Two coupled components of the mitogen-activated protein kinase cascade MdMPK1 and MdMKK1 from apple function in ABA signal transduction

Apple E3 ligase MdPUB23 mediates ubiquitin-dependent degradation of MdABI5 to delay ABA-triggered leaf senescence

ABSCISIC ACID-INSENSITIVE5 (ABI5) is a core regulatory factor that mediates the ABA signaling response and leaf senescence. However, the molecular mechanism underlying the synergistic regulation of leaf senescence by ABI5 with interacting partners and the homeostasis of ABI5 in the ABA signaling response remain to be further investigated. In this study, we found that the accelerated effect of MdABI5 on leaf senescence is partly dependent on MdbHLH93, an activator of leaf senescence in apple. MdABI5 directly interacted with MdbHLH93 and improved the transcriptional activation of the senescence-associated gene MdSAG18 by MdbHLH93. MdPUB23, a U-box E3 ubiquitin ligase, physically interacted with MdABI5 and delayed ABA-triggered leaf senescence. Genetic and biochemical analyses suggest that MdPUB23 inhibited MdABI5-promoted leaf premature senescence by targeting MdABI5 for ubiquitin-dependent degradation. In conclusion, our results verify that MdABI5 accelerates leaf senescence through the MdABI5-MdbHLH93-MdSAG18 regulatory module, and MdPUB23 is responsible for the dynamic regulation of ABA-triggered leaf senescence by modulating the homeostasis of MdABI5.

Genome-Wide Identification and Expression of MAPK Gene Family in Cultivated Strawberry and Their Involvement in Fruit Developing and Ripening

Studies on many plants have shown that mitogen-activated protein kinases (MAPKs) are key proteins involved in regulating plant responses to biotic and abiotic stresses. However, their involvement in cultivated strawberry development and ripening remains unclear. In this study, 43 FaMAPK gene family members were identified in the genome of cultivated strawberry (Fragaria × ananassa), phylogenetic analysis indicated that FaMAPKs could be classified into four groups. Systematic analysis of the conserved motif, exon-intron structure showed that there were significant varieties between different groups in structure, but in the same group they were similar. Multiple cis-regulatory elements associated with phytohormone response, and abiotic and biotic stresses were predicted in the promoter regions of FaMAPK genes. Transcriptional analysis showed that all FaMAPK genes were expressed at all developmental stages. Meanwhile, the effect of exogenous ABA and sucrose on the expression profile of FaMAPKs was investigated. Exogenous ABA, sucrose, and ABA plus sucrose treatments upregulated the expression of FaMAPK genes and increased the content of endogenous ABA, sucrose, and anthocyanin in strawberry fruits, suggesting that ABA and sucrose might be involved in the FaMAPK-mediated regulation of strawberry fruit ripening. Based on the obtained results, MAPK genes closely related to the ripening of strawberries were screened to provide a theoretical basis and support for future research on strawberries.

The mitogen-activated protein kinase kinase TaMKK5 mediates immunity via the TaMKK5-TaMPK3-TaERF3 module

Sharp eyespot disease, caused by the soil-borne fungus Rhizoctonia cerealis, seriously threatens production of wheat (Triticum aestivum). Despite considerable advances in understanding the mechanisms of mitogen-activated protein kinase (MAPK) cascades in innate immunity in model plant species, the roles of MAPK cascades in wheat are unknown. In this study, we identified a wheat MAPK kinase TaMKK5, located on chromosome 6B, and deciphered its functional role in the innate immune responses to R. cerealis attack. The TaMKK5-6B transcript level was elevated after R. cerealis infection and was higher in resistant wheat genotypes compared to susceptible genotypes. Overexpressing TaMKK5-6B increased resistance to sharp eyespot and upregulated the expression of multiple defense-related genes in wheat, including the MAPK gene TaMPK3, the ethylene response factor gene TaERF3, the calcium-dependent protein kinase gene TaCPK7-D, the glutathione s-transferase-1 gene TaGST1, Defensin, and Chitinase 2, while TaMKK5 knock-down compromised the resistance and repressed the expression of these defense-related genes. Bimolecular fluorescence complementation, yeast two-hybrid, pull-down, and phosphorylation assays showed that TaMKK5 physically interacted with TaMPK3, and phosphorylated and activated TaMPK3, and that TaMPK3 interacted with and phosphorylated TaERF3. The TaMKK5-TaMPK3 cascade modulates the expression of TaGST1, Defensin, and Chitinase 2 through TaERF3. Collectively, TaMKK5 mediates resistance to sharp eyespot through the TaMKK5-TaMPK3-TaERF3 module and by upregulating the expression of defense-related genes in wheat. This study provides insights into the role of the wheat MAPK cascades in innate immunity. TaMKK5-6B is a promising gene for breeding wheat cultivars that are resistant to sharp eyespot.

A celery transcriptional repressor AgERF8 negatively modulates abscisic acid and salt tolerance

Ethylene response factors (ERFs) widely exist in plants and have been reported to be an important regulator of plant abiotic stress. Celery, a common economic vegetable of Apiaceae, contains lots of ERF transcription factors (TFs) with various functions. AP2/ERF TFs play positive or negative roles in plant growth and stress response. Here, AgERF8, a gene encoding EAR-type AP2/ERF TF, was identified. The AgERF8 mRNA accumulated in response to both abscisic acid (ABA) signaling and salt treatment. AgERF8 was proving to be a nucleus-located protein and could bind to GCC-box. The overexpression of AgERF8 in Arabidopsis repressed the transcription of downstream genes, AtBGL and AtBCH. Arabidopsis overexpressing AgERF8 gene showed inhibited root growth under ABA and NaCl treatments. AgERF8 transgenic lines showed low tolerance to ABA and salt stress than wild-type plants. Low increment in SOD and POD activities, increased accumulation of MDA, and significantly decreased plant fresh weights and chlorophyll levels were detected in AgERF8 hosting lines after treated with ABA and NaCl. Furthermore, the overexpression of AgERF8 also inhibited the levels of ascorbic acid and antioxidant-related genes (AtCAT1, AtSOD1, AtPOD, AtSOS1, AtAPX1, and AtP5CS1) expression in transgenic Arabidopsis. This finding indicated that AgERF8 negatively affected the resistance of transgenic Arabidopsis to ABA and salt stress through regulating downstream genes expression and relevant physiological changes. It will provide a potential sight to further understand the functions of ERF TFs in celery.

The cotton MAPK kinase GhMPK20 negatively regulates resistance to Fusarium oxysporum by mediating the MKK4-MPK20-WRKY40 cascade

Fusarium wilt is one of the most serious diseases affecting cotton. However, the pathogenesis and mechanism by which Fusarium oxysporum overcomes plant defence responses are unclear. Here, a new group D mitogen-activated protein kinase (MAPK) gene, GhMPK20, was identified and functionally analysed in cotton. GhMPK20 expression was significantly induced by F. oxysporum. Virus-induced gene silencing (VIGS) of GhMPK20 in cotton increased the tolerance to F. oxysporum, whereas ectopic GhMPK20 overexpression in Nicotiana benthamiana reduced F. oxysporum resistance via disruption of the salicylic acid (SA)-mediated defence pathway. More importantly, an F. oxysporum-induced MAPK cascade pathway composed of GhMKK4, GhMPK20 and GhWRKY40 was identified. VIGS of GhMKK4 and GhWRKY40 also enhanced F. oxysporum resistance in cotton, and the function of GhMKK4-GhMPK20 was shown to be essential for F. oxysporum-induced GhWRKY40 expression. Together, our results indicate that the GhMKK4-GhMPK20-GhWRKY40 cascade in cotton plays an important role in the pathogenesis of F. oxysporum. This research broadens our knowledge of the negative role of the MAPK cascade in disease resistance in cotton and provides an important scientific basis for the formulation of Fusarium wilt prevention strategies.

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.

The grapevine kinome: annotation, classification and expression patterns in developmental processes and stress responses

Protein kinases (PKs) have evolved as the largest family of molecular switches that regulate protein activities associated with almost all essential cellular functions. Only a fraction of plant PKs, however, have been functionally characterized even in model plant species. In the present study, the entire grapevine kinome was identified and annotated using the most recent version of the grapevine genome. A total of 1168 PK-encoding genes were identified and classified into 20 groups and 121 families, with the RLK-Pelle group being the largest, with 872 members. The 1168 kinase genes were unevenly distributed over all 19 chromosomes, and both tandem and segmental duplications contributed to the expansion of the grapevine kinome, especially of the RLK-Pelle group. Ka/Ks values indicated that most of the tandem and segmental duplication events were under purifying selection. The grapevine kinome families exhibited different expression patterns during plant development and in response to various stress treatments, with many being coexpressed. The comprehensive annotation of grapevine kinase genes, their patterns of expression and coexpression, and the related information facilitate a more complete understanding of the roles of various grapevine kinases in growth and development, responses to abiotic stress, and evolutionary history.

Transcriptome-based gene expression profiling identifies differentially expressed genes critical for salt stress response in radish (Raphanus sativus L.)

Transcriptome-based gene expression analysis identifies many critical salt-responsive genes in radish and facilitates further dissecting the molecular mechanism underlying salt stress response. Salt stress severely impacts plant growth and development. Radish, a moderately salt-sensitive vegetable crop, has been studied for decades towards the physiological and biochemical performances under salt stress. However, no systematic study on isolation and identification of genes involved in salt stress response has been performed in radish, and the molecular mechanism governing this process is still indistinct. Here, the RNA-Seq technique was applied to analyze the transcriptomic changes on radish roots treated with salt (200 mM NaCl) for 48 h in comparison with those cultured in normal condition. Totally 8709 differentially expressed genes (DEGs) including 3931 up- and 4778 down-regulated genes were identified. Functional annotation analysis indicated that many genes could be involved in several aspects of salt stress response including stress sensing and signal transduction, osmoregulation, ion homeostasis and ROS scavenging. The association analysis of salt-responsive genes and miRNAs exhibited that 36 miRNA-mRNA pairs had negative correlationship in expression trends. Reverse-transcription quantitative PCR (RT-qPCR) analysis revealed that the expression profiles of DEGs were in line with results from the RNA-Seq analysis. Furthermore, the putative model of DEGs and miRNA-mediated gene regulation was proposed to elucidate how radish sensed and responded to salt stress. This study represents the first comprehensive transcriptome-based gene expression profiling under salt stress in radish. The outcomes of this study could facilitate further dissecting the molecular mechanism underlying salt stress response and provide a valuable platform for further genetic improvement of salt tolerance in radish breeding programs.

Arabidopsis ABA-Activated Kinase MAPKKK18 is Regulated by Protein Phosphatase 2C ABI1 and the Ubiquitin-Proteasome Pathway

Phosphorylation and dephosphorylation events play an important role in the transmission of the ABA signal. Although SnRK2 [sucrose non-fermenting1-related kinase2] protein kinases and group A protein phosphatase type 2C (PP2C)-type phosphatases constitute the core ABA pathway, mitogen-activated protein kinase (MAPK) pathways are also involved in plant response to ABA. However, little is known about the interplay between MAPKs and PP2Cs or SnRK2 in the regulation of ABA pathways. In this study, an effort was made to elucidate the role of MAP kinase kinase kinase18 (MKKK18) in relation to ABA signaling and response. The MKKK18 knockout lines showed more vigorous root growth, decreased abaxial stomatal index and increased stomatal aperture under normal growth conditions, compared with the control wild-type Columbia line. In addition to transcriptional regulation of the MKKK18 promoter by ABA, we demonstrated using in vitro and in vivo kinase assays that the kinase activity of MKKK18 was regulated by ABA. Analysis of the cellular localization of MKKK18 showed that the active kinase was targeted specifically to the nucleus. Notably, we identified abscisic acid insensitive 1 (ABI1) PP2C as a MKKK18-interacting protein, and demonstrated that ABI1 inhibited its activity. Using a cell-free degradation assay, we also established that MKKK18 was unstable and was degraded by the proteasome pathway. The rate of MKKK18 degradation was delayed in the ABI1 knockout line. Overall, we provide evidence that ABI1 regulates the activity and promotes proteasomal degradation of MKKK18.

A cotton Raf-like MAP3K gene, GhMAP3K40, mediates reduced tolerance to biotic and abiotic stress in Nicotiana benthamiana by negatively regulating growth and development

Mitogen-activated protein kinase (MAPK) cascades mediate various responses in plants. As the top component, MAP3Ks deserve more attention; however, little is known about the role of MAP3Ks, especially in cotton, a worldwide economic crop. In this study, a gene encoding a putative Raf-like MAP3K, GhMAP3K40, was isolated. GhMAP3K40 expression was induced by stress and multiple signal molecules. The plants overexpressing GhMAP3K40 had an enhanced tolerance to drought and salt stress at the germination stage. However, at the seedling stage, the transgenic plants suffered more severe damage after drought, exposure to pathogens and oxidative stress. The defence-related genes and the antioxidant system were activated in transgenic palnts, suggesting that GhMAP3K40 positively regulate the defence response. The transgenic plants were less able to prevent pathogenic invasion, which was due to defects in the cell structure of the leaves. The root system of the control plants were stronger compared with the transgenic plants. These results indicated a negative role of GhMAP3K40 in growth and development and GhMAP3K40 possibly caused the defects by down-regulating the lignin biosynthesis. Overall, these results suggest that GhMAP3K40 may positively regulate defence response but cause reduced tolerance to biotic and abiotic stress by negatively regulating growth and development.

Transcriptomic analysis of Litchi chinensis pericarp during maturation with a focus on chlorophyll degradation and flavonoid biosynthesis

Background

The fruit of litchi (Litchi chinensis) comprises a white translucent edible aril surrounded by a pericarp. The pericarp of litchi has been the focus of studies associated with fruit size, coloration, cracking and shelf life. However, research at the molecular level has been limited by the lack of genomic and transcriptomic information. In this study, an analysis of the transcriptome of litchi pericarp was performed to obtain information regarding the molecular mechanisms underlying the physiological changes in the pericarp, including those leading to fruit surface coloration.

Results

Coincident with the rapid break down of chlorophyll, but substantial increase of anthocyanins in litchi pericarp as fruit developed, two major physiological changes, degreening and pigmentation were visually apparent. In this study, a cDNA library of litchi pericarp with three different coloration stages was constructed. A total of 4.7 Gb of raw RNA-Seq data was generated and this was then de novo assembled into 51,089 unigenes with a mean length of 737 bp. Approximately 70% of the unigenes (34,705) could be annotated based on public protein databases and, of these, 3,649 genes were significantly differentially expressed between any two coloration stages, while 156 genes were differentially expressed among all three stages. Genes encoding enzymes involved in chlorophyll degradation and flavonoid biosynthesis were identified in the transcriptome dataset. The transcript expression patterns of the Stay Green (SGR) protein suggested a key role in chlorophyll degradation in the litchi pericarp, and this conclusion was supported by the result of an assay over-expressing LcSGR protein in tobacco leaves. We also found that the expression levels of most genes especially late anthocyanin biosynthesis genes were co-ordinated up-regulated coincident with the accumulation of anthocyanins, and that candidate MYB transcription factors that likely regulate flavonoid biosynthesis were identified.

Conclusions

This study provides a large collection of transcripts and expression profiles associated with litchi fruit maturation processes, including coloration. Since most of the unigenes were annotated, they provide a platform for litchi functional genomic research within this species.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1433-4) contains supplementary material, which is available to authorized users.

Mitogen-activated protein kinase cascades in Vitis vinifera

Protein phosphorylation is one of the most important mechanisms to control cellular functions in response to external and endogenous signals. Mitogen-activated protein kinases (MAPK) are universal signaling molecules in eukaryotes that mediate the intracellular transmission of extracellular signals resulting in the induction of appropriate cellular responses. MAPK cascades are composed of four protein kinase modules: MAPKKK kinases (MAPKKKKs), MAPKK kinases (MAPKKKs), MAPK kinases (MAPKKs), and MAPKs. In plants, MAPKs are activated in response to abiotic stresses, wounding, and hormones, and during plant pathogen interactions and cell division. In this report, we performed a complete inventory of MAPK cascades genes in Vitis vinifera, the whole genome of which has been sequenced. By comparison with MAPK, MAPK kinases, MAPK kinase kinases and MAPK kinase kinase kinase kinase members of Arabidopsis thaliana, we revealed the existence of 14 MAPKs, 5 MAPKKs, 62 MAPKKKs, and 7 MAPKKKKs in Vitis vinifera. We identified orthologs of V. vinifera putative MAPKs in different species, and ESTs corresponding to members of MAPK cascades in various tissues. This work represents the first complete inventory of MAPK cascades in V. vinifera and could help elucidate the biological and physiological functions of these proteins in V. vinifera.

GhMPK17, a cotton mitogen-activated protein kinase, is involved in plant response to high salinity and osmotic stresses and ABA signaling

Mitogen-activated protein kinase (MAPK) cascades play pivotal roles in mediating biotic and abiotic stress responses. Cotton (Gossypium hirsutum) is the most important textile crop in the world, and often encounters abiotic stress during its growth seasons. In this study, a gene encoding a mitogen-activated protein kinase (MAPK) was isolated from cotton, and designated as GhMPK17. The open reading frame (ORF) of GhMPK17 gene is 1494 bp in length and encodes a protein with 497 amino acids. Quantitative RT-PCR analysis indicated that GhMPK17 expression was up-regulated in cotton under NaCl, mannitol and ABA treatments. The transgenic Arabidopsis plants expressing GhMPK17 gene showed higher seed germination, root elongation and cotyledon greening/expansion rates than those of the wild type on MS medium containing NaCl, mannitol and exogenous ABA, suggesting that overexpression of GhMPK17 in Arabidopsis increased plant ABA-insensitivity, and enhanced plant tolerance to salt and osmotic stresses. Furthermore, overexpression of GhMPK17 in Arabidopsis reduced H2O2 level and altered expression of ABA- and abiotic stress-related genes in the transgenic plants. Collectively, these data suggested that GhMPK17 gene may be involved in plant response to high salinity and osmotic stresses and ABA signaling.

Arabidopsis SAG protein containing the MDN1 domain participates in seed germination and seedling development by negatively regulating ABI3 and ABI5

Three proteins containing a midasin homologue 1 (MDN1) domain from the yeast Solanum chacoense and Arabidopsis thaliana have important functions in yeast survival, seed development, and female gametogenesis. In this study, a novel protein containing the MDN1 domain from Arabidopsis negatively regulated abscisic acid (ABA) signalling during seed germination. Seeds of a T-DNA insertion line of this gene exhibited increased sensitivity to ABA during seed germination and seedling development (named sag). By contrast, seeds with overexpressed AtSAG (OX2) were less sensitive to ABA. The seeds of the sag mutant showed similar sensitivity to high concentrations of mannitol and NaCl during these stages. AtSAG was also highly expressed in germinating seeds. However, ABA-induced AtSAG expression remained almost unchanged. ABA-responsive marker genes, including ABI3, ABI5, Em1, Em6, RD29A, and RAB18, were upregulated in sag mutants but were downregulated in OX2. Genetic analyses indicated that the function of AtSAG in ABA signalling depended on ABI3 and ABI5. The expression of some target genes of ABI3 and ABI5, such as seed storage protein and oleosin genes, was induced higher by ABA in sag mutants than in wild-type germinated seeds, even higher than in abi5 mutants. This finding indicated that other regulators similar to ABI3 or ABI5 played a role during these stages. Taken together, these results indicate that AtSAG is an important negative regulator of ABA signalling during seed germination and seedling development.

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.

Overexpression of cotton GhMKK4 enhances disease susceptibility and affects abscisic acid, gibberellin and hydrogen peroxide signalling in transgenic Nicotiana benthamiana

Mitogen-activated protein kinase (MAPK) cascades are involved in plant development, stress responses and hormonal signal transduction. MAPK kinases (MAPKKs), as the key nodes in these cascades, link MAPKs and MAPKK kinases (MAPKKKs). In this study, GhMKK4, a novel group C MAPKK gene from cotton (Gossypium hirsutum), was isolated and identified. Its expression can be induced by various stresses and signalling molecules. The overexpression of GhMKK4 in Nicotiana benthamiana enhanced its susceptibility to bacterial and fungal pathogens, but had no significant effects on salt or drought tolerance. Notably, the overexpressing plants showed increased sensitivity to abscisic acid (ABA) and gibberellin A3 (GA3), and ABA and gibberellin (GA) signalling were affected on infection with Ralstonia solanacearum bacteria. Furthermore, the overexpressing plants showed more reactive oxygen species (ROS) accumulation and stronger inhibition of catalase (CAT), a ROS-scavenging enzyme, than control plants after salicylic acid (SA) treatment. Interestingly, two genes encoding ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (SAMDC), the key enzymes in polyamine synthesis, exhibited reduced R. solanacearum-induced expression in overexpressing plants. These findings broaden our knowledge about the functions of MAPKKs in diverse signalling pathways and the negative regulation of disease resistance in the cotton crop.

Identification of mitogen-activated protein kinase kinase gene family and MKK-MAPK interaction network in maize

Plant mitogen-activated protein kinases (MAPK) are involved in important processes, including stress signaling and development. MAPK kinases (MAPKK, MKK) have been investigated in several plant species including Arabidopsis thaliana, Oryza sativa, Populus trichocarpa, and Brachypodium distachyon. In the present study, nine putative maize MKK genes have been identified. Analysis of the conserved protein motifs, exon-intron junctions and intron phase has revealed high levels of conservation within the phylogenetic groups. Next, we defined four new ZmMKK-ZmMPK interactions using yeast two-hybrid. Finally, we examined the biological functions of the ZmMKK4 gene. Overexpression of ZmMKK4 in Arabidopsis conferred tolerance to oxidative stress by increased germination rate and early seedling growth compared with WT plants. Taken together, we provide a comprehensive bioinformatics analysis of the MKK gene family in maize genome and our data provide an important foundation for further functional study of MAPK and MKK families in maize.

Silencing the SpMPK1, SpMPK2, and SpMPK3 genes in tomato reduces abscisic acid-mediated drought tolerance

Drought is a major threat to agriculture production worldwide. Mitogen-activated protein kinases (MAPKs) play a pivotal role in sensing and converting stress signals into appropriate responses so that plants can adapt and survive. To examine the function of MAPKs in the drought tolerance of tomato plants, we silenced the SpMPK1, SpMPK2, and SpMPK3 genes in wild-type plants using the virus-induced gene silencing (VIGS) method. The results indicate that silencing the individual genes or co-silencing SpMPK1, SpMPK2, and SpMPK3 reduced the drought tolerance of tomato plants by varying degrees. Co-silencing SpMPK1 and SpMPK2 impaired abscisic acid (ABA)-induced and hydrogen peroxide (H₂O₂)-induced stomatal closure and enhanced ABA-induced H₂O₂ production. Similar results were observed when silencing SpMPK3 alone, but not when SpMPK1 and SpMPK2 were individually silenced. These data suggest that the functions of SpMPK1 and SpMPK2 are redundant, and they overlap with that of SpMPK3 in drought stress signaling pathways. In addition, we found that SpMPK3 may regulate H₂O₂ levels by mediating the expression of CAT1. Hence, SpMPK1, SpMPK2, and SpMPK3 may play crucial roles in enhancing tomato plants’ drought tolerance by influencing stomatal activity and H₂O₂ production via the ABA-H₂O₂ pathway.

Cotton GhMPK6a negatively regulates osmotic tolerance and bacterial infection in transgenic Nicotiana benthamiana, and plays a pivotal role in development

Mitogen-activated protein kinase (MAPK) cascades play important roles in the perception of external signals and the generation of suitable responses. Cotton (Gossypium hirsutum) is an important fibre-producing and oil-producing crop worldwide. However, few MAPKs and their interaction partners have been functionally characterized in cotton. In the present study, the group A MAPK G. hirsutum (Gh)MPK6a was identified and characterized. GhMPK6a expression can be induced through multiple defence-related signal molecules and abiotic and biotic stresses. The ectopic expression of GhMPK6a in Nicotiana benthamiana reduced drought and salt tolerance, with elevated malondialdehyde content, higher reactive oxygen species content and lower abscisic acid content than in wild-type plants. Moreover, plants overexpressing GhMPK6a were sensitive to the bacterial pathogen Ralstonia solanacearum. Histochemical analysis of β-glucuronidase activity revealed that GhMPK6a showed tissue-specific expression during postgermination development, mixed bud differentiation, and pollination. Most importantly, GhMPK6a interacts with the upstream MAPK kinase GhMKK4, as shown by the use of yeast two-hybrid and bimolecular fluorescence complementation systems, compensating for a deficiency of MAPK interaction partners in cotton crops. Taken together, these results suggest that GhMPK6a negatively regulates osmotic stress and bacterial infection, and plays an important role in developmental processes. These results provide useful information for elucidating the roles of MAPK cascades in cotton crops.

STRUCTURED DIGITAL ABSTRACT

GhMPK6a physically interacts with GhMPK4 by two hybrid (View interaction) GhMPK6a and GhMPK4 physically interact by bimolecular fluorescence complementation (View interaction).

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.

Arabidopsis ALTERED MERISTEM PROGRAM 1 negatively modulates plant responses to abscisic acid and dehydration stress

Abscisic acid (ABA) is an important signaling molecule with multiple biological functions in seed germination, plant development and stress responses. ALTERED MERISTEM PROGRAM 1 (AMP1), encoding a putative glutamate carboxypeptidase, is involved in plant growth, morphogenesis and seed dormancy. In this study, we assigned new role for AtAMP1 in ABA signaling and dehydration stress. AtAMP1 was transcriptionally down-regulated by ABA. Loss-of-function mutant of AtAMP1 (amp1-1, encoding a premature stop codon in AtAMP1) resulted in hypersensitive phenotypes toward ABA-mediated seed germination and primary root elongation. The amp1-1 mutant also exhibited enhanced dehydration resistance, as evidenced by the changes of electrolyte leakage (EL), water loss rate and survival rate. Notably, the amp1-1 lines exhibited higher expression levels of ABA-responsive genes (RAB18, RD29A and RD29B), higher concentration of proline and lower reactive oxygen species (ROS) levels (H2O2 and O2(-)) after ABA and dehydration treatments than those of wild type. Taken together, these observations indicated a negative role for AtAMP1 in ABA-mediated seed germination, seedling development and dehydration stress response.

ZmMKK3, a novel maize group B mitogen-activated protein kinase kinase gene, mediates osmotic stress and ABA signal responses

Mitogen-activated protein kinase (MAPK) cascades are important intracellular signaling modules and function as a convergent point for crosstalk during abiotic stress signaling. In this article, we isolated a novel group B MAPKK gene, ZmMKK3, from Zea mays. ZmMKK3 protein might be localized in both the cytoplasm and the nucleus. RNA blot analysis indicated that the ZmMKK3 transcription was up-regulated by abscisic acid (ABA), hydrogen peroxide (H(2)O(2)) and PEG, and that H(2)O(2) mediated PEG-induced expression of ZmMKK3. Constitutive expression of ZmMKK3 in Nicotiana tabacum reduced H(2)O(2) accumulation under osmotic stress by affecting antioxidant defense systems and alleviated reactive oxygen species-mediated injury under oxidative stress. Transgenic tobacco exhibited attenuated ABA sensitivity by means of an increased germination rate and main root growth. Taken together, these results indicate that ZmMKK3 is a positive regulator of osmotic tolerance and ABA signaling in plants.

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.

ZmMKK4, a novel group C mitogen-activated protein kinase kinase in maize (Zea mays), confers salt and cold tolerance in transgenic Arabidopsis

Mitogen-activated protein kinase (MAPK) cascades are signalling modules that transduce extracellular signalling to a range of cellular responses. Plant MAPK cascades have been implicated in development and stress response. In this study, we isolated a novel group C MAPKK gene, ZmMKK4, from maize. Northern blotting analysis revealed that the ZmMKK4 transcript expression was up-regulated by cold, high salt and exogenous H(2)O(2,) but down-regulated by exogenous abscisic acid (ABA). Over-expression of ZmMKK4 in Arabidopsis conferred tolerance to cold and salt stresses by increased germination rate, lateral root numbers, plant survival rate, chlorophyll, proline and soluble sugar contents, and antioxidant enzyme [peroxidase (POD), catalase (CAT)] activities compared with control plants. Furthermore, ZmMKK4 enhanced a 37 kDa kinase activity after cold and salt stresses. RT-PCR analysis revealed that the transcript levels of stress-responsive transcription factors and functional genes were higher in ZmMKK4-over-expressing plants than in control plants. In addition, ZmMKK4 protein is localized in the nucleus. Taken together, these results indicate that ZmMKK4 is a positive regulator of salt and cold tolerance in plants.