CRISPR-targeted mutagenesis of mitogen-activated protein kinase phosphatase 1 improves both immunity and yield in wheat

Plants have evolved a sophisticated immunity system for specific detection of pathogens and rapid induction of measured defences. Over- or constitutive activation of defences would negatively affect plant growth and development. Hence, the plant immune system is under tight positive and negative regulation. MAP kinase phosphatase1 (MKP1) has been identified as a negative regulator of plant immunity in model plant Arabidopsis. However, the molecular mechanisms by which MKP1 regulates immune signalling in wheat (Triticum aestivum) are poorly understood. In this study, we investigated the role of TaMKP1 in wheat defence against two devastating fungal pathogens and determined its subcellular localization. We demonstrated that knock-down of TaMKP1 by CRISPR/Cas9 in wheat resulted in enhanced resistance to rust caused by Puccinia striiformis f. sp. tritici (Pst) and powdery mildew caused by Blumeria graminis f. sp. tritici (Bgt), indicating that TaMKP1 negatively regulates disease resistance in wheat. Unexpectedly, while Tamkp1 mutant plants showed increased resistance to the two tested fungal pathogens they also had higher yield compared with wild-type control plants without infection. Our results suggested that TaMKP1 interacts directly with dephosphorylated and activated TaMPK3/4/6, and TaMPK4 interacts directly with TaPAL. Taken together, we demonstrated TaMKP1 exert negative modulating roles in the activation of TaMPK3/4/6, which are required for MAPK-mediated defence signalling. This facilitates our understanding of the important roles of MAP kinase phosphatases and MAPK cascades in plant immunity and production, and provides germplasm resources for breeding for high resistance and high yield.

Salinity stress-induced phosphorylation of INDETERMINATE-DOMAIN 4 (IDD4) by MPK6 regulates plant growth adaptation in Arabidopsis

The INDETERMINATE DOMAIN (IDD) family belongs to a group of plant-specific transcription factors that coordinates plant growth/development and immunity. However, the function and mode of action of IDDs during abiotic stress, such as salt, are poorly understood. We used idd4 transgenic lines and screened them under salt stress to find the involvement of IDD4 in salinity stress tolerance The genetic disruption of IDD4 increases salt-tolerance, characterized by sustained plant growth, improved Na+/K+ ratio, and decreased stomatal density/aperture. Yet, IDD4 overexpressing plants were hypersensitive to salt-stress with an increase in stomatal density and pore size. Transcriptomic and ChIP-seq analyses revealed that IDD4 directly controls an important set of genes involved in abiotic stress/salinity responses. Interestingly, using anti-IDD4-pS73 antibody we discovered that IDD4 is specifically phosphorylated at serine-73 by MPK6 in vivo under salinity stress. Analysis of plants expressing the phospho-dead and phospho-mimicking IDD4 versions proved that phosphorylation of IDD4 plays a crucial role in plant transcriptional reprogramming of salt-stress genes. Altogether, we show that salt stress adaption involves MPK6 phosphorylation of IDD4 thereby regulating IDD4 DNA-binding and expression of target genes.

Rice Mitogen-Activated Protein Kinase regulates serotonin accumulation and interacts with cell cycle regulators under prolonged UV-B exposure

Stress conditions such as UV-B exposure activates MAPKs in Arabidopsis and rice. UV-B radiation is hazardous to plant as it causes photosystem disruption, DNA damage and ROS generation. Here we report its effect on biological pathways by studying the global changes in transcript profile in rice seedling exposed to UV-B radiation for 1 h and 16 h. Short UV-B exposure (1 h) led to moderate changes, while a drastic change in transcript landscape was observed after long term UV-B exposure (16 h) in rice seedlings. Prolonged UV-B exposure negatively impacts the expression of cell cycle regulating genes and several other metabolic pathways in developing seedlings. MAP kinase signaling cascade gets activated upon UV-B exposure similar to reports in Arabidopsis indicating conservation of its function in both dicot and monocot. Expression analysis in inducible overexpression transgenic lines of MPK3 and MPK6 shows higher transcript abundance of phytoalexin biosynthesis gene like Oryzalexin D synthase and Momilactone A synthase, along with serotonin biosynthesis genes. An accumulation of serotonin was observed upon UV-B exposure and its abundance positively correlates with the MPK3 and MPK6 transcript level in the respective over-expression lines. Interestingly, multiple cell cycle inhibitor proteins including WEE1 and SMR1 interact with MPK3 and MPK6 thus, implying a major role of this pathway in cell cycle regulation under stress condition. Overall overexpression of MPK3 and MPK6 found to be detrimental for rice as overexpression lines shows higher cell death and compromised tolerance to UV-B.

Regulation of photosynthesis by mitogen-activated protein kinase in rice: antagonistic adjustment by OsMPK3 and OsMPK6

Photosynthesis is the basis of almost all life on earth and is the main component of crop yield that contributes to the carbohydrate partitioning to the grains. Maintaining the photosynthetic efficiency of plants in challenging environmental conditions by regulating the associated factors is a potential research arena which will help in the improvement of crop yield. Phosphorylation is known to play a pivotal role in the regulation of photosynthesis. Mitogen Activated Protein Kinases (MAPKs) cascade although known to regulate a diverse range of processes does not have any exact reported function in the regulation of photosynthesis. To elucidate the regulatory role of MAPKs in photosynthesis we investigated the changes in net photosynthesis rate and related parameters in DEX inducible over-expressing (OE) lines of two members of MAPK gene family namely, OsMPK3 and OsMPK6 in rice. Interestingly, significant changes were found in net photosynthesis rate and related physiological parameters in OsMPK3 and OsMPK6-OE lines compared to its wild-type relatives. OsMPK3 and OsMPK6 have regulatory effects on nuclear-encoded photosynthetic genes. Untargeted metabolite profiling reveals a higher accumulation of sugars and their derivatives in MPK6 overexpressing plants and a lower accumulation of sugars and organic acids in MPK3 overexpressing plants. The accumulation of amino acids was found in abundance in both MPK3 and MPK6 overexpressing plants. Understanding the effects of MPK3 and MPK6 on the CO2 assimilation of rice plants under normal growth conditions, will help in devising strategies that can be extended for crop improvement.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-023-01383-9.

JA-induced TaMPK6 enhanced the freeze tolerance of Arabidopsis thaliana through regulation of ICE-CBF-COR module and antioxidant enzyme system

Mitogen-activated protein kinases (MAPKs) play important roles in the stress response of plants. However, the function of MPK proteins in freeze-resistance in wheat remains unclear. Dongnongdongmai No.1 (Dn1) is a winter wheat variety with a strong freezing resistance at extremely low temperature. In this study, we demonstrated that TaMPK6 is induced by JA signaling and is involved in the modulation of Dn1 freeze resistance. Overexpression of TaMPK6 in Arabidopsis increased the survival rate of plant at -10 ℃. The scavenging ability of reactive oxygen species (ROS) and the expression of cold-responsive genes CBFs and CORs were significantly enhanced in TaMPK6-overexpressed Arabidopsis, suggesting a role of TaMPK6 in activating the ICE-CBF-COR module and antioxidant enzyme system to resist freezing stress. Furthermore, TaMPK6 is localized in the nucleus and TaMPK6 interacts with TaICE41, TaCBF14, and TaMYC2 proteins, the key components in JA signaling and the ICE-CBF-COR pathway. These results suggest that JA-induced TaMPK6 may regulate freezing-resistance in wheat by interacting with the TaICE41, TaCBF14, and TaMYC2 proteins, which in turn enhances the ICE-CBF-COR pathway. Our study revealed the molecular mechanism of TaMPK6 involvement in the cold resistance pathway in winter wheat under cold stress, which provides a basis for enriching the theory of wheat cold resistance.

Post-embryonic function of GLOBULAR EMBRYO 4 (GLE4)/OsMPK6 in rice development

In plants, mitogen activated protein kinases (MPKs) are involved in various signaling pathways that lead to biotic and abiotic responses as well as that regulate developmental processes. Among them, MPK6 and its closely related homologue, MPK3, act redundantly and are known to be involved in asymmetric cell divisions of meristemoid mother cells in stomata development and of zygotes in Arabidopsis. Loss-of-function mutants of GLE4/OsMPK6, which is an orthologue of MPK6 in rice, showed a defect in polarity establishment in early stage of embryogenesis. However, because of the embryo lethality of the mutations, the function of GLE4/OsMPK6 in post-embryonic development is not clarified. Here, we report the analysis of post embryonic function of GLE4/OsMPK6 in vegetative stage of rice using regenerated gle4/osmpk6 homozygous plants from tissue culture. The regenerated plants are dwarf and produce multiple shoots with small leaves. These shoots never develop into reproductive stage, instead, proliferate vegetative shoots repeatedly. Leaves of gle4/osmpk6 have small leaf blade at the tip and blade-sheath boundary become obscure. Stomata arrangement is also disturbed in gle4/osmpk6 leaf blade. The shape of shoot apical meristem of gle4/osmpk6 become disorganized. Thus, GLE4/OsMPK6 functions in shoot organization and stomata patterning in the post embryonic development in rice.

ROOT MERISTEM GROWTH FACTOR1 (RGF1)-RGF1 INSENSITIVE 1 peptide-receptor pair inhibits lateral root development via the MPK6-PUCHI module in Arabidopsis

ROOT MERISTEM GROWTH FACTOR1 (RGF1) and its receptors RGF1 INSENSITIVEs (RGIs) regulate primary root meristem activity via a mitogen-activated protein kinase (MPK) signaling cascade in Arabidopsis. However, it is unknown how RGF1 regulates lateral root (LR) development. Here, we show that the RGF1-RGI1 peptide-receptor pair negatively regulates LR development via activation of PUCHI encoding AP2/EREBP. Exogenous RGF1 peptides inhibited LR development of the wild type. However, the rgi1 mutants were partially or fully insensitive to RGF1 during LR development, whereas four other rgi single mutants, namely rgi2, rgi3, rgi4, and rgi5, were sensitive to RGF1 in inhibiting LR formation. Consistent with this, the red fluorescent protein (RFP) signals driven by the RGF1 promoter were detected at stage I and the following stages, overlapping with RGI1 expression. PUCHI expression was significantly up-regulated by RGF1 but completely inhibited in rgi1. LR development of puchi1-1 was insensitive to RGF1. PUCHI expression driven by the RGI1 promoter reduced LR density in both the wild type and rgi1,2,3. Further, mpk6, but not mpk3, displayed significantly down-regulated PUCHI expression and insensitive LR development in response to RGF1. Collectively, these results suggest that the RGF1-RGI1 module negatively regulates LR development by activating PUCHI expression via MPK6.

CRK41 Modulates Microtubule Depolymerization in Response to Salt Stress in Arabidopsis

The pivotal role of cysteine-rich receptor-like kinases (CRKs) in modulating growth, development, and responses to stress has been widely acknowledged in Arabidopsis. However, the function and regulation of CRK41 has remained unclear. In this study, we demonstrate that CRK41 is critical for modulating microtubule depolymerization in response to salt stress. The crk41 mutant exhibited increased tolerance, while overexpression of CRK41 led to hypersensitivity to salt. Further analysis revealed that CRK41 interacts directly with the MAP kinase3 (MPK3), but not with MPK6. Inactivation of either MPK3 or MPK6 could abrogate the salt tolerance of the crk41 mutant. Upon NaCl treatment, microtubule depolymerization was heightened in the crk41 mutant, yet alleviated in the crk41mpk3 and crk41mpk6 double mutants, indicating that CRK41 suppresses MAPK-mediated microtubule depolymerizations. Collectively, these results reveal that CRK41 plays a crucial role in regulating microtubule depolymerization triggered by salt stress through coordination with MPK3/MPK6 signalling pathways, which are key factors in maintaining microtubule stability and conferring salt stress resistance in plants.

MPK6-mediated HY5 phosphorylation regulates light-induced anthocyanin accumulation in apple fruit

Light is known to regulate anthocyanin pigment biosynthesis in plants on several levels, but the significance of protein phosphorylation in light-induced anthocyanin accumulation needs further investigation. In this study, we investigated the dynamics of the apple fruit phosphoproteome in response to light, using high-performance liquid chromatography-tandem mass spectrometry analysis. Among the differentially phosphorylated proteins, the bZIP (basic leucine zipper) transcription factor, HY5, which has been identified as an anthocyanin regulator, was rapidly activated by light treatment of the fruit. We hypothesized that phosphorylated MdHY5 may play a role in light-induced anthocyanin accumulation of apple fruit. Protein interaction and phosphorylation assays showed that mitogen-activated protein kinase MdMPK6 directly interacted with, and activated, MdHY5 via phosphorylation under light conditions, thereby increasing its stability. Consistent with this finding, the suppression of the mitogen-activated protein kinase genes MdMPK6 or MdHY5 resulted in an inhibition of anthocyanin accumulation, and further showed that light-induced anthocyanin accumulation is dependent on MdMPK6 kinase activity, and is required for maximum MdHY5 activity. Under light conditions, active MdMPK6 phosphorylated MdHY5 leading to accumulation of phospho-MdHY5, which enhanced the binding of MdHY5 to its target anthocyanin related genes in fruit. Our findings reveal an MdMPK6-MdHY5 phosphorylation pathway in light-induced anthocyanin accumulation, providing new insights into the regulation of light-induced anthocyanin biosynthesis in apple fruit at both the transcriptional and post-translational levels.

Arabidopsis MPK3 and MPK6 regulates D-glucose signaling and interacts with G-protein, RGS1

Sugar as a signaling molecule has attracted lots of attention. Even though several kinases have been shown to play a crucial role in the sugar signaling and response to exogenous D-glucose (Glc), the information on the involvement of MAP kinase cascade in sugar signaling has remain largely unexplored. In this report we demonstrate that MAP kinase signaling is essential for sensitivity to higher concentrations of D-Glc in Arabidopsis. We found that D-Glc activates MAP kinases, MPK3 and MPK6 in a concentration and time-dependent manner. The mutants of mpk3 and mpk6 display hyposensitivity to 6% D-Glc during seed germination, cotyledon greening and root growth. Interestingly, the altered sensitivity to increased D-Glc is severely enhanced by addition of 1% Sucrose in the media. Our study also deciphered the role of one of the Glc sensor proteins, RGS1 that interacts and gets phosphorylated at its C-terminal domain by MPK3 and MPK6. Overall our study provides a new insight on the involvement of MAP kinases in association with G-proteins that might regulate sugar signaling and sugar responsive growth and development in Arabidopsis.

Two phylogenetically unrelated peptide-receptor modules jointly regulate lateral root initiation via a partially shared signaling pathway in Arabidopsis thaliana

Peptide-receptor signaling is an important system for intercellular communication, regulating many developmental processes. A single process can be controlled by several distinct signaling peptides. However, since peptide-receptor modules are usually studied separately, their mechanistic interactions remain largely unexplored. Two phylogenetically unrelated peptide-receptor modules, GLV6/GLV10-RGI and TOLS2/PIP2-RLK7, independently described as inhibitors of lateral root initiation, show striking similarities between their expression patterns and gain- and loss-of-function phenotypes, suggesting a common function during lateral root spacing and initiation. The GLV6/GLV10-RGI and TOLS2/PIP2-RLK7 modules trigger similar transcriptional changes, likely in part via WRKY transcription factors. Their overlapping set of response genes includes PUCHI and PLT5, both required for the effect of GLV6/10, as well as TOLS2, on lateral root initiation. Furthermore, both modules require the activity of MPK6 and can independently trigger MPK3/MPK6 phosphorylation. The GLV6/10 and TOLS2/PIP2 signaling pathways seem to converge in the activation of MPK3/MPK6, leading to the induction of a similar transcriptional response in the same target cells, thereby regulating lateral root initiation through a (partially) common mechanism. Convergence of signaling pathways downstream of phylogenetically unrelated peptide-receptor modules adds an additional, and hitherto unrecognized, level of complexity to intercellular communication networks in plants.

Effects of MPK3 and MPK6 kinases on the chloroplast architecture and function induced by cold acclimation in Arabidopsis

Exposure to low, non-freezing temperatures develops freezing tolerance in many plant species. Such process is called cold acclimation. Molecular changes undergone during cold acclimation are orchestrated by signalling networks including MAP kinases. Structure and function of chloroplasts are affected by low temperatures. The aim of this work was to study how the MAP kinases MPK3 and MPK6 are involved in the chloroplast performance upon a long period of cold acclimation. We used Arabidopsis thaliana wild type and mpk3 and mpk6 mutants. Adult plants were acclimated during 7 days at 4 °C and then measurements of PSII performance and chloroplast ultrastructure were carried out. Only the mpk6 acclimated plants showed a high freezing sensitivity. No differences in the PSII function were observed in the plants from the three genotypes exposed to non-acclimated or acclimated conditions. The acclimation of wild-type plants produced severe alterations in the ultrastructure of chloroplast and thylakoids, which was more accentuated in the mpk plants. However, only the mpk6 mutant was unable to internalize the damaged chloroplasts into the vacuole. These results indicate that cold acclimation induces alterations in the chloroplast architecture leading to preserve an optimal performance of PSII. MPK3 and MPK6 are necessary to regulate these morphological changes, but besides, MPK6 is needed to the vacuolization of the damaged chloroplasts, suggesting a role in the chloroplast recycling during cold acclimation. The latter could be quite relevant, since it could explain why this mutant is the only one showing an extremely low freezing tolerance.

MPK6 Kinase Regulates Plasma Membrane H+-ATPase Activity in Cold Acclimation

Cold and freezing stresses severely affect plant growth, development, and survival rate. Some plant species have evolved a process known as cold acclimation, in which plants exposed to temperatures above 0 °C trigger biochemical and physiological changes to survive freezing. During this response, several signaling events are mediated by transducers, such as mitogen activated protein kinase (MAPK) cascades. Plasma membrane H+-ATPase is a key enzyme for the plant cell life under regular and stress conditions. Using wild type and mpk3 and mpk6 knock out mutants in Arabidopsis thaliana, we explored the transcriptional, translational, and 14-3-3 protein regulation of the plasma membrane H+-ATPase activity under the acclimation process. The kinetic analysis revealed a differential profiling of the H+-ATPase activity depending on the presence or absence of MPK3 or MPK6 under non-acclimated or acclimated conditions. Negative regulation of the plasma membrane H+-ATPase activity was found to be exerted by MPK3 in non-acclimated conditions and by MPK6 in acclimated conditions, describing a novel form of regulation of this master ATPase. The MPK6 regulation involved changes in plasma membrane fluidity. Moreover, our results indicated that MPK6 is a critical regulator in the process of cold acclimation that leads to freezing tolerance and further survival.

A dual-specificity phosphatase, MAP kinase phosphatase 1, positively regulates blue light-mediated seedling development in Arabidopsis

A dual-specificity phosphatase MKP1 negatively regulates the activity of MPK6 by dephosphorylating it and acts as a positive regulator of blue light (BL)-mediated photomorphogenic development in Arabidopsis. Reversible phosphorylation of proteins is one of the major post-translational modifications in nearly all signaling pathways in plants. MAP kinase phosphatases are very crucial in the regulation of MAPKs as they dephosphorylate both threonine (Thr) and tyrosine (Tyr) residues within the T-X-Y motif of active MAPKs. Therefore, to gain insight of involvement of MAP kinase phosphatases in the regulation of light signaling, we searched for the potential phosphatase which may regulate the function of MPK6, a negative regulator of blue light (BL)-mediated photomorphogenic development. We report here the identification of a dual-specificity phosphatase, MAP kinase phosphatase 1 (MKP1) as a positive regulator of BL-mediated seedling development. Overexpression of MKP1 enhances the BL-induced inhibition of hypocotyl elongation and displays more opened cotyledons. We also show that MKP1OE accumulates more pigments and positively affects the expression of downstream light-related genes in response to BL. In vitro and in vivo evidences also demonstrate that MKP1 not only interacts with but also dephosphorylates MPK6 in BL. In addition, MKP1 regulates stability as well as activity of MPK6 upon BL. Taken together our study highlights the important role of phosphatases in the regulation of a signaling pathway and identifies the role of MKP1 in the negative regulation of MPK6 activity leading to a change in BL-induced photomorphogenic responses.

ZmMPK6 and ethylene signalling negatively regulate the accumulation of anti-insect metabolites DIMBOA and DIMBOA-Glc in maize inbred line A188

2,4-Dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) and DIMBOA-glucoside (DIMBOA-Glc) are anti-insect benzoxazinoids in maize, yet very little information is known about how they are regulated. Reverse genetics, kinase activity analysis, phytohormone and DIMBOA/DIMBOA-Glc quantification, bioassays and transcriptome analysis were employed to study the function of ZmMPK6, a mitogen-activated protein kinase, in maize response to herbivory. ZmMPK6 was rapidly activated by wounding and simulated herbivory. Silencing ZmMPK6 in maize A188 compromised simulated herbivory-induced ethylene levels but not those of jasmonic acid or salicylic acid, and the ZmMPK6-silenced plants exhibited elevated DIMBOA/DIMBOA-Glc and insect resistance. An ethylene complementation experiment revealed that ZmMPK6 repressed the accumulation of DIMBOA/DIMBOA-Glc in an ethylene-dependent manner. Transcriptome analysis revealed that ZmMPK6 might meditate the transcription of BX1 by controlling a MYB transcription factor that is likely to be located in the ethylene signalling pathway and, furthermore, ZmMPK6 and ethylene signalling also specifically and commonly regulate the transcription of other benzoxazinoid biosynthetic genes. We also show that different maize lines have very different responses to simulated herbivory in terms of ZmMPK6 activation, ethylene emission and benzoxazinoid levels. These results uncover that ZmMPK6 and ethylene pathway are novel repressors of DIMBOA/DIMBOA-Glc and provide new insight into the regulatory mechanisms underlying these two pathways.

CRISPR/Cas9-Induced Loss-of-Function Mutation in the Barley Mitogen-Activated Protein Kinase 6 Gene Causes Abnormal Embryo Development Leading to Severely Reduced Grain Germination and Seedling Shootless Phenotype

The diverse roles of mitogen-activated protein kinases (MAPKs, MPKs) in plant development could be efficiently revealed by reverse genetic studies. In Arabidopsis, mpk6 knockout mutants complete the life cycle; however, ~40% of their embryos show defects in the development leading to abnormal phenotypes of seeds and seedlings' roots. Contrary to the Arabidopsis MPK6, the rice MPK6 (OsMPK6) is an essential gene as transfer DNA (T-DNA) insertion and CRISPR/Cas9 induced loss-of-function mutations in the OsMPK6 cause early embryo arrest. In this study, we successfully developed a viable transgenic barley line with the CRISPR/Cas9-induced heterozygous single base pair cytosine-guanine (CG) deletion [wild type (WT)/-1C] in the third exon of the HvMPK6 gene, a barley ortholog of the Arabidopsis and rice MPK6. There were no obvious macroscopic phenotype differences between the WT/-1C plants and WT plants. All the grains collected from the WT/-1C plants were of similar size and appearance. However, seedling emergence percentage (SEP) from these grains was substantially decreased in the soil in the T2 and T3 generation. The mutation analysis of the 248 emerged T2 and T3 generation plants showed that none of them was a biallelic mutant in the HvMPK6 gene, suggesting lethality of the -1C/-1C homozygous knockout mutation. In the soil, the majority of the -1C/-1C grains did not germinate and the minority of them developed into abnormal seedlings with a shootless phenotype and a reduced root system. Some of the -1C/-1C seedlings also developed one or more small chlorotic leaf blade-like structure/structures. The -1C/-1C grains contained the late-stage developed abnormal embryos with the morphologically obvious scutellum and root part of the embryonic axis but with the missing or substantially reduced shoot part of the embryonic axis. The observed embryonic abnormalities correlated well with the shootless phenotype of the seedlings and suggested that the later-stage defect is predetermined already during the embryo development. In conclusion, our results indicate that barley MPK6 is essential for the embryologically predetermined shoot formation, but not for the most aspects of the embryo and early seedling development.

The growth of Arabidopsis primary root is repressed by several and diverse amino acids through auxin-dependent and independent mechanisms and MPK6 kinase activity

Amino acids serve as structural monomers for protein synthesis and are considered important biostimulants for plants. In this report, the effects of all 20-L amino acids in Arabidopsis primary root growth were evaluated. 15 amino acids inhibited growth, being l-leucine (l-Leu), l-lysine (l-Lys), l-tryptophan (l-Trp), and l-glutamate (l-Glu) the most active, which repressed both cell division and elongation in primary roots. Comparisons of DR5:GFP expression and growth of WT Arabidopsis seedlings and several auxin response mutants including slr, axr1 and axr2 single mutants, arf7/arf19 double mutant and tir1/afb2/afb3 triple mutant, treated with inhibitory concentrations of l-Glu, l-Leu, l-Lys and l-Trp revealed gene-dependent, specific changes in auxin response. In addition, l- isomers of Glu, Leu and Lys, but not l-Trp diminished the GFP fluorescence of pPIN1::PIN1:GFP, pPIN2::PIN2:GFP, pPIN3::PIN3:GFP and pPIN7::PIN7:GFP constructs in root tips. MPK6 activity in roots was enhanced by amino acid treatment, being greater in response to l-Trp while mpk6 mutants supported cell division and elongation at high doses of l-Glu, l-Leu, l-Lys and l-Trp. We conclude that independently of their auxin modulating properties, amino acids signals converge in MPK6 to alter the Arabidopsis primary root growth.

A bHLH transcription factor, MYC2, imparts salt intolerance by regulating proline biosynthesis in Arabidopsis

MYC2, a bHLH TF, acts as regulatory hub within several signaling pathways by integration of various endogenous and exogenous signals which shape plant growth and development. However, its involvement in salt stress regulation is still elusive. This study has deciphered a novel role of MYC2 in imparting salt stress intolerance by regulating delta1 -pyrroline-5-carboxylate synthase1 (P5CS1) gene and hence proline synthesis. P5CS1 is a rate-limiting enzyme in the biosynthesis of proline. Y-1-H and EMSA studies confirmed the binding of MYC2 with the 5'UTR region of P5CS1. Transcript and biochemical studies have revealed MYC2 as a negative regulator of proline biosynthesis. Proline is necessary for imparting tolerance toward abiotic stress; however, its overaccumulation is toxic for the plants. Hence, studying the regulation of proline biosynthesis is requisite to understand the mechanism of stress tolerance. We have also studied that MYC2 is regulated by mitogen-activated protein kinase (MAPK) cascade mitogen-activated protein kinase kinase 3-MPK6 and vice versa. Altogether, this study demonstrates salt stress-mediated activation of MYC2 by MAPK cascade, regulating proline biosynthesis and thus salt stress.

A PAMP-triggered MAPK cascade inhibits phosphatidylinositol 4,5-bisphosphate production by PIP5K6 in Arabidopsis thaliana

The phosphoinositide kinase PIP5K6 has recently been identified as a target for the mitogen-activated protein kinase (MAPK) MPK6. Phosphorylation of PIP5K6 inhibited the production of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂ ), impacting membrane trafficking and cell expansion in pollen tubes. Here, we analyzed whether MPK6 regulated PIP5K6 in vegetative Arabidopsis cells in response to the pathogen-associated molecular pattern (PAMP) flg22. Promoter-β-glucuronidase analyses and quantitative real-time reverse transcription polymerase chain reaction data show PIP5K6 expressed throughout Arabidopsis tissues. Upon flg22 treatment of transgenic protoplasts, the PIP5K6 protein was phosphorylated, and this modification was reduced for a PIP5K6 variant lacking MPK6-targeted residues, or in protoplasts from mpk6 mutants. Upon flg22 treatment of Arabidopsis plants, phosphoinositide levels mildly decreased and a fluorescent reporter for PtdIns(4,5)P₂ displayed reduced plasma membrane association, contrasting with phosphoinositide increases reported for abiotic stress responses. Flg22 treatment and chemical induction of the upstream MAPK kinase, MKK5, decreased phosphatidylinositol 4-phosphate 5-kinase activity in mesophyll protoplasts, indicating that the flg22-activated MAPK cascade limited PtdIns(4,5)P₂ production. PIP5K6 expression or PIP5K6 protein abundance changed only marginally upon flg22 treatment, consistent with post-translational control of PIP5K6 activity. PtdIns(4,5)P₂ -dependent endocytosis of FM 4-64, PIN2 and the NADPH-oxidase RbohD were reduced upon flg22 treatment or MKK5 induction. Reduced RbohD-endocytosis was correlated with enhanced ROS production. We conclude that MPK6-mediated phosphorylation of PIP5K6 limits the production of a functional PtdIns(4,5)P₂ pool upon PAMP perception.

Specification of basal region identity after asymmetric zygotic division requires mitogen-activated protein kinase 6 in rice

Asymmetric cell division is a key step in cellular differentiation in multicellular organisms. In plants, asymmetric zygotic division produces the apical and basal cells. The mitogen-activated protein kinase (MPK) cascade in Arabidopsis acts in asymmetric divisions such as zygotic division and stomatal development, but whether the effect on cellular differentiation of this cascade is direct or indirect following asymmetric division is not clear. Here, we report the analysis of a rice mutant, globular embryo 4 (gle4). In two- and four-cell-stage embryos, asymmetric zygotic division and subsequent cell division patterns were indistinguishable between the wild type and gle4 mutants. However, marker gene expression and transcriptome analyses showed that specification of the basal region was compromised in gle4 We found that GLE4 encodes MPK6 and that GLE4/MPK6 is essential in cellular differentiation rather than in asymmetric zygotic division. Our findings provide a new insight into the role of MPK in plant development. We propose that the regulation of asymmetric zygotic division is separate from the regulation of cellular differentiation that leads to apical-basal polarity.

Mitogen activated protein kinase 6 and MAP kinase phosphatase 1 are involved in the response of Arabidopsis roots to L-glutamate

The function and components of L-glutamate signaling pathways in plants have just begun to be elucidated. Here, using a combination of genetic and biochemical strategies, we demonstrated that a MAPK module is involved in the control of root developmental responses to this amino acid. Root system architecture plays an essential role in plant adaptation to biotic and abiotic factors via adjusting signal transduction and gene expression. L-Glutamate (L-Glu), an amino acid with neurotransmitter functions in animals, inhibits root growth, but the underlying genetic mechanisms are poorly understood. Through a combination of genetic analysis, in-gel kinase assays, detailed cell elongation and division measurements and confocal analysis of expression of auxin, quiescent center and stem cell niche related genes, the critical roles of L-Glu in primary root growth acting through the mitogen-activated protein kinase 6 (MPK6) and the dual specificity serine-threonine-tyrosine phosphatase MKP1 could be revealed. In-gel phosphorylation assays revealed a rapid and dose-dependent induction of MPK6 and MPK3 activities in wild-type Arabidopsis seedlings in response to L-Glu. Mutations in MPK6 or MKP1 reduced or increased root cell division and elongation in response to L-Glu, possibly modulating auxin transport and/or response, but in a PLETHORA1 and 2 independent manner. Our data highlight MPK6 and MKP1 as components of an L-Glu pathway linking the auxin response, and cell division for primary root growth.

Genetic dissection of Arabidopsis MAP kinase phosphatase 1-dependent PAMP-induced transcriptional responses

Plant immunity is initiated by extracellular detection of pathogen-associated molecular patterns (PAMPs) through surface-localized pattern recognition receptors (PRRs). PRR activation induces many responses including the activation of mitogen-activated protein kinases (MAPKs) that ultimately limit bacterial growth. Previous work identified Arabidopsis MAP kinase phosphatase 1 (MKP1) as a negative regulator of signaling pathways required for some, but not all, of PAMP-initiated responses. Specifically, loss of MAPK MPK6 in an mkp1 background suppressed a subset of the mkp1-dependent biological phenotypes, indicating the requirement for MPK6 in MKP1-dependent signaling. To further genetically separate the outputs of PAMP-responsive signaling pathways, we performed a transcriptome analysis in Arabidopsis wild type, mkp1 and mkp1 mpk6 seedlings treated with the bacterially derived PAMP elf26 for 0, 30, and 90 min. Using differential genetic and temporal clustering analyses between and within genotypes, we identified and separated 6963 elf26-responsive transcripts based on both genetic requirements of MKP1 (with or without a requirement for MPK6) and temporal transcriptional accumulation patterns, and some of these novel response markers were validated by qRT-PCR over a more extended time course. Taken together, our transcriptome analysis provides novel information for delineating PAMP signaling pathways.

Nitric oxide is involved in stomatal development by modulating the expression of stomatal regulator genes in Arabidopsis

As sessile organisms, plants require many flexible strategies to adapt to the environment. Although some environmental signaling pathways regulating stomatal development have been identified, how stomatal regulators are modulated by internal and external signals to determine the final stomatal abundance requires further exploration. In our studies, we found that nitric oxide (NO) promotes stomatal development with increased stomatal index as well as the relative number of meristemoids and guard mother cells [%(M+GMC)] in NO-treated wild-type Arabidopsis plants; this role of NO was further verified in the nox1 mutant, which exhibits higher NO levels, and the noa1 mutant, which exhibits low NO accumulation. To gain insight into the molecular mechanisms underlying the effect of NO, we further assayed the expression of genes involved in stomatal development and found that NO induces the expression of the master regulators SPCH, MUTE and SCRM2 to initiate stomatal development. In addition, MPK6 is also involved in NO-promoted stomatal development, as MPK6 expression was repressed in nox1 and NO-treated plants, and transgenic plants overexpressing MPK6 were less sensitive to SNP treatment in terms of changes in the%(M+GMC). Thus, our study shows that NO promotes the production of stomata by up-regulating the expression of SPCH, MUTE and SCRM2 and down-regulating MPK6 expression.

Mitogen-activated protein kinase pathways are required for melatonin-mediated defense responses in plants

Melatonin enhances pathogen resistance by inducing the expression of a number of plant defense-related genes. To examine whether the melatonin-mediated pathogen resistance is associated with mitogen-activated protein kinase (MAPK) cascades, Arabidopsis and tobacco leaves were treated with melatonin and investigated for MAPK activation using an antiphospho-p44/42 MAPK (Erk1/2) monoclonal antibody. Two MAPKs, MPK3 and MPK6, were activated rapidly and transiently by 1 μm melatonin treatment in Arabidopsis. Its tobacco ortholog MAPKs were also activated. The activation of MPK3 and MPK6 by 2-hydroxymelatonin and N-acetylserotonin was also observed, albeit to a lesser degree than that by melatonin. Furthermore, MAPK activation by melatonin was uncoupled from G-protein signaling, because melatonin efficiently activated two MAPKs in a G-protein β knockout mutant (agb1). Suppression of both MPK3 and MPK6 in transgenic Arabidopsis exhibited significant decreases in the induction of defense-related gene expression and pathogen resistance relative to wild-type plants. Using an array of MAP kinase kinase (MKK) knockout mutants, we found that four MKKs, namely MKK4, MKK5, MKK7, and MKK9, are responsible for the activation of MPK3 and MPK6 by melatonin, indicating that melatonin-mediated innate immunity is triggered by MAPK signaling through MKK4/5/7/9-MPK3/6 cascades.

The Arabidopsis mitogen-activated protein kinase 6 is associated with γ-tubulin on microtubules, phosphorylates EB1c and maintains spindle orientation under nitrosative stress

Stress-activated plant mitogen-activated protein (MAP) kinase pathways play roles in growth adaptation to the environment by modulating cell division through cytoskeletal regulation, but the mechanisms are poorly understood. We performed protein interaction and phosphorylation experiments with cytoskeletal proteins, mass spectrometric identification of MPK6 complexes and immunofluorescence analyses of the microtubular cytoskeleton of mitotic cells using wild-type, mpk6-2 mutant and plants overexpressing the MAP kinase-inactivating phosphatase, AP2C3. We showed that MPK6 interacted with γ-tubulin and co-sedimented with plant microtubules polymerized in vitro. It was the active form of MAP kinase that was enriched with microtubules and followed similar dynamics to γ-tubulin, moving from poles to midzone during the anaphase-to-telophase transition. We found a novel substrate for MPK6, the microtubule plus end protein, EB1c. The mpk6-2 mutant was sensitive to 3-nitro-l-tyrosine (NO2 -Tyr) treatment with respect to mitotic abnormalities, and root cells overexpressing AP2C3 showed defects in chromosome segregation and spindle orientation. Our data suggest that the active form of MAP kinase interacts with γ-tubulin on specific subsets of mitotic microtubules during late mitosis. MPK6 phosphorylates EB1c, but not EB1a, and has a role in maintaining regular planes of cell division under stress conditions.

NPR1 is Instrumental in Priming for the Enhanced flg22-induced MPK3 and MPK6 Activation

Pathogen-associated molecular patterns (PAMPs) activate mitogen-activated protein kinases (MAPKs), essential components of plant defense signaling. Salicylic acid (SA) is also central to plant resistance responses, but its specific role in regulation of MAPK activation is not completely defined. We have investigated the role of SA in PAMP-triggered MAPKs pathways in Arabidopsis SA-related mutants, specifically in the flg22-triggered activation of MPK3 and MPK6. cim6, sid2, and npr1 mutants exhibited wild-type-like flg22-triggered MAPKs activation, suggesting that impairment of SA signaling has no effect on the flg22-triggered MAPKs activation. Pretreatment with low concentrations of SA enhanced flg22-induced MPK3 and MPK6 activation in all seedlings except npr1, indicating that NPR1 is involved in SA-mediated priming that enhanced flg22-induced MAPKs activation.

Mitogen-activated protein kinase 6 regulates NPR1 gene expression and activation during leaf senescence induced by salicylic acid

Plant senescence is a highly regulated process that can be induced by a range of factors. The nonexpressor of pathogenesis-related genes 1 (npr1) mutant is defective in the salicylic acid (SA) signalling pathway, displaying delayed yellowing during developmental senescence. However, the regulating mechanism of NPR1 on exogenous SA-induced senescence in detached Arabidopsis leaves has not yet been clarified. It was shown here that mitogen-activated protein kinase 6 (MPK6) is involved in promoting exogenous SA-induced detached leaf senescence. During the process of SA-induced senescence, the expression of NPR1 and senescence-related transcription factor WRKY6 was suppressed in mpk6 mutant plants. Further analyses showed that the NPR1 mRNA level is reduced in wrky6 mutants and enhanced in WRKY6 overexpressing lines. Meanwhile, chromatin immunoprecipitation experiments revealed that WRKY6 binds directly to the NPR1 promoter containing W-box motifs. Moreover, inhibition of MPK6 function diminished SA-induced monomerization and nuclear localization of NPR1. In addition, the expression of Trx h5, which catalyses the SA-induced NPR1 activation, was suppressed in the mpk6 mutant, suggesting that MPK6 promotes NPR1 activation, possibly by regulating the expression of Trx h5. Collectively, MPK6-mediated WRKY6 and Trx h5 transcriptional activation co-regulated the expression of the NPR1 gene and the monomerization of NPR1 protein, allowing it to enter the nucleus, thereby promoting SA-induced leaf senescence. These results provide new insight into the mechanism of exogenous SA-induced detached leaf senescence.

Activation of MKK9-MPK3/MPK6 enhances phosphate acquisition in Arabidopsis thaliana

Despite the abundance of phosphorus in soil, very little is available as phosphate (Pi) for plants. Plants often experience low Pi (LP) stress. Intensive studies have been conducted to reveal the mechanism used by plants to deal with LP; however, Pi sensing and signal transduction pathways are not fully understood. Using in-gel kinase assays, we determined the activities of MPK3 and MPK6 in Arabidopsis thaliana seedlings under both LP and Pi-sufficient (Murashige and Skoog, MS) conditions. Using MKK9 mutant transgenic and crossed mutants, we analyzed the functions of MPK3 and MPK6 in regulating Pi responses of seedlings. The regulation of Pi responses by downstream components of MKK9-MPK3/MPK6 was also screened. LP treatment activated MPK3 and MPK6. Under both LP and MS conditions, mpk3 and mpk6 seedlings took up and accumulated less Pi than the wild-type; activation of MKK9-MPK3/MPK6 in transgenic seedlings induced the transcription of Pi acquisition-related genes and enhanced Pi uptake and accumulation, whereas its activation suppressed the transcription of anthocyanin biosynthetic genes and anthocyanin accumulation; WRKY75 was downstream of MKK9-MPK3/MPK6 when regulating the accumulation of Pi and anthocyanin, and the transcription of Pi acquisition-related and anthocyanin biosynthetic genes. These results suggest that the MKK9-MPK3/MPK6 cascade is part of the Pi signaling pathway in plants.

Involvement of YODA and mitogen activated protein kinase 6 in Arabidopsis post-embryogenic root development through auxin up-regulation and cell division plane orientation

The role of YODA MITOGEN ACTIVATED PROTEIN KINASE KINASE KINASE 4 (MAPKKK4) upstream of MITOGEN ACTIVATED PROTEIN KINASE 6 (MPK6) was studied during post-embryonic root development of Arabidopsis thaliana. Loss- and gain-of-function mutants of YODA (yda1 and ΔNyda1) were characterized in terms of root patterning, endogenous auxin content and global proteomes. We surveyed morphological and cellular phenotypes of yda1 and ΔNyda1 mutants suggesting possible involvement of auxin. Endogenous indole-3-acetic acid (IAA) levels were up-regulated in both mutants. Proteomic analysis revealed up-regulation of auxin biosynthetic enzymes tryptophan synthase and nitrilases in these mutants. The expression, abundance and phosphorylation of MPK3, MPK6 and MICROTUBULE ASSOCIATED PROTEIN 65-1 (MAP65-1) were characterized by quantitative polymerase chain reaction (PCR) and western blot analyses and interactions between MAP65-1, microtubules and MPK6 were resolved by quantitative co-localization studies and co-immunoprecipitations. yda1 and ΔNyda1 mutants showed disoriented cell divisions in primary and lateral roots, abortive cytokinesis, and differential subcellular localization of MPK6 and MAP65-1. They also showed deregulated expression of TANGLED1 (TAN1), PHRAGMOPLAST ORIENTING KINESIN 1 (POK1), and GAMMA TUBULIN COMPLEX PROTEIN 4 (GCP4). The findings that MPK6 localized to preprophase bands (PPBs) and phragmoplasts while the mpk6-4 mutant transformed with MPK6AEF (alanine (A)-glutamic acid (E)-phenylanine (F)) showed a root phenotype similar to that of yda1 demonstrated that MPK6 is an important player downstream of YODA. These data indicate that YODA and MPK6 are involved in post-embryonic root development through an auxin-dependent mechanism regulating cell division and mitotic microtubule (PPB and phragmoplast) organization.

Arabidopsis thaliana mitogen-activated protein kinase 6 is involved in seed formation and modulation of primary and lateral root development

Mitogen-activated protein kinase (MAPKs) cascades are signal transduction modules highly conserved in all eukaryotes regulating various aspects of plant biology, including stress responses and developmental programmes. In this study, we characterized the role of MAPK 6 (MPK6) in Arabidopsis embryo development and in post-embryonic root system architecture. We found that the mpk6 mutation caused altered embryo development giving rise to three seed phenotypes that, post-germination, correlated with alterations in root architecture. In the smaller seed class, mutant seedlings failed to develop the primary root, possibly as a result of an earlier defect in the division of the hypophysis cell during embryo development, but they had the capacity to develop adventitious roots to complete their life cycle. In the larger class, the MPK6 loss of function did not cause any evident alteration in seed morphology, but the embryo and the mature seed were bigger than the wild type. Seedlings developed from these bigger seeds were characterized by a primary root longer than that of the wild type, accompanied by significantly increased lateral root initiation and more and longer root hairs. Apparently, the increment in primary root growth resulted from an enhanced cell production and cell elongation. Our data demonstrated that MPK6 plays an important role during embryo development and acts as a repressor of primary and lateral root development.

MAPK signaling in plant hormone ethylene signal transduction

The signal transduction pathway of the plant stress and defense hormone, ethylene, has been extensively elucidated using the plant genetic model Arabidopsis over the last two decades. Among others, a MAPKKK CTR1 was identified as a negative regulator that has led to the speculation of MAPK involvement in ethylene signaling. However, it remained unclear how the MAPK modules acting downstream of the receptors to mediate ethylene signaling. We have recently presented new evidence that the MKK9-MPK3/6 modules identified by combined functional genomic and genetic screens mediate ethylene signaling, which is negatively regulated by the genetically identified CTR1-dependent cascades. Our genetic studies show consistently that the MKK9-MPK3/MPK6 modules act downstream of the ethylene receptors. Biochemical and transgenic analyses further demonstrated that the positive-acting and negative-acting MAPK activities are integrated and act simultaneously to control the key transcription factor EIN3 through dual phosphorylations to regulate the EIN3 protein stability and downstream transcription cascades. This study has revealed a novel molecular mechanism that defines the specificity of complex MAPK signaling. Comprehensive elucidation of MAPK cascades and the underlying molecular mechanisms would provide more precise explanations for how plant cells utilize MAPK cascades to control specific downstream outputs in response to distinct stimuli.

Surviving the passage: Non-canonical stromal targeting of an Arabidopsis mitogen-activated protein kinase kinase

In plants, mitogen-activated protein kinases (MAPK) have been implicated in signalling associated with many processes, including cellular differentiation, organ development, cell death and stress/hormone signalling. While MAPK cascades are known to act in the cytosol and the nucleus, sequence analysis of the Arabidopsis MAPK cascade proteins predicts the presence of import signals that would target some of them to other organelles. In vitro uptake experiments confirm the predicted import of an oxidant-responsive MAPKK, AtMKK4, into the chloroplast. Unexpectedly, the imported MKK4 protein was not processed through stromal peptidase-dependent cleavage of the N-terminal signal peptide, thus leaving the pre-protein intact. Nevertheless, the N-terminal region was shown to be essential both for the import process and for the ability of MKK4 to activate its cognate MAPK targets in vivo. MKK4 import also occurred irrespective of the activation status of the kinase. The import of this primarily cytosolic oxidant-stimulated AtMKK4 into the chloroplasts, organelles with high redox fluxes, suggests that one of the functions of MKK4 might be to help coordinate intercompartment responses to cellular redox imbalances.