Modulation of nitrosative stress by S-nitrosoglutathione reductase is critical for thermotolerance and plant growth in Arabidopsis

Nitric oxide in plants: an insight on redox activity and responses toward abiotic stress signaling

Plants, as sessile organisms, are subjected to diverse abiotic stresses, including salinity, desiccation, metal toxicity, thermal fluctuations, and hypoxia at different phases of plant growth. Plants can activate messenger molecules to initiate a signaling cascade of response toward environmental stresses that results in either cell death or plant acclimation. Nitric oxide (NO) is a small gaseous redox-active molecule that exhibits a plethora of physiological functions in growth, development, flowering, senescence, stomata closure and responses to environmental stresses. It can also facilitate alteration in protein function and reprogram the gene profiling by direct or indirect interaction with different target molecules. The bioactivity of NO can be manifested through different redox-based protein modifications including S-nitrosylation, protein nitration, and metal nitrosylation in plants. Although there has been considerable progress in the role of NO in regulating stress signaling, still the physiological mechanisms regarding the abiotic stress tolerance in plants remain unclear. This review summarizes recent advances in understanding the emerging knowledge regarding NO function in plant tolerance against abiotic stresses. The manuscript also highlighted the importance of NO as an abiotic stress modulator and developed a rational design for crop cultivation under a stress environment.

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

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

Nitric oxide is involved in the regulation of guard mother cell division by inhibiting the synthesis of ACC

A stoma forms by a series of asymmetric divisions of stomatal lineage precursor cell and the terminal division of a guard mother cell (GMC). GMC division is restricted to once through genetic regulation mechanisms. Here, we show that nitric oxide (NO) is involved in the regulation of the GMC division. NO donor treatment results in the formation of single guard cells (SGCs). SGCs are also produced in plants that accumulate high NO, whereas clustered guard cells (GCs) appear in plants with low NO accumulation. NO treatment promotes the formation of SGCs in the stomatal signalling mutants sdd1, epf1 epf2, tmm1, erl1 erl2 and er erl1 erl2, reduces the cell number per stomatal cluster in the fama-1 and flp1 myb88, but has no effect on stomatal of cdkb1;1 cyca2;234. Aminocyclopropane-1-carboxylic acid (ACC), a positive regulator of GMC division, reduces the NO-induced SGC formation. Further investigation found NO inhibits ACC synthesis by repressing the expression of several ACC SYNTHASE (ACS) genes, and in turn ACC represses NO accumulation by promoting the expression of HEMOGLOBIN 1 (HB1) encoding a NO scavenger. This work shows NO plays a role in the regulation of GMC division by modulating ACC accumulation in the Arabidopsis cotyledon.

Nitric oxide alleviates programmed cell death induced by cadmium in Solanum lycopersicum seedlings through protein S-nitrosylation

Cadmium (Cd), as a non-essential and toxic heavy metal in plants, has deleterious effects on plant physiological and biochemical processes. Nitric oxide (NO) is one of the most important signaling molecules for plants to response diverse stresses. Here, we found that Cd-induced programmed cell death (PCD) was accompanied by NO bursts, which exacerbated cell death when NO was removed and vice versa. Proteomic analysis of S-nitrosylated proteins showed that the differential proteins in Cd-induced PCD and in NO-alleviated PCD mainly exist together in carbohydrate metabolism and amino acid metabolism, while some of the differential proteins exist alone in metabolism of cofactors and vitamins and lipid metabolism. Meanwhile, S-nitrosylation of proteins in porphyrin and chlorophyll metabolism and starch and sucrose metabolism could explain the leaf chlorosis induced by PCD. Moreover, protein transport protein SEC23, ubiquitinyl hydrolase 1 and pathogenesis-related protein 1 were identified to be S-nitrosylated in vivo, and their expressions were increased in Cd-induced PCD while decreased in NO treatment. Similar results were obtained in tomato seedlings with higher S-nitrosylation. Taken together, our results indicate that NO might be involved in the regulation of Cd-induced PCD through protein S-nitrosylation, especially proteins involved in PCD response.

Maternal nitric oxide homeostasis impacts female gametophyte development under optimal and stress conditions

In adverse environments, the number of fertilizable female gametophytes (FGs) in plants is reduced, leading to increased survival of the remaining offspring. How the maternal plant perceives internal growth cues and external stress conditions to alter FG development remains largely unknown. We report that homeostasis of the stress signaling molecule nitric oxide (NO) plays a key role in controlling FG development under both optimal and stress conditions. NO homeostasis is precisely regulated by S-nitrosoglutathione reductase (GSNOR). Prior to fertilization, GSNOR protein is exclusively accumulated in sporophytic tissues and indirectly controls FG development in Arabidopsis (Arabidopsis thaliana). In GSNOR null mutants, NO species accumulated in the degenerating sporophytic nucellus, and auxin efflux into the developing FG was restricted, which inhibited FG development, resulting in reduced fertility. Importantly, restoring GSNOR expression in maternal, but not gametophytic tissues, or increasing auxin efflux substrate significantly increased the proportion of normal FGs and fertility. Furthermore, GSNOR overexpression or added auxin efflux substrate increased fertility under drought and salt stress. These data indicate that NO homeostasis is critical to normal auxin transport and maternal control of FG development, which in turn determine seed yield. Understanding this aspect of fertility control could contribute to mediating yield loss under adverse conditions.

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

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

Genome-Wide Identification of PYL/RCAR ABA Receptors and Functional Analysis of LbPYL10 in Heat Tolerance in Goji (Lycium barbarum)

The characterization of the PYL/RCAR ABA receptors in a great deal of plant species has dramatically advanced the study of ABA functions involved in key physiological processes. However, the genes in this family are still unclear in Lycium (Goji) plants, one of the well-known economically, medicinally, and ecologically valuable fruit crops. In the present work, 12 homologs of Arabidopsis PYL/RCAR ABA receptors were first identified and characterized from Lycium (L.) barbarum (LbPYLs). The quantitative real-time PCR (qRT-PCR) analysis showed that these genes had clear tissue-specific expression patterns, and most of them were transcribed in the root with the largest amount. Among the three subfamilies, while the Group I and Group III members were down-regulated by extraneous ABA, the Group II members were up-regulated. At 42 °C, most transcripts showed a rapid and violent up-regulation response to higher temperature, especially members of Group II. One of the genes in the Group II members, LbPYL10, was further functionally validated by virus-induced gene silencing (VIGS) technology. LbPYL10 positively regulates heat stress tolerance in L. barbarum by alleviating chlorophyll degradation, thus maintaining chlorophyll stability. Integrating the endogenous ABA level increase following heat stress, it may be concluded that LbPYL-mediated ABA signaling plays a vital role in the thermotolerance of L. barbarum plants. Our results highlight the strong potential of LbPYL genes in breeding genetically modified L. barbarum crops that acclimate to climate change.

Protein S-nitrosylation under abiotic stress: Role and mechanism

Abiotic stress is one of the main threats affecting crop growth and production. Nitric oxide (NO), an important signaling molecule involved in wide range of plant growth and development as well as in response to abiotic stress. NO can exert its biological functions through protein S-nitrosylation, a redox-based posttranslational modification by covalently adding NO moiety to a reactive cysteine thiol of a target protein to form an S-nitrosothiol (SNO). Protein S-nitrosylation is an evolutionarily conserved mechanism regulating multiple aspects of cellular signaling in plant. Recently, emerging evidence have elucidated protein S-nitrosylation as a modulator of plant in responses to abiotic stress, including salt stress, extreme temperature stress, light stress, heavy metal and drought stress. In addition, significant mechanism has been made in functional characterization of protein S-nitrosylated candidates, such as changing protein conformation, and the subcellular localization of proteins, regulating protein activity and influencing protein interactions. In this study, we updated the data related to protein S-nitrosylation in plants in response to adversity and gained a deeper understanding of the functional changes of target proteins after protein S-nitrosylation.

Brassinosteroid enhances salt tolerance via S-nitrosoglutathione reductase and nitric oxide signaling pathway in mangrove Kandelia obovata

Brassinosteroid (BR) has been shown to modulate plant tolerance to various stresses. S-nitrosoglutathione reductase (GSNOR) is involved in the plant response to environment stress by fine-turning the level of nitric oxide (NO). However, whether GSNOR is involved in BR-regulated Na+ /K+ homeostasis to improve the salt tolerance in halophyte is unknown. Here, we firstly reported that high salinity increases the expression of BR-biosynthesis genes and the endogenous levels of BR in mangrove Kandelia obovata. Then, salt-induced BR triggers the activities and gene expressions of GSNOR and antioxidant enzymes, thereafter decrease the levels of malondialdehyde, hydrogen peroxide. Subsequently, BR-mediated GSNOR negatively regulates NO contributions to the reduction of reactive oxygen species generation and induction of the gene expression related to Na+ and K+ transport, leading to the decrease of Na+ /K+ ratio in the roots of K. obovata. Finally, the applications of exogenous BR, NO scavenger, BR biosynthetic inhibitor and GSNOR inhibitor further confirm the function of BR. Taken together, our result provides insight into the mechanism of BR in the response of mangrove K. obovata to high salinity via GSNOR and NO signaling pathway by reducing oxidative damage and modulating Na+ /K+ homeostasis.

Differential S-nitrosylation and characterization of purified S-nitrosoglutathione reductase (GSNOR) from Brassica juncea shows multiple forms of the enzyme

S-nitrosoglutathione reductase (GSNOR). a master regulator of NO homeostasis, is a single-copy gene in most plants. In Lotus japonicus, two GSNOR isoforms were identified exhibiting similar kinetic properties but differential tissue-specific expressions. Previously, a genome-wide identification in Brassica juncea revealed four copies of GSNOR, each encoding proteins that vary in subunit molecular weights and pI. Here, we report multiple forms of GSNOR using 2D immunoblot which showed 4 immunopositive spots of 41.5 kDa (pl 5.79 and 6.78) and 43 kDa (pl 6.16 and 6.23). To confirm, purification of GSNOR using anion-exchange chromatography yielded 2 distinct pools (GSNOR-A & GSNOR-B) with GSNOR activities. Subsequently, affinity-based purification resulted in 1 polypeptide from GSNOR-A and 2 polypeptides from GSNOR-B. Size exclusion-HPLC confirmed 3 GSNORs with molecular weight of 87.48 ± 2.74 KDa (GSNOR-A); 87.36 ± 3.25 and 82.74 ± 2.75 kDa (GSNOR-B). Kinetic analysis showed Km of 118 ± 11 μM and Vmax of 287 ± 22 nkat/mg for GSNOR-A, whereas Km of 96.4 ± 8 μM and Vmax of 349 ± 15 nkat/mg for GSNOR-B. S-nitrosylation and inhibition by NO showed redox regulation of all BjGSNORs. Both purified GSNORs exhibited variable denitrosylation efficiency as depicted by Biotin Switch assay. To the best of our knowledge, this is the first report confirming multiple isoforms of GSNOR in B. juncea.

The Roles of Functional Amino Acids in Plant Growth and Development

Plants incorporate acquired carbon and nitrogen into amino acid metabolism, whereby the building blocks of proteins and the precursors of various metabolites are produced. This fundamental demand requires tight amino acid metabolism to sustain physiological homeostasis. There is increasing evidence that amino acid metabolism undergoes plastic alteration to orchestrate specific growth and developmental events. Consequently, there has been a gradual exploration of the interface at which amino acid metabolism and plant morphogenesis are mutually affected. This research progress offers an opportunity to explore amino acid metabolism, with the goal to understand how it can be modulated to serve special cellular needs and regulate specific growth and developmental pathways. Continuous improvements in the sensitivity and coverage of metabolomics technology, along with the development of chemoinformatics, have allowed the investigation of these research questions. In this review, we summarize the roles of threonine, serine, arginine and γ-aminobutyric acid as representative examples of amino acids relevant to specific developmental processes in plants ('functional amino acids'). Our objective is to expand perspectives regarding amino acid metabolism beyond the conventional view that it is merely life-supporting machinery.

Loss of S-nitrosoglutathione reductase disturbs phytohormone homeostasis and regulates shoot side branching and fruit growth in tomato

S-Nitrosoglutathione plays a central role in nitric oxide (NO) homeostasis, and S-nitrosoglutathione reductase (GSNOR) regulates the cellular levels of S-nitrosoglutathione across kingdoms. Here, we investigated the role of endogenous NO in shaping shoot architecture and controlling fruit set and growth in tomato (Solanum lycopersicum). SlGSNOR silencing promoted shoot side branching and led to reduced fruit size, negatively impacting fruit yield. Greatly intensified in slgsnor knockout plants, these phenotypical changes were virtually unaffected by SlGSNOR overexpression. Silencing or knocking out of SlGSNOR intensified protein tyrosine nitration and S-nitrosation and led to aberrant auxin production and signaling in leaf primordia and fruit-setting ovaries, besides restricting the shoot basipetal polar auxin transport stream. SlGSNOR deficiency triggered extensive transcriptional reprogramming at early fruit development, reducing pericarp cell proliferation due to restrictions on auxin, gibberellin, and cytokinin production and signaling. Abnormal chloroplast development and carbon metabolism were also detected in early-developing NO-overaccumulating fruits, possibly limiting energy supply and building blocks for fruit growth. These findings provide new insights into the mechanisms by which endogenous NO fine-tunes the delicate hormonal network controlling shoot architecture, fruit set, and post-anthesis fruit development, emphasizing the relevance of NO-auxin interaction for plant development and productivity.

Supersulfide catalysis for nitric oxide and aldehyde metabolism

Abundant formation of endogenous supersulfides, which include reactive persulfide species and sulfur catenated residues in thiols and proteins (supersulfidation), has been observed. We found here that supersulfides catalyze S-nitrosoglutathione (GSNO) metabolism via glutathione-dependent electron transfer from aldehydes by exploiting alcohol dehydrogenase 5 (ADH5). ADH5 is a highly conserved bifunctional enzyme serving as GSNO reductase (GSNOR) that down-regulates NO signaling and formaldehyde dehydrogenase (FDH) that detoxifies formaldehyde in the form of glutathione hemithioacetal. C174S mutation significantly reduced the supersulfidation of ADH5 and almost abolished GSNOR activity but spared FDH activity. Notably, Adh5C174S/C174S mice manifested improved cardiac functions possibly because of GSNOR elimination and consequent increased NO bioavailability. Therefore, we successfully separated dual functions (GSNOR and FDH) of ADH5 (mediated by the supersulfide catalysis) through the biochemical analysis for supersulfides in vitro and characterizing in vivo phenotypes of the GSNOR-deficient organisms that we established herein. Supersulfides in ADH5 thus constitute a substantial catalytic center for GSNO metabolism mediating electron transfer from aldehydes.

Nitric oxide working: no worries about heat stress

Nitric oxide (NO) has multifaceted roles in plants. He et al. report that NO produced in the shoot apex causes S-nitrosation of transcription factor GT-1. This mediator of NO signal perception subsequently regulates the expression of the HEAT SHOCK TRANSCRIPTION FACTOR A2 (HSFA2) gene, thus leading to thermotolerance in Arabidopsis thaliana.

Multiple Ways of Nitric Oxide Production in Plants and Its Functional Activity under Abiotic Stress Conditions

Nitric oxide (NO) is an endogenous signaling molecule that plays an important role in plant ontogenesis and responses to different stresses. The most widespread abiotic stress factors limiting significantly plant growth and crop yield are drought, salinity, hypo-, hyperthermia, and an excess of heavy metal (HM) ions. Data on the accumulation of endogenous NO under stress factors and on the alleviation of their negative effects under exogenous NO treatments indicate the perspectives of its practical application to improve stress resistance and plant productivity. This requires fundamental knowledge of the NO metabolism and the mechanisms of its biological action in plants. NO generation occurs in plants by two main alternative mechanisms: oxidative or reductive, in spontaneous or enzymatic reactions. NO participates in plant development by controlling the processes of seed germination, vegetative growth, morphogenesis, flower transition, fruit ripening, and senescence. Under stressful conditions, NO contributes to antioxidant protection, osmotic adjustment, normalization of water balance, regulation of cellular ion homeostasis, maintenance of photosynthetic reactions, and growth processes of plants. NO can exert regulative action by inducing posttranslational modifications (PTMs) of proteins changing the activity of different enzymes or transcriptional factors, modulating the expression of huge amounts of genes, including those related to stress tolerance. This review summarizes the current data concerning molecular mechanisms of NO production and its activity in plants during regulation of their life cycle and adaptation to drought, salinity, temperature stress, and HM ions.

Nitric oxide-mediated S-nitrosylation of IAA17 protein in intrinsically disordered region represses auxin signaling

The phytohormone auxin plays crucial roles in nearly every aspect of plant growth and development. Auxin signaling is activated through the phytohormone-induced proteasomal degradation of the Auxin/INDOLE-3-ACETIC ACID (Aux/IAA) family of transcriptional repressors. Notably, many auxin-modulated physiological processes are also regulated by nitric oxide (NO) that executes its biological effects predominantly through protein S-nitrosylation at specific cysteine residues. However, little is known about the molecular mechanisms in regulating the interactive NO and auxin networks. Here, we show that NO represses auxin signaling by inhibiting IAA17 protein degradation. NO induces the S-nitrosylation of Cys-70 located in the intrinsically disordered region of IAA17, which inhibits the TIR1-IAA17 interaction and consequently the proteasomal degradation of IAA17. The accumulation of a higher level of IAA17 attenuates auxin response. Moreover, an IAA17C70W nitrosomimetic mutation renders the accumulation of a higher level of the mutated protein, thereby causing partial resistance to auxin and defective lateral root development. Taken together, these results suggest that S-nitrosylation of IAA17 at Cys-70 inhibits its interaction with TIR1, thereby negatively regulating auxin signaling. This study provides unique molecular insights into the redox-based auxin signaling in regulating plant growth and development.

FAT-switch-based quantitative S-nitrosoproteomics reveals a key role of GSNOR1 in regulating ER functions

Reversible protein S-nitrosylation regulates a wide range of biological functions and physiological activities in plants. However, it is challenging to quantitively determine the S-nitrosylation targets and dynamics in vivo. In this study, we develop a highly sensitive and efficient fluorous affinity tag-switch (FAT-switch) chemical proteomics approach for S-nitrosylation peptide enrichment and detection. We quantitatively compare the global S-nitrosylation profiles in wild-type Arabidopsis and gsnor1/hot5/par2 mutant using this approach, and identify 2,121 S-nitrosylation peptides in 1,595 protein groups, including many previously unrevealed S-nitrosylated proteins. These are 408 S-nitrosylated sites in 360 protein groups showing an accumulation in hot5-4 mutant when compared to wild type. Biochemical and genetic validation reveal that S-nitrosylation at Cys337 in ER OXIDOREDUCTASE 1 (ERO1) causes the rearrangement of disulfide, resulting in enhanced ERO1 activity. This study offers a powerful and applicable tool for S-nitrosylation research, which provides valuable resources for studies on S-nitrosylation-regulated ER functions in plants.

S-Nitrosylation of the histone deacetylase HDA19 stimulates its activity to enhance plant stress tolerance in Arabidopsis

Arabidopsis histone deacetylase HDA19 is required for gene expression programs of a large spectrum of plant developmental and stress-responsive pathways. How this enzyme senses cellular environment to control its activity remains unclear. In this work, we show that HDA19 is post-translationally modified by S-nitrosylation at 4 Cysteine (Cys) residues. HDA19 S-nitrosylation depends on the cellular nitric oxide level, which is enhanced under oxidative stress. We find that HDA19 is required for cellular redox homeostasis and plant tolerance to oxidative stress, which in turn stimulates its nuclear enrichment, S-nitrosylation and epigenetic functions including binding to genomic targets, histone deacetylation and gene repression. The Cys137 of the protein is involved in basal and stress-induced S-nitrosylation, and is required for HDA19 functions in developmental, stress-responsive and epigenetic controls. Together, these results indicate that S-nitrosylation regulates HDA19 activity and is a mechanism of redox-sensing for chromatin regulation of plant tolerance to stress.

Glutathione-mediated thermomorphogenesis and heat stress responses in Arabidopsis thaliana

In the context of climate change, the global rise of temperature and intense heat waves affect plant development and productivity. Among the molecular perturbations that high temperature induces in living cells is the accumulation of reactive oxygen species (ROS), which perturbs the cellular redox state. In plants, the dynamics of the cellular and subcellular redox state have been poorly investigated under high temperature. Glutathione plays a major role in maintaining the cellular redox state. We investigated its contribution in adaptation of Arabidopsis thaliana to contrasting high temperature regimes: high ambient temperature inducing thermomorphogenesis and heat stress affecting plant viability. Using the genetically encoded redox marker roGFP2, we show that high temperature regimes lead to cytoplasmic and nuclear oxidation and impact the glutathione pool. This pool is restored within a few hours, which probably contributes to plant adaptation to high temperatures. Moreover, low glutathione mutants fail to adapt to heat stress and to induce thermomorphogenesis, suggesting that glutathione is involved in both heat adaptation mechanisms. We also evaluate the transcriptomic signature in the two high temperature regimes and identified gene expression deviations in low glutathione mutants, which might contribute to their sensitivity to high temperature. Thus, we define glutathione as a major player in the adaptation of Arabidopsis to contrasting high temperature regimes.

Functions of nitric oxide-mediated post-translational modifications under abiotic stress

Environmental conditions greatly impact plant growth and development. In the current context of both global climate change and land degradation, abiotic stresses usually lead to growth restriction limiting crop production. Plants have evolved to sense and respond to maximize adaptation and survival; therefore, understanding the mechanisms involved in the different converging signaling networks becomes critical for improving plant tolerance. In the last few years, several studies have shown the plant responses against drought and salinity, high and low temperatures, mechanical wounding, heavy metals, hypoxia, UV radiation, or ozone stresses. These threats lead the plant to coordinate a crosstalk among different pathways, highlighting the role of phytohormones and reactive oxygen and nitrogen species (RONS). In particular, plants sense these reactive species through post-translational modification (PTM) of macromolecules such as nucleic acids, proteins, and fatty acids, hence triggering antioxidant responses with molecular implications in the plant welfare. Here, this review compiles the state of the art about how plant systems sense and transduce this crosstalk through PTMs of biological molecules, highlighting the S-nitrosylation of protein targets. These molecular mechanisms finally impact at a physiological level facing the abiotic stressful traits that could lead to establishing molecular patterns underlying stress responses and adaptation strategies.

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.

Nitric oxide induces S-nitrosylation of CESA1 and CESA9 and increases cellulose content in Arabidopsis hypocotyls

Nitric oxide (NO), a small signaling gas molecule, participates in several growth and developmental processes in plants. However, how NO regulates cell wall biosynthesis remains unclear. Here, we demonstrate a positive effect of NO on cellulose content that may be related to S-nitrosylation of cellulose synthase 1 (CESA1) and CESA9. Two S-nitrosylated cysteine (Cys) residues, Cys562 and Cys641, which are exposed on the surface of CESA1 and CESA9 and located in the cellulose synthase catalytic domain, were identified to be S-nitrosylated. Meanwhile, Cys641 was located on the binding surface of CESA1 and CESA9, and Cys562 was very close to the binding surface. Cellulose synthase complexes (CSCs) dynamics are closely associated with cellulose content. S-nitrosylation of CESA1 and CESA9 improved particles mobility and thus increased the accumulation of cellulose in Arabidopsis hypocotyl cells. An increase in hemicellulose content as well as an alteration in pectin content facilitated cell wall extension and contributed to cell growth, finally promoting elongation of Arabidopsis hypocotyls. Overall, our work provides a path to investigate the way NO affects the cellulose content of plants.

The role of nitric oxide and hydrogen sulfide in regulation of redox homeostasis at extreme temperatures in plants

Nitric oxide and hydrogen sulfide, as important signaling molecules (gasotransmitters), are involved in many functions of plant organism, including adaptation to stress factors of various natures. As redox-active molecules, NO and H₂S are involved in redox regulation of functional activity of many proteins. They are also involved in maintaining cell redox homeostasis due to their ability to interact directly and indirectly (functionally) with ROS, thiols, and other molecules. The review considers the involvement of nitric oxide and hydrogen sulfide in plant responses to low and high temperatures. Particular attention is paid to the role of gasotransmitters interaction with other signaling mediators (in particular, with Ca²⁺ ions and ROS) in the formation of adaptive responses to extreme temperatures. Pathways of stress-induced enhancement of NO and H₂S synthesis in plants are considered. Mechanisms of the NO and H₂S effect on the activity of some proteins of the signaling system, as well as on the state of antioxidant and osmoprotective systems during adaptation to stress temperatures, were analyzed. Possibilities of practical use of nitric oxide and hydrogen sulfide donors as inductors of plant adaptive responses are discussed.

The role of sulfur nutrition in plant response to metal(loid) stress: Facilitating biofortification and phytoremediation

Metal(loid)s contamination poses a serious threat to ecosystem biosafety and human health. Phytoremediation is a cost-effective and eco-friendly technology with good public acceptance, although the process does require a significant amount of time for success. To enhance the phytoremediation efficiency, numerous approaches have been explored, including soil amendments application with chelators to facilitate remediation. Sulfur (S), a macronutrient for plant growth, plays vital roles in several metabolic pathways that can actively affect metal(loid)s phytoextraction, as well as attenuate metal(loid) toxicity. In this review, different forms of S-amendments (fertilizers) on uptake and translocation in plants upon exposure to various metal(loid) are evaluated. Possible mechanisms for S application alleviating metal(loid) toxicity are documented at the physiological, biochemical and molecular levels. Furthermore, this review highlights the crosstalk between S-assimilation and other biomolecules, such as phytohormones, polyamines and nitric oxide, which are also important for metal(loid) stress tolerance. Given the effectiveness and potential of S amendments on phytoremediation, future studies should focus on optimizing phytoremediation efficiency in long-term field studies and on investigating the appropriate S dose to maximize the food safety and ecosystem health.

Stigma receptors control intraspecies and interspecies barriers in Brassicaceae

Flowering plants have evolved numerous intraspecific and interspecific prezygotic reproductive barriers to prevent production of unfavourable offspring¹. Within a species, self-incompatibility (SI) is a widely utilized mechanism that rejects self-pollen^2,3 to avoid inbreeding depression. Interspecific barriers restrain breeding between species and often follow the SI × self-compatible (SC) rule, that is, interspecific pollen is unilaterally incompatible (UI) on SI pistils but unilaterally compatible (UC) on SC pistils^1,4-6. The molecular mechanisms underlying SI, UI, SC and UC and their interconnections in the Brassicaceae remain unclear. Here we demonstrate that the SI pollen determinant S-locus cysteine-rich protein/S-locus protein 11 (SCR/SP11)^2,3 or a signal from UI pollen binds to the SI female determinant S-locus receptor kinase (SRK)^2,3, recruits FERONIA (FER)^7-9 and activates FER-mediated reactive oxygen species production in SI stigmas^10,11 to reject incompatible pollen. For compatible responses, diverged pollen coat protein B-class^12-14 from SC and UC pollen differentially trigger nitric oxide, nitrosate FER to suppress reactive oxygen species in SC stigmas to facilitate pollen growth in an intraspecies-preferential manner, maintaining species integrity. Our results show that SRK and FER integrate mechanisms underlying intraspecific and interspecific barriers and offer paths to achieve distant breeding in Brassicaceae crops.

Regulatory roles of selective autophagy through targeting of native proteins in plant adaptive responses

Selective autophagy functions as a regulatory mechanism by targeting native and functional proteins to ensure their proper levels and activities in plant adaptive responses. Autophagy is a cellular degradation and recycling pathway with a key role in cellular homeostasis and metabolism. Autophagy is initiated with the biogenesis of autophagosomes, which fuse with the lysosomes or vacuoles to release their contents for degradation. Under nutrient starvation or other adverse environmental conditions, autophagy usually targets unwanted or damaged proteins, organelles and other cellular components for degradation and recycling to promote cell survival. Over the past decade, however, a substantial number of studies have reported that autophagy in plants also functions as a regulatory mechanism by targeting enzymes, structural and regulatory proteins that are not necessarily damaged or dysfunctional to ensure their proper abundance and function to facilitate cellular changes required for response to endogenous and environmental conditions. During plant-pathogen interactions in particular, selective autophagy targets specific pathogen components as a defense mechanism and pathogens also utilize autophagy to target functional host factors to suppress defense mechanisms. Autophagy also targets native and functional protein regulators of plant heat stress memory, hormone signaling, and vesicle trafficking associated with plant responses to abiotic and other conditions. In this review, we discuss advances in the regulatory roles of selective autophagy through targeting of native proteins in plant adaptive responses, what questions remain and how further progress in the analysis of these special regulatory roles of autophagy can help understand biological processes important to plants.

Phytochrome B enhances seed germination tolerance to high temperature by reducing S-nitrosylation of HFR1

Light and ambient high temperature (HT) have opposite effects on seed germination. Light induces seed germination through activating the photoreceptor phytochrome B (phyB), resulting in the stabilization of the transcription factor HFR1, which in turn sequesters the suppressor PIF1. HT suppresses seed germination and triggers protein S-nitrosylation. Here, we find that HT suppresses seed germination by inducing the S-nitrosylation of HFR1 at C164, resulting in its degradation, the release of PIF1, and the activation of PIF1-targeted SOMNUS (SOM) expression to alter gibberellin (GA) and abscisic acid (ABA) metabolism. Active phyB (phyBY276H ) antagonizes HFR1 S-nitrosylation and degradation by increasing S-nitrosoglutathione reductase (GSNOR) activity. In line with this, substituting cysteine-164 of HFR1 with serine (HFR1C164S ) abolishes the S-nitrosylation of HFR1 and decreases the HT-induced degradation of HFR1. Taken together, our study suggests that HT and phyB antagonistically modulate the S-nitrosylation level of HFR1 to coordinate seed germination, and provides the possibility to enhance seed thermotolerance through gene-editing of HFR1.

Interactions of melatonin, reactive oxygen species, and nitric oxide during fruit ripening: an update and prospective view

Fruit ripening is a physiological process that involves a complex network of signaling molecules that act as switches to activate or deactivate certain metabolic pathways at different levels, not only by regulating gene and protein expression but also through post-translational modifications of the involved proteins. Ethylene is the distinctive molecule that regulates the ripening of fruits, which can be classified as climacteric or non-climacteric according to whether or not, respectively, they are dependent on this phytohormone. However, in recent years it has been found that other molecules with signaling potential also exert regulatory roles, not only individually but also as a result of interactions among them. These observations imply the existence of mutual and hierarchical regulations that sometimes make it difficult to identify the initial triggering event. Among these 'new' molecules, hydrogen peroxide, nitric oxide, and melatonin have been highlighted as prominent. This review provides a comprehensive outline of the relevance of these molecules in the fruit ripening process and the complex network of the known interactions among them.

Determination of Reactive Oxygen or Nitrogen Species and Novel Volatile Organic Compounds in the Defense Responses of Tomato Plants against Botrytis cinerea Induced by Trichoderma virens TRS 106

In the present study, Trichoderma virens TRS 106 decreased grey mould disease caused by Botrytis cinerea in tomato plants (S. lycopersicum L.) by enhancing their defense responses. Generally, plants belonging to the ‘Remiz’ variety, which were infected more effectively by B. cinerea than ‘Perkoz’ plants, generated more reactive molecules such as superoxide (O₂⁻) and peroxynitrite (ONOO⁻), and less hydrogen peroxide (H₂O₂), S-nitrosothiols (SNO), and green leaf volatiles (GLV). Among the new findings, histochemical analyses revealed that B. cinerea infection caused nitric oxide (NO) accumulation in chloroplasts, which was not detected in plants treated with TRS 106, while treatment of plants with TRS 106 caused systemic spreading of H₂O₂ and NO accumulation in apoplast and nuclei. SPME-GCxGC TOF-MS analysis revealed 24 volatile organic compounds (VOC) released by tomato plants treated with TRS 106. Some of the hexanol derivatives, e.g., 4-ethyl-2-hexynal and 1,5-hexadien-3-ol, and salicylic acid derivatives, e.g., 4-hepten-2-yl and isoamyl salicylates, are considered in the protection of tomato plants against B. cinerea for the first time. The results are valuable for further studies aiming to further determine the location and function of NO in plants treated with Trichoderma and check the contribution of detected VOC in plant protection against B. cinerea.

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

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

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.

NUCLEOCYTOPLASMIC shuttling of ETHYLENE RESPONSE FACTOR 5 mediated by nitric oxide suppresses ethylene biosynthesis in apple fruit

Nitric oxide (NO) is known to modulate the action of several phytohormones. This includes the gaseous hormone ethylene, but the molecular mechanisms underlying the effect of NO on ethylene biosynthesis are unclear. Here, we observed a decrease in endogenous NO abundance during apple (Malus domestica) fruit development and exogenous treatment of apple fruit with a NO donor suppressed ethylene production, suggesting that NO is a ripening suppressor. Expression of the transcription factor MdERF5 was activated by NO donor treatment. NO induced the nucleocytoplasmic shuttling of MdERF5 by modulating its interaction with the protein phosphatase, MdPP2C57. MdPP2C57-induced dephosphorylation of MdERF5 at Ser260 is sufficient to promote nuclear export of MdERF5. As a consequence of this export, MdERF5 proteins in the cytoplasm interacted with and suppressed the activity of MdACO1, an enzyme that converts 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene. The NO-activated MdERF5 was observed to increase in abundance in the nucleus and bind to the promoter of the ACC synthase gene MdACS1 and directly suppress its transcription. Together, these results suggest that NO-activated nucleocytoplasmic MdERF5 suppresses the action of ethylene biosynthetic genes, thereby suppressing ethylene biosynthesis and limiting fruit ripening.

S-Nitrosoglutathione Reductase Contributes to Thermotolerance by Modulating High Temperature-Induced Apoplastic H2O2 in Solanum lycopersicum

S-nitrosoglutathione reductase (GSNOR) is considered as a critical regulator of plant stress tolerance for its impacts on protein S-nitrosylation through regulation of the S-nitrosothiol (SNO) level. However, the mechanism of GSNOR-mediated stress tolerance is still obscure. Here, we found that GSNOR activity was induced by high temperature in tomato (Solanum lycopersicum) plants, whereas mRNA level of SlGSNOR1 exhibited little response. Suppressing SlGSNOR1 expression by virus-induced gene silencing (VIGS) increased accumulation of SNO and nitrites under high temperature and reduced thermotolerance. The compromised thermotolerance was associated with less accumulation of abscisic acid (ABA) and salicylic acid (SA), attenuated activation of mitogen-activated protein kinase (MAPK) and reduced expression of heat shock protein. Intriguingly, SlGSNOR1 silencing impaired upregulation of RESPIRATORY BURST OXIDASE HOMOLOG1 (SlRBOH1) and apoplastic H₂O₂ accumulation in response to high temperature, whereas SlRBOH1 silencing abolished activation of GSNOR and led to a similar decline in thermotolerance as in SlGSNOR1-silenced plants. Importantly, H₂O₂ treatment recovered the thermotolerance and improved antioxidant capacity in SlGSNOR1-silenced plants. Our results suggest that GSNOR plays a role in regulating the SlRBOH1-dependent apoplastic H₂O₂ production in response to high temperature, while a balanced interaction between SNO and H₂O₂ is critical for maintaining the cellular redox homeostasis and thermotolerance.

Role of Nitric Oxide in Plant Senescence

In plants senescence is the final stage of plant growth and development that ultimately leads to death. Plants experience age-related as well as stress-induced developmental ageing. Senescence involves significant changes at the transcriptional, post-translational and metabolomic levels. Furthermore, phytohormones also play a critical role in the programmed senescence of plants. Nitric oxide (NO) is a gaseous signalling molecule that regulates a plethora of physiological processes in plants. Its role in the control of ageing and senescence has just started to be elucidated. Here, we review the role of NO in the regulation of programmed cell death, seed ageing, fruit ripening and senescence. We also discuss the role of NO in the modulation of phytohormones during senescence and the significance of NO-ROS cross-talk during programmed cell death and senescence.

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.

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.

ADH2/GSNOR1 is a key player in limiting genotoxic damage mediated by formaldehyde and UV-B in Arabidopsis

Maintenance of genome stability is an essential requirement for all living organisms. Formaldehyde and UV-B irradiation cause DNA damage and affect genome stability, growth and development, but the interplay between these two genotoxic factors is poorly understood in plants. We show that Arabidopsis adh2/gsnor1 mutant, which lacks alcohol dehydrogenase 2/S-nitrosoglutathione reductase 1 (ADH2/GSNOR1), are hypersensitive to low fluence UV-B irradiation or UV-B irradiation-mimetic chemicals. Although the ADH2/GSNOR1 enzyme can act on different substrates, notably on S-hydroxymethylglutathione (HMG) and S-nitrosoglutathione (GSNO), our study provides several lines of evidence that the sensitivity of gsnor1 to UV-B is caused mainly by UV-B-induced formaldehyde accumulation rather than other factors such as alteration of the GSNO concentration. Our results demonstrate an interplay between formaldehyde and UV-B that exacerbates genome instability, leading to severe DNA damage and impaired growth and development in Arabidopsis, and show that ADH2/GSNOR1 is a key player in combating these effects.

Genomics Associated Interventions for Heat Stress Tolerance in Cool Season Adapted Grain Legumes

Cool season grain legumes occupy an important place among the agricultural crops and essentially provide multiple benefits including food supply, nutrition security, soil fertility improvement and revenue for farmers all over the world. However, owing to climate change, the average temperature is steadily rising, which negatively affects crop performance and limits their yield. Terminal heat stress that mainly occurred during grain development phases severely harms grain quality and weight in legumes adapted to the cool season, such as lentils, faba beans, chickpeas, field peas, etc. Although, traditional breeding approaches with advanced screening procedures have been employed to identify heat tolerant legume cultivars. Unfortunately, traditional breeding pipelines alone are no longer enough to meet global demands. Genomics-assisted interventions including new-generation sequencing technologies and genotyping platforms have facilitated the development of high-resolution molecular maps, QTL/gene discovery and marker-assisted introgression, thereby improving the efficiency in legumes breeding to develop stress-resilient varieties. Based on the current scenario, we attempted to review the intervention of genomics to decipher different components of tolerance to heat stress and future possibilities of using newly developed genomics-based interventions in cool season adapted grain legumes.

Quantitative Proteome Profiling of a S-Nitrosoglutathione Reductase (GSNOR) Null Mutant Reveals a New Class of Enzymes Involved in Nitric Oxide Homeostasis in Plants

Nitric oxide (NO) is a short-lived radical gas that acts as a signaling molecule in all higher organisms, and that is involved in multiple plant processes, including germination, root growth, and fertility. Regulation of NO-levels is predominantly achieved by reaction of oxidation products of NO with glutathione to form S-nitrosoglutathione (GSNO), the principal bioactive form of NO. The enzyme S-nitrosoglutathione reductase (GSNOR) is a major route of NADH-dependent GSNO catabolism and is critical to NO homeostasis. Here, we performed a proteomic analysis examining changes in the total leaf proteome of an Arabidopsis thaliana GSNOR null mutant (hot5-2/gsnor1-3). Significant increases or decreases in proteins associated with chlorophyll metabolism and with redox and stress metabolism provide insight into phenotypes observed in hot5-2/gsnor1-3 plants. Importantly, we identified a significant increase in proteins that belong to the aldo-keto reductase (AKR) protein superfamily, AKR4C8 and 9. Because specific AKRs have been linked to NO metabolism in mammals, we expressed and purified A. thaliana AKR4C8 and 9 and close homologs AKR4C10 and 11 and determined that they have NADPH-dependent activity in GSNO and S-nitroso-coenzyme A (SNO-CoA) reduction. Further, we found an increase of NADPH-dependent GSNO reduction activity in hot5-2/gsnor1-3 mutant plants. These data uncover a new, NADPH-dependent component of NO metabolism that may be integrated with NADH-dependent GSNOR activity to control NO homeostasis in plants.

Characteristics and functions of glyceraldehyde 3-phosphate dehydrogenase S-nitrosylation during controlled aging of elm and Arabidopsis seeds

Seed aging is the gradual decline in seed vigor, during which programmed cell death (PCD) occurs. The functions of nitric oxide (NO) are exerted through protein S-nitrosylation, a reversible post-translational modification. During seed aging, more than 80 proteins are S-nitrosylated, but the particular role of individual proteins is unknown. Here, we showed that the S-nitrosylation level of glyceraldehyde 3-phosphate dehydrogenase (UpGAPDH) in elm (Ulmus pumila L.) seeds increased after controlled deterioration treatment. UpGAPDH was S-nitrosylated at Cys154 during S-nitrosoglutathione (GSNO) treatment, and its oligomerization was triggered both in vitro and in elm seeds. Interestingly, UpGAPDH interacted with the mitochondrial voltage-dependent anion channel in an S-nitrosylation-dependent way. Some UpGAPDH-green fluorescent protein in Arabidopsis protoplasts co-localized with mitochondria during the GSNO treatment, while the S-nitrosylation-defective UpGAPDH C154S-GFP protein did not. Seeds of oxUpGAPDH lines showed cell death and lost seed vigor rapidly during controlled deterioration treatment-triggered seed aging, while those overexpressing S-nitrosylation-defective UpGAPDH-Cys154 did not. Our results suggest that S-nitrosylation of UpGAPDH may accelerate cell death and seed deterioration during controlled deterioration treatment. These results provide new insights into the effects of UpGAPDH S-nitrosylation on protein interactions and seed aging.

Nitric oxide coordinates growth, development, and stress response via histone modification and gene expression

Nitric oxide (NO) is a signaling molecule with multiple regulatory functions in plant physiology and stress response. In addition to direct effects on transcriptional machinery, NO executes its signaling function via epigenetic mechanisms. We report that light intensity-dependent changes in NO correspond to changes in global histone acetylation (H3, H3K9, and H3K9/K14) in Arabidopsis (Arabidopsis thaliana) wild-type leaves, and that this relationship depends on S-nitrosoglutathione reductase and histone deacetylase 6 (HDA6). The activity of HDA6 was sensitive to NO, demonstrating that NO participates in regulation of histone acetylation. Chromatin immunoprecipitation sequencing and RNA-seq analyses revealed that NO participates in the metabolic switch from growth and development to stress response. This coordinating function of NO might be particularly important in plant ability to adapt to a changing environment, and is therefore a promising foundation for mitigating the negative effects of climate change on plant productivity.

Perturbations in nitric oxide homeostasis promote Arabidopsis disease susceptibility towards Phytophthora parasitica

Phytophthora species can infect hundreds of different plants, including many important crops, causing a number of agriculturally relevant diseases. A key feature of attempted pathogen infection is the rapid production of the redox active molecule nitric oxide (NO). However, the potential role(s) of NO in plant resistance against Phytophthora is relatively unexplored. Here we show that the level of NO accumulation is crucial for basal resistance in Arabidopsis against Phytophthora parasitica. Counterintuitively, both relatively low or relatively high NO accumulation leads to reduced resistance against P. parasitica. S-nitrosylation, the addition of a NO group to a protein cysteine thiol to form an S-nitrosothiol, is an important route for NO bioactivity and this process is regulated predominantly by S-nitrosoglutathione reductase 1 (GSNOR1). Loss-of-function mutations in GSNOR1 disable both salicylic acid accumulation and associated signalling, and also the production of reactive oxygen species, leading to susceptibility towards P. parasitica. Significantly, we also demonstrate that secreted proteins from P. parasitica can inhibit Arabidopsis GSNOR1 activity.

Molecular responses of legumes to abiotic stress: post-translational modifications of proteins and redox signaling

Legumes include several major crops that can fix atmospheric nitrogen in symbiotic root nodules, thus reducing the demand for nitrogen fertilizers and contributing to sustainable agriculture. Global change models predict increases in temperature and extreme weather conditions. This scenario might increase plant exposure to abiotic stresses and negatively affect crop production. Regulation of whole plant physiology and nitrogen fixation in legumes during abiotic stress is complex, and only a few mechanisms have been elucidated. Reactive oxygen species (ROS), reactive nitrogen species (RNS), and reactive sulfur species (RSS) are key players in the acclimation and stress tolerance mechanisms of plants. However, the specific redox-dependent signaling pathways are far from understood. One mechanism by which ROS, RNS, and RSS fulfil their signaling role is the post-translational modification (PTM) of proteins. Redox-based PTMs occur in the cysteine thiol group (oxidation, S-nitrosylation, S-glutathionylation, persulfidation), and also in methionine (oxidation), tyrosine (nitration), and lysine and arginine (carbonylation/glycation) residues. Unraveling PTM patterns under different types of stress and establishing the functional implications may give insight into the underlying mechanisms by which the plant and nodule respond to adverse conditions. Here, we review current knowledge on redox-based PTMs and their possible consequences in legume and nodule biology.

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.

S-nitrosoglutathione Reductase-Mediated Nitric Oxide Affects Axillary Buds Outgrowth of Solanum lycopersicum L. by Regulating Auxin and Cytokinin Signaling

Auxin and cytokinin are two kinds of important phytohormones that mediate outgrowth of axillary buds in plants. How nitric oxide and its regulator of S-nitrosoglutathione reductase (GSNOR) take part in auxin and cytokinin signaling for controlling axillary buds outgrowth remains elusive. We investigated the roles of GSNOR during tomato axillary bud outgrowth by using physiological, biochemical and genetic approaches. GSNOR negatively regulated NO homeostasis. Suppression of GSNOR promoted axillary bud outgrowth by inhibiting the expression of FZY in both apical and axillary buds. Meanwhile, AUX1 and PIN1 were down-regulated in apical buds but up-regulated in axillary buds in GSNOR-suppressed plants. Thus, reduced IAA accumulation was shown in both apical buds and axillary buds of GSNOR-suppressed plants. GSNOR-mediated changes of NO and auxin affected cytokinin biosynthesis, transport, and signaling. And a decreased ratio of auxin: cytokinin was shown in axillary buds of GSNOR-suppressed plants, leading to bud dormancy breaking. We also found that the original NO signaling was generated by nitrate reductase (NR) catalyzing nitrate as substrate. NR-mediated NO reduced the GSNOR activity through S-nitrosylation of Cys-10, then induced a further NO burst, which played the above roles to promote axillary buds outgrowth. Together, GSNOR-mediated NO played important roles in controlling axillary buds outgrowth by altering the homeostasis and signaling of auxin and cytokinin in tomato plants.

GSNOR Contributes to Demethylation and Expression of Transposable Elements and Stress-Responsive Genes

In the past, reactive nitrogen species (RNS) were supposed to be stress-induced by-products of disturbed metabolism that cause oxidative damage to biomolecules. However, emerging evidence demonstrates a substantial role of RNS as endogenous signals in eukaryotes. In plants, S-nitrosoglutathione (GSNO) is the dominant RNS and serves as the ^•NO donor for S-nitrosation of diverse effector proteins. Remarkably, the endogenous GSNO level is tightly controlled by S-nitrosoglutathione reductase (GSNOR) that irreversibly inactivates the glutathione-bound NO to ammonium. Exogenous feeding of diverse RNS, including GSNO, affected chromatin accessibility and transcription of stress-related genes, but the triggering function of RNS on these regulatory processes remained elusive. Here, we show that GSNO reductase-deficient plants (gsnor1-3) accumulate S-adenosylmethionine (SAM), the principal methyl donor for methylation of DNA and histones. This SAM accumulation triggered a substantial increase in the methylation index (MI = [SAM]/[S-adenosylhomocysteine]), indicating the transmethylation activity and histone methylation status in higher eukaryotes. Indeed, a mass spectrometry-based global histone profiling approach demonstrated a significant global increase in H3K9me2, which was independently verified by immunological detection using a selective antibody. Since H3K9me2-modified regions tightly correlate with methylated DNA regions, we also determined the DNA methylation status of gsnor1-3 plants by whole-genome bisulfite sequencing. DNA methylation in the CG, CHG, and CHH contexts in gsnor1-3 was significantly enhanced compared to the wild type. We propose that GSNOR1 activity affects chromatin accessibility by controlling the transmethylation activity (MI) required for maintaining DNA methylation and the level of the repressive chromatin mark H3K9me2.

S-nitrosated proteomic analysis reveals the regulatory roles of protein S-nitrosation and S-nitrosoglutathione reductase during Al-induced PCD in peanut root tips

Nitric oxide-mediated S-nitrosation through S-nitrosoglutathione reductase (GSNOR) plays important roles in cellular processes and signaling of plants; however, the regulatory mechanism of programmed cell death (PCD) by S-nitrosation remains unclear. In this study, the S-nitrosated proteomic and functions of GSNOR during Al-induced PCD in peanut were investigated. Al stress induced an increase of S-nitrosothiol (SNO) content and GSNOR activity in Al-induced PCD. There was significant positive correlation between SNO content and hydrogen peroxide content. The S-nitrosated proteomic analysis identified 402 S-nitrosated proteins containing 551 S-nitrosated sites during Al-induced PCD in the root tips of peanut. These S-nitrosated proteins were involved in regulation of various biological processes including energy metabolism, maintenance of cell wall function and organic acid secretion. Among them, 128 S-nitrosated proteins were up-regulated and one was down-regulated after Al stress. Experiments with recombinant AhGSNOR revealed that activity of the enzyme was inhibited by its S-nitrosation, with a moderate decrease of 17.9 % after 100 μM GSNO incubation. These data provide novel insights to understanding the functional mechanism of NO-mediated S-nitrosation during plant PCD.

Overexpression of SlGSNOR impairs in vitro shoot proliferation and developmental architecture in tomato but confers enhanced disease resistance

The pervasive presence of nitric oxide (NO) in cells and its role in modifying cystein residues through protein S-nitrosylation is a remarkable redox based signalling mechanism regulating a variety of cellular processes. S-NITROSOGLUTATHIONE REDUCTASE (GSNOR) governs NO bioavailability by the breakdown of S-nitrosoglutathione (GSNO), fine-tunes NO signalling and controls total cellular S-nitrosylated proteins. Most of the published data on GSNOR functional analysis is based on the model plant Arabidopsis with no previous report for its effect on in vitro regeneration of tissue cultured plants. Moreover, the effect of GSNOR overexpression (O.E) on tomato growth, development and disease resistance remains enigmatic. Here we show that SlGSNOR O.E in tomato alters multiple developmental programs from in vitro culture establishment to plant growth and fruit set. Moreover, constitutive SlGSNOR O.E in tomato showed enhanced resistance against early blight (EB) disease caused by Alternaria solani and reduction in hypersensitive response (HR)-mediated cell death after Pseudomonas syringae pv. tomato DC3000 (Pst DC3000) infiltrations. High GSNOR transcript levels led to the inhibition of in vitro shoot proliferation in transformed explants as revealed by the fluorescence microscopy after YFP labelling. Transgenic tomato lines overexpressing SlGSNOR showed defective phenotypes exhibiting stunted plant growth and bushy-type plants due to loss of apical dominance, along with reduced seed germination and delayed flowering. Furthermore, SlGSNOR O.E plants exhibited altered leaf arrangement, fruit shape and modified locules number in tomato fruit. These findings give a novel insight into a multifaceted regulatory role of SlGSNOR in tomato plant development, reproduction and response to pathogens.