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.

Targeting alternative splicing by RNAi: from the differential impact on splice variants to triggering artificial pre-mRNA splicing.

Alternative splicing generates multiple transcript and protein isoforms from a single gene and controls transcript intracellular localization and stability by coupling to mRNA export and nonsense-mediated mRNA decay (NMD). RNA interference (RNAi) is a potent mechanism to modulate gene expression. However, its interactions with alternative splicing are poorly understood. We used artificial microRNAs (amiRNAs, also termed shRNAmiR) to knockdown all splice variants of selected target genes in Arabidopsis thaliana. We found that splice variants, which vary by their protein-coding capacity, subcellular localization and sensitivity to NMD, are affected differentially by an amiRNA, although all of them contain the target site. Particular transcript isoforms escape amiRNA-mediated degradation due to their nuclear localization. The nuclear and NMD-sensitive isoforms mask RNAi action in alternatively spliced genes. Interestingly, Arabidopsis SPL genes, which undergo alternative splicing and are targets of miR156, are regulated in the same manner. Moreover, similar results were obtained in mammalian cells using siRNAs, indicating cross-kingdom conservation of these interactions among RNAi and splicing isoforms. Furthermore, we report that amiRNA can trigger artificial alternative splicing, thus expanding the RNAi functional repertoire. Our findings unveil novel interactions between different post-transcriptional processes in defining transcript fates and regulating gene expression.

Protein phosphatase AP2C1 negatively regulates basal resistance and defense responses to Pseudomonas syringae.

Mitogen-activated protein kinases (MAPKs) mediate plant immune responses to pathogenic bacteria. However, less is known about the cell autonomous negative regulatory mechanism controlling basal plant immunity. We report the biological role of Arabidopsis thaliana MAPK phosphatase AP2C1 as a negative regulator of plant basal resistance and defense responses to Pseudomonas syringae. AP2C2, a closely related MAPK phosphatase, also negatively controls plant resistance. Loss of AP2C1 leads to enhanced pathogen-induced MAPK activities, increased callose deposition in response to pathogen-associated molecular patterns or to P. syringae pv. tomato (Pto) DC3000, and enhanced resistance to bacterial infection with Pto. We also reveal the impact of AP2C1 on the global transcriptional reprogramming of transcription factors during Pto infection. Importantly, ap2c1 plants show salicylic acid-independent transcriptional reprogramming of several defense genes and enhanced ethylene production in response to Pto. This study pinpoints the specificity of MAPK regulation by the different MAPK phosphatases AP2C1 and MKP1, which control the same MAPK substrates, nevertheless leading to different downstream events. We suggest that precise and specific control of defined MAPKs by MAPK phosphatases during plant challenge with pathogenic bacteria can strongly influence plant resistance.

Plant resistance against the parasitic nematode Heterodera schachtii is mediated by MPK3 and MPK6 kinases, which are controlled by the MAPK phosphatase AP2C1 in Arabidopsis.

Plant-parasitic cyst nematodes infect plants and form highly sophisticated feeding sites in roots. It is not known which plant cell signalling mechanisms trigger plant defence during the early stages of nematode parasitism. Mitogen-activated protein kinases (MAPKs) are central components of protein phosphorylation cascades transducing extracellular signals to plant defence responses. MAPK phosphatases control kinase activities and the signalling outcome. The involvement and the role of MPK3 and MPK6, as well as the MAPK phosphatase AP2C1, is demonstrated during parasitism of the beet cyst nematode Heterodera schachtii in Arabidopsis. Our data reveal notable activation patterns of plant MAPKs and the induction of AP2C1 suggesting the attenuation of defence signalling in plant cells during early nematode infection. It is demonstrated that the ap2c1 mutant that is lacking AP2C1 is more attractive but less susceptible to nematodes compared with the AP2C1-overexpressing line. This implies that the function of AP2C1 is a negative regulator of nematode-induced defence. By contrast, the enhanced susceptibility of mpk3 and mpk6 plants indicates a positive role of stress-activated MAPKs in plant immunity against nematodes. Evidence is provided that phosphatase AP2C1, as well as AP2C1-targeted MPK3 and MPK6, are important regulators of plant-nematode interaction, where the co-ordinated action of these signalling components ensures the timely activation of plant defence.

Phosphatases in plants.

Reversible protein phosphorylation is an essential posttranslational modification mechanism executed by opposing actions of protein phosphatases and protein kinases. About 1,000 predicted kinases in Arabidopsis thaliana kinome predominate the number of protein phosphatases, of which there are only ~150 members in Arabidopsis. Protein phosphatases were often referred to as "housekeeping" enzymes, which act to keep eukaryotic systems in balance by counteracting the activity of protein kinases. However, recent investigations reveal the crucial and specific regulatory functions of phosphatases in cell signaling. Phosphatases operate in a coordinated manner with the protein kinases, to execute their important function in determining the cellular response to a physiological stimulus. Closer examination has established high specificity of phosphatases in substrate recognition and important roles in plant signaling pathways, such as pathogen defense and stress regulation, light and hormonal signaling, cell cycle and differentiation, metabolism, and plant growth. In this minireview we provide a compact overview about Arabidopsis protein phosphatase families, as well as members of phosphoglucan and lipid phosphatases, and highlight the recent discoveries in phosphatase research.

Bimolecular fluorescent complementation (BiFC) by MAP kinases and MAPK phosphatases.

The adaptation of plants to the environment is a key property for survival. Adaptation responses to environmental cues are generated in cells by signaling initiated from cell receptors. Signal transduction is based on protein phosphorylation that is employed in mitogen-activated protein kinase (MAPK) cascades to integrate signals from receptors to cellular responses. MAPK activity is determined by phosphorylation of amino acid residues within the kinase activation loop and their dephosphorylation by phosphatases is essential to control signal duration and intensity.Monitoring protein-protein interactions (PPIs) of MAPKs with MAPK phosphatases in vivo provides valuable information about specificity and intracellular localization of the protein complex. Here, we report studying PPIs between Arabidopsis MAPKs and PP2C-type MAPK phosphatases using bimolecular fluorescent complementation (BiFC) in suspension cell protoplasts. The interactions of the MAPKs MPK3, MKP4 and MPK6 with the phosphatases AP2C1 and AP2C3 have been tested.

Salt-induced subcellular kinase relocation and seedling susceptibility caused by overexpression of Medicago SIMKK in Arabidopsis.

Dual-specificity mitogen-activated protein kinases kinases (MAPKKs) are the immediate upstream activators of MAPKs. They simultaneously phosphorylate the TXY motif within the activation loop of MAPKs, allowing them to interact with and regulate multiple substrates. Often, the activation of MAPKs triggers their nuclear translocation. However, the spatiotemporal dynamics and the physiological consequences of the activation of MAPKs, particularly in plants, are still poorly understood. Here, we studied the activation and localization of the Medicago sativa stress-induced MAPKK (SIMKK)-SIMK module after salt stress. In the inactive state, SIMKK and SIMK co-localized in the cytoplasm and in the nucleus. Upon salt stress, however, a substantial part of the nuclear pool of both SIMKK and SIMK relocated to cytoplasmic compartments. The course of nucleocytoplasmic shuttling of SIMK correlated temporally with the dual phosphorylation of the pTEpY motif. SIMKK function was further studied in Arabidopsis plants overexpressing SIMKK-yellow fluorescent protein (YFP) fusions. SIMKK-YFP plants showed enhanced activation of Arabidopsis MPK3 and MPK6 kinases upon salt treatment and exhibited high sensitivity against salt stress at the seedling stage, although they were salt insensitive during seed germination. Proteomic analysis of SIMKK-YFP overexpressors indicated the differential regulation of proteins directly or indirectly involved in salt stress responses. These proteins included catalase, peroxiredoxin, glutathione S-transferase, nucleoside diphosphate kinase 1, endoplasmic reticulum luminal-binding protein 2, and finally plasma membrane aquaporins. In conclusion, Arabidopsis seedlings overexpressing SIMKK-YFP exhibited higher salt sensitivity consistent with their proteome composition and with the presumptive MPK3/MPK6 hijacking of the salt response pathway.

The TPLATE adaptor complex drives clathrin-mediated endocytosis in plants.

Clathrin-mediated endocytosis is the major mechanism for eukaryotic plasma membrane-based proteome turn-over. In plants, clathrin-mediated endocytosis is essential for physiology and development, but the identification and organization of the machinery operating this process remains largely obscure. Here, we identified an eight-core-component protein complex, the TPLATE complex, essential for plant growth via its role as major adaptor module for clathrin-mediated endocytosis. This complex consists of evolutionarily unique proteins that associate closely with core endocytic elements. The TPLATE complex is recruited as dynamic foci at the plasma membrane preceding recruitment of adaptor protein complex 2, clathrin, and dynamin-related proteins. Reduced function of different complex components severely impaired internalization of assorted endocytic cargoes, demonstrating its pivotal role in clathrin-mediated endocytosis. Taken together, the TPLATE complex is an early endocytic module representing a unique evolutionary plant adaptation of the canonical eukaryotic pathway for clathrin-mediated endocytosis.

Type 2C protein phosphatases in plants.

Type 2C protein phosphatases (PP2Cs) form a structurally unique class of Mg(2+)-/Mn(2+)-dependent enzymes. PP2Cs are evolutionary conserved from prokaryotes to higher eukaryotes and play a prominent role in stress signalling. In this review, we focus on the evolution, function and regulation of the plant PP2Cs. Members of a subclass of plant PP2Cs counteract mitogen-activated protein kinase pathways, whereas members of other subfamilies function as co-receptors for the phytohormone abscisic acid. Recent structural analyses of abscisic acid receptors have elucidated the mode of ligand-dependent regulation and substrate targeting.

Substrate analysis of Arabidopsis PP2C-type protein phosphatases.

Protein phosphorylation by protein kinases can be reversed by the action of protein phosphatases. In plants, the Ser/Thr-specific phosphatases dominate among the protein phosphatase families with the type 2C protein phosphatases (PP2Cs) being the most abundant among them. PP2Cs are monomeric enzymes that require metal cations for their activity and are insensitive to known phosphatase inhibitors. PP2Cs were shown to counteract the mitogen-activated protein kinase (MAP kinase/MAPK) activities in plants and to regulate developmental and stress signaling pathways. Studies of PP2C activities can be performed in vitro using recombinant proteins. The potential substrates of PP2Cs can be tested for dephosphorylation by the phosphatase in vitro. We have found that the stress-induced PP2Cs from alfalfa and Arabidopsis interact with stress-activated MAPKs in yeast two-hybrid (Y2H) screens. Consequently, recombinant MAPKs were employed as substrates for dephosphorylation by selected PP2Cs from different family clusters. The members of the PP2C phosphatase family demonstrated specificity toward the substrate already in vitro, supporting the notion that protein phosphatases are specific enzymes. The PP2C from Arabidopsis thaliana cluster B, Arabidopsis PP2C-type phosphatase (AP2C1), and its homolog from Medicago sativa, Medicago PP2C-type phosphatase (MP2C), were able to dephosphorylate and inactivate MAPKs, whereas the ABSCISIC ACID (ABA)-INSENSITIVE 2 (ABI2) and HOMOLOGY TO ABI1 (HAB1) PP2Cs from the distinct Arabidopsis cluster A were not able to do so. The method described here can be used for the determination of PP2C protein activity and for studying the effect of mutations introduced into their catalytic domains.

MAPK phosphatase AP2C3 induces ectopic proliferation of epidermal cells leading to stomata development in Arabidopsis.

In plant post-embryonic epidermis mitogen-activated protein kinase (MAPK) signaling promotes differentiation of pavement cells and inhibits initiation of stomata. Stomata are cells specialized to modulate gas exchange and water loss. Arabidopsis MAPKs MPK3 and MPK6 are at the core of the signaling cascade; however, it is not well understood how the activity of these pleiotropic MAPKs is constrained spatially so that pavement cell differentiation is promoted only outside the stomata lineage. Here we identified a PP2C-type phosphatase termed AP2C3 (Arabidopsis protein phosphatase 2C) that is expressed distinctively during stomata development as well as interacts and inactivates MPK3, MPK4 and MPK6. AP2C3 co-localizes with MAPKs within the nucleus and this localization depends on its N-terminal extension. We show that other closely related phosphatases AP2C2 and AP2C4 are also MAPK phosphatases acting on MPK6, but have a distinct expression pattern from AP2C3. In accordance with this, only AP2C3 ectopic expression is able to stimulate cell proliferation leading to excess stomata development. This function of AP2C3 relies on the domains required for MAPK docking and intracellular localization. Concomitantly, the constitutive and inducible AP2C3 expression deregulates E2F-RB pathway, promotes the abundance and activity of CDKA, as well as changes of CDKB1;1 forms. We suggest that AP2C3 downregulates the MAPK signaling activity to help maintain the balance between differentiation of stomata and pavement cells.

Phosphatase activities analyzed by in vivo expressions.

Protein phosphatases act to reverse phosphorylation-related modifications induced by protein kinases. Type 2C protein phosphatases (PP2C) are monomeric Ser/Thr phosphatases that require a metal for their activity and are abundant in prokaryotes and eukaryotes. In plants, such as Medicago and Arabidopsis PP2Cs control several essential processes, including ABA signaling, development, and wound-induced mitogen-activated protein kinase (MAPK) pathways. In vitro assays with recombinant proteins and yeast two-hybrid systems usually provide initial information about putative PP2C substrates; however, these observations have to be verified in vivo. Therefore, a method for transient expression in isolated Arabidopsis suspension cell protoplasts was developed to assay PP2C action in living cells. This system has proven to be very useful in producing active enzymes and their substrates and in performing enzymatic reactions in vivo. Transient gene expression in isolated cells enabled assembly of functional protein kinase cascades and the creation of phosphorylated targets for PP2Cs. The method is based on the co-transformation and transient co-expression of different PP2C proteins with MAPK. It shows that epitope-tagged PP2C and MAPK proteins exhibit high enzymatic activities and produce substantial protein amounts easily monitored by Western blot analysis. Additionally, PP2C phosphatase activities can be directly tested in protein extracts from protoplasts, suggesting a possibility for analysis of activities of new PP2C family members.

Regulation of stress hormones jasmonates and ethylene by MAPK pathways in plants.

Plant stress hormones, such as jasmonates (JAs) and ethylene (ET) are essential in plant defence against stress conditions. JAs are used in cosmetics and food flavouring, and the recently demonstrated anti-cancer activity of JAs highlights their potential in health protection. It reinforces the need for a better understanding of biosynthetic regulation of JAs. Which mechanisms are involved in the regulation of the biosynthesis of JAs and ET? Production of stress hormones is induced in plants after wounding or herbivore attack. ET is a gaseous compound and plant JAs are oxylipins structurally similar to prostaglandins that are induced upon inflammation or injury in mammals. Wounding activates protein phosphorylation cascades involving mitogen-activated protein kinases (MAPKs). MAPKs regulate ET production. The induction of JA biosynthesis was suggested to require MAPK activation; however the defined roles of MAPKs in JA production remain unclear. Here we will highlight the most recent findings suggesting the regulation of JA biosynthesis and ethylene production by stress activated MAPKs and phosphatases that inactivate these MAPKs.

Antagonistic regulation of PIN phosphorylation by PP2A and PINOID directs auxin flux.

In plants, cell polarity and tissue patterning are connected by intercellular flow of the phytohormone auxin, whose directional signaling depends on polar subcellular localization of PIN auxin transport proteins. The mechanism of polar targeting of PINs or other cargos in plants is largely unidentified, with the PINOID kinase being the only known molecular component. Here, we identify PP2A phosphatase as an important regulator of PIN apical-basal targeting and auxin distribution. Genetic analysis, localization, and phosphorylation studies demonstrate that PP2A and PINOID both partially colocalize with PINs and act antagonistically on the phosphorylation state of their central hydrophilic loop, hence mediating PIN apical-basal polar targeting. Thus, in plants, polar sorting by the reversible phosphorylation of cargos allows for their conditional delivery to specific intracellular destinations. In the case of PIN proteins, this mechanism enables switches in the direction of intercellular auxin fluxes, which mediate differential growth, tissue patterning, and organogenesis.

The PP2C-type phosphatase AP2C1, which negatively regulates MPK4 and MPK6, modulates innate immunity, jasmonic acid, and ethylene levels in Arabidopsis.

Wound signaling pathways in plants are mediated by mitogen-activated protein kinases (MAPKs) and stress hormones, such as ethylene and jasmonates. In Arabidopsis thaliana, the transmission of wound signals by MAPKs has been the subject of detailed investigations; however, the involvement of specific phosphatases in wound signaling is not known. Here, we show that AP2C1, an Arabidopsis Ser/Thr phosphatase of type 2C, is a novel stress signal regulator that inactivates the stress-responsive MAPKs MPK4 and MPK6. Mutant ap2c1 plants produce significantly higher amounts of jasmonate upon wounding and are more resistant to phytophagous mites (Tetranychus urticae). Plants with increased AP2C1 levels display lower wound activation of MAPKs, reduced ethylene production, and compromised innate immunity against the necrotrophic pathogen Botrytis cinerea. Our results demonstrate a key role for the AP2C1 phosphatase in regulating stress hormone levels, defense responses, and MAPK activities in Arabidopsis and provide evidence that the activity of AP2C1 might control the plant's response to B. cinerea.

Stress-induced protein phosphatase 2C is a negative regulator of a mitogen-activated protein kinase.

Protein phosphatases of type 2C (PP2Cs) play important roles in eukaryotic signal transduction. In contrast to other eukaryotes, plants such as Arabidopsis have an unusually large group of 69 different PP2C genes. At present, little is known about the functions and substrates of plant PP2Cs. We have previously shown that MP2C, a wound-induced alfalfa PP2C, is a negative regulator of mitogen-activated protein kinase (MAPK) pathways in yeast and plants. In this report, we provide evidence that alfalfa salt stress-inducible MAPK (SIMK) and stress-activated MAPK (SAMK) are activated by wounding and that MP2C is a MAPK phosphatase that directly inactivates SIMK but not the wound-activated MAPK, SAMK. SIMK is inactivated through threonine dephosphorylation of the pTEpY motif, which is essential for MAPK activity. Mutant analysis indicated that inactivation of SIMK depends on the catalytic activity of MP2C. A comparison of MP2C with two other PP2Cs, ABI2 and AtP2CHA, revealed that although all three phosphatases have similar activities toward casein as a substrate, only MP2C is able to dephosphorylate and inactivate SIMK. In agreement with the notion that MP2C interacts directly with SIMK, the MAPK was identified as an interacting partner of MP2C in a yeast two-hybrid screen. MP2C can be immunoprecipitated with SIMK in a complex in vivo and shows direct binding to SIMK in vitro in protein interaction assays. Wound-induced MP2C expression correlates with the time window when SIMK is inactivated, corroborating the notion that MP2C is involved in resetting the SIMK signaling pathway.