A mitogen-activated protein kinase gene, AhMPK3 of peanut: molecular cloning, genomic organization, and heterologous expression conferring resistance against Spodoptera litura in tobacco

Advances in omics research on peanut response to biotic stresses

Peanut growth, development, and eventual production are constrained by biotic and abiotic stresses resulting in serious economic losses. To understand the response and tolerance mechanism of peanut to biotic and abiotic stresses, high-throughput Omics approaches have been applied in peanut research. Integrated Omics approaches are essential for elucidating the temporal and spatial changes that occur in peanut facing different stresses. The integration of functional genomics with other Omics highlights the relationships between peanut genomes and phenotypes under specific stress conditions. In this review, we focus on research on peanut biotic stresses. Here we review the primary types of biotic stresses that threaten sustainable peanut production, the multi-Omics technologies for peanut research and breeding, and the recent advances in various peanut Omics under biotic stresses, including genomics, transcriptomics, proteomics, metabolomics, miRNAomics, epigenomics and phenomics, for identification of biotic stress-related genes, proteins, metabolites and their networks as well as the development of potential traits. We also discuss the challenges, opportunities, and future directions for peanut Omics under biotic stresses, aiming sustainable food production. The Omics knowledge is instrumental for improving peanut tolerance to cope with various biotic stresses and for meeting the food demands of the exponentially growing global population.

GmMKK4-activated GmMPK6 stimulates GmERF113 to trigger resistance to Phytophthora sojae in soybean

Phytophthora root and stem rot is a worldwide soybean (Glycine max) disease caused by the soil-borne pathogen Phytophthora sojae. This disease is devastating to soybean production, so improvement of resistance to P. sojae is a major target in soybean breeding. Mitogen-activated protein kinase (MAPK) cascades are important signaling modules that convert environmental stimuli into cellular responses. Compared with extensive studies in Arabidopsis, the molecular mechanism of MAPK cascades in soybean disease resistance is barely elucidated. In this work, we found that the gene expression of mitogen-activated protein kinase 6 (GmMPK6) was potently induced by P. sojae infection in the disease-resistant soybean cultivar 'Suinong 10'. Overexpression of GmMPK6 in soybean resulted in enhanced resistance to P. sojae and silencing of GmMPK6 led to the opposite phenotype. In our attempt to dissect the role of GmMPK6 in soybean resistance to phytophthora disease, we found that MAPK kinase 4 (GmMKK4) and the ERF transcription factor GmERF113 physically interact with GmMPK6, and we determined that GmMKK4 could phosphorylate and activate GmMPK6, which could subsequently phosphorylate GmERF113 upon P. sojae infection, suggesting that P. sojae can stimulate the GmMKK4-GmMPK6-GmERF113 signaling pathway in soybean. Moreover, phosphorylation of GmERF113 by the GmMKK4-GmMPK6 module promoted GmERF113 stability, nuclear localization and transcriptional activity, which significantly enhanced expression of the defense-related genes GmPR1 and GmPR10-1 and hence improved disease resistance of the transgenic soybean seedlings. In all, our data reveal that the GmMKK4-GmMPK6-GmERF113 cascade triggers resistance to P. sojae in soybean and shed light on functions of MAPK kinases in plant disease resistance.

Nuclear Translocation of Soybean MPK6, GmMPK6, Is Mediated by Hydrogen Peroxide in Salt Stress

The plant mitogen-activated protein kinase (MPK) cascade, a highly conserved signal transduction system in eukaryotes, plays a crucial role in the plant's response to environmental stimuli and phytohormones. It is well-known that nuclear translocation of MPKs is necessary for their activities in mammalian cells. However, the mechanism underlying nuclear translocation of plant MPKs is not well elucidated. In the previous study, it has been shown that soybean MPK6 (GmMPK6) is activated by phosphatidic acid (PA) and hydrogen peroxide (H₂O₂), which are two signaling molecules generated during salt stress. Using the two signaling molecules, we investigated how salt stress triggers its translocation to the nucleus. Our results show that the translocation of GmMPK6 to the nucleus is mediated by H₂O₂, but not by PA. Furthermore, the translocation was interrupted by diphenylene iodonium (DPI) (an inhibitor of RBOH), confirming that H₂O₂ is the signaling molecule for the nuclear translocation of GmMPK6 during salt stress.

Plant Unsaturated Fatty Acids: Multiple Roles in Stress Response

Land plants are exposed to not only biotic stresses such as pathogen infection and herbivore wounding, but abiotic stresses such as cold, heat, drought, and salt. Elaborate strategies have been developed to avoid or abide the adverse effects, with unsaturated fatty acids (UFAs) emerging as general defenders. In higher plants, the most common UFAs are three 18-carbon species, namely, oleic (18:1), linoleic (18:2), and α-linolenic (18:3) acids. These simple compounds act as ingredients and modulators of cellular membranes in glycerolipids, reserve of carbon and energy in triacylglycerol, stocks of extracellular barrier constituents (e.g., cutin and suberin), precursors of various bioactive molecules (e.g., jasmonates and nitroalkenes), and regulators of stress signaling. Nevertheless, they are also potential inducers of oxidative stress. In this review, we will present an overview of these roles and then shed light on genetic engineering of FA synthetic genes for improving plant/crop stress tolerance.

OsMKK3, a Stress-Responsive Protein Kinase, Positively Regulates Rice Resistance to Nilaparvata lugens via Phytohormone Dynamics

Plants undergo several but very precise molecular, physiological, and biochemical modulations in response to biotic stresses. Mitogen-activated protein kinase (MAPK) cascades orchestrate multiple cellular processes including plant growth and development as well as plant responses against abiotic and biotic stresses. However, the role of MAPK kinases (MAPKKs/MKKs/MEKs) in the regulation of plant resistance to herbivores has not been extensively investigated. Here, we cloned a rice MKK gene, OsMKK3, and investigated its function. It was observed that mechanical wounding, infestation of brown planthopper (BPH) Nilaparvata lugens, and treatment with methyl jasmonate (MeJA) or salicylic acid (SA) could induce the expression of OsMKK3. The over-expression of OsMKK3 (oe-MKK3) increased levels of jasmonic acid (JA), jasmonoyl-L-isoleucine (JA-Ile), and abscisic acid (ABA), and decreased SA levels in rice after BPH attack. Additionally, the preference for feeding and oviposition, the hatching rate of BPH eggs, and BPH nymph survival rate were significantly compromised due to over-expression of OsMKK3. Besides, oe-MKK3 also augmented chlorophyll content but impaired plant growth. We confirm that MKK3 plays a pivotal role in the signaling pathway. It is proposed that OsMKK3 mediated positive regulation of rice resistance to BPH by means of herbivory-induced phytohormone dynamics.

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.

Nuclear Signaling of Plant MAPKs

Mitogen-activated protein kinases (MAPKs) are conserved protein kinases in eukaryotes that establish signaling modules where MAPK kinase kinases (MAPKKKs) activate MAPK kinases (MAPKKs) which in turn activate MAPKs. In plants, they are involved in the signaling of multiple environmental stresses and developmental programs. MAPKs phosphorylate their substrates and this post-translational modification (PTM) contributes to the regulation of proteins. PTMs may indeed modify the activity, subcellular localization, stability or trans-interactions of modified proteins. Plant MAPKs usually localize to the cytosol and/or nucleus, and in some instances they may also translocate from the cytosol to the nucleus. Upon the detection of environmental changes at the cell surface, MAPKs participate in the signal transduction to the nucleus, allowing an adequate transcriptional reprogramming. The identification of plant MAPK substrates largely contributed to a better understanding of the underlying signaling mechanisms. In this review, we highlight the nuclear signaling of plant MAPKs. We discuss the activation, regulation and activity of plant MAPKs, as well as their nuclear re-localization. We also describe and discuss known nuclear substrates of plant MAPKs in the context of biotic stress, abiotic stress and development and consider future research directions in the field of plant MAPKs.

SlMAPK3 enhances tolerance to tomato yellow leaf curl virus (TYLCV) by regulating salicylic acid and jasmonic acid signaling in tomato (Solanum lycopersicum)

Several recent studies have reported on the role of mitogen-activated protein kinase (MAPK3) in plant immune responses. However, little is known about how MAPK3 functions in tomato (Solanum lycopersicum L.) infected with tomato yellow leaf curl virus (TYLCV). There is also uncertainty about the connection between plant MAPK3 and the salicylic acid (SA) and jasmonic acid (JA) defense-signaling pathways. The results of this study indicated that SlMAPK3 participates in the antiviral response against TYLCV. Tomato seedlings were inoculated with TYLCV to investigate the possible roles of SlMAPK1, SlMAPK2, and SlMAPK3 against this virus. Inoculation with TYLCV strongly induced the expression and the activity of all three genes. Silencing of SlMAPK1, SlMAPK2, and SlMAPK3 reduced tolerance to TYLCV, increased leaf H2O2 concentrations, and attenuated expression of defense-related genes after TYLCV infection, especially in SlMAPK3-silenced plants. Exogenous SA and methyl jasmonic acid (MeJA) both significantly induced SlMAPK3 expression in tomato leaves. Over-expression of SlMAPK3 increased the transcript levels of SA/JA-mediated defense-related genes (PR1, PR1b/SlLapA, SlPI-I, and SlPI-II) and enhanced tolerance to TYLCV. After TYLCV inoculation, the leaves of SlMAPK3 over-expressed plants compared with wild type plants showed less H2O2 accumulation and greater superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) activity. Overall, the results suggested that SlMAPK3 participates in the antiviral response of tomato to TYLCV, and that this process may be through either the SA or JA defense-signaling pathways.

Molecular characterization of biotic and abiotic stress-responsive MAP kinase genes, IbMPK3 and IbMPK6, in sweetpotato

Plants are continually exposed to numerous environmental stresses. To decrease damage caused by these potentially detrimental factors, various stress-related signaling cascades are activated in plants. One such stress-responsive signaling pathway, the mitogen-activated protein kinase (MAPK) module, plays a critical role in diverse plant stress responses. Here, we functionally characterized biotic and abiotic stress-responsive MAPK genes, IbMPK3 and IbMPK6, from sweetpotato. IbMPK3/6 contain totally 11 MAPK conserved subdomains and the phosphorylating motif TEY. Bacterially expressed IbMPK3/6 could be autophosphorylated in vitro, and these proteins phosphorylated universal kinase substrate, such as myelin basic protein. IbMPK3/6 transcripts were expressed in leaf, stem, and root of sweetpotato cultivars with storage roots of various colors. IbMPK3 and IbMPK6 were induced by various biotic/abiotic stress treatments. Furthermore, the kinase activity of IbMPK3/6 was induced during early NaCl, SA, H2O2, and ABA treatment. IbMPK3/6 were predominantly localized to the nucleus. To determine the biological functions of IbMPK3/6, we transiently expressed the IbMPK genes in tobacco (Nicotiana benthamiana) leaves, which resulted in enhanced tolerance to bacterial pathogen and increased expression of pathogenesis-related (PR) genes. These data demonstrate that IbMPK3 and IbMPK6 play significant roles in plant responses to environmental stress.

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.

ZmMKK1, a novel group A mitogen-activated protein kinase kinase gene in maize, conferred chilling stress tolerance and was involved in pathogen defense in transgenic tobacco

As an important intracellular signaling module, the mitogen-activated protein kinase (MAPK) cascades have been previously implicated in signal transduction during plants responsing to various environmental stresses as well as pathogen attack. The mitogen-activated protein kinase kinase acts as the convergent point of MAPK cascades during a variety of stress signaling. In this study, a novel MAPKK gene, ZmMKK1, in maize (Zea mays L.) belonging to group A MAPKK was isolated and functionally characterized. ZmMKK1 was mainly localized in the cytoplasm and its constitutive kinase-active form ZmMKK1DD was localized in both cytoplasm and nucleus. QRT-PCR analysis uncovered that ZmMKK1 expression was triggered by abiotic and biotic stresses and exogenous signaling molecules. Moreover, hydrogen peroxide (H2O2) and Ca(2+) mediated 12°C-induced up-regulated expressing of ZmMKK1 at mRNA level. Ectopic expression of ZmMKK1 in tobacco (Nicotiana tabacum) conferred tolerance to chilling stress by higher antioxidant enzyme activities, more accumulation of osmoregulatory substances and more significantly up-expression of ROS-related and stress-responsive genes compared with empty vector control plants. Furthermore, ZmMKK1 played differential functions in biotrophic versus necrotrophic pathogen-induced responses. These results suggested ZmMKK1 played a crucial role in chilling stress and pathogen defense in plants.

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.

OsMPK3 positively regulates the JA signaling pathway and plant resistance to a chewing herbivore in rice

KEY MESSAGE : Silencing OsMPK3 decreased elicited JA levels, which subsequently reduced levels of herbivore-induced trypsin protease inhibitors (TrypPIs) and improved the performance of SSB larvae, but did not influence BPH. Mitogen-activated protein kinases (MPKs) are known to play an important role in plant defense by transferring biotic and abiotic signals into programmed cellular responses. However, their functions in the herbivore-induced defense response in rice remain largely unknown. Here, we identified a MPK3 gene from rice, OsMPK3, and found that its expression levels were up-regulated in response to infestation by the larvae of the striped stem borer (SSB) (Chilo suppressalis), to mechanical wounding and to treatment with jasmonic acid (JA), but not to infestation by the brown planthopper (BPH) Nilaparvata lugens or to treatment with salicylic acid. Moreover, mechanical wounding and SSB infestation induced the expression of OsMPK3 strongly and quickly, whereas JA treatment induced the gene more weakly and slowly. Silencing OsMPK3 (ir-mpk3) reduced the expression of the gene by 50-70 %, decreased elicited levels of JA and diminished the expression of a lipoxygenase gene OsHI-LOX and an allene oxide synthase gene OsAOS1. The reduced JA signaling in ir-mpk3 plants decreased the levels of herbivore-induced trypsin protease inhibitors (TrypPIs) and improved the performance of SSB larvae, but did not influence BPH. Our findings suggest that the gene OsMPK3 responds early in herbivore-induced defense and can be regulated by rice plants to activate a specific and appropriate defense response to different herbivores.

Overexpression of a multiple stress-responsive gene, ZmMPK4, enhances tolerance to low temperature in transgenic tobacco

Mitogen-activated protein kinase (MAPK) cascades play important roles in mediating biotic and abiotic stress responses. In this study, we found that ZmMPK4 protein was predominantly localized in the nucleus. Semi-quantitative RT-PCR analysis revealed that the ZmMPK4 transcription in maize leaves was up-regulated by low temperature, high temperature and exogenous signaling molecules such as hydrogen peroxide, methyl jasmonate and ethephon. Hydrogen peroxide acted as second messenger to mediate 4°C-induced up-regulation of ZmMPK4 mRNA. Transgenic tobacco of overexpressing ZmMPK4 accumulated less reactive oxygen species (ROS), more peroxidase and catalase activities, more proline and soluble sugar contents, and more stress-responsive genes expression, leading to enhancing low temperature stress tolerance compared to the control plants. Taken together, these results strongly suggest that ZmMPK4 positively regulates low temperature stress tolerance in plants.

ZmMPK17, a novel maize group D MAP kinase gene, is involved in multiple stress responses

Plant mitogen-activated protein kinase (MAPK) cascades play a pivotal role in a range of biotic and abiotic stress responses. In this study, we isolated a novel group D MAPK gene, ZmMPK17, from maize (Zea mays L.). ZmMPK17 is localized mainly to the nucleus and its C-terminal domain extension is believed to be essential for this. Northern-blot analysis indicated that ZmMPK17 transcription is involved in response to exogenous signaling molecules such as abscisic acid, hydrogen peroxide, salicylic acid, jasmonic acid and ethylene and induced by low temperature and osmotic stress. Hydrogen peroxide and Ca²⁺ mediate PEG-induced downregulation of ZmMPK17 at transcription level and Ca²⁺ also mediates low temperature-induced expression of ZmMPK17. Overexpression of ZmMPK17 in tobacco (Nicotonia tobaccum) accumulated less reactive oxygen species under osmotic stress by affecting antioxidant defense systems. Transgenic tobacco exhibited enhanced tolerance to cold by means of an increased germination rate, and increased proline and soluble sugar levels relative to control plants. The transcription levels of NtERD10 genes were higher in ZmMPK17-overexpressing lines than in control plants under cold and osmotic stress conditions. ZmMPK17-overexpressing plants displayed enhanced resistance to viral pathogens, and the expression of the pathogenesis-related gene PR1a was significantly increased, indicating that ZmMPK17 might be involved in SA-mediated pathogen defense-signaling pathways.

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.

Transgenic tobacco plants constitutively expressing Arabidopsis NPR1 show enhanced resistance to root-knot nematode, Meloidogyne incognita

In Arabidopsis, non-expressor of pathogenesis related genes-1, NPR1 has been shown to be a positive regulator of the salicylic acid controlled systemic acquired resistance pathway and modulates the cross talk between SA and JA signaling. Transgenic plants expressing AtNPR1 constitutively exhibited resistance against pathogens as well as herbivory. In the present study, tobacco transgenic plants expressing AtNPR1 were studied further for their response to infection by the sedentary endoparasitic root knot nematode, Meloidogyne incognita. Transgenic plants showed enhanced resistance against the root-knot nematode infection. Prominent differences in the shoot and root weights of wild type and transgenic plants were observed post-inoculation with M. incognita. This was associated with a decrease in the number of root galls and egg masses in transgenic plants compared to WT. The transgenic plants also showed constitutive and induced expression of some PR protein genes, when challenged with M. incognita.

Differential gene expression in Arachis diogoi upon interaction with peanut late leaf spot pathogen, Phaeoisariopsis personata and characterization of a pathogen induced cyclophilin

The wild relatives of peanut are resistant to various economically important diseases including late leaf spot (LLS) caused by Phaeoisariopsis personata, compared with the susceptible cultivated peanut (Arachis hypogaea L.). The interaction of the late leaf spot pathogen, Phaeoisariopsis personata and the highly resistant, diploid peanut wild species, Arachis diogoi was analyzed at the molecular level by differential gene expression studies. Genes up-regulated with in 48 h of pathogen challenge were isolated as partial cDNAs. Some of the isolated genes, which are shown to be involved in the first line of defense in plants, were further characterized with respect to their transcriptional regulation in response to pathogen. Among the isolated clones, two were found to encode cyclophilin like proteins. One of the two isolated partial cDNAs encoding cyclophilin like proteins was extended using 5' RACE. The full length cDNA, designated as AdCyp, was 886 bp in length and encodes a polypeptide of 172 amino acids. Southern hybridization suggests that AdCyp is possibly coded by a single gene and at least one more identical gene is present in Arachis diogoi genome. AdCyp exhibits evolutionary conservation across the kingdoms. Phylogenetic analysis showed that AdCyp belongs to the subgroup I of Group I in cyclophilins. A translational fusion of GFP-AdCyp was found to localize to both cytosol and nucleus. AdCyp transcripts were found to accumulate in response to the treatments with pathogen as well as phytohormones. Constitutive heterologous expression of AdCyp resulted in enhanced resistance to Ralstonia solanacearum and reduced susceptibility towards Phytophthora parasitica var. nicotianae in transgenic tobacco and the resistance was associated with higher transcript levels of various defense related genes.