Mitogen-activated protein kinase 4 is involved in the regulation of mitotic and cytokinetic microtubule transitions in Arabidopsis thaliana

? A mitogen-activated protein kinase kinase kinase (MAPKKK) double mutant, Arabidopsis homologue of nucleus and phragmoplast associated kinase (anp) anp2anp3, and the mitogen-activated protein kinase (MAPK) 4 mutant mpk4 of Arabidopsis thaliana show prominent cytokinetic defects. This prompted the analysis of mitotic and cytokinetic progression as a function of MAPK signalling. Mutants were compared with wild types untreated or treated with the specific MAPKK inhibitor PD98059. ? This study included phenotype analysis, expression analysis of the MPK4 promoter, immunofluorescent localization of MPK4, tubulin and MAP65-1, and time-lapse microscopic visualization of the mitotic microtubule (MT) transitions in control, mutant and inhibitor-treated cells. ? Mutant and inhibitor-treated cells showed defects in mitosis and cytokinesis, including aberrant spindle and phragmoplast formation and drastically delayed or abortive mitosis and cytokinesis. As a result, bi- and multinucleate cells were formed, ultimately disturbing the vegetative tissue patterning. MPK4 was localized to all stages of the expanding phragmoplast, in a pattern similar to that of its putative substrate MAP65-1. ? In this study, MPK4 is shown to be involved in the regulation of mitosis/cytokinesis through modulation of the cell division plane and cytokinetic progression.     

The MEKK1-MKK1/MKK2-MPK4 kinase cascade negatively regulates immunity mediated by a mitogen-activated protein kinase kinase kinase in Arabidopsis

In Arabidopsis thaliana, the MEKK1-MKK1/MKK2-MPK4 mitogen-activated protein (MAP) kinase cascade represses cell death and immune responses. In mekk1, mkk1 mkk2, and mpk4 mutants, programmed cell death and defense responses are constitutively activated, but the mechanism by which MEKK1, MKK1/MKK2, and MPK4 negatively regulate cell death and immunity was unknown. From a screen for suppressors of mkk1 mkk2, we found that mutations in suppressor of mkk1 mkk2 1 (summ1) suppress the cell death and defense responses not only in mkk1 mkk2 but also in mekk1 and mpk4. SUMM1 encodes the MAP kinase kinase kinase MEKK2. It interacts with MPK4 and is phosphorylated by MPK4 in vitro. Overexpression of SUMM1 activates cell death and defense responses that are dependent on the nucleotide binding-leucine-rich repeat protein SUMM2. Taken together, our data suggest that the MEKK1-MKK1/MKK2-MPK4 kinase cascade negatively regulates MEKK2 and activation of MEKK2 triggers SUMM2-mediated immune responses.     

EDR1 Physically Interacts with MKK4/MKK5 and Negatively Regulates a MAP Kinase Cascade to Modulate Plant Innate Immunity

Mitogen-activated protein (MAP) kinase signaling cascades play important roles in the regulation of plant defense. The Raf-like MAP kinase kinase kinase (MAPKKK) EDR1 negatively regulates plant defense responses and cell death. However, how EDR1 functions, and whether it affects the regulation of MAPK cascades, are not well understood. Here, we showed that EDR1 negatively regulates the MKK4/MKK5-MPK3/MPK6 kinase cascade in Arabidopsis. We found that edr1 mutants have highly activated MPK3/MPK6 kinase activity and higher levels of MPK3/MPK6 proteins than wild type. EDR1 physically interacts with MKK4 and MKK5, and this interaction requires the N-terminal domain of EDR1. EDR1 also negatively affects MKK4/MKK5 protein levels. In addition, the mpk3, mkk4 and mkk5 mutations suppress edr1-mediated resistance, and over-expression of MKK4 or MKK5 causes edr1-like resistance and mildew-induced cell death. Taken together, our data indicate that EDR1 physically associates with MKK4/MKK5 and negatively regulates the MAPK cascade to fine-tune plant innate immunity.     

An Arabidopsis kinase cascade influences auxin‐responsive cell expansion

Mitogen‐activated protein kinase (MPK) cascades are conserved mechanisms of signal transduction across eukaryotes. Despite the importance of MPK proteins in signaling events, specific roles for many Arabidopsis MPK proteins remain unknown. Multiple studies have suggested roles for MPK signaling in a variety of auxin‐related processes. To identify MPK proteins with roles in auxin response, we screened mpk insertional alleles and identified mpk1‐1 as a mutant that displays hypersensitivity in auxin‐responsive cell expansion assays. Further, mutants defective in the upstream MAP kinase kinase MKK3 also display hypersensitivity in auxin‐responsive cell expansion assays, suggesting that this MPK cascade affects auxin‐influenced cell expansion. We found that MPK1 interacts with and phosphorylates ROP BINDING PROTEIN KINASE 1 (RBK1), a protein kinase that interacts with members of the Rho‐like GTPases from Plants (ROP) small GTPase family. Similar to mpk1‐1 and mkk3‐1 mutants, rbk1 insertional mutants display auxin hypersensitivity, consistent with a possible role for RBK1 downstream of MPK1 in influencing auxin‐responsive cell expansion. We found that RBK1 directly phosphorylates ROP4 and ROP6, supporting the possibility that RBK1 effects on auxin‐responsive cell expansion are mediated through phosphorylation‐dependent modulation of ROP activity. Our data suggest a MKK3 ? MPK1 ? RBK1 phosphorylation cascade that may provide a dynamic module for altering cell expansion.     

MEKK1, MKK1/MKK2 and MPK4 function together in a mitogen-activated protein kinase cascade to regulate innate immunity in plants

Mitogen-activated protein kinase (MAPK) cascades play important roles in regulating plant innate immune responses. In a genetic screen to search for mutants with constitutive defense responses, we identified multiple alleles of mpk4 and mekk1 that exhibit cell death and constitutive defense responses. Bimolecular fluorescence complementation (BiFC) analysis showed that both MPK4 and MEKK1 interact with MKK1 and MKK2, two closely related MAPK kinases. mkk1 and mkk2 single mutant plants do not have obvious mutant phenotypes. To test whether MKK1 and MKK2 function redundantly, mkk1 mkk2 double mutants were generated. The mkk1 mkk2 double mutant plants die at seedling stage and the seedling-lethality phenotype is temperature-dependent. Similar to the mpk4 and mekk1 mutants, the mkk1 mkk2 double mutant seedlings accumulate high levels of H2O2, display spontaneous cell death, constitutively express Pathogenesis Related (PR) genes and exhibit pathogen resistance. In addition, activation of MPK4 by flg22 is impaired in the mkk1 mkk2 double mutants, suggesting that MKK1 and MKK2 function together with MPK4 and MEKK1 in a MAP kinase cascade to negatively regulate innate immune responses in plants.     

Maternal control of embryogenesis by MPK6 and its upstream MKK4/MKK5 in Arabidopsis

In flowering plants, developing embryos reside in maternal sporophytes. It is known that maternal generation influences the development of next‐generation embryos; however, little is known about the signaling components in the process. Previously, we demonstrated that Arabidopsis mitogen‐activated protein kinase 6 (MPK6) and MPK3 play critical roles in plant reproduction. In addition, we noticed that a large fraction of seeds from mpk6 single‐mutant plants showed a wrinkled seed coat or a burst‐out embryo phenotype. Here, we report that these seed phenotypes can be traced back to defective embryogenesis. The defective embryos have shorter suspensors and reduced growth along the longitudinal axis. Furthermore, the cotyledons fail to bend over to progress to the bent‐cotyledon stage. As a result of the uneven circumference along the axis, the seed coat wrinkles to develop raisin‐like morphology after dehydration. In more severe cases, the embryo can be pushed out from the micropylar end, resulting in the burst‐out embryo seed phenotype. Genetic analyses demonstrated that the defective embryogenesis of the mpk6 mutant is a maternal effect. Heterozygous or homozygous mpk6 embryos have defects only in mpk6 homozygous maternal plants, but not in wild‐type or heterozygous maternal plants. The loss of function of MKK4/MKK5 also results in the same phenotypes, suggesting that MKK4/MKK5 might act upstream of MPK6 in this pathway. The maternal‐mediated embryo defects are associated with changes in auxin activity maxima and PIN localization. In summary, this research demonstrates that the Arabidopsis MKK4/MKK5–MPK6 cascade is an important player in the maternal control of embryogenesis.     

The NLR protein SUMM2 senses the disruption of an immune signaling MAP kinase cascade via CRCK3

MAP kinase signaling is an integral part of plant immunity. Disruption of the MEKK1‐MKK1/2‐MPK4 kinase cascade results in constitutive immune responses mediated by the NLR protein SUMM2, but the molecular mechanism is so far poorly characterized. Here, we report that SUMM2 monitors a substrate protein of MPK4, CALMODULIN‐BINDING RECEPTOR‐LIKE CYTOPLASMIC KINASE 3 (CRCK3). Similar to SUMM2, CRCK3 was isolated from a suppressor screen of mkk1 mkk2 and is required for the autoimmunity phenotypes in mekk1, mkk1 mkk2, and mpk4 mutants. In wild‐type plants, CRCK3 is mostly phosphorylated. MPK4 interacts with CRCK3 and can phosphorylate CRCK3 in vitro. In mpk4 mutant plants, phosphorylation of CRCK3 is substantially reduced, suggesting that MPK4 phosphorylates CRCK3 in vivo. Further, CRCK3 associates with SUMM2 in planta, suggesting SUMM2 senses the disruption of the MEKK1‐MKK1/2‐MPK4 kinase cascade through CRCK3. Our study suggests that a MAP kinase substrate is used as a guardee or decoy for monitoring the integrity of MAP kinase signaling.     

MPK9 and MPK12 function in SA-induced stomatal closure in Arabidopsis thaliana

Salicylic acid (SA) induces stomatal closure sharing several components with abscisic acid (ABA) and methyl jasmonate (MeJA) signaling. We have previously shown that two guard cell-preferential mitogen-activated protein kinases (MAPKs), MPK9 and MPK12, positively regulate ABA signaling and MeJA signaling in Arabidopsis thaliana. In this study, we examined whether these two MAPKs are involved in SA-induced stomatal closure using genetic mutants and a pharmacological, MAPKK inhibitor. Salicylic acid induced stomatal closure in mpk9 and mpk12 single mutants but not in mpk9 mpk12 double mutants. The MAPKK inhibitor PD98059 inhibited SA-induced stomatal closure in wild-type plants. Salicylic acid induced extracellular reactive oxygen species (ROS) production, intracellular ROS accumulation, and cytosolic alkalization in the mpk9, mpk12, and mpk9 mpk12 mutants. Moreover, SA-activated S-type anion channels in guard cells of wild-type plants but not in guard cells of mpk9 mpk12 double mutants. These results imply that MPK9 and MPK12 are positive regulators of SA signaling in Arabidopsis guard cells.     

AtMPK4 is required for male-specific meiotic cytokinesis in Arabidopsis

Mitogen-activated protein kinase (MAPK) cascades have been implicated in regulating various aspects of plant development, including somatic cytokinesis. The evolution of expanded plant MAPK gene families has enabled the diversification of potential MAPK cascades, but functionally overlapping components are also well documented. Here we report that Arabidopsis MPK4, an MAPK that was previously described as a regulator of disease resistance, can interact with and be phosphorylated by the cytokinesis-related MAP kinase kinase, AtMKK6. In mpk4 mutant plants, anthers can develop normal microspore mother cells (MMCs) and peripheral supporting tissues, but the MMCs fail to form a normal intersporal callose wall after male meiosis, and thus cannot complete meiotic cytokinesis. Nevertheless, the multinucleate mpk4 microspores subsequently proceed through mitotic cytokinesis, resulting in enlarged mature pollen grains that possess increased sets of the tricellular structure. This pollen development phenotype is reminiscent of those observed in both atnack2/tes/stud and anq1/mkk6 mutants, and protein-protein interaction analysis defines a putative signalling module linking AtNACK2/TES/STUD, AtANP3, AtMKK6 and AtMPK4 together as a cascade that facilitates male-specific meiotic cytokinesis in Arabidopsis.     

The Arabidopsis MAP kinase kinase MKK1 participates in defence responses to the bacterial elicitor flagellin

Plants sense pathogens through both pathogen-associated molecular patterns and recognition of race-specific virulence factors, which induce basal defence or an accelerated defence (often manifest in the form of local cell death), respectively. A mitogen-activated protein kinase (MAPK) module in Arabidopsis was previously proposed to signal from perception of the bacterial elicitor flagellin to the activation of basal defence-related genes. Here, we present evidence for a parallel MAPK-signalling pathway involved in the response to flg22, a peptide corresponding to the most conserved domain of flagellin. The endogenous Arabidopsis MAP kinase kinase MKK1 is activated in cells treated with flg22, phosphorylates the MAPK MPK4 in vitro, and activates it in vivo in protoplasts. In mkk1 mutant plants, the activation by flg22 of MPK4 and two other flg22-induced MAPKs (MPK3 and MPK6) is impaired. In the mkk1 mutant, a battery of both flg22-induced and flg22-repressed genes show altered expression, indicating that MKK1 negatively regulates the activity of flagellin-responsive genes. Intriguingly, in contrast to the mpk4 mutant, mkk1 shows no morphological anomalies and is compromised in resistance to both virulent and avirulent Pseudomonas syringae strains. Thus, the MKK1 signalling pathway modulates the expression of genes responding to elicitors and plays an important role in pathogen defence.     

Arabidopsis thaliana MAP65-1 and MAP65-2 function redundantly with MAP65-3/PLEIADE in cytokinesis downstream of MPK4

Plant cytokinesis occurs by the growth of cell plates from the interior to the periphery of the cell. These dynamic events in cytokinesis are mediated by a plant-specific microtubule (MT) array called the phragmoplast, which consists of bundled antiparallel MTs between the two daughter nuclei. The NACK-PQR pathway, a NACK1 kinesin-like protein and mitogen activated protein kinase (MAPK) cascade, is a key regulator of plant cytokinesis through the regulation of phragmoplast MTs. The MT-associated protein MAP65 has been identified as one of the structural components of MT assays involved in cell division, and we recently showed that Arabidopsis AtMAP65-3/PLEIADE (PLE) is a substrate of MPK4 that is a component of the NACK-PQR pathway in Arabidopsis. Here we show that AtMAP65-1 and AtMAP65-2 are also phosphorylated by MPK4. AtMAP65-1 and AtMAP65-2 that localize to the phragmoplast were phosphorylated by MPK4 in vitro. Although mutants of the Arabidopsis AtMAP65-1 and AtMAP65-2 genes exhibited a wild-type phenotype, double mutations of AtMAP65-3 and AtMAP65-1 or AtMAP65-2 caused more severe growth and cytokinetic defects than the single atmap65-3/ple mutation. These results suggest that AtMAP65-1 and AtMAP65-2 also function in cytokinesis downstream of MPK4.Key words: MAP65, microtubule-associated protein, MAPK, cytokinesis, phragmoplast, microtubule, arabidopsisMitogen-activated protein kinase (MAPK) cascades are highly conserved signaling modules in eukaryotes, and are involved in various signaling processes in plant development, cell division and responses to endogenous or exogenous stimuli.¹ The NACK-PQR pathway, one of the best-characterized MAPK cascades in plants, has been identified as a key regulator of plant cytokinesis in tobacco. This pathway is composed of NPK1 MAPK kinase kinase (MAPKKK), NQK1/NtMEK1 MAPK kinase (MAPKK), NRK1/NTF6 MAPK and NACK1 kinesin-like protein, an activator of NPK1 MAPKKK.^2–5 During cytokinesis, all these components are localized on the equator of the phragmoplast, which is the plant-specific cytokinetic apparatus organized by microtubules (MTs). Downstream of this pathway, tobacco MAP65-1, an MT-associated protein, is phosphorylated by NRK1/NTF6 MAPK and phosphorylated MAP65-1 is localized to the equator of the phragmoplast.⁶ Phosphorylation of MAP65-1 by NRK1/NTF6 decreases the ability of MAP65-1 to bundle MTs, suggesting that the NACK-PQR pathway regulates expansion of the phragmoplast through the phosphorylation of MAP65.⁶The NACK-PQR pathway also seems to be conserved in Arabidopsis and rice. Several orthologs of components of the NACK-PQR pathway except for MAPK have been identified independently as regulators of cytokinesis in these plants.^3,5,7–14 Recently we reported that ANP MAPKKKs, MPK6/ANQ MAPKK and MPK4 MAPK biochemically constitute the MAPK pathway and HINKEL/AtNACK1 functions as an activator of ANP MAPKKKs.¹⁵ In addition, we revealed that MPK4 MAPK is localized to cell plates during cytokinesis, is required for cytokinesis in Arabidopsis and phosphorylates AtMAP65-3.¹⁶ Although AtMAP65-3 is proposed to be involved in cytokinesis,^17,18 and AtMAP65-1 is supposed to be a substrate of MPK4 based on a series of experiments,^6,19,20 the involvement in cytokinesis of other closely related members of the Arabidopsis MAP65 family, AtMAP65-1 and AtNAP65-2, has yet to be tested. In this report, we suggest redundant functions of these MAP65 molecules in cytokinesis of Arabidopsis.     

Two guard cell mitogen‐activated protein kinases,MPK9 and MPK12, function in methyl jasmonate‐induced stomatal closure in Arabidopsis thaliana

Methyl jasmonate (MeJA) and abscisic acid (ABA) signalling cascades share several signalling components in guard cells. We previously showed that two guard cell‐preferential mitogen‐activated protein kinases (MAPKs), MPK9 and MPK12, positively regulate ABA signalling in Arabidopsis thaliana. In this study, we examined whether these two MAP kinases function in MeJA signalling using genetic mutants for MPK9 and MPK12 combined with a pharmacological approach. MeJA induced stomatal closure in mpk9‐1 and mpk12‐1 single mutants as well as wild‐type plants, but not in mpk9‐1 mpk12‐1 double mutants. Consistently, the MAPKK inhibitor PD98059 inhibited the MeJA‐induced stomatal closure in wild‐type plants. MeJA elicited reactive oxygen species (ROS) production and cytosolic alkalisation in guard cells of the mpk9‐1, mpk12‐1 and mpk9‐1 mpk12‐1 mutants, as well in wild‐type plants. Furthermore, MeJA triggered elevation of cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt) in the mpk9‐1 mpk12‐1 double mutant as well as wild‐type plants. Activation of S‐type anion channels by MeJA was impaired in mpk9‐1 mpk12‐1. Together, these results indicate that MPK9 and MPK12 function upstream of S‐type anion channel activation and downstream of ROS production, cytosolic alkalisation and [Ca²⁺]_cyt elevation in guard cell MeJA signalling, suggesting that MPK9 and MPK12 are key regulators mediating both ABA and MeJA signalling in guard cells.     

An Arabidopsis Mitogen-Activated Protein Kinase Cascade, MKK9-MPK6, Plays a Role in Leaf Senescence

Leaf senescence is a developmentally programmed cell death process that constitutes the final step of leaf development, and it can be regulated by multiple environmental cues and endogenous signals. The mitogen-activated protein kinase (MAPK) cascades play diverse roles in intracellular and extracellular signaling in plants. Roles of the MAPK signaling module in leaf senescence are unknown. Here, a MAPK cascade involving MKK9-MPK6 is shown to play an important role in regulating leaf senescence in Arabidopsis (Arabidopsis thaliana). Both MKK9 and MPK6 possess kinase activities, with MPK6 an immediate target of MKK9, as revealed by in vitro, in vivo, and in planta assays. The constitutive and inducible overexpression of MKK9 causes premature senescence in leaves and in whole Arabidopsis plants. The premature senescence phenotype is suppressed when MKK9 is overexpressed in the mpk6 null background. When either MKK9 or MPK6 is knocked out, leaf senescence is delayed.     

Arabidopsis map kinase 4 negatively regulates systemic acquired resistance

Transposon inactivation of Arabidopsis MAP kinase 4 produced the mpk4 mutant exhibiting constitutive systemic acquired resistance (SAR) including elevated salicylic acid (SA) levels, increased resistance to virulent pathogens, and constitutive pathogenesis-related gene expression shown by Northern and microarray hybridizations. MPK4 kinase activity is required to repress SAR, as an inactive MPK4 form failed to complement mpk4. Analysis of mpk4 expressing the SA hydroxylase NahG and of mpk4/npr1 double mutants indicated that SAR expression in mpk4 is dependent upon elevated SA levels but is independent of NPR1. PDF1.2 and THI2.1 gene induction by jasmonate was blocked in mpk4 expressing NahG, suggesting that MPK4 is required for jasmonic acid-responsive gene expression.     

Genetic interaction and phenotypic analysis of the Arabidopsis MAP kinase pathway mutations mekk1 and mpk4 suggests signaling pathway complexity

It has been shown that the Arabidopsis MEK kinase MEKK1 acts upstream of the MAP kinase MPK4 to negatively regulate salicylic acid-dependent defense-response pathways. Here, we report that the mekk1;mpk4 double-mutant combination causes seedling lethality. In addition, we demonstrate that mekk1 and mpk4 single-mutant plants have significantly different phenotypes. mekk1 plants are defective for lateral root formation, while mpk4 plants are not. In addition, treatment with elevated levels of sodium chloride improves the growth of mekk1 plants, while it inhibits the growth of mpk4 plants. Our results suggest that MEKK1 and MPK4 functions are not limited to a single, linear signaling pathway. Instead there appears to be more complexity to the signaling pathways in which these two proteins function.     

Calmodulin-dependent activation of MAP kinase for ROS homeostasis in Arabidopsis

Rapid recognition and signal transduction of mechanical wounding through various signaling molecules, including calcium (Ca2+), protein phosphorylation, and reactive oxygen species (ROS), are necessary early events leading to stress resistance in plants. Here we report that an Arabidopsis mitogen-activated protein kinase 8 (MPK8) connects protein phosphorylation, Ca2+, and ROS in the wound-signaling pathway. MPK8 is activated through mechanical wounding, and this activation requires direct binding of calmodulins (CaMs) in a Ca2+-dependent manner. MPK8 is also phosphorylated and activated by a MAPKK MKK3 in the prototypic kinase cascade, and full activation of MPK8 needs both CaMs and MKK3 in planta. The MPK8 pathway negatively regulates ROS accumulation through controlling expression of the Rboh D gene. These findings suggest that two major activation modes in eukaryotes, Ca2+/CaMs and the MAP kinase phosphorylation cascade, converge at MPK8 to monitor or maintain an essential part of ROS homeostasis.     

Two guard cell‐preferential MAPKs,MPK9 and MPK12, regulate YEL signalling in Arabidopsis guard cells

We report that two mitogen‐activated protein kinases (MAPKs), MPK9 and MPK12, positively regulate abscisic acid (ABA)‐induced stomatal closure in Arabidopsis thaliana. Yeast elicitor (YEL) induced stomatal closure accompanied by intracellular reactive oxygen species (ROS) accumulation and cytosolic free calcium concentration ([Ca²⁺]_cyt) oscillation. In this study, we examined whether these two MAP kinases are involved in YEL‐induced stomatal closure using MAPKK inhibitors, PD98059 and U0126, and MAPK mutants, mpk9, mpk12 and mpk9 mpk12. Both PD98059 and U0126 inhibited YEL‐induced stomatal closure. YEL induced stomatal closure in the mpk9 and mpk12 mutants but not in the mpk9 mpk12 mutant, suggesting that a MAPK cascade involving MPK9 and MPK12 functions in guard cell YEL signalling. However, YEL induced extracellular ROS production, intracellular ROS accumulation and cytosolic alkalisation in the mpk9, mpk12 and mpk9 mpk12 mutants. YEL induced [Ca²⁺]_cyt oscillations in both wild type and mpk9 mpk12 mutant. These results suggest that MPK9 and MPK12 function redundantly downstream of extracellular ROS production, intracellular ROS accumulation, cytosolic alkalisation and [Ca²⁺]_cyt oscillation in YEL‐induced stomatal closure in Arabidopsis guard cells and are shared with ABA signalling.     

Activation of the Arabidopsis thaliana mitogen-activated protein kinase MPK11 by the flagellin-derived elicitor peptide, flg22

Mitogen-activated protein kinases (MAPK) mediate cellular signal transduction during stress responses, as well as diverse growth and developmental processes in eukaryotes. Pathogen infection or treatments with conserved pathogen-associated molecular patterns (PAMPs) such as the bacterial flagellin-derived flg22 peptide are known to activate three Arabidopsis thaliana MAPK: MPK3, MPK4, and MPK6. Several stresses, including flg22 treatment, are known to increase MPK11 expression but activation of MPK11 has not been shown. Here, we show that MPK11 activity can, indeed, be increased through flg22 elicitation. A small-scale microarray for profiling defense-related genes revealed that cinnamyl alcohol dehyrogenase 5 requires MPK11 for full flg22-induced expression. An mpk11 mutant showed increased flg22-mediated growth inhibition but no altered susceptibility to Pseudomonas syringae, Botrytis cinerea, or Alternaria brassicicola. In mpk3, mpk6, or mpk4 backgrounds, MPK11 is required for embryo or seed development or general viability. Although this developmental deficiency in double mutants and the lack of or only subtle mpk11 phenotypes suggest functional MAPK redundancies, comparison with the paralogous MPK4 reveals distinct functions. Taken together, future investigations of MAPK roles in stress signaling should include MPK11 as a fourth PAMP-activated MAPK.