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

PP2C Phosphatase Pic6 Suppresses MAPK Activation and Disease Resistance in Tomato

Type 2C protein phosphatases (PP2Cs) are essential for regulating plant immune responses to pathogens. Our study focuses on the tomato PP2C-immunity associated candidate 6 (Pic6), elucidating its role in negatively regulating pattern-triggered immunity (PTI) signaling pathways in tomato. Using reverse-transcription quantitative polymerase chain reaction (RT-qPCR), we observed that treatment with microbe-associated molecular patterns (MAMPs)-flg22 and flgII-28-significantly increased Pic6 mRNA levels in wild-type (RG-PtoR) tomato plants. Pic6 features a conserved N-terminal kinase-interacting motif (KIM) and a C-terminal PP2C domain. We produced variants of Pic6 with mutations in these regions, demonstrating their involvements in negatively regulating tomato immunity. Agrobacterium-mediated transient overexpression of Pic6 resulted in enhanced growth of the bacterial pathogen Pseudomonas syringae pathovar tomato (Pst) strain DC3000ΔhopQ1-1 compared with a yellow fluorescent protein (YFP) control. Additionally, Pic6 overexpression inhibited mitogen-activated protein kinase (MAPK) activation in response to flg22 and flgII-28 treatments. Importantly, Pic6 exhibited phosphatase activity and interacted with tomato Mkk1/Mkk2 proteins and dephosphorylated them in a KIM-dependent manner. Furthermore, we generated RG-pic6 loss-of-function mutants by CRISPR/Cas9, revealing that the absence of Pic6 heightened MAPK activity and increased resistance to Xanthomonas euvesicatoria strain 85-10 (Xe 85-10) when compared with the wild-type (RG-PtoR) plants. Transcript analyses showed that after flg22/flgII-28 treatment, PTI-reporter genes NAC and Osmotin were significantly upregulated in RG-pic6 mutants in comparison to the wild-type (RG-PtoR) plants. Overall, our findings indicate that Pic6 acts as a negative regulator of MAPK signaling and plays a pivotal role in modulating tomato immunity against bacterial pathogens. [Formula: see text] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Emerging Functions of Protein Tyrosine Phosphatases in Plants

Reversible protein phosphorylation, known as the "switch" of the cell, is controlled by protein kinases (PKs) and protein phosphatases (PPs). Based on substrate specificity, PPs are classified into protein serine/threonine phosphatases and protein tyrosine phosphatases (PTPs). PTPs can dephosphorylate phosphotyrosine and phosphoserine/phosphothreonine. In plants, PTPs monitor plant physiology, growth, and development. This review summarizes an overview of the PTPs' classification and describes how PTPs regulate various plant processes, including plant growth and development, plant hormone responses, and responses to abiotic and biotic stresses. Then, future research directions on the PTP family in plants are discussed. This summary will serve as a reference for researchers studying PTPs in plants.

PP2C phosphatase Pic14 negatively regulates tomato Pto/Prf-triggered immunity by inhibiting MAPK activation

Type 2C protein phosphatases (PP2Cs) are emerging as important regulators of plant immune responses, although little is known about how they might impact nucleotide-binding, leucine-rich repeat (NLR)-triggered immunity (NTI). We discovered that expression of the PP2C immunity-associated candidate 14 gene (Pic14) is induced upon activation of the Pto/Prf-mediated NTI response in tomato. Pto/Prf recognizes the effector AvrPto translocated into plant cells by the pathogen Pseudomonas syringae pv. tomato (Pst) and activate a MAPK cascade and other responses which together confer resistance to bacterial speck disease. Pic14 encodes a PP2C with an N-terminal kinase-interacting motif (KIM) and a C-terminal phosphatase domain. Upon inoculation with Pst-AvrPto, Pto/Prf-expressing tomato plants with loss-of-function mutations in Pic14 developed less speck disease, specifically in older leaves, compared to wild-type plants. Transient expression of Pic14 in leaves of Nicotiana benthamiana and tomato inhibited cell death typically induced by Pto/Prf and the MAPK cascade members M3Kα and Mkk2. The cell death-suppressing activity of Pic14 was dependent on the KIM and the catalytic phosphatase domain. Pic14 inhibited M3Kα- and Mkk2-mediated activation of immunity-associated MAPKs and Pic14 was shown to be an active phosphatase that physically interacts with and dephosphorylates Mkk2 in a KIM-dependent manner. Together, our results reveal Pic14 as an important negative regulator of Pto/Prf-triggered immunity by interacting with and dephosphorylating Mkk2.

CaMAPK1 Plays a Vital Role in the Regulation of Resistance to Ralstonia solanacearum Infection and Tolerance to Heat Stress

As an important member of mitogen-activated protein kinase (MAPK) cascades, MAPKs play an important role in plant defense response against biotic and abiotic stresses; however, the involvement of the majority of the MAPK family members against Ralstonia solanacearum and heat stress (HS) remains poorly understood. In the present study, CaMAPK1 was identified from the genome of pepper and its function against R. solanacearum and HS was analyzed. The transcript accumulations of CaMAPK1 and the activities of its native promoter were both significantly induced by R. solanacearum inoculation, HS, and the application of exogenous hormones, including SA, MeJA, and ABA. Transient expression of CaMAPK1 showed that CaMAPK1 can be targeted throughout the whole cells in Nicotiana benthamiana and triggered chlorosis and hypersensitive response-like cell death in pepper leaves, accompanied by the accumulation of H2O2, and the up-regulations of hormones- and H2O2-associated marker genes. The knock-down of CaMAPK1 enhanced the susceptibility to R. solanacearum partially by down-regulating the expression of hormones- and H2O2-related genes and impairing the thermotolerance of pepper probably by attenuating CaHSFA2 and CaHSP70-1 transcripts. Taken together, our results revealed that CaMAPK1 is regulated by SA, JA, and ABA signaling and coordinates responses to R. solanacearum infection and HS in pepper.

Functional screening of the Arabidopsis 2C protein phosphatases family identifies PP2C15 as a negative regulator of plant immunity by targeting BRI1-associated receptor kinase 1

Genetic engineering using negative regulators of plant immunity has the potential to provide a huge impetus in agricultural biotechnology to achieve a higher degree of disease resistance without reducing yield. Type 2C protein phosphatases (PP2Cs) represent the largest group of protein phosphatases in plants, with a high potential for negative regulatory functions by blocking the transmission of defence signals through dephosphorylation. Here, we established a PP2C functional protoplast screen using pFRK1::luciferase as a reporter and found that 14 of 56 PP2Cs significantly inhibited the immune response induced by flg22. To verify the reliability of the system, a previously reported MAPK3/4/6-interacting protein phosphatase, PP2C5, was used; it was confirmed to be a negative regulator of PAMP-triggered immunity (PTI). We further identified PP2C15 as an interacting partner of BRI1-associated receptor kinase 1 (BAK1), which is the most well-known co-receptor of plasma membrane-localized pattern recognition receptors (PRRs), and a central component of PTI. PP2C15 dephosphorylates BAK1 and negatively regulates BAK1-mediated PTI responses such as MAPK3/4/6 activation, defence gene expression, reactive oxygen species bursts, stomatal immunity, callose deposition, and pathogen resistance. Although plant growth and 1000-seed weight of pp2c15 mutants were reduced compared to those of wild-type plants, pp2c5 mutants did not show any adverse effects. Thus, our findings strengthen the understanding of the mechanism by which PP2C family members negatively regulate plant immunity at multiple levels and indicate a possible approach to enhance plant resistance by eliminating specific PP2Cs without affecting plant growth and yield.

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

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

Insights into the early transcriptomic response against watermelon mosaic virus in melon

BACKGROUND

Watermelon mosaic virus (WMV) is one of the most prevalent viruses affecting melon worldwide. Recessive resistance to WMV in melon has previously been reported in the African accession TGR-1551. Moreover, the genomic regions associated to the resistance have also been described. Nevertheless, the transcriptomic response that might infer the resistance to this potyvirus has not been explored.

RESULTS

We have performed a comparative transcriptomic analysis using mock and WMV-inoculated plants of the susceptible cultivar "Bola de oro" (BO) and a resistant RIL (Recombinant inbred line) derived from the initial cross between "TGR-1551" and BO. In total, 616 genes were identified as differentially expressed and the weighted gene co-expression network analysis (WGCNA) detected 19 gene clusters (GCs), of which 7 were differentially expressed for the genotype x treatment interaction term. SNPs with a predicted high impact on the protein function were detected within the coding regions of most of the detected DEGs. Moreover, 3 and 16 DEGs were detected within the QTL regions previously described in chromosomes 11 and 5, respectively. In addition to these two specific genomic regions, we also observde large transcriptomic changes from genes spread across the genome in the resistant plants in response to the virus infection. This early response against WMV implied genes involved in plant-pathogen interaction, plant hormone signal transduction, the MAPK signaling pathway or ubiquitin mediated proteolysis, in detriment to the photosynthetic and basal metabolites pathways. Moreover, the gene MELO3C021395, which coded a mediator of RNA polymerase II transcription subunit 33A (MED33A), has been proposed as the candidate gene located on chromosome 11 conferring resistance to WMV.

CONCLUSIONS

The comparative transcriptomic analysis presented here showed that, even though the resistance to WMV in TGR-1551 has a recessive nature, it triggers an active defense response at a transcriptomic level, which involves broad-spectrum resistance mechanisms. Thus, this study represents a step forward on our understanding of the mechanisms underlaying WMV resistance in melon. In addition, it sheds light into a broader topic on the mechanisms of recessive resistances.

VvWRKY5 enhances white rot resistance in grape by promoting the jasmonic acid pathway

Grape white rot is a disease caused by Coniella diplodiella (Speg.) Sacc. (Cd) can drastically reduce the production and quality of grape (Vitis vinifera). WRKY transcription factors play a vital role in the regulation of plant resistance to pathogens, but their functions in grape white rot need to be further explored. Here, we found that the expression of the WRKY IIe subfamily member VvWRKY5 was highly induced by Cd infection and jasmonic acid (JA) treatment. Transient injection and stable overexpression (in grape calli and Arabidopsis) demonstrated that VvWRKY5 positively regulated grape resistance to white rot. We also determined that VvWRKY5 regulated the JA response by directly binding to the promoters of VvJAZ2 (a JA signaling suppressor) and VvMYC2 (a JA signaling activator), thereby inhibiting and activating the transcription of VvJAZ2 and VvMYC2, respectively. Furthermore, the interaction between VvJAZ2 and VvWRKY5 enhanced the suppression and promotion of VvJAZ2 and VvMYC2 activities by VvWRKY5, respectively. When VvWRKY5 was overexpressed in grape, JA content was also increased. Overall, our results suggested that VvWRKY5 played a key role in regulating JA biosynthesis and signal transduction as well as enhancing white rot resistance in grape. Our results also provide theoretical guidance for the development of elite grape cultivars with enhanced pathogen resistance.

Signaling in plant development and immunity through the lens of the stomata

The proper development and function of stomata - turgor-driven valves for efficient gas-exchange and water control - impact plant survival and productivity. It has become apparent that various receptor kinases regulate stomatal development and immunity. Although stomatal development and immunity occur over different cellular time scales, their signaling components and regulatory modules are strikingly similar, and often shared. In this review, we survey the current knowledge of stomatal development and immunity signaling components, and provide a synthesis and perspectives on the key concepts to further understand the conservation and specificity of these two signaling pathways.

Genome-wide characterization of the PP2C gene family in peanut (Arachis hypogaea L.) and the identification of candidate genes involved in salinity-stress response

Plant protein phosphatase 2C (PP2C) play important roles in response to salt stress by influencing metabolic processes, hormone levels, growth factors, etc. Members of the PP2C family have been identified in many plant species. However, they are rarely reported in peanut. In this study, 178 PP2C genes were identified in peanut, which were unevenly distributed across the 20 chromosomes, with segmental duplication in 78 gene pairs. AhPP2Cs could be divided into 10 clades (A-J) by phylogenetic analysis. AhPP2Cs had experienced segmental duplications and strong purifying selection pressure. 22 miRNAs from 14 different families were identified, targeting 57 AhPP2C genes. Gene structures and motifs analysis exhibited PP2Cs in subclades AI and AII had high structural and functional similarities. Phosphorylation sites of AhPP2C45/59/134/150/35/121 were predicted in motifs 2 and 4, which located within the catalytic site at the C-terminus. We discovered multiple MYB binding factors and ABA response elements in the promoter regions of the six genes (AhPP2C45/59/134/150/35/121) by cis-elements analysis. GO and KEGG enrichment analysis confirmed AhPP2C-A genes in protein binding, signal transduction, protein modification process response to abiotic stimulus through environmental information processing. Based on RNA-Seq data of 22 peanut tissues, clade A AhPP2Cs showed a varying degree of tissue specificity, of which, AhPP2C35 and AhPP2C121 specifically expressed in seeds, while AhPP2C45/59/134/150 expressed in leaves and roots. qRT-PCR indicated that AhPP2C45 and AhPP2C134 displayed significantly up-regulated expression in response to salt stress. These results indicated that AhPP2C45 and AhPP2C134 could be candidate PP2Cs conferring salt tolerance. These results provide further insights into the peanut PP2C gene family and indicate PP2Cs potentially involved in the response to salt stress, which can now be further investigated in peanut breeding efforts to obtain cultivars with improved salt tolerance.

Protein phosphatases and their targets: Comprehending the interactions in plant signaling pathways

Protein phosphorylation is a vital reversible post-translational modification. This process is established by two classes of enzymes: protein kinases and protein phosphatases. Protein kinases phosphorylate proteins while protein phosphatases dephosphorylate phosphorylated proteins, thus, functioning as 'critical regulators' in signaling pathways. The eukaryotic protein phosphatases are classified as phosphoprotein phosphatases (PPP), metallo-dependent protein phosphatases (PPM), protein tyrosine (Tyr) phosphatases (PTP), and aspartate (Asp)-dependent phosphatases. The PPP and PPM families are serine (Ser)/threonine (Thr) specific phosphatases (STPs) that dephosphorylate Ser and Thr residues. The PTP family dephosphorylates Tyr residues while dual-specificity phosphatases (DsPTPs/DSPs) dephosphorylate Ser, Thr, and Tyr residues. The composition of these enzymes as well as their substrate specificity are important determinants of their functional significance in a number of cellular processes and stress responses. Their role in animal systems is well-understood and characterized. The functional characterization of protein phosphatases has been extensively covered in plants, although the comprehension of their mechanistic basis is an ongoing pursuit. The nature of their interactions with other key players in the signaling process is vital to our understanding. The substrates or targets determine their potential as well as magnitude of the impact they have on signaling pathways. In this article, we exclusively overview the various substrates of protein phosphatases in plant signaling pathways, which are a critical determinant of the outcome of various developmental and stress stimuli.

Opposite roles of MAPKKK17 and MAPKKK21 against Tetranychus urticae in Arabidopsis

After recognizing a biotic stress, plants activate signalling pathways to fight against the attack. Typically, these signalling pathways involve the activation of phosphorylation cascades mediated by Mitogen-Activated Protein Kinases (MAPKs). In the Arabidopsis thaliana-Tetranychus urticae plant-herbivore model, several Arabidopsis MAP kinases are induced by the mite attack. In this study, we demonstrate the participation of the MEKK-like kinases MAPKKK17 and MAPKKK21. Leaf damage caused by the mite was assessed using T-DNA insertion lines. Differential levels of damage were found when the expression of MAPKKK17 was increased or reduced. In contrast, reduced expression of MAPKKK21 resulted in less damage caused by the mite. Whereas the expression of several genes associated with hormonal responses did not suffer significant variations in the T-DNA insertion lines, the expression of one of these kinases depends on the expression of the other one. In addition, MAPKKK17 and MAPKKK21 are coexpressed with different sets of genes and encode proteins with low similarity in the C-terminal region. Overall, our results demonstrate that MAPKKK17 and MAPKKK21 have opposite roles. MAPKKK17 and MAPKKK21 act as positive and negative regulators, respectively, on the plant response. The induction of MAPKKK17 and MAPKKK21 after mite infestation would be integrated into the bulk of signalling pathways activated to balance the response of the plant to a biotic stress.

MITOGEN-ACTIVATED PROTEIN KINASE PHOSPHATASE 1 mediates root sensing of serotonin through jasmonic acid signaling and modulating reactive oxygen species

Serotonin (5-hydroxytryptamine) acts as a neurotransmitter in mammals and is widely distributed in the plant kingdom, where it influences root growth and defense. Mitogen-Activated Protein Kinases (MAPKs) and MAPK phosphatases (MKPs) play critical functions in decoding hormonal signalling, but their possible roles in mediating serotonin responses await investigation. In this report, we unveiled positive roles for the MITOGEN-ACTIVATED PROTEIN KINASE PHOSPHATASE1 (MKP1) in the inhibition of the primary root growth, cell division, meristem structure, and differentiation events in Arabidopsis seedlings. mkp1 mutants were less sensitive to jasmonic acid applications that halted primary root growth in wild-type (WT) plants, and consistently, the neurotransmitter activated the expression of the JASMONATE ZIM-domain (JAZ) proteins JAZ1 and JAZ10, two critical proteins orchestrating jasmonic acid signalling. This effect correlated with exacerbated production of endogenous reactive oxygen species (ROS) in the WT, a process constitutively manifested in mkp1 mutants. These data help to clarify the relationship between serotonin and growth/defense trade-offs, and reveal the importance of the MAPK pathway in root development through ROS production.

Genome-Wide Identification and Expression Analysis of MAPK Gene Family in Lettuce (Lactuca sativa L.) and Functional Analysis of LsMAPK4 in High- Temperature-Induced Bolting

The mitogen-activated protein kinase (MAPK) pathway is a widely distributed signaling cascade in eukaryotes and is involved in regulating plant growth, development, and stress responses. High temperature, a frequently occurring environmental stressor, causes premature bolting in lettuce with quality decline and yield loss. However, whether MAPKs play roles in thermally induced bolting remains poorly understood. In this study, 17 LsMAPK family members were identified from the lettuce genome. The physical and chemical properties, subcellular localization, chromosome localization, phylogeny, gene structure, family evolution, cis-acting elements, and phosphorylation sites of the LsMAPK gene family were evaluated via in silico analysis. According to phylogenetic relationships, LsMAPKs can be divided into four groups, A, B, C, and D, which is supported by analyses of gene structure and conserved domains. The collinearity analysis showed that there were 5 collinearity pairs among LsMAPKs, 8 with AtMAPKs, and 13 with SlMAPKs. The predicted cis-acting elements and potential phosphorylation sites were closely associated with hormones, stress resistance, growth, and development. Expression analysis showed that most LsMAPKs respond to high temperatures, among which LsMAPK4 is significantly and continuously upregulated upon heat treatments. Under heat stress, the stem length of the LsMAPK4-knockdown lines was significantly shorter than that of the control plants, and the microscope observations demonstrated that the differentiation time of flower buds at the stem apex was delayed accordingly. Therefore, silencing of LsMAPK4 significantly inhibited the high- temperature-accelerated bolting in lettuce, indicating that LsMPAK4 might be a potential regulator of lettuce bolting. This study provides a theoretical basis for a better understanding of the molecular mechanisms underlying the MAPK genes in high-temperature-induced bolting.