK activity of the protein band. This suggests that there was a very low fluence response (VLFR) of phys. In addition, 15 min of R light irradiation or a sequential treatment of 15 min of R plus 5 min of FR also increased MAPK activity. The stimulatory effect of R light was also attributed to the same photoreceptor, which suggests that MAPKs are involved in phytochrome signal transduction. Protein immunoprecipitation and immunoblotting analysis with the polyclonal antibody anti-pERK1/2 (Tyr 204) and the monoclonal antibody anti-phosphotyrosine PY20 allowed us to recognize the above mentioned protein band as two proteins with molecular masses (M(r)) of approximately 47 and 45 kDa, and MAPK activity. The biochemical and immunological properties showed by the proteins detected indicated that they were members of the MAPK family phosphorylated in tyrosine residues.

GsAPK, an ABA-activated and calcium-independent SnRK2-type kinase from G. soja, mediates the regulation of plant tolerance to salinity and ABA stress

Plant Snf1 (sucrose non-fermenting-1) related protein kinase (SnRK), a subfamily of serine/threonine kinases, has been implicated as a crucial upstream regulator of ABA and osmotic signaling as in many other signaling cascades. In this paper, we have isolated a novel plant specific ABA activated calcium independent protein kinase (GsAPK) from a highly salt tolerant plant, Glycine soja (50109), which is a member of the SnRK2 family. Subcellular localization studies using GFP fusion protein indicated that GsAPK is localized in the plasma membrane. We found that autophosphorylation and Myelin Basis Protein phosphorylation activity of GsAPK is only activated by ABA and the kinase activity also was observed when calcium was replaced by EGTA, suggesting its independence of calcium in enzyme activity. We also found that cold, salinity, drought, and ABA stress alter GsAPK gene transcripts and heterogonous overexpression of GsAPK in Arabidopsis alters plant tolerance to high salinity and ABA stress. In summary, we demonstrated that GsAPK is a Glycine soja ABA activated calcium independent SnRK-type kinase presumably involved in ABA mediated stress signal transduction.

Roles of mitogen-activated protein kinase cascades in ABA signaling

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

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

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

ABA signaling in stomatal guard cells: lessons from Commelina and Vicia

Abscisic acid (ABA) signaling mechanisms have been studied in a broad variety of plant species using complementary analyses, taking advantage of different methodologies suitable for each plant species. Early studies on ABA biosynthesis using Solanum lycopersicum mutants suggested an importance of ABA synthesis in stomatal closure. To understand ABA signaling in guard cells, cellular, biochemical and electrophysiological studies in Vicia faba and Commelina communis have been conducted, providing fundamental knowledge that was further reconfirmed by molecular genetic studies of Arabidopsis. In this article, examples of stomatal studies in several plants and prospects in ABA research are discussed.

Stomatal development and movement: the roles of MAPK signaling

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

A novel mitogen-activated protein kinase gene in maize (Zea mays), ZmMPK3, is involved in response to diverse environmental cues

In search for components of mitogen-activated protein kinase (MAPK) cascades in maize (Zea mays) involved in response to abscisic acid (ABA) stimulus, a novel MAPK gene, ZmMPK3, from ABA-treated maize leaves cDNA was isolated and characterized. The full length of the ZmMPK3 gene is 1 520 bp and encodes a 376 amino acid protein with a predicted molecular mass of 43.5 kD and a pI of 5.83. ZmMPK3 contains all 11 MAPK conserved subdomains and the phosphorylation motif TEY. Amino acid sequence alignment revealed that ZmMPK3 shared high identity with group-A MAPK in plants. A time course (30-360 min) experiment using a variety of signal molecules and stresses revealed that the transcripts level of ZmMPK3 accumulated markedly and rapidly when maize seedlings were subjected to exogenous signaling molecules: ABA, H2O2, jasmonic acid and salicylic acid, various abiotic stimuli such as cold, drought, ultraviolet light, salinity, heavy metal and mechanical wounding. Its transcription was also found to be tissue-specific regulated. Here, we show that ABA and H2O2 induced a significant increase in the ZmMPK3 activity using immunoprecipitation and in-gel kinase assay. Furthermore, the results showed that the ZmMPK3 protein is localized mainly to the nucleus. These results suggest that the ZmMPK3 may play an important role in response to environmental stresses.

The endogenous hormones in soybean seedlings under the joint actions of rare earth element La(III) and ultraviolet-B stress

The dynamic state of endogenous hormone content in soybean seedlings was investigated for a further demonstration of alleviating the damage of the ultraviolet ultraviolet-B (UV-B) radiation in the La(III)-treated soybean seedlings under UV-B stress. Using hydroponics culture, the effects of lanthanum(III) on the contents of endogenous hormone under elevated ultraviolet-B radiation (280–320 nm) was studied. The results showed that the content of indole-3-acetic acid (IAA) in soybean seedlings decreased initially and then increased when the seedlings underwent UV-B treatment during the stress and convalescent period; this was compared with a control; acetic acid oxidase (IAAO) activity increased at first (first to fifth day) and then decreased (sixth to 11th day). A similar change of abscisic acid content and IAAO content in soybean seedlings occurred; gibberellic acid (GA) content decreased during the experiment compared with control. The content of IAA and GA in soybean seedlings with La(III) + UV-B treatment was higher than those of UV-B treatment; IAAO activity and GA content in soybean seedlings with La (III) + UV-B treatment were lower than those of UV-B treatment. It suggested that the regulative effect of La(III) at the optimum concentration on endogenous hormone improved the ability of plant stress resistance, and its protective effect against low UV-B radiation was superior to high UV-B radiation. The defensive effect of La(III) on soybean seedlings under UV-B stress was carried out on the layer of defense system.

MAP kinases MPK9 and MPK12 are preferentially expressed in guard cells and positively regulate ROS-mediated ABA signaling

Reactive oxygen species (ROS) mediate abscisic acid (ABA) signaling in guard cells. To dissect guard cell ABA-ROS signaling genetically, a cell type-specific functional genomics approach was used to identify 2 MAPK genes, MPK9 and MPK12, which are preferentially and highly expressed in guard cells. To provide genetic evidence for their function, Arabidopsis single and double TILLING mutants that carry deleterious point mutations in these genes were isolated. RNAi-based gene-silencing plant lines, in which both genes are silenced simultaneously, were generated also. Mutants carrying a mutation in only 1 of these genes did not show any altered phenotype, indicating functional redundancy in these genes. ABA-induced stomatal closure was strongly impaired in 2 independent RNAi lines in which both MPK9 and MPK12 transcripts were significantly silenced. Consistent with this result, mpk9-1/12-1 double mutants showed an enhanced transpirational water loss and ABA- and H(2)O(2)-insensitive stomatal response. Furthermore, ABA and calcium failed to activate anion channels in guard cells of mpk9-1/12-1, indicating that these 2 MPKs act upstream of anion channels in guard cell ABA signaling. An MPK12-YFP fusion construct rescued the ABA-insensitive stomatal response phenotype of mpk9-1/12-1, demonstrating that the phenotype was caused by the mutations. The MPK12 protein is localized in the cytosol and the nucleus, and ABA and H(2)O(2) treatments enhance the protein kinase activity of MPK12. Together, these results provide genetic evidence that MPK9 and MPK12 function downstream of ROS to regulate guard cell ABA signaling positively.

Identity of an ABA-activated 46 kDa mitogen-activated protein kinase from Zea mays leaves: partial purification, identification and characterization

Mitogen-activated protein kinase (MAPK) cascades have been shown to be important components in abscisic acid (ABA) signal transduction pathway. In this study, a 46 kDa MAPK (p46MAPK) induced by ABA was partially purified from maize (Zea mays) by Q-Sepharose FF, Phenyl-Sepharose FF, Resource Q, Mono QTM 5/50 GL, poly-L-lysine-agarose, and Superdex 75 prep-grade columns, and was identified as ZmMAPK5 (gi|4239889) by the matrix-assisted laser desorption/ionization time-of-flight/time-of-flight (MALDI-TOF/TOF) mass spectrometry. Furthermore, the kinase showed optimal activity at pH 8.0, 30 degrees C, and 10 mM MgCl(2); the K(m) for myelin basic protein (MBP) substrate and ATP were 0.13 microg microl(-1) and 62 microM, respectively. MBP was the preferred substrate, of which the threonine residue was phosphorylated. Finally, the kinase was found to respond to diverse extracellular stimuli. These results enable us to further reveal the function of the ZmMAPK5 in ABA signaling.

Positive feedback regulation of maize NADPH oxidase by mitogen-activated protein kinase cascade in abscisic acid signalling

In maize (Zea mays), abscisic acid (ABA)-induced H(2)O(2) production activates a 46 kDa mitogen-activated protein kinase (p46MAPK), and the activation of p46MAPK also regulates the production of H(2)O(2). However, the mechanism for the regulation of H(2)O(2) production by MAPK in ABA signalling remains to be elucidated. In this study, four reactive oxygen species (ROS)-producing NADPH oxidase (rboh) genes (ZmrbohA-D) were isolated and characterized in maize leaves. ABA treatment induced a biphasic response (phase I and phase II) in the expression of ZmrbohA-D and the activity of NADPH oxidase. Phase II induced by ABA was blocked by pretreatments with two MAPK kinase (MPKKK) inhibitors and two H(2)O(2) scavengers, but phase I was not affected by these inhibitors or scavengers. Treatment with H(2)O(2) alone also only induced phase II, and the induction was arrested by the MAPKK inhibitors. Furthermore, the ABA-activated p46MAPK was partially purified. Using primers corresponding to the sequences of internal tryptic peptides, the p46MAPK gene was cloned. Analysis of the tryptic peptides and the p46MAPK sequence indicate it is the known ZmMPK5. Treatments with ABA and H(2)O(2) led to a significant increase in the activity of ZmMPK5, although ABA treatment only induced a slight increase in the expression of ZmMPK5. The data indicate that H(2)O(2)-activated ZmMPK5 is involved in the activation of phase II in ABA signalling, but not in phase I. The results suggest that there is a positive feedback loop involving NADPH oxidase, H(2)O(2), and ZmMPK5 in ABA signalling.

MEK1/2 and p38-like MAP kinase successively mediate H(2)O(2) signaling in Vicia guard cell

As a second messenger, H(2)O(2) generation and signal transduction is subtly controlled and involves various signal elements, among which are the members of MAP kinase family. The increasing evidences indicate that both MEK1/2 and p38-like MAP protein kinase mediate ABA-induced H(2)O(2) signaling in plant cells. Here we analyze the mechanisms of similarity and difference between MEK1/2 and p38-like MAP protein kinase in mediating ABA-induced H(2)O(2) generation, inhibition of inward K(+) currents, and stomatal closure. These data suggest that activation of MEK1/2 is prior to p38-like protein kinase in Vicia guard cells.

Nitric oxide signaling in plant responses to abiotic stresses

Nitric oxide (NO) plays important roles in diverse physiological processes in plants. NO can provoke both beneficial and harmful effects, which depend on the concentration and location of NO in plant cells. This review is focused on NO synthesis and the functions of NO in plant responses to abiotic environmental stresses. Abiotic stresses mostly induce NO production in plants. NO alleviates the harmfulness of reactive oxygen species, and reacts with other target molecules, and regulates the expression of stress responsive genes under various stress conditions.

Arabidopsis MAPKs: a complex signalling network involved in multiple biological processes

Many changes in environmental conditions and hormones are mediated by MAPK (mitogen-activated protein kinase) cascades in all eukaryotes, including plants. Studies of MAPK pathways in genetic model organisms are especially informative in revealing the molecular mechanisms by means of which MAPK cascades are controlled and modulate cellular processes. The present review highlights recent insights into MAPK-based signalling in Arabidopsis thaliana (thale cress), revealing the complexity and future challenges to understanding signal-transduction networks on a global scale.

The involvement of a P38-like MAP kinase in ABA-induced and H2O2-mediated stomatal closure in Vicia faba L

SB203580 is a specific inhibitor of p38 mitogen-activated protein (MAP) kinase and has been widely used to investigate the physiological roles of p38 in animal and yeast cells. Here by using an epidermal strip bioassay, laser-scanning confocal microscopy and whole-cell patch clamp analysis, we assess the effects of pyridinyl imidazoles-like SB203580 on the H(2)O(2) signaling in guard cells of Vicia faba L. The results indicated that SB203580 blocks H(2)O(2)- or ABA-induced stomatal closure, ABA-induced H(2)O(2) generation, and decrease in K(+) fluxing across plasma membrane of Vicia guard cells by application of ABA and H(2)O(2), whereas its analog SB202474 had no effect on these events. Thus, these results suggest that activation of p38-like MAP kinase modulates guard cell ROS signaling in response to stress.