S-nitrosylation positively regulates ascorbate peroxidase activity during plant stress responses

Dimethyloxalylglycine (DMOG), a Hypoxia Mimetic Agent, Does Not Replicate a Rat Pheochromocytoma (PC12) Cell Biological Response to Reduced Oxygen Culture

Cells respond to reduced oxygen availability predominately by activation of the hypoxia-inducible factor (HIF) pathway. HIF activation upregulates hundreds of genes that help cells survive in the reduced oxygen environment. The aim of this study is to determine whether chemical-induced HIF accumulation mimics all aspects of the hypoxic response of cells. We compared the effects of dimethyloxalylglycine (DMOG) (a HIF stabiliser) on PC12 cells cultured in air oxygen (20.9% O_2, AO) with those cultured in either intermittent 20.9% O₂ to 2% O₂ (IH) or constant 2% O₂ (CN). Cell viability, cell cycle, HIF accumulation, reactive oxygen species (ROS) formation, mitochondrial function and differentiation were used to characterise the PC12 cells and evaluate the impact of DMOG. IH and CN culture reduced the increase in cell numbers after 72 and 96 h and MTT activity after 48 h compared to AO culture. Further, DMOG supplementation in AO induced a dose-dependent reduction in the increase in PC12 cell numbers and MTT activity. IH-cultured PC12 cells displayed increased and sustained HIF-1 expression over 96 h. This was accompanied by increased ROS and mitochondrial burden. PC12 cells in CN displayed little changes in HIF-1 expression or ROS levels. DMOG (0.1 mM) supplementation resulted in an IH-like HIF-1 profile. The mitochondrial burden and action potential of DMOG-supplemented PC12 cells did not mirror those seen in other conditions. DMOG significantly increased S phase cell populations after 72 and 96 h. No significant effect on PC12 cell differentiation was noted with IH and CN culture without induction by nerve growth factor (NGF), while DMOG significantly increased PC12 cell differentiation with and without NGF. In conclusion, DMOG and reduced oxygen levels stabilise HIF and affect mitochondrial activity and cell behaviour. However, DMOG does not provide an accurate replication of the reduced oxygen environments.

A nitric oxide burst at the shoot apex triggers a heat-responsive pathway in Arabidopsis

When confronted with heat stress, plants depend on the timely activation of cellular defences to survive by perceiving the rising temperature. However, how plants sense heat at the whole-plant level has remained unanswered. Here we demonstrate that shoot apical nitric oxide (NO) bursting under heat stress as a signal triggers cellular heat responses at the whole-plant level on the basis of our studies mainly using live-imaging of transgenic plants harbouring pHsfA2::LUC, micrografting, NO accumulation mutants and liquid chromatography-tandem mass spectrometry analysis in Arabidopsis. Furthermore, we validate that S-nitrosylation of the trihelix transcription factor GT-1 by S-nitrosoglutathione promotes its binding to NO-responsive elements in the HsfA2 promoter and that loss of function of GT-1 disrupts the activation of HsfA2 and heat tolerance, revealing that GT-1 is the long-sought mediator linking signal perception to the activation of cellular heat responses. These findings uncover a heat-responsive mechanism that determines the timing and execution of cellular heat responses at the whole-plant level.

Activation of ascorbate metabolism by nitrogen starvation and its physiological impacts in Arabidopsis thaliana

Redox homeostasis is crucial for plant acclimation to nutrient-deficient conditions, but its molecular mechanisms remain largely unknown. In this study, the effects of nutrient deficiencies on antioxidant systems in Arabidopsis thaliana were investigated. We found that ascorbate content in the plants grown with nitrogen starvation was higher than those with complete nutrition. The higher ascorbate levels were associated with enhanced gene expression of ascorbate biosynthesis enzymes and cytosolic isozymes of the ascorbate-glutathione cycle, suggesting that nitrogen starvation facilitated both consumption and biosynthesis of ascorbate. Nevertheless, we did not identify any phenotypic differences between wild type and ascorbate-deficient mutants (vtc2) under nitrogen starvation. Under high-light stress, the vtc2 mutants suffered severer photoinhibition than wild type. Interestingly, when high-light stress and nitrogen starvation were combined, wild type and vtc2 plants exhibited photoinhibition to the same extent. Based on these findings, we discuss the regulation and role of ascorbate metabolism under nitrogen starvation.

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.

Nitric oxide negatively regulates gibberellin signaling to coordinate growth and salt tolerance in Arabidopsis

In response to dynamically altered environments, plants must finely coordinate the balance between growth and stress responses for their survival. However, the underpinning regulatory mechanisms remain largely elusive. The phytohormone gibberellin promotes growth via a derepression mechanism by proteasomal degradation of the DELLA transcription repressors. Conversely, the stress-induced burst of nitric oxide (NO) enhances stress tolerance, largely relaying on NO-mediated S-nitrosylation, a redox-based posttranslational modification. Here, we show that S-nitrosylation of Cys-374 in the Arabidopsis RGA protein, a key member of DELLAs, inhibits its interaction with the F-box protein SLY1, thereby preventing its proteasomal degradation under salinity condition. The accumulation of RGA consequently retards growth but enhances salt tolerance. We propose that NO negatively regulates gibberellin signaling via S-nitrosylation of RGA to coordinate the balance of growth and stress responses when challenged by adverse environments.

Nitric oxide regulation of plant metabolism

Nitric oxide (NO) has emerged as an important signal molecule in plants, having myriad roles in plant development. In addition, NO also orchestrates both biotic and abiotic stress responses, during which intensive cellular metabolic reprogramming occurs. Integral to these responses is the location of NO biosynthetic and scavenging pathways in diverse cellular compartments, enabling plants to effectively organize signal transduction pathways. NO regulates plant metabolism and, in turn, metabolic pathways reciprocally regulate NO accumulation and function. Thus, these diverse cellular processes are inextricably linked. This review addresses the numerous redox pathways, located in the various subcellular compartments that produce NO, in addition to the mechanisms underpinning NO scavenging. We focus on how this molecular dance is integrated into the metabolic state of the cell. Within this context, a reciprocal relationship between NO accumulation and metabolite production is often apparent. We also showcase cellular pathways, including those associated with nitrate reduction, that provide evidence for this integration of NO function and metabolism. Finally, we discuss the potential importance of the biochemical reactions governing NO levels in determining plant responses to a changing environment.

Nitric Oxide Signaling and Its Association with Ubiquitin-Mediated Proteasomal Degradation in Plants

Nitric oxide (NO) is a versatile signaling molecule with diverse roles in plant biology. The NO-mediated signaling mechanism includes post-translational modifications (PTMs) of target proteins. There exists a close link between NO-mediated PTMs and the proteasomal degradation of proteins via ubiquitylation. In some cases, ubiquitin-mediated proteasomal degradation of target proteins is followed by an NO-mediated post-translational modification on them, while in other cases NO-mediated PTMs can regulate the ubiquitylation of the components of ubiquitin-mediated proteasomal machinery for promoting their activity. Another pathway that links NO signaling with the ubiquitin-mediated degradation of proteins is the N-degron pathway. Overall, these mechanisms reflect an important mechanism of NO signal perception and transduction that reflect a close association of NO signaling with proteasomal degradation via ubiquitylation. Therefore, this review provides insight into those pathways that link NO-PTMs with ubiquitylation.

GSNOR regulates ganoderic acid content in Ganoderma lucidum under heat stress through S-nitrosylation of catalase

As a master regulator of the balance between NO signaling and protein S-nitrosylation, S-nitrosoglutathione (GSNO) reductase (GSNOR) is involved in various developmental processes and stress responses. However, the proteins and specific sites that can be S-nitrosylated, especially in microorganisms, and the physiological functions of S-nitrosylated proteins remain unclear. Herein, we show that the ganoderic acid (GA) content in GSNOR-silenced (GSNORi) strains is significantly lower (by 25%) than in wild type (WT) under heat stress (HS). Additionally, silencing GSNOR results in an 80% increase in catalase (CAT) activity, which consequently decreases GA accumulation via inhibition of ROS signaling. The mechanism of GSNOR-mediated control of CAT activity may be via protein S-nitrosylation. In support of this possibility, we show that CAT is S-nitrosylated (as shown via recombinant protein in vitro and via GSNORi strains in vivo). Additionally, Cys (cysteine) 401, Cys642 and Cys653 in CAT are S-nitrosylation sites (assayed via mass spectrometry analysis), and Cys401 may play a pivotal role in CAT activity. These findings indicate a mechanism by which GSNOR responds to stress and regulates secondary metabolite content through protein S-nitrosylation. Our results also define a new S-nitrosylation site and the function of an S-nitrosylated protein regulated by GSNOR in microorganisms.

Liu et al. identify catalase as a target of S-nitrosylation by GSNOR and the specific sites of S-nitrosylation critical for its role in regulating secondary metabolite production in Ganoderma lucidum under heat stress. This study suggests that GSNOR regulates other metabolic pathways in microorganisms through S-nitrosylation of target proteins in response to environmental changes.

Phytohormonal Regulation Through Protein S-Nitrosylation Under Stress

The liaison between Nitric oxide (NO) and phytohormones regulates a myriad of physiological processes at the cellular level. The interaction between NO and phytohormones is mainly influenced by NO-mediated post-translational modifications (PTMs) under basal as well as induced conditions. Protein S-nitrosylation is the most prominent and widely studied PTM among others. It is the selective but reversible redox-based covalent addition of a NO moiety to the sulfhydryl group of cysteine (Cys) molecule(s) on a target protein to form S-nitrosothiols. This process may involve either direct S-nitrosylation or indirect S-nitrosylation followed by transfer of NO group from one thiol to another (transnitrosylation). During S-nitrosylation, NO can directly target Cys residue (s) of key genes involved in hormone signaling thereby regulating their function. The phytohormones regulated by NO in this manner includes abscisic acid, auxin, gibberellic acid, cytokinin, ethylene, salicylic acid, jasmonic acid, brassinosteroid, and strigolactone during various metabolic and physiological conditions and environmental stress responses. S-nitrosylation of key proteins involved in the phytohormonal network occurs during their synthesis, degradation, or signaling roles depending upon the response required to maintain cellular homeostasis. This review presents the interaction between NO and phytohormones and the role of the canonical NO-mediated post-translational modification particularly, S-nitrosylation of key proteins involved in the phytohormonal networks under biotic and abiotic stresses.

Nitro-fatty acids: electrophilic signaling molecules in plant physiology

MAIN CONCLUSION

Nitro fatty acids (NO2-FA)have relevant physiological roles as signaling molecules in biotic and abiotic stress, growth, and development, but the mechanism of action remains controversial. The two main mechanisms involving nitric oxide release and thiol modification are discussed. Fatty acids (FAs) are major components of membranes and contribute to cellular energetic demands. Besides, FAs are precursors of signaling molecules, including oxylipins and other oxidized fatty acids derived from the activity of lipoxygenases. In addition, non-canonical modified fatty acids, such as nitro-fatty acids (NO2-FAs), are formed in animals and plants. The synthesis NO2-FAs involves a nitration reaction between unsaturated fatty acids and reactive nitrogen species (RNS). This review will focus on recent findings showing that, in plants, NO2-FAs such as nitro-linolenic acid (NO2-Ln) and nitro-oleic acid (NO2-OA) have relevant physiological roles as signaling molecules in biotic and abiotic stress, growth, and development. Moreover, since there is controversy on mechanisms of action of NO2-FAs as signaling molecules, we will provide evidence showing why this aspect needs further evaluation.

Nitric oxide could allay arsenic phytotoxicity in tomato (Solanum lycopersicum L.) by modulating photosynthetic pigments, phytochelatin metabolism, molecular redox status and arsenic sequestration

Plants do not always have the genetic capacity to tolerate high levels of arsenic (As), which may not only arrest their growth but pose potential health risks through dietary bioaccumulation. Meanwhile, the interplay between the tomato plants and As-NO-driven molecular cell dynamics is obscure. Accordingly, seedlings were treated with As (10 mg/L) alone or in combination with 100 μM sodium nitroprusside (SNP, NO donor) and 200 μM 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO, NO scavenger). Sodium nitroprusside immobilized As in the roots and reduced the shoot translocation by up-regulating the transcriptional expression of the PCS, GSH1, MT2, and ABC1. SNP further restored the growth retardation through modulating the chlorophyll and proline metabolism, increasing NO accumulation and stomatal conductance along with clear crosstalk between the antioxidant activity as well as glyoxalase I and II leading to endogenous H₂O₂ and MG reduction. Higher PCs and glutathione accumulation helped protect photosynthetic apparatus; however, cPTIO reversed the protective effects of SNP, confirming the role of NO in the As toxicity alleviation.

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.

Molecular Mechanisms of Nitric Oxide (NO) Signaling and Reactive Oxygen Species (ROS) Homeostasis during Abiotic Stresses in Plants

Abiotic stressors, such as drought, heavy metals, and high salinity, are causing huge crop losses worldwide. These abiotic stressors are expected to become more extreme, less predictable, and more widespread in the near future. With the rapidly growing human population and changing global climate conditions, it is critical to prevent global crop losses to meet the increasing demand for food and other crop products. The reactive gaseous signaling molecule nitric oxide (NO) is involved in numerous plant developmental processes as well as plant responses to various abiotic stresses through its interactions with various molecules. Together, these interactions lead to the homeostasis of reactive oxygen species (ROS), proline and glutathione biosynthesis, post-translational modifications such as S-nitrosylation, and modulation of gene and protein expression. Exogenous application of various NO donors positively mitigates the negative effects of various abiotic stressors. In view of the multidimensional role of this signaling molecule, research over the past decade has investigated its potential in alleviating the deleterious effects of various abiotic stressors, particularly in ROS homeostasis. In this review, we highlight the recent molecular and physiological advances that provide insights into the functional role of NO in mediating various abiotic stress responses in plants.

Cross-Talk between Hydrogen Peroxide and Nitric Oxide during Plant Development and Responses to Stress

Nitric oxide (NO) and hydrogen peroxide (H₂O₂) are gradually becoming established as critical regulators in plants under physiological and stressful conditions. Strong spatiotemporal correlations in their production and distribution have been identified in various plant biological processes. In this context, NO and H₂O₂ act synergistically or antagonistically as signals or stress promoters depending on their respective concentrations, engaging in processes such as the hypersensitive response, stomatal movement, and abiotic stress responses. Moreover, proteins identified as potential targets of NO-based modifications include a number of enzymes related to H₂O₂ metabolism, reinforcing their cross-talk. In this review, several processes of well-characterized functional interplay between H₂O₂ and NO are discussed with respect to the most recent reported evidence on hypersensitive response-induced programmed cell death, stomatal movement, and plant responses to adverse conditions and, where known, the molecular mechanisms and factors underpinning their cross-talk.

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.

Crosstalk between abscisic acid and nitric oxide under heat stress: exploring new vantage points

Heat stress adversely affects plants growth potential. Global warming is reported to increase in the intensity, frequency, and duration of heatwaves, eventually affecting ecology, agriculture and economy. With an expected increase in average temperature by 2-3 °C over the next 30-50 years, crop production is facing a severe threat to sub-optimum growth conditions. Abscisic acid (ABA) and nitric oxide (NO) are growth regulators that are involved in the adaptation to heat stress by affecting each other and changing the adaptation process. The interaction between these molecules has been discussed in various studies in general or under stress conditions; however, regarding high temperature, their interaction has little been worked out. In the present review, the focus is shifted on the role of these molecules under heat stress emphasizing the different possible interactions between ABA and NO as both regulate stomatal closure and other molecules including hydrogen peroxide (H₂O₂), hydrogen sulfide (H₂S), antioxidants, proline, glycine betaine, calcium (Ca²⁺) and heat shock protein (HSP). Exploring the crosstalk between ABA and NO with other molecules under heat stress will provide us with a comprehensive knowledge of plants mechanism of heat tolerance which could be useful to develop heat stress-resistant varieties.

The protein kinase CPK28 phosphorylates ascorbate peroxidase and enhances thermotolerance in tomato

High temperatures are a major threat to plant growth and development, leading to yield losses in crops. Calcium-dependent protein kinases (CPKs) act as critical components of Ca2+ sensing in plants that transduce rapid stress-induced responses to multiple environmental stimuli. However, the role of CPKs in plant thermotolerance and their mechanisms of action remain poorly understood. To address this issue, tomato (Solanum lycopersicum) cpk28 mutants were generated using a CRISPR-Cas9 gene-editing approach. The responses of mutant and wild-type plants to normal (25°C) and high temperatures (45°C) were documented. Thermotolerance was significantly decreased in the cpk28 mutants, which showed increased heat stress-induced accumulation of reactive oxygen species (ROS) and levels of protein oxidation, together with decreased activities of ascorbate peroxidase (APX) and other antioxidant enzymes. The redox status of ascorbate and glutathione were also modified. Using a yeast two-hybrid library screen and protein interaction assays, we provide evidence that CPK28 directly interacts with cytosolic APX2. Mutations in APX2 rendered plants more sensitive to high temperatures, whereas the addition of exogenous reduced ascorbate (AsA) rescued the thermotolerance phenotype of the cpk28 mutants. Moreover, protein phosphorylation analysis demonstrated that CPK28 phosphorylates the APX2 protein at Thr-59 and Thr-164. This process is suggested to be responsive to Ca2+ stimuli and may be required for CPK28-mediated thermotolerance. Taken together, these results demonstrate that CPK28 targets APX2, thus improving thermotolerance. This study suggests that CPK28 is an attractive target for the development of improved crop cultivars that are better adapted to heat stress in a changing climate.

Contemporary proteomic strategies for cysteine redoxome profiling

Protein cysteine residues are susceptible to oxidative modifications that can affect protein functions. Proteomic techniques that comprehensively profile the cysteine redoxome, the repertoire of oxidized cysteine residues, are pivotal towards a better understanding of the protein redox signaling. Recent technical advances in chemical tools and redox proteomic strategies have greatly improved selectivity, in vivo applicability, and quantification of the cysteine redoxome. Despite this substantial progress, still many challenges remain. Here, we provide an update on the recent advances in proteomic strategies for cysteine redoxome profiling, compare the advantages and disadvantages of current methods and discuss the outstanding challenges and future perspectives for plant redoxome research.

Mapping the plant proteome: tools for surveying coordinating pathways

Plants rapidly respond to environmental fluctuations through coordinated, multi-scalar regulation, enabling complex reactions despite their inherently sessile nature. In particular, protein post-translational signaling and protein-protein interactions combine to manipulate cellular responses and regulate plant homeostasis with precise temporal and spatial control. Understanding these proteomic networks are essential to addressing ongoing global crises, including those of food security, rising global temperatures, and the need for renewable materials and fuels. Technological advances in mass spectrometry-based proteomics are enabling investigations of unprecedented depth, and are increasingly being optimized for and applied to plant systems. This review highlights recent advances in plant proteomics, with an emphasis on spatially and temporally resolved analysis of post-translational modifications and protein interactions. It also details the necessity for generation of a comprehensive plant cell atlas while highlighting recent accomplishments within the field.

Alleviative effects of nitric oxide on Vigna radiata seedlings under acidic rain stress

Although nitric oxide (NO) is a key regulatory molecule in plants, its function in plants under conditions of simulated acid rain (SAR) has not been fully established yet. In this study, exogenous sodium nitroprusside (SNP) at three different concentrations were applied to mung bean seedlings. Malondialdehyde (MDA), NO, hydrogen peroxide (H₂O₂), antioxidant enzyme activities, and nitrate reductases (NR) were measured. Real time PCR was used to measure the NR expression. Compared to the control, the NR activity and NO content under the pH 2 SAR decreased by 79% and 85.6% respectively. Meanwhile, the SAR treatment reduced the activities of superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX), while increased MDA content. Application of SNP could potentially reverse the adverse impact of SAR, depending on its concentration. For plants under the pH 2 SAR and 0.25 mM SNP condition, the activities of SOD, POD, APX increased by 123%, 291%, and 135.7% respectively, meanwhile, MDA concentration decreased by 43%, NR activities increased by 269%, and NO concentration increased by 123.6% compared with plants undergoing only pH 2 SAR. The relative expression of the NR1 gene was 2.69 times higher than that of pH 2 SAR alone. Overall, the application of 0.25 mM SNP eliminated reactive oxygen species (ROS) by stimulating antioxidant enzyme activities, reducing oxidative stress and mitigating the toxic effects of SAR on mung bean seedlings. This research provides a foundation for further research on the mechanism of NO on plants under SAR conditions.

Methionine Sulfoxide Reductase B Regulates the Activity of Ascorbate Peroxidase of Banana Fruit

Ascorbate peroxidase (APX) is a key antioxidant enzyme that is involved in diverse developmental and physiological process and stress responses by scavenging H₂O₂ in plants. APX itself is also subjected to multiple posttranslational modifications (PTMs). However, redox-mediated PTM of APX in plants remains poorly understood. Here, we identified and confirmed that MaAPX1 interacts with methionine sulfoxide reductase B2 (MsrB2) in bananas. Ectopic overexpression of MaAPX1 delays the detached leaf senescence induced by darkness in Arabidopsis. Sulfoxidation of MaAPX1, i.e., methionine oxidation, leads to loss of the activity, which is repaired partially by MaMsrB2. Moreover, mimicking sulfoxidation by mutating Met36 to Gln also decreases its activity in vitro and in vivo, whereas substitution of Met36 with Val36 to mimic the blocking of sulfoxidation has little effect on APX activity. Spectral analysis showed that mimicking sulfoxidation of Met36 hinders the formation of compound I, the first intermediate between APX and H₂O₂. Our findings demonstrate that the redox state of methionine in MaAPX1 is critical to its activity, and MaMsrB2 can regulate the redox state and activity of MaAPX1. Our results revealed a novel post-translational redox modification of APX.

Role of electrophilic nitrated fatty acids during development and response to abiotic stress processes in plants

Nitro-fatty acids are generated from the interaction of unsaturated fatty acids and nitric oxide (NO)-derived molecules. The endogenous occurrence and modulation throughout plant development of nitro-linolenic acid (NO2-Ln) and nitro-oleic acid (NO2-OA) suggest a key role for these molecules in initial development stages. In addition, NO2-Ln content increases significantly in stress situations and induces the expression of genes mainly related to abiotic stress, such as genes encoding members of the heat shock response family and antioxidant enzymes. The promoter regions of NO2-Ln-induced genes are also involved mainly in stress responses. These findings confirm that NO2-Ln is involved in plant defense processes against abiotic stress conditions via induction of the chaperone network and antioxidant systems. NO2-Ln signaling capacity lies mainly in its electrophilic nature and allows it to mediate a reversible post-translational modification called nitroalkylation, which is capable of modulating protein function. NO2-Ln is a NO donor that may be involved in NO signaling events and is able to generate S-nitrosoglutathione, the major reservoir of NO in cells and a key player in NO-mediated abiotic stress responses. This review describes the current state of the art regarding the essential role of nitro-fatty acids as signaling mediators in development and abiotic stress processes.

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.

Dual Roles of GSNOR1 in Cell Death and Immunity in Tetraploid Nicotiana tabacum

S-nitrosoglutathione reductase 1 (GSNOR1) is the key enzyme that regulates cellular homeostasis of S-nitrosylation. Although extensively studied in Arabidopsis, the roles of GSNOR1 in tetraploid Nicotiana species have not been investigated previously. To study the function of NtGSNOR1, we knocked out two NtGSNOR1 genes simultaneously in Nicotiana tabacum using clustered regularly interspaced short palindromic repeats (CRISPR)/caspase 9 (Cas9) technology. To our surprise, spontaneous cell death occurred on the leaves of the CRISPR/Cas9 lines but not on those of the wild-type (WT) plants, suggesting that NtGSNOR1 negatively regulates cell death. The natural cell death on the CRISPR/Cas9 lines could be a result from interactions between overaccumulated nitric oxide (NO) and hydrogen peroxide (H₂O₂). This spontaneous cell death phenotype was not affected by knocking out two Enhanced disease susceptibility 1 genes (NtEDS11a/1b) and thus was independent of the salicylic acid (SA) pathway. Unexpectedly, we found that the NtGSNOR1a/1b knockout plants displayed a significantly (p < 0.001) enhanced resistance to paraquat-induced cell death compared to WT plants, suggesting that NtGSNOR1 functions as a positive regulator of the paraquat-induced cell death. The increased resistance to the paraquat-induced cell death of the NtGSNOR1a/1b knockout plants was correlated with the reduced level of H₂O₂ accumulation. Interestingly, whereas the N gene-mediated resistance to Tobacco mosaic virus (TMV) was significantly enhanced (p < 0.001), the resistance to Pseudomonas syringae pv. tomato DC3000 was significantly reduced (p < 0.01) in the NtGSNOR1a/1b knockout lines. In summary, our results indicate that NtGSNOR1 functions as both positive and negative regulator of cell death under different conditions and displays distinct effects on resistance against viral and bacterial pathogens.

Nitric oxide affects seed oil accumulation and fatty acid composition through protein S-nitrosation

Nitric oxide (NO) is a key signaling molecule regulating several plant developmental and stress responses. Here, we report that NO plays an important role in seed oil content and fatty acid composition. RNAi silencing of Arabidopsis S-nitrosoglutathione reductase 1 (GSNOR1) led to reduced seed oil content. In contrast, nitrate reductase double mutant nia1nia2 had increased seed oil content, compared with wild-type plants. Moreover, the concentrations of palmitic acid (C16:0), linoleic acid (C18:2), and linolenic acid (C18:3) were higher, whereas those of stearic acid (C18:0), oleic acid (C18:1), and arachidonic acid (C20:1) were lower, in seeds of GSNOR1 RNAi lines. Similar results were obtained with rapeseed embryos cultured in vitro with the NO donor sodium nitroprusside (SNP), and the NO inhibitor NG-Nitro-L-arginine Methyl Ester (L-NAME). Compared with non-treated embryos, the oil content decreased in SNP-treated embryos, and increased in L-NAME-treated embryos. Relative concentrations of C16:0, C18:2 and C18:3 were higher, whereas C18:1 concentration decreased in rapeseed embryos treated with SNP. Proteomics and transcriptome analysis revealed that three S-nitrosated proteins and some key genes involved in oil synthesis, were differentially regulated in SNP-treated embryos. Therefore, regulating NO content could be a novel approach to increasing seed oil content in cultivated oil crops.

Signaling Toward Reactive Oxygen Species-Scavenging Enzymes in Plants

Reactive oxygen species (ROS) are signaling molecules essential for plant responses to abiotic and biotic stimuli as well as for multiple developmental processes. They are produced as byproducts of aerobic metabolism and are affected by adverse environmental conditions. The ROS content is controlled on the side of their production but also by scavenging machinery. Antioxidant enzymes represent a major ROS-scavenging force and are crucial for stress tolerance in plants. Enzymatic antioxidant defense occurs as a series of redox reactions for ROS elimination. Therefore, the deregulation of the antioxidant machinery may lead to the overaccumulation of ROS in plants, with negative consequences both in terms of plant development and resistance to environmental challenges. The transcriptional activation of antioxidant enzymes accompanies the long-term exposure of plants to unfavorable environmental conditions. Fast ROS production requires the immediate mobilization of the antioxidant defense system, which may occur via retrograde signaling, redox-based modifications, and the phosphorylation of ROS detoxifying enzymes. This review aimed to summarize the current knowledge on signaling processes regulating the enzymatic antioxidant capacity of plants.

The Emerging Role of GSNOR in Oxidative Stress Regulation

Oxidative stress is a common event in aerobic organisms and a fundamental and unavoidable cost of the aerobic lifestyle. Reactive oxygen and nitrogen species (ROS/RNS) and iron (Fe) are the most common agents that trigger oxidative stress. A conserved enzyme in the S-nitrosoglutathione (GSNO) metabolism, GSNO reductase (GSNOR), modulates a multitude of abiotic and biotic stress responses. In this review, we focus on the emerging role of GSNOR as a master regulator in oxidative stress through its regulation of the interaction of ROS, RNS, and Fe, and highlight recent discoveries in post-translational modifications of GSNOR and functional variations of natural GSNOR variants during oxidative stress. Recent advances in understanding GSNOR regulation show promise for the modulation of oxidative stress in plants.

Protein S-nitrosation differentially modulates tomato responses to infection by hemi-biotrophic oomycetes of Phytophthora spp

Regulation of protein function by reversible S-nitrosation, a post-translational modification based on the attachment of nitroso group to cysteine thiols, has emerged among key mechanisms of NO signalling in plant development and stress responses. S-nitrosoglutathione is regarded as the most abundant low-molecular-weight S-nitrosothiol in plants, where its intracellular concentrations are modulated by S-nitrosoglutathione reductase. We analysed modulations of S-nitrosothiols and protein S-nitrosation mediated by S-nitrosoglutathione reductase in cultivated Solanum lycopersicum (susceptible) and wild Solanum habrochaites (resistant genotype) up to 96 h post inoculation (hpi) by two hemibiotrophic oomycetes, Phytophthora infestans and Phytophthora parasitica. S-nitrosoglutathione reductase activity and protein level were decreased by P. infestans and P. parasitica infection in both genotypes, whereas protein S-nitrosothiols were increased by P. infestans infection, particularly at 72 hpi related to pathogen biotrophy-necrotrophy transition. Increased levels of S-nitrosothiols localised in both proximal and distal parts to the infection site, which suggests together with their localisation to vascular bundles a signalling role in systemic responses. S-nitrosation targets in plants infected with P. infestans identified by a proteomic analysis include namely antioxidant and defence proteins, together with important proteins of metabolic, regulatory and structural functions. Ascorbate peroxidase S-nitrosation was observed in both genotypes in parallel to increased enzyme activity and protein level during P. infestans pathogenesis, namely in the susceptible genotype. These results show important regulatory functions of protein S-nitrosation in concerting molecular mechanisms of plant resistance to hemibiotrophic pathogens.

Regulation of Dual Activity of Ascorbate Peroxidase 1 From Arabidopsis thaliana by Conformational Changes and Posttranslational Modifications

Ascorbate peroxidase (APX) is an important reactive oxygen species (ROS)-scavenging enzyme, which catalyzes the removal of hydrogen peroxide (H₂O₂) to prevent oxidative damage. The peroxidase activity of APX is regulated by posttranslational modifications (PTMs), such as S-nitrosylation, tyrosine nitration, and S-sulfhydration. In addition, it has been recently reported that APX functions as a molecular chaperone, protecting rice against heat stress. In this study, we attempted to identify the various functions of APX in Arabidopsis and the effects of PTMs on these functions. Cytosol type APX1 from Arabidopsis thaliana (AtAPX1) exists in multimeric forms ranging from dimeric to high-molecular-weight (HMW) complexes. Similar to the rice APX2, AtAPX1 plays a dual role behaving both as a regular peroxidase and a chaperone molecule. The dual activity of AtAPX1 was strongly related to its structural status. The main dimeric form of the AtAPX1 protein showed the highest peroxidase activity, whereas the HMW form exhibited the highest chaperone activity. Moreover, in vivo studies indicated that the structure of AtAPX1 was regulated by heat and salt stresses, with both involved in the association and dissociation of complexes, respectively. Additionally, we investigated the effects of S-nitrosylation, S-sulfhydration, and tyrosine nitration on the protein structure and functions using gel analysis and enzymatic activity assays. S-nitrosylation and S-sulfhydration positively regulated the peroxidase activity, whereas tyrosine nitration had a negative impact. However, no effects were observed on the chaperone function and the oligomeric status of AtAPX1. Our results will facilitate the understanding of the role and regulation of APX under abiotic stress and posttranslational modifications.

Reactive Oxygen Species in Plant Interactions With Aphids

Reactive oxygen species (ROS) such as hydrogen peroxide and superoxide are produced in plants in response to many biotic and abiotic stressors, and they can enhance stress adaptation in certain circumstances or mediate symptom development in others. The roles of ROS in plant-pathogen interactions have been extensively studied, but far less is known about their involvement in plant-insect interactions. A growing body of evidence, however, indicates that ROS accumulate in response to aphids, an economically damaging group of phloem-feeding insects. This review will cover the current state of knowledge about when, where, and how ROS accumulate in response to aphids, which salivary effectors modify ROS levels in plants, and how microbial associates influence ROS induction by aphids. We will also explore the potential adaptive significance of intra- and extracellular oxidative responses to aphid infestation in compatible and incompatible interactions and highlight knowledge gaps that deserve further exploration.

Overexpression of SlMDHAR in transgenic tobacco increased salt stress tolerance involving S-nitrosylation regulation

Protein S-nitrosylation, which refers to the redox-based posttranslational modification of a cysteine thiol by the attachment of a nitric oxide (NO) group, modulates a variety of enzyme activities. Monodehydroascorbate reductase (MDHAR) is essential for ascorbic acid (AsA) regeneration, which protects plant cells against damage by detoxifying reactive oxygen species (ROS). However, the relationship between S-nitrosylation and the role of tomato MDHAR (SlMDHAR) under salt stress remains unclear. In this paper, we show that the SlMDHAR mRNA expression, enzyme activity, protein level, total S-nitrosylated proteins and S-nitrosylated SlMDHAR protein level in tomato leaves significantly increase after NaCl treatment. To further evaluate the function of SlMDHAR under salt stress, overexpressed transgenic tobacco plants were used. The germination rate and root length of the overexpressed plants under NaCl stress were significantly higher than those of wild-type (WT) plants. Meanwhile, the transgenic plants had lower ROS accumulation, higher antioxidant enzyme activities and AsA-DHA ratio, more proline and soluble sugar contents than those in WT plants under salt stress. With a higher expression of stress-related genes, the transgenic plants demonstrated lower Na+ and higher K+ accumulation compared with WT plants. The NO accumulation and S-nitrosylated MDHAR level were higher in transgenic plants than in WT plants after NaCl treatment. In contrast, virus-induced gene silencing (VIGS) of SlMDHAR tomato plants showed enhanced sensitivity to salt stress and have lower S-nitrosylated MDHAR protein. These results suggested that SlMDHAR confers salt stress tolerance by alleviating oxidative damage probably involving the S-nitrosylation of MDHAR.

The outcomes of the functional interplay of nitric oxide and hydrogen sulfide in metal stress tolerance in plants

Phytotoxicity of metals constraints plants health, metabolism and productivity. The sustainable approaches for minimizing major metals-accrued phytotoxicity have been least explored. The gasotransmitters signaling molecules such as nitric oxide (NO) and hydrogen sulfide (H₂S) play a significant role in the mitigation of major consequences of metals stress. Versatile gaseous signaling molecules, NO and H₂S are involved in the regulation of various physiological processes in plants and their tolerance to abiotic stresses. However, literature available on NO or H₂S stand alone, and the major insights into the roles of NO and/or H₂S in plant tolerance, particularly to metals, remained unclear. Given above, this paper aimed to (a) briefly overview metals and highlight their major phytotoxicity; (b) appraises literature reporting potential mechanisms underlying the roles of NO and H₂S in plant-metal tolerance; (c) crosstalk on NO and H₂S in relation to plant metal tolerance. Additionally, major aspects so far unexplored in the current context have also been mentioned.

Regulating the regulator: nitric oxide control of post-translational modifications

Nitric oxide (NO) is perfectly suited for the role of a redox signalling molecule. A key route for NO bioactivity occurs via protein S-nitrosation, and involves the addition of a NO moiety to a protein cysteine (Cys) thiol (-SH) to form an S-nitrosothiol (SNO). This process is thought to underpin a myriad of cellular processes in plants that are linked to development, environmental responses and immune function. Here we collate emerging evidence showing that NO bioactivity regulates a growing number of diverse post-translational modifications including SUMOylation, phosphorylation, persulfidation and acetylation. We provide examples of how NO orchestrates these processes to mediate plant adaptation to a variety of cellular cues.

Plant catalases as NO and H2S targets

Catalase is a powerful antioxidant metalloenzyme located in peroxisomes which also plays a central role in signaling processes under physiological and adverse situations. Whereas animals contain a single catalase gene, in plants this enzyme is encoded by a multigene family providing multiple isoenzymes whose number varies depending on the species, and their expression is regulated according to their tissue/organ distribution and the environmental conditions. This enzyme can be modulated by reactive oxygen and nitrogen species (ROS/RNS) as well as by hydrogen sulfide (H2S). Catalase is the major protein undergoing Tyr-nitration [post-translational modification (PTM) promoted by RNS] during fruit ripening, but the enzyme from diverse sources is also susceptible to undergo other activity-modifying PTMs. Data on S-nitrosation and persulfidation of catalase from different plant origins are given and compared here with results from obese children where S-nitrosation of catalase occurs. The cysteine residues prone to be S-nitrosated in catalase from plants and from bovine liver have been identified. These evidences assign to peroxisomes a crucial statement in the signaling crossroads among relevant molecules (NO and H2S), since catalase is allocated in these organelles. This review depicts a scenario where the regulation of catalase through PTMs, especially S-nitrosation and persulfidation, is highlighted.

Nitric oxide regulates ganoderic acid biosynthesis by the S-nitrosylation of aconitase under heat stress in Ganoderma lucidum

Nitric oxide (NO) is an important signalling molecule in stress response of organisms. We previously reported that NO decreases heat stress (HS)-induced ganoderic acid (GA) accumulation in Ganoderma lucidum. To explore the mechanisms by which NO modulates GA biosynthesis under HS, the effect of NO on the reactive oxygen species (ROS) content was examined. The results showed that NO decreased the production of mitochondrial ROS (mitROS) by 60% under HS. Further research revealed that NO reduced the mitROS content by inhibiting aconitase (Acon) activity. The GA content in Acon-silenced (Aconi) strains treated with NO donor did not differ significantly from that in untreated Aconi strains. To study the mechanism by which Acon activity is inhibited, the S-nitrosylation level of Acon was determined. Biotin-switch technology and mass spectrometry analysis were used to show that Acon is S-nitrosylated at the Cys-594 amino acid residue. Substitution of Cys-594 with a Ser, which cannot be S-nitrosylated, abolished the responsiveness of Acon to the NO-induced reduction in its enzymatic activity. These findings demonstrate that NO inhibits Acon activity through S-nitrosylation at Cys-594. In summary, these findings describe mechanism by which NO regulates GA biosynthesis via S-nitrosylation of Acon under HS condition in G. lucidum.

Hydrogen sulfide: a multi-tasking signal molecule in the regulation of oxidative stress responses

Accumulating evidence suggests that hydrogen sulfide (H2S) is an important signaling molecule in plant environmental interactions. The consensus view amongst plant scientists is that environmental stress leads to enhanced production and accumulation of reactive oxygen species (ROS). H2S interacts with the ROS-mediated oxidative stress response network at multiple levels, including the regulation of ROS-processing systems by transcriptional or post-translational modifications. H2S-ROS crosstalk also involves other interacting factors, including nitric oxide, and can affect key cellular processes like autophagy. While H2S often functions to prevent ROS accumulation, it can also act synergistically with ROS signals in processes such as stomatal closure. In this review, we summarize the mechanisms of H2S action and the multifaceted roles of this molecule in plant stress responses. Emphasis is placed on the interactions between H2S, ROS, and the redox signaling network that is crucial for plant defense against environmental threats.

Redox-based protein persulfidation in guard cell ABA signaling

Hydrogen sulfide (H₂S) is a versatile signaling molecule that regulates multiple physiological processes in plants, including growth and development, immunity, and stress response as well. Signaling triggered by H₂S is proposed to occur via persulfidation, an oxidative post-translational modification (PTM) of cysteine residues (-SH) to persulfides (-SSH). Notwithstanding the growing body of information for the plant persulfidation proteome, the gap between the molecular mechanism of H₂S and physiological functions of protein persulfidation remains large. In this mini-review, we discussed the specific regulatory mechanism of persulfidation on guard cell abscisic acid (ABA) signaling and the possible link of persulfidation, sulfenylation, and S-nitrosylation within the framework of redox-based regulation.

Transnitrosylation Mediated by the Non-canonical Catalase ROG1 Regulates Nitric Oxide Signaling in Plants

The redox-based protein S-nitrosylation is a conserved mechanism modulating nitric oxide (NO) signaling and has been considered mainly as a non-enzymatic reaction. S-nitrosylation is regulated by the intracellular NO level that is tightly controlled by S-nitrosoglutathione reductase (GSNOR). However, the molecular mechanisms regulating S-nitrosylation selectivity remain elusive. Here, we characterize an Arabidopsis "repressor of" gsnor1 (rog1) mutation that specifically suppresses the gsnor1 mutant phenotype. ROG1, identical to the non-canonical catalase, CAT3, is a transnitrosylase that specifically modifies GSNOR1 at Cys-10. The transnitrosylase activity of ROG1 is regulated by a unique and highly conserved Cys-343 residue. A ROG1^C343T mutant displays increased catalase but decreased transnitrosylase activities. Consistent with these results, the rog1 mutation compromises responses to NO under both normal and stress conditions. We propose that ROG1 functions as a transnitrosylase to regulate the NO-based redox signaling in plants.

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

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

Sodium nitroprusside (SNP) improves tolerance to arsenic (As) toxicity in Vicia faba through the modifications of biochemical attributes, antioxidants, ascorbate-glutathione cycle and glyoxalase cycle

The present study was conducted to evaluate the effect of arsenic (As) toxicity and the mitigating role of nitric oxide (NO) donor sodium nitroprusside (SNP) on Vicia faba. Arsenics stress decreased the growth and biomass yield, and photosynthetic pigments, but it enhanced As accumulation. Supplementation of NO enhanced the afore-mentioned parameters except As accumulation which decreased in both shoot and root. Supplementation of NO enhanced the shoot tolerance index (Shoot TI%), root tolerance index (Root TI%) but it declined the As translocation factor (TF). Application of NO alleviated the As-induced decline in net assimilation rate, stomatal conductance, transpiration and leaf relative water content. The levels of proline and glycine betaine (GB) further increased due to NO application, whereas malondialdehyde (MDA), hydrogen peroxide (H₂O₂), electrolyte leakage (EL) and methylglyoxal (MG) declined considerably. Activities of enzymatic antioxidants such as superoxide dismutase (SOD) and catalase (CAT) increased under As stress. Supplementation of NO up-regulated the enzymes involved in Asc-Glu cycle and glyoxalase cycle under As toxicity. Another experiment was setup to authenticate whether NO was certainly able to alleviate As toxicity. For this purpose, the NO scavenger [2-(4-carboxy-2 phenyl)-4,4,5,5-tertamethylimidazoline-1-oxyl-3-oxide (cPTIO)] was added to As and NO supplemented plants. Addition of cPTIO to NO supplemented As-treated plants showed the same effect when As alone was supplied to plants. In conclusion, addition of NO to the growth medium maintained the plant performance under As toxicity through modulation of physio-biochemical attributes, antioxidant enzymes, and the Asc-Glu and glyoxalase systems.

S-Nitroso-Proteome Revealed in Stomatal Guard Cell Response to Flg22

Nitric oxide (NO) plays an important role in stomata closure induced by environmental stimuli including pathogens. During pathogen challenge, nitric oxide (NO) acts as a second messenger in guard cell signaling networks to activate downstream responses leading to stomata closure. One means by which NO’s action is achieved is through the posttranslational modification of cysteine residue(s) of target proteins. Although the roles of NO have been well studied in plant tissues and seedlings, far less is known about NO signaling and, more specifically, protein S-nitrosylation (SNO) in stomatal guard cells. In this study, using iodoTMTRAQ quantitative proteomics technology, we analyzed changes in protein SNO modification in guard cells of reference plant Arabidopsis thaliana in response to flg22, an elicitor-active peptide derived from bacterial flagellin. A total of 41 SNO-modified peptides corresponding to 35 proteins were identified. The proteins cover a wide range of functions, including energy metabolism, transport, stress response, photosynthesis, and cell–cell communication. This study creates the first inventory of previously unknown NO responsive proteins in guard cell immune responses and establishes a foundation for future research toward understanding the molecular mechanisms and regulatory roles of SNO in stomata immunity against bacterial pathogens.

The role of methane in plant physiology: a review

Methane (CH₄), one of the most important greenhouse gases, has conventionally been considered as a physiologic inert gas. However, this perspective has been challenged by the observation that CH₄ has diverse biological functions in animals, such as anti-inflammatory, antioxidant, and anti-apoptosis. Meanwhile, it has now been identified as a possible candidate of gaseous signaling molecule in plants, although its biosynthetic and metabolic pathways as well as the mechanism(s) of CH₄ signaling have not fully understood yet. This paper aims to review the available evidence for the biological roles of CH₄ in regulating plant physiology. Although currently available reports do not fully support the notion of CH₄ as a gasotransmitter, they do show that CH₄ might be produced by an aerobic, non-microbial pathway from plants, and plays important roles in enhancing plant tolerance against abiotic stresses, such as salinity, drought, heavy metal exposure, and promoting root development, as well as delaying senescence and browning. Further results showed that CH₄ could interact with reactive oxygen species (ROS), other gaseous signaling molecules [e.g., nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H₂S)], and glutathione (GSH). These reports thus support the idea that plant-produced CH₄ might be a component of a survival strategy of plants. Finally, the possibility of CH₄ application in agriculture is preliminarily discussed.

Regulation of physiological aspects in plants by hydrogen sulfide and nitric oxide under challenging environment

Plants are exposed to a plethora of abiotic stresses such as drought, salinity, heavy metal and temperature stresses at different stages of their life cycle, from germination to seedling till the reproductive phase. As protective mechanisms, plants release signaling molecules that initiate a cascade of stress-signaling events, leading either to programmed cell death or plant acclimation. Hydrogen sulfide (H₂ S) and nitric oxide (NO) are considered as new 'gasotransmitter' molecules that play key roles in regulating gene expression, posttranslational modification (PTM), as well as cross-talk with other hormones. Although the exact role of NO in plants remains unclear and is species dependent, various studies have suggested a positive correlation between NO accumulation and environmental stress in plants. These molecules are also involved in a large array of stress responses and act synergistically or antagonistically as signaling components, depending on their respective concentration. This study provides a comprehensive update on the signaling interplay between H₂ S and NO in the regulation of various physiological processes under multiple abiotic stresses, modes of action and effects of exogenous application of these two molecules under drought, salt, heat and heavy metal stresses. However, the complete picture of the signaling cascades mediated by H₂ S and NO is still elusive. Recent researches indicate that during certain plant processes, such as stomatal closure, H₂ S could act upstream of NO signaling or downstream of NO in response to abiotic stresses by improving antioxidant activity in most plant species. In addition, PTMs of antioxidative pathways by these two molecules are also discussed.

Thioredoxin Network in Plant Mitochondria: Cysteine S-Posttranslational Modifications and Stress Conditions

Plants are sessile organisms presenting different adaptation mechanisms that allow their survival under adverse situations. Among them, reactive oxygen and nitrogen species (ROS, RNS) and H₂S are emerging as components not only of cell development and differentiation but of signaling pathways involved in the response to both biotic and abiotic attacks. The study of the posttranslational modifications (PTMs) of proteins produced by those signaling molecules is revealing a modulation on specific targets that are involved in many metabolic pathways in the different cell compartments. These modifications are able to translate the imbalance of the redox state caused by exposure to the stress situation in a cascade of responses that finally allow the plant to cope with the adverse condition. In this review we give a generalized vision of the production of ROS, RNS, and H₂S in plant mitochondria. We focus on how the principal mitochondrial processes mainly the electron transport chain, the tricarboxylic acid cycle and photorespiration are affected by PTMs on cysteine residues that are produced by the previously mentioned signaling molecules in the respiratory organelle. These PTMs include S-oxidation, S-glutathionylation, S-nitrosation, and persulfidation under normal and stress conditions. We pay special attention to the mitochondrial Thioredoxin/Peroxiredoxin system in terms of its oxidation-reduction posttranslational targets and its response to environmental stress.

Identification of Sulfenylated Cysteines in Arabidopsis thaliana Proteins Using a Disulfide-Linked Peptide Reporter

In proteins, hydrogen peroxide (H₂O₂) reacts with redox-sensitive cysteines to form cysteine sulfenic acid, also known as S-sulfenylation. These cysteine oxidation events can steer diverse cellular processes by altering protein interactions, trafficking, conformation, and function. Previously, we had identified S-sulfenylated proteins by using a tagged proteinaceous probe based on the yeast AP-1-like (Yap1) transcription factor that specifically reacts with sulfenic acids and traps them through a mixed disulfide bond. However, the identity of the S-sulfenylated amino acid residues within a protein remained enigmatic. By using the same transgenic YAP1C probe, we present here a technological advancement to identify in situ sulfenylated cysteine sites in Arabidopsis thaliana cells under control condition and oxidative stress. Briefly, the total extract of transgenic YAP1C A. thaliana cells was initially purified on IgG-Sepharose beads, followed by a tryptic digest. Then, the mixed disulfide-linked peptides were further enriched at the peptide level on an anti-YAP1C-derived peptide (C₅₉₈SEIWDR) antibody. Subsequent mass spectrometry analysis with pLink 2 identified 1,745 YAP1C cross-linked peptides, indicating sulfenylated cysteines in over 1,000 proteins. Approximately 55% of these YAP1C-linked cysteines had previously been reported as redox-sensitive cysteines (S-sulfenylation, S-nitrosylation, and reversibly oxidized cysteines). The presented methodology provides a noninvasive approach to identify sulfenylated cysteines in any species that can be genetically modified.

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.

Extensive protein S-nitrosylation associated with human pancreatic ductal adenocarcinoma pathogenesis

NO (nitric oxide)-mediated protein S-nitrosylation has been established as one major signaling mechanism underlying cancer initiation and development, but its roles in PDAC (pancreatic ductal adenocarcinoma) pathogenesis still remain largely unexplored. In this study, we identified 585 unique S-nitrosylation sites among 434 proteins in PDAC patients and PANC-1 cell line by a site-specific proteomics. Larger number of S-nitrosylated proteins were identified in PDAC tissues and PANC-1 cells than adjacent non-cancerous tissues. These S-nitrosylated proteins are significantly enriched in a multitude of biological processes associated with tumorigenesis, including carbohydrate metabolism, cytoskeleton regulation, cell cycle, focal adhesion, adherent junctions, and cell migration. Components of the pancreatic cancer pathway were extensively S-nitrosylated, such as v-raf-1 murine leukemia viral oncogene homolog 1 (Raf-1) and Signal transducer and activator of transcription 3 (STAT3). Moreover, NOS (NO synthase) inhibitor significantly repressed STAT3 S-nitrosylation in PANC-1 cells, which caused significant increase of STAT3 phosphorylation and PANC-1 cell viability, suggesting important roles of protein S-nitrosylation in PDAC development. These results revealed extensive protein S-nitrosylation associated with PDAC pathogenesis, which provided a basis for protein modification-based cancer diagnosis and targeted therapy.

Fructan and antioxidant metabolisms in plants of Lolium perenne under drought are modulated by exogenous nitric oxide

Drought is a major environmental factor that can trigger oxidative stress and affect plant growth and productivity. Previous studies have shown that exogenous nitric oxide (NO) can minimize oxidative stress-related damage through the modulation of antioxidant enzyme activity. Fructan accumulation also has an important role in drought tolerance, since these carbohydrates participate in osmoregulation, membrane protection and oxidant scavenging. Currently, there are few studies investigating NO-regulated fructan metabolism in response to abiotic stresses. In the present study, we sought to determine if treating plants of Lolium perenne with S-nitrosoglutathione (GSNO), a NO donor, improved drought tolerance. Two-month-old plants received water (control), GSNO and reduced glutathione (GSH) as foliar spray treatments and were then maintained under drought or well-watered conditions for 23 days. At the end of drought period, we evaluated growth, pigment content and antioxidant and fructan metabolisms. None of these conditions influenced dry mass accumulation, but the leaves of plants treated with GSNO exhibited a slight increase in pigment content under drought. GSNO treatment also induced 1-SST activity, which was associated with a 3-fold increase in fructan content. GSNO-treated plants presented higher GR activity and, consequently, increased GSH levels. L. perenne cv. AberAvon was relatively tolerant to the water stress condition employed herein, maintaining ROS homeostasis and mitigating oxidative stress, possibly due to fructan, ascorbate and glutathione pools.

Protein S-Nitrosylation in plants: Current progresses and challenges

Nitric oxide (NO) is an important signaling molecule regulating diverse biological processes in all living organisms. A major physiological function of NO is executed via protein S-nitrosylation, a redox-based posttranslational modification by covalently adding a NO molecule to a reactive cysteine thiol of a target protein. S-nitrosylation is an evolutionarily conserved mechanism modulating multiple aspects of cellular signaling. During the past decade, significant progress has been made in functional characterization of S-nitrosylated proteins in plants. Emerging evidence indicates that protein S-nitrosylation is ubiquitously involved in the regulation of plant development and stress responses. Here we review current understanding on the regulatory mechanisms of protein S-nitrosylation in various biological processes in plants and highlight key challenges in this field.