Nitric oxide-mediated regulation of oxidative stress in plants under metal stress: a review on molecular and biochemical aspects

Exploring Sustainable Agriculture with Nitrogen-Fixing Cyanobacteria and Nanotechnology

The symbiotic relationship between nitrogen-fixing cyanobacteria and plants offers a promising avenue for sustainable agricultural practices and environmental remediation. This review paper explores the molecular interactions between nitrogen-fixing cyanobacteria and nanoparticles, shedding light on their potential synergies in agricultural nanotechnology. Delving into the evolutionary history and specialized adaptations of cyanobacteria, this paper highlights their pivotal role in fixing atmospheric nitrogen, which is crucial for ecosystem productivity. The review discusses the unique characteristics of metal nanoparticles and their emerging applications in agriculture, including improved nutrient delivery, stress tolerance, and disease resistance. It delves into the complex mechanisms of nanoparticle entry into plant cells, intracellular transport, and localization, uncovering the impact on root-shoot translocation and systemic distribution. Furthermore, the paper elucidates cellular responses to nanoparticle exposure, emphasizing oxidative stress, signaling pathways, and enhanced nutrient uptake. The potential of metal nanoparticles as carriers of essential nutrients and their implications for nutrient-use efficiency and crop yield are also explored. Insights into the modulation of plant stress responses, disease resistance, and phytoremediation strategies demonstrate the multifaceted benefits of nanoparticles in agriculture. Current trends, prospects, and challenges in agricultural nanotechnology are discussed, underscoring the need for responsible and safe nanoparticle utilization. By harnessing the power of nitrogen-fixing cyanobacteria and leveraging the unique attributes of nanoparticles, this review paves the way for innovative, sustainable, and efficient agricultural practices.

Sodium nitroprusside modulates oxidative and nitrosative processes in Lycopersicum esculentum L. under drought stress

KEY MESSAGE

Sodium nitroprusside mediates drought stress responses in tomatoes by modulating nitrosative and oxidative pathways, highlighting the interplay between nitric oxide, hydrogen sulfide, and antioxidant systems for enhanced drought tolerance. While nitric oxide (NO), a signalling molecule, enhances plant tolerance to abiotic stresses, its precise contribution to improving tomato tolerance to drought stress (DS) through modulating oxide-nitrosative processes is not yet fully understood. We aimed to examine the interaction of NO and nitrosative signaling, revealing how sodium nitroprusside (SNP) could mitigate the effects of DS on tomatoes. DS-seedlings endured 12% polyethylene glycol (PEG) in a 10% nutrient solution (NS) for 2 days, then transitioned to half-strength NS for 10 days alongside control plants. DS reduced total plant dry weight, chlorophyll a and b, Fv/Fm, leaf water potential (ΨI), and relative water content, but improved hydrogen peroxide (H2O2), proline, and NO content. The SNP reduced the DS-induced H2O2 generation by reducing thiol (-SH) and the carbonyl (-CO) groups. SNP increased not only NO but also the activity of L-cysteine desulfhydrase (L-DES), leading to the generation of H2S. Decreases in S-nitrosoglutathione reductase (GSNOR) and NADPH oxidase (NOX) suggest a potential regulatory mechanism in which S-nitrosylation [formation of S-nitrosothiol (SNO)] may influence protein function and signaling pathways during DS. Moreover, SNP improved ascorbate (AsA) and glutathione (GSH) and reduced oxidized glutathione (GSSG) levels in tomato plants under drought. Furthermore, the interaction of NO and H2S, mediated by L-DES activity, may serve as a vital cross-talk mechanism impacting plant responses to DS. Understanding these signaling interactions is crucial for developing innovative drought-tolerance strategies in crops.

Silicon regulates phosphate deficiency through involvement of auxin and nitric oxide in barley roots

MAIN CONCLUSION

Silicon application mitigates phosphate deficiency in barley through an interplay with auxin and nitric oxide, enhancing growth, photosynthesis, and redox balance, highlighting the potential of silicon as a fertilizer for overcoming nutritional stresses. Silicon (Si) is reported to attenuate nutritional stresses in plants, but studies on the effect of Si application to plants grown under phosphate (Pi) deficiency are still very scarce, especially in barley. Therefore, the present work was undertaken to investigate the potential role of Si in mitigating the adverse impacts of Pi deficiency in barley Hordeum vulgare L. (var. BH902). Further, the involvement of two key regulatory signaling molecules--auxin and nitric oxide (NO)--in Si-induced tolerance against Pi deficiency in barley was tested. Morphological attributes, photosynthetic parameters, oxidative stress markers (O2·-, H2O2, and MDA), antioxidant system (enzymatic--APX, CAT, SOD, GR, DHAR, MDHAR as well as non-enzymatic--AsA and GSH), NO content, and proline metabolism were the key traits that were assessed under different treatments. The P deficiency distinctly declined growth of barley seedlings, which was due to enhancement in oxidative stress leading to inhibition of photosynthesis. These results were also in parallel with an enhancement in antioxidant activity, particularly SOD and CAT, and endogenous proline level and its biosynthetic enzyme (P5CS). The addition of Si exhibited beneficial effects on barley plants grown in Pi-deficient medium as reflected in increased growth, photosynthetic activity, and redox balance through the regulation of antioxidant machinery particularly ascorbate-glutathione cycle. We noticed that auxin and NO were also found to be independently participating in Si-mediated improvement of growth and other parameters in barley roots under Pi deficiency. Data of gene expression analysis for PHOSPHATE TRANSPORTER1 (HvPHT1) indicate that Si helps in increasing Pi uptake as per the need of Pi-deficient barley seedlings, and also auxin and NO both appear to help Si in accomplishing this task probably by inducing lateral root formation. These results are suggestive of possible application of Si as a fertilizer to correct the negative effects of nutritional stresses in plants. Further research at genetic level to understand Si-induced mechanisms for mitigating Pi deficiency can be helpful in the development of new varieties with improved tolerance against Pi deficiency, especially for cultivation in areas with Pi-deficient soils.

Differential response of blueberry to the application of bacterial inoculants to improve yield, organoleptic qualities and concentration of bioactive compounds

The application of bacterial biofortifiers is an increasingly common technique. In recent years, some strains have been shown to improve the nutraceutical qualities of crops. This work analyses the impact of biofortification with 3 bacterial strains of the genera Rhizobium, Paenibacillus and Lactiplantibacillus on the nutritional characteristics and organic composition of blueberry in Portugal. Paenibacillus sp. VMFR46 treatment showed increase of 71.36 % and 79.88 % in total production. Biofortified treatments were able to increase Brix degree, maturity index (up to 48.05 % for cv. Legacy and up to 26.04 % for cv. Duke) and CIEL*a*b* index respect to uninoculated control. In this way, (poly)phenolic compounds concentration increased in biofortified treatment, and their (poly)phenolic profile was modified, some compounds such as myricetin aglycone or myricetin derivative are exclusive of the fruits from biofortified plants, with increases in (poly)phenolic concentrations related with R. laguerreae PEPV16 or Paenibacillus sp. VMFR46 inoculation in cv. Legacy. These modifications resulted in the improvement of the nutraceutical characteristics of the fruits obtained.

Nitrate-induced AHb1 expression aggravates Cd toxicity in plants

Cadmium (Cd) causes severe toxicity in plants. However, the molecular mechanisms underlying plant resistance to Cd in relation to nitrogen (N) supply remain unclear. The non-symbiotic hemoglobin gene Hb1 plays an important role in scavenging nitric oxide (NO) in plants. In this study, there was no differential effect of Cd on the biomass of wild-type (WT) and AHb1-overexpressing (H7) plants when NH4+-N was used as a nitrogen source. However, under NO3--N conditions, Cd exerted less biomass stress on AHb1-silenced (L3) plants and more stress on H7 plants than on WT plants. The Cd tolerance index followed the order: L3 > WT > H7. However, there was no difference in Cd concentrations in the roots or shoots of the WT, L3, and H7 plants, indicating that differences in AHb1 expression were unrelated to Cd uptake. Further investigation showed that Cd exposure enhanced H2O2 accumulation and aggravated oxidative damage in H7 plants. The application of an NO donor effectively reversed growth inhibition, H2O2 burst, and oxidative stress induced by Cd in H7 plants. Thus, we suggest that NO3--induced AHb1 expression suppresses Cd-induced NO production in plants, increasing the ROS burst and exacerbating Cd toxicity.

Meta-Analysis of the Effect of Nitric Oxide Application on Heavy Metal Stress Tolerance in Plants

Substantial single-species studies have reported the facility of nitric oxide (NO) in alleviating heavy metal-induced stress in plants. Understanding the mechanisms of NO-involved stress alleviation is progressing; however, a quantitative description of the alleviative capacity of NO against heavy metal stress is still lacking. We combined the results of 86 studies using meta-analysis to statistically assess the responses of heavy metal-stressed plants to NO supply across several metal stresses and plant families. The results showed that plant biomass was consistently improved following NO supply to metal-stressed plants. NO played an important role in mitigating oxidative damage caused by heavy metal stress by significantly stimulating the activities of antioxidant enzymes. Moreover, NO supply consistently increased the Ca, Fe, and Mg contents in both leaves and roots. Plant tissues accumulated less heavy metals when exposed to heavy metal stress after NO addition. Additionally, the best concentration of SNP (an NO donor) for hydroponic culture is in the range of 75-150 μM. We further confirmed that NO application can generally alleviate plant heavy metal stress and its action pathway. The results presented here can help guide future applications of NO as a plant growth regulator in agriculture and breeding plants for heavy metal stress tolerance.

Molecular mechanisms underlying the toxicity and detoxification of trace metals and metalloids in plants

Plants take up a wide range of trace metals/metalloids (hereinafter referred to as trace metals) from the soil, some of which are essential but become toxic at high concentrations (e.g., Cu, Zn, Ni, Co), while others are non-essential and toxic even at relatively low concentrations (e.g., As, Cd, Cr, Pb, and Hg). Soil contamination of trace metals is an increasing problem worldwide due to intensifying human activities. Trace metal contamination can cause toxicity and growth inhibition in plants, as well as accumulation in the edible parts to levels that threatens food safety and human health. Understanding the mechanisms of trace metal toxicity and how plants respond to trace metal stress is important for improving plant growth and food safety in contaminated soils. The accumulation of excess trace metals in plants can cause oxidative stress, genotoxicity, programmed cell death, and disturbance in multiple physiological processes. Plants have evolved various strategies to detoxify trace metals through cell-wall binding, complexation, vacuolar sequestration, efflux, and translocation. Multiple signal transduction pathways and regulatory responses are involved in plants challenged with trace metal stresses. In this review, we discuss the recent progress in understanding the molecular mechanisms involved in trace metal toxicity, detoxification, and regulation, as well as strategies to enhance plant resistance to trace metal stresses and reduce toxic metal accumulation in food crops.

Selenium seed priming enhanced the growth of salt-stressed Brassica rapa L. through improving plant nutrition and the antioxidant system

Various abiotic stresses may affect the germination, growth, and yield of direct-seeded vegetable crops. Seed priming with effective antioxidant mediators may alleviate these environmental stresses by maintaining uniformity in seed germination and improving the subsequent health of developing seedlings. Salt-induced stress has become a limiting factor for the successful cultivation of Brassica rapa L., especially in Southeast Asian countries. The present study was performed to elucidate the efficacy of seed priming using selenium (Se) in mitigating salt-induced oxidative stress in turnip crops by reducing the uptake of Na⁺. In this study, we administered three different levels of Se (Se-1, 75 μmol L^-1; Se-2, 100 μmol L^-1; and Se-3, 125 μmol L^-1) alone or in combination with NaCl (200 mM). Conspicuously, salinity and Se-2 modulated the expression levels of the antioxidant genes, including catalase (CAT), peroxidase (POD), superoxide dismutase (SOD), and ascorbate peroxidase (APX). The upregulated expression of stress-responsive genes alleviated salt stress by scavenging the higher reactive oxygen species (ROS) level. The stress ameliorative potential of Se (Se-2 = 100 μmol L^-1) enhanced the final seed germination percentage, photosynthetic content, and seedling biomass production up to 48%, 56%, and 51%, respectively, under stress. The advantageous effects of Se were attributed to the alleviation of salinity stress through the reduction of the levels of malondialdehyde (MDA), proline, and H₂O₂. Generally, treatment with Se-2 (100 μmo L^-1) was more effective in enhancing the growth attributes of B. rapa compared to Se-1 (75 μmo L^-1) and Se-3 (125 μmo L^-1) under salt-stressed and non-stressed conditions. The findings of the current study advocate the application of the Se seed priming technique as an economical and eco-friendly approach for salt stress mitigation in crops grown under saline conditions.

Mitigation of Negative Effects of Chromium (VI) Toxicity in Faba Bean (Vicia faba) Plants through the Supplementation of Kinetin (KN) and Gibberellic Acid (GA3)

The present study was carried out to explore the possible role of kinetin and gibberellic acid (GA3) on faba bean under chromium (Cr) stress. Cr treatment negatively affected growth and biomass production, reduced photosynthetic pigments, and inhibited photosynthesis, gas exchange parameters, antioxidant enzymes, and the glyoxylase cycle. Moreover, Cr stress enhanced the production of malondialdehyde (MDA, 216.11%) and hydrogen peroxide (H₂O₂, 230.16%), electrolyte leakage (EL, 293.30%), and the accumulation of proline and glycine betaine. Exogenous application of kinetin and GA3 increased growth and biomass, improved pigment contents and photosynthesis, as well as up-regulated the antioxidant system by improving the antioxidant enzyme activities and the content of nonenzymatic components, and the glyoxylase cycle. Additionally, kinetin and GA3 application displayed a considerable enhancement in proline (602.61%) and glycine betaine (423.72), which help the plants to maintain water balance under stress. Furthermore, a decline in Cr uptake was also observed due to kinetin and GA3 application. Exogenous application of kinetin and GA3 ameliorated the toxic effects of Cr in faba bean plants, up-shooting the tolerance mechanisms, including osmolyte metabolism and the antioxidant system.

Heavy metal toxicity in plants and the potential NO-releasing novel techniques as the impending mitigation alternatives

Environmental pollutants like heavy metals are toxic, persistent, and bioaccumulative in nature. Contamination of agricultural fields with heavy metals not only hampers the quality and yield of crops but also poses a serious threat to human health by entering the food chain. Plants generally cope with heavy metal stress by regulating their redox machinery. In this context, nitric oxide (NO) plays a potent role in combating heavy metal toxicity in plants. Studies have shown that the exogenous application of NO donors protects plants against the deleterious effects of heavy metals by enhancing their antioxidative defense system. Most of the studies have used sodium nitroprusside (SNP) as a NO donor for combating heavy metal stress despite the associated concerns related to cyanide release. Recently, NO-releasing nanoparticles have been tested for their efficacy in a few plants and other biomedical research applications suggesting their use as an alternative to chemical NO donors with the advantage of safe, slow and prolonged release of NO. This suggests that they may also serve as potential candidates in mitigating heavy metal stress in plants. Therefore, this review presents the role of NO, the application of chemical NO donors, potential advantages of NO-releasing nanoparticles, and other NO-release strategies in biomedical research that may be useful in mitigating heavy metal stress in plants.

Cross Talk between Hydrogen Peroxide and Nitric Oxide in the Unicellular Green Algae Cell Cycle: How Does It Work?

The regulatory role of some reactive oxygen species (ROS) and reactive nitrogen species (RNS), such as hydrogen peroxide or nitric oxide, has been demonstrated in some higher plants and algae. Their involvement in regulation of the organism, tissue and single cell development can also be seen in many animals. In green cells, the redox potential is an important photosynthesis regulatory factor that may lead to an increase or decrease in growth rate. ROS and RNS are important signals involved in the regulation of photoautotrophic growth that, in turn, allow the cell to attain the commitment competence. Both hydrogen peroxide and nitric oxide are directly involved in algal cell development as the signals that regulate expression of proteins required for completing the cell cycle, such as cyclins and cyclin-dependent kinases, or histone proteins and E2F complex proteins. Such regulation seems to relate to the direct interaction of these signaling molecules with the redox-sensitive transcription factors, but also with regulation of signaling pathways including MAPK, G-protein and calmodulin-dependent pathways. In this paper, we aim to elucidate the involvement of hydrogen peroxide and nitric oxide in algal cell cycle regulation, considering the role of these molecules in higher plants. We also evaluate the commercial applicability of this knowledge. The creation of a simple tool, such as a precisely established modification of hydrogen peroxide and/or nitric oxide at the cellular level, leading to changes in the ROS-RNS cross-talk network, can be used for the optimization of the efficiency of algal cell growth and may be especially important in the context of increasing the role of algal biomass in science and industry. It could be a part of an important scientific challenge that biotechnology is currently focused on.

Multivariate analysis of morpho-physiological traits in Amaranthus tricolor as affected by nitric oxide and cadmium stress

Edible amaranth (Amaranthus tricolor L.) is used as a food-medicine or ornamental plant, and despite its importance, there are few reports associated with cadmium (Cd) stress. This study aimed to appraise the crosstalk between sodium nitroprusside (SNP), as a source of nitric oxide (NO), and cadmium toxicity on growth and physiological traits in edible amaranth by using different multivariate statistical methods. The results showed that growth-related traits of A. tricolor were significantly reduced under Cd stress. Contrarily, Cd treatments increased lipid peroxidation and reduced total protein content. Delving on the results of SNP application showed the suitability of its medium level (100 µM) on increasing the growth-related traits and also plant tolerance to Cd stress via lowering the lipid peroxidation and radical molecules production due to the higher activities of superoxide dismutase and catalase. Increasing the amount of Cd in roots and shoots, as the result of Cd treatment, reduced the growth and production of A. tricolor plants by high rates (over 50% in 60 mg kg^-1 Cd level), indicating its susceptibility to high Cd toxicity. Contrarily, treating plants with SNP showed no effect on shoot Cd content, while it significantly increased Cd allocation in the root, which might be attributable to the protective effect of NO on Cd toxicity by trapping Cd in the root. Subsequently, the application of a medium level of SNP (around 100 µM) is recommendable for A. tricolor plant to overcome the negative impacts of Cd toxicity. Moreover, according to the results of heatmap and biplot, under no application of Cd, the application of 100 µM SNP showed a great association with growth-related traits indicating the effectiveness of SNP on the productivity of this species even under no stress situations.

Differential Impact of Nitric Oxide and Abscisic Acid on the Cellular and Physiological Functioning of sub1A QTL Bearing Rice Genotype under Salt Stress

Hydroponic culture containing 200 mM NaCl was used to induce oxidative stress in seedlings of cultivars initially primed with 1 mM SNP and 10 µM ABA. Exogenous application of sodium nitroprusside (SNP - a nitric oxide donor) and abscisic acid (ABA) was well sensitized more in cv. Swarna Sub1 than cv. Swarna and also reflected in different cellular responses. The major effects of salinity, irrespective of the cultivar, were lowering the water relation, including relative water content and osmotic potential, and decreasing the compatible solutes like alanine, gamma-aminobutyric acid, and glycine betaine. The accumulated polyamines were reduced more in cv. Swarna with a concomitant decrease in photosynthetic reserves. NADP-malic enzyme activity, sucrose accumulation, ascorbate peroxidase, and glutathione S-transferase activities gradually declined under NaCl stress and the catabolizing enzymes like invertase (both wall and cytosolic forms) also declined. On the contrary, plants suffered from oxidative stress through superoxide, hydrogen peroxide, and their biosynthetic enzymes like NADP(H) oxidase. Moderation of Na⁺/K⁺ by both SNP and ABA were correlated with other salt sensitivities in the plants. The maximum effects of SNP and ABA were found in the recovery of antioxidation pathways, osmotic tolerance, and carbohydrate metabolism. Findings predict the efficacy of SNP and ABA either independently or cumulatively in overcoming NaCl toxicity in rice.

Transcriptome Profiling Reveals the Effects of Nitric Oxide on the Growth and Physiological Characteristics of Watermelon under Aluminum Stress

Excessive aluminum ions (Al³⁺) in acidic soil can have a toxic effect on watermelons, restricting plant growth and reducing yield and quality. In this study, we found that exogenous application of nitric oxide (NO) could increase the photochemical efficiency of watermelon leaves under aluminum stress by promoting closure of leaf stomata, reducing malondialdehyde and superoxide anion in leaves, and increasing POD and CAT activity. These findings showed that the exogenous application of NO improved the ability of watermelon to withstand aluminum stress. To further reveal the mitigation mechanism of NO on watermelons under aluminum stress, the differences following different types of treatments—normal growth, Al, and Al + NO—were shown using de novo sequencing of transcriptomes. In total, 511 differentially expressed genes (DEGs) were identified between the Al + NO and Al treatment groups. Significantly enriched biological processes included nitrogen metabolism, phenylpropane metabolism, and photosynthesis. We selected 23 genes related to antioxidant enzymes and phenylpropane metabolism for qRT-PCR validation. The results showed that after exogenous application of NO, the expression of genes encoding POD and CAT increased, consistent with the results of the physiological indicators. The expression patterns of genes involved in phenylpropanoid metabolism were consistent with the transcriptome expression abundance. These results indicate that aluminum stress was involved in the inhibition of the photosynthetic pathway, and NO could activate the antioxidant enzyme defense system and phenylpropane metabolism to protect cells and scavenge reactive oxygen species. This study improves our current understanding by comprehensively analyzing the molecular mechanisms underlying NO-induced aluminum stress alleviation in watermelons.

Response to Antimony Toxicity in Dittrichia viscosa Plants: ROS, NO, H2S, and the Antioxidant System

Dittrichia viscosa plants were grown hydroponically with different concentrations of Sb. There was preferential accumulation of Sb in roots. Fe and Cu decreased, while Mn decreased in roots but not in leaves. Chlorophyll content declined, but the carotenoid content increased, and photosynthetic efficiency was unaltered. O₂^●− generation increased slightly, while lipid peroxidation increased only in roots. H₂O₂, NO, ONOO⁻, S-nitrosothiols, and H₂S showed significant increases, and the enzymatic antioxidant system was altered. In roots, superoxide dismutase (SOD) and monodehydroascorbate reductase (MDAR) activities declined, dehydroscorbate reductase (DHAR) rose, and ascorbate peroxidase (APX), peroxidase (POX), and glutathione reductase (GR) were unaffected. In leaves, SOD and POX increased, MDAR decreased, and APX was unaltered, while GR increased. S-nitrosoglutathione reductase (GSNOR) and l-cysteine desulfhydrilase (l-DES) increased in activity, while glutathione S-transferase (GST) decreased in leaves but was enhanced in roots. Components of the AsA/GSH cycle decreased. The great capacity of Dittrichia roots to accumulate Sb is the reason for the differing behaviour observed in the enzymatic antioxidant systems of the two organs. Sb appears to act by binding to thiol groups, which can alter free GSH content and SOD and GST activities. The coniferyl alcohol peroxidase activity increased, possibly to lignify the roots’ cell walls. Sb altered the ROS balance, especially with respect to H₂O₂. This led to an increase in NO and H₂S acting on the antioxidant system to limit that Sb-induced redox imbalance. The interaction NO, H₂S and H₂O₂ appears key to the response to stress induced by Sb. The interaction between ROS, NO, and H₂S appears to be involved in the response to Sb.

Foliar Application of Sodium Nitroprusside Boosts Solanum lycopersicum L. Tolerance to Glyphosate by Preventing Redox Disorders and Stimulating Herbicide Detoxification Pathways

Strategies to minimize the effects of glyphosate (GLY), the most used herbicide worldwide, on non-target plants need to be developed. In this context, the current study was designed to evaluate the potential of nitric oxide (NO), provided as 200 µM sodium nitroprusside (SNP), to ameliorate GLY (10 mg kg⁻¹ soil) phytotoxicity in tomato plants. Upon herbicide exposure, plant development was majorly inhibited in shoots and roots, followed by a decrease in flowering and fruit set; however, the co-application of NO partially prevented these symptoms, improving plant growth. Concerning redox homeostasis, lipid peroxidation (LP) and reactive oxygen species (ROS) levels rose in response to GLY in shoots of tomato plants, but not in roots. Additionally, GLY induced the overaccumulation of proline and glutathione, and altered ascorbate redox state, but resulted in the inhibition of the antioxidant enzymes. Upon co-treatment with NO, the non-enzymatic antioxidants were not particularly changed, but an upregulation of all antioxidant enzymes was found, which helped to keep ROS and LP under control. Overall, data point towards the benefits of NO against GLY in tomato plants by reducing the oxidative damage and stimulating detoxification pathways, while also preventing GLY-induced impairment of flowering and fruit fresh mass.

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

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

Improved physiological defense responses by application of sodium nitroprusside in Isatis cappadocica Desv. under cadmium stress

Isatis cappadocica is a well-known arsenic-hyperaccumulator, but there are no reports of its responses to cadmium (Cd). Nitric oxide (NO) is a signaling molecule, which induces cross-stress tolerance and mediates several physio-biochemical processes related to heavy metal toxicity. In this study, the effects of Cd and sodium nitroprusside (SNP as NO donor) on the growth, defense responses and Cd accumulation in I. cappadocica were investigated. When I. cappadocica was treated with 100 and 200 μM Cd, there was an insignificant inhibition of shoot growth. However, Cd stress at Cd400 treatment decreased significantly the dry weight of root and shoot by 73 and 38%, respectively, as compared to control. The application of SNP significantly improved the growth parameters and mitigated Cd toxicity. In addition, SNP decreased reactive oxygen species (ROS) production induced by Cd. The increased total thiol and glutathione (GSH) concentrations after SNP application may play a decisive role in maintaining cellular redox homeostasis, thereby protecting plants against oxidative damage under Cd stress. Bovine hemoglobin (Hb as NO scavenger) reduced the protective role of SNP, suggesting a major role of NO in the defensive effect of SNP. Furthermore, the reduction in shoot growth and the increase of oxidative damage were more severe after the addition of Hb, which confirms the protective role of NO against Cd-induced oxidative stress. The protective role of SNP in decreasing Cd-induced oxidative stress may be related to NO production, which can lead to stimulation of the thiols synthesis and improve defense system.

Priming of tomato seedlings with 2-oxoglutarate induces arsenic toxicity alleviatory responses by involving endogenous nitric oxide

Metal toxicity in crop plants is a matter of scientific concern. Therefore, in recent years efforts have been made to minimize metal toxicity in crop plants. Out of various strategies, priming of seedlings with certain chemicals, like e.g. donors of signaling molecules, nutrients, metabolites or plant hormones has shown encouraging results. However, mechanisms related with the priming-induced mitigation of metal toxicity are still poorly known. Hence, we have tested the potential of 2-oxoglutarate (2-OG) priming in enhancing the arsenate (As^V ) toxicity tolerance in tomato seedlings along with deciphering the probable role of nitric oxide (NO) in accomplishing this task. Arsenate decreased growth, endogenous NO and nitric oxide synthase-like activity but enhanced the accumulation of As, which collectively led to root cell death. Arsenate toxicity also decreased some photosynthetic characteristics (i.e. F_v /F_m, qP, F_v /F₀ and F_m /F₀ , and total chlorophyll content) but enhanced NPQ. However, priming with 2-OG alleviated the toxic effect of As^V on growth, endogenous NO, cell death and photosynthesis. Moreover, arsenate inhibited the activities of enzymes of nitrogen metabolism (i.e. nitrate reductase, nitrite reductase, glutamine synthetase and glutamine 2-oxoglutarate aminotransferase) but increased the activity of glutamate dehydrogenase and NH₄ ⁺ content. Superoxide radicals, hydrogen peroxide, lipid peroxidation, protein oxidation and membrane damage increased upon As^V exposure, but the antioxidant enzymes (i.e. superoxide dismutase, catalase and glutathione-S-transferase) showed differential responses. Overall, our results showed that 2-OG is capable of alleviating As^V toxicity in tomato seedlings but the involvement of endogenous NO is probably required.

Methyl Jasmonate and Sodium Nitroprusside Jointly Alleviate Cadmium Toxicity in Wheat (Triticum aestivum L.) Plants by Modifying Nitrogen Metabolism, Cadmium Detoxification, and AsA-GSH Cycle

The principal intent of the investigation was to examine the influence of joint application of methyl jasmonate (MeJA, 10 μM) and a nitric oxide-donor sodium nitroprusside (SNP, 100 μM) to wheat plants grown under cadmium (Cd as CdCl2, 100 μM) stress. Cd stress suppressed plant growth, chlorophylls (Chl), and PSII maximum efficiency (F v /F m ), but it elevated leaf and root Cd, and contents of leaf proline, phytochelatins, malondialdehyde, and hydrogen peroxide, as well as the activity of lipoxygenase. MeJA and SNP applied jointly or singly improved the concentrations of key antioxidant biomolecules, e.g., reduced glutathione and ascorbic acid and the activities of the key oxidative defense system enzymes such as catalase, superoxide dismutase, dehydroascorbate reductase, glutathione S-transferase, and glutathione reductase. Exogenously applied MeJA and SNP jointly or singly also improved nitrogen metabolism by activating the activities of glutamine synthetase, glutamate synthase, and nitrate and nitrite reductases. Compared with individual application of MeJA or SNP, the combined application of both showed better effect in terms of improving plant growth and key metabolic processes and reducing tissue Cd content, suggesting a putative interactive role of both compounds in alleviating Cd toxicity in wheat plants.

MAIN FINDINGS

The main findings are that exogenous application of methyl jasmonate and nitric oxide-donor sodium nitroprusside alleviated the cadmium (Cd)-induced adverse effects on growth of wheat plants grown under Cd by modulating key physiological processes and up-regulating enzymatic antioxidants and the ascorbic acid-glutathione cycle-related enzymes.

Nitric oxide, crosstalk with stress regulators and plant abiotic stress tolerance

Nitric oxide is a dynamic gaseous molecule involved in signalling, crosstalk with stress regulators, and plant abiotic-stress responses. It has great exploratory potentials for engineering abiotic stress tolerance in crops. Nitric oxide (NO), a redox-active gaseous signalling molecule, though present uniformly through the eukaryotes, maintain its specificity in plants with respect to its formation, signalling, and functions. Its cellular concentrations are decisive for its function, as a signalling molecule at lower concentrations, but triggers nitro-oxidative stress and cellular damage when produced at higher concentrations. Besides, it also acts as a potent stress alleviator. Discovered in animals as neurotransmitter, NO has come a long way to being a stress radical and growth regulator in plants. As a key redox molecule, it exhibits several key cellular and molecular interactions including with reactive chemical species, hydrogen sulphide, and calcium. Apart from being a signalling molecule, it is emerging as a key player involved in regulations of plant growth, development and plant-environment interactions. It is involved in crosstalk with stress regulators and is thus pivotal in these stress regulatory mechanisms. NO is getting an unprecedented attention from research community, being investigated and explored for its multifaceted roles in plant abiotic stress tolerance. Through this review, we intend to present the current knowledge and updates on NO biosynthesis and signalling, crosstalk with stress regulators, and how biotechnological manipulations of NO pathway are leading towards developing transgenic crop plants that can withstand environmental stresses and climate change. The targets of various stress responsive miRNA signalling have also been discussed besides giving an account of current approaches used to characterise and detect the NO.

Nitrate reductase is required for salicylic acid-induced water stress tolerance of pepper by upraising the AsA-GSH pathway and glyoxalase system

A trial was conducted to evaluate whether nitrate reductase (NR) participates in salicylic acid (SA)-improved water stress (WS) tolerance in pepper (Capsicum annuum L.) plants. Before starting WS treatment, 0.5 mM SA was applied to half of the well-watered (WW) plants as well as to WS-plants as a foliar spray once a day for a week. The soil water holding capacity was maintained at 40 and 80% of the full water storing capacity for WS and and well-watered (WW) plants, respectively. Water stress caused substantial decreases in total plant dry weight, F_v /F_m , chlorophyll a and b, relative water content, leaf water potential (ΨI) by 53, 37, 49, 21, 36 and 33%, respectively relative to control, but significant increases in malondialdehyde (MDA), hydrogen peroxide (H₂ O₂ ), electrolyte leakage (EL), methylglyoxal (MG), proline, key antioxidant enzymes' activities, NO and NR activity. The SA reduced oxidative stress, but improved antioxidant defence system, ascorbate-glutathione (AsA-GSH) cycle enzymes, glyoxalase system-related enzymes, glyoxalase I (Gly I) and glyoxalase II (Gly II), plant growth, photosynthetic traits, NO, NR and proline. SA-induced WS tolerance was further improved by supplementation of sodium nitroprusside (SNP), a donor of NO. NR inhibitor, sodium tungstate (ST) was applied in conjunction with SA and SA + SNP to the WW and WS-plants to assess whether NR contributes to SA-improved WS tolerance. ST abolished the beneficial effects of SA by reducing NO and NR activity in WS-pepper, but the application of SNP along with SA + ST reversed negative effects of ST, showing that NO and NR are jointly needed for SA-induced WS tolerance of pepper plants.

Mechanistic insights of CRISPR/Cas-mediated genome editing towards enhancing abiotic stress tolerance in plants

Abiotic stresses such as temperature (high/low), drought, salinity, and others make the environment hostile to plants. Abiotic stressors adversely affect plant growth and development; and thereby makes a direct impact on overall plant productivity. Plants confront stress by developing an internal defense system orchestrated by compatible solutes, reactive oxygen species scavengers and phytohormones. However, routine exposure to unpredictable environmental stressors makes it essential to equip plants with a system that contributes to sustainable agricultural productivity, besides imparting multi-stress tolerance. The sustainable approach against abiotic stress is accomplished through breeding of tolerant cultivars. Though eco-friendly, tedious screening and crossing protocol limits its usage to overcome stress and in attaining the goal of global food security. Advancement on the technological front has enabled adoption of genomic engineering approaches to perform site-specific modification in the plant genome for improving adaptability, increasing the yield and in attributing resilience against different stressors. Of the different genome editing approaches, CRISPR/Cas has revolutionized biological research with wider applicability to crop plants. CRISPR/Cas emerged as a versatile tool in editing genomes for desired traits in highly accurate and precise manner. The present study summarizes advancement of the CRISPR/Cas genome editing tool in its adoption to manipulate plant genomes for novel traits towards developing high-yielding and climate-resilient crop varieties.

Downstream Signalling from Molecular Hydrogen

Molecular hydrogen (H2) is now considered part of the suite of small molecules that can control cellular activity. As such, H2 has been suggested to be used in the therapy of diseases in humans and in plant science to enhance the growth and productivity of plants. Treatments of plants may involve the creation of hydrogen-rich water (HRW), which can then be applied to the foliage or roots systems of the plants. However, the molecular action of H2 remains elusive. It has been suggested that the presence of H2 may act as an antioxidant or on the antioxidant capacity of cells, perhaps through the scavenging of hydroxyl radicals. H2 may act through influencing heme oxygenase activity or through the interaction with reactive nitrogen species. However, controversy exists around all the mechanisms suggested. Here, the downstream mechanisms in which H2 may be involved are critically reviewed, with a particular emphasis on the H2 mitigation of stress responses. Hopefully, this review will provide insight that may inform future research in this area.

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.

Exogenously supplied silicon (Si) improves cadmium tolerance in pepper (Capsicum annuum L.) by up-regulating the synthesis of nitric oxide and hydrogen sulfide

The current research was aimed to observe the interactive role of silicon-generated hydrogen sulfide (H₂S) and nitric oxide (NO) on tolerance of pepper (Capsicum annum L.) plants to cadmium (Cd). Thus, the pepper plants were subjected to control (no Cd) or cadmium stress with and without Si supplementation. Significant decreases were found in plant dry weights, water potential, PSII maximum efficiency, glutathione (GSH), total chlorophyll, relative water content, Ca²⁺ and K⁺ concentrations and ascorbate, but there was a significant increase in H₂O₂, MDA, electron leakage (EL), proline, key antioxidant enzymes' activities, and endogenous Cd, NO and H₂S in the Cd-stressed plants. Silicon enhanced Cd tolerance of the pepper plants by lowering the leaf Cd concentration, oxidative stress, enhancing the antioxidant defence system, leaf Si content, photosynthetic traits and plant growth as well as the contents of NO, proline and H₂S. Furthermore, foliar-applied NO scavenger, cPTIO, and that of H₂S, hypotaurine (HT), significantly decreased the levels of H₂S alone, but cPTIO effectively reduced the concentrations of NO and H₂S accumulated by Si in the Cd-stressed plants. The positive effect of Si was eliminated by cPTIO, but not by HT, suggesting that both molecules were involved in Si-induced improvement in Cd tolerance of the pepper plants.

Roles of nitric oxide in heavy metal stress in plants: Cross-talk with phytohormones and protein S-nitrosylation

Heavy metal (HM) stress is a major hazard, which significantly affects plant growth and development. In order to confront HM stress, plants directly or indirectly regulate the levels of endogenous nitric oxide (NO), a redox-related signaling molecule involved in wide range of plant growth and development as well as in response to HM stress. In addition, there is now compelling experimental evidence that NO usually mediates signaling processes through interactions with different biomolecules like phytohormones to regulate HM tolerance. Apart from phytohormones, NO partly operates through posttranslational modification of proteins, notably via S-nitrosylation in response to HM stress. Recently, the roles of S-nitrosylation as a regulator of plant responses to HM stress and S-nitrosylated candidates have also been established and detected. Here, we describe the roles of NO in confronting HM phytotoxicity in plants with a particular focus on the presentation and discussion of recent data obtained in this field, which involves in the function of various phytohormones and S-nitrosylation during plant responses to HM stress. Additionally, both importance and challenges of future work are outlined in order to further elucidate the specific mechanisms underlying the roles of NO in plant responses to HM stress.

Involvement of nitrate reductase-dependent nitric oxide production in magnetopriming-induced salt tolerance in soybean

In the present study, experiments were performed to investigate the role of nitric oxide (NO) in magnetopriming-induced seed germination and early growth characteristics of soybean (Glycine max) seedlings under salt stress. The NO donor (sodium nitroprusside, SNP), NO scavenger (2-[4-carboxyphenyl]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, CPTIO), inhibitors of nitrate reductase (sodium tungstate, ST) or NO synthase (N-nitro-L-Arg-methyl ester, LNAME) and NADPH oxidase inhibitor (diphenylene iodonium, DPI) have been used to measure the role of NO in the alleviation of salinity stress by static magnetic field (SMF of 200 mT, 1 h). Salt stress (50 mM NaCl) significantly reduced germination and early growth of seedlings emerged from non-primed seeds. Pre-treatment of seeds with SMF positively stimulated the germination and consequently promoted the seedling growth. ST, LNAME, CPTIO and DPI significantly decreased the growth of seedling, activities of α-amylase, protease and nitrate reductase (NR), hydrogen peroxide (H2 O2 ), superoxide (O2 •- ) and NO content in roots of seedlings emerged from non-primed and SMF-primed seeds. However, the extent of reduction was higher with ST in seedlings of SMF-primed seeds under both conditions, whereas SNP promoted all the studied parameters. Moreover, the generation of NO was also confirmed microscopically using a membrane permanent fluorochrome (4-5-diaminofluorescein diacetate [DAF-2 DA]). Further, analysis showed that SMF enhanced the NR activity and triggered the NO production and NR was maximally decreased by ST as compared to LNAME, CPTIO and DPI. Thus, in addition to ROS, NO might be one of the important signaling molecules in magnetopriming-induced salt tolerance in soybean and NR may be responsible for SMF-triggered NO generation in roots of soybean.

Role of Exogenous and Endogenous Hydrogen Sulfide (H2S) on Functional Traits of Plants Under Heavy Metal Stresses: A Recent Perspective

Improving growth and productivity of plants that are vulnerable to environmental stresses, such as heavy metals, is of significant importance for meeting global food and energy demands. Because heavy metal toxicity not only causes impaired plant growth, it has also posed many concerns related to human well-being, so mitigation of heavy metal pollution is a necessary priority for a cleaner environment and healthier world. Hydrogen sulfide (H₂S), a gaseous signaling molecule, is involved in metal-related oxidative stress mitigation and increased stress tolerance in plants. It performs multifunctional roles in plant growth regulation while reducing the adverse effects of abiotic stress. Most effective function of H₂S in plants is to eliminate metal-related oxidative toxicity by regulating several key physiobiochemical processes. Soil pollution by heavy metals presents significant environmental challenge due to the absence of vegetation cover and the resulting depletion of key soil functions. However, the use of stress alleviators, such as H₂S, along with suitable crop plants, has considerable potential for an effective management of these contaminated soils. Overall, the present review examines the imperative role of exogenous application of different H₂S donors in reducing HMs toxicity, by promoting plant growth, stabilizing their physiobiochemical processes, and upregulating antioxidative metabolic activities. In addition, crosstalk of different growth regulators with endogenous H₂S and their contribution to the mitigation of metal phytotoxicity have also been explored.

Modulation of the Functional Components of Growth, Photosynthesis, and Anti-Oxidant Stress Markers in Cadmium Exposed Brassica juncea L

Abstract: Heavy metals (including Cadmium) are being entered into the environment through various sources and cause toxicity to plants. Response of Brassica juncea L. var. RLC-1 was evaluated after exposing them to different concentration of cadmium (Cd) for seven days. Seeds of B. juncea were treated with different concentrations of Cd like 0.2-0.6 mM for 7 days, allowing them to grow in Petri-dishes, and seedlings were examined for different physiological responses. Following exposure to Cd, in the seedlings of B. juncea, growth parameters (root and shoot length), stress markers (lipid peroxidation and H2O2 content), secondary metabolites, photosynthetic pigments, and ion analysis, were estimated along with enzymatic and non-enzymatic antioxidants. We observed a significant reduction in root and shoot length after Cd treatment as compared to control seedlings. Malondialdehyde and H2O2 contents were increased accompanied by enhanced Cd uptake. Activities of antioxidative enzymes were also significantly altered following Cd exposure to the seedlings of B. juncea. Conclusively, we suggest that Cd exposure to the seedlings triggered an induction of several defense responses in B. juncea including major metabolites.

Phytohormones Regulate Accumulation of Osmolytes Under Abiotic Stress

Plants face a variety of abiotic stresses, which generate reactive oxygen species (ROS), and ultimately obstruct normal growth and development of plants. To prevent cellular damage caused by oxidative stress, plants accumulate certain compatible solutes known as osmolytes to safeguard the cellular machinery. The most common osmolytes that play crucial role in osmoregulation are proline, glycine-betaine, polyamines, and sugars. These compounds stabilize the osmotic differences between surroundings of cell and the cytosol. Besides, they also protect the plant cells from oxidative stress by inhibiting the production of harmful ROS like hydroxyl ions, superoxide ions, hydrogen peroxide, and other free radicals. The accumulation of osmolytes is further modulated by phytohormones like abscisic acid, brassinosteroids, cytokinins, ethylene, jasmonates, and salicylic acid. It is thus important to understand the mechanisms regulating the phytohormone-mediated accumulation of osmolytes in plants during abiotic stresses. In this review, we have discussed the underlying mechanisms of phytohormone-regulated osmolyte accumulation along with their various functions in plants under stress conditions.