Nitric oxide production mediates oligogalacturonide-triggered immunity and resistance to Botrytis cinerea in Arabidopsis thaliana

Oligogalacturonide application increases resistance to Fusarium head blight in durum wheat

Fusariosis causes substantial yield losses in the wheat crop worldwide and compromises food safety because of the presence of toxins associated with the fungal disease. Among the current approaches to crop protection, the use of elicitors able to activate natural defense mechanisms in plants is a strategy gaining increasing attention. Several studies indicate that applications of plant cell-wall-derived elicitors, such as oligogalacturonides (OGs) derived from partial degradation of pectin, induce local and systemic resistance against plant pathogens. The aim of this study was to establish the efficacy of OGs in protecting durum wheat (Triticum turgidum subsp. durum), which is characterized by an extreme susceptibility to Fusarium graminearum. To evaluate the functionality of OGs, spikes and seedlings of cv. Svevo were inoculated with OGs, F. graminearum spores, and a co-treatment of both. Results demonstrated that OGs are active elicitors of wheat defenses, triggering typical immune marker genes and determining regulation of fungal genes. Moreover, bioassays on spikes and transcriptomic analyses on seedlings showed that OGs can regulate relevant physiological processes in Svevo with dose-dependent specificity. Thus, the OG sensing system plays an important role in fine tuning immune signaling pathways in durum wheat.

Adaptive responses of nitric oxide (NO) and its intricate dialogue with phytohormones during salinity stress

Nitric oxide (NO) is a gaseous free radical that acts as a messenger for various plant phenomena corresponding to photomorphogenesis, fertilisation, flowering, germination, growth, and productivity. Recent developments have suggested the critical role of NO in inducing adaptive responses in plants during salinity. NO minimises salinity-induced photosynthetic damage and improves plant-water relation, nutrient uptake, stomatal conductance, electron transport, and ROS and antioxidant metabolism. NO contributes active participation in ABA-mediated stomatal regulation. Similar crosstalk of NO with other phytohormones such as auxins (IAAs), gibberellins (GAs), cytokinins (CKs), ethylene (ET), salicylic acid (SA), strigolactones (SLs), and brassinosteroids (BRs) were also observed. Additionally, we discuss NO interaction with other gaseous signalling molecules such as reactive oxygen species (ROS) and reactive sulphur species (RSS). Conclusively, the present review traces critical events in NO-induced morpho-physiological adjustments under salt stress and discusses how such modulations upgrade plant resilience.

Natural variation in the pattern-triggered immunity response in plants: Investigations, implications and applications

The pattern-triggered immunity (PTI) response is triggered at the plant cell surface by the recognition of microbe-derived molecules known as microbe- or pathogen-associated molecular patterns or molecules derived from compromised host cells called damage-associated molecular patterns. Membrane-localized receptor proteins, known as pattern recognition receptors, are responsible for this recognition. Although much of the machinery of PTI is conserved, natural variation for the PTI response exists within and across species with respect to the components responsible for pattern recognition, activation of the response, and the strength of the response induced. This review describes what is known about this variation. We discuss how variation in the PTI response can be measured and how this knowledge might be utilized in the control of plant disease and in developing plant varieties with enhanced disease resistance.

Join the green team: Inducers of plant immunity in the plant disease sustainable control toolbox

BACKGROUND

Crops are constantly attacked by various pathogens. These pathogenic microorganisms, such as fungi, oomycetes, bacteria, viruses, and nematodes, threaten global food security by causing detrimental crop diseases that generate tremendous quality and yield losses worldwide. Chemical pesticides have undoubtedly reduced crop damage; however, in addition to increasing the cost of agricultural production, the extensive use of chemical pesticides comes with environmental and social costs. Therefore, it is necessary to vigorously develop sustainable disease prevention and control strategies to promote the transition from traditional chemical control to modern green technologies. Plants possess sophisticated and efficient defense mechanisms against a wide range of pathogens naturally. Immune induction technology based on plant immunity inducers can prime plant defense mechanisms and greatly decrease the occurrence and severity of plant diseases. Reducing the use of agrochemicals is an effective way to minimize environmental pollution and promote agricultural safety.

AIM OF REVIEW

The purpose of this workis to offer valuable insights into the current understanding and future research perspectives of plant immunity inducers and their uses in plant disease control, ecological and environmental protection, and sustainable development of agriculture.

KEY SCIENTIFIC CONCEPTS OF REVIEW

In this work, we have introduced the concepts of sustainable and environment-friendly concepts of green disease prevention and control technologies based on plant immunity inducers. This article comprehensively summarizes these recent advances, emphasizes the importance of sustainable disease prevention and control technologies for food security, and highlights the diverse functions of plant immunity inducers-mediated disease resistance. The challenges encountered in the potential applications of plant immunity inducers and future research orientation are also discussed.

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.

Phytohormone ethylene mediates oligogalacturonic acid-induced growth inhibition in tomato etiolated seedlings

Plant growth and immunity are tightly interconnected. Oligogalacturonic acids (OGs) are pectic fragments and have been well investigated in plant immunity as a damage-associated molecular pattern. However, little is known regarding how OGs affect plant growth. Here, we reveal that OGs inhibit the growth of intact etiolated seedling by using the horticultural crop tomato as a model. This inhibitory effect is partially suppressed by the action of ethylene biosynthesis inhibitors, or the gene silencing of SlACS2, an essential rate-limiting enzyme for ethylene biosynthesis, suggesting that SlACS2-mediated ethylene production promotes OG-induced growth inhibition. Furthermore, OGs treatment elevates the SlACS2 protein phosphorylation, and its decrease by the kinase inhibitor K252a partially rescue OG-induced growth inhibition, indicating that SlACS2 phosphorylation involves in OG-induced growth inhibition. Moreover, the mitogen-activated protein kinase SlMPK3 could be activated by OGs treatment and can directly phosphorylate SlACS2 in vitro, and the bimolecular fluorescence complementation combining with the yeast two-hybrid assay shows that SlMPK3 interacts with SlACS2, indicating that SlMPK3 may participate in modulating the OG-induced SlACS2 phosphorylation and growth inhibition. Our results reveal a regulatory mechanism at both the transcriptional and post-transcriptional levels by which OGs inhibit the growth of intact plant seedlings.

A leucine-rich repeat receptor kinase gene confers quantitative susceptibility to maize southern leaf blight

Southern leaf blight (SLB), caused by the necrotrophic fungal pathogen Cochliobolus heterostrophus (anamorph Bipolaris maydis), is a major foliar disease which causes significant yield losses in maize worldwide. A major quantitative trait locus, qSLB_3.04 , conferring recessive resistance to SLB was previously mapped on maize chromosome 3. Using a combination of map-based cloning, association analysis, ethyl methanesulfonate and transposon mutagenesis, and CRISPR-Cas9 editing, we demonstrate that a leucine-rich repeat receptor-like kinase gene which we have called ChSK1 (Cochliobolus heterostrophus Susceptibility Kinase 1) at qSLB_3.04 causes increased susceptibility to SLB. Genes of this type have generally been associated with the defense response. We present evidence that ChSK1 may be associated with suppression of the basal immune response. These findings contribute to our understanding of plant disease susceptibility genes and the potential to use them for engineering durable disease resistance.

Glutamate receptor like channels: Emerging players in calcium mediated signaling in plants

Glutamate receptors like channels (GLRs) are ligand gated non-selective cation channels and are multigenic in nature. They are homologs of mammalian ionic glutamate receptors (iGLRs) that play an important role in neurotransmission. It has been more than 25 years of discovery of plant GLRs, since then, significant progress has been made to unravel their structure and function in plants. Recently, the first crystal structure of plant GLR has been resolved that suggests that, though, plant GLRs contain the conserved signature domains of iGLRs, their unique features enable agonist/antagonist-dependent change in their activity. GLRs exhibit diverse subcellular localization and undergo dynamic expression variation in response to developmental and environmental stress conditions in plants. The combined use of genetic, electrophysiology and calcium imaging using different genetically encoded calcium indicators has revealed that GLRs are involved in generating calcium (Ca²⁺) influx across the plasma membrane and are involved in shaping the Ca²⁺ signature in response to different developmental and environmental stimuli. These findings indicate that GLRs influence cytosolic Ca²⁺ dynamics, thus, highlighting "GLR-Ca²⁺-crosstalk (GCC)" in developmental and stress-responsive signaling pathways. With this background, the present review summarises the recent developments pertaining to GLR function, in the broader context of regulation of stress tolerance in plants.

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.

Bacterial Volatiles (mVOC) Emitted by the Phytopathogen Erwinia amylovora Promote Arabidopsis thaliana Growth and Oxidative Stress

Phytopathogens are well known for their devastating activity that causes worldwide significant crop losses. However, their exploitation for crop welfare is relatively unknown. Here, we show that the microbial volatile organic compound (mVOC) profile of the bacterial phytopathogen, Erwinia amylovora, enhances Arabidopsis thaliana shoot and root growth. GC-MS head-space analyses revealed the presence of typical microbial volatiles, including 1-nonanol and 1-dodecanol. E. amylovora mVOCs triggered early signaling events including plasma transmembrane potential Vm depolarization, cytosolic Ca2+ fluctuation, K+-gated channel activity, and reactive oxygen species (ROS) and nitric oxide (NO) burst from few minutes to 16 h upon exposure. These early events were followed by the modulation of the expression of genes involved in plant growth and defense responses and responsive to phytohormones, including abscisic acid, gibberellin, and auxin (including the efflux carriers PIN1 and PIN3). When tested, synthetic 1-nonanol and 1-dodecanol induced root growth and modulated genes coding for ROS. Our results show that E. amylovora mVOCs affect A. thaliana growth through a cascade of early and late signaling events that involve phytohormones and ROS.

Cell Wall Integrity Signaling in Fruit Ripening

Plant cell walls are essential structures for plant growth and development as well as plant adaptation to environmental stresses. Thus, plants have evolved signaling mechanisms to monitor the changes in the cell wall structure, triggering compensatory changes to sustain cell wall integrity (CWI). CWI signaling can be initiated in response to environmental and developmental signals. However, while environmental stress-associated CWI signaling has been extensively studied and reviewed, less attention has been paid to CWI signaling in relation to plant growth and development under normal conditions. Fleshy fruit development and ripening is a unique process in which dramatic alternations occur in cell wall architecture. Emerging evidence suggests that CWI signaling plays a pivotal role in fruit ripening. In this review, we summarize and discuss the CWI signaling in relation to fruit ripening, which will include cell wall fragment signaling, calcium signaling, and NO signaling, as well as Receptor-Like Protein Kinase (RLKs) signaling with an emphasis on the signaling of FERONIA and THESEUS, two members of RLKs that may act as potential CWI sensors in the modulation of hormonal signal origination and transduction in fruit development and ripening.

Oligogalacturonides Enhance Resistance against Aphids through Pattern-Triggered Immunity and Activation of Salicylic Acid Signaling

The remarkable capacity of the generalist aphid Myzus persicae to resist most classes of pesticides, along with the environmental and human health risks associated with these agrochemicals, has necessitated the development of safer and greener solutions to control this agricultural pest. Oligogalacturonides (OGs) are pectin-derived molecules that can be isolated from fruit industry waste. OGs have been shown to efficiently stimulate plant defenses against pathogens such as Pseudomonas syringae and Botrytis cinerea. However, whether OGs confer resistance against phytophagous insects such as aphids remains unknown. Here, we treated Arabidopsis plants with OGs and recorded their effects on the feeding performance and population of M. persicae aphids. We also identified the defense mechanism triggered by OGs in plants through the analysis of gene expression and histological approaches. We found that OG treatments increased their resistance to M. persicae infestation by reducing the offspring number and feeding performance. Furthermore, this enhanced resistance was related to a substantial accumulation of callose and reactive oxygen species and activation of the salicylic acid signaling pathway.

Effect of Oligogalacturonides on Seed Germination and Disease Resistance of Sugar Beet Seedling and Root

Oligogalacturonides (OGs) are a bioactive carbohydrate derived from homogalacturonan. The OGs synthesized in this study significantly inhibited the mycelial growth of Rhizoctonia solani AG-4HGI in vitro, even at a low concentration (10 mg/L). The seed vigor test demonstrated that the application of 50 mg/L OGs to sugar beet seeds significantly increased average germination percentage, germination energy, germination index, and seedling vigor index. The same concentration of OGs also improved the seedling emergence percentage of sugar beet when seeds were sown in soil inoculated with D2 and D31 isolates, respectively. The lesion diameter on mature sugar beet roots caused by R. solani AG-4HGI isolates D2 and D31 also decreased by 40.60% and 39.86%, respectively, in sugar beets roots first treated with 50 mg/mL OGs in the wound site, relative to lesion size in untreated/pathogen inoculated wounds. Sugar beet roots treated with 50 mg/mL OGs prior to inoculation with the D2 isolate exhibited up-regulation of the defense-related genes glutathione peroxidase (GPX) and superoxide dismutase (SOD) by 2.4- and 1.6-fold, respectively, relative to control roots. Sugar beet roots treated with 50 mg/mL OGs prior to inoculation with D31 exhibited a 2.0- and 1.6-fold up-regulation of GPX and SOD, respectively, relative to the control. Our results indicate that OGs have the potential to be used for the protection of sugar beet against R. solani AG-4HGI.

Alginate-Induced Disease Resistance in Plants

Plants are continuously exposed to a wide range of pathogens, including fungi, bacteria, nematodes, and viruses; therefore, survival under these conditions requires a sophisticated defense system. The activation of defense responses and related signals in plants is regulated mainly by the hormones salicylic acid, jasmonic acid, and ethylene. Resistance to pathogen infection can be induced in plants by various biotic and abiotic agents. For many years, the use of abiotic plant resistance inducers has been considered in integrated disease management programs. Recently, natural inducer compounds, such as alginates, have become a focus of interest due to their environmentally friendly nature and their ability to stimulate plant defense mechanisms and enhance growth. Polysaccharides and the oligosaccharides derived from them are examples of eco-compatible compounds that can enhance plant growth while also inducing plant resistance against pathogens and triggering the expression of the salicylic acid-dependent defense pathway.

Mechanisms of plant cell wall surveillance in response to pathogens, cell wall-derived ligands and the effect of expansins to infection resistance or susceptibility

Cell wall integrity is tightly regulated and maintained given that non-physiological modification of cell walls could render plants vulnerable to biotic and/or abiotic stresses. Expansins are plant cell wall-modifying proteins active during many developmental and physiological processes, but they can also be produced by bacteria and fungi during interaction with plant hosts. Cell wall alteration brought about by ectopic expression, overexpression, or exogenous addition of expansins from either eukaryote or prokaryote origin can in some instances provide resistance to pathogens, while in other cases plants become more susceptible to infection. In these circumstances altered cell wall mechanical properties might be directly responsible for pathogen resistance or susceptibility outcomes. Simultaneously, through membrane receptors for enzymatically released cell wall fragments or by sensing modified cell wall barrier properties, plants trigger intracellular signaling cascades inducing defense responses and reinforcement of the cell wall, contributing to various infection phenotypes, in which expansins might also be involved. Here, we review the plant immune response activated by cell wall surveillance mechanisms, cell wall fragments identified as responsible for immune responses, and expansin's roles in resistance and susceptibility of plants to pathogen attack.

Recent Advances in Understanding the Roles of Pectin as an Active Participant in Plant Signaling Networks

Pectin is an abundant cell wall polysaccharide with essential roles in various biological processes. The structural diversity of pectins, along with the numerous combinations of the enzymes responsible for pectin biosynthesis and modification, plays key roles in ensuring the specificity and plasticity of cell wall remodeling in different cell types and under different environmental conditions. This review focuses on recent progress in understanding various aspects of pectin, from its biosynthetic and modification processes to its biological roles in different cell types. In particular, we describe recent findings that cell wall modifications serve not only as final outputs of internally determined pathways, but also as key components of intercellular communication, with pectin as a major contributor to this process. The comprehensive view of the diverse roles of pectin presented here provides an important basis for understanding how cell wall-enclosed plant cells develop, differentiate, and interact.

High-CO2 Treatment Prolongs the Postharvest Shelf Life of Strawberry Fruits by Reducing Decay and Cell Wall Degradation

Improved methods are needed to extend the shelf life of strawberry fruits. The objective of this study was to determine the postharvest physiological mechanism of high-CO₂ treatment in strawberries. Harvested strawberries were stored at 10 °C after 3 h of exposure to a treatment with 30% CO₂ or air. Pectin and gene expression levels related to cell wall degradation were measured to assess the high-CO₂ effects on the cell wall and lipid metabolism. Strawberries subjected to high-CO₂ treatment presented higher pectin content and firmness and lower decay than those of control fruits. Genes encoding cell wall-degrading enzymes (pectin methylesterase, polygalacturonase, and pectate lyase) were downregulated after high-CO₂ treatment. High-CO₂ induced the expression of oligogalacturonides, thereby conferring defense against Botrytis cinerea in strawberry fruits, and lowering the decay incidence at seven days after its inoculation. Our findings suggest that high-CO₂ treatment can maintain strawberry quality by reducing decay and cell wall degradation.

Selective redox signaling shapes plant-pathogen interactions

A review of recent progress in understanding the mechanisms whereby plants utilize selective and reversible redox signaling to establish immunity.

GhMYB4 downregulates lignin biosynthesis and enhances cotton resistance to Verticillium dahliae

GhMYB4 acts as a negative regulator in lignin biosynthesis, which results in alteration of cell wall integrity and activation of cotton defense response. Verticillium wilt of cotton (Gossypium hirsutum) caused by the soil-borne fungus Verticillium dahliae (V. dahliae) represents one of the most important constraints of cotton production worldwide. Mining of the genes involved in disease resistance and illuminating the molecular mechanisms that underlie this resistance is of great importance in cotton breeding programs. Defense-induced lignification in plants is necessary for innate immunity, and there are reports of a correlation between increased lignification and disease resistance. In this study, we present an example in cotton whereby plants with reduced lignin content also exhibit enhanced disease resistance. We identified a negative regulator of lignin synthesis, in cotton encoded in GhMYB4. Overexpression of GhMYB4 in cotton and Arabidopsis enhanced resistance to V. dahliae  with reduced lignin deposition. Moreover, GhMYB4 could bind the promoters of several genes involved in lignin synthesis, such as GhC4H-1, GhC4H-2, Gh4CL-4, and GhCAD-3, and impair their expression. The reduction of lignin content in GhMYB4-overexpressing cotton led to alterations of cell wall integrity (CWI) and released more oligogalacturonides (OGs) which may act as damage-associated molecular patterns (DAMPs) to stimulate plant defense responses. In support of this hypothesis, exogenous application with polygalacturonic acid (PGA) in cotton activated biosynthesis of jasmonic acid (JA) and JA-mediated defense against V. dahliae, similar to that described for cotton plants overexpressing GhMYB4. This study provides a new candidate gene for cotton disease-resistant breeding and an increased understanding of the relationship between lignin synthesis, OG release, and plant immunity.

NO Network for Plant-Microbe Communication Underground: A Review

Mechanisms governing plant-microbe interaction in the rhizosphere attracted a lot of investigative attention in the last decade. The rhizosphere is not simply a source of nutrients and support for the plants; it is rather an ecosystem teeming with diverse flora and fauna including different groups of microbes that are useful as well as harmful for the plants. Plant-microbe interaction occurs via a highly complex communication network that involves sophisticated machinery for the recognition of friend and foe at both sides. On the other hand, nitric oxide (NO) is a key, signaling molecule involved in plant development and defense. Studies on legume-rhizobia symbiosis suggest the involvement of NO during recognition, root hair curling, development of infection threads, nodule development, and nodule senescence. A similar role of NO is also suggested in the case of plant interaction with the mycorrhizal fungi. Another, insight into the plant-microbe interaction in the rhizosphere comes from the recognition of pathogen-associated molecular patterns (PAMPs)/microbe-associated molecular patterns (MAMPs) by the host plant and thereby NO-mediated activation of the defense signaling cascade. Thus, NO plays a major role in mediating the communication between plants and microbes in the rhizosphere. Interestingly, reports suggesting the role of silicon in increasing the number of nodules, enhancing nitrogen fixation, and also the combined effect of silicon and NO may indicate a possibility of their interaction in mediating microbial communication underground. However, the exact role of NO in mediating plant-microbe interaction remains elusive. Therefore, understanding the role of NO in underground plant physiology is very important, especially in relation to the plant's interaction with the rhizospheric microbiome. This will help devise new strategies for protection against phytopathogens and enhancing plant productivity by promoting symbiotic interaction. This review focuses on the role of NO in plant-microbe communication underground.

Nitric oxide signalling in plant interactions with pathogenic fungi and oomycetes

Nitric oxide (NO) and reactive nitrogen species have emerged as crucial signalling and regulatory molecules across all organisms. In plants, fungi, and fungi-like oomycetes, NO is involved in the regulation of multiple processes during their growth, development, reproduction, responses to the external environment, and biotic interactions. It has become evident that NO is produced and used as a signalling and defence cue by both partners in multiple forms of plant interactions with their microbial counterparts, ranging from symbiotic to pathogenic modes. This review summarizes current knowledge on the role of NO in plant-pathogen interactions, focused on biotrophic, necrotrophic, and hemibiotrophic fungi and oomycetes. Actual advances and gaps in the identification of NO sources and fate in plant and pathogen cells are discussed. We review the decisive role of time- and site-specific NO production in germination, oriented growth, and active penetration by filamentous pathogens of the host tissues, as well in pathogen recognition, and defence activation in plants. Distinct functions of NO in diverse interactions of host plants with fungal and oomycete pathogens of different lifestyles are highlighted, where NO in interplay with reactive oxygen species governs successful plant colonization, cell death, and establishment of resistance.

Botrytis cinerea induces local hypoxia in Arabidopsis leaves

Low oxygen availability often is associated with soil waterlogging or submergence, but may occur also as hypoxic niches in otherwise aerobic tissues. Experimental evidence assigns a role in Botrytis cinerea resistance to a group of oxygen-unstable Ethylene Response Factors (ERF-VII). Given that infection by B. cinerea often occurs in aerobic organs such as leaves, where ERF-VII stability should be compromised, we explored the possibility of local leaf hypoxia at the site of infection. We analyzed the expression of hypoxia-responsive genes in infected leaves. Confocal microscopy was utilized to verify the localization of the ERF-VII protein RAP2.12. Oxygen concentration was measured to evaluate the availability of oxygen (O₂ ). We discovered that infection by B. cinerea induces increased respiration, leading to a drastic drop in the O₂ concentration in an otherwise fully aerobic leaf. The establishment of a local hypoxic area results in stabilization and nuclear relocalization of RAP2.12. The possible roles of defence elicitors, ABA and ethylene were evaluated. Local hypoxia at the site of B. cinerea infection allows the stabilization of ERF-VII proteins. Hypoxia at the site of pathogen infection generates a nearly O₂ -free environment that may affect the stability of other N-degron-regulated proteins as well as the metabolism of elicitors.

Dampening the DAMPs: How Plants Maintain the Homeostasis of Cell Wall Molecular Patterns and Avoid Hyper-Immunity

Several oligosaccharide fragments derived from plant cell walls activate plant immunity and behave as typical damage-associated molecular patterns (DAMPs). Some of them also behave as negative regulators of growth and development, and due to their antithetic effect on immunity and growth, their concentrations, activity, time of formation, and localization is critical for the so-called "growth-defense trade-off." Moreover, like in animals, over accumulation of DAMPs in plants provokes deleterious physiological effects and may cause hyper-immunity if the cellular mechanisms controlling their homeostasis fail. Recently, a mechanism has been discovered that controls the activity of two well-known plant DAMPs, oligogalacturonides (OGs), released upon hydrolysis of homogalacturonan (HG), and cellodextrins (CDs), products of cellulose breakdown. The potential homeostatic mechanism involves specific oxidases belonging to the family of berberine bridge enzyme-like (BBE-like) proteins. Oxidation of OGs and CDs not only inactivates their DAMP activity, but also makes them a significantly less desirable food source for microbial pathogens. The evidence that oxidation and inactivation of OGs and CDs may be a general strategy of plants for controlling the homeostasis of DAMPs is discussed. The possibility exists of discovering additional oxidative and/or inactivating enzymes targeting other DAMP molecules both in the plant and in animal kingdoms.

Breaking Bad News: Dynamic Molecular Mechanisms of Wound Response in Plants

Recognition and repair of damaged tissue are an integral part of life. The failure of cells and tissues to appropriately respond to damage can lead to severe dysfunction and disease. Therefore, it is essential that we understand the molecular pathways of wound recognition and response. In this review, we aim to provide a broad overview of the molecular mechanisms underlying the fate of damaged cells and damage recognition in plants. Damaged cells release the so-called damage associated molecular patterns to warn the surrounding tissue. Local signaling through calcium (Ca2+), reactive oxygen species (ROS), and hormones, such as jasmonic acid, activates defense gene expression and local reinforcement of cell walls to seal off the wound and prevent evaporation and pathogen colonization. Depending on the severity of damage, Ca2+, ROS, and electrical signals can also spread throughout the plant to elicit a systemic defense response. Special emphasis is placed on the spatiotemporal dimension in order to obtain a mechanistic understanding of wound signaling in plants.

Oligogalacturonides induce resistance in Arabidopsis thaliana by triggering salicylic acid and jasmonic acid pathways against Pst DC3000

Oligogalacturonides (OGAs) are a biologically active carbohydrate derived from homogalacturonan, a major element of cell wall pectin. OGAs induced resistance and mechanism were assessed in Arabidopsis thaliana-Pseudomonas syringae pv. tomato DC3000 (Pst DC3000) interaction. The effective resistance was mainly observed at 25 mg/L OGAs with reduced disease index, bacterial multiplication, higher transcript level of salicylic acid (SA) pathway related genes (PR1, PR2, PR5) and jasmonic acid (JA) pathway related genes (PDF1.2, VSP2) as well as SA, JA content and production of reactive oxygen species (ROS), nitric oxide (NO). In SA (NahG, sid2) and JA (jar1) deficient mutants, disease severity indicated that both SA and JA pathways are necessary for Arabidopsis response to Pst DC3000. OGAs triggered less resistance to Pst DC3000 in JA-deficient mutant, and SA-deficient mutants signifying that SA and JA play redundant roles in OGAs induced resistance. Therefore, these evidences further reveal the signaling pathways of OGAs resistance, which is conducive to its application in agriculture to protect plants from diseases.

Nitric Oxide and Hydrogen Peroxide Are Involved in Methyl Jasmonate-Regulated Response against Botrytis cinerea in Postharvest Blueberries

The involvement and the relationship between nitric oxide (NO) and hydrogen peroxide (H₂O₂) in methyl jasmonate (MeJA)-induced immune responses in blueberries against Botrytis cinerea was explored using diphenylene iodonium (DPI, NADPH oxidase inhibitor) and 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO, NO scavenger). MeJA induced NO and H₂O₂ burst and enhanced the resistance of blueberries by elevating defense-related enzymes and the phenylpropanoid pathway. However, the above impacts stimulated by MeJA were weakened by DPI and destroyed by cPTIO. Furthermore, cPTIO abolished the increment in H₂O₂ by regulating the activities of NADPH oxidase, superoxide dismutase, catalase, and ascorbate peroxidase, whereas DPI weakened the increase in H₂O₂ but barely affected the generation of NO and the activity of nitric oxide synthase elevated by MeJA. These results indicated that NO and H₂O₂ were involved in the MeJA-induced resistance in blueberries, and NO worked upstream of H₂O₂ in this process.

Nitrate reductase-dependent nitric oxide plays a key role on MeJA-induced ganoderic acid biosynthesis in Ganoderma lucidum

Ganoderma lucidum, which contains numerous biologically active compounds, is known worldwide as a medicinal basidiomycete. Because of its application for the prevention and treatment of various diseases, most of artificially cultivated G. lucidum is output to many countries as food, tea, and dietary supplements for further processing. Methyl jasmonate (MeJA) has been reported as a compound that can induce ganoderic acid (GA) biosynthesis, an important secondary metabolite of G. lucidum. Herein, MeJA was found to increase the intracellular level of nitric oxide (NO). In addition, upregulation of GA biosynthesis in the presence of MeJA was abolished when NO was depleted from the culture. This result demonstrated that MeJA-regulated GA biosynthesis might occur via NO signaling. To elucidate the underlying mechanism, we used gene-silenced strains of nitrate reductase (NR) and the inhibitor of NR to illustrate the role of NO in MeJA induction. The results indicated that the increase in GA biosynthesis induced by MeJA was activated by NR-generated NO. Furthermore, the findings indicated that the reduction of NO could induce GA levels in the control group, but NO could also activate GA biosynthesis upon MeJA treatment. Further results indicated that NR silencing reversed the increased enzymatic activity of NOX to generate ROS due to MeJA induction. Importantly, our results highlight the NR-generated NO functions in signaling crosstalk between reactive oxygen species and MeJA. These results provide a good opportunity to determine the potential pathway linking NO to the ROS signaling pathway in fungi treated with MeJA. KEY POINTS: • MeJA increased the intracellular level of nitric oxide (NO) in G. lucidum. • The increase in GA biosynthesis induced by MeJA is activated by NR-generated NO. • NO acts as a signaling molecule between reactive oxygen species (ROS) and MeJA.

Differential modulation of S-nitrosoglutathione reductase and reactive nitrogen species in wild and cultivated tomato genotypes during development and powdery mildew infection

Nitric oxide plays an important role in the pathogenesis of Pseudoidium neolycopersici, the causative agent of tomato powdery mildew. S-nitrosoglutathione reductase, the key enzyme of S-nitrosothiol homeostasis, was investigated during plant development and following infection in three genotypes of Solanum spp. differing in their resistance to P. neolycopersici. Levels and localization of reactive nitrogen species (RNS) including NO, S-nitrosoglutathione (GSNO) and peroxynitrite were studied together with protein nitration and the activity of nitrate reductase (NR). GSNOR expression profiles and enzyme activities were modulated during plant development and important differences among Solanum spp. genotypes were observed, accompanied by modulation of NO, GSNO, peroxynitrite and nitrated proteins levels. GSNOR was down-regulated in infected plants, with exception of resistant S. habrochaites early after inoculation. Modulations of GSNOR activities in response to pathogen infection were found also on the systemic level in leaves above and below the inoculation site. Infection strongly increased NR activity and gene expression in resistant S. habrochaites in contrast to susceptible S. lycopersicum. Obtained data confirm the key role of GSNOR and modulations of RNS during plant development under normal conditions and point to their involvement in molecular mechanisms of tomato responses to biotrophic pathogens on local and systemic levels.

Salmonella enterica Elicits and Is Restricted by Nitric Oxide and Reactive Oxygen Species on Tomato

The enteric pathogen Salmonella enterica can interact with parts of the plant immune system despite not being a phytopathogen. Previous transcriptomic profiling of S. enterica associating with tomato suggested that Salmonella was responding to oxidative and nitrosative stress in the plant niche. We aimed to investigate whether Salmonella was eliciting generation of reactive oxygen species (ROS) and nitric oxide (NO), two components of the microbe-associated molecular pattern (MAMP)-triggered immunity (MTI) of plants. We also sought to determine whether this interaction had any measurable effects on Salmonella colonization of plants. Biochemical, gene expression and on-plant challenge assays of tomato vegetative and fruit organs were conducted to assess the elicitation of ROS and NO in response to Salmonella Newport association. The counter bacterial response and the effect of NO and ROS on Salmonella colonization was also investigated. We detected H₂O₂ in leaves and fruit following challenge with live S. Newport (p < 0.05). Conversely, NO was detected on leaves but not on fruit in response to S. Newport (p < 0.05). We found no evidence of plant defense attenuation by live S. Newport. Bacterial gene expression of S. Newport associating with leaves and fruit were indicative of adaptation to biotic stress in the plant niche. The nitrosative stress response genes hmpA and yoaG were significantly up-regulated in S. Newport on leaves and fruit tissue compared to tissue scavenged of NO or ROS (p < 0.05). Chemical modulation of these molecules in the plant had a restrictive effect on bacterial populations. Significantly higher S. Newport titers were retrieved from H₂O₂ scavenged leaves and fruit surfaces compared to controls (p < 0.05). Similarly, S. Newport counts recovered from NO-scavenged leaves, but not fruit, were higher compared to control (p < 0.05), and significantly lower on leaves pre-elicited to produce endogenous NO. We present evidence of Salmonella elicitation of ROS and NO in tomato, which appear to have a restricting effect on the pathogen. Moreover, bacterial recognition of ROS and NO stress was detected. This work shows that tomato has mechanisms to restrict Salmonella populations and ROS and NO detoxification may play an important role in Salmonella adaptation to the plant niche.

Recommendations on terminology and experimental best practice associated with plant nitric oxide research

Nitric oxide (NO) emerged as a key signal molecule in plants. During the last two decades impressive progress has been made in plant NO research. This small, redox-active molecule is now known to play an important role in plant immunity, stress responses, environmental interactions, plant growth and development. To more accurately and robustly establish the full spectrum of NO bioactivity in plants, it will be essential to apply methodological best practice. In addition, there are some instances of conflicting nomenclature within the field, which would benefit from standardization. In this context, we attempt to provide some helpful guidance for best practice associated with NO research and also suggestions for the cognate terminology.

TaWAK6 encoding wall-associated kinase is involved in wheat resistance to leaf rust similar to adult plant resistance

In wheat, adult plant resistance (APR) to leaf rust (Puccinia triticina), is effective in restricting pathogen growth and provides durable resistance against a wide range of virulent forms of P. triticina. Despite the importance, there is limited knowledge on the molecular basis of this type of resistance. We isolated and characterized the wall-associated kinase encoding gene in wheat, and assigned it as TaWAK6. Localization of TaWAK6 homeologs in A and B wheat subgenomes was consistent with the presence of the gene's orthologs in T. urartu (AA) and T. dicoccoides (AABB) and with the absence of its orthologs in Aegilops tauschii (DD). Overexpression of TaWAK6 did not change the wheat phenotype, nor did it affect seedling resistance. However, the adult plants overexpressing TaWAK6 showed that important parameters of APR were significantly elevated. Infection types scored on the first (flag), second and third leaves indicated elevated resistance, which significantly correlated with expression of TaWAK6. Analysis of plant-pathogen interactions showed a lower number of uredinia and higher rates of necrosis at the infection sites and this was associated with smaller size of uredinia and a longer latent period. The results indicated a role of TaWAK6 in quantitative partial resistance similar to APR in wheat. It is proposed that TaWAK6, which is a non-arginine-aspartate (non-RD) kinase, represents a novel class of quantitative immune receptors in monocots.

OsPGIP1-Mediated Resistance to Bacterial Leaf Streak in Rice is Beyond Responsive to the Polygalacturonase of Xanthomonas oryzae pv. oryzicola

Polygalacturonase-inhibiting proteins (PGIPs) have been shown to recognize fungal polygalacturonases (PGs), which initiate innate immunity in various plant species. Notably, the connection between rice OsPGIPs and PGs in Xanthomonas oryzae pv. oryzicola (Xoc), which causes bacterial leaf streak (BLS), remains unclear. Here, we show that OsPGIP1 was strongly induced after inoculating rice with the Xoc strain RS105. Furthermore, OsPGIP1-overexpressing (OV) and RNA interference (RNAi) rice lines increased and decreased, respectively, the resistance of rice to RS105, indicating that OsPGIP1 contributes to BLS resistance. Subsequently, we generated the unique PG mutant RS105Δpg, the virulence of which is attenuated compared to that of RS105. Surprisingly, the lesion lengths caused by RS105Δpg were similar to those caused by RS105 in the OV lines compared with wild-type ZH11 with reduced Xoc susceptibility. However, the lesion lengths caused by RS105Δpg were still significantly shorter in the OV lines than in ZH11, implying that OsPGIP1-mediated BLS resistance could respond to other virulence factors in addition to PGs. To explore the OsPGIP1-mediated resistance, RNA-seq analysis were performed and showed that many plant cell wall-associated genes and several MYB transcription factor genes were specifically expressed or more highly induced in the OV lines compared to ZH11 postinoculation with RS105. Consistent with the expression of the differentially expressed genes, the OV plants accumulated a higher content of jasmonic acid (JA) than ZH11 postinoculation with RS105, suggesting that the OsPGIP1-mediated resistance to BLS is mainly dependent on the plant cell wall-associated immunity and the JA signaling pathway.

Novel and conserved functions of S-nitrosoglutathione reductase in tomato

Nitric oxide (NO) is emerging as a key signalling molecule in plants. The chief mechanism for the transfer of NO bioactivity is thought to be S-nitrosylation, the addition of an NO moiety to a protein cysteine thiol to form an S-nitrosothiol (SNO). The enzyme S-nitrosoglutathione reductase (GSNOR) indirectly controls the total levels of cellular S-nitrosylation, by depleting S-nitrosoglutathione (GSNO), the major cellular NO donor. Here we show that depletion of GSNOR function impacts tomato (Solanum lycopersicum. L) fruit development. Thus, reduction of GSNOR expression through RNAi modulated both fruit formation and yield, establishing a novel function for GSNOR. Further, depletion of S. lycopersicum GSNOR (SlGSNOR) additionally impacted a number of other developmental processes, including seed development, which also has not been previously linked with GSNOR activity. In contrast to Arabidopsis, depletion of GSNOR function did not influence root development. Further, reduction of GSNOR transcript abundance compromised plant immunity. Surprisingly, this was in contrast to previous data in Arabidopsis that reported that reducing Arabidopsis thaliana GSNOR (AtGSNOR) expression by antisense technology increased disease resistance. We also show that increased SlGSNOR expression enhanced pathogen protection, uncovering a potential strategy to enhance disease resistance in crop plants. Collectively, our findings reveal, at the genetic level, that some but not all GSNOR activities are conserved outside the Arabidopsis reference system. Thus, manipulating the extent of GSNOR expression may control important agricultural traits in tomato and possibly other crop plants.

Nitric oxide in plant-fungal interactions

Whilst many interactions with fungi are detrimental for plants, others are beneficial and result in improved growth and stress tolerance. Thus, plants have evolved sophisticated mechanisms to restrict pathogenic interactions while promoting mutualistic relationships. Numerous studies have demonstrated the importance of nitric oxide (NO) in the regulation of plant defence against fungal pathogens. NO triggers a reprograming of defence-related gene expression, the production of secondary metabolites with antimicrobial properties, and the hypersensitive response. More recent studies have shown a regulatory role of NO during the establishment of plant-fungal mutualistic associations from the early stages of the interaction. Indeed, NO has been recently shown to be produced by the plant after the recognition of root fungal symbionts, and to be required for the optimal control of mycorrhizal symbiosis. Although studies dealing with the function of NO in plant-fungal mutualistic associations are still scarce, experimental data indicate that different regulation patterns and functions for NO exist between plant interactions with pathogenic and mutualistic fungi. Here, we review recent progress in determining the functions of NO in plant-fungal interactions, and try to identify common and differential patterns related to pathogenic and mutualistic associations, and their impacts on plant health.

The evolution of nitric oxide signalling diverges between animal and green lineages

Nitric oxide (NO) is a ubiquitous signalling molecule with widespread distribution in prokaryotes and eukaryotes where it is involved in countless physiological processes. While the mechanisms governing nitric oxide (NO) synthesis and signalling are well established in animals, the situation is less clear in the green lineage. Recent investigations have shown that NO synthase, the major enzymatic source for NO in animals, is absent in land plants but present in a limited number of algae. The first detailed analysis highlighted that these new NO synthases are functional but display specific structural features and probably original catalytic activities. Completing this picture, analyses were undertaken in order to investigate whether major components of the prototypic NO/cyclic GMP signalling cascades mediating many physiological effects of NO in animals were also present in plants. Only a few homologues of soluble guanylate cyclases, cGMP-dependent protein kinases, cyclic nucleotide-gated channels, and cGMP-regulated phosphodiesterases were identified in some algal species and their presence did not correlate with that of NO synthases. In contrast, S-nitrosoglutathione reductase, a critical regulator of S-nitrosothiols, was recurrently found. Overall, these findings highlight that plants do not mediate NO signalling through the classical NO/cGMP signalling module and support the concept that S-nitrosation is a ubiquitous NO-dependent signalling mechanism.

Nitric oxide and ROS mediate autophagy and regulate Alternaria alternata toxin-induced cell death in tobacco BY-2 cells

Synergistic interaction of nitric oxide (NO) and reactive oxygen species (ROS) is essential to initiate cell death mechanisms in plants. Though autophagy is salient in either restricting or promoting hypersensitivity response (HR)-related cell death, the crosstalk between the reactive intermediates and autophagy during hypersensitivity response is paradoxical. In this investigation, the consequences of Alternaria alternata toxin (AaT) in tobacco BY-2 cells were examined. At 3 h, AaT perturbed intracellular ROS homeostasis, altered antioxidant enzyme activities, triggered mitochondrial depolarization and induced autophagy. Suppression of autophagy by 3-Methyladenine caused a decline in cell viability in AaT treated cells, which indicated the vital role of autophagy in cell survival. After 24 h, AaT facilitated Ca2+ influx with an accumulation of reactive oxidant intermediates and NO, to manifest necrotic cell death. Inhibition of NO accumulation by 2-(4-Carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) decreased the level of necrotic cell death, and induced autophagy, which suggests NO accumulation represses autophagy and facilitates necrotic cell death at 24 h. Application of N-acetyl-L-cysteine at 3 h, confirmed ROS to be the key initiator of autophagy, and together with cPTIO for 24 h, revealed the combined effects of NO and ROS is required for necrotic HR cell death.

Salt tolerance in indica rice cell cultures depends on a fine tuning of ROS signalling and homeostasis

Among cereal crops, salinity tolerance is rare and complex. Multiple genes control numerous pathways, which constitute plant's response to salinity. Cell cultures act as model system and are useful to investigate the salinity response which can possibly mimic a plant's response to stress. In the present study two indica rice varieties, KS-282 and Super Basmati which exhibited contrasting sodium chloride (NaCl) stress response were used to establish cell cultures. The cell cultures showed a contrasting response to salt stress at 100 mM NaCl. High level of intracellular hydrogen peroxide (H2O2) and nitric oxide (NO) were observed in sensitive cell culture for prolonged period as compared to the tolerant cells in which an extracellular H2O2 burst along with controlled intracellular H2O2 and NO signal was seen. To evaluate the role of NO in inducing cell death under salt stress, cell death percentage (CDP) was measured after 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) pre-treatment. CDP was reduced significantly in both tolerant and sensitive cell cultures emphasizing NO's possible role in programmed cell death. Expression analysis of apoplastic NADPH oxidase, i.e. OsRbohA and recently characterised OSCA family members i.e. OsOSCA 1.2 and OsOSCA 3.1 was done. Intracellular H2O2/NO levels displayed an interplay between Ca2+ influx and ROS/RNS signal. Detoxifying enzyme (i.e. ascorbate peroxidase and catalase) activity was considerably higher in tolerant KS-282 while the activity of superoxide dismutase was significantly prominent in the sensitive cells triggering greater oxidative damage owing to the prolonged presence of intracellular H2O2. Salt stress and ROS responsive TFs i.e. OsSERF1 and OsDREB2A were expressed exclusively in the tolerant cells. Similarly, the expression of genes involved in maintaining high [K+]/[Na+] ratio was considerably higher and earlier in the tolerant variety. Overall, we suggest that a control over ROS production, and an increase in the expression of genes important for potassium homeostasis play a dynamic role in salinity tolerance in rice cell cultures.

SNF1-Related Protein Kinases SnRK2.4 and SnRK2.10 Modulate ROS Homeostasis in Plant Response to Salt Stress

In response to salinity and various other environmental stresses, plants accumulate reactive oxygen species (ROS). The ROS produced at very early stages of the stress response act as signaling molecules activating defense mechanisms, whereas those produced at later stages in an uncontrolled way are detrimental to plant cells by damaging lipids, DNA, and proteins. Multiple systems are involved in ROS generation and also in ROS scavenging. Their level and activity are tightly controlled to ensure ROS homeostasis and protect the plant against the negative effects of the environment. The signaling pathways responsible for maintaining ROS homeostasis in abiotic stress conditions remain largely unknown. Here, we show that in Arabidopsis thaliana, two abscisic acid- (ABA)-non-activated SNF1-releted protein kinases 2 (SnRK2) kinases, SnRK2.4 and SnRK2.10, are involved in the regulation of ROS homeostasis in response to salinity. They regulate the expression of several genes responsible for ROS generation at early stages of the stress response as well as those responsible for their removal. Moreover, the SnRK2.4 regulate catalase levels and its activity and the level of ascorbate in seedlings exposed to salt stress.

Recognition of Elicitors in Grapevine: From MAMP and DAMP Perception to Induced Resistance

In a context of a sustainable viticulture, the implementation of innovative eco-friendly strategies, such as elicitor-triggered immunity, requires a deep knowledge of the molecular mechanisms underlying grapevine defense activation, from pathogen perception to resistance induction. During plant-pathogen interaction, the first step of plant defense activation is ensured by the recognition of microbe-associated molecular patterns, which are elicitors directly derived from pathogenic or beneficial microbes. Vitis vinifera, like other plants, can perceive elicitors of different nature, including proteins, amphiphilic glycolipid, and lipopeptide molecules as well as polysaccharides, thanks to their cognate pattern recognition receptors, the discovery of which recently began in this plant species. Furthermore, damage-associated molecular patterns are another class of elicitors perceived by V. vinifera as an invader's hallmark. They are mainly polysaccharides derived from the plant cell wall and are generally released through the activity of cell wall-degrading enzymes secreted by microbes. Elicitor perception and subsequent activation of grapevine immunity end in some cases in efficient grapevine resistance against pathogens. Using complementary approaches, several molecular markers have been identified as hallmarks of this induced resistance stage. This review thus focuses on the recognition of elicitors by Vitis vinifera describing the molecular mechanisms triggered from the elicitor perception to the activation of immune responses. Finally, we discuss the fact that the link between elicitation and induced resistance is not so obvious and that the formulation of resistance inducers remains a key step before their application in vineyards.

The Three Members of the Arabidopsis Glycosyltransferase Family 92 are Functional β-1,4-Galactan Synthases

Pectin is a major component of primary cell walls and performs a plethora of functions crucial for plant growth, development and plant-defense responses. Despite the importance of pectic polysaccharides their biosynthesis is poorly understood. Several genes have been implicated in pectin biosynthesis by mutant analysis, but biochemical activity has been shown for very few. We used reverse genetics and biochemical analysis to study members of Glycosyltransferase Family 92 (GT92) in Arabidopsis thaliana. Biochemical analysis gave detailed insight into the properties of GALS1 (Galactan synthase 1) and showed galactan synthase activity of GALS2 and GALS3. All proteins are responsible for adding galactose onto existing galactose residues attached to the rhamnogalacturonan-I (RG-I) backbone. Significant GALS activity was observed with galactopentaose as acceptor but longer acceptors are favored. Overexpression of the GALS proteins in Arabidopsis resulted in accumulation of unbranched β-1, 4-galactan. Plants in which all three genes were inactivated had no detectable β-1, 4-galactan, and surprisingly these plants exhibited no obvious developmental phenotypes under standard growth conditions. RG-I in the triple mutants retained branching indicating that the initial Gal substitutions on the RG-I backbone are added by enzymes different from GALS.

Integration between ROS Regulatory Systems and Other Signals in the Regulation of Various Types of Heat Responses in Plants

Because of their sessile lifestyle, plants cannot escape from heat stress and are forced to alter their cellular state to prevent damage. Plants, therefore, evolved complex mechanisms to adapt to irregular increases in temperature in the natural environment. In addition to the ability to adapt to an abrupt increase in temperature, plants possess strategies to reprogram their cellular state during pre-exposure to sublethal heat stress so that they are able to survive under subsequent severe heat stress. Such an acclimatory response to heat, i.e., acquired thermotolerance, might depend on the maintenance of heat memory and propagation of long-distance signaling. In addition, plants are able to tailor their specific cellular state to adapt to heat stress combined with other abiotic stresses. Many studies revealed significant roles of reactive oxygen species (ROS) regulatory systems in the regulation of these various heat responses in plants. However, the mode of coordination between ROS regulatory systems and other pathways is still largely unknown. In this review, we address how ROS regulatory systems are integrated with other signaling networks to control various types of heat responses in plants. In addition, differences and similarities in heat response signals between different growth stages are also addressed.

Nitric oxide alleviates cell death through protein S-nitrosylation and transcriptional regulation during the ageing of elm seeds

Seed ageing is a major problem in the conservation of germplasm resources. The involvement of possible signalling molecules during seed deterioration needs to be identified. In this study, we confirmed that nitric oxide (NO), a key signalling molecule in plants, plays a positive role in the resistance of elm seeds to deterioration. To explore which metabolic pathways were affected by NO, an untargeted metabolomic analysis was conducted, and 163 metabolites could respond to both NO and the ageing treatment. The primary altered pathways include glutathione, methionine, and carbohydrate metabolism. The genes involved in glutathione and methionine metabolism were up-regulated by NO at the transcriptional level. Using a biotin switch method, proteins with an NO-dependent post-translational modification were screened during seed deterioration, and 82 putative S-nitrosylated proteins were identified. Eleven of these proteins were involved in carbohydrate metabolism, and the activities of the three enzymes were regulated by NO. In combination, the results of the metabolomic and S-nitrosoproteomic studies demonstrated that NO could activate glycolysis and inhibit the pentose phosphate pathway. In summary, the combination of these results demonstrated that NO could modulate carbohydrate metabolism at the post-translational level and regulate glutathione and methionine metabolism at the transcriptional level. It provides initial insights into the regulatory mechanisms of NO in seed deterioration.

A semi-dominant mutation in a CC-NB-LRR-type protein leads to a short-root phenotype in rice

The mechanisms of plant defense against pathogen attack in plant leaves have been extensively studied. However, our understanding of plant defense mechanisms in plant roots is still limited. In this study, a semi-dominant mutant nrtp1-D (necrotic root tip 1), with a short-root phenotype, was characterized in rice. Map-based cloning revealed that NRTP1 encoded a typical coiled-coil nucleotide binding leucine rich repeat (CC-NB-LRR) type protein and the mutation caused an amino acid substitution in the Nucleotide-Binding adaptor shared by Apaf1, certain R genes and CED4 (NB-ARC) domain, which may cause constitutive auto-activation of the NRTP1 protein. Gene expression analysis revealed that NRTP1 was preferentially expressed in rice roots. Expression of mutant nrtp1-D in tobacco leaves induced necrotic lesions, which indicated a common mechanism of plant defense response between leaves and roots. Transcriptome analysis revealed that many typical defense-response genes were differentially expressed in homozygous nrtp1-D. In addition, we also found differential expression of genes in pathways which had not previously been described as being associated with pathogen response. Histochemical analysis showed that the level of nitric oxide (NO), but not reactive oxygen species (ROS), was increased in homozygous nrtp1-D mutant roots. These results indicate that, in addition to the mechanism of defense response common to both roots and shoots, a novel pathway may also exist in rice roots, which does not operate in shoots.

Impact of galactoglucomannan oligosaccharides and Cd stress on maize root growth parameters, morphology, and structure

Biologically active oligosaccharides, including galactoglucomannan oligosaccharides (GGMOs), affect plant growth and development. The impact of GGMOs is dependent on their concentration, and the plant species and plant parts affected. The aim of this article is to ascertain the effects of GGMOs, GGMOs + Cd²⁺, on growth parameters, morphology, and the structure of maize (Zea mays L.) roots. We undertook this research because, in monocots, the effect of these oligosaccharides is so far unknown. In our study, GGMOs stimulated primary root elongation, induction and elongation of lateral roots, and biomass production. Their effect was dependent on the concentration used. Simultaneously, GGMOs moderated the negative effect of Cd²⁺ on root elongation growth. Besides, GGMOs affected the primary root structure, proven in the earlier development of xylem and Casparian bands, but not of suberin lamellae (compared to the control). The presence of Cd²⁺ shifted the apoplasmic barriers closer to the root apex in comparison to samples treated with GGMOs + Cd²⁺. GGMOs do not inhibit Cd uptake into the root directly, but they moderate its effect, and therefore their influence at the structural and metabolic level seems possible. Their positive impact on plant vitality, even in contaminated conditions, strongly indicates their potential application in remediation technologies.

Dickeya dadantii pectic enzymes necessary for virulence are also responsible for activation of the Arabidopsis thaliana innate immune system

Soft-rot diseases of plants attributed to Dickeya dadantii result from lysis of the plant cell wall caused by pectic enzymes released by the bacterial cell by a type II secretion system (T2SS). Arabidopsis thaliana can express several lines of defence against this bacterium. We employed bacterial mutants with defective envelope structures or secreted proteins to examine early plant defence reactions. We focused on the production of AtrbohD-dependent reactive oxygen species (ROS), callose deposition and cell death as indicators of these reactions. We observed a significant reduction in ROS and callose formation with a bacterial mutant in which genes encoding five pectate lyases (Pels) were disrupted. Treatment of plant leaves with bacterial culture filtrates containing Pels resulted in ROS and callose production, and both reactions were dependent on a functional AtrbohD gene. ROS and callose were produced in response to treatment with a cellular fraction of a T2SS-negative mutant grown in a Pels-inducing medium. Finally, ROS and callose were produced in leaves treated with purified Pels that had also been shown to induce the expression of jasmonic acid-dependent defence genes. Pel catalytic activity is required for the induction of ROS accumulation. In contrast, cell death observed in leaves infected with the wild-type strain appeared to be independent of a functional AtrbohD gene. It was also independent of the bacterial production of pectic enzymes and the type III secretion system (T3SS). In conclusion, the work presented here shows that D. dadantii is recognized by the A. thaliana innate immune system through the action of pectic enzymes secreted by bacteria at the site of infection. This recognition leads to AtrbohD-dependent ROS and callose accumulation, but not cell death.

The Cell Wall-Derived Xyloglucan Is a New DAMP Triggering Plant Immunity in Vitis vinifera and Arabidopsis thaliana

Damage-associated molecular patterns (DAMPs) are endogenous molecules that can activate the plant innate immunity. DAMPs can derive from the plant cell wall, which is composed of a complex mixture of cellulose, hemicellulose, and pectin polysaccharides. Fragments of pectin, called oligogalacturonides (OG), can be released after wounding or by pathogen-encoded cell wall degrading enzymes (CWDEs) such as polygalacturonases (PGs). OG are known to induce innate immune responses, including the activation of mitogen-activated protein kinases (MAPKs), production of H₂O₂, defense gene activation, and callose deposition. Thus, we hypothesized that xyloglucans (Xh), derived from the plant cell wall hemicellulose, could also act as an endogenous elicitor and trigger a signaling cascade similar to OG. Our results indicate that purified Xh elicit MAPK activation and immune gene expression in grapevine (Vitis vinifera) and Arabidopsis (Arabidopsis thaliana) to trigger induced resistance against necrotrophic (Botrytis cinerea) or biotrophic (Hyaloperonospora arabidopsidis) pathogens. Xh also induce resveratrol production in grapevine cell suspension and callose deposition in Arabidopsis which depends on the callose synthase PMR4. In addition, we characterized some signaling components of Xh-induced immunity using Arabidopsis mutants. Our data suggest that Xh-induced resistance against B. cinerea is dependent on the phytoalexin, salicylate, jasmonate, and ethylene pathways.

Damage-associated responses of the host contribute to defence against cyst nematodes but not root-knot nematodes

When nematodes invade and subsequently migrate within plant roots, they generate cell wall fragments (in the form of oligogalacturonides; OGs) that can act as damage-associated molecular patterns and activate host defence responses. However, the molecular mechanisms mediating damage responses in plant-nematode interactions remain unexplored. Here, we characterized the role of a group of cell wall receptor proteins in Arabidopsis, designated as polygalacturonase-inhibiting proteins (PGIPs), during infection with the cyst nematode Heterodera schachtii and the root-knot nematode Meloidogyne incognita. PGIPs are encoded by a family of two genes in Arabidopsis, and are involved in the formation of active OG elicitors. Our results show that PGIP gene expression is strongly induced in response to cyst nematode invasion of roots. Analyses of loss-of-function mutants and overexpression lines revealed that PGIP1 expression attenuates infection of host roots by cyst nematodes, but not root-knot nematodes. The PGIP1-mediated attenuation of cyst nematode infection involves the activation of plant camalexin and indole-glucosinolate pathways. These combined results provide new insights into the molecular mechanisms underlying plant damage perception and response pathways during infection by cyst and root-knot nematodes, and establishes the function of PGIP in plant resistance to cyst nematodes.