Nitric oxide and phytohormone interactions in the response of Lactuca sativa to salinity stress

Adaptive responses of nitric oxide (NO) and its intricate dialogue with phytohormones during salinity stress

Nitric oxide (NO) is a gaseous free radical that acts as a messenger for various plant phenomena corresponding to photomorphogenesis, fertilisation, flowering, germination, growth, and productivity. Recent developments have suggested the critical role of NO in inducing adaptive responses in plants during salinity. NO minimises salinity-induced photosynthetic damage and improves plant-water relation, nutrient uptake, stomatal conductance, electron transport, and ROS and antioxidant metabolism. NO contributes active participation in ABA-mediated stomatal regulation. Similar crosstalk of NO with other phytohormones such as auxins (IAAs), gibberellins (GAs), cytokinins (CKs), ethylene (ET), salicylic acid (SA), strigolactones (SLs), and brassinosteroids (BRs) were also observed. Additionally, we discuss NO interaction with other gaseous signalling molecules such as reactive oxygen species (ROS) and reactive sulphur species (RSS). Conclusively, the present review traces critical events in NO-induced morpho-physiological adjustments under salt stress and discusses how such modulations upgrade plant resilience.

Nitric oxide: An emerging warrior of plant physiology under abiotic stress

The natural environment of plants comprises a complex set of various abiotic stresses and their capability to react and survive under this anticipated changing climate is highly flexible and involves a series of balanced interactions between signaling molecules where nitric oxide becomes a crucial component. In this article, we focussed on the role of nitric oxide (NO) in various signal transduction pathways of plants and its positive impact on maintaining cellular homeostasis under various abiotic stresses. Besides this, the recent data on interactions of NO with various phytohormones to control physiological and biochemical processes to attain abiotic stress tolerance have also been considered. These crosstalks modulate the plant's defense mechanism and help in alleviating the negative impact of stress. While focusing on the diverse functions of NO, an effort has been made to explore the functions of NO-mediated post-translational modifications, such as the N-end rule pathway, tyrosine nitration, and S-nitrosylation which revealed the exact mechanism and characterization of proteins that modify various metabolic processes in stressed conditions. Considering all of these factors, the present review emphasizes the role of NO and its interlinking with various phytohormones in maintaining developmental processes in plants, specifically under unfavorable environments.

Deciphering the biochemical regulation of metabolic pathways in abating stress-induced damages during co-exposure of arsenic and fluoride in a non-aromatic and an aromatic rice variety

The aim of the current work was to decipher the systemic damage and biochemical defense machinery due to combined arsenic (5 mg L-1 Na3AsO4) and fluoride (50 mg L-1 NaF) stress in two rice cultivars viz., IR-64 (non-aromatic) and Gobindobhog (aromatic), grown for 14 days, under 16/8 h light/dark photoperiodic cycle at 32 °C. Higher accumulation of arsenic and fluoride in Gobindobhog generated higher levels of H2O2 that caused higher electrolyte leakage, along with malondialdehyde and methylglyoxal formation. Higher oxidative damages severely compromised seed germination and led to chlorophyll loss, inhibition of root and shoot growth and fresh and dry weight of the seedlings. On the contrary, oxidative damage was less pronounced in IR-64, as compared to that of Gobindobhog, which can be attributed to higher accumulation of protective metabolites, i.e., osmolytes and antioxidants. Higher levels of osmolytes (proline, glycine betaine and amino acids) in IR-64 helped in maintaining the osmotic balance of the cells and the integrity of the cell membrane. Additionally, up regulated activity of enzymatic antioxidants (catalase, superoxide dismutase, ascorbate peroxidase, glutathione peroxidase and glutathione S-transferase) along with elevated levels of non-enzymatic antioxidants (flavonoids, phenolics, xanthophylls and carotenoids) played a pivotal role in controlling oxidative damages and strengthening the defense machinery in IR-64, as compared to that of Gobindobhog where lesser enhancement in the level of the above mentioned protective metabolites was noted. The present work illustrated differential phytotoxicity in rice seedlings and elucidated the yet uncharacterized biohazard associated with arsenic and fluoride co-contamination, with better adaptive features of IR-64, compared to Gobindobhog, which appeared as the sensitive variety.

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

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

Nitric oxide, calmodulin and calcium protein kinase interactions in the response of Brassica napus to salinity stress

Involvement of nitric oxide (NO) in plant metabolism and its connection with phytohormones has not been fully described, thus information about the role of this molecule in signalling pathways remains fragmented. In this study, the effects of NO on calmodulin (CAM), calcium protein kinase (CPK), content of phytohormones and secondary metabolites in canola plants under salinity stress were investigated. We applied 100 μM sodium nitroprusside as an NO source to canola plants grown under saline (100 mM NaCl) and non-saline conditions at the vegetative stage. Plant growth was negatively affected by salinity, but exogenous NO treatment improved growth. NO caused a significant increase in activity of CAT, SOD and POX through their enhanced gene expression in stressed canola. Salinity-responsive genes, namely CAM and CPK, were induced by NO in plants grown under salinity. NO application enhanced phenolic compounds, such as gallic acid and coumaric acid and flavonoid compound,s catechin, diadzein and kaempferol, in plants subjected to salinity. NO treatment enhanced abscisic acid and brassinosteroids but decreased auxin and gibberellin in stressed canola plants. The impacts of NO in improving stress tolerance in canola required CAM and CPK. Also, NO signalling re-established the phytohormone balance and resulted in enhanced tolerance to salt stress. Furthermore, NO improved salinity tolerance in canola by increasing enzymatic and non-enzymatic antioxidant content.

Regulation of Reactive Oxygen Species during Salt Stress in Plants and Their Crosstalk with Other Signaling Molecules-Current Perspectives and Future Directions

Salt stress is a severe type of environmental stress. It adversely affects agricultural production worldwide. The overproduction of reactive oxygen species (ROS) is the most frequent phenomenon during salt stress. ROS are extremely reactive and, in high amounts, noxious, leading to destructive processes and causing cellular damage. However, at lower concentrations, ROS function as secondary messengers, playing a critical role as signaling molecules, ensuring regulation of growth and adjustment to multifactorial stresses. Plants contain several enzymatic and non-enzymatic antioxidants that can detoxify ROS. The production of ROS and their scavenging are important aspects of the plant's normal response to adverse conditions. Recently, this field has attracted immense attention from plant scientists; however, ROS-induced signaling pathways during salt stress remain largely unknown. In this review, we will discuss the critical role of different antioxidants in salt stress tolerance. We also summarize the recent advances on the detrimental effects of ROS, on the antioxidant machinery scavenging ROS under salt stress, and on the crosstalk between ROS and other various signaling molecules, including nitric oxide, hydrogen sulfide, calcium, and phytohormones. Moreover, the utilization of "-omic" approaches to improve the ROS-regulating antioxidant system during the adaptation process to salt stress is also described.

Salicylic acid-mediated alleviation of fluoride toxicity in rice by restricting fluoride bioaccumulation and strengthening the osmolyte, antioxidant and glyoxalase systems

The aim of the manuscript was to demonstrate the efficacy of salicylic acid (SA) in abrogating the fluoride-induced oxidative damages in the susceptible rice cultivar, MTU1010. Prolonged exposure of seedlings to sodium fluoride (25 mg L^-1) severely impaired growth and overall physiological parameters like germination percentage, biomass and root and shoot length and incited the formation of hydrogen peroxide that enhanced electrolyte leakage, formation of cytotoxic products like malondialdehyde and methylglyoxal and lipoxygenase activity. Exogenous application of SA (0.5 mM) enhanced the endogenous level of SA that restored the chlorophyll content and catalase activity and further escalated the activity of other enzymatic antioxidants (superoxide dismutase, guaiacol peroxidase, ascorbate peroxidase, glutathione peroxidase and glutathione S-transferase), formation of non-enzymatic antioxidants (anthocyanins, carotenoids, flavonoids, phenolics, ascorbate and reduced glutathione) and osmolytes (proline, amino acids and glycine betaine) that cumulatively maintained the integrity of membrane structure and homeostatic balance of the cells by scavenging the accumulated hydrogen peroxide. SA-mediated formation of proline and flavonoids was linked with the enhanced activity of Δ¹-pyrroline-5-carboxylate synthetase and phenylalanine ammonia lyase. Fluoride stress enhanced the activity of enzymes like glyoxalase I and glyoxalase II which were further aggravated in the seedlings upon treatment with SA, effectively detoxifying the methylglyoxal formed during stress. Overall, the manuscript depicts the pivotal role played by exogenous SA in ameliorating the effects of fluoride-induced damages in the seedlings and proves its potentiality as a protective chemical against fluoride stress when applied exogenously in rice.

Role of Sodium Nitroprusside on Potential Mitigation of Salt Stress in Centaury (Centaurium erythraea Rafn) Shoots Grown In Vitro

Soil salinity is one of the most common abiotic stressors that affects plant growth and development. The aim of this work was to investigate the influence of sodium nitroprusside (SNP), a donor of nitric oxide (NO), on the physiological response of common centaury (Centaurium erythraea) shoots grown under stress conditions caused by sodium chloride (NaCl) in vitro. Centaury shoots were first grown on nutrient medium containing different SNP concentrations (50, 100 and 250 μM) during the pretreatment phase. After three weeks, the shoots were transferred to nutrient media supplemented with NaCl (150 mM) and/or SNP (50, 100 or 250 μM) for one week. The results showed that salinity decreased photosynthetic pigments, total phenolic content and DPPH (1,1-diphenyl-2-picrylhydrazyl radical) concentration. The activities of antioxidant enzymes, namely superoxide dismutase (SOD), catalase (CAT) and peroxidase (POX), were also reduced under salt stress. However, MDA concentration was decreased, while H₂O₂ and proline content did not drastically change under the stress conditions caused by NaCl. Exogenous application of SNP altered the biochemical parameters of centaury shoots grown under salt stress. In this case, increased photosynthetic pigment content, total phenolics and proline content were noted, with reduced MDA, but not H₂O₂, concentration was observed. In addition, the exogenous application of SNP increased the degree of DPPH reduction as well as SOD, CAT and POX activities.

Say "NO" to plant stresses: Unravelling the role of nitric oxide under abiotic and biotic stress

Nitric oxide (NO) is a diatomic gaseous molecule, which plays different roles in different strata of organisms. Discovered as a neurotransmitter in animals, NO has now gained a significant place in plant signaling cascade. NO regulates plant growth and several developmental processes including germination, root formation, stomatal movement, maturation and defense in plants. Due to its gaseous state, it is unchallenging for NO to reach different parts of cell and counterpoise antioxidant pool. Various abiotic and biotic stresses act on plants and affect their growth and development. NO plays a pivotal role in alleviating toxic effects caused by various stressors by modulating oxidative stress, antioxidant defense mechanism, metal transport and ion homeostasis. It also modulates the activity of some transcriptional factors during stress conditions in plants. Besides its role during stress conditions, interaction of NO with other signaling molecules such as other gasotransmitters (hydrogen sulfide), phytohormones (abscisic acid, salicylic acid, jasmonic acid, gibberellin, ethylene, brassinosteroids, cytokinins and auxin), ions, polyamines, etc. has been demonstrated. These interactions play vital role in alleviating plant stress by modulating defense mechanisms in plants. Taking all these aspects into consideration, the current review focuses on the role of NO and its interaction with other signaling molecules in regulating plant growth and development, particularly under stressed conditions.

Nitric oxide and hydrogen sulfide work together to improve tolerance to salinity stress in wheat plants by upraising the AsA-GSH cycle

The participation of nitric oxide (NO) in wheat plant tolerance to salinity stress (SS) brought about by hydrogen sulphide (H₂S) via modifying the ascorbate-glutathione (AsA-GSH) cycle was studied. The SS-plants received either 0.2 mM sodium hydrosulfide (NaHS; H₂S donor), or NaHS plus 0.1 mM sodium nitroprusside (SNP; a NO donor) through the nutrient solution. Salinity stress decreased plant growth, leaf water status, leaf K⁺, and glyoxalase II (gly II), while it elevated proline content, leaf Na⁺ content, oxidative stress, methylglyoxal (MG), glyoxalase I (gly I), the superoxide dismutase, catalase and peroxidase activities, contents of endogenous NO and H₂S. The NaHS supplementation elevated plant development, decreased leaf Na⁺ content and oxidative stress, and altered leaf water status, leaf K⁺ and involved enzymes in AsA-GSH, H₂S and NO levels. The SNP supplementation boosted the positive impact of NaHS on these traits in the SS-plants. Moreover, 0.1 mM cPTIO, scavenger of NO, countered the beneficial effect of NaHS by lowering NO levels. SNP and NaHS + cPTIO together restored the beneficial effects of NaHS by increasing NO content, implying that NO may have been a major factor in SS tolerance in wheat plants induced by H₂S via activating enzymes connected to the AsA-GSH cycle.

Nitric oxide mediated alleviation of abiotic challenges in plants

Agriculture and ecosystem are negatively influenced by the abiotic stresses which create solemn pressures on plants as they are sessile in nature leading to excessive losses in economy. For maintenance of sustainable agriculture and to fulfil the cumulative call of food for rapidly growing population worldwide, it becomes crucial to protects the crop plants from climate fluctuations. Plants fight back against these challenges by generation of redox molecules comprising reactive oxygen species (ROS) and reactive nitrogen species (RNS) and cause modulation at cellular, physiological and molecular levels. Nitric oxide (NO) deliver tolerance to several biotic and abiotic stresses in plants by acting as signalling molecule or free radicals. It is also intricated in several developmental processes in plants using different mechanisms. Supplementation of exogenous NO reduce toxicity of abiotic stresses and provide resistance. In this review article, we summarize the recent research studies (five years) depicting the functional role of NO in alleviation of abiotic stresses such as drought, cold, heat, heavy metals and flooding. Moreover, by investigating studies found that among heavy metals works associated with Hg, Pb, and Cr is limited comparatively. Additionally, role of NO in abiotic stress resistance such as cold, freezing and heat stress less/poorly investigated. Consequently, further emphasis should be diverted towards how NO can facilitate protection against these stresses. In recent studies mostly beneficial role of NO against abiotic challenges have been elucidated by observing physiological/biochemical parameters but relatively inadequate research done at the transcripts level or gene regulation subsequently researchers should include it in future. Lastly, brief outline and an evaluative discussion on the present information and future prospective provided. Altogether, these inclusive experimental agendas could facilitate in future to produce climate tolerant plants. This will help to confront the constant fluctuations in the environment and to reduce the challenges in way of agriculture productivity and global food demands.

Mercury stress tolerance in wheat and maize is achieved by lignin accumulation controlled by nitric oxide

Nitric oxide (NO) is an important phytohormone for plant adaptation to mercury (Hg) stress. The effect of Hg on lignin synthesis, NO production in leaf, sheath and root and their relationship were investigated in two members of the grass family - wheat and maize. Hg stress decreased growth and lignin contents, significantly affected phenylpropanoid and monolignol pathways (PAL, phenylalanine ammonia-lyase; 4-coumarate: CoA ligase, 4CL; cinnamyl alcohol dehydrogenase, CAD), with maize identified to be more sensitive to Hg stress than wheat. Among the tissue types, sheath encountered severe damage compared to leaves and roots. Hg translocation in maize was about twice that in wheat. Interestingly, total NO produced under Hg stress was significantly decreased compared to control, with maximum reduction of 43.4% and 42.9% in wheat and maize sheath, respectively. Regression analysis between lignin and NO contents or the activities of three enzymes including CAD, 4CL and PAL displayed the importance of NO contents, CAD, 4CL and PAL for lignin synthesis. Further, the gene expression profiles encoding CAD, 4CL and PAL provided support for the damaging effect of Hg on wheat sheath, and maize shoot. To validate NO potential to mitigate Hg toxicity in maize and wheat, NO donor and NO synthase inhibitor were supplemented along with Hg. The resulting phenotype, histochemical analysis and lignin contents showed that NO mitigated Hg toxicity by improving growth and lignin synthesis and accumulation. In summary, Hg sensitivity was higher in maize seedlings compared to wheat, which was associated with the lower lignin contents and reduced NO contents. External supplementation of NO is proposed as a sustainable approach to mitigate Hg toxicity in maize and wheat.

Cytokinin Production by Azospirillum brasilense Contributes to Increase in Growth, Yield, Antioxidant, and Physiological Systems of Wheat (Triticum aestivum L.)

Plant growth-promoting rhizobacteria are known to associate with several cereal crops. The rhizobacterium exerts its function by synthesizing diverse arrays of phytohormones, such as cytokinin (Ck). However, it is difficult to determine the plant growth promotion when a bacterium produces many different kinds of phytohormones. Therefore, to assess the involvement of Ck in growth promotion and activation of antioxidant and physiological systems, we set up this experiment. Wheat seeds (Triticum aestivum L.) were inoculated with Azospirillum brasilense RA−17 (which produces zeatin type Ck) and RA−18 (which failed to produce Ck). Results showed that seed inoculation with RA−17 significantly improved growth and yield-related parameters compared with RA−18. The activity of enzymes, proline contents, and endogenous hormonal levels in wheat kernels were improved considerably with RA−17 than with RA−18. Strain RA−17 enhanced grain assimilation more than strain RA−18 resulting in a higher crop yield. These results suggest that microbial Ck production may be necessary for stimulating plant growth promotion and activating antioxidant and physiological systems in wheat.

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.

Nitric Oxide Crosstalk With Phytohormone Is Involved in Enhancing Photosynthesis of Tetrastigma hemsleyanum for Photovoltaic Adaptation

Photovoltaic agriculture is a newly emerging ecological planting pattern. In view of the adverse effect on production, a better understanding of photovoltaic adaptation responses is essential for the development of the innovative agriculture mode in sustainable crop production. Here, we investigated the impact of photovoltaic condition on endogenous hormone composition and transcriptome profile of Tetrastigma hemsleyanum. A total of 16 differentially accumulated phytohormones and 12,615 differentially expressed genes (DEGs) were identified. Photovoltaic adaptation significantly decreased the contents of phytohormones especially salicylic acid (SA) and jasmonic acid (JA). DEGs were the most relevant to photosynthesis and mitogen-activated protein kinase (MAPK) signaling pathway especially the key genes encoding proteins involved in photosystem I (PS I) and photosystem II (PS II) reaction center. Nitric oxide (NO), JA, and SA treatment alone significantly enhanced the photosynthetic efficiency which was decreased by exposure to photovoltaic condition, but the combined treatment of "NO + SA" could weaken the enhancement effect by regulating the expression level of psaL, CHIL, petF1, psbQ, and psaE genes. Exogenous phytohormones and NO treatment mitigated the accumulation of reactive oxygen species (ROS) and potentiated antioxidant capacity, which would be weakened by the combined treatment of "NO + SA." SA and JA significantly decreased endogenous NO burst triggered by photovoltaic adaptation. SA might be a potent scavenger of NO and counter the restoration effect of NO on growth and photosynthetic potential in T. hemsleyanum. The results could provide reference for the application of phytohormones/other signaling molecules in photovoltaic agriculture.

Nitric oxide regulation of plant metabolism

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

Phytotoxicity of the chiral herbicide dichlorprop: Cross-talk between nitric oxide, reactive oxygen species and phytohormones

Nitric oxide (NO), reactive oxygen species (ROS), and phytohormones in plants often initiate responses to sources of abiotic stress. However, we have a poor understanding of the cross-talk between NO, ROS, and phytohormones during exogenous chiral auxin-induced phytotoxicity. In this study, the toxicity of the chiral synthetic auxin herbicide dichlorprop (DCPP) to Arabidopsis thaliana, as well as the mutual regulation of NO, hydrogen peroxide (H₂O₂), superoxide anion (O₂^.-), and phytohormones at the enantiomeric level was investigated. The ROS production exhibited an enantioselective manner, further, that was positively correlated with the change of the morphological indicators. This confirmed that ROS played an important role in the enantioselective effect of DCPP. The distribution of ROS and NO was partially overlapped, indicating that the production of NO may be affected by ROS, and also related to the degree of plant damage. In terms of phytohormones, the level of salicylic acid (SA), jasmonic acid (JA), and abscisic acid (ABA) in the whole plant increased as the (R)-DCPP concentration applied increased, however, the trend has changed, when the data of leaves and roots was discussed separately. The results revealed that the redistribution of phytohormones may exist between leaves and roots, caused by the joint action of ROS and NO. The differences in the biological activity identified between the two enantiomers in this study enhance our understanding of the toxicity mechanism of exogenous auxin via their effects on phytohormones.

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.

Crucial Cell Signaling Compounds Crosstalk and Integrative Multi-Omics Techniques for Salinity Stress Tolerance in Plants

In the era of rapid climate change, abiotic stresses are the primary cause for yield gap in major agricultural crops. Among them, salinity is considered a calamitous stress due to its global distribution and consequences. Salinity affects plant processes and growth by imposing osmotic stress and destroys ionic and redox signaling. It also affects phytohormone homeostasis, which leads to oxidative stress and eventually imbalances metabolic activity. In this situation, signaling compound crosstalk such as gasotransmitters [nitric oxide (NO), hydrogen sulfide (H2S), hydrogen peroxide (H2O2), calcium (Ca), reactive oxygen species (ROS)] and plant growth regulators (auxin, ethylene, abscisic acid, and salicylic acid) have a decisive role in regulating plant stress signaling and administer unfavorable circumstances including salinity stress. Moreover, recent significant progress in omics techniques (transcriptomics, genomics, proteomics, and metabolomics) have helped to reinforce the deep understanding of molecular insight in multiple stress tolerance. Currently, there is very little information on gasotransmitters and plant growth regulator crosstalk and inadequacy of information regarding the integration of multi-omics technology during salinity stress. Therefore, there is an urgent need to understand the crucial cell signaling crosstalk mechanisms and integrative multi-omics techniques to provide a more direct approach for salinity stress tolerance. To address the above-mentioned words, this review covers the common mechanisms of signaling compounds and role of different signaling crosstalk under salinity stress tolerance. Thereafter, we mention the integration of different omics technology and compile recent information with respect to salinity stress tolerance.

Lipoic Acid Combined with Melatonin Mitigates Oxidative Stress and Promotes Root Formation and Growth in Salt-Stressed Canola Seedlings (Brassica napus L.)

Lipoic acid (LA) and melatonin (MT) are pleiotropic molecules participating in plant stress resistance by modulating cellular biochemical changes, ion homeostasis, and antioxidant enzyme activities. However, the combined role of these two molecules in counteracting the detrimental impacts of salinity stress is still unknown. In the present study, we determined the effects of exogenous LA (0.5 µM), MT (1 µM) and their combination (LA + MT) on growth performance and biomass accumulation, photosynthetic pigments, enzymatic and non-enzymatic antioxidant activities, and ions homeostatic in canola (Brassica napus L.) seedlings under salinity stress (0, 100 mM) for 40 days. The results indicate that exogenous application of LA + MT improved the phenotypic growth (by 25 to 45%), root thickness (by 68%), number of later lateral roots (by 52%), root viability (by 44%), and root length (by 50%) under salinity stress. Moreover, total soluble protein, chlorophyll pigments, the concentration of superoxide dismutase (SOD), catalase peroxidase (CAT), and ascorbic peroxidase (ASA) increased with the presence of salt concentration into the growth media and then decreased with the addition of LA + MT to saline solution. Leaf protein contents and the degradation of photosynthetic pigments were lower when LA + MT treatments were added into NaCl media. The proline and phenol contents decreased in the exogenous application of LA + MT treatments more than individual LA or MT treatments under the salinity stress. The incorporation of LA or MT or a combination of LA + MT to saline solution decreased salinity-induced malondialdehyde and electrolyte leakage. In conclusion, the alteration of metabolic pathways, redox modulation, and ions homeostasis in plant tissues by the combined LA and MT application are helpful towards the adaptation of Brassica napus L. seedlings in a saline environment. The results of this study provide, for the first time, conclusive evidence about the protective role of exogenous LA + MT in canola seedlings under salinity stress.

Exogenous Sodium Nitroprusside Mitigates Salt Stress in Lentil (Lens culinaris Medik.) by Affecting the Growth, Yield, and Biochemical Properties

Soil salinity disrupts the physiological and biochemical processes of crop plants and ultimately leads to compromising future food security. Sodium nitroprusside (SNP), a contributor to nitric oxide (NO), holds the potential to alleviate abiotic stress effects and boost tolerance in plants, whereas less information is available on its role in salt-stressed lentils. We examined the effect of exogenously applied SNP on salt-stressed lentil plants by monitoring plant growth and yield-related attributes, biochemistry of enzymes (superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD)) amassing of leaf malondialdehyde (MDA) and hydrogen peroxide (H2O2). Salinity stress was induced by NaCl application at concentrations of 50 mM (moderate salinity) and 100 mM (severe salinity), while it was alleviated by SNP application at concentrations of 50 µM and 100 µM. Salinity stress severely inhibited the length of roots and shoots, the relative water content, and the chlorophyll content of the leaves, the number of branches, pods, seeds, seed yield, and biomass per plant. In addition, MDA, H2O2 as well as SOD, CAT, and POD activities were increased with increasing salinity levels. Plants supplemented with SNP (100 µM) showed a significant improvement in the growth- and yield-contributing parameters, especially in plants grown under moderate salinity (50 mM NaCl). Essentially, the application of 100 µM SNP remained effective to rescue lentil plants under moderate salinity by regulating plant growth and biochemical pathways. Thus, the exogenous application of SNP could be developed as a useful strategy for improving the performance of lentil plants in salinity-prone environments.

H2S induces NO in the regulation of AsA-GSH cycle in wheat seedlings by water stress

In current study, we investigated the relationship between hydrogen sulfide (H₂S) and nitric oxide (NO) in the regulation of ascorbate-glutathione (AsA-GSH) cycle in wheat seedlings by water stress. Findings showed that water stress significantly stimulated the production of H₂S and NO, the transcript levels and activities of enzymes in AsA-GSH cycle, as well as malondialdehyde (MDA) production and electrolyte leakage, but significantly decreased AsA/DHA and GSH/GSSG. Meanwhile, water stress significantly decreased plant height and dry biomass. Except MDA and electrolyte leakage, above changes induced by water stress were reversed by H₂S synthesis inhibitor aminooxyacetic acid (AOA) and NO synthesis inhibitor N^G-nitro-L-arginine methyl ester (L-NAME). However, AOA and L-NAME significantly enhanced MDA production and electrolyte leakage, which further decreased plant height and dry biomass of wheat seedlings under water stress. Application of exogenous H₂S donor sodium hydrosulfide (NaHS) to AOA-treated plants and application of exogenous NO donor sodium nitroprusside (SNP) to L-NAME-treated plants reversed above effects of AOA and L-NAME, respectively. Application of L-NAME plus water stress significantly decreased NO production induced by water stress. However, application of L-NAME plus water stress had no obvious influence on H₂S production induced by water stress, while application of AOA plus water stress significantly reduced the production of H₂S and NO induced by water stress. Current findings suggested that H₂S acted upstream of NO in the regulation of AsA-GSH cycle in wheat seedlings by water stress.

Nitric oxide alleviates salt stress in seed germination and early seedling growth of pakchoi (Brassica chinensis L.) by enhancing physiological and biochemical parameters

The germination and seedling vigor of crops is negatively affected by soil salinity. Nitric oxide (NO) has emerged as a key molecule involved in many physiological events in plants. The objective of present study was to evaluate the impact of exogenous sodium nitroprusside (SNP, a NO donor) at different concentrations on the seed germination and early seedling growth characteristics of pakchoi (Brassica chinensis L.) under NaCl stress. 100 mM NaCl stress markedly inhibited the seed germination potential, germination index, vitality index and growth of radicles and plumules. SNP pretreatment attenuated the salt stress effects in a dose-dependent manner, as indicated by enhancing the characteristics of seed germination and early seedling growth parameters, and the mitigating effect was most pronounced at 10 μM SNP. Efficient antioxidant systems were activated by SNP pre-treatment, and which effectively increased the activities of superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX), and reduced contents of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) and the production rate of superoxide anion radical (O₂^·-) in radicles and plumules, thereby preventing oxidative damage from NaCl stress. SNP pre-treatment also increased the contents of proline and soluble sugar in radicles and plumules under NaCl stress. In addition, SNP pre-treatment significantly increased the K⁺ contents and decreased Na⁺ contents in radicles and plumules, resulting in the increased level of K⁺/Na⁺ ratio. Our results demonstrated that SNP application on pakchoi seeds may be a good option to improve seed germination and seedling growth under NaCl stress by modulating the physiological responses resulting in better seed germination and seedling growth.