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

Identification and analysis of MAPK cascade gene families of Camellia oleifera and their roles in response to cold stress

BACKGROUND

Low-temperature severely limits the growth and development of Camellia oleifera (C. oleifera). The mitogen-activated protein kinase (MAPK) cascade plays a key role in the response to cold stress.

METHODS AND RESULTS

Our study aims to identify MAPK cascade genes in C. oleifera and reveal their roles in response to cold stress. In our study, we systematically identified and analyzed the MAPK cascade gene families of C. oleifera, including their physical and chemical properties, conserved motifs, and multiple sequence alignments. In addition, we characterized the interacting networks of MAPKK kinase (MAPKKK)-MAPK kinase (MAPKK)-MAPK in C. oleifera. The molecular mechanism of cold stress resistance of MAPK cascade genes in wild C. oleifera was analyzed by differential gene expression and real-time quantitative reverse transcription-PCR (qRT-PCR).

CONCLUSION

In this study, 21 MAPKs, 4 MAPKKs and 55 MAPKKKs genes were identified in the leaf transcriptome of C. oleifera. According to the phylogenetic results, MAPKs were divided into 4 groups (A, B, C and D), MAPKKs were divided into 3 groups (A, B and D), and MAPKKKs were divided into 2 groups (MEKK and Raf). Motif analysis showed that the motifs in each subfamily were conserved, and most of the motifs in the same subfamily were basically the same. The protein interaction network based on Arabidopsis thaliana (A. thaliana) homologs revealed that MAPK, MAPKK, and MAPKKK genes were widely involved in C. oleifera growth and development and in responses to biotic and abiotic stresses. Gene expression analysis revealed that the CoMAPKKK5/CoMAPKKK43/CoMAPKKK49-CoMAPKK4-CoMAPK8 module may play a key role in the cold stress resistance of wild C. oleifera at a high-elevation site in Lu Mountain (LSG). This study can facilitate the mining and utilization of genetic resources of C. oleifera with low-temperature tolerance.

Molecular mechanisms underlying the signal perception and transduction during seed germination

QuerySeed germination is a vital step in the life cycle of a plant, playing a significant role in seedling establishment and crop yield potential. It is also an important factor in the conservation of plant germplasm resources. This complex process is influenced by a myriad of factors, including environmental conditions, the genetic makeup of the seed, and endogenous hormones. The perception of these environmental signals triggers a cascade of intricate signal transduction events that determine whether a seed germinates or remains dormant. Despite considerable progress in uncovering the molecular mechanisms governing these processes, many questions remain unanswered. In this review, we summarize the current progress in the molecular mechanisms underlying the perception of environmental signals and consequent signal transduction during seed germination, and discuss questions that need to be addressed to better understand the process of seed germination and develop novel strategies for germplasm improvement.

Dephosphorylation of the MAP kinases MPK6 and MPK3 fine-tunes responses to wounding and herbivory in Arabidopsis

The Arabidopsis MAP Kinases (MAPKs) MPK6 and MPK3 and orthologs in other plants function as major stress signaling hubs. MAPKs are activated by phosphorylation and are negatively regulated by MAPK-inactivating phosphatases (MIPPs), which alter the intensity and duration of MAPK signaling via dephosphorylation. Unlike in other plant species, jasmonic acid (JA) accumulation in Arabidopsis is apparently not MPK6- and MPK3-dependent, so their role in JA-mediated defenses against herbivorous insects is unclear. Here we explore whether changes in MPK6/3 phosphorylation kinetics in Arabidopsis MIPP mutants lead to changes in hormone synthesis and resistance against herbivores. The MIPPs MKP1, DsPTP1, PP2C5, and AP2C1 have been implicated in responses to infection, drought, and osmotic stress, which all impinge on JA-mediated defenses. In loss-of-function mutants, we found that the four MIPPs alter wound-induced MPK6/3 phosphorylation kinetics and affect the accumulation of the defense hormones JA, abscisic acid, and salicylic acid, as compared to wild type plants (Col-0). Moreover, MPK6/3 misregulation in MIPP or MAPK mutant plants resulted in slight changes in the resistance to Trichoplusia ni and Spodoptera exigua larvae as compared to Col-0. Our data indicate that MPK6/3 and the four MIPPs moderately contribute to wound signaling and defense against herbivorous insects in Arabidopsis.

MKK3 Cascade Regulates Seed Dormancy Through a Negative Feedback Loop Modulating ABA Signal in Rice

BACKGROUND

With the increasing frequency of climatic anomalies, high temperatures and long-term rain often occur during the rice-harvesting period, especially for early rice crops in tropical and subtropical regions. Seed dormancy directly affects the resistance to pre-harvest sprouting (PHS). Therefore, in order to increase rice production, it is critical to enhance seed dormancy and avoid yield losses to PHS. The elucidation and utilization of the seed dormancy regulation mechanism is of great significance to rice production. Preliminary results indicated that the OsMKKK62-OsMKK3-OsMPK7/14 module might regulate ABA sensitivity and then control seed dormancy. The detailed mechanism is still unclear.

RESULTS

The overexpression of OsMKK3 resulted in serious PHS. The expression levels of OsMKK3 and OsMPK7 were upregulated by ABA and GA at germination stage. OsMKK3 and OsMPK7 are both located in the nucleus and cytoplasm. The dormancy level of double knockout mutant mkk3/mft2 was lower than that of mkk3, indicating that OsMFT2 functions in the downstream of MKK3 cascade in regulating rice seeds germination. Biochemical results showed that OsMPK7 interacted with multiple core ABA signaling components according to yeast two-hybrid screening and luciferase complementation experiments, suggesting that MKK3 cascade regulates ABA signaling by modulating the core ABA signaling components. Moreover, the ABA response and ABA responsive genes of mpk7/14 were significantly higher than those of wild-type ZH11 when subjected to ABA treatment.

CONCLUSION

MKK3 cascade mediates the negative feedback loop of ABA signal through the interaction between OsMPK7 and core ABA signaling components in rice.

The Abscisic Acid-Responsive Element Binding Factors (ABFs)-MAPKKK18 module regulates ABA- induced leaf senescence in Arabidopsis

The Mitogen-Activated Protein Kinase Kinase Kinase 18 (MAPKKK18) has been reported to play a role in abiotic stress priming in long-term abscisic acid (ABA) response including drought tolerance and leaf senescence. However, the upstream transcriptional regulators of MAPKKK18 remain to be determined. Here, we report ABA-Responsive Element (ABRE) Binding Factors (ABFs) as upstream transcription factors (TFs) of MAPKKK18 expression. Mutants of abf2, abf3, abf4 and abf2abf3abf4 dramatically reduced the transcription of MAPKKK18. Our electrophoresis mobility shift assay (EMSA) and dual-luciferase reporter assay demonstrated that ABF2, ABF3, and ABF4 bound to ABRE cis-elements within the promoter of MAPKKK18 to transactivate its expression. Furthermore, enrichments of the promoter region of MAPKKK18 by ABF2, ABF3, and ABF4 were confirmed by in vivo chromatin immunoprecipitation coupled with qPCR (ChIP-qPCR). Additionally, we found that mutants of mapkkk18 exhibited obvious delayed leaf senescence. Moreover, a genetic study showed that overexpression of ABF2, ABF3, and ABF4 in the background of mapkkk18 mostly phenocopied the stay-green phenotype of mapkkk18 and, expression levels of five target genes of ABFs, that is, NYE1, NYE2, NYC1, PAO, and SAG29 were attenuated as a result of MAPKKK18 mutation. These findings demonstrate that ABF2, ABF3, and ABF4 act as transcription regulators of MAPKKK18, and also suggest that, at least in part, ABA acts in priming leaf senescence via ABF-induced expression of MAPKKK18.

Arabidopsis MPK3 and MPK6 regulates D-glucose signaling and interacts with G-protein, RGS1

Sugar as a signaling molecule has attracted lots of attention. Even though several kinases have been shown to play a crucial role in the sugar signaling and response to exogenous D-glucose (Glc), the information on the involvement of MAP kinase cascade in sugar signaling has remain largely unexplored. In this report we demonstrate that MAP kinase signaling is essential for sensitivity to higher concentrations of D-Glc in Arabidopsis. We found that D-Glc activates MAP kinases, MPK3 and MPK6 in a concentration and time-dependent manner. The mutants of mpk3 and mpk6 display hyposensitivity to 6% D-Glc during seed germination, cotyledon greening and root growth. Interestingly, the altered sensitivity to increased D-Glc is severely enhanced by addition of 1% Sucrose in the media. Our study also deciphered the role of one of the Glc sensor proteins, RGS1 that interacts and gets phosphorylated at its C-terminal domain by MPK3 and MPK6. Overall our study provides a new insight on the involvement of MAP kinases in association with G-proteins that might regulate sugar signaling and sugar responsive growth and development in Arabidopsis.

Dynamics of Protein Phosphorylation during Arabidopsis Seed Germination

Seed germination is critical for early plantlet development and is tightly controlled by environmental factors. Nevertheless, the signaling networks underlying germination control remain elusive. In this study, the remodeling of Arabidopsis seed phosphoproteome during imbibition was investigated using stable isotope dimethyl labeling and nanoLC-MS/MS analysis. Freshly harvested seeds were imbibed under dark or constant light to restrict or promote germination, respectively. For each light regime, phosphoproteins were extracted and identified from dry and imbibed (6 h, 16 h, and 24 h) seeds. A large repertoire of 10,244 phosphopeptides from 2546 phosphoproteins, including 110 protein kinases and key regulators of seed germination such as Delay Of Germination 1 (DOG1), was established. Most phosphoproteins were only identified in dry seeds. Early imbibition led to a similar massive downregulation in dormant and non-dormant seeds. After 24 h, 411 phosphoproteins were specifically identified in non-dormant seeds. Gene ontology analyses revealed their involvement in RNA and protein metabolism, transport, and signaling. In addition, 489 phosphopeptides were quantified, and 234 exhibited up or downregulation during imbibition. Interaction networks and motif analyses revealed their association with potential signaling modules involved in germination control. Our study provides evidence of a major role of phosphosignaling in the regulation of Arabidopsis seed germination.

Identification of OsPK5 involved in rice glycolytic metabolism and GA/ABA balance for improving seed germination via genome-wide association study

Seed germination plays a pivotal role in the plant life cycle, and its precise regulatory mechanisms are not clear. In this study, 19 quantitative trait loci (QTLs) associated with rice seed germination were identified through genome-wide association studies (GWAS) of the following traits in 2016 and 2017: germination rate (GR) at 3, 5, and 7 days after imbibition (DAI) and germination index (GI). Two major stable QTLs, qSG4 and qSG11.1, were found to be associated with GR and GI over 2 continuous years. Furthermore, OsPK5, encoding a pyruvate kinase, was shown to be a crucial regulator of seed germination in rice, and might be a causal gene of the key QTL qSG11.1, on chromosome 11. Natural variation in OsPK5 function altered the activity of pyruvate kinase. The disruption of OsPK5 function resulted in slow germination and seedling growth during seed germination, blocked glycolytic metabolism, caused glucose accumulation, decreased energy levels, and affected the GA/ABA balance. Taken together, our results provide novel insights into the roles of OsPK5 in seed germination, and facilitate its application in rice breeding to improve seed vigour.

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.

Phosphorylation of Serine 114 of the transcription factor ABSCISIC ACID INSENSITIVE 4 is essential for activity

The transcription factor ABA-INSENSITIVE(ABI)4 has diverse roles in regulating plant growth, including inhibiting germination and reserve mobilization in response to ABA and high salinity, inhibiting seedling growth in response to high sugars, inhibiting lateral root growth, and repressing light-induced gene expression. ABI4 activity is regulated at multiple levels, including gene expression, protein stability, and activation by phosphorylation. Although ABI4 can be phosphorylated at multiple residues by MAPKs, we found that S114 is the preferred site of MPK3. To examine the possible biological role of S114 phosphorylation, we transformed abi4-1 mutant plants with ABI4pro::ABI4 constructs encoding wild type (114S), phosphorylation-null (S114A) or phosphomimetic (S114E) forms of ABI4. Phosphorylation of S114 is necessary for the response to ABA, glucose, salt stress, and lateral root development, where the abi4 phenotype could be complemented by expressing ABI4 (114S) or ABI4 (S114E) but not ABI4 (S114A). Comparison of root transcriptomes in ABA-treated roots of abi4-1 mutant plants transformed with constructs encoding the different phosphorylation-forms of S114 of ABI4 revealed that 85 % of the ABI4-regulated genes whose expression pattern could be restored by expressing ABI4 (114S) are down-regulated by ABI4. Phosphorylation of S114 was required for regulation of 35 % of repressed genes, but only 17 % of induced genes. The genes whose repression requires the phosphorylation of S114 are mainly involved in embryo and seedling development, growth and differentiation, and regulation of gene expression.

The intracellular ROS accumulation in elicitor-induced immunity requires the multiple organelle-targeted Arabidopsis NPK1-related protein kinases

Recognition at the plasma membrane of danger signals (elicitors) belonging to the classes of the microbe/pathogen- and damage-associated molecular patterns is a key event in pathogen sensing by plants and is associated with a rapid activation of immune responses. Different cellular compartments, including plasma membrane, chloroplasts, nuclei and mitochondria, are involved in the immune cellular program. However, how pathogen sensing is transmitted throughout the cell remains largely to be uncovered. Arabidopsis NPK1-related Proteins (ANPs) are mitogen-activated protein kinase kinase kinases previously shown to have a role in immunity. In this article, we studied the in vivo intracellular dynamics of ANP1- and ANP3-GFP fusions and found that under basal physiological conditions both proteins are present in the cytosol, while ANP3 is also localized in mitochondria. After elicitor perception, both proteins are present also in the plastids and nuclei, revealing a localization pattern that is so far unique. The N-terminal region of the protein kinases is responsible for their localization in mitochondria and plastids. Moreover, we found that the localization of ANPs coincides with the sites of elicitor-induced ROS accumulation and that plants lacking ANP function do not accumulate intracellular ROS.

MxMPK6-2-bHLH104 interaction is involved in reactive oxygen species signaling in response to iron deficiency in apple rootstock

Iron (Fe) is a trace element necessary for plant growth. Many land plants have evolved a set of mechanisms associated with the Fe absorption process to deal with the problem of insufficient Fe supply in the soil. During Fe absorption, reactive oxygen species (ROS) can be used as a signal to initiate a response to stress caused by Fe deficiency. However, the molecular mechanisms underlying the involvement of ROS in the Fe deficiency stress response remains unclear. In this study, we have identified a kinase, MxMPK6-2, from Malus xiaojinensis, an apple rootstock that is highly efficient at Fe absorption. MxMPK6-2 has been shown to be responsive to ROS signals during Fe deficiency, and MxMPK6-2 overexpression in apple calli enhanced its tolerance to Fe deficiency. We further screened for proteins in the Fe absorption pathway and identified MxbHLH104, a transcription factor which interacts with MxMPK6-2. MxbHLH104 can be phosphorylated by MxMPK6-2 in vivo, and we confirmed that its phosphorylation increased Fe absorption in apple calli under Fe deficiency, with the presence of ROS promoting this process. Overall, we have demonstrated that MxMPK6-2 is responsive to ROS signaling during Fe deficiency, and is able to control its response by regulating MxbHLH104.

Receptor-Like Protein Kinases Function Upstream of MAPKs in Regulating Plant Development

Mitogen-activated protein kinases (MAPKs) are a group of protein kinase broadly involved in various signal pathways in eukaryotes. In plants, MAPK cascades regulate growth, development, stress responses and immunity by perceiving signals from the upstream regulators and transmitting the phosphorylation signals to the downstream signaling components. To reveal the interactions between MAPK cascades and their upstream regulators is important for understanding the functional mechanisms of MAPKs in the life span of higher plants. Typical receptor-like protein kinases (RLKs) are plasma membrane-located to perceive endogenous or exogenous signal molecules in regulating plant growth, development and immunity. MAPK cascades bridge the extracellular signals and intracellular transcription factors in many RLK-mediated signaling pathways. This review focuses on the current findings that RLKs regulate plant development through MAPK cascades and discusses questions that are worth investigating in the near future.

A bHLH transcription factor, MYC2, imparts salt intolerance by regulating proline biosynthesis in Arabidopsis

MYC2, a bHLH TF, acts as regulatory hub within several signaling pathways by integration of various endogenous and exogenous signals which shape plant growth and development. However, its involvement in salt stress regulation is still elusive. This study has deciphered a novel role of MYC2 in imparting salt stress intolerance by regulating delta1 -pyrroline-5-carboxylate synthase1 (P5CS1) gene and hence proline synthesis. P5CS1 is a rate-limiting enzyme in the biosynthesis of proline. Y-1-H and EMSA studies confirmed the binding of MYC2 with the 5'UTR region of P5CS1. Transcript and biochemical studies have revealed MYC2 as a negative regulator of proline biosynthesis. Proline is necessary for imparting tolerance toward abiotic stress; however, its overaccumulation is toxic for the plants. Hence, studying the regulation of proline biosynthesis is requisite to understand the mechanism of stress tolerance. We have also studied that MYC2 is regulated by mitogen-activated protein kinase (MAPK) cascade mitogen-activated protein kinase kinase 3-MPK6 and vice versa. Altogether, this study demonstrates salt stress-mediated activation of MYC2 by MAPK cascade, regulating proline biosynthesis and thus salt stress.

The MPK8-TCP14 pathway promotes seed germination in Arabidopsis

The accurate control of dormancy release and germination is critical for successful plantlet establishment. Investigations in cereals hypothesized a crucial role for specific MAP kinase (MPK) pathways in promoting dormancy release, although the identity of the MPK involved and the downstream events remain unclear. In this work, we characterized mutants for Arabidopsis thaliana MAP kinase 8 (MPK8). Mpk8 seeds presented a deeper dormancy than wild-type (WT) at harvest that was less efficiently alleviated by after-ripening and gibberellic acid treatment. We identified Teosinte Branched1/Cycloidea/Proliferating cell factor 14 (TCP14), a transcription factor regulating germination, as a partner of MPK8. Mpk8 tcp14 double-mutant seeds presented a deeper dormancy at harvest than WT and mpk8, but similar to that of tcp14 seeds. MPK8 interacted with TCP14 in the nucleus in vivo and phosphorylated TCP14 in vitro. Furthermore, MPK8 enhanced TCP14 transcriptional activity when co-expressed in tobacco leaves. Nevertheless, the stimulation of TCP14 transcriptional activity by MPK8 could occur independently of TCP14 phosphorylation. The comparison of WT, mpk8 and tcp14 transcriptomes evidenced that whereas no effect was observed in dry seeds, mpk8 and tcp14 mutants presented dramatic transcriptomic alterations after imbibition with a sustained expression of genes related to seed maturation. Moreover, both mutants exhibited repression of genes involved in cell wall remodeling and cell cycle G1/S transition. As a whole, this study unraveled a role for MPK8 in promoting seed germination, and suggested that its interaction with TCP14 was critical for regulating key processes required for germination completion.

The MKKK62-MKK3-MAPK7/14 module negatively regulates seed dormancy in rice

BACKGROUND

Seed dormancy directly affects the phenotype of pre-harvest sprouting, and ultimately affects the quality and yield of rice seeds. Although many genes controlling seed dormancy have been cloned from cereals, the regulatory mechanisms controlling this process are complex, and much remains unknown. The MAPK cascade is involved in many signal transduction pathways. Recently, MKK3 has been reported to be involved in the regulation of seed dormancy, but its mechanism of action is unclear.

RESULTS

We found that MKKK62-overexpressing rice lines (OE) lost seed dormancy. Further analyses showed that the abscisic acid (ABA) sensitivity of OE lines was decreased. In yeast two-hybrid experiments, MKKK62 interacted with MKK3, and MKK3 interacted with MAPK7 and MAPK14. Knock-out experiments confirmed that MKK3, MAPK7, and MAPK14 were involved in the regulation of seed dormancy. The OE lines showed decreased transcript levels of OsMFT, a homolog of a gene that controls seed dormancy in wheat. The up-regulation of OsMFT in MKK3-knockout lines (OE/mkk3) and MAPK7/14-knockout lines (OE/mapk7/mapk14) indicated that the MKKK62-MKK3-MAPK7/MAPK14 system controlled seed dormancy by regulating the transcription of OsMFT.

CONCLUSION

Our results showed that MKKK62 negatively controls seed dormancy in rice, and that during the germination stage and the late stage of seed maturation, ABA sensitivity and OsMFT transcription are negatively controlled by MKKK62. Our results have clarified the entire MAPK cascade controlling seed dormancy in rice. Together, these results indicate that protein modification by phosphorylation plays a key role in controlling seed dormancy.

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.

Identification and Analysis of Mitogen-Activated Protein Kinase (MAPK) Cascades in Fragaria vesca

Mitogen-activated protein kinase (MAPK) cascades are highly conserved signaling modules in eukaryotes, including yeasts, plants and animals. MAPK cascades are responsible for protein phosphorylation during signal transduction events, and typically consist of three protein kinases: MAPK, MAPK kinase, and MAPK kinase kinase. In this current study, we identified a total of 12 FvMAPK, 7 FvMAPKK, 73 FvMAPKKK, and one FvMAPKKKK genes in the recently published Fragaria vesca genome sequence. This work reported the classification, annotation and phylogenetic evaluation of these genes and an assessment of conserved motifs and the expression profiling of members of the gene family were also analyzed here. The expression profiles of the MAPK and MAPKK genes in different organs and fruit developmental stages were further investigated using quantitative real-time reverse transcription PCR (qRT-PCR). Finally, the MAPK and MAPKK expression patterns in response to hormone and abiotic stresses (salt, drought, and high and low temperature) were investigated in fruit and leaves of F. vesca. The results provide a platform for further characterization of the physiological and biochemical functions of MAPK cascades in strawberry.

Hydrogen sulfide toxicity inhibits primary root growth through the ROS-NO pathway

High concentrations of hydrogen sulfide (H₂S) are toxic to plants and inhibit their growth. Previous research indicated that high concentrations of H₂S modulate the root system architecture (RSA) by affecting auxin transport; however, the signaling pathway underlying this process remains unclear. Here, we investigated the effects of exogenous sodium hydrosulfide (NaHS), an H₂S donor, on primary root (PR) growth in Arabidopsis using pharmacological, physiological, and genetic approaches. H₂S toxicity repressed PR growth by triggering a signal transduction pathway involving reactive oxygen species (ROS) accumulation, MITOGEN-ACTIVATED PROTEIN KINASE 6 (MPK6) activation, and nitric oxide (NO) production. Respiratory burst oxidase homolog mutants and an NO synthase mutant were less sensitive to NaHS, suggesting that both ROS and NO mediate the inhibitory effects of H₂S on PR growth. We found that exogenous H₂S-activated ROS production was required for NO generation and that MPK6 mediated H₂S-induced NO production. MPK6 was shown to function downstream of ROS and upstream of NO. Finally, we demonstrated that exogenous H₂S repressed the distribution of auxin and reduced the meristematic cell division potential in root tips, and NO was involved in this process.

Arabidopsis MAP3K16 and Other Salt-Inducible MAP3Ks Regulate ABA Response Redundantly

In the Arabidopsis genome, approximately 80 MAP3Ks (mitogen-activated protein kinase kinase kinases) have been identified. However, only a few of them have been characterized, and the functions of most MAP3Ks are largely unknown. In this paper, we report the function of MAP3K16 and several other MAP3Ks, MAP3K14/15/17/18, whose expression is salt-inducible. We prepared MAP3K16 overexpression (OX) lines and analyzed their phenotypes. The result showed that the transgenic plants were ABA-insensitive during seed germination and cotyledon greening stage but their root growth was ABA-hypersensitive. The OX lines were more susceptible to water-deficit condition at later growth stage in soil. A MAP3K16 knockout (KO) line, on the other hand, exhibited opposite phenotypes. In similar transgenic analyses, we found that MAP3K14/15/17/18 OX and KO lines displayed similar phenotypes to those of MA3K16, suggesting the functional redundancy among them. MAP3K16 possesses in vitro kinase activity, and we carried out two-hybrid analyses to identify MAP3K16 substrates. Our results indicate that MAP3K16 interacts with MKK3 and the negative regulator of ABA response, ABR1, in yeast. Furthermore, MAP3K16 recombinant protein could phosphorylate MKK3 and ABR1, suggesting that they might be MAP3K16 substrates. Collectively, our results demonstrate that MAP3K16 and MAP3K14/15/17/18 are involved in ABA response, playing negative or positive roles depending on developmental stage and that MAP3K16 may function via MKK3 and ABR1.

Environmental and genetic effects on tomato seed metabolic balance and its association with germination vigor

BACKGROUND

The metabolite content of a seed and its ability to germinate are determined by genetic makeup and environmental effects during development. The interaction between genetics, environment and seed metabolism and germination was studied in 72 tomato homozygous introgression lines (IL) derived from Solanum pennelli and S. esculentum M82 cultivar. Plants were grown in the field under saline and fresh water irrigation during two consecutive seasons, and collected seeds were subjected to morphological analysis, gas chromatograph-mass spectrometry (GC-MS) metabolic profiling and germination tests.

RESULTS

Seed weight was under tight genetic regulation, but it was not related to germination vigor. Salinity significantly reduced seed number but had little influence on seed metabolites, affecting only 1% of the statistical comparisons. The metabolites negatively correlated to germination were simple sugars and most amino acids, while positive correlations were found for several organic acids and the N metabolites urea and dopamine. Germination tests identified putative loci for improved germination as compared to M82 and in response to salinity, which were also characterized by defined metabolic changes in the seed.

CONCLUSIONS

An integrative analysis of the metabolite and germination data revealed metabolite levels unambiguously associated with germination percentage and rate, mostly conserved in the different tested seed development environments. Such consistent relations suggest the potential for developing a method of germination vigor prediction by metabolic profiling, as well as add to our understanding of the importance of primary metabolic processes in germination.

Mitogen-activated protein kinase kinase kinase (MAPKKK) 4 from rapeseed (Brassica napus L.) is a novel member inducing ROS accumulation and cell death

MAPKKK is the largest family of MAPK cascade, which is known to play important roles in plant growth, development and immune responses. So far, only a few have been functionally characterized even in the model plant, Arabidopsis due to the potential functional redundancy of MAPKKK. We previously identified and cloned a few MAPKKK family genes from rapeseed. In this study, BnaMAPKKK4 was characterized as a member in eliciting accumulation of reactive oxygen species (ROS) and hypersensitive response (HR)-like cell death. This is accompanied with accumulation of malondialdehyde (MDA), anthocyanin as well as nuclear DNA fragmentation. The transcript abundance of a series of ROS accumulation, cell death, and defense response related genes were up-regulated by the expression of MAPKKK4. Further investigation identified BnaMAPKKK4 elicited ROS through the downstream MPK3. These results indicate that BnaMAPKKK4 and its downstream components function in the ROS-induced cell death.

A G-protein β subunit, AGB1, negatively regulates the ABA response and drought tolerance by down-regulating AtMPK6-related pathway in Arabidopsis

Heterotrimeric G-proteins are versatile regulators involved in diverse cellular processes in eukaryotes. In plants, the function of G-proteins is primarily associated with ABA signaling. However, the downstream effectors and the molecular mechanisms in the ABA pathway remain largely unknown. In this study, an AGB1 mutant (agb1-2) was found to show enhanced drought tolerance, indicating that AGB1 might negatively regulate drought tolerance in Arabidopsis. Data showed that AGB1 interacted with protein kinase AtMPK6 that was previously shown to phosphorylate AtVIP1, a transcription factor responding to ABA signaling. Our study found that transcript levels of three ABA responsive genes, AtMPK6, AtVIP1 and AtMYB44 (downstream gene of AtVIP1), were significantly up-regulated in agb1-2 lines after ABA or drought treatments. Other ABA-responsive and drought-inducible genes, such as RD29A (downstream gene of AtMYB44), were also up-regulated in agb1-2 lines. Furthermore, overexpression of AtVIP1 resulted in hypersensitivity to ABA at seed germination and seedling stages, and significantly enhanced drought tolerance in transgenic plants. These results suggest that AGB1 was involved in the ABA signaling pathway and drought tolerance in Arabidopsis through down-regulating the AtMPK6, AtVIP1 and AtMYB44 cascade.

MPK6 controls H2 O2-induced root elongation by mediating Ca2+ influx across the plasma membrane of root cells in Arabidopsis seedlings

Mitogen-activated protein kinases (MPKs) play critical roles in signalling and growth, and Ca(2+) and H2 O2 control plant growth processes associated with abscisic acid (ABA). However, it remains unclear how MPKs are involved in H2 O2 - and Ca(2+) -mediated root elongation. Root elongation in seedlings of the loss-of-function mutant Atmpk6 (Arabidopsis thaliana MPK6) was less sensitive to moderate H2 O2 or ABA than that in wild-type (WT) plants. The enhanced elongation was a result of root cell expansion. This effect disappeared when ABA-induced H2 O2 accumulation or the cytosolic Ca(2+) increase were defective. Molecular and biochemical evidence showed that increased expression of the cell wall peroxidase PRX34 in Atmpk6 root cells enhanced apoplastic H2 O2 generation; this promoted a cytosolic Ca(2+) increase and Ca(2+) influx across the plasma membrane. The plasma membrane damage caused by high levels of H2 O2 was ameliorated in a Ca(2+) -dependent manner. These results suggested that there was intensified PRX34-mediated H2 O2 generation in the apoplast and increased Ca(2+) flux into the cytosol of Atmpk6 root cells; that is, the spatial separation of apoplastic H2 O2 from cytosolic Ca(2+) in root cells prevented H2 O2 -induced inhibition of root elongation in Atmpk6 seedlings.

Positive and negative roles for soybean MPK6 in regulating defense responses

It has been well established that MPK6 is a positive regulator of defense responses in model plants such as Arabidopsis and tobacco. However, the functional importance of soybean MPK6 in disease resistance has not been investigated. Here, we showed that silencing of GmMPK6 in soybean using virus-induced gene silencing mediated by Bean pod mottle virus (BPMV) caused stunted growth and spontaneous cell death on the leaves, a typical phenotype of activated defense responses. Consistent with this phenotype, expression of pathogenesis-related (PR) genes and the conjugated form of salicylic acid were significantly increased in GmMPK6-silenced plants. As expected, GmMPK6-silenced plants were more resistant to downy mildew and Soybean mosaic virus compared with vector control plants, indicating a negative role of GmMPK6 in disease resistance. Interestingly, overexpression of GmMPK6, either transiently in Nicotiana benthamiana or stably in Arabidopsis, resulted in hypersensitive response (HR)-like cell death. The HR-like cell death was accompanied by increased PR gene expression, suggesting that GmMPK6, like its counterpart in other plant species, also plays a positive role in cell death induction and defense response. Using bimolecular fluorescence complementation analysis, we determined that GmMKK4 might function upstream of GmMPK6 and GmMKK4 could interact with GmMPK6 independent of its phosphorylation status. Taken together, our results indicate that GmMPK6 functions as both repressor and activator in defense responses of soybean.

Putting the brakes on: abscisic acid as a central environmental regulator of stomatal development

Stomata are produced by a controlled series of epidermal cell divisions. The molecular underpinnings of this process are becoming well understood, but mechanisms that determine plasticity of stomatal patterning to many exogenous and environmental cues remain less clear. Light quantity and quality, vapour pressure deficit, soil water content, and CO2 concentration are detected by the plant, and new leaves adapt their stomatal densities accordingly. Mature leaves detect these environmental signals and relay messages to immature leaves to tell them how to adapt and grow. Stomata on mature leaves may act as stress signal-sensing and transduction centres, locally by aperture adjustment, and at long distance by optimizing stomatal density to maximize future carbon gain while minimizing water loss. Although mechanisms of stomatal aperture responses are well characterized, the pathways by which mature stomata integrate environmental signals to control immature epidermal cell fate, and ultimately stomatal density, are not. Here we evaluate current understanding of the latter through the influence of the former. We argue that mature stomata, as key portals by which plants coordinate their carbon and water relations, are controlled by abscisic acid (ABA), both metabolically and hydraulically, and that ABA is also a core regulator of environmentally determined stomatal development.

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.

A genomic-scale artificial microRNA library as a tool to investigate the functionally redundant gene space in Arabidopsis

Traditional forward genetic screens are limited in the identification of homologous genes with overlapping functions. Here, we report the analyses and assembly of genome-wide protein family definitions that comprise the largest estimate for the potentially redundant gene space in Arabidopsis thaliana. On this basis, a computational design of genome-wide family-specific artificial microRNAs (amiRNAs) was performed using high-performance computing resources. The amiRNA designs are searchable online (http://phantomdb.ucsd.edu). A computationally derived library of 22,000 amiRNAs was synthesized in 10 sublibraries of 1505 to 4082 amiRNAs, each targeting defined functional protein classes. For example, 2964 amiRNAs target annotated DNA and RNA binding protein families and 1777 target transporter proteins, and another sublibrary targets proteins of unknown function. To evaluate the potential of an amiRNA-based screen, we tested 122 amiRNAs targeting transcription factor, protein kinase, and protein phosphatase families. Several amiRNA lines showed morphological phenotypes, either comparable to known phenotypes of single and double/triple mutants or caused by overexpression of microRNAs. Moreover, novel morphological and abscisic acid-insensitive seed germination mutants were identified for amiRNAs targeting zinc finger homeodomain transcription factors and mitogen-activated protein kinase kinase kinases, respectively. These resources provide an approach for genome-wide genetic screens of the functionally redundant gene space in Arabidopsis.

Nicotiana attenuata MPK4 suppresses a novel jasmonic acid (JA) signaling-independent defense pathway against the specialist insect Manduca sexta, but is not required for the resistance to the generalist Spodoptera littoralis

How plants tailor their defense responses to attack from different insects remains largely unknown. Here, we studied the role of a mitogen-activated protein kinase (MAPK), MPK4, in the resistance of a wild tobacco Nicotiana attenuata to two herbivores, the specialist Manduca sexta and the generalist Spodoptera littoralis. Stably transformed N. attenuata plants silenced in MPK4 (irMPK4) were generated and characterized for traits important for defense against herbivores. Only the oral secretions (OS) from M. sexta, but not the OS from S. littoralis or mechanical wounding, induced elevated levels of jasmonic acid (JA) in irMPK4 plants relative to the wild-type plants. Moreover, silencing of MPK4 strongly increased the resistance of N. attenuata to M. sexta in a fashion that was independent of COI1 (CORONATINE INSENSITIVE1)-mediated JA signaling. Untargeted metabolomic screening identified several new MPK4-dependent putative defensive compounds against M. sexta. By contrast, silencing of MPK4 did not affect the growth of the generalist insect S. littoralis, and we propose that this was because of the very low levels of fatty acid-amino acid conjugates (FACs) in S. littoralis OS. Thus, MPK4 is likely to be a key signaling element that enables plants to tailor defense responses to different attackers.

Towards the identification of new genes involved in ABA-dependent abiotic stresses using Arabidopsis suppressor mutants of abh1 hypersensitivity to ABA during seed germination

Abscisic acid plays a pivotal role in the abiotic stress response in plants. Although great progress has been achieved explaining the complexity of the stress and ABA signaling cascade, there are still many questions to answer. Mutants are a valuable tool in the identification of new genes or new alleles of already known genes and in elucidating their role in signaling pathways. We applied a suppressor mutation approach in order to find new components of ABA and abiotic stress signaling in Arabidopsis. Using the abh1 (ABA hypersensitive 1) insertional mutant as a parental line for EMS mutagenesis, we selected several mutants with suppressed hypersensitivity to ABA during seed germination. Here, we present the response to ABA and a wide range of abiotic stresses during the seed germination and young seedling development of two suppressor mutants—soa2 (suppressor of abh1 hypersensitivity to ABA 2) and soa3 (suppressor of abh1 hypersensitivity to ABA 3). Generally, both mutants displayed a suppression of the hypersensitivity of abh1 to ABA, NaCl and mannitol during germination. Both mutants showed a higher level of tolerance than Columbia-0 (Col-0—the parental line of abh1) in high concentrations of glucose. Additionally, soa2 exhibited better root growth than Col-0 in the presence of high ABA concentrations. soa2 and soa3 were drought tolerant and both had about 50% fewer stomata per mm² than the wild-type but the same number as their parental line—abh1. Taking into account that suppressor mutants had the same genetic background as their parental line—abh1, it was necessary to backcross abh1 with Landsberg erecta four times for the map-based cloning approach. Mapping populations, derived from the cross of abh1 in the Landsberg erecta background with each suppressor mutant, were created. Map based cloning in order to identify the suppressor genes is in progress.

Identification and analysis of MKK and MPK gene families in canola (Brassica napus L.)

BACKGROUND

Eukaryotic mitogen-activated protein kinase (MAPK/MPK) signaling cascades transduce and amplify environmental signals via three types of reversibly phosphorylated kinases to activate defense gene expression. Canola (oilseed rape, Brassica napus) is a major crop in temperate regions. Identification and characterization of MAPK and MAPK kinases (MAPKK/MKK) of canola will help to elucidate their role in responses to abiotic and biotic stresses.

RESULTS

We describe the identification and analysis of seven MKK (BnaMKK) and 12 MPK (BnaMPK) members from canola. Sequence alignments and phylogenetic analyses of the predicted amino acid sequences of BnaMKKs and BnaMPKs classified them into four different groups. We also examined the subcellular localization of four and two members of BnaMKK and BnaMPK gene families, respectively, using green fluorescent protein (GFP) and, found GFP signals in both nuclei and cytoplasm. Furthermore, we identified several interesting interaction pairs through yeast two-hybrid (Y2H) analysis of interactions between BnaMKKs and BnaMPKs, as well as BnaMPK and BnaWRKYs. We defined contiguous signaling modules including BnaMKK9-BnaMPK1/2-BnaWRKY53, BnaMKK2/4/5-BnaMPK3/6-BnaWRKY20/26 and BnaMKK9-BnaMPK5/9/19/20. Of these, several interactions had not been previously described in any species. Selected interactions were validated in vivo by a bimolecular fluorescence complementation (BiFC) assay. Transcriptional responses of a subset of canola MKK and MPK genes to stimuli including fungal pathogens, hormones and abiotic stress treatments were analyzed through real-time RT-PCR and we identified a few of BnaMKKs and BnaMPKs responding to salicylic acid (SA), oxalic acid (OA), Sclerotinia sclerotiorum or other stress conditions. Comparisons of expression patterns of putative orthologs in canola and Arabidopsis showed that transcript expression patterns were generally conserved, with some differences suggestive of sub-functionalization.

CONCLUSIONS

We identified seven MKK and 12 MPK genes from canola and examined their phylogenetic relationships, transcript expression patterns, subcellular localization, and protein-protein interactions. Not all expression patterns and interactions were conserved between canola and Arabidopsis, highlighting the limitations of drawing inferences about crops from model species. The data presented here provide the first systematic description of MKK-MPK-WRKY signaling modules in canola and will further improve our understanding of defense responses in general and provide a basis for future crop improvement.

The rice MAPKK-MAPK interactome: the biological significance of MAPK components in hormone signal transduction

Mitogen-activated protein kinase (MAPK) signaling cascades are evolutionarily conserved fundamental signal transduction pathways. A MAPK cascade consists of many distinct MAPKKK-MAPKK-MAPK modules linked to various upstream receptors and downstream targets through sequential phosphorylation and activation of the cascade components. These cascades collaborate in transmitting a variety of extracellular signals and in controlling cellular responses and processes such as growth, differentiation, cell death, hormonal signaling, and stress responses. Although MAPK proteins play central roles in signal transduction pathways, our knowledge of MAPK signaling in hormonal responses in rice has been limited to a small subset of specific upstream and downstream interacting targets. However, recent studies revealing direct MAPK and MAPKK interactions have provided the basis for elucidating interaction specificities, functional divergence, and functional modulation during hormonal responses. In this review, we highlight current insights into MAPKK-MAPK interaction patterns in rice, with emphasis on the biological significance of these interacting pairs in SA (salicylic acid), JA (jasmonic acid), ET (ethylene), and ABA (abscisic acid) responses, and discuss the challenges in understanding functional signal transduction networks mediated by these hormones.

Regulation of the heat stress response in Arabidopsis by MPK6-targeted phosphorylation of the heat stress factor HsfA2

So far little is known on the functional role of phosphorylation in the heat stress response of plants. Here we present evidence that heat stress activates the Arabidopsis mitogen-activated protein kinase MPK6. In vitro and in vivo evidence is provided that MPK6 specifically targets the major heat stress transcription factor HsfA2. Activation of MPK6 results in complex formation with HsfA2. MPK6 phosphorylates HsfA2 on T249 and changes its intracellular localisation. Protein kinase and phosphatase inhibitor studies indicate that HsfA2 protein stability is regulated in a phosphorylation-dependent manner, but this mechanism is independent of MPK6. Overall, our data show that heat stress-induced targeting of HsfA2 by MPK6 participates in the complex regulatory mechanism how plants respond to heat stress.

An abscisic acid-independent oxylipin pathway controls stomatal closure and immune defense in Arabidopsis

In Arabidopsis the stomatal defense response, a feature of the innate immunity in plants, involves oxylipin-mediated mechanisms that are independent of the phytohormone abscisic acid.

Plant stomata function in innate immunity against bacterial invasion and abscisic acid (ABA) has been suggested to regulate this process. Using genetic, biochemical, and pharmacological approaches, we demonstrate that (i) the Arabidopsis thaliana nine-specific-lipoxygenase encoding gene, LOX1, which is expressed in guard cells, is required to trigger stomatal closure in response to both bacteria and the pathogen-associated molecular pattern flagellin peptide flg22; (ii) LOX1 participates in stomatal defense; (iii) polyunsaturated fatty acids, the LOX substrates, trigger stomatal closure; (iv) the LOX products, fatty acid hydroperoxides, or reactive electrophile oxylipins induce stomatal closure; and (v) the flg22-mediated stomatal closure is conveyed by both LOX1 and the mitogen-activated protein kinases MPK3 and MPK6 and involves salicylic acid whereas the ABA-induced process depends on the protein kinases OST1, MPK9, or MPK12. Finally, we show that the oxylipin and the ABA pathways converge at the level of the anion channel SLAC1 to regulate stomatal closure. Collectively, our results demonstrate that early biotic signaling in guard cells is an ABA-independent process revealing a novel function of LOX1-dependent stomatal pathway in plant immunity.

Stomata are microscopic pores that are present in the epidermis of the aerial parts of higher plants, such as the leaves. These pores, which are flanked by a pair of cells called guard cells, regulate transpiration and the exchange of gas between leaves and the atmosphere. It is well documented that the phytohormone abscisic acid (ABA) is a key regulator that controls the osmotic pressure in guard cells, allowing pore size to be adjusted in response to environmental conditions. Recently, stomata have also been shown to play an important role in the innate immune response. Indeed, upon contact with microbes, plants actively close stomata to prevent the entry of microbes and the consequent colonization of host tissue. This response is known as the stomatal defense response. However, the molecular mechanisms that regulate this defense response are not well understood. Using a variety of approaches, we show in this study that LOX1, a gene that encodes lipoxygenase (LOX) in guard cells, plays a major role in stomatal defense in the model plant Arabidopsis thaliana. Mutations in LOX1 impair stomatal closure and make plants more susceptible to the bacterium Pseudomonas syringae pv. tomato. We also show that several LOX-derived metabolites, the oxylipins, are potent inducers of stomatal closure. Finally, we provide evidence to show that ABA plays only a minor role in stomatal defense response, specifically by modulating this response.

MAP Kinase 6-mediated activation of vacuolar processing enzyme modulates heat shock-induced programmed cell death in Arabidopsis

• Vacuolar processing enzyme (VPE), a cysteine protease, has been intensively studied in plant hypersensitive cell death, but the role and molecular mechanism of VPEs in response to abiotic stresses remain unclear. This work investigated the involvement of VPEs in Arabidopsis response to heat stress. • Under heat shock (HS), Arabidopsis VPE activity and the transcript level of γVPE were both upregulated, and γVPE deficiency suppressed vacuolar disruption and delayed caspase-3-like activation in HS-induced programmed cell death (PCD). Moreover, the change of VPE activity generally paralleled the alteration of caspase-1-like activity under HS treatment, indicating that HS-induced VPE activity might exhibit the caspase-1-like activity. • Further studies showed that MAP Kinase 6 (MPK6) activity was increased after HS treatment, and experiments with inhibitors and mutants suggested that MPK6 was responsible for the γVPE activation after HS treatment. In response to HS stress, reactive oxygen species (ROS) production, increase of cytoplasmic calcium concentration ([Ca(2+) ](cyt)) and the upregulation of calmodulin 3 (CaM3) transcript level occurred upstream of MPK6 activation. • Our results suggested that activation of Arabidopsis γVPE was mediated by MPK6 and played an important role in HS-induced Arabidopsis PCD, providing new insight into the mechanistic study of plant VPEs.

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.

Improvement of stress tolerance in plants by genetic manipulation of mitogen-activated protein kinases

Plant stress tolerance depends on many factors among which signaling by mitogen-activated protein-kinase (MAPK) modules plays a crucial role. Reversible phosphorylation of MAPKs, their upstream activators and downstream targets such as transcription factors can trigger a myriad of transcriptomic, cellular and physiological responses. Genetic manipulation of abundance and/or activity of some of these modular MAPK components can lead to better stress tolerance in Arabidopsis and crop plant species such as tobacco and cereals. The main focus of this review is devoted to the MAPK-related signaling components which show the most promising biotechnological potential. Additionally, recent studies identified MAPK components to be involved both in plant development as well as in stress responses, suggesting that these processes are tightly linked in plants.

Involvement of the abscisic acid catabolic gene CYP707A2 in the glucose-induced delay in seed germination and post-germination growth of Arabidopsis

Earlier studies showed that sugars as signaling molecules play pivotal roles in the regulation of seed germination. ABA biosynthesis upregulation is suggested as one of the possible mechanisms mediating the glucose-induced delay in seed germination. In this study, the role of ABA catabolism in glucose-induced inhibition was investigated. Using Arabidopsis thaliana seeds, the results show that the repression of ABA catabolism by diniconazole aggravated the glucose-induced delay in seed germination. The transcript and protein profiles of CYP707A2, a key gene encoding ABA 8'-hydroxylase in ABA catabolism in Arabidopsis, were significantly decreased by exogenous glucose treatment. Transgenic experiments confirmed that the over-expression of the CYP707A2 gene alleviated the glucose-induced inhibition effect, whereas the cyp707a2 mutant seeds displayed a hypersensitivity to glucose during imbibition. Exogenous glucose also arrested the early seedling development of Arabidopsis. The CYP707A2 over-expression seedlings exhibited lower ABA levels and seemed less sensitive to exogenous glucose compared with wild type seedlings. In summary, the glucose-induced delay in seed germination and seedling development is directly related to the suppression of ABA catabolism through the repression of the CYP707A2 expression.

Microtubules and mitogen-activated protein kinase signalling

Subcellular signalling by mitogen-activated protein kinases (MAPKs) was originally regarded as a means to regulate microtubule (MT) organization and dynamics, but with time MAPKs were assigned to broader roles concerning biotic and abiotic signal transductions. MAPKs, which regulate a broad spectrum of substrates including transcription factors and cytoskeletal proteins, belong to complex MAPK cascades, which are mainly involved in plant development and in plant stress responses. The fact that single MAPK can be regulated by more than a single MAPKKK/MAPKK pair make MAPK signalling modules versatile tools in the regulation of microtubule organization. Until recently, the best-studied MAPK module implicated in cytoskeletal regulation is the NACK-PQR pathway in tobacco (Nicotiana tabacum). Homologues of each constituent of this pathway were also discovered in Arabidopsis thaliana. So far, direct phosphorylation of tubulins by MAPKs has not been shown. However, the first MAPK-related substrate involved in the regulation of MT dynamics to have been identified is MT-associated protein MAP65-1.

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.