The redox switch: dynamic regulation of protein function by cysteine modifications

Liver epigenomic signature associated with chronic oxidative stress in a mouse model of glutathione deficiency

Oxidative stress is intimately involved in the pathogenesis of fatty liver disease (FLD). A major factor contributing to oxidative stress is the depletion of the ubiquitous antioxidant glutathione (GSH). Unexpectedly, chronic GSH deficiency renders glutamate-cysteine ligase modifier subunit (Gclm)-null mice protected from fatty liver injuries. Epigenetic regulation serves as an important cellular mechanism in modulating gene expression and disease outcome in FLD, although it is not well understood how systemic redox imbalance modifies the liver epigenome. In the current study, utilizing the Gclm-null mouse model, we aimed to elucidate redox-associated epigenomic changes and their implications in liver stress response. We performed high-throughput array-based DNA methylation profiling (MeDIP array) in 22,327 gene promoter regions (from -1300 bp to +500 bp of the Transcription Start Sites) in the liver and peripheral blood cells. Results from the MeDIP array demonstrate that, although global methylation enrichment in gene promoters did not change, low GSH resulted in prevalent demethylation at the individual promoter level. Such an effect likely attributed to a declined availability of the methyl donor S-adenosyl methionine (SAM) in Gclm-null liver. Functional enrichment analysis of liver target genes is suggestive of a potential role of epigenetic mechanisms in promoting cellular survival and lipid homeostasis in Gclm-null liver. In comparison with the liver tissue, MeDIP array in peripheral blood cells revealed a panel of 19 gene promoters that are candidate circulating biomarkers for hepatic epigenomic changes associated with chronic GSH deficiency. Collectively, our results provided new insights into the in vivo interplay between liver redox state and DNA methylation status. The current study laid the groundwork for future epigenetic/epigenomic investigations in experimental settings or human populations under conditions of liver oxidative stress induced by environmental or dietary challenges.

Chemobiosis reveals tardigrade tun formation is dependent on reversible cysteine oxidation

Tardigrades, commonly known as 'waterbears', are eight-legged microscopic invertebrates renowned for their ability to withstand extreme stressors, including high osmotic pressure, freezing temperatures, and complete desiccation. Limb retraction and substantial decreases to their internal water stores results in the tun state, greatly increasing their ability to survive. Emergence from the tun state and/or activity regain follows stress removal, where resumption of life cycle occurs as if stasis never occurred. However, the mechanism(s) through which tardigrades initiate tun formation is yet to be uncovered. Herein, we use chemobiosis to demonstrate that tardigrade tun formation is mediated by reactive oxygen species (ROS). We further reveal that tuns are dependent on reversible cysteine oxidation, and that this reversible cysteine oxidation is facilitated by the release of intracellular reactive oxygen species (ROS). We provide the first empirical evidence of chemobiosis and map the initiation and survival of tardigrades via osmobiosis, chemobiosis, and cryobiosis. In vivo electron paramagnetic spectrometry suggests an intracellular release of reactive oxygen species following stress induction; when this release is quenched through the application of exogenous antioxidants, the tardigrades can no longer survive osmotic stress. Together, this work suggests a conserved dependence of reversible cysteine oxidation across distinct tardigrade cryptobioses.

Agonists and hydrogen peroxide mediate hyperoxidation of β2-adrenergic receptor in airway epithelial cells: Implications for tachyphylaxis to β2-agonists in constrictive airway disorders

Asthma and other airway obstructive disorders are characterized by heightened inflammation and excessive airway epithelial cell reactive oxygen species (ROS), which give rise to a highly oxidative environment. After decades of use, β2-adrenergic receptor (β2AR) agonists remain at the forefront of treatment options for asthma, however, chronic use of β2-agonists leads to tachyphylaxis to the bronchorelaxant effects, a phenomenon that remains mechanistically unexplained. We have previously demonstrated that β2AR agonism increases ROS generation in airway epithelial cells, which upholds proper receptor function via feedback oxidation of β2AR cysteine thiolates to Cys-S-sulfenic acids (Cys-SOH). Our previous results also demonstrate that prevention of normal redox cycling of this post-translational oxi-modification back to the thiol prevents proper receptor function. Given that Cys-S-sulfenic acids can be irreversibly overoxidized to Cys-S-sulfinic (Cys-SO2H) or S-sulfonic (Cys-SO3H) acids, which are incapable of further participation in redox reactions, we hypothesized that β2-agonist tachyphylaxis may be explained by hyperoxidation of β2AR to S-sulfinic acids. Here, using airway epithelial cell lines and primary small airway epithelial cells from healthy and asthma-diseased donors, we show that β2AR agonism generates H2O2 in a receptor and NAPDH oxidase-dependent manner. We also demonstrate that acute and chronic receptor agonism can facilitate β2AR S-sulfination, and that millimolar H2O2 concentrations are deleterious to β2AR-mediated cAMP formation, an effect that can be rescued to a degree in the presence of the cysteine-donating antioxidant N-acetyl-L-cysteine. Our results reveal that the oxidative state of β2AR may contribute to receptor functionality and may, at least in part, explain β2-agonist tachyphylaxis.

Cysteinyl and methionyl redox switches: Structural prerequisites and consequences

Redox modifications of specific cysteinyl and methionyl residues regulate key enzymes and signal-transducing proteins in various pathways. Here, we analyzed the effect of redox modifications on protein structure screening the RCSB protein data bank for oxidative modifications of proteins, i.e. protein disulfides, mixed disulfides with glutathione, cysteinyl sulfenic acids, cysteinyl S-nitrosylation, and methionyl sulfoxide residues. When available, these structures were compared to the structures of the same proteins in the reduced state with respect to both pre-requirements for the oxidative modifications as well as the structural consequences of the modifications. In general, the conformational changes induced by the redox modification are small, i.e. within the range of normal fluctuations. Some redox modifications, disulfides in particular, induces alterations in the electrostatic properties of the proteins. Solvent accessibility does not seem to be a strict pre-requirement for the redox modification of a particular residue. We identified an enrichment of certain other amino acid residues in the vicinity of the susceptible residues, for disulfide and sulfenic acid modifications, for instance, histidyl and tyrosyl residues. These motifs, as well as the specific features of the susceptible sulfur-containing amino acids, may become helpful for the prediction of redox modifications.

Redox Signaling in Plant Heat Stress Response

The increase in environmental temperature due to global warming is a critical threat to plant growth and productivity. Heat stress can cause impairment in several biochemical and physiological processes. Plants sense and respond to this adverse environmental condition by activating a plethora of defense systems. Among them, the heat stress response (HSR) involves an intricate network of heat shock factors (HSFs) and heat shock proteins (HSPs). However, a growing amount of evidence suggests that reactive oxygen species (ROS), besides potentially being responsible for cellular oxidative damage, can act as signal molecules in HSR, leading to adaptative responses. The role of ROS as toxic or signal molecules depends on the fine balance between their production and scavenging. Enzymatic and non-enzymatic antioxidants represent the first line of defense against oxidative damage and their activity is critical to maintaining an optimal redox environment. However, the HS-dependent ROS burst temporarily oxidizes the cellular environment, triggering redox-dependent signaling cascades. This review provides an overview of the redox-activated mechanisms that participate in the HSR.

Interaction between Melatonin and NO: Action Mechanisms, Main Targets, and Putative Roles of the Emerging Molecule NOmela

Melatonin (MEL), a ubiquitous indolamine molecule, has gained interest in the last few decades due to its regulatory role in plant metabolism. Likewise, nitric oxide (NO), a gasotransmitter, can also affect plant molecular pathways due to its function as a signaling molecule. Both MEL and NO can interact at multiple levels under abiotic stress, starting with their own biosynthetic pathways and inducing a particular signaling response in plants. Moreover, their interaction can result in the formation of NOmela, a very recently discovered nitrosated form of MEL with promising roles in plant physiology. This review summarizes the role of NO and MEL molecules during plant development and fruit ripening, as well as their interactions. Due to the impact of climate-change-related abiotic stresses on agriculture, this review also focuses on the role of these molecules in mediating abiotic stress tolerance and the main mechanisms by which they operate, from the upregulation of the entire antioxidant defense system to the post-translational modifications (PTMs) of important molecules. Their individual interaction and crosstalk with phytohormones and H₂S are also discussed. Finally, we introduce and summarize the little information available about NOmela, an emerging and still very unknown molecule, but that seems to have a stronger potential than MEL and NO separately in mediating plant stress response.

Computational prediction of NO-dependent posttranslational modifications in plants: Current status and perspectives

The perception and transduction of nitric oxide (NO) signal is achieved by NO-dependent posttranslational modifications (PTMs) among which S-nitrosation and tyrosine nitration has biological significance. In plants, 100-1000 S-nitrosated and tyrosine nitrated proteins have been identified so far by mass spectrometry. The determination of NO-modified protein targets/amino acid residues is often methodologically challenging. In the past decade, the growing demand for the knowledge of S-nitrosated or tyrosine nitrated sites has motivated the introduction of bioinformatics tools. For predicting S-nitrosation seven computational tools have been developed (GPS-SNO, SNOSite, iSNO-PseACC, iSNO-AAPAir, PSNO, PreSNO, RecSNO). Four predictors have been developed for indicating tyrosine nitration sites (GPS-YNO2, iNitro-Tyr, PredNTS, iNitroY-Deep), and one tool (DeepNitro) predicts both NO-dependent PTMs. The advantage of these computational tools is the fast provision of large amount of information. In this review, the available software tools have been tested on plant proteins in which S-nitrosated or tyrosine nitrated sites have been experimentally identified. The predictors showed distinct performance and there were differences from the experimental results partly due to the fact that the three-dimensional protein structure is not taken into account by the computational tools. Nevertheless, the predictors excellently establish experiments, and it is suggested to apply all available tools on target proteins and compare their results. In the future, computational prediction must be developed further to improve the precision with which S-nitrosation/tyrosine nitration-sites are identified.

Redox sensor QSOX1 regulates plant immunity by targeting GSNOR to modulate ROS generation

Reactive oxygen signaling regulates numerous biological processes, including stress responses in plants. Redox sensors transduce reactive oxygen signals into cellular responses. Here, we present biochemical evidence that a plant quiescin sulfhydryl oxidase homolog (QSOX1) is a redox sensor that negatively regulates plant immunity against a bacterial pathogen. The expression level of QSOX1 is inversely correlated with pathogen-induced reactive oxygen species (ROS) accumulation. Interestingly, QSOX1 both senses and regulates ROS levels by interactingn with and mediating redox regulation of S-nitrosoglutathione reductase, which, consistent with previous findings, influences reactive nitrogen-mediated regulation of ROS generation. Collectively, our data indicate that QSOX1 is a redox sensor that negatively regulates plant immunity by linking reactive oxygen and reactive nitrogen signaling to limit ROS production.

A Novel DUF569 Gene Is a Positive Regulator of the Drought Stress Response in Arabidopsis

In the last two decades, global environmental change has increased abiotic stress on plants and severely affected crops. For example, drought stress is a serious abiotic stress that rapidly and substantially alters the morphological, physiological, and molecular responses of plants. In Arabidopsis, several drought-responsive genes have been identified; however, the underlying molecular mechanism of drought tolerance in plants remains largely unclear. Here, we report that the “domain of unknown function” novel gene DUF569 (AT1G69890) positively regulates drought stress in Arabidopsis. The Arabidopsis loss-of-function mutant atduf569 showed significant sensitivity to drought stress, i.e., severe wilting at the rosette-leaf stage after water was withheld for 3 days. Importantly, the mutant plant did not recover after rewatering, unlike wild-type (WT) plants. In addition, atduf569 plants showed significantly lower abscisic acid accumulation under optimal and drought-stress conditions, as well as significantly higher electrolyte leakage when compared with WT Col-0 plants. Spectrophotometric analyses also indicated a significantly lower accumulation of polyphenols, flavonoids, carotenoids, and chlorophylls in atduf569 mutant plants. Overall, our results suggest that novel DUF569 is a positive regulator of the response to drought in Arabidopsis.

Central Metabolism in Mammals and Plants as a Hub for Controlling Cell Fate

Significance: The importance of oxidoreductases in energy metabolism together with the occurrence of enzymes of central metabolism in the nucleus gave rise to the active research field aiming to understand moonlighting enzymes that undergo post-translational modifications (PTMs) before carrying out new tasks. Recent Advances: Cytosolic enzymes were shown to induce gene transcription after PTM and concomitant translocation to the nucleus. Changed properties of the oxidized forms of cytosolic glyceraldehyde 3-phosphate dehydrogenase, and also malate dehydrogenases and others, are the basis for a hypothesis suggesting moonlighting functions that directly link energy metabolism to adaptive responses required for maintenance of redox-homeostasis in all eukaryotes. Critical Issues: Small molecules, such as metabolic intermediates, coenzymes, or reduced glutathione, were shown to fine-tune the redox switches, interlinking redox state, metabolism, and induction of new functions via nuclear gene expression. The cytosol with its metabolic enzymes connecting energy fluxes between the various cell compartments can be seen as a hub for redox signaling, integrating the different signals for graded and directed responses in stressful situations. Future Directions: Enzymes of central metabolism were shown to interact with p53 or the assumed plant homologue suppressor of gamma response 1 (SOG1), an NAM, ATAF, and CUC transcription factor involved in the stress response upon ultraviolet exposure. Metabolic enzymes serve as sensors for imbalances, their inhibition leading to changed energy metabolism, and the adoption of transcriptional coactivator activities. Depending on the intensity of the impact, rerouting of energy metabolism, proliferation, DNA repair, cell cycle arrest, immune responses, or cell death will be induced. Antioxid. Redox Signal. 34, 1025-1047.

Oxidative stress in phenylketonuria-evidence from human studies and animal models, and possible implications for redox signaling

Phenylketonuria (PKU) is one of the commonest inborn error of amino acid metabolism. Before mass neonatal screening was possible, and the success of introducing diet therapy right after birth, the typical clinical finds in patients ranged from intellectual disability, epilepsy, motor deficits to behavioral disturbances and other neurological and psychiatric symptoms. Since early diagnosis and treatment became widespread, usually only those patients who do not strictly follow the diet present psychiatric, less severe symptoms such as anxiety, depression, sleep pattern disturbance, and concentration and memory problems. Despite the success of low protein intake in preventing otherwise severe outcomes, PKU's underlying neuropathophysiology remains to be better elucidated. Oxidative stress has gained acceptance as a disturbance implicated in the pathogenesis of PKU. The conception of oxidative stress has evolved to comprehend how it could interfere and ultimately modulate metabolic pathways regulating cell function. We summarize the evidence of oxidative damage, as well as compromised antioxidant defenses, from patients, animal models of PKU, and in vitro experiments, discussing the possible clinical significance of these findings. There are many studies on oxidative stress and PKU, but only a few went further than showing macromolecular damage and disturbance of antioxidant defenses. In this review, we argue that these few studies may point that oxidative stress may also disturb redox signaling in PKU, an aspect few authors have explored so far. The reported effect of phenylalanine on the expression or activity of enzymes participating in metabolic pathways known to be responsive to redox signaling might be mediated through oxidative stress.

The Arabidopsis zinc finger proteins SRG2 and SRG3 are positive regulators of plant immunity and are differentially regulated by nitric oxide

Nitric oxide (NO) regulates the deployment of a phalanx of immune responses, chief among which is the activation of a constellation of defence-related genes. However, the underlying molecular mechanisms remain largely unknown. The Arabidopsis thaliana zinc finger transcription factor (ZF-TF), S-nitrosothiol (SNO) Regulated 1 (SRG1), is a central target of NO bioactivity during plant immunity. Here we characterize the remaining members of the SRG gene family. Both SRG2 and, especially, SRG3 were positive regulators of salicylic acid-dependent plant immunity. Analysis of SRG single, double and triple mutants implied that SRG family members have additive functions in plant immunity and, surprisingly, are under reciprocal regulation. SRG2 and SRG3 localized to the nucleus and functioned as ethylene-responsive element binding factor-associated amphiphilic repression (EAR) domain-dependent transcriptional repressors: NO abolished this activity for SRG3 but not for SRG2. Consistently, loss of GSNOR function, resulting in increased (S)NO concentrations, fully suppressed the disease resistance phenotype established from SRG3 but not SRG2 overexpression. Remarkably, SRG3 but not SRG2 was S-nitrosylated in vitro and in vivo. Our findings suggest that the SRG family has separable functions in plant immunity, and, surprisingly, these ZF-TFs exhibit reciprocal regulation. It is remarkable that, through neofunctionalization, the SRG family has evolved to become differentially regulated by the key immune-related redox cue, NO.

Molecular functions of nitric oxide and its potential applications in horticultural crops

Nitric oxide (NO) regulates plant growth, enhances nutrient uptake, and activates disease and stress tolerance mechanisms in most plants, making NO a potential tool for use in improving the yield and quality of horticultural crop species. Although the use of NO in horticulture is still in its infancy, research on NO in model plant species has provided an abundance of valuable information on horticultural crop species. Emerging evidence implies that the bioactivity of NO can occur through many potential mechanisms but occurs mainly through S-nitrosation, the covalent and reversible attachment of NO to cysteine thiol. In this context, NO signaling specifically affects crop development, immunity, and environmental interactions. Moreover, NO can act as a fumigant against a wide range of postharvest diseases and pests. However, for effective use of NO in horticulture, both understanding and exploring the biological significance and potential mechanisms of NO in horticultural crop species are critical. This review provides a picture of our current understanding of how NO is synthesized and transduced in plants, and particular attention is given to the significance of NO in breaking seed dormancy, balancing root growth and development, enhancing nutrient acquisition, mediating stress responses, and guaranteeing food safety for horticultural production.

Nitric oxide signaling in the plant nucleus: the function of nitric oxide in chromatin modulation and transcription

Nitric oxide (NO) is involved in a vast number of physiologically important processes in plants, such as organ development, stress resistance, and immunity. Transduction of NO bioactivity is generally achieved by post-translational modification of proteins, with S-nitrosation of cysteine residues as the predominant form. While traditionally the subcellular location of the factors involved was of lesser importance, recent studies identified the connection between NO and transcriptional activity and thereby raised the question about the route of NO into the nuclear sphere. Identification of NO-affected transcription factors and chromatin-modifying histone deacetylases implicated the important role of NO signaling in the plant nucleus as a regulator of epigenetic mechanisms and gene transcription. Here, we discuss the relationship between NO and its directly regulated protein targets in the nuclear environment, focusing on S-nitrosated chromatin modulators and transcription factors.

Recent advances in the regulation of plant immunity by S-nitrosylation

S-nitrosylation, the addition of a nitric oxide (NO) moiety to a reactive protein cysteine (Cys) thiol, to form a protein S-nitrosothiol (SNO), is emerging as a key regulatory post-translational modification (PTM) to control the plant immune response. NO also S-nitrosylates the antioxidant tripeptide, glutathione, to form S-nitrosoglutathione (GSNO), both a storage reservoir of NO bioactivity and a natural NO donor. GSNO and, by extension, S-nitrosylation, are controlled by GSNO reductase1 (GSNOR1). The emerging data suggest that GSNOR1 itself is a target of NO-mediated S-nitrosylation, which subsequently controls its selective autophagy, regulating cellular protein SNO levels. Recent findings also suggest that S-nitrosylation may be deployed by pathogen-challenged host cells to counteract the effect of delivered microbial effector proteins that promote pathogenesis and by the pathogens themselves to augment virulence. Significantly, it also appears that S-nitrosylation may regulate plant immune functions by controlling SUMOylation, a peptide-based PTM. In this context, global SUMOylation is regulated by S-nitrosylation of SUMO conjugating enzyme 1 (SCE1) at Cys139. This redox-based PTM has also been shown to control the function of a key zinc finger transcriptional regulator during the establishment of plant immunity. Here, we provide an update of these recent advances.

Nitric oxide and hydrogen sulfide modulate the NADPH-generating enzymatic system in higher plants

Nitric oxide (NO) and hydrogen sulfide (H2S) are two key molecules in plant cells that participate, directly or indirectly, as regulators of protein functions through derived post-translational modifications, mainly tyrosine nitration, S-nitrosation, and persulfidation. These post-translational modifications allow the participation of both NO and H2S signal molecules in a wide range of cellular processes either physiological or under stressful circumstances. NADPH participates in cellular redox status and it is a key cofactor necessary for cell growth and development. It is involved in significant biochemical routes such as fatty acid, carotenoid and proline biosynthesis, and the shikimate pathway, as well as in cellular detoxification processes including the ascorbate-glutathione cycle, the NADPH-dependent thioredoxin reductase (NTR), or the superoxide-generating NADPH oxidase. Plant cells have diverse mechanisms to generate NADPH by a group of NADP-dependent oxidoreductases including ferredoxin-NADP reductase (FNR), NADP-glyceraldehyde-3-phosphate dehydrogenase (NADP-GAPDH), NADP-dependent malic enzyme (NADP-ME), NADP-dependent isocitrate dehydrogenase (NADP-ICDH), and both enzymes of the oxidative pentose phosphate pathway, designated as glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH). These enzymes consist of different isozymes located in diverse subcellular compartments (chloroplasts, cytosol, mitochondria, and peroxisomes) which contribute to the NAPDH cellular pool. We provide a comprehensive overview of how post-translational modifications promoted by NO (tyrosine nitration and S-nitrosation), H2S (persulfidation), and glutathione (glutathionylation), affect the cellular redox status through regulation of the NADP-dependent dehydrogenases.

Leaf isoprene emission as a trait that mediates the growth-defense tradeoff in the face of climate stress

Plant isoprene emissions are known to contribute to abiotic stress tolerance, especially during episodes of high temperature and drought, and during cellular oxidative stress. Recent studies have shown that genetic transformations to add or remove isoprene emissions cause a cascade of cellular modifications that include known signaling pathways, and interact to remodel adaptive growth-defense tradeoffs. The most compelling evidence for isoprene signaling is found in the shikimate and phenylpropanoid pathways, which produce salicylic acid, alkaloids, tannins, anthocyanins, flavonols and other flavonoids; all of which have roles in stress tolerance and plant defense. Isoprene also influences key gene expression patterns in the terpenoid biosynthetic pathways, and the jasmonic acid, gibberellic acid and cytokinin signaling networks that have important roles in controlling inducible defense responses and influencing plant growth and development, particularly following defoliation. In this synthesis paper, using past studies of transgenic poplar, tobacco and Arabidopsis, we present the evidence for isoprene acting as a metabolite that coordinates aspects of cellular signaling, resulting in enhanced chemical defense during periods of climate stress, while minimizing costs to growth. This perspective represents a major shift in our thinking away from direct effects of isoprene, for example, by changing membrane properties or quenching ROS, to indirect effects, through changes in gene expression and protein abundances. Recognition of isoprene's role in the growth-defense tradeoff provides new perspectives on evolution of the trait, its contribution to plant adaptation and resilience, and the ecological niches in which it is most effective.

Removal of sulfamethazine and Cu2+ by Sakaguchia cladiensis A5: Performance and transcriptome analysis

To reduce the potential risks of contamination of antibiotics and heavy metals to ecological environment and human safety, biological removal of these composite pollutants is the focus of much study. One previously identified isolate, Sakaguchia cladiensis A5, was used to decompose sulfamethazine (SMZ) and adsorb Cu²⁺. The ability of A5 to remove SMZ was enhanced by pre-induced culture, which reached 49.8% on day 9. The removal of SMZ could be also increased to 37.6% on day 3 in the presence of Cu²⁺, but only to 12.2% in the system without Cu²⁺. The biosorption of Cu²⁺ mainly occurred on the cell walls, while the biodegradation of SMZ was inside the cells. By comparative transcriptome analysis for A5, 1270 and 2220 differentially expressed genes (DEGs) were identified after treating single SMZ and SMZ/Cu²⁺, respectively. The Gene expression pattern analysis suggested a suppression of transcriptional changes in A5 responding to SMZ/Cu²⁺ as compared to under the sole stress of SMZ. The DEGs functional enrichment analysis suggested that the antioxidant and sulfate assimilation pathways played a key role on SMZ biodegradation and Cu²⁺ biosorption. The DEGs of proteins CAT, PRDX5, SAT, and CYSC were up-regulated to facilitate the resistance of A5 against oxidative toxicity of Cu²⁺. Moreover, the protein MET30 activated by Cu²⁺ was also overexpressed to promote the transmembrane transport of SMZ, such that A5 could decompose SMZ more effectively in SMZ/Cu²⁺ system. The results of this study would provide new insights into the mechanism of biodegradation and biosorption of SMZ/Cu²⁺.

Hydrogen peroxide as a signalling molecule in plants and its crosstalk with other plant growth regulators under heavy metal stress

Hydrogen peroxide (H₂O₂) acts as a significant regulatory component interrelated with signal transduction in plants. The positive role of H₂O₂ in plants subjected to myriad of abiotic factors has led us to comprehend that it is not only a free radical, generated as a product of oxidative stress, but also helpful in the maintenance of cellular homeostasis in crop plants. Studies over the last two centuries has indicated that H₂O₂ is a key molecule which regulate photosynthesis, stomatal movement, pollen growth, fruit and flower development and leaf senescence. Exogenously-sourced H₂O₂ at nanomolar levels functions as a signalling molecule, facilitates seed germination, chlorophyll content, stomatal opening, and delays senescence, while at elevated levels, it triggers oxidative burst to organic molecules, which could lead to cell death. Furthermore, H₂O₂ is also known to interplay synergistically or antagonistically with other plant growth regulators such as auxins, gibberellins, cytokinins, abscisic acid, jasmonic acid, ethylene and salicylic acid, nitric oxide and Ca²⁺ (as signalling molecules), and brassinosteroids (steroidal PGRs) under myriad of environmental stresses and thus, mediate plant growth and development and reactions to abiotic factors. The purpose of this review is to specify accessible knowledge on the role and dynamic mechanisms of H₂O₂ in mediating growth responses and plant resilience to HM stresses, and its crosstalk with other significant PGRs in controlling various processes. More recently, signal transduction by mitogen activated protein kinases and other transcription factors which attenuate HM stresses in plants have also been dissected.

MicroRNA and ROS Crosstalk in Cardiac and Pulmonary Diseases

Reactive oxygen species (ROS) affect many cellular functions and the proper redox balance between ROS and antioxidants contributes substantially to the physiological welfare of the cell. During pathological conditions, an altered redox equilibrium leads to increased production of ROS that in turn may cause oxidative damage. MicroRNAs (miRNAs) regulate gene expression at the post-transcriptional level contributing to all major cellular processes, including oxidative stress and cell death. Several miRNAs are expressed in response to ROS to mediate oxidative stress. Conversely, oxidative stress may lead to the upregulation of miRNAs that control mechanisms to buffer the damage induced by ROS. This review focuses on the complex crosstalk between miRNAs and ROS in diseases of the cardiac (i.e., cardiac hypertrophy, heart failure, myocardial infarction, ischemia/reperfusion injury, diabetic cardiomyopathy) and pulmonary (i.e., idiopathic pulmonary fibrosis, acute lung injury/acute respiratory distress syndrome, asthma, chronic obstructive pulmonary disease, lung cancer) compartments. Of note, miR-34a, miR-144, miR-421, miR-129, miR-181c, miR-16, miR-31, miR-155, miR-21, and miR-1/206 were found to play a role during oxidative stress in both heart and lung pathologies. This review comprehensively summarizes current knowledge in the field.

Cysteine Oxidations in Mitochondrial Membrane Proteins: The Case of VDAC Isoforms in Mammals

Cysteine residues are reactive amino acids that can undergo several modifications driven by redox reagents. Mitochondria are the source of an abundant production of radical species, and it is surprising that such a large availability of highly reactive chemicals is compatible with viable and active organelles, needed for the cell functions. In this work, we review the results highlighting the modifications of cysteines in the most abundant proteins of the outer mitochondrial membrane (OMM), that is, the voltage-dependent anion selective channel (VDAC) isoforms. This interesting protein family carries several cysteines exposed to the oxidative intermembrane space (IMS). Through mass spectrometry (MS) analysis, cysteine posttranslational modifications (PTMs) were precisely determined, and it was discovered that such cysteines can be subject to several oxidization degrees, ranging from the disulfide bridge to the most oxidized, the sulfonic acid, one. The large spectra of VDAC cysteine oxidations, which is unique for OMM proteins, indicate that they have both a regulative function and a buffering capacity able to counteract excess of mitochondrial reactive oxygen species (ROS) load. The consequence of these peculiar cysteine PTMs is discussed.

Cytosolic/Plastid Glyceraldehyde-3-Phosphate Dehydrogenase Is a Negative Regulator of Strawberry Fruit Ripening

Cytosolic glyceraldehyde-3-phosphate dehydrogenase (GAPC) and plastid glyceraldehyde-3-phosphate dehydrogenase (GAPCp) are key enzymes in glycolysis. Besides their catalytic function, GAPC/GAPCp participates in the regulation of plant stress response and growth and development. However, the involvement of GAPC/GAPCp in the regulation of fruit ripening is unclear. In this study, FaGAPC2 and FaGAPCp1 in strawberries were isolated and analyzed. FaGAPC2 and FaGAPCp1 transcripts showed high transcript levels in the fruit. Transient overexpression of FaGAPC2 and FaGAPCp1 delayed fruit ripening, whereas RNA interference promoted fruit ripening and affected fruit anthocyanins and sucrose levels. Change in the expression patterns of FaGAPC2 and FaGAPCp1 also influenced the expression of several glycolysis-related and ripening-related genes such as CEL1, CEL2, SS, ANS, MYB5, NCED1, ABI1, ALDO, PK, and G6PDH, and H₂O₂ level and reduced glutathione (GSH)/glutathione disulfide (GSSG) redox potential. Meanwhile, metabolomics experiments showed that transient overexpression of FaGAPCp1 resulted in a decrease in anthocyanins, flavonoids, organic acid, amino acids, and their derivatives. In addition, abscisic acid (ABA) and sucrose treatment induced the production of large amounts of H₂O₂ and inhibited the expression of FaGAPC2/FaGAPCp1 in strawberry fruit. These results revealed that FaGAPC2/FaGAPCp1 is a negative regulator of ABA and sucrose mediated fruit ripening which can be regulated by oxidative stress.

Exercise, redox homeostasis and the epigenetic landscape

Physical exercise represents one of the strongest physiological stimuli capable to induce functional and structural modifications in all biological systems. Indeed, beside the traditional genetic mechanisms, physical exercise can modulate gene expression through epigenetic modifications, namely DNA methylation, post-translational histone modification and non-coding RNA transcripts.

Initially considered as merely damaging molecules, it is now well recognized that both reactive oxygen (ROS) and nitrogen species (RNS) produced under voluntary exercise play an important role as regulatory mediators in signaling processes. While robust scientific evidences highlight the role of exercise-associated redox modifications in modulating gene expression through the genetic machinery, the understanding of their specific impact on epigenomic profile is still at an early stage. This review will provide an overview of the role of ROS and RNS in modulating the epigenetic landscape in the context of exercise-related adaptations.

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Physical exercise can modulate gene expression through epigenetic modifications.

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Epigenetic regulation of ROS/RNS generating, sensing and neutralizing enzymes can impact the cellular levels of ROS and RNS.

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ROS might act as modulators of epigenetic machinery, interfering with DNA methylation, hPTMs and ncRNAs expression.

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Redox homeostasis might hold a relevant role in the epigenetic landscape modulating exercise-related adaptations.

The progeny of Chernobyl Arabidopsis thaliana plants does not exhibit changes in morphometric parameters and cellular antioxidant defence system of shoots

Morphometric parameters and functional state of the cellular antioxidant defence system of shoots were studied in the progeny of Arabidopsis thaliana (L.) Heynh. (A. thaliana) plants, previously and chronically exposed in the Chernobyl Exclusion Zone (ChEZ). Changes in cellular antioxidant enzyme activities in the progeny of exposed plants were assumed because antioxidant status of cell may be altered by inherited epigenetic changes, resulting in changes in antioxidant-response genes expression. These changes can be inferred as induced expression of CAT and SOD genes was found previously for A. thaliana plants by another group of scientists. It is well-known that ionizing radiation may induce changes in hormonal-signalling net-work, shifting balance in growth factors that may cause changes in morphometric parameters of plants. Seeds from A. thaliana plants were collected in the ChEZ at different levels of the external dose rate from 0.28 ± 0.01 to 12.93 ± 0.08 μGy/h. Internal dose rate for parent plants was calculated on the basis of the activity concentration of ⁹⁰Sr and ¹³⁷Cs in the plants, using dose conversion coefficients for wild grass. Total dose rate, absorbed by parent plants, was calculated as the sum of the external and internal dose rate and was in a range between 2.8 ± 0.2 and 99 ± 8 μGy/h. Seeds were then grown in the standard laboratory conditions (nutrient-agar, light-dark cycle and appropriate temperature) to analyse morphometric parameters of seedlings and final germination percentage. No significant changes in the morphometric parameters (root length and rosette diameter of shoots) of the seedlings were observed. Changes in the final germination percentage of the studied seeds were found, but low correlation was observed between found changes and the dose rate, absorbed by parent plants. In contrast to the results obtained in A. thaliana plants directly sampled in the field, no effect on the functional state of the cellular antioxidant defence system of shoots in the progeny of Chernobyl A. thaliana plants was observed.

Melatonin-Nitric Oxide Crosstalk and Their Roles in the Redox Network in Plants

Melatonin, an amine hormone highly conserved during evolution, has a wide range of physiological functions in animals and plants. It is involved in plant growth, development, maturation, and aging, and also helps ameliorate various types of abiotic and biotic stresses, including salt, drought, heavy metals, and pathogens. Melatonin-related growth and defense responses of plants are complex, and involve many signaling molecules. Among these, the most important one is nitric oxide (NO), a freely diffusing amphiphilic biomolecule that can easily cross the cell membrane, produce rapid signal responses, and participate in a wide variety of physiological reactions. NO-induced S-nitrosylation is also involved in plant defense responses. NO interacts with melatonin as a long-range signaling molecule, and helps regulate plant growth and maintain oxidative homeostasis. Exposure of plants to abiotic stresses causes the increase of endogenous melatonin levels, with the consequent up-regulation of melatonin synthesis genes, and further increase of melatonin content. The application of exogenous melatonin causes an increase in endogenous NO and up-regulation of defense-related transcription factors, resulting in enhanced stress resistance. When plants are infected by pathogenic bacteria, NO acts as a downstream signal to lead to increased melatonin levels, which in turn induces the mitogen-activated protein kinase (MAPK) cascade and associated defense responses. The application of exogenous melatonin can also promote sugar and glycerol production, leading to increased levels of salicylic acid and NO. Melatonin and NO in plants can function cooperatively to promote lateral root growth, delay aging, and ameliorate iron deficiency. Further studies are needed to clarify certain aspects of the melatonin/NO relationship in plant physiology.

Activity of the yeast cytoplasmic Hsp70 nucleotide-exchange factor Fes1 is regulated by reversible methionine oxidation

Cells employ a vast network of regulatory pathways to manage intracellular levels of reactive oxygen species (ROS). An effectual means used by cells to control these regulatory systems are sulfur-based redox switches, which consist of protein cysteine or methionine residues that become transiently oxidized when intracellular ROS levels increase. Here, we describe a methionine-based oxidation event involving the yeast cytoplasmic Hsp70 co-chaperone Fes1. We show that Fes1 undergoes reversible methionine oxidation during excessively-oxidizing cellular conditions, and we map the site of this oxidation to a cluster of three methionine residues in the Fes1 core domain. Making use of recombinant proteins and a variety of in vitro assays, we establish that oxidation inhibits Fes1 activity and, correspondingly, alters Hsp70 activity. Moreover, we demonstrate in vitro and in cells that Fes1 oxidation is reversible and is regulated by the cytoplasmic methionine sulfoxide reductase Mxr1 (MsrA) and a previously unidentified cytoplasmic pool of the reductase Mxr2 (MsrB). We speculate that inactivation of Fes1 activity during excessively-oxidizing conditions may help maintain protein-folding homeostasis in a suboptimal cellular folding environment. The characterization of Fes1 oxidation during cellular stress provides a new perspective as to how the activities of the cytoplasmic Hsp70 chaperones may be attuned by fluctuations in cellular ROS levels and provides further insight into how cells use methionine-based redox switches to sense and respond to oxidative stress.

Redox Proteomes in Human Physiology and Disease Mechanisms

Redox proteomics is a field of proteomics that is concerned with the characterization of the oxidation state of proteins to gain information about their modulated structure, function, activity, and involvement in different physiological pathways. Oxidative modifications of proteins have been shown to be implicated in normal physiological processes of cells as well as in pathomechanisms leading to the development of cancer, diabetes, neurodegenerative diseases, and some rare hereditary metabolic diseases, like classic galactosemia. Reactive oxygen species generate a variety of reversible and irreversible modifications in amino acid residue side chains and within the protein backbone. These oxidative post-translational modifications (Ox-PTMs) can participate in the activation of signal transduction pathways and mediate the toxicity of harmful oxidants. Thus the application of advanced redox proteomics technologies is important for gaining insights into molecular mechanisms of diseases. Mass-spectrometry-based proteomics is one of the most powerful methods that can be used to give detailed qualitative and quantitative information on protein modifications and allows us to characterize redox proteomes associated with diseases. This Review illustrates the role and biological consequences of Ox-PTMs under basal and oxidative stress conditions by focusing on protein carbonylation and S-glutathionylation, two abundant modifications with an impact on cellular pathways that have been intensively studied during the past decade.

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.

The function of S-nitrosothiols during abiotic stress in plants

Nitric oxide (NO) is an active redox molecule involved in the control of a wide range of functions integral to plant biology. For instance, NO is implicated in seed germination, floral development, senescence, stomatal closure, and plant responses to stress. NO usually mediates signaling events via interactions with different biomolecules, for example the modulation of protein functioning through post-translational modifications (NO-PTMs). S-nitrosation is a reversible redox NO-PTM that consists of the addition of NO to a specific thiol group of a cysteine residue, leading to formation of S-nitrosothiols (SNOs). SNOs are more stable than NO and therefore they can extend and spread the in vivo NO signaling. The development of robust and reliable detection methods has allowed the identification of hundreds of S-nitrosated proteins involved in a wide range of physiological and stress-related processes in plants. For example, SNOs have a physiological function in plant development, hormone metabolism, nutrient uptake, and photosynthesis, among many other processes. The role of S-nitrosation as a regulator of plant responses to salinity and drought stress through the modulation of specific protein targets has also been well established. However, there are many S-nitrosated proteins that have been identified under different abiotic stresses for which the specific roles have not yet been identified. In this review, we examine current knowledge of the specific role of SNOs in the signaling events that lead to plant responses to abiotic stress, with a particular focus on examples where their functions have been well characterized at the molecular level.

Nitric oxide and hydrogen sulfide in plants: which comes first?

Nitric oxide (NO) is a signal molecule regarded as being involved in myriad functions in plants under physiological, pathogenic, and adverse environmental conditions. Hydrogen sulfide (H2S) has also recently been recognized as a new gasotransmitter with a diverse range of functions similar to those of NO. Depending on their respective concentrations, both these molecules act synergistically or antagonistically as signals or damage promoters in plants. Nevertheless, available evidence shows that the complex biological connections between NO and H2S involve multiple pathways and depend on the plant organ and species, as well as on experimental conditions. Cysteine-based redox switches are prone to reversible modification; proteomic and biochemical analyses have demonstrated that certain target proteins undergo post-translational modifications such as S-nitrosation, caused by NO, and persulfidation, caused by H2S, both of which affect functionality. This review provides a comprehensive update on NO and H2S in physiological processes (seed germination, root development, stomatal movement, leaf senescence, and fruit ripening) and under adverse environmental conditions. Existing data suggest that H2S acts upstream or downstream of the NO signaling cascade, depending on processes such as stomatal closure or in response to abiotic stress, respectively.

Sulfur: the heart of nitric oxide-dependent redox signalling

Nitric oxide (NO), more benign than its more reactive and damaging related molecules, reactive oxygen species (ROS), is perfectly suited for duties as a redox signalling molecule. A key route for NO bioactivity is through S-nitrosation, the addition of an NO moiety to a protein Cys thiol (-SH). This redox-based, post-translational modification (PTM) can modify protein function analogous to more well established PTMs such as phosphorylation, for example by modulating enzyme activity, localization, or protein-protein interactions. At the heart of the underpinning chemistry associated with this PTM is sulfur. The emerging evidence suggests that S-nitrosation is integral to a myriad of plant biological processes embedded in both development and environmental relations. However, a role for S-nitrosation is perhaps most well established in plant-pathogen interactions.

Nitric Oxide as a Beneficial Signaling Molecule in Trichoderma atroviride TRS25-Induced Systemic Defense Responses of Cucumber Plants Against Rhizoctonia solani

In the present study, Trichoderma atroviride TRS25 is presented as a biological control agent, which significantly limits the development of infection and reduces the disease caused by the pathogenic fungus Rhizoctonia solani in cucumber plants (Cucumis sativus L.). The systemic disease suppression is related to oxidative, signaling, and biochemical changes, that are triggered in response to a pathogen. Induction of systemic defense in cucumber by TRS25 greatly depends on the accumulation of signaling molecules including hydrogen peroxide (H2O2) and nitric oxide (NO) as well as salicylic acid (SA) and its derivatives including methyl salicylate (MeSA) and octyl salicylate (OSA). The study established that NO was accumulated in leaves and shoots of the cucumber plants, especially those pretreated with Trichoderma and inoculated with R. solani, where the compound was accumulated mainly in the cells localized in the vascular bundles and in epidermal tissues. We suggest, for the first time, that in the plants pretreated with TRS25, the accumulation of H2O2 and NO may be related to catalase (CAT) and S-nitrosoglutathione reductase (GSNOR) activity decrease. On the other hand, excessive accumulation of NO and SA may be controlled by forming their inactive forms, S-nitrosothiols (SNO) and salicylic acid glucosylated conjugates (SAGC), respectively. The obtained results suggest that the mentioned molecules may be an important component of the complex signaling network activated by TRS25, which is positively involved in systemic defense responses of cucumber plants against R. solani.

Fluorescent Imaging for Cysteine Detection In Vivo with High Selectivity

As an essential amino acid, cysteine is involved in various biosynthetic and metabolic processes, such as protein synthesis, hormone synthesis, and redox homeostatic maintenance. Inordinate cysteine levels are often associated with serious diseases. Thus, designing and synthesizing a novel fluorescent probe for determining the concentration of cellular cysteine, which could indirectly monitor the prevalence of these diseases, is essential. We developed a florescence probe P−Cy with good sensitivity for cysteine detection in vivo. P−Cy only exhibited good response toward cysteine but did not show response toward other biothiols, such as homocysteine (Hcy) and glutathione (GSH). In this study, we used P−Cy by successfully imaging cellular endogenous and exogenous cysteine levels. Furthermore, P−Cy was also performed in mice to detect cysteine level, indicating that P−Cy is a powerful tool for cysteine detection in situ.

Trichoderma asperellum T42 induces local defense against Xanthomonas oryzae pv. oryzae under nitrate and ammonium nutrients in tobacco

Trichoderma has been explored and found to play a vital role in the defense mechanism of plants. However, its effects on host disease management in the presence of N nutrients remains elusive. The present study aimed to assess the latent effects of Trichoderma asperellum T42 on oxidative burst-mediated defense mechanisms against Xanthomonas oryzae pv. oryzae (Xoo) in tobacco plants fed 10 mM NO₃⁻ and 3 mM NH₄⁺ nutrients. The nitrate-fed tobacco plants displayed an increased HR when Xoo infected, which was enhanced in the Trichoderma-treated plants. This mechanism was enhanced by the involvement of Trichoderma, which elicited NO production and enhanced the expression pattern of NO-modulating genes (NR, NOA and ARC). The real-time NO fluorescence intensity was alleviated in the NH₄⁺-fed tobacco plants compared to that fed NO₃⁻ nutrient, suggesting the significant role of Trichoderma-elicited NO. The nitrite content and NR activity demonstration further confirmed that nitrate metabolism led to NO generation. The production of ROS (H₂O₂) in the plant leaves well-corroborated that the disease resistance was mediated through the oxidative burst mechanism. Nitrate application resulted in greater ROS production compared to NH₄⁺ nutrient after Xoo infection at 12 h post-infection (hpi). Additionally, the mechanism of enhanced plant defense under NO₃⁻ and NH₄⁺ nutrients mediated by Trichoderma involved NO, ROS production and induction of PR1a MEK3 and antioxidant enzyme transcription level. Moreover, the use of sodium nitroprusside (100 μM) with Xoo suspension in the leaves matched the disease resistance mediated via NO burst. Altogether, this study provides novel insights into the fundamental mechanism behind the role of Trichoderma in the activation of plant defense against non-host pathogens under N nutrients.

A hypothetical proposed defense pathway activated during interactions between bacterial pathogen (Xoo) with tobacco plant leaves among treatments.

Recurrent water deficit causes alterations in the profile of redox proteins in citrus plants

Plant acclimation to recurrent stress involves profound alterations in multiple genetic, metabolic and physiological processes. Stressful conditions usually implicate imbalance in reactive oxygen species (ROS) production and removal rates, which may lead to oxidative stress. However, the primary cellular targets of oxidative stress and their relevance in plant acclimation to abiotic stresses remains poorly characterized. By comparing redox proteomic and sugar profiles in citrus Valencia (VO) scions grafted onto two rootstocks with different soil water extraction capacities - Rangpur Lime (RL) and Sunki Maravilha (SM) - here we demonstrate that both ROS-mediated post-translational protein modification and changes in sugar composition are associated with acclimation to recurrent drought in citrus. The redox proteomic analysis of the distinct scion/rootstock combinations exposed to one (WD1), two (WD2) or three (WD3) water deficit episodes revealed a total of 32 and 55 redox protein spots present in VO/RL and VO/SM plants, respectively. Mass spectrometry analysis of these protein spots revealed essential targets of ROS-mediated posttranslational protein modification in citrus plants challenged by recurrent drought. The oxidation of cysteine thiol groups into glyceraldehyde 3-phosphate dehydrogenase (GAPDH) was shown to increase in WD3 samples of the VO/RL combination, whereas the opposite was observed for the VO/SM combination. Similarly, recurrent drought promoted the oxidation of catalase thiol groups in VO/SM, but not in VO/RL. Carbohydrate profiling revealed that glucose, fructose and galactose may also contribute to the phenotypic differences observed between the citrus genotypes exposed to drought. These findings reveal for the first time that recurrent drought differentially affects the profile of redox proteomics of citrus, suggesting that this alteration may be part of the stress memory in perennial plants.

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.

S-nitrosylation of the zinc finger protein SRG1 regulates plant immunity

Nitric oxide (NO) orchestrates a plethora of incongruent plant immune responses, including the reprograming of global gene expression. However, the cognate molecular mechanisms remain largely unknown. Here we show a zinc finger transcription factor (ZF-TF), SRG1, is a central target of NO bioactivity during plant immunity, where it functions as a positive regulator. NO accumulation promotes SRG1 expression and subsequently SRG1 occupies a repeated canonical sequence within target promoters. An EAR domain enables SRG1 to recruit the corepressor TOPLESS, suppressing target gene expression. Sustained NO synthesis drives SRG1 S-nitrosylation predominantly at Cys87, relieving both SRG1 DNA binding and transcriptional repression activity. Accordingly, mutation of Cys87 compromises NO-mediated control of SRG1-dependent transcriptional suppression. Thus, the SRG1-SNO formation may contribute to a negative feedback loop that attenuates the plant immune response. SRG1 Cys87 is evolutionary conserved and thus may be a target for redox regulation of ZF-TF function across phylogenetic kingdoms.

Genome-wide identification of the entire 90 glutathione S-transferase (GST) subfamily genes in four rotifer Brachionus species and transcriptional modulation in response to endocrine disrupting chemicals

Genome-wide identification of glutathione S-transferase (GST), a major phase II detoxification enzyme, was investigated in four different aquatic model rotifer species Brachionus koreanus, B. plicatilis, B. rotundiformis, and B. calyciflorus. GSTs are ubiquitous antioxidant enzymes that play versatile function including cellular detoxification, stress alleviation, and production of the radical conjugates. Among the four rotifers, B. rotundiformis was found with the least number of GST genes (total 19 GST genes), whereas the other three species shared 23 to 24 GST genes. Among the identified GST genes, belonging to the cytosolic GST superfamily, the expansion of GST sigma classes mainly occurs through tandem duplication, resulting in tandem-arrayed gene clusters on the chromosomes. Overall, the number of genes discovered in this study was highest in the sigma class, zeta, alpha, and omega in descending order. With integration of phylogenetic analysis and xenobiotic-mediated GST mRNA expression patterns along with previous enzymatic activities, the functional divergence among species-specific GST genes was clearly observed. This study covers full identification of GST classes in three marine rotifer and one fresh-water rotifer species and their important role in marine environmental ecotoxicology.

Specificity in nitric oxide signalling

Reactive nitrogen species (RNS) and their cognate redox signalling networks pervade almost all facets of plant growth, development, immunity, and environmental interactions. The emerging evidence implies that specificity in redox signalling is achieved by a multilayered molecular framework. This encompasses the production of redox cues in the locale of the given protein target and protein tertiary structures that convey the appropriate local chemical environment to support redox-based, post-translational modifications (PTMs). Nascent nitrosylases have also recently emerged that mediate the formation of redox-based PTMs. Reversal of these redox-based PTMs, rather than their formation, is also a major contributor of signalling specificity. In this context, the activities of S-nitrosoglutathione (GSNO) reductase and thioredoxin h5 (Trxh5) are a key feature. Redox signalling specificity is also conveyed by the unique chemistries of individual RNS which is overlaid on the structural constraints imposed by tertiary protein structure in gating access to given redox switches. Finally, the interactions between RNS and ROS (reactive oxygen species) can also indirectly establish signalling specificity through shaping the formation of appropriate redox cues. It is anticipated that some of these insights might function as primers to initiate their future translation into agricultural, horticultural, and industrial biological applications.

Brassinosteroid-mediated apoplastic H2 O2 -glutaredoxin 12/14 cascade regulates antioxidant capacity in response to chilling in tomato

Brassinosteroids (BRs) regulate plant development and stress response. Although much has been learned about their roles in plant development, the mechanisms by which BRs regulate plant stress tolerance remain unclear. Chilling is a major stress that adversely affects plant growth. Here, we report that BR positively regulates chilling tolerance in tomato. BR partial deficiency aggravated chilling-induced oxidized protein accumulation, membrane lipid peroxidation, and decrease of maximum quantum efficiency of photosystem II (Fv/Fm). By contrast, overexpression of BR biosynthetic gene Dwarf or treatment with 24-epibrassinolide (EBR) attenuated chilling-induced oxidative damages and resulted in an increase of Fv/Fm. BR increased transcripts of RESPIRATORY BURST OXIDASE HOMOLOG1 (RBOH1) and GLUTAREDOXIN (GRX) genes, and BR-induced chilling tolerance was associated with an increase in the ratio of reduced/oxidized 2-cysteine peroxiredoxin (2-Cys Prx) and activation of antioxidant enzymes. However, RBOH1-RNAi plants failed to respond to EBR as regards to the induction of GRX genes, activation of antioxidant capacity, and attenuation of chilling-induced oxidative damages. Furthermore, silencing of GRXS12 and S14 compromised EBR-induced increases in the ratio of reduced/oxidized 2-Cys Prx and activities of antioxidant enzymes. Our study suggests that BR enhances chilling tolerance through a signalling cascade involving RBOH1, GRXs, and 2-Cys Prx in tomato.

Transcriptome profile of NO-induced Arabidopsis transcription factor genes suggests their putative regulatory role in multiple biological processes

TFs are important proteins regulating plant responses during environmental stresses. These insults typically induce changes in cellular redox tone driven in part by promoting the production of reactive nitrogen species (RNS). The main source of these RNS is nitric oxide (NO), which serves as a signalling molecule, eliciting defence and resistance responses. To understand how these signalling molecules regulate key biological processes, we performed a large scale S-nitrosocysteine (CySNO)-mediated RNA-seq analysis. The DEGs were analysed to identify potential regulatory TFs. We found a total of 673 (up- and down-regulated) TFs representing a broad range of TF families. GO-enrichment and MapMan analysis suggests that more than 98% of TFs were mapped to the Arabidopsis thaliana genome and classified into pathways like hormone signalling, protein degradation, development, biotic and abiotic stress, etc. A functional analysis of three randomly selected TFs, DDF1, RAP2.6, and AtMYB48 identified a regulatory role in plant growth and immunity. Loss-of-function mutations within DDF1 and RAP2.6 showed compromised basal defence and effector triggered immunity, suggesting their positive role in two major plant defence systems. Together, these results imply an important data representing NO-responsive TFs that will help in exploring the core mechanisms involved in biological processes in plants.

Glutathione, Glutaredoxins, and Iron

SIGNIFICANCE

Glutathione (GSH) is the most abundant cellular low-molecular-weight thiol in the majority of organisms in all kingdoms of life. Therefore, functions of GSH and disturbed regulation of its concentration are associated with numerous physiological and pathological situations. Recent Advances: The function of GSH as redox buffer or antioxidant is increasingly being questioned. New functions, especially functions connected to the cellular iron homeostasis, were elucidated. Via the formation of iron complexes, GSH is an important player in all aspects of iron metabolism: sensing and regulation of iron levels, iron trafficking, and biosynthesis of iron cofactors. The variety of GSH coordinated iron complexes and their functions with a special focus on FeS-glutaredoxins are summarized in this review. Interestingly, GSH analogues that function as major low-molecular-weight thiols in organisms lacking GSH resemble the functions in iron homeostasis.

CRITICAL ISSUES

Since these iron-related functions are most likely also connected to thiol redox chemistry, it is difficult to distinguish between mechanisms related to either redox or iron metabolisms.

FUTURE DIRECTIONS

The ability of GSH to coordinate iron in different complexes with or without proteins needs further investigation. The discovery of new Fe-GSH complexes and their physiological functions will significantly advance our understanding of cellular iron homeostasis. Antioxid. Redox Signal. 27, 1235-1251.

Redox-dependent dimerization of p38α mitogen-activated protein kinase with mitogen-activated protein kinase kinase 3

The kinase p38α MAPK (p38α) plays a pivotal role in many biological processes. p38α is activated by canonical upstream kinases that phosphorylate the activation region. The purpose of our study was to determine whether such activation may depend on redox-sensing cysteines within p38α. p38α was activated and formed a disulfide-bound heterodimer with MAP2K3 (MKK3) in rat cardiomyocytes and isolated hearts exposed to H₂O₂. This disulfide heterodimer was sensitive to reduction by mercaptoethanol and was enhanced by the thioredoxin-reductase inhibitor auranofin. We predicted that Cys-119 or Cys-162 of p38α, close to the known MKK3 docking domain, were relevant for these redox characteristics. The C119S mutation decreased whereas the C162S mutation increased the dimer formation, suggesting that these two Cys residues act as vicinal thiols, consistent with C119S/C162S being incapable of sensing H₂O₂. Similarly, disulfide heterodimer formation was abolished in H9C2 cells expressing both MKK3 and p38α C119S/C162S and subjected to simulated ischemia and reperfusion. However, the p38α C119S/C162S mutants did not exhibit appreciable alteration in activating dual phosphorylation. In contrast, the anti-inflammatory agent 10-nitro-oleic acid (NO₂-OA), a component of the Mediterranean diet, reduced p38α activation and covalently modified Cys-119/Cys-162, probably obstructing MKK3 access. Moreover, NO₂-OA reduced the dephosphorylation of p38α by hematopoietic tyrosine phosphatase (HePTP). Furthermore, steric obstruction of Cys-119/Cys-162 by NO₂-OA pretreatment in Langendorff-perfused murine hearts prevented the p38-MKK3 disulfide dimer formation and attenuated H₂O₂-induced contractile dysfunction. Our findings suggest that cysteine residues within p38α act as redox sensors that can dynamically regulate the association between p38 and MKK3.

Regulatory Role of Redox Balance in Determination of Neural Precursor Cell Fate

In 1990s, reports of discovery of a small group of cells capable of proliferation and contribution to formation of new neurons in the central nervous system (CNS) reversed a century-old concept on lack of neurogenesis in the adult mammalian brain. These cells are found in all stages of human life and contribute to normal cellular turnover of the CNS. Therefore, the identity of regulating factors that affect their proliferation and differentiation is a highly noteworthy issue for basic scientists and their clinician counterparts for therapeutic purposes. The cues for such control are embedded in developmental and environmental signaling through a highly regulated tempo-spatial expression of specific transcription factors. Novel findings indicate the importance of reactive oxygen species (ROS) in the regulation of this signaling system. The elusive nature of ROS signaling in many vital processes from cell proliferation to cell death creates a complex literature in this field. Here, we discuss the emerging thoughts on the importance of redox regulation of proliferation and maintenance in mammalian neural stem and progenitor cells under physiological and pathological conditions. The current knowledge on ROS-mediated changes in redox-sensitive proteins that govern the molecular mechanisms in proliferation and differentiation of these cells is reviewed.

NOXious gases and the unpredictability of emerging plant pathogens under climate change

Emerging pathogens of crops threaten food security and are increasingly problematic due to intensive agriculture and high volumes of trade and transport in plants and plant products. The ability to predict pathogen risk to agricultural regions would therefore be valuable. However, predictions are complicated by multi-faceted relationships between crops, their pathogens, and climate change. Climate change is related to industrialization, which has brought not only a rise in greenhouse gas emissions but also an increase in other atmospheric pollutants. Here, we consider the implications of rising levels of reactive nitrogen gases and their manifold interactions with crops and crop diseases.

Transport of chemical signals in systemic acquired resistance

Systemic acquired resistance (SAR) is a form of broad-spectrum resistance induced in response to local infections that protects uninfected parts against subsequent secondary infections by related or unrelated pathogens. SAR signaling requires two parallel branches, one regulated by salicylic acid (SA), and the other by azelaic acid (AzA) and glycerol-3-phosphate (G3P). AzA and G3P function downstream of the free radicals nitric oxide (NO) and reactive oxygen species (ROS). During SAR, SA, AzA and G3P accumulate in the infected leaves, but only a small portion of these is transported to distal uninfected leaves. SA is preferentially transported via the apoplast, whereas phloem loading of AzA and G3P occurs via the symplast. The symplastic transport of AzA and G3P is regulated by gating of the plasmodesmata (PD). The PD localizing proteins, PDLP1 and PDLP5, regulate SAR by regulating PD gating as well as the subcellular partitioning of a SAR-associated protein.