Molecular steps in the immune signaling pathway evoked by plant elicitor peptides: Ca2+-dependent protein kinases, nitric oxide, and reactive oxygen species are downstream from the early Ca2+ signal

Carbon dot unravels accumulation of triterpenoid in Evolvulus alsinoides hairy roots culture by stimulating growth, redox reactions and ANN machine learning model prediction of metabolic stress response

Evolvulus alsinoides, a therapeutically valuable shrub can provide consistent supply of secondary metabolites (SM) with pharmaceutical significance. Nonetheless, because of its short life cycle, fresh plant material for research and medicinal diagnostics is severely scarce throughout the year. The effects of exogenous carbon quantum dot (CD) application on metabolic profiles, machine learning (ML) prediction of metabolic stress response, and SM yields in hairy root cultures of E. alsinoides were investigated and quantified. The range of the particle size distribution of the CDs was between 3 and 7 nm. The CDs EPR signal and spin trapping experiments demonstrated the formation of O2-•spin-adducts at (g = 2.0023). Carbon dot treatment increased the levels of hydrogen peroxide and malondialdehyde concentrations as well as increased antioxidant enzyme activity. CD treatments (6 μg mL-1) significantly enhanced the accumulation of squalene and stigmasterol (7 and 5-fold respectively). The multilayer perceptron (MLP) algorithm demonstrated remarkable prediction accuracy (MSE value = 1.99E-03 and R2 = 0.99939) in both the training and testing sets for modelling. Based on the prediction, the maximum oxidative stress index and enzymatic activities were highest in the medium supplemented with 10 μg mL-1 CDs. The outcome of this study indicated that, for the first time, using CD could serve as a novel elicitor for the production of valuable SM. MLP may also be used as a forward-thinking tool to optimize and predict SM with high pharmaceutical significance. This study would be a touchstone for understanding the use of ML and luminescent nanomaterials in the production and commercialization of important SM.

S-nitrosylation of a receptor-like cytoplasmic kinase regulates plant immunity

Perception of pathogen/microbial-associated molecular patterns (P/MAMPs) by plant cell surface receptors leads to a sustained burst of reactive oxygen species (ROS), a key feature of P/MAMP-triggered immunity (PTI). Here we report that P/MAMP recognition leads to a rapid nitrosative burst, initiating the accumulation of nitric oxide (NO), subsequently leading to S-nitrosylation of the receptor-like cytoplasmic kinase (RLCK), botrytis-induced kinase 1 (BIK1), at Cys80. This redox-based, posttranslational modification, promotes the phosphorylation of BIK1, subsequently resulting in BIK1 activation and stabilization. Further, BIK1 S-nitrosylation increases its physical interaction with RBOHD, the source of the apoplastic oxidative burst, promoting ROS formation. Our data identify mechanistic links between rapid NO accumulation and the expression of PTI, providing insights into plant immunity.

Damage-Associated Molecular Patterns and Systemic Signaling

Cellular damage inflicted by wounding, pathogen infection, and herbivory releases a variety of host-derived metabolites, degraded structural components, and peptides into the extracellular space that act as alarm signals when perceived by adjacent cells. These so-called damage-associated molecular patterns (DAMPs) function through plasma membrane localized pattern recognition receptors to regulate wound and immune responses. In plants, DAMPs act as elicitors themselves, often inducing immune outputs such as calcium influx, reactive oxygen species generation, defense gene expression, and phytohormone signaling. Consequently, DAMP perception results in a priming effect that enhances resistance against subsequent pathogen infections. Alongside their established function in local tissues, recent evidence supports a critical role of DAMP signaling in generation and/or amplification of mobile signals that induce systemic immune priming. Here, we summarize the identity, signaling, and synergy of proposed and established plant DAMPs, with a focus on those with published roles in systemic signaling.

The calcium connection: exploring the intricacies of calcium signaling in plant-microbe interactions

The process of plant immune response is orchestrated by intracellular signaling molecules. Since plants are devoid of a humoral system, they develop extensive mechanism of pathogen recognition, signal perception, and intricate cell signaling for their protection from biotic and abiotic stresses. The pathogenic attack induces calcium ion accumulation in the plant cells, resulting in calcium signatures that regulate the synthesis of proteins of defense system. These calcium signatures induct different calcium dependent proteins such as calmodulins (CaMs), calcineurin B-like proteins (CBLs), calcium-dependent protein kinases (CDPKs) and other signaling molecules to orchestrate the complex defense signaling. Using advanced biotechnological tools, the role of Ca2+ signaling during plant-microbe interactions and the role of CaM/CMLs and CDPKs in plant defense mechanism has been revealed to some extent. The Emerging perspectives on calcium signaling in plant-microbe interactions suggest that this complex interplay could be harnessed to improve plant resistance against pathogenic microbes. We present here an overview of current understanding in calcium signatures during plant-microbe interaction so as to imbibe a future direction of research.

Plant Elicitor Peptide (Pep) Signaling and Pathogen Defense in Tomato

Endogenous signaling compounds are intermediaries in signaling pathways that plants use to respond to the perception of harmful and beneficial organisms. The plant elicitor peptides (Peps) of plants are important endogenous signaling molecules that induce elements of defense responses such as hormone production, increased expression of defensive genes, the activation of phosphorelays, and the induction of cell secondary messenger synthesis. The processes by which Peps confer resistance to pathogenic microorganisms have been extensively studied in Arabidopsis but are less known in crop plants. Tomato and many other solanaceous plants have an endogenous signaling polypeptide, systemin, that is involved in the defense against herbivorous insects and necrotrophic pathogens. This paper explores the similarity of the effects and chemical properties of Pep and systemin in tomato. Additionally, the relationship of the Pep receptor and systemin receptors is explored, and the identification of a second tomato Pep receptor in the literature is called into question. We suggest future directions for research on Pep signaling in solanaceous crops during interactions with microbes.

Phenotypes of cyclic nucleotide-gated cation channel mutants: probing the nature of native channels

Plant cyclic nucleotide gated channels (CNGCs) facilitate cytosolic Ca²⁺ influx as an early step in numerous signaling cascades. CNGC-mediated Ca²⁺ elevations are essential for plant immune defense and high temperature thermosensing. In the present study, we evaluated phenotypes of CNGC2, CNGC4, CNGC6, and CNGC12 null mutants in these two pathways. It is shown CNGC2, CNGC4, and CNGC6 physically interact in vivo, whereas CNGC12 does not. CNGC involvement in immune signaling was evaluated by monitoring mutant response to elicitor peptide Pep3. Pep3 response cascades involving CNGCs included mitogen-activated kinase activation mediated by Ca²⁺ -dependent protein kinase phosphorylation. Pep3-induced reactive oxygen species generation was impaired in cngc2, cngc4, and cngc6, but not in cngc12, suggesting that CNGC2, CNGC4, and CNGC6 (which physically interact) may be components of a multimeric CNGC channel complex for immune signaling. However, unlike cngc2 and cngc4, cngc6 is not sensitive to high Ca²⁺ and displays no pleiotropic dwarfism. All four cngc mutants showed thermotolerance compared to wild-type, although CNGC12 does not interact with the other three CNGCs. These results imply that physically interacting CNGCs may, in some cases, function in a signaling cascade as components of a heteromeric channel complex, although this may not be the case in other signaling pathways.

Small Signals Lead to Big Changes: The Potential of Peptide-Induced Resistance in Plants

The plant immunity system is being revisited more and more and new elements and roles are attributed to participating in the response to biotic stress. The new terminology is also applied in an attempt to identify different players in the whole scenario of immunity: Phytocytokines are one of those elements that are gaining more attention due to the characteristics of processing and perception, showing they are part of a big family of compounds that can amplify the immune response. This review aims to highlight the latest findings on the role of phytocytokines in the whole immune response to biotic stress, including basal and adaptive immunity, and expose the complexity of their action in plant perception and signaling events.

Flagellin C decreases the expression of the Gossypium hirsutum cation/proton exchanger 3 gene to promote calcium ion, hydrogen peroxide, and nitric oxide and synergistically regulate the resistance of cotton to Verticillium wilt

To date, no ideal effective method for controlling Verticillium wilt in upland cotton (Gossypium hirsutum) has been defined. The purpose of this study was to determine the effects and mechanism through which flagellin C (FLiC) regulates the Gossypium hirsutum cation/proton exchanger 3 gene (GhCAX3), induces plant immunity, and increases resistance to Verticillium wilt. The FLiC gene was cloned from an endophytic bacterium (Pseudomonas) isolated from roots of the upland cotton cultivar Zhongmiansuo 41. The biocontrol effects of FLiC purified in vitro on resistant and susceptible upland cotton cultivars were 47.50 and 32.42%, respectively. FLiC induced a hypersensitive response (HR) in leaves of tobacco and immune responses in upland cotton. Transcriptome data showed that treatment with FLiC significantly enriched the calcium antiporter activity-associated disease-resistant metabolic pathway in seedlings. Moreover, FLiC downregulated GhCAX3 expression to increase intracellular calcium ion (Ca²⁺) content and stimulate increases in the intracellular hydrogen peroxide (H₂O₂) and nitric oxide (NO) contents. The coordinated regulation of Ca²⁺, H₂O₂, and NO enhanced cotton resistance to Verticillium wilt. Furthermore, transgenic Arabidopsis plants overexpressing FLiC showed significantly improved resistance to Verticillium wilt. FLiC may be used as a resistance gene and a regulator to improve resistance to Verticillium dahliae (VD) in upland cotton.

In vivo identification of putative CPK5 substrates in Arabidopsis thaliana

Calcium signaling mediates most developmental processes and stress responses in plants. Among plant calcium sensors, the calcium-dependent protein kinases display a unique structure harboring both calcium sensing and kinase responding activities. AtCPK5 is an essential member of this family in Arabidopsis that regulates immunity and abiotic stress tolerance. To understand the underlying molecular mechanisms, we implemented a biochemical approach to identify in vivo substrates of AtCPK5. We generated transgenic lines expressing a constitutively active form of AtCPK5 under the control of a dexamethasone-inducible promoter. Lines expressing a kinase-dead version were used as a negative control. By comparing the phosphoproteome of the kinase-active and kinase-dead lines upon dexamethasone treatment, we identified 5 phosphopeptides whose abundance increased specifically in the kinase-active lines. Importantly, we showed that all 5 proteins were phosphorylated in vitro by AtCPK5 in a calcium-dependent manner, suggesting that they are direct targets of AtCPK5. We also detected several interaction patterns between the kinase and the candidates in the cytosol, membranes or nucleus, consistent with the ubiquitous localization of AtCPK5. Finally, we further validated the two phosphosites S245 and S280 targeted by AtCPK5 in the E3 ubiquitin ligase ATL31. Altogether, those results open new perspectives to decipher AtCPK5 biological functions.

Flg22-induced Ca2+ increases undergo desensitization and resensitization

The flagellin epitope flg22, a pathogen-associated molecular pattern (PAMP), binds to the receptor-like kinase FLAGELLIN SENSING2 (FLS2), and triggers Ca²⁺ influx across the plasma membrane (PM). The flg22-induced increases in cytosolic Ca²⁺ concentration ([Ca²⁺ ]_i ) (FICA) play a crucial role in plant innate immunity. It's well established that the receptor FLS2 and reactive oxygen species (ROS) burst undergo sensitivity adaptation after flg22 stimulation, referred to as desensitization and resensitization, to prevent over responses to pathogens. However, whether FICA also mount adaptation mechanisms to ensure appropriate and efficient responses against pathogens remains poorly understood. Here, we analysed systematically [Ca²⁺ ]_i increases upon two successive flg22 treatments, recorded and characterized rapid desensitization but slow resensitization of FICA in Arabidopsis thaliana. Pharmacological analyses showed that the rapid desensitization might be synergistically regulated by ligand-induced FLS2 endocytosis as well as the PM depolarization. The resensitization of FICA might require de novo FLS2 protein synthesis. FICA resensitization appeared significantly slower than FLS2 protein recovery, suggesting additional regulatory mechanisms of other components, such as flg22-related Ca²⁺ permeable channels. Taken together, we have carefully defined the FICA sensitivity adaptation, which will facilitate further molecular and genetic dissection of the Ca²⁺ -mediated adaptive mechanisms in PAMP-triggered immunity.

Calcium Signaling in Plant Programmed Cell Death

Programmed cell death (PCD) is a process intended for the maintenance of cellular homeostasis by eliminating old, damaged, or unwanted cells. In plants, PCD takes place during developmental processes and in response to biotic and abiotic stresses. In contrast to the field of animal studies, PCD is not well understood in plants. Calcium (Ca²⁺) is a universal cell signaling entity and regulates numerous physiological activities across all the kingdoms of life. The cytosolic increase in Ca²⁺ is a prerequisite for the induction of PCD in plants. Although over the past years, we have witnessed significant progress in understanding the role of Ca²⁺ in the regulation of PCD, it is still unclear how the upstream stress perception leads to the Ca²⁺ elevation and how the signal is further propagated to result in the onset of PCD. In this review article, we discuss recent advancements in the field, and compare the role of Ca²⁺ signaling in PCD in biotic and abiotic stresses. Moreover, we discuss the upstream and downstream components of Ca²⁺ signaling and its crosstalk with other signaling pathways in PCD. The review is expected to provide new insights into the role of Ca²⁺ signaling in PCD and to identify gaps for future research efforts.

A mis-regulated cyclic nucleotide-gated channel mediates cytosolic calcium elevation and activates immunity in Arabidopsis

Calcium (Ca²⁺ ) is a second messenger for plant cell surface and intracellular receptors mediating pattern-triggered and effector-triggered immunity (respectively, PTI and ETI). Several CYCLIC NUCLEOTIDE-GATED CHANNELS (CNGCs) were shown to control transient cytosolic Ca²⁺ influx upon PTI activation. The contributions of specific CNGC members to PTI and ETI remain unclear. ENHANCED DISEASE SUSCEPTIBLITY1 (EDS1) regulates ETI signaling. In an Arabidopsis genetic screen for suppressors of eds1, we identify a recessive gain-of-function mutation in CNGC20, denoted cngc20-4, which partially restores disease resistance in eds1. cngc20-4 enhances PTI responses and ETI hypersensitive cell death. A cngc20-4 single mutant exhibits autoimmunity, which is dependent on genetically parallel EDS1 and salicylic acid (SA) pathways. CNGC20 self-associates, forms heteromeric complexes with CNGC19, and is phosphorylated and stabilized by BOTRYTIS INDUCED KINASE1 (BIK1). The cngc20-4 L371F exchange on a predicted transmembrane channel inward surface does not disrupt these interactions but leads to increased cytosolic Ca²⁺ accumulation, consistent with mis-regulation of CNGC20 Ca²⁺ -permeable channel activity. Our data show that ectopic Ca²⁺ influx caused by a mutant form of CNGC20 in cngc20-4 affects both PTI and ETI responses. We conclude that tight control of the CNGC20 Ca²⁺ ion channel is important for regulated immunity.

Possible role of small secreted peptides (SSPs) in immune signaling in bryophytes

Plants utilize a plethora of peptide signals to regulate their immune response. Peptide ligands and their cognate receptors involved in immune signaling share common motifs among many species of vascular plants. However, the origin and evolution of immune peptides is still poorly understood. Here, we searched for genes encoding small secreted peptides in the genomes of three bryophyte lineages-mosses, liverworts and hornworts-that occupy a critical position in the study of land plant evolution. We found that bryophytes shared common predicted small secreted peptides (SSPs) with vascular plants. The number of SSPs is higher in the genomes of mosses than in both the liverwort Marchantia polymorpha and the hornwort Anthoceros sp. The synthetic peptide elicitors-AtPEP and StPEP-specific for vascular plants, triggered ROS production in the protonema of the moss Physcomitrella patens, suggesting the possibility of recognizing peptide ligands from angiosperms by moss receptors. Mass spectrometry analysis of the moss Physcomitrella patens, both the wild type and the Δcerk mutant secretomes, revealed peptides that specifically responded to chitosan treatment, suggesting their role in immune signaling.

Danger-Associated Peptide Regulates Root Immune Responses and Root Growth by Affecting ROS Formation in Arabidopsis

Plant elicitor peptides (Peps) are damage/danger-associated molecular patterns (DAMPs) that are perceived by a pair of receptor-like kinases, PEPR1 and PEPR2, to enhance innate immunity and induce the growth inhibition of root in Arabidopsis thaliana. In this study, we show that PEPR1 and PEPR2 function vitally in roots to regulate the root immune responses when treating the roots with bacterial pathogen Pst DC3000. PEPR2, rather than PEPR1, played a predominant role in the perception of Pep1 in the roots and further triggered a strong ROS accumulation—the substance acts as an antimicrobial agent or as a secondary messenger in plant cells. Consistently, seedlings mutating two major ROS-generating enzyme genes, respiratory burst oxidase homologs D and F (RBOHD and RBOHF), abolished the root ROS accumulation and impaired the growth inhibition of the roots induced by Pep1. Furthermore, we revealed that botrytis-induced kinase 1 (BIK1) physically interacted with PEPRs and RBOHD/F, respectively, and served downstream of the Pep1-PEPRs signaling pathway to regulate Pep1-induced ROS production and root growth inhibition. In conclusion, this study demonstrates a previously unrecognized signaling crosstalk between Pep1 and ROS signaling to regulate root immune response and root growth.

Identification and Functional Investigation of Genome-Encoded, Small, Secreted Peptides in Plants

Hundreds to thousands of small secreted peptides (SSPs) are encoded in plant genomes but have been overlooked, and most remain unannotated and unstudied. Despite their low profile, they have been found to confer dramatic effects on growth and development of plants. With the growing appreciation of their significance, the development of appropriate methods to identify and functionally assess the myriad SSPs encoded in plant genomes has become critical. Here, we provide protocols for the computational and physiological analysis of SSPs in plant genomes. We first describe our methodology successfully used for genome-wide identification and annotation of SSP-coding genes in the model legume Medicago truncatula, which can be readily adapted for other plant species. We then provide protocols for the functional analysis of SSPs using various synthetic peptide screens. Considerations for the design and handling of peptides are included. © 2019 by John Wiley & Sons, Inc.

Arabidopsis RETICULON-LIKE4 (RTNLB4) Protein Participates in Agrobacterium Infection and VirB2 Peptide-Induced Plant Defense Response

Agrobacterium tumefaciens uses the type IV secretion system, which consists of VirB1-B11 and VirD4 proteins, to deliver effectors into plant cells. The effectors manipulate plant proteins to assist in T-DNA transfer, integration, and expression in plant cells. The Arabidopsis reticulon-like (RTNLB) proteins are located in the endoplasmic reticulum and are involved in endomembrane trafficking in plant cells. The rtnlb4 mutants were recalcitrant to A. tumefaciens infection, but overexpression of RTNLB4 in transgenic plants resulted in hypersusceptibility to A. tumefaciens transformation, which suggests the involvement of RTNLB4 in A. tumefaciens infection. The expression of defense-related genes, including FRK1, PR1, WRKY22, and WRKY29, were less induced in RTNLB4 overexpression (O/E) transgenic plants after A. tumefaciens elf18 peptide treatment. Pretreatment with elf18 peptide decreased Agrobacterium-mediated transient expression efficiency more in wild-type seedlings than RTNLB4 O/E transgenic plants, which suggests that the induced defense responses in RTNLB4 O/E transgenic plants might be affected after bacterial elicitor treatments. Similarly, A. tumefaciens VirB2 peptide pretreatment reduced transient T-DNA expression in wild-type seedlings to a greater extent than in RTNLB4 O/E transgenic seedlings. Furthermore, the VirB2 peptides induced FRK1, WRKY22, and WRKY29 gene expression in wild-type seedlings but not efr-1 and bak1 mutants. The induced defense-related gene expression was lower in RTNLB4 O/E transgenic plants than wild-type seedlings after VirB2 peptide treatment. These data suggest that RTNLB4 may participate in elf18 and VirB2 peptide-induced defense responses and may therefore affect the A. tumefaciens infection process.

Hydrogen peroxide sensor HPCA1 is an LRR receptor kinase in Arabidopsis

Hydrogen peroxide (H₂O₂) is a major reactive oxygen species in unicellular and multicellular organisms, and is produced extracellularly in response to external stresses and internal cues^1-4. H₂O₂ enters cells through aquaporin membrane proteins and covalently modifies cytoplasmic proteins to regulate signalling and cellular processes. However, whether sensors for H₂O₂ also exist on the cell surface remains unknown. In plant cells, H₂O₂ triggers an influx of Ca²⁺ ions, which is thought to be involved in H₂O₂ sensing and signalling. Here, by using forward genetic screens based on Ca²⁺ imaging, we isolated hydrogen-peroxide-induced Ca²⁺ increases (hpca) mutants in Arabidopsis, and identified HPCA1 as a leucine-rich-repeat receptor kinase belonging to a previously uncharacterized subfamily that features two extra pairs of cysteine residues in the extracellular domain. HPCA1 is localized to the plasma membrane and is activated by H₂O₂ via covalent modification of extracellular cysteine residues, which leads to autophosphorylation of HPCA1. HPCA1 mediates H₂O₂-induced activation of Ca²⁺ channels in guard cells and is required for stomatal closure. Our findings help to identify how the perception of extracellular H₂O₂ is integrated with responses to various external stresses and internal cues in plants, and have implications for the design of crops with enhanced fitness.

Oxidative post-translational modifications controlling plant-pathogen interaction

Pathogen recognition is linked to the perception of microbe/pathogen-associated molecular patterns triggering a specific and transient accumulation of reactive oxygen species (ROS) at the pathogen attack site. The apoplastic oxidative "burst" generated at the pathogen attack site depends on the ROS-generator systems including enzymes such as plasma membrane NADP (H) oxidases, cell wall peroxidases and lipoxygenase. ROS are cytotoxic molecules that inhibit invading pathogens or signalling molecules that control the local and systemic induction of defence genes. Post-translational modifications induced by ROS are considered as a potential signalling mechanism that can modify protein structure and/or function, localisation and cellular stability. Thus, this review focuses on how ROS are essential molecules regulating the function of proteins involved in the plant response to a pathogen attack through post-translational modifications.

Properties and functions of calcium-dependent protein kinases and their relatives in Arabidopsis thaliana

Calcium is a ubiquitous second messenger that mediates plant responses to developmental and environmental cues. Calcium-dependent protein kinases (CDPKs) are key actors of plant signaling that convey calcium signals into physiological responses by phosphorylating various substrates including ion channels, transcription factors and metabolic enzymes. This large diversity of targets confers pivotal roles of CDPKs in shoot and root development, pollen tube growth, stomatal movements, hormonal signaling, transcriptional reprogramming and stress tolerance. On the one hand, specificity in CDPK signaling is achieved by differential calcium sensitivities, expression patterns, subcellular localizations and substrates. On the other hand, CDPKs also target some common substrates to ensure key cellular processes indispensable for plant growth and survival in adverse environmental conditions. In addition, the CDPK-related protein kinases (CRKs) might be closer to some CDPKs than previously anticipated and could contribute to calcium signaling despite their inability to bind calcium. This review highlights the regulatory properties of Arabidopsis CDPKs and CRKs that coordinate their multifaceted functions in development, immunity and abiotic stress responses.

A physiological perspective on targets of nitration in NO-based signaling networks in plants

Although peroxynitrite (ONOO-) has been well documented as a nitrating cognate of nitric oxide (NO) in plant cells, modifications of proteins, fatty acids, and nucleotides by nitration are relatively under-explored topics in plant NO research. As a result, they are seen mainly as hallmarks of redox processes or as markers of nitro-oxidative stress under unfavorable conditions, similar to those observed in human and other animal systems. Protein tyrosine nitration is the best-known nitrative modification in the plant system and can be promoted by the action of both ONOO- and related NO-derived oxidants within the cell environment. Recent progress in 'omics' and modeling tools have provided novel biochemical insights into the physiological and pathophysiological fate of nitrated proteins. The nitration process can be specifically involved in various cell regulatory mechanisms that control redox signaling via nitrated cGMP or nitrated fatty acids. In addition, there is evidence to suggest that nitrative modifications of nucleotides embedded in DNA and RNA can be considered as smart switches of gene expression that fine-tune adaptive cellular responses to stress. This review highlights recent advances in our understanding of the potential implications of biotargets in the regulation of intracellular traffic and plant biological processes.

Calcium-Dependent Protein Kinase Genes in Glycyrrhiza Uralensis Appear to be Involved in Promoting the Biosynthesis of Glycyrrhizic Acid and Flavonoids under Salt Stress

As calcium signal sensors, calcium-dependent protein kinases (CPKs) play vital roles in stimulating the production of secondary metabolites to participate in plant development and response to environmental stress. However, investigations of the Glycyrrhiza uralensis CPK family genes and their multiple functions are rarely reported. In this study, a total of 23 GuCPK genes in G. uralensis were identified, and their phylogenetic relationships, evolutionary characteristics, gene structure, motif distribution, and promoter cis-acting elements were analyzed. Ten GuCPKs showed root-specific preferential expressions, and GuCPKs indicated different expression patterns under treatments of CaCl2 and NaCl. In addition, under 2.5 mM of CaCl2 and 30 mM of NaCl treatments, the diverse, induced expression of GuCPKs and significant accumulations of glycyrrhizic acid and flavonoids suggested the possible important function of GuCPKs in regulating the production of glycyrrhizic acid and flavonoids. Our results provide a genome-wide characterization of CPK family genes in G. uralensis, and serve as a foundation for understanding the potential function and regulatory mechanism of GuCPKs in promoting the biosynthesis of glycyrrhizic acid and flavonoids under salt stress.

Regulation of Plant Immune Signaling by Calcium-Dependent Protein Kinases

Activation of Ca²⁺ signaling is a universal response to stress that allows cells to quickly respond to environmental cues. Fluctuations in cytosolic Ca²⁺ are decoded in plants by Ca²⁺-sensing proteins such as Ca²⁺-dependent protein kinases (CDPKs). The perception of microbes results in an influx of Ca²⁺ that activates numerous CDPKs responsible for propagating immune signals required for resistance against disease-causing pathogens. This review describes our current understanding of CDPK activation and regulation, and provides a comprehensive overview of CDPK-mediated immune signaling through interaction with various substrates.

Damage-Associated Molecular Pattern-Triggered Immunity in Plants

As a universal process in multicellular organisms, including animals and plants, cells usually emit danger signals when suffering from attacks of microbes and herbivores, or physical damage. These signals, termed as damage-associated molecular patterns (DAMPs), mainly include cell wall or extracellular protein fragments, peptides, nucleotides, and amino acids. Once exposed on cell surfaces, DAMPs are detected by plasma membrane-localized receptors of surrounding cells to regulate immune responses against the invading organisms and promote damage repair. DAMPs may also act as long-distance mobile signals to mediate systemic wounding responses. Generation, release, and perception of DAMPs, and signaling events downstream of DAMP perception are all rigorously modulated by plants. These processes integrate together to determine intricate mechanisms of DAMP-triggered immunity in plants. In this review, we present an extensive overview on our current understanding of DAMPs in plant immune system.

CaSK23, a Putative GSK3/SHAGGY-Like Kinase of Capsicum annuum, Acts as a Negative Regulator of Pepper's Response to Ralstonia solanacearum Attack

GSK3-like kinases have been mainly implicated in the brassinosteroids (BR) pathway and, therefore, in plant growth, development, and responses to abiotic stresses; however, their roles in plant immunity remain poorly understood. Herein, we present evidence that CaSK23, a putative GSK3/SHAGGY-like kinase in pepper, acts as a negative regulator in pepper’s response to Ralstonia solanacearum (R. solanacearum) inoculation (RSI). Data from quantitative RT-PCR (qRT-PCR) showed that the constitutively-expressed CaSK23 in pepper leaves was down-regulated by RSI, as well as by exogenously-applied salicylic acid (SA) or methyl jasomonate (MeJA). Silencing of CaSK23 by virus-induced gene silencing (VIGS) decreased the susceptibility of pepper plants to RSI, coupled with up-regulation of the tested genes encoding SA-, JA-, and ethylene (ET)-dependent pathogenesis-related (PR) proteins. In contrast, ectopic overexpression (OE) of CaSK23 conferred a compromised resistance of tobacco plants to RSI, accompanied by down-regulation of the tested immunity-associated SA-, JA-, and ET-dependent PR genes. In addition, transient overexpression of CaSK23 in pepper plants consistently led to down-regulation of the tested SA-, JA-, and ET-dependent PR genes. We speculate that CaSK23 acts as a negative regulator in pepper immunity and its constitutive expression represses pepper immunity in the absence of pathogens. On the other hand, its decreased expression derepresses immunity when pepper plants are attacked by pathogens.

Danger-Associated Peptides Close Stomata by OST1-Independent Activation of Anion Channels in Guard Cells

The plant elicitor peptides (Peps), a family of damage/danger-associated molecular patterns (DAMPs), are perceived by two receptors, PEPR1 and PEPR2, and contribute to plant defense against pathogen attack and abiotic stress. Here, we show that the Peps-PEPR signaling pathway functions in stomatal immunity by activating guard cell anion channels in Arabidopsis thaliana The mutant plants lacking both PEPR1 and PEPR2 (pepr1 pepr2) displayed enhanced bacterial growth after being sprayed with Pseudomonas syringae pv tomato (Pst) DC3000, but not after pathogen infiltration into leaves, implicating PEPR function in stomatal immunity. Indeed, synthetic Arabidopsis Peps (AtPeps) effectively induced stomatal closure in wild-type but not pepr1 pepr2 mutant leaves, suggesting that the AtPeps-PEPR signaling pathway triggers stomatal closure. Consistent with this finding, patch-clamp recording revealed AtPep1-induced activation of anion channels in the guard cells of wild-type but not pepr1 pepr2 mutant plants. We further identified two guard cell-expressed anion channels, SLOW ANION CHANNEL1 (SLAC1) and its homolog SLAH3, as functionally overlapping components responsible for AtPep1-induced stomatal closure. The slac1 slah3 double mutant, but not slac1 or slah3 single mutants, failed to respond to AtPep1 in stomatal closure assays. Interestingly, disruption of OPEN STOMATA1 (OST1), an essential gene for abscisic acid-triggered stomatal closure, did not affect the AtPep1-induced anion channel activity and stomatal response. Together, these results illustrate a DAMP-triggered signaling pathway that, unlike the flagellin22-FLAGELLIN-SENSITIVE2 pathway, triggers stomata immunity through an OST1-independent mechanism.

Plant elicitor peptides promote plant defences against nematodes in soybean

Plant elicitor peptides (Peps) are widely distributed among angiosperms, and have been shown to amplify immune responses in multiple plant families. Here, we characterize three Peps from soybean (Glycine max) and describe their effects on plant defences against two damaging agricultural pests, the root-knot nematode (Meloidogyne incognita) and the soybean cyst nematode (Heterodera glycines). Seed treatments with exogenous GmPep1, GmPep2 or GmPep3 significantly reduced the reproduction of both nematodes. Pep treatment also protected plants from the inhibitory effects of root-knot nematodes on above-ground growth, and up-regulated basal expression levels of nematode-responsive defence genes. GmPep1 induced the expression of its propeptide precursor (GmPROPEP1), a nucleotide-binding site leucine-rich repeat protein (NBS-LRR), a pectin methylesterase inhibitor (PMEI), Respiratory Burst Oxidase Protein D (RBOHD) and the accumulation of reactive oxygen species (ROS) in leaves. In addition, GmPep2 and GmPep3 seed treatments up-regulated RBOHD expression and ROS accumulation in roots and leaves. These results suggest that GmPeps activate plant defences through systemic transcriptional reprogramming and ROS signalling, and that Pep seed treatments represent a potential strategy for nematode management.

Novel Rosaceae plant elicitor peptides as sustainable tools to control Xanthomonas arboricola pv. pruni in Prunus spp

Fruit crops are regarded as important health promoters and constitute a major part of global agricultural production, and Rosaceae species are of high economic impact. Their culture is threatened by bacterial diseases, whose control is based on preventative treatments using compounds of limited efficacy and negative environmental impact. One of the most economically relevant examples is the pathogen Xanthomonas arboricola pv. pruni (Xap) affecting Prunus spp. The plant immune response against pathogens can be triggered and amplified by plant elicitor peptides (Peps), perceived by specific receptors (PEPRs). Although they have been described in various angiosperms, scarce information is available on Rosaceae species. Here, we identified the Pep precursor (PROPEP), Pep and PEPR orthologues of 10 Rosaceae species and confirmed the presence of the Pep/PEPR system in this family. We showed the perception and elicitor activity of Rosaceae Peps using the Prunus-Xap pathosystem as proof-of-concept. Treatment with nanomolar doses of Peps induced the corresponding PROPEP and a set of defence-related genes in Prunus leaves, and enhanced resistance against Xap. Peps from the same species had the highest efficiencies. Rosaceae Peps could potentially be used to develop natural, targeted and environmentally friendly strategies to enhance the resistance of Prunus species against biotic attackers.

SlCNGC1 and SlCNGC14 Suppress Xanthomonas oryzae pv. oryzicola-Induced Hypersensitive Response and Non-host Resistance in Tomato

Mechanisms underlying plant non-host resistance to Xanthomonas oryzae pv. oryzicola (Xoc), the pathogen causing rice leaf streak disease, are largely unknown. Cyclic nucleotide-gated ion channels (CNGCs) are calcium-permeable channels that are involved in various biological processes including plant resistance. In this study, functions of two tomato CNGC genes SlCNGC1 and SlCNGC14 in non-host resistance to Xoc were analyzed. Silencing of SlCNGC1 and SlCNGC14 in tomato significantly enhanced Xoc-induced hypersensitive response (HR) and non-host resistance, demonstrating that both SlCNGC1 and SlCNGC14 negatively regulate non-host resistance related HR and non-host resistance to Xoc in tomato. Silencing of SlCNGC1 and SlCNGC14 strikingly increased Xoc-induced callose deposition and strongly promoted both Xoc-induced and flg22-elicited H₂O₂, indicating that these two SlCNGCs repress callose deposition and ROS accumulation to attenuate non-host resistance and PAMP-triggered immunity (PTI). Importantly, silencing of SlCNGC1 and SlCNGC14 apparently compromised cytosolic Ca²⁺ accumulation, implying that SlCNGC1 and SlCNGC14 function as Ca²⁺ channels and negatively regulate non-host resistance and PTI-related responses through modulating cytosolic Ca²⁺ accumulation. SlCNGC14 seemed to play a stronger regulatory role in the non-host resistance and PTI compared to SlCNGC1. Our results reveal the contribution of CNGCs and probably also Ca²⁺ signaling pathway to non-host resistance and PTI.

Plant ion channels and transporters in herbivory-induced signalling

In contrast to many biotic stresses that plants face, feeding by herbivores produces unique mechanical and chemical signatures. Plants have evolved effective systems to recognise these mechanical stimuli and chemical elicitors at the plasma membrane (PM), where this recognition generates ion fluxes, including an influx of Ca2+ that elicits cellular Ca2+ signalling, production of reactive oxygen species (ROS), and variation in transmembrane potential. These signalling events also function in propagation of long-distance signals (Ca2+ waves, ROS waves, and electrical signals), which contribute to rapid, systemic induction of defence responses. Recent studies have identified several candidate channels or transporters that likely produce these ion fluxes at the PM. Here, we describe the important roles of these channels/transporters in transduction or transmission of herbivory-induced early signalling events, long-distance signals, and jasmonic acid and green leaf volatile signalling in plants.

cGMP signalling in plants: from enigma to main stream

All living organisms communicate with their environment, and part of this dialogue is mediated by secondary messengers such as cyclic guanosine mono phosphate (cGMP). In plants, most of the specific components that allow production and breakdown of cGMP have now been identified apart from cGMP dependent phosphodiesterases, enzymes responsible for cGMP catabolism. Irrespectively, the role of cGMP in plant signal transductions is now firmly established with involvement of this nucleotide in development, stress response, ion homeostasis and hormone function. Within these areas, several consistent themes where cGMP may be particularly relevant are slowly emerging: these include regulation of cation fluxes, for example via cyclic nucleotide gated channels and in stomatal functioning. Many details of signalling pathways that incorporate cGMP remain to be unveiled. These include downstream targets other than a small number of ion channels, in particular cGMP dependent kinases. Improved genomics tools may help in this respect, especially since many proteins involved in cGMP signalling appear to have multiple and often overlapping functional domains which hampers identification on the basis of simple homology searches. Another open question regards the topographical distribution of cGMP signals are they cell limited? Does long distance cGMP signalling occur and if so, by what mechanisms? The advent of non-disruptive fluorescent reporters with high spatial and temporal resolution will provide a tool to accelerate progress in all these areas. Automation can facilitate large scale screens of mutants or the action of effectors that impact on cGMP signalling.

The Peach RGF/GLV Signaling Peptide pCTG134 Is Involved in a Regulatory Circuit That Sustains Auxin and Ethylene Actions

In vascular plants the cell-to-cell interactions coordinating morphogenetic and physiological processes are mediated, among others, by the action of hormones, among which also short mobile peptides were recognized to have roles as signals. Such peptide hormones (PHs) are involved in defense responses, shoot and root growth, meristem homeostasis, organ abscission, nutrient signaling, hormone crosstalk and other developmental processes and act as both short and long distant ligands. In this work, the function of CTG134, a peach gene encoding a ROOT GROWTH FACTOR/GOLVEN-like PH expressed in mesocarp at the onset of ripening, was investigated for its role in mediating an auxin-ethylene crosstalk. In peach fruit, where an auxin-ethylene crosstalk mechanism is necessary to support climacteric ethylene synthesis, CTG134 expression peaked before that of ACS1 and was induced by auxin and 1-methylcyclopropene (1-MCP) treatments, whereas it was minimally affected by ethylene. In addition, the promoter of CTG134 fused with the GUS reporter highlighted activity in plant parts in which the auxin-ethylene interplay is known to occur. Arabidopsis and tobacco plants overexpressing CTG134 showed abnormal root hair growth, similar to wild-type plants treated with a synthetic form of the sulfated peptide. Moreover, in tobacco, lateral root emergence and capsule size were also affected. In Arabidopsis overexpressing lines, molecular surveys demonstrated an impaired hormonal crosstalk, resulting in a re-modulated expression of a set of genes involved in both ethylene and auxin synthesis, transport and perception. These data support the role of pCTG134 as a mediator in an auxin-ethylene regulatory circuit and open the possibility to exploit this class of ligands for the rational design of new and environmental friendly agrochemicals able to cope with a rapidly changing environment.

The Hippeastrum hybridum PepR1 gene (HpPepR1) encodes a functional guanylyl cyclase and is involved in early response to fungal infection

It is generally known that cyclic GMP widespread in prokaryotic and eukaryotic cells, is involved in essential cellular processes and stress signal transduction. However, in contrast to animals the knowledge about plant guanylyl cyclases (GCs) which catalyze the formation of cGMP from GTP is still quite obscure. Recent studies of plant GCs are focused on identification and functional analysis of a new family of membrane proteins called "moonlighting kinases with GC activity" with guanylyl cyclase catalytic center encapsulated within intracellular kinase domain. Here we report identification and characterization of plasma membrane receptor of peptide signaling molecules - HpPepR1 in Hippeastrum hybridum. Both bioinformatic analysis of amimo acid sequence and in vitro studies revealed that the protein can act as guanylyl cyclase. The predicted amino acid sequence contains highly conserved 14 aa-long search motif in the catalytic center of GCs from lower and higher eukaryotes. Here, we provide experimental evidence to show that the intracellular domain of HpPepR1 can generate cGMP in vitro. Moreover, it was shown that the accumulation of HpPepR1 transcript was sharply increased after Peyronellaea curtisii (=Phoma narcissi) fungal infection, whereas mechanical wounding has no influence on expression profile of studied gene. These results may indicate the participation of cGMP-dependent pathway in rapid, alarm plant reactions induced by pathogen infection.

From Chaos to Harmony: Responses and Signaling upon Microbial Pattern Recognition

Pathogen- or microbe-associated molecular patterns (PAMPs/MAMPs) are detected as nonself by host pattern recognition receptors (PRRs) and activate pattern-triggered immunity (PTI). Microbial invasions often trigger the production of host-derived endogenous signals referred to as danger- or damage-associated molecular patterns (DAMPs), which are also perceived by PRRs to modulate PTI responses. Collectively, PTI contributes to host defense against infections by a broad range of pathogens. Remarkable progress has been made toward demonstrating the cellular and physiological responses upon pattern recognition, elucidating the molecular, biochemical, and genetic mechanisms of PRR activation, and dissecting the complex signaling networks that orchestrate PTI responses. In this review, we present an update on the current understanding of how plants recognize and respond to nonself patterns, a process from which the seemingly chaotic responses form into a harmonic defense.

Calcium signatures and signaling events orchestrate plant-microbe interactions

Calcium (Ca²⁺) acts as an essential second messenger connecting the perception of microbe signals to the establishment of appropriate immune and symbiotic responses in plants. Accumulating evidence suggests that plants distinguish different microorganisms through plasma membrane-localized pattern recognition receptors. The particular recognition events are encoded into Ca²⁺ signatures, which are sensed by diverse intracellular Ca²⁺ binding proteins. The Ca²⁺ signatures are eventually decoded to distinct downstream responses through transcriptional reprogramming of the defense or symbiosis-related genes. Recent observations further reveal that Ca²⁺-mediated signaling is also involved in negative regulation of plant immunity. This review is intended as an overview of Ca²⁺ signaling during immunity and symbiosis, including Ca²⁺ responses in the nucleus and cytosol.

Intracellular Ca2+ is important for flagellin-triggered defense in Arabidopsis and involves inositol polyphosphate signaling

Cytosolic Ca2+ increase is a crucial and early step of plant immunity evoked by pathogen-associated molecular patterns (PAMPs) such as flagellin (flg). Components responsible for this increase are still not uncovered, although current models of plant immune signaling portray extracellular Ca2+ influx as paramount to flg activation of defense pathways. Work presented here provides new insights into cytosolic Ca2+ increase associated with flg-induced defense responses. We show that extracellular Ca2+ contributes more to immune responses evoked by plant elicitor peptide (Pep3) than that evoked by flg, indicating an intracellular Ca2+ source responsible for immune responses evoked by flg. Genetic impairment of the inositol polyphosphate (InsP) and G-protein signal associated with flg perception reduced flg-dependent immune responses. Previous work indicates that prior exposure of Arabidopsis plants to flg leads to an immune response reflected by less vigorous growth of a pathogenic microbe. We found that this immune response to flg was compromised in mutants lacking the ability to generate an InsP or G-protein signal. We conclude that the recruitment of intracellular Ca2+ stores by flg may involve InsP and G-protein signaling. We also found a notable difference in contribution of intracellular stores of Ca2+ to the immune signaling evoked by another PAMP, elf18 peptide, which had a very different response profile to impairment of InsP signaling. Although Ca2+ signaling is at the core of the innate immune as well as hypersensitive response to plant pathogens, it appears that the molecular mechanisms generating the Ca2+ signal in response to different PAMPs are different.

Cellulose-Derived Oligomers Act as Damage-Associated Molecular Patterns and Trigger Defense-Like Responses

The plant cell wall, often the site of initial encounters between plants and their microbial pathogens, is composed of a complex mixture of cellulose, hemicellulose, and pectin polysaccharides as well as proteins. The concept of damage-associated molecular patterns (DAMPs) was proposed to describe plant elicitors like oligogalacturonides (OGs), which can be derived by the breakdown of the pectin homogalacturon by pectinases. OGs act via many of the same signaling steps as pathogen- or microbe-associated molecular patterns (PAMPs) to elicit defenses and provide protection against pathogens. Given both the complexity of the plant cell wall and the fact that many pathogens secrete a wide range of cell wall-degrading enzymes, we reasoned that the breakdown products of other cell wall polymers may be similarly biologically active as elicitors and may help to reinforce the perception of danger by plant cells. Our results indicate that oligomers derived from cellulose are perceived as signal molecules in Arabidopsis (Arabidopsis thaliana), triggering a signaling cascade that shares some similarities to responses to well-known elicitors such as chitooligomers and OGs. However, in contrast to other known PAMPs/DAMPs, cellobiose stimulates neither detectable reactive oxygen species production nor callose deposition. Confirming our idea that both PAMPs and DAMPs are likely to cooccur at infection sites, cotreatments of cellobiose with flg22 or chitooligomers led to synergistic increases in gene expression. Thus, the perception of cellulose-derived oligomers may participate in cell wall integrity surveillance and represents an additional layer of signaling following plant cell wall breakdown during cell wall remodeling or pathogen attack.

Role of Cyclic Nucleotide Gated Channels in Stress Management in Plants

Tolerance of plants to a number of biotic and abiotic stresses such as pathogen and herbivore attack, drought, salinity, cold and nutritional limitations is ensued by complex multimodule signaling pathways. The outcome of this complex signaling pathways results in adaptive responses by restoring the cellular homeostasis and thus promoting survival. Functions of many plant cation transporter and channel protein families such as glutamate receptor homologs (GLRs), cyclic nucleotide-gated ion channel (CNGC) have been implicated in providing biotic and abiotic stress tolerance. Ion homeostasis regulated by several transporters and channels is one of the crucial parameters for the optimal growth, development and survival of all living organisms. The CNGC family members are known to be involved in the uptake of cations such as Na(+), K(+) and Ca(2+) and regulate plant growth and development. Detail functional genomics approaches have given an emerging picture of CNGCs wherein these protein are believed to play crucial role in pathways related to cellular ion homeostasis, development and as a 'guard' in defense against biotic and abiotic challenges. Here, we discuss the current knowledge of role of CNGCs in mediating stress management and how they aid plants in survival under adverse conditions.

Regulation of plant reactive oxygen species (ROS) in stress responses: learning from AtRBOHD

Reactive oxygen species (ROS) are constantly produced in plants, as the metabolic by-products or as the signaling components in stress responses. High levels of ROS are harmful to plants. In contrast, ROS play important roles in plant physiology, including abiotic and biotic tolerance, development, and cellular signaling. Therefore, ROS production needs to be tightly regulated to balance their function. Respiratory burst oxidase homologue (RBOH) proteins, also known as plant nicotinamide adenine dinucleotide phosphate oxidases, are well studied enzymatic ROS-generating systems in plants. The regulatory mechanisms of RBOH-dependent ROS production in stress responses have been intensively studied. This has greatly advanced our knowledge of the mechanisms that regulate plant ROS production. This review attempts to integrate the regulatory mechanisms of RBOHD-dependent ROS production by discussing the recent advance. AtRBOHD-dependent ROS production could provide a valuable reference for studying ROS production in plant stress responses.

TMT-based quantitative proteomics analyses reveal novel defense mechanisms of Brassica napus against the devastating necrotrophic pathogen Sclerotinia sclerotiorum

The white mould disease, caused by Sclerotinia sclerotiorum, is one of the most important diseases in the vital oil crop Brassica napus. Nevertheless, the defense mechanisms of B. napus against S. sclerotiorum are poorly understood. In this study, we performed comparative quantitative proteomics analyses to reveal B. napus defense mechanisms against S. sclerotiorum. The proteomes of B. napus leaves inoculated with S. sclerotiorum wild-type strain 1980 and nonpathogenic mutant strain Ep-1PB as well as empty agar plug as the control were analyzed using TMT label-based quantitative analysis technique. A total of 79, 299 and 173 proteins consistently differentially expressed between Ep-1PB- and mock-inoculated leaves, 1980- and mock-inoculated leaves, as well as 1980- and Ep-1PB-inoculated leaves, respectively, were identified. The differential expression of 12 selected proteins was confirmed by qRT-PCR analyses. The Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) and protein-protein interaction prediction analyses revealed that redox homeostasis, lipid signaling, calcium signaling, histone and DNA methylation-mediated transcription regulation and defense-related proteins such as defensin and defensin-like proteins and cyanate lyase, contribute to defense against S. sclerotiorum. Our results provide new insights into molecular mechanisms that may be involved in defense responses of B. napus to S. sclerotiorum.

SIGNIFICANCE

The Sclerotinia white mould disease is one of the most important diseases in the significant oil crop Brassica napus. Nevertheless, the defense mechanisms of B. napus against S. sclerotiorum are still largely unknown to date. In this study, we addressed this issue by performing TMT label-based comparative quantitative analyses of the proteomes of B. napus leaves inoculated with S. sclerotiorum wild-type strain 1980 and nonpathogenic mutant strain Ep-1PB as well as empty agar plug as the control. Through comparative analyses on 79, 299, and 173 proteins that are consistently differentially expressed in between Ep-1PB-inoculated and the control leaves, 1980-inoculated and the control leaves, as well as 1980-inoculated and Ep-1PB-inoculated leaves, respectively, we revealed that redox homeostasis, lipid signaling, calcium signaling, histone and DNA methylation-mediated transcription regulation and defense-related proteins such as defensin and defensin-like proteins as well as cyanate lyase, contribute to B. napus defenses against S. sclerotiorum. Notably, the potential role of lipid signaling, calcium signaling, histone and DNA methylation-mediated transcription regulation and cyanate lyase in B. napus defense against S. sclerotiorum are not reported previously but rather unveiled for the first time in this study. The current study represents the most extensive analysis of the protein profile of B. napus in response to S. sclerotiorum inoculation and includes for the first time the results from comparison between plants inoculated with the wild-type strain and a nonpathogenic mutant strain of S. sclerotiorum. Collectively, our results provide new insights into the molecular mechanisms of interactions between B. napus and S. sclerotiorum.

In Search of Enzymes with a Role in 3', 5'-Cyclic Guanosine Monophosphate Metabolism in Plants

In plants, nitric oxide (NO)-mediated 3', 5'-cyclic guanosine monophosphate (cGMP) synthesis plays an important role during pathogenic stress response, stomata closure upon osmotic stress, the development of adventitious roots and transcript regulation. The NO-cGMP dependent pathway is well characterized in mammals. The binding of NO to soluble guanylate cyclase enzymes (GCs) initiates the synthesis of cGMP from guanosine triphosphate. The produced cGMP alters various cellular responses, such as the function of protein kinase activity, cyclic nucleotide gated ion channels and cGMP-regulated phosphodiesterases. The signal generated by the second messenger is terminated by 3', 5'-cyclic nucleotide phosphodiesterase (PDEs) enzymes that hydrolyze cGMP to a non-cyclic 5'-guanosine monophosphate. To date, no homologues of mammalian cGMP-synthesizing and degrading enzymes have been found in higher plants. In the last decade, six receptor proteins from Arabidopsis thaliana have been reported to have guanylate cyclase activity in vitro. Of the six receptors, one was shown to be a NO dependent guanylate cyclase enzyme (NOGC1). However, the role of these proteins in planta remains to be elucidated. Enzymes involved in the degradation of cGMP remain elusive, albeit, PDE activity has been detected in crude protein extracts from various plants. Additionally, several research groups have partially purified and characterized PDE enzymatic activity from crude protein extracts. In this review, we focus on presenting advances toward the identification of enzymes involved in the cGMP metabolism pathway in higher plants.

Microbe Associated Molecular Pattern Signaling in Guard Cells

Stomata, formed by pairs of guard cells in the epidermis of terrestrial plants, regulate gas exchange, thus playing a critical role in plant growth and stress responses. As natural openings, stomata are exploited by microbes as an entry route. Recent studies reveal that plants close stomata upon guard cell perception of molecular signatures from microbes, microbe associated molecular patterns (MAMPs), to prevent microbe invasion. The perception of MAMPs induces signal transduction including recruitment of second messengers, such as Ca(2+) and H2O2, phosphorylation events, and change of transporter activity, leading to stomatal movement. In the present review, we summarize recent findings in signaling underlying MAMP-induced stomatal movement by comparing with other signalings.

Quo vadis, Pep? Plant elicitor peptides at the crossroads of immunity, stress, and development

The first line of inducible plant defence, pattern-triggered immunity (PTI), is activated by the recognition of exogenous as well as endogenous elicitors. Exogenous elicitors, also called microbe-associated molecular patterns, signal the presence of microbes. In contrast, endogenous elicitors seem to be generated and recognized under more diverse circumstances, making the evaluation of their biological relevance much more complex. Plant elicitor peptides (Peps) are one class of such endogenous elicitors, which contribute to immunity against attack by bacteria, fungi, as well as herbivores. Recent studies indicate that the Pep-triggered signalling pathways also operate during the response to a more diverse set of stresses including starvation stress. In addition, in silico data point to an involvement in the regulation of plant development, and a study on Pep-mediated inhibition of root growth supports this indication. Importantly, Peps are neither limited to the model plant Arabidopsis nor to a specific plant family like the previously intensively studied systemin peptides. On the contrary, they are present and active in angiosperms all across the phylogenetic tree, including many important crop plants. Here we summarize the progress made in research on Peps from their discovery in 2006 until now. We discuss the two main models which describe their likely function in plant immunity, highlight the studies supporting additional roles of Pep-triggered signalling and identify urgent research tasks to further uncover their biological relevance.

The Plant Peptidome: An Expanding Repertoire of Structural Features and Biological Functions

Peptides fulfill a plethora of functions in plant growth, development, and stress responses. They act as key components of cell-to-cell communication, interfere with signaling and response pathways, or display antimicrobial activity. Strikingly, both the diversity and amount of plant peptides have been largely underestimated. Most characterized plant peptides to date acting as small signaling peptides or antimicrobial peptides are derived from nonfunctional precursor proteins. However, evidence is emerging on peptides derived from a functional protein, directly translated from small open reading frames (without the involvement of a precursor) or even encoded by primary transcripts of microRNAs. These novel types of peptides further add to the complexity of the plant peptidome, even though their number is still limited and functional characterization as well as translational evidence are often controversial. Here, we provide a comprehensive overview of the reported types of plant peptides, including their described functional and structural properties. We propose a novel, unifying peptide classification system to emphasize the enormous diversity in peptide synthesis and consequent complexity of the still expanding knowledge on the plant peptidome.

Changes of Nitric Oxide and Its Relationship with H2O2 and Ca2+ in Defense Interactions between Wheat and Puccinia Triticina

In this research, the wheat cultivar 'Lovrin 10' and Puccinia triticina races 165 and 260 were used to constitute compatible and incompatible combinations to investigate the relationship between NO and H2O2 and between NO and calcium (Ca(2+)) signaling in the cell defense process by pharmacological means. The specific fluorescent probe DAF-FM DA was coupled with confocal laser scanning microscopy and used to label intracellular nitric oxide (NO) and monitoring the real-time NO dynamics during the processes of wheat defense response triggered by P. triticina infection. The results showed that at 4 h after inoculation, weak green fluorescence was observed in the stomatal guard cells at the P. triticina infection site in the incompatible combination, which indicates a small amount of NO production. Twelve hours after inoculation, the fluorescence of NO in- cell adjacent to the stomata gradually intensified, and the NO fluorescent area also expanded continuously; the green fluorescence primarily occurred in the cells undergoing a hypersensitive response (HR) at 24-72 h after inoculation. For the compatible combination, however, a small amount of green fluorescence was observed in stomata where the pathogenic contact occurred at 4 h after inoculation, and fluorescence was not observed thereafter. Injections of the NO scavenger c-PTIO prior to inoculation postponed the onset of NO production to 48 h after inoculation and suppressed HR advancement. The injection of imidazole, a NADPH oxidase inhibitor, or EGTA, an extracellular calcium chelator, in the leaves prior to inoculation, delayed the onset of NO production in the incompatible combination and suppressed HR advancement. Combined with our previous results, it could be concluded that, Ca(2+) and hydrogen peroxide (H2O2) are involved in upstream of NO production to induce the HR cell death during P. triticina infection, and Ca(2+), NO and H2O2 are jointly involved in the signal transduction process of HR in the interaction system.

Genome-wide Identification and Expression Analysis of the CDPK Gene Family in Grape, Vitis spp

BACKGROUND

Calcium-dependent protein kinases (CDPKs) play vital roles in plant growth and development, biotic and abiotic stress responses, and hormone signaling. Little is known about the CDPK gene family in grapevine.

RESULTS

In this study, we performed a genome-wide analysis of the 12X grape genome (Vitis vinifera) and identified nineteen CDPK genes. Comparison of the structures of grape CDPK genes allowed us to examine their functional conservation and differentiation. Segmentally duplicated grape CDPK genes showed high structural conservation and contributed to gene family expansion. Additional comparisons between grape and Arabidopsis thaliana demonstrated that several grape CDPK genes occured in the corresponding syntenic blocks of Arabidopsis, suggesting that these genes arose before the divergence of grapevine and Arabidopsis. Phylogenetic analysis divided the grape CDPK genes into four groups. Furthermore, we examined the expression of the corresponding nineteen homologous CDPK genes in the Chinese wild grape (Vitis pseudoreticulata) under various conditions, including biotic stress, abiotic stress, and hormone treatments. The expression profiles derived from reverse transcription and quantitative PCR suggested that a large number of VpCDPKs responded to various stimuli on the transcriptional level, indicating their versatile roles in the responses to biotic and abiotic stresses. Moreover, we examined the subcellular localization of VpCDPKs by transiently expressing six VpCDPK-GFP fusion proteins in Arabidopsis mesophyll protoplasts; this revealed high variability consistent with potential functional differences.

CONCLUSIONS

Taken as a whole, our data provide significant insights into the evolution and function of grape CDPKs and a framework for future investigation of grape CDPK genes.

Plant elicitor peptides in induced defense against insects

Over 20 years ago the peptide systemin was discovered to be an integral regulator of anti-herbivore defense responses in Solanaceous plants. However, other peptides of similar function have remained elusive. Plant Elicitor Peptides (Peps) were initially discovered in Arabidopsis as mediators of basal immune responses protective against invading pathogens. Recently a Pep from maize, ZmPep3, was demonstrated to be a potent regulator of anti-herbivore defenses. ZmPep3 was as active as the Lepidopteran elicitor N-linolenoyl-l-glutamine (Gln-18:3) in stimulating volatile emission and accumulation of defense transcripts and metabolites, resulting in both attraction of the parasitoid Cotesia marginiventris and suppressed growth of Spodoptera exigua larvae. Orthologues of Peps in Solanaceous and Fabaceous plants also trigger emission of herbivore-associated volatiles, indicating that Peps have a conserved role as regulators of plant defense against herbivores in diverse species. This conservation of a peptide signal and cognate receptor for activation of plant defense responses reveals a widespread regulatory motif and provides opportunities for manipulation of plant resistance.

Reactive oxygen species and plant resistance to fungal pathogens

Reactive oxygen species (ROS) have been studied for their role in plant development as well as in plant immunity. ROS were consistently observed to accumulate in the plant after the perception of pathogens and microbes and over the years, ROS were postulated to be an integral part of the defence response of the plant. In this article we will focus on recent findings about ROS involved in the interaction of plants with pathogenic fungi. We will describe the ways to detect ROS, their modes of action and their importance in relation to resistance to fungal pathogens. In addition we include some results from works focussing on the fungal interactor and from studies investigating roots during pathogen attack.

A sophisticated network of signaling pathways regulates stomatal defenses to bacterial pathogens

Guard cells are specialized cells forming stomatal pores at the leaf surface for gas exchanges between the plant and the atmosphere. Stomata have been shown to play an important role in plant defense as a part of the innate immune response. Plants actively close their stomata upon contact with microbes, thereby preventing pathogen entry into the leaves and the subsequent colonization of host tissues. In this review, we present current knowledge of molecular mechanisms and signaling pathways implicated in stomatal defenses, with particular emphasis on plant-bacteria interactions. Stomatal defense responses begin from the perception of pathogen-associated molecular patterns (PAMPs) and activate a signaling cascade involving the production of secondary messengers such as reactive oxygen species, nitric oxide, and calcium for the regulation of plasma membrane ion channels. The analyses on downstream molecular mechanisms implicated in PAMP-triggered stomatal closure have revealed extensive interplays among the components regulating hormonal signaling pathways. We also discuss the strategies deployed by pathogenic bacteria to counteract stomatal immunity through the example of the phytotoxin coronatine.

NO signaling in plant immunity: a tale of messengers

Nitric oxide (NO) is a free radical gas involved in a myriad of plant physiological processes including immune responses. How NO mediates its biological effects in plant facing microbial pathogen attack is an unresolved question. Insights into the molecular mechanisms by which it propagates signals reveal the contribution of this simple gas in complex signaling pathways shared with reactive oxygen species (ROS) and the second messenger Ca(2+). Understanding of the subtle cross-talks operating between these signals was greatly improved by the recent identification and the functional analysis of proteins regulated through S-nitrosylation, a major NO-dependent post-translational protein modification. Overall, these findings suggest that NO is probably an important component of the mechanism coordinating and regulating Ca(2+) and ROS signaling in plant immunity.