The evolution of nitric oxide signalling diverges between animal and green lineages

Emerging concern of nano-pollution in agro-ecosystem: Flip side of nanotechnology

Nanomaterials (NMs) have proven to be a game-changer in agriculture, showcasing their potential to boost plant growth and safeguarding crops. The agricultural sector has widely adopted NMs, benefiting from their small size, high surface area, and optical properties to augment crop productivity and provide protection against various stressors. This is attributed to their unique characteristics, contributing to their widespread use in agriculture. Human exposure from various components of agro-environmental sectors (soil, crops) NMs residues are likely to upsurge with exposure paths may stimulates bioaccumulation in food chain. With the aim to achieve sustainability, nanotechnology (NTs) do exhibit its potentials in various domains of agriculture also have its flip side too. In this review article we have opted a fusion approach using bibliometric based analysis of global research trend followed by a holistic assessment of pros and cons i.e. toxicological aspect too. Moreover, we have also tried to analyse the current scenario of policy associated with the application of NMs in agro-environment.

Auxin Crosstalk with Reactive Oxygen and Nitrogen Species in Plant Development and Abiotic Stress

The phytohormone auxin acts as an important signaling molecule having regulatory functions during the growth and development of plants. Reactive oxygen species (ROS) are also known to perform signaling functions at low concentrations; however, over-accumulation of ROS due to various environmental stresses damages the biomolecules and cell structures and leads to cell death, and therefore, it can be said that ROS act as a double-edged sword. Nitric oxide (NO), a gaseous signaling molecule, performs a wide range of favorable roles in plants. NO displays its positive role in photomorphogenesis, root growth, leaf expansion, seed germination, stomatal closure, senescence, fruit maturation, mitochondrial activity and metabolism of iron. Studies have revealed the early existence of these crucial molecules during evolution. Moreover, auxin, ROS and NO together show their involvement in various developmental processes and abiotic stress tolerance. Redox signaling is a primary response during exposure of plants to stresses and shows a link with auxin signaling. This review provides updated information related to crosstalk between auxin, ROS and NO starting from their evolution during early Earth periods and their interaction in plant growth and developmental processes as well as in the case of abiotic stresses to plants.

Nitric Oxide in Seed Biology

Nitric oxide (NO) has been recognized as a gasotransmitter in the mainstream of plant research since the beginning of the 21st century. It is produced in plant tissue and the environment. It influences plant physiology during every ontogenetic stage from seed germination to plant senescence. In this review, we demonstrate the increased interest in NO as a regulatory molecule in combination with other signalling molecules and phytohormones in the information network of plant cells. This work is a summary of the current knowledge on NO action in seeds, starting from seed pretreatment techniques applied to increase seed quality. We describe mode of action of NO in the regulation of seed dormancy, germination, and aging. During each stage of seed physiology, NO appears to act as a key agent with a predominantly beneficial effect.

Reactive Oxygen, Nitrogen, and Sulfur Species (RONSS) as a Metabolic Cluster for Signaling and Biostimulation of Plants: An Overview

This review highlights the relationship between the metabolism of reactive oxygen species (ROS), reactive nitrogen species (RNS), and H₂S-reactive sulfur species (RSS). These three metabolic pathways, collectively termed reactive oxygen, nitrogen, and sulfur species (RONSS), constitute a conglomerate of reactions that function as an energy dissipation mechanism, in addition to allowing environmental signals to be transduced into cellular information. This information, in the form of proteins with posttranslational modifications or signaling metabolites derived from RONSS, serves as an inducer of many processes for redoxtasis and metabolic adjustment to the changing environmental conditions to which plants are subjected. Although it is thought that the role of reactive chemical species was originally energy dissipation, during evolution they seem to form a cluster of RONSS that, in addition to dissipating excess excitation potential or reducing potential, also fulfils essential signaling functions that play a vital role in the stress acclimation of plants. Signaling occurs by synthesizing many biomolecules that modify the activity of transcription factors and through modifications in thiol groups of enzymes. The result is a series of adjustments in plants' gene expression, biochemistry, and physiology. Therefore, we present an overview of the synthesis and functions of the RONSS, considering the importance and implications in agronomic management, particularly on the biostimulation of crops.

Nitric Oxide Metabolism Affects Germination in Botrytis cinerea and Is Connected to Nitrate Assimilation

Nitric oxide regulates numerous physiological processes in species from all taxonomic groups. Here, its role in the early developmental stages of the fungal necrotroph Botrytis cinerea was investigated. Pharmacological analysis demonstrated that NO modulated germination, germ tube elongation and nuclear division rate. Experimental evidence indicates that exogenous NO exerts an immediate but transitory negative effect, slowing down germination-associated processes, and that this effect is largely dependent on the flavohemoglobin BCFHG1. The fungus exhibited a “biphasic response” to NO, being more sensitive to low and high concentrations than to intermediate levels of the NO donor. Global gene expression analysis in the wild-type and ΔBcfhg1 strains indicated a situation of strong nitrosative and oxidative stress determined by exogenous NO, which was much more intense in the mutant strain, that the cells tried to alleviate by upregulating several defense mechanisms, including the simultaneous upregulation of the genes encoding the flavohemoglobin BCFHG1, a nitronate monooxygenase (NMO) and a cyanide hydratase. Genetic evidence suggests the coordinated expression of Bcfhg1 and the NMO coding gene, both adjacent and divergently arranged, in response to NO. Nitrate assimilation genes were upregulated upon exposure to NO, and BCFHG1 appeared to be the main enzymatic system involved in the generation of the signal triggering their induction. Comparative expression analysis also showed the influence of NO on other cellular processes, such as mitochondrial respiration or primary and secondary metabolism, whose response could have been mediated by NmrA-like domain proteins.

The Evolution of Nitric Oxide Function: From Reactivity in the Prebiotic Earth to Examples of Biological Roles and Therapeutic Applications

Nitric oxide was once considered to be of marginal interest to the biological sciences and medicine; however, there is now wide recognition, but not yet a comprehensive understanding, of its functions and effects. NO is a reactive, toxic free radical with numerous biological targets, especially metal ions. However, NO and its reaction products also play key roles as reductant and oxidant in biological redox processes, in signal transduction, immunity and infection, as well as other roles. Consequently, it can be sensed, metabolized and modified in biological systems. Here, we present a brief overview of the chemistry and biology of NO—in particular, its origins in geological time and in contemporary biology, its toxic consequences and its critical biological functions. Given that NO, with its intrinsic reactivity, appeared in the early Earth’s atmosphere before the evolution of complex lifeforms, we speculate that the potential for toxicity preceded biological function. To examine this hypothesis, we consider the nature of non-biological and biological targets of NO, the evolution of biological mechanisms for NO detoxification, and how living organisms generate this multifunctional gas.

Tryptophan Levels as a Marker of Auxins and Nitric Oxide Signaling

The aromatic amino acid tryptophan is the main precursor for indole-3-acetic acid (IAA), which involves various parallel routes in plants, with indole-3-acetaldoxime (IAOx) being one of the most common intermediates. Auxin signaling is well known to interact with free radical nitric oxide (NO) to perform a more complex effect, including the regulation of root organogenesis and nitrogen nutrition. To fathom the link between IAA and NO, we use a metabolomic approach to analyze the contents of low-molecular-mass molecules in cultured cells of Arabidopsis thaliana after the application of S-nitrosoglutathione (GSNO), an NO donor or IAOx. We separated the crude extracts of the plant cells through ion-exchange columns, and subsequent fractions were analyzed by gas chromatography-mass spectrometry (GC-MS), thus identifying 26 compounds. A principal component analysis (PCA) was performed on N-metabolism-related compounds, as classified by the Kyoto Encyclopedia of Genes and Genomes (KEGG). The differences observed between controls and treatments are mainly explained by the differences in Trp contents, which are much higher in controls. Thus, the Trp is a shared response in both auxin- and NO-mediated signaling, evidencing some common signaling mechanism to both GSNO and IAOx. The differences in the low-molecular-mass-identified compounds between GSNO- and IAOx-treated cells are mainly explained by their concentrations in benzenepropanoic acid, which is highly associated with IAA levels, and salicylic acid, which is related to glutathione. These results show that the contents in Trp can be a marker for the study of auxin and NO signaling.

Arginine-Dependent Nitric Oxide Generation and S-Nitrosation in the Non-Photosynthetic Unicellular Alga Polytomella parva

Nitric oxide (NO) acts as a key signaling molecule in higher plants, regulating many physiological processes. Several photosynthetic algae from different lineages are also known to produce NO. However, it remains unclear whether this messenger is produced by non-photosynthetic algae. Among these organisms, the colorless alga Polytomella parva is a special case, as it has lost not only its plastid genome, but also nitrate reductase and nitrite reductase. Up to now, the question of whether NO synthesis occurs in the absence of functional nitrate reductase (NR) and the assimilation of nitrates/nitrites in P. parva has not been elucidated. Using spectrofluorometric assays and confocal microscopy with NO-sensitive fluorescence dye, we demonstrate L-arginine-dependent NO synthesis by P. parva cells. Based on a pharmacological approach, we propose the existence of arginine-dependent NO synthase-like activity in this non-photosynthetic alga. GC-MS analysis provides primary evidence that P. parva synthesizes putrescine, which is not an NO source in this alga. Moreover, the generated NO causes the S-nitrosation of protein cysteine thiol groups. Together, our data argue for NR-independent NO synthesis and its active role in S-nitrosation as an essential post-translational modification in P. parva.

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.

Nitric oxide represses photosystem II and NDH-1 in the cyanobacterium Synechocystis sp. PCC 6803

Photosynthetic electron transfer comprises a series of light-induced redox reactions catalysed by multiprotein machinery in the thylakoid. These protein complexes possess cofactors susceptible to redox modifications by reactive small molecules. The gaseous radical nitric oxide (NO), a key signalling molecule in green algae and plants, has earlier been shown to bind to Photosystem (PS) II and obstruct electron transfer in plants. The effects of NO on cyanobacterial bioenergetics however, have long remained obscure. In this study, we exposed the model cyanobacterium Synechocystis sp. PCC 6803 to NO under anoxic conditions and followed changes in whole-cell fluorescence and oxidoreduction of P700 in vivo. Our results demonstrate that NO blocks photosynthetic electron transfer in cells by repressing PSII, PSI, and likely the NDH dehydrogenase-like complex 1 (NDH-1). We propose that iron‑sulfur clusters of NDH-1 complex may be affected by NO to such an extent that ferredoxin-derived electron injection to the plastoquinone pool, and thus cyclic electron transfer, may be inhibited. These findings reveal the profound effects of NO on Synechocystis cells and demonstrate the importance of controlled NO homeostasis in cyanobacteria.

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.

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.

Blue and UV-B light synergistically induce anthocyanin accumulation by co-activating nitrate reductase gene expression in Anthocyanin fruit (Aft) tomato

The tomato accession LA1996, which carries a dominant allele of anthocyanin fruit (Aft) locus, accumulates anthocyanins in the epidermis of fruits when exposed to sunlight. The involvement of blue, UV-A, UV-B and a combination of these wavelengths on anthocyanin accumulation and the molecular mechanism of their regulation was investigated in LA1996. The most effective treatment for inducing anthocyanin biosynthesis in Aft fruits was co-irradiation with blue and UV-B (blue + UV-B) light. Finding the correlated genes is an important approach towards understanding their molecular mechanisms. In the present study, the nitrate reductase (NR) gene SlNIA was isolated using RNA-seq profiling of Aft fruits given different light treatments. The functions of NR-mediated anthocyanin induction by blue + UV-B were confirmed using a series of chemical treatments, followed by assessment of NR activity and nitric oxide (NO) detection. The expression of NR was highly induced by blue + UV-B, and this specificity was also confirmed with the enzyme activity of NR and the NO concentration. The NR inhibitors, which reduce NO generation, the expression levels of anthocyanin related genes and decreased anthocyanin accumulation in LA1996. Our results suggest that NR plays a key role in blue + UV-B-mediated anthocyanin accumulation in LA1996 fruits.

Downstream Signalling from Molecular Hydrogen

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

Nitric oxide signalling in plant interactions with pathogenic fungi and oomycetes

Nitric oxide (NO) and reactive nitrogen species have emerged as crucial signalling and regulatory molecules across all organisms. In plants, fungi, and fungi-like oomycetes, NO is involved in the regulation of multiple processes during their growth, development, reproduction, responses to the external environment, and biotic interactions. It has become evident that NO is produced and used as a signalling and defence cue by both partners in multiple forms of plant interactions with their microbial counterparts, ranging from symbiotic to pathogenic modes. This review summarizes current knowledge on the role of NO in plant-pathogen interactions, focused on biotrophic, necrotrophic, and hemibiotrophic fungi and oomycetes. Actual advances and gaps in the identification of NO sources and fate in plant and pathogen cells are discussed. We review the decisive role of time- and site-specific NO production in germination, oriented growth, and active penetration by filamentous pathogens of the host tissues, as well in pathogen recognition, and defence activation in plants. Distinct functions of NO in diverse interactions of host plants with fungal and oomycete pathogens of different lifestyles are highlighted, where NO in interplay with reactive oxygen species governs successful plant colonization, cell death, and establishment of resistance.

Nitric oxide, other reactive signalling compounds, redox, and reductive stress

Nitric oxide (NO) and other reactive nitrogen species (RNS) are key signalling molecules in plants, but they do not work in isolation. NO is produced in cells, often increased in response to stress conditions, but many other reactive compounds used in signalling are generated and accumulate spatially and temporally together. This includes the reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), and hydrogen sulfide (H2S). Here, the interactions with such other reactive molecules is briefly reviewed. Furthermore, along with ROS and H2S, NO will potentially contribute to the overall intracellular redox of the cell. However, RNS will exist in redox couples and therefore the influence of the cellular redox on such couples will be explored. In discussions of the aberrations in intracellular redox it is usually oxidation, so-called oxidative stress, which is discussed. Here, we consider the notion of reductive stress and how this may influence the signalling which may be mediated by NO. By getting a more holistic view of NO biology, the influence on cell activity of NO and other RNS can be more fully understood, and may lead to the elucidation of methods for NO-based manipulation of plant physiology, leading to better stress responses and improved crops in the future.

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.

Nitric Oxide Signaling in Plants

Nitric oxide (NO) is an integral part of cell signaling mechanisms in animals and plants. In plants, its enzymatic generation is still controversial. Evidence points to nitrate reductase being important, but the presence of a nitric oxide synthase-like enzyme is still contested. Regardless, NO has been shown to mediate many developmental stages in plants, and to be involved in a range of physiological responses, from stress management to stomatal aperture closure. Downstream from its generation are alterations of the actions of many cell signaling components, with post-translational modifications of proteins often being key. Here, a collection of papers embraces the differing aspects of NO metabolism in plants.

Hydrogen sulfide in horticulture: Emerging roles in the era of climate change

Future climate change will present many plants with environmental challenges, including extreme temperatures and drought. Hydrogen sulfide (H₂S) has emerged as an important signal transmitting molecule in plants, especially important in many stress responses and it is known to regulate numerous physiological and developmental processes. Being recently suggested as a signaling molecule, research exploring the regulatory functions is continuously progressing regarding the role of H₂S in plant science, agriculture and horticulture. Biosynthesis of H₂S occurs in different cellular compartments from where it can freely translocate via membranes to where needed or be excluded where not required. H₂S interacts with related signaling molecules which together mediate stress tolerance against a plethora of harsh conditions. The H₂S induced tolerance against stresses occurs via regulation of antioxidants activities, endogenous levels of GSH, osmoregulator accumulation, cell signaling proteins, and stress-related gene expression. Overall this efficiently eliminates excessive reactive oxygen species (ROS) and maintains the intracellular redox balance. The current review summarizes the recent progress on H₂S or H₂S donor-mediated abiotic stress tolerance with special reference to climate change and horticulture crops, pre- and post-harvest. Elucidating the role of H₂S in cell signaling pathways may open new horizons towards understanding how exogenous treatments with H₂S in horticulture plants may aid in the tolerance to stress, especially as environmental conditions change, and can secure better crop yields and avoid post-harvest losses.

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

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

The diversification and lineage-specific expansion of nitric oxide signaling in Placozoa: insights in the evolution of gaseous transmission

Nitric oxide (NO) is a ubiquitous gaseous messenger, but we know little about its early evolution. Here, we analyzed NO synthases (NOS) in four different species of placozoans-one of the early-branching animal lineages. In contrast to other invertebrates studied, Trichoplax and Hoilungia have three distinct NOS genes, including PDZ domain-containing NOS. Using ultra-sensitive capillary electrophoresis assays, we quantified nitrites (products of NO oxidation) and L-citrulline (co-product of NO synthesis from L-arginine), which were affected by NOS inhibitors confirming the presence of functional enzymes in Trichoplax. Using fluorescent single-molecule in situ hybridization, we showed that distinct NOSs are expressed in different subpopulations of cells, with a noticeable distribution close to the edge regions of Trichoplax. These data suggest both the compartmentalized release of NO and a greater diversity of cell types in placozoans than anticipated. NO receptor machinery includes both canonical and novel NIT-domain containing soluble guanylate cyclases as putative NO/nitrite/nitrate sensors. Thus, although Trichoplax and Hoilungia exemplify the morphologically simplest free-living animals, the complexity of NO-cGMP-mediated signaling in Placozoa is greater to those in vertebrates. This situation illuminates multiple lineage-specific diversifications of NOSs and NO/nitrite/nitrate sensors from the common ancestor of Metazoa and the preservation of conservative NOS architecture from prokaryotic ancestors.

Arabidopsis DIACYLGLYCEROL KINASE4 is involved in nitric oxide-dependent pollen tube guidance and fertilization

Nitric oxide (NO) is a key signaling molecule that regulates diverse biological processes in both animals and plants, including important roles in male gamete physiology. In plants, NO is generated in pollen tubes (PTs) and affects intracellular responses through the modulation of Ca²⁺ signaling, actin organization, vesicle trafficking and cell wall deposition, bearing consequences in pollen-stigma interactions and PT guidance. In contrast, the NO-responsive proteins that mediate these responses remain elusive. Here, we show that PTs of Arabidopsis thaliana mutants impaired in the pollen-specific DIACYLGLYCEROL KINASE4 (DGK4) grow slower and become partially insensitive to NO-dependent growth inhibition and re-orientation responses. Recombinant DGK4 protein yields NO-responsive spectral and catalytic changes in vitro that are compatible with a role in NO perception and signaling in PTs. In addition to the expected phosphatidic acid-producing kinase activity, DGK4 recombinant protein also revealed guanylyl cyclase activity, as inferred by sequence analysis. Our results are compatible with a role for the fast-diffusible NO gas in signaling and cell-cell communication via the modulation of DGK4 activity during the progamic phase of angiosperm reproduction.

Nitric Oxide, an Essential Intermediate in the Plant-Herbivore Interaction

Reactive nitrogen species (RNS), mainly nitric oxide (NO), are highly reactive molecules with a prominent role in plant response to numerous stresses including herbivores, although the information is still very limited. This perspective article compiles the current progress in determining the NO function, as either a signal molecule, a metabolic intermediate, or a toxic oxidative product, as well as the contribution of molecules associated with NO metabolic pathway in the generation of plant defenses against phytophagous arthropods, in particular to insects and acari.

Present knowledge and controversies, deficiencies, and misconceptions on nitric oxide synthesis, sensing, and signaling in plants

After 30 years of intensive work, nitric oxide (NO) has just started to be characterized as a relevant regulatory molecule on plant development and responses to stress. Its reactivity as a free radical determines its mode of action as an inducer of posttranslational modifications of key target proteins through cysteine S-nitrosylation and tyrosine nitration. Many of the NO-triggered regulatory actions are exerted in tight coordination with phytohormone signaling. This review not only summarizes and updates the information accumulated on how NO is synthesized, sensed, and transduced in plants but also makes emphasis on controversies, deficiencies, and misconceptions that are hampering our present knowledge on the biology of NO in plants. The development of noninvasive accurate tools for the endogenous NO quantitation as well as the implementation of genetic approaches that overcome misleading pharmacological experiments will be critical for getting significant advances in better knowledge of NO homeostasis and regulatory actions in plants.