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

Vinyl sulfone silica: application of an open preactivated support to the study of transnitrosylation of plant proteins by S-nitrosoglutathione

BACKGROUND

S-nitrosylaton is implicated in the regulation of numerous signaling pathways with a diversity of regulatory roles. The high lability of the S-NO bond makes the study of proteins regulated by S-nitrosylation/denitrosylation a challenging task and most studies have focused on already S-nitrosylated proteins. We hypothesize that: i) S-nitrosoglutathione (GSNO) transnitrosylation is a feasible mechanism to account for the physiological S-nitrosylation of rather electropositive sulfur atoms from proteins, ii) affinity chromatography is a suitable approach to isolate proteins that are prone to undergo S-transnitrosylation and iii) vinyl sulfone silica is a suitable chromatographic bead.

RESULTS

The combination of vinyl sulfone silica with GSNO yielded an affinity resin that withstood high ionic strength without shrinking or deforming and that it was suitable to isolate potential GSNO transnitrosylation target candidates. Fractions eluted at 1500 mM NaCl resulted in a symmetrical peak for both, protein and S-nitrosothiols, supporting the idea of transnitrosylation by GSNO as a selective process that involves strong and specific interactions with the target protein. Proteomic analysis led to the identification of 22 physiological significant enzymes that differ with the tissue analyzed, being regulatory proteins the most abundant group in hypocotyls. The identification of chloroplastidic FBPase, proteasome, GTP-binding protein, heat shock Hsp70, syntaxin, catalase I, thioredoxin peroxidase and cytochrome P450 that have already been reported as S-nitrosylated by other techniques can be considered as internal positive controls that validate our experimental approach. An additional validation was provided by the prediction of the S-nitrosylation sites in 19 of the GSNO transnitrosylation target candidates.

CONCLUSIONS

Vinyl sulfone silica is an open immobilization support that can be turned ad hoc and in a straightforward manner into an affinity resin. Its potential in omic sciences was successfully put to test in the context of the analysis of post-translational modification by S-nitrosylation with two different tissues: mature pea leaves and embryogenic sunflower hypocotyls. The identified proteins reveal an intriguing overlap among S-nitrosylation and both tyrosine nitration and thioredoxin regulation. Chloroplastidic FBPase is a paradigm of such overlap of post-translational modifications since it is reversible modified by thioredoxin and S-nitrosylation and irreversibly by tyrosine nitration. Our results suggest a complex interrelation among different modulation mechanisms mediated by NO-derived molecules.

[Nitric oxide is a major player in plant immune system]

In animals, nitric oxide (NO) functions as a ubiquitous signaling molecule involved in diverse physiological processes such as immunity. Recent studies provided evidence that plants challenged by pathogenic microorganisms also produce NO. The emerging picture is that NO functions as a signal in plant immunity and executes part of its effects through posttranslational protein modifications. Notably, the characterization of S-nitrosylated proteins provided insights into the molecular mechanisms by which NO exerts its activities. Based on these findings, it appears that NO is involved in both the activation and the negative control of the signaling pathways related to plant immunity.

Nitric oxide as a mediator for defense responses

Sequential recognition of invading microbes and rapid induction of plant immune responses comprise at least two recognition systems. Early basal defenses are initiated by pathogen-associated molecular patterns and pattern recognition receptors (PRR) in the plasma membrane. Pathogens produce effectors to suppress defense but plants, in turn, can sense such effectors by dominant plant resistance (R) gene products. Plant PRR and R proteins modulate signaling networks for defense responses that rely on rapid production of reactive nitrogen species (RNS) and reactive oxygen species (ROS). Recent research has shown that nitric oxide (NO) mainly mediates biological function through chemical reactions between locally controlled accumulation of RNS and proteins leading to potential alteration of protein function. Many proteins specifically regulated by NO and participating in signaling during plant defense response have been identified, highlighting the physiological relevance of these modifications in plant immunity. ROS function independently or in cooperation with NO during defense, modulating the RNS signaling functions through the entire process. This review provides an overview of current knowledge about regulatory mechanisms for NO burst and signaling, and crosstalk with ROS in response to pathogen attack.

Salinity-induced changes in S-nitrosylation of pea mitochondrial proteins

Together with reactive oxygen species, nitric oxide is an essential part of the signal transduction induced by stress conditions. In this work we study the pattern of S-nitrosylated proteins from mitochondria of pea plants subjected to 150mM NaCl for 5 and 14days. A differential pattern of target proteins was found during plant development and salt stress, with a minor number of S-nitrosylated proteins at 14 days specifically some key enzymes related to respiration and photorespiration. At this time of stress, only ATP synthase β subunit, peroxiredoxin and Hsp90 were S-nitrosylated and no changes in protein levels were observed, although the activity of PrxII F may be reduced by S-nitrosylation. The NADH/NAD(+) ratio was also high at 14days but not the NADPH/NADP(+). An enhancement in NO measured by fluorimetry and confocal microscopy was observed in leaves, being part of the NO localized in mitochondria. An increase in mitochondrial GSNOR activity was produced in response to short and long-term NaCl treatment, where a higher number of nitrated proteins were also observed. The results indicated that posttranslational modifications seem to modulate respiratory and photorespiratory pathways, as well as some antioxidant enzymes, through differential S-nitrosylation/denitrosylation in control conditions and under salt stress.

Nitric oxide generated by the rice blast fungus Magnaporthe oryzae drives plant infection

Plant-derived nitric oxide (NO) triggers defence, priming the onset of the hypersensitive response and restricting pathogen ingress during incompatibility. However, little is known about the role of pathogen-produced NO during pre-infection development and infection. We sought evidence for NO production by the rice blast fungus during early infection. NO production was measured using fluorescence of DAR-4M and the role of NO assessed using NO scavengers. The synthesis of NO was investigated by targeted knockout of genes potentially involved in NO synthesis, including nitric oxide synthase-like genes (NOL2 and NOL3) and nitrate (NIA1) and nitrite reductase (NII1), generating single and double Δnia1Δnii1, Δnia1Δnol3, and Δnol2Δnol3 mutants. We demonstrate that Magnaporthe oryzae generates NO during germination and in early development. Removal of NO delays germling development and reduces disease lesion numbers. NO is not generated by the candidate proteins tested, nor by other arginine-dependent NO systems, by polyamine oxidase activity or non-enzymatically by low pH. Furthermore, we show that, while NIA1 and NII1 are essential for nitrate assimilation, NIA1, NII1, NOL2 and NOL3 are all dispensable for pathogenicity. Development of M. oryzae and initiation of infection are critically dependent on fungal NO synthesis, but its mode of generation remains obscure.

Endogenous nitric oxide generation in protoplast chloroplasts

KEY MESSAGE : NO generation is studied in the protoplast chloroplasts. NO, ONOO ( - ) and ROS (O ( 2 ) ( - ) and H ( 2 ) O ( 2 ) ) are generated in chloroplasts. Nitric oxide synthase-like protein appears to be involved in NO generation. Nitric oxide stimulates chlorophyll biosynthesis and chloroplast differentiation. The present study was conducted to better understand the process of NO generation in the leaf chloroplasts and protoplasts. NO, peroxynitrite and superoxide anion were investigated in the protoplasts and isolated chloroplasts using specific dyes, confocal laser scanning and light microscopy. The level of NO was highest after protoplast isolation and subsequently decreased during culture. Suppression of NO signal in the presence of PTIO, suggests that diaminofluorescein-2 diacetate (DAF-2DA) detected NO. Detection of peroxynitrite, a reaction product of NO and superoxide anion, further suggests NO generation. Moreover, generation of NO and peroxynitrite in the chloroplasts of wild-type Arabidopsis and their absence or weak signals in the leaf-derived protoplasts of Atnoa1 mutants confirmed the reactivity of DAF-2DA and aminophenyl fluorescein to NO and peroxynitrite, respectively. Isolated chloroplasts also showed signal of NO. Suppression of NO signal in the presence of 100 μM nitric oxide synthase inhibitors [L-NNA, Nω-nitro-L-arginine and PBIT, S,S'-1,3-phenylene-bis(1,2-ethanediyl)-bis-isothiourea] revealed that nitric oxide synthase-like system is involved in NO synthesis. Suppression of NO signal in the protoplasts isolated in the presence of cycloheximide suggests de novo synthesis of NO generating protein during the process of protoplast isolation. Furthermore, the lack of inhibition of NO production by sodium tungstate (250 μM) and inhibition by L-NNA, and PBIT suggest involvement NOS-like protein, but not nitrate reductase, in NO generation in the leaf chloroplasts and protoplasts.

Peroxisomes as cell generators of reactive nitrogen species (RNS) signal molecules

Nitric oxide is a gaseous free radical with a wide range of direct and indirect actions in plant cells. However, the enzymatic sources of NO and its subcellular localization in plants are still under debate. Among the different subcellular compartments where NO has been found to be produced, peroxisomes are the best characterized since in these organelles it has been demonstrated the presence of NO and it has been biochemically characterized a L-arginine-dependent nitric oxide synthase activity. This chapter summarizes the present knowledge of the NO metabolism and its derived reactive nitrogen species (RNS) in plant peroxisomes and how this gaseous free radical is involved in natural senescence, and is released to the cytosol under salinity stress conditions acting as a signal molecule.

Regulatory role of nitric oxide in lipopolysaccharides-triggered plant innate immunity

Recent studies have suggested that lipopolysaccharides (LPS) induce nitric oxide (NO) production and defense gene expression in plants. Our current work investigated the signaling mechanism of NO and the role of NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 (NPR1) in LPS-induced innate immunity of Arabidopsis (Arabidopsis thaliana). We have provided evidence that LPS-elicited NO generation as well as increased antioxidant enzyme activities capable of maintaining the redox state could be important to protect plants against oxidative damage from pathogen attack. In addition, LPS-activated defense responses, including callose deposition and defense-related gene expression, are regulated through an NPR1-dependent signaling pathway. Our results contribute to elucidation of the signaling mechanism of NO and highlight an important role of NPR1 in modulating LPS-triggered innate immunity in plants. However, further research is necessary to clarify the cross-talk between mitochondria and NO on activating LPS-induced defense responses, and the regulatory mechanism of NO in LPS-induced innate immunity needs further improvement.

Peroxisomes and photomorphogenesis

In higher plants, light-grown seedlings exhibit photomorphogenesis, a developmental program controlled by a complex web of interactions between photoreceptors, central repressors, and downstream effectors that leads to changes in gene expression and physiological changes. Light induces peroxisomal proliferation through a phytochrome A-mediated pathway, in which the transcription factor HYH activates the peroxisomal proliferation factor gene PEX11b. Microarray analysis revealed that light activates the expression of a number of peroxisomal genes, especially those involved in photorespiration, a process intimately associated with photosynthesis. In contrast, light represses the expression of genes involved in β-oxidation and the glyoxylate cycle, peroxisomal pathways essential for seedling establishment before photosynthesis begins. Furthermore, the peroxisome is a source of signaling molecules, notably nitric oxide, which promotes photomorphogenesis. Lastly, a gain-of-function mutant of the peroxisomal membrane-tethered RING-type E3 ubiquitin ligase PEX2 partially suppresses the phenotype of the photomorphogenic mutant det1. Possible mechanisms underlying this phenomenon are discussed.

Nitric oxide in plants: an assessment of the current state of knowledge

BACKGROUND AND AIMS

After a series of seminal works during the last decade of the 20th century, nitric oxide (NO) is now firmly placed in the pantheon of plant signals. Nitric oxide acts in plant-microbe interactions, responses to abiotic stress, stomatal regulation and a range of developmental processes. By considering the recent advances in plant NO biology, this review will highlight certain key aspects that require further attention.

SCOPE AND CONCLUSIONS

The following questions will be considered. While cytosolic nitrate reductase is an important source of NO, the contributions of other mechanisms, including a poorly defined arginine oxidizing activity, need to be characterized at the molecular level. Other oxidative pathways utilizing polyamine and hydroxylamine also need further attention. Nitric oxide action is dependent on its concentration and spatial generation patterns. However, no single technology currently available is able to provide accurate in planta measurements of spatio-temporal patterns of NO production. It is also the case that pharmaceutical NO donors are used in studies, sometimes with little consideration of the kinetics of NO production. We here include in planta assessments of NO production from diethylamine nitric oxide, S-nitrosoglutathione and sodium nitroprusside following infiltration of tobacco leaves, which could aid workers in their experiments. Further, based on current data it is difficult to define a bespoke plant NO signalling pathway, but rather NO appears to act as a modifier of other signalling pathways. Thus, early reports that NO signalling involves cGMP-as in animal systems-require revisiting. Finally, as plants are exposed to NO from a number of external sources, investigations into the control of NO scavenging by such as non-symbiotic haemoglobins and other sinks for NO should feature more highly. By crystallizing these questions the authors encourage their resolution through the concerted efforts of the plant NO community.

Nitric oxide participates in cold-inhibited Camellia sinensis pollen germination and tube growth partly via cGMP in vitro

Nitric oxide (NO) plays essential roles in many biotic and abiotic stresses in plant development procedures, including pollen tube growth. Here, effects of NO on cold stress inhibited pollen germination and tube growth in Camellia sinensis were investigated in vitro. The NO production, NO synthase (NOS)-like activity, cGMP content and proline (Pro) accumulation upon treatment with NO scavenger cPTIO, NOS inhibitor L-NNA, NO donor DEA NONOate, guanylate cyclase (GC) inhibitor ODQ or phosphodiesterase (PDE) inhibitor Viagra at 25°C (control) or 4°C were analyzed. Exposure to 4°C for 2 h reduced pollen germination and tube growth along with increase of NOS-like activity, NO production and cGMP content in pollen tubes. DEA NONOate treatment inhibited pollen germination and tube growth in a dose-dependent manner under control and reinforced the inhibition under cold stress, during which NO production and cGMP content promoted in pollen tubes. L-NNA and cPTIO markedly reduced the generation of NO induced by cold or NO donor along with partly reverse of cold- or NO donor-inhibited pollen germination and tube growth. Furthermore, ODQ reduced the cGMP content under cold stress and NO donor treatment in pollen tubes. Meanwhile, ODQ disrupted the reinforcement of NO donor on the inhibition of pollen germination and tube growth under cold condition. Additionally, Pro accumulation of pollen tubes was reduced by ODQ compared with that receiving NO donor under cold or control condition. Effects of cPTIO and L-NNA in improving cold-treated pollen germination and pollen tube growth could be lowered by Viagra. Moreover, the inhibitory effects of cPTIO and L-NNA on Pro accumulation were partly reversed by Viagra. These data suggest that NO production from NOS-like enzyme reaction decreased the cold-responsive pollen germination, inhibited tube growth and reduced Pro accumulation, partly via cGMP signaling pathway in C. sinensis.

Constitutive expression of mammalian nitric oxide synthase in tobacco plants triggers disease resistance to pathogens

Nitric oxide (NO) is known for its role in the activation of plant defense responses. To examine the involvement and mode of action of NO in plant defense responses, we introduced calmodulin-dependent mammalian neuronal nitric oxide synthase (nNOS), which controls the CaMV35S promoter, into wild-type and NahG tobacco plants. Constitutive expression of nNOS led to NO production and triggered spontaneous induction of leaf lesions. Transgenic plants accumulated high amounts of H(2)O(2), with catalase activity lower than that in the wild type. nNOS transgenic plants contained high levels of salicylic acid (SA), and they induced an array of SA-, jasmonic acid (JA)-, and/or ethylene (ET)-related genes. Consequently, NahG co-expression blocked the induction of systemic acquired resistance (SAR)-associated genes in transgenic plants, implying SA is involved in NO-mediated induction of SAR genes. The transgenic plants exhibited enhanced resistance to a spectrum of pathogens, including bacteria, fungi, and viruses. Our results suggest a highly ranked regulatory role for NO in SA-, JA-, and/or ET-dependent pathways that lead to disease resistance.

Nitric oxide-mediated maintenance of redox homeostasis contributes to NPR1-dependent plant innate immunity triggered by lipopolysaccharides

The perception of lipopolysaccharides (LPS) by plant cells can lead to nitric oxide (NO) production and defense gene induction. However, the signaling cascades underlying these cellular responses have not yet been resolved. This work investigated the biosynthetic origin of NO and the role of NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 (NPR1) to gain insight into the mechanism involved in LPS-induced resistance of Arabidopsis (Arabidopsis thaliana). Analysis of inhibitors and mutants showed that LPS-induced NO synthesis was mainly mediated by an arginine-utilizing source of NO generation. Furthermore, LPS-induced NO caused transcript accumulation of alternative oxidase genes and increased antioxidant enzyme activity, which enhanced antioxidant capacity and modulated redox state. We also analyzed the subcellular localization of NPR1 to identify the mechanism for protein-modulated plant innate immunity triggered by LPS. LPS-activated defense responses, including callose deposition and defense-related gene expression, were found to be regulated through an NPR1-dependent pathway. In summary, a significant NO synthesis induced by LPS contributes to the LPS-induced defense responses by up-regulation of defense genes and modulation of cellular redox state. Moreover, NPR1 plays an important role in LPS-triggered plant innate immunity.

ABA says NO to UV-B: a universal response?

Abscisic acid (ABA) signaling pathways have been widely characterized in plants, whereas the function of ABA in animals is less well understood. However, recent advances show ABA production by a wide range of lower animals and higher mammals. This enables a new evaluation of ABA signaling pathways in different organisms in response to common environmental stress, such as ultraviolet (UV)-B. In this opinion article, we propose that the induction of common signaling components, such as ABA, nitric oxide (NO) and Ca(2+), in plant and animal cells in response to high doses of UV-B, suggests that the evolution of a general mechanism activated by UV-B is conserved in divergent multicellular organisms challenged by a changing common environment.

NADP-dependent isocitrate dehydrogenase from Arabidopsis roots contributes in the mechanism of defence against the nitro-oxidative stress induced by salinity

NADPH regeneration appears to be essential in the mechanism of plant defence against oxidative stress. Plants contain several NADPH-generating dehydrogenases including isocitrate dehydrogenase (NADP-ICDH), glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), and malic enzyme (ME). In Arabidopsis seedlings grown under salinity conditions (100 mM NaCl) the analysis of physiological parameters, antioxidant enzymes (catalase and superoxide dismutase) and content of superoxide radical (O₂^∙−), nitric oxide (NO), and peroxynitrite (ONOO⁻) indicates a process of nitro-oxidative stress induced by NaCl. Among the analysed NADPH-generating dehydrogenases under salinity conditions, the NADP-ICDH showed the maximum activity mainly attributable to the root NADP-ICDH. Thus, these data provide new insights on the relevance of the NADP-ICDH which could be considered as a second barrier in the mechanism of response against the nitro-oxidative stress generated by salinity.

Metabolism of reactive oxygen species and reactive nitrogen species in pepper (Capsicum annuum L.) plants under low temperature stress

Low temperature is an environmental stress that affects crop production and quality and regulates the expression of many genes, and the level of a number of proteins and metabolites. Using leaves from pepper (Capsicum annum L.) plants exposed to low temperature (8 °C) for different time periods (1 to 3 d), several key components of the metabolism of reactive nitrogen and oxygen species (RNS and ROS, respectively) were analysed. After 24 h of exposure at 8 °C, pepper plants exhibited visible symptoms characterized by flaccidity of stems and leaves. This was accompanied by significant changes in the metabolism of RNS and ROS with an increase of both protein tyrosine nitration (NO(2) -Tyr) and lipid peroxidation, indicating that low temperature induces nitrosative and oxidative stress. During the second and third days at low temperature, pepper plants underwent cold acclimation by adjusting their antioxidant metabolism and reverting the observed nitrosative and oxidative stress. In this process, the levels of the soluble non-enzymatic antioxidants ascorbate and glutathione, and the activity of the main NADPH-generating dehydrogenases were significantly induced. This suggests that ascorbate, glutathione and the NADPH-generating dehydrogenases have a role in the process of cold acclimation through their effect on the redox state of the cell.

Increasing nitric oxide content in Arabidopsis thaliana by expressing rat neuronal nitric oxide synthase resulted in enhanced stress tolerance

Nitric oxide (NO) plays essential roles in many physiological and developmental processes in plants, including biotic and abiotic stresses, which have adverse effects on agricultural production. However, due to the lack of findings regarding nitric oxide synthase (NOS), many difficulties arise in investigating the physiological roles of NO in vivo and thus its utilization for genetic engineering. Here, to explore the possibility of manipulating the endogenous NO level, rat neuronal NOS (nNOS) was expressed in Arabidopsis thaliana. The 35S::nNOS plants showed higher NOS activity and accumulation of NO using the fluorescent probe 3-amino, 4-aminomethyl-2', 7'-difluorescein, diacetate (DAF-FM DA) assay and the hemoglobin assay. Compared with the wild type, the 35S::nNOS plants displayed improved salt and drought tolerance, which was further confirmed by changes in physiological parameters including reduced water loss rate, reduced stomatal aperture, and altered proline and malondialdehyde content. Quantitative real-time PCR analyses revealed that the expression of several stress-regulated genes was up-regulated in the transgenic lines. Furthermore, the transgenic lines also showed enhanced disease resistance against Pseudomonas syringae pv. tomato (Pst) DC3000 by activating the expression of defense-related genes. In addition, we found that the 35S::nNOS lines flowered late by regulating the expression of CO, FLC and LFY genes. Together, these results demonstrated that it is a useful strategy to exploit the roles of plant NO in various processes by the expression of rat nNOS. The approach may also be useful for genetic engineering of crops with increased environmental adaptations.

Nitric oxide and protein S-nitrosylation are integral to hydrogen peroxide-induced leaf cell death in rice

Nitric oxide (NO) is a key redox-active, small molecule involved in various aspects of plant growth and development. Here, we report the identification of an NO accumulation mutant, nitric oxide excess1 (noe1), in rice (Oryza sativa), the isolation of the corresponding gene, and the analysis of its role in NO-mediated leaf cell death. Map-based cloning revealed that NOE1 encoded a rice catalase, OsCATC. Furthermore, noe1 resulted in an increase of hydrogen peroxide (H(2)O(2)) in the leaves, which consequently promoted NO production via the activation of nitrate reductase. The removal of excess NO reduced cell death in both leaves and suspension cultures derived from noe1 plants, implicating NO as an important endogenous mediator of H(2)O(2)-induced leaf cell death. Reduction of intracellular S-nitrosothiol (SNO) levels, generated by overexpression of rice S-nitrosoglutathione reductase gene (GSNOR1), which regulates global levels of protein S-nitrosylation, alleviated leaf cell death in noe1 plants. Thus, S-nitrosylation was also involved in light-dependent leaf cell death in noe1. Utilizing the biotin-switch assay, nanoliquid chromatography, and tandem mass spectrometry, S-nitrosylated proteins were identified in both wild-type and noe1 plants. NO targets identified only in noe1 plants included glyceraldehyde 3-phosphate dehydrogenase and thioredoxin, which have been reported to be involved in S-nitrosylation-regulated cell death in animals. Collectively, our data suggest that both NO and SNOs are important mediators in the process of H(2)O(2)-induced leaf cell death in rice.

Tungstate: is it really a specific nitrate reductase inhibitor in plant nitric oxide research?

Nitrate reductase (NR) is an enzymatic source of nitric oxide (NO) in plants, and it needs Mo for the Mo-cofactor to be activated. Because NR-deficient mutants are not always available in some species, a cheap and simple pharmacological application of tungstate, which substitutes for Mo in the Mo-cofactor as a competitive antagonist, is widely used as a NR inhibitor in plant NO research. However, evidence indicates that tungstate not only inactivates NR but also inhibits other molybdate-dependent enzymes in plants. In addition, a number of investigations have shown that tungstate also inhibits root growth, affects cortical microtubule formation, and induces programmed cell death (PCD) in plants, just like other heavy metals do. Therefore, tungstate has been shown to exert many other effects that are not connected with the inhibition of NR activity. The origin and mechanism of using tungstate as a NR inhibitor in plants is reviewed here and the progress regarding tungstate toxicity to plants and the possible problems involved in using tungstate as a NR inhibitor in plant NO research are analysed. In summary, the use of tungstate as a NR inhibitor in plant NO research must be treated with caution, keeping in mind that it is not completely specific. It is necessary to search for more NR-deficient mutants and new, specific NR inhibitors. A combination of pharmacological and biochemical analysis with a genetic approach will be necessary in order to investigate the roles of NO in plants.

Discovering the role of mitochondria in the iron deficiency-induced metabolic responses of plants

In plants, iron (Fe) deficiency-induced chlorosis is a major problem, affecting both yield and quality of crops. Plants have evolved multifaceted strategies, such as reductase activity, proton extrusion, and specialised storage proteins, to mobilise Fe from the environment and distribute it within the plant. Because of its fundamental role in plant productivity, several issues concerning Fe homeostasis in plants are currently intensively studied. The activation of Fe uptake reactions requires an overall adaptation of the primary metabolism because these activities need the constant supply of energetic substrates (i.e., NADPH and ATP). Several studies concerning the metabolism of Fe-deficient plants have been conducted, but research focused on mitochondrial implications in adaptive responses to nutritional stress has only begun in recent years. Mitochondria are the energetic centre of the root cell, and they are strongly affected by Fe deficiency. Nevertheless, they display a high level of functional flexibility, which allows them to maintain the viability of the cell. Mitochondria represent a crucial target of studies on plant homeostasis, and it might be of interest to concentrate future research on understanding how mitochondria orchestrate the reprogramming of root cell metabolism under Fe deficiency. In this review, I summarise what it is known about the effect of Fe deficiency on mitochondrial metabolism and morphology. Moreover, I present a detailed view of the possible roles of mitochondria in the development of plant responses to Fe deficiency, integrating old findings with new and discussing new hypotheses for future investigations.

Nitric oxide imbalance provokes a nitrosative response in plants under abiotic stress

Nitric oxide (NO), a free radical generated in plant cells, belongs to a family of related molecules designated as reactive nitrogen species (RNS). When an imbalance of RNS takes place for any adverse environmental circumstances, some of these molecules can cause direct or indirect damage at the cellular or molecular level, promoting a phenomenon of nitrosative stress. Thus, this review will emphasize the recent progress in understanding the function of NO and its production under adverse environmental conditions.

Nitric oxide in legume-rhizobium symbiosis

Nitric oxide (NO) is a gaseous signaling molecule with a broad spectrum of regulatory functions in plant growth and development. NO has been found to be involved in various pathogenic or symbiotic plant-microbe interactions. During the last decade, increasing evidence of the occurrence of NO during legume-rhizobium symbioses has been reported, from early steps of plant-bacteria interaction, to the nitrogen-fixing step in mature nodules. This review focuses on recent advances on NO production and function in nitrogen-fixing symbiosis. First, the potential plant and bacterial sources of NO, including NO synthase-like, nitrate reductase or electron transfer chains of both partners, are presented. Then responses of plant and bacterial cells to the presence of NO are presented in the context of the N(2)-fixing symbiosis. Finally, the roles of NO as either a regulatory signal of development, or a toxic compound with inhibitory effects on nitrogen fixation, or an intermediate involved in energy metabolism, during symbiosis establishment and nodule functioning are discussed.

Nitric oxide and ABA in the control of plant function

Abscisic acid (ABA) and nitric oxide (NO) are both extremely important signalling molecules employed by plants to control many aspects of physiology. ABA has been extensively studied in the mechanisms which control stomatal movement as well as in seed dormancy and germination and plant development. The addition of either ABA or NO to plant cells is known to instigate the actions of many signal transduction components. Both may have an influence on the phosphorylation of proteins in cells mediated by effects on protein kinases and phosphatases, as well as recruiting a wide range of other signal transduction molecules to mediate the final effects. Both ABA and NO may also lead to the regulation of gene expression. However, it is becoming more apparent that NO may be acting downstream of ABA, with such action being mediated by reactive oxygen species such as hydrogen peroxide in some cases. However not all ABA responses require the action of NO. Here, examples of where ABA and NO have been put together into the same signal transduction pathways are discussed.

The role of nitric oxide in the germination of plant seeds and pollen

Two complex physiological processes, with opposite positions in the plant's life-cycle, seed and pollen germination, are vital to the accomplishment of successful plant growth and reproduction. This review summarizes the current state of knowledge of the intersection of NO signalling with the signalling pathways of ABA, GA, and ethylene; plant hormones that control the release of plant seeds from dormancy and germination. The cross-talk of NO and ROS is involved in the light- and hormone-specific regulation of seeds' developmental processes during the initiation of plant ontogenesis. Similarly to seed germination, the mechanisms of plant pollen hydration, germination, tube growth, as well as pollen-stigma recognition are tightly linked to the proper adjustment of NO and ROS levels. The interaction of NO with ROS and secondary messengers such as Ca(2+), cAMP and cGMP discovered in pollen represent a common mechanism of NO signalling. The involvement of NO in both breakpoints of plant physiology, as well as in the germination of spores within fungi and oomycetes, points toward NO as a component of an evolutionary conserved signalling pathway.

Methods of nitric oxide detection in plants: a commentary

Over the last decade nitric oxide (NO) has been shown to influence a range of processes in plants. However, when, where and even if NO production occurs is controversial in several physiological scenarios in plants. This arises from a series of causes: (a) doubts have arisen over the specificity of widely used 4,5-diaminofluorescein diacetate (DAF-2DA)/4-amino-5-methylamino-2,7-difluorofluorescein (DAF-FM) dyes for NO, (b) no plant nitric oxide synthase (NOS) has been cloned, so that the validity of using mammalian NOS inhibitors to demonstrate that NO is being measured is debatable, (c) the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-l-oxyl-3-oxide (cPTIO) needs to be used with caution, and (d) some discrepancies between assays for in planta measurements and another based on sampling NO from the gas phase have been reported. This review will outline some commonly used methods to determine NO, attempt to reconcile differing results obtained by different laboratories and suggest appropriate approaches to unequivocally demonstrate the production of NO.

The emerging roles of nitric oxide (NO) in plant mitochondria

In recent years nitric oxide (NO) has been recognized as an important signal molecule in plants. Both, reductive and oxidative pathways and different subcellular compartments appear involved in NO production. The reductive pathway uses nitrite as substrate, which is exclusively generated by cytosolic nitrate reductase (NR) and can be converted to NO by the same enzyme. The mitochondrial electron transport chain is another site for nitrite to NO reduction, operating specifically when the normal electron acceptor, O(2), is low or absent. Under these conditions, the mitochondrial NO production contributes to hypoxic survival by maintaining a minimal ATP formation. In contrast, excessive NO production and concomitant nitrosative stress may be prevented by the operation of NO-scavenging mechanisms in mitochondria and cytosol. During pathogen attacks, mitochondrial NO serves as a nitrosylating agent promoting cell death; whereas in symbiotic interactions as in root nodules, the turnover of mitochondrial NO helps in improving the energy status similarly as under hypoxia/anoxia. The contribution of NO turnover during pathogen defense, symbiosis and hypoxic stress is discussed in detail.

Roles of Ca2+ and cyclic nucleotide gated channel in plant innate immunity

The increase of cytosolic Ca(2+) is a vital event in plant pathogen signaling cascades. Molecular components linking pathogen signal perception to cytosolic Ca(2+) increase have not been well characterized. Plant cyclic nucleotide gated channels (CNGCs) play important roles in the pathogen signaling cascade, in terms of facilitating Ca(2+) uptake into the cytosol in response to pathogen and pathogen associated molecular pattern (PAMP) signals. Perception of pathogens leads to cyclic nucleotide production and the activation of CNGCs. The Ca(2+) signal is transduced through Ca(2+) sensors (Calmodulin (CaM) and CaM-like proteins (CMLs)), which regulates the production of nitric oxide (NO). In addition, roles of Ca(2+)/CaM interacting proteins such as CaM binding Protein (CBP) and CaM-binding transcription activators (CAMTAs)) have been recently identified in the plant defense signaling cascade as well. Furthermore, Ca(2+)-dependent protein kinases (CDPKs) have been found to function as components in terms of transcriptional activation in response to a pathogen (PAMP) signal. Although evidence shows that Ca(2+) is an essential signaling component upstream from many vital signaling molecules (such as NO), some work also indicates that these downstream signaling components can also regulate Ca(2+) homeostasis. NO can induce cytosolic Ca(2+) increase (through activation of plasma membrane- and intracellular membrane-localized Ca(2+) channels) during pathogen signaling cascades. Thus, much work is needed to further elucidate the complexity of the plant pathogen signaling network in the future.

Redox-based protein modifications: the missing link in plant immune signalling

Activation of plant immunity is associated with dramatic changes in the cellular redox status. Both oxidative and reductive bursts have been described that trigger a set of down stream responses resulting in reprogramming of the transcriptome and establishment of disease resistance. Nonetheless, how these redox changes are sensed and signal to downstream regulators remained a missing link in studies of plant immunity. Emerging evidence now indicates that pathogen-induced changes in the cellular redox environment are sensed by reactive cysteine residues of key regulatory proteins. Varying degrees of reversible, oxidative cysteine modifications control the activity, localization, protein-interaction and stability of regulatory proteins. These diverse effects on protein function make post-translational redox-based modifications potent modulators of plant immunity.

Nitric oxide increases the enzymatic activity of three ascorbate peroxidase isoforms in soybean root nodules

Ascorbate peroxidase is one of the major enzymes regulating the levels of H2O2 in plants and plays a crucial role in maintaining root nodule redox status. We used fully developed and mature nitrogen fixing root nodules from soybean plants to analyze the effect of exogenously applied nitric oxide, generated from the nitric oxide donor 2,2'-(hydroxynitrosohydrazono)bis-ethanimine, on the enzymatic activity of soybean root nodule ascorbate peroxidase. Nitric oxide caused an increase in the total enzymatic activity of ascorbate peroxidase. The nitric oxide-induced changes in ascorbate peroxidase enzymatic activity were coupled to altered nodule H2O2 content. Further analysis of ascorbate peroxidase enzymatic activity identified three ascorbate peroxidase isoforms for which augmented enzymatic activity occurred in response to nitric oxide. Our results demonstrate that nitric oxide regulates soybean root nodule ascorbate peroxidase activity. We propose a role of nitric oxide in regulating ascorbate-dependent redox status in soybean root nodule tissue.

NO way! Is nitric oxide level in tomato regulated by a mammalian IKK/NF-κB like signaling pathway?

Nitric oxide (NO) is an essential signaling molecule in plants. However little is known about signaling pathways regulating NO levels in plants. Recently we reported a NO overproducing mutant of tomato that had extremely short roots (shr) at seedling stage. The scavenging of NO restored root elongation in the shr mutant providing us with a convenient bioassay to analyze the signaling pathway upstream of NO production. The application of previously reported pharmacological inhibitors of ubiquitin-proteasome signaling caused a drastic reduction in NO levels and restored root elongation in the mutant. Since these pharmacological inhibitors specifically inhibit mammalian IKK/NF-κB signaling, we propose that a pathway functionally similar to IKK/NF-κB pathway regulates NO levels in tomato.

Cyclic nucleotide gated channel and Ca²⁺-mediated signal transduction during plant senescence signaling

Previous studies reveal that both Ca(2+) and nitric oxide (NO) play pivotal roles in the plant senescence signaling cascade. However, not much is known about the molecular identity of the Ca(2+) entry during senescence programming and its relationship to the downstream NO signal. Our recent study shows that Arabidopsis cyclic nucleotide gated channel2 (CNGC2) contributes to Ca(2+) uptake and senescence signaling. The CNGC2 loss-of-function mutant dnd1 displays reduced Ca(2+) accumulation in leaves and a series of early senescence phenotypes compared to wild type (WT). Notably, endogenous NO content in dnd1 leaves is lower than leaves of WT. Application of an NO donor can effectively rescue a number of early senescence phenotypes found in the dnd1 plants. Current evidence supports the notion that NO functions as a negative regulator in senescence signaling and our model supports this point. In this article, we expand our discussion of CNGC2 mediated Ca(2+) uptake and other related signaling components involved in the plant senescence signaling cascade.

Mechanical wounding induces a nitrosative stress by down-regulation of GSNO reductase and an increase in S-nitrosothiols in sunflower (Helianthus annuus) seedlings

Nitric oxide (NO) and related molecules such as peroxynitrite, S-nitrosoglutathione (GSNO), and nitrotyrosine, among others, are involved in physiological processes as well in the mechanisms of response to stress conditions. In sunflower seedlings exposed to five different adverse environmental conditions (low temperature, mechanical wounding, high light intensity, continuous light, and continuous darkness), key components of the metabolism of reactive nitrogen species (RNS) and reactive oxygen species (ROS), including the enzyme activities L-arginine-dependent nitric oxide synthase (NOS), S-nitrosogluthathione reductase (GSNOR), nitrate reductase (NR), catalase, and superoxide dismutase, the content of lipid hydroperoxide, hydrogen peroxide, S-nitrosothiols (SNOs), the cellular level of NO, GSNO, and GSNOR, and protein tyrosine nitration [nitrotyrosine (NO(2)-Tyr)] were analysed. Among the stress conditions studied, mechanical wounding was the only one that caused a down-regulation of NOS and GSNOR activities, which in turn provoked an accumulation of SNOs. The analyses of the cellular content of NO, GSNO, GSNOR, and NO(2)-Tyr by confocal laser scanning microscopy confirmed these biochemical data. Therefore, it is proposed that mechanical wounding triggers the accumulation of SNOs, specifically GSNO, due to a down-regulation of GSNOR activity, while NO(2)-Tyr increases. Consequently a process of nitrosative stress is induced in sunflower seedlings and SNOs constitute a new wound signal in plants.

Both plant and bacterial nitrate reductases contribute to nitric oxide production in Medicago truncatula nitrogen-fixing nodules

Nitric oxide (NO) is a signaling and defense molecule of major importance in living organisms. In the model legume Medicago truncatula, NO production has been detected in the nitrogen fixation zone of the nodule, but the systems responsible for its synthesis are yet unknown and its role in symbiosis is far from being elucidated. In this work, using pharmacological and genetic approaches, we explored the enzymatic source of NO production in M. truncatula-Sinorhizobium meliloti nodules under normoxic and hypoxic conditions. When transferred from normoxia to hypoxia, nodule NO production was rapidly increased, indicating that NO production capacity is present in functioning nodules and may be promptly up-regulated in response to decreased oxygen availability. Contrary to roots and leaves, nodule NO production was stimulated by nitrate and nitrite and inhibited by tungstate, a nitrate reductase inhibitor. Nodules obtained with either plant nitrate reductase RNA interference double knockdown (MtNR1/2) or bacterial nitrate reductase-deficient (napA) and nitrite reductase-deficient (nirK) mutants, or both, exhibited reduced nitrate or nitrite reductase activities and NO production levels. Moreover, NO production in nodules was found to be inhibited by electron transfer chain inhibitors, and nodule energy state (ATP-ADP ratio) was significantly reduced when nodules were incubated in the presence of tungstate. Our data indicate that both plant and bacterial nitrate reductase and electron transfer chains are involved in NO synthesis. We propose the existence of a nitrate-NO respiration process in nodules that could play a role in the maintenance of the energy status required for nitrogen fixation under oxygen-limiting conditions.

MtNOA1/RIF1 modulates Medicago truncatula-Sinorhizobium meliloti nodule development without affecting its nitric oxide content

AtNoa1/Rif1 (formerly referred to as AtNos1) has been shown to modulate nitric oxide (NO) content in Arabidopsis. As NO generation in the legume-rhizobium symbiosis has been shown, the involvement of an AtNoa1/Rif1 orthologue from Medicago truncatula (MtNoa1/Rif1) during its symbiotic interaction with Sinorhizobium meliloti has been studied. The expression of MtNoa1/Rif1 appeared to occur mainly in nodule vascular bundles and the meristematic zone. Using an RNA interference strategy, transgenic roots exhibiting a significantly decreased level of MtNoa1/Rif1 expression were analysed. NO production was assessed using a fluorescent probe, and the symbiotic capacities of the composite plants upon infection with Sinorhizobium meliloti were determined. The decrease in MtNoa1/Rif1 expression level resulted in a decrease in NO production in roots, but not in symbiotic nodules, indicating a different regulation of NO synthesis in these organs. However, it significantly lowered the nodule number and the nitrogen fixation capacity of the functional nodules. Although having no influence on NO production in nodules, MtNOA1/RIF1 significantly affected the establishment and the functioning of the symbiotic interaction. The impairment of plastid functioning may explain this phenotype.

Nitric oxide synthase activity is required for development of functional nodules in soybean

The effects of a nitric oxide synthase inhibitor (N(ω)-nitro-L-arginine) on soybean growth parameters and nodule functioning were investigated, along with soybean nodule cell viability and cysteine endopeptidase activity. N(ω)-nitro-L-arginine reduced soybean growth parameters, inhibited nodule nitrogenase activity, and caused a decrease in nodule cell viability. The negative effects of N(ω)-nitro-L-arginine were reversed by the nitric oxide donor 2,2'-(hydroxynitrosohydrazono)bis-ethanimine. Cysteine endopeptidase activity was higher in plants treated with N(ω)-nitro-L-arginine than untreated plants (controls), but decreased to levels similar to the controls when plants were exposed to a combination of N(ω)-nitro-L-arginine and 2,2'-(hydroxynitrosohydrazono)bis-ethanimine. These results suggest that nitric oxide, resulting from nitric oxide synthase activity, is required for development of functional soybean nodules.

Characterization of a nitric oxide synthase from the plant kingdom: NO generation from the green alga Ostreococcus tauri is light irradiance and growth phase dependent

The search for a nitric oxide synthase (NOS) sequence in the plant kingdom yielded two sequences from the recently published genomes of two green algae species of the Ostreococcus genus, O. tauri and O. lucimarinus. In this study, we characterized the sequence, protein structure, phylogeny, biochemistry, and expression of NOS from O. tauri. The amino acid sequence of O. tauri NOS was found to be 45% similar to that of human NOS. Folding assignment methods showed that O. tauri NOS can fold as the human endothelial NOS isoform. Phylogenetic analysis revealed that O. tauri NOS clusters together with putative NOS sequences of a Synechoccocus sp strain and Physarum polycephalum. This cluster appears as an outgroup of NOS representatives from metazoa. Purified recombinant O. tauri NOS has a K(m) for the substrate l-Arg of 12 ± 5 μM. Escherichia coli cells expressing recombinant O. tauri NOS have increased levels of NO and cell viability. O. tauri cultures in the exponential growth phase produce 3-fold more NOS-dependent NO than do those in the stationary phase. In O. tauri, NO production increases in high intensity light irradiation and upon addition of l-Arg, suggesting a link between NOS activity and microalgal physiology.

Leaf senescence signaling: the Ca2+-conducting Arabidopsis cyclic nucleotide gated channel2 acts through nitric oxide to repress senescence programming

Ca(2+) and nitric oxide (NO) are essential components involved in plant senescence signaling cascades. In other signaling pathways, NO generation can be dependent on cytosolic Ca(2+). The Arabidopsis (Arabidopsis thaliana) mutant dnd1 lacks a plasma membrane-localized cation channel (CNGC2). We recently demonstrated that this channel affects plant response to pathogens through a signaling cascade involving Ca(2+) modulation of NO generation; the pathogen response phenotype of dnd1 can be complemented by application of a NO donor. At present, the interrelationship between Ca(2+) and NO generation in plant cells during leaf senescence remains unclear. Here, we use dnd1 plants to present genetic evidence consistent with the hypothesis that Ca(2+) uptake and NO production play pivotal roles in plant leaf senescence. Leaf Ca(2+) accumulation is reduced in dnd1 leaves compared to the wild type. Early senescence-associated phenotypes (such as loss of chlorophyll, expression level of senescence-associated genes, H(2)O(2) generation, lipid peroxidation, tissue necrosis, and increased salicylic acid levels) were more prominent in dnd1 leaves compared to the wild type. Application of a Ca(2+) channel blocker hastened senescence of detached wild-type leaves maintained in the dark, increasing the rate of chlorophyll loss, expression of a senescence-associated gene, and lipid peroxidation. Pharmacological manipulation of Ca(2+) signaling provides evidence consistent with genetic studies of the relationship between Ca(2+) signaling and senescence with the dnd1 mutant. Basal levels of NO in dnd1 leaf tissue were lower than that in leaves of wild-type plants. Application of a NO donor effectively rescues many dnd1 senescence-related phenotypes. Our work demonstrates that the CNGC2 channel is involved in Ca(2+) uptake during plant development beyond its role in pathogen defense response signaling. Work presented here suggests that this function of CNGC2 may impact downstream basal NO production in addition to its role (also linked to NO signaling) in pathogen defense responses and that this NO generation acts as a negative regulator during plant leaf senescence signaling.

Nitric oxide synthase-like dependent NO production enhances heme oxygenase up-regulation in ultraviolet-B-irradiated soybean plants

Heme oxygenase (HO) has antioxidant properties and is up-regulated by reactive oxygen species (ROS) in ultraviolet-B-irradiated soybean plants. This study shows that nitric oxide (NO) protects against oxidative damage and that nitric oxide synthase (NOS)-like activity is also required for HO-1 induction under UV-B radiation. Pre-treatments with sodium nitroprussiate (SNP), a NO-donor, prevented chlorophyll loss, H(2)O(2) and O(2)(*-) accumulation, and ion leakage in UV-B-treated plants. HO activity was significantly enhanced by NO and showed a positive correlation with HO-1 transcript levels. In fact, HO-1 mRNA levels were increased 2.1-fold in 0.8 mM SNP-treated plants, whereas subsequent UV-B irradiation augmented this expression up to 3.5-fold with respect to controls. This response was not observed using ferrocyanide, a SNP inactive analog, and was effectively blocked by 2-(4-carboxyphenil)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), a specific NO-scavenger. In addition, experiments carried out in the presence of N(G)-nitro-L-arginine methyl ester (L-NAME) or tungsten, well-known inhibitors of NOS and nitrate reductase, showed that NOS is the endogenous source of NO that mediates HO-1 expression. In summary, we found that NO is involved in the signaling pathway leading to HO-1 up-regulation under UV-B, and that a balance between NO and ROS is important to trigger the antioxidant response against oxidative stress.

Inhibition of the ubiquitin-proteasome pathway alters cellular levels of nitric oxide in tomato seedlings

Nitric oxide (NO) is involved in diverse plant growth processes; however, little is known about pathways regulating NO levels in plants. In this study, we isolated a NO-overproducing mutant of tomato (Solanum lycopersicum) in which hyper-accumulation of NO, associated with increase in nitric oxide synthase (NOS)-like activity, caused diminished vegetative growth of plants and showed delayed flowering. The hyper-accumulation of NO caused drastic shortening of primary root (shr) in the seedlings, while the scavenging of NO restored root elongation in shr mutant. Inhibition of NOS-like activity reduced NO levels and stimulated root elongation in the shr mutant seedlings, while inhibition of nitrate reductase (NR) activity could not rescue shr phenotype. The stimulation of NO levels in shr mutant also conferred increased resistance to pathogen Pseudomonas syringae. Application of pharmacological inhibitors regulating ubiquitin-proteasome pathway reduced NO levels and NOS-like activity and stimulated shr root elongation. Our data indicate that a signaling pathway involving regulated protein degradation likely regulates NO synthesis in tomato.

New insights into the mitochondrial nitric oxide production pathways

Considerable evidence has appeared over the past few years that nitric oxide (NO) is an important anoxic metabolite and a potent signal molecule in plants. Several pathways operative in different cell compartments, lead to NO production. Mitochondria, being a major NO producing compartment, can generate it by either nitrite reduction occurring at nearly anoxic conditions or by the oxidative route via nitric oxide synthase (NOS). Recently we compared both pathways by ozone collision chemiluminescence and by DAF fluorescence. We found that nitrite reduction to NO is associated with the mitochondrial membrane fraction but not with the matrix. In case of the nitric oxide synthase pathway, an L-arginine dependent fluorescence was detected but its response to NOS inhibitors and substrates was untypical. Therefore the existence of NOS or NOS-like activity in barley root mitochondria is very doubtful. We also found that mitochondria scavenge NO. In addition, we found indirect evidence that mitochondria are able to convert NO to gaseous intermediates like NO2, N2O and N2O3.

Reactive oxygen species and nitric oxide in plant mitochondria: origin and redundant regulatory systems

Plant mitochondria differ from their mammalian counterparts in many respects, which are due to the unique and variable surroundings of plant mitochondria. In green leaves, plant mitochondria are surrounded by ample respiratory substrates and abundant molecular oxygen, both resulting from active photosynthesis, while in roots and bulky rhizomes and fruit carbohydrates may be plenty, whereas oxygen levels are falling. Several enzymatic complexes in mitochondrial electron transport chain (ETC) are capable of reactive oxygen species (ROS) formation under physiological and pathological conditions. Inherently connected parameters such as the redox state of electron carriers in the ETC, ATP synthase activity and inner mitochondrial membrane potential, when affected by external stimuli, can give rise to ROS formation via complexes I and III, and by reverse electron transport (RET) from complex II. Superoxide radicals produced are quickly scavenged by superoxide dismutase (MnSOD), and the resulting H(2)O(2) is detoxified by peroxiredoxin-thioredoxin system or by the enzymes of ascorbate-glutathione cycle, found in the mitochondrial matrix. Arginine-dependent nitric oxide (NO)-releasing activity of enzymatic origin has been detected in plant mitochondria. The molecular identity of the enzyme is not clear but the involvement of mitochondria-localized enzymes responsible for arginine catabolism, arginase and ornithine aminotransferase has been shown in the regulation of NO efflux. Besides direct control by antioxidants, mitochondrial ROS production is tightly controlled by multiple redundant systems affecting inner membrane potential: NAD(P)H-dependent dehydrogenases, alternative oxidase (AOX), uncoupling proteins, ATP-sensitive K(+) channel and a number of matrix and intermembrane enzymes capable of direct electron donation to ETC. NO removal, on the other hand, takes place either by reactions with molecular oxygen or superoxide resulting in peroxynitrite, nitrite or nitrate ions or through interaction with non-symbiotic hemoglobins or glutathione. Mitochondrial ROS and NO production is tightly controlled by multiple redundant systems providing the regulatory mechanism for redox homeostasis and specific ROS/NO signaling.

Production and scavenging of nitric oxide by barley root mitochondria

We examined whether root mitochondria and mitochondrial membranes produce nitric oxide (NO) exclusively by reduction of nitrite or also via a nitric oxide synthase (NOS), and to what extent direct NO measurements could become undetectable due to NO oxidation. Chemiluminescence detection of NO in the gas phase was used to monitor NO emission from suspensions (i.e. direct chemiluminescence). For comparison, diaminofluorescein (DAF) and diaminorhodamine (DAR) were used as NO indicators. NO oxidation to nitrite and nitrate was quantified after reduction of nitrite + nitrate to NO by vandium (III) with subsequent chemiluminescence detection (i.e. indirect chemiluminescence). Nitrite and NADH consumption were also measured. Anaerobic nitrite-dependent NO emission was exclusively associated with the membrane fraction, without participation of matrix components. Rates of nitrite and NADH consumption matched, whereas the rate of NO emission was lower. In air, mitochondria apparently produced no nitrite-dependent NO, and no NOS activity was detected by direct or indirect chemiluminescence. In contrast, with DAF-2 or DAR-4M, an l-arginine-dependent fluorescence increase took place. However, the response of this apparent low NOS activity to inhibitors, substrates and cofactors was untypical when compared with commercial inducible NOS (iNOS), and the existence of NOS in root mitochondria is therefore doubtful. In a solution of commercial iNOS, about two-thirds of the NO (measured as NADPH consumption) were oxidized to nitrite + nitrate. Addition of mitochondria to iNOS decreased the apparent NO emission, but without a concomitant increase in nitrite + nitrate formation. Thus, mitochondria appeared to accelerate oxidation of NO to volatile intermediates.

The redox switch: dynamic regulation of protein function by cysteine modifications

Reactive oxygen intermediates (ROIs) and reactive nitrogen intermediates (RNIs) have now become well established as important signalling molecules in physiological settings within microorganisms, mammals and plants. These intermediates are routinely synthesised in a highly controlled and transient fashion by NADPH-dependent enzymes, which constitute key regulators of redox signalling. Mild oxidants such as hydrogen peroxide (H(2)O(2)) and especially nitric oxide (NO) signal through chemical reactions with specific atoms of target proteins that result in covalent protein modifications. Specifically, highly reactive cysteine (Cys) residues of low pK(a) are a major site of action for these intermediates. The oxidation of target Cys residues can result in a number of distinct redox-based, post-translational modifications including S-nitrosylation, S-glutathionylation; and sulphenic acid, sulphinic acid and disulphide formation. Importantly, such modifications precisely regulate protein structure and function. Cys-based redox switches are now increasingly being found to underpin many different signalling systems and regulate physiological outputs across kingdoms.

Cellular localization of ROS and NO in olive reproductive tissues during flower development

BACKGROUND

Recent studies have shown that reactive oxygen species (ROS) and nitric oxide (NO) are involved in the signalling processes taking place during the interactions pollen-pistil in several plants. The olive tree (Olea europaea L.) is an important crop in Mediterranean countries. It is a dicotyledonous species, with a certain level of self-incompatibility, fertilisation preferentially allogamous, and with an incompatibility system of the gametophytic type not well determined yet. The purpose of the present study was to determine whether relevant ROS and NO are present in the stigmatic surface and other reproductive tissues in the olive over different key developmental stages of the reproductive process. This is a first approach to find out the putative function of these signalling molecules in the regulation of the interaction pollen-stigma.

RESULTS

The presence of ROS and NO was analyzed in the olive floral organs throughout five developmental stages by using histochemical analysis at light microscopy, as well as different fluorochromes, ROS and NO scavengers and a NO donor by confocal laser scanning microscopy. The "green bud" stage and the period including the end of the "recently opened flower" and the "dehiscent anther" stages displayed higher concentrations of the mentioned chemical species. The stigmatic surface (particularly the papillae and the stigma exudate), the anther tissues and the pollen grains and pollen tubes were the tissues accumulating most ROS and NO. The mature pollen grains emitted NO through the apertural regions and the pollen tubes. In contrast, none of these species were detected in the style or the ovary.

CONCLUSION

The results obtained clearly demonstrate that both ROS and NO are produced in the olive reproductive organs in a stage- and tissue- specific manner. The biological significance of the presence of these products may differ between early flowering stages (defence functions) and stages where there is an intense interaction between pollen and pistil which may determine the presence of a receptive phase in the stigma. The study confirms the enhanced production of NO by pollen grains and tubes during the receptive phase, and the decrease in the presence of ROS when NO is actively produced.

Peroxisomes are required for in vivo nitric oxide accumulation in the cytosol following salinity stress of Arabidopsis plants

Peroxisomes are unique organelles involved in multiple cellular metabolic pathways. Nitric oxide (NO) is a free radical active in many physiological functions under normal and stress conditions. Using Arabidopsis (Arabidopsis thaliana) wild type and mutants expressing green fluorescent protein through the addition of peroxisomal targeting signal 1 (PTS1), which enables peroxisomes to be visualized in vivo, this study analyzes the temporal and cell distribution of NO during the development of 3-, 5-, 8-, and 11-d-old Arabidopsis seedlings and shows that Arabidopsis peroxisomes accumulate NO in vivo. Pharmacological analyses using nitric oxide synthase (NOS) inhibitors detected the presence of putative calcium-dependent NOS activity. Furthermore, peroxins Pex12 and Pex13 appear to be involved in transporting the putative NOS protein to peroxisomes, since pex12 and pex13 mutants, which are defective in PTS1- and PTS2-dependent protein transport to peroxisomes, registered lower NO content. Additionally, we show that under salinity stress (100 mM NaCl), peroxisomes are required for NO accumulation in the cytosol, thereby participating in the generation of peroxynitrite (ONOO(-)) and in increasing protein tyrosine nitration, which is a marker of nitrosative stress.