Glutathione-dependent denitrosation of GSNOR1 promotes oxidative signalling downstream of H2 O2

ROS and RNS production, subcellular localization, and signaling triggered by immunogenic danger signals

Plants continuously monitor the environment to detect changing conditions and to properly respond, avoiding deleterious effects on their fitness and survival. An enormous number of cell surface and intracellular immune receptors are deployed to perceive danger signals associated with microbial infections. Ligand binding by cognate receptors represents the first essential event in triggering plant immunity and determining the outcome of the tissue invasion attempt. Reactive oxygen and nitrogen species (ROS/RNS) are secondary messengers rapidly produced in different subcellular localizations upon the perception of immunogenic signals. Danger signal transduction inside the plant cells involves cytoskeletal rearrangements as well as several organelles and interactions between them to activate key immune signaling modules. Such immune processes depend on ROS and RNS accumulation, highlighting their role as key regulators in the execution of the immune cellular program. In fact, ROS and RNS are synergic and interdependent intracellular signals required for decoding danger signals and for the modulation of defense-related responses. Here we summarize current knowledge on ROS/RNS production, compartmentalization, and signaling in plant cells that have perceived immunogenic danger signals.

Glutathione: a key modulator of plant defence and metabolism through multiple mechanisms

Redox reactions are fundamental to energy conversion in living cells, and also determine and tune responses to the environment. Within this context, the tripeptide glutathione plays numerous roles. As an important antioxidant, glutathione confers redox stability on the cell and also acts as an interface between signalling pathways and metabolic reactions that fuel growth and development. It also contributes to the assembly of cell components, biosynthesis of sulfur-containing metabolites, inactivation of potentially deleterious compounds, and control of hormonal signalling intensity. The multiplicity of these roles probably explains why glutathione status has been implicated in influencing plant responses to many different conditions. In particular, there is now a considerable body of evidence showing that glutathione is a crucial player in governing the outcome of biotic stresses. This review provides an overview of glutathione synthesis, transport, degradation, and redox turnover in plants. It examines the expression of genes associated with these processes during pathogen challenge and related conditions, and considers the diversity of mechanisms by which glutathione can influence protein function and gene expression.

Structural and biochemical characterization of Arabidopsis alcohol dehydrogenases reveals distinct functional properties but similar redox sensitivity

Alcohol dehydrogenases (ADHs) are a group of zinc-binding enzymes belonging to the medium-length dehydrogenase/reductase (MDR) protein superfamily. In plants, these enzymes fulfill important functions involving the reduction of toxic aldehydes to the corresponding alcohols (as well as catalyzing the reverse reaction, i.e., alcohol oxidation; ADH1) and the reduction of nitrosoglutathione (GSNO; ADH2/GSNOR). We investigated and compared the structural and biochemical properties of ADH1 and GSNOR from Arabidopsis thaliana. We expressed and purified ADH1 and GSNOR and determined two new structures, NADH-ADH1 and apo-GSNOR, thus completing the structural landscape of Arabidopsis ADHs in both apo- and holo-forms. A structural comparison of these Arabidopsis ADHs revealed a high sequence conservation (59% identity) and a similar fold. In contrast, a striking dissimilarity was observed in the catalytic cavity supporting substrate specificity and accommodation. Consistently, ADH1 and GSNOR showed strict specificity for their substrates (ethanol and GSNO, respectively), although both enzymes had the ability to oxidize long-chain alcohols, with ADH1 performing better than GSNOR. Both enzymes contain a high number of cysteines (12 and 15 out of 379 residues for ADH1 and GSNOR, respectively) and showed a significant and similar responsivity to thiol-oxidizing agents, indicating that redox modifications may constitute a mechanism for controlling enzyme activity under both optimal growth and stress conditions.

Sulfate Availability and Hormonal Signaling in the Coordination of Plant Growth and Development

Sulfur (S), one of the crucial macronutrients, plays a pivotal role in fundamental plant processes and the regulation of diverse metabolic pathways. Additionally, it has a major function in plant protection against adverse conditions by enhancing tolerance, often interacting with other molecules to counteract stresses. Despite its significance, a thorough comprehension of how plants regulate S nutrition and particularly the involvement of phytohormones in this process remains elusive. Phytohormone signaling pathways crosstalk to modulate growth and developmental programs in a multifactorial manner. Additionally, S availability regulates the growth and development of plants through molecular mechanisms intertwined with phytohormone signaling pathways. Conversely, many phytohormones influence or alter S metabolism within interconnected pathways. S metabolism is closely associated with phytohormones such as abscisic acid (ABA), auxin (AUX), brassinosteroids (BR), cytokinins (CK), ethylene (ET), gibberellic acid (GA), jasmonic acid (JA), salicylic acid (SA), and strigolactones (SL). This review provides a summary of the research concerning the impact of phytohormones on S metabolism and, conversely, how S availability affects hormonal signaling. Although numerous molecular details are yet to be fully understood, several core signaling components have been identified at the crossroads of S and major phytohormonal pathways.

Wounding induces a peroxisomal H2 O2 decrease via glycolate oxidase-catalase switch dependent on glutamate receptor-like channel-supported Ca2+ signaling in plants

Sensing of environmental challenges, such as mechanical injury, by a single plant tissue results in the activation of systemic signaling, which attunes the plant's physiology and morphology for better survival and reproduction. As key signals, both calcium ions (Ca2+ ) and hydrogen peroxide (H2 O2 ) interplay with each other to mediate plant systemic signaling. However, the mechanisms underlying Ca2+ -H2 O2 crosstalk are not fully revealed. Our previous study showed that the interaction between glycolate oxidase and catalase, key enzymes of photorespiration, serves as a molecular switch (GC switch) to dynamically modulate photorespiratory H2 O2 fluctuations via metabolic channeling. In this study, we further demonstrate that local wounding induces a rapid shift of the GC switch to a more interactive state in systemic leaves, resulting in a sharp decrease in peroxisomal H2 O2 levels, in contrast to a simultaneous outburst of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-derived apoplastic H2 O2 . Moreover, the systemic response of the two processes depends on the transmission of Ca2+ signaling, mediated by glutamate-receptor-like Ca2+ channels 3.3 and 3.6. Mechanistically, by direct binding and/or indirect mediation by some potential biochemical sensors, peroxisomal Ca2+ regulates the GC switch states in situ, leading to changes in H2 O2 levels. Our findings provide new insights into the functions of photorespiratory H2 O2 in plant systemic acclimation and an optimized systemic H2 O2 signaling via spatiotemporal interplay between the GC switch and NADPH oxidases.

Deciphering peroxisomal reactive species interactome and redox signalling networks

Plant peroxisomes are highly dynamic organelles with regard to metabolic pathways, number and morphology and participate in different metabolic processes and cell responses to their environment. Peroxisomes from animal and plant cells house a complex system of reactive oxygen species (ROS) production associated to different metabolic pathways which are under control of an important set of enzymatic and non enzymatic antioxidative defenses. Nitric oxide (NO) and its derivate reactive nitrogen species (RNS) are also produced in these organelles. Peroxisomes can regulate ROS and NO/RNS levels to allow their role as signalling molecules. The metabolism of other reactive species such as carbonyl reactive species (CRS) and sulfur reactive species (SRS) in peroxisomes and their relationship with ROS and NO have not been explored in depth. In this review, we define a peroxisomal reactive species interactome (PRSI), including all reactive species ROS, RNS, CRS and SRS, their interaction and effect on target molecules contributing to the dynamic redox/ROS homeostasis and plasticity of peroxisomes, enabling fine-tuned regulation of signalling networks associated with peroxisome-dependent H₂O₂. Particular attention will be paid to update the information available on H₂O₂-dependent peroxisomal retrograde signalling and to discuss a specific peroxisomal footprint.

Expanding roles for S-nitrosylation in the regulation of plant immunity

Following pathogen recognition, plant cells produce a nitrosative burst resulting in a striking increase in nitric oxide (NO), altering the redox state of the cell, which subsequently helps orchestrate a plethora of immune responses. NO is a potent redox cue, efficiently relayed between proteins through its co-valent attachment to highly specific, powerfully reactive protein cysteine (Cys) thiols, resulting in formation of protein S-nitrosothiols (SNOs). This process, known as S-nitrosylation, can modulate the function of target proteins, enabling responsiveness to cellular redox changes. Key targets of S-nitrosylation control the production of reactive oxygen species (ROS), the transcription of immune-response genes, the triggering of the hypersensitive response (HR) and the establishment of systemic acquired resistance (SAR). Here, we bring together recent advances in the control of plant immunity by S-nitrosylation, furthering our appreciation of how changes in cellular redox status reprogramme plant immune function.

Dynamic and fluctuating generation of hydrogen peroxide via photorespiratory metabolic channeling in plants

The homeostasis of hydrogen peroxide (H₂ O₂ ), a key regulator of basic biological processes, is a result of the cooperation between its generation and scavenging. However, the mechanistic basis of this balance is not fully understood. We previously proposed that the interaction between glycolate oxidase (GLO) and catalase (CAT) may serve as a molecular switch that modulates H₂ O₂ levels in plants. In this study, we demonstrate that the GLO-CAT complex in plants exists in different states, which are dynamically interchangeable in response to various stimuli, typically salicylic acid (SA), at the whole-plant level. More crucially, changes in the state of the complex were found to be closely linked to peroxisomal H₂ O₂ fluctuations, which were independent of the membrane-associated NADPH oxidase. Furthermore, evidence suggested that H₂ O₂ channeling occurred even in vitro when GLO and CAT worked cooperatively. These results demonstrate that dynamic changes in H₂ O₂ levels are physically created via photorespiratory metabolic channeling in plants, and that such H₂ O₂ fluctuations may serve as signals that are mechanistically involved in the known functions of photorespiratory H₂ O₂ . In addition, our study also revealed a new way for SA to communicate with H₂ O₂ in plants.

Focus on Nitric Oxide Homeostasis: Direct and Indirect Enzymatic Regulation of Protein Denitrosation Reactions in Plants

Protein cysteines (Cys) undergo a multitude of different reactive oxygen species (ROS), reactive sulfur species (RSS), and/or reactive nitrogen species (RNS)-derived modifications. S-nitrosation (also referred to as nitrosylation), the addition of a nitric oxide (NO) group to reactive Cys thiols, can alter protein stability and activity and can result in changes of protein subcellular localization. Although it is clear that this nitrosative posttranslational modification (PTM) regulates multiple signal transduction pathways in plants, the enzymatic systems that catalyze the reverse S-denitrosation reaction are poorly understood. This review provides an overview of the biochemistry and regulation of nitro-oxidative modifications of protein Cys residues with a focus on NO production and S-nitrosation. In addition, the importance and recent advances in defining enzymatic systems proposed to be involved in regulating S-denitrosation are addressed, specifically cytosolic thioredoxins (TRX) and the newly identified aldo-keto reductases (AKR).

Reactive oxygen species signalling in plant stress responses

Reactive oxygen species (ROS) are key signalling molecules that enable cells to rapidly respond to different stimuli. In plants, ROS play a crucial role in abiotic and biotic stress sensing, integration of different environmental signals and activation of stress-response networks, thus contributing to the establishment of defence mechanisms and plant resilience. Recent advances in the study of ROS signalling in plants include the identification of ROS receptors and key regulatory hubs that connect ROS signalling with other important stress-response signal transduction pathways and hormones, as well as new roles for ROS in organelle-to-organelle and cell-to-cell signalling. Our understanding of how ROS are regulated in cells by balancing production, scavenging and transport has also increased. In this Review, we discuss these promising developments and how they might be used to increase plant resilience to environmental stress.

Genetic Manipulation of Reactive Oxygen Species (ROS) Homeostasis Utilizing CRISPR/Cas9-Based Gene Editing in Rice

Reactive oxygen species (ROS) are now recognized as key signals in plant stress responses. Adverse environmental conditions can either promote ROS production or downregulate antioxidative enzymes, leading to the alteration of redox homeostasis and activation of ROS-linked stress signaling. To uncover their signaling mechanisms and to characterize related components, genetic modification of ROS homeostasis is a central approach. CRISPR/Cas9-based genome editing system has become a powerful tool for gene mutation in a variety of organisms, including plants. Within this chapter, we describe a method that can be applied to manipulate ROS homeostasis in rice (Oryza sativa L.) utilizing CRISPR/Cas9 technology. Step-by-step protocols including the design and construction of Cas9/sgRNA, agrobacterium-mediated transformation, and mutation characterization are described. Application of this system in editing a rice catalase gene CatC, a key antioxidative enzyme in controlling ROS homeostasis, is also presented.

Modification of Chloroplast Antioxidant Capacity by Plastid Transformation

As immobile organisms, green plants must be frequently challenged by a broad range of environmental stresses. During these constantly adverse conditions, reactive oxygen species (ROS) levels can rise extremely in plants, leading to cellular dysfunction and cell death presumably due to irreversible protein overoxidation. Once considered merely as deleterious molecules, cells seek to remove them as efficiently as possible. To enhance ROS scavenging capacity, genes encoding antioxidative enzymes can be directly expressed from the genome of plastid (chloroplast), a major compartment for ROS production in photosynthetic organisms. Thus, overexpression of antioxidant enzymes by plastid engineering may provide an alternative to enhance plant's tolerance to stressful conditions specifically related with chloroplast-derived ROS. Here, we describe basic procedures for expressing glutathione reductase, a vital component of ascorbate-glutathione pathway, in tobacco via plastid transformation technology.

Decoding the potential of a new Pseudomonas putida strain for inducing drought tolerance of tomato (Solanum lycopersicum) plants through seed biopriming

A total of 11 potential plant growth promoting rhizobacteria previously isolated from naturally stressed environments were evaluated for various traits of interest for a beneficial symbiosis with plants, including colonization ability, biofilm formation, motility, exopolysaccharide production and salt tolerance. The vast majority of the strains were found to possess multiple plant growth promoting traits. Nevertheless, the intensity varied among isolates, with those originated from tomato plants being more efficient colonizers. The strain SAESo11, genetically characterized as a Pseudomonas putida member was selected for further investigation of its potential to alleviate drought stress in tomato seedlings. Inoculation with SAESo11 mitigated the negative effects of drought stress as indicated by growth and photosynthetic indices. Furthermore, bacterial inoculation enhanced H₂O₂ content and malondialdehyde levels in colonized plants. Drought treatment did not further alter the oxidative status of these plants. Similarly, total phenolic content and antioxidant enzyme activity were induced in plant tissues in response to drought stress only at the absence of inoculum. These results indicated that inoculation with the selected strain imposed plants at a priming state, that enabled them to respond more robustly at the exposure to drought stress and efficiently attenuated the drought-induced injury. This state of plant alertness mediated by SAESo11 occurred at no cost to growth, highlighting its role as a potential plant priming agent.

Abiotic stress-triggered oxidative challenges: Where does H2S act?

Hydrogen sulfide (H₂S) was once principally considered the perpetrator of plant growth cessation and cell death. However, this has become an antiquated view, with cumulative evidence showing that the H₂S serves as a biological signaling molecule notably involved in abiotic stress response and adaptation, such as defense by phytohormone activation, stomatal movement, gene reprogramming, and plant growth modulation. Reactive oxygen species (ROS)-dependent oxidative stress is involved in these responses. Remarkably, an ever-growing body of evidence indicates that H₂S can directly interact with ROS processing systems in a redox-dependent manner, while it has been gradually recognized that H₂S-based posttranslational modifications of key protein cysteine residues determine stress responses. Furthermore, the reciprocal interplay between H₂S and nitric oxide (NO) in regulating oxidative stress has significant importance. The interaction of H₂S with NO and ROS during acclimation to abiotic stress may vary from synergism to antagonism. However, the molecular pathways and factors involved remain to be identified. This review not only aims to provide updated information on H₂S action in regulating ROS-dependent redox homeostasis and signaling, but also discusses the mechanisms of H₂S-dependent regulation in the context of oxidative stress elicited by environmental cues.

Hydrogen sulfide attenuates intracellular oxidative stress via repressing glycolate oxidase activities in Arabidopsis thaliana

BACKGROUND

Hydrogen sulfide (H₂S) has been proposed to exert anti-oxidative effect under many environmental stresses; however, how it influences oxidative stress remains largely unclear.

RESULTS

Here, we assessed the effects of H₂S on oxidative stress responses such as salicylic acid (SA)-dependent cell death, which triggered by increased H₂O₂ availability in Arabidopsis thaliana catalase-deficient mutants cat2 displaying around 20% wild-type catalase activity. H₂S generation and its producing enzyme _L-cysteine desulfhydrase (LCD/DES) were found to transient increase in response to intracellular oxidative stress. Although introducing the mutation of des1, an important LCD, into the cat2 background produced little effect, H₂S fumigation not only rescued the cell death phenotype of cat2 plant, but also attenuated SA accumulation and oxidation of the glutathione pool. Unexpectedly, the activities of major components of ascorbate-glutathione pathway were less affected by the presence of H₂S treatment, but decreased glycolate oxidase (GOX) in combination with accumulation of glycolate implied H₂S treatment impacts the cellular redox homeostasis by repressing the GOX-catalyzed reaction likely via altering the major GOX transcript levels.

CONCLUSIONS

Our findings reveal a link between H₂S and peroxisomal H₂O₂ production that has implications for the understanding of the multifaceted roles of H₂S in the regulation of oxidative stress responses.

Glutathione contributes to resistance responses to TMV through a differential modulation of salicylic acid and reactive oxygen species

Systemic acquired resistance (SAR) is induced by pathogens and confers protection against a broad range of pathogens. Several SAR signals have been characterized, but the nature of the other unknown signalling by small metabolites in SAR remains unclear. Glutathione (GSH) has long been implicated in the defence reaction against biotic stress. However, the mechanism that GSH increases plant tolerance against virus infection is not entirely known. Here, a combination of a chemical, virus-induced gene-silencing-based genetics approach, and transgenic technology was undertaken to investigate the role of GSH in plant viral resistance in Nicotiana benthamiana. Tobacco mosaic virus (TMV) infection results in increasing the expression of GSH biosynthesis genes NbECS and NbGS, and GSH content. Silencing of NbECS or NbGS accelerated oxidative damage, increased accumulation of reactive oxygen species (ROS), compromised plant resistance to TMV, and suppressed the salicylic acid (SA)-mediated signalling pathway. Application of GSH or l-2-oxothiazolidine-4-carboxylic acid (a GSH activator) alleviated oxidative damage, decreased accumulation of ROS, elevated plant local and systemic resistance, enhanced the SA-mediated signalling pathway, and increased the expression of ROS scavenging-related genes. However, treatment with buthionine sulfoximine (a GSH inhibitor) accelerated oxidative damage, elevated ROS accumulation, compromised plant systemic resistance, suppressed the SA-mediated signalling pathway, and reduced the expression of ROS-regulating genes. Overexpression of NbECS reduced oxidative damage, decreased accumulation of ROS, increased resistance to TMV, activated the SA-mediated signalling pathway, and increased the expression of the ROS scavenging-related genes. We present molecular evidence suggesting GSH is essential for both local and systemic resistance of N. benthamiana to TMV through a differential modulation of SA and ROS.

Overexpressing the N-terminus of CATALASE2 enhances plant jasmonic acid biosynthesis and resistance to necrotrophic pathogen Botrytis cinerea B05.10

Salicylic acid (SA) acts antagonistically to jasmonic acid (JA) in plant immunity. We previously reported that CATALASE2 (CAT2) promotes JA-biosynthetic acyl-CoA oxidase (ACX) activity to enhance plant resistance to necrotrophic Botrytis cinerea, and SA represses JA biosynthesis through inhibiting CAT2 activity, while the underlying mechanism remains to be further elucidated. Here, we report that the truncated CAT2 N-terminus (CAT2-N) interacts with and promotes ACX2/3, and CAT2-N-overexpressing plants have increased JA accumulation and enhanced resistance to B. cinerea B05.10, but compromised antagonism of SA on JA. Catalase inhibitor treatment or mutating CAT2 active amino acids abolished CAT2 H₂ O₂ -decomposing activity but did not affect its promotion of ACX2/3 activity via interaction. CAT2-N, a truncated protein with no catalase activity, interacted with and promoted ACX2/3. Overexpressing CAT2-N in Arabidopsis plants resulted in increased ACX activity, higher JA accumulation, and stronger resistance to B. cinerea B05.10 infection. Additionally, SA dramatically repressed JA biosynthesis and resistance to B. cinerea in the wild type but not in the CAT2-N-overexpressing plants. Together, our study reveals that CAT2-N can be utilized as an accelerator for JA biosynthesis during plant resistance to B. cinerea B05.10, and this truncated protein partly relieves SA repression of JA biosynthesis in plant defence responses.

Peroxisomes as redox-signaling nodes in intracellular communication and stress responses

Peroxisomes are redox nodes playing a diverse range of roles in cell functionality and in the perception of and responses to changes in their environment.

The Versatile Roles of Sulfur-Containing Biomolecules in Plant Defense-A Road to Disease Resistance

Sulfur (S) is an essential plant macronutrient and the pivotal role of sulfur compounds in plant disease resistance has become obvious in recent decades. This review attempts to recapitulate results on the various functions of sulfur-containing defense compounds (SDCs) in plant defense responses to pathogens. These compounds include sulfur containing amino acids such as cysteine and methionine, the tripeptide glutathione, thionins and defensins, glucosinolates and phytoalexins and, last but not least, reactive sulfur species and hydrogen sulfide. SDCs play versatile roles both in pathogen perception and initiating signal transduction pathways that are interconnected with various defense processes regulated by plant hormones (salicylic acid, jasmonic acid and ethylene) and reactive oxygen species (ROS). Importantly, ROS-mediated reversible oxidation of cysteine residues on plant proteins have profound effects on protein functions like signal transduction of plant defense responses during pathogen infections. Indeed, the multifaceted plant defense responses initiated by SDCs should provide novel tools for plant breeding to endow crops with efficient defense responses to invading pathogens.

Structural and functional insights into nitrosoglutathione reductase from Chlamydomonas reinhardtii

Protein S-nitrosylation plays a fundamental role in cell signaling and nitrosoglutathione (GSNO) is considered as the main nitrosylating signaling molecule. Enzymatic systems controlling GSNO homeostasis are thus crucial to indirectly control the formation of protein S-nitrosothiols. GSNO reductase (GSNOR) is the key enzyme controlling GSNO levels by catalyzing its degradation in the presence of NADH. Here, we found that protein extracts from the microalga Chlamydomonas reinhardtii catabolize GSNO via two enzymatic systems having specific reliance on NADPH or NADH and different biochemical features. Scoring the Chlamydomonas genome for orthologs of known plant GSNORs, we found two genes encoding for putative and almost identical GSNOR isoenzymes. One of the two, here named CrGSNOR1, was heterologously expressed and purified. Its kinetic properties were determined and the three-dimensional structures of the apo-, NAD⁺- and NAD⁺/GSNO-forms were solved. These analyses revealed that CrGSNOR1 has a strict specificity towards GSNO and NADH, and a conserved folding with respect to other plant GSNORs. The catalytic zinc ion, however, showed an unexpected variability of the coordination environment. Furthermore, we evaluated the catalytic response of CrGSNOR1 to thermal denaturation, thiol-modifying agents and oxidative modifications as well as the reactivity and position of accessible cysteines. Despite being a cysteine-rich protein, CrGSNOR1 contains only two solvent-exposed/reactive cysteines. Oxidizing and nitrosylating treatments have null or limited effects on CrGSNOR1 activity and folding, highlighting a certain resistance of the algal enzyme to redox modifications. The molecular mechanisms and structural features underlying the response to thiol-based modifications are discussed.

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Chlamydomonas protein extracts catalyze NAD(P)H-dependent GSNO degradation.

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Chlamydomonas GSNOR1 is a zinc-containing protein strictly relying on GSNO and NADH.

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The 3D-structure of CrGSNOR1 revealed a conserved folding with other plant GSNORs.

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CrGSNOR1 contains only two solvent-exposed/reactive cysteines.

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Oxidizing and nitrosylating treatments have limited effects on CrGSNOR1 activity.