Alleviation of Cr(VI)-induced oxidative stress in maize (Zea mays L.) seedlings by NO and H2S donors through differential organ-dependent regulation of ROS and NADPH-recycling metabolisms

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.

NADP-Dependent Malic Enzyme Genes in Sweet Pepper Fruits: Involvement in Ripening and Modulation by Nitric Oxide (NO)

NADPH is an indispensable cofactor in a wide range of physiological processes that is generated by a family of NADPH dehydrogenases, of which the NADP-dependent malic enzyme (NADP-ME) is a member. Pepper (Capsicum annuum L.) fruit is a horticultural product consumed worldwide that has great nutritional and economic relevance. Besides the phenotypical changes that pepper fruit undergoes during ripening, there are many associated modifications at transcriptomic, proteome, biochemical and metabolic levels. Nitric oxide (NO) is a recognized signal molecule with regulatory functions in diverse plant processes. To our knowledge, there is very scarce information about the number of genes encoding for NADP-ME in pepper plants and their expression during the ripening of sweet pepper fruit. Using a data mining approach to evaluate the pepper plant genome and fruit transcriptome (RNA-seq), five NADP-ME genes were identified, and four of them, namely CaNADP-ME2 to CaNADP-ME5, were expressed in fruit. The time course expression analysis of these genes during different fruit ripening stages, including green immature (G), breaking point (BP) and red ripe (R), showed that they were differentially modulated. Thus, while CaNADP-ME3 and CaNADP-ME5 were upregulated, CaNADP-ME2 and CaNADP-ME4 were downregulated. Exogenous NO treatment of fruit triggered the downregulation of CaNADP-ME4. We obtained a 50-75% ammonium-sulfate-enriched protein fraction containing CaNADP-ME enzyme activity, and this was assayed via non-denaturing polyacrylamide gel electrophoresis (PAGE). The results allow us to identify four isozymes designated from CaNADP-ME I to CaNADP-ME IV. Taken together, the data provide new pieces of information on the CaNADP-ME system with the identification of five CaNADP-ME genes and how the four genes expressed in pepper fruits are modulated during ripening and exogenous NO gas treatment.

Nitric Oxide and Strigolactone Alleviate Mercury-Induced Oxidative Stress in Lens culinaris L. by Modulating Glyoxalase and Antioxidant Defense System

Developmental activities have escalated mercury (Hg) content in the environment and caused food security problems. The present investigation describes mercury-incited stress in Lens culinaris (lentil) and its mitigation by supplementation of sodium nitroprusside (SNP) and strigolactone (GR24). Lentil exposure to Hg decreased root and shoot length, relative water content and biochemical variables. Exogenous application of SNP and GR24 alone or in combination enhanced all of the aforementioned growth parameters. Hg treatment increased electrolyte leakage and malondialdehyde content, but this significantly decreased with combined application (Hg + SNP + GR24). SNP and GR24 boosted mineral uptake and reduced Hg accumulation, thus minimizing the adverse impacts of Hg. An increase in mineral accretion was recorded in lentil roots and shoots in the presence of SNP and GR24, which might support the growth of lentil plants under Hg stress. Hg accumulation was decreased in lentil roots and shoots by supplementation of SNP and GR24. The methylglyoxal level was reduced in lentil plants with increase in glyoxalase enzymes. Antioxidant and glyoxylase enzyme activities were increased by the presence of SNP and GR24. Therefore, synergistic application of nitric oxide and strigolactone protected lentil plants against Hg-incited oxidative pressure by boosting antioxidant defense and the glyoxalase system, which assisted in biochemical processes regulation.

Soybean (Glycine max L.) Lipoxygenase 1 (LOX 1) Is Modulated by Nitric Oxide and Hydrogen Sulfide: An In Vitro Approach

Hydrogen sulfide (H₂S) and nitric oxide (NO) are two relevant signal molecules that can affect protein function throughout post-translational modifications (PTMs) such as persulfidation, S-nitrosation, metal-nitrosylation, and nitration. Lipoxygenases (LOXs) are a group of non-heme iron enzymes involved in a wide range of plant physiological functions including seed germination, plant growth and development, and fruit ripening and senescence. Likewise, LOXs are also involved in the mechanisms of response to diverse environmental stresses. Using purified soybean (Glycine max L.) lipoxygenase type 1 (LOX 1) and nitrosocysteine (CysNO) and sodium hydrosulfide (NaHS) as NO and H₂S donors, respectively, the present study reveals that both compounds negatively affect LOX activity, suggesting that S-nitrosation and persulfidation are involved. Mass spectrometric analysis of nitrated soybean LOX 1 using a peroxynitrite (ONOO^-) donor enabled us to identify that, among the thirty-five tyrosine residues present in this enzyme, only Y214 was exclusively nitrated by ONOO^-. The nitration of Y214 seems to affect its interaction with W500, a residue involved in the substrate binding site. The analysis of the structure 3PZW demonstrates the existence of several tunnels that directly communicate the surface of the protein with different internal cysteines, thus making feasible their potential persulfidation, especially C429 and C127. On the other hand, the CysNO molecule, which is hydrophilic and bulkier than H₂S, can somehow be accommodated throughout the tunnel until it reaches C127, thus facilitating its nitrosation. Overall, a large number of potential persulfidation targets and the ease by which H₂S can reach them through the diffuse tunneling network could be behind their efficient inhibition.

Rootstock increases the physiological defence of tomato plants against Pseudomonas syringae pv. tomato infection

Climate change has intensified the infection of tomato plants by pathogens such as Pseudomonas syringae pv. tomato (Pst). Rootstocks may increase plant tolerance to leaf phytopathogens. The aim of this study was to evaluate the effects of the tolerant Poncho Negro (R) tomato rootstock on physiological defence and the role of hydrogen sulfide (H2S) in susceptible Limachino (L) tomato plant responses to Pst attack. Ungrafted (L), self-grafted (L/L), and grafted (L/R) plants were infected with Pst. Rootstock increased the concentration of antioxidant compounds including ascorbate in the scion. Tolerant rootstock induced an increase of H2S in the scion, which correlated with enhanced expression of the SlAPX2 gene. A high accumulation of salicylic acid was observed in Pst-inoculated grafted L/L and L/R plants, but this was higher in L/R plants. The increase of H2S during Pst infection was associated with a reduction of ethylene in L/R plants. Our study indicates that the Poncho Negro rootstock reduced the symptoms of bacterial speck disease in the Limachino tomato plants, conferring tolerance to Pst infection. This study provides new knowledge about the impact of rootstock in the defence of tomato plants against leaf pathogens that could be used in sustainable management of tomato cultivation.

Nitric oxide and brassinosteroids enhance chromium stress tolerance in Glycine max L. (Merr.) by modulating antioxidative defense and glyoxalase systems

Chromium (Cr) contamination of agricultural soils is a major threat to human and plant health worldwide and causes reductions in plant growth and crop yields. 24-epibrassinolide (EBL) and nitric oxide (NO) have been shown to ameliorate the reductions in growth caused by the stresses induced by heavy metals; however, the interactions between EBL and NO on the alleviation of Cr-induced phytotoxicity have been poorly studied. Hence, this study was undertaken to examine any beneficial effects of EBL (0.01 µM) and NO (100 µM), applied alone or in combination, on the mitigation of stress induced by Cr (100 µM) in soybean seedlings. Although EBL and NO applied alone reduced the toxic effects of Cr, the combined treatment had the greatest effect. Mitigation of Cr intoxication occurred via reduced Cr uptake and translocation and by ameliorating reductions in water contents, light-harvesting pigments, and other photosynthetic parameters. In addition, the two hormones increased the activity of enzymatic and non-enzymatic defense mechanisms increasing the scavenging of reactive oxygen species, thereby reducing membrane damage and electrolyte leakage. Furthermore, the hormones reduced the accumulation of the toxic compound, methylglyoxal, by amplifying activities of glyoxalase I and glyoxalase II. Thus, applications of NO and EBL can significantly mitigate Cr-phytotoxicity when cultivating soybean plants in Cr-contaminated soils. However, further more-in depth studies including field investigations parallel with calculations of cost to profit ratios and yield losses are requested to validate the effectiveness of NO and/or EBL for remediation agents in Cr-contaminated soils with using key biomarkers (i.e., oxidative stress, antioxidant defense, and osmoprotectants) involved in the uptake, accumulation, and attenuation of Cr toxicity tested in our study.

Regulation of chromium translocation to shoot and physiological, metabolomic, and ionomic adjustments confer chromium stress tolerance in the halophyte Suaeda maritima

Chromium (Cr) is a highly toxic element adversely affecting the environment, cultivable lands, and human populations. The present study investigated the effects of Cr (VI) (100-400 μM) on plant morphology and growth, photosynthetic pigments, organic osmolytes, ionomics, and metabolomic dynamics of the halophyte Suaeda maritima to decipher the Cr tolerance mechanisms. Cr exposure reduced the growth and biomass in S. maritima. The photosynthetic pigments content significantly declined at higher Cr concentrations (400 μM). However, at lower Cr concentrations (100-300 μM), the photosynthetic pigments remained unaffected or increased. The results suggest that a high concentration of Cr exposure might have adverse effects on PS II in S. maritima. The enhanced uptake of Na⁺ in S. maritima imposed to Cr stress indicates that Na⁺ might have a pivotal role in osmotic adjustment, thereby maintaining water status under Cr stress. The proline content was significantly upregulated in Cr-treated plants suggesting its role in maintaining osmotic balance and scavenging ROS. The metabolomic analysis of control and 400 μM Cr treated plants led to the identification of 62 metabolites. The fold chain analysis indicated the upregulation of several metabolites, including phytohormones (SA and GA₃), polyphenols (cinnamic acid, sinapic acid, coumaric acid, vanillic acid, and syringic acid), and amino acids (alanine, leucine, proline, methionine, and cysteine) under Cr stress. The upregulation of these metabolites suggests the enhanced metal chelation and sequestration in vacuoles, reducing oxidative stress by scavenging ROS and promoting photosynthesis by maintaining the chloroplast membrane structure and photosynthetic pigments. Furthermore, in S. maritima, Cr tolerance index (T_i) was more than 60% in all the treatments, and Cr bio-concentration factor (BCF) and translocation factor (T_f) values were all greater than 1.0, which clearly indicates the Cr-hyperaccumulator characteristics of this halophyte.

H2 O2 , NO, and H2 S networks during root development and signalling under physiological and challenging environments: Beneficial or toxic?

Hydrogen peroxide (H₂ O₂ ) is a reactive oxygen species (ROS) and a key modulator of the development and architecture of the root system under physiological and adverse environmental conditions. Nitric oxide (NO) and hydrogen sulphide (H₂ S) also exert myriad functions on plant development and signalling. Accumulating pieces of evidence show that depending upon the dose and mode of applications, NO and H₂ S can have synergistic or antagonistic actions in mediating H₂ O₂ signalling during root development. Thus, H₂ O₂ -NO-H₂ S crosstalk might essentially impart tolerance to elude oxidative stress in roots. Growth and proliferation of root apex involve crucial orchestration of NO and H₂ S-mediated ROS signalling which also comprise other components including mitogen-activated protein kinase, cyclins, cyclin-dependent kinases, respiratory burst oxidase homolog (RBOH), and Ca²⁺ flux. This assessment provides a comprehensive update on the cooperative roles of NO and H₂ S in modulating H₂ O₂ homoeostasis during root development, abiotic stress tolerance, and root-microbe interaction. Furthermore, it also analyses the scopes of some fascinating future investigations associated with strigolactone and karrikins concerning H₂ O₂ -NO-H₂ S crosstalk in plant roots.

Transcriptome reveals the crucial role of exogenous hydrogen sulfide in alleviation of thiocyanate (SCN-) toxicity in rice seedlings

Application of exogenous hydrogen sulfide (H₂S) is a novel strategy for alleviation of the adverse effects caused by abiotic stresses. However, little is known about H₂S-mediated global molecular response of rice seedlings to thiocyanate (SCN^-) exposure. Herein, a hydroponic experiment was carried out to investigate the crucial role of exogenous H₂S in alleviation of SCN^- toxicity generated at different effective concentrations (EC₂₀: 24.0 mg SCN/L, EC₅₀: 96.0 mg SCN/L, and EC₇₅: 300.0 mg SCN/L) in rice seedlings through transcriptome analysis. The results showed that the total numbers of differentially expressed genes (DEGs, upregulated genes/downregulated genes) in rice roots were 755/313, 1114/3303, and 2184/7427, while they were 427/292, 2134/526, and 2378/890 in rice shoots at EC₂₀, EC₅₀, and EC₇₅ of SCN^-, respectively. When exogenous H₂S was supplied to rice seedlings exposed to SCN^-, the total number of DEGs (upregulated genes/downregulated genes) in rice roots was 1158/316, 1943/2959, and 1737/5392, while it was 2067/937, 2689/683, and 3492/1062 in rice shoots at EC₂₀, EC₅₀, and EC₇₅ of SCN^-, respectively. Upregulated DEGs in shoots were positively correlated with SCN^- concentration in the presence of exogenous H₂S, suggesting its crucial role in regulating the phytotoxicity of SCN^-. Gene function and pathway enrichment analyses showed that exogenous H₂S triggered "secondary metabolite synthesis," "metabolic pathways," and "signal transduction mechanisms" in rice seedlings corresponding to different effective concentrations of SCN^- exposure.

Regulation of enzymatic and non-enzymatic antioxidants in rice seedlings against chromium stress through sodium hydrosulfide and sodium nitroprusside

Hydrogen sulfide (H₂S) and nitric oxide (NO) play a well-organized protective mechanism in coping with oxidative stress induced by toxic metals. However, the comparative effects of H₂S and NO on enzymatic and non-enzymatic antioxidants in plants under Cr(III) stress have not been defined. In this study, we mathematically evaluate the importance of sodium hydrosulfide (NaHS) and sodium nitroprusside (SNP) on these two antioxidant systems in rice seedlings under Cr(III) stress. The results displayed that the optimal dose of NaHS and SNP was 100 μM and 75 μM, respectively, in rice tissues under Cr(III) stress, judging by the reactive oxygen species (ROS) levels in rice tissues. When rice seedlings were exposed to Cr(III) at concentrations of 3.57, 7.24, 26.52 mg Cr/l, Cr-induced ROS accumulation had a significant (p < 0.05) effect on the lipid peroxidation of cell membrane in rice tissues, and decreased the response of SOD, CAT, Pro, GSH, and AsA in rice tissues. Application of exogenous NaHS and SNP effectively (p < 0.05) alleviated the toxic effects of Cr(III) in rice seedlings by activating different antioxidants. A novel physiological response model suggested that the regulatory effect of NaHS and SNP on non-enzymatic antioxidants was stronger than that of enzymatic antioxidants. Moreover, NaHS-mediated response intensity of both enzymatic and non-enzymatic antioxidants was greater than that of SNP, suggesting the importance of utilizing NaHS over SNP as antioxidant regulators during detoxification of Cr(III) in rice seedlings.

Silicon Palliates Chromium Toxicity through the Formation of Root Hairs in Rice (Oryza sativa) Mediated by GSH and IAA

Along with the rapidly increasing environmental contamination by heavy metals, the exposure of plants to chromium has also magnified, resulting in a declined productivity. Hexavalent chromium [Cr(VI)], the most toxic form of Cr, brings about changes in plant processes at morpho-physiological and biochemical levels. However, silicon (Si) is known to mitigate the impact of abiotic stresses in plants. Here, we demonstrate Si-mediated alleviation of Cr(VI) toxicity and its effects on root hair formation in rice seedlings. Reduced glutathione (GSH) and indole-3 acetic acid (IAA, an important auxin) were assessed for their involvement in root hair formation after the application of Si to Cr(VI)-stressed plants, and our results confirmed their crucial significance in such developmental processes. The expression analysis of genes involved in GSH biosynthesis (OsGS2) and regeneration (OsGR1), and auxin biosynthesis (OsTAA1 and OsYUCCA1) and transport (OsAUX1 and OsPIN1) corroborated their positive role in Si-mediated root hair formation in Cr(VI)-stressed rice seedlings. Moreover, the results indicated that nitric oxide (NO) seems a probable but not fundamental component in Si-mediated formation of roots in rice during exposure to Cr(VI) stress. In this study, the indispensable role of GSH and IAA, redox homeostasis of GSH and IAA biosynthesis and transport are discussed with regard to Si-mediated formation of root hairs in rice under Cr(VI) stress. The results of the study suggest that Si is a protective agent against Cr(VI) stress in rice, and the findings can be used to develop Cr(VI) stress-tolerant varieties of rice with enhanced productivity.

Functions of NO and H2S Signal Molecules Against Plant Abiotic Stress

Nitric oxide (NO) and hydrogen sulfide (H₂S) are two recognized signal molecules in higher plants involved in a wide range of physiological processes and the mechanisms of response against adverse environmental conditions. These molecules can interact to provide an adequate response to palliate the negative impact exerted by stressful conditions, particularly by regulating key components of the metabolism of reactive oxygen species (ROS) to avoid their overproduction and further oxidative damage which, finally, affects cellular functioning. NO and H₂S can exert the regulation over the function of susceptible proteins by posttranslational modifications (PTMs) including nitration, S-nitrosation, and persulfidation but also through the regulation of gene expression by the induction of specific transcription factors which modulate the expression of genes encoding proteins related to stress resistance. This chapter encompasses a wide perspective of the signaling and functional relationships between NO and H₂S to modulate the overproduction of reactive oxygen species, particularly under abiotic stress conditions.

Plant responses to metals stress: microRNAs in focus

Metal toxicity can largely affect the growth and yield of numerous plant species. Plants have developed specific mechanisms to withstand the varying amounts of metals. One approach involves utilization of microRNAs (miRNAs) that are known for cleaving transcripts or inhibiting translation to mediate post-transcriptional control. Use of transcription factors (TFs) or gene regulation in metal detoxification largely depends on metal-responsive miRNAs. Moreover, systemic signals and physiological processes for plants response to metal toxicities are likewise controlled by miRNAs. Therefore, it is necessary to understand miRNAs and their regulatory networks in relation to metal stress. The miRNA-based approach can be important to produce metal-tolerant plant species. Here, we have reviewed the importance of plant miRNAs and their role in mitigating metal toxicities. The current review also discusses the specific advances that have occurred as a result of the identification and validation of several metal stress-responsive miRNAs.

Beneficial role of methyl jasmonate on morphological, physiological and phytochemical responses of Calendula officinalis L. under Chromium toxicity

Contamination of soil with chromium (Cr) is a rising problem in terms of agricultural sustainability and food safety. Here, the effects of methyl jasmonate (MJ; 0, 5, and 10 µM) on alleviating Cr stress (0, 100, and 200 µM) were surveyed in pot marigold (Calendula officinalis L.). The results showed that Cr stress significantly reduced photosynthetic pigments and leaf accumulation of total soluble sugars, total starch, and mineral nutrients and, consequently, lowered the height and biomass of pot marigold plants. Chromium toxicity also increased the leaf levels of oxidative stress markers and induced oxidative stress, which was associated with damage to bio-membranes and increased levels of malondialdehyde. However, MJ supplementation reduced the leaf accumulation of Cr, increased the content of photosynthetic pigments, and improved the performance of the photosynthetic machinery in Cr-stressed plants. MJ supplementation boosted the antioxidant defense system by upregulating antioxidant enzymes, glyoxalase enzymes, and the ascorbate-glutathione (AsA-GSH) pool redox, which significantly diminished Cr-induced oxidative stress. Hence, MJ supplementation might be a practicable approach for reducing Cr absorption and its negative impacts on pot marigold plants growing under Cr-contaminated conditions. Clinical trials registration Not applicable.

Crosstalk between H2 S and NO: an emerging signalling pathway during waterlogging stress in legume crops

In legumes, waterlogging is a major detrimental factor leading to huge yield losses. Generally, legumes lack tolerance to submergence, and conventional breeding to develop tolerant varieties are limited due to the lack of tolerant germplasm and potential target genes. Moreover, our understanding of the various signalling cascades, their interactions and key pathways induced during waterlogging is limited. Here, we focus on the role of two important plant signalling molecules, viz. hydrogen sulphide (H₂ S) and nitric oxide (NO), during waterlogging stress in legumes. Plants and soil microbes produce these signalling molecules both endogenously and exogenously under various stresses, including waterlogging. NO and H₂ S are known to regulate key physiological pathways, such as stomatal closure, leaf senescence and regulation of numerous stress signalling pathways, while NO plays a pivotal role in adventitious root formation during waterlogging. The crosstalk between H₂ S and NO is synergistic because of the resemblance of their physiological effects and proteomic functions, which mainly operate through cysteine-dependent post-translational modifications via S-nitrosation and persulfidation. Such knowledge has provided novel platforms for researchers to unravel the complexity associated with H₂ S-NO signalling and interactions with plant stress hormones. This review provides an overall summary on H₂ S and NO, including biosynthesis, biological importance, crosstalk, transporter regulation as well as understanding their role during waterlogging using 'multi-omics' approach. Understanding H₂ S and NO signalling will help in deciphering the metabolic interactions and identifying key regulatory genes that could be used for developing waterlogging tolerance in legumes.

Hydrogen sulfide: an emerging component against abiotic stress in plants

As a result of climate change, abiotic stresses are the most common cause of crop losses worldwide. Abiotic stresses significantly impair plants' physiological, biochemical, molecular and cellular mechanisms, limiting crop productivity under adverse climate conditions. However, plants can implement essential mechanisms against abiotic stressors to maintain their growth and persistence under such stressful environments. In nature, plants have developed several adaptations and defence mechanisms to mitigate abiotic stress. Moreover, recent research has revealed that signalling molecules like hydrogen sulfide (H₂ S) play a crucial role in mitigating the adverse effects of environmental stresses in plants by implementing several physiological and biochemical mechanisms. Mainly, H₂ S helps to implement antioxidant defence systems, and interacts with other molecules like nitric oxide (NO), reactive oxygen species (ROS), phytohormones, etc. These molecules are well-known as the key players that moderate the adverse effects of abiotic stresses. Currently, little progress has been made in understanding the molecular basis of the protective role of H₂ S; however, it is imperative to understand the molecular basis using the state-of-the-art CRISPR-Cas gene-editing tool. Subsequently, genetic engineering could provide a promising approach to unravelling the molecular basis of stress tolerance mediated by exogenous/endogenous H₂ S. Here, we review recent advances in understanding the beneficial roles of H₂ S in conferring multiple abiotic stress tolerance in plants. Further, we also discuss the interaction and crosstalk between H₂ S and other signal molecules; as well as highlighting some genetic engineering-based current and future directions.

Potassium (K+) Starvation-Induced Oxidative Stress Triggers a General Boost of Antioxidant and NADPH-Generating Systems in the Halophyte Cakile maritima

Potassium (K+) is an essential macro-element for plant growth and development given its implication in major processes such as photosynthesis, osmoregulation, protein synthesis, and enzyme function. Using 30-day-old Cakile maritima plants as halophyte model grown under K+ deprivation for 15 days, it was analyzed at the biochemical level to determine the metabolism of reactive oxygen species (ROS), key photorespiratory enzymes, and the main NADPH-generating systems. K+ starvation-induced oxidative stress was noticed by high malondialdehyde (MDA) content associated with an increase of superoxide radical (O2•−) in leaves from K+-deficient plants. K+ shortage led to an overall increase in the activity of hydroxypyruvate reductase (HPR) and glycolate oxidase (GOX), as well as of antioxidant enzymes catalase (CAT), those of the ascorbate-glutathione cycle, peroxidase (POX), and superoxide dismutase (SOD), and the main enzymes involved in the NADPH generation in both leaves and roots. Especially remarkable was the induction of up to seven CuZn-SOD isozymes in leaves due to K+ deficiency. As a whole, data show that the K+ starvation has associated oxidative stress that boosts a biochemical response leading to a general increase of the antioxidant and NADPH-generating systems that allow the survival of the halophyte Cakile maritima.

Nitric oxide donor, sodium nitroprusside modulates hydrogen sulfide metabolism and cysteine homeostasis to aid the alleviation of chromium toxicity in maize seedlings (Zea mays L.)

The current research aimed to assess the protective role of nitric oxide (NO) against chromium (Cr) toxicity in maize seedlings. Chromium (200 µM) lowered osmotic potential in epicotyls and mostly in radicles (by 38% and 63%, respectively) as compared to the control. Sodium nitroprusside (SNP, NO donor) restored seedling biomass (+90% for both organs) and water potential, whereas application of Nω-nitro-L-arginine methylester (_L-NAME, a NOS inhibitor) increased sensitivity to Cr. SNP suppressed Cr-triggered proline accumulation by inhibiting Δ¹-pyrroline-5-carboxylate synthetase activity and stimulating proline dehydrogenase activity, leading to glutamate over-accumulation (~30% for both organs). Cr stimulated cysteine metabolism and this was further enhanced by SNP which stimulated serine acetyl-transferase and O-acetylserine (thiol) lyase activities. This was followed by an increase in endogenous hydrogen sulfide (H₂S) generation by up-regulating L-cysteine desulfhydrase (+205%), D-cysteine desulfhydrase (+150%) and cyanoalanine synthase (+65%) activities in radicles compared to Cr-treatments plants. These positive effects were reduced in _L-NAME compared to control. Combined Cr+SNP affected the levels of compounds involved in glutathione metabolism (γ-glutamyl-cysteinyl, γ-glutamyl-cysteinyl-clycine, γ-cysteinyl-glycine, and glycine.). All together, our findings indicate that NO and elicited cellular H₂S act synergistically to alleviate Cr stress in maize seedlings by influencing a metabolic interplay between cysteine, proline, and glutathione.

Hydrogen Sulfide in Plants: Crosstalk with Other Signal Molecules in Response to Abiotic Stresses

Hydrogen sulfide (H₂S) has recently been considered as a crucial gaseous transmitter occupying extensive roles in physiological and biochemical processes throughout the life of plant species. Furthermore, plenty of achievements have been announced regarding H₂S working in combination with other signal molecules to mitigate environmental damage, such as nitric oxide (NO), abscisic acid (ABA), calcium ion (Ca²⁺), hydrogen peroxide (H₂O₂), salicylic acid (SA), ethylene (ETH), jasmonic acid (JA), proline (Pro), and melatonin (MT). This review summarizes the current knowledge within the mechanism of H₂S and the above signal compounds in response to abiotic stresses in plants, including maintaining cellular redox homeostasis, exchanging metal ion transport, regulating stomatal aperture, and altering gene expression and enzyme activities. The potential relationship between H₂S and other signal transmitters is also proposed and discussed.

The Modus Operandi of Hydrogen Sulfide(H2S)-Dependent Protein Persulfidation in Higher Plants

Protein persulfidation is a post-translational modification (PTM) mediated by hydrogen sulfide (H₂S), which affects the thiol group of cysteine residues from target proteins and can have a positive, negative or zero impact on protein function. Due to advances in proteomic techniques, the number of potential protein targets identified in higher plants, which are affected by this PTM, has increased considerably. However, its precise impact on biological function needs to be evaluated at the experimental level in purified proteins in order to identify the specific cysteine(s) residue(s) affected. It also needs to be evaluated at the cellular redox level given the potential interactions among different oxidative post-translational modifications (oxiPTMs), such as S-nitrosation, glutathionylation, sulfenylation, S-cyanylation and S-acylation, which also affect thiol groups. This review aims to provide an updated and comprehensive overview of the important physiological role exerted by persulfidation in higher plants, which acts as a cellular mechanism of protein protection against irreversible oxidation.

Hydrogen sulfide alleviates salinity stress in Cyclocarya paliurus by maintaining chlorophyll fluorescence and regulating nitric oxide level and antioxidant capacity

Cyclocarya paliurus is commonly used to treat diabetes in China. However, the natural habitats of C. paliurus are typically affected by salt stress. Hydrogen sulfide (H₂S) is a growth regulator that is widely used to enhance plant stress tolerance, but the possible mechanism underlying H₂S-alleviated salt stress in C. paliurus remains unclear. C. paliurus seedlings pretreated with NaHS (an H₂S donor) were exposed to salt stress, and then, the leaf and total biomass, chlorophyll fluorescence parameters, nitric oxide (NO) content, oxidative damage, and proline and phenolic content were investigated to test the hypothesis that H₂S and NO were involved in the salt tolerance of C. paliurus. The results showed that H₂S pretreatment maintained chlorophyll fluorescence and attenuated the loss of plant biomass. We also found that H₂S pretreatment further increased the endogenous NO content and nitrate reductase activity compared with salt treatment. Moreover, H₂S pretreatment alleviated salt-induced oxidative damage, as indicated by lowered lipid peroxidation, through an enhanced antioxidant system including more proline and phenolic accumulation and increased antioxidant enzyme activities. However, C. paliurus leaves treated with the NO scavenger significantly diminished H₂S-mediated NO production and alleviation of membrane lipid peroxidation. Thus, we concluded that H₂S-induced NO was involved in C. paliurus salt tolerance.

Influence of foliar glutathione and putrescine on metabolism and mineral status of genetically diverse rapeseed cultivars under hexavalent chromium stress

We studied the physio-biochemical involvement of exogenous signaling compounds, glutathione and putrescine (alone and in combination), on three contrasting genotypes (cvs. Shiralee, Rainbow, and Dunkled) of canola (Brassica napus L.) of plants exposed to chromium stress. Seeds were germinated in Cr-contaminated soil (0 and 50 μg/g Cr⁶⁺), and both signaling compounds were applied as a foliar spray to 20-day-old plants. Changes in root, stem, and leaf nitro-oxidative metabolism, endogenous GSH level, secondary metabolites, and mineral nutrients were investigated from 60-day-old plants. Exposure to Cr⁶⁺ increased stem GSH and NO concentrations in all cultivars. Maximum root Cr⁶⁺ bioaccumulation was recorded in cv. Rainbow and the least in cv. Shiralee. Also, Cr⁶⁺ stress decreased number and weight of seeds and pod length. Disturbances in root and shoot mineral profile were evident; however, its magnitude varied in all cultivars. The exogenous GSH improved root and shoot P, Fe, S, and Zn concentrations; however, the effect was cultivar specific. Leaf endogenous GSH was increased by exogenous GSH while NO levels remained unaffected. The GSH application also promoted shoot Cr⁶⁺ bioaccumulation while PUT application caused a recovery in seed number and seed weight. Both PUT and GSH differentially affected tissue-specific secondary metabolite profile. Overall, the exogenous GSH was much more effective in alleviating the Cr⁺⁶ toxicity in canola.

Hydrogen Sulfide: A Robust Combatant against Abiotic Stresses in Plants

Hydrogen sulfide (H2S) is predominantly considered as a gaseous transmitter or signaling molecule in plants. It has been known as a crucial player during various plant cellular and physiological processes and has been gaining unprecedented attention from researchers since decades. They regulate growth and plethora of plant developmental processes such as germination, senescence, defense, and maturation in plants. Owing to its gaseous state, they are effectively diffused towards different parts of the cell to counterbalance the antioxidant pools as well as providing sulfur to cells. H2S participates actively during abiotic stresses and enhances plant tolerance towards adverse conditions by regulation of the antioxidative defense system, oxidative stress signaling, metal transport, Na+/K+ homeostasis, etc. They also maintain H2S-Cys-cycle during abiotic stressed conditions followed by post-translational modifications of cysteine residues. Besides their role during abiotic stresses, crosstalk of H2S with other biomolecules such as NO and phytohormones (abscisic acid, salicylic acid, melatonin, ethylene, etc.) have also been explored in plant signaling. These processes also mediate protein post-translational modifications of cysteine residues. We have mainly highlighted all these biological functions along with proposing novel relevant issues that are required to be addressed further in the near future. Moreover, we have also proposed the possible mechanisms of H2S actions in mediating redox-dependent mechanisms in plant physiology.

Hydrogen sulfide (H2S) underpins the beneficial silicon effects against the copper oxide nanoparticles (CuO NPs) phytotoxicity in Oryza sativa seedlings

Nanoparticle-pollution has associated severe negative effects on crop productivity. Hence, methods are needed to alleviate nano-toxicity in crop plants. The present study aims to evaluate if the exogenous hydrogen sulfide (H₂S) application in combination with silicon (Si) could palliate the harmful effects of copper oxide nanoparticles (CuO NPs). Fifteen day-old rice (Oryza sativa L.) seedlings were used as a model plant. The results indicate that simultaneous exogenous addition of 10 μM Si and 100 μM NaHS (as an H₂S donor) provided tolerance and enhanced defence mechanism of the rice seedlings against 100 μM CuO NPs. Thus, it was observed in terms of their growth, photosynthetic pigments, antioxidant enzyme activities, the content of non-enzymatic components, chlorophyll fluorescence and up-regulation of antioxidant genes. Si and NaHS stimulated gene expression of silicon (Lsi1 and Lsi2) and auxin (PIN5 and PIN10) transporters. Taken together, data indicate that H₂S underpins the beneficial Si effects in rice seedlings against the oxidative stress triggers by CuO NPs, and stimulation of enzymatic components of the ascorbate-glutathione cycle being the main factor for the beneficial effects triggered by the couple of Si and H₂S. Therefore, it could be concluded that the simultaneous application of Si and H₂S promote the resilience of the rice seedlings against the oxidative stress induced by CuO NPs.

Interplay between hydrogen sulfide and other signaling molecules in the regulation of guard cell signaling and abiotic/biotic stress response

Stomatal aperture controls the balance between transpirational water loss and photosynthetic carbon dioxide (CO2) uptake. Stomata are surrounded by pairs of guard cells that sense and transduce environmental or stress signals to induce diverse endogenous responses for adaptation to environmental changes. In a recent decade, hydrogen sulfide (H2S) has been recognized as a signaling molecule that regulates stomatal movement. In this review, we summarize recent progress in research on the regulatory role of H2S in stomatal movement, including the dynamic regulation of phytohormones, ion homeostasis, and cell structural components. We focus especially on the cross talk among H2S, nitric oxide (NO), and hydrogen peroxide (H2O2) in guard cells, as well as on H2S-mediated post-translational protein modification (cysteine thiol persulfidation). Finally, we summarize the mechanisms by which H2S interacts with other signaling molecules in plants under abiotic or biotic stress. Based on evidence and clues from existing research, we propose some issues that need to be addressed in the future.

Uptake of hexavalent chromium by Lactuca sativa and Triticum aestivum plants and mediated effects on their performance, linked with associated public health risks

Plants exposed to toxic Cr(VI) concentrations show reduced biomass production and yield. The present study examined (1) the impacts of increasing Cr(VI) exposure (0.05, 0.5, 1, 5 and 10 mg L-1 K2Cr2O7 in irrigation water) on the growth and development, yield, and stress physiology of two important crop species, such as wheat (Triticum aestivum L.) and lettuce (Lactuca sativa L.), and (2) the associated human health risks due to the consumption of these commodities. Plants in all Cr(VI) treatments preserved similar growth rates and phenotypes with control, untreated plants, with no remarkable modulations in biomass yield (fresh/dry weight, plant height, straw/grain weight). Grains harvested from plants exposed to all Cr(VI) treatments had increased size and 1000 grain weight, partially suggesting hormetic effect. Elevated K2Cr2O7 concentrations (5 and 10 mg L-1) resulted in increased photosynthetic pigments' levels in lettuce leaves. Lipid peroxidation and H2O2 content also revealed the absence of oxidative stress in lettuce plants. Non-carcinogenic (target hazard quotient) and carcinogenic risks from the consumption of these commodities grown under the conditions described in this study were found de minimis. However, more studies are needed in order to obtain more solid information on the safety of Cr(VI)-contaminated water irrigation.

Loss of function of the chloroplast membrane K+/H+ antiporters AtKEA1 and AtKEA2 alters the ROS and NO metabolism but promotes drought stress resilience

Potassium (K⁺) exerts key physiological functions such as osmoregulation, stomatal movement, membrane transport, protein synthesis and photosynthesis among others. Previously, it was demonstrated in Arabidopsis thaliana that the loss of function of the chloroplast K⁺Efflux Antiporters KEA1 and KEA2, located in the inner envelope membrane, provokes inefficient photosynthesis. Therefore, the main goal of this study was to evaluate the potential impact of the loss of function of those cation transport systems in the metabolism of reactive oxygen and nitrogen species (ROS and RNS). Using 14-day-old seedlings from Arabidopsis double knock-out kea1kea2 mutants, ROS metabolism and NO content in roots and green cotyledons were studied at the biochemical level. The loss of function of AtKEA1 and AtKEA2 did not cause oxidative stress but it provoked an alteration of the ROS homeostasis affecting some ROS-generating enzymes. These included glycolate oxidase (GOX) and NADPH-dependent superoxide generation activity, enzymatic and non-enzymatic antioxidants and both NADP-isocitrate dehydrogenase and NADP-malic enzyme activities. NO content, analyzed by confocal laser scanning microscopy (CLSM), was negatively affected in both photosynthetic and non-photosynthetic organs in kea1kea2 mutant seedlings. Furthermore, the S-nitrosoglutathione reductase (GSNOR) protein expression and activity were downregulated in kea1kea2 mutants, whereas the tyrosine nitrated protein profile, analyzed by immunoblot, was unaffected but the relative expression of each immunoreactive band changed. Moreover, kea1kea2 mutants showed an increased photorespiratory pathway and stomata closure, thus promoting a higher resilience to drought stress. Data suggest that the chloroplast osmotic balance and integrity maintained by AtKEA1 and AtKEA2 are necessary to keep the balance of ROS/RNS metabolism. Moreover, these data open new questions about how endogenous NO generation might be affected by the K⁺/H⁺ transport located in the chloroplasts.

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

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

Evidences for antioxidant response and biosorption potential of Bacillus simplex strain 115 against lead

In this study, we investigated effects of lead on growth response and antioxidant defense protection in a new identified strain isolated from a soil, in the rhizosphere of Sainfoin Hedysarum coronarium L. Different concentrations of lead (0, 0.2, 1.5 and 3 g L^-1) added to Bacillus simplex strain 115 cultures surprisingly did not inhibit its growth. However, a resulting oxidative stress as attested by overproduction of H₂O₂ (+ 6.2 fold) and malondialdehyde (+ 2.3 fold) concomitantly to the enhancement of proteins carbonylation (+ 221%) and lipoxygenase activity (+ 59%) was observed in presence of 3 g L^-1 of lead. Intrinsic antioxidant defenses were revealed by the coupled up-regulation of catalase (+ 416%) and superoxide dismutase (+ 4 fold) activities, with a more important Fe-SOD increase in comparison to the other isoforms. Bioaccumulation assays showed both intracellular and extracellular lead accumulation. Biosorption was confirmed as a particularly lead resistance mechanism for Bacillus simplex strain 115 as the metal sequestration in cell wall accounted for 88.5% to 98.5% of the total endogenous metal accumulation. Potentiality of this new isolated microorganism as a biotechnological tool for agricultural soil lead bioremediation was thus proposed.

Nitric Oxide and Hydrogen Sulfide Coordinately Reduce Glucose Sensitivity and Decrease Oxidative Stress via Ascorbate-Glutathione Cycle in Heat-Stressed Wheat (Triticum aestivum L.) Plants

The involvement of nitric oxide (NO) and hydrogen sulfide (H₂S) in countermanding heat-inhibited photosynthetic features were studied in wheat (Triticum aestivum L.). Heat stress (HS) was employed at 40 °C after establishment for 6 h daily, and then plants were allowed to recover at 25 °C and grown for 30 days. Glucose (Glc) content increased under HS and repressed plant photosynthetic ability, but the application of sodium nitroprusside (SNP, as NO donor) either alone or with sodium hydrosulfide (NaHS, as H₂S donor) reduced Glc-mediated photosynthetic suppression by enhancing ascorbate-glutathione (AsA-GSH) metabolism and antioxidant system, which reduced oxidative stress with decreased H₂O₂ and TBARS content. Oxidative stress reduction or inhibiting Glc repression was maximum with combined SNP and NaHS treatment, which was substantiated by 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) and hypotaurine (HT), scavengers for NO and H₂S, respectively. The scavenge of H₂S reduced NO-mediated alleviation of HS suggesting of its downstream action in NO-mediated heat-tolerance. However, a simultaneous decrease of both (NO and H₂S) led to higher Glc-mediated repression of photosynthesis and oxidative stress in terms of increased H₂O₂ content that was comparable to HS plants. Thus, NO and H₂S cooperate to enhance photosynthesis under HS by reducing H₂O₂-induced oxidative stress and excess Glc-mediated photosynthetic suppression.

Hydrogen sulfide: Roles in plant abiotic stress response and crosstalk with other signals

Hydrogen sulfide (H₂S) has been recently recognized as an endogenous gas transmitter alongside nitric oxide and carbon monoxide. Exposure of plants to H₂S, for example through applicating H₂S donors, reveals that H₂S play important roles in plant response to abiotic stresses such as heavy metals, salinity, drought and extreme temperatures. Sodium hydrosulfide is the most widely used donor in plants due to its direct and instantaneous release of H₂S, followed by GYY4137. H₂S can enhance plant tolerance to salt and heavy metal stresses through regulating Na⁺/K⁺ homeostasis and the uptake and transport of metal ions. H₂S also promotes the H₂S-Cys cycle balance under abiotic stress and enhances its roles in regulation of the antioxidant system, alternative respiratory pathway, and heavy metal chelators synthesis. H₂S coordinates with gaseous signal molecules, reactive oxygen species and nitric oxide to respond to stress directly through influencing their generation or competing for the regulation of the downstream signaling. Moreover, H₂S interacts with phytohormones including abscisic acid, ethylene, salicylic acid and melatonin as well as polyamines to regulate plant response to abiotic stresses. In this review, the application of H₂S donors and their functional mechanism are summarized. We propose promising new research directions, which can lead to new insights on the role of this gastrasmitter during plant growth and development.

Nitric oxide and hydrogen sulfide protect plasma membrane integrity and mitigate chromium-induced methylglyoxal toxicity in maize seedlings

The present study aims to analyse the potential crosstalk between nitric oxide (NO) and hydrogen sulfide (H₂S) in triggering resilience of maize (Zea mays L.) seedlings to hexavalent chromium (Cr VI). Exogenous application of 500 μM sodium nitroprusside (SNP, as a NO donor) or sodium hydrosulfide (NaHS, as a H₂S donor) to 9-day-old maize seedlings, countered a Cr (200 μM) -elicited reduction in embryonic axis biomass. Cr caused cellular membrane injury by enhancing the levels of superoxide and hydroxyl radicals as well as methylglyoxal, and 4-hydroxy-2-nonenal. The application of SNP or NaHS considerably improved the endogenous NO and H₂S pool, decreased oxidative stress and lipid peroxidation by suppressing lipoxygenase activity and improving some antioxidant enzymes activities in radicles and epicotyls. Radicles were more affected than epicotyls by Cr-stress with enhanced electrolyte leakage and decreased proton extrusion as indicated by lesser H⁺-ATPase activity. H₂S appeared to mitigate Cr toxicity through up-regulated H⁺-ATPase and glyoxalase pathways and by maintaining optimal GSH levels as downstream effects of ROS and MG suppression. Hence, H₂S-mediated the regeneration of GSH pool is associated with the attenuation of MG toxicity by enhancing S-lactoglutathione and D-lactate production. Taken together, our results indicate complementary roles for H₂S and GSH to strengthen membrane integrity against Cr stress in maize seedlings.

Nitric oxide: A radical molecule with potential biotechnological applications in fruit ripening

Nitric oxide (NO) is a short-life and free radical molecule involved in a wide range of cellular, physiological and stressful processes in higher plants. In recent years it has been observed that exogenous NO application can palliate adverse damages against abiotic and biotic stresses. Conversely, there is accumulating information indicating that endogenous NO participates significantly in the mechanism of modulation of the ripening in climacteric and non-climacteric fruits. Even more, when NO is exogenously applied, it can mediate beneficial effects during ripening and postharvest storage being one of the main effects the increase of antioxidant systems. Consequently, NO could be a promising biotechnological tool to improve crops through ameliorating nutritional indexes and to alleviate damages during fruit ripening and postharvest management. Thus, this approach should be complementary to previous strategies to allow preserving the quality and healthiness of fruits with a view of enhancing their added value. The present mini-review aims to provide an overview of NO biochemistry in plants and updated information on the relevance of NO in fruit ripening and postharvest stages with a view to its biotechnological applications.

Appraisal of H2S metabolism in Arabidopsis thaliana: In silico analysis at the subcellular level

Hydrogen sulfide (H₂S) has become a new signal molecule in higher plants which seems to be involved in almost all physiological processes from seed germination, root and plant growth until flowering and fruit ripening. Moreover, H₂S also participates in the mechanism of response against adverse environmental stresses. However, its basic biochemistry in plant cells can be considered in a nascent stage. Using the available information of the model plant Arabidopsis thaliana, the goal of the present study is to provide a broad overview of H₂S metabolism and to display an in silico analysis of the 26 enzymatic components involved in the metabolism of H₂S and their subcellular compartmentation (cytosol, chloroplast and mitochondrion) thus providing a wide picture of the cross-talk inside the organelles and amongst them and, consequently, to get a better understanding of the cellular and tissue implications of H₂S. This information will be also relevant for other crop species, especially those whose whole genome is not yet available.

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

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

Crosstalk among hydrogen sulfide (H2S), nitric oxide (NO) and carbon monoxide (CO) in root-system development and its rhizosphere interactions: A gaseous interactome

Root development in higher plants is achieved by a precise intercellular communication which determines cell fate in the primary embryonic meristem where the gasotransmitters H₂S, NO and CO participate dynamically. Furthermore, the rhizosphere interaction of these molecules with microbial and soil metabolism also affects root development. NO regulates root growth and architecture in association with several other biomolecules like auxin indole-3-acetic acid (IAA), ethylene, jasmonic acid (JA), strigolactones, alkamides and melatonin. The CO-mediated signal transduction pathway in roots is closely linked to the NO-mediated signal cascades. Interestingly, H₂S acts also as an upstream component in IAA and NO-mediated crosstalk during root development. Heme oxygenase (HO) 1 generates CO and functions as a downstream component in H₂S-mediated adventitious rooting and H₂S-CO crosstalk. Likewise, reactive oxygen species (ROS), H₂S and NO crosstalk are important components in the regulation of root architecture. Deciphering these interactions will be a potential biotechnological tool which could provide benefits in crop management in soils, especially under adverse environmental conditions. This review aims to provide a comprehensive update of the complex networks of these gasotransmitters during the development of roots.

Exogenous application of hydrogen sulfide reduces chromium toxicity in maize seedlings by suppressing NADPH oxidase activities and methylglyoxal accumulation

Chromium (Cr) represents an important source of metallic stress in plants. Working with maize (Zea mays) seedlings, we characterize the suppressive effects of exogenously applied NaHS (a hydrogen sulfide; [H₂S] donor) on the toxic effects of Cr (VI). Heavy metal treatment reduced radicle and epicotyl lengths and fresh weights in seedlings at 6 and 9 days following germination. The negative Cr (200 μM) effect was countered by application with NaHS (500 μM) but this countering was reduced with the co-application of the H₂S generation inhibitor hydroxylamine (HA) or the H₂S scavenger hypotaurine (HT). The Cr-elicited H₂O₂ production was suppressed by NaHS and also by an inhibitor of the reactive oxygen species (ROS) generating NADPH oxidase (NOX). These effects were correlated with relative changes in carbomyl (-CO) and thiol (-SH) groups. Nitric oxide (NO) production increased by NaHS application with associated increase in S-nitrosoglutathione (GSNO) level, but low S-nitrosoglutathione reductase (GSNOR) activities indicating an elevated S-nitrosylation. Assessment of the role of the ascorbate-glutathione antioxidant cycle indicated that whilst ascorbate played at a best minor role, glutathione was more prominent. Methylglyoxal (MG) production was increased by Cr but reduced by NaHS through a mechanism which could be based on glutathione-S-transferase (GST) detoxification. Taken together data suggest that H₂S acts to counter Cr effect in maize by NOX suppression, mostly likely by the well-characterised S-nitrosylation mechanism, as well as a reduction of MG accumulation.

Involvement of L-Cysteine Desulfhydrase and Hydrogen Sulfide in Glutathione-Induced Tolerance to Salinity by Accelerating Ascorbate-Glutathione Cycle and Glyoxalase System in Capsicum

The aim of this study is to assess the role of l-cysteine desulfhydrase (l-DES) and endogenous hydrogen sulfide (H₂S) in glutathione (GSH)-induced tolerance to salinity stress (SS) in sweet pepper (Capsicum annuum L.). Two weeks after germination, before initiating SS, half of the pepper seedlings were retained for 12 h in a liquid solution containing H₂S scavenger, hypotaurine (HT), or the l-DES inhibitor dl-propargylglycine (PAG). The seedlings were then exposed for three weeks to control or SS (100 mmol L⁻¹ NaCl) and supplemented with or without GSH or GSH+NaHS (sodium hydrosulfide, H₂S donor). Salinity suppressed dry biomass, leaf water potential, chlorophyll contents, maximum quantum efficiency, ascorbate, and the activities of dehydroascorbate reductase, monodehydroascorbate reductase, and glyoxalase II in plants. Contrarily, it enhanced the accumulation of hydrogen peroxide, malondialdehyde, methylglyoxal, electrolyte leakage, proline, GSH, the activities of glutathione reductase, peroxidase, catalase, superoxide dismutase, ascorbate peroxidase, glyoxalase I, and l-DES, as well as endogenous H₂S content. Salinity enhanced leaf Na⁺ but reduced K⁺; however, the reverse was true with GSH application. Overall, the treatments, GSH and GSH+NaHS, effectively reversed the oxidative stress and upregulated salt tolerance in pepper plants by controlling the activities of the AsA-GSH and glyoxalase-system-related enzymes as well as the levels of osmolytes.

Nitric oxide alleviates cadmium- but not arsenic-induced damages in rice roots

Nitric oxide (NO) has signalling roles in plant stress responses. Cadmium (Cd) and arsenic (As) soil pollutants alter plant development, mainly the root-system, by increasing NO-content, triggering reactive oxygen species (ROS), and forming peroxynitrite by NO-reaction with the superoxide anion. Interactions of NO with ROS and peroxynitrite seem important for plant tolerance to heavy metal(oid)s, but the mechanisms underlying this process remain unclear. Our goal was to investigate NO-involvement in rice (Oryza sativa L.) root-system after exposure to Cd or As, to highlight possible differences in NO-behaviour between the two pollutants. To the aim, morpho-histological, chemical and epifluorescence analyses were carried out on roots of different origin in the root-system, under exposure to Cd or As, combined or not with sodium nitroprusside (SNP), a NO-donor compound. Results show that increased intracellular NO levels alleviate the root-system alterations induced by Cd, i.e., inhibition of adventitious root elongation and lateral root formation, increment in lignin deposition in the sclerenchyma/endodermis cell-walls, but, even if reducing As-induced endodermis lignification, do not recover the majority of the As-damages, i.e., enhancement of AR-elongation, reduction of LR-formation, anomalous tissue-proliferation. However, NO decreases both Cd and As uptake, without affecting the pollutants translocation-capability from roots to shoots. Moreover, NO reduces the Cd-induced, but not the As-induced, ROS levels by triggering peroxynitrite production. Altogether, results highlight a different behaviour of NO in modulating rice root-system response to the toxicity of the heavy metal Cd and the metalloid As, which depends by the NO-interaction with the specific pollutant.

H2S signaling in plants and applications in agriculture

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Hydrogen sulfide (H₂S) plays a signaling role in higher plants.

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It mediates persulfidation, a post-translational modification.

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It regulates physiological functions ranging from seed germination to fruit ripening.

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The beneficial effects of exogenous H₂S are mainly caused by the stimulation of antioxidant systems.

The signaling properties of the gasotransmitter molecule hydrogen sulfide (H₂S), which is endogenously generated in plant cells, are mainly observed during persulfidation, a protein post-translational modification (PTM) that affects redox-sensitive cysteine residues. There is growing experimental evidence that H₂S in higher plants may function as a mechanism of response to environmental stress conditions. In addition, exogenous applications of H₂S to plants appear to provide additional protection against stresses, such as salinity, drought, extreme temperatures and heavy metals, mainly through the induction of antioxidant systems, in order to palliate oxidative cellular damage. H₂S also appears to be involved in regulating physiological functions, such as seed germination, stomatal movement and fruit ripening, as well as molecules that maintain post-harvest quality and rhizobium–legume symbiosis. These properties of H₂S open up new challenges in plant research to better understand its functions as well as new opportunities for biotechnological treatments in agriculture in a changing environment.

Inhibition of NADP-malic enzyme activity by H2 S and NO in sweet pepper (Capsicum annuum L.) fruits

NADPH is an essential cofactor in many physiological processes. Fruit ripening is caused by multiple biochemical pathways in which, reactive oxygen and nitrogen species (ROS/RNS) metabolism is involved. Previous studies have demonstrated the differential modulation of nitric oxide (NO) and hydrogen sulfide (H₂ S) content during sweet pepper (Capsicum annuum L.) fruit ripening, both of which regulate NADP-isocitrate dehydrogenase activity. To gain a deeper understanding of the potential functions of other NADPH-generating components, we analyzed glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH), which are involved in the oxidative phase of the pentose phosphate pathway (OxPPP) and NADP-malic enzyme (NADP-ME). During fruit ripening, G6PDH activity diminished by 38%, while 6PGDH and NADP-ME activity increased 1.5- and 2.6-fold, respectively. To better understand the potential regulation of these NADP-dehydrogenases by H₂ S, we obtained a 50-75% ammonium-sulfate-enriched protein fraction containing these proteins. With the aid of in vitro assays, in the presence of H₂ S, we observed that, while NADP-ME activity was inhibited by up to 29-32% using 2 and 5 mM Na₂ S as H₂ S donor, G6PDH and 6PGDH activities were unaffected. On the other hand, NO donors, S-nitrosocyteine (CysNO) and DETA NONOate also inhibited NADP-ME activity by 35%. These findings suggest that both NADP-ME and 6PGDH play an important role in maintaining the supply of NADPH during pepper fruit ripening and that H₂ S and NO partially modulate the NADPH-generating system.

Chromium-Induced Reactive Oxygen Species Accumulation by Altering the Enzymatic Antioxidant System and Associated Cytotoxic, Genotoxic, Ultrastructural, and Photosynthetic Changes in Plants

Chromium (Cr) is one of the top seven toxic heavy metals, being ranked 21st among the abundantly found metals in the earth’s crust. A huge amount of Cr releases from various industries and Cr mines, which is accumulating in the agricultural land, is significantly reducing the crop development, growth, and yield. Chromium mediates phytotoxicity either by direct interaction with different plant parts and metabolic pathways or it generates internal stress by inducing the accumulation of reactive oxygen species (ROS). Thus, the role of Cr-induced ROS in the phytotoxicity is very important. In the current study, we reviewed the most recent publications regarding Cr-induced ROS, Cr-induced alteration in the enzymatic antioxidant system, Cr-induced lipid peroxidation and cell membrane damage, Cr-induced DNA damage and genotoxicity, Cr-induced ultrastructural changes in cell and subcellular level, and Cr-induced alterations in photosynthesis and photosynthetic apparatus. Taken together, we conclude that Cr-induced ROS and the suppression of the enzymatic antioxidant system actually mediate Cr-induced cytotoxic, genotoxic, ultrastructural, and photosynthetic changes in plants.

Assessment of Subcellular ROS and NO Metabolism in Higher Plants: Multifunctional Signaling Molecules

Reactive oxygen species (ROS) and nitric oxide (NO) are produced in all aerobic life forms under both physiological and adverse conditions. Unregulated ROS/NO generation causes nitro-oxidative damage, which has a detrimental impact on the function of essential macromolecules. ROS/NO production is also involved in signaling processes as secondary messengers in plant cells under physiological conditions. ROS/NO generation takes place in different subcellular compartments including chloroplasts, mitochondria, peroxisomes, vacuoles, and a diverse range of plant membranes. This compartmentalization has been identified as an additional cellular strategy for regulating these molecules. This assessment of subcellular ROS/NO metabolisms includes the following processes: ROS/NO generation in different plant cell sites; ROS interactions with other signaling molecules, such as mitogen-activated protein kinases (MAPKs), phosphatase, calcium (Ca²⁺), and activator proteins; redox-sensitive genes regulated by the iron-responsive element/iron regulatory protein (IRE-IRP) system and iron regulatory transporter 1(IRT1); and ROS/NO crosstalk during signal transduction. All these processes highlight the complex relationship between ROS and NO metabolism which needs to be evaluated from a broad perspective.

Hydrogen Sulfide: A New Warrior against Abiotic Stress

Nitro-oxidative stress in plants involves reactive oxygen and nitrogen species overproduction, which could trigger irreversible cellular damage. In this context, a novel signaling gaseous molecule, hydrogen sulfide (H₂S), has emerged, which appears to stimulate the antioxidant system and is capable of mediating effective mechanisms of response to these abiotic stresses.

Reactive oxygen species, auxin and nitric oxide in metal-stressed roots: toxicity or defence

The presented review is a summary on the current knowledge about metal induced stress response in plants, focusing on the roles of reactive oxygen species, auxin and nitric oxide in roots. The article focuses mainly on the difference between defence and toxicity symptoms of roots during metal-induced stress. Nowadays, pollution of soils by heavy metals is a rapidly growing issue, which affects agriculture and human health. In order to deal with these problems, we must first understand the basic mechanisms and responses to environmental conditions in plants growing under such conditions. Studies so far show somewhat conflicting data, interpreting the same stress responses as both symptoms of defence and toxicity. Therefore, the aim of this review is to give a report about current knowledge of heavy metal-induced stress research, and also to differentiate between toxicity and defence, and outline the challenges of research, focusing on reactive oxygen and nitrogen species, auxin, and the interplay among them. There are still remaining questions on how reactive oxygen and nitrogen species, as well as auxin, can activate either symptoms of toxicity or defence, and adaptation responses.

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

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

Nitric oxide-dependent regulation of sweet pepper fruit ripening

Ripening is a complex physiological process that involves changes in reactive nitrogen and oxygen species that govern the shelf-life and quality of fruits. Nitric oxide (NO)-dependent changes in the sweet pepper fruit transcriptome were determined by treating fruits at the initial breaking point stage with NO gas. Fruits were also harvested at the immature (green) and ripe (red) stages. Fruit ripening in the absence of NO resulted in changes in the abundance of 8805 transcripts whose function could be identified. Among these, functional clusters associated with reactive oxygen/nitrogen species and lipid metabolism were significantly modified. NO treatment resulted in the differential expression of 498 genes framed within these functional categories. Biochemical analysis revealed that NO treatment resulted in changes in fatty acid profiling, glutathione and proline contents, and the extent of lipid peroxidation, as well as increases in the activity of ascorbate peroxidase and lipoxygenase. These data provide supporting evidence for the crucial role of NO in the ripening of pepper fruit.

Signaling by hydrogen sulfide and cyanide through post-translational modification

Two cysteine metabolism-related molecules, hydrogen sulfide and hydrogen cyanide, which are considered toxic, have now been considered as signaling molecules. Hydrogen sulfide is produced in chloroplasts through the activity of sulfite reductase and in the cytosol and mitochondria by the action of sulfide-generating enzymes, and regulates/affects essential plant processes such as plant adaptation, development, photosynthesis, autophagy, and stomatal movement, where interplay with other signaling molecules occurs. The mechanism of action of sulfide, which modifies protein cysteine thiols to form persulfides, is related to its chemical features. This post-translational modification, called persulfidation, could play a protective role for thiols against oxidative damage. Hydrogen cyanide is produced during the biosynthesis of ethylene and camalexin in non-cyanogenic plants, and is detoxified by the action of sulfur-related enzymes. Cyanide functions include the breaking of seed dormancy, modifying the plant responses to biotic stress, and inhibition of root hair elongation. The mode of action of cyanide is under investigation, although it has recently been demonstrated to perform post-translational modification of protein cysteine thiols to form thiocyanate, a process called S-cyanylation. Therefore, the signaling roles of sulfide and most probably of cyanide are performed through the modification of specific cysteine residues, altering protein functions.

Hydrogen sulfide: A novel component in Arabidopsis peroxisomes which triggers catalase inhibition

Plant peroxisomes have the capacity to generate different reactive oxygen and nitrogen species (ROS and RNS), such as H₂ O₂ , superoxide radical (O₂ ^· - ), nitric oxide and peroxynitrite (ONOO^- ). These organelles have an active nitro-oxidative metabolism which can be exacerbated by adverse stress conditions. Hydrogen sulfide (H₂ S) is a new signaling gasotransmitter which can mediate the posttranslational modification (PTM) persulfidation. We used Arabidopsis thaliana transgenic seedlings expressing cyan fluorescent protein (CFP) fused to a canonical peroxisome targeting signal 1 (PTS1) to visualize peroxisomes in living cells, as well as a specific fluorescent probe which showed that peroxisomes contain H₂ S. H₂ S was also detected in chloroplasts under glyphosate-induced oxidative stress conditions. Peroxisomal enzyme activities, including catalase, photorespiratory H₂ O₂ -generating glycolate oxidase (GOX) and hydroxypyruvate reductase (HPR), were assayed in vitro with a H₂ S donor. In line with the persulfidation of this enzyme, catalase activity declined significantly in the presence of the H₂ S donor. To corroborate the inhibitory effect of H₂ S on catalase activity, we also assayed pure catalase from bovine liver and pepper fruit-enriched samples, in which catalase activity was inhibited. Taken together, these data provide evidence of the presence of H₂ S in plant peroxisomes which appears to regulate catalase activity and, consequently, the peroxisomal H₂ O₂ metabolism.

Impact of Nitric Oxide (NO) on the ROS Metabolism of Peroxisomes

Nitric oxide (NO) is a gaseous free radical endogenously generated in plant cells. Peroxisomes are cell organelles characterized by an active metabolism of reactive oxygen species (ROS) and are also one of the main cellular sites of NO production in higher plants. In this mini-review, an updated and comprehensive overview is presented of the evidence available demonstrating that plant peroxisomes have the capacity to generate NO, and how this molecule and its derived products, peroxynitrite (ONOO⁻) and S-nitrosoglutathione (GSNO), can modulate the ROS metabolism of peroxisomes, mainly throughout protein posttranslational modifications (PTMs), including S-nitrosation and tyrosine nitration. Several peroxisomal antioxidant enzymes, such as catalase (CAT), copper-zinc superoxide dismutase (CuZnSOD), and monodehydroascorbate reductase (MDAR), have been demonstrated to be targets of NO-mediated PTMs. Accordingly, plant peroxisomes can be considered as a good example of the interconnection existing between ROS and reactive nitrogen species (RNS), where NO exerts a regulatory function of ROS metabolism acting upstream of H₂O₂.