Inhibitory effects of nitric oxide on oxidative phosphorylation in plant mitochondria

Physiological Role of Aerobic Fermentation Constitutively Expressed in an Aluminum-Tolerant Cell Line of Tobacco (Nicotiana tabacum)

Aluminum (Al)-tolerant tobacco cell line ALT301 derived from SL (wild-type) hardly exhibits Al-triggered reactive oxygen species (ROS) compared with SL. Molecular mechanism leading to this phenotype was investigated comparatively with SL. Under normal growth condition, metabolome data suggested the activation of glycolysis and lactate fermentation but the repression of the tricarboxylic acid (TCA) cycle in ALT301, namely aerobic fermentation, which seemed to be transcriptionally controlled partly by higher expression of genes encoding lactate dehydrogenase and pyruvate dehydrogenase kinase. Microarray and gene ontology analyses revealed the upregulation of the gene encoding related to APETALA2.3 (RAP2.3)-like protein, one of the group VII ethylene response factors (ERFVIIs), in ALT301. ERFVII transcription factors are known to be key regulators for hypoxia response that promotes substrate-level ATP production by glycolysis and fermentation. ERFVIIs are degraded under normoxia by the N-end rule pathway of proteolysis depending on both oxygen and nitric oxide (NO), and NO is produced mainly by nitrate reductase (NR) in plants. In ALT301, levels of the NR gene expression (NIA2), NR activity and NO production were all lower compared with SL. Consistently, the known effects of NO on respiratory pathways were also repressed in ALT301. Under Al-treatment condition, NO level increased in both lines but was lower in ALT301. These results suggest that the upregulation of the RAP2.3-like gene and the downregulation of the NIA2 gene and resultant NO depletion in ALT301 coordinately enhance aerobic fermentation, which seems to be related to a higher capacity to prevent ROS production in mitochondria under Al stress.

The intracellular ROS accumulation in elicitor-induced immunity requires the multiple organelle-targeted Arabidopsis NPK1-related protein kinases

Recognition at the plasma membrane of danger signals (elicitors) belonging to the classes of the microbe/pathogen- and damage-associated molecular patterns is a key event in pathogen sensing by plants and is associated with a rapid activation of immune responses. Different cellular compartments, including plasma membrane, chloroplasts, nuclei and mitochondria, are involved in the immune cellular program. However, how pathogen sensing is transmitted throughout the cell remains largely to be uncovered. Arabidopsis NPK1-related Proteins (ANPs) are mitogen-activated protein kinase kinase kinases previously shown to have a role in immunity. In this article, we studied the in vivo intracellular dynamics of ANP1- and ANP3-GFP fusions and found that under basal physiological conditions both proteins are present in the cytosol, while ANP3 is also localized in mitochondria. After elicitor perception, both proteins are present also in the plastids and nuclei, revealing a localization pattern that is so far unique. The N-terminal region of the protein kinases is responsible for their localization in mitochondria and plastids. Moreover, we found that the localization of ANPs coincides with the sites of elicitor-induced ROS accumulation and that plants lacking ANP function do not accumulate intracellular ROS.

Hydrogen sulfide (H2S) signaling in plant development and stress responses

ABSTRACT

Hydrogen sulfide (H₂S) was initially recognized as a toxic gas and its biological functions in mammalian cells have been gradually discovered during the past decades. In the latest decade, numerous studies have revealed that H₂S has versatile functions in plants as well. In this review, we summarize H₂S-mediated sulfur metabolic pathways, as well as the progress in the recognition of its biological functions in plant growth and development, particularly its physiological functions in biotic and abiotic stress responses. Besides direct chemical reactions, nitric oxide (NO) and hydrogen peroxide (H₂O₂) have complex relationships with H₂S in plant signaling, both of which mediate protein post-translational modification (PTM) to attack the cysteine residues. We also discuss recent progress in the research on the three types of PTMs and their biological functions in plants. Finally, we propose the relevant issues that need to be addressed in the future research.

GRAPHIC ABSTRACT

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s42994-021-00035-4.

Boletus aereus protects against acute alcohol-induced liver damage in the C57BL/6 mouse via regulating the oxidative stress-mediated NF-κB pathway

CONTEXT

Alcoholic liver disease, caused by abuse and consumption of alcohol, exhibits high morbidity and mortality. Boletus aereus Bull. (Boletaceae) (BA) shows antioxidant, anti-inflammatory and antimicrobial effects.

OBJECTIVES

To investigate the hepatoprotective effects of BA using an acute alcohol-induced hepatotoxicity mice model.

MATERIALS AND METHODS

The composition of BA fruit body was first systematically analyzed. Subsequently, a C57BL/6 mice model of acute alcohol-induced liver injury was established by intragastrically administration of alcohol, which was intragastrically received with BA powder at 200 mg/kg and 800 mg/kg for 2 weeks, 60 mg/kg silybin treatment was used as positive control group. By employing the pathological examination, ELISA, RT-PCR and western blot, the regulation of BA on oxidative stress signals was investigated.

RESULTS

The LD₅₀ of BA was much higher than 4 g/kg/p.o. In acute alcohol-damaged mice, BA reduced the levels of alanine aminotransferase (>18.3%) and aspartate aminotransferase (>27.6%) in liver, increased the activity of liver alcohol dehydrogenase (>35.0%) and serum acetaldehyde dehydrogenase (>18.9%). BA increased the activity of superoxide dismutase (>13.4%), glutathione peroxidase (>11.0%) and 800 mg/kg BA strongly reduced chemokine (C-X-C motif) ligand 13 (14.9%) and chitinase-3 like-1 protein (13.4%) in serum. BA reversed mRNA over-expression (>70%) and phosphor-stimulated expression (>45.0%) of an inhibitor of nuclear factor κ-B kinase (NF-κB, an inhibitor of nuclear factor κ-B α and nuclear factor κ-B in the liver.

CONCLUSIONS

BA is effective in ameliorating alcohol-induced liver injury through regulating oxidative stress-mediated NF-κB signalling, which provides a scientific basis for further research on its clinical applications.

Differential Roles of the mTOR-STAT3 Signaling in Dermal γδ T Cell Effector Function in Skin Inflammation

Dermal γδT cells play critical roles in skin homeostasis and inflammation. However, the underlying molecular mechanisms by which these cells are activated have not been fully understood. Here, we show that the mechanistic or mammalian target of rapamycin (mTOR) and STAT3 pathways are activated in dermal γδT cells in response to innate stimuli such as interleukin-1β (IL-1β) and IL-23. Although both mTOR complex 1 (mTORC1) and mTORC2 are essential for dermal γδT cell proliferation, mTORC2 deficiency leads to decreased dermal γδT17 cells. It appears that mitochondria-mediated oxidative phosphorylation is critical in this process. Notably, although the STAT3 pathway is critical for dermal Vγ4T17 effector function, it is not required for γδ6T17 cells. Transcription factor IRF-4 activation promotes dermal γδT cell IL-17 production by linking IL-1β and IL-23 signaling. The absence of mTORC2 in dermal γδT cells, but not STAT3, ameliorates skin inflammation. Taken together, our results demonstrate that the mTOR-STAT3 signaling differentially regulates dermal γδT cell effector function in skin inflammation.

Cai et al. demonstrate that the mTOR and STAT3 signaling pathways differentially regulate dermal Vγ4 and Vγ6 T cell effector function, leading to distinct outcomes in skin inflammation.

Stress signalling dynamics of the mitochondrial electron transport chain and oxidative phosphorylation system in higher plants

BACKGROUND

Mitochondria play a diversity of physiological and metabolic roles under conditions of abiotic or biotic stress. They may be directly subjected to physico-chemical constraints, and they are also involved in integrative responses to environmental stresses through their central position in cell nutrition, respiration, energy balance and biosyntheses. In plant cells, mitochondria present various biochemical peculiarities, such as cyanide-insensitive alternative respiration, and, besides integration with ubiquitous eukaryotic compartments, their functioning must be coupled with plastid functioning. Moreover, given the sessile lifestyle of plants, their relative lack of protective barriers and present threats of climate change, the plant cell is an attractive model to understand the mechanisms of stress/organelle/cell integration in the context of environmental stress responses.

SCOPE

The involvement of mitochondria in this integration entails a complex network of signalling, which has not been fully elucidated, because of the great diversity of mitochondrial constituents (metabolites, reactive molecular species and structural and regulatory biomolecules) that are linked to stress signalling pathways. The present review analyses the complexity of stress signalling connexions that are related to the mitochondrial electron transport chain and oxidative phosphorylation system, and how they can be involved in stress perception and transduction, signal amplification or cell stress response modulation.

CONCLUSIONS

Plant mitochondria are endowed with a diversity of multi-directional hubs of stress signalling that lead to regulatory loops and regulatory rheostats, whose functioning can amplify and diversify some signals or, conversely, dampen and reduce other signals. Involvement in a wide range of abiotic and biotic responses also implies that mitochondrial stress signalling could result in synergistic or conflicting outcomes during acclimation to multiple and complex stresses, such as those arising from climate change.

Photosynthesis is sensitive to nitric oxide and respiration sensitive to hydrogen peroxide: Studies with pea mesophyll protoplasts

Reports on the effect of nitric oxide (NO) or reactive oxygen species (ROS) on photosynthesis and respiration in leaf tissues are intriguing; therefore, the effects of exogenous addition of sodium nitroprusside (SNP, releases NO) or H2O2 on the photosynthetic O2 evolution and respiratory O2 uptake by mesophyll protoplasts in pea (Pisum sativum) were evaluated in the present study. Low concentrations of SNP or H2O2 were used to minimize nonspecific effects. The effects of NO or H2O2 on respiration and photosynthesis were different. The presence of NO decreased the rate of photosynthesis but caused a marginal stimulation of dark respiration. Conversely, externally administered H2O2 drastically decreased the rate of respiration but only slightly decreased photosynthesis. The PS I activity was more sensitive to NO than PS II. On the other hand, 100 μM H2O2 had no effect on the photochemical reactions of either PS I or PS II. The sensitivity of photosynthesis to antimycin A or SHAM (reflecting the interplay between chloroplasts and mitochondria) was not affected by NO. By contrast, H2O2 markedly decreased the sensitivity of photosynthesis to antimycin A and SHAM. It can be concluded that chloroplasts are the primary targets of NO, while mitochondria are the primary targets of ROS in plant cells. We propose that H2O2 can be an important signal to modulate the crosstalk between chloroplasts and mitochondria.

Inonotus obliquus polysaccharide ameliorates impaired reproductive function caused by Toxoplasma gondii infection in male mice via regulating Nrf2-PI3K/AKT pathway

This study was carried out to investigate the effects of Inonotus obliquus polysaccharide (IOP) on impaired reproductive function and its mechanisms in Toxoplasma gondii (T. gondii)-infected male mice. Results showed that IOP significantly improved the spermatogenic capacity and ameliorated pathological damage of testis, increased serum testosterone (T), luteinizing hormone (LH) and follicular-stimulating hormone (FSH) levels in T. gondii-infected male mice. IOP effectively up-regulated testicular steroidogenic acute regulatory protein (StAR), P450scc and 17β-HSD expressions. IOP also significantly decreased the levels of malondialdehyde (MDA) and nitric oxide (NO), but increased the activities of antioxidant enzyme superoxide dismutase (SOD) and glutathione (GSH). Furthermore, IOP up-regulated the expressions of nuclear factor erythroid 2-related factor 2 (Nrf2), heme oxygenase-1 (HO-1) and NADPH quinoneoxidoreductase-1 (NQO-1), and suppressed the apoptosis of testicular cells by decreasing Bcl-2 associated x protein (Bax) and cleaved caspase-3 expressions. IOP further enhanced testicular phosphatidylinositol 3-kinase (PI3K), phospho-protein kinase B (p-AKT) and phospho-mammalian target of rapamycin (p-mTOR) expression levels. It demonstrates the beneficial effects of IOP on impaired reproductive function in T. gondii-infected male mice due to its anti-oxidative stress and anti-apoptosis via regulating Nrf2-PI3K/AKT signaling pathway.

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

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

Visualization of Nitric Oxide, Measurement of Nitrosothiols Content, Activity of NOS and NR in Wheat Seedlings

Nitric oxide (NO), is a redox-active, endogenous signalling molecule involved in the regulation of numerous processes. It plays a crucial role in adaptation and tolerance to various abiotic and biotic stresses. In higher plants, NO is produced either by enzymatic or non-enzymatic reduction of nitrite and an oxidative pathway requiring a putative nitric oxide synthase (NOS)-like enzyme. There are several methods to measure NO production: mass spectrometry, tissue localization by DAF-FM dye. Electron paramagnetic resonance (EPR) also known as electron spin resonance (ESR) and spectrophotometric assays. The activity of NOS can be measured by L-citrulline based assay and spectroscopic method (NADPH utilization method). A major route for the transfer of NO bioactivity is S-nitrosylation, the addition of a NO moiety to a protein cysteine thiol forming an S-nitrosothiol (SNO). This experimental method describes visualization of NO using DAF-FM dye by fluorescence microscopy (Zeiss AXIOSKOP 2). The whole procedure is simplified, so it is easy to perform but has a high sensitivity for NO detection. In addition, spectrophotometry based protocols for assay of NOS, Nitrate Reductase (NR) and the content of S-nitrosothiols are also described. These spectrophotometric protocols are easy to perform, less expensive and sufficiently sensitive assays which provide adequate information on NO based regulation of physiological processes depending on the treatments of interest.

Nitric oxide accelerates germination via the regulation of respiration in chickpea

Seed germination is crucial for the plant life cycle. We investigated the role of nitric oxide (NO) in two chickpea varieties that differ in germination capacity: Kabuli, which has a low rate of germination and germinates slowly, and Desi, which shows improved germination properties. Desi produced more NO than Kabuli and had lower respiratory rates. As a result of the high respiration rates, Kabuli had higher levels of reactive oxygen species (ROS). Treatment with the NO donor S-nitroso-N-acetyl-D,L-penicillamine (SNAP) reduced respiration in Kabuli and decreased ROS levels, resulting in accelerated germination rates. These findings suggest that NO plays a key role in the germination of Kabuli. SNAP increased the levels of transcripts encoding enzymes involved in carbohydrate metabolism and the cell cycle. Moreover, the levels of amino acids and organic acids were increased in Kabuli as a result of SNAP treatment. 1H-nuclear magnetic resonance analysis revealed that Kabuli has a higher capacity for glucose oxidation than Desi. An observed SNAP-induced increase in 13C incorporation into soluble alanine may result from enhanced oxidation of exogenous [13C]glucose via glycolysis and the pentose phosphate pathway. A homozygous hybrid that originated from a recombinant inbred line population of a cross between Desi and Kabuli germinated faster and had increased NO levels and a reduced accumulation of ROS compared with Kabuli. Taken together, these findings demonstrate the importance of NO in chickpea germination via the control of respiration and ROS accumulation.

Nitric oxide synthase-mediated early nitric oxide burst alleviates water stress-induced oxidative damage in ammonium-supplied rice roots

BACKGROUND

Nutrition with ammonium (NH4+) can enhance the drought tolerance of rice seedlings in comparison to nutrition with nitrate (NO3-). However, there are still no detailed studies investigating the response of nitric oxide (NO) to the different nitrogen nutrition and water regimes. To study the intrinsic mechanism underpinning this relationship, the time-dependent production of NO and its protective role in the antioxidant defense system of NH4+- or NO3--supplied rice seedlings were studied under water stress.

RESULTS

An early NO burst was induced by 3 h of water stress in the roots of seedlings subjected to NH4+ treatment, but this phenomenon was not observed under NO3- treatment. Root oxidative damage induced by water stress was significantly higher for treatment with NO3- than with NH4+ due to reactive oxygen species (ROS) accumulation in the former. Inducing NO production by applying the NO donor 3 h after NO3- treatment alleviated the oxidative damage, while inhibiting the early NO burst by applying the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO) increased root oxidative damage in NH4+ treatment. Application of the nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester(L-NAME) completely suppressed NO synthesis in roots 3 h after NH4+ treatment and aggravated water stress-induced oxidative damage. Therefore, the aggravation of oxidative damage by L-NAME might have resulted from changes in the NOS-mediated early NO burst. Water stress also increased the activity of root antioxidant enzymes (catalase, superoxide dismutase, and ascorbate peroxidase). These were further induced by the NO donor but repressed by the NO scavenger and NOS inhibitor in NH4+-treated roots.

CONCLUSION

These findings demonstrate that the NOS-mediated early NO burst plays an important role in alleviating oxidative damage induced by water stress by enhancing the antioxidant defenses in roots supplemented with NH4+.

The polysaccharide from Inonotus obliquus protects mice from Toxoplasma gondii-induced liver injury

The study aimed to explore the protective effects and mechanism of Inonotus obliquus polysaccharide (IOP) on liver injury caused by Toxoplasma gondii (T. gondii) infection in mice. The results showed that treatment with IOP significantly decreased the liver coefficient, the levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), malondialdehyde (MDA) and nitric oxide (NO), and increased the contents of antioxidant enzyme superoxide dismutase (SOD) and glutathione (GSH). IOP effectively decreased the expression of serum tumor necrosis factor alpha (TNF-α), interleukin-6 (IL-6), interleukin-1β (IL-1β), interferon-γ (IFN-γ) and interluekin-4 (IL-4) in T. gondii-infected mice. In agreement with these observations, IOP also alleviated hepatic pathological damages caused by T. gondii. Furthermore, we found that IOP down-regulated the levels of toll-like receptor 2 (TLR2) and toll-like receptor 4 (TLR4), phosphorylations of nuclear factor-κappaB (NF-κB) p65 and inhibitor kappaBα (IκBα), whereas up-regulated the expressions of nuclear factor erythroid 2-related factor 2 (Nrf2) and heme oxygenase-1 (HO-1). These findings suggest that IOP possesses hepatoprotective effects against T. gondii-induced liver injury in mice, and such protection is at least in part due to its anti-inflammatory effects through inhibiting the TLRs/NF-κB signaling axis and the activation of an antioxidant response by inducing the Nrf2/HO-1 signaling.

The EPR Method for Detecting Nitric Oxide in Plant Senescence

Nitric oxide (NO) is gaining increasing attention as a central molecule with diverse signaling functions. It has been shown that NO acts as a negative regulator of leaf senescence. In this chapter, we describe a highly selective method, electron paramagnetic resonance ([EPR], also known as electron spin resonance [ESR]), for NO determination in leaf senescence. An iron complex of ferrous and mononitrosyl dithiocarbamate (Fe²⁺(DETC)₂) is used as a chelating agent for NO. Using ethyl acetate as extracting solvent, the NOFe²⁺(DETC)₂ complex is extracted and determined by EPR spectrometer.

Regulation of Respiratory Pathway and Electron Transport Chain in Relation to Senescence of Postharvest White Mushroom (Agaricus bisporus) under High O2/CO2 Controlled Atmospheres

In order to study the respiration metabolism mechanism based on the generation of adenosine triphosphate (ATP) and reactive oxygen species (ROS) and nitric oxide (NO) by the electron transport chain (ETC) of the white mushroom under high O₂/CO₂ controlled atmospheres, the treatments of 100% O₂, 80% O₂ + 20% CO₂, 60% O₂ + 40% CO₂, and 40% O₂ + 60% CO₂ at 2 ± 1 °C were employed and natural air was used as the control. ATP and energy charge can maintain the membrane integrity and function, life activities, and physicochemical reactions of higher plants. The results showed that the 80% O₂ + 20% CO₂ treatment inhibited the respiration rate, embden-meyerhof-parnas or glycolysis pathway, and ROS and NO contents. It significantly delayed the reduction of the ATP content and energy charge level, tricarboxyfic-acid-cycle and cytochrome pathway proportion, and their key enzymes activity and gene expression. It also maintained a high phosphopentose pathway and moderate alternative pathway. Results indicated that the 80% O₂ + 20% CO₂ prolonged the storage time of mushrooms to 24 days and retarded the senescence through retaining the higher energy, suppressing the ROS contents, enhancing the endurance capability in adversity, and regulating the respiration pathways and ETC metabolism.

Hepatoprotective Effects of Antrodia cinnamomea: The Modulation of Oxidative Stress Signaling in a Mouse Model of Alcohol-Induced Acute Liver Injury

In the present study, the components of A. cinnamomea (AC) mycelia were systematically analyzed. Subsequently, its hepatoprotective effects and the underlying mechanisms were explored using a mouse model of acute alcohol-induced liver injury. AC contained 25 types of fatty acid, 16 types of amino acid, 3 types of nucleotide, and 8 types of mineral. The hepatoprotective effects were observed after 2 weeks of AC treatment at doses of 75 mg/kg, 225 mg/kg, and 675 mg/kg in the mouse model. These effects were indicated by the changes in the levels of aspartate aminotransferase, alanine aminotransferase, several oxidation-related factors, and inflammatory cytokines in serum and/or liver samples. AC reduced the incidence rate of necrosis, inflammatory infiltration, fatty droplets formation, and cell apoptosis in liver detecting via histological and TUNEL assay. In addition, AC reduced the expression of cleaved caspase-3, -8, and -9 and the levels of phosphor-protein kinase B (Akt) and phosphor-nuclear factor-κB (NF-κB) in the liver samples. Collectively, AC-mediated hepatoprotective effects in a mouse model of acute alcohol-induced liver injury are the result of reduction in oxidative stress. This may be associated with Akt/NF-κB signaling. These results provide valuable evidence to support the use of A. cinnamomea as a functional food and/or medicine.

Abscisic acid-induced nitric oxide and proline accumulation in independent pathways under water-deficit stress during seedling establishment in Medicago truncatula

Nitric oxide (NO) production and amino acid metabolism modulation, in particular abscisic acid (ABA)-dependent proline accumulation, are stimulated in planta by most abiotic stresses. However, the relationship between NO production and proline accumulation under abiotic stress is still poorly understood, especially in the early phases of plant development. To unravel this question, this work investigated the tight relationship between NO production and proline metabolism under water-deficit stress during seedling establishment. Endogenous nitrate reductase-dependent NO production in Medicago truncatula seedlings increased in a time-dependent manner after short-term water-deficit stress. This water-deficit-induced endogenous NO accumulation was mediated through a ABA-dependent pathway and accompanied by an inhibition of seed germination, a loss of water content, and a decrease in elongation of embryo axes. Interestingly, a treatment with a specific NO scavenger (cPTIO) alleviated these water-deficit detrimental effects. However, the content of total amino acids, in particular glutamate and proline, as well as the expression of genes encoding enzymes of synthesis and degradation of proline were not affected by cPTIO treatment under water-deficit stress. Under normal conditions, exogenous NO donor stimulated neither the expression of P5CS2 nor the proline content, as observed after PEG treatment. These results strongly suggest that the modulation of proline metabolism is independent of NO production under short-term water-deficit stress during seedling establishment.

Phytoextraction potential of wild type and 35S-gshI transgenic poplar trees (Populus x Canescens) for environmental pollutants herbicide paraquat, salt sodium, zinc sulfate and nitric oxide in vitro

Phytoextraction potentials of two transgenic (TR) poplar (Populus x canescens) clones TRggs11 and TRlgl6 were compared with that of wild-type (WT) following exposure to paraquat, zinc sulfate, common salt and nitric oxide (NO), using a leaf-disc system incubated for 21 days on EDTA-containing nutritive WPM media in vitro. Glutathione (GSH) contents of leaf discs of TRlgl6 and TRggs11 showed increments to 296% and 190%, respectively, compared with WT. NO exposure led to a twofold GSH content in TRlgl6, which was coupled with a significantly increased sulfate uptake when exposed to 10(-3) M ZnSO4. The highest mineral contents of Na, Zn, Mn, Cu, and Mo was observed in the TRggs11 clone. Salt-induced activity of catalase enzyme increased in both TR clones significantly compared with WT under NaCl (0.75% and 1.5%) exposure. The in silico sequence analyses of gsh1 genes revealed that P. x canadensis and Salix sachalinensis show the closest sequence similarity to that of P. x canescens, which predicted an active GSH production with high phytoextraction potentials of these species with indication for their use where P. x canescens can not be grown.

Nitric oxide signaling in aluminum stress in plants

Nitric oxide (NO) is a ubiquitous signal molecule involved in multiple plant responses to environmental stress. In the recent years, the regulating role of NO on heavy metal toxicity in plants is realized increasingly, but knowledge of NO in alleviating aluminum (Al) toxicity is quite limited. In this article, NO homeostasis between its biosynthesis and elimination in plants is presented. Some genes involved in NO/Al network and their expressions are also introduced. Furthermore, the role of NO in Al toxicity and the functions in Al tolerance are discussed. It is proposed that Al toxicity may disrupt NO homeostasis, leading to endogenous NO concentration being lower than required for root elongation in plants. There are many evidences that pointed out that the exogenous NO treatments improve Al tolerance in plants through activating antioxidative capacity to eliminate reactive oxygen species. Most of the work with respect to NO regulating pathways and functions still has to be done in the future.

Inhibition of aconitase by nitric oxide leads to induction of the alternative oxidase and to a shift of metabolism towards biosynthesis of amino acids

Nitric oxide (NO) is a free radical molecule involved in signalling and in hypoxic metabolism. This work used the nitrate reductase double mutant of Arabidopsis thaliana (nia) and studied metabolic profiles, aconitase activity, and alternative oxidase (AOX) capacity and expression under normoxia and hypoxia (1% oxygen) in wild-type and nia plants. The roots of nia plants accumulated very little NO as compared to wild-type plants which exhibited ∼20-fold increase in NO emission under low oxygen conditions. These data suggest that nitrate reductase is involved in NO production either directly or by supplying nitrite to other sites of NO production (e.g. mitochondria). Various studies revealed that NO can induce AOX in mitochondria, but the mechanism has not been established yet. This study demonstrates that the NO produced in roots of wild-type plants inhibits aconitase which in turn leads to a marked increase in citrate levels. The accumulating citrate enhances AOX capacity, expression, and protein abundance. In contrast to wild-type plants, the nia double mutant failed to show AOX induction. The overall induction of AOX in wild-type roots correlated with accumulation of glycine, serine, leucine, lysine, and other amino acids. The findings show that NO inhibits aconitase under hypoxia which results in accumulation of citrate, the latter in turn inducing AOX and causing a shift of metabolism towards amino acid biosynthesis.

Role of nitric oxide in tolerance of plants to abiotic stress

Nitric oxide (NO) has now gained significant place in plant science, mainly due to its properties (free radical, small size, no charge, short-lived, and highly diffusible across biological membranes) and multifunctional roles in plant growth, development, and regulation of remarkable spectrum of plant cellular mechanisms. In the last few years, the role of NO in tolerance of plants to abiotic stress has established much consideration. As it is evident from the present review, recent progress on NO potentiality in tolerance of plants to environmental stresses has been impressive. These investigations suggest that NO, itself, possesses antioxidant properties and might act as a signal in activating ROS-scavenging enzyme activities under abiotic stress. NO plays an important role in resistance to salt, drought, temperature (high and low), UV-B, and heavy metal stress. Rapidly increasing evidences indicate that NO is essentially involve in several physiological processes; however, there has been much disagreement regarding the mechanism(s) by which NO reduces abiotic stress.

Seed-specific elevation of non-symbiotic hemoglobin AtHb1: beneficial effects and underlying molecular networks in Arabidopsis thaliana

BACKGROUND

Seed metabolism is dynamically adjusted to oxygen availability. Processes underlying this auto-regulatory mechanism control the metabolic efficiency under changing environmental conditions/stress and thus, are of relevance for biotechnology. Non-symbiotic hemoglobins have been shown to be involved in scavenging of nitric oxide (NO) molecules, which play a key role in oxygen sensing/balancing in plants and animals. Steady state levels of NO are suggested to act as an integrator of energy and carbon metabolism and subsequently, influence energy-demanding growth processes in plants.

RESULTS

We aimed to manipulate oxygen stress perception in Arabidopsis seeds by overexpression of the non-symbiotic hemoglobin AtHb1 under the control of the seed-specific LeB4 promoter. Seeds of transgenic AtHb1 plants did not accumulate NO under transient hypoxic stress treatment, showed higher respiratory activity and energy status compared to the wild type. Global transcript profiling of seeds/siliques from wild type and transgenic plants under transient hypoxic and standard conditions using Affymetrix ATH1 chips revealed a rearrangement of transcriptional networks by AtHb1 overexpression under non-stress conditions, which included the induction of transcripts related to ABA synthesis and signaling, receptor-like kinase- and MAP kinase-mediated signaling pathways, WRKY transcription factors and ROS metabolism. Overexpression of AtHb1 shifted seed metabolism to an energy-saving mode with the most prominent alterations occurring in cell wall metabolism. In combination with metabolite and physiological measurements, these data demonstrate that AtHb1 overexpression improves oxidative stress tolerance compared to the wild type where a strong transcriptional and metabolic reconfiguration was observed in the hypoxic response.

CONCLUSIONS

AtHb1 overexpression mediates a pre-adaptation to hypoxic stress. Under transient stress conditions transgenic seeds were able to keep low levels of endogenous NO and to maintain a high energy status, in contrast to wild type. Higher weight of mature transgenic seeds demonstrated the beneficial effects of seed-specific overexpression of AtHb1.

Integrative response of plant mitochondrial electron transport chain to nitrogen source

Nitrogen (N) availability is widely known as a determinant of plant growth and respiration rate. However, less attention has been paid to the effect of the type of N source (nitrate, nitrite or ammonium) on the respiratory system. This review summarizes the latest findings on this topic, with an emphasis on the effect of ammonium and nitric oxide (NO) on the respiratory system, and the physiological role of alternative oxidase (AOX). First, concentrated ammonium has been found to increase plant respiration rate (ammonium-dependent respiratory increase, ARI). We will introduce two hypotheses to explain ARI, futile ammonium cycling and excess reducing equivalents, and verify the validity of each hypothesis. We suggest that these two hypotheses are not necessarily mutually exclusive. Second, gene expression of AOX is suppressed when N is predominately available as nitrate instead of ammonium. We will discuss possible signaling pathways leading to this expression pattern. Third, while AOX expression is induced by NO, AOX activity itself is insensitive to NO. In contrast, activity of cytochrome c oxidase (COX) is sensitive to NO. We outline the NO production pathway, focusing on nitrite-dependent NO production, and discuss the physiological significance of the fact that AOX activity is insensitive to NO. Finally, this review aims to build an integrated scheme of the respiratory response to the type of N source, considering leaves in high light conditions or hypoxic roots.

Chemical induction of rapid and reversible plastid filamentation in Arabidopsis thaliana roots

Plastids assume various morphologies depending on their developmental status, but the basis for developmentally regulated plastid morphogenesis is poorly understood. Chemical induction of alterations in plastid morphology would be a useful tool for studying this; however, no such chemicals have been identified. Here, we show that antimycin A, an effective respiratory inhibitor, can change plastid morphology rapidly and reversibly in Arabidopsis thaliana. In the root cortex, hypocotyls, cotyledon epidermis and true leaf epidermis, significant differences in mitochondrial morphology were not observed between antimycin-treated and untreated tissues. In contrast, antimycin caused extreme filamentation of plastids in the mature cortices of main roots. This phenomenon was specifically observed in the mature root cortex. Other mitochondrial respiratory inhibitors (rotenone and carbonyl cyanide m-chlorophenylhydrazone), hydrogen peroxide, S-nitroso-N-acetylpenicillamine [a nitric oxide (NO) donor] and 3-(3,4-dichlorophenyl)-1,1-dimethylurea did not mimic the phenomenon under the present study conditions. Antimycin-induced plastid filamentation was initiated within 5 min after the onset of chemical treatment and appeared to complete within 1 h. Plastid morphology was restored within 7 h after the washout of antimycin, suggesting that the filamentation was reversible. Co-applications of antimycin and cytoskeletal inhibitors (demecolcine or latrunculin B) or protein synthesis inhibitors (cycloheximide or chloramphenicol) still caused plastid filamentation. Antimycin A was also effective for plastid filamentation in the chloroplast division mutants atftsZ1-1 and atminE1. Salicylhydroxamic acid, an alternative oxidase inhibitor, was solely found to suppress the filamentation, implying the possibility that this phenomenon was partly mediated by an antimycin-activated alternative oxidase in the mitochondria.

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.

Regulation of plant glycine decarboxylase by s-nitrosylation and glutathionylation

Mitochondria play an essential role in nitric oxide (NO) signal transduction in plants. Using the biotin-switch method in conjunction with nano-liquid chromatography and mass spectrometry, we identified 11 candidate proteins that were S-nitrosylated and/or glutathionylated in mitochondria of Arabidopsis (Arabidopsis thaliana) leaves. These included glycine decarboxylase complex (GDC), a key enzyme of the photorespiratory C(2) cycle in C3 plants. GDC activity was inhibited by S-nitrosoglutathione due to S-nitrosylation/S-glutathionylation of several cysteine residues. Gas-exchange measurements demonstrated that the bacterial elicitor harpin, a strong inducer of reactive oxygen species and NO, inhibits GDC activity. Furthermore, an inhibitor of GDC, aminoacetonitrile, was able to mimic mitochondrial depolarization, hydrogen peroxide production, and cell death in response to stress or harpin treatment of cultured Arabidopsis cells. These findings indicate that the mitochondrial photorespiratory system is involved in the regulation of NO signal transduction in Arabidopsis.

Nitric oxide: promoter or suppressor of programmed cell death?

Nitric oxide (NO) is a short-lived gaseous free radical that predominantly functions as a messenger and effector molecule. It affects a variety of physiological processes, including programmed cell death (PCD) through cyclic guanosine monophosphate (cGMP)-dependent and - independent pathways. In this field, dominant discoveries are the diverse apoptosis networks in mammalian cells, which involve signals primarily via death receptors (extrinsic pathway) or the mitochondria (intrinsic pathway) that recruit caspases as effector molecules. In plants, PCD shares some similarities with animal cells, but NO is involved in PCD induction via interacting with pathways of phytohormones. NO has both promoting and suppressing effects on cell death, depending on a variety of factors, such as cell type, cellular redox status, and the flux and dose of local NO. In this article, we focus on how NO regulates the apoptotic signal cascade through protein S-nitrosylation and review the recent progress on mechanisms of PCD in both mammalian and plant cells.

Systemic induction and role of mitochondrial alternative oxidase and nitric oxide in a compatible tomato-Tobacco mosaic virus interaction

The role of mitochondrial alternative oxidase (AOX) and the relationship between AOX and nitric oxide (NO) in virus-induced systemic defense to Tobacco mosaic virus (TMV) were investigated in susceptible tomato (Solanum lycopersicum) plants. TMV inoculation to the lower leaves induced a rapid NO synthesis and AOX activation in upper uninoculated leaves as early as 0.5 day postinoculation. Application of exogenous potassium cyanide (KCN, a cytochrome pathway inhibitor) at nonlethal concentrations and NO donor diethylamine NONOate (DEA/NO) to the upper uninoculated leaves greatly induced accumulation of AOX transcript, reduced TMV viral RNA accumulation, and increased the leaf photochemical quantum yield at photosystem II. Pretreatment with NO scavenger almost completely blocked TMV-induced AOX induction and substantially increased TMV susceptibility. Salicylhydroxamic acid (SHAM, an AOX inhibitor) pretreatment reduced the DEA/NO-induced cyanide-resistant respiration and partially compromised induced resistance to TMV. Conversely, KCN and SHAM pretreatment had very little effect on generation of NO, and pretreatment with NO scavenger did not affect KCN-induced AOX induction and TMV resistance. These results suggest that TMV-induced NO generation acts upstream and mediates AOX induction which, in turn, induces mitochondrial alternative electron transport and triggers systemic basal defense against the viral pathogen.

Nitrite reduction and superoxide-dependent nitric oxide degradation by Arabidopsis mitochondria: influence of external NAD(P)H dehydrogenases and alternative oxidase in the control of nitric oxide levels

Mitochondria recently have emerged as important sites in controlling NO levels within the cell. In this study, the synthesis of nitric oxide (NO) from nitrite and its degradation by mitochondria isolated from Arabidopsis thaliana were examined. Oxygen and NO concentrations in the reaction medium were measured with specific electrodes. Nitrite inhibited the respiration of isolated A. thaliana mitochondria, in competition with oxygen, an effect that was abolished or potentiated when electron flow occurred via alternative oxidase (AOX) or cytochrome c oxidase (COX), respectively. The production of NO from nitrite was detected electrochemically only under anaerobiosis because of a superoxide-dependent process of NO degradation. Electron leakage from external NAD(P)H dehydrogenases contributed the most to NO degradation as higher rates of Amplex Red-detected H(2)O(2) production and NO consumption were observed in NAD(P)H-energized mitochondria. Conversely, the NO-insensitive AOX diminished electron leakage from the respiratory chain, allowing the increase of NO half-life without interrupting oxygen consumption. These results show that the accumulation of nitric oxide derived from nitrite reduction and the superoxide-dependent mechanism of NO degradation in isolated A. thaliana mitochondria are influenced by the external NAD(P)H dehydrogenases and AOX, revealing a role for these alternative proteins of the mitochondrial respiratory chain in the control of NO levels in plant cells.

Methodology and Significance of Microsensor-based Oxygen Mapping in Plant Seeds - an Overview

Oxygen deficiency is commonplace in seeds, and limits both their development and their germination. It is, therefore, of considerable relevance to crop production. While the underlying physiological basis of seed hypoxia has been known for some time, the lack of any experimental means of measuring the global or localized oxygen concentration within the seed has hampered further progress in this research area. The development of oxygen-sensitive microsensors now offers the capability to determine the localized oxygen status within a seed, and to study its dynamic adjustment both to changes in the ambient environment, and to the seed's developmental stage. This review illustrates the use of oxygen microsensors in seed research, and presents an overview of existing data with an emphasis on crop species. Oxygen maps, both static and dynamic, should serve to increase our basic understanding of seed physiology, as well as to facilitate upcoming breeding and biotechnology-based approaches for crop improvement.

The oxygen status of the developing seed

Recent applications of oxygen-sensitive microsensors have demonstrated steep oxygen gradients in developing seeds of various crops. Here, we present an overview on oxygen distribution, major determinants of the oxygen status in the developing seed and implications for seed physiology. The steady-state oxygen concentration in different seed tissues depends on developmental parameters, and is determined to a large extent by environmental factors. Photosynthetic activity of the seed significantly diminishes hypoxic constraints, and can even cause transient, local hyperoxia. Changes in oxygen availability cause rapid adjustments in mitochondrial respiration and global metabolism. We argue that nitric oxide (NO) is a key player in the oxygen balancing process in seeds, avoiding fermentation and anoxia in vivo. Molecular approaches aiming to increase oxygen availability within the seed are discussed.

Plant mitochondrial function during anaerobiosis

BACKGROUND

Under hypoxic conditions, plant mitochondria preserve the capacity to oxidize external NADH, NADPH and tricarboxylic acid cycle substrates. Nitrite serves as an alternative electron acceptor at the level of cytochrome oxidase, with possibly complex III and the alternative oxidase also being involved. Nitric oxide is a significant product of the reaction, which has a high affinity for cytochrome c oxidase, inhibiting it. The excess NO is scavenged by hypoxically induced class 1 haemoglobin in the reaction involving ascorbate.

SCOPE

By using nitrite, mitochondria retain a limited capacity for ATP synthesis. NADH, produced from glycolysis during anaerobiosis and oxidized in the mitochondrial electron transport chain, should shift the composition of metabolites formed during anaerobiosis with increased conversion of pyruvate to alanine and greater involvement of other transamination reactions, such as those involving gamma-aminobutyric acid formation.

CONCLUSIONS

Anaerobic mitochondrial metabolism may have a more significant role than previously thought in alleviating the effects of anoxia on plant cells. There is a need to re-examine mitochondrial carbon and nitrogen metabolism under anoxia to establish the extent of this involvement.

Plant polyamine catabolism: The state of the art

Polyamines have long been implicated in plant growth and development, as well as adaptation to abiotic and biotic stress. As a general rule of thumb the higher the polyamine titers the better. However, their molecular roles in plant stress responses still remain obscure. It has been postulated that they could act through their catabolism, which generates molecules which may act as secondary messengers signalling networks of numerous developmental and stress adaptation processes. Recently it was shown that plant and mammalian polyamine catabolism share critical features, giving new insight in plant polyamine catabolism. In this review, the advances in genes and proteins of polyamine catabolism in plants is presented and compared to other models.

Comparative genomic and physiological analysis of nutrient response to NH4+, NH4+:NO3- and NO3- in barley seedlings

Long-term differences in photosynthesis, respiration and growth of plants receiving distinct nitrogen (N) sources imply that N metabolism generates signals that regulate metabolism and development. The molecular basis of these signals remains unclear. Here we studied the gene expression profiles of barley (Hordeum vulgare L. cv. Graphic) seedlings fertilized either with ammonium (NH4+), with ammonium and nitrate (NH4+:NO3-), or with nitrate (NO3-) only. Our transcriptome analysis after 48 h of growth in these N sources showed major changes in the expression of genes involved in N metabolism (nitrate reductase), signalling (protein kinases and protein phosphatases), photosynthesis (chlorophyll a/b-binding protein and a PsbQ domain), where increases in NO3- as compared with NH4+ were observed. Moreover, NH4+ assimilation induced genes participating in C and sugars metabolism (phosphoglycerate kinase, glucosyltranferase and galactokinase), respiration (cytochrome c oxidase), protein fate (heat shock proteins) and development (MTN3-like protein). These changes in gene expression could well explain the long-term growth depression observed in NH4+ plants. Even if a few genes participating in protein fate (proteases) and development (OsNAC5) were upregulated in NH4+ as compared with NH4+:NO3-, the general pattern of expression was quite similar between these two N sources. Taken together, these results indicated that other downstream mechanisms should be involved in the synergetic long-term response of NH4+:NO3-.

Polyamines: essential factors for growth and survival

Polyamines are low molecular weight, aliphatic polycations found in the cells of all living organisms. Due to their positive charges, polyamines bind to macromolecules such as DNA, RNA, and proteins. They are involved in diverse processes, including regulation of gene expression, translation, cell proliferation, modulation of cell signalling, and membrane stabilization. They also modulate the activities of certain sets of ion channels. Because of these multifaceted functions, the homeostasis of polyamines is crucial and is ensured through regulation of biosynthesis, catabolism, and transport. Through isolation of the genes involved in plant polyamine biosynthesis and loss-of-function experiments on the corresponding genes, their essentiality for growth is reconfirmed. Polyamines are also involved in stress responses and diseases in plants, indicating their importance for plant survival. This review summarizes the recent advances in polyamine research in the field of plant science compared with the knowledge obtained in microorganisms and animal systems.

Nitric oxide is a versatile sensor of low oxygen stress in plants

The plant response to low levels of oxygen involves an interplay of transcriptional, translational and post-translational signaling. However, in plants, the sensing mechanism itself remains obscure. The role of nitric oxide (NO) in oxygen sensing and balancing has been extensively explored in our laboratory. We suggest that NO is generated within the mitochondria from nitrite in response to hypoxia, and that this small gaseous molecule can reversibly modify both the respiratory oxygen consumption and the oxygen availability within the seed. We further propose that hemoglobins play a central role in the detoxification of excess NO.

Nitric oxide degradation by potato tuber mitochondria: evidence for the involvement of external NAD(P)H dehydrogenases

The mechanisms of nitric oxide (NO) synthesis in plants have been extensively investigated. NO degradation can be just as important as its synthesis in controlling steady-state levels of NO. Here, we examined NO degradation in mitochondria isolated from potato tubers and the contribution of the respiratory chain to this process. NO degradation was faster in mitochondria energized with NAD(P)H than with succinate or malate. Oxygen consumption and the inner membrane potential were transiently inhibited by NO in NAD(P)H-energized mitochondria, in contrast to the persistent inhibition seen with succinate. NO degradation was abolished by anoxia and superoxide dismutase, which suggested that NO was consumed by its reaction with superoxide anion (O2(-)). Antimycin-A stimulated and myxothiazol prevented NO consumption in succinate- and malate-energized mitochondria. Although favored by antimycin-A, NAD(P)H-mediated NO consumption was not abolished by myxothiazol, indicating that an additional site of O2(-) generation, besides complex III, stimulated NO degradation. Larger amounts of O2(-) were generated in NAD(P)H- compared to succinate- or malate-energized mitochondria. NAD(P)H-mediated NO degradation and O2(-) production were stimulated by free Ca2+ concentration. Together, these results indicate that Ca2+-dependent external NAD(P)H dehydrogenases, in addition to complex III, contribute to O2(-) production that favors NO degradation in potato tuber mitochondria.

Function of nitric oxide and superoxide anion in the adventitious root development and antioxidant defence in Panax ginseng

The involvement of NO in O(2)(.-) generation, rootlet development and antioxidant defence were investigated in the adventitious root cultures of mountain ginseng. Treatments of NO producers (SNP, sodium nitroprusside; SNAP, S-nitroso-N-acetylpenicillamine; and sodium nitrite with ascorbic acid), and NO scavenger (PTIO, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl3-oxide) revealed that NO is involved in the induction of new rootlets. Severe decline in number of new rootlets compared to the control under PTIO treatment indicates that NO acts downstream of auxin action in the process. NO producers (SNP, SNAP and sodium nitrite with ascorbic acid) activated NADPH oxidase activity, resulting in greater O(2)(.-) generation and higher number of new rootlets in the adventitious root explants. Moreover, treatment of diphenyliodonium chloride, a NADPH oxidase inhibitor, individually or along with SNP, inhibited root growth, NADPH oxidase activity and O(2)(.-) anion generation. NO supply also enhanced the activities of antioxidant enzymes that are likely to be responsible for reducing H(2)O(2)levels and lipid peroxidation as well as modulation of ascorbate and non-protein thiol concentrations in the adventitious roots. Our results suggest that NO-induced generation of O(2) (.-) by activating NADPH oxidase activity is related to adventitious root formation in mountain ginseng.

New insights into nitric oxide signaling in plants

A decade-long investigation of nitric oxide (NO) functions in plants has led to its characterization as a biological mediator involved in key physiological processes. Despite the wealth of information gathered from the analysis of its functions, until recently little was known about the mechanisms by which NO exerts its effects. In the past few years, part of the gap has been bridged. NO modulates the activity of proteins through nitrosylation and probably tyrosine nitration. Furthermore, NO can act as a Ca(2+)-mobilizing messenger, and researchers are beginning to unravel the mechanisms underlying the cross talk between NO and Ca(2+). Nonetheless, progress in this area of research is hindered by our ignorance of the pathways for NO production in plants. This review summarizes the basic concepts of NO signaling in animals and discusses new insights into NO enzymatic sources and molecular signaling in plants.

Low oxygen sensing and balancing in plant seeds: a role for nitric oxide

Storage product accumulation in seeds of major crop species is limited by their low internal oxygen concentration. Adjustment of energy and storage metabolism to oxygen deficiency (hypoxia) in seeds is highly relevant for agriculture and biotechnology. However, the mechanisms of low-oxygen sensing and balancing remain a mystery. Here, it is shown that normal hypoxia in seeds of soybean (Glycine max) and pea (Pisum sativum) triggers a nitrite-dependent increase in endogenous nitric oxide (NO) concentrations. NO, in turn, reduces the oxygen consumption of seeds, generating a localized decrease in both ATP availability and biosynthetic activity. Increasing oxygen availability reduces endogenous NO concentrations, thereby abolishing mitochondrial and metabolic inhibition. This auto-regulatory and reversible oxygen balancing, via NO, avoids seed anoxia and suggests a key role for NO in regulating storage activity. This hypothesis is reinforced by changes in energy status (ATP:ADP ratio), steady-state metabolite concentrations and biosynthetic fluxes under NO treatment. The proposed mechanism of low-oxygen sensing and balancing in plants offers the prospect of a new field of study in crop biotechnology.

Glucose-6-phosphate dehydrogenase plays a pivotal role in nitric oxide-involved defense against oxidative stress under salt stress in red kidney bean roots

The pivotal role of glucose-6-phosphate dehydrogenase (G-6-PDH)-mediated nitric oxide (NO) production in the tolerance to oxidative stress induced by 100 mM NaCl in red kidney bean (Phaseolus vulgaris) roots was investigated. The results show that the G-6-PDH activity was enhanced rapidly in the presence of NaCl and reached a maximum at 100 mM. Western blot analysis indicated that the increase of G-6-PDH activity in the red kidney bean roots under 100 mM NaCl was mainly due to the increased content of the G-6-PDH protein. NO production and nitrate reductase (NR) activity were also induced by 100 mM NaCl. The NO production was reduced by NaN(3) (an NR inhibitor), but not affected by N(omega)-nitro-L-arginine (L-NNA) (an NOS inhibitor). Application of 2.5 mM Na(3)PO(4), an inhibitor of G-6-PDH, blocked the increase of G-6-PDH and NR activity, as well as NO production in red kidney bean roots under 100 mM NaCl. The activities of antioxidant enzymes in red kidney bean roots increased in the presence of 100 mM NaCl or sodium nitroprusside (SNP), an NO donor. The increased activities of all antioxidant enzymes tested at 100 mM NaCl were completely inhibited by 2.5 mM Na(3)PO(4). Based on these results, we conclude that G-6-PDH plays a pivotal role in NR-dependent NO production, and in establishing tolerance of red kidney bean roots to salt stress.