The Green MAPKs

Harnessing the role of mitogen-activated protein kinases against abiotic stresses in plants

Crop plants are vulnerable to various biotic and abiotic stresses, whereas plants tend to retain their physiological mechanisms by evolving cellular regulation. To mitigate the adverse effects of abiotic stresses, many defense mechanisms are induced in plants. One of these mechanisms is the mitogen-activated protein kinase (MAPK) cascade, a signaling pathway used in the transduction of extracellular stimuli into intercellular responses. This stress signaling pathway is activated by a series of responses involving MAPKKKs→MAPKKs→MAPKs, consisting of interacting proteins, and their functions depend on the collaboration and activation of one another by phosphorylation. These proteins are key regulators of MAPK in various crop plants under abiotic stress conditions and also related to hormonal responses. It is revealed that in response to stress signaling, MAPKs are characterized as multigenic families and elaborate the specific stimuli transformation as well as the antioxidant regulation system. This pathway is directed by the framework of proteins and stopping domains confer the related associates with unique structure and functions. Early studies of plant MAPKs focused on their functions in model plants. Based on the results of whole-genome sequencing, many MAPKs have been identified in plants, such as Arbodiposis, tomato, potato, alfalfa, poplar, rice, wheat, maize, and apple. In this review, we summarized the recent work on MAPK response to abiotic stress and the classification of MAPK cascade in crop plants. Moreover, we highlighted the modern research methodologies such as transcriptomics, proteomics, CRISPR/Cas technology, and epigenetic studies, which proposed, identified, and characterized the novel genes associated with MAPKs and their role in plants under abiotic stress conditions. In-silico-based identification of novel MAPK genes also facilitates future research on MAPK cascade identification and function in crop plants under various stress conditions.

Molecular cloning and sequence analysis of a mitogen-activated protein kinase gene in the Antarctic yeast Rhodotorula mucilaginosa AN5

BACKGROUND

The mitogen-activated protein kinase (MAPK) cascades play important roles in various signaling transduction networks of biotic and abiotic stress responses. However, MAPK signaling pathways in cold-active yeast Rhodotorula mucilaginosa have not been reported comprehensively.

METHODS AND RESULTS

In the present study, MAPK gene (RmMAPK) was first cloned and characterized from Antarctic sea ice yeast R. mucilaginosa AN5. The full length of the RmMAPK gene is 1086 bp and encodes a 361 amino acids protein with a predicted molecular mass of 40.9 kDa and a pI of 5.25. The RmMAPK contains 11 MAPK conserved subdomains and the phosphorylation motif TGY located in the activation loop of the kinase. Quantitative real-time PCR and western blot assay revealed that the expression and phosphorylation level of RmMAPK up-regulated rapidly and significantly when yeast cells were subjected to low temperature (4 °C), high salinity (120‰ NaCl) and heavy metal (2 mmol/L CuCl₂).

CONCLUSIONS

All data suggested that the MAPK cascades might act as a key function in response to extreme stresses, such as low temperature, high salinity and heavy metal.

ROS Homeostasis in Abiotic Stress Tolerance in Plants

Climate change-induced abiotic stress results in crop yield and production losses. These stresses result in changes at the physiological and molecular level that affect the development and growth of the plant. Reactive oxygen species (ROS) is formed at high levels due to abiotic stress within different organelles, leading to cellular damage. Plants have evolved mechanisms to control the production and scavenging of ROS through enzymatic and non-enzymatic antioxidative processes. However, ROS has a dual function in abiotic stresses where, at high levels, they are toxic to cells while the same molecule can function as a signal transducer that activates a local and systemic plant defense response against stress. The effects, perception, signaling, and activation of ROS and their antioxidative responses are elaborated in this review. This review aims to provide a purview of processes involved in ROS homeostasis in plants and to identify genes that are triggered in response to abiotic-induced oxidative stress. This review articulates the importance of these genes and pathways in understanding the mechanism of resistance in plants and the importance of this information in breeding and genetically developing crops for resistance against abiotic stress in plants.

Elicitor and Receptor Molecules: Orchestrators of Plant Defense and Immunity

Pathogen-associated molecular patterns (PAMPs), microbe-associated molecular patterns (MAMPs), herbivore-associated molecular patterns (HAMPs), and damage-associated molecular patterns (DAMPs) are molecules produced by microorganisms and insects in the event of infection, microbial priming, and insect predation. These molecules are then recognized by receptor molecules on or within the plant, which activates the defense signaling pathways, resulting in plant’s ability to overcome pathogenic invasion, induce systemic resistance, and protect against insect predation and damage. These small molecular motifs are conserved in all organisms. Fungi, bacteria, and insects have their own specific molecular patterns that induce defenses in plants. Most of the molecular patterns are either present as part of the pathogen’s structure or exudates (in bacteria and fungi), or insect saliva and honeydew. Since biotic stresses such as pathogens and insects can impair crop yield and production, understanding the interaction between these organisms and the host via the elicitor–receptor interaction is essential to equip us with the knowledge necessary to design durable resistance in plants. In addition, it is also important to look into the role played by beneficial microbes and synthetic elicitors in activating plants’ defense and protection against disease and predation. This review addresses receptors, elicitors, and the receptor–elicitor interactions where these components in fungi, bacteria, and insects will be elaborated, giving special emphasis to the molecules, responses, and mechanisms at play, variations between organisms where applicable, and applications and prospects.

Rootstock-induced molecular responses associated with drought tolerance in sweet orange as revealed by RNA-Seq

Background

Citrus plants are commercially propagated by grafting, with the rootstock variety influencing a number of horticultural traits, including drought tolerance. Among the different rootstock varieties available for citrus propagation, ‘Rangpur’ lime is known to confer enhanced tolerance to drought as compared to other citrus rootstocks. The objective of this study was to investigate the poorly understood molecular responses underlying the rootstock-induced drought tolerance in sweet orange.

Results

RNA-Seq transcriptome analysis was carried out in leaves of sweet orange grafted on ‘Rangpur’ lime subjected to control and drought-stress treatments, under greenhouse conditions, using the Illumina HiSeq platform. A total of 41,827 unique transcripts were identified, among which 1764 transcripts showed significant variation (P ≤ 0.001) between the treatments, with 1081 genes induced and 683 repressed by drought-stress treatment. The transcripts were distributed in 44 different categories of cellular component, molecular function and biological process. Several genes related to cell metabolism, including those involved in the metabolisms of cell wall, carbohydrates and antioxidants, light reactions, biotic and abiotic stress responses, as well as genes coding for transcription factors (TFs), protein kinases (PKs) and proteins involved in the abscisic acid (ABA) and ethylene signaling pathways, were differentially regulated by drought stress. RNA-Seq data were validated by quantitative real-time PCR (qPCR) analysis and comparative analysis of expression of the selected genes between sweet orange grafted on drought-tolerant and -sensitive rootstocks revealed new candidate genes for drought tolerance in citrus.

Conclusions

In conclusion, our results showed that only a relatively small but functionally diverse fraction of the sweet orange transcriptome, with functions in metabolism, cellular responses and regulation, was differentially regulated by drought stress. The data suggest that the rootstock-induced drought tolerance in sweet orange includes the transcriptional activation of genes related to the cell wall, soluble carbohydrate and antioxidant metabolisms, biotic and abiotic stress responses, TFs, PKs and ABA signaling pathway, and the downregulation of genes involved in the starch metabolism, light reactions and ethylene signaling. Future efforts to elucidate their functional roles and explore their potential in the citrus genetic improvement should benefit from this data.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5481-z) contains supplementary material, which is available to authorized users.

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.

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.

Transcriptome-wide profiling and expression analysis of diploid and autotetraploid Paulownia tomentosa × Paulownia fortunei under drought stress

Paulownia is a fast-growing deciduous hardwood species native to China, which has high ecological and economic value. In an earlier study, we reported ploidy-dependent differences in Paulownia drought tolerance by the microscopic observations of the leaves. Autotetraploid Paulownia has a higher resistance to drought stress than their diploid relatives. In order to obtain genetic information on molecular mechanisms responses of Paulownia plants to drought, Illumina/Solexa Genome sequencing platform was used to de novo assemble the transcriptomes of leaves from diploid and autotetraploid Paulownia tomentosa × Paulownia fortunei seedlings (PTF2 and PTF4 respectively) grown under control conditions and under drought stress and obtained 98,671 nonredundant unigenes. A comparative transcriptome analysis revealed that hundreds of unigenes were predicted to be involved mainly in ROS-scavenging system, amino acid and carbohydrate metabolism, plant hormone biosynthesis and signal transduction, while these unigenes exhibited differential transcript alteration of the two accessions. This study provides a comprehensive map of how P. tomentosa × P. fortunei responds to drought stress at physiological and molecular levels, which may help in understanding the mechanisms involve in water-deficit response and will be useful for further study of drought tolerance in woody plants.

Polyphenols and the human brain: plant “secondary metabolite” ecologic roles and endogenous signaling functions drive benefits

Flavonoids and other polyphenols are ubiquitous plant chemicals that fulfill a range of ecologic roles for their home plant, including protection from a range of biotic and abiotic stressors and a pivotal role in the management of pathogenic and symbiotic soil bacteria and fungi. They form a natural part of the human diet, and evidence suggests that their consumption is associated with the beneficial modulation of a number of health-related variables, including those related to cardiovascular and brain function. Over recent years, the consensus as to the mechanisms responsible for these effects in humans has shifted away from polyphenols having direct antioxidant effects and toward their modulation of cellular signal transduction pathways. To date, little consideration has been given to the question of why, rather than how, these plant-derived chemicals might exert these effects. Therefore, this review summarizes the evidence suggesting that polyphenols beneficially affect human brain function and describes the current mechanistic hypotheses explaining these effects. It then goes on to describe the ecologic roles and potential endogenous signaling functions that these ubiquitous phytochemicals play within their home plant and discusses whether these functions drive their beneficial effects in humans via a process of “cross-kingdom” signaling predicated on the many conserved similarities in plant, microbial, and human cellular signal transduction pathways.

The overexpression of a maize mitogen-activated protein kinase gene (ZmMPK5) confers salt stress tolerance and induces defence responses in tobacco

As sessile organisms, plants are exposed to potential dangers, including multiple biotic and abiotic stresses. The mitogen-activated protein kinase (MAPK) is a universal signalling pathways involved in these processes. A previous study showed that maize ZmMPK5 is induced by various stimuli at transcriptional and post-translational levels. In this study, ZmMPK5 was overexpressed in tobacco to further analyse its biological functions. Under salt and oxidative stresses, ZmMPK5-overexpressing lines displayed less severe damage and stronger growth phenotypes corresponding to a series of physiological changes. In addition, the transgenic lines accumulated less reactive oxygen species (ROS) and had higher levels of antioxidant enzyme activity and metabolites than wild-type (WT) plants following NaCl treatment. Quantitative RT-PCR revealed that the expression of ROS-related and stress-responsive genes was higher in transgenic plants than in WT plants. Furthermore, transgenic lines exhibited enhanced resistance to viral pathogens, and expressed constitutively higher transcript levels of pathogenesis-related genes, such as PR1a, PR4, PR5 and EREBP. Taken together, these results demonstrated that ZmMPK5 is involved in salt stress, oxidative stress and pathogen defence signalling pathways, and its function may be at least partly devoted to efficiently eliminating ROS accumulation under salt stress.

Ectopic expression of ZmSIMK1 leads to improved drought tolerance and activation of systematic acquired resistance in transgenic tobacco

The mitogen-activated protein kinase (MAPK) cascades play pivotal roles in diverse signaling pathways related to plant biotic and abiotic stress responses. In this study, a group B MAPK gene in Zea mays, ZmSIMK1, was functionally analyzed. Quantitative real-time PCR (qRT-PCR) analysis indicated that ZmSIMK1 transcript could be induced by drought, salt, Pseudomonas syringae pv. tomato DC3000 (Pst DC3000) and certain exogenous signaling molecules. Analysis of the ZmSIMK1 promoter revealed a group of putative cis-acting elements related to drought and defense responses. β-Glucuronidase (GUS) staining produced similar results as qRT-PCR. ZmSIMK1 was mainly localized in the nucleus, and further study indicated that the C-terminal domain (CD) was essential for targeting to the nucleus. Transgenic tobacco accumulated less reactive oxygen species (ROS), had higher levels of antioxidant enzyme activity and osmoregulatory substances and exhibited an increased germination rate compared with wild-type (WT) tobacco under drought stress. ROS-related and drought stress-responsive genes in transgenic tobacco were significantly upregulated compared with the same genes in WT lines under drought stress. Moreover, overexpression of ZmSIMK1 promoted the hypersensitive response (HR) and pathogen-related gene (PR) transcription in addition to triggering systemic acquired resistance (SAR) in tobacco.

Cloning and molecular characterization of a mitogen-activated protein kinase gene from Poncirus trifoliata whose ectopic expression confers dehydration/drought tolerance in transgenic tobacco

The mitogen-activated protein kinase (MAPK) cascade plays pivotal roles in diverse signalling pathways related to plant development and stress responses. In this study, the cloning and functional characterization of a group-I MAPK gene, PtrMAPK, in Poncirus trifoliata (L.) Raf are reported. PtrMAPK contains 11 highly conserved kinase domains and a phosphorylation motif (TEY), and is localized in the nucleus of transformed onion epidermal cells. The PtrMAPK transcript level was increased by dehydration and cold, but was unaffected by salt. Transgenic overexpression of PtrMAPK in tobacco confers dehydration and drought tolerance. The transgenic plants exhibited better water status, less reactive oxygen species (ROS) generation, and higher levels of antioxidant enzyme activity and metabolites than the wild type. Interestingly, the stress tolerance capacity of the transgenic plants was compromised by inhibitors of antioxidant enzymes. In addition, overexpression of PtrMAPK enhanced the expression of ROS-related and stress-responsive genes under normal or drought conditions. Taken together, these data demonstrate that PtrMAPK acts as a positive regulator in dehydration/drought stress responses by either regulating ROS homeostasis through activation of the cellular antioxidant systems or modulating transcriptional levels of a variety of stress-associated genes.