Constitutively Active Arabidopsis MAP Kinase 3 Triggers Defense Responses Involving Salicylic Acid and SUMM2 Resistance Protein

Combined metabolome and transcriptome reveal HmF6'H1 regulating simple coumarin accumulation against powdery mildew infection in Heracleum moellendorffii Hance

BACKGROUND

Powdery mildew, caused by Eeysiphe heraclei, seriously threatens Heracleum moellendorffii Hance. Plant secondary metabolites are essential to many activities and are necessary for defense against biotic stress. In order to clarify the functions of these metabolites in response to the pathogen, our work concentrated on the variations in the accumulation of secondary metabolites in H. moellendorffii during E. heraclei infection.

RESULTS

Following E. heraclei infection, a significant upregulation of coumarin metabolites-particularly simple coumarins and associated genes was detected by RNA-seq and UPLC-MS/MS association analysis. Identifying HmF6'H1, a Feruloyl CoA 6'-hydroxylase pivotal in the biosynthesis of the coumarin basic skeleton through ortho-hydroxylation, was a significant outcome. The cytoplasmic HmF6'H1 protein was shown to be able to catalyze the ortho-hydroxylation of p-coumaroyl-CoA and caffeoyl-CoA, resulting in the formation of umbelliferone and esculetin, respectively. Over-expression of the HmF6'H1 gene resulted in increased levels of simple coumarins, inhibiting the biosynthesis of furanocoumarins and pyranocoumarins by suppressing PT gene expression, enhancing H. moellendorffii resistance to powdery mildew.

CONCLUSIONS

These results established HmF6'H1 as a resistance gene aiding H. moellendorffii in combatting E. heraclei infection, offering additional evidence of feruloyl-CoA 6'-hydroxylase role in catalyzing various types of simple coumarins. Therefore, this work contributes to our understanding of the function of simple coumarins in plants' defense against powdery mildew infection.

VqMAPK3/VqMAPK6, VqWRKY33, and VqNSTS3 constitute a regulatory node in enhancing resistance to powdery mildew in grapevine

Grapevine powdery mildew is caused by Erysiphe necator, which seriously harms grape production in the world. Stilbene synthase makes phytoalexins that contribute to the resistance of grapevine against powdery mildew. A novel VqNSTS3 was identified and cloned from Chinese wild Vitis quinquangularis accession Danfeng-2. The novel VqNSTS3 was transferred into susceptible 'Thompson Seedless' by Agrobacterium-mediated transformation. The transgenic plants showed resistance to the disease and activated other resistance-related genes. VqNSTS3 expression in grapevine is regulated by VqWRKY33, and which binds to TTGACC in the VqNSTS3 promoter. Furthermore, VqWRKY33 was phosphorylated by VqMAPK3/VqMAPK6 and thus led to enhanced signal transduction and increased VqNSTS3 expression. ProVqNSTS3::VqNSTS3-GFP of transgenic VqNSTS3 in Arabidopsis thaliana was observed to move to and wrap the pathogen's haustoria and block invasion by Golovinomyces cichoracearum. These results demonstrate that stilbene accumulation of novel VqNSTS3 of the Chinese wild Vitis quinquangularis accession Danfeng-2 prevented pathogen invasion and enhanced resistance to powdery mildew. Therefore, VqNSTS3 can be used in generating powdery mildew-resistant grapevines.

EDR1 associates with its homologs to synergistically regulate plant immunity in Arabidopsis

ENHANCED DISEASE RESISTANCE 1 (EDR1), a Raf-like mitogen-activated protein kinase (MAPK) kinase kinase (MAPKKK), is a negative regulator of resistance. There are three homologs, RAF3/4/5, of EDR1 in Arabidopsis. However, the roles of RAF3/4/5 in resistance and their functional link with EDR1 in plant immunity remain unclear. Here, we showed that the raf3/4/5 triple mutant displayed wild-type-like phenotypes to the powdery mildew pathogen Golovinomyces cichoracearum UCSC1 and the bacterial pathogen Pseudomonas syringae pv. tomato (Pto) DC3000. However, the edr1 raf3/4/5 quadruple mutant exhibited enhanced resistance to G. cichoracearum UCSC1 and Pto DC3000 compared to edr1. Consistently, MPK3/6 kinase activity was more highly activated in edr1 raf3/4/5 than that in edr1. Moreover, the enhanced resistance of edr1 raf3/4/5 required SALICYLIC ACID INDUCTION DEFICIENT 2 (SID2), an isochorismate synthase required for salicylic acid (SA) synthesis. Additionally, unlike EDR1, RAF3/4/5 weakly and indirectly associated with MKK4/5, and EDR1 was directly associated with RAF3/4/5. Taken together, these data indicate that EDR1 associates with RAF3/4/5, and they may function together to synergistically suppress MAPK cascades activation, which reveal the complexity and importance of Raf-like MAPKKKs in plant immunity regulation.

Soybean GmSAUL1, a Bona Fide U-Box E3 Ligase, Negatively Regulates Immunity Likely through Repressing the Activation of GmMPK3

E3 ubiquitin ligases play important roles in plant immunity, but their role in soybean has not been investigated previously. Here, we used Bean pod mottle virus (BPMV)-mediated virus-induced gene silencing (VIGS) to investigate the function of GmSAUL1 (Senescence-Associated E3 Ubiquitin Ligase 1) homologs in soybean. When two closely related SAUL1 homologs were silenced simultaneously, the soybean plants displayed autoimmune phenotypes, which were significantly alleviated by high temperature, suggesting that GmSAUL1a/1b might be guarded by an R protein. Interestingly, silencing GmSAUL1a/1b resulted in the decreased activation of GmMPK6, but increased activation of GmMPK3 in response to flg22, suggesting that the activation of GmMPK3 is most likely responsible for the activated immunity observed in the GmSAUL1a/1b-silenced plants. Furthermore, we provided evidence that GmSAUL1a is a bona fide E3 ligase. Collectively, our results indicated that GmSAUL1 plays a negative role in regulating cell death and immunity in soybean.

Exogenous application of salicylic acid improves freezing stress tolerance in alfalfa

Freezing stress is one of the most detrimental environmental factors that can seriously impact the growth, development, and distribution of alfalfa (Medicago sativa L.). Exogenous salicylic acid (SA) has been revealed as a cost-effective method of improving defense against freezing stress due to its predominant role in biotic and abiotic stress resistance. However, how the molecular mechanisms of SA improve freezing stress resistance in alfalfa is still unclear. Therefore, in this study, we used leaf samples of alfalfa seedlings pretreatment with 200 μM and 0 μM SA, which were exposed to freezing stress (-10°C) for 0, 0.5, 1, and 2h and allowed to recover at normal temperature in a growth chamber for 2 days, after which we detect the changes in the phenotypical, physiological, hormone content, and performed a transcriptome analysis to explain SA influence alfalfa in freezing stress. The results demonstrated that exogenous SA could improve the accumulation of free SA in alfalfa leaves primarily through the phenylalanine ammonia-lyase pathway. Moreover, the results of transcriptome analysis revealed that the mitogen-activated protein kinase (MAPK) signaling pathway-plant play a critical role in SA alleviating freezing stress. In addition, the weighted gene co-expression network analysis (WGCNA) found that MPK3, MPK9, WRKY22 (downstream target gene of MPK3), and TGACG-binding factor 1 (TGA1) are candidate hub genes involved in freezing stress defense, all of which are involved in the SA signaling pathway. Therefore, we conclude that SA could possibly induce MPK3 to regulate WRKY22 to participate in freezing stress to induced gene expression related to SA signaling pathway (NPR1-dependent pathway and NPR1-independent pathway), including the genes of non-expresser of pathogenesis-related gene 1 (NPR1), TGA1, pathogenesis-related 1 (PR1), superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX), glutathione-S-transferase (GST), and heat shock protein (HSP). This enhanced the production of antioxidant enzymes such as SOD, POD, and APX, which increases the freezing stress tolerance of alfalfa plants.

Bacillus amyloliquefaciens PMB05 Increases Resistance to Bacterial Wilt by Activating Mitogen-Activated Protein Kinase and Reactive Oxygen Species Pathway Crosstalk in Arabidopsis thaliana

Bacterial wilt caused by Ralstonia solanacearum can infect many crops, causing significant losses worldwide. The use of beneficial microorganisms is considered a feasible method for controlling this disease. Our previous study showed that Bacillus amyloliquefaciens PMB05 can control bacterial wilt through intensifying immune signals triggered by a pathogen-associated molecular pattern (PAMP) from R. solanacearum. It is still uncertain whether induction of the mitogen-activated protein kinase (MAPK) pathway during PAMP-triggered immunity (PTI) is responsible for enhancing disease resistance. To gain more insights on how the presence of PMB05 regulates PTI signaling, its association with the MAPK pathway was assayed. Our results showed that the activation of MPK3/6 and expression of wrky22 upon treatment with the PAMP, PopW, was increased during co-treatment with PMB05. Moreover, the disease resistance conferred by PMB05 to bacterial wilt was abolished in mekk1, mkk5, and mpk6 mutants. To determine the relationship between the MAPK pathway and plant immune signals, the assay on reactive oxygen species (ROS) generation and callose deposition showed that only the ROS generation was strongly reduced in these mutants. Because ROS generation is highly correlated with RbohD, the results revealed that the effects of PMB05 on both PopW-induced ROS generation and disease resistance to bacterial wilt were eliminated in the rbohD mutant, suggesting that the generation of ROS is also required for PMB05-enhanced disease resistance. Taken together, we concluded that the crosstalk between the initiation of ROS generation and further activation of the MAPK pathway is necessary when PMB05 is used to improve disease resistance to bacterial wilt. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Ambivalent response in pathogen defense: A double-edged sword?

Plants possess effective immune systems that defend against most microbial attackers. Recent plant immunity research has focused on the classic binary defense model involving the pivotal role of small-molecule hormones in regulating the plant defense signaling network. Although most of our current understanding comes from studies that relied on information derived from a limited number of pathosystems, newer studies concerning the incredibly diverse interactions between plants and microbes are providing additional insights into other novel mechanisms. Here, we review the roles of both classical and more recently identified components of defense signaling pathways and stress hormones in regulating the ambivalence effect during responses to diverse pathogens. Because of their different lifestyles, effective defense against biotrophic pathogens normally leads to increased susceptibility to necrotrophs, and vice versa. Given these opposing forces, the plant potentially faces a trade-off when it mounts resistance to a specific pathogen, a phenomenon referred to here as the ambivalence effect. We also highlight a novel mechanism by which translational control of the proteins involved in the ambivalence effect can be used to engineer durable and broad-spectrum disease resistance, regardless of the lifestyle of the invading pathogen.

The Sclerotinia sclerotiorum-inducible promoter pBnGH17D7 in Brassica napus: isolation, characterization, and application in host-induced gene silencing

Sclerotinia stem rot (SSR), caused by Sclerotinia sclerotiorum, is among the most devastating diseases in Brassica napus worldwide. Conventional breeding for SSR resistance in Brassica species is challenging due to the limited availability of resistant germplasm. Therefore, genetic engineering is an attractive approach for developing SSR-resistant Brassica crops. Compared with the constitutive promoter, an S. sclerotiorum-inducible promoter would avoid ectopic expression of defense genes that may cause plant growth deficits. In this study, we generated a S. sclerotiorum-inducible promoter. pBnGH17D7, from the promoter of B. napus glycosyl hydrolase 17 gene (pBnGH17). Specifically, 5'-deletion and promoter activity analyses in transgenic Arabidopsis thaliana plants defined a 189 bp region of pBnGH17 which was indispensable for S. sclerotiorum-induced response. Compared with pBnGH17, pBnGH17D7 showed a similar response upon S. sclerotiorum infection, but lower activity in plant tissues in the absence of S. sclerotiorum infection. Moreover, we revealed that the transcription factor BnTGA7 directly binds to the TGACG motif in pBnGH17D7 to activate BnGH17. Ultimately, pBnGH17D7 was exploited for engineering Sclerotinia-resistant B. napus via host-induced gene silencing. It induces high expression of siRNAs against the S. sclerotiorum pathogenic factor gene specifically during infection, leading to increased resistance.

The MKK2a Gene Involved in the MAPK Signaling Cascades Enhances Populus Salt Tolerance

Mitogen-activated protein kinase (MAPK) cascades are highly conserved signal transduction modules, which transmit environmental signals in plant cells through stepwise phosphorylation and play indispensable roles in a wide range of physiological and biochemical processes. Here, we isolated and characterized a gene encoding MKK2 protein from poplar through the rapid amplification of cDNA ends (RACE). The full-length PeMKK2a gene was 1571 bp, including a 1068 bp open reading frame (ORF) encoding 355 amino acids, and the putative PeMKK2a protein belongs to the PKc_like (protein kinase domain) family (70–336 amino acids) in the PKc_MAPKK_plant subfamily and contains 62 sites of possible phosphorylation and two conserved domains, DLK and S/T-xxxxx-S/T. Detailed information about its gene structure, sequence similarities, subcellular localization, and transcript profiles under salt-stress conditions was revealed. Transgenic poplar lines overexpressing PeMKK2a exhibited higher activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) than non-transgenic poplar under salt stress conditions. These results will provide insight into the roles of MAPK signaling cascades in poplar response to salt stress.

CYSTEINE-RICH RECEPTOR-LIKE KINASE5 (CRK5) and CRK22 regulate the response to Verticillium dahliae toxins

Cysteine-rich receptor-like kinases (CRKs) play critical roles in responses to biotic and abiotic stresses. However, the molecular mechanisms of CRKs in plant defense responses remain unknown. Here, we demonstrated that two CRKs, CRK5 and CRK22, are involved in regulating defense responses to Verticillium dahliae toxins (Vd-toxins) in Arabidopsis (Arabidopsis thaliana). Biochemical and genetic analyses showed that CRK5 and CRK22 may act upstream of MITOGEN-ACTIVATED PROTEIN KINASE3 (MPK3) and MPK6 to regulate the salicylic acid (SA)-signaling pathway in response to Vd-toxins. In addition, MPK3 and MPK6 interact with the transcription factor WRKY70 to modulate defense responses to Vd-toxins. WRKY70 directly binds the promoter domains of the SA-signaling-related transcription factor genes TGACG SEQUENCE-SPECIFIC BINDING PROTEIN (TGA2) and TGA6 to regulate their expression in response to Vd-toxins. Thus, our study reveals a mechanism by which CRK5 and CRK22 regulate SA signaling through the MPK3/6-WRKY70-TGA2/6 pathway in response to Vd-toxins.

MAPKK2/4/5/7-MAPK3-JAZs modulate phenolic acid biosynthesis in Salvia miltiorrhiza

Phenolic acids are the major bioactive metabolites produced in Salvia miltiorrhiza, a traditional Chinese medicine called Danshen. Many phytohormone elicitor treatments induce phenolic acid biosynthesis, even though the underlying mechanism remains obscure. Expression pattern analysis showed that SmMAPK3 was highly expressed in leaves, and SmMAPK3 was significantly induced by salicylic acid (SA) and methyl jasmonate (JA). Bioinformatics analysis revealed that SmMAPK3 belongs to group A and contains a TEY motif in the activation loop together with three conserved regions (P-loop, C-loop and CD-domain). A previous study speculated that SmMAPK3 is likely a positive regulator in the biosynthesis of phenolic acids in S. miltiorrhiza. In this study, overexpression of SmMAPK3 increased phenolic acid biosynthetic gene expression and enhanced the accumulation of phenolic acids in S. miltiorrhiza plantlets. Yeast two-hybrid (Y2H) analysis and firefly luciferase complementation imaging (LCI) assays revealed that SmMAPKK2/4/5/7-SmMAPK3-SmJAZs form a cascade that regulates the accumulation of phenolic acids. In summary, this work deepens our understanding of the posttranscriptional regulatory mechanisms of phenolic acid biosynthesis and sheds new light on metabolic engineering in S. miltiorrhiza.

Con-Ca2+ -tenating plant immune responses via calcium-permeable cation channels

Calcium serves as a second messenger in a variety of developmental and physiological processes and has long been identified as important for plant immune responses. We discuss recent discoveries regarding plant immune-related calcium-permeable channels and how the two intertwined branches of the plant immune system are intricately linked to one another through calcium signalling. Cell surface immune receptors carefully tap the immense calcium gradient that exists between apoplast and cytoplasm in a short burst via tightly regulated plasma membrane (PM)-resident cation channels. Intracellular immune receptors form atypical calcium-permeable cation channels at the PM and mediate a prolonged calcium influx, overcoming the deleterious influence of pathogen effectors and enhancing plant immune responses.

Dual control of MAPK activities by AP2C1 and MKP1 MAPK phosphatases regulates defence responses in Arabidopsis

Mitogen-activated protein kinase (MAPK) cascades transmit environmental signals and induce stress and defence responses in plants. These signalling cascades are negatively controlled by specific Ser/Thr protein phosphatases of the type 2C (PP2C) and dual-specificity phosphatase (DSP) families that inactivate stress-induced MAPKs; however, the interplay between phosphatases of these different types has remained unknown. This work reveals that different Arabidopsis MAPK phosphatases, the PP2C-type AP2C1 and the DSP-type MKP1, exhibit both specific and overlapping functions in plant stress responses. Each single mutant, ap2c1 and mkp1, and the ap2c1 mkp1 double mutant displayed enhanced stress-induced activation of the MAPKs MPK3, MPK4, and MPK6, as well as induction of a set of transcription factors. Moreover, ap2c1 mkp1 double mutants showed an autoimmune-like response, associated with increased levels of the stress hormones salicylic acid and ethylene, and of the phytoalexin camalexin. This phenotype was reduced in the ap2c1 mkp1 mpk3 and ap2c1 mkp1 mpk6 triple mutants, suggesting that the autoimmune-like response is due to MAPK misregulation. We conclude that the evolutionarily distant MAPK phosphatases AP2C1 and MKP1 contribute crucially to the tight control of MAPK activities, ensuring appropriately balanced stress signalling and suppression of autoimmune-like responses during plant growth and development.

MPK3 and MPK6 control salicylic acid signaling by up-regulating NLR receptors during pattern- and effector-triggered immunity

Arabidopsis thaliana mitogen-activated protein kinases 3 and 6 (MPK3/6) are activated transiently during pathogen-associated molecular pattern-triggered immunity (PTI) and durably during effector-triggered immunity (ETI). The functional differences between these two kinds of activation kinetics and how they coordinate the two layers of plant immunity remain poorly understood. Here, by suppressor analyses, we demonstrate that ETI-mediating nucleotide-binding domain leucine-rich repeat receptors (NLRs) and the NLR signaling components NDR1 and EDS1 can promote the salicylic acid sector of defense downstream of MPK3 activity. Moreover, we provide evidence that both sustained and transient MPK3/6 activities positively control the expression of several NLR genes, including AT3G04220 and AT4G11170. We further show that NDR1 and EDS1 contribute to the up-regulation of these two NLRs in both an ETI and a PTI context. Remarkably, whereas in ETI MPK3/6 activities are dependent on NDR1 and EDS1, they are not in PTI, suggesting crucial differences in the two signaling pathways. Finally, we demonstrate that expression of the NLR AT3G04220 is sufficient to induce expression of defense genes from the salicylic acid branch. Overall, this study expands our knowledge of MPK3/6 functions during immunity and provides new insights into the intricate interplay of PTI and ETI.

Phosphoproteomics Reveals Regulation of Secondary Metabolites in Mahonia bealei Exposed to Ultraviolet-B Radiation

Mahonia bealei (M. bealei) is a traditional Chinese medicine containing a high alkaloid content used to treat various diseases. Generally, only dried root and stem are used as medicines, considering that the alkaloid content in M. bealei leaves is lower than in the stems and roots. Some previous research found that alkaloid and flavonoid contents in the M. bealei leaves may increase when exposed to ultraviolet B (UV-B) radiation. However, the underlying mechanism of action is still unclear. In this study, we used titanium dioxide material enrichment and mass-based label-free quantitative proteomics techniques to explore the effect and mechanism of M. bealei leaves when exposed to UV-B treatment. Our data suggest that UV-B radiation increases the ATP content, photosynthetic pigment content, and some enzymatic/nonenzymatic indicators in the leaves of M. bealei. Moreover, phosphoproteomics suggests phosphoproteins related to mitogen-activated protein kinase (MAPK) signal transduction and the plant hormone brassinosteroid signaling pathway as well as phosphoproteins related to photosynthesis, glycolysis, the tricarboxylic acid cycle, and the amino acid synthesis/metabolism pathway are all affected by UV-B radiation. These results suggest that the UV-B radiation activates the oxidative stress response, MAPK signal transduction pathway, and photosynthetic energy metabolism pathway, which may lead to the accumulation of secondary metabolites in M. bealei leaves.

MKK4/5-MPK3/6 Cascade Regulates Agrobacterium-Mediated Transformation by Modulating Plant Immunity in Arabidopsis

Agrobacterium tumefaciens is a specialized plant pathogen that causes crown gall disease and is commonly used for Agrobacterium-mediated transformation. As a pathogen, Agrobacterium triggers plant immunity, which affects transformation. However, the signaling components and pathways in plant immunity to Agrobacterium remain elusive. We demonstrate that two Arabidopsis mitogen-activated protein kinase kinases (MAPKKs) MKK4/MKK5 and their downstream mitogen-activated protein kinases (MAPKs) MPK3/MPK6 play major roles in both Agrobacterium-triggered immunity and Agrobacterium-mediated transformation. Agrobacteria induce MPK3/MPK6 activity and the expression of plant defense response genes at a very early stage. This process is dependent on the MKK4/MKK5 function. The loss of the function of MKK4 and MKK5 or their downstream MPK3 and MPK6 abolishes plant immunity to agrobacteria and increases transformation frequency, whereas the activation of MKK4 and MKK5 enhances plant immunity and represses transformation. Global transcriptome analysis indicates that agrobacteria induce various plant defense pathways, including reactive oxygen species (ROS) production, ethylene (ET), and salicylic acid- (SA-) mediated defense responses, and that MKK4/MKK5 is essential for the induction of these pathways. The activation of MKK4 and MKK5 promotes ROS production and cell death during agrobacteria infection. Based on these results, we propose that the MKK4/5-MPK3/6 cascade is an essential signaling pathway regulating Agrobacterium-mediated transformation through the modulation of Agrobacterium-triggered plant immunity.

Understanding and Exploiting Post-Translational Modifications for Plant Disease Resistance

Plants are constantly threatened by pathogens, so have evolved complex defence signalling networks to overcome pathogen attacks. Post-translational modifications (PTMs) are fundamental to plant immunity, allowing rapid and dynamic responses at the appropriate time. PTM regulation is essential; pathogen effectors often disrupt PTMs in an attempt to evade immune responses. Here, we cover the mechanisms of disease resistance to pathogens, and how growth is balanced with defence, with a focus on the essential roles of PTMs. Alteration of defence-related PTMs has the potential to fine-tune molecular interactions to produce disease-resistant crops, without trade-offs in growth and fitness.

TALEN-Based HvMPK3 Knock-Out Attenuates Proteome and Root Hair Phenotypic Responses to flg22 in Barley

Mitogen activated protein kinases (MAPKs) integrate elicitor perception with both early and late responses associated with plant defense and innate immunity. Much of the existing knowledge on the role of plant MAPKs in defense mechanisms against microbes stems from extensive research in the model plant Arabidopsis thaliana. In the present study, we investigated the involvement of barley (Hordeum vulgare) MPK3 in response to flagellin peptide flg22, a well-known bacterial elicitor. Using differential proteomic analysis we show that TALEN-induced MPK3 knock-out lines of barley (HvMPK3 KO) exhibit constitutive downregulation of defense related proteins such as PR proteins belonging to thaumatin family and chitinases. Further analyses showed that the same protein families were less prone to flg22 elicitation in HvMPK3 KO plants compared to wild types. These results were supported and validated by chitinase activity analyses and immunoblotting for HSP70. In addition, differential proteomes correlated with root hair phenotypes and suggested tolerance of HvMPK3 KO lines to flg22. In conclusion, our study points to the specific role of HvMPK3 in molecular and root hair phenotypic responses of barley to flg22.

Plant Mitogen-Activated Protein Kinase Cascades in Environmental Stresses

Due to global warming and population growth, plants need to rescue themselves, especially in unfavorable environments, to fulfill food requirements because they are sessile organisms. Stress signal sensing is a crucial step that determines the appropriate response which, ultimately, determines the survival of plants. As important signaling modules in eukaryotes, plant mitogen-activated protein kinase (MAPK) cascades play a key role in regulating responses to the following four major environmental stresses: high salinity, drought, extreme temperature and insect and pathogen infections. MAPK cascades are involved in responses to these environmental stresses by regulating the expression of related genes, plant hormone production and crosstalk with other environmental stresses. In this review, we describe recent major studies investigating MAPK-mediated environmental stress responses. We also highlight the diverse function of MAPK cascades in environmental stress. These findings help us understand the regulatory network of MAPKs under environmental stress and provide another strategy to improve stress resistance in crops to ensure food security.

Sustained Incompatibility between MAPK Signaling and Pathogen Effectors

In plants, Mitogen-Activated Protein Kinases (MAPKs) are important signaling components involved in developemental processes as well as in responses to biotic and abiotic stresses. In this review, we focus on the roles of MAPKs in Effector-Triggered Immunity (ETI), a specific layer of plant defense responses dependent on the recognition of pathogen effector proteins. Having inspected the literature, we synthesize the current state of knowledge concerning this topic. First, we describe how pathogen effectors can manipulate MAPK signaling to promote virulence, and how in parallel plants have developed mechanisms to protect themselves against these interferences. Then, we discuss the striking finding that the recognition of pathogen effectors can provoke a sustained activation of the MAPKs MPK3/6, extensively analyzing its implications in terms of regulation and functions. In line with this, we also address the question of how a durable activation of MAPKs might affect the scope of their substrates, and thereby mediate the emergence of possibly new ETI-specific responses. By highlighting the sometimes conflicting or missing data, our intention is to spur further research in order to both consolidate and expand our understanding of MAPK signaling in immunity.

N-acyl homoserine lactone-mediated modulation of plant growth and defense against Pseudoperonospora cubensis in cucumber

N-acyl-homoserine lactones (AHLs), a well-described group of quorum sensing molecules, may modulate plant defense responses and plant growth. However, there is limited knowledge regarding the defense responses of non-model crops to AHLs and the mechanism of action responsible for the modulation of defense responses against microbial pathogens. In the present study, long-chain N-3-oxo-tetradecanoyl-l-homoserine lactone (oxo-C14-HSL) was shown to have a distinct potential to prime cucumber for enhanced defense responses against the biotrophic oomycete pathogen Pseudoperonospora cubensis and the hemibiotrophic bacterium Pseudomonas syringae pv. lachrymans. We provide evidence that AHL-mediated enhanced defense against downy mildew disease is based on cell wall reinforcement by lignin and callose deposition, the activation of defense-related enzymes (peroxidase, β-1,3-glucanase, phenylalanine ammonia-lyase), and the accumulation of reactive oxygen species (hydrogen peroxide, superoxide) and phenolic compounds. Quantitative analysis of salicylic acid and jasmonic acid, and transcriptional analysis of several of genes associated with these phytohormones, revealed that defense priming with oxo-C14-HSL is commonly regulated by the salicylic acid signaling pathway. We also show that treatment with short- (N-hexanoyl-l-homoserine lactone) and medium-chain (N-3-oxo-decanoyl-l-homoserine lactone) AHLs promoted primary root elongation and modified root architecture, respectively, resulting in enhanced plant growth.

A holistic view on plant effector-triggered immunity presented as an iceberg model

The immune system of plants is highly complex. It involves pattern-triggered immunity (PTI), which is signaled and manifested through branched multi-step pathways. To counteract this, pathogen effectors target and inhibit individual PTI steps. This in turn can cause specific plant cytosolic nucleotide-binding leucine-rich repeat (NLR) receptors to activate effector-triggered immunity (ETI). Plants and pathogens have many genes encoding NLRs and effectors, respectively. Yet, only a few segregate genetically as resistance (R) genes and avirulence (Avr) effector genes in wild-type populations. In an attempt to explain this contradiction, a model is proposed where far most of the NLRs, the effectors and the effector targets keep one another in a silent state. In this so-called "iceberg model", a few NLR-effector combinations are genetically visible above the surface, while the vast majority is hidden below. Besides, addressing the existence of many NLRs and effectors, the model also helps to explain why individual downregulation of many effectors causes reduced virulence and why many lesion-mimic mutants are found. Finally, the iceberg model accommodates genuine plant susceptibility factors as potential effector targets.

Phosphorylation of the CAMTA3 Transcription Factor Triggers Its Destabilization and Nuclear Export

The Arabidopsis (Arabidopsis thaliana) calmodulin-binding transcription activator3 (CAMTA3) is a repressor of immunity-related genes but an activator of cold-induced or general stress-responsive genes in plants. Post-transcriptional or posttranslational mechanisms have been proposed to control CAMTA3 functions in different stress responses. Here, we show that treatment with the bacterial flg22 elicitor induces CAMTA3 phosphorylation, which is accompanied by its destabilization and nuclear export. Two flg22-responsive mitogen-activated protein kinases (MAPKs), MPK3 and MPK6, directly phosphorylate CAMTA3, with the phospho-sites contributing to CAMTA3 degradation and suppression of downstream target gene expression. However, the flg22-induced nuclear export and phospho-mobility shift can still be observed for the CAMTA3 phospho-null variant of the MAPK-modified sites, suggesting additional flg22-responsive kinases might be involved. Taken together, we propose that flg22-induced CAMTA3 depletion facilitates de-repression of downstream defense target genes, which involves phosphorylation, increased protein turnover, and nucleo-cytoplasmic trafficking.

Genome-wide characterization and expression profiling of MAPK cascade genes in Salvia miltiorrhiza reveals the function of SmMAPK3 and SmMAPK1 in secondary metabolism

BACKGROUND

The contribution of mitogen-activated protein kinase (MAPK) cascades to plant growth and development has been widely studied, but this knowledge has not yet been extended to the medicinal plant Salvia miltiorrhiza, which produces a number of pharmacologically active secondary metabolites.

RESULTS

In this study, we performed a genome-wide survey and identified six MAPKKK kinases (MAPKKKKs), 83 MAPKK kinases (MAPKKKs), nine MAPK kinases (MAPKKs) and 18 MAPKs in the S. miltiorrhiza genome. Within each class of genes, a small number of subfamilies were recognized. A transcriptional analysis revealed differences in the genes' behaviour with respect to both their site of transcription and their inducibility by elicitors and phytohormones. Two genes were identified as strong candidates for playing roles in phytohormone signalling. A gene-to-metabolite network was constructed based on correlation analysis, highlighting the likely involvement of two of the cascades in the synthesis of two key groups of pharmacologically active secondary metabolites: phenolic acids and tanshinones.

CONCLUSION

The data provide insight into the functional diversification and conservation of MAPK cascades in S. miltiorrhiza.

The Proteomics of Resistance to Halo Blight in Common Bean

Halo blight disease of beans is caused by a gram-negative bacterium, Pseudomonas syringae pv. phaseolicola. The disease is prevalent in South America and Africa and causes crop loss for indigent people who rely on beans as a primary source of daily nutrition. In susceptible beans, P. syringae pv. phaseolicola causes water-soaking at the site of infection and produces phaseolotoxin, an inhibitor of bean arginine biosynthesis. In resistant beans, P. syringae pv. phaseolicola triggers a hypersensitive response that limits the spread of infection. Here, we used high-throughput mass spectrometry to interrogate the responses to two different P. syringae pv. phaseolicola isolates on a single line of common bean, Phaseolus vulgaris PI G19833, with a reference genome sequence. We obtained quantitative information for 4,135 bean proteins. A subset of 160 proteins with similar accumulation changes during both susceptible and resistant reactions included salicylic acid responders EDS1 and NDR1, ethylene and jasmonic acid biosynthesis enzymes, and proteins enabling vesicle secretion. These proteins revealed the activation of a basal defense involving hormonal responses and the mobilization of extracellular proteins. A subset of 29 proteins specific to hypersensitive immunity included SOBIR1, a G-type lectin receptor-like kinase, and enzymes needed for glucoside and phytoalexin production. Virus-induced gene silencing revealed that the G-type lectin receptor-like kinase suppresses bacterial infection. Together, the results define the proteomics of disease resistance to P. syringae pv. phaseolicola in beans and support a model whereby the induction of hypersensitive immunity reinstates defenses targeted by P. syringae pv. phaseolicola.

Genome-wide identification of mitogen-activated protein kinase (MAPK) cascade and expression profiling of CmMAPKs in melon (Cucumis melo L.)

Mitogen-activated protein kinase (MAPK) is a form of serine/threonine protein kinase that activated by extracellular stimulation acting through the MAPK cascade (MAPKKK-MAPKK-MAPK). The MAPK cascade gene family, an important family of protein kinases, plays a vital role in responding to various stresses and hormone signal transduction processes in plants. In this study, we identified 14 CmMAPKs, 6 CmMAPKKs and 64 CmMAPKKKs in melon genome. Based on structural characteristics and a comparison of phylogenetic relationships of MAPK gene families from Arabidopsis, cucumber and watermelon, CmMAPKs and CmMAPKKs were categorized into 4 groups, and CmMAPKKKs were categorized into 3 groups. Furthermore, chromosome location revealed an unevenly distribution on chromosomes of MAPK cascade genes in melon, respectively. Eventually, qRT-PCR analysis showed that all 14 CmMAPKs had different expression patterns under drought, salt, salicylic acid (SA), methyl jasmonate (MeJA), red light (RL), and Podosphaera xanthii (P. xanthii) treatments. Overall, the expression levels of CmMAPK3 and CmMAPK7 under different treatments were higher than those in control. Our study provides an important basis for future functional verification of MAPK genes in regulating responses to stress and signal substance in melon.

Envisioning the immune interactome in Arabidopsis

During plant-pathogen interaction, immune targets were regulated by protein-protein interaction events such as ligand-receptor/co-receptor, kinase-substrate, protein sequestration, activation or repression via post-translational modification and homo/oligo/hetro-dimerisation of proteins. A judicious use of molecular machinery requires coordinated protein interaction among defence components. Immune signalling in Arabidopsis can be broadly represented in successive or simultaneous steps; pathogen recognition at cell surface, Ca2+ and reactive oxygen species signalling, MAPK signalling, post-translational modification, transcriptional regulation and phyto-hormone signalling. Proteome wide interaction studies have shown the existence of interaction hubs associated with physiological function. So far, a number of protein interaction events regulating immune targets have been identified, but their understanding in an interactome view is lacking. We focussed specifically on the integration of protein interaction signalling in context to plant-pathogenesis and identified the key targets. The present review focuses towards a comprehensive view of the plant immune interactome including signal perception, progression, integration and physiological response during plant pathogen interaction.

Plant immunity in signal integration between biotic and abiotic stress responses

Plants constantly monitor and cope with the fluctuating environment while hosting a diversity of plant-inhabiting microbes. The mode and outcome of plant-microbe interactions, including plant disease epidemics, are dynamically and profoundly influenced by abiotic factors, such as light, temperature, water and nutrients. Plants also utilize associations with beneficial microbes during adaptation to adverse conditions. Elucidation of the molecular bases for the plant-microbe-environment interactions is therefore of fundamental importance in the plant sciences. Following advances into individual stress signaling pathways, recent studies are beginning to reveal molecular intersections between biotic and abiotic stress responses and regulatory principles in combined stress responses. We outline mechanisms underlying environmental modulation of plant immunity and emerging roles for immune regulators in abiotic stress tolerance. Furthermore, we discuss how plants coordinate conflicting demands when exposed to combinations of different stresses, with attention to a possible determinant that links initial stress response to broad-spectrum stress tolerance or prioritization of specific stress tolerance.

Salicylic acid application alleviates the adverse effects of triclosan stress in tobacco plants through the improvement of plant photosynthesis and enhancing antioxidant system

Triclosan (TCS) is a chlorophenol which is highly bacteriostatic and used in a wide array of consumer products. TCS is now one of the most commonly detected organic pollutants in the sewage sludges. The sludge utilization for fertilizers on agricultural land would pose the risk of causing adverse effects on plant growth and yield by TCS. However, the toxicity of TCS toward plants is comparatively less understood. In this study, we assessed the effects of TCS on tobacco plants which were grown in MS medium or soils containing various concentrations of TCS. Our results indicated that TCS at the concentration of 2 mg/L could strongly inhibit the tobacco seed germination. TCS could suppress tobacco plant growth in soil with different concentrations (10, 20, and 50 mg/kg) of TCS through the downregulation of chlorophyll contents, restricting photosynthesis and increasing generation of reactive oxygen species (ROS). Salicylic acid (SA) plays important roles in the stress response of plants. The role of exogenous SA application in protecting tobacco plants from TCS stress was also investigated in this study. SA application could significantly increase net photosynthesis, enhance antioxidant enzyme activity, and thereby enhancing tobacco plant tolerance to TCS. Moreover, the activation of MPK3 and MPK6 induced by TCS was downregulated in plants with the treatment of SA. It was thus referred that mitogen-activated protein kinases (MAPKs) might play a key role in the signal transduction of TCS stress, and this process might be regulated by SA signaling. Overall, our results demonstrated that TCS had negative impacts on tobacco plants and SA played a protective role on tobacco plants against TCS stress.

Oregano essential oil vapour prevents Plasmopara viticola infection in grapevine (Vitis Vinifera) and primes plant immunity mechanisms

The reduction of synthetic fungicides in agriculture is necessary to guarantee a sustainable production that protects the environment and consumers' health. Downy mildew caused by the oomycete Plasmopara viticola is the major pathogen in viticulture worldwide and responsible for up to 60% of pesticide treatments. Alternatives to reduce fungicides are thus utterly needed to ensure sustainable vineyard-ecosystems, consumer health and public acceptance. Essential oils (EOs) are amongst the most promising natural plant protection alternatives and have shown their antibacterial, antiviral and antifungal properties on several agricultural crops. However, the efficiency of EOs highly depends on timing, application method and the molecular interactions between the host, the pathogen and EO. Despite proven EO efficiency, the underlying processes are still not understood and remain a black box. The objectives of the present study were: a) to evaluate whether a continuous fumigation of a particular EO can control downy mildew in order to circumvent the drawbacks of direct application, b) to decipher molecular mechanisms that could be triggered in the host and the pathogen by EO application and c) to try to differentiate whether essential oils directly repress the oomycete or act as plant resistance primers. To achieve this a custom-made climatic chamber was constructed that enabled a continuous fumigation of potted vines with different EOs during long-term experiments. The grapevine (Vitis vinifera) cv Chasselas was chosen in reason of its high susceptibility to Plasmopara viticola. Grapevine cuttings were infected with P. viticola and subsequently exposed to continuous fumigation of different EOs at different concentrations, during 2 application time spans (24 hours and 10 days). Experiments were stopped when infection symptoms were clearly observed on the leaves of the control plants. Plant physiology (photosynthesis and growth rate parameters) were recorded and leaves were sampled at different time points for subsequent RNA extraction and transcriptomics analysis. Strikingly, the Oregano vulgare EO vapour treatment during 24h post-infection proved to be sufficient to reduce downy mildew development by 95%. Total RNA was extracted from leaves of 24h and 10d treatments and used for whole transcriptome shotgun sequencing (RNA-seq). Sequenced reads were then mapped onto the V. vinifera and P. viticola genomes. Less than 1% of reads could be mapped onto the P. viticola genome from treated samples, whereas up to 30% reads from the controls mapped onto the P. viticola genome, thereby confirming the visual observation of P. viticola absence in the treated plants. On average, 80% of reads could be mapped onto the V. vinifera genome for differential expression analysis, which yielded 4800 modulated genes. Transcriptomic data clearly showed that the treatment triggered the plant's innate immune system with genes involved in salicylic, jasmonic acid and ethylene synthesis and signaling, activating Pathogenesis-Related-proteins as well as phytoalexin synthesis. These results elucidate EO-host-pathogen interactions for the first time and indicate that the antifungal efficiency of EO is mainly due to the triggering of resistance pathways inside the host plants. This is of major importance for the production and research on biopesticides, plant stimulation products and for resistance-breeding strategies.

The Systemin Signaling Cascade As Derived from Time Course Analyses of the Systemin-responsive Phosphoproteome

Systemin is a small peptide with important functions in plant wound response signaling. Although the transcriptional responses of systemin action are well described, the signaling cascades involved in systemin perception and signal transduction at the protein level are poorly understood. Here we used a tomato cell suspension culture system to profile phosphoproteomic responses induced by systemin and its inactive Thr17Ala analog, allowing us to reconstruct a systemin-specific kinase/phosphatase signaling network. Our time-course analysis revealed early phosphorylation events at the plasma membrane, such as dephosphorylation of H+-ATPase, rapid phosphorylation of NADPH-oxidase and Ca2+-ATPase. Later responses involved transient phosphorylation of small GTPases, vesicle trafficking proteins and transcription factors. Based on a correlation analysis of systemin-induced phosphorylation profiles, we predicted substrate candidates for 44 early systemin-responsive kinases, which includes receptor kinases and downstream kinases such as MAP kinases, as well as nine phosphatases. We propose a regulatory module in which H+-ATPase LHA1 is rapidly de-phosphorylated at its C-terminal regulatory residue T955 by phosphatase PLL5, resulting in the alkalization of the growth medium within 2 mins of systemin treatment. We found the MAP kinase MPK2 to have increased phosphorylation level at its activating TEY-motif at 15 min post-treatment. The predicted interaction of MPK2 with LHA1 was confirmed by in vitro kinase assays, suggesting that the H+-ATPase LHA1 is re-activated by MPK2 later in the systemin response. Our data set provides a resource of proteomic events involved in systemin signaling that will be valuable for further in-depth functional studies in elucidation of systemin signaling cascades.

The Age of Coumarins in Plant-Microbe Interactions

Coumarins are a family of plant-derived secondary metabolites that are produced via the phenylpropanoid pathway. In the past decade, coumarins have emerged as iron-mobilizing compounds that are secreted by plant roots and aid in iron uptake from iron-deprived soils. Members of the coumarin family are found in many plant species. Besides their role in iron uptake, coumarins have been extensively studied for their potential to fight infections in both plants and animals. Coumarin activities range from antimicrobial and antiviral to anticoagulant and anticancer. In recent years, studies in the model plant species tobacco and Arabidopsis have significantly increased our understanding of coumarin biosynthesis, accumulation, secretion, chemical modification and their modes of action against plant pathogens. Here, we review current knowledge on coumarins in different plant species. We focus on simple coumarins and provide an overview on their biosynthesis and role in environmental stress responses, with special attention for the recently discovered semiochemical role of coumarins in aboveground and belowground plant-microbe interactions and the assembly of the root microbiome.

A Chimeric IDD4 Repressor Constitutively Induces Immunity in Arabidopsis via the Modulation of Salicylic Acid and Jasmonic Acid Homeostasis

INDETERMINATE DOMAIN (IDD)/BIRD proteins belong to a highly conserved plant-specific group of transcription factors with dedicated functions in plant physiology and development. Here, we took advantage of the chimeric repressor gene-silencing technology (CRES-T, SRDX) to widen our view on the role of IDD4/IMPERIAL EAGLE and IDD family members in plant immunity. The hypomorphic idd4SRDX lines are compromised in growth and show a robust autoimmune phenotype. Hormonal measurements revealed the concomitant accumulation of salicylic acid and jasmonic acid suggesting that IDDs are involved in regulating the metabolism of these biotic stress hormones. The analysis of immunity-pathways showed enhanced activation of immune MAP kinase-signaling pathways, the accumulation of hydrogen peroxide and spontaneous programmed cell death. The transcriptome of nonelicited idd4SRDX lines can be aligned to approximately 40% of differentially expressed genes (DEGs) in flg22-treated wild-type plants. The pattern of DEGs implies IDDs as pivotal repressors of flg22-dependent gene induction. Infection experiments showed the increased resistance of idd4SRDX lines to Pseudomonas syringae and Botrytis cinerea implying a function of IDDs in defense adaptation to hemibiotrophs and necrotrophs. Genome-wide IDD4 DNA-binding studies (DAP-SEQ) combined with DEG analysis of idd4SRDX lines identified IDD4-regulated functional gene clusters that contribute to plant growth and development. In summary, we discovered that the expression of idd4SRDX activates a wide range of defense-related traits opening up the possibility to apply idd4SRDX as a powerful tool to stimulate innate immunity in engineered crops.

Salicylic Acid Alleviated Salt Damage of Populus euphratica: A Physiological and Transcriptomic Analysis

Populus euphratica Oliv. is a model tree for studying abiotic stress, especially salt stress response. Salt stress is one of the most extensive abiotic stresses, which has an adverse effect on plant growth and development. Salicylic acid (SA) is an important signaling molecule that plays an important role in modulating the plant responses to abiotic stresses. To answer whether the endogenous SA can be induced by salt stress, and whether SA effectively alleviates the negative effects of salt on poplar growth is the main purpose of the study. To elucidate the effects of SA and salt stress on the growth of P. euphratica, we examined the morphological and physiological changes of P. euphratica under 300 mM NaCl after treatment with different concentrations of SA. A pretreatment of P. euphratica with 0.4 mM SA for 3 days effectively improved the growth status of plants under subsequent salt stress. These results indicate that appropriate concentrations of exogenous SA can effectively counteract the negative effect of salt stress on growth and development. Subsequently, transcripts involved in salt stress response via SA signaling were captured by RNA sequencing. The results indicated that numerous specific genes encoding mitogen-activated protein kinase, calcium-dependent protein kinase, and antioxidant enzymes were upregulated. Potassium transporters and Na+/H+ antiporters, which maintain K+/Na+ balance, were also upregulated after SA pretreatment. The transcriptome changes show that the ion transport and antioxidant enzymes were the early enhanced systems in response of P. euphratica to salt via SA, expanding our knowledge about SA function in salt stress defense in P. euphratica. This provides a solid foundation for future study of functional genes controlling effective components in metabolic pathways of trees.

The mitogen activated protein kinase (MAPK) gene family functions as a cohort during the Glycine max defense response to Heterodera glycines

Mitogen activated protein kinases (MAPKs) play important signal transduction roles. However, little is known regarding how they influence the gene expression of other family members and the relationship to a biological process, including the Glycine max defense response to Heterodera glycines. Transcriptomics have identified MAPK gene expression occurring within root cells undergoing a defense response to a pathogenic event initiated by H. glycines in the allotetraploid Glycine max. Functional analyses are presented for its 32 MAPKs revealing 9 have a defense role, including homologs of Arabidopsis thaliana MAPK (MPK) MPK2, MPK3, MPK4, MPK5, MPK6, MPK13, MPK16 and MPK20. Defense signaling occurring through pathogen activated molecular pattern (PAMP) triggered immunity (PTI) and effector triggered immunity (ETI) have been determined in relation to these MAPKs. Five different types of gene expression relate to MAPK expression, influencing PTI and ETI gene expression and proven defense genes including an ABC-G transporter, 20S membrane fusion particle components, glycoside biosynthesis, carbon metabolism, hemicellulose modification, transcription and secretion. The experiments show MAPKs broadly influence defense MAPK gene expression, including the co-regulation of parologous MAPKs and reveal its relationship to proven defense genes. The experiments reveal each defense MAPK induces the expression of a G. max homolog of a PATHOGENESIS RELATED1 (PR1), itself shown to function in defense in the studied pathosystem.

INDETERMINATE-DOMAIN 4 (IDD4) coordinates immune responses with plant-growth in Arabidopsis thaliana

INDETERMINATE DOMAIN (IDD)/ BIRD proteins are a highly conserved plant-specific family of transcription factors which play multiple roles in plant development and physiology. Here, we show that mutation in IDD4/IMPERIAL EAGLE increases resistance to the hemi-biotrophic pathogen Pseudomonas syringae, indicating that IDD4 may act as a repressor of basal immune response and PAMP-triggered immunity. Furthermore, the idd4 mutant exhibits enhanced plant-growth indicating IDD4 as suppressor of growth and development. Transcriptome comparison of idd4 mutants and IDD4ox lines aligned to genome-wide IDD4 DNA-binding studies revealed major target genes related to defense and developmental-biological processes. IDD4 is a phospho-protein that interacts and becomes phosphorylated on two conserved sites by the MAP kinase MPK6. DNA-binding studies of IDD4 after flg22 treatment and with IDD4 phosphosite mutants show enhanced binding affinity to ID1 motif-containing promoters and its function as a transcriptional regulator. In contrast to the IDD4-phospho-dead mutant, the IDD4 phospho-mimicking mutant shows altered susceptibility to PstDC3000, salicylic acid levels and transcriptome reprogramming. In summary, we found that IDD4 regulates various hormonal pathways thereby coordinating growth and development with basal immunity.

The MAPK Kinase Kinase GmMEKK1 Regulates Cell Death and Defense Responses

MAPK signaling pathways play critical roles in plant immunity. Here, we silenced multiple genes encoding MAPKs using virus-induced gene silencing mediated by Bean pod mottle virus to identify MAPK genes involved in soybean (Glycine max) immunity. Surprisingly, a strong hypersensitive response (HR) cell death was observed when soybean MAPK KINASE KINASE1 (GmMEKK1), a homolog of Arabidopsis (Arabidopsis thaliana) MEKK1, was silenced. The HR was accompanied by the overaccumulation of defense signaling molecules, salicylic acid (SA) and hydrogen peroxide. Genes involved in primary metabolism, translation/transcription, photosynthesis, and growth/development were down-regulated in GmMEKK1-silenced plants, while the expression of defense-related genes was activated. Accordingly, GmMEKK1-silenced plants were more resistant to downy mildew (Peronospora manshurica) and Soybean mosaic virus compared with control plants. Silencing GmMEKK1 reduced the activation of GmMPK6 but enhanced the activation of GmMPK3 in response to flg22 peptide. Unlike Arabidopsis MPK4, GmMPK4 was not activated by either flg22 or SA. Interestingly, transient overexpression of GmMEKK1 in Nicotiana benthamiana also induced HR. Our results indicate that GmMEKK1 plays both positive and negative roles in immunity and appears to differentially activate downstream MPKs by promoting GmMPK6 activation but suppressing GmMPK3 activation in response to flg22. The involvement of GmMPK4 kinase activity in cell death and in flg22- or SA-triggered defense responses in soybean requires further investigation.

Cell and Developmental Biology of Plant Mitogen-Activated Protein Kinases

Plant mitogen-activated protein kinases (MAPKs) constitute a network of signaling cascades responsible for transducing extracellular stimuli and decoding them to dedicated cellular and developmental responses that shape the plant body. Over the last decade, we have accumulated information about how MAPK modules control the development of reproductive tissues and gametes and the embryogenic and postembryonic development of vegetative organs such as roots, root nodules, shoots, and leaves. Of key importance to understanding how MAPKs participate in developmental and environmental signaling is the characterization of their subcellular localization, their interactions with upstream signal perception mechanisms, and the means by which they target their substrates. In this review, we summarize the roles of MAPK signaling in the regulation of key plant developmental processes, and we survey what is known about the mechanisms guiding the subcellular compartmentalization of MAPK modules.

Interaction points in plant stress signaling pathways

Plants live in a world where they are challenged by abiotic and biotic stresses. In response to unfavorable conditions or an acute challenge like a pathogen attack, plants use various signaling pathways that regulate expression of defense genes and other mechanisms to provide resistance or stress adaptation. Identification of the regulatory steps in defense signaling has seen much progress in recent years. Many of the identified signaling pathways show interactions with each other, exemplified by the modulation of the jasmonic acid response by salicylic acid. Accordingly, defense regulation is more appropriately thought of as a web of interactions, rather than linear pathways. Here we describe various regulatory components and how they interact to provide an appropriate defense response. One of the common assays to monitor the output of defense signaling, as well as interaction between signaling pathways, is the measurement of altered gene expression. We illustrate that, while this is a suitable assay to monitor defense regulation, it can also inadvertently provide overstated conclusions about interaction among signaling pathways.

Cellular Complexity in MAPK Signaling in Plants: Questions and Emerging Tools to Answer Them

Mitogen activated protein kinase (MAPK) cascades play an important role in many aspects of plant growth, development, and environmental response. Because of their central role in many important processes, MAPKs have been extensively studied using biochemical and genetic approaches. This work has allowed for the identification of the MAPK genes and proteins involved in a number of different signaling pathways. Less well developed, however, is our understanding of how MAPK cascades and their corresponding signaling pathways are organized at subcellular levels. In this review, we will provide an overview of plant MAPK signaling, including a discussion of what is known about cellular mechanisms for achieving signaling specificity. Then we will explore what is currently known about the subcellular localization of MAPK proteins in resting conditions and after pathway activation. Finally, we will discuss a number of new experimental methods that have not been widely deployed in plants that have the potential to provide a deeper understanding of the spatial and temporal dynamics of MAPK signaling.

The MAPKKK gene family in cassava: Genome-wide identification and expression analysis against drought stress

Mitogen-activated protein kinase kinase kinases (MAPKKKs), an important unit of MAPK cascade, play crucial roles in plant development and response to various stresses. However, little is known concerning the MAPKKK family in the important subtropical and tropical crop cassava. In this study, 62 MAPKKK genes were identified in the cassava genome, and were classified into 3 subfamilies based on phylogenetic analysis. Most of MAPKKKs in the same subfamily shared similar gene structures and conserved motifs. The comprehensive transcriptome analysis showed that MAPKKK genes participated in tissue development and response to drought stress. Comparative expression profiles revealed that many MAPKKK genes were activated in cultivated varieties SC124 and Arg7 and the function of MeMAPKKKs in drought resistance may be different between SC124/Arg7 and W14. Expression analyses of the 7 selected MeMAPKKK genes showed that most of them were significantly upregulated by osmotic, salt and ABA treatments, whereas slightly induced by H₂O₂ and cold stresses. Taken together, this study identified candidate MeMAPKKK genes for genetic improvement of abiotic stress resistance and provided new insights into MAPKKK -mediated cassava resistance to drought stress.

Host-Mediated S-Nitrosylation Disarms the Bacterial Effector HopAI1 to Reestablish Immunity

Pathogens deliver effectors into plant cells to suppress immunity-related signaling. However, effector recognition by the host elicits a hypersensitive response (HR) that overcomes the inhibition of host signaling networks, restoring disease resistance. Signaling components are shared between the pathogen-associated molecular pattern-triggered immunity and effector-triggered immunity, and it is unclear how plants inactivate these effectors to execute the HR. Here, we report that, in Arabidopsis thaliana, during the onset of the HR, the bacterial effector HopAI1 is S-nitrosylated and that this modification inhibits its phosphothreonine lyase activity. HopAI1 targets and suppresses mitogen-activated protein kinases (MAPKs). The S-nitrosylation of HopAI1 restores MAPK signaling and is required during the HR for activation of the associated cell death. S-nitrosylation is therefore revealed here as a nitric oxide-dependent host strategy involved in plant immunity that works by directly disarming effector proteins.

Constitutive activity of the Arabidopsis MAP Kinase 3 confers resistance to Pseudomonas syringae and drives robust immune responses

Mitogen Activated Protein Kinases (MAPKs) are known to be important mediators of plant responses to biotic and abiotic stresses. In a recent report, we enlarged the understanding of the Arabidopsis thaliana MPK3 functions showing that the expression of a constitutively active (CA) form of the protein led to auto-immune phenotypes. CA-MPK3 plants are dwarf and display defense responses that are characterized by the accumulation of salicylic acid and phytoalexins as well as by the upregulation of several defense genes. Consistently with these data, we present here results demonstrating that, compared with wild type controls, CA-MPK3 plants are more resistant to the hemibiotrophic pathogen Pseudomonas syringae DC3000. Based on our previous work, we also discuss the mechanisms of robust plant immunity controlled by sustained MPK3 activity, focusing especially on the roles of disease resistance proteins.