Hydrogen sulfide and cell signaling: team player or referee?

Considerations of the importance of redox state for reactive nitrogen species action

Nitric oxide (NO) and other reactive nitrogen species (RNS) are immensely important signalling molecules in plants, being involved in a range of physiological responses. However, the exact way in which NO fits into signal transduction pathways is not always easy to understand. Here, some of the issues that should be considered are discussed. This includes how NO may interact directly with other reactive signals, such as reactive oxygen and sulfur species, how NO metabolism is almost certainly compartmentalized, that threshold levels of RNS may need to be reached to have effects, and how the intracellular redox environment may impact on NO signalling. Until better tools are available to understand how NO is generated in cells, where it accumulates, and to what levels it reaches, it will be hard to get a full understanding of NO signalling. The interaction of RNS metabolism with the intracellular redox environment needs further investigation. A changing redox poise will impact on whether RNS species can thrive in or around cells. Such mechanisms will determine whether specific RNS can indeed control the responses needed by a cell.

Rhodanese domain-containing sulfurtransferases: multifaceted proteins involved in sulfur trafficking in plants

Sulfur is an essential element for the growth and development of plants, which synthesize cysteine and methionine from the reductive assimilation of sulfate. Besides its incorporation into proteins, cysteine is the building block for the biosynthesis of numerous sulfur-containing molecules and cofactors. The required sulfur atoms are extracted either directly from cysteine by cysteine desulfurases or indirectly after its catabolic transformation to 3-mercaptopyruvate, a substrate for sulfurtransferases (STRs). Both enzymes are transiently persulfidated in their reaction cycle, i.e. the abstracted sulfur atom is bound to a reactive cysteine residue in the form of a persulfide group. Trans-persulfidation reactions occur when sulfur atoms are transferred to nucleophilic acceptors such as glutathione, proteins, or small metabolites. STRs form a ubiquitous, multigenic protein family. They are characterized by the presence of at least one rhodanese homology domain (Rhd), which usually contains the catalytic, persulfidated cysteine. In this review, we focus on Arabidopsis STRs, presenting the sequence characteristics of all family members as well as their biochemical and structural features. The physiological functions of particular STRs in the biosynthesis of molybdenum cofactor, thio-modification of cytosolic tRNAs, arsenate tolerance, cysteine catabolism, and hydrogen sulfide formation are also discussed.

The Role of Redox in Signal Transduction

It is the functioning of efficient cell signaling which is vital for the survival of cells, whether it is a simple prokaryote or a complex eukaryote, including both animals and plants. Over many years various components have been identified and recognized as crucial for the transduction of signals in cells, including small organic molecules and ions. Many of the mechanisms allow for a relatively rapid switching of signals, on or off, with common examples being the G proteins and protein phosphorylation. However, it has become apparent that other amino acid modifications are also vitally important. This includes reactions with nitric oxide, for example S-nitrosation (S-nitrosylation), and, of particular relevance here, oxidation of cysteine residues. Such oxidation will be dependent on the redox status of the intracellular environment in which that protein resides, and this will in turn be dictated by the presence of pro-oxidants and antioxidants, either produced by the cell itself or from the cell's environment. Here, the chemistry of redox modification of amino acids is introduced, and a general overview of the role of redox in mediating signal transduction is given.

Nitric Oxide: Its Generation and Interactions with Other Reactive Signaling Compounds

Nitric oxide (NO) is an immensely important signaling molecule in animals and plants. It is involved in plant reproduction, development, key physiological responses such as stomatal closure, and cell death. One of the controversies of NO metabolism in plants is the identification of enzymatic sources. Although there is little doubt that nitrate reductase (NR) is involved, the identification of a nitric oxide synthase (NOS)-like enzyme remains elusive, and it is becoming increasingly clear that such a protein does not exist in higher plants, even though homologues have been found in algae. Downstream from its production, NO can have several potential actions, but none of these will be in isolation from other reactive signaling molecules which have similar chemistry to NO. Therefore, NO metabolism will take place in an environment containing reactive oxygen species (ROS), hydrogen sulfide (H₂S), glutathione, other antioxidants and within a reducing redox state. Direct reactions with NO are likely to produce new signaling molecules such as peroxynitrite and nitrosothiols, and it is probable that chemical competitions will exist which will determine the ultimate end result of signaling responses. How NO is generated in plants cells and how NO fits into this complex cellular environment needs to be understood.

Hydrogen sulfide directs metabolic flux towards the lignan biosynthesis in Linum album hairy roots

Hydrogen sulfide (H₂S) has been recently found as an important signaling molecule especially in root system architecture of plants. The regulation of root formation through H₂S has been reported in previous works; while the profiling of metabolites in response to H₂S is not clearly discussed. To this end, different concentrations of sodium hydrosulfide (an H₂S donor) were applied to the culture of Linum album hairy roots. Subsequently, the amino acid profiles, soluble carbohydrates, and central intermediates of phenylpropanoid pathway with two branches of lignans and flavonoids were assessed by spectroscopy and high performance liquid chromatography techniques. An analysis of the signaling molecules (nitric oxide, hydrogen peroxide, and salicylic acid) was also conducted as they proposed to act in conjunction with H₂S. The H₂S activated antioxidant systems and caused a shift from flavonoid to lignan production (podophyllotoxin and 6-methoxypodophyllotoxin); although, some of the flavonoids increased in a dose-dependent manner. The H₂S decreased the contents of phenylalanine and tyrosine as substrates of the phenylpropanoid pathway, but increased proline and histidine as an osmolyte and antioxidant, respectively. These findings propose that H₂S modulates other signaling molecules, regulates free amino acids, and mediates biosynthesis of lignans and flavonoids in the phenylpropanoids biosynthesis pathway.

Sulfate-Induced Stomata Closure Requires the Canonical ABA Signal Transduction Machinery

Phytohormone abscisic acid (ABA) is the canonical trigger for stomatal closure upon abiotic stresses like drought. Soil-drying is known to facilitate root-to-shoot transport of sulfate. Remarkably, sulfate and sulfide—a downstream product of sulfate assimilation—have been independently shown to promote stomatal closure. For induction of stomatal closure, sulfate must be incorporated into cysteine, which triggers ABA biosynthesis by transcriptional activation of NCED3. Here, we apply reverse genetics to unravel if the canonical ABA signal transduction machinery is required for sulfate-induced stomata closure, and if cysteine biosynthesis is also mandatory for the induction of stomatal closure by the gasotransmitter sulfide. We provide genetic evidence for the importance of reactive oxygen species (ROS) production by the plasma membrane-localized NADPH oxidases, RBOHD, and RBOHF, during the sulfate-induced stomatal closure. In agreement with the established role of ROS as the second messenger of ABA-signaling, the SnRK2-type kinase OST1 and the protein phosphatase ABI1 are essential for sulfate-induced stomata closure. Finally, we show that sulfide fails to close stomata in a cysteine-biosynthesis depleted mutant. Our data support the hypothesis that the two mobile signals, sulfate and sulfide, induce stomatal closure by stimulating cysteine synthesis to trigger ABA production.

Equations to Support Redox Experimentation

Working with redox compounds needs to take into account the oxidation and reduction state of the compound under study. This redox state can be influenced by the media in which the compound is found, but will also be influenced by local environments. For example, this may be dictated perhaps by the locality of amino acids in the three dimensional structure of a protein. Therefore, historically, equations have been developed to enable either the redox poise of the environment to be determined, or the redox state of the compound of interest. If a compound is found in the wrong redox state-perhaps inactive-in a cell this has significant ramifications for its role, for example in cell signaling. Here, the use of such equations is discussed, with examples of the relevance to modern redox biology.

Investigating ROS, RNS, and H2S-Sensitive Signaling Proteins

The modification of proteins is a key way to alter their activity and function. Often thiols, cysteine residues, on proteins are attractive targets for such modification. Assuming that the thiol group is accessible then reactions may take place with a range of chemicals found in cells. These may include reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), reactive nitrogen species such as nitric oxide (NO), hydrogen sulfide (H2S), or glutathione. Such modifications often are instrumental to important cellular signaling processes, which ultimately result in modification of physiology of the organism. Therefore, there is a need to be able to identify such modifications. There are a variety of techniques to find proteins which may be altered in this way but here the focus is on two approaches: firstly, the use of fluorescent thiol derivatives and the subsequent use of mass spectrometry to identify the thiols involved; secondly the confirmation of such changes using biochemical assays and genetic mutants. The discussion will be based on the use of two model organisms: firstly the plant Arabidopsis thaliana (both as cell cultures and whole plants) and secondly the nematode worm Caenorhabditis elegans. However, these tools, as described, may be used in a much wider range of biological systems, including human and human tissue cultures.

Hydrogen Sulfide: A Therapeutic Option in Systemic Sclerosis

Systemic sclerosis (SSc) is a lethal disease that is characterized by auto-immunity, vascular injury, and progressive fibrosis of multiple organ systems. Despite the fact that the exact etiology of SSc remains unknown, oxidative stress has been associated with a large range of SSc-related complications. In addition to the well-known detrimental properties of reactive oxygen species (ROS), gasotransmitters (e.g., nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H₂S)) are also thought to play an important role in SSc. Accordingly, the diverse physiologic actions of NO and CO and their role in SSc have been previously studied. Recently, multiple studies have also shown the importance of the third gasotransmitter H₂S in both vascular physiology and pathophysiology. Interestingly, homocysteine (which is converted into H₂S through the transsulfuration pathway) is often found to be elevated in SSc patients; suggesting defects in the transsulfuration pathway. Hydrogen sulfide, which is known to have several effects, including a strong antioxidant and vasodilator effect, could potentially play a prominent role in the initiation and progression of vasculopathy. A better understanding of the actions of gasotransmitters, like H₂S, in the development of SSc-related vasculopathy, could help to create early interventions to attenuate the disease course. This paper will review the role of H₂S in vascular (patho-)physiology and potential disturbances in SSc. Moreover, current data from experimental animal studies will be reviewed. Lastly, we will evaluate potential interventional strategies.

Hydrogen Sulfide-Mediated Activation of O-Acetylserine (Thiol) Lyase and l/d-Cysteine Desulfhydrase Enhance Dehydration Tolerance in Eruca sativa Mill

Hydrogen sulfide (H₂S) has emerged as an important signaling molecule and plays a significant role during different environmental stresses in plants. The present work was carried out to explore the potential role of H₂S in reversal of dehydration stress-inhibited O-acetylserine (thiol) lyase (OAS-TL), l-cysteine desulfhydrase (LCD), and d-cysteine desulfhydrase (DCD) response in arugula (Eruca sativa Mill.) plants. Dehydration-stressed plants exhibited reduced water status and increased levels of hydrogen peroxide (H₂O₂) and superoxide (O₂•-) content that increased membrane permeability and lipid peroxidation, and caused a reduction in chlorophyll content. However, H₂S donor sodium hydrosulfide (NaHS), at the rate of 2 mM, substantially reduced oxidative stress (lower H₂O₂ and O₂•-) by upregulating activities of antioxidant enzymes (superoxide dismutase, peroxidase, and catalase) and increasing accumulation of osmolytes viz. proline and glycine betaine (GB). All these, together, resulted in reduced membrane permeability, lipid peroxidation, water loss, and improved hydration level of plants. The beneficial role of H₂S in the tolerance of plants to dehydration stress was traced with H₂S-mediated activation of carbonic anhydrase activity and enzyme involved in the biosynthesis of cysteine (Cys), such as OAS-TL. H₂S-treated plants showed maximum Cys content. The exogenous application of H₂S also induced the activity of LCD and DCD enzymes that assisted the plants to synthesize more H₂S from accumulated Cys. Therefore, an adequate concentration of H₂S was maintained, that improved the efficiency of plants to mitigate dehydration stress-induced alterations. The central role of H₂S in the reversal of dehydration stress-induced damage was evident with the use of the H₂S scavenger, hypotaurine.

Abiotic Stresses: General Defenses of Land Plants and Chances for Engineering Multistress Tolerance

Abiotic stresses, such as low or high temperature, deficient or excessive water, high salinity, heavy metals, and ultraviolet radiation, are hostile to plant growth and development, leading to great crop yield penalty worldwide. It is getting imperative to equip crops with multistress tolerance to relieve the pressure of environmental changes and to meet the demand of population growth, as different abiotic stresses usually arise together in the field. The feasibility is raised as land plants actually have established more generalized defenses against abiotic stresses, including the cuticle outside plants, together with unsaturated fatty acids, reactive species scavengers, molecular chaperones, and compatible solutes inside cells. In stress response, they are orchestrated by a complex regulatory network involving upstream signaling molecules including stress hormones, reactive oxygen species, gasotransmitters, polyamines, phytochromes, and calcium, as well as downstream gene regulation factors, particularly transcription factors. In this review, we aimed at presenting an overview of these defensive systems and the regulatory network, with an eye to their practical potential via genetic engineering and/or exogenous application.

Hydrogen sulphide trapeze: Environmental stress amelioration and phytohormone crosstalk

Hydrogen sulphide (H₂S) is recognized as the third endogenous gasotransmitter in plants after nitric oxide (NO) and carbon monoxide (CO). Though initially visualized as a toxic gaseous molecule, recent studies have illustrated its diverse role in regulating plant growth and developmental physiology. H₂S is also a potent inducer of osmolytes and cellular antioxidants of enzymatic and non-enzymatic origins. It interacts with the Ca²⁺ and NO signaling pathways. Exogenous fumigation of H₂S or application of the H₂S donor, sodium hydrosulphide (NaHS) has been found to be beneficial in the amelioration of multiple abiotic stresses like salinity, drought, temperature, hypoxia and heavy metal toxicity. H₂S also protects stress-sensitive proteins via persulphidation of cysteine residues, prone to reactive oxygen species (ROS)-mediated oxidation. It is well established that plants are highly dependent on phytohormone signaling during any physiological process. By virtue of the diversity of the H₂S-mediated signaling network, interactions and crosstalks of this gasotransmitter with the plant hormones are evident. This article presents a detailed summary regarding the role of H₂S in oxidative and environmental stress tolerance; and furthermore illustrates the reported interactions with crucial hormones like abscisic acid, auxins, gibberellic acid, ethylene and salicylic acid under physiologically differing circumstances.

Hydrogen Sulfide: A Potent Tool in Postharvest Fruit Biology and Possible Mechanism of Action

Hydrogen sulfide (H2S), an endogenous gaseous molecule, is considered as a signaling agent, in parallel with other low molecular weight reactive substances, mainly hydrogen peroxide (H2O2) and nitric oxide (NO), in various plant systems. New studies are now revealing that the postharvest application of H2S, through H2S donors such as sodium hydrosulfide (NaSH) or sodium sulfide (Na2S), can inhibit fruit ripening and senescence programs in numerous fruits. We discuss here current knowledge on the impact of H2S in postharvest physiology of several climacteric and non-climacteric fruits such as banana, apple, pear, kiwifruit, strawberry, mulberry fruit, and grape. Although there is still a considerable lack of studies establishing the mechanisms by which H2S signaling is linked to fruit metabolism, we highlight several candidate mechanisms, including a putative cross-talk between H2S and ethylene, reactive oxygen and nitrogen species, oxidative/nitrosative stress signaling, sulfate metabolism, and post-translational modification of protein cysteine residues (S-sulfhydration) as being functional in this H2S postharvest action. Understanding H2S metabolism and signaling during postharvest storage and the interplay with other key player molecules would therefore provide new, improved strategies for better fruit postharvest storage. To achieve this understanding, postharvest fruit physiology research will need to focus increasingly on the spatial interaction between H2S and ethylene perception as well as on the interplay between S-sulfhydration/desulfhydration and S-nitrosylation/denitrosylation under several postharvest conditions.

The central role of hydrogen sulfide in plant responses to toxic metal stress

With the increase of industrial wastes, sewage irrigation, chemical fertilizers and pesticides, metal contamination is increasingly serious. How to reduce the environmental risk has become a compelling problem in cultivated land. As a gaseous signal molecule, hydrogen sulfide (H₂S) is involved in multiple plant responses to toxic metal stress. Metal stress rapidly triggers endogenous H₂S production and exogenous H₂S alleviates metal toxicity in plants. To elucidate the role of H₂S in metal tolerance, the physiological and molecular mechanisms of H₂S in alleviating metal toxicity is necessary to be reviewed. Here, the latest progress on endogenous H₂S metabolism and the role of H₂S in plant responses to toxic metal stress were summarized and discussed. The mechanisms of exogenous H₂S in alleviating metal toxicity is proposed.

Dynamic mapping of spontaneously produced H2S in the entire cell space and in live animals using a rationally designed molecular switch

A rationally designed molecular switch created to detect and dynamically map spontaneous production of H₂S in whole cells and the organs of live zebrafish.

Functional analysis of the role of hydrogen sulfide in the regulation of dark-induced leaf senescence in Arabidopsis

There is growing evidence that hydrogen sulfide (H₂S) is involved in many physiological processes in plants, but the role of H₂S in dark-induced leaf senescence remains unknown. In this work, we found that H₂S not only inhibited chlorophyll degradation but also caused the accumulation of photoreactive pheide a in detached leaves under extended darkness. Despite this, transcript levels of senescence-associated genes (SAGs) were less affected in H₂S-treated detached leaves compared with those in H₂S-untreated detached leaves. Furthermore, cell death/rapid bleaching occurred in both H₂S-treated detached and attached leaves after transfer from extended darkness to light. Unlike the lack of effect of H₂S on SAG transcripts in darkened detached leaves, exogenous H₂S induced higher SAG transcript levels in attached leaves than untreated attached leaves. Genetic evidence further underlined the positive correlation between SAG expression in attached leaves and H₂S. In addition, effects of H₂S on SAG expression in attached leaves were compromised in the S-nitrosoglutathione reductase-deficient mutant, gsnor1. Taken together, our results suggest that H₂S suppresses chlorophyll degradation of detached leaves by regulating a dark-dependent reaction, and that this gas positively modulates SAG expression in attached leaves under prolonged darkness in a GSNOR1-dependent manner.

β-N-Oxalyl-l-α,β-diaminopropionic Acid (β-ODAP) Content in Lathyrus sativus: The Integration of Nitrogen and Sulfur Metabolism through β-Cyanoalanine Synthase

Grass pea (Lathyrus sativus L.) is an important legume crop grown mainly in South Asia and Sub-Saharan Africa. This underutilized legume can withstand harsh environmental conditions including drought and flooding. During drought-induced famines, this protein-rich legume serves as a food source for poor farmers when other crops fail under harsh environmental conditions; however, its use is limited because of the presence of an endogenous neurotoxic nonprotein amino acid β-N-oxalyl-l-α,β-diaminopropionic acid (β-ODAP). Long-term consumption of Lathyrus and β-ODAP is linked to lathyrism, which is a degenerative motor neuron syndrome. Pharmacological studies indicate that nutritional deficiencies in methionine and cysteine may aggravate the neurotoxicity of β-ODAP. The biosynthetic pathway leading to the production of β-ODAP is poorly understood, but is linked to sulfur metabolism. To date, only a limited number of studies have been conducted in grass pea on the sulfur assimilatory enzymes and how these enzymes regulate the biosynthesis of β-ODAP. Here, we review the current knowledge on the role of sulfur metabolism in grass pea and its contribution to β-ODAP biosynthesis. Unraveling the fundamental steps and regulation of β-ODAP biosynthesis in grass pea will be vital for the development of improved varieties of this underutilized legume.

Harnessing Evolutionary Toxins for Signaling: Reactive Oxygen Species, Nitric Oxide and Hydrogen Sulfide in Plant Cell Regulation

During the early periods of evolution, as well as in niche environments today, organisms have had to learn to tolerate the presence of many reactive compounds, such as reactive oxygen species, nitric oxide, and hydrogen sulfide. It is now known that such compounds are instrumental in the signaling processes in plant cells. There are enzymes which can make them, while downstream of their signaling pathways are coming to light. These include the production of cGMP, the activation of MAP kinases and transcription factors, and the modification of thiol groups on many proteins. However, organisms have also had to tolerate other reactive compounds such as ammonia, methane, and hydrogen gas, and these too are being found to have profound effects on signaling in cells. Before a holistic view of how such signaling works, the full effects and interactions of all such reactive compounds needs to be embraced. A full understanding will be beneficial to both agriculture and future therapeutic strategies.

Hydrogen Sulfide Protects Renal Grafts Against Prolonged Cold Ischemia-Reperfusion Injury via Specific Mitochondrial Actions

Ischemia-reperfusion injury is unavoidably caused by loss and subsequent restoration of blood flow during organ procurement, and prolonged ischemia-reperfusion injury IRI results in increased rates of delayed graft function and early graft loss. The endogenously produced gasotransmitter, hydrogen sulfide (H₂ S), is a novel molecule that mitigates hypoxic tissue injury. The current study investigates the protective mitochondrial effects of H₂ S during in vivo cold storage and subsequent renal transplantation (RTx) and in vitro cold hypoxic renal injury. Donor allografts from Brown Norway rats treated with University of Wisconsin (UW) solution + H₂ S (150 μM NaSH) during prolonged (24-h) cold (4°C) storage exhibited significantly (p < 0.05) decreased acute necrotic/apoptotic injury and significantly (p < 0.05) improved function and recipient Lewis rat survival compared to UW solution alone. Treatment of rat kidney epithelial cells (NRK-52E) with the mitochondrial-targeted H₂ S donor, AP39, during in vitro cold hypoxic injury improved the protective capacity of H₂ S >1000-fold compared to similar levels of the nonspecific H₂ S donor, GYY4137 and also improved syngraft function and survival following prolonged cold storage compared to UW solution. H₂ S treatment mitigates cold IRI-associated renal injury via mitochondrial actions and could represent a novel therapeutic strategy to minimize the detrimental clinical outcomes of prolonged cold IRI during RTx.

Nitric oxide-induced synthesis of hydrogen sulfide alleviates osmotic stress in wheat seedlings through sustaining antioxidant enzymes, osmolyte accumulation and cysteine homeostasis

Nitric oxide (NO) and hydrogen sulfide (H₂S) have been shown to act as signaling molecules in various physiological processes, play significant roles in plant cellular processes, and also mediate responses to both biotic and abiotic stresses in plants. The present investigation was carried out to test the effect of exogenous NO on endogenous synthesis of H₂S in osmotic-stressed wheat (Triticum aestivum L.) seedlings. The results show that application of NO to wheat seedlings, suffered from PEG8000-induced osmotic stress, considerably enhanced the activities of H₂S-synthesizing enzymes l-cysteine desulfhydrase (LCD) and d-cysteine desulfhydrase (DCD) leading to enhanced level of endogenous H₂S content. At the same time exogenous NO also enhanced the activity of cysteine (Cys)-synthesizing enzyme O-acetylserine(thiol)lyase (OAS-TL) and maintained Cys homeostasis under osmotic stress. NO and H₂S together markedly improved the activities of antioxidant enzymes viz. ascorbate peroxidase (APX), glutathione reductase (GR), peroxidase (POX), superoxide dismutase (SOD) and catalase (CAT). Furthermore, NO and H₂S caused additional accumulation of osmolytes proline (Pro) and glycine betaine (GB), all these collectively resulted in the protection of plants against osmotic stress-induced oxidative stress. On the other hand, NO scavenger cPTIO [2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide] and H₂S scavenger HT (hypotaurine) invalidated the effect of NO on endogenous H₂S levels and Cys homeostasis which resulted in weak protection against osmotic stress. Application of N-ethylmaleimide (NEM) suppressed GR activity and caused an increase in oxidative stress. We concluded that NO in association with endogenous H₂S activates the defense system to the level required to counter osmotic stress and maintains normal functioning of cellular machinery.

Thiolysis of NBD-based dyes for colorimetric and fluorescence detection of H2S and biothiols: design and biological applications

H₂S-specific fluorescent/colorimetric probes based on the thiolysis of NBD dyes are summarized.

Cadmium Disrupts the Balance between Hydrogen Peroxide and Superoxide Radical by Regulating Endogenous Hydrogen Sulfide in the Root Tip of Brassica rapa

Cd (cadmium) stress always alters the homeostasis of ROS (reactive oxygen species) including H₂O₂ (hydrogen sulfide) and [Formula: see text] (superoxide radical), leading to the oxidative injury and growth inhibition in plants. In addition to triggering oxidative injury, ROS has been suggested as important regulators modulating root elongation. However, whether and how Cd stress induces the inhibition of root elongation by differentially regulating endogenous H₂O₂ and [Formula: see text], rather than by inducing oxidative injury, remains elusive. To address these gaps, histochemical, physiological, and biochemical approaches were applied to investigate the mechanism for Cd to fine-tune the balance between H₂O₂ and [Formula: see text] in the root tip of Brassica rapa. Treatment with Cd at 4 and 16 μM significantly inhibited root elongation, while only 16 μM but not 4 μM of Cd induced oxidative injury and cell death in root tip. Fluorescent and pharmaceutical tests suggested that H₂O₂ and [Formula: see text] played negative and positive roles, respectively, in the regulation of root elongation in the presence of Cd (4 μM) or not. Treatment with Cd at 4 μM led to the increase in H₂O₂ and the decrease in [Formula: see text] in root tip, which may be attributed to the up-regulation of Br_UPB1s and the down-regulation of their predicted targets (four peroxidase genes). Cd at 4 μM resulted in the increase in endogenous H₂S in root tip by inducing the up-regulation of LCDs and DCDs. Treatment with H₂S biosynthesis inhibitor or H₂S scavenger significantly blocked Cd (4 μM)-induced increase in endogenous H₂S level, coinciding with the recovery of root elongation, the altered balance between H₂O₂ and [Formula: see text], and the expression of Br_UPB1s and two peroxidase genes. Taken together, it can be proposed that endogenous H₂S mediated the phytotoxicity of Cd at low concentration by regulating Br_UPB1s-modulated balance between H₂O₂ and [Formula: see text] in root tip. Such findings shed new light on the regulatory role of endogenous H₂S in plant adaptions to Cd stress.

Hydrogen sulfide enhances nitric oxide-induced tolerance of hypoxia in maize (Zea mays L.)

Our data present H ₂ S in a new role, serving as a multi-faceted transducer to different response mechanisms during NO-induced acquisition of tolerance to flooding-induced hypoxia in maize seedling roots. Nitric oxide (NO), serving as a secondary messenger, modulates physiological processes in plants. Recently, hydrogen sulfide (H₂S) has been demonstrated to have similar signaling functions. This study focused on the effects of treatment with H₂S on NO-induced hypoxia tolerance in maize seedlings. The results showed that treatment with the NO donor sodium nitroprusside (SNP) enhanced survival rate of submerged maize roots through induced accumulation of endogenous H₂S. The induced H₂S then enhanced endogenous Ca²⁺ levels as well as the Ca²⁺-dependent activity of alcohol dehydrogenase (ADH), improving the capacity for antioxidant defense and, ultimately, the hypoxia tolerance in maize seedlings. In addition, NO induced the activities of key enzymes in H₂S biosynthesis, such as L-cysteine desulfhydrases (L-CDs), O-acetyl-L-serine (thiol)lyase (OAS-TL), and β-Cyanoalanine Synthase (CAS). SNP-induced hypoxia tolerance was enhanced by the application of NaHS, but was eliminated by the H₂S-synthesis inhibitor hydroxylamine (HA) and the H₂S-scavenger hypotaurine (HT). H₂S concurrently enhanced the transcriptional levels of relative hypoxia-induced genes. Together, our findings indicated that H₂S serves as a multi-faceted transducer that enhances the nitric oxide-induced hypoxia tolerance in maize (Zea mays L.).

The β-cyanoalanine synthase pathway: beyond cyanide detoxification

Production of cyanide through biological and environmental processes requires the detoxification of this metabolic poison. In the 1960s, discovery of the β-cyanoalanine synthase (β-CAS) pathway in cyanogenic plants provided the first insight on cyanide detoxification in nature. Fifty years of investigations firmly established the protective role of the β-CAS pathway in cyanogenic plants and its role in the removal of cyanide produced from ethylene synthesis in plants, but also revealed the importance of this pathway for plant growth and development and the integration of nitrogen and sulfur metabolism. This review describes the β-CAS pathway, its distribution across and within higher plants, and the diverse biological functions of the pathway in cyanide assimilation, plant growth and development, stress tolerance, regulation of cyanide and sulfide signalling, and nitrogen and sulfur metabolism. The collective roles of the β-CAS pathway highlight its potential evolutionary and ecological importance in plants.

Emerging role of hydrogen sulfide-microRNA crosstalk in cardiovascular diseases

Despite an obnoxious smell and toxicity at a high dose, hydrogen sulfide (H2S) is emerging as a cardioprotective gasotransmitter. H2S mitigates pathological cardiac remodeling by regulating several cellular processes including fibrosis, hypertrophy, apoptosis, and inflammation. These encouraging findings in rodents led to initiation of a clinical trial using a H2S donor in heart failure patients. However, the underlying molecular mechanisms by which H2S mitigates cardiac remodeling are not completely understood. Empirical evidence suggest that H2S may regulate signaling pathways either by directly influencing a gene in the cascade or interacting with nitric oxide (another cardioprotective gasotransmitter) or both. Recent studies revealed that H2S may ameliorate cardiac dysfunction by up- or downregulating specific microRNAs. MicroRNAs are noncoding, conserved, regulatory RNAs that modulate gene expression mostly by translational inhibition and are emerging as a therapeutic target for cardiovascular disease (CVD). Few microRNAs also regulate H2S biosynthesis. The inter-regulation of microRNAs and H2S opens a new avenue for exploring the H2S-microRNA crosstalk in CVD. This review embodies regulatory mechanisms that maintain the physiological level of H2S, exogenous H2S donors used for increasing the tissue levels of H2S, H2S-mediated regulation of CVD, H2S-microRNAs crosstalk in relation to the pathophysiology of heart disease, clinical trials on H2S, and future perspectives for H2S as a therapeutic agent for heart failure.

Different Modes of Hydrogen Peroxide Action During Seed Germination

Hydrogen peroxide was initially recognized as a toxic molecule that causes damage at different levels of cell organization and thus losses in cell viability. From the 1990s, the role of hydrogen peroxide as a signaling molecule in plants has also been discussed. The beneficial role of H2O2 as a central hub integrating signaling network in response to biotic and abiotic stress and during developmental processes is now well established. Seed germination is the most pivotal phase of the plant life cycle, affecting plant growth and productivity. The function of hydrogen peroxide in seed germination and seed aging has been illustrated in numerous studies; however, the exact role of this molecule remains unknown. This review evaluates evidence that shows that H2O2 functions as a signaling molecule in seed physiology in accordance with the known biology and biochemistry of H2O2. The importance of crosstalk between hydrogen peroxide and a number of signaling molecules, including plant phytohormones such as abscisic acid, gibberellins, and ethylene, and reactive molecules such as nitric oxide and hydrogen sulfide acting on cell communication and signaling during seed germination, is highlighted. The current study also focuses on the detrimental effects of H2O2 on seed biology, i.e., seed aging that leads to a loss of germination efficiency. The dual nature of hydrogen peroxide as a toxic molecule on one hand and as a signal molecule on the other is made possible through the precise spatial and temporal control of its production and degradation. Levels of hydrogen peroxide in germinating seeds and young seedlings can be modulated via pre-sowing seed priming/conditioning. This rather simple method is shown to be a valuable tool for improving seed quality and for enhancing seed stress tolerance during post-priming germination. In this review, we outline how seed priming/conditioning affects the integrative role of hydrogen peroxide in seed germination and aging.

Hydrogen Sulfide: A Signal Molecule in Plant Cross-Adaptation

For a long time, hydrogen sulfide (H₂S) has been considered as merely a toxic by product of cell metabolism, but nowadays is emerging as a novel gaseous signal molecule, which participates in seed germination, plant growth and development, as well as the acquisition of stress tolerance including cross-adaptation in plants. Cross-adaptation, widely existing in nature, is the phenomenon in which plants expose to a moderate stress can induce the resistance to other stresses. The mechanism of cross-adaptation is involved in a complex signal network consisting of many second messengers such as Ca²⁺, abscisic acid, hydrogen peroxide and nitric oxide, as well as their crosstalk. The cross-adaptation signaling is commonly triggered by moderate environmental stress or exogenous application of signal molecules or their donors, which in turn induces cross-adaptation by enhancing antioxidant system activity, accumulating osmolytes, synthesizing heat shock proteins, as well as maintaining ion and nutrient balance. In this review, based on the current knowledge on H₂S and cross-adaptation in plant biology, H₂S homeostasis in plant cells under normal growth conditions; H₂S signaling triggered by abiotic stress; and H₂S-induced cross-adaptation to heavy metal, salt, drought, cold, heat, and flooding stress were summarized, and concluded that H₂S might be a candidate signal molecule in plant cross-adaptation. In addition, future research direction also has been proposed.

H2S: A Novel Gasotransmitter that Signals by Sulfhydration

Hydrogen sulfide (H2S) is a member of the growing family of gasotransmitters. Once regarded as a noxious molecule predominantly present in the atmosphere, H2S is now known to be synthesized endogenously in mammals. H2S participates in a myriad of physiological processes ranging from regulation of blood pressure to neuroprotection. Its chemical nature precludes H2S from being stored in vesicles and acting on receptor proteins in the fashion of other chemical messengers. Thus, novel cellular mechanisms have evolved to mediate its effects. This review focuses on sulfhydration (or persulfidation), which appears to be the principal post-translational modification elicited by H2S.

Endogenous and Exogenous Calcium Involved in the Betulin Production from Submerged Culture of Phellinus linteus Induced by Hydrogen Sulfide

Using pharmacological and biochemical approaches, Ca(2+) involved in the betulin production in mycelia of Phellinus linteus induced by hydrogen sulfide (H2S) were investigated. The results showed that 2 mM H2S donor NaHS or 10 mM CaCl2 was found to enhance the betulin content in the mycelia of Phellinus to the maximum, which were 112.43 and 93.24% higher than that in the control, respectively. Further, NaHS and CaCl2 co-treatment also showed positive outcome, which were 128.95 or 24.52% higher than that in the control or NaHS treatment. At the same time, NaHS also enhanced the content of Ca(2+) and CaM. But, the above positive inductive effects for Ca(2+), CaM, and betulin production can be blocked with either Ca(2+) channel blocker (LaCl3, 2-aminoethoxydiphenyl borate) or Ca(2+) chelator (ethylenediaminetetraacetic acid (EDTA)). Among of them, betulin content was reduced 35.06% by NaHS and EGTA to the minimum, and this reduction could be reversed by the application of CaCl2 (NaHS + EGTA + CaCl2). From above results, it can be concluded that endogenous and exogenous calcium involved in the betulin production from submerged culture of P. linteus induced by hydrogen sulfide.

Sulfurtransferase and thioredoxin specifically interact as demonstrated by bimolecular fluorescence complementation analysis and biochemical tests

Sulfurtransferases (Strs) and thioredoxins (Trxs) are members of large protein families. Trxs are disulfide reductases and play an important role in redox-related cellular processes. They interact with a broad range of proteins. Strs catalyze the transfer of a sulfur atom from a suitable sulfur donor to nucleophilic sulfur acceptors in vitro, but the physiological roles of these enzymes are not well defined. Several studies in different organisms demonstrate protein-protein interactions of Strs with members of the Trx family. We are interested in investigating the specificity of the interaction between Str and Trx isoforms. In order to use the bimolecular fluorescence complementation (BiFC), several Str and Trx sequences from Arabidopsis thaliana were cloned into the pUC-SPYNE and pUC-SPYCE split-YFP vectors, respectively. Each couple of plasmids containing the sequences for the putative interaction partners were transformed into Arabidopsis protoplasts and screened using a confocal laser scanning microscope. Compartment- and partner-specific interactions could be observed in transformed protoplasts. Replacement of cysteine residues in the redox-active site of Trxs abolished the interaction signal. Therefore, the redox site is not only involved in the redox reaction but also responsible for the interaction with partner proteins. Biochemical assays support a specific interaction among Strs and certain Trxs. Based on the results obtained, the interaction of Strs and Trxs indicates a role of Strs in the maintenance of the cellular redox homeostasis.

Hydrogen sulfide regulates abiotic stress tolerance and biotic stress resistance in Arabidopsis

Hydrogen sulfide (H2S) is an important gaseous molecule in various plant developmental processes and plant stress responses. In this study, the transgenic Arabidopsis thaliana plants with modulated expressions of two cysteine desulfhydrases, and exogenous H2S donor (sodium hydrosulfide, NaHS) and H2S scavenger (hypotaurine, HT) pre-treated plants were used to dissect the involvement of H2S in plant stress responses. The cysteine desulfhydrases overexpressing plants and NaHS pre-treated plants exhibited higher endogenous H2S level and improved abiotic stress tolerance and biotic stress resistance, while cysteine desulfhydrases knockdown plants and HT pre-treated plants displayed lower endogenous H2S level and decreased stress resistance. Moreover, H2S upregulated the transcripts of multiple abiotic and biotic stress-related genes, and inhibited reactive oxygen species (ROS) accumulation. Interestingly, MIR393-mediated auxin signaling including MIR393a/b and their target genes (TIR1, AFB1, AFB2, and AFB3) was transcriptionally regulated by H2S, and was related with H2S-induced antibacterial resistance. Moreover, H2S regulated 50 carbon metabolites including amino acids, organic acids, sugars, sugar alcohols, and aromatic amines. Taken together, these results indicated that cysteine desulfhydrase and H2S conferred abiotic stress tolerance and biotic stress resistance, via affecting the stress-related gene expressions, ROS metabolism, metabolic homeostasis, and MIR393-targeted auxin receptors.

Hydrogen sulfide alleviates toxic effects of arsenate in pea seedlings through up-regulation of the ascorbate-glutathione cycle: Possible involvement of nitric oxide

In plants, hydrogen sulfide (H2S) is an emerging novel signaling molecule that is involved in growth regulation and abiotic stress responses. However, little is known about its role in the regulation of arsenate (As(V)) toxicity. Therefore, hydroponic experiments were conducted to investigate whether sodium hydrosulfide (NaHS; a source of H2S) is involved in the regulation of As(V) toxicity in pea seedlings. Results showed that As(V) caused decreases in growth, photosynthesis (measured as chlorophyll fluorescence) and nitrogen content, which was accompanied by the accumulation of As. As(V) treatment also reduced the activities of cysteine desulfhydrase and nitrate reductase, and contents of H2S and nitric oxide (NO). However, addition of NaHS ameliorated As(V) toxicity in pea seedlings, which coincided with the increased contents of H2S and NO. The cysteine level was higher under As(V) treatment in comparison to all other treatments (As-free; NaHS; As(V)+NaHS). The content of reactive oxygen species (ROS) and damage to lipids, proteins and membranes increased by As(V) while NaHS alleviated these effects. Enzymes of the ascorbate-glutathione cycle (AsA-GSH cycle) showed inhibition of their activities following As(V) treatment while their activities were increased by application of NaHS. The redox status of ascorbate and glutathione was disturbed by As(V) as indicated by a steep decline in their reduced/oxidized ratios. However, simultaneous NaHS application restored the redox status of the ascorbate and glutathione pools. The results of this study demonstrated that H2S and NO might both be involved in reducing the accumulation of As and triggering up-regulation of the AsA-GSH cycle to counterbalance ROS-mediated damage to macromolecules. Furthermore, the results suggest a crucial role of H2S in plant priming, and in particular for pea seedlings in mitigating As(V) stress.

Cytosolic thiol switches regulating basic cellular functions: GAPDH as an information hub?

Cytosolic glyceraldehyde 3-phosphate dehydrogenase (GAPDH, E.C. 1.2.1.12) is present in all organisms and catalyzes the oxidation of triose phosphate during glycolysis. GAPDH is one of the most prominent cellular targets of oxidative modifications when reactive oxygen and nitrogen species are formed during metabolism and under stress conditions. GAPDH harbors a strictly conserved catalytic cysteine, which is susceptible to a variety of thiol modifications, including S-sulfenylation, S-glutathionylation, S-nitrosylation, and S-sulfhydration. Upon reversible oxidative thiol modification of GAPDH, glycolysis is inhibited leading to a diversion of metabolic flux through the pentose-phosphate cycle to increase NADPH production. Furthermore, oxidized GAPDH may adopt new functions in different cellular compartments including the nucleus, as well as in new microcompartments associated with the cytoskeleton, mitochondria and plasma membrane. This review focuses on the recently discovered mechanism underlying the eminent reactivity between GAPDH and hydrogen peroxide and the subsequent redox-dependent moonlighting functions discriminating between the induction either of adaptive responses and adjustment of metabolism or of cell death in yeast, plants, and mammals. In light of the summarized results, cytosolic GAPDH might function as a sensor for redox signals and an information hub to transduce these signals for appropriate responses.

Hydrogen sulfide acts as a downstream signal molecule in salicylic acid-induced heat tolerance in maize (Zea mays L.) seedlings

Salicylic acid (SA), 2-hydroxy benzoic acid, is a small phenolic compound with multifunction that is involved in plant growth, development, and the acquisition of stress tolerance. In recent years, hydrogen sulfide (H2S) has been found to have similar functions, but cross talk between SA and H2S in the acquisition of heat tolerance is not clear. In this study, pretreatment of maize seedlings with SA improved the survival percentage of seedlings under heat stress, indicating that SA pretreatment could improve the heat tolerance of maize seedlings. In addition, treatment with SA enhanced the activity of L-cysteine desulfhydrase (L-DES), a key enzyme in H2S biosynthesis, which in turn induced accumulation of endogenous H2S. Interestingly, SA-induced heat tolerance was enhanced by addition of NaHS, a H2S donor, but weakened by specific inhibitors of H2S biosynthesis DL-propargylglycine (PAG) and its scavenger hydroxylamine (HT). Furthermore, pretreatment with paclobutrazol (PAC) and 2-aminoindan-2-phosphonic acid (AIP), inhibitors of SA biosynthesis, had no significant effect on NaHS-induced heat tolerance of maize seedlings. Similarly, significant change in the activities of phenylalanine ammonia lyase (PAL) and benzoic-acid-2-hydroxylase (BA2H), the key enzymes in SA biosynthesis, and the content of endogenous SA, was not observed in maize seedlings by NaHS treatment. All of the above-mentioned results suggest that SA pretreatment could improve the heat tolerance of maize seedlings, and H2S might be a novel downstream signal molecule in SA-induced heat tolerance.

Comparative proteomic analysis reveals the role of hydrogen sulfide in the adaptation of the alpine plant Lamiophlomis rotata to altitude gradient in the Northern Tibetan Plateau

We found the novel role of hydrogen sulfide in the adaptation of the alpine plant to altitude gradient in the Northern Tibetan Plateau. Alpine plants have developed strategies to survive the extremely cold conditions prevailing at high altitudes; however, the mechanism underlying the evolution of these strategies remains unknown. Hydrogen sulfide (H2S) is an essential messenger that enhances plant tolerance to environmental stress; however, its role in alpine plant adaptation to environmental stress has not been reported until now. In this work, we conducted a comparative proteomics analysis to investigate the dynamic patterns of protein expression in Lamiophlomis rotata plants grown at three different altitudes. We identified and annotated 83 differentially expressed proteins. We found that the levels and enzyme activities of proteins involved in H2S biosynthesis markedly increased at higher altitudes, and that H2S accumulation increased. Exogenous H2S application increased antioxidant enzyme activity, which reduced ROS (reactive oxygen species) damage, and GSNOR (S-nitrosoglutathione reductase) activity, which reduced RNS (reactive nitrogen species) damage, and activated the downstream defense response, resulting in protein degradation and proline and sugar accumulation. However, such defense responses could be reversed by applying H2S biosynthesis inhibitors. Based on these findings, we conclude that L. rotata uses multiple strategies to adapt to the alpine stress environment and that H2S plays a central role during this process.

Trapping hydrogen sulfide (H₂S) with diselenides: the application in the design of fluorescent probes

Here we report a unique reaction between phenyl diselenide-ester substrates and H₂S to form 1,2-benzothiaselenol-3-one. This reaction proceeded rapidly under mild conditions. Thiols could also react with the diselenide substrates. However, the resulted S–Se intermediate retained high reactivity toward H₂S and eventually led to the same cyclized product 1,2-benzothiaselenol-3-one. Based on this reaction two fluorescent probes were developed and showed high selectivity and sensitivity for H₂S. The presence of thiols was found not to interfere with the detection process.