A singular nitric oxide synthase with a globin domain found in Synechococcus PCC 7335 mobilizes N from arginine to nitrate

Nitric oxide synthase expression in Pseudomonas koreensis MME3 improves plant growth promotion traits

The development of novel biotechnologies that promote a better use of N to optimize crop yield is a central goal for sustainable agriculture. Phytostimulation, biofertilization, and bioprotection through the use of bio-inputs are promising technologies for this purpose. In this study, the plant growth-promoting rhizobacteria Pseudomonas koreensis MME3 was genetically modified to express a nitric oxide synthase of Synechococcus SyNOS, an atypical enzyme with a globin domain that converts nitric oxide to nitrate. A cassette for constitutive expression of synos was introduced as a single insertion into the genome of P. koreensis MME3 using a miniTn7 system. The resulting recombinant strain MME3:SyNOS showed improved growth, motility, and biofilm formation. The impact of MME3:SyNOS inoculation on Brachypodium distachyon growth and N uptake and use efficiencies under different N availability situations was analyzed, in comparison to the control strain MME3:c. After 35 days of inoculation, plants treated with MME3:SyNOS had a higher root dry weight, both under semi-hydroponic and greenhouse conditions. At harvest, both MME3:SyNOS and MME3:c increased N uptake and use efficiency of plants grown under low N soil. Our results indicate that synos expression is a valid strategy to boost the phytostimulatory capacity of plant-associated bacteria and improve the adaptability of plants to N deficiency. KEY POINTS: • synos expression improves P. koreensis MME3 traits important for rhizospheric colonization • B. distachyon inoculated with MME3:SyNOS shows improved root growth • MME3 inoculation improves plant N uptake and use efficiencies in N-deficient soil.

A new oxidative pathway of nitric oxide production from oximes in plants

Nitric oxide (NO) is an essential reactive oxygen species and a signal molecule in plants. Although several studies have proposed the occurrence of oxidative NO production, only reductive routes for NO production, such as the nitrate (NO-3) -upper-reductase pathway, have been evidenced to date in land plants. However, plants grown axenically with ammonium as the sole source of nitrogen exhibit contents of nitrite and NO3-, evidencing the existence of a metabolic pathway for oxidative production of NO. We hypothesized that oximes, such as indole-3-acetaldoxime (IAOx), a precursor to indole-3-acetic acid, are intermediate oxidation products in NO synthesis. We detected the production of NO from IAOx and other oximes catalyzed by peroxidase (POD) enzyme using both 4-amino-5-methylamino-2',7'-difluorescein fluorescence and chemiluminescence. Flavins stimulated the reaction, while superoxide dismutase inhibited it. Interestingly, mouse NO synthase can also use IAOx to produce NO at a lower rate than POD. We provided a full mechanism for POD-dependent NO production from IAOx consistent with the experimental data and supported by density functional theory calculations. We showed that the addition of IAOx to extracts from Medicago truncatula increased the in vitro production of NO, while in vivo supplementation of IAOx and other oximes increased the number of lateral roots, as shown for NO donors, and a more than 10-fold increase in IAOx dehydratase expression. Furthermore, we found that in vivo supplementation of IAOx increased NO production in Arabidopsis thaliana wild-type plants, while prx33-34 mutant plants, defective in POD33-34, had reduced production. Our data show that the release of NO by IAOx, as well as its auxinic effect, explain the superroot phenotype. Collectively, our study reveals that plants produce NO utilizing diverse molecules such as oximes, POD, and flavins, which are widely distributed in the plant kingdom, thus introducing a long-awaited oxidative pathway to NO production in plants. This knowledge has essential implications for understanding signaling in biological systems.

A putative bifunctional CPD/ (6-4) photolyase from the cyanobacteria Synechococcus sp. PCC 7335 is encoded by a UV-B inducible operon: New insights into the evolution of photolyases

Photosynthetic organisms are continuously exposed to solar ultraviolet radiation-B (UV-B) because of their autotrophic lifestyle. UV-B provokes DNA damage, such as cyclobutane pyrimidine dimers (CPD) or pyrimidine (6-4) pyrimidone photoproducts (6-4 PPs). The cryptochrome/photolyase family (CPF) comprises flavoproteins that can bind damaged or undamaged DNA. Photolyases (PHRs) are enzymes that repair either CPDs or 6-4 PPs. A natural bifunctional CPD/(6-4)- PHR (PhrSph98) was recently isolated from the UV-resistant bacteria Sphingomonas sp. UV9. In this work, phylogenetic studies of bifunctional CPD/(6-4)- photolyases and their evolutionary relationship with other CPF members were performed. Amino acids involved in electron transfer and binding to FAD cofactor and DNA lesions were conserved in proteins from proteobacteria, planctomycete, bacteroidete, acidobacteria and cyanobacteria clades. Genome analysis revealed that the cyanobacteria Synechococcus sp. PCC 7335 encodes a two-gene assembly operon coding for a PHR and a bifunctional CPD/(6-4) PHR- like. Operon structure was validated by RT-qPCR analysis and the polycistronic transcript accumulated after 15 min of UV-B irradiation. Conservation of structure and evolution is discussed. This study provides evidence for a UV-B inducible PHR operon that encodes a CPD/(6-4)- photolyase homolog with a putative bifunctional role in the repair of CPDs and 6-4 PPs damages in oxygenic photosynthetic organisms.

Focus on Nitric Oxide Homeostasis: Direct and Indirect Enzymatic Regulation of Protein Denitrosation Reactions in Plants

Protein cysteines (Cys) undergo a multitude of different reactive oxygen species (ROS), reactive sulfur species (RSS), and/or reactive nitrogen species (RNS)-derived modifications. S-nitrosation (also referred to as nitrosylation), the addition of a nitric oxide (NO) group to reactive Cys thiols, can alter protein stability and activity and can result in changes of protein subcellular localization. Although it is clear that this nitrosative posttranslational modification (PTM) regulates multiple signal transduction pathways in plants, the enzymatic systems that catalyze the reverse S-denitrosation reaction are poorly understood. This review provides an overview of the biochemistry and regulation of nitro-oxidative modifications of protein Cys residues with a focus on NO production and S-nitrosation. In addition, the importance and recent advances in defining enzymatic systems proposed to be involved in regulating S-denitrosation are addressed, specifically cytosolic thioredoxins (TRX) and the newly identified aldo-keto reductases (AKR).

Nitric oxide synthases from photosynthetic organisms improve growth and confer nitrosative stress tolerance in E. coli. Insights on the pterin cofactor

Nitric oxide synthase (NOS) catalyzes NO formation from the substrate l-arginine (Arg). Previously, NOS with distinct biochemical properties were characterized from two photosynthetic microorganisms, the unicellular algae Ostreococcus tauri (OtNOS) and the cyanobacteria Synechococcus PCC 7335 (SyNOS). In this work we studied the effect of recombinant OtNOS and SyNOS expressed under IPTG-induced promoter in E. coli, a bacterium that lacks NOS. Results show that OtNOS and SyNOS expression promote E. coli growth in a nutrient replete medium and allow to better metabolize Arg as N source. In LB medium, OtNOS induces the expression of the NO dioxygenase hmp in E. coli, in accordance with high NO levels visualized with the probe DAF-FM DA. In contrast, SyNOS expression does not induce hmp and show a slight increase of NO production compared to OtNOS. NOS expression reduces ROS production and increases viability of E. coli cultures growing in LB. A strong nitrosative stress provoked by the addition of 1 mM of the NO donors sodium nitroprusside (SNP) and nitrosoglutathione (GSNO) inhibits bacterial growth rate. Under these conditions, the expression of OtNOS or SyNOS counteracts NO donor toxicity restoring bacterial growth. Finally, using bioinformatic tools and ligand docking analyses, we postulate that tetrahydromonapterin (MH4), an endogenous pterin found in E. coli, could act as cofactor required for NOS catalytic activity. Our findings could be useful for the development of biotechnological applications using NOS expression to improve growth in NOS-lacking bacteria.

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.

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.

The evolutionary origins of peroxynitrite signalling

Peroxynitrite is a reactive intermediate formed in vivo through uncatalysed reaction of superoxide and nitric oxide radicals. Despite significant interest in detecting peroxynitrite in vivo and understanding its production, little attention has been given to the evolutionary origins of peroxynitrite signalling. Herein we focus on two enzymes that are key to the biosynthesis of superoxide and nitric oxide, NADPH oxidase 5 (NOX5) and endothelial nitric oxide synthase (eNOS), respectively. Multiple sequence alignments of both enzymes including homologues from all domains of life, coupled with a phylogenetic analysis of NOX5, suggest eNOS and NOX5 are present in animals as the result of horizontal gene transfer from ancestral cyanobacteria to ancestral eukaryotes. Therefore, biochemical studies from other laboratories on a NOX5 homologue in Cylindrospermum stagnale and an eNOS homologue in Synechococcus sp. PCC 7335 are likely to be of relevance to human NOX5 and eNOS and to the production of superoxide, nitric oxide and peroxynitrite in humans.

Cyanobacterial NOS expression improves nitrogen use efficiency, nitrogen-deficiency tolerance and yield in Arabidopsis

Developing strategies to improve nitrogen (N) use efficiency (NUE) in plants is a challenge to reduce environmental problems linked to over-fertilization. The nitric oxide synthase (NOS) enzyme from the cyanobacteria Synechococcus PCC 7335 (SyNOS) has been recently identified and characterized. SyNOS catalyzes the conversion of arginine to citrulline and nitric oxide (NO), and then approximately 75 % of the produced NO is rapidly oxidized to nitrate by an unusual globin domain in the N-terminus of the enzyme. In this study, we assessed whether SyNOS expression in plants affects N metabolism, NUE and yield. Our results showed that SyNOS-expressing transgenic Arabidopsis plants have greater primary shoot length and shoot branching when grown under N-deficient conditions and higher seed production both under N-sufficient and N-deficient conditions. Moreover, transgenic plants showed significantly increased NUE in both N conditions. Although the uptake of N was not modified in the SyNOS lines, they showed an increase in the assimilation/remobilization of N under conditions of low N availability. In addition, SyNOS lines have greater N-deficiency tolerance compared to control plants. Our results support that SyNOS expression generates a positive effect on N metabolism and seed production in Arabidopsis, and it might be envisaged as a strategy to improve productivity in crops under adverse N environments.

Proteome-wide modulation of S-nitrosylation in Trypanosoma cruzi trypomastigotes upon interaction with the host extracellular matrix

Trypanosoma cruzi trypomastigotes adhere to extracellular matrix (ECM) to invade mammalian host cells regulating intracellular signaling pathways. Herein, resin-assisted enrichment of thiols combined with mass spectrometry were employed to map site-specific S-nitrosylated (SNO) proteins from T. cruzi trypomastigotes incubated (MTy) or not (Ty) with ECM. We confirmed the reduction of S-nitrosylation upon incubation with ECM, associated with a rewiring of the subcellular distribution and intracellular signaling pathways. Forty, 248 and 85 SNO-peptides were identified only in MTy, Ty or in both conditions, respectively. SNO proteins were enriched in ribosome, transport, carbohydrate and lipid metabolisms. Nitrosylation of histones H2B and H3 on Cys64 and Cys126, respectively, is described. Protein-protein interaction networks revealed ribosomal proteins, proteins involved in carbon and fatty acid metabolism to be among the enriched protein complexes. Kinases, phosphatases and enzymes involved in the metabolism of carbohydrates, lipids and amino acids were identified as nitrosylated and phosphorylated, suggesting a post-translational modifications crosstalk. In silico mapping of nitric oxide synthase (NOS) genes, previously uncharacterized, matched to four putative T. cruzi proteins expressing C-terminal NOS domain. Our results provide the first site-specific characterization of S-nitrosylated proteins in T. cruzi and their modulation upon ECM incubation before infection of the mammalian hosts. SIGNIFICANCE: Protein S-nitrosylation represents a major molecular mechanism for signal transduction by nitric oxide. We present for the first time a proteomic profile of S-nitrosylated proteins from infective forms of T. cruzi, showing a decrease in SNO proteins after incubation of the parasite with the extracellular matrix, a necessary step for the parasite invasion of the host mammalian cells. We also show for the first time nitrosylation of H2B (Cys64) and H3 (Cys126) histones, sites not conserved in higher eukaryotic cells, and suggest that some specific histone isoforms are sensitive to NO signaling. S-nitrosylation in H2B and H3 histones are more abundant in MTy. Moreover, proteins involved in translation, glycolytic pathway and fatty acid metabolism are enriched in the present dataset. Comparison of the SNO proteome and the phosphoproteome, obtained previously under the same experimental conditions, show that most of the proteins sharing both modifications are involved in metabolic pathways, transport and ribosome function. The data suggest that both PTMs are involved in reprogramming the metabolism of T. cruzi in response to environmental changes. Although NO synthesis was detected in T. cruzi, the identification of NOS remains elusive. Analysis in silico showed two genes similar in domains to NADPH-dependent cytochrome-P450 reductase and two putative oxidoreductases, but no oxygenase domain of NOS was mapped in the T. cruzi genome. It is tempting to speculate that NO synthase-like from T. cruzi and its early NO-mediated pathways triggered in response to host interaction constitute potential diagnostic and therapeutic targets.

Nitrogen Depletion Blocks Growth Stimulation Driven by the Expression of Nitric Oxide Synthase in Tobacco

Nitric oxide (NO) is a messenger molecule widespread studied in plant physiology. Latter evidence supports the lack of a NO-producing system involving a NO synthase (NOS) activity in higher plants. However, a NOS gene from the unicellular marine alga Ostreococcus tauri (OtNOS) was characterized in recent years. OtNOS is a genuine NOS, with similar spectroscopic fingerprints to mammalian NOSs and high NO producing capacity. We are interested in investigating whether OtNOS activity alters nitrogen metabolism and nitrogen availability, thus improving growth promotion conditions in tobacco. Tobacco plants were transformed with OtNOS under the constitutive CaMV 35S promoter. Transgenic tobacco plants expressing OtNOS accumulated higher NO levels compared to siblings transformed with the empty vector, and displayed accelerated growth in different media containing sufficient nitrogen availability. Under conditions of nitrogen scarcity, the growth promoting effect of the OtNOS expression is diluted in terms of total leaf area, protein content and seed production. It is proposed that OtNOS might possess a plant growth promoting effect through facilitating N remobilization and nitrate assimilation with potential to improve crop plants performance.

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

Nitric oxide (NO) is a ubiquitous signalling molecule with widespread distribution in prokaryotes and eukaryotes where it is involved in countless physiological processes. While the mechanisms governing nitric oxide (NO) synthesis and signalling are well established in animals, the situation is less clear in the green lineage. Recent investigations have shown that NO synthase, the major enzymatic source for NO in animals, is absent in land plants but present in a limited number of algae. The first detailed analysis highlighted that these new NO synthases are functional but display specific structural features and probably original catalytic activities. Completing this picture, analyses were undertaken in order to investigate whether major components of the prototypic NO/cyclic GMP signalling cascades mediating many physiological effects of NO in animals were also present in plants. Only a few homologues of soluble guanylate cyclases, cGMP-dependent protein kinases, cyclic nucleotide-gated channels, and cGMP-regulated phosphodiesterases were identified in some algal species and their presence did not correlate with that of NO synthases. In contrast, S-nitrosoglutathione reductase, a critical regulator of S-nitrosothiols, was recurrently found. Overall, these findings highlight that plants do not mediate NO signalling through the classical NO/cGMP signalling module and support the concept that S-nitrosation is a ubiquitous NO-dependent signalling mechanism.

A nitric oxide synthase-like protein from Synechococcus produces NO/NO3- from l-arginine and NADPH in a tetrahydrobiopterin- and Ca2+-dependent manner

Nitric oxide synthases (NOSs) are heme-based monooxygenases that convert l-Arg to l-citrulline and nitric oxide (NO), a key signaling molecule and cytotoxic agent in mammals. Bacteria also contain NOS proteins, but the role of NO production within these organisms, where understood, differs considerably from that of mammals. For example, a NOS protein in the marine cyanobacterium Synechococcus sp. PCC 7335 (syNOS) has recently been proposed to function in nitrogen assimilation from l-Arg. syNOS retains the oxygenase (NOSox) and reductase (NOSred) domains present in mammalian NOS enzymes (mNOSs), but also contains an N-terminal globin domain (NOSg) homologous to bacterial flavohemoglobin proteins. Herein, we show that syNOS functions as a dimer and produces NO from l-Arg and NADPH in a tetrahydrobiopterin (H4B)-dependent manner at levels similar to those produced by other NOSs but does not require Ca2+-calmodulin, which regulates NOSred-mediated NOSox reduction in mNOSs. Unlike other bacterial NOSs, syNOS cannot function with tetrahydrofolate and requires high Ca2+ levels (>200 μm) for its activation. NOSg converts NO to NO3- in the presence of O2 and NADPH; however, NOSg did not protect Escherichia coli strains against nitrosative stress, even in a mutant devoid of NO-protective flavohemoglobin. We also found that syNOS does not have NOS activity in E. coli (which lacks H4B) and that the recombinant protein does not confer growth advantages on l-Arg as a nitrogen source. Our findings indicate that syNOS has both NOS and NO oxygenase activities, requires H4B, and may play a role in Ca2+-mediated signaling.

Gasotransmitters in Action: Nitric Oxide-Ethylene Crosstalk during Plant Growth and Abiotic Stress Responses

Since their first description as atmospheric gases, it turned out that both nitric oxide (NO) and ethylene (ET) are multifunctional plant signals. ET and polyamines (PAs) use the same precursor for their synthesis, and NO can be produced from PA oxidation. Therefore, an indirect metabolic link between NO and ET synthesis can be considered. NO signal is perceived primarily through S-nitrosation without the involvement of a specific receptor, while ET signal is sensed by a well-characterized receptor complex. Both NO and ET are synthetized by plants at various developmental stages (e.g., seeds, fruits) and as a response to numerous environmental factors (e.g., heat, heavy metals) and they mutually regulate each other's levels. Most of the growth and developmental processes (e.g., fruit ripening, de-etiolation) are regulated by NO-ET antagonism, while in abiotic stress responses, both antagonistic (e.g., dark-induced stomatal opening, cadmium-induced cell death) and synergistic (e.g., UV-B-induced stomatal closure, iron deficiency-induced expression of iron acquisition genes) NO-ET interplays have been revealed. Despite the numerous pieces of experimental evidence revealing NO-ET relationships in plants, the picture is far from complete. Understanding the mechanisms of NO-ET interactions may contribute to the increment of yield and intensification of stress tolerance of crop plants in changing environments.