Cell and Developmental Biology of Plant Mitogen-Activated Protein Kinases

Mitogen-activated protein kinase MxMPK3-2 mediated phosphorylation of MxZR3.1 participates in regulating iron homoeostasis in apple rootstocks

The micronutrient iron plays a crucial role in the growth and development of plants, necessitating meticulous regulation for its absorption by plants. Prior research has demonstrated that the transcription factor MxZR3.1 restricts iron absorption in apple rootstocks; however, the precise mechanism by which MxZR3.1 contributes to the regulation of iron homoeostasis in apple rootstocks remains unexplored. Here, MxMPK3-2, a protein kinase, was discovered to interact with MxZR3.1. Y2H, bimolecular fluorescence complementation and pull down experiments were used to confirm the interaction. Phosphorylation and cell semi-degradation tests have shown that MxZR3.1 can be used as a substrate of MxMPK3-2, which leads to the MxZR3.1 protein being more stable. In addition, through tobacco transient transformation (LUC and GUS) experiments, it was confirmed that MxZR3.1 significantly inhibited the activity of the MxHA2 promoter, while MxMPK3-2 mediated phosphorylation at the Ser94 site of MxZR3.1 further inhibited the activity of the MxHA2 promoter. It is tightly controlled to absorb iron during normal growth and development of apple rootstocks due to the regulatory effect of the MxMPK3-2-MxZR3.1 module on MxHA2 transcription level. Consequently, this research has revealed the molecular basis of how the MxMPK3-2-MxZR3.1 module in apple rootstocks controls iron homoeostasis by regulating the MxHA2 promoter's activity.

Glutamine induces lateral root initiation, stress responses, and disease resistance in Arabidopsis

The production of glutamine (Gln) from NO3- and NH4+ requires ATP, reducing power, and carbon skeletons. Plants may redirect these resources to other physiological processes using Gln directly. However, feeding Gln as the sole nitrogen (N) source has complex effects on plants. Under optimal concentrations, Arabidopsis (Arabidopsis thaliana) seedlings grown on Gln have similar primary root lengths, more lateral roots, smaller leaves, and higher amounts of amino acids and proteins compared to those grown on NH4NO3. While high levels of Gln accumulate in Arabidopsis seedlings grown on Gln, the expression of GLUTAMINE SYNTHETASE1;1 (GLN1;1), GLN1;2, and GLN1;3 encoding cytosolic GS1 increases and expression of GLN2 encoding chloroplastic GS2 decreases. These results suggest that Gln has distinct effects on regulating GLN1 and GLN2 gene expression. Notably, Arabidopsis seedlings grown on Gln have an unexpected gene expression profile. Compared with NH4NO3, which activates growth-promoting genes, Gln preferentially induces stress- and defense-responsive genes. Consistent with the gene expression data, exogenous treatment with Gln enhances disease resistance in Arabidopsis. The induction of Gln-responsive genes, including PATHOGENESIS-RELATED1, SYSTEMIC ACQUIRED RESISTANCE DEFICIENT1, WRKY54, and WALL ASSOCIATED KINASE1, is compromised in salicylic acid (SA) biosynthetic and signaling mutants under Gln treatments. Together, these results suggest that Gln may partly interact with the SA pathway to trigger plant immunity.

Arabidopsis α-Aurora kinase plays a role in cytokinesis through regulating MAP65-3 association with microtubules at phragmoplast midzone

The α-Aurora kinase is a crucial regulator of spindle microtubule organization during mitosis in plants. Here, we report a post-mitotic role for α-Aurora in reorganizing the phragmoplast microtubule array. In Arabidopsis thaliana, α-Aurora relocated from spindle poles to the phragmoplast midzone, where it interacted with the microtubule cross-linker MAP65-3. In a hypomorphic α-Aurora mutant, MAP65-3 was detected on spindle microtubules, followed by a diffuse association pattern across the phragmoplast midzone. Simultaneously, phragmoplast microtubules remained belatedly in a solid disk array before transitioning to a ring shape. Microtubules at the leading edge of the matured phragmoplast were often disengaged, accompanied by conspicuous retentions of MAP65-3 at the phragmoplast interior edge. Specifically, α-Aurora phosphorylated two residues towards the C-terminus of MAP65-3. Mutation of these residues to alanines resulted in an increased association of MAP65-3 with microtubules within the phragmoplast. Consequently, the expansion of the phragmoplast was notably slower compared to wild-type cells or cells expressing a phospho-mimetic variant of MAP65-3. Moreover, mimicking phosphorylation reinstated disrupted MAP65-3 behaviors in plants with compromised α-Aurora function. Overall, our findings reveal a mechanism in which α-Aurora facilitates cytokinesis progression through phosphorylation-dependent restriction of MAP65-3 associating with microtubules at the phragmoplast midzone.

MAPK Cascades in Plant Microbiota Structure and Functioning

Mitogen-activated protein kinase (MAPK) cascades are highly conserved signaling modules that coordinate diverse biological processes such as plant innate immunity and development. Recently, MAPK cascades have emerged as pivotal regulators of the plant holobiont, influencing the assembly of normal plant microbiota, essential for maintaining optimal plant growth and health. In this review, we provide an overview of current knowledge on MAPK cascades, from upstream perception of microbial stimuli to downstream host responses. Synthesizing recent findings, we explore the intricate connections between MAPK signaling and the assembly and functioning of plant microbiota. Additionally, the role of MAPK activation in orchestrating dynamic changes in root exudation to shape microbiota composition is discussed. Finally, our review concludes by emphasizing the necessity for more sophisticated techniques to accurately decipher the role of MAPK signaling in establishing the plant holobiont relationship.

Transcriptional insights of citrus defense response against Diaporthe citri

Citrus melanose, caused by Diaporthe citri, is one of the most important and widespread fungal diseases of citrus. Previous studies demonstrated that the citrus host was able to trigger the defense response to restrict the spread of D. citri. However, the molecular mechanism underlying this defense response has yet to be elucidated. Here, we used RNA-Seq to explore the gene expression pattern at the early (3 days post infection, dpi) and late (14 dpi) infection stages of citrus leaves in response to D. citri infection, and outlined the differences in transcriptional regulation associated with defense responses. The functional enrichment analysis indicated that the plant cell wall biogenesis was significantly induced at the early infection stage, while the callose deposition response was more active at the late infection stage. CYP83B1 genes of the cytochrome P450 family were extensively induced in the callus deposition-mediated defense response. Remarkably, the gene encoding pectin methylesterase showed the highest upregulation and was only found to be differentially expressed at the late infection stage. Genes involved in the synthesis and regulation of phytoalexin coumarin were effectively activated. F6'H1 and S8H, encoding key enzymes in the biosynthesis of coumarins and their derivatives, were more strongly expressed at the late infection stage than at the early infection stage. Collectively, our study profiled the response pattern of citrus leaves against D. citri infection and provided the transcriptional evidence to support the defense mechanism.

Deciphering the regulatory network of the NAC transcription factor FvRIF, a key regulator of strawberry (Fragaria vesca) fruit ripening

The NAC transcription factor ripening inducing factor (RIF) was previously reported to be necessary for the ripening of octoploid strawberry (Fragaria × ananassa) fruit, but the mechanistic basis of RIF-mediated transcriptional regulation and how RIF activity is modulated remains elusive. Here, we show that FvRIF in diploid strawberry, Fragaria vesca, is a key regulator in the control of fruit ripening and that knockout mutations of FvRIF result in a complete block of fruit ripening. DNA affinity purification sequencing coupled with transcriptome deep sequencing suggests that 2,080 genes are direct targets of FvRIF-mediated regulation, including those related to various aspects of fruit ripening. We provide evidence that FvRIF modulates anthocyanin biosynthesis and fruit softening by directly regulating the related core genes. Moreover, we demonstrate that FvRIF interacts with and serves as a substrate of MAP kinase 6 (FvMAPK6), which regulates the transcriptional activation function of FvRIF by phosphorylating FvRIF at Thr-310. Our findings uncover the FvRIF-mediated transcriptional regulatory network in controlling strawberry fruit ripening and highlight the physiological significance of phosphorylation modification on FvRIF activity in ripening.

Genome-Wide Identification of the MPK Gene Family and Expression Analysis under Low-Temperature Stress in the Banana

Mitogen-activated protein kinases (MAPKs and MPKs) are important in the process of resisting plant stress. In this study, 21, 12, 18, 16, and 10 MPKs were identified from Musa acuminata, Musa balbisiana, Musa itinerans, Musa schizocarpa, and Musa textilis, respectively. These MPKs were divided into Group A, B, C, and D. Phylogenetic analysis revealed that this difference in number was due to the gene shrinkage of the Group B subfamily of Musa balbisiana and Musa textilis. KEGG annotations revealed that K14512, which is involved in plant hormone signal transduction and the plant-pathogen interaction, was the most conserved pathway of the MPKs. The results of promoter cis-acting element prediction and focTR4 (Fusarium oxysporum f. sp. cubense tropical race 4) transcriptome expression analysis preliminarily confirmed that MPKs were relevant to plant hormone and biotic stress, respectively. The expression of MPKs in Group A was significantly upregulated at 4 °C, and dramatically, the MPKs in the root were affected by low temperature. miR172, miR319, miR395, miR398, and miR399 may be the miRNAs that regulate MPKs during low-temperature stress, with miR172 being the most critical. miRNA prediction and qRT-PCR results indicated that miR172 may negatively regulate MPKs. Therefore, we deduced that MPKs might coordinate with miR172 to participate in the process of the resistance to low-temperature stress in the roots of the banana. This study will provide a theoretical basis for further analysis of the mechanism of MPKs under low-temperature stress of bananas, and this study could be applied to molecular breeding of bananas in the future.

Comparative transcriptome profiling to unravel the key molecular signalling pathways and drought adaptive plasticity in shoot borne root system of sugarcane

Sugarcane root system comprises of superficial sett roots as well as deeply-penetrating shoot borne roots (SBR) with latter being the permanent root system. In sugarcane, the healthy SBR contributes to a better crop yield and it also helps to produce multiple ratoon crops after the harvest. There is a dearth of in-depth knowledge on SBR system architecture and its functional role in modern day commercial hybrids. A comprehensive phenotypic, anatomical and whole transcriptome profiling, conducted between the commercial sugarcane hybrids and a wild germplasm Erianthus, found a developmental delay in both initiation and establishment of the SBR in commercial hybrid compared to Erianthus. The SBR system in Erianthus proved to be an extensive drought-adaptive root system architecture that significantly contributes to drought tolerance. On the other hand, SBRs in the commercial hybrids showed an irreversible collapse and damage of the root cells under drought stress. The outcomes from the comparative analysis of the transcriptome data showed a significant upregulation of the genes that regulate important stress signalling pathways viz., sugar, calcium, hormone signalling and phenylpropanoid biosynthesis in the SBRs of Erianthus. It was found that through these key signalling pathways, Erianthus SBRs triggered the downstream signalling cascade to impart physiological responses like osmoprotection, modification of the cell walls, detoxification of reactive oxygen species, expression of drought responsive transcription factors, maintenance of cell stability and lateral root development. The current study forms a basis for further exploration of the Shoot Borne Root system as a valuable breeding target to develop drought tolerant sugarcane genotypes.

Transcriptomic analysis of humic acid in relieving the inhibitory effect of high nitrogen on soybean nodulation

Introduction

Nitrogen fertilizer intake promotes soybean growth before the formation of nodules, but excess nitrogen has an inhibitory effect on soybean nodulation. It is important to balance nitrogen levels to meet both growth and nodulation needs.

Methods

the nitrogen level suitable for soybean growth and nodulation was studied, the role of humic acid (HA) in alleviating the inhibition of high nitrogen on soybean nodulation was analyzed, and transcriptomic analysis was performed to understand its mechanism.

Results

The results showed that a lower level of nitrogen with 36.4 mg urea per pot could increase the number of nodules of soybean, and a higher level of nitrogen with 145.9 mg urea per pot (U4 group) had the best growth indicators but inhibited nodulation significantly. HA relieved the inhibitory effect at high nitrogen level, and the number of nodules increased by 122.1% when 1.29 g HA was added (H2 group) compared with the U4 group. The transcriptome analysis was subsequently performed on the H2 and U4 groups, showing that there were 2995 differentially expressed genes (DEGs) on the 25th day, accounting for 6.678% of the total annotated genes (44,848) under the test conditions. These DEGs were enriched in mitogen-activated protein kinase signaling pathway-plant, flavonoid biosynthesis, and plant hormone signal transduction based on the -log10 (P adjusted) value in the Kyoto Encyclopedia of Genes and Genomes pathway (KEGG).

Discussion

HA balanced the nitrogen level through the above pathways in soybean planting to control the number of nodules.

FvWRKY50 is an important gene that regulates both vegetative growth and reproductive growth in strawberry

The WRKY transcription factors play important roles in plant growth and resistance, but only a few members have been identified in strawberry. Here we identified a WRKY transcription factor, FvWRKY50, in diploid strawberry which played essential roles in strawberry vegetative growth, and reproductive growth. Knocking out FvWRKY50 by genome editing accelerated flowering time and leaf senescence but delayed anthocyanin accumulation in fruit. Further analysis showed that FvWRKY50 acted as a transcriptional repressor to negatively regulate the expression of flowering- and leaf senescence-related genes, including FvFT2, FvCO, FvFT3, and FvSAUR36. Notably, FvWRKY50 directly upregulated the expression of FvCHI and FvDFR by binding their promoter under normal conditions, but at low temperature FvWRKY50 was phosphorylated by FvMAPK3 and then induced protein degradation by ubiquitination, delaying anthocyanin accumulation. In addition, the homozygous mutant of FvWRKY50 was smaller while the biallelic mutant showed normal size. These new findings provide important clues for us to further reveal the regulatory mechanisms of strawberry growth and fruit ripening.

The potato StMKK5-StSIPK module enhances resistance to Phytophthora pathogens through activating the salicylic acid and ethylene signalling pathways

Mitogen-activated protein kinase (MAPK) cascades play pivotal roles in plant responses to both biotic and abiotic stress. A screen of a Nicotiana benthamiana cDNA virus-induced gene silencing (VIGS) library for altered plant responses to inoculation with Phytophthora infestans previously identified an NbMKK gene, encoding a clade D MAPKK that we renamed as NbMKK5, which is involved in immunity to P. infestans. To study the role of the potato orthologous gene, referred to as StMKK5, in the response to P. infestans, we transiently overexpressed StMKK5 in N. benthamiana and observed that cell death occurred at 2 days postinfiltration. Silencing of the highly conserved eukaryotic protein SGT1 delayed the StMKK5-induced cell death, whereas silencing of the MAPK-encoding gene NbSIPK completely abolished the cell death response. Further investigations showed that StMKK5 interacts with, and directly phosphorylates, StSIPK. Furthermore, both StMKK5 and StSIPK trigger salicylic acid (SA)- and ethylene (Eth)-related gene expression, and co-expression of the salicylate hydroxylase NahG with the negative regulator of Eth signalling CTR1 hampers StSIPK-triggered cell death. This observation indicates that the cell death triggered by StMKK5-StSIPK is dependent on the combination of SA- and Eth-signalling. By introducing point mutations, we showed that the kinase activity of both StMKK5 and StSIPK is required for triggering cell death. Genetic analysis showed that StMKK5 depends on StSIPK to trigger plant resistance. Thus, our results define a potato StMKK5-SIPK module that positively regulates immunity to P. infestans via activation of both the SA and Eth signalling pathways.

Advanced microscopy resolves dynamic localization patterns of stress-induced mitogen-activated protein kinase SIMK during alfalfa root hair interactions with Ensifer meliloti

Leguminous plants have established a mutualistic endosymbiotic interaction with nitrogen-fixing rhizobia to secure nitrogen sources in new specialised organs called root nodules. Before nodule formation, the development of early symbiotic structures is essential for rhizobia docking, internalization, targeted delivery and intracellular accommodation. We have recently reported that overexpression of stress-induced mitogen-activated protein kinase (SIMK) in alfalfa affects root hair, nodule and shoot formation, which raised the questions how SIMK may modulate these processes. In particular, detailed subcellular spatial distribution, activation and developmental relocation of SIMK during the early stages of alfalfa nodulation remain unclear. Here, we qualitatively and quantitatively characterised SIMK distribution patterns in Ensifer meliloti-infected root hairs using live-cell imaging and immunolocalization, employing alfalfa stable transgenic lines with genetically manipulated SIMK abundance and kinase activity. In the SIMKK-RNAi line, showing downregulation of SIMKK and SIMK, we found considerably decreased accumulation of phosphorylated SIMK around infection pockets and infection threads. However, this was strongly increased in the GFP-SIMK line, constitutively overexpressing GFP-tagged SIMK. Thus, genetically manipulated SIMK modulates root hair capacity to form infection pockets and infection threads. Employment of advanced light-sheet fluorescence microscopy (LSFM) on intact plants allowed gentle and non-invasive imaging of spatiotemporal interactions between root hairs and symbiotic Ensifer meliloti, while immunofluorescence detection confirmed that SIMK was activated in these locations. Our results shed new light on SIMK spatiotemporal participation in early interactions between alfalfa and Ensifer meliloti, and its internalization into root hairs, showing that local accumulation of active SIMK indeed modulates early nodulation in alfalfa.

Harnessing the role of mitogen-activated protein kinases against abiotic stresses in plants

Crop plants are vulnerable to various biotic and abiotic stresses, whereas plants tend to retain their physiological mechanisms by evolving cellular regulation. To mitigate the adverse effects of abiotic stresses, many defense mechanisms are induced in plants. One of these mechanisms is the mitogen-activated protein kinase (MAPK) cascade, a signaling pathway used in the transduction of extracellular stimuli into intercellular responses. This stress signaling pathway is activated by a series of responses involving MAPKKKs→MAPKKs→MAPKs, consisting of interacting proteins, and their functions depend on the collaboration and activation of one another by phosphorylation. These proteins are key regulators of MAPK in various crop plants under abiotic stress conditions and also related to hormonal responses. It is revealed that in response to stress signaling, MAPKs are characterized as multigenic families and elaborate the specific stimuli transformation as well as the antioxidant regulation system. This pathway is directed by the framework of proteins and stopping domains confer the related associates with unique structure and functions. Early studies of plant MAPKs focused on their functions in model plants. Based on the results of whole-genome sequencing, many MAPKs have been identified in plants, such as Arbodiposis, tomato, potato, alfalfa, poplar, rice, wheat, maize, and apple. In this review, we summarized the recent work on MAPK response to abiotic stress and the classification of MAPK cascade in crop plants. Moreover, we highlighted the modern research methodologies such as transcriptomics, proteomics, CRISPR/Cas technology, and epigenetic studies, which proposed, identified, and characterized the novel genes associated with MAPKs and their role in plants under abiotic stress conditions. In-silico-based identification of novel MAPK genes also facilitates future research on MAPK cascade identification and function in crop plants under various stress conditions.

The MEKK1-MKK1/2-MPK4 cascade phosphorylates and stabilizes STOP1 to confer aluminum resistance in Arabidopsis

Aluminum (Al) toxicity can seriously restrict crop production on acidic soils, which comprise 40% of the world's potentially arable land. The zinc finger transcription factor STOP1 has a conserved and essential function in mediating plant Al resistance. Al stress induces STOP1 accumulation via post-transcriptional regulatory mechanisms. However, the upstream signaling pathway involved in Al-triggered STOP1 accumulation remains unclear. Here, we report that the MEKK1-MKK1/2-MPK4 cascade positively regulates STOP1 phosphorylation and stability. Mutations of MEKK1, MKK1/2, or MPK4 lead to decreased STOP1 stability and Al resistance. Al stress induces the kinase activity of MPK4, which interacts with and phosphorylates STOP1. The phosphorylation of STOP1 reduces its interaction with the F-box protein RAE1 that mediates STOP1 degradation, thereby leading to enhanced STOP1 stability and Al resistance. Taken together, our results suggest that the MEKK1-MKK1/2-MPK4 cascade is important for Al signaling and confers Al resistance through phosphorylation-mediated enhancement of STOP1 accumulation in Arabidopsis.

Genome-wide association analysis for drought tolerance and associated traits in faba bean (Vicia faba L.)

Faba bean (Vicia faba L.) is an important high protein legume adapted to diverse climatic conditions with multiple benefits for the overall sustainability of the cropping systems. Plant-based protein demand is being expanded and faba bean is a good candidate to cover this need. However, the crop is very sensitive to abiotic stresses, especially drought, which severely affects faba bean yield and development worldwide. Therefore, identifying genes associated with drought stress tolerance is a major challenge in faba bean breeding. Although the faba bean response to drought stress has been widely studied, the molecular approaches to improve drought tolerance in this crop are still limited. Here we built on recent genomic advances such as the development of the first high-density SNP genotyping array, to conduct a genome-wide association study (GWAS) using thousands of genetic polymorphisms throughout the entire faba bean genome. A worldwide collection of 100 faba bean accessions was grown under control and drought conditions and 10 morphological, phenological and physiological traits were evaluated to identify single nucleotide polymorphism (SNP) markers associated with drought tolerance. We identified 29 SNP markers significantly correlated with these traits under drought stress conditions. The flanking sequences were blasted to the Medicago truncatula reference genomes in order to annotate potential candidate genes underlying the causal variants. Three of the SNPs for chlorophyll content after the stress, correspond to uncharacterized proteins indicating the presence of novel genes associated with drought tolerance in faba bean. The significance of stress-inducible signal transducers provides valuable information on the possible mechanisms underlying the faba bean response to drought stress, thus providing a foundation for future marker-assisted breeding in the crop.

Genome-wide identification of Arabidopsis non-AUG-initiated upstream ORFs with evolutionarily conserved regulatory sequences that control protein expression levels

This study identified four novel regulatory non-AUG-initiated upstream ORFs (uORFs) with evolutionarily conserved sequences in Arabidopsis and elucidated the mechanism by which a non-AUG-initiated uORF promotes main ORF translation. Upstream open reading frames (uORFs) are short ORFs found in the 5'-untranslated regions (5'-UTRs) of eukaryotic transcripts and can influence the translation of protein-coding main ORFs (mORFs). Recent genome-wide ribosome profiling studies have revealed that hundreds or thousands of uORFs initiate translation at non-AUG start codons. However, the physiological significance of these non-AUG uORFs has so far been demonstrated for only a few of them. In this study, to identify physiologically important regulatory non-AUG uORFs in Arabidopsis, we took an approach that combined bioinformatics and experimental analysis. Since physiologically important non-AUG uORFs are likely to be conserved across species, we first searched the Arabidopsis genome for non-AUG-initiated uORFs with evolutionarily conserved sequences. Then, we examined the effects of the conserved non-AUG uORFs on the expression of the downstream mORFs using transient expression assays. As a result, three inhibitory and one promotive non-AUG uORFs were identified. Among the inhibitory non-AUG uORFs, two exerted repressive effects on mORF expression in an amino acid sequence-dependent manner. These two non-AUG uORFs are likely to encode regulatory peptides that cause ribosome stalling, thereby enhancing their repressive effects. In contrast, one of the identified regulatory non-AUG uORFs promoted mORF expression by alleviating the inhibitory effect of a downstream AUG-initiated uORF. These findings provide insights into the mechanisms that enable non-AUG uORFs to play regulatory roles despite their low translation initiation efficiencies.

Knockout of MITOGEN-ACTIVATED PROTEIN KINASE 3 causes barley root resistance against Fusarium graminearum

The roles of mitogen-activated protein kinases (MAPKs) in plant-fungal pathogenic interactions are poorly understood in crops. Here, microscopic, phenotypic, proteomic, and biochemical analyses revealed that roots of independent transcription activator-like effector nuclease (TALEN)-based knockout lines of barley (Hordeum vulgare L.) MAPK 3 (HvMPK3 KO) were resistant against Fusarium graminearum infection. When co-cultured with roots of the HvMPK3 KO lines, F. graminearum hyphae were excluded to the extracellular space, the growth pattern of extracellular hyphae was considerably deregulated, mycelia development was less efficient, and number of appressoria-like structures and their penetration potential were substantially reduced. Intracellular penetration of hyphae was preceded by the massive production of reactive oxygen species (ROS) in attacked cells of the wild-type (WT), but ROS production was mitigated in the HvMPK3 KO lines. Suppression of ROS production in these lines coincided with elevated abundance of catalase (CAT) and ascorbate peroxidase (APX). Moreover, differential proteomic analysis revealed downregulation of several defense-related proteins in WT, and the upregulation of pathogenesis-related protein 1 (PR-1) and cysteine proteases in HvMPK3 KO lines. Proteins involved in suberin formation, such as peroxidases, lipid transfer proteins (LTPs), and the GDSL esterase/lipase (containing "GDSL" aminosequence motif) were differentially regulated in HvMPK3 KO lines after F. graminearum inoculation. Consistent with proteomic analysis, microscopic observations showed enhanced suberin accumulation in roots of HvMPK3 KO lines, most likely contributing to the arrested infection by F. graminearum. These results suggest that TALEN-based knockout of HvMPK3 leads to barley root resistance against Fusarium root rot.

Conuping BSA-Seq and RNA-Seq Reveal the Molecular Pathway and Genes Associated with the Plant Height of Foxtail Millet (Setaria italica)

Foxtail millet (Setaria italica) plays an important role in C4 crop research and agricultural development in arid areas due to its short growth period, drought tolerance, and barren tolerance. Exploration of the dwarfing mechanism and the dwarf genes of foxtail millet can provide a reference for dwarf breeding and dwarf research of other C4 crops. In this study, genetic analysis was performed using phenotypic data, candidate genes were screened by bulk segregant analysis sequencing (BSA-Seq); differentially expressed genes and metabolic pathways in different strains of high samples were analyzed by RNA sequencing (RNA-Seq). The association analysis of BSA-Seq and RNA-Seq further narrowed the candidate range. As a result, a total of three quantitative trait loci (QTLs) and nine candidate genes related to plant height were obtained on chromosomes I and IX. Based on the functional prediction of the candidate genes, we propose a hypothetical mechanism for the formation of millet dwarfing, in which, metabolism and MAPK signaling play important roles in the formation of foxtail millet plant height.

Concerted actions of PRR- and NLR-mediated immunity

Plants utilise cell-surface immune receptors (functioning as pattern recognition receptors, PRRs) and intracellular nucleotide-binding leucine-rich repeat receptors (NLRs) to detect pathogens. Perception of pathogens by these receptors activates immune signalling and resistance to infections. PRR- and NLR-mediated immunity have primarily been considered parallel processes contributing to disease resistance. Recent studies suggest that these two pathways are interdependent and converge at multiple nodes. This review summarises and provides a perspective on these convergent points.

Beauty and the pathogens: A leaf-less control presents a better image of Cymbidium orchids defense strategy

Biological control is a safe way of combating plant diseases using the living organisms. For the precise use of microbial biological control agents, the genetic information on the hypersensitive response (HR), and defense-related gene induction pathways of plants are necessary. Orchids are the most prominent stakeholders of floriculture industry, and owing to their long-awaited flowering pattern, disease control is imperative to allow healthy vegetative growth that spans more than 2 years in most of the orchids. We observed leaf-less flowering in three orchid species (Cymbidium ensifolium, C. goeringii and C. sinense). Using these materials as reference, we performed transcriptome profiling for healthy leaves from non-infected plants to identify genes specifically involved in plant-pathogen interaction pathway. For this pathway, a total of 253 differentially expressed genes (DEGs) were identified in C. ensifolium, 189 DEGs were identified in C. goeringii and 119 DEGs were found in C. sinense. These DEGs were mainly related to bacterial secretion systems, FLS2, CNGCs and EFR, regulating HR, stomatal closure and defense-related gene induction. FLS2 (LRR receptor-like serine/threonine kinase) contained the highest number of DEGs among three orchid species, followed by calmodulin. Highly upregulated gene sets were found in C. sinense as compared to other species. The great deal of DEGs, mainly the FLS2 and EFR families, related to defense and immunity responses can effectively direct the future of biological control of diseases for orchids.

Crystal structure of the phosphorylated Arabidopsis MKK5 reveals activation mechanism of MAPK kinases

The mitogen-activated protein kinase (MAPK) signaling pathways are highly conserved in eukaryotes, regulating various cellular processes. The MAPK kinases (MKKs) are dual specificity kinases, serving as convergence and divergence points of the tripartite MAPK cascades. Here, we investigate the biochemical characteristics and three-dimensional structure of MKK5 in Arabidopsis (AtMKK5). The recombinant full-length AtMKK5 is phosphorylated and can activate its physiological substrate AtMPK6. There is a conserved kinase interacting motif (KIM) at the N-terminus of AtMKK5, indispensable for specific recognition of AtMPK6. The kinase domain of AtMKK5 adopts active conformation, of which the extended activation segment is stabilized by the phosphorylated Ser221 and Thr215 residues. In line with sequence divergence from other MKKs, the αD and αK helices are missing in AtMKK5, suggesting that the AtMKK5 may adopt distinct modes of upstream kinase/substrate binding. Our data shed lights on the molecular mechanisms of MKK activation and substrate recognition, which may help design specific inhibitors targeting human and plant MKKs.

Mucilaginibacter sp. K Improves Growth and Induces Salt Tolerance in Nonhost Plants via Multilevel Mechanisms

Soil salinity negatively modulates plant growth and development, contributing to severe decreases in the growth and production of crops. Mucilaginibacter sp. K is a root endophytic bacterium that was previously reported by our laboratory to stimulate growth and confer salt tolerance in Arabidopsis (Arabidopsis thaliana). The main purpose of the present study is to elucidate the physiological and molecular machinery responsible for the prospective salt tolerance as imparted by Mucilaginibacter sp. K. We first report that auxin, gibberellin, and MPK6 signalings were required for strain K-induced growth promotion and salt tolerance in Arabidopsis. Then, this strain was assessed as a remediation strategy to improve maize performance under salinity stress. Under normal growth conditions, the seed vigor index, nitrogen content, and plant growth were significantly improved in maize. After NaCl exposure, strain K significantly promoted the growth of maize seedlings, ameliorated decline in chlorophyll content and reduced accretion of MDA and ROS compared with the control. The possible mechanisms involved in salt resistance in maize could be the improved activities of SOD and POD (antioxidative system) and SPS (sucrose biosynthesis), upregulated content of total soluble sugar and ABA, and reduced Na⁺ accumulation. These physiological changes were then confirmed by induced gene expression for ion transportation, photosynthesis, ABA biosynthesis, and carbon metabolism. In summary, these results suggest that strain K promotes plant growth through increases in photosynthesis and auxin- and MPK6-dependent pathways; it also bestows salt resistance on plants through protection against oxidative toxicity, Na⁺ imbalance, and osmotic stress, along with the activation of auxin-, gibberellin-, and MPK6-dependent signaling pathways. This is the first detailed report of maize growth promotion by a Mucilaginibacter sp. strain from wild plant. This strain could be used as a favorable biofertilizer and a salinity stress alleviator for maize, with further ascertainment as to its reliability of performance under field conditions and in the presence of salt stress.

Functional analysis of mitogen-activated protein kinases (MAPKs) in potato under biotic and abiotic stress

Biotic and abiotic stresses are the main constrain of potato (Solanum tuberosum L.) production all over the world. To overcome these hurdles, many techniques and mechanisms have been used for increasing food demand for increasing population. One of such mechanism is mitogen-activated protein kinase (MAPK) cascade, which is significance regulators of MAPK pathway under various biotic and abiotic stress conditions in plants. However, the acute role in potato for various biotic and abiotic resistance is not fully understood. In eukaryotes including plants, MAPK transfer information from sensors to responses. In potato, biotic and abiotic stresses, as well as a range of developmental responses including differentiation, proliferation, and cell death in plants, MAPK plays an essential role in transduction of diverse extracellular stimuli. Different biotic and abiotic stress stimuli such as pathogen (bacteria, virus, and fungi, etc.) infections, drought, high and low temperatures, high salinity, and high or low osmolarity are induced by several MAPK cascade and MAPK gene families in potato crop. The MAPK cascade is synchronized by numerous mechanisms, including not only transcriptional regulation but also through posttranscriptional regulation such as protein-protein interactions. In this review, we will discuss the recent detailed functional analysis of certain specific MAPK gene families which are involved in resistance to various biotic and abiotic stresses in potato. This study will also provide new insights into functional analysis of various MAPK gene families in biotic and abiotic stress response as well as its possible mechanism.

Echinacoside Induces Mitochondria-Mediated Pyroptosis through Raf/MEK/ERK Signaling in Non-Small Cell Lung Cancer Cells

Background

Various natural compounds are effective in cancer prevention and treatment with fewer side effects than conventional radiotherapy and chemotherapy. Considering the uncertainty of the antitumor mechanism of Echinacoside (Ech) and the fact that no study on Ech against non-small cell lung cancer (NSCLC) has been explored previously, this study inquired into the anti-NSCLC effect of Ech and explored its potential mechanisms.

Methods

The IC₅₀ to Ech of the NSCLC cells was calculated based on a series of cell viability assays. Different concentrations of Ech were used to treat the cells; the proliferation activity of the cells was evaluated using EdU staining. Mitochondrial membrane potential was detected by JC-1 staining. Levels of cytokines IL-1β and IL-18 were measured by ELISA. GSH and MDA levels were measured by microplate reader. Expression of cytochrome c, NLRP3, caspase-1, IL-1β, c-Myc, c-Fos, and Raf/MEK/ERK pathway proteins was evaluated by western blot. Meanwhile, we used xenograft, immunohistochemical staining, and H&E staining to evaluate the pharmacological effects of Ech in mice in vivo.

Results

ECH inhibited the proliferation of NSCLC cells. Ech increased the expression of pyroptosis-related proteins. Besides, Ech perturbed the mitochondrial membrane potential with the release of mitochondrial cytochrome c, accompanied by increased oxidative stress. Ech inhibited the phosphorylation levels of Raf/MEK/ERK signaling pathway and subsequently reduced c-myc and c-fos protein expression. In addition, Ech effectively restrained the growth of tumors in vivo.

Conclusions

Ech inhibited the Raf/MEK/ERK signaling. Impaired mitochondria activated inflammasome, which in turn led to the pyroptosis of NSCLC cells. These findings can provide some ideas on how to use pyroptosis to treat NSCLC.

Phosphorylation of MdERF17 by MdMPK4 promotes apple fruit peel degreening during light/dark transitions

As apple fruits (Malus domestica) mature, they accumulate anthocyanins concomitantly with losing chlorophyll (Chl); however, the molecular pathways and events that coordinate Chl degradation and fruit coloration have not been elucidated. We showed previously that the transcription factor ETHYLENE RESPONSE FACTOR17 (MdERF17) modulates Chl degradation in apple fruit peels and that variation in the pattern of MdERF17 serine (Ser) residues is responsible for differences in its transcriptional regulatory activity. Here, we report that MdERF17 interacts with and is phosphorylated by MAP KINASE4 (MdMPK4-14G). Phosphorylation of MdERF17 at residue Thr67 by MdMPK4-14G is necessary for its transcriptional regulatory activity and its regulation of Chl degradation. We also show that MdERF17 mutants with different numbers of Ser repeat insertions exhibit altered phosphorylation profiles, with more repeats increasing its interaction with MdMPK4. MdMPK4-14G can be activated by exposure to darkness and is involved in the dark-induced degreening of fruit peels. We also demonstrate that greater phosphorylation of MdERF17 by MdMPK4-14G is responsible for the regulation of Chl degradation during light/dark transitions. Overall, our findings reveal the mechanism by which MdMPK4 controls fruit peel coloration.

MAPK Cascades and Transcriptional Factors: Regulation of Heavy Metal Tolerance in Plants

In nature, heavy metal (HM) stress is one of the most destructive abiotic stresses for plants. Heavy metals produce toxicity by targeting key molecules and important processes in plant cells. The mitogen-activated protein kinase (MAPK) cascade transfers the signals perceived by cell membrane surface receptors to cells through phosphorylation and dephosphorylation and targets various effector proteins or transcriptional factors so as to result in the stress response. Signal molecules such as plant hormones, reactive oxygen species (ROS), and nitric oxide (NO) can activate the MAPK cascade through differentially expressed genes, the activation of the antioxidant system and synergistic crosstalk between different signal molecules in order to regulate plant responses to HMs. Transcriptional factors, located downstream of MAPK, are key factors in regulating plant responses to heavy metals and improving plant heavy metal tolerance and accumulation. Thus, understanding how HMs activate the expression of the genes related to the MAPK cascade pathway and then phosphorylate those transcriptional factors may allow us to develop a regulation network to increase our knowledge of HMs tolerance and accumulation. This review highlighted MAPK pathway activation and responses under HMs and mainly focused on the specificity of MAPK activation mediated by ROS, NO and plant hormones. Here, we also described the signaling pathways and their interactions under heavy metal stresses. Moreover, the process of MAPK phosphorylation and the response of downstream transcriptional factors exhibited the importance of regulating targets. It was conducive to analyzing the molecular mechanisms underlying heavy metal accumulation and tolerance.

Low temperature inhibits anthocyanin accumulation in strawberry fruit by activating FvMAPK3-induced phosphorylation of FvMYB10 and degradation of Chalcone Synthase 1

Low temperature causes poor coloration of strawberry (Fragaria sp.) fruits, thus greatly reducing their commercial value. Strawberry fruits accumulate anthocyanins during ripening, but how low temperature modulates anthocyanin accumulation in plants remains largely unknown. We identified MITOGEN-ACTIVATED PROTEIN KINASE3 (FvMAPK3) as an important negative regulator of anthocyanin accumulation that mediates the poor coloration of strawberry fruits in response to low temperature. FvMAPK3 activity was itself induced by low temperature, leading to the repression of anthocyanin accumulation via two mechanisms. Activated FvMAPK3 acted as the downstream target of MAPK KINASE4 (FvMKK4) and SUCROSE NONFERMENTING1-RELATED KINASE2.6 (FvSnRK2.6) to phosphorylate the transcription factor FvMYB10 and reduce its transcriptional activity. In parallel, FvMAPK3 phosphorylated CHALCONE SYNTHASE1 (FvCHS1) to enhance its proteasome-mediated degradation. These results not only provide an important reference to elucidate the molecular mechanisms underlying low-temperature-mediated repression of anthocyanin accumulation in plants, but also offer valuable candidate genes for generating strawberry varieties with high tolerance to low temperature and good fruit quality.

Effective Use of Water in Crop Plants in Dryland Agriculture: Implications of Reactive Oxygen Species and Antioxidative System

Under dryland conditions, annual and perennial food crops are exposed to dry spells, severely affecting crop productivity by limiting available soil moisture at critical and sensitive growth stages. Climate variability continues to be the primary cause of uncertainty, often making timing rather than quantity of precipitation the foremost concern. Therefore, mitigation and management of stress experienced by plants due to limited soil moisture are crucial for sustaining crop productivity under current and future harsher environments. Hence, the information generated so far through multiple investigations on mechanisms inducing drought tolerance in plants needs to be translated into tools and techniques for stress management. Scope to accomplish this exists in the inherent capacity of plants to manage stress at the cellular level through various mechanisms. One of the most extensively studied but not conclusive physiological phenomena is the balance between reactive oxygen species (ROS) production and scavenging them through an antioxidative system (AOS), which determines a wide range of damage to the cell, organ, and the plant. In this context, this review aims to examine the possible roles of the ROS-AOS balance in enhancing the effective use of water (EUW) by crops under water-limited dryland conditions. We refer to EUW as biomass produced by plants with available water under soil moisture stress rather than per unit of water (WUE). We hypothesize that EUW can be enhanced by an appropriate balance between water-saving and growth promotion at the whole-plant level during stress and post-stress recovery periods. The ROS-AOS interactions play a crucial role in water-saving mechanisms and biomass accumulation, resulting from growth processes that include cell division, cell expansion, photosynthesis, and translocation of assimilates. Hence, appropriate strategies for manipulating these processes through genetic improvement and/or application of exogenous compounds can provide practical solutions for improving EUW through the optimized ROS-AOS balance under water-limited dryland conditions. This review deals with the role of ROS-AOS in two major EUW determining processes, namely water use and plant growth. It describes implications of the ROS level or content, ROS-producing, and ROS-scavenging enzymes based on plant water status, which ultimately affects photosynthetic efficiency and growth of plants.

CsAtf1, a bZIP transcription factor, is involved in fludioxonil sensitivity and virulence in the rubber tree anthracnose fungus Colletotrichum siamense

In phytopathogenic fungi, the HOG MAPK pathway has roles in osmoregulation, fungicide sensitivity, and other processes. The ATF1/CREB-activating transcription factor Atf1 is a regulator that functions downstream of the HOG MAPK pathway. Here, we identified a gene, designated CsAtf1, that encodes a bZIP transcription factor in Colletotrichum siamense, which is the main pathogen that causes Colletotrichum leaf fall disease in rubber trees in China. CsAtf1 localizes to the nucleus. Its mRNA expression correlates positively with that of CsPbs2 and CsHog1 in the HOG MAPK pathway in response to activator (anisomycin), inhibitor (SB203580) and fludioxonil treatments. The CsAtf1 deletion mutant showed slightly retarded mycelial growth, small conidia, slow spore germination, and abnormal appressorium formation. This mutant showed the increased spore germination rate after fludioxonil treatment and more resistance to the fungicide fludioxonil than did the wild-type fungus. However, unlike deletion of Pbs2 or Hog1, which resulted in greater sensitivity to osmotic stress, the CsAtf1 deletion induced slightly increased resistance to osmotic stress and the cell wall stress response. The ΔCsAtf1 strain also exhibited significantly reduced virulence on rubber tree leaves. These data revealed that CsAtf1 plays a key role in the regulation of fludioxonil sensitivity and in pathogenicity regulation in C. siamense.

The involvement of gaseous signaling molecules in plant MAPK cascades: function and signal transduction

This review describes the interaction of gaseous signaling molecules and MAPK cascade components, which further reveals the specific mechanism of the crosstalk between MAPK cascade components and gaseous signaling molecules. Plants have evolved complex and sophisticated mitogen-activated protein kinase (MAPK) signaling cascades that are engaged in response to environmental stress. There is currently compelling experimental evidence that gaseous signaling molecules are involved in MAPK cascades. During stress, nitric oxide (NO) activates MAPK cascades to transmit stimulus signals, and MAPK cascades also regulate NO biosynthesis to mediate NO-dependent physiological processes. Activated MAPK cascades lead to phosphorylation of specific sites of aminocyclopropane carboxylic acid synthase to regulate the ethylene biosynthesis-signaling pathway. Hydrogen sulfide functions upstream of MAPKs and regulates the MAPK signaling pathway at the transcriptional level. Here, we describe the function and signal transduction of gaseous signaling molecules involved in MAPK cascades and focus on introducing and discussing the recent data obtained in this field concerning the interaction of gaseous signaling molecules and MAPK cascades. In addition, this article outlines the direction and challenges of future work and further reveals the specific mechanism of the crosstalk between MAPK cascade components and gaseous signaling molecules.

Characterization of Selected Plant Growth-Promoting Rhizobacteria and Their Non-Host Growth Promotion Effects

Plant growth-promoting rhizobacteria (PGPR) are a functionally diverse group of microbes having immense potential as biostimulants and biopesticides. We isolated four PGPR (designated n, L, K, and Y) that confer growth-promoting effects on Arabidopsis thaliana. The present study describes the detailed polyphasic characterization of these PGPR. Classical methods of bacterial identification and biochemical test kits (API20E, API20NE, API ZYM, and API 50CH) revealed their metabolic versatility. All rhizobacterial isolates were positive for 1-aminocyclopropane-1-carboxylate (ACC) deaminase (ACCD) and indole acetic acid production and phosphorous solubilization. PCR analysis confirmed the presence of the nifH gene in strains n, L, and Y, showing their N2-fixation potential. In vitro dual culture methods and bacterial infestation in planta demonstrated that strains n and L exerted antagonistic effects on Pseudomonas syringae pv. tomato DC3000 and Botrytis cinerea 191 and provided protection to Arabidopsis plants against both phytopathogens. Short- or long-term bacterial treatment revealed significant changes in transcript levels of genes annotated to stress response and hormone metabolism in A. thaliana. In particular, the expression of stress-responsive genes in A. thaliana showed an upregulation under salinity stress. MAP kinase 6 (MPK6) was involved in the growth promotion induced by the four bacterial strains. Furthermore, these strains caused a significant increase in root dry weight of maize seedlings under gnotobiotic conditions. We conclude that the four rhizobacteria are good candidates as biofertilizers for enhancing growth of maize, among which strains n and L showed marked plant growth-promoting attributes and the potential to be exploited as functional biostimulants and biopesticides for sustainable agriculture. IMPORTANCE There are pressing needs to reduce the use of agrochemicals, and PGPR are receiving increasing interest in plant growth promotion and disease protection. This study follows up our previous report that the four newly isolated rhizobacteria promote the growth of Arabidopsis thaliana. We test the hypothesis that they have multiple PGP traits and that they can be used as biofertilizers and biopesticides. In vitro assays indicated that these four strains have various PGP properties related to nutrient availability, stress resistance, and/or pest organism antagonism. They significantly influenced the transcript levels of genes involved in stress response and hormone metabolism in A. thaliana. MPK6 is indispensable to the growth stimulation effects. Strains n and L protected A. thaliana seedlings against phytopathogens. Three strains significantly increased maize growth in vitro. In summary, introducing these four strains onto plant roots provides a benefit to the plants. This is the first study regarding the potential mechanism(s) applied by Mucilaginibacter sp. as biostimulants.

The role of quercetin in plants

Flavonoids are a special category of hydroxylated phenolic compounds having an aromatic ring structure. Quercetin is aspecial subclass of flavonoid. It is a bioactive natural compound built upon the flavon structure nC6(ring A)-C3(ring C)-C6(ring B). Quercetin facilitates several plant physiological processes, such as seed germination, pollen growth, antioxidant machinery, and photosynthesis, as well as induces proper plant growth and development. Quercetin is a powerful antioxidant, so it potently provides plant tolerance against several biotic and abiotic stresses. This review highlights quercetin's role in increasing several physiological and biochemical processes under stress and non-stress environments. Additionally, this review briefly assesses quercetin's role in mitigating biotic and abiotic stresses (e.g., salt, heavy metal, and UV stress). The biosynthesis of flavonoids, their signaling pathways, and quercetin's role in plant signaling are also discussed.

HEAT SHOCK PROTEIN 90 proteins and YODA regulate main body axis formation during early embryogenesis

The YODA (YDA) kinase pathway is intimately associated with the control of Arabidopsis (Arabidopsis thaliana) embryo development, but little is known regarding its regulators. Using genetic analysis, HEAT SHOCK PROTEIN 90 (HSP90) proteins emerge as potent regulators of YDA in the process of embryo development and patterning. This study is focused on the characterization and quantification of early embryonal traits of single and double hsp90 and yda mutants. HSP90s genetic interactions with YDA affected the downstream signaling pathway to control the development of both basal and apical cell lineage of embryo. Our results demonstrate that the spatiotemporal expression of WUSCHEL-RELATED HOMEOBOX 8 (WOX8) and WOX2 is changed when function of HSP90s or YDA is impaired, suggesting their essential role in the cell fate determination and possible link to auxin signaling during early embryo development. Hence, HSP90s together with YDA signaling cascade affect transcriptional networks shaping the early embryo development.

Chromatin phosphoproteomics unravels a function for AT-hook motif nuclear localized protein AHL13 in PAMP-triggered immunity

In many eukaryotic systems during immune responses, mitogen-activated protein kinases (MAPKs) link cytoplasmic signaling to chromatin events by targeting transcription factors, chromatin remodeling complexes, and the RNA polymerase machinery. So far, knowledge on these events is scarce in plants and no attempts have been made to focus on phosphorylation events of chromatin-associated proteins. Here we carried out chromatin phosphoproteomics upon elicitor-induced activation of Arabidopsis The events in WT were compared with those in mpk3, mpk4, and mpk6 mutant plants to decipher specific MAPK targets. Our study highlights distinct signaling networks involving MPK3, MPK4, and MPK6 in chromatin organization and modification, as well as in RNA transcription and processing. Among the chromatin targets, we characterized the AT-hook motif containing nuclear localized (AHL) DNA-binding protein AHL13 as a substrate of immune MAPKs. AHL13 knockout mutant plants are compromised in pathogen-associated molecular pattern (PAMP)-induced reactive oxygen species production, expression of defense genes, and PAMP-triggered immunity. Transcriptome analysis revealed that AHL13 regulates key factors of jasmonic acid biosynthesis and signaling and affects immunity toward Pseudomonas syringae and Botrytis cinerea pathogens. Mutational analysis of the phosphorylation sites of AHL13 demonstrated that phosphorylation regulates AHL13 protein stability and thereby its immune functions.

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

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

Initiation and amplification of SnRK2 activation in abscisic acid signaling

The phytohormone abscisic acid (ABA) is crucial for plant responses to environmental challenges. The SNF1-regulated protein kinase 2s (SnRK2s) are key components in ABA-receptor coupled core signaling, and are rapidly phosphorylated and activated by ABA. Recent studies have suggested that Raf-like protein kinases (RAFs) participate in ABA-triggered SnRK2 activation. In vitro kinase assays also suggest the existence of autophosphorylation of SnRK2s. Thus, how SnRK2 kinases are quickly activated during ABA signaling still needs to be clarified. Here, we show that both B2 and B3 RAFs directly phosphorylate SnRK2.6 in the kinase activation loop. This transphosphorylation by RAFs is essential for SnRK2 activation. The activated SnRK2s then intermolecularly trans-phosphorylate other SnRK2s that are not yet activated to amplify the response. High-order Arabidopsis mutants lacking multiple B2 and B3 RAFs show ABA hyposensitivity. Our findings reveal a unique initiation and amplification mechanism of SnRK2 activation in ABA signaling in higher plants.

Cross Inhibition of MPK10 and WRKY10 Participating in the Growth of Endosperm in Arabidopsis thaliana

The product of double fertilization produces seed, which contains three components: triploid endosperm, diploid embryo, and maternal seed coat. Amongst them, the endosperm plays a crucial role in coordinating seed growth. Mitogen-activated protein kinase (MAPK) cascades are conserved in eukaryotes and involved in signal transduction of plant development. MPK3, MPK6, and MPK10 form a small group of MPKs family in Arabidopsis thaliana. MPK3 and MPK6 are extensively studied and were found to be involved in diverse processes including plant reproduction. However, less is known about the function of MPK10. Here, we found WRKY10/MINI3, a member of HAIKU (IKU) pathway engaging in endosperm development, and MPK10 is high-specifically expressed in the early developmental endosperm but with opposite gradients. We further proved that MPK10 and WRKY10 cross-inhibit the expression of each other. The inhibition effect of MPK10 on gene expression of WRKY10 and the downstream targets is supported by the fact that MPK10 interacts with WRKY10 and suppresses the transcriptional activity of WRKY10. Constantly, mpk10 mutants produce big seeds while WRKY10/MINI3 positively regulate seed growth. Altogether, our data provides a model of WRKY10 and MPK10 regulating endosperm development with a unique cross inhibitory mechanism.

The intracellular ROS accumulation in elicitor-induced immunity requires the multiple organelle-targeted Arabidopsis NPK1-related protein kinases

Recognition at the plasma membrane of danger signals (elicitors) belonging to the classes of the microbe/pathogen- and damage-associated molecular patterns is a key event in pathogen sensing by plants and is associated with a rapid activation of immune responses. Different cellular compartments, including plasma membrane, chloroplasts, nuclei and mitochondria, are involved in the immune cellular program. However, how pathogen sensing is transmitted throughout the cell remains largely to be uncovered. Arabidopsis NPK1-related Proteins (ANPs) are mitogen-activated protein kinase kinase kinases previously shown to have a role in immunity. In this article, we studied the in vivo intracellular dynamics of ANP1- and ANP3-GFP fusions and found that under basal physiological conditions both proteins are present in the cytosol, while ANP3 is also localized in mitochondria. After elicitor perception, both proteins are present also in the plastids and nuclei, revealing a localization pattern that is so far unique. The N-terminal region of the protein kinases is responsible for their localization in mitochondria and plastids. Moreover, we found that the localization of ANPs coincides with the sites of elicitor-induced ROS accumulation and that plants lacking ANP function do not accumulate intracellular ROS.

Restructuring the Cellular Responses: Connecting Microbial Intervention With Ecological Fitness and Adaptiveness to the Maize (Zea mays L.) Grown in Saline-Sodic Soil

Salt stress hampers plant growth and development. It is now becoming one of the most important threats to agricultural productivity. Rhizosphere microorganisms play key roles in modulating cellular responses and enable plant tolerant to salt stress, but the detailed mechanisms of how this occurs need in-depth investigation. The present study elucidated that the microbe-mediated restructuring of the cellular responses leads to ecological fitness and adaptiveness to the maize (Zea mays L.) grown in saline-sodic soil. In the present study, effects of seed biopriming with B. safensis MF-01, B. altitudinis MF-15, and B. velezensis MF-08 singly and in consortium on different growth parameters were recorded. Soil biochemical and enzymatic analyses were performed. The activity and gene expression of High-Affinity K+ Transporter (ZmHKT-1), Sodium/Hydrogen exchanger 1 (zmNHX1), and antioxidant enzymes (ZmAPX1.2, ZmBADH-1, ZmCAT, ZmMPK5, ZmMPK7, and ZmCPK11) were studied. The expression of genes related to lateral root development (ZmHO-1, ZmGSL-1, and ZmGSL-3) and root architecture were also carried out. Seeds bioprimed with consortium of all three strains have been shown to confer increased seed germination (23.34-26.31%) and vigor indices (vigor index I: 38.71-53.68% and vigor index II: 74.11-82.43%) as compared to untreated control plant grown in saline-sodic soil at 30 days of sowing. Results indicated that plants treated with consortium of three strains induced early production of adventitious roots (tips: 4889.29, forks: 7951.57, and crossings: 2296.45) in maize compared to plants primed with single strains and untreated control (tips: 2019.25, forks: 3021.45, and crossings: 388.36), which was further confirmed by assessing the transcript level of ZmHO-1 (7.20 folds), ZmGSL-1 (4.50 folds), and ZmGSL-3 (12.00 folds) genes using the qPCR approach. The uptake and translocation of Na+, K+, and Ca2+ significantly varied in the plants treated with bioagents alone or in consortium. qRT-PCR analysis also revealed that the ZmHKT-1 and zmNHX1 expression levels varied significantly in the maize root upon inoculation and showed a 6- to 11-fold increase in the plants bioprimed with all the three strains in combination. Further, the activity and gene expression levels of antioxidant enzymes were significantly higher in the leaves of maize subjected seed biopriming with bioagents individually or in combination (3.50- to 12.00-fold). Our research indicated that ZmHKT-1 and zmNHX1 expression could effectively enhance salt tolerance by maintaining an optimal Na+/K+ balance and increasing the antioxidant activity that keeps reactive oxygen species at a low accumulation level. Interestingly, up-regulation of ZmHKT-1, NHX1, ZmHO-1, ZmGSL-1, and ZmGSL-3 and genes encoding antioxidants regulates the cellular responses that could effectively enhance the adaptiveness and ultimately leads to better plant growth and grain production in the maize crop grown in saline-sodic soil.

Update on the Roles of Rice MAPK Cascades

The mitogen-activated protein kinase (MAPK) cascades have been validated playing critical roles in diverse aspects of plant biology, from growth and developmental regulation, biotic and abiotic stress responses, to phytohormone signal transduction or responses. A classical MAPK cascade consists of a MAPK kinase kinase (MAPKKK), a MAPK kinase (MAPKK), and a MAPK. From the 75 MAPKKKs, eight MAPKKs, and 15 MAPKs of rice, a number of them have been functionally deciphered. Here, we update recent advances in knowledge of the roles of rice MAPK cascades, including their components and complicated action modes, their diversified functions controlling rice growth and developmental responses, coordinating resistance to biotic and abiotic stress, and conducting phytohormone signal transduction. Moreover, we summarize several complete MAPK cascades that harbor OsMAPKKK-OsMAPKK-OsMAPK, their interaction with different upstream components and their phosphorylation of diverse downstream substrates to fulfill their multiple roles. Furthermore, we state a comparison of networks of rice MAPK cascades from signal transduction crosstalk to the precise selection of downstream substrates. Additionally, we discuss putative concerns for elucidating the underlying molecular mechanisms and molecular functions of rice MAPK cascades in the future.

Plant Mitogen-Activated Protein Kinase Cascades in Environmental Stresses

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

Signaling Toward Reactive Oxygen Species-Scavenging Enzymes in Plants

Reactive oxygen species (ROS) are signaling molecules essential for plant responses to abiotic and biotic stimuli as well as for multiple developmental processes. They are produced as byproducts of aerobic metabolism and are affected by adverse environmental conditions. The ROS content is controlled on the side of their production but also by scavenging machinery. Antioxidant enzymes represent a major ROS-scavenging force and are crucial for stress tolerance in plants. Enzymatic antioxidant defense occurs as a series of redox reactions for ROS elimination. Therefore, the deregulation of the antioxidant machinery may lead to the overaccumulation of ROS in plants, with negative consequences both in terms of plant development and resistance to environmental challenges. The transcriptional activation of antioxidant enzymes accompanies the long-term exposure of plants to unfavorable environmental conditions. Fast ROS production requires the immediate mobilization of the antioxidant defense system, which may occur via retrograde signaling, redox-based modifications, and the phosphorylation of ROS detoxifying enzymes. This review aimed to summarize the current knowledge on signaling processes regulating the enzymatic antioxidant capacity of plants.

Exogenous NO induces apoptosis of hepatocellular carcinoma cells via positive p38/JNK signaling pathway and negative ERK signaling pathways

JS-K as an exogenous NO donor could release NO after activation by glutathione S-transferases (GSTs). The present study explores the effects of JS-K on MAPK pathway in HepG2 and Bel-7402 cells. JS-K significantly prompted apoptosis and SB203580 (a p38 inhibitor) and SP600125 (a JNK inhibitor) prior to JS-K could partly reverse apoptosis and activation of cleaved-caspase-3 and cleaved PARP. However, U0126 (a MEK inhibitor) strengthened the cell apoptosis and the expressions of cleaved-caspase-3 and cleaved PARP. JS-K caused phosphorylation of p38 MAPK and JNK but attenuated phosphorylation of ERK, which were reversed by Carboxy-PTIO (a NO scavenger). Meanwhile, the phosphorylation of HSP27, c-JUN and ATF-2 were activated in JS-K-treated cells. SB203580 and SP600125 could attenuate phosphorylation of p38 MAPK and JNK, respectively. The phosphorylation in downstream substrates of p38 MAPK and JNK was also abolished by SB203580 and SP600125 in JS-K-treated cells. Additionally, JS-K decreased phosphorylation of c-Raf, which subsequently caused a decrease of MEK1/2 phosphorylation. Several downstream targets of ERK1/2 including p90RSK and transcription factors (e.g., Elk-1, c-Myc and c-Fos) were inhibited. U0126 potentiated JS-K-induced inhibitory effect of Raf/MEK/ERK pathway. The same results were also observed in the downstream substrates of ERK1/2 including p90RSK, Elk-1, c-Myc and c-Fos. Moreover, Carboxy-PTIO abolished the inhibitory effect of Raf/MEK/ERK pathway triggered by JS-K. Finally, JS-K significantly suppressed the growth of rat primary hepatic carcinoma via MAPK pathway in vivo. Taken together, JS-K can induce hepatocellular carcinoma cells apoptosis through its activation of JNK and p38 MAPK and inactivation of Raf/MEK/ERK signaling 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.

Receptor-Like Protein Kinases Function Upstream of MAPKs in Regulating Plant Development

Mitogen-activated protein kinases (MAPKs) are a group of protein kinase broadly involved in various signal pathways in eukaryotes. In plants, MAPK cascades regulate growth, development, stress responses and immunity by perceiving signals from the upstream regulators and transmitting the phosphorylation signals to the downstream signaling components. To reveal the interactions between MAPK cascades and their upstream regulators is important for understanding the functional mechanisms of MAPKs in the life span of higher plants. Typical receptor-like protein kinases (RLKs) are plasma membrane-located to perceive endogenous or exogenous signal molecules in regulating plant growth, development and immunity. MAPK cascades bridge the extracellular signals and intracellular transcription factors in many RLK-mediated signaling pathways. This review focuses on the current findings that RLKs regulate plant development through MAPK cascades and discusses questions that are worth investigating in the near future.

HSP90 chaperones regulate stomatal differentiation under normal and heat stress conditions

Stomatal development is tightly connected with the overall plant growth, while changes in environmental conditions, like elevated temperature, affect negatively stomatal formation. Stomatal ontogenesis follows a well-defined series of cell developmental transitions in the cotyledon and leaf epidermis that finally lead to the production of mature stomata. YODA signaling cascade regulates stomatal development mainly through the phosphorylation and inactivation of SPEECHLESS (SPCH) transcription factor, while HSP90 chaperones have a central role in the regulation of YODA cascade. Here, we report that acute heat stress affects negatively stomatal differentiation, leads to high phosphorylation levels of MPK3 and MPK6, and alters the expression of SPCH and MUTE transcription factors. Genetic depletion of HSP90 leads to decreased stomatal differentiation rates. Thus, HSP90 chaperones safeguard the completion of distinct stomatal differentiation steps depending on these two transcription factors under normal and heat stress conditions.

A Kinase-Phosphatase-Transcription Factor Module Regulates Adventitious Root Emergence in Arabidopsis Root-Hypocotyl Junctions

Adventitious roots form from non-root tissues as part of normal development or in response to stress or wounding. The root primordia form in the source tissue, and during emergence the adventitious roots penetrate the inner cell layers and the epidermis; however, the mechanisms underlying this emergence remain largely unexplored. Here, we report that a regulatory module composed of the AP2/ERF transcription factor ABSCISIC ACID INSENSITIVE 4 (ABI4), the MAP kinases MPK3 and MPK6, and the phosphatase PP2C12 plays an important role in the emergence of junction adventitious roots (J-ARs) from the root-hypocotyl junctions in Arabidopsis thaliana. ABI4 negatively regulates J-AR emergence, preventing the accumulation of reactive oxygen species and death of epidermal cells, which would otherwise facilitate J-AR emergence. Phosphorylation by MPK3/MPK6 activates ABI4 and dephosphorylation by PP2C12 inactivates ABI4. MPK3/MPK6 also directly phosphorylate and inactivate PP2C12 during J-AR emergence. We propose that this "double-check" mechanism increases the robustness of MAP kinase signaling and finely regulates the local programmed cell death required for J-AR emergence.

The Function of MAPK Cascades in Response to Various Stresses in Horticultural Plants

The mitogen-activated protein kinase (MAPK) cascade is a highly conserved signaling transduction module that transduces extracellular stimuli into intracellular responses in plants. Early studies of plant MAPKs focused on their functions in model plants. Based on the results of whole-genome sequencing, many MAPKs have been identified in horticultural plants, such as tomato and apple. Recent studies revealed that the MAPK cascade also plays crucial roles in the biotic and abiotic stress responses of horticultural plants. In this review, we summarize the composition and classification of MAPK cascades in horticultural plants and recent research on this cascade in responses to abiotic stresses (such as drought, extreme temperature and high salinity) and biotic stresses (such as pathogen infection). In addition, we discuss the most advanced research themes related to plant MAPK cascades, thus facilitating research on MAPK cascade functions in horticultural plants.