Nitric oxide function in plant abiotic stress

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

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

Comparative physiological, transcriptomic, and WGCNA analyses reveal the key genes and regulatory pathways associated with drought tolerance in Tartary buckwheat

Drought stress is one of the major abiotic stress factors that affect plant growth and crop productivity. Tartary buckwheat is a nutritionally balanced and flavonoid-rich pseudocereal crop and also has strong adaptability to different adverse environments including drought. However, little is known about its drought tolerance mechanism. In this study, we performed comparative physiological and transcriptomic analyses of two contrasting drought-resistant Tartary buckwheat genotypes under nature drought treatment in the reproductive stage. Under drought stress, the drought-tolerant genotype XZSN had significantly higher contents of relative water, proline, and soluble sugar, as well as lower relative electrolyte leakage in the leaves than the drought-susceptible LK3. A total of 5,058 (2,165 upregulated and 2,893 downregulated) and 5,182 (2,358 upregulated and 2,824 downregulated) potential drought-responsive genes were identified in XZSN and LK3 by transcriptome sequencing analysis, respectively. Among the potential drought-responsive genes of XZSN, 1,206 and 1,274 genes were identified to be potential positive and negative contributors for XZSN having higher drought resistance ability than LK3. Furthermore, 851 out of 1,206 positive drought-resistant genes were further identified to be the core drought-resistant genes of XZSN based on WGCNA analysis, and most of them were induced earlier and quicker by drought stress than those in LK3. Functional annotation of the 851 core drought-resistant genes found that a large number of stress-responsive genes were involved in TFs, abscisic acid (ABA) biosynthesis, signal transduction and response, non-ABA signal molecule biosynthesis, water holding, oxygen species scavenging, osmotic adjustment, cell damage prevention, and so on. Transcriptional regulatory network analyses identified the potential regulators of these drought-resistant functional genes and found that the HD-ZIP and MYB TFs might be the key downstream TFs of drought resistance in Tartary buckwheat. Taken together, these results indicated that the XZSN genotype was more drought-tolerant than the LK3 genotype as evidenced by triggering the rapid and dramatic transcriptional reprogramming of drought-resistant genes to reduce water loss, prevent cell damage, and so on. This research expands our current understanding of the drought tolerance mechanisms of Tartary buckwheat and provides important information for its further drought resistance research and variety breeding.

Crosstalk of nitro-oxidative stress and iron in plant immunity

Accumulation of oxygen and nitrogen radicals and their derivatives, known as reactive oxygen species (ROS) and reactive nitrogen species (RNS), occurs throughout various phases of plant growth in association with biotic and abiotic stresses. One of the consequences of environmental stresses is disruption of homeostasis between production and scavenging of ROS and RNS, which leads to nitro-oxidative burst and affects other defense-related mechanisms, such as polyamines levels, phenolics, lignin and callose as defense components related to plant cell wall reinforcement. Although this subject has attracted huge interest, the cross-talk between these signaling molecules and iron, as a main metal element involved in the activity of various enzymes and numerous vital processes in the living cells, remains largely unexplored. Therefore, it seems necessary to pay more in depth attention to the mechanisms of plant resistance against various environmental stimuli for designing novel and effective plant protection strategies. This review is focused on advances in recent knowledge related to the role of ROS, RNS, and association of these signaling molecules with iron in plant immunity. Furthermore, the role of cell wall fortification as a main physical barrier involved in plant defense have been discussed in association with reactive species and iron ions.

Nitrate–Nitrite–Nitric Oxide Pathway: A Mechanism of Hypoxia and Anoxia Tolerance in Plants

Oxygen (O₂) is the most crucial substrate for numerous biochemical processes in plants. Its deprivation is a critical factor that affects plant growth and may lead to death if it lasts for a long time. However, various biotic and abiotic factors cause O₂ deprivation, leading to hypoxia and anoxia in plant tissues. To survive under hypoxia and/or anoxia, plants deploy various mechanisms such as fermentation paths, reactive oxygen species (ROS), reactive nitrogen species (RNS), antioxidant enzymes, aerenchyma, and adventitious root formation, while nitrate (NO₃⁻), nitrite (NO₂⁻), and nitric oxide (NO) have shown numerous beneficial roles through modulating these mechanisms. Therefore, in this review, we highlight the role of reductive pathways of NO formation which lessen the deleterious effects of oxidative damages and increase the adaptation capacity of plants during hypoxia and anoxia. Meanwhile, the overproduction of NO through reductive pathways during hypoxia and anoxia leads to cellular dysfunction and cell death. Thus, its scavenging or inhibition is equally important for plant survival. As plants are also reported to produce a potent greenhouse gas nitrous oxide (N₂O) when supplied with NO₃⁻ and NO₂⁻, resembling bacterial denitrification, its role during hypoxia and anoxia tolerance is discussed here. We point out that NO reduction to N₂O along with the phytoglobin-NO cycle could be the most important NO-scavenging mechanism that would reduce nitro-oxidative stress, thus enhancing plants’ survival during O₂-limited conditions. Hence, understanding the molecular mechanisms involved in reducing NO toxicity would not only provide insight into its role in plant physiology, but also address the uncertainties seen in the global N₂O budget.

Biodegradable active packaging: Components, preparation, and applications in the preservation of postharvest perishable fruits and vegetables

The consumption of fresh fruits and vegetables is restricted by the susceptibility of fresh produce to deterioration caused by postharvest physiological and metabolic activities. Developing efficient preservation strategies is thus among the most important scientific issues to be urgently addressed in the field of food science. The incorporation of active agents into a polymer matrix to prepare biodegradable active packaging is being increasingly explored to mitigate the postharvest spoilage of fruits and vegetables during storage. This paper reviews the composition of biodegradable polymers and the methods used to prepare biodegradable active packaging. In addition, the interactions between bioactive ingredients and biodegradable polymers that can lead to plasticizing or cross-linking effects are summarized. Furthermore, the applications of biodegradable active (i.e., antibacterial, antioxidant, ethylene removing, barrier, and modified atmosphere) packaging in the preservation of fruits and vegetables are illustrated. These films may increase sensory acceptability, improve quality, and prolong the shelf life of postharvest products. Finally, the challenges and trends of biodegradable active packaging in the preservation of fruits and vegetables are discussed. This review aims to provide new ideas and insights for developing novel biodegradable active packaging materials and their practical application in the preservation of postharvest fruits and vegetables.

Role of Nitric Oxide in Postharvest Senescence of Fruits

Nitric oxide (NO) acts as a gaseous signalling molecule and is considered to be a key regulator in the postharvest storage of fruits. Postharvest senescence is one of the most serious threats affecting the usage and economic value of fruits. Most recent studies have found that exogenous NO application can effectively improve the quality and prolong the shelf life of fruit postharvest by inhibiting postharvest diseases and alleviating chilling injury. Understanding the roles of NO is essential to elucidating how NO activates the appropriate set of responses to postharvest senescence. Here, we concluded that exogenous NO treatment alleviated senescence in postharvest fruit and attributed this to the following factors: (1) ethylene biosynthesis, (2) the antioxidant system, (3) polyamine metabolism and γ-aminobutyric acid (GABA) shunting, (4) cell wall metabolism, (5) sugar metabolism, (6) energy metabolism, (7) the CRT/DRE-binding factor (CBF) pathway and (8) S-nitrosylation. Moreover, crosstalk between NO and hydrogen sulfide (H₂S), hydrogen peroxide (H₂O₂), oxalic acid (OA), arginine (Arg), GATA or plant hormone abscisic acid (ABA), melatonin (MT), and methyl jasmonate (MeJA), along with the regulation of key genes, were found to be very important in responses to postharvest senescence. In this study, we focus on the recent knowledge concerning the alleviative effect of NO on postharvest senescence, covering ethylene biosynthesis, the antioxidant system and related gene and protein expression.

Silicon and nitric oxide-mediated mechanisms of cadmium toxicity alleviation in wheat seedlings

The individual impact of silicon (Si) and nitric oxide (NO, as sodium nitroprusside) on metal toxicity in various plant species has been well documented; however, their combined action in the regulation of metal stress has never been tested yet. Therefore, this study investigates the effects of the combined application of Si and NO in the mitigation of Cd toxicity in wheat seedlings. Seedlings grown on Cd has a significantly declined growth due to an increased accumulation of Cd and oxidative stress markers (due to downregulation of antioxidant defense system particularly ascorbate-glutathione cycle) and a decreased accumulation of NO and Si. Additionally, the altered leaf and root structures resulted into a declined photosynthetic efficiency. However, the addition of Si and NO alone as well as combined significantly alleviated Cd toxicity in wheat seedlings by lowering the accumulation of Cd and oxidative stress markers and improving leaf and root structures, which are collectively responsible for a better photosynthetic rate under Cd toxicity, and hence an improved growth was noticed. Particularly, the application of Si and NO in combination lowered the oxidative stress markers via upregulating the antioxidant defense system (particularly AsA-GSH cycle) suggesting the increased efficacy of Si + NO against the Cd toxicity in wheat seedlings as compared to their alone treatments.

Cold adaptation strategies in plants—An emerging role of epigenetics and antifreeze proteins to engineer cold resilient plants

Cold stress adversely affects plant growth, development, and yield. Also, the spatial and geographical distribution of plant species is influenced by low temperatures. Cold stress includes chilling and/or freezing temperatures, which trigger entirely different plant responses. Freezing tolerance is acquired via the cold acclimation process, which involves prior exposure to non-lethal low temperatures followed by profound alterations in cell membrane rigidity, transcriptome, compatible solutes, pigments and cold-responsive proteins such as antifreeze proteins. Moreover, epigenetic mechanisms such as DNA methylation, histone modifications, chromatin dynamics and small non-coding RNAs play a crucial role in cold stress adaptation. Here, we provide a recent update on cold-induced signaling and regulatory mechanisms. Emphasis is given to the role of epigenetic mechanisms and antifreeze proteins in imparting cold stress tolerance in plants. Lastly, we discuss genetic manipulation strategies to improve cold tolerance and develop cold-resistant plants.

Melatonin-Induced Inhibition of Shiraia Hypocrellin A Biosynthesis Is Mediated by Hydrogen Peroxide and Nitric Oxide

Melatonin (MLT), an evolutionarily conserved pleiotropic molecule, is implicated in numerous physiological processes in plants and animals. However, the effects of MLT on microbes have seldom been reported. In this study, we examined the influence of exogenous MLT on the growth and hypocrellin biosynthesis of bambusicolous fungus Shiraia sp. S9. Hypocrellin A (HA) is a photoactivated and photoinduced perylenequinone (PQ) toxin in Shiraia. Exogenous MLT at 100.00 μM not only decreased fungal conidiation and spore germination but inhibited HA contents significantly in fungal cultures under a light/dark (24 h:24 h) shift. MLT treatment was associated with higher activity of antioxidant enzymes (superoxide dismutase, catalase and peroxidase) and a marked decline in reactive oxygen species (ROS) production in the mycelia. Moreover, MLT induced endogenous nitric oxide (NO) production during the culture. The NO donor sodium nitroprusside (SNP) potentiated MLT-induced inhibition of O₂⁻ production, but NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) enhanced O₂⁻ production, whereas MLT-induced NO level was increased by the ROS scavenger vitamin C (Vc). The changes in NO and H₂O₂ were proved to be involved in the MLT-induced downregulation of the expressions of HA biosynthetic genes, leading to the suppression of HA production. This study provides new insight into the regulatory roles of MLT on fungal secondary metabolism activities and a basis for understanding self-resistance in phototoxin-producing fungi.

Nitric oxide, salicylic acid and oxidative stress: Is it a perfect equilateral triangle?

Nitric oxide (NO) is an endogenous free radical involved in the regulation of a wide array of physio-biochemical phenomena in plants. The biological activity of NO directly depend on its cellular concentration which usually changes under stress conditions, it participates in maintaining cellular redox equilibrium and regulating target checkpoints which control switches among development and stress. It is one of the key players in plant signalling and a plethora of evidence supports its crosstalk with other phytohormones. NO and salicylic acid (SA) cooperation is also of great physiological relevance, where NO modulates the immune response by regulating SA linked target proteins i.e., non-expressor of pathogenesis-related genes (NPR-1 and NPR-2) and Group D bZIP (basic leucine zipper domain transcription factor). Many experimental data suggest a functional cooperative role between NO and SA in mitigating the plant oxidative stress which suggests that these relationships could constitute a metabolic "equilateral triangle".

ELO2 Participates in the Regulation of Osmotic Stress Response by Modulating Nitric Oxide Accumulation in Arabidopsis

The ELO family is involved in synthesizing very-long-chain fatty acids (VLCFAs) and VLCFAs play a crucial role in plant development, protein transport, and disease resistance, but the physiological function of the plant ELO family is largely unknown. Further, while nitric oxide synthase (NOS)-like activity acts in various plant environmental responses by modulating nitric oxide (NO) accumulation, how the NOS-like activity is regulated in such different stress responses remains misty. Here, we report that the yeast mutant Δelo3 is defective in H₂O₂-triggered cell apoptosis with decreased NOS-like activity and NO accumulation, while its Arabidopsis homologous gene ELO2 (ELO HOMOLOG 2) could complement such defects in Δelo3. The expression of this gene is enhanced and required in plant osmotic stress response because the T-DNA insertion mutant elo2 is more sensitive to the stress than wild-type plants, and ELO2 expression could rescue the sensitivity phenotype of elo2. In addition, osmotic stress-promoted NOS-like activity and NO accumulation are significantly repressed in elo2, while exogenous application of NO donors can rescue this sensitivity of elo2 in terms of germination rate, fresh weight, chlorophyll content, and ion leakage. Furthermore, stress-responsive gene expression, proline accumulation, and catalase activity are also repressed in elo2 compared with the wild type under osmotic stress. In conclusion, our study identifies ELO2 as a pivotal factor involved in plant osmotic stress response and reveals its role in regulating NOS-like activity and NO accumulation.

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.

Alleviating excess boron stress in tomato calli by applying benzoic acid to various biochemical strategies

Benzoic acid (BA) represents vital roles in plant activity and response to diverse unfavorable conditions. However, its participation in mitigating excess boron (EB) stress in plants is elusive. Herein, we have examined the impacts of BA (1 μM) in controlling boron (B) uptake in tomato (Solanum lycopersicum L.) calli exposed to various EB levels (0, 1, 2, and 3 mM). The free, semi-bound, and bound B forms were stimulated by EB, while these forms were reduced in B-stressed calli by BA supplementation (40.37%, 36.08%, and 66.91%, respectively, less than 3 mM B-stressed calli alone). EB caused a reduction in the uptake of potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺), and nitrite (NO₂^-) while increasing the concentration of phosphorus (P), nitrate (NO₃^-), sulfur (S), and sulfate (SO₄^2-) in B-stressed calli. BA application induced the uptake of K⁺, Ca²⁺, Mg²⁺, NO₃^-, S, and SO₄^2-; however, it reduced P and NO₂^- concentrations in B-stressed calli. EB reduced nitrate reductase activity (NR), while BA application did not alleviate this reduction. EB treatments significantly, in most cases, increased sulfite oxidase (SO) activity. Supplementation of BA along with EB further enhanced SO activity. Cell wall components (cellulose, hemicellulose, and pectin) were decreased under EB treatments but considerably increased in B-stressed calli by BA application. Fourier Transform Infrared Spectrometer (FT-IR) output showed that EB treatments with/without BA led to alterations in cell wall functional groups of calli. Our findings indicated that BA application enabled tomato callus to counteract the harmful effect of EB, leading to improved callus growth.

Exogenous nitric oxide alleviates manganese toxicity in bean plants by modulating photosynthesis in relation to leaf lipid composition

Nitric oxide (NO) is a signaling molecule controlling several steps of plant development and defense process under stress conditions. NO-induced alleviation of manganese (Mn) toxicity was investigated on bean plants submitted for 28 days to 500 µM MnCl₂. Manganese excess decreased plant dry weight and elongation and increased levels of reactive oxygen species and lipid peroxidation leading to up-regulation of superoxide dismutase, catalase, and ascorbate peroxidase activities. The inhibitory effects of Mn on plant growth were associated to reduction of light-saturated carbon assimilation (A_max), stomatal conductance (g_s), and transpiration (E). By contrast, Mn induced significant increase in the apparent quantum yield (ɸ) and light compensation point (LCP). Interestingly, intracellular CO₂ (Ci) remains stable under Mn stress. Concomitantly, leaf membrane lipids have drastically reduced under high Mn concentration. After Mn exposition, leaf fatty acids exhibited a significant loss of linolenic acid, accompanied by an accumulation of palmitoleic, stearic, and linoleic acids leading to alteration of lipid desaturation. NO supply reversed Mn toxicity as evidenced by enhancement of growth biomass and recovery of A_max, E, ɸ, and LCP. Similarly, NO addition has positive effects on leaf lipid content and composition leading to restoration of lipid unsaturation. The modulation of fatty acid composition can be a way to reduce leaf membrane damages and maintain optimal photosynthesis and plant growth. Despite the absence of enough evidences in how NO is involved in lipid and photosynthesis recovery under Mn stress conditions, it is assumed that NO beneficial effects are attributable to NO/Mn cross-talk.

Harnessing the power of hydrogen sulphide (H2 S) for improving fruit quality traits

Hydrogen sulphide (H₂ S) is a gaseous molecule and originates endogenously in plants. It is considered a potential signalling agent in various physiological processes of plants. Numerous reports have examined the role of H₂ S in fruit ripening and in enhancing fruit quality traits. H₂ S coordinates the fruit antioxidant system, fruit ripening phytohormones, such as ethylene and abscisic acid, together with other ripening-related signalling molecules, including nitric oxide and hydrogen peroxide. Although many studies have increased understanding of various aspects of this complex network, there is a gap in understanding crosstalk of H₂ S with key players of fruit ripening, postharvest senescence and fruit metabolism. This review focused on deciphering fruit H₂ S metabolism, signalling and its interaction with other ripening-related signalling molecules during fruit ripening and postharvest storage. Moreover, we also discuss how H₂ S can be used as a tool for improving fruit quality and productivity and reducing postharvest loss of perishable fruits.

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

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

Combating Salinity Through Natural Plant Extracts Based Biostimulants: A Review

Enhanced crop growth and yield are the recurring concerns in agricultural field, considering the soaring world population and climate change. Abiotic stresses are one of the major limiting factors for constraining crop production, for several economically important horticultural crops, and contribute to almost 70% of yield gap. Salt stress is one of these unsought abiotic stresses that has become a consistent problem in agriculture over the past few years. Salinity further induces ionic, osmotic, and oxidative stress that result in various metabolic perturbations (including the generation of reactive oxygen, carbonyl, and nitrogen species), reduction in water potential (ψw), distorted membrane potential, membrane injury, altered rates of photosynthesis, leaf senescence, and reduced nitrogen assimilation, among others); thereby provoking a drastic reduction in crop growth and yield. One of the strategies to mitigate salt stress is the use of natural plant extracts (PEs) instead of chemical fertilizers, thus limiting water, soil, and environmental pollution. PEs mainly consist of seeds, roots, shoots, fruits, flowers, and leaves concentrates employed either individually or in mixtures. Since PEs are usually rich in bioactive compounds (e.g., carotenoids, flavonoids, phenolics, etc.), therefore they are effective in regulating redox metabolism, thereby promoting plant growth and yield. However, various factors like plant growth stage, doses applied, application method, soil, and environmental conditions may greatly influence their impact on plants. PEs have been reported to enhance salt tolerance in plants primarily through modulation of signaling signatures and pathways (e.g., Na+, ANNA4, GIPC, SOS3, and SCaBP8 Ca2+ sensors, etc.), and regulation of redox machinery [e.g., superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), non-specific peroxidase (POX), glutathione peroxidase (GPX), peroxiredoxin (Prx), ascorbic acid (AsA), glutathione (GSH), α-tocopherol, etc.]. The current study highlights the role of PEs in terms of their sources, methods of preparation, and mode of action with subsequent physiological changes induced in plants against salinity. However, an explicit mode of action of PEs remains nebulous, which might be explicated utilizing transcriptomics, proteomics, metabolomics, and bioinformatics approaches. Being ecological and economical, PEs might pave the way for ensuring the food security in this challenging era of climate change.

Methylglyoxal detoxification pathway - Explored first time for imazethapyr tolerance in lentil (Lens culinaris L.)

Lentil is an important pulses crop but it's short stature and slow growth rate make it vulnerable to weed competition, limiting crop productivity. There is need to identify herbicide tolerant genotypes and their tolerance mechanism. The present investigation was conducted to understand the effect of imazethapyr (IM) treatment on accumulation of methylglyoxal (MG) and its detoxification mechanism in IM-tolerant (LL1397 and LL1612) susceptible (FLIP2004-7L and PL07) genotypes sown under control (weed free), weedy check (weeds were growing with crop) and sprayed with imazethapyr. The enzymes of glyoxalase pathway (glyoxalase I, II and III) and non glyoxalase pathway (methylglyoxal reductase), lactate dehydrogenase (LDH), glutathione content, gamma-glutamyl-cysteine synthetase (γ-GCS) were estimated in lentil genotypes at different days after spray. Higher activities of glyoxalase I, II and III and MGR along with the increased glutathione content (GSH) content in LL1397 and LL1612 under IM treatment as compared to FLIP2004-7L and PL07 might be responsible for lowering MG accumulation and increasing lactate content, which is end product of these pathways. Enhanced LDH activity in LL1397 and LL1612 might be responsible for energy production via TCA cycle that might be responsible for growth and recovery of tolerant genotypes after IM treatment. Higher γ-GCS activity in tolerant genotypes led to increased glutathione content required for glyoxalase pathway. However, decreased activities of glyoxalase enzymes and MGR in susceptible genotypes result in MG accumulation which limit plant growth. This is the first ever study elucidating the role of MG detoxification pathway conferring IM tolerance in lentil.

Transcriptome analysis of Tamarix ramosissima leaves in response to NaCl stress

Halophyte Tamarix ramosissima. Lcdcb (T. ramosissima) are known as the representative of Tamarix plants that are widely planted in salinized soil. However, molecular mechanisms towards salt tolerance and adaptation are largely rare. In this study, we carried out RNA-sequence and transcriptome analysis of T. ramosissima in response to NaCl stress, screened differentially expressed genes (DEGs) and further verified by qRT-PCR. Results showed that 105702 unigenes were spliced from the raw data of transcriptome sequencing, where 54238 unigenes were retrieved from KEGG, KOG, NR, and SwissProt. After 48 hours of NaCl treatment, the expression levels of 6374 genes were increased, and 5380 genes were decreased in leaves. After 168 hours, the expression levels of 3837 genes were up-regulated and 7808 genes were down-regulated. In particular, 8 transcription factors annotated to the KEGG Pathway were obtained, involving the WRKY and bZIP transcription family. In addition, KEGG pathway annotation showed that expression of 39 genes involved in ROS scavenging mechanisms were significantly changed, in which 21 genes were up-regulated and 18 genes were down-regulated after 48 hours as well as 15 genes were up-regulated and 24 genes were down-regulated after 168h. Simultaneously, the enzyme activities of SOD and POD were significantly enhanced under NaCl treatment, but the enzyme activity of CAT was not significantly enhanced. Moreover, WRKY, MYB and bZIP may participate in the process of salt resistance in T. ramosissima. This study provides gene resources and a theoretical basis for further molecular mechanisms of salt tolerance in T. ramosissima.

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

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

The Interplay between Hydrogen Sulfide and Phytohormone Signaling Pathways under Challenging Environments

Hydrogen sulfide (H₂S) serves as an important gaseous signaling molecule that is involved in intra- and intercellular signal transduction in plant–environment interactions. In plants, H₂S is formed in sulfate/cysteine reduction pathways. The activation of endogenous H₂S and its exogenous application has been found to be highly effective in ameliorating a wide variety of stress conditions in plants. The H₂S interferes with the cellular redox regulatory network and prevents the degradation of proteins from oxidative stress via post-translational modifications (PTMs). H₂S-mediated persulfidation allows the rapid response of proteins in signaling networks to environmental stimuli. In addition, regulatory crosstalk of H₂S with other gaseous signals and plant growth regulators enable the activation of multiple signaling cascades that drive cellular adaptation. In this review, we summarize and discuss the current understanding of the molecular mechanisms of H₂S-induced cellular adjustments and the interactions between H₂S and various signaling pathways in plants, emphasizing the recent progress in our understanding of the effects of H₂S on the PTMs of proteins. We also discuss future directions that would advance our understanding of H₂S interactions to ultimately mitigate the impacts of environmental stresses in the plants.

A nitric oxide burst at the shoot apex triggers a heat-responsive pathway in Arabidopsis

When confronted with heat stress, plants depend on the timely activation of cellular defences to survive by perceiving the rising temperature. However, how plants sense heat at the whole-plant level has remained unanswered. Here we demonstrate that shoot apical nitric oxide (NO) bursting under heat stress as a signal triggers cellular heat responses at the whole-plant level on the basis of our studies mainly using live-imaging of transgenic plants harbouring pHsfA2::LUC, micrografting, NO accumulation mutants and liquid chromatography-tandem mass spectrometry analysis in Arabidopsis. Furthermore, we validate that S-nitrosylation of the trihelix transcription factor GT-1 by S-nitrosoglutathione promotes its binding to NO-responsive elements in the HsfA2 promoter and that loss of function of GT-1 disrupts the activation of HsfA2 and heat tolerance, revealing that GT-1 is the long-sought mediator linking signal perception to the activation of cellular heat responses. These findings uncover a heat-responsive mechanism that determines the timing and execution of cellular heat responses at the whole-plant level.

NLP2-NR Module Associated NO Is Involved in Regulating Seed Germination in Rice under Salt Stress

Salt stress has the most severe impact on plant growth and development, including seed germination. However, little is known about the mechanism of NR (nitrate reductase)-associated nitric oxide (NO) regulates salt tolerance during seed germination in rice. Herein, we shown that inhibition of seed germination by salt stress was significantly impaired by sodium nitroferricyanide (SNP), a NO donor. Then a triple mutant, nr1/nr2/nr3, was generated. Results shown that germination of triple mutants were delayed and were much more sensitive to salt stress than WT plant, which can be rescued by application of SNP. qPCR analysis revealed that expressions of abscisic acid (ABA) catabolism gene, OsABA8ox1, was suppressed in triple mutants under salt stress, resulting in an elevated ABA content. Similar to SNP, application of nitrate also rescued seed germination under salt stress, which, however, was blocked in the triple mutants. Further study revealed that a nitrate responsive transcript factor, OsNLP2, was induced by salt stress, which thus up-regulates the expression of OsNRs and NR activity, resulting in promoted salt tolerance during seed germination. In addition, nitrate-mediated salt tolerance was impaired in mutant of aba8ox1, a target gene for NLP2. Transient trans-activation assays further revealed NLP2 can significantly activate the expression of OsABA8ox1 and OsNR1, suggesting that NLP2 activates expression of ABA catabolism gene directly or indirectly via NR-associated NO. Taken together, our results demonstrate that NLP2-NR associated NO was involved in salt response by increasing ABA catabolism during seed germination and highlight the importance of NO for stress tolerance of plants.

Ethylene involvement in the regulation of heat stress tolerance in plants

Because of the rise in global temperature, heat stress has become a major concern for crop production. Heat stress deteriorates plant productivity and alters phenological and physiological responses that aid in precise monitoring and sensing of mild-to-severe transient heat stress. Plants have evolved several sophisticated mechanisms including hormone-signaling pathways to sense heat stimuli and acquire heat stress tolerance. In response to heat stress, ethylene, a gaseous hormone, is produced which is indispensable for plant growth and development and tolerance to various abiotic stresses including heat stress. The manipulation of ethylene in developing heat stress tolerance targeting ethylene biosynthesis and signaling pathways has brought promising out comes. Conversely increased ethylene biosynthesis and signaling seem to exhibit inhibitory effects in plant growth responses from primitive to maturity stages. This review mainly focuses on the recent studies of ethylene involvement in plant responses to heat stress and its functional regulation, and molecular mechanism underlying the plant responses in the mitigation of heat-induced damages. Furthermore, this review also describes the crosstalk between ethylene and other signaling molecules under heat stress and approaches to improve heat stress tolerance in plants.

Strigolactone is involved in nitric oxide-enhanced the salt resistance in tomato seedlings

Both strigolactones (SLs) and nitric oxide (NO) are regulatory signals with diverse roles during stress responses. At present, the interaction and mechanism of SLs and NO in tomato salt tolerance remain unclear. In the current study, tomato 'Micro-Tom' was used to study the roles and interactions of SLs and NO in salinity stress tolerance. The results show that 15 μM SLs synthetic analogs GR24 and 10 μM NO donor S-nitrosoglutathione (GSNO) promoted seedling growth under salt stress. TIS108 (an inhibitor of strigolactone synthesis) suppressed the positive roles of NO in tomato growth under salt stress, indicating that endogenous SLs might be involved in NO-induced salt response in tomato seedlings. Meanwhile, under salt stress, GSNO or GR24 treatment induced the increase of endogenous SLs content in tomato seedlings. Moreover, GR24 or GSNO treatment effectively increased the content of chlorophyll, carotenoids and ascorbic acid (ASA), and enhanced the activities of antioxidant enzymes (superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase), glutathione reductase (GR) and cleavage dioxygenase (CCD) enzyme. Additionally, GSNO or GR24 treatment also up-regulated the expression of SLs synthesis genes (SlCCD7, SlCCD8, SlD27 and SlMAX1) and its signal transduction genes (SlD14 and SlMAX2) in tomato seedlings under salt stress. While, a strigolactone synthesis inhibitor TIS108 blocked the increase of endogenous SLs, chlorophyll, carotenoids and ASA content, and antioxidant enzyme, GR, CCD enzyme activity and SLs-related gene expression levels induced by GSNO. Thus, SLs may play an important role in NO-enhanced salinity tolerance in tomato seedlings by increasing photosynthetic pigment content, enhancing antioxidant capacity and improving endogenous SLs synthesis.

Nitric oxide alleviates salt-induced stress damage by regulating the ascorbate-glutathione cycle and Na+/K+ homeostasis in Nitraria tangutorum Bobr

Nitric oxide (NO) is an important signaling molecule involved in mediation of salt stress induced physiological responses in plants. In this study, we investigated the effect of NO on Nitraria tangutorum seedlings exposed to salt stress. Exogenous application of NO donor, sodium nitroprusside (SNP) increased fresh weight, shoot and root elongation and decreased electrolyte leakage and malondialdehyde (MDA) content in N. tangutorum seedlings under salt stress. Simultaneously, leaf senescence and root damage induced by salt stress were alleviated. SNP effectively increased NO content both in leaves and roots of plants under salt stress. Meanwhile, SNP activated the ascorbate-glutathione (AsA-GSH) cycle by increasing antioxidants contents, antioxidant enzymes activities, and related genes expression, thereby scavenging reactive oxygen species (ROS) and alleviating oxidative damage caused by salt stress. SNP alleviated salt stress induced ion toxicity by promoting Na⁺ efflux and ion transporter gene expression and reducing Na⁺ content and the Na⁺/K⁺ ratio. In addition, application of NO specific scavenger cPTIO and mammalian NO synthase inhibitor _L-NAME sifnificantly aggravated stress damage in plant under salt stress. These results show the beneficial impacts of NO as a stress-signaling molecule that positively regulates defense response in N. tangutorum to salt stress.

Brassinosteroids Mitigate Cadmium Effects in Arabidopsis Root System without Any Cooperation with Nitric Oxide

The heavy metal cadmium (Cd) affects root system development and quiescent center (QC)-definition in Arabidopsis root-apices. The brassinosteroids-(BRs)-mediated tolerance to heavy metals has been reported to occur by a modulation of nitric oxide (NO) and root auxin-localization. However, how BRs counteract Cd-action in different root types is unknown. This research aimed to find correlations between BRs and NO in response to Cd in Arabidopsis’s root system, monitoring their effects on QC-definition and auxin localization in root-apices. To this aim, root system developmental changes induced by low levels of 24-epibrassinolide (eBL) or by the BR-biosynthesis inhibitor brassinazole (Brz), combined or not with CdSO₄, and/or with the NO-donor nitroprusside (SNP), were investigated using morpho-anatomical and NO-epifluorescence analyses, and monitoring auxin-localization by the DR5::GUS system. Results show that eBL, alone or combined with Cd, enhances lateral (LR) and adventitious (AR) root formation and counteracts QC-disruption and auxin-delocalization caused by Cd in primary root/LR/AR apices. Exogenous NO enhances LR and AR formation in Cd-presence, without synergism with eBL. The NO-signal is positively affected by eBL, but not in Cd-presence, and BR-biosynthesis inhibition does not change the low NO-signal caused by Cd. Collectively, results show that BRs ameliorate Cd-effects on all root types acting independently from NO.

GSNOR regulates ganoderic acid content in Ganoderma lucidum under heat stress through S-nitrosylation of catalase

As a master regulator of the balance between NO signaling and protein S-nitrosylation, S-nitrosoglutathione (GSNO) reductase (GSNOR) is involved in various developmental processes and stress responses. However, the proteins and specific sites that can be S-nitrosylated, especially in microorganisms, and the physiological functions of S-nitrosylated proteins remain unclear. Herein, we show that the ganoderic acid (GA) content in GSNOR-silenced (GSNORi) strains is significantly lower (by 25%) than in wild type (WT) under heat stress (HS). Additionally, silencing GSNOR results in an 80% increase in catalase (CAT) activity, which consequently decreases GA accumulation via inhibition of ROS signaling. The mechanism of GSNOR-mediated control of CAT activity may be via protein S-nitrosylation. In support of this possibility, we show that CAT is S-nitrosylated (as shown via recombinant protein in vitro and via GSNORi strains in vivo). Additionally, Cys (cysteine) 401, Cys642 and Cys653 in CAT are S-nitrosylation sites (assayed via mass spectrometry analysis), and Cys401 may play a pivotal role in CAT activity. These findings indicate a mechanism by which GSNOR responds to stress and regulates secondary metabolite content through protein S-nitrosylation. Our results also define a new S-nitrosylation site and the function of an S-nitrosylated protein regulated by GSNOR in microorganisms.

Liu et al. identify catalase as a target of S-nitrosylation by GSNOR and the specific sites of S-nitrosylation critical for its role in regulating secondary metabolite production in Ganoderma lucidum under heat stress. This study suggests that GSNOR regulates other metabolic pathways in microorganisms through S-nitrosylation of target proteins in response to environmental changes.

Glycogen synthase kinases in model and crop plants - From negative regulators of brassinosteroid signaling to multifaceted hubs of various signaling pathways and modulators of plant reproduction and yield

Glycogen synthase kinases, also known as SHAGGY-like Kinases (GSKs/SKs), are highly conserved serine/threonine protein kinases present both in animals and plants. Plant genomes contain multiple homologs of the GSK3 genes which participate in various biological processes. Plant GSKs/SKs, and their best known representative in Arabidopsis thaliana - Brassinosteroid Insentisive2 (BIN2/SK21) in particular, were first identified as components of the brassinosteroid (BR) signaling pathway. As phytohormones, BRs regulate a wide range of physiological processes in plants - from germination, cell division, elongation and differentiation to leaf senescence, and response to environmental stresses. The GSKs/SKs proteins belong to a group of several highly conserved components of the BR signaling which evolved early during evolution of this molecular relay. However, recent reports indicated that the GSKs/SKs proteins are also implicated in signaling pathways of other phytohormones and stress-response processes. As a consequence, the GSKs/SKs proteins became hubs of various signaling pathways and modulators of plant development and reproduction. Thus, it is very important to understand molecular mechanisms regulating activity of the GSKs/SKs proteins, but also to get insights into role of the GSKs/SKs proteins in modulation of stability and activity of various substrate proteins which participate in the numerous signaling pathways. Although elucidation of these aspects is still in progress, this review presents a comprehensive and detailed description of these processes and their implications for regulation of development, stress response, and reproduction of model and crop species. The GSKs/SKs proteins and their activity are modulated through phosphorylation and de-phosphorylation reactions which are regulated by various proteins. Importantly, both phosphorylations and de-phosphorylations may have positive and negative effects on the activity of the GSKs/SKs proteins. Additionally, the activity of the GSKs/SKs proteins is positively regulated by reactive oxygen species, whereas it is negatively regulated through ubiquitylation, deacetylation, and nitric oxide-mediated nitrosylation. On the other hand, the GSKs/SKs proteins interact with proteins representing various signaling pathways, and on the basis of the complicated network of interactions the GSKs/SKs proteins differentially regulate various physiological, developmental, stress response, and yield-related processes.

Nanoencapsulation improves the protective effects of a nitric oxide donor on drought-stressed Heliocarpus popayanensis seedlings

Despite the important role played by nitric oxide (NO) in plants subjected to abiotic stress, NO donors application to induce drought tolerance in neotropical tree seedlings has not yet been tested. It is also worth investigating whether NO bioactivity in drought-stressed seedlings could be potentiated by NO donors nanoencapsulation. The aim of the current study is to evaluate the effects of chitosan nanoparticles (NPs) containing S-nitroso-mercaptosuccinic acid (S-nitroso-MSA) on drought-stressed seedlings of neotropical tree species Heliocarpus popayanensis Kunth in comparison to free NO donor and NPs loaded with non-nitrosated MSA. Nanoencapsulation slowed down NO release from S-nitroso-MSA, and nanoencapsulated S-nitroso-MSA yielded 2- and 1.6-fold higher S-nitrosothiol levels in H. popayanensis roots and leaves, respectively, than the free NO donor. S-nitroso-MSA has prevented drought-induced CO₂ assimilation inhibition, regardless of nanoencapsulation, but the nanoencapsulated NO donor has induced earlier ameliorative effect. Both NO and MSA have decreased oxidative stress in H. popayanensis roots, but this effect was not associated with antioxidant enzyme induction, with higher seedling biomass, or with proline and glycine betaine accumulation. Nanoencapsulated S-nitroso-MSA was the only formulation capable of increasing leaf relative water content in drought-stressed plants (from 32.3% to 60.5%). In addition, it induced root hair formation (increase by 36.6% in comparison to well-hydrated plants). Overall, results have evidenced that nanoencapsulation was capable of improving the protective effect of S-nitroso-MSA on H. popayanensis seedlings subjected to drought stress, a fact that highlighted the potential application of NO-releasing NPs to obtain drought-tolerant tree seedlings for reforestation programs.

Alleviating aluminum toxicity in plants: Implications of reactive oxygen species signaling and crosstalk with other signaling pathways

Aluminum (Al) toxicity is a major limiting factor for plant growth and productivity in acidic soil. At pH lower than 5.0 (pH < 5.0), the soluble and toxic form of Al (Al³⁺ ions) enters root cells and inhibits root growth and uptake of water and nutrients. The organic acids malate, citrate, and oxalate are secreted by the roots and chelate Al³⁺ to form a non-toxic Al-OA complex, which decreases the entry of Al³⁺ into the root cells. When Al³⁺ enters, it leads to the production of reactive oxygen species (ROS) in cells, which are toxic and cause damage to biomolecules like lipids, carbohydrates, proteins, and nucleic acids. When ROS levels rise beyond the threshold, plants activate an antioxidant defense system that comprises of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glutathione S-transferase (GST), ascorbic acid (ASA), phenolics and alkaloids etc., which protect plant cells from oxidative damage by scavenging and neutralizing ROS. Besides, ROS also play an important role in signal transduction and influence many molecular and cellular process like hormone signaling, gene expression, cell wall modification, cell cycle, programed cell death (PCD), and development. In the present review, the mechanisms of Al-induced ROS generation, ROS signaling, and crosstalk with other signaling pathways helping to combat Al toxicity have been summarized, which will help researchers to understand the intricacies of Al-induced plant response at cellular level and plan research for developing Al-toxicity tolerant crops for sustainable agriculture in acid soil-affected regions of the world.

Interplay between gasotransmitters and potassium is a K+ey factor during plant response to abiotic stress

Carbon monoxide (CO), nitric oxide (NO) and hydrogen sulfide (H₂S) are gasotransmitters known for their roles in plant response to (a)biotic stresses. The crosstalk between these gasotransmitters and potassium ions (K⁺) has received considerable attention in recent years, particularly due to the dual role of K⁺ as an essential mineral nutrient and a promoter of plant tolerance to abiotic stress. This review brings together what it is known about the interplay among NO, CO, H₂S and K⁺ in plants with focus on the response to high salinity. Some findings obtained for plants under water deficit and metal stress are also presented and discussed since both abiotic stresses share similarities with salt stress. The molecular targets of the gasotransmitters NO, CO and H₂S in root and guard cells that drive plant tolerance to salt stress are highlighted as well.

Salicylic acid reduces cadmium (Cd) accumulation in rice (Oryza sativa L.) by regulating root cell wall composition via nitric oxide signaling

The effects of salicylic acid (SA) on cadmium (Cd) accumulation, Cd subcellular distribution, cell wall composition and Cd adsorption in Cd-stressed rice seedlings were examined. The interaction between SA and nitric oxide (NO) signaling in regulating cell wall composition under Cd exposure was also investigated. Our results showed that 5 μmol·L^-1 Cd treatment significantly decreased plant height, root length and plant dry weight by 40.1%, 46.1% and 21.3% (p < 0.05), respectively, and the inhibitory effects of Cd on the growth parameters were alleviated by exogenous SA. Application of SA remarkably decreased Cd concentrations in roots and shoots of rice seedlings by 48.0% and 19.6%, respectively, and increased the distribution ratio of Cd in the root cell wall fraction (from 35.7% to 40.6%) compared with Cd treatment alone. The reduced Cd accumulation in rice plants could be attributed to that SA application promoted pectin synthesis and demethylesterification, thereby increasing Cd deposition in the root cell wall. Moreover, SA application promoted lignin biosynthesis to strengthen the cell wall and prevent Cd from entering the root cells. In addition, NO might be involved in SA-induced pectin synthesis, pectin demethylesterification and lignin biosynthesis as a downstream signaling molecule, contributing to reduced Cd accumulation in Cd-stressed rice seedlings. The results provide deep insights into the mechanisms of exogenous SA action in reducing Cd accumulation in rice plants.

Persulfidation of Nitrate Reductase 2 Is Involved in l-Cysteine Desulfhydrase-Regulated Rice Drought Tolerance

Hydrogen sulfide (H₂S) is an important signaling molecule that regulates diverse cellular signaling pathways through persulfidation. Our previous study revealed that H₂S is involved in the improvement of rice drought tolerance. However, the corresponding enzymatic sources of H₂S and its regulatory mechanism in response to drought stress are not clear. Here, we cloned and characterized a putative l-cysteine desulfhydrase (LCD) gene in rice, which encodes a protein possessing H₂S-producing activity and was named OsLCD1. Overexpression of OsLCD1 results in enhanced H₂S production, persulfidation of total soluble protein, and confers rice drought tolerance. Further, we found that nitrate reductase (NR) activity was decreased under drought stress, and the inhibition of NR activity was controlled by endogenous H₂S production. Persulfidation of NIA2, an NR isoform responsible for the main NR activity, led to a decrease in total NR activity in rice. Furthermore, drought stress-triggered inhibition of NR activity and persulfidation of NIA2 was intensified in the OsLCD1 overexpression line. Phenotypical and molecular analysis revealed that mutation of NIA2 enhanced rice drought tolerance by activating the expression of genes encoding antioxidant enzymes and ABA-responsive genes. Taken together, our results showed the role of OsLCD1 in modulating H₂S production and provided insight into H₂S-regulated persulfidation of NIA2 in the control of rice drought stress.

Small signaling molecules in plant response to cold stress

Cold stress is one of the harsh environmental stresses that adversely affect plant growth and crop yields in the Qinghai-Tibet Plateau. However, plants have evolved mechanisms to overcome the impact of cold stress. Progress has been made in understanding how plants perceive and transduce low-temperature signals to tolerate cold stress. Small signaling molecules are crucial for cellular signal transduction by initiating the downstream signaling cascade that helps plants to respond to cold stress. These small signaling molecules include calcium, reactive oxygen species, nitric oxide, hydrogen sulfide, cyclic guanosine monophosphate, phosphatidic acid, and sphingolipids. The small signaling molecules are involved in many aspects of cellular and physiological functions, such as inducing gene expression and activating hormone signaling, resulting in upregulation of the antioxidant enzyme activities, osmoprotectant accumulation, malondialdehyde reduction, and photosynthesis improvement. We summarize our current understanding of the roles of the small signaling molecules in cold stress in plants, and highlight their crosstalk in cold signaling transduction. These discoveries help us understand how the plateau plants adapt to the severe alpine environment as well as to develop new crops tolerating cold stress in the Qinghai-Tibet Plateau.

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

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

Plants Saline Environment in Perception with Rhizosphere Bacteria Containing 1-Aminocyclopropane-1-Carboxylate Deaminase

Soil salinity stress has become a serious roadblock for food production worldwide since it is one of the key factors affecting agricultural productivity. Salinity and drought are predicted to cause considerable loss of crops. To deal with this difficult situation, a variety of strategies have been developed, including plant breeding, plant genetic engineering, and a wide range of agricultural practices, including the use of plant growth-promoting rhizobacteria (PGPR) and seed biopriming techniques, to improve the plants' defenses against salinity stress, resulting in higher crop yields to meet future human food demand. In the present review, we updated and discussed the negative effects of salinity stress on plant morphological parameters and physio-biochemical attributes via various mechanisms and the beneficial roles of PGPR with 1-Aminocyclopropane-1-Carboxylate(ACC) deaminase activity as green bio-inoculants in reducing the impact of saline conditions. Furthermore, the applications of ACC deaminase-producing PGPR as a beneficial tool in seed biopriming techniques are updated and explored. This strategy shows promise in boosting quick seed germination, seedling vigor and plant growth uniformity. In addition, the contentious findings of the variation of antioxidants and osmolytes in ACC deaminase-producing PGPR treated plants are examined.

Protein Persulfidation in Plants: Function and Mechanism

As an endogenous gaseous transmitter, the function of hydrogen sulfide (H2S) has been extensively studied in plants. Once synthesized, H2S may be involved in almost all life processes of plants. Among them, a key route for H2S bioactivity occurs via protein persulfidation, in which process oxidizes cysteine thiol (R-SH) groups into persulfide (R-SSH) groups. This process is thought to underpin a myriad of cellular processes in plants linked to growth, development, stress responses, and phytohormone signaling. Multiple lines of emerging evidence suggest that this redox-based reversible post-translational modification can not only serve as a protective mechanism for H2S in oxidative stress, but also control a variety of biochemical processes through the allosteric effect of proteins. Here, we collate emerging evidence showing that H2S-mediated persulfidation modification involves some important biochemical processes such as growth and development, oxidative stress, phytohormone and autophagy. Additionally, the interaction between persulfidation and S-nitrosylation is also discussed. In this work, we provide beneficial clues for further exploration of the molecular mechanism and function of protein persulfidation in plants in the future.

Involvement of nitric oxide (NO) in plant responses to metalloids

Plants respond to the limited or excess supply of metalloids, boron (B), silicon (Si), selenium (Se), arsenic (As), and antimony (Sb) via complex signaling pathways that are mainly regulated by nitric oxide (NO). The absorption of metalloids from the soil is facilitated by pathways that involve aquaporins, aquaglyceroporins, phosphate, and sulfate transporters; however, their regulation by NO is poorly understood. Using in silico software, we predicted the S-nitrosation of known metalloid transporters, proposing NO-dependent regulation of metalloid transport systems at the posttranslational level. NO intensifies the stress-mitigating effect of Si, whereas in the case of Se, As, and Sb, the accumulation of NO or reactive nitrogen species contributes to toxicity. NO promotes the beneficial effect of low Se concentrations and mitigates the damage caused by B deficiency. In addition, the exogenous application of NO donor, sodium nitroprusside, reduces B, Se, and As toxicity. The primary role of NO in metalloid stress response is to mitigate oxidative stress by activating antioxidant defense at the level of protein activity and gene expression. This review discusses the role of NO in plant responses to metalloids and suggests future research directions.

Alfalfa (Medicago sativa L.) MsCML46 gene encoding calmodulin-like protein confers tolerance to abiotic stress in tobacco

KEY MESSAGE

MsCML46 enhances tolerance to abiotic stresses through alleviating osmotic stress and oxidative damage by regulating the expression of stress-related genes to optimize osmolytes levels and antioxidant enzyme activity in transgenic tobacco. Abiotic stresses are major environmental factors that constraint crop productivity worldwide. Various stimuli regulate intracellular calcium levels and calcium-mediated signal transduction, and cellular responses. Ca2+ signals are perceived by different Ca2+ receptors. Calmodulin-like protein (CML) is one of the best-characterized Ca2+ sensors which shares sequence similarity with highly conserved calmodulin (CaM) ubiquitously expressed in plants. Currently, the molecular and physiological functions of CMLs are largely unknown. In this study, the MsCML46 was characterized in alfalfa (Medicago sativa cv. Zhaodong) under freezing stress. Results showed that MsCML46 was localized to the cytoplasm of Arabidopsis, and its expression was strongly elevated by cold, drought, salt, saline-alkali, and ABA treatments. Overexpressing MsCML46 in tobacco enhanced tolerance to freezing, drought, and salt stresses as evidenced by improved contents of osmotic regulatory solutes and antioxidant enzyme activity but decreased reactive oxygen species (ROS) accumulation. Furthermore, cold, drought, and salt stresses increased the expression of stress-related genes in transgenic tobacco. MsCML46 binds free Ca2+ to promote signal transduction and maintain higher K+/Na+ ratio. In this way, it protects intracellular homeostasis under sodium ion toxicity. These results suggest that MsCML46 plays a crucial role in resisting abiotic stresses and can be exploited in genetic engineering for crops.

What signals the glyoxalase pathway in plants?

Glyoxalase (GLY) system, comprising of GLYI and GLYII enzymes, has emerged as one of the primary methylglyoxal (MG) detoxification pathways with an indispensable role during abiotic and biotic stresses. MG homeostasis is indeed very closely guarded by the cell as its higher levels are cytotoxic for the organism. The dynamic responsiveness of MG-metabolizing GLY pathway to both endogenous cues such as, phytohormones, nutrient status, etc., as well as external environmental fluctuations (abiotic and biotic stresses) indicates that a tight regulation occurs in the cell to maintain physiological levels of MG in the system. Interestingly, GLY pathway is also manipulated by its substrates and reaction products. Hence, an investigation of signalling and regulatory aspects of GLY pathway would be worthwhile. Herein, we have attempted to converge all known factors acting as signals or directly regulating GLYI/II enzymes in plants. Further, we also discuss how crosstalk between these different signal molecules might facilitate the regulation of glyoxalase pathway. We believe that MG detoxification is controlled by intricate mechanisms involving a plethora of signal molecules.

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

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

Computational prediction of NO-dependent posttranslational modifications in plants: Current status and perspectives

The perception and transduction of nitric oxide (NO) signal is achieved by NO-dependent posttranslational modifications (PTMs) among which S-nitrosation and tyrosine nitration has biological significance. In plants, 100-1000 S-nitrosated and tyrosine nitrated proteins have been identified so far by mass spectrometry. The determination of NO-modified protein targets/amino acid residues is often methodologically challenging. In the past decade, the growing demand for the knowledge of S-nitrosated or tyrosine nitrated sites has motivated the introduction of bioinformatics tools. For predicting S-nitrosation seven computational tools have been developed (GPS-SNO, SNOSite, iSNO-PseACC, iSNO-AAPAir, PSNO, PreSNO, RecSNO). Four predictors have been developed for indicating tyrosine nitration sites (GPS-YNO2, iNitro-Tyr, PredNTS, iNitroY-Deep), and one tool (DeepNitro) predicts both NO-dependent PTMs. The advantage of these computational tools is the fast provision of large amount of information. In this review, the available software tools have been tested on plant proteins in which S-nitrosated or tyrosine nitrated sites have been experimentally identified. The predictors showed distinct performance and there were differences from the experimental results partly due to the fact that the three-dimensional protein structure is not taken into account by the computational tools. Nevertheless, the predictors excellently establish experiments, and it is suggested to apply all available tools on target proteins and compare their results. In the future, computational prediction must be developed further to improve the precision with which S-nitrosation/tyrosine nitration-sites are identified.

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.

Coordinated Role of Nitric Oxide, Ethylene, Nitrogen, and Sulfur in Plant Salt Stress Tolerance

Salt stress significantly contributes to major losses in agricultural productivity worldwide. The sustainable approach for salinity-accrued toxicity has been explored. The use of plant growth regulators/phytohormones, mineral nutrients and other signaling molecules is one of the major approaches for reversing salt-induced toxicity in plants. Application of the signaling molecules such as nitric oxide (NO) and ethylene (ETH) and major mineral nutrient such as nitrogen (N) and sulfur (S) play significant roles in combatting the major consequences of salt stress impacts in plants. However, the literature available on gaseous signaling molecules (NO/ETH) or/and mineral nutrients (N/S) stands alone, and major insights into the role of NO or/and ETH along with N and S in plant-tolerance to salt remained unclear. Thus, this review aimed to (a) briefly overview salt stress and highlight salt-induced toxicity, (b) appraise the literature reporting potential mechanisms underlying the role of gaseous signaling molecules and mineral nutrient in salt stress tolerance, and (c) discuss NO and ETH along with N and S in relation to salt stress tolerance. In addition, significant issues that have still to be investigated in this context have been mentioned.

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.

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

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

Nitric oxide sensing revisited

Nitric oxide (NO) sensing is an ancient trait enabled by hemoproteins harboring a highly conserved Heme-Nitric oxide/OXygen (H-NOX) domain that operates throughout bacteria, fungi, and animal kingdoms including in humans, but that has long thought to be absent in plants. Recently, H-NOX-containing plant hemoproteins mediating crucial NO-dependent responses such as stomatal closure and pollen tube guidance have been reported. There are indications that the detection method that led to these discoveries will uncover many more heme-based NO sensors that operate as regulatory sites in complex proteins. Their characterizations will in turn offer a much more complete picture of plant NO responses at both the molecular and systems level.

Legume-rhizobium dance: an agricultural tool that could be improved?

The specific interaction between rhizobia and legume roots leads to the development of a highly regulated process called nodulation, by which the atmospheric nitrogen is converted into an assimilable plant nutrient. This capacity is the basis for the use of bacterial inoculants for field crop cultivation. Legume plants have acquired tools that allow the entry of compatible bacteria. Likewise, plants can impose sanctions against the maintenance of nodules occupied by rhizobia with low nitrogen-fixing capacity. At the same time, bacteria must overcome different obstacles posed first by the environment and then by the legume. The present review describes the mechanisms involved in the regulation of the entire legume-rhizobium symbiotic process and the strategies and tools of bacteria for reaching the nitrogen-fixing state inside the nodule. Also, we revised different approaches to improve the nodulation process for a better crop yield.