Nitric oxide in plants. To NO or not to NO

The current knowledge on the occurrence and activity of NO in plants is reviewed. The multiplicity of nitrogen monoxide species and implications for differentiated reactivity are indicated. Possible sources of NO are evaluated, and the evidence for the presence of nitric oxide synthase in plants is summarised. The regulatory role of NO. in plant development and in plant interactions with microorganisms, involving an interplay with other molecules, like ethylene or reactive oxygen species is demonstrated. Finally, some other suggestions on potential functions of NO. in plants are indicated.     

Nitric Oxide Signaling in Plant Responses to Abiotic Stresses

Nitric oxide (NO) plays important roles in diverse physiological processes In plants. NO can provoke both beneficial and harmful effects, which depend on the concentration and location of NO in plant cells. This review is focused on NO synthesis and the functions of NO in plant responses to abiotic environmental stresses. Abiotic stresses mostly induce NO production in plants. NO alleviates the harmfulness of reactive oxygen species, and reacts with other target molecules, and regulates the expression of stress responsive genes under various stress conditions.     

The role of nitric oxide in the maintenance of vasoactive balance

Endothelial dysfunction may be considered as the interstage between risk factors and cardiovascular pathology. An imbalance between the production of vasorelaxing and vasoconstricting factors plays a decisive role in the development of hypertension, atherosclerosis and target organ damage. Except vasorelaxing and antiproliferative properties per se, nitric oxide participates in antagonizing vasoconstrictive and growth promoting effects of angiotensin II, endothelins and reactive oxygen species. Angiotensin II is a potent activator of NAD(P)H oxidase contributing to the production of reactive oxygen species. Numerous signaling pathways activated in response to angiotensin II and endothelin-1 are mediated through the increased level of oxidative stress, which seems to be in casual relation to a number of cardiovascular disturbances including hypertension. With respect to the oxidative stress, the NO molecule seems to be of ambivalent nature. On the one hand, NO is able to reduce generation of reactive oxygen species by inhibiting association of NAD(P)H oxidase subunits. On the other hand, when excessively produced, NO reacts with superoxides resulting in the formation of peroxynitrite, which is a free radical deteriorating endothelial function. The balance between vasorelaxing and vasoconstricting substances appears to be the principal issue for the physiological functioning of the vascular bed.     

Nitric Oxide Signaling in Plant Responses to Abiotic Stresses

Nitric oxide (NO) plays important roles in diverse physiological processes In plants. NO can provoke both beneficial and harmful effects, which depend on the concentration and location of NO in plant cells. This review is focused on NO synthesis and the functions of NO in plant responses to abiotic environmental stresses. Abiotic stresses mostly induce NO production in plants. NO alleviates the harmfulness of reactive oxygen species, and reacts with other target molecules, and regulates the expression of stress responsive genes under various stress conditions.     

Interplay between ferritin metabolism, reactive oxygen species and nitric oxide

 The biological relevance of each of the three inorganic species – iron, oxygen, and nitric oxide (NO) – is crucial. Moreover, their metabolic pathways cross each other and thus create a complex network of connections responsible for the regulation of many essential biological processes. The iron storage protein ferritin, one of the main regulators of iron homeostasis, influences oxygen and NO metabolism. Here, examples are given of the biological interactions of the ferritin molecule (ferritin iron and ferritin shell) with reactive oxygen species (ROS) and NO. The focus is the regulation of ferritin expression by ROS and NO. From these data, ferritin emerges as an important cytoprotective component of the cellular response to ROS and NO. Also, by its ability to alter the amount of intracellular "free" iron, ferritin may affect the metabolism of ROS and NO. It is proposed that this putative activity of ferritin may constitute a missing link in the regulatory loop between iron, ROS, and NO. Received: 2 January 1997 / Accepted: 9 June 1997     

Cell signalling following plant/pathogen interactions involves the generation of reactive oxygen and reactive nitrogen species

It is now clear that reactive oxygen species (ROS), such as hydrogen peroxide (H₂O₂), and reactive nitrogen species, such as nitric oxide (NO), are produced by plant cells in response to a variety of stresses, including pathogen challenge. Such molecules may be involved in direct defence mechanisms, such as cross-linking of plant cell walls, or as antimicrobial agents. However, it is also apparent that cells generate such reactive species as signalling molecules, produced at controlled levels, and leading to defined responses. Signalling responses to ROS and NO include the activation of mitogen-activated protein kinases, and the up- and down-regulation of gene expression, often leading to localised programmed cell death, characteristic of the hypersensitive response. Therefore, ROS and NO are key molecules which may help to orchestrate events following pathogen challenge. Here we review the generation and role of both reactive oxygen and reactive nitrogen species in plant cells.     

Protection of lung tissue against ischemia/reperfusion injury by preconditioning with inhaled nitric oxide in an in situ pig model of normothermic pulmonary ischemia

Topical administration of nitric oxide (NO) by inhalation is currently used as therapy in various pulmonary diseases, but preconditioning with NO to ameliorate lung ischemia/reperfusion (I/R) injury has not been fully evaluated. In this study, we investigated the effects of NO inhalation on functional pulmonary parameters using an in situ porcine model of normothermic pulmonary ischemia. After left lateral thoracotomy, left lung ischemia was maintained for 90 min, followed by a 5h reperfusion period (group I, n = 7). In group II (n = 6), I/R was preceded by inhalation of NO (10 min, 15 ppm). Animals in group III (n = 7) underwent sham surgery without NO inhalation or ischemia. In order to evaluate the effects of NO preconditioning, lung functional and hemodynamic parameters were measured, and the zymosan-stimulated release of reactive oxygen species in arterial blood was determined. Animals in group I developed significant pulmonary I/R injury, including pulmonary hypertension, a decreased pO(2) level in pulmonary venous blood of the ischemic lung, and a significant increase of the stimulated release of reactive oxygen species. All these effects were prevented, or the onset (release of reactive oxygen species) was delayed, by NO inhalation. These results indicate that preconditioning by NO inhalation before lung ischemia is protective against I/R injury in the porcine lung.     

Production of nitric oxide-derived reactive nitrogen species in human oral cavity and their scavenging by salivary redox components

Nitrite is reduced to nitric oxide (NO) in the oral cavity. The NO generated can react with molecular oxygen producing reactive nitrogen species. In this study, reduction of nitrite to NO was observed in bacterial fractions of saliva and whole saliva. Formation of reactive nitrogen species from NO was detected by measuring the transformation of 4,5-diaminofluorescein (DAF-2) to triazolfluorescein (DAF-2T). The transformation was fast in bacterial fractions but slow in whole saliva. Salivary components such as ascorbate, glutathione, uric acid and thiocyanate inhibited the transformation of DAF-2 to DAF-2T in bacterial fractions without affecting nitrite-dependent NO production. The inhibition was deduced to be due to scavenging of reactive nitrogen species, which were formed from NO, by the above reagents. The transformation of DAF-2 to DAF-2T was faster in bacterial fractions and whole saliva which were prepared 1-4 h after tooth brushing than those prepared immediately after toothbrushing. Increase in the rate as a function of time after toothbrushing seemed to be due to the increase in population of bacteria which could reduce nitrite to NO. The results obtained in this study suggest that reactive nitrogen species derived from NO are continuously formed in the oral cavity and that the reactive nitrogen species are effectively scavenged by salivary redox components in saliva but the scavenging is not complete.     

Production of nitric oxide-derived reactive nitrogen species in human oral cavity and their scavenging by salivary redox components

Nitrite is reduced to nitric oxide (NO) in the oral cavity. The NO generated can react with molecular oxygen producing reactive nitrogen species. In this study, reduction of nitrite to NO was observed in bacterial fractions of saliva and whole saliva. Formation of reactive nitrogen species from NO was detected by measuring the transformation of 4,5-diaminofluorescein (DAF-2) to triazolfluorescein (DAF-2T). The transformation was fast in bacterial fractions but slow in whole saliva. Salivary components such as ascorbate, glutathione, uric acid and thiocyanate inhibited the transformation of DAF-2 to DAF-2T in bacterial fractions without affecting nitrite-dependent NO production. The inhibition was deduced to be due to scavenging of reactive nitrogen species, which were formed from NO, by the above reagents. The transformation of DAF-2 to DAF-2T was faster in bacterial fractions and whole saliva which were prepared 1–4?h after tooth brushing than those prepared immediately after toothbrushing. Increase in the rate as a function of time after toothbrushing seemed to be due to the increase in population of bacteria which could reduce nitrite to NO. The results obtained in this study suggest that reactive nitrogen species derived from NO are continuously formed in the oral cavity and that the reactive nitrogen species are effectively scavenged by salivary redox components in saliva but the scavenging is not complete.     

The effect of different antioxidants on nitric oxide production in hypertensive rats

The imbalance between nitric oxide (NO) and reactive oxygen species (ROS) production appears to be a common feature of experimental and human hypertension. Previously, different antioxidants and/or scavengers of oxygen free radicals were shown to activate nitric oxide synthase (NO synthase, NOS) and to increase the expression of both endothelial and neuronal NO synthase isoforms leading to blood pressure reduction. On the other hand, various antihypertensive drugs have been documented to possess antioxidant properties, which may contribute to their beneficial effect on blood pressure. This review is focused on the effects of antioxidant treatment in different models of experimental hypertension with a special attention to the prevention of oxidative damage and the augmentation of NO synthase activity and expression of NOS isoforms.     

植物细胞一氧化氮信号转导研究进展

一氧化氮(nitric oxide, NO)作为重要的信号分子, 调控植物的种子萌发、根形态建成和花器官发生等许多生长发育过程, 并参与气孔运动的调节以及植物对多种非生物胁迫和病原体侵染的应答过程。已经知道, 精氨酸依赖的NOS途径和亚硝酸盐依赖的NR途径是植物细胞NO产生的主要酶促合成途径。NO及其衍生物能够直接修饰底物蛋白的金属基团、半胱氨酸和酪氨酸残基, 通过金属亚硝基化、巯基亚硝基化和Tyr-硝基化等化学修饰方式, 调节靶蛋白的活性, 并影响cGMP和Ca²⁺信使系统等下游信号途径, 调控相应的生理过程。最新的一些研究结果也显示, MAPK级联系统与NO信号转导途径之间存在复杂的交叉调控。此外, 作为活跃的小分子信号, NO和活性氧相互依赖并相互影响, 共同介导了植物的胁迫应答和激素响应过程。文章综述了植物NO信号转导研究领域中一些新的研究进展, 对NO与活性氧信号途径间的交叉作用等也作了简要介绍。     

To NO or not to NO: 'where?' is the question

Nitric oxide (NO) is a gaseous radical with unique biological functions essential for the cardiovascular system, host defense and neuro-transmission. For two decades it was thought that NO was able to diffuse freely across relatively long distances and to traverse major parts of the cell, if not multiple cell layers. However, NO has been proven to be extremely reactive: it reacts with other reactive oxygen species, heavy metals, as well as with cysteine and tyrosine residues in proteins. In accordance, it is now widely accepted that once NO is generated, it is very short-lived and diffuses only over a short distance. This urges for the local production of NO and the localization of NO synthases in the proximity of their downstream targets. This review discusses the highly organized localization of NO synthases, with the endothelial isoform (eNOS) as its main focus, since from this synthase most is known about its subcellular localization and regulation.     

活性氮、活性氧及植物激素在种子休眠解除中的作用及相互关系研究进展

休眠是植物种子对环境变化的适应机制,其机理至今未完全清楚阐明。前期对种子休眠机制的研究主要集中在激素调节上,近期的研究结果表明,一氧化氮（nitric oxide,NO）参与打破种子的休眠,并与其所引起的种子中活性氧的变化有关。本文简要综述活性氮（reactive nitrogen species,RNS）、活性氧（reactive oxygen species,R0s）和植物激素在种子休眠解除中的作用及相互关系研究进展。     

Critical O2 and NO concentrations in NO-induced cell death in a rat liver sinusoidal endothelial cell line

Nitric oxide (NO) plus oxygen (O2) are known to cause cell damage via formation of reactive nitrogen species. NO itself directly inhibits cytochrome oxidase of the mitochondrial respiratory chain in competition with O2, thus inducing a hypoxic-like injury. To assess the critical NO and O2 concentrations for both mechanisms of NO-induced cell injury, cells of a rat liver sinusoidal endothelial cell line were incubated in the presence of the NO donor spermineNONOate at different O2 concentrations, and their loss of viability was determined by the release of lactate dehydrogenase. Protection by ascorbic acid was used as indication for the involvement of reactive nitrogen species, whereas a hypoxic-like injury was indicated by the protective effects of glycine and glucose and the increase in NAD(P)H fluorescence. High concentrations of NO (approx. 10 microM NO) and O2 (21% O2) were required to induce endothelial cell death mediated by formation of reactive nitrogen species. On the other hand, pathophysiologically relevant NO concentrations at low but physiological O2 concentrations (ca. 2 microM NO at 5% O2 and about 1 microM NO at 2% O2) induced hypoxic-like cell death in the endothelial cells that was prevented by the presence of glucose.     

Free radical biology and medicine: it's a gas, man!

We review gases that can affect oxidative stress and that themselves may be radicals. We discuss O(2) toxicity, invoking superoxide, hydrogen peroxide, and the hydroxyl radical. We also discuss superoxide dismutase (SOD) and both ground-state, triplet oxygen ((3)O(2)), and the more energetic, reactive singlet oxygen ((1)O(2)). Nitric oxide ((*)NO) is a free radical with cell signaling functions. Besides its role as a vasorelaxant, (*)NO and related species have other functions. Other endogenously produced gases include carbon monoxide (CO), carbon dioxide (CO(2)), and hydrogen sulfide (H(2)S). Like (*)NO, these species impact free radical biochemistry. The coordinated regulation of these species suggests that they all are used in cell signaling. Nitric oxide, nitrogen dioxide, and the carbonate radical (CO(3)(*-)) react selectively at moderate rates with nonradicals, but react fast with a second radical. These reactions establish "cross talk" between reactive oxygen (ROS) and reactive nitrogen species (RNS). Some of these species can react to produce nitrated proteins and nitrolipids. It has been suggested that ozone is formed in vivo. However, the biomarkers that were used to probe for ozone reactions may be formed by non-ozone-dependent reactions. We discuss this fascinating problem in the section on ozone. Very low levels of ROS or RNS may be mitogenic, but very high levels cause an oxidative stress that can result in growth arrest (transient or permanent), apoptosis, or necrosis. Between these extremes, many of the gasses discussed in this review will induce transient adaptive responses in gene expression that enable cells and tissues to survive. Such adaptive mechanisms are thought to be of evolutionary importance.     

Two faces of nitric oxide: implications for cellular mechanisms of oxygen toxicity

Recent investigations have elucidated some of the diverse roles played by reactive oxygen and nitrogen species in events that lead to oxygen toxicity and defend against it. The focus of this review is on toxic and protective mechanisms in hyperoxia that have been investigated in our laboratories, with an emphasis on interactions of nitric oxide (NO) with other endogenous chemical species and with different physiological systems. It is now emerging from these studies that the anatomical localization of NO release, which depends, in part, on whether the oxygen exposure is normobaric or hyperbaric, strongly influences whether toxicity emerges and what form it takes, for example, acute lung injury, central nervous system excitation, or both. Spatial effects also contribute to differences in the susceptibility of different cells in organs at risk from hyperoxia, especially in the brain and lungs. As additional nodes are identified in this interactive network of toxic and protective responses, future advances may open up the possibility of novel pharmacological interventions to extend both the time and partial pressures of oxygen exposures that can be safely tolerated. The implications of a better understanding of the mechanisms by which NO contributes to central nervous system oxygen toxicity may include new insights into the pathogenesis of seizures of diverse etiologies. Likewise, improved knowledge of NO-based mechanisms of pulmonary oxygen toxicity may enhance our understanding of other types of lung injury associated with oxidative or nitrosative stress.     

植物细胞一氧化氮信号转导研究进展

一氧化氮（nitric oxide,NO）作为重要的信号分子,调控植物的种子萌发、根形态建成和花器官发生等许多生长发育过程,并参与气孔运动的调节以及植物对多种非生物胁迫和病原体侵染的应答过程。已经知道,精氨酸依赖的NOS途径和亚硝酸盐依赖的NR途径是植物细胞NO产生的主要酶促合成途径。NO及其衍生物能够直接修饰底物蛋白的金属基团、半胱氨酸和酪氨酸残基,通过金属亚硝基化、巯基亚硝基化和Tyr．硝基化等化学修饰方式,调节靶蛋白的活性,并影响cGMP和Ca2＋信使系统等下游信号途径,调控相应的生理过程。最新的一些研究结果也显示,MAPK级联系统与NO信号转导途径之间存在复杂的交叉调控。此外,作为活跃的小分子信号,NO和活性氧相互依赖并相互影响,共同介导了植物的胁迫应答和激素响应过程。文章综述了植物NO信号转导研究领域中一些新的研究进展,对NO与活性氧信号途径间的交叉作用等也作了简要介绍。     

Reactive Oxygen and Nitrogen Species Regulate Mitochondrial Ca2+ Homeostasis and Respiration

The reduction of molecular oxygen to water provides most of the biologically useful energy. However, oxygen reduction is a mixed blessing because incompletely reduced oxygen species such as superoxide or peroxides are quite reactive and can, when out of control, cause damage. In mitochondria, where most of the oxygen utilized by eukaryotic cells is reduced, the dichotomy of oxygen shows itself best. Thus, reactive oxygen is a threat to them, as is evident from oxidative damage to mitochondrial lipids, proteins, and nucleic acids. Reactive oxygen, in the form of peroxides, also serves useful functions in mitochondria. This is exemplified by the control of mitochondrial and cellular calcium homeostasis, whose understanding has improved greatly during the last few years. An exciting new aspect is the discovery that nitric oxide and congeners have an enormous impact on mitochondria. Physiological concentrations of nitrogen monoxide (NO) at physiological cellular oxygen pressure inhibit cytochrome oxidase and thereby respiration. A transient inhibition of cytochrome oxidase by NO appears to be used in at least some forms of cell signalling. Peroxynitrite, the product of the reaction between superoxide and NO, can stimulate the specific calcium release pathway from mitochondria by oxidizing some vicinal thiols in mitochondria. There is evidence mounting that mitochondrial calcium handling and its modulation by reactive oxygen and nitrogen species is important for necrotic and apoptotic cell death.