Activation of a mitogen-activated protein kinase pathway is involved in disease resistance in tobacco

The Apple Mitogen-Activated Protein Kinase MdMAPK6 Increases Drought, Salt, and Disease Resistance in Plants

As sessile organisms, plants are exposed to a variety of environmental stresses caused by biotic and abiotic factors during their life cycle; as a result, plants have evolved complex defense mechanisms to cope with these stresses, among which the mitogen-activated protein kinase cascade signaling pathway is particularly critical. This study focused on MdMAPK6, a specific mitogen-activated protein kinase gene in Malus domestica, to illuminate its functions in stress responses. MdMAPK6 was successfully cloned from apple and shown to respond to various stressors, including drought, salt, and abscisic acid. Overexpressing MdMAPK6 in apple calli resulted in enhanced resistance to drought, salt, and Botryosphaeria dothidea. Ectopic expression of MdMAPK6 in Arabidopsis thaliana enhanced the resistance to drought, salt, and Pseudomonas syringae pathovar tomato DC3000. These results indicated that MdMAPK6 in apples is a traditional mitogen-activated protein kinase, which plays an important role in both biotic and abiotic stresses.

Alginate oligosaccharides trigger multiple defense responses in tobacco and induce resistance to Phytophthora infestans

Alginate oligosaccharides (AOSs), important plant immunity inducers, are widely used in agriculture because of their important role in the biological control of crop diseases. However, the mechanism by which AOSs induce plant resistance to pathogens is not clear. Here, we report AOS with a degree of polymerization of 2-5, which was obtained by a newly reported enzyme Aly2. AOS treatment exhibited high activity in enhancing resistance to Phytophthora infestans (P. infestans). AOS significantly induced reactive oxygen species (ROS) accumulation, calcium influx, stomata closure, and callose deposition. The salicylic acid (SA) synthesis-related gene and the defense-related genes were upregulated after AOS treatment. A transcriptome file generated from AOS-treated seedlings verified the SA pathway and suggested the presence of chitin elicitor receptor kinase (CERK). The subsequent results showed that AtCERK1 binds AOS tightly, suggesting that AtCERK1 is responsible for AOS recognition. This study laid a theoretical foundation for the broad application of AOS.

Identification of the fructose 1,6-bisphosphate aldolase (FBA) family genes in maize and analysis of the phosphorylation regulation of ZmFBA8

Fructose 1,6-bisphosphate aldolase (FBA) is a class of aldolase that functions as enzyme participating in carbohydrate metabolism of the Calvin-Benson cycle, gluconeogenesis, and glycolysis, and also as non-enzymatic protein involving in protein binding, gene transcription, signal transduction. FBAs have been identified in a few plant species, however, limited information is known regarding FBA family genes, their biological functions and posttranslational regulations in maize (Zea mays). In this study, nine class I FBAs (ZmFBA1 to ZmFBA9) and one class II FBA (ZmFBA10) in maize were identified. Phosphoproteomic analysis further revealed that multiple ZmFBAs were phosphorylated. We showed that phosphorylation at Ser32 in ZmFBA8 inhibited its FBP binding and enzyme activity. Loss of ZmFBA8 function reduced the growth of maize seedlings. Our results suggest that the phosphorylation is an important regulatory mechanism of ZmFBA8 function.

Chemical-sensitized MITOGEN-ACTIVATED PROTEIN KINASE 4 provides insights into its functions in plant growth and immunity

Two mitogen-activated protein kinase (MAPK) cascades with MPK4 and MPK3/MPK6 as the bottommost kinases are key to plant growth/development and immune signaling. Disruption of the MPK4 cascade leads to severe dwarfism and autoimmunity, complicating the study of MPK4 in plant growth/development and immunity. In this study, we successfully rescued the Arabidopsis (Arabidopsis thaliana) mpk4 mutant using a chemical-sensitized MPK4 variant, MPK4YG, creating a conditional activity-null mpk4 mutant named MPK4SR (genotype: PMPK4:MPK4YG mpk4) that could be used to examine the functions of MPK4 in plant growth/development and immunity. We discovered that the duration of the loss of MPK4 activity is important to plant immune responses. Short-term loss of MPK4 activity did not impact flg22-induced ROS burst or resistance against Pseudomonas syringae (Pst). Enhanced Pst resistance was only observed in the MPK4SR plants with stunted growth following prolonged inhibition of MPK4 activity. Transcriptome analyses in plants with short-term loss of MPK4 activity revealed a vital role of MPK4 in regulating several housekeeping processes, including mitosis, transcription initiation, and cell wall macromolecule catabolism. Furthermore, the constitutive weak activation of MPK4GA in the MPK4CA plants (genotype: PMPK4:MPK4GA mpk4) led to early flowering and premature senescence, which was associated with its compromised resistance against Pst. These findings suggest that MPK4 plays important roles in plant growth and development and in maintaining the delicate balance between growth/development and immune adaptation in plants.

Phosphorylation of auxin signaling repressor IAA8 by heat-responsive MPKs causes defective flower development

Heat stress is a substantial and imminent threat to plant growth and development. Understanding its adverse effects on plant development at the molecular level is crucial for sustainable agriculture. However, the molecular mechanism underlying how heat stress causes developmental defects in flowers remains poorly understood. Here, we identified Indole-3-Acetic Acid 8 (IAA8), a repressor of auxin signaling, as a substrate of mitogen-activated protein kinases (MPKs) in Arabidopsis thaliana, and found that MPK-mediated phosphorylation of IAA8 inhibits flower development. MPKs phosphorylated three residues of IAA8: S74, T77, and S135. Interestingly, transgenic plants overexpressing a phospho-mimicking mutant of IAA8 (IAA8DDD OX) exhibited defective flower development due to high IAA8 levels. Furthermore, MPK-mediated phosphorylation inhibited IAA8 polyubiquitination, thereby significantly increasing its stability. Additionally, the expression of key transcription factors involved in flower development, such as bZIP and MYB genes, was significantly perturbed in the IAA8DDD OX plants. Collectively, our study demonstrates that heat stress inhibits flower development by perturbing the expression of flower development genes through the MPK-mediated phosphorylation of IAA8, suggesting that Aux/IAA phosphorylation enables plants to fine-tune their development in response to environmental stress.

Dual roles of the MPK3 and MPK6 mitogen-activated protein kinases in regulating Arabidopsis stomatal development

An Arabidopsis (Arabidopsis thaliana) mitogen-activated protein kinase (MAPK) cascade composed of YODA (YDA)-MKK4/MKK5-MPK3/MPK6 plays an essential role downstream of the ERECTA (ER)/ER-LIKE (ERL) receptor complex in regulating stomatal development in the leaf epidermis. STOMAGEN (STO), a peptide ligand produced in mesophyll cells, competes with EPIDERMAL PATTERNING FACTOR2 (EPF2) for binding ER/ERL receptors to promote stomatal formation. In this study, we found that activation of MPK3/MPK6 suppresses STO expression. Using MUTE and STO promoters that confer epidermis- and mesophyll-specific expression, respectively, we generated lines with cell-specific activation and suppression of MPK3/MPK6. The activation or suppression of MPK3/MPK6 in either epidermis or mesophyll cells is sufficient to alter stomatal differentiation. Epistatic analyses demonstrated that STO overexpression can rescue the suppression of stomatal formation conferred by the mesophyll-specific expression of the constitutively active MKK4DD or MKK5DD, but not by the epidermis-specific expression of these constitutively active MKKs. These data suggest that STO is downstream of MPK3/MPK6 in mesophyll cells, but upstream of MPK3/MPK6 in epidermal cells in stomatal development signaling. This function of the MPK3/MPK6 cascade allows it to coordinate plant epidermis development based on its activity in mesophyll cells during leaf development.

Functional characterization of WsPR-1 reveals its interplay with cytokinin and gibberellin signaling pathways

Pathogenesis-related protein 1 (PR-1) is an antimicrobial protein involved in systemic acquired resistance (SAR) in plants, but its regulatory role and interactions with other pathways remain unclear. In this study, we functionally characterize WsPR-1 gene of Withania somnifera in Nicotiana tabacum to elucidate its role in plant defense, growth, and development. Interestingly, transgenic tobacco plants with increased levels of cytokinin (CK) and decreased gibberellins (GAs) exhibited stunted shoot growth, an underdeveloped root system, modified leaf morphology, reduced seed pod production, and delayed leaf senescence. Transcriptional analysis revealed that WsPR-1 overexpression downregulated the GA 20-oxidase (GA20ox) gene involved in GA biosynthesis while upregulating GA 2-oxidase (GA2ox), a GA catabolic enzyme. Moreover, transcript levels of FRUITFULL (FUL) and LEAFY (NFL2) flowering genes exhibited a decrease in WsPR-1 plants, which could explain the delayed flowering and reduced seed pod development in transgenic plants. Confocal microscopy confirmed increased lignin deposition in stem cross-sections of WsPR-1 transgenic plants, supported by gene expression analysis and lignin content quantification. Additionally, our findings also suggest the involvement of Knotted1-like homeobox (KNOX) gene in enhancing cytokinin levels. This study highlights PR-1's regulatory role in plant growth and development, with potential to boost crop yields and enhance resilience.

NADPH oxidase-mediated sulfenylation of cysteine derivatives regulates plant immunity

Reactive oxygen species (ROS) are rapidly generated during plant immune responses by respiratory burst oxidase homolog (RBOH), which is a plasma membrane-localized NADPH oxidase. Although regulatory mechanisms of RBOH activity have been well documented, the ROS-mediated downstream signaling is unclear. We here demonstrated that ROS sensor proteins play a central role in ROS signaling via oxidative post-translational modification of cysteine residues, sulfenylation. To detect protein sulfenylation, we used dimedone, which specifically and irreversibly binds to sulfenylated proteins. The sulfenylated proteins were labeled by dimedone in Nicotiana benthamiana leaves, and the conjugates were detected by immunoblot analyses. In addition, a reductant dissociated H2O2-induced conjugates, suggesting that cysteine persulfide and/or polysulfides are involved in sulfenylation. These sulfenylated proteins were continuously increased during both pattern-triggered immunity (PTI) and effector-triggered immunity (ETI) in a RBOH-dependent manner. Pharmacological inhibition of ROS sensor proteins by dimedone perturbated cell death, ROS accumulation induced by INF1 and MEK2DD, and defense against fungal pathogens. On the other hand, Rpi-blb2-mediated ETI responses were enhanced by dimedone. These results suggest that the sulfenylation of cysteine and its derivatives in various ROS sensor proteins are important events downstream of the RBOH-dependent ROS burst to regulate plant immune responses.

Molecular Insights into the Defense of Dioscorea opposita Cultivar Tiegun Callus Against Pathogenic and Endophytic Fungal Infection Through Transcriptome Analysis

Dioscorea opposita cultivar Tiegun is an economically important crop with high nutritional and medicinal value. Plants can activate complex and diverse defense mechanisms after infection by pathogenic fungi. Moreover, endophytic fungi can also trigger the plant immune system to resist pathogen invasion. However, the study of the effects of endophytic fungi on plant infection lags far behind that of pathogenic fungi, and the underlying mechanism is not fully understood. Here, the black spot pathogen Alternaria alternata and the endophytic fungus Penicillium halotolerans of Tiegun were identified and used to infect calli. The results showed that A. alternata could cause more severe membrane lipid peroxidation, whereas P. halotolerans could rapidly increase the activity of the plant antioxidant enzymes superoxide dismutase, peroxidase, and catalase; thus, the degree of damage to the callus caused by P. halotolerans was weaker than that caused by A. alternata. RNA sequencing analysis revealed that various plant defense pathways, such as phenylpropanoid biosynthesis, flavonoid biosynthesis, plant hormone signal transduction, and the mitogen-activated protein kinase signaling pathway, play important roles in triggering the plant immune response during fungal infection. Furthermore, the tryptophan metabolism, betalain biosynthesis, fatty acid degradation, flavonoid biosynthesis, tyrosine metabolism, and isoquinoline alkaloid biosynthesis pathways may accelerate the infection of pathogenic fungi, and the ribosome biogenesis pathway in eukaryotes may retard the damage caused by endophytic fungi. This study lays a foundation for exploring the infection mechanism of yam pathogens and endophytic fungi and provides insight for effective fungal disease control in agriculture.

MAPK20-mediated ATG6 phosphorylation is critical for pollen development in Solanum lycopersicum L

In flowering plants, male gametogenesis is tightly regulated by numerous genes. Mitogen-activated protein kinase (MAPK) plays a critical role in plant development and stress response, while its role in plant reproductive development is largely unclear. The present study demonstrated MAPK20 phosphorylation of ATG6 to mediate pollen development and germination in tomato (Solanum lycopersicum L.). MAPK20 was preferentially expressed in the stamen of tomato, and mutation of MAPK20 resulted in abnormal pollen grains and inhibited pollen viability and germination. MAPK20 interaction with ATG6 mediated the formation of autophagosomes. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis showed that ATG6 was phosphorylated by MAPK20 at Ser-265. Mutation of ATG6 in wild-type (WT) or in MAPK20 overexpression plants resulted in malformed and inviable pollens. Meanwhile, the number of autophagosomes in mapk20 and atg6 mutants was significantly lower than that of WT plants. Our results suggest that MAPK20-mediated ATG6 phosphorylation and autophagosome formation are critical for pollen development and germination.

HvMPK4 phosphorylates HvWRKY1 to enhance its suppression of barley immunity to powdery mildew fungus

Mitogen-activated protein kinase (MAPK) cascades play important roles in disease resistance in model plant species. However, the functions of MAPK signaling pathways in crop disease resistance are largely unknown. Here we report the function of HvMKK1-HvMPK4-HvWRKY1 module in barley immune system. HvMPK4 is identified to play a negative role in barley immune response against Bgh, as virus-induced gene silencing of HvMPK4 results in enhanced disease resistance whilst stably overexpressing HvMPK4 leads to super-susceptibility to Bgh infection. Furthermore, the barley MAPK kinase HvMKK1 is found to specifically interact with HvMPK4, and the activated HvMKK1DD variant specifically phosphorylates HvMPK4 in vitro. Moreover, the transcription factor HvWRKY1 is identified to be a downstream target of HvMPK4 and phosphorylated by HvMPK4 in vitro in the presence of HvMKK1DD. Phosphorylation assay coupled with mutagenesis analyses identifies S122, T284, and S347 in HvWRKY1 as the major residues phosphorylated by HvMPK4. HvWRKY1 is phosphorylated in barley at the early stages of Bgh infection, which enhances its suppression on barley immunity likely due to enhanced DNA-binding and transcriptional repression activity. Our data suggest that the HvMKK1-HvMPK4 kinase pair acts upstream of HvWRKY1 to negatively regulate barley immunity against powdery mildew.

Defense signaling pathways in resistance to plant viruses: Crosstalk and finger pointing

Resistance to infection by plant viruses involves proteins encoded by plant resistance (R) genes, viz., nucleotide-binding leucine-rich repeats (NLRs), immune receptors. These sensor NLRs are activated either directly or indirectly by viral protein effectors, in effector-triggered immunity, leading to induction of defense signaling pathways, resulting in the synthesis of numerous downstream plant effector molecules that inhibit different stages of the infection cycle, as well as the induction of cell death responses mediated by helper NLRs. Early events in this process involve recognition of the activation of the R gene response by various chaperones and the transport of these complexes to the sites of subsequent events. These events include activation of several kinase cascade pathways, and the syntheses of two master transcriptional regulators, EDS1 and NPR1, as well as the phytohormones salicylic acid, jasmonic acid, and ethylene. The phytohormones, which transit from a primed, resting states to active states, regulate the remainder of the defense signaling pathways, both directly and by crosstalk with each other. This regulation results in the turnover of various suppressors of downstream events and the synthesis of various transcription factors that cooperate and/or compete to induce or suppress transcription of either other regulatory proteins, or plant effector molecules. This network of interactions results in the production of defense effectors acting alone or together with cell death in the infected region, with or without the further activation of non-specific, long-distance resistance. Here, we review the current state of knowledge regarding these processes and the components of the local responses, their interactions, regulation, and crosstalk.

Pseudomonas syringae Type III Secretion Protein HrpP Manipulates Plant Immunity To Promote Infection

The bacterial plant pathogen Pseudomonas syringae deploys a type III secretion system (T3SS) to deliver effector proteins into plant cells to facilitate infection, for which many effectors have been characterized for their interactions. However, few T3SS Hrp (hypersensitive response and pathogenicity) proteins from the T3SS secretion apparatus have been studied for their direct interactions with plants. Here, we show that the P. syringae pv. tomato DC3000 T3SS protein HrpP induces host cell death, suppresses pattern-triggered immunity (PTI), and restores the effector translocation ability of the hrpP mutant. The hrpP-transgenic Arabidopsis lines exhibited decreased PTI responses to flg22 and elf18 and enhanced disease susceptibility to P. syringae pv. tomato DC3000. Transcriptome analysis reveals that HrpP sensing activates salicylic acid (SA) signaling while suppressing jasmonic acid (JA) signaling, which correlates with increased SA accumulation and decreased JA biosynthesis. Both yeast two-hybrid and bimolecular fluorescence complementation assays show that HrpP interacts with mitogen-activated protein kinase kinase 2 (MKK2) on the plant membrane and in the nucleus. The HrpP truncation HrpP1-119, rather than HrpP1-101, retains the ability to interact with MKK2 and suppress PTI in plants. In contrast, HrpP1-101 continues to cause cell death and electrolyte leakage. MKK2 silencing compromises SA signaling but has no effect on cell death caused by HrpP. Overall, our work highlights that the P. syringae T3SS protein HrpP facilitates effector translocation and manipulates plant immunity to facilitate bacterial infection. IMPORTANCE The T3SS is required for the virulence of many Gram-negative bacterial pathogens of plants and animals. This study focuses on the sensing and function of the T3SS protein HrpP during plant interactions. Our findings show that HrpP and its N-terminal truncation HrpP1-119 can interact with MKK2, promote effector translocation, and manipulate plant immunity to facilitate bacterial infection, highlighting the P. syringae T3SS component involved in the fine-tuning of plant immunity.

Two Candidate Meloidogyne javanica Effector Genes, MjShKT and MjPUT3: A Functional Investigation of Their Roles in Regulating Nematode Parasitism

During parasitism, root-knot nematode Meloidogyne spp. inject molecules termed effectors that have multifunctional roles in construction and maintenance of nematode feeding sites. As an outcome of transcriptomic analysis of Meloidogyne javanica, we identified and characterized two differentially expressed genes encoding the predicted proteins MjShKT, carrying a Stichodactyla toxin (ShKT) domain, and MjPUT3, carrying a ground-like domain, both expressed during nematode parasitism of the tomato plant. Fluorescence in-situ hybridization revealed expression of MjShKT and MjPUT3 in the dorsal esophageal glands, suggesting their injection into host cells. MjShKT expression was upregulated during the parasitic life stages, to a maximum at the mature female stage, whereas MjPUT3 expression increased in third- to fourth-stage juveniles. Subcellular in-planta localization of MjShKT and MjPUT3 using a fused fluorescence marker indicated MjShKT co-occurrence with the endoplasmic reticulum, the perinuclear endoplasmatic reticulum, and the Golgi organelle markers, while MjPUT3 localized, to some extent, within the endoplasmatic reticulum and was clearly observed within the nucleoplasm. MjShKT inhibited programmed cell death induced by overexpression of MAPKKKα and Gpa2/RBP-1. Overexpression of MjShKT in tomato hairy roots allowed an increase in nematode reproduction, as indicated by the high number of eggs produced on roots overexpressing MjShKT. Roots overexpressing MjPUT3 were characterized by enhanced root growth, with no effect on nematode development on those roots. Investigation of the two candidate effectors suggested that MjShKT is mainly involved in manipulating the plant effector-triggered immune response toward establishment and maintenance of active feeding sites, whereas MjPUT3 might modulate roots morphology in favor of nematode fitness in the host roots. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Natural immunity stimulation using ELICE16INDURES® plant conditioner in field culture of soybean

Recently, climate change has had an increasing impact on the world. Innate defense mechanisms operating in plants - such as PAMP-triggered Immunity (PTI) - help to reduce the adverse effects caused by various abiotic and biotic stressors. In this study, the effects of ELICE16INDURES® plant conditioner for organic farming, developed by the Research Institute for Medicinal Plants and Herbs Ltd. Budakalász Hungary, were studied in a soybean population in Northern Hungary. The active compounds and ingredients of this product were selected in such a way as to facilitate the triggering of general plant immunity without the presence and harmful effects of pathogens, thereby strengthening the healthy plant population and preparing it for possible stress effects. In practice, treatments of this agent were applied at two different time points and two concentrations. The conditioning effect was well demonstrated by using agro-drone and ENDVI determination in the soybean field. The genetic background of healthier plants was investigated by NGS sequencing, and by the expression levels of genes encoding enzymes involved in the catalysis of metabolic pathways regulating PTI. The genome-wide transcriptional profiling resulted in 13 contigs related to PAMP-triggered immunity and activated as a result of the treatments. Further analyses showed 16 additional PTI-related contigs whose gene expression changed positively as a result of the treatments. The gene expression values of genes encoded in these contigs were determined by in silico mRNA quantification and validated by RT-qPCR. Both - relatively low and high treatments - showed an increase in gene expression of key genes involving AOC, IFS, MAPK4, MEKK, and GST. Transcriptomic results indicated that the biosyntheses of jasmonic acid (JA), salicylic acid (SA), phenylpropanoid, flavonoid, phytoalexin, and cellular detoxification processes were triggered in the appropriate molecular steps and suggested that plant immune reactions may be activated also artificially, and innate immunity can be enhanced with proper plant biostimulants.

Genome-wide association study uncovers major genetic loci associated with flowering time in response to active accumulated temperature in wild soybean population

Flowering time and active accumulated temperature (AAT) are two key factors that limit the expanded production especially for soybean across different regions. Wild soybean provides an important germplasm for functional genomics study in cultivar soybean. However, the studies on genetic basis underlying flowering time in response to AAT especially in wild soybean were rarely reported. In this study, we used 294 wild soybean accessions derived from major soybean production region characterized by different AAT in Northeast of China. Based on genome-wide association study (GWAS), we identified 96 SNPs corresponded to 342 candidate genes that significantly associated with flowering time recorded in two-year experiments. Gene Ontology enrichment analysis suggests that the pathways of photosynthesis light reaction and actin filament binding were significantly enriched. We found three lead SNPs with -log10(p-value) > 32 across the two-year experiments, i.e., Chr02:9490318, Chr04:8545910 and Chr09:49553555. Linkage disequilibrium block analysis shows 28 candidate genes within the genomic region centered on the lead SNPs. Among them, expression levels of three genes (aspartic peptidase 1, serine/threonine-protein kinase and protein SCAR2-like) were significantly differed between two subgroups possessing contrasting flowering time distributed at chromosome 2, 4 and 9, respectively. There are 6, 7 and 3 haplotypes classified on the coding regions of the three genes, respectively. Collectively, accessions with late flowering time phenotype are typically derived from AAT zone 1, which is associated with the haplotypic distribution and expression levels of the three genes. This study provides an insight into a potential mechanism responsible for flowering time in response to AAT in wild soybean, which could promote the understanding of genetic basis for other major crops.

Phosphorylation of the auxin signaling transcriptional repressor IAA15 by MPKs is required for the suppression of root development under drought stress in Arabidopsis

Since plants are sessile organisms, developmental plasticity in response to environmental stresses is essential for their survival. Upon exposure to drought, lateral root development is suppressed to induce drought tolerance. However, the molecular mechanism by which the development of lateral roots is inhibited by drought is largely unknown. In this study, the auxin signaling repressor IAA15 was identified as a novel substrate of mitogen-activated protein kinases (MPKs) and was shown to suppress lateral root development in response to drought through stabilization by phosphorylation. Both MPK3 and MPK6 directly phosphorylated IAA15 at the Ser-2 and Thr-28 residues. Transgenic plants overexpressing a phospho-mimicking mutant of IAA15 (IAA15^DD OX) showed reduced lateral root development due to a higher accumulation of IAA15. In addition, MPK-mediated phosphorylation strongly increased the stability of IAA15 through the inhibition of polyubiquitination. Furthermore, IAA15^DD OX plants showed the transcriptional downregulation of two key transcription factors LBD16 and LBD29, responsible for lateral root development. Overall, this study provides the molecular mechanism that explains the significance of the MPK-Aux/IAA module in suppressing lateral root development in response to drought.

Cell death signalling is competitively but coordinately regulated by repressor-type and activator-type ethylene response factors in tobacco (Nicotiana tabacum) plants

Ethylene response factors (ERFs) comprise one of the largest transcription factor families in many plant species. Tobacco (Nicotiana tabacum) ERF3 (NtERF3) and other ERF-associated amphiphilic repression (EAR) motif-containing ERFs are known to function as transcriptional repressors. NtERF3 and several repressor-type ERFs induce cell death in tobacco leaves and are also associated with a defence response against tobacco mosaic virus (TMV). We investigated whether transcriptional activator-type NtERFs function together with NtERF3 in the defence response against TMV infection by performing transient ectopic expression, together with gene expression, chromatin immunoprecipitation (ChIP) and promoter analyses. Transient overexpression of NtERF2 and NtERF4 induced cell death in tobacco leaves, albeit later than that induced by NtERF3. Fusion of the EAR motif to the C-terminal end of NtERF2 and NtERF4 abolished their cell death-inducing ability. The expression of NtERF2 and NtERF4 was upregulated at the early phase of N gene-triggered hypersensitive response (HR) against TMV infection. The cell death phenotype induced by overexpression of wild-type NtERF2 and NtERF4 was suppressed by co-expression of an EAR motif-deficient form of NtERF3. Furthermore, ChIP and promoter analyses suggested that NtERF2, NtERF3 and NtERF4 positively or negatively regulate the expression of NtERF3 by binding to its promoter region. Overall, our results revealed the cell death-inducing abilities of genes encoding activator-type NtERFs, including NtERF2 and NtERF4, suggesting that the HR-cell death signalling via the repressor-type NtERF3 is competitively but coordinately regulated by these NtERFs.

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.

Thirty years of resistance: Zig-zag through the plant immune system

Understanding the plant immune system is crucial for using genetics to protect crops from diseases. Plants resist pathogens via a two-tiered innate immune detection-and-response system. The first plant Resistance (R) gene was cloned in 1992 . Since then, many cell-surface pattern recognition receptors (PRRs) have been identified, and R genes that encode intracellular nucleotide-binding leucine-rich repeat receptors (NLRs) have been cloned. Here, we provide a list of characterized PRRs and NLRs. In addition to immune receptors, many components of immune signaling networks were discovered over the last 30 years. We review the signaling pathways, physiological responses, and molecular regulation of both PRR- and NLR-mediated immunity. Recent studies have reinforced the importance of interactions between the two immune systems. We provide an overview of interactions between PRR- and NLR-mediated immunity, highlighting challenges and perspectives for future research.

Regulation of Arabidopsis Matrix Metalloproteinases by Mitogen-Activated Protein Kinases and Their Function in Leaf Senescence

Leaf senescence is a developmentally programmed cell death process that is influenced by a variety of endogenous signals and environmental factors. Here, we report that MPK3 and MPK6, two Arabidopsis mitogen-activated protein kinases (MAPKs or MPKs), and their two upstream MAPK kinases (MAPKKs or MKKs), MKK4 and MKK5, are key regulators of leaf senescence. Weak induction of constitutively active MAPKKs driven by steroid-inducible promoter, which activates endogenous MPK3 and MPK6, induces leaf senescence. This gain-of-function phenotype requires functional endogenous MPK3 and MPK6. Furthermore, loss of function of both MKK4 and MKK5 delays leaf senescence. Expression profiling leads to the identification of matrix metalloproteinases (MMPs), a family of zinc- and calcium-dependent endopeptidases, as the downstream target genes of MPK3/MPK6 cascade. MPK3/MPK6 activation-triggered leaf senescence is associated with rapid and strong induction of At3-MMP and At2-MMP. Expression of Arabidopsis MMP genes is strongly induced during leaf senescence, qualifying them as senescence-associated genes (SAGs). In addition, either constitutive or inducible overexpression of At3-MMP is sufficient to trigger leaf senescence. Based on these findings, we conclude that MPK3/MPK6 MAPK cascade and MMP target genes further downstream are involved in regulating leaf senescence in Arabidopsis.

Plant immune networks

Plants have both cell-surface and intracellular receptors to recognize diverse self- and non-self molecules. Cell-surface pattern recognition receptors (PRRs) recognize extracellular pathogen-/damage-derived molecules or apoplastic pathogen-derived effectors. Intracellular nucleotide-binding leucine-rich repeat proteins (NLRs) recognize pathogen effectors. Activation of both PRRs and NLRs elevates defense gene expression and accumulation of the phytohormone salicylic acid (SA), which results in SA-dependent transcriptional reprogramming. These receptors, together with their coreceptors, form networks to mediate downstream immune responses. In addition, cell-surface and intracellular immune systems are interdependent and function synergistically to provide robust resistance against pathogens. Here, we summarize the interactions between these immune systems and attempt to provide a holistic picture of plant immune networks. We highlight current challenges and discuss potential new research directions.

BnaA03.MKK5-BnaA06.MPK3/BnaC03.MPK3 Module Positively Contributes to Sclerotinia sclerotiorum Resistance in Brassica napus

Brassica napus (oilseed rape) is one of the most important oil crops worldwide, but its growth is seriously threatened by Sclerotinia sclerotiorum. The mechanism of oilseed rape response to this pathogen has rarely been studied. Here, it was identified that BnaA03.MKK5 whose expression was induced by S. sclerotiorum infection was involved in plant immunity. BnaA03.MKK5 overexpression lines exhibited decreased disease symptoms compared to wild-type plants, accompanied by the increased expression of camalexin-biosynthesis-related genes, including BnPAD3 and BnCYP71A13. In addition, two copies of BnMPK3 (BnA06.MPK3 and BnC03.MPK3) were induced by Sclerotinia incubation, and BnaA03.MKK5 interacted with BnaA06.MPK3/BnaC03.MPK3 in yeast. These interactions were confirmed using in vivo co-immunoprecipitation assays. In vitro phosphorylation assays showed that BnaA06.MPK3 and BnaC03.MPK3 were the direct phosphorylation substrates of BnaA03.MKK5. The transgenic oilseed rape plants including BnaA06.MPK3 and BnaC03.MPK3 overexpression lines and BnMPK3 gene editing lines mediated by CRISPR/Cas9 were generated; the results of the genetic transformation of BnaA06.MPK3/BnaC03.MPK3 indicate that BnMPK3 also has a positive role in Sclerotinia resistance. This study provides information about the potential mechanism of B. napus defense against S. Sclerotiorum mediated by a detailed BnaA03.MKK5-BnaA06.MPK3/BnaC03.MPK3 module.

Coat proteins of necroviruses target 14-3-3a to subvert MAPKKKα-mediated antiviral immunity in plants

Mitogen-activated protein kinase (MAPK) cascades play an important role in innate immunity against various pathogens in plants and animals. However, we know very little about the importance of MAPK cascades in plant defense against viral pathogens. Here, we used a positive-strand RNA necrovirus, beet black scorch virus (BBSV), as a model to investigate the relationship between MAPK signaling and virus infection. Our findings showed that BBSV infection activates MAPK signaling, whereas viral coat protein (CP) counteracts MAPKKKα-mediated antiviral defense. CP does not directly target MAPKKKα, instead it competitively interferes with the binding of 14-3-3a to MAPKKKα in a dose-dependent manner. This results in the instability of MAPKKKα and subversion of MAPKKKα-mediated antiviral defense. Considering the conservation of 14-3-3-binding sites in the CPs of diverse plant viruses, we provide evidence that 14-3-3-MAPKKKα defense signaling module is a target of viral effectors in the ongoing arms race of defense and viral counter-defense.

The hybrid lethality of interspecific F1 hybrids of Nicotiana: a clue to understanding hybrid inviability-a major obstacle to wide hybridization and introgression breeding of plants

Reproductive isolation poses a major obstacle to wide hybridization and introgression breeding of plants. Hybrid inviability in the postzygotic isolation barrier inevitably reduces hybrid fitness, consequently causing hindrances in the establishment of novel genotypes from the hybrids among genetically divergent parents. The idea that the plant immune system is involved in the hybrid problem is applicable to the intra- and/or interspecific hybrids of many different taxa. The lethality characteristics and expression profile of genes associated with the hypersensitive response of the hybrids, along with the suppression of causative genes, support the deleterious epistatic interaction of parental NB-LRR protein genes, resulting in aberrant hyper-immunity reactions in the hybrid. Moreover, the cellular, physiological, and biochemical reactions observed in hybrid cells also corroborate this hypothesis. However, the difference in genetic backgrounds of the respective hybrids may contribute to variations in lethality phenotypes among the parental species combinations. The mixed state in parental components of the chaperone complex (HSP90-SGT1-RAR1) in the hybrid may also affect the hybrid inviability. This review article discusses the facts and hypothesis regarding hybrid inviability, alongside the findings of studies on the hybrid lethality of interspecific hybrids of the genus Nicotiana. A possible solution for averting the hybrid problem has also been scrutinized with the aim of improving the wide hybridization and introgression breeding program in plants.

The mitogen-activated protein kinase kinase TaMKK5 mediates immunity via the TaMKK5-TaMPK3-TaERF3 module

Sharp eyespot disease, caused by the soil-borne fungus Rhizoctonia cerealis, seriously threatens production of wheat (Triticum aestivum). Despite considerable advances in understanding the mechanisms of mitogen-activated protein kinase (MAPK) cascades in innate immunity in model plant species, the roles of MAPK cascades in wheat are unknown. In this study, we identified a wheat MAPK kinase TaMKK5, located on chromosome 6B, and deciphered its functional role in the innate immune responses to R. cerealis attack. The TaMKK5-6B transcript level was elevated after R. cerealis infection and was higher in resistant wheat genotypes compared to susceptible genotypes. Overexpressing TaMKK5-6B increased resistance to sharp eyespot and upregulated the expression of multiple defense-related genes in wheat, including the MAPK gene TaMPK3, the ethylene response factor gene TaERF3, the calcium-dependent protein kinase gene TaCPK7-D, the glutathione s-transferase-1 gene TaGST1, Defensin, and Chitinase 2, while TaMKK5 knock-down compromised the resistance and repressed the expression of these defense-related genes. Bimolecular fluorescence complementation, yeast two-hybrid, pull-down, and phosphorylation assays showed that TaMKK5 physically interacted with TaMPK3, and phosphorylated and activated TaMPK3, and that TaMPK3 interacted with and phosphorylated TaERF3. The TaMKK5-TaMPK3 cascade modulates the expression of TaGST1, Defensin, and Chitinase 2 through TaERF3. Collectively, TaMKK5 mediates resistance to sharp eyespot through the TaMKK5-TaMPK3-TaERF3 module and by upregulating the expression of defense-related genes in wheat. This study provides insights into the role of the wheat MAPK cascades in innate immunity. TaMKK5-6B is a promising gene for breeding wheat cultivars that are resistant to sharp eyespot.

The Plant-Beneficial Rhizobacterium Bacillus velezensis FZB42 Controls the Soybean Pathogen Phytophthora sojae Due to Bacilysin Production

Soybean root rot caused by the oomycete Phytophthora sojae is a serious soilborne disease threatening soybean production in China. Bacillus velezensis FZB42 is a model strain for Gram-positive plant growth-promoting rhizobacteria and is able to produce multiple antibiotics. In this study, we demonstrated that B. velezensis FZB42 can efficiently antagonize P. sojae. The underlying mechanism for the inhibition was then investigated. The FZB42 mutants deficient in the synthesis of lipopeptides (bacillomycin D and fengycin), known to have antifungal activities, and polyketides (bacillaene, difficidin, and macrolactin), known to have antibacterial activities, were not impaired in their antagonism toward P. sojae; in contrast, mutants deficient in bacilysin biosynthesis completely lost their antagonistic activities toward P. sojae, indicating that bacilysin was responsible for the activity. Isolated pure bacilysin confirmed this inference. Bacilysin was previously shown to be antagonistic mainly toward prokaryotic bacteria rather than eukaryotes. Here, we found that bacilysin could severely damage the hyphal structures of P. sojae and lead to the loss of its intracellular contents. A device was invented allowing interactions between P. sojae and B. velezensis FZB42 on nutrient agar. In this manner, the effect of FZB42 on P. sojae was studied by transcriptomics. FZB42 significantly inhibited the expression of P. sojae genes related to growth, macromolecule biosynthesis, pathogenicity, and ribosomes. Among them, the genes for pectate lyase were the most significantly downregulated. Additionally, we showed that bacilysin effectively prevents soybean sprouts from being infected by P. sojae and could antagonize diverse Phytophthora species, such as Phytophthora palmivora, P. melonis, P. capsici, P. litchi, and, most importantly, P. infestans. IMPORTANCEPhytophthora spp. are widespread eukaryotic phytopathogens and often extremely harmful. Phytophthora can infect many types of plants important to agriculture and forestry and thus cause large economic losses. Perhaps due to inappropriate recognition of Phytophthora as a common pathogen in history, research on the biological control of Phytophthora is limited. This study shows that B. velezensis FZB42 can antagonize various Phytophthora species and prevent the infection of soybean seedlings by P. sojae. The antibiotic produced by FZB42, bacilysin, which was already known to have antibacterial effectiveness, is responsible for the inhibitory action against Phytophthora. We further showed that some Phytophthora genes and pathways may be targeted in future biocontrol studies. Therefore, our data provide a basis for the development of new tools for the prevention and control of root and stem rot in soybean and other plant diseases caused by Phytophthora.

Pharmacoinformatics-based investigation of bioactive compounds of Rasam (South Indian recipe) against human cancer

Spice-rich recipes are referred to as "functional foods" because they include a variety of bioactive chemicals that have health-promoting properties, in addition to their nutritional value. Using pharmacoinformatics-based analysis, we explored the relevance of bioactive chemicals found in Rasam (a South Indian cuisine) against oxidative stress-induced human malignancies. The Rasam is composed of twelve main ingredients, each of which contains a variety of bioactive chemicals. Sixty-six bioactive compounds were found from these ingredients, and their structures were downloaded from Pubchem. To find the right target via graph theoretical analysis (mitogen-activated protein kinase 6 (MAPK6)) and decipher their signaling route, a network was built. Sixty-six bioactive compounds were used for in silico molecular docking study against MAPK6 and compared with known MAPK6 inhibitor drug (PD-173955). The top four compounds were chosen for further study based on their docking scores and binding energies. In silico analysis predicted ADMET and physicochemical properties of the selected compounds and were used to assess their drug-likeness. Molecular dynamics (MD) simulation modelling methodology was also used to analyse the effectiveness and safety profile of selected bioactive chemicals based on the docking score, as well as to assess the stability of the MAPK6-ligand complex. Surprisingly, the discovered docking scores against MAPK6 revealed that the selected bioactive chemicals exhibit varying binding ability ranges between - 3.5 and - 10.6 kcal mol-1. MD simulation validated the stability of four chemicals at the MAPK6 binding pockets, including Assafoetidinol A (ASA), Naringin (NAR), Rutin (RUT), and Tomatine (TOM). According to the results obtained, fifty of the sixty-six compounds showed higher binding energy (- 6.1 to - 10.6 kcal mol-1), and four of these compounds may be used as lead compounds to protect cells against oxidative stress-induced human malignancies.

Engineering Resistance Against Viruses in Field Crops Using CRISPR- Cas9

Food security is threatened by various biotic stresses that affect the growth and production of agricultural crops. Viral diseases have become a serious concern for crop plants as they incur huge yield losses. The enhancement of host resistance against plant viruses is a priority for the effective management of plant viral diseases. However, in the present context of the climate change scenario, plant viruses are rapidly evolving, resulting in the loss of the host resistance mechanism. Advances in genome editing techniques, such as CRISPR-Cas9 [clustered regularly interspaced palindromic repeats-CRISPR-associated 9], have been recognized as promising tools for the development of plant virus resistance. CRISPR-Cas9 genome editing tool is widely preferred due to high target specificity, simplicity, efficiency, and reproducibility. CRISPR-Cas9 based virus resistance in plants has been successfully achieved by gene targeting and cleaving the viral genome or altering the plant genome to enhance plant innate immunity. In this article, we have described the CRISPR-Cas9 system, mechanism of plant immunity against viruses and highlighted the use of the CRISPR-Cas9 system to engineer virus resistance in plants. We also discussed prospects and challenges on the use of CRISPR-Cas9-mediated plant virus resistance in crop improvement.

MKK4/5-MPK3/6 Cascade Regulates Agrobacterium-Mediated Transformation by Modulating Plant Immunity in Arabidopsis

Agrobacterium tumefaciens is a specialized plant pathogen that causes crown gall disease and is commonly used for Agrobacterium-mediated transformation. As a pathogen, Agrobacterium triggers plant immunity, which affects transformation. However, the signaling components and pathways in plant immunity to Agrobacterium remain elusive. We demonstrate that two Arabidopsis mitogen-activated protein kinase kinases (MAPKKs) MKK4/MKK5 and their downstream mitogen-activated protein kinases (MAPKs) MPK3/MPK6 play major roles in both Agrobacterium-triggered immunity and Agrobacterium-mediated transformation. Agrobacteria induce MPK3/MPK6 activity and the expression of plant defense response genes at a very early stage. This process is dependent on the MKK4/MKK5 function. The loss of the function of MKK4 and MKK5 or their downstream MPK3 and MPK6 abolishes plant immunity to agrobacteria and increases transformation frequency, whereas the activation of MKK4 and MKK5 enhances plant immunity and represses transformation. Global transcriptome analysis indicates that agrobacteria induce various plant defense pathways, including reactive oxygen species (ROS) production, ethylene (ET), and salicylic acid- (SA-) mediated defense responses, and that MKK4/MKK5 is essential for the induction of these pathways. The activation of MKK4 and MKK5 promotes ROS production and cell death during agrobacteria infection. Based on these results, we propose that the MKK4/5-MPK3/6 cascade is an essential signaling pathway regulating Agrobacterium-mediated transformation through the modulation of Agrobacterium-triggered plant immunity.

Resistance induction based on the understanding of molecular interactions between plant viruses and host plants

BACKGROUND

Viral diseases cause significant damage to crop yield and quality. While fungi- and bacteria-induced diseases can be controlled by pesticides, no effective approaches are available to control viruses with chemicals as they use the cellular functions of their host for their infection cycle. The conventional method of viral disease control is to use the inherent resistance of plants through breeding. However, the genetic sources of viral resistance are often limited. Recently, genome editing technology enabled the publication of multiple attempts to artificially induce new resistance types by manipulating host factors necessary for viral infection.

MAIN BODY

In this review, we first outline the two major (R gene-mediated and RNA silencing) viral resistance mechanisms in plants. We also explain the phenomenon of mutations of host factors to function as recessive resistance genes, taking the eIF4E genes as examples. We then focus on a new type of virus resistance that has been repeatedly reported recently due to the widespread use of genome editing technology in plants, facilitating the specific knockdown of host factors. Here, we show that (1) an in-frame mutation of host factors necessary to confer viral resistance, sometimes resulting in resistance to different viruses and that (2) certain host factors exhibit antiviral resistance and viral-supporting (proviral) properties.

CONCLUSION

A detailed understanding of the host factor functions would enable the development of strategies for the induction of a new type of viral resistance, taking into account the provision of a broad resistance spectrum and the suppression of the appearance of resistance-breaking strains.

Mitogen-Activated Protein Kinase OsMEK2 and OsMPK1 Signaling Is Required for Ferroptotic Cell Death in Rice-Magnaporthe oryzae Interactions

Mitogen-activated protein kinase (MAPK) signaling is required for plant cell death responses to invading microbial pathogens. Iron- and reactive oxygen species (ROS)-dependent ferroptotic cell death occurs in rice (Oryza sativa) during an incompatible rice-Magnaporthe oryzae interaction. Here, we show that rice MAP kinase (OsMEK2 and OsMPK1) signaling cascades are involved in iron- and ROS-dependent ferroptotic cell death responses of rice to M. oryzae infection using OsMEK2 knock-out mutant and OsMEK2 and OsMPK1 overexpression rice plants. The OsMPK1:GFP and OsWRKY90:GFP transcription factor were localized to the nuclei, suggesting that OsMPK1 in the cytoplasm moves into the nuclei to interact with the WRKY90. M. oryzae infection in ΔOsmek2 knock-out plants did not trigger iron and ROS accumulation and lipid peroxidation, and also downregulated OsMPK1, OsWRKY90, OsRbohB, and OsPR-1b expression. However, 35S:OsMEK2 overexpression induced ROS- and iron-dependent cell death in rice. The downstream MAP kinase (OsMPK1) overexpression induced ROS- and iron-dependent ferroptotic cell death response to virulent M. oryzae infection. The small-molecule ferroptosis inhibitor ferrostatin-1 suppressed iron- and ROS-dependent ferroptotic cell death in 35S:OsMPK1 overexpression plants. However, the small-molecule inducer erastin triggered iron- and lipid ROS-dependent, but OsMEK2-independent, ferroptotic cell death during M. oryzae infection. Disease (susceptibility)-related cell death was lipid ROS-dependent, but iron-independent in the ΔOsmek2 knock-out mutant during the late M. oryzae infection stage. These combined results suggest that OsMEK2 and OsMPK1 expression positively regulates iron- and ROS-dependent ferroptotic cell death, and blast disease (susceptibility)-related cell death was ROS-dependent but iron-independent in rice-M. oryzae interactions.

The role of the MAP kinase-kinase protein StMKK1 in potato immunity to different pathogens

Mitogen-activated protein kinase (MAPK) cascades play important roles in plant immunity. Previously, we reported that the potato StMKK1 protein negatively regulates Nicotiana benthamiana resistance to Phytophthora infestans. However, the functions of StMKK1 in potato immunity are unknown. To investigate the roles of StMKK1 in potato resistance to different pathogens, such as the potato late-blight pathogen P. infestans, the bacterial wilt pathogen Ralstonia solanacearum, and the gray-mold fungal pathogen Botrytis cinerea, we generated StMKK1 transgenic lines and investigated the response of potato transformants to destructive oomycete, bacterial, and fungal pathogens. The results showed that overexpression and silencing of StMKK1 do not alter plant growth and development. Interestingly, we found that StMKK1 negatively regulated potato resistance to the hemibiotrophic/biotrophic pathogens P. infestans and R. solanacearum, while it positively regulated potato resistance to the necrotrophic pathogen B. cinerea. Further investigation showed that overexpression of StMKK1 suppressed potato pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and salicylic acid (SA)-related responses, while silencing of StMKK1 enhanced PTI and SA-related immune responses. Taken together, our results showed that StMKK1 plays dual roles in potato defense against different plant pathogens via negative regulation of PTI and SA-related signaling pathways.

Phytophthora infestans RXLR effector PITG20303 targets a potato MKK1 protein to suppress plant immunity

Pathogens secret a plethora of effectors into the host cell to modulate plant immunity. Analysing the role of effectors in altering the function of their host target proteins will reveal critical components of the plant immune system. Here we show that Phytophthora infestans RXLR effector PITG20303, a virulent variant of AVRblb2 (PITG20300) that escapes recognition by the resistance protein Rpi-blb2, suppresses PAMP-triggered immunity (PTI) and promotes pathogen colonization by targeting and stabilizing a potato MAPK cascade protein, StMKK1. Both PITG20300 and PITG20303 target StMKK1, as confirmed by multiple in vivo and in vitro assays, and StMKK1 was shown to be a negative regulator of plant immunity, as determined by overexpression and gene silencing. StMKK1 is a negative regulator of plant PTI, and the kinase activities of StMKK1 are required for its suppression of PTI and effector interaction. PITG20303 depends partially on MKK1, PITG20300 does not depend on MKK1 for suppression of PTI-induced reactive oxygen species burst, while the full virulence activities of nuclear targeted PITG20303 and PITG20300 are dependent on MKK1. Our results show that PITG20303 and PITG20300 target and stabilize the plant MAPK cascade signalling protein StMKK1 to negatively regulate plant PTI response.

Sustained Incompatibility between MAPK Signaling and Pathogen Effectors

In plants, Mitogen-Activated Protein Kinases (MAPKs) are important signaling components involved in developemental processes as well as in responses to biotic and abiotic stresses. In this review, we focus on the roles of MAPKs in Effector-Triggered Immunity (ETI), a specific layer of plant defense responses dependent on the recognition of pathogen effector proteins. Having inspected the literature, we synthesize the current state of knowledge concerning this topic. First, we describe how pathogen effectors can manipulate MAPK signaling to promote virulence, and how in parallel plants have developed mechanisms to protect themselves against these interferences. Then, we discuss the striking finding that the recognition of pathogen effectors can provoke a sustained activation of the MAPKs MPK3/6, extensively analyzing its implications in terms of regulation and functions. In line with this, we also address the question of how a durable activation of MAPKs might affect the scope of their substrates, and thereby mediate the emergence of possibly new ETI-specific responses. By highlighting the sometimes conflicting or missing data, our intention is to spur further research in order to both consolidate and expand our understanding of MAPK signaling in immunity.

Redox Status, JA and ET Signaling Pathway Regulating Responses to Botrytis cinerea Infection Between the Resistant Cucumber Genotype and Its Susceptible Mutant

Botrytis cinerea is an important necrotrophic fungal pathogen with a broad host range and the ability to causing great economic losses in cucumber. However, the resistance mechanism against this pathogen in cucumber was not well understood. In this study, the microscopic observation of the spore growth, redox status measurements and transcriptome analysis were carried out after Botrytis cinerea infection in the resistant genotype No.26 and its susceptible mutant 26M. Results revealed shorter hypha, lower rate of spore germination, less acceleration of H₂O₂, O₂ ^-, and lower total glutathione content (GSH+GSSG) in No.26 than that in 26M, which were identified by the staining result of DAB and NBT. Transcriptome data showed that after pathogen infection, a total of 3901 and 789 different expression genes (DEGs) were identified in No.26 and 26M respectively. These DEGs were highly enriched in redox regulation pathway, hormone signaling pathway and plant-pathogen interaction pathway. The glutathione S-transferase genes, putative peroxidase gene, and NADPH oxidase were up-regulated in No.26 whereas these genes changed little in 26M after Botrytis cinerea infection. Jasmonic acid and ethylene biosynthesis and signaling pathways were distinctively activated in No.26 comparing with 26M upon infection. Much more plant defense related genes including mitogen-activated protein kinases, calmodulin, calmodulin-like protein, calcium-dependent protein kinase, and WRKY transcription factor were induced in No.26 than 26M after pathogen infection. Finally, a model was established which elucidated the resistance difference between resistant cucumber genotype and susceptible mutant after B. cinerea infection.

The wild strawberry kinome: identification, classification and transcript profiling of protein kinases during development and in response to gray mold infection

Background

Protein kinases (PKs) play an important role in signaling cascades and are one of the largest and most conserved protein super families in plants. Despite their importance, the woodland strawberry (Fragaria vesca) kinome and expression patterns of PK genes remain to be characterized.

Results

Here, we report on the identification and classification of 954 Fragaria vesca PK genes, which were classified into nine groups and 124 gene families. These genes were distributed unevenly among the seven chromosomes, and the number of introns per gene varied from 0 to 47. Almost half of the putative PKs were predicted to localize to the nucleus and 24.6% were predicted to localize to the cell membrane. The expansion of the woodland strawberry PK gene family occurred via different duplication mechanisms and tandem duplicates occurred relatively late as compared to other duplication types. Moreover, we found that tandem and transposed duplicated PK gene pairs had undergone stronger diversifying selection and evolved relatively faster than WGD genes. The GO enrichment and transcriptome analysis implicates the involvement of strawberry PK genes in multiple biological processes and molecular functions in differential tissues, especially in pollens. Finally, 109 PKs, mostly the receptor-like kinases (RLKs), were found transcriptionally responsive to Botrytis cinerea infection.

Conclusions

The findings of this research expand the understanding of the evolutionary dynamics of PK genes in plant species and provide a potential link between cell signaling pathways and pathogen attack.

ERECTA1 Acts Upstream of the OsMKKK10-OsMKK4-OsMPK6 Cascade to Control Spikelet Number by Regulating Cytokinin Metabolism in Rice

Grain number is a flexible trait that strongly contributes to grain yield. In rice (Oryza sativa), the OsMKKK10-OsMKK4-OsMPK6 cascade, which is negatively regulated by the dual-specificity phosphatase GSN1, coordinates the trade-off between grain number and grain size. However, the specific components upstream and downstream of the GSN1-MAPK module that regulate spikelet number per panicle remain obscure. Here, we report that ERECTA1 (OsER1), a negative regulator of spikelet number per panicle, acts upstream of the OsMKKK10-OsMKK4-OsMPK6 cascade and that the OsER1-OsMKKK10-OsMKK4-OsMPK6 pathway is required to maintain cytokinin homeostasis. OsMPK6 directly interacts with and phosphorylates the zinc finger transcription factor DST to enhance its transcriptional activation of CYTOKININ OXIDASE2 (OsCKX2), indicating that the OsER1-OsMKKK10-OsMKK4-OsMPK6 pathway shapes panicle morphology by regulating cytokinin metabolism. Furthermore, overexpression of either DST or OsCKX2 rescued the spikelet number phenotype of the oser1, osmkkk10, osmkk4, and osmpk6 mutants, suggesting that the DST-OsCKX2 module genetically functions downstream of the OsER1-OsMKKK10-OsMKK4-OsMPK6 pathway. These findings reveal specific crosstalk between a MAPK signaling pathway and cytokinin metabolism, shedding light on how developmental signals modulate phytohormone homeostasis to shape the inflorescence.

Functional analysis of the SlERF01 gene in disease resistance to S. lycopersici

BACKGROUND

Tomato gray leaf spot caused by Stemphylium lycopersici (S. lycopersici) is a serious disease that can severely hinder tomato production. To date, only Sm has been reported to provide resistance against this disease, and the molecular mechanism underlying resistance to this disease in tomato remains unclear. To better understand the mechanism of tomato resistance to S. lycopersici, real-time quantitative reverse transcription-polymerase chain reaction (qRT-PCR)-based analysis, physiological indexes, microscopy observations and transgenic technology were used in this study.

RESULTS

Our results showed that the expression of SlERF01 was strongly induced by S. lycopersici and by exogenous applications of the hormones salicylic acid (SA) and jasmonic acid (JA). Furthermore, overexpression of SlERF01 enhanced the hypersensitive response (HR) to S. lycopersici and elevated the expression of defense genes in tomato. Furthermore, the accumulation of lignin, callose and hydrogen peroxide (H₂O₂) increased in the transgenic lines after inoculation with S. lycopersici. Taken together, our results showed that SlERF01 played an indispensable role in multiple SA, JA and reactive oxygen species (ROS) signaling pathways to provide resistance to S. lycopersici invasion. Our findings also indicated that SlERF01 could activate the expression of the PR1 gene and enhance resistance to S. lycopersici.

CONCLUSIONS

We identified the SlERF01 gene, which encodes a novel tomato AP2/ERF transcription factor (TF). Functional analysis revealed that SlERF01 positively regulates tomato resistance to S. lycopersici. Our findings indicate that SlERF01 plays a key role in multiple SA, JA and ROS signaling pathways to provide resistance to invasion by S. lycopersici. The findings of this study not only help to better understand the mechanisms of response to pathogens but also enable targeted breeding strategies for tomato resistance to S. lycopersici.

Cloning, molecular and functional characterization by overexpression in Arabidopsis of MAPKK genes from grapevine (Vitis vinifera)

BACKGROUND

The mitogen-activated protein kinases (MAPKs), as a part of the MAPKKK-MAPKK-MAPK cascade, play crucial roles in plant development as an intracellular signal transduction pathway to respond various environmental signals. However, few MAPKK have been functionally characterized in grapevine.

RESULTS

In the study, five MAPKK (MKK) members were identified in grapevine (cultivar 'Pinot Noir'), cloned and designated as VvMKK1-VvMKK5. A phylogenetic analysis grouped them into four sub-families based on the similarity of their conserved motifs and gene structure to Arabidopsis MAPKK members. qRT-PCR results indicated that the expression of VvMKK1, VvMKK2, VvMKK4, and VvMKK5 were up-regulated in mature leaf and young blades, and roots, but exhibited low expression in leaf petioles. VvMKK2, VvMKK3, and VvMKK5 genes were differentially up-regulated when grapevine leaves were inoculated with spores of Erisyphe necator, or treated with salicylic acid (SA), ethylene (ETH), H2O2, or exposed to drought, indicating that these genes may be involved in a variety of signaling pathways. Over expression of VvMKK2 and VvMKK4 genes in transgenic Arabidopsis plants resulted in the production of seeds with a significantly higher germination and survival rate, and better seedling growth under stress conditions than wild-type plants. Overexpression of VvMKK2 in Arabidopsis improved salt and drought stress tolerance while overexpression of VvMKK4 only improved salt stress tolerance.

CONCLUSIONS

Results of the present investigation provide a better understanding of the interaction and function of MAPKKK-MAPKK-MAPK genes at the transcriptional level in grapevine and led to the identification of candidate genes for drought and salt stress in grapes.

In-depth Understanding of Camellia oleifera Self-incompatibility by Comparative Transcriptome, Proteome and Metabolome

Oil-tea tree (Camellia oleifera) is the most important edible oil tree species in China with late-acting self-incompatibility (LSI) properties. The mechanism of LSI is uncertain, which seriously hinders the research on its genetic characteristics, construction of genetic map, selection of cross breeding parents and cultivar arrangement. To gain insights into the LSI mechanism, we performed cytological, transcriptomic, proteomic and metabolomic studies on self- and cross-pollinated pistils. The studies identified 166,591 transcripts, 6851 proteins and 6455 metabolites. Transcriptomic analysis revealed 1197 differentially expressed transcripts between self- and cross-pollinated pistils and 47 programmed cell death (PCD)-control transcripts. Trend analysis by Pearson correlation categorized nine trend graphs linked to 226 differentially expressed proteins and 38 differentially expressed metabolites. Functional enrichment analysis revealed that the LSI was closely associated with PCD-related genes, mitogen-activated protein kinase (MAPK) signaling pathway, plant hormone signal transduction, ATP-binding cassette (ABC) transporters and ubiquitin-mediated proteolysis. These particular trends in transcripts, proteins and metabolites suggested the involvement of PCD in LSI. The results provide a solid genetic foundation for elucidating the regulatory network of PCD-mediated self-incompatibility in C. oleifera.

Apple ALMT9 Requires a Conserved C-Terminal Domain for Malate Transport Underlying Fruit Acidity

Malate accumulation in the vacuole largely determines apple (Malus domestica) fruit acidity, and low fruit acidity is strongly associated with truncation of Ma1, an ortholog of ALUMINUM-ACTIVATED MALATE TRANSPORTER9 (ALMT9) in Arabidopsis (Arabidopsis thaliana). A mutation at base 1,455 in the open reading frame of Ma1 leads to a premature stop codon that truncates the protein by 84 amino acids at its C-terminal end. Here, we report that both the full-length protein, Ma1, and its naturally occurring truncated protein, ma1, localize to the tonoplast; when expressed in Xenopus laevis oocytes and Nicotiana benthamiana cells, Ma1 mediates a malate-dependent inward-rectifying current, whereas the ma1-mediated transmembrane current is much weaker, indicating that ma1 has significantly lower malate transport activity than Ma1. RNA interference suppression of Ma1 expression in 'McIntosh' apple leaves, 'Empire' apple fruit, and 'Orin' apple calli results in a significant decrease in malate level. Genotyping and phenotyping of 186 apple accessions from a diverse genetic background of 17 Malus species combined with the functional analyses described above indicate that Ma1 plays a key role in determining fruit acidity and that the truncation of Ma1 to ma1 is genetically responsible for low fruit acidity in apple. Furthermore, we identified a C-terminal domain conserved in all tonoplast-localized ALMTs essential for Ma1 function; protein truncations into this conserved domain significantly lower Ma1 transport activity. We conclude that the truncation of Ma1 to ma1 reduces its malate transport function by removing a conserved C-terminal domain, leading to low fruit acidity in apple.

Transcriptional induction of capsidiol synthesis genes by wounding can promote pathogen signal-induced capsidiol synthesis

BACKGROUND

Plants are exposed to various forms of environmental stress. Penetration by pathogens is one of the most serious environmental insults. Wounding caused by tissue damage or herbivory also affects the growth and reproduction of plants. Moreover, wounding disrupts physical barriers present at the plant surface and increases the risk of pathogen invasion. Plants cope with environmental stress by inducing a variety of responses. These stress responses must be tightly controlled, because their unnecessary induction is detrimental to plant growth. In tobacco, WIPK and SIPK, two wound-responsive mitogen-activated protein kinases, have been shown to play important roles in regulating wound responses. However, their contribution to downstream wound responses such as gene expression is not well understood.

RESULTS

To identify genes regulated by WIPK and SIPK, the transcriptome of wounded WIPK/SIPK-suppressed plants was analyzed. Among the genes down-regulated in WIPK/SIPK-suppressed plants, the largest group consisted of those involved in the production of antimicrobial phytoalexins. Almost all genes involved in the biosynthesis of capsidiol, a major phytoalexin in tobacco, were transcriptionally induced by wounding in WIPK/SIPK-dependent and -independent manners. 5-epi-aristolochene synthase (EAS) is the committing enzyme for capsidiol synthesis, and the promoter of EAS4, a member of the EAS family, was analyzed. Reporter gene analysis revealed that at least two regions each 40-50 bp length were involved in activation of the EAS4 promoter by wounding, as well as by artificial activation of WIPK and SIPK. Unlike transcripts of the capsidiol synthesis genes, accumulation of EAS protein and capsidiol itself were not induced by wounding; however, wounding significantly enhanced their subsequent induction by a pathogen-derived elicitor.

CONCLUSIONS

Our results suggest a so-called priming phenomenon since the induction of EAS by wounding is only visible at the transcript level. By inducing transcripts, not the proteins, of EAS and possibly other capsidiol synthesis genes at wound sites, plants can produce large quantities of capsidiol quickly if pathogens invade the wound site, whereas plants can minimize energy loss and avoid the cytotoxic effects of capsidiol where pathogens do not gain entry during wound healing.

A human pathogenic bacterium Shigella proliferates in plants through adoption of type III effectors for shigellosis

Shigella, which infects primates, can be transmitted via fresh vegetables; however, its molecular interactions with plants have not been elucidated. Here, we show that four Shigella strains, Shigella boydii, Shigella sonnei, Shigella flexneri 2a, and S. flexneri 5a, proliferate at different levels in Arabidopsis thaliana. Microscopic studies revealed that these bacteria were present inside leaves and damaged plant cells. Green fluorescent protein (GFP)-tagged S. boydii and S. flexneri 5a colonized leaves only, whereas S. flexneri 2a colonized both leaves and roots. Using Shigella mutants lacking type III secretion systems (T3SSs), we found that T3SSs that regulate the pathogenesis of shigellosis in humans also play a central role in bacterial proliferation in Arabidopsis. Strikingly, the immunosuppressive activity of two T3S effectors, OspF and OspG, was required for proliferation of Shigella in Arabidopsis. Of note, delivery of OspF or OspG effectors inside plant cells upon Shigella inoculation was confirmed using a split GFP system. These findings demonstrate that the human pathogen Shigella can proliferate in plants by adapting immunosuppressive machinery used in the original host human.

Identification of Genes/Proteins Related to Submergence Tolerance by Transcriptome and Proteome Analyses in Soybean

Flooding can lead to yield reduction of soybean. Therefore, identification of flooding tolerance genes has great significance in production practice. In this study, Qihuang 34, a highly-resistant variety to flooding stress, was selected for submergence treatments. Transcriptome and proteome analyses were conducted, by which twenty-two up-regulated differentially expressed genes (DEGs)/differentially expressed proteins (DEPs) associated with five KEGG pathways were isolated. The number of the DEGs/DEPs enriched in glycolysis/gluconeogenesis was the highest. Four of these genes were confirmed by RT-qPCR, suggesting that glycolysis/gluconeogenesis may be activated to generate energy for plant survival under anaerobic conditions. Thirty-eight down-regulated DEGs/DEPs associated with six KEGG pathways were identified under submergence stress. Eight DEGs/DEPs enriched in phenylpropanoid biosynthesis were assigned to peroxidase, which catalyzes the conversion of coumaryl alcohol to hydroxy-phenyl lignin in the final step of lignin biosynthesis. Three of these genes were confirmed by RT-qPCR. The decreased expression of these genes led to the inhibition of lignin biosynthesis, which may be the cause of plant softening under submergence stress for a long period of time. This study revealed a number of up-/down-regulated pathways and the corresponding DEGs/DEPs, by which, a better understanding of the mechanisms of submergence tolerance in soybean may be achieved.

Identification of Regulatory Networks of MicroRNAs and Their Targets in Response to Colletotrichum gloeosporioides in Tea Plant (Camellia sinensis L.)

Anthracnose disease is caused by Colletotrichum gloeosporioides, and is common in leaves of the tea plant (Camellia sinensis). MicroRNAs (miRNAs) have been known as key modulators of gene expression in response to environmental stresses, disease resistance, defense responses, and plant immunity. However, the role of miRNAs in responses to C. gloeosporioides remains unexplored in tea plant. Therefore, in the present study, six miRNA sequencing data sets and two degradome data sets were generated from C. gloeosporioides-inoculated and control tea leaves. A total of 485 conserved and 761 novel miRNAs were identified. Of those, 239 known and 369 novel miRNAs exhibited significantly differential expression under C. gloeosporioides stress. One thousand one hundred thirty-four and 596 mRNAs were identified as targets of 389 conserved and 299 novel miRNAs by degradome analysis, respectively. Based on degradome analysis, most of the predicted targets are negatively correlated with their corresponding conserved and novel miRNAs. The expression levels of 12 miRNAs and their targets were validated by quantitative real-time PCR. A negative correlation between expression profiles of five miRNAs (PC-5p-80764_22, csn-miR160c, csn-miR828a, csn-miR164a, and csn-miR169e) and their targets (WRKY, ARF, MYB75, NAC, and NFY transcription factor) was observed. The predicted targets of five interesting miRNAs were further validated through 5'RLM-RACE. Furthermore, Gene Ontology and metabolism pathway analysis revealed that most of the target genes were involved in the regulation of auxin pathway, ROS scavenging pathway, salicylic acid mediated pathway, receptor kinases, and transcription factors for plant growth and development as well as stress responses in tea plant against C. gloeosporioides stress. This study enriches the resources of stress-responsive miRNAs and their targets in C. sinensis and thus provides novel insights into the miRNA-mediated regulatory mechanisms, which could contribute to the enhanced susceptibility of C. gloeosporioides in tea plant.

Comparative Transcriptome Analysis of Gene Expression Patterns in Tomato Under Dynamic Light Conditions

Plants grown under highly variable natural light regimes differ strongly from plants grown under constant light (CL) regimes. Plant phenotype and adaptation responses are important for plant biomass and fitness. However, the underlying regulatory mechanisms are still poorly understood, particularly from a transcriptional perspective. To investigate the influence of different light regimes on tomato plants, three dynamic light (DL) regimes were designed, using a CL regime as control. Morphological, photosynthetic, and transcriptional differences after five weeks of treatment were compared. Leaf area, plant height, shoot /root weight, total chlorophyll content, photosynthetic rate, and stomatal conductance all significantly decreased in response to DL regimes. The biggest expression difference was found between the treatment with the highest light intensity at the middle of the day with a total of 1080 significantly up-/down-regulated genes. A total of 177 common differentially expressed genes were identified between DL and CL conditions. Finally, significant differences were observed in the levels of gene expression between DL and CL treatments in multiple pathways, predominantly of plant–pathogen interactions, plant hormone signal transductions, metabolites, and photosynthesis. These results expand the understanding of plant development and photosynthetic regulations under DL conditions by multiple pathways.

Acquiring control: The evolution of ROS-Induced oxidative stress and redox signaling pathways in plant stress responses

Reactive oxygen species (ROS) - the byproducts of aerobic metabolism - influence numerous aspects of the plant life cycle and environmental response mechanisms. In plants, ROS act like a double-edged sword; they play multiple beneficial roles at low concentrations, whereas at high concentrations ROS and related redox-active compounds cause cellular damage through oxidative stress. To examine the dual role of ROS as harmful oxidants and/or crucial cellular signals, this review elaborates that (i) how plants sense and respond to ROS in various subcellular organelles and (ii) the dynamics of subsequent ROS-induced signaling processes. The recent understanding of crosstalk between various cellular compartments in mediating their redox state spatially and temporally is discussed. Emphasis on the beneficial effects of ROS in maintaining cellular energy homeostasis, regulating diverse cellular functions, and activating acclimation responses in plants exposed to abiotic and biotic stresses are described. The comprehensive view of cellular ROS dynamics covering the breadth and versatility of ROS will contribute to understanding the complexity of apparently contradictory ROS roles in plant physiological responses in less than optimum environments.

Bipartite anchoring of SCREAM enforces stomatal initiation by coupling MAP kinases to SPEECHLESS

Cell fate in eukaryotes is controlled by mitogen-activated protein kinases (MAPKs) that translate external cues into cellular responses. In plants, two MAPKs-MPK3 and MPK6-regulate diverse processes of development, environmental response and immunity. However, the mechanism that bridges these shared signalling components with a specific target remains unresolved. Focusing on the development of stomata-epidermal valves that are essential for gas exchange and transpiration-here, we report that the basic helix-loop-helix protein SCREAM functions as a scaffold that recruits MPK3/6 to downregulate SPEECHLESS, a transcription factor that initiates stomatal cell lineages. SCREAM directly binds to MPK3/6 through an evolutionarily conserved, yet unconventional, bipartite motif. Mutations in this motif abrogate association, phosphorylation and degradation of SCREAM, unmask hidden non-redundancies between MPK3 and MPK6, and result in uncontrolled stomatal differentiation. Structural analyses of MPK6 with a resolution of 2.75 Å showed bipartite binding of SCREAM to MPK6 that is distinct from an upstream MAPKK. Our findings elucidate, at the atomic resolution, the mechanism that directly links extrinsic signals to transcriptional reprogramming during the establishment of stomatal cell fate, and highlight a unique substrate-binding mode adopted by plant MAPKs.

Regulation of GDSL Lipase Gene Expression by the MPK3/MPK6 Cascade and Its Downstream WRKY Transcription Factors in Arabidopsis Immunity

Mitogen-activated protein kinase (MAPK) cascades serve as unified signaling modules in plant development and defense response. Previous reports demonstrated an essential role of Arabidopsis GLIP1, a member of the GDSL-like-motif lipase family, in both local and systemic resistance. GLIP1 expression is highly induced by pathogen attack. However, the one or more signaling pathways involved are unknown. Here, we report that two pathogen-responsive MAPKs, MPK3 and MPK6, are implicated in regulating gene expression of GLIP1 as well as GLIP3 and GLIP4. After gain-of-function activation, MPK3 and MPK6 can strongly induce the expression of GLIP1, GLIP3, and GLIP4. Both GLIP1 and GLIP3 contribute to the plant resistance to Botrytis cinerea. WRKY33, a MPK3/MPK6 substrate, is essential for the MPK3/MPK6-dependent GLIP1 induction. In addition, WRKY2 and WRKY34, two close homologs of WRKY33, have a minor effect in MPK3/MPK6-regulated GLIP1 expression in B. cinerea-infected plants. Chromatin immunoprecipitation-quantitative polymerase chain reaction analysis demonstrated that the GLIP1 gene is a direct target of WRKY33. In addition, we demonstrated that MPK3/MPK6-induced GLIP1 expression is independent of ethylene and jasmonic acid, two important hormones in plant defense. Our results provide insights into the regulation of the GLIP family at the transcriptional level in plant immunity.