Evidence supporting the existence of L-arginine-dependent nitric oxide synthase activity in plants

Melatonin: A Small Molecule but Important for Salt Stress Tolerance in Plants

Salt stress is one of the most serious limiting factors in worldwide agricultural production, resulting in huge annual yield loss. Since 1995, melatonin (N-acetyl-5-methoxytryptamine)—an ancient multi-functional molecule in eukaryotes and prokaryotes—has been extensively validated as a regulator of plant growth and development, as well as various stress responses, especially its crucial role in plant salt tolerance. Salt stress and exogenous melatonin lead to an increase in endogenous melatonin levels, partly via the phyto-melatonin receptor CAND2/PMTR1. Melatonin plays important roles, as a free radical scavenger and antioxidant, in the improvement of antioxidant systems under salt stress. These functions improve photosynthesis, ion homeostasis, and activate a series of downstream signals, such as hormones, nitric oxide (NO) and polyamine metabolism. Melatonin also regulates gene expression responses to salt stress. In this study, we review recent literature and summarize the regulatory roles and signaling networks involving melatonin in response to salt stress in plants. We also discuss genes and gene families involved in the melatonin-mediated salt stress tolerance.

Nitric oxide buffering and conditional nitric oxide release in stress response

Nitric oxide (NO) has emerged as an essential biological messenger in plant biology that usually transmits its bioactivity by post-translational modifications such as S-nitrosylation, the reversible addition of an NO group to a protein cysteine residue leading to S-nitrosothiols (SNOs). In recent years, SNOs have risen as key signalling molecules mainly involved in plant response to stress. Chief among SNOs is S-nitrosoglutathione (GSNO), generated by S-nitrosylation of the key antioxidant glutathione (GSH). GSNO is considered the major NO reservoir and a phloem mobile signal that confers to NO the capacity to be a long-distance signalling molecule. GSNO is able to regulate protein function and gene expression, resulting in a key role for GSNO in fundamental processes in plants, such as development and response to a wide range of environmental stresses. In addition, GSNO is also able to regulate the total SNO pool and, consequently, it could be considered the storage of NO in cells that may control NO signalling under basal and stress-related responses. Thus, GSNO function could be crucial during plant response to environmental stresses. Besides the importance of GSNO in plant biology, its mode of action has not been widely discussed in the literature. In this review, we will first discuss the GSNO turnover in cells and secondly the role of GSNO as a mediator of physiological and stress-related processes in plants, highlighting those aspects for which there is still some controversy.

S-nitrosocysteine-responsive genes modulate diverse regulatory pathways in Oryza sativa: a transcriptome profiling study

Rice (Oryza sativa L.) is a major food crop and also a well-established genetic model. Nitric oxide (NO) and its derivatives are important signalling molecules that actively participate in various signalling pathways in response to different stresses. In this study, we performed RNA-seq mediated transcriptomic analysis of rice after treatment with the nitric oxide donor, S-nitroso-L-cysteine (CySNO), generating an average of 37.5 and 41.5 million reads from control and treated leaf samples respectively. More than 95% of the reads were successfully mapped to the O. sativa reference genome yielding a total of 33539 differentially expressed genes (DEGs, P < 0.05). Further analyses identified 825 genes with at least 2-fold change in the expression following treatment with CySNO (P < 0.01). The DEGs identified were involved in diverse molecular functions such as catalytic activity, binding, transport, and receptor activity and were mostly located in the membrane, organelles such as nucleus, Golgi apparatus and mitochondria. DEGs also contained several genes that regulate responses to abiotic stresses such as drought, heat, cold and salt stress and biotic stresses. We also found significantly similar expression patterns of CySNO-responsive DEGs of rice with the CySNO-responsive DEGs of Arabidopsis in a previous study. Expression patterns of genes involved in key biological functions were verified using quantitative real time (qRT)-PCR. The findings of this study suggest that NO regulates the transcriptional control of genes involved in a wide variety of physiological functions in rice, and that NO-mediated transcriptional networks are highly conserved across the plant kingdom. This study provides useful information regarding the transcriptional response of plants to nitrosative stress.

Nitric oxide alleviates wheat yield reduction by protecting photosynthetic system from oxidation of ozone pollution

Accelerated industrialization has been increasing releases of chemical precursors of ozone. Ozone concentration has risen nowadays, and it's predicted that this trend will continue in the next few decades. The yield of many ozone-sensitive crops suffers seriously from ozone pollution, and there are abundant reports exploring the damage mechanisms of ozone to these crops, such as winter wheat. However, little is known on how to alleviate these negative impacts to increase grain production under elevated ozone. Nitric oxide, as a bioactive gaseous, mediates a variety of physiological processes and plays a central role in response to biotic and abiotic stresses. In the present study, the accumulation of endogenous nitric oxide in wheat leaves was found to increase in response to ozone. To study the functions of nitric oxide, its precursor sodium nitroprusside was spayed to wheat leaves under ozone pollution. Wheat leaves spayed with sodium nitroprusside accumulated less hydrogen peroxide, malondialdehyde and electrolyte leakage under ozone pollution, which can be accounted for by the higher activities of superoxide dismutase and peroxidase than in leaves treated without sodium nitroprusside. Consequently, net photosynthetic rate of wheat treated using sodium nitroprusside was much higher, and yield reduction was alleviated under ozone fumigation. These findings are important for our understanding of the potential roles of nitric oxide in responses of crops in general and wheat in particular to ozone pollution, and provide a viable method to mitigate the detrimental effects on crop production induced by ozone pollution, which is valuable for keeping food security worldwide.

A Role for RNS in the Communication of Plant Peroxisomes with Other Cell Organelles?

Plant peroxisomes are organelles with a very active participation in the cellular regulation of the metabolism of reactive oxygen species (ROS). However, during the last two decades peroxisomes have been shown to be also a relevant source of nitric oxide (NO) and other related molecules designated as reactive nitrogen species (RNS). ROS and RNS have been mainly associated to nitro-oxidative processes; however, some members of these two families of molecules such as H₂O₂, NO or S-nitrosoglutathione (GSNO) are also involved in the mechanism of signaling processes mainly through post-translational modifications. Peroxisomes interact metabolically with other cell compartments such as chloroplasts, mitochondria or oil bodies in different pathways including photorespiration, glyoxylate cycle or β-oxidation, but peroxisomes are also involved in the biosynthesis of phytohormones including auxins and jasmonic acid (JA). This review will provide a comprehensive overview of peroxisomal RNS metabolism with special emphasis in the identified protein targets of RNS inside and outside these organelles. Moreover, the potential interconnectivity between peroxisomes and other plant organelles, such as mitochondria or chloroplasts, which could have a regulatory function will be explored, with special emphasis on photorespiration.

Nitric oxide associated protein 1 is associated with chloroplast perturbation and disease symptoms in Nicotiana benthamiana infected with South African cassava mosaic virus

Nitric oxide associated 1 (NOA1) in plants is a cyclic GTPase involved in protein translation in the chloroplast and has been indirectly linked to nitric oxide (NO) accumulation and response to biotic stress. The association between NOA1 and NO accumulation in Arabidopsis noa1 mutants has been linked to the inability of noa1 mutants to accumulate carbon reserves such as fumarate, leading to chloroplast dysfunction and a pale green leaf phenotype. To understand the role played by NOA1 in response to South African cassava mosaic virus infection in Nicotiana benthamiana, the expression of NbNOA1 and the accumulation of NO in leaf samples was compared between south african cassava mosaic (SACMV)-infected and mock-infected plants at 14 and 28 dpi. Real-time qPCR was used to measure SACMV viral load which increased significantly by 20% from 14 to 28 dpi as chlorosis and symptom severity progressed. At 14 and 28 dpi, NbNOA1 expression was significantly lower than mock inoculated plants (2-fold lower at 14 dpi, p-value=0.01 and 5-fold lower at 28, p-value=0.00). At 14 dpi, NO accumulation remained unchanged in infected leaf tissue compared to mock inoculated, while at 28 dpi, NO accumulation was 40% lower (p-value=0.01). At 28 dpi, the decrease in NbNOA1 expression and NO accumulation was accompanied by chloroplast dysfunction, evident from the significant reduction in chlorophylls a and b and carotenoids in SACMV-infected leaves. Furthermore, the expression of chloroplast translation factors (chloroplast RNA binding, chloroplast elongation factor G, translation elongation factor Tu, translation initiation factor 3-2, plastid-specific ribosomal protein 6 and plastid ribosome recycling factor) were found to be repressed in infected N. benthamiana. GC-MS analysis showed a decrease in fumarate and an increase in glucose in SACMV-infected N. benthamiana in comparison to mock samples suggesting a decrease in carbon stores. Collectively, these results provide evidence that in response to SACMV infection, a decrease in photopigments and carbon stores, accompanied by an increase in glucose and decrease in fumarate, leads to a decline in NbNOA1expression and NO levels. This is manifested by suppressed translation factors and disruption of chloroplast function, thereby contributing to chlorotic disease symptoms.

Ethylene and nitric oxide interact to regulate the magnesium deficiency-induced root hair development in Arabidopsis

Nitric oxide (NO) and ethylene respond to biotic and abiotic stresses through either similar or independent processes. This study examines the mechanism underlying the effects of NO and ethylene on promoting root hair development in Arabidopsis under magnesium (Mg) deficiency. The interaction between NO and ethylene in the regulation of Mg deficiency-induced root hair development was investigated using NO- and ethylene-related mutants and pharmacological methods. Mg deficiency triggered a burst of NO and ethylene, accompanied by a stimulated development of root hairs. Interestingly, ethylene facilitated NO generation by activation of both nitrate reductase and nitric oxide synthase-like (NOS-L) in the roots of Mg-deficient plants. In turn, NO enhanced ethylene synthesis through stimulating the activities of 1-aminocyclopropane-1-carboxylate (ACC) oxidase and ACC synthase (ACS). These two processes constituted an NO-ethylene feedback loop. Blocking either of these two processes inhibited the stimulation of root hair development under Mg deficiency. In conclusion, we suggest that Mg deficiency increases the production of NO and ethylene in roots, each influencing the accumulation and role of the other, and thus these two signals interactively regulate Mg deficiency-induced root hair morphogenesis.

Nitric oxide-induced synthesis of hydrogen sulfide alleviates osmotic stress in wheat seedlings through sustaining antioxidant enzymes, osmolyte accumulation and cysteine homeostasis

Nitric oxide (NO) and hydrogen sulfide (H₂S) have been shown to act as signaling molecules in various physiological processes, play significant roles in plant cellular processes, and also mediate responses to both biotic and abiotic stresses in plants. The present investigation was carried out to test the effect of exogenous NO on endogenous synthesis of H₂S in osmotic-stressed wheat (Triticum aestivum L.) seedlings. The results show that application of NO to wheat seedlings, suffered from PEG8000-induced osmotic stress, considerably enhanced the activities of H₂S-synthesizing enzymes l-cysteine desulfhydrase (LCD) and d-cysteine desulfhydrase (DCD) leading to enhanced level of endogenous H₂S content. At the same time exogenous NO also enhanced the activity of cysteine (Cys)-synthesizing enzyme O-acetylserine(thiol)lyase (OAS-TL) and maintained Cys homeostasis under osmotic stress. NO and H₂S together markedly improved the activities of antioxidant enzymes viz. ascorbate peroxidase (APX), glutathione reductase (GR), peroxidase (POX), superoxide dismutase (SOD) and catalase (CAT). Furthermore, NO and H₂S caused additional accumulation of osmolytes proline (Pro) and glycine betaine (GB), all these collectively resulted in the protection of plants against osmotic stress-induced oxidative stress. On the other hand, NO scavenger cPTIO [2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide] and H₂S scavenger HT (hypotaurine) invalidated the effect of NO on endogenous H₂S levels and Cys homeostasis which resulted in weak protection against osmotic stress. Application of N-ethylmaleimide (NEM) suppressed GR activity and caused an increase in oxidative stress. We concluded that NO in association with endogenous H₂S activates the defense system to the level required to counter osmotic stress and maintains normal functioning of cellular machinery.

Nitric oxide-polyamines cross-talk during dormancy release and germination of apple embryos

Nitric oxide (NO) and polyamines (PAs) belong to plant growth and development regulators. These compounds play a key role in numerous physiological processes e.g. seed germination. Based on the suggestion of overlapping of NO and PAs biosynthetic pathways, we demonstrated a cross-talk of NO and PAs in regulation of embryonic dormancy release. The aim of the work was to investigate an impact of PAs (Put, Spd and Spm) or NO short-term fumigation on nitrite, urea, Arg and ornithine (Orn) content, NO synthase-like (NOS-like) and arginase activity in axes of apple (Malus domestica Borkh.) embryos during dormancy alleviation and at the stage of termination of germination sensu stricto. NO, Put/Spd induced dormancy breakage and germination of apple embryos corresponded to stimulation of urea cycle and high free Arg pool in seedlings roots. After two days of the culture Put and Spd stimulated Arg dependent NO formation, inhibition of which was observed after Spm application. Put or Spd application as well as NO short-term pretreatment of apple embryos influenced level of ubiquitin-conjugated proteins. Higher abundance of such modified proteins correlated well to the declined content of nitrated proteins, suggesting their important role in regulation of embryo germination. NO led to stimulation of embryos germination by increasing level of free PAs (mostly Put). While transcriptomic approach showed down regulation of Spm synthesis and up-regulation of Spm degradation by NO, confirming negative role of Spm over-accumulation in embryo dormancy removal. Our data clearly indicate positive relationship of NO-Put/Spd acting as dormancy removing factors.

Modification of the endogenous NO level influences apple embryos dormancy by alterations of nitrated and biotinylated protein patterns

NO donors and Arg remove dormancy of apple embryos and stimulate germination. Compounds lowering NO level (cPTIO, L -NAME, CAN) strengthen dormancy. Embryo transition from dormancy state to germination is linked to increased nitric oxide synthase (NOS)-like activity. Germination of embryos is associated with declined level of biotin containing proteins and nitrated proteins in soluble protein fraction of root axis. Pattern of nitrated proteins suggest that storage proteins are putative targets of nitration. Nitric oxide (NO) acts as a key regulatory factor in removal of seed dormancy and is a signal necessary for seed transition from dormant state into germination. Modulation of NO concentration in apple (Malus domestica Borkh.) embryos by NO fumigation, treatment with NO donor (S-nitroso-N-acetyl-D,L-penicillamine, SNAP), application of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), N ω-nitro-L-arginine methyl ester (L-NAME), canavanine (CAN) or arginine (Arg) allowed us to investigate the NO impact on seed dormancy status. Arg analogs and NO scavenger strengthened embryo dormancy by lowering reactive nitrogen species level in embryonic axes. This effect was accompanied by strong inhibition of NOS-like activity, without significant influence on tissue NO2 (-) concentration. Germination sensu stricto of apple embryos initiated by dormancy breakage via short term NO treatment or Arg supplementation were linked to a reduced level of biotinylated proteins in root axis. Decrease of total soluble nitrated proteins was observed at the termination of germination sensu stricto. Also modulation of NO tissue status leads to modification in nitrated protein pattern. Among protein bands that correspond to molecular mass of approximately 95 kDa, storage proteins (legumin A-like and seed biotin-containing protein) were identified, and can be considered as good markers for seed dormancy status. Moreover, pattern of nitrated proteins suggest that biotin containing proteins are also targets of nitration.

Nitric oxide synthase in plants: Where do we stand?

Over the past twenty years, nitric oxide (NO) has emerged as an important player in various plant physiological processes. Although many advances in the understanding of NO functions have been made, the question of how NO is produced in plants is still challenging. It is now generally accepted that the endogenous production of NO is mainly accomplished through the reduction of nitrite via both enzymatic and non-enzymatic mechanisms which remain to be fully characterized. Furthermore, experimental arguments in favour of the existence of plant nitric oxide synthase (NOS)-like enzymes have been reported. However, recent investigations revealed that land plants do not possess animal NOS-like enzymes while few algal species do. Phylogenetic and structural analyses reveals interesting features specific to algal NOS-like proteins.

Low concentrations of salicylic acid delay methyl jasmonate-induced leaf senescence by up-regulating nitric oxide synthase activity

In plants, extensive efforts have been devoted to understanding the crosstalk between salicylic acid (SA) and jasmonic acid (JA) signaling in pathogen defenses, but this crosstalk has scarcely been addressed during senescence. In this study, the effect of SA application on methyl jasmonate (MeJA)-induced leaf senescence was assessed. We found that low concentrations of SA (1-50 μM) played a delayed role against the senescence promoted by MeJA. Furthermore, low concentrations of SA enhanced plant antioxidant defenses and restricted reactive oxygen species (ROS) accumulation in MeJA-treated leaves. When applied simultaneously with MeJA, low concentrations of SA triggered a nitric oxide (NO) burst, and the elevated NO levels were linked to the nitric oxide associated 1 (NOA1)-dependent pathway via nitric oxide synthase (NOS) activity. The ability of SA to up-regulate plant antioxidant defenses, reduce ROS accumulation, and suppress leaf senescence was lost in NO-deficient Atnoa1 plants. In a converse manner, exogenous addition of NO donors increased the plant antioxidant capacity and lowered the ROS levels in MeJA-treated leaves. Taken together, the results indicate that SA at low concentrations counteracts MeJA-induced leaf senescence through NOA1-dependent NO signaling and strengthening of the antioxidant defense.

A liposome-encapsulated spin trap for the detection of nitric oxide

Electron paramagnetic resonance (EPR) is one of the few methods that allows for the unambiguous detection of nitric oxide (NO). However, the dithiocarbamate-iron spin traps employed with this method inhibit the activity of nitric oxide synthase and catalyze NO production from nitrite. These disadvantages limit EPR's application to biological NO detection. We present a liposome-encapsulated spin-trap (LEST) method for the capture and in situ detection of NO by EPR. The method shows a linear response for [NO]≥4µM and can detect [NO]≥40nM in a 500µL sample (≥20 pmol). The kinetics of NO production can be followed in real time over minutes to hours. LEST does not inhibit the activity of inducible nitric oxide synthase or nitrate reductase and shows minimal abiotic NO production in the presence of nitrite and NADH. Nitrate reductase-like activity is detected in cell lysates of the coccolithophore Emiliania huxleyi and is elevated in virus-infected culture. This method shows particular promise for NO detection in cell lysates and crude preparations of NO-producing tissues.

Nitric Oxide Mediated Transcriptome Profiling Reveals Activation of Multiple Regulatory Pathways in Arabidopsis thaliana

Imbalance between the accumulation and removal of nitric oxide and its derivatives is a challenge faced by all plants at the cellular level, and is especially important under stress conditions. Exposure of plants to various biotic and abiotic stresses causes rapid changes in cellular redox tone potentiated by the rise in reactive nitrogen species that serve as signaling molecules in mediating defensive responses. To understand mechanisms mediated by these signaling molecules, we performed a large-scale analysis of the Arabidopsis transcriptome induced by nitrosative stress. We generated an average of 84 and 91 million reads from three replicates each of control and 1 mM S-nitrosocysteine (CysNO)-infiltrated Arabidopsis leaf samples, respectively. After alignment, more than 95% of all reads successfully mapped to the reference and 32,535 genes and 55,682 transcripts were obtained. CysNO infiltration caused differential expression of 6436 genes (3448 up-regulated and 2988 down-regulated) and 6214 transcripts (3335 up-regulated and 2879 down-regulated) 6 h post-infiltration. These differentially expressed genes were found to be involved in key physiological processes, including plant defense against various biotic and abiotic stresses, hormone signaling, and other developmental processes. After quantile normalization of the FPKM values followed by student's T-test (P < 0.05) we identified 1165 DEGs (463 up-regulated and 702 down-regulated) with at least 2-folds change in expression after CysNO treatment. Expression patterns of selected genes involved in various biological pathways were verified using quantitative real-time PCR. This study provides comprehensive information about plant responses to nitrosative stress at transcript level and would prove helpful in understanding and incorporating mechanisms associated with nitrosative stress responses in plants.

Toxicity of canavanine in tomato (Solanum lycopersicum L.) roots is due to alterations in RNS, ROS and auxin levels

Canavanine (CAN) is non-proteinogenic aminoacid and a structural analog of arginine (Arg). Naturally, CAN occurs in legumes e.g. jack bean and is considered as a strong allelochemical. As a selective inhibitor of inducible nitric oxide synthase in mammalians, it could act as a modifier of nitric oxide (NO) concentration in plants. Modifications in the content of endogenous reactive nitrogen species (RNS) and reactive oxygen species (ROS) influence root structure and architecture, being also under hormonal control. The aim of the work was to investigate regulation of root growth in tomato (Solanum lycopersicum L. cv. Malinowy Ożarowski) seedling by application of CAN at concentration (10 and 50 μM) leading to 50% or 100% restriction of root elongation. CAN at higher concentration led to slight DNA fragmentation, increased total RNA and protein level. Decline in total respiration rate after CAN supplementation was not associated with enhanced membrane permeability. Malformations in root morphology (shorter and thicker roots, limited number of lateral roots) were accompanied by modification in NO and ONOO(-) localization; determined mainly in peridermal cells and some border cells. Although, CAN resulted in low RNS production, addition of exogenous NO by usage of NO donors did not reverse its negative effect, nor recovery effect was detected after roots imbibition in Arg. To build up a comprehensive view on mode of action of CAN as root growth inhibitor, it was shown an elevated level of auxin. To summarize, we demonstrated several secondary mode of action of CAN, indicating its toxicity in plants linked to restriction in RNS formation accompanied by simultaneous overaccumulation of ROS.

Activation of NADPH-recycling systems in leaves and roots of Arabidopsis thaliana under arsenic-induced stress conditions is accelerated by knock-out of Nudix hydrolase 19 (AtNUDX19) gene

NADPH is an important cofactor in cell growth, proliferation and detoxification. Arabidopsis thaliana Nudix hydrolase 19 (AtNUDX19) belongs to a family of proteins defined by the conserved amino-acid sequence GX5-EX7REUXEEXGU which has the capacity to hydrolyze NADPH as a physiological substrate in vivo. Given the importance of NADPH in the cellular redox homeostasis of plants, the present study compares the responses of the main NADPH-recycling systems including NADP-isocitrate dehydrogenase (ICDH), glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH) and NADP-malic enzyme (ME) in the leaves and roots of Arabidopsis wild-type (Wt) and knock-out (KO) AtNUDX19 mutant (Atnudx19) plants under physiological and arsenic-induced stress conditions. Two major features were observed in the behavior of the main NADPH-recycling systems: (i) under optimal conditions in both organs, the levels of these activities were higher in nudx19 mutants than in Wt plants; and, (ii) under 500μM AsV conditions, these activities increase, especially in nudx19 mutant plants. Moreover, G6PDH activity in roots was the most affected enzyme in both Wt and nudx19 mutant plants, with a 4.6-fold and 5.0-fold increase, respectively. In summary, the data reveals a connection between the absence of chloroplastic AtNUDX19 and the rise in all NADP-dehydrogenase activities under physiological and arsenic-induced stress conditions, particularly in roots. This suggests that AtNUDX19 could be a key factor in modulating the NADPH pool in plants and consequently in redox homeostasis.

Canavanine Alters ROS/RNS Level and Leads to Post-translational Modification of Proteins in Roots of Tomato Seedlings

Canavanine (CAN), a structural analog of arginine (Arg), is used as a selective inhibitor of inducible NOS in mammals. CAN is incorporated into proteins' structure in the place of Arg, leading to the formation of aberrant compounds. This non-protein amino acid is found in legumes, e.g., Canavalia ensiformis (L.) DC. or Sutherlandia frutescens (L.) R.Br. and acts as a strong toxin against herbivores or plants. Tomato (Solanum lycopersicum L.) seedlings were treated for 24-72 h with CAN (10 or 50 μM) inhibiting root growth by 50 or 100%, without lethal effect. We determined ROS level/production in root extracts, fluorescence of DAF-FM and APF derivatives corresponding to RNS level in roots of tomato seedlings and linked CAN-induced restriction of root growth to the post-translational modifications (PTMs) of proteins: carbonylation and nitration. Both PTMs are stable markers of nitro-oxidative stress, regarded as the plant's secondary response to phytotoxins. CAN enhanced H2O2 content and superoxide radicals generation in extracts of tomato roots and stimulated formation of protein carbonyl groups. An elevated level of carbonylated proteins was characteristic for the plants after 72 h of the culture, mainly for the roots exposed to 10 μM CAN. The proteolytic activity was stimulated by tested non-protein amino acid. CAN treatment led to decline of fluorescence of DAF-FM derivatives, and transiently stimulated fluorescence of APF derivatives. Short-term exposure of tomato seedlings to CAN lowered the protein nitration level. Activity of peroxidase, polyamine oxidase and NADPH oxidase, enzymes acting as modulators of H2O2 concentration and governing root architecture and growth were determined. Activities of all enzymes were stimulated by CAN, but no strict CAN concentration dependence was observed. We conclude, that although CAN treatment led to a decline in the nitric oxide level, PTMs observed in roots of plants exposed to CAN are linked rather to the formation of carbonyl groups than to nitration, and are detected particularly after 24 h. Thus, oxidative stress and oxidative modifications of proteins seems to be of significant importance in the rapid response of plants to CAN.

Promoting Roles of Melatonin in Adventitious Root Development of Solanum lycopersicum L. by Regulating Auxin and Nitric Oxide Signaling

Melatonin (MT) plays integral roles in regulating several biological processes including plant growth, seed germination, flowering, senescence, and stress responses. This study investigated the effects of MT on adventitious root formation (ARF) of de-rooted tomato seedlings. Exogenous MT positively or negatively influenced ARF, which was dependent on the concentration of MT application. In the present experiment, 50 μM MT showed the best effect on inducing ARF. Interestingly, exogenous MT promoted the accumulation of endogenous nitric oxide (NO) by down-regulating the expression of S-nitrosoglutathione reductase (GSNOR). To determine the interaction of MT and NO in ARF, MT synthesis inhibitor p-chlorophenylalanine, NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt as well as GSNOR-overexpression plants with low NO levels were used. The function of MT was removed by NO scavenger or GSNOR-overexpression plants. However, application of MT synthesis inhibitor did little to abolish the function of NO. These results indicate that NO, as a downstream signal, was involved in the MT-induced ARF. Concentrations of indole-3-acetic acid and indole-3-butyric acid, as well as the expression of several genes related to the auxin signaling pathway (PIN1, PIN3, PIN7, IAA19, and IAA24), showed that MT influenced auxin transport and signal transduction as well as auxin accumulation through the NO signaling pathway. Collectively, these strongly suggest that elevated NO levels resulting from inhibited GSNOR activity and auxin signaling were involved in the MT-induced ARF in tomato plants. This can be applied in basic research and breeding.

The effect of cryptogein with changed abilities to transfer sterols and altered charge distribution on extracellular alkalinization, ROS and NO generation, lipid peroxidation and LOX gene transcription in Nicotiana tabacum

Cryptogein, a protein from oomycete Phytophthora cryptogea, induces a hypersensitive cell death in Nicotiana tabacum. We prepared a new series of cryptogein mutant proteins with altered abilities to bind sterols and with altered charge distribution in the proteins. The effect of the mutations on the cryptogein ability to induce plant defence mechanisms associated with hypersensitive cell death were examined. Our results with new mutants support the previous findings that the sterol binding does not influence synthesis of ROS, cytosol acidification and development of leaf necrosis as these events seem to be more likely affected by the charge distribution and the overall protein structure. This hypothesis was also applicable on other mechanisms involved in the execution of plant cell death such as the NO generation, the stimulation of lipid peroxidation (determination of malondialdehyde and hydroxy fatty acids levels) and LOX gene transcription. In addition, the ability to bind sterols was found to serve not only for pathogen utilisation in its own metabolism but also to have an important function for the destabilization of plant membrane facilitating the pathogen spread inside the plant tissue as well as intensively contributing to the development of plant cell death. Considering the insertion of charged amino acid residues in the protein structure, the change localized in the protein surface affected its biological activity more effectively than that change inside the protein cavity. Moreover, the insertion of negative charged amino acids influenced mainly the events involved in the early phase of defence reaction, while the positive residues affected especially the necrotic activity of cryptogein.

Understanding nitrate assimilation and its regulation in microalgae

Nitrate assimilation is a key process for nitrogen (N) acquisition in green microalgae. Among Chlorophyte algae, Chlamydomonas reinhardtii has resulted to be a good model system to unravel important facts of this process, and has provided important insights for agriculturally relevant plants. In this work, the recent findings on nitrate transport, nitrate reduction and the regulation of nitrate assimilation are presented in this and several other algae. Latest data have shown nitric oxide (NO) as an important signal molecule in the transcriptional and posttranslational regulation of nitrate reductase and inorganic N transport. Participation of regulatory genes and proteins in positive and negative signaling of the pathway and the mechanisms involved in the regulation of nitrate assimilation, as well as those involved in Molybdenum cofactor synthesis required to nitrate assimilation, are critically reviewed.

Differential molecular response of monodehydroascorbate reductase and glutathione reductase by nitration and S-nitrosylation

The ascorbate-glutathione cycle is a metabolic pathway that detoxifies hydrogen peroxide and involves enzymatic and non-enzymatic antioxidants. Proteomic studies have shown that some enzymes in this cycle such as ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR), and glutathione reductase (GR) are potential targets for post-translational modifications (PMTs) mediated by nitric oxide-derived molecules. Using purified recombinant pea peroxisomal MDAR and cytosolic and chloroplastic GR enzymes produced in Escherichia coli, the effects of peroxynitrite (ONOO(-)) and S-nitrosoglutathione (GSNO) which are known to mediate protein nitration and S-nitrosylation processes, respectively, were analysed. Although ONOO(-) and GSNO inhibit peroxisomal MDAR activity, chloroplastic and cytosolic GR were not affected by these molecules. Mass spectrometric analysis of the nitrated MDAR revealed that Tyr213, Try292, and Tyr345 were exclusively nitrated to 3-nitrotyrosine by ONOO(-). The location of these residues in the structure of pea peroxisomal MDAR reveals that Tyr345 is found at 3.3 Å of His313 which is involved in the NADP-binding site. Site-directed mutagenesis confirmed Tyr345 as the primary site of nitration responsible for the inhibition of MDAR activity by ONOO(-). These results provide new insights into the molecular regulation of MDAR which is deactivated by nitration and S-nitrosylation. However, GR was not affected by ONOO(-) or GSNO, suggesting the existence of a mechanism to conserve redox status by maintaining the level of reduced GSH. Under a nitro-oxidative stress induced by salinity (150mM NaCl), MDAR expression (mRNA, protein, and enzyme activity levels) was increased, probably to compensate the inhibitory effects of S-nitrosylation and nitration on the enzyme. The present data show the modulation of the antioxidative response of key enzymes in the ascorbate-glutathione cycle by nitric oxide (NO)-PTMs, thus indicating the close involvement of NO and reactive oxygen species metabolism in antioxidant defence against nitro-oxidative stress situations in plants.

Transcriptome sequencing of three Pseudo-nitzschia species reveals comparable gene sets and the presence of Nitric Oxide Synthase genes in diatoms

Diatoms are among the most diverse eukaryotic microorganisms on Earth, they are responsible for a large fraction of primary production in the oceans and can be found in different habitats. Pseudo-nitzschia are marine planktonic diatoms responsible for blooms in coastal and oceanic waters. We analyzed the transcriptome of three species, Pseudo-nitzschia arenysensis, Pseudo-nitzschia delicatissima and Pseudo-nitzschia multistriata, with different levels of genetic relatedness. These species have a worldwide distribution and the last one produces the neurotoxin domoic acid. We were able to annotate about 80% of the sequences in each transcriptome and the analysis of the relative functional annotations allowed comparison of the main metabolic pathways, pathways involved in the biosynthesis of isoprenoids (MAV and MEP pathways), and pathways putatively involved in domoic acid synthesis. The search for homologous transcripts among the target species and other congeneric species resulted in the discovery of a sequence annotated as Nitric Oxide Synthase (NOS), found uniquely in Pseudo-nitzschia multistriata. The predicted protein product contained all the domains of the canonical metazoan sequence. Putative NOS sequences were found in other available diatom datasets, supporting a role for nitric oxide as signaling molecule in this group of microalgae.

Nitric Oxide Mediates 5-Aminolevulinic Acid-Induced Antioxidant Defense in Leaves of Elymus nutans Griseb. Exposed to Chilling Stress

Nitric oxide (NO) and 5-aminolevulinic acid (ALA) are both extremely important signalling molecules employed by plants to control many aspects of physiology. In the present study, the role of NO in ALA-induced antioxidant defense in leaves of two sources of Elymus nutans Griseb. (Damxung, DX and Zhengdao, ZD) was investigated. Chilling stress enhanced electrolyte leakage, accumulation of malondialdehyde (MDA), hydrogen peroxide (H2O2) and superoxide radical in two E. nutans, which were substantially alleviated by exogenous ALA and NO application. Pretreatment with NO scavenger PTIO or NOS inhibitor L-NNA alone and in combination with ALA induced enhancements in electrolyte leakage and the accumulation of MDA, H2O2 and superoxide radical in leaves of DX and ZD exposed to chilling stress, indicating that the inhibition of NO biosynthesis reduced the chilling resistance of E. nutans and the ALA-enhanced chilling resistance. Further analyses showed that ALA and NO enhanced antioxidant defense and activated plasma membrane (PM) H+-ATPase and decreased the accumulation of ROS induced by chilling stress. A pronounced increase in nitric oxide synthase (NOS) activity and NO release by exogenous ALA treatment was found in chilling-resistant DX plants exposed to chilling stress, while only a little increase was observed in chilling-sensitive ZD. Furthermore, inhibition of NO accumulation by PTIO or L-NNA blocked the protective effect of exogenous ALA, while both exogenous NO treatment and inhibition of endogenous NO accumulation did not induce ALA production. These results suggested that NO might be a downstream signal mediating ALA-induced chilling resistance in E. nutans.

Expression of the tetrahydrofolate-dependent nitric oxide synthase from the green alga Ostreococcus tauri increases tolerance to abiotic stresses and influences stomatal development in Arabidopsis

Nitric oxide (NO) is a signaling molecule with diverse biological functions in plants. NO plays a crucial role in growth and development, from germination to senescence, and is also involved in plant responses to biotic and abiotic stresses. In animals, NO is synthesized by well-described nitric oxide synthase (NOS) enzymes. NOS activity has also been detected in higher plants, but no gene encoding an NOS protein, or the enzymes required for synthesis of tetrahydrobiopterin, an essential cofactor of mammalian NOS activity, have been identified so far. Recently, an NOS gene from the unicellular marine alga Ostreococcus tauri (OtNOS) has been discovered and characterized. Arabidopsis thaliana plants were transformed with OtNOS under the control of the inducible short promoter fragment (SPF) of the sunflower (Helianthus annuus) Hahb-4 gene, which responds to abiotic stresses and abscisic acid. Transgenic plants expressing OtNOS accumulated higher NO concentrations compared with siblings transformed with the empty vector, and displayed enhanced salt, drought and oxidative stress tolerance. Moreover, transgenic OtNOS lines exhibited increased stomatal development compared with plants transformed with the empty vector. Both in vitro and in vivo experiments indicate that OtNOS, unlike mammalian NOS, efficiently uses tetrahydrofolate as a cofactor in Arabidopsis plants. 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 to improve plant fitness under adverse growth conditions.

The role of reactive oxygen species and nitric oxide in programmed cell death associated with self-incompatibility

Successful sexual reproduction often relies on the ability of plants to recognize self- or genetically-related pollen and prevent pollen tube growth soon after germination in order to avoid self-fertilization. Angiosperms have developed different reproductive barriers, one of the most extended being self-incompatibility (SI). With SI, pistils are able to reject self or genetically-related pollen thus promoting genetic variability. There are basically two distinct systems of SI: gametophytic (GSI) and sporophytic (SSI) based on their different molecular and genetic control mechanisms. In both types of SI, programmed cell death (PCD) has been found to play an important role in the rejection of self-incompatible pollen. Although reactive oxygen species (ROS) were initially recognized as toxic metabolic products, in recent years, a new role for ROS has become apparent: the control and regulation of biological processes such as growth, development, response to biotic and abiotic environmental stimuli, and PCD. Together with ROS, nitric oxide (NO) has become recognized as a key regulator of PCD. PCD is an important mechanism for the controlled elimination of targeted cells in both animals and plants. The major focus of this review is to discuss how ROS and NO control male-female cross-talk during fertilization in order to trigger PCD in self-incompatible pollen, providing a highly effective way to prevent self-fertilization.

Nitric oxide is a ubiquitous signal for maintaining redox balance in plant cells: regulation of ascorbate peroxidase as a case study

Oxidative and nitrosative stresses and their respective antioxidant responses are common metabolic adjustments operating in all biological systems. These stresses result from an increase in reactive oxygen species (ROS) and reactive nitrogen species (RNS) and an imbalance in the antioxidant response. Plants respond to ROS and RNS accumulation by increasing the level of the antioxidant molecules glutathione and ascorbate and by activating specific antioxidant enzymes. Nitric oxide (NO) is a free radical considered to be toxic or protective depending on its concentration, combination with ROS compounds, and subcellular localization. In this review we focus on the mechanisms of NO action in combination with ROS on the regulation of the antioxidant system in plants. In particular, we describe the redox post-translational modifications of cytosolic ascorbate peroxidase and its influence on enzyme activity. The regulation of ascorbate peroxidase activity by NO as a redox sensor of acute oxidative stress or as part of a hormone-induced signalling pathway leading to lateral root development is presented and discussed.

Interplays between nitric oxide and reactive oxygen species in cryptogein signalling

Nitric oxide (NO) has many functions in plants. Here, we investigated its interplays with reactive oxygen species (ROS) in the defence responses triggered by the elicitin cryptogein. The production of NO induced by cryptogein in tobacco cells was partly regulated through a ROS-dependent pathway involving the NADPH oxidase NtRBOHD. In turn, NO down-regulated the level of H2O2. Both NO and ROS synthesis appeared to be under the control of type-2 histone deacetylases acting as negative regulators of cell death. Occurrence of an interplay between NO and ROS was further supported by the finding that cryptogein triggered a production of peroxynitrite (ONOO(-)). Next, we showed that ROS, but not NO, negatively regulate the intensity of activity of the cryptogein-induced protein kinase NtOSAK. Furthermore, using a DNA microarray approach, we identified 15 genes early induced by cryptogein via NO. A part of these genes was also modulated by ROS and encoded proteins showing sequence identity to ubiquitin ligases. Their expression appeared to be negatively regulated by ONOO(-), suggesting that ONOO(-) mitigates the effects of NO and ROS. Finally, we provided evidence that NO required NtRBOHD activity for inducing cell death, thus confirming previous assumption that ROS channel NO through cell death pathways.

Nitric oxide from a "green" perspective

The molecule nitric oxide (NO) which is involved in practically all biochemical and physiological plant processes has become a subject for plant research. However, there remain many unanswered questions concerning how, where and when this molecule is enzymatically generated in higher plants. This mini-review aims to provide an overview of NO in plants for those readers unfamiliar with this field of research. The review will therefore discuss the importance of NO in higher plants at the physiological and biochemical levels, its involvement in designated nitro-oxidative stresses in response to adverse abiotic and biotic environmental conditions, NO emission/uptake from plants, beneficial plant-microbial interactions, and its potential application in the biotechnological fields of agriculture and food nutrition.

Protein S-nitrosylation: specificity and identification strategies in plants

The role of nitric oxide (NO) as a major regulator of plant physiological functions has become increasingly evident. To further improve our understanding of its role, within the last few years plant biologists have begun to embrace the exciting opportunity of investigating protein S-nitrosylation, a major reversible NO-dependent post-translational modification (PTM) targeting specific Cys residues and widely studied in animals. Thanks to the development of dedicated proteomic approaches, in particular the use of the biotin switch technique (BST) combined with mass spectrometry, hundreds of plant protein candidates for S-nitrosylation have been identified. Functional studies focused on specific proteins provided preliminary comprehensive views of how this PTM impacts the structure and function of proteins and, more generally, of how NO might regulate biological plant processes. The aim of this review is to detail the basic principle of protein S-nitrosylation, to provide information on the biochemical and structural features of the S-nitrosylation sites and to describe the proteomic strategies adopted to investigate this PTM in plants. Limits of the current approaches and tomorrow's challenges are also discussed.

Arginine metabolism of Arabidopsis thaliana is modulated by Heterodera schachtii infection

The plant-parasitic cyst nematode Heterodera schachtii induces syncytial feeding structures in the roots of host plants. These syncytia provide all required nutrients, water and solutes to the parasites. Previous studies on the composition of primary metabolites in syncytia revealed significantly increased amino acid levels. However, mainly due to technical limitations, little is known about the role of arginine in plant-nematode interactions. This free amino acid plays a central role in the plant primary metabolism and serves as substrate for metabolites involved in plant stress responses. Thus, in the present work, expression of genes coding for the enzymes of arginine metabolism were studied in nematode-induced syncytia compared to non-infected control roots of Arabidopsis thaliana. Further, amiRNA lines were constructed and T-DNA lines were isolated to test their effects on nematode development. While the silencing of genes involved in arginine synthesis increased nematode development, most T-DNA lines did not show any significant difference from the wild type. Amino acid analyses of syncytia showed that they accumulate high arginine levels. In addition, manipulating arginine cycling had a global effect on the local amino acid composition in syncytia as well as on the systemic amino acid levels in roots and shoots.

Indole acetic acid is responsible for protection against oxidative stress caused by drought in soybean plants: the role of heme oxygenase induction

Objectives This study was focused on the role of indole acetic acid (IAA) in the defense against oxidative stress damage caused by drought in soybean plants and to elucidate whether heme oxygenase-1 (HO-1) and nitric oxide (NO) are involved in this mechanism. IAA is an auxin that participates in many plant processes including oxidative stress defense, but to the best of our knowledge no information is yet available about its possible action in drought stress. Methods To this end, soybean plants were treated with 8% polyethylene glycol (PEG) or 100 µM IAA. To evaluate the behavior of IAA, plants were pretreated with this compound previous to PEG addition. Lipid peroxidation levels (thiobarbituric acid reactive substances (TBARS)), glutathione (GSH) and ascorbate (AS) contents, catalase (CAT), superoxide dismutase (SOD), and guaiacol peroxidase (POD) activities were determined to evaluate oxidative damage. Results Drought treatment (8% PEG) caused a significant increase in TBARS levels as well as a marked decrease in the non-enzymatic (GSH and AS) and enzymatic (CAT, SOD, and POD) antioxidant defense systems. Pre-treatment with IAA prevented the alterations of stress parameters caused by drought, while treatment with IAA alone did not produce changes in TBARS levels, or GSH and AS contents. Moreover, the activities of the classical enzymes involved in the enzymatic defense system (SOD, CAT, and POD) remained similar to control values. Furthermore, this hormone could enhance HO-1 activity (75% with respect to controls), and this increase was positively correlated with protein content as well as gene expression. The direct participation of HO-1 as an antioxidant enzyme was established by performing experiments in the presence of Zn-protoporphyrin IX, a well-known irreversible inhibitor of this enzyme. It was also demonstrated that HO-1 is modulated by NO, as shown by experiments performed in the presence of an NO donor (sodium nitroprusside), an NO scavenger (2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide), or an NO synthesis inhibitor (N-nitro-l-arginine methyl ester, NAME). Discussion It is concluded that IAA is responsible, at least in part, for the protection against oxidative stress caused by drought in soybean plants through the modulation of NO levels which, in turn, enhances HO-1 synthesis and activity.

Circadian redox signaling in plant immunity and abiotic stress

SIGNIFICANCE

Plant crops are critically important to provide quality food and bio-energy to sustain a growing human population. Circadian clocks have been shown to deliver an adaptive advantage to plants, vastly increasing biomass production by efficient anticipation to the solar cycle. Plant stress, on the other hand, whether biotic or abiotic, prevents crops from reaching maximum productivity.

RECENT ADVANCES

Stress is associated with fluctuations in cellular redox and increased phytohormone signaling. Recently, direct links between circadian timekeeping, redox fluctuations, and hormone signaling have been identified. A direct implication is that circadian control of cellular redox homeostasis influences how plants negate stress to ensure growth and reproduction.

CRITICAL ISSUES

Complex cellular biochemistry leads from perception of stress via hormone signals and formation of reactive oxygen intermediates to a physiological response. Circadian clocks and metabolic pathways intertwine to form a confusing biochemical labyrinth. Here, we aim to find order in this complex matter by reviewing current advances in our understanding of the interface between these networks.

FUTURE DIRECTIONS

Although the link is now clearly defined, at present a key question remains as to what extent the circadian clock modulates redox, and vice versa. Furthermore, the mechanistic basis by which the circadian clock gates redox- and hormone-mediated stress responses remains largely elusive.

Nitric oxide function in plant biology: a redox cue in deconvolution

Nitric oxide (NO), a gaseous, redox-active small molecule, is gradually becoming established as a central regulator of growth, development, immunity and environmental interactions in plants. A major route for the transfer of NO bioactivity is S-nitrosylation, the covalent attachment of an NO moiety to a protein cysteine thiol to form an S-nitrosothiol (SNO). This chemical transformation is rapidly emerging as a prototypic, redox-based post-translational modification integral to the life of plants. Here we review the myriad roles of NO and SNOs in plant biology and, where known, the molecular mechanisms underpining their activity.

Action and target sites of nitric oxide in chloroplasts

Nitric oxide (NO) is an important signalling molecule in plants under physiological and stress conditions. Here we review the influence of NO on chloroplasts which can be directly induced by interaction with the photosynthetic apparatus by influencing photophosphorylation, electron transport activity and oxido-reduction state of the Mn clusters of the oxygen-evolving complex or by changes in gene expression. The influence of NO-induced changes in the photosynthetic apparatus on its functions and sensitivity to stress factors are discussed.

Trichoderma asperelloides suppresses nitric oxide generation elicited by Fusarium oxysporum in Arabidopsis roots

Inoculations with saprophytic fungus Trichoderma spp. are now extensively used both to promote plant growth and to suppress disease development. The underlying mechanisms for both roles have yet to be fully described so that the use of Trichoderma spp. could be optimized. Here, we show that Trichoderma asperelloides effects include the manipulation of host nitric oxide (NO) production. NO was rapidly formed in Arabidopsis roots in response to the soil-borne necrotrophic pathogen Fusarium oxysporum and persisted for about 1 h but is only transiently produced (approximately 10 min) when roots interact with T. asperelloides (T203). However, inoculation of F. oxysporum-infected roots with T. asperelloides suppressed F. oxysporum-initiated NO production. A transcriptional study of 78 NO-modulated genes indicated most genes were suppressed by single and combinational challenge with F. oxysporum or T. asperelloides. Only two F. oxysporum-induced genes were suppressed by T. asperelloides inoculation undertaken either 10 min prior to or after pathogen infection: a concanavlin A-like lectin protein kinase (At4g28350) and the receptor-like protein RLP30. Thus, T. asperelloides can actively suppress NO production elicited by F. oxysporum and impacts on the expression of some genes reported to be NO-responsive. Of particular interest was the reduced expression of receptor-like genes that may be required for F. oxysporum-dependent necrotrophic disease development.

Nitric oxide functions as a signal in ultraviolet-B-induced baicalin accumulation in Scutellaria baicalensis suspension cultures

Stress induced by ultraviolet-B (UV-B) irradiation stimulates the accumulation of various secondary metabolites in plants. Nitric oxide (NO) serves as an important secondary messenger in UV-B stress-induced signal transduction pathways. NO can be synthesized in plants by either enzymatic catalysis or an inorganic nitrogen pathway. The effects of UV-B irradiation on the production of baicalin and the associated molecular pathways in plant cells are poorly understood. In this study, nitric oxide synthase (NOS) activity, NO release and the generation of baicalin were investigated in cell suspension cultures of Scutellaria baicalensis exposed to UV-B irradiation. UV-B irradiation significantly increased NOS activity, NO release and baicalin biosynthesis in S. baicalensis cells. Additionally, exogenous NO supplied by the NO donor, sodium nitroprusside (SNP), led to a similar increase in the baicalin content as the UV-B treatment. The NOS inhibitor, N^ω-nitro-l-arginine (LNNA), and NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) partially inhibited UV-B-induced NO release and baicalin accumulation. These results suggest that NO is generated by NOS or NOS-like enzymes and plays an important role in baicalin biosynthesis as part of the defense response of S. baicalensis cells to UV-B irradiation.

Plant mitochondria: source and target for nitric oxide

Plant mitochondria generate nitric oxide (NO) under anoxia through the action of cytochrome c oxidase and other electron transport chain components on nitrite. This reductive mechanism operates under aerobic conditions at high electron transport rates. Indirect evidence also indicates that the oxidative pathway of NO production may be associated with mitochondria. We review the consequences of mitochondrial NO production, including the inhibition of oxygen uptake by cytochrome c oxidase, the inhibition of aconitase and succinate dehydrogenase, the induction of alternative oxidase, and the nitrosylation of several proteins, including glycine decarboxylase. The importance of these events in adaptation to abiotic and biotic stresses is discussed.

Inhibition of peroxisomal hydroxypyruvate reductase (HPR1) by tyrosine nitration

BACKGROUND

Protein tyrosine nitration is a post-translational modification (PTM) mediated by nitric oxide-derived molecules. Peroxisomes are oxidative organelles in which the presence of nitric oxide (NO) has been reported.

METHODS

We studied peroxisomal nitroproteome of pea leaves by high-performance liquid chromatography with tandem mass spectrometry (LC-MS/MS) and proteomic approaches.

RESULTS

Proteomic analysis of peroxisomes from pea leaves detected a total of four nitro-tyrosine immunopositive proteins by using an antibody against nitrotyrosine. One of these proteins was found to be the NADH-dependent hydroxypyruvate reductase (HPR). The in vitro nitration of peroxisomal samples caused a 65% inhibition of HPR activity. Analysis of recombinant peroxisomal NADH-dependent HPR1 activity from Arabidopsis in the presence of H2O2, NO, GSH and peroxynitrite showed that the ONOO(-) molecule caused the highest inhibition of activity (51% at 5mM SIN-1), with 5mM H2O2 having no inhibitory effect. Mass spectrometric analysis of the nitrated recombinant HPR1 enabled us to determine that, among the eleven tyrosine present in this enzyme, only Tyr-97, Tyr-108 and Tyr-198 were exclusively nitrated to 3-nitrotyrosine by peroxynitrite. Site-directed mutagenesis confirmed Tyr198 as the primary site of nitration responsible for the inhibition on the enzymatic activity by peroxynitrite.

CONCLUSION

These findings suggest that peroxisomal HPR is a target of peroxynitrite which provokes a loss of function.

GENERAL SIGNIFICANCE

This is the first report demonstrating the peroxisomal NADH-dependent HPR activity involved in the photorespiration pathway is regulated by tyrosine nitration, indicating that peroxisomal NO metabolism may contribute to the regulation of physiological processes under no-stress conditions.

There's more to the picture than meets the eye: nitric oxide cross talk with Ca2+ signaling

Calcium and nitric oxide (NO) are two important biological messengers. Increasing evidence indicates that Ca(2+) and NO work together in mediating responses to pathogenic microorganisms and microbe-associated molecular patterns. Ca(2+) fluxes were recognized to account for NO production, whereas evidence gathered from a number of studies highlights that NO is one of the key messengers mediating Ca(2+) signaling. Here, we present a concise description of the current understanding of the molecular mechanisms underlying the cross talk between Ca(2+) and NO in plant cells exposed to biotic stress. Particular attention will be given to the involvement of cyclic nucleotide-gated ion channels and Ca(2+) sensors. Notably, we provide new evidence that calmodulin might be regulated at the posttranslational level by NO through S-nitrosylation. Furthermore, we report original transcriptomic data showing that NO produced in response to oligogalacturonide regulates the expression of genes related to Ca(2+) signaling. Deeper insight into the molecules involved in the interplay between Ca(2+) and NO not only permits a better characterization of the Ca(2+) signaling system but also allows us to further understand how plants respond to pathogen attack.

Nitrate reductase is required for the transcriptional modulation and bactericidal activity of nitric oxide during the defense response of Arabidopsis thaliana against Pseudomonas syringae

Nitrate reductase (NR) has emerged as a potential NO source in plants. Indeed, the Arabidopsis thaliana NR double-deficient mutant (nia1 nia2) produces low NO and develops abnormal susceptibility to bacterial infection. We have employed quantitative real-time polymerase chain reactions to analyze the effects of NO gas on the expression of defense-related genes in wild-type and nia1 nia2 A. thaliana plants that were inoculated with an avirulent strain of Pseudomonas syringae pv. tomato. The pathogenesis-related gene 1 (PR1) was up-regulated by bacterial infection, and its expression was higher in the wild type than in nia1 nia2. Fumigation with NO attenuated the expression of PR1 and other salicylic acid-related genes in plants that had been inoculated with P. syringae. Nevertheless, NO inhibited the most intense bacterial growth and disease symptoms in nia1 nia2 leaves. The NO fumigation also directly modulated lignin biosynthesis-related gene expression (CAD1) and parts of the auxin (TIR1, ILL1, GH3) and ethylene (ACCS7) pathways, among other defense-related genes, and their modulation was more intense in the NR-deficient mutant. Pathogen inoculation induced delayed but intense H2O2 production in mutant leaves in comparison with the wild type. Hydrogen peroxide potentiated the microbicidal effects of NO against bacterial cultures. These results suggest that NO has a direct microbicidal effect in combination with H2O2 to allow for the attenuation of the SA-mediated defense response, thereby reducing the energy expenditure associated with defense-related gene transcription. Overall, these results highlight the importance of NR-dependent NO production in the establishment of disease resistance.

Nitric oxide controls nitrate and ammonium assimilation in Chlamydomonas reinhardtii

Nitrate and ammonium are major inorganic nitrogen sources for plants and algae. These compounds are assimilated by means of finely regulated processes at transcriptional and post-translational levels. In Chlamydomonas, the expression of several genes involved in high-affinity ammonium (AMT1.1, AMT1.2) and nitrate transport (NRT2.1) as well as nitrate reduction (NIA1) are downregulated by ammonium through a nitric oxide (NO)-dependent mechanism. At the post-translational level, nitrate/nitrite uptake and nitrate reductase (NR) are also inhibited by ammonium, but the mechanisms implicated in this regulation are scarcely known. In this work, the effect of NO on nitrate assimilation and the high-affinity ammonium uptake was addressed. NO inhibited the high-affinity uptake of ammonium and nitrate/nitrite, as well as the NR activity, in a reversible form. In contrast, nitrite reductase and glutamine synthetase activities were not affected. The in vivo and in vitro studies suggested that NR enzyme is inhibited by NO in a mediated process that requires the cell integrity. These data highlight a role of NO in inorganic nitrogen assimilation and suggest that this signalling molecule is an important regulator for the first steps of the pathway.

Pharmacological and genetical evidence supporting nitric oxide requirement for 2,4-epibrassinolide regulation of root architecture in Arabidopsis thaliana

Brassinosteroids (BRs) regulate various physiological processes, such as tolerance to stresses and root growth. Recently, a connection was reported between BRs and nitric oxide (NO) in plant responses to abiotic stress. Here we present evidence supporting NO functions in BR signaling during root growth process. Arabidopsis seedlings treated with BR 24-epibrassinolide (BL) show increased lateral roots (LR) density, inhibition of primary root (PR) elongation and NO accumulation. Similar effects were observed adding the NO donor GSNO to BR-receptor mutant bri1-1. Furthermore, BL-induced responses in the root were abolished by the specific NO scavenger c-PTIO. The activities of nitrate reductase (NR) and nitric oxide synthase (NOS)-like, two NO generating enzymes were involved in BR signaling. These results demonstrate that BR increases the NO concentration in root cells, which is required for BR-induced changes in root architecture.

Hydrogen peroxide and nitric oxide: key regulators of the Legume-Rhizobium and mycorrhizal symbioses

SIGNIFICANCE

During the Legume-Rhizobium symbiosis, hydrogen peroxide (H(2)O(2)) and nitric oxide (NO) appear to play an important signaling role in the establishment and the functioning of this interaction. Modifications of the levels of these reactive species in both partners impair either the development of the nodules (new root organs formed on the interaction) or their N(2)-fixing activity.

RECENT ADVANCES

NADPH oxidases (Noxs) have been recently described as major sources of H(2)O(2) production, via superoxide anion dismutation, during symbiosis. Nitrate reductases (NR) and electron transfer chains from both partners were found to significantly contribute to NO production in N(2)-fixing nodules. Both S-sulfenylated and S-nitrosylated proteins have been detected during early interaction and in functioning nodules, linking reactive oxygen species (ROS)/NO production to redox-based protein regulation. NO was also found to play a metabolic role in nodule energy metabolism.

CRITICAL ISSUES

H(2)O(2) may control the infection process and the subsequent bacterial differentiation into the symbiotic form. NO is required for an optimal establishment of symbiosis and appears to be a key player in nodule senescence.

FUTURE DIRECTIONS

A challenging question is to define more precisely when and where reactive species are generated and to develop adapted tools to detect their production in vivo. To investigate the role of Noxs and NRs in the production of H(2)O(2) and NO, respectively, the use of mutants under the control of organ-specific promoters will be of crucial interest. The balance between ROS and NO production appears to be a key point to understand the redox regulation of symbiosis.