Nitric oxide production in plants: an update

Protein Tyrosine Nitration in Plant Nitric Oxide Signaling

Nitric oxide (NO), which is ubiquitously present in living organisms, regulates many developmental and stress-activated processes in plants. Regulatory effects exerted by NO lies mostly in its chemical reactivity as a free radical. Proteins are main targets of NO action as several amino acids can undergo NO-related post-translational modifications (PTMs) that include mainly S-nitrosylation of cysteine, and nitration of tyrosine and tryptophan. This review is focused on the role of protein tyrosine nitration on NO signaling, making emphasis on the production of NO and peroxynitrite, which is the main physiological nitrating agent; the main metabolic and signaling pathways targeted by protein nitration; and the past, present, and future of methodological and strategic approaches to study this PTM. Available information on identification of nitrated plant proteins, the corresponding nitration sites, and the functional effects on the modified proteins will be summarized. However, due to the low proportion of in vivo nitrated peptides and their inherent instability, the identification of nitration sites by proteomic analyses is a difficult task. Artificial nitration procedures are likely not the best strategy for nitration site identification due to the lack of specificity. An alternative to get artificial site-specific nitration comes from the application of genetic code expansion technologies based on the use of orthogonal aminoacyl-tRNA synthetase/tRNA pairs engineered for specific noncanonical amino acids. This strategy permits the programmable site-specific installation of genetically encoded 3-nitrotyrosine sites in proteins expressed in Escherichia coli, thus allowing the study of the effects of specific site nitration on protein structure and function.

Identification of Partner Proteins of the Algae Klebsormidium nitens NO Synthases: Toward a Better Understanding of NO Signaling in Eukaryotic Photosynthetic Organisms

In animals, NO is synthesized from L-arginine by three isoforms of nitric oxide synthase (NOS) enzyme. NO production and effects have also been reported in plants but the identification of its sources, especially the enzymatic ones, remains one of the critical issues in the field. NOS-like activities have been reported, although there are no homologs of mammalian NOS in the land plant genomes sequenced so far. However, several NOS homologs have been found in algal genomes and transcriptomes. A first study has characterized a functional NOS in the chlorophyte Ostreococcus tauri and the presence of NOS homologs was later confirmed in a dozen algae. These results raise the questions of the significance of the presence of NOS and their molecular diversity in algae. We hypothesize that comparisons among protein structures of the two KnNOS, together with the identification of their interacting partner proteins, might allow a better understanding of the molecular diversification and functioning of NOS in different physiological contexts and, more generally, new insights into NO signaling in photosynthetic organisms. We recently identified two NOS homologs sequences in the genome of the streptophyte Klebsormidium nitens, a model alga in the study of plant adaptation to terrestrial life. The first sequence, named KnNOS1, contains canonical NOS signatures while the second, named KnNOS2, presents a large C-ter extension including a globin domain. In order to identify putative candidates for KnNOSs partner proteins, we draw the protein-protein interaction networks of the three human NOS using the BioGRID database and hypothesized on the biological role of K. nitens orthologs. Some of these conserved partners are known to be involved in mammalian NOSs regulation and functioning. In parallel, our methodological strategy for the identification of partner proteins of KnNOS1 and KnNOS2 by in vitro pull-down assay is presented.

NIN-like protein7 and PROTEOLYSIS6 functional interaction enhances tolerance to sucrose, ABA, and submergence

Nitrate (NO3) assimilation and signaling regulate plant growth through the relevant function of the transcription factor NIN-like Protein7 (NLP7). NO3 is also the main source for plants to produce nitric oxide (NO), which regulates growth and stress responses. NO-mediated regulation requires efficient sensing via the PROTEOLYSIS6 (PRT6)-mediated proteasome-triggered degradation of group VII of ethylene response transcription factors through the Cys/Arg N-degron pathway. The convergence of NO3 signaling and N-degron proteolysis on NO-mediated regulation remains largely unknown. Here, we investigated the functional interaction between NLP7 and PRT6 using Arabidopsis (Arabidopsis thaliana) double prt6 nlp7 mutant plants as well as complementation lines overexpressing NLP7 in different mutant genetic backgrounds. prt6 nlp7 mutant plants displayed several potentiated prt6 characteristic phenotypes, including slower vegetative growth, increased NO content, and diminished tolerance to abiotic stresses such as high-sucrose concentration, abscisic acid, and hypoxia-reoxygenation. Although NLP7 has an N-terminus that could be targeted by the N-degron proteolytic pathway, it was not a PRT6 substrate. The potential PRT6- and NO-regulated nucleocytoplasmic translocation of NLP7, which is likely modulated by posttranslational modifications, is proposed to act as a regulatory loop to control NO homeostasis and action.

Nitric oxide and hydrogen sulfide: an indispensable combination for plant functioning

Nitric oxide (NO) and hydrogen sulfide (H₂S) are gasotransmitters, which are involved in almost all plant physiological and stress-related processes. With its antioxidant regulatory properties, NO on its own ameliorates plant stress, while H₂S, a foul-smelling gas, has differential effects. Recent studies have shown that these signaling molecules are involved in intertwined pathway networks. This is due to the contrasting effects of NO and H₂S depending on cell type, subcellular compartment, and redox status, as well as the flux and dosage of NO and H₂S in different plant species and cellular contexts. Here, we provide a comprehensive review of the complex networks of these molecules, with particular emphasis on root development, stomatal movement, and plant cell death.

Transcriptomic and Biochemical Analysis Reveal Integrative Pathways Between Carbon and Nitrogen Metabolism in Guzmania monostachia (Bromeliaceae) Under Drought

Most epiphytes are found in low-nutrient environments with an intermittent water supply. To deal with water limitation, many bromeliads perform crassulacean acid metabolism (CAM), such as Guzmania monostachia, which shifts from C₃ to CAM and can recycle CO₂ from the respiration while stomata remain closed during daytime and nighttime (CAM-idling mode). Since the absorbing leaf trichomes can be in contact with organic (urea) and inorganic nutrients (NO₃ ^-, NH₄ ⁺) and the urea hydrolysis releases NH₄ ⁺ and CO₂, we hypothesized that urea can integrate the N and C metabolism during periods of severe drought. Under this condition, NH₄ ⁺ can be assimilated into amino acids through glutamine synthetase (GS), while the CO₂ can be pre-fixated by phosphoenolpyruvate carboxylase (PEPC). In this context, we evaluated the foliar transcriptome of G. monostachia to compare the relative gene expression of some genes involved with CAM and the N metabolism when bromeliads were submitted to 7days of drought. We also conducted a controlled experiment with an extended water deficit period (21days) in which bromeliads were cultivated in different N sources (urea, NH₄ ⁺, and NO₃ ^-). Our transcriptome results demonstrated an increment in the expression of genes related to CAM, particularly those involved in the carboxylation metabolism (PEPC1, PPCK, and NAD-MDH), the movement of malate through vacuolar membrane (ALMT9), and the decarboxylation process (PEPCK). Urea stimulated the expression of PEPC1 and ALMT9, while Urease transcripts increased under water deficit. Under this same condition, GS1 gene expression increased, indicating that the NH₄ ⁺ from urea hydrolysis can be assimilated in the cytosol. We suggest that the link between C and N metabolism occurred through the supply of carbon skeleton (2-oxoglutarate, 2-OG) by the cytosolic isocitrate dehydrogenase since the number of NADP-ICDH transcripts was also higher under drought conditions. These findings indicate that while urea hydrolysis provides NH₄ ⁺ that can be consumed by glutamine synthetase-cytosolic/glutamate synthase (GS1/GOGAT) cycle, the CO₂ can be used by CAM, maintaining photosynthetic efficiency even when most stomata remain closed 24h (CAM-idling) as in the case of a severe water deficit condition. Thus, we suggest that urea could be used by G. monostachia as a strategy to increase its survival under drought, integrating N and C metabolism.

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.

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.

Nitrate Regulates Maize Root Transcriptome through Nitric Oxide Dependent and Independent Mechanisms

Maize root responds to nitrate by modulating its development through the coordinated action of many interacting players. Nitric oxide is produced in primary root early after the nitrate provision, thus inducing root elongation. In this study, RNA sequencing was applied to discover the main molecular signatures distinguishing the response of maize root to nitrate according to their dependency on, or independency of, nitric oxide, thus discriminating the signaling pathways regulated by nitrate through nitric oxide from those regulated by nitrate itself of by further downstream factors. A set of subsequent detailed functional annotation tools (Gene Ontology enrichment, MapMan, KEGG reconstruction pathway, transcription factors detection) were used to gain further information and the lateral root density was measured both in the presence of nitrate and in the presence of nitrate plus cPTIO, a specific NO scavenger, and compared to that observed for N-depleted roots. Our results led us to identify six clusters of transcripts according to their responsiveness to nitric oxide and to their regulation by nitrate provision. In general, shared and specific features for the six clusters were identified, allowing us to determine the overall root response to nitrate according to its dependency on nitric oxide.

Ascorbate and glutathione independently alleviate arsenate toxicity in brinjal but both require endogenous nitric oxide

In this study, we have explored the possible role of ascorbic acid (ASC) and glutathione (GSH) in alleviating arsenate (As^V ) toxicity in brinjal roots. Moreover, we have also focused our attention on the possible involvement of endogenous nitric oxide (NO) in accomplishing this task. As^V treatment negatively impacts the length and fresh weight of roots and shoots as well as the dry weight and fitness of roots, and this was accompanied by greater As accumulation in roots and shoots of brinjal. As^V treatment also declined the endogenous NO level by inhibiting Nitric Oxide Synthase-like (NOS-like) activity. Furthermore, As^V stimulated oxidative stress markers, caused protein damage by their carbonylation due to downregulation in antioxidants [particularly ascorbate (AsA)-GSH cycle], leading to disturbed cellular redox status. This, collectively, led to root cell death in brinjal. However, the addition of either ASC or GSH rescued brinjal roots from the toxic effects of As^V in. Interestingly, lycorine (an inhibitor of ASC biosynthesis) further increased As^V toxicity, while ASC rescued its effects. Moreover, buthionine sulphoximine (BSO, an inhibitor of GSH biosynthesis) interestingly increased further As^V toxicity, while GSH rescued the plant from the As toxic effects. An interesting notion of this study was that GSH rescued the toxic effect of lycorine, while ASC rescued the toxic effect of BSO, though the As^V toxicity mediated by either ASC or GSH was always accompanied by high endogenous NO level and NOS-like activity. All together, these results suggest that ASC and GSH independently mitigate As^V toxicity in brinjal roots, but both might be dependent on endogenous NO for accomplishing the As^V toxicity alleviatory tasks.

Comparative transcriptomic analysis uncovers conserved pathways involved in adventitious root formation in poplar

Cutting propagation is widely used in establishing poplar plantations, and this approach requires efficient adventitious root (AR) forming capacities. Although poplar species are considered to form roots easily, interspecific variations in AR formation are still observed. To better understand the gene regulatory network underlying the conserved modified pathways that are essential for AR formation in poplar species, comparative transcriptomic approaches were applied to identify the conserved common genes that were differentially expressed during the AR formation processes in two poplar species (Populus × euramericana and P. simonii) in woody plant medium (WPM). A total of 2146 genes were identified as conserved genes that shared similar gene expression profiles in at least one comparison. These conserved genes were enriched in diverse hormone signaling pathways, as well as the mitogen-associated protein kinase (MAPK) signaling pathway, suggesting an important role for signaling transduction in coordinating external stimuli and endogenous physiological status during AR regulation in poplar. Furthermore, the co-expression network analysis of conserved genes allowed identification of several co-expressed modules (CM) that are co-expressed with distinct biological functions, for instance, CM1 was enriched in defense response and hormone signaling, CM2 and CM3 were overrepresented in defense response-related pathways and for cell cycle, respectively. These results suggest that the AR formation processes in poplar were finely tuned at the transcriptomic level by integrating multiple biological processes essential for AR formation. Our results suggest conserved machinery for AR formation in poplar and generated informative gene co-expression networks that describe the basis of AR formation in these species.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01054-7.

The scope of flavin-dependent reactions and processes in the model plant Arabidopsis thaliana

Flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) are utilized as coenzymes in many biochemical reduction-oxidation reactions owing to the ability of the tricyclic isoalloxazine ring system to employ the oxidized, radical and reduced state. We have analyzed the genome of Arabidopsis thaliana to establish an inventory of genes encoding flavin-dependent enzymes (flavoenzymes) as a basis to explore the range of flavin-dependent biochemical reactions that occur in this model plant. Expectedly, flavoenzymes catalyze many pivotal reactions in primary catabolism, which are connected to the degradation of basic metabolites, such as fatty and amino acids as well as carbohydrates and purines. On the other hand, flavoenzymes play diverse roles in anabolic reactions most notably the biosynthesis of amino acids as well as the biosynthesis of pyrimidines and sterols. Importantly, the role of flavoenzymes goes much beyond these basic reactions and extends into pathways that are equally crucial for plant life, for example the production of natural products. In this context, we outline the participation of flavoenzymes in the biosynthesis and maintenance of cofactors, coenzymes and accessory plant pigments (e. g. carotenoids) as well as phytohormones. Moreover, several multigene families have emerged as important components of plant immunity, for example the family of berberine bridge enzyme-like enzymes, flavin-dependent monooxygenases and NADPH oxidases. Furthermore, the versatility of flavoenzymes is highlighted by their role in reactions leading to tRNA-modifications, chromatin regulation and cellular redox homeostasis. The favorable photochemical properties of the flavin chromophore are exploited by photoreceptors to govern crucial processes of plant adaptation and development. Finally, a sequence- and structure-based approach was undertaken to gain insight into the catalytic role of uncharacterized flavoenzymes indicating their involvement in unknown biochemical reactions and pathways in A. thaliana.

An update on redox signals in plant responses to biotic and abiotic stress crosstalk: insights from cadmium and fungal pathogen interactions

Complex signalling pathways are involved in plant protection against single and combined stresses. Plants are able to coordinate genome-wide transcriptional reprogramming and display a unique programme of transcriptional responses to a combination of stresses that differs from the response to single stresses. However, a significant overlap between pathways and some defence genes in the form of shared and general stress-responsive genes appears to be commonly involved in responses to multiple biotic and abiotic stresses. Reactive oxygen and nitrogen species, as well as redox signals, are key molecules involved at the crossroads of the perception of different stress factors and the regulation of both specific and general plant responses to biotic and abiotic stresses. In this review, we focus on crosstalk between plant responses to biotic and abiotic stresses, in addition to possible plant protection against pathogens caused by previous abiotic stress. Bioinformatic analyses of transcriptome data from cadmium- and fungal pathogen-treated plants focusing on redox gene ontology categories were carried out to gain a better understanding of common plant responses to abiotic and biotic stresses. The role of reactive oxygen and nitrogen species in the complex network involved in plant responses to changes in their environment is also discussed.

Molecular responses of legumes to abiotic stress: post-translational modifications of proteins and redox signaling

Legumes include several major crops that can fix atmospheric nitrogen in symbiotic root nodules, thus reducing the demand for nitrogen fertilizers and contributing to sustainable agriculture. Global change models predict increases in temperature and extreme weather conditions. This scenario might increase plant exposure to abiotic stresses and negatively affect crop production. Regulation of whole plant physiology and nitrogen fixation in legumes during abiotic stress is complex, and only a few mechanisms have been elucidated. Reactive oxygen species (ROS), reactive nitrogen species (RNS), and reactive sulfur species (RSS) are key players in the acclimation and stress tolerance mechanisms of plants. However, the specific redox-dependent signaling pathways are far from understood. One mechanism by which ROS, RNS, and RSS fulfil their signaling role is the post-translational modification (PTM) of proteins. Redox-based PTMs occur in the cysteine thiol group (oxidation, S-nitrosylation, S-glutathionylation, persulfidation), and also in methionine (oxidation), tyrosine (nitration), and lysine and arginine (carbonylation/glycation) residues. Unraveling PTM patterns under different types of stress and establishing the functional implications may give insight into the underlying mechanisms by which the plant and nodule respond to adverse conditions. Here, we review current knowledge on redox-based PTMs and their possible consequences in legume and nodule biology.

Interplay Among Hydrogen Sulfide, Nitric Oxide, Reactive Oxygen Species, and Mitochondrial DNA Oxidative Damage

Hydrogen sulfide (H2S), nitric oxide (NO), and reactive oxygen species (ROS) play essential signaling roles in cells by oxidative post-translational modification within suitable ranges of concentration. All of them contribute to the balance of redox and are involved in the DNA damage and repair pathways. However, the damage and repair pathways of mitochondrial DNA (mtDNA) are complicated, and the interactions among NO, H2S, ROS, and mtDNA damage are also intricate. This article summarized the current knowledge about the metabolism of H2S, NO, and ROS and their roles in maintaining redox balance and regulating the repair pathway of mtDNA damage in plants. The three reactive species may likely influence each other in their generation, elimination, and signaling actions, indicating a crosstalk relationship between them. In addition, NO and H2S are reported to be involved in epigenetic variations by participating in various cell metabolisms, including (nuclear and mitochondrial) DNA damage and repair. Nevertheless, the research on the details of NO and H2S in regulating DNA damage repair of plants is in its infancy, especially in mtDNA.

GSNOR Contributes to Demethylation and Expression of Transposable Elements and Stress-Responsive Genes

In the past, reactive nitrogen species (RNS) were supposed to be stress-induced by-products of disturbed metabolism that cause oxidative damage to biomolecules. However, emerging evidence demonstrates a substantial role of RNS as endogenous signals in eukaryotes. In plants, S-nitrosoglutathione (GSNO) is the dominant RNS and serves as the ^•NO donor for S-nitrosation of diverse effector proteins. Remarkably, the endogenous GSNO level is tightly controlled by S-nitrosoglutathione reductase (GSNOR) that irreversibly inactivates the glutathione-bound NO to ammonium. Exogenous feeding of diverse RNS, including GSNO, affected chromatin accessibility and transcription of stress-related genes, but the triggering function of RNS on these regulatory processes remained elusive. Here, we show that GSNO reductase-deficient plants (gsnor1-3) accumulate S-adenosylmethionine (SAM), the principal methyl donor for methylation of DNA and histones. This SAM accumulation triggered a substantial increase in the methylation index (MI = [SAM]/[S-adenosylhomocysteine]), indicating the transmethylation activity and histone methylation status in higher eukaryotes. Indeed, a mass spectrometry-based global histone profiling approach demonstrated a significant global increase in H3K9me2, which was independently verified by immunological detection using a selective antibody. Since H3K9me2-modified regions tightly correlate with methylated DNA regions, we also determined the DNA methylation status of gsnor1-3 plants by whole-genome bisulfite sequencing. DNA methylation in the CG, CHG, and CHH contexts in gsnor1-3 was significantly enhanced compared to the wild type. We propose that GSNOR1 activity affects chromatin accessibility by controlling the transmethylation activity (MI) required for maintaining DNA methylation and the level of the repressive chromatin mark H3K9me2.

Magneto-priming promotes nitric oxide via nitric oxide synthase to ameliorate the UV-B stress during germination of soybean seedlings

We have evaluated the contribution of nitric oxide (NO) in static magnetic field (SMF-200 mT for 1h) induced tolerance towards UV-B stress in soybean seedlings using various NO modulators like sodium nitroprusside (SNP), inhibitor of nitrate reductase (NR) sodium tungstate (ST), NO synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) and diphenylene iodonium (DPI) a NADPH oxidase inhibitor. The UV-B exposure significantly reduced germination, seedling growth together with activities of total amylase, NOS and NR in seedlings from un-primed seeds whereas SMF-primed seedlings showed significant enhancement in all these parameters along with higher level of NO/ROS. The supply of NO donor, SNP further improved all the seedlings parameters in un-primed and SMF-primed seeds after UV-B exposure. While ST, L-NAME and DPI significantly reduced the SMF-induced seedling performance after UV-B exposure. The gene expression study also showed significant up-regulation of α-amylase (GmAMY1, GmAMY2), nitric oxide synthase (GmNOS2) and nitrate reductase (GmNR2) encoding genes in UV-B exposed SMF-primed seedlings over un-primed seedlings. In particular, SNP+UV-B treatment enhanced the GmNOS2 expression in both unprimed (31.9-fold) and SMF-primed (93.2-fold) seedlings in comparison to their respective controls of CK+UV-B. In contrast, L-NAME+UV-B treatment reduced the SMF-induced GmNOS2 expression (4.8-fold) and NOS activity (76%). It confirmed that NO may be the key signaling molecule in SMF stimulated tolerance towards UV-B stress during early seedling growth and NOS may possibly be accountable for SMF-triggered NO production in soybean seedlings exposed to UV-B irradiations.

Ozone Induced Stomatal Regulations, MAPK and Phytohormone Signaling in Plants

Ozone (O₃) is a gaseous environmental pollutant that can enter leaves through stomatal pores and cause damage to foliage. It can induce oxidative stress through the generation of reactive oxygen species (ROS) like hydrogen peroxide (H₂O₂) that can actively participate in stomatal closing or opening in plants. A number of phytohormones, including abscisic acid (ABA), ethylene (ET), salicylic acid (SA), and jasmonic acid (JA) are involved in stomatal regulation in plants. The effects of ozone on these phytohormones’ ability to regulate the guard cells of stomata have been little studied, however, and the goal of this paper is to explore and understand the effects of ozone on stomatal regulation through guard cell signaling by phytohormones. In this review, we updated the existing knowledge by considering several physiological mechanisms related to stomatal regulation after response to ozone. The collected information should deepen our understanding of the molecular pathways associated with response to ozone stress, in particular, how it influences stomatal regulation, mitogen-activated protein kinase (MAPK) activity, and phytohormone signaling. After summarizing the findings and noting the gaps in the literature, we present some ideas for future research on ozone stress in plants

Bacterial nitric oxide metabolism: Recent insights in rhizobia

Nitric oxide (NO) is a reactive gaseous molecule that has several functions in biological systems depending on its concentration. At low concentrations, NO acts as a signaling molecule, while at high concentrations, it becomes very toxic due to its ability to react with multiple cellular targets. Soil bacteria, commonly known as rhizobia, have the capacity to establish a N₂-fixing symbiosis with legumes inducing the formation of nodules in their roots. Several reports have shown NO production in the nodules where this gas acts either as a signaling molecule which regulates gene expression, or as a potent inhibitor of nitrogenase and other plant and bacteria enzymes. A better understanding of the sinks and sources of NO in rhizobia is essential to protect symbiotic nitrogen fixation from nitrosative stress. In nodules, both the plant and the microsymbiont contribute to the production of NO. From the bacterial perspective, the main source of NO reported in rhizobia is the denitrification pathway that varies significantly depending on the species. In addition to denitrification, nitrate assimilation is emerging as a new source of NO in rhizobia. To control NO accumulation in the nodules, in addition to plant haemoglobins, bacteroids also contribute to NO detoxification through the expression of a NorBC-type nitric oxide reductase as well as rhizobial haemoglobins. In the present review, updated knowledge about the NO metabolism in legume-associated endosymbiotic bacteria is summarized.

Effect of Individual, Simultaneous and Sequential Inoculation of Pseudomonas fluorescens and Meloidogyne incognita on Growth, Biochemical, Enzymatic and Nonenzymatic Antioxidants of Tomato (Solanum lycopersicum L.)

This study was conducted on tomato (Solanum lycopersicum cv. K-21) to investigate the bioprotective nature of Pseudomonas fluorescens and its interactive effects with Meloidogyne incognita in terms of growth biomarkers, changes in biochemical attributes and modulation in antioxidant enzymes of the tomato plant. In this study, we grew tomato plants with M. incognita and P. fluorescens in separate pots, simultaneously and sequentially (15 days prior or post) after 15 days of seed sowing. The sequential inoculation of Mi15→Pf maximally increased the root-knot index and decreased the nematode population. It was also noted that inoculation suppressed the plant growth biomarkers in comparison to control. However, maximum suppression in nematode reproduction and increment in growth and physiological attributes were observed when P. fluorescens was applied 15 days prior to the nematode (Pf15→Mi) as compared to control. All the treatments showed an increase in antioxidant enzymes. Expression of phenol content and defensive enzymes such as peroxidase (POX) and superoxide dismutase (SOD) increased, in contrast to a significant reduction in malondialdehyde (MDA) and hydrogen peroxide (H2O2) contents when compared with the untreated inoculated plants. However, the highest levels of POX and SOD, and a lowest of phenol, MDA and H2O2 were displayed in the treatment Pf15→Mi, followed by Mi+Pf and Mi15→Pf.

Endogenous • NO accumulation in soybean is associated with initial stomatal response to water deficit

Drought is the main cause of productivity losses in soybean plants, triggering physiological and biochemical responses, stomatal closure being essential to prevent water losses and thus mitigate the negative effects of drought. Abscisic acid (ABA) is the main molecule involved in stomatal closure under drought conditions along with nitric oxide (• NO). However, the role of • NO in this process is not yet fully understood and contrasting findings about its role have been reported. Most of the assays in the literature have been carried out under in vitro conditions using • NO donors or scavengers, but little is known about the effects of endogenously produced • NO under drought conditions. This study is aimed to determine the pattern of endogenous • NO accumulation from the establishment of water stress and how this relates to stomatal closure and other biochemical and physiological responses. The analysis of soybean plant responses to drought revealed no correlation between whole-leaf • NO accumulation and typical water-deficit stress markers. Moreover, • NO accumulation did not explain oxidative damage induced by drought. However, endogenous • NO content correlated with the early stomatal closure. Analysis of stomatal behavior and endogenous • NO content in guard cells through epidermal peel technique showed a stomatal population with high variation in stomatal opening and • NO content under the initial stages of water stress, even when ABA responses are activated. Our data suggest that upon early stress perception, soybean plants respond by accumulating • NO in the guard cells to inhibit stomatal closure, potentially through the inhibition of ABA responses.

Redox and low-oxygen stress: signal integration and interplay

Plants are aerobic organisms relying on oxygen to serve their energy needs. The amount of oxygen available to sustain plant growth can vary significantly due to environmental constraints or developmental programs. In particular, flooding stress, which negatively impacts crop productivity, is characterized by a decline in oxygen availability. Oxygen fluctuations result in an altered redox balance and the formation of reactive oxygen/nitrogen species (ROS/RNS) during the onset of hypoxia and upon re-oxygenation. In this update, we provide an overview of the current understanding of the impact of redox and ROS/RNS on low-oxygen signaling and adaptation. We first focus on the formation of ROS and RNS during low-oxygen conditions. Following this, we examine the impact of hypoxia on cellular and organellar redox systems. Finally, we describe how redox and ROS/RNS participate in signaling events during hypoxia through potential post-translational modifications (PTMs) of hypoxia-relevant proteins. The aim of this update is to define our current understanding of the field and to provide avenues for future research directions.

Selective redox signaling shapes plant-pathogen interactions

A review of recent progress in understanding the mechanisms whereby plants utilize selective and reversible redox signaling to establish immunity.

Lessons from Comparison of Hypoxia Signaling in Plants and Mammals

Hypoxia is an important stress for organisms, including plants and mammals. In plants, hypoxia can be the consequence of flooding and causes important crop losses worldwide. In mammals, hypoxia stress may be the result of pathological conditions. Understanding the regulation of responses to hypoxia offers insights into novel approaches for crop improvement, particularly for the development of flooding-tolerant crops and for producing better therapeutics for hypoxia-related diseases such as inflammation and cancer. Despite their evolutionary distance, plants and mammals deploy strikingly similar mechanisms to sense and respond to the different aspects of hypoxia-related stress, including low oxygen levels and the resulting energy crisis, nutrient depletion, and oxidative stress. Over the last two decades, the ubiquitin/proteasome system and the ubiquitin-like protein SUMO have been identified as key regulators that act in concert to regulate core aspects of responses to hypoxia in plants and mammals. Here, we review ubiquitin and SUMO-dependent mechanisms underlying the regulation of hypoxia response in plants and mammals. By comparing and contrasting these mechanisms in plants and mammals, this review seeks to pinpoint conceptually similar mechanisms but also highlight future avenues of research at the junction between different fields of research.

Regulation of ascorbate-glutathione cycle by exogenous nitric oxide and hydrogen peroxide in soybean roots under arsenate stress

The role of nitric oxide (NO) and hydrogen peroxide (H2O2) is well known for regulating plant abiotic stress responses. However, underlying mechanisms are still poorly understood. Therefore, the present study investigated the involvement of NO and H2O2 signalling in the regulation of arsenate toxicity (AsV) in soybean roots employing a pharmacological approach. Results show that AsV toxicity declined root length and biomass due to greater As accumulation in the cell wall and cellular organelles. Arsenate induced cell death due to enhanced levels of reactive oxygen species, lipid and protein oxidation and down-regulation in ascorbate-glutathione cycle and redox states of ascorbate and glutathione. These results correlate with lower endogenous level of NO. Interestingly, addition of L-NAME increased AsV toxicity. However, addition of SNP reverses effect of L-NAME, suggesting that endogenous NO has a role in mitigating AsV toxicity. Exogenous H2O2 also demonstrated capability of alleviating AsV stress, while NAC reversed the protective effect of H2O2. Furthermore, DPI application further increased AsV toxicity, suggesting that endogenous H2O2 is also implicated in mitigating AsV stress. SNP was not able to mitigate AsV toxicity in the presence of DPI, suggesting that H2O2 might have acted downstream of NO in accomplishing amelioration of AsV toxicity.

Leaf arginine spraying improves leaf gas exchange under water deficit and root antioxidant responses during the recovery period

Arginine (Arg) metabolism is associated with many cellular and developmental processes in plants and proline, nitric oxide (NO) and polyamines (PAs) have a wide range of physiological functions in plants, including increased tolerance to environmental stresses. This study aimed to test the hypothesis that Arg spraying would stimulate the synthesis of proline, NO and PAs, reducing the oxidative damage caused by water deficit (WD) and increasing drought tolerance of sugarcane plants. Sugarcane plants were sprayed with water or Arg 1 mM, and subjected to WD by gradual addition of polyethylene glycol (PEG-8000) to the nutrient solution. As references, sugarcane plants were grown in nutrient solution without PEG-8000 and sprayed or not with Arg. Our data indicate that exogenous Arg supply improved leaf gas exchange during water deficit and enhanced the root antioxidative protection of sugarcane plants during the recovery period. Arg supply prevented the proline accumulation induced by water deficit and then the main pathway for proline synthesis is likely through glutamate instead of arginine. Although Arg is a substrate for NO and PAs production, supplying Arg had only slight effects in both NO and PAs levels. The spraying of amino acids capable of reducing the harmful effects of drought, such as Arg, can be an alternative to improve crop growth under field conditions.

Silicon- induced nitric oxide burst modulates systemic defensive responses of Salvia officinalis under copper toxicity

In this study, the role of nitric oxide (NO) burst in modulating Si-induced defensive responses in leaves and roots of Salvia officinalis under copper (Cu) stress were investigated. The result showed that 400 μM Cu markedly reduced shoot dry weight, but increased electrolyte leakage (EL) in leaves and both Si and sodium nitroprusside (SNP as the NO donor) improved these attributes in a dose-dependent manner. Interestingly, Cu toxicity systemically boosted a NO burst in both roots and shoots and applying Si and SNP markedly intensified it. The application of Si and SNP alone as well as their combination improved growth parameters and systemically alleviated Cu-induced lipid peroxidation and H₂O₂ accumulation through lowering Cu accumulation, increasing proline content, enhancing the activities of catalase (CAT) and superoxide dismutase (SOD) in both roots and leaves and up-regulating expression of SOD gene in leaves of S. officinalis. NO generation was substantially arrested and the responses induced by Si were significantly suppressed by pretreatment with 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxy l-3-oxide (cPTIO) as a NO scavenger, Nx-Nitro- L-arginine methyl ester hydrochloride (L-NAME) as a nitric oxide synthase inhibitor, and tungstate as a nitrate reductase inhibitor. These novel results indicate that Si can induce Cu tolerance through triggering NO generation which systemically modulates defensive reactions in both roots and leaves of Salvia officinalis.

Nitric Oxide Alters the Pattern of Auxin Maxima and PIN-FORMED1 During Shoot Development

Hormone patterns tailor cell fate decisions during plant organ formation. Among them, auxins and cytokinins are critical phytohormones during early development. Nitric oxide (NO) modulates root architecture by the control of auxin spatial patterns. However, NO involvement during the coordination of shoot organogenesis remains unclear. Here, we explore the effect of NO during shoot development by using a phenotypic, cellular, and genetic analysis in Arabidopsis thaliana and get new insights into the characterization of NO-mediated leaf-related phenotypes. NO homeostasis mutants are impaired in several shoot architectural parameters, including phyllotactic patterns, inflorescence stem elongation, silique production, leaf number, and margin. Auxin distribution is a key feature for tissue differentiation and need to be controlled at different levels (i.e., synthesis, transport, and degradation mechanisms). The phenotypes resulting from the introduction of the cue1 mutation in the axr1 auxin resistant and pin1 backgrounds exacerbate the relationship between NO and auxins. Using the auxin reporter DR5:GUS, we observed an increase in auxin maxima under NO-deficient mutant backgrounds and NO scavenging, pointing to NO-ASSOCIATED 1 (NOA1) as the main player related to NO production in this process. Furthermore, polar auxin transport is mainly regulated by PIN-FORMED 1 (PIN1), which controls the flow along leaf margin and venations. Analysis of PIN1 protein levels shows that NO controls its accumulation during leaf development, impacting the auxin mediated mechanism of leaf building. With these findings, we also provide evidence for the NO opposite effects to determine root and shoot architecture, in terms of PIN1 accumulation under NO overproduction.

Root NRT, NiR, AMT, GS, GOGAT and GDH expression levels reveal NO and ABA mediated drought tolerance in Brassica juncea L

Little is known about the interactive effects of exogenous nitric oxide (NO) and abscisic acid (ABA) on nitrogen (N) metabolism and related changes at molecular and biochemical levels under drought stress. The present study highlights the independent and combined effect of NO and ABA (grouped as "nitrate agonists") on expression profiles of representative key genes known to be involved in N-uptake and assimilation, together with proline metabolism, N-NO metabolism enzyme's activity and nutrient content in polyethylene glycol (PEG) treated roots of Indian mustard (B. juncea cv. Varuna). Here we report that PEG mediated drought stress negatively inhibited growth performance, as manifested by reduced biomass (fresh and dry weight) production. Total N content and other nitrogenous compounds (NO3-, NO2-) were decreased; however, NH4+, NH4+/ NO3- ratio and total free amino acids content were increased. These results were positively correlated with the PEG induced changes in expression of genes and enzymes involved in N-uptake and assimilation. Also, PEG supply lowered the content of macro- and micro-nutrients but proline level and the activity of ∆1-pyrroline-5-carboxylate synthetase increased indicating increased oxidative stress. However, all these responses were reversed upon the exogenous application of nitrate agonists (PEG + NO, PEG + NO + ABA, and PEG + ABA) where NO containing nitrate agonist treatment i.e. PEG + NO was significantly more effective than PEG + ABA in alleviating drought stress. Further, increases in activities of L-arginine dependent NOS-like enzyme and S-nitrosoglutathione reductase were observed under nitrate agonist treatments. This indicates that the balanced endogenous change in NO and ABA levels together during synthesis and degradation of NO mitigated the oxidative stress in Indian mustard seedlings. Overall, our results reveal that NO independently or together with ABA may contribute to improved crop growth and productivity under drought stress.

Mitochondrial dysfunction and potential mitochondrial protectant treatments in tendinopathy

Tendinopathy is a common musculoskeletal condition that affects a wide range of patients, including athletes, laborers, and older patients. Tendinopathy is often characterized by pain, swelling, and impaired performance and function. The etiology of tendinopathy is multifactorial, including both intrinsic and extrinsic mechanisms. Various treatment strategies have been described, but outcomes are often variable, as tendons have poor intrinsic healing potential compared with other tissues. Therefore, several novel targets for tendon regeneration have been identified and are being explored. Mitochondria are organelles that generate adenosine triphosphate, and they are considered to be the power generators of the cell. Recently, mitochondrial dysfunction verified by increased reactive oxygen species (ROS), decreased superoxide dismutase activity, cristae disorganization, and decreased number of mitochondria has been identified as a mechanism that may contribute to tendinopathy. This has provided new insights for studying tendinopathy pathogenesis and potential treatments via antioxidant, metabolic modulation, or ROS inhibition. In this review, we present the current understanding of mitochondrial dysfunction in tendinopathy. The review summarizes the potential mechanism by which mitochondrial dysfunction contributes to the development of tendinopathy, as well as the potential therapeutic benefits of mitochondrial protectants in the treatment of tendinopathy.

Nitric oxide effectively curtails neck bending and mitigates senescence in isolated flowers of Calendula officinalis L

In recent years, there has been a considerable and renewed upsurge in research to ascertain the physiological and biochemical role of Nitric oxide (NO) in plants. The present investigation is focused to study the role of NO on neck bending associated with senescence and postharvest performance in isolated flowers of Calendula officinalis. The flower buds harvested at one day before anthesis stage were supplied with sodium nitroprusside (SNP) as a source of NO at different concentrations viz., 50, 100, 150 and 200 µM. A distinct set of flowers held in distilled water designated the control. The investigation revealed that SNP delayed the senescence in flowers of C. officinalis significantly manifested by prolonged longevity. The maximum longevity of 12 days was recorded in flowers supplemented with 100 µM SNP. The flowers held in distilled water (control) displayed early senescence symptoms and lasted for 6 days only. Our research suggested that improved flower longevity by SNP was commensurate with delayed neck bending, inhibition of bacterial growth in the vase, increased solution uptake, high membrane stability, besides an up-regulated activities of antioxidant enzymes in the tissue samples. In addition, the treated flowers exhibited increased content of  sugar fractions, total phenols and soluble proteins in the petal tissues compared to control. Further, 100 µM SNP was observed as most effective treatment and increased the longevity of flowers by 6 days. The concentration above 150 µM provoked early senescence compared to control, whereas concentration lower than 100 µM was less efficacious in improving the postharvest life and longevity of cut Calendula flowers.

Stress-induced reactive oxygen species compartmentalization, perception and signalling

Reactive oxygen species (ROS) are essential for life and are involved in the regulation of almost all biological processes. ROS production is critical for plant development, response to abiotic stresses and immune responses. Here, we focus on recent discoveries in ROS biology emphasizing abiotic and biotic stress responses. Recent advancements have resulted in the identification of one of the first sensors for extracellular ROS and highlighted waves of ROS production during stress signalling in Arabidopsis. Enzymes that produce ROS, including NADPH oxidases, exhibit precise regulation through diverse post-translational modifications. Discoveries highlight the importance of both amino- and carboxy-terminal regulation of NADPH oxidases through protein phosphorylation and cysteine oxidation. Here, we discuss advancements in ROS compartmentalization, systemic ROS waves, ROS sensing and post-translational modification of ROS-producing enzymes and identify areas where foundational gaps remain.

Molecular functions of nitric oxide and its potential applications in horticultural crops

Nitric oxide (NO) regulates plant growth, enhances nutrient uptake, and activates disease and stress tolerance mechanisms in most plants, making NO a potential tool for use in improving the yield and quality of horticultural crop species. Although the use of NO in horticulture is still in its infancy, research on NO in model plant species has provided an abundance of valuable information on horticultural crop species. Emerging evidence implies that the bioactivity of NO can occur through many potential mechanisms but occurs mainly through S-nitrosation, the covalent and reversible attachment of NO to cysteine thiol. In this context, NO signaling specifically affects crop development, immunity, and environmental interactions. Moreover, NO can act as a fumigant against a wide range of postharvest diseases and pests. However, for effective use of NO in horticulture, both understanding and exploring the biological significance and potential mechanisms of NO in horticultural crop species are critical. This review provides a picture of our current understanding of how NO is synthesized and transduced in plants, and particular attention is given to the significance of NO in breaking seed dormancy, balancing root growth and development, enhancing nutrient acquisition, mediating stress responses, and guaranteeing food safety for horticultural production.

Short-term drought triggers defence mechanisms faster than ABA accumulation in the epiphytic bromeliad Acanthostachys strobilacea

Epiphytic bromeliads might experience drought after a few hours without water, which is especially critical during early life stages. Consequently, juvenile epiphytic bromeliads probably rely on short-term activation of drought tolerance strategies, although the biochemical processes involved are still poorly understood. In this study, we aimed to evaluate the short-term drought response of juvenile plants of the epiphytic bromeliad Acanthostachys strobilacea (Schult. & Schult. f.) Klotzsch. We hypothesized that short-term drought would induce the accumulation of abscisic acid (ABA) and secondary messengers such as reactive oxygen and nitrogen species (ROS and RNS, respectively) before the activation of defence mechanisms. Three-month-old plants were transferred from well-watered to dry substrates and stress markers were assessed at 0, 2, 5, 10, 24, 48, and 72 h. Drought caused a 27.3% decrease in relative water content compared to the well-watered control at 72 h. A nearly 5-fold increment in the ABA content occurred at 72 h of stress, which was about two days after the first detection of peaks in RNS levels and defence mechanisms activation. Indeed, ascorbate peroxidase (EC 1.11.1.11) activities and proline content increased after 10 h, whereas after 24 h a higher catalase (EC 1.11.1.6) activity and osmotic adjustment occurred. Oxidative stress markers and photochemical efficiency of photosystem II indicated no significant damage induced by drought. We concluded that defence mechanisms activation during early drought in juvenile A. strobilacea might be regulated initially by ABA-independent pathways and RNS, while ABA-induced responses are triggered at subsequent stages of stress.

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

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

Evidence for a role of nitric oxide in iron homeostasis in plants

Nitric oxide (NO), once regarded as a poisonous air pollutant, is now understood as a regulatory molecule essential for several biological functions in plants. In this review, we summarize NO generation in different plant organs and cellular compartments, and also discuss the role of NO in iron (Fe) homeostasis, particularly in Fe-deficient plants. Fe is one of the most limiting essential nutrient elements for plants. Plants often exhibit Fe deficiency symptoms despite sufficient tissue Fe concentrations. NO appears to not only up-regulate Fe uptake mechanisms but also makes Fe more bioavailable for metabolic functions. NO forms complexes with Fe, which can then be delivered into target cells/tissues. NO generated in plants can alleviate oxidative stress by regulating antioxidant defense processes, probably by improving functional Fe status and by inducing post-translational modifications in the enzymes/proteins involved in antioxidant defense responses. It is hypothesized that NO acts in cooperation with transcription factors such as bHLHs, FIT, and IRO to regulate the expression of enzymes and proteins essential for Fe homeostasis. However, further investigations are needed to disentangle the interaction of NO with intracellular target molecules that leads to enhanced internal Fe availability in plants.

Plant Nitric Oxide Signaling under Drought Stress

Water deficit caused by drought is a significant threat to crop growth and production. Nitric oxide (NO), a water- and lipid-soluble free radical, plays an important role in cytoprotection. Apart from a few studies supporting the role of NO in drought responses, little is known about this pivotal molecular amendment in the regulation of abiotic stress signaling. In this review, we highlight the knowledge gaps in NO roles under drought stress and the technical challenges underlying NO detection and measurements, and we provide recommendations regarding potential avenues for future investigation. The modulation of NO production to alleviate abiotic stress disturbances in higher plants highlights the potential of genetic manipulation to influence NO metabolism as a tool with which plant fitness can be improved under adverse growth conditions.

Recent advances in the regulation of plant immunity by S-nitrosylation

S-nitrosylation, the addition of a nitric oxide (NO) moiety to a reactive protein cysteine (Cys) thiol, to form a protein S-nitrosothiol (SNO), is emerging as a key regulatory post-translational modification (PTM) to control the plant immune response. NO also S-nitrosylates the antioxidant tripeptide, glutathione, to form S-nitrosoglutathione (GSNO), both a storage reservoir of NO bioactivity and a natural NO donor. GSNO and, by extension, S-nitrosylation, are controlled by GSNO reductase1 (GSNOR1). The emerging data suggest that GSNOR1 itself is a target of NO-mediated S-nitrosylation, which subsequently controls its selective autophagy, regulating cellular protein SNO levels. Recent findings also suggest that S-nitrosylation may be deployed by pathogen-challenged host cells to counteract the effect of delivered microbial effector proteins that promote pathogenesis and by the pathogens themselves to augment virulence. Significantly, it also appears that S-nitrosylation may regulate plant immune functions by controlling SUMOylation, a peptide-based PTM. In this context, global SUMOylation is regulated by S-nitrosylation of SUMO conjugating enzyme 1 (SCE1) at Cys139. This redox-based PTM has also been shown to control the function of a key zinc finger transcriptional regulator during the establishment of plant immunity. Here, we provide an update of these recent advances.

The light and dark sides of nitric oxide: multifaceted roles of nitric oxide in plant responses to light

Light drives photosynthesis and informs plants about their surroundings. Regarded as a multifunctional signaling molecule in plants, nitric oxide (NO) has been repeatedly demonstrated to interact with light signaling cascades to control plant growth, development and metabolism. During early plant development, light-triggered NO accumulation counteracts negative regulators of photomorphogenesis and modulates the abundance of, and sensitivity to, plant hormones to promote seed germination and de-etiolation. In photosynthetically active tissues, NO is generated at distinct rates under light or dark conditions and acts at multiple target sites within chloroplasts to regulate photosynthetic reactions. Moreover, changes in NO concentrations in response to light stress promote plant defenses against oxidative stress under high light or ultraviolet-B radiation. Here we review the literature on the interaction of NO with the complicated light and hormonal signaling cascades controlling plant photomorphogenesis and light stress responses, focusing on the recently identified molecular partners and action mechanisms of NO in these events. We also discuss the versatile role of NO in regulating both photosynthesis and light-dependent stomatal movements, two key determinants of plant carbon gain. The regulation of nitrate reductase (NR) by light is highlighted as vital to adjust NO production in plants living under natural light conditions.

Nitric oxide production and signalling in algae

Nitric oxide (NO) was the first identified gaseous messenger and is now well established as a major ubiquitous signalling molecule. The rapid development of our understanding of NO biology in embryophytes came with the partial characterization of the pathways underlying its production and with the decrypting of signalling networks mediating its effects. Notably, the identification of proteins regulated by NO through nitrosation greatly enhanced our perception of NO functions. In comparison, the role of NO in algae has been less investigated. Yet, studies in Chlamydomonas reinhardtii have produced key insights into NO production through the identification of NO-forming nitrite reductase and of S-nitrosated proteins. More intriguingly, in contrast to embryophytes, a few algal species possess a conserved nitric oxide synthase, the main enzyme catalysing NO synthesis in metazoans. This latter finding paves the way for a deeper characterization of novel members of the NO synthase family. Nevertheless, the typical NO-cyclic GMP signalling module transducing NO effects in metazoans is not conserved in algae, nor in embryophytes, highlighting a divergent acquisition of NO signalling between the green and the animal lineages.

Effect of Engineered Nickel Oxide Nanoparticle on Reactive Oxygen Species-Nitric Oxide Interplay in the Roots of Allium cepa L

Scientists anxiously follow instances of heavy metals augmenting in the environment and undergoing bioaccumulation and trace their biomagnification across food webs, wary of their potent toxicity on biological entities. Engineered nanoparticles supplement natural pools of respective heavy metals and can mimic their effects, exerting toxicity at higher concentrations. Thus, a thorough understanding of the underlying mechanism of this precarious interaction is mandatory. Most urban and industrial environments contain considerable quantities of nickel oxide nanoparticles. These in excess can cause considerable damage to plant metabolism through a significant increase in cellular reactive oxygen species and perturbation of its cross-talk with the reactive nitrogen species. In the present work, the authors have demonstrated how the intrusion of nickel oxide nanoparticles (NiO-NP) affected the exposed roots of Allium cepa: starting with disruption of cell membranes, before being interiorized within cell organelles, effectively disrupting cellular homeostasis and survival. A major shift in the reactive oxygen species (ROS) and nitric oxide (NO) equanimity was also observed, unleashing major altercations in several crucial biochemical profiles. Altered antioxidant contents and upregulation of stress-responsive genes, namely, Catalase, Ascorbate peroxidase, Superoxide dismutase, and Rubisco activase, showing on average 50-250% rise across NiO-NP concentrations tested, also entailed increased cellular hydrogen peroxide contents, with tandem rise in cellular NO. Increased NO content was evinced from altered concentrations of nitric oxide synthase and nitrate reductase, along with NADPH oxidase, when compared with the negative control. Though initially showing a dose-dependent concomitant rise, a significant decrease of NO was observed at higher concentrations of NiO-NP, while cellular ROS continued to increase. Modified K/Na ratios, with increased proline concentrations and GABA contents, all hallmarks of cellular stress, correlated with ROS-NO perturbations. Detailed studies showed that NiO-NP concentration had a significant role in inducing toxicity, perturbing the fine balance of ROS-NO, which turned lethal for the cell at higher dosages of the ENP precipitating in the accumulation of stress markers and an inevitable shutdown of cellular mechanisms.

Rhizobia: highways to NO

The interaction between rhizobia and their legume host plants conduces to the formation of specialized root organs called nodules where rhizobia differentiate into bacteroids which fix atmospheric nitrogen to the benefit of the plant. This beneficial symbiosis is of importance in the context of sustainable agriculture as legumes do not require the addition of nitrogen fertilizer to grow. Interestingly, nitric oxide (NO) has been detected at various steps of the rhizobium-legume symbiosis where it has been shown to play multifaceted roles. Both bacterial and plant partners are involved in NO synthesis in nodules. To better understand the role of NO, and in particular the role of bacterial NO, at all steps of rhizobia-legumes interaction, the enzymatic sources of NO have to be elucidated. In this review, we discuss different enzymatic reactions by which rhizobia may potentially produce NO. We argue that there is most probably no NO synthase activity in rhizobia, and that instead the NO2- reductase nirK, which is part of the denitrification pathway, is the main bacterial source of NO. The nitrate assimilation pathway might contribute to NO production but only when denitrification is active. The different approaches to measure NO in rhizobia are also addressed.

The role of alanine synthesis and nitrate-induced nitric oxide production during hypoxia stress in Cucurbita pepo nectaries

Floral nectar is a sugary solution produced by nectaries to attract and reward pollinators. Nectar metabolites, such as sugars, are synthesized within the nectary during secretion from both pre-stored and direct phloem-derived precursors. In addition to sugars, nectars contain nitrogenous compounds such as amino acids; however, little is known about the role(s) of nitrogen (N) compounds in nectary function. In this study, we investigated N metabolism in Cucurbita pepo (squash) floral nectaries in order to understand how various N-containing compounds are produced and determine the role of N metabolism in nectar secretion. The expression and activity of key enzymes involved in primary N assimilation, including nitrate reductase (NR) and alanine aminotransferase (AlaAT), were induced during secretion in C. pepo nectaries. Alanine (Ala) accumulated to about 35% of total amino acids in nectaries and nectar during peak secretion; however, alteration of vascular nitrate supply had no impact on Ala accumulation during secretion, suggesting that nectar(y) amino acids are produced by precursors other than nitrate. In addition, nitric oxide (NO) is produced from nitrate and nitrite, at least partially by NR, in nectaries and nectar. Hypoxia-related processes are induced in nectaries during secretion, including lactic acid and ethanolic fermentation. Finally, treatments that alter nitrate supply affect levels of hypoxic metabolites, nectar volume and nectar sugar composition. The induction of N metabolism in C. pepo nectaries thus plays an important role in the synthesis and secretion of nectar sugar.

Post-Translational Modifications of Nitrate Reductases Autoregulates Nitric Oxide Biosynthesis in Arabidopsis

Nitric oxide (NO) is a regulator of growth, development, and stress responses in living organisms. Plant nitrate reductases (NR) catalyze the reduction of nitrate to nitrite or, alternatively, to NO. In plants, NO action and its targets remain incompletely understood, and the way NO regulates its own homeostasis remains to be elucidated. A significant transcriptome overlapping between NO-deficient mutant and NO-treated wild type plants suggests that NO could negatively regulate its biosynthesis. A significant increase in NO content was detected in transgenic plants overexpressing NR1 and NR2 proteins. In turn, NR protein and activity as well as NO content, decreased in wild-type plants exposed to a pulse of NO gas. Tag-aided immunopurification procedures followed by tandem mass spectrometry allowed identifying NO-triggered post-translational modifications (PTMs) and ubiquitylation sites in NRs. Nitration of tyrosine residues and S-nitrosation of cysteine residues affected key amino acids involved in binding the essential FAD and molybdenum cofactors. NO-related PTMs were accompanied by ubiquitylation of lysine residues flanking the nitration and S-nitrosation sites. NO-induced PTMs of NRs potentially inhibit their activities and promote their proteasome-mediated degradation. This auto-regulatory feedback loop may control nitrate assimilation to ammonium and nitrite-derived production of NO under complex environmental conditions.

Cultivar-Dependent Responses in Plant Growth, Leaf Physiology, Phosphorus Use Efficiency, and Tuber Quality of Potatoes Under Limited Phosphorus Availability Conditions

The limited availability of phosphorus (P) in soils causes a major constraint in the productivity of potatoes, which requires increased knowledge of plant adaptation responses in this condition. In this study, six potato cultivars, namely, Agria, Lady Claire, Milva, Lilly, Sieglinde, and Verdi, were assessed for their responses on plant growth, leaf physiology, P use efficiency (PUE), and tuber quality with three P levels (P_low, P_med, and P_high). The results reveal a significant variation in the cultivars in response to different P availabilities. P-efficient cultivars, Agria, Milva, and Lilly, possessed substantial plant biomass, tuber yield, and high P uptake efficiency (PUpE) under low P supply conditions. The P-inefficient cultivars, Lady Claire, Sieglinde, and Verdi, could not produce tubers under P deprivation conditions, as well as the ability to efficiently uptake P under low-level conditions, but they were efficient in P uptake under high soil P conditions. Improved PUpE is important for plant tolerance with limited P availability, which results in the efficient use of the applied P. At the leaf level, increased accumulations of nitrate, sulfate, sucrose, and proline are necessary for a plant to acclimate to P deficiency-induced stress and to mobilize leaf inorganic phosphate to increase internal PUE and photosynthesis. The reduction in plant biomass and tuber yield under P-deficient conditions could be caused by reduced CO₂ assimilation. Furthermore, P deficiency significantly reduced tuber yield, dry matter, and starch concentration in Agria, Milva, and Lilly. However, contents of tuber protein, sugars, and minerals, as well as antioxidant capacity, were enhanced under these conditions in these cultivars. These results highlight the important traits contributing to potato plant tolerance under P-deficient conditions and indicate an opportunity to improve the P efficiency and tuber quality of potatoes under deficient conditions using more efficient cultivars. Future research to evaluate molecular mechanisms related to P and sucrose translocation, and minimize tuber yield reduction under limited P availability conditions is necessary.

Nitrogen and Phosphorus Signaling and Transport During Legume-Rhizobium Symbiosis

Nitrogen (N) and phosphorus (P) are the two predominant mineral elements, which are not only essential for plant growth and development in general but also play a key role in symbiotic N fixation in legumes. Legume plants have evolved complex signaling networks to respond to both external and internal levels of these macronutrients to optimize symbiotic N fixation in nodules. Inorganic phosphate (Pi) and nitrate (NO₃ ^-) are the two major forms of P and N elements utilized by plants, respectively. Pi starvation and NO₃ ^- application both reduce symbiotic N fixation via similar changes in the nodule gene expression and invoke local and long-distance, systemic responses, of which N-compound feedback regulation of rhizobial nitrogenase activity appears to operate under both conditions. Most of the N and P signaling and transport processes have been investigated in model organisms, such as Medicago truncatula, Lotus japonicus, Glycine max, Phaseolus vulgaris, Arabidopsis thaliana, Oryza sativa, etc. We attempted to discuss some of these processes wherever appropriate, to serve as references for a better understanding of the N and P signaling and transport during symbiosis.

Regulation of antioxidant enzymes and osmo-protectant molecules by salt and drought responsive genes in Bambusa balcooa

Bio-energy crops need to be grown on marginal salt and drought lands in India as per policy. Understanding environmental stress response in bio-energy crops might help in promoting cultivation of bio-energy feedstock on marginal salty and drought land. This is one of the first report for vegetative propagation of Bamboo (Bambusa balcooa) under salt and drought stress to understand antioxidant enzymes' gene regulations to combat stress through activation of antioxidant enzymes and osmo-protectant molecules to scavenge reactive oxygen species as measured by physiological changes. Morphological, physiological, and biochemical traits were noted as indicators of plant health upon different sodium chloride (NaCl) salt-stress while various drought conditions with correlation analysis. A significant up-regulation of genes related to most of the antioxidant enzymes was observed up to salinity of 14 mS cm^- 1 electric conductivity (EC) at 150 mM NaCl experimental salt stress which declined with higher salt-stress. While in the case of drought-stress, all genes remained up-regulated while proline dehydrogenase (PDH) remained down-regulated up-to 100% drought-stress having 4% soil moisture. The gene expressions of antioxidant enzymes were significantly correlated with their corresponding gene-products namely super-oxide dismutase (SOD), catalase (CAT), glutathione reductase (GR) and ascorbate peroxidase (APX) activities. Biochemical parameters such as, soluble sugar, proline, malondialdehyde (MDA), total amino acids, hydrogen peroxide and electrolyte leakage ratio also showed positive correlation (p = 0.001) with salt condition. Genetic and biochemical test parameters were significantly correlated with physiological attributes of plant health at soil EC of 14 mS cm^- 1 shown as 150 mM NaCl salt stress and 60% drought-stress having 17% soil moisture content, were the optimum stress tolerance limits observed. Application of these data would be useful to cultivate 0.63 million ha of salinity affected land and 10.05 million ha of drought affected land among wastelands in India to meet biofuel need.

Mechanistic Insight of Allantoin in Protecting Tomato Plants Against Ultraviolet C Stress

Allantoin ((AT) a purine metabolite)-mediated ultraviolet C (UVC) stress mitigation has not been studied to date. Here, we reported the physicochemical mechanisms of UVC-induced stress in tomato (Solanum lycopersicum L.) plants, including an AT-directed mitigation strategy. UVC stress reduced plant growth and photosynthetic pigments. Heatmap and principal component analysis (PCA) revealed that these toxic impacts were triggered by the greater oxidative damage and disruption of osmolyte homeostasis. However, pre-treatment of AT noticeably ameliorated the stress-induced toxicity as evident by enhanced chlorophyll, soluble protein, and soluble carbohydrate contents in AT-pretreated UVC-stressed plants relative to only stressed plants leading to the improvement of the plant growth and biomass. Moreover, AT pre-treatment enhanced endogenous AT and allantoate content, phenylalanine ammonia-lyase, non-enzymatic antioxidants, and the enzymatic antioxidants leading to reduced oxidative stress markers compared with only stressed plants, indicating the protective effect of AT against oxidative damage. Moreover, PCA displayed that the protective roles of AT strongly associate with the improved antioxidants. On the other hand, post-treatment of AT showed less efficacy in UVC stress mitigation relative to pre-treatment of AT. Overall, this finding illustrated that AT pre-treatment could be an effective way to counteract the UVC stress in tomato, and perhaps in other crop plants.

Canavanine Increases the Content of Phenolic Compounds in Tomato (Solanum lycopersicum L.) Roots

Canavanine (CAN) is a nonproteinogenic amino acid, and its toxicity comes from its utilization instead of arginine in many cellular processes. As presented in previous experiments, supplementation of tomato (Solanum lycopersicum L.) with CAN led to decreased nitric oxide (NO) level and induced secondary oxidative stress. CAN improved total antioxidant capacity in roots, with parallel inhibition of enzymatic antioxidants. The aim of this work was to determine how CAN-dependent limitation of NO emission and reactive oxygen species overproduction impact content, localization, and metabolism of phenolic compounds (PCs) in tomato roots. Tomato seedlings were fed with CAN (10 and 50 µM) for 24 or 72 h. Inhibition of root growth due to CAN supplementation correlated with increased concentration of total PCs; CAN (50 µM) led to the homogeneous accumulation of PCs all over the roots. CAN increased also flavonoids content in root tips. The activity of polyphenol oxidases and phenylalanine ammonia-lyase increased only after prolonged treatment with 50 µM CAN, while expressions of genes encoding these enzymes were modified variously, irrespectively of CAN dosage and duration of the culture. PCs act as the important elements of the cellular antioxidant system under oxidative stress induced by CAN.