Nitric oxide signalling in plants

Action and target sites of nitric oxide in chloroplasts

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

A fungal protein elicitor PevD1 induces Verticillium wilt resistance in cotton

KEY MESSAGE

We found that the elicitor PevD1 triggered innate immunity in cotton, which plays an important role in future cotton wilt disease control.

ABSTRACT

Elicitors can induce defense responses in plants and improve pathogen resistance. PevD1 is a secreted protein from Verticillium dahliae and activates the hypersensitive response and systemic acquired resistance to tobacco mosaic virus in tobacco plants. To investigate the PevD1-induced disease resistance mechanisms in cotton (Gossypium hirsutum), we report that Escherichia coli expressing PevD1 enhanced cotton resistance and the defense response to the fungal pathogen V. dahliae. The results showed that recombinant PevD1 improved cotton resistance when infiltrated at a concentration as low as 4 μg ml(-1), and the highest disease reduction was 38.16 % on the 15th day post V. dahliae inoculation. This protein was able to systemically induce hydrogen peroxide production, nitric oxide generation, lignin deposition, vessel reinforcement and defense enzymes, including phenylalanine ammonia-lyase, peroxidase, and polyphenol oxidase. PevD1 also enhanced the expression of three pathogenesis-related genes, namely, β-1,3-glucanase, chitinase, and cadinene synthase, and three key genes, PAL, C4H1, and 4CL, from the cotton defense phenylpropanoid metabolism pathway. Our results demonstrated that PevD1 acted as an effector in cotton and V. dahliae interactions and triggered innate immunity in cotton, resulting in the upregulation of defense-related genes, metabolic substance deposition and cell wall modifications. PevD1 is a candidate plant defense activator for cotton wilt disease control.

Nitrate reductase-mediated early nitric oxide burst alleviates oxidative damage induced by aluminum through enhancement of antioxidant defenses in roots of wheat (Triticum aestivum)

• Nitric oxide (NO) is an important signaling molecule involved in the physiological processes of plants. The role of NO release in the tolerance strategies of roots of wheat (Triticum aestivum) under aluminum (Al) stress was investigated using two genotypes with different Al resistances. • An early NO burst at 3 h was observed in the root tips of the Al-tolerant genotype Jian-864, whereas the Al-sensitive genotype Yang-5 showed no NO accumulation at 3 h but an extremely high NO concentration after 12 h. Stimulating NO production at 3 h in the root tips of Yang-5 with the NO donor relieved Al-induced root inhibition and callose production, as well as oxidative damage and ROS accumulation, while elimination of the early NO burst by NO scavenger aggravated root inhibition in Jian-864. • Synthesis of early NO in roots of Jian-864 was mediated through nitrate reductase (NR) but not through NO synthase. Elevated antioxidant enzyme activities were induced by Al stress in both wheat genotypes and significantly enhanced by NO donor, but suppressed by NO scavenger or NR inhibitor. • These results suggest that an NR-mediated early NO burst plays an important role in Al resistance of wheat through modulating enhanced antioxidant defense to adapt to Al stress.

Waterlogging and nitric oxide induce gene expression and increase antioxidant enzyme activity in wheat (Triticum aestivum L.)

The effects of waterlogging (WL) and WL plus nitric oxide (WL+NO) were investigated in seedlings of one wheat cultivars (Triticum aestivum cv. Dogankent) and one wheat line (Triticum aestivum cv. Ducula-4). Under WL conditions, catalase activity was greater in Ducula-4 than in Dogankent. Glutathione reductase activity increased in Ducula-4 seedlings under WL+NO conditions, especially at 48 and 72 hours of treatment. Myb2 expression increased during the early hours of treatment in both wheat varieties exposed to WL, with 40-fold higher levels in Ducula-4, gradually decreasing to control levels. Under WL+NO treatment, Myb2 expression increased 44-fold at 12 hours and high levels of expression were still observed at 72 hours. When Ducula-4 seedlings were subjected to WL+NO treatment, PDPK expression increased approximately 15-fold at 3 hours and decreased to control levels at 72 hours. Under the same conditions, SST1 expression increased 3-fold at 3 and 12 hours and reached control levels during the subsequent hours. Among the genes studied, the highest level of expression was observed for Myb2. Moreover, gene expression was altered most by waterlogging in Ducula-4 seedlings.

Diverse functional interactions between nitric oxide and abscisic acid in plant development and responses to stress

The extensive support for abscisic acid (ABA) involvement in the complex regulatory networks controlling stress responses and development in plants contrasts with the relatively recent role assigned to nitric oxide (NO). Because treatment with exogenous ABA leads to enhanced production of NO, it has been widely considered that NO participates downstream of ABA in controlling processes such as stomata movement, seed dormancy, and germination. However, data on leaf senescence and responses to stress suggest that the functional interaction between ABA and NO is more complex than previously thought, including not only cooperation but also antagonism. The functional relationship is probably determined by several factors including the time- and place-dependent pattern of accumulation of both molecules, the threshold levels, and the regulatory factors important for perception. These factors will determine the actions exerted by each regulator. Here, several examples of well-documented functional interactions between NO and ABA are analysed in light of the most recent reported data on seed dormancy and germination, stomata movements, leaf senescence, and responses to abiotic and biotic stresses.

cGMP and ethylene are involved in maintaining ion homeostasis under salt stress in Arabidopsis roots

KEY MESSAGE

cGMP promotes ethylene production and enhances the perception of ethylene. Endogenous ethylene or cGMP accumulation maintains ion homeostasis to enhancing salt resistance. etr1 - 3 is insensitive to cGMP under salt stress.

ABSTRACT

In the present study, we presented a signaling network involving ethylene and cGMP in salt resistance pathway of Arabidopsis roots. Results showed that the ethylene-insensitive mutant etr1-3 was more sensitive to salt stress than the wild type (WT). etr1-3 displayed a greater electrolyte leakage, thiobarbituric acid reactive substances and Na(+)/K(+) ratio, but a lower plasma membrane (PM) H(+)-ATPase activity compared to WT under the different NaCl contents. Application of 1-aminocyclopropane-1-carboxylic acid (ACC, an ethylene precursor) or 8-Br-cGMP (the cGMP analog) alleviated NaCl-induced injury by maintaining a lower Na(+)/K(+) ratio and increasing PM H(+)-ATPase activity in WT, but not in etr1-3. Roots treated with 8-Br-cGMP could promote ethylene production and enhance the expression of ACC synthase gene in WT. In addition, the 8-Br-cGMP action in NaCl stress was inhibited by aminooxyacetic acid (an inhibitor of ethylene biosynthesis), but 6-Anilino-5,8-quinolinedione (Ly83583, a guanylate cyclase inhibitor) could not affect ACC action in WT. These results suggest that ethylene functions as a downstream signal of cGMP that stimulates the PM H(+)-ATPase activity, which finally results in regulating ion homeostasis in Arabidopsis tolerance to salt. Moreover, cGMP enhanced the perception of ethylene in Arabidopsis under salt stress, which reversed the salt-induced increase of ETR1 and increased ERF1 at the transcript levels in WT. In a word, cGMP modulates salt resistance pathway of ethylene through regulating biosynthesis and perception of ethylene in Arabidopsis roots.

Plant mitochondria: source and target for nitric oxide

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

Signal regulation involved in sulfur dioxide-induced guard cell apoptosis in Hemerocallis fulva

Chronic and acute exposure to SO₂ is associated with increased risks of various damages to plants. In the present study, epidermal strip experiment was employed to investigate SO₂-induced guard cells apoptosis and the signal regulation in Hemerocallis fulva. The results showed that with the increase of treatment concentrate of SO₂ derivates (a mixture of sodium sulfite and sodium bisulfite, 3:1, mmol L⁻¹/mmol L⁻¹, 1.0-5.0 mmol L⁻¹), the physiological activity of the guard cells declined and cell death occurred. While the concentration of SO₂ derivatives exceeded 2.0 mmol L⁻¹, the percentage of cell death increased significantly (P<0.05). Typical features of apoptosis including nuclear condensation, nuclear elongation, fragmentation etc. were found. Meanwhile, concomitant presence of nitric oxide (NO), reactive oxygen species (ROS) and Ca²⁺ level increment appeared. However, SO₂-induced cell death can be effectively blocked by either of the following substances with their respective optimal concentrations: antioxidant ascorbic acid (Asc; 0.05 mmol L⁻¹) or catalase (CAT; 200 U mL⁻¹), nitric oxide (NO) scavenger 2-(4-carboxyphenyl)-4, 4, 5, 5- tetramethylmidiazoline-1-oxyl-3-oxide (c-PTIO; 0.20 mmol L⁻¹), nitrate reductase inhibitor NaN₃ (0.20 mmol L⁻¹), Ca²⁺ chelating agent EGTA (0.05 mmol L⁻¹) or plasma membrane Ca²⁺ channel blocker LaCl₃ (0.05 mmol L⁻¹). In addition to a significant decrease in cell death rate, a reduction in the levels of reactive oxygen species (ROS), NO and Ca²⁺ was observed. Further study showed that compared to treatment with SO₂ alone, Asc treatment led to a decrease in NO and Ca²⁺ levels and NaN₃ treatment led to a decrease in ROS and Ca²⁺ levels, but the NO and ROS levels of the LaCl₃ treatment changed little. All results suggested that NO, ROS and Ca²⁺ were involved in the apoptosis induced by SO₂ in H. fulva. The process might be related to the burst of NO or ROS, which would activate the plasma Ca²⁺ channel and result in the increase of intercellular Ca²⁺.

The oxidative damage initiation hypothesis for meiosis

The maintenance of sexual reproduction in eukaryotes is still a major enigma in evolutionary biology. Meiosis represents the only common feature of sex in all eukaryotic kingdoms, and thus, we regard it a key issue for discussing its function. Almost all asexuality modes maintain meiosis either in a modified form or as an alternative pathway, and facultatively apomictic plants increase frequencies of sexuality relative to apomixis after abiotic stress. On the physiological level, abiotic stress causes oxidative stress. We hypothesize that repair of oxidative damage on nuclear DNA could be a major driving force in the evolution of meiosis. We present a hypothetical model for the possible redox chemistry that underlies the binding of the meiosis-specific protein Spo11 to DNA. During prophase of meiosis I, oxidized sites at the DNA molecule are being targeted by the catalytic tyrosine moieties of Spo11 protein, which acts like an antioxidant reducing the oxidized target. The oxidized tyrosine residues, tyrosyl radicals, attack the phosphodiester bonds of the DNA backbone causing DNA double strand breaks that can be repaired by various mechanisms. Polyploidy in apomictic plants could mitigate oxidative DNA damage and decrease Spo11 activation. Our hypothesis may contribute to explaining various enigmatic phenomena: first, DSB formation outnumbers crossovers and, thus, effective recombination events by far because the target of meiosis may be the removal of oxidative lesions; second, it offers an argument for why expression of sexuality is responsive to stress in many eukaryotes; and third, repair of oxidative DNA damage turns meiosis into an essential characteristic of eukaryotic reproduction.

Nitric oxide and brassinosteroids mediated fungal endophyte-induced volatile oil production through protein phosphorylation pathways in Atractylodes lancea plantlets

Fungal endophytes have been isolated from almost every plant, infecting their hosts without causing visible disease symptoms, and yet have still proved to be involved in plant secondary metabolites accumulation. To decipher the possible physiological mechanisms of the endophytic fungus-host interaction, the role of protein phosphorylation and the relationship between endophytic fungus-induced kinase activity and nitric oxide (NO) and brassinolide (BL) in endophyte-enhanced volatile oil accumulation in Atractylodes lancea plantlets were investigated using pharmacological and biochemical approaches. Inoculation with the endophytic fungus Gilmaniella sp. AL12 enhanced the activities of total protein phosphorylation, Ca²⁺-dependent protein kinase, and volatile oil accumulation in A. lancea plantlets. The upregulation of protein kinase activity could be blocked by the BL inhibitor brassinazole. Furthermore, pretreatments with the NO-specific scavenger cPTIO significantly reduced the increased activities of protein kinases in A. lancea plantlets inoculated with endophytic fungus. Pretreatments with different protein kinase inhibitors also reduced fungus-induced NO production and volatile oil accumulation, but had barely no effect on the BL level. These data suggest that protein phosphorylation is required for endophyte-induced volatile oil production in A. lancea plantlets, and that crosstalk between protein phosphorylation and the NO pathway may occur and act as a downstream signaling event of the BL pathway.

Comparative proteomic analysis on wild type and nitric oxide-overproducing mutant (nox1) of Arabidopsis thaliana

Nitric oxide (NO) as a ubiquitous signal molecule plays an important role in plant development and growth. Here, we compared the proteomic changes between NO-overproducing mutant (nox1) and wild-type (WT) of Arabidopsis thaliana using two-dimensional electrophoresis coupled with MALDI-TOF MS. We successfully identified 59 differentially expressed proteins in nox1 mutant, which are predicted to play potential roles in specific cellular processes, such as post-translational modification, energy production and conversion, metabolism, transcription and signal transduction, cell rescue and defense, development and differentiation. Particularly, expression levels of five anti-oxidative enzymes were altered by the mutation; and assays of their respective enzymatic activities indicated an enhanced level of oxidative stress in nox1 mutant. Finally, some important proteins were further confirmed at transcriptional level using quantitative real-time PCR revealing the systemic changes between WT and nox1. The result suggests that obvious morphological changes in the nox1 mutant may be regulated by different mechanisms and factors, while excess endogenous NO maybe one of the possible reasons.

Nitric oxide in guard cells as an important secondary messenger during stomatal closure

The modulation of guard cell function is the basis of stomatal closure, essential for optimizing water use and CO2 uptake by leaves. Nitric oxide (NO) in guard cells plays a very important role as a secondary messenger during stomatal closure induced by effectors, including hormones. For example, exposure to abscisic acid (ABA) triggers a marked increase in NO of guard cells, well before stomatal closure. In guard cells of multiple species, like Arabidopsis, Vicia and pea, exposure to ABA or methyl jasmonate or even microbial elicitors (e.g., chitosan) induces production of NO as well as reactive oxygen species (ROS). The role of NO in stomatal closure has been confirmed by using NO donors (e.g., SNP) and NO scavengers (like cPTIO) and inhibitors of NOS (L-NAME) or NR (tungstate). Two enzymes: a L-NAME-sensitive, nitric oxide synthase (NOS)-like enzyme and a tungstate-sensitive nitrate reductase (NR), can mediate ABA-induced NO rise in guard cells. However, the existence of true NOS in plant tissues and its role in guard cell NO-production are still a matter of intense debate. Guard cell signal transduction leading to stomatal closure involves the participation of several components, besides NO, such as cytosolic pH, ROS, free Ca(2+), and phospholipids. Use of fluorescent dyes has revealed that the rise in NO of guard cells occurs after the increase in cytoplasmic pH and ROS. The rise in NO causes an elevation in cytosolic free Ca(2+) and promotes the efflux of cations as well as anions from guard cells. Stomatal guard cells have become a model system to study the signaling cascade mechanisms in plants, particularly with NO as a dominant component. The interrelationships and interactions of NO with cytosolic pH, ROS, and free Ca(2+) are quite complex and need further detailed examination. While assessing critically the available literature, the present review projects possible areas of further work related to NO-action in stomatal guard cells.

Nitric oxide and phytohormone interactions: current status and perspectives

Nitric oxide (NO) is currently considered a ubiquitous signal in plant systems, playing significant roles in a wide range of responses to environmental and endogenous cues. During the signaling events leading to these plant responses, NO frequently interacts with plant hormones and other endogenous molecules, at times originating remarkably complex signaling cascades. Accumulating evidence indicates that virtually all major classes of plant hormones may influence, at least to some degree, the endogenous levels of NO. In addition, studies conducted during the induction of diverse plant responses have demonstrated that NO may also affect biosynthesis, catabolism/conjugation, transport, perception, and/or transduction of different phytohormones, such as auxins, gibberellins, cytokinins, abscisic acid, ethylene, salicylic acid, jasmonates, and brassinosteroids. Although still not completely elucidated, the mechanisms underlying the interaction between NO and plant hormones have recently been investigated in a number of species and plant responses. This review specifically focuses on the current knowledge of the mechanisms implicated in NO-phytohormone interactions during the regulation of developmental and metabolic plant events. The modifications triggered by NO on the transcription of genes encoding biosynthetic/degradative enzymes as well as proteins involved in the transport and signal transduction of distinct plant hormones will be contextualized during the control of developmental, metabolic, and defense responses in plants. Moreover, the direct post-translational modification of phytohormone biosynthetic enzymes and receptors through S-nitrosylation will also be discussed as a key mechanism for regulating plant physiological responses. Finally, some future perspectives toward a more complete understanding of NO-phytohormone interactions will also be presented and discussed.

Heuristic aspect of the lateral root initiation index: A case study of the role of nitric oxide in root branching

PREMISE OF THE STUDY

Lateral root (LR) initiation (LRI) is a central process in root branching. Based on LR and/or LR primordium densities, it has been shown that nitric oxide (NO) promotes LRI. However, because NO inhibits primary root growth, we hypothesized that NO may have an opposite effect if the analysis is performed on a cellular basis. Using a previously proposed parameter, the LRI index (which measures how many LRI events take place along a root portion equivalent to the length of a single file of 100 cortical cells of average length), we addressed this hypothesis and illustrate here that the LRI index provides a researcher with a tool to uncover hidden but important information about root initiation. •

METHODS AND RESULTS

Arabidopsis thaliana roots were treated with an NO donor (sodium nitroprusside [SNP]) and/or an NO scavenger (2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide [cPTIO]). LRI was analyzed separately in the root portions formed before and during the treatment. In the latter, SNP caused root growth inhibition and an increase in the LR density accompanied by a decrease in LRI index, indicating overall inhibitory outcome of the NO donor on branching. The inhibitory effect of SNP was reversed by cPTIO, showing the NO-specific action of SNP on LRI. •

CONCLUSIONS

Analysis of the LRI index permits the discovery of otherwise unknown modes of action of a substance on the root system formation. NO has a dual action on root branching, slightly promoting it in the root portion formed before the treatment and strongly inhibiting it in the root portion formed during the treatment.

Protein S-nitrosylation in plants under abiotic stress: an overview

Abiotic stress is one of the main problems affecting agricultural losses, and understanding the mechanisms behind plant tolerance and stress response will help us to develop new means of strengthening fruitful agronomy. The mechanisms of plant stress response are complex. Data obtained by experimental procedures are sometimes contradictory, depending on the species, strength, and timing applied. In recent years nitric oxide has been identified as a key signaling molecule involved in most plant responses to abiotic stress, either indirectly through gene activation or interaction with reactive oxygen species and hormones; or else directly, as a result of modifying enzyme activities mainly by nitration and S-nitrosylation. While the functional relevance of the S-nitrosylation of certain proteins has been assessed in response to biotic stress, it has yet to be characterized under abiotic stress. Here, we review initial works about S-nitrosylation in response to abiotic stress to conclude with a brief overview, and discuss further perspectives to obtain a clear outlook of the relevance of S-nitrosylation in plant response to abiotic stress.

Developmental stage- and concentration-specific sodium nitroprusside application results in nitrate reductase regulation and the modification of nitrate metabolism in leaves of Medicago truncatula plants

Nitric oxide (NO) is a bioactive molecule involved in numerous biological events that has been reported to display both pro-oxidant and antioxidant properties in plants. Several reports exist which demonstrate the protective action of sodium nitroprusside (SNP), a widely used NO donor, which acts as a signal molecule in plants responsible for the expression regulation of many antioxidant enzymes. This study attempts to provide a novel insight into the effect of application of low (100 μΜ) and high (2.5 mM) concentrations of SNP on the nitrosative status and nitrate metabolism of mature (40 d) and senescing (65 d) Medicago truncatula plants. Higher concentrations of SNP resulted in increased NO content, cellular damage levels and reactive oxygen species (ROS) concentration, further induced in older tissues. Senescing M. truncatula plants demonstrated greater sensitivity to SNP-induced oxidative and nitrosative damage, suggesting a developmental stage-dependent suppression in the plant's capacity to cope with free oxygen and nitrogen radicals. In addition, measurements of the activity of nitrate reductase (NR), a key enzyme involved in the generation of NO in plants, indicated a differential regulation in a dose and time-dependent manner. Furthermore, expression levels of NO-responsive genes (NR, nitrate/nitrite transporters) involved in nitrogen assimilation and NO production revealed significant induction of NR and nitrate transporter during long-term 2.5 mM SNP application in mature plants and overall gene suppression in senescing plants, supporting the differential nitrosative response of M. truncatula plants treated with different concentrations of SNP.

From Zinnia to Arabidopsis: approaching the involvement of peroxidases in lignification

Zinnia elegans constitutes one of the most useful model systems for studying xylem differentiation, which simultaneously involves secondary cell wall synthesis, cell wall lignification, and programmed cell death. Likewise, the in vitro culture system of Z. elegans has been the best characterized as the differentiation of mesophyll cells into tracheary elements allows study of the biochemistry and physiology of xylogenesis free from the complexity that heterogeneous plant tissues impose. Moreover, Z. elegans has emerged as an excellent plant model to study the involvement of peroxidases in cell wall lignification. This is due to the simplicity and duality of the lignification pattern shown by the stems and hypocotyls, and to the basic nature of the peroxidase isoenzyme. This protein is expressed not only in hypocotyls and stems but also in mesophyll cells transdifferentiating into tracheary elements. Therefore, not only does this peroxidase fulfil all the catalytic requirements to be involved in lignification overcoming all restrictions imposed by the polymerization step, but also its expression is inherent in lignification. In fact, its basic nature is not exceptional since basic peroxidases are differentially expressed during lignification in other model systems, showing unusual and unique biochemical properties such as oxidation of syringyl moieties. This review focuses on the experiments which led to a better understanding of the lignification process in Zinnia, starting with the basic knowledge about the lignin pattern in this plant, how lignification takes place, and how a sole basic peroxidase with unusual catalytic properties is involved and regulated by hormones, H2O2, and nitric oxide.

Pre-exposure to nitric oxide modulates the effect of ozone on oxidative defenses and volatile emissions in lima bean

The roles that ozone and nitric oxide (NO), the chief O₃ precursor, play in the antioxidative balance and inducible volatile emissions of lima bean were assessed. Exposure to O₃ inhibited APX, CAT, and GR, decreased GSH content and induced emissions of (E)-β-ocimene, limonene, 1,8-cineole, linalool, (E)-4,8-dimethyl-1,3,7-nonatriene (E)-DMNT, 2-butanone and nonanal. O₃ did not induce emissions of (E)-β-caryophyllene and appeared to reduce the antioxidative capacity of plants to a greater extent than NO and NO followed by O₃ (NO/O₃) treatments. There were significant differences in emissions of (E)-β-ocimene and linalool between NO/O₃ treated plants and controls, but no differences in antioxidant concentrations. A model to explain the relationships between the ascorbate-glutathione cycle and O₃ and NO inducible volatiles was proposed. Our findings suggest that prior exposure to NO modulates the oxidative effect of ozone by the process of cross-tolerance, which might regulate the antioxidative system and induction of volatile organic compounds.

Nitric oxide mediates cytokinin functions in cell proliferation and meristem maintenance in Arabidopsis

Cytokinin and nitric oxide (NO) have been characterized as signaling molecules to trigger cell division in tissue culture. Here, we show that the hypocotyl and root explants of Arabidopsis NO-deficient mutant nos1/noa1 exhibit severe defects in callus induction and shoot regeneration in response to cytokinin. Accordingly, depletion of NO caused by a NO scavenger leads to a severe inhibitory effect on callus induction. Moreover, cytokinin-induced NO production is impaired in nos1/noa1 in which cytokinin-triggered activation of cell cycle gene CYCD3;1 is inhibited, indicating that NO may act downstream of cytokinin in the control of cell proliferation through CYCD3;1. This hypothesis is further confirmed by the genetic evidence that constitutive expression of CYCD3;1 complements the defects of nos1/noa1 mutant in meristematic activity in shoot, root, and floral tissues as well as in cytokinin-induced callus initiation and shoot regeneration. Furthermore, we show that NO deficiency caused by loss of NOS1/NOA1 impairs cellular development such as the duration of the mitotic phase and timing of the transition to endocycles in nos1/noa1 mutant leaves, which can be reverted by constitutive expression of CYCD3;1. Taken together, these results demonstrate that NO mediates transcriptional activation of CYCD3;1 in regulating the mitotic cycles in response to cytokinins.

Nitric oxide as a key component in hormone-regulated processes

Nitric oxide (NO) is a small gaseous molecule, with a free radical nature that allows it to participate in a wide spectrum of biologically important reactions. NO is an endogenous product in plants, where different biosynthetic pathways have been proposed. First known in animals as a signaling molecule in cardiovascular and nervous systems, it has turned up to be an essential component for a wide variety of hormone-regulated processes in plants. Adaptation of plants to a changing environment involves a panoply of processes, which include the control of CO2 fixation and water loss through stomatal closure, rearrangements of root architecture as well as growth restriction. The regulation of these processes requires the concerted action of several phytohormones, as well as the participation of the ubiquitous molecule NO. This review analyzes the role of NO in relation to the signaling pathways involved in stomatal movement, plant growth and senescence, in the frame of its interaction with abscisic acid, auxins, gibberellins, and ethylene.

Nitric oxide-cold stress signalling cross-talk, evolution of a novel regulatory mechanism

Plants enhance their cold stress tolerance by cold acclimation, a process which results in vast reprogramming of transcriptome, proteome and metabolome. Evidence suggests nitric oxide (NO) production during cold stress which regulates genes (especially the C-repeat binding factor (CBF) cold stress signalling pathway), diverse proteins including transcription factors (TFs) and phosphosphingolipids. About 59% (redox), 50% (defence/stress) and 30% (signalling) cold responsive proteins are modulated by NO-based post translational modifications (PTMs) namely S-nitrosylation, tyrosine nitration and S-glutathionylation, suggesting a cross-talk between NO and cold. Analysis of cold stress responsive deep proteome in apoplast, mitochondria, chloroplast and nucleus suggested continuation of this cross-talk in sub-cellular systems. Modulation of cold responsive proteins by these PTMs right from cytoskeletal elements in plasma membrane to TFs in nucleus suggests a novel regulation of cold stress signalling. NO-mediated altered protein transport in nucleus seems an important stress regulatory mechanism. This review addresses the NO and cold stress signalling cross-talk to present the overview of this novel regulatory mechanism.

Chloroplast functionality has a positive effect on nitric oxide level in soybean cotyledons

The subcellular localization of NO generation in soybean cotyledons, and the relationship between NO synthesis and in vivo chloroplast performance were studied. Employing the NO probe 4-aminomethyl-2',7'-difluorofluorescein diacetate (DAF-FM DA) and fluorescence microscopy, a strongly punctuated fluorescence was detected in mesophyll cells. The co-localization of DAF-FM and chlorophyll fluorescence, in confocal laser microscopy images, indicated the presence of NO in the chloroplasts. NO visualization was dependent on light, seedling age, and chloroplast function throughout cotyledons lifespan. The addition of herbicides with action in chloroplasts (DCMU and paraquat) dramatically reduced the quantum yield of photosystem II (φ(PSII)), and lead to images with absence of punctuated green fluorescence. Moreover, electron paramagnetic resonance signals corresponding to NO-spin trap adduct observed in cotyledon homogenates decreased significantly by the treatment with herbicides, as compared to controls. Neither chloroplast function nor NO content were significantly different in cotyledons from plants growing in the presence of ammonium or nitrate as the nitrogen source. These findings suggest that chloroplasts are organelles that contribute to NO synthesis in vivo, and that their proper functionality is essential for maintaining NO levels in soybean cotyledons.

Co-application of selenite and phosphate reduces arsenite uptake in hydroponically grown rice seedlings: toxicity and defence mechanism

The study empirically evaluates the abatement of As(III) uptake in rice seedlings (7d), in presence of Se and phosphate (P) under hydroponic condition. Positive correlation between As(III) translocation to the shoots of As(III) and P treated seedlings, shows P dependent As(III) translocation in rice. Whereas, presence of both P (5 and 10μgml(-1)) and (0.75μgml(-1)) of Se significantly reduces the As(III) uptake in rice seedlings. Application of Se alone also reduces As(III) uptake both in shoots and roots significantly, however, the seedlings suffers from lipid peroxidation. Among all the studied treatments, lower rates of P (5μgml(-1)) and Se (0.75μgml(-1)) when co-applied, significantly reduced As(III) translocation to the shoots without inflicting much toxicity in the seedlings which is manifested as significant increase in biomass with lower thio-barbituric reactive substances (TBARS). Also, significantly lower TBARS in seedlings receiving As(4)+Se(0.75) and higher TBARS in As(4)+Se(1.5), demonstrates that Se applied at lower rates (0.75μgml(-1)), lowers As induced toxicity. Higher SOD, APX and guaiacol peroxidase (POD) activities in As(4)+P(5)+Se(0.75) compared to that of As(4)+P(5) and As(4)+Se(0.75), supports that lower rate of P and Se provides tolerance towards As induced stress. The nitrogen metabolism in As(4)+P+Se treated seedlings is affected adversely at higher rates of Se and P application. Overall study concluded that application of lower rates of P (5μgml(-1)) and Se (0.75μgml(-1)) provides maximum amelioration of As(III) toxicity in rice seedlings.

Saccharomyces cerevisiae-induced stomatal closure mainly mediated by salicylhydroxamic acid-sensitive peroxidases in Vicia faba

Saccharomyces cerevisiae induced stomatal closure in a dose-dependent manner on Vicia faba L. (cv. Daqingpi). Using pharmacological inhibitors in this study, we found that stomatal closure was completely inhibited by salicylhydroxamic acid (SHAM) and reduced glutathione (GSH), whereas slightly inhibited by diphenyleneiodonium chloride (DPI), suggesting that H2O2 was mostly produced by cell wall peroxidases. The specific NO scavenger (cPTIO), NO synthase (NOS) inhibitor NG-nitro-l-arginine methyl ester (l-NAME) and sodium azide (NaN3; inhibitor of nitrate reductase) prevented yeast-induced stomatal closure, suggesting that NO in guard cells of V. faba is derived from both NOS-like enzyme and nitrate reductase. Results of HgCl2 and β-mercaptoethanol (ME) treatment (as a functional inhibitor of water channels and its reversing agent, respectively) suggest that water channels are involved in yeast-induced stomatal movements. CoCl2 (the blocker of calcium channel), LaCl3 (Ca(2+) antagonist) and EGTA (Ca(2+) chelator) also impaired yeast-induced stomatal closure. Thus, it is concluded that H2O2, NO, water channels and Ca(2+) are involved in yeast-induced stomatal closure.

Nitric oxide (NO) in alleviation of heavy metal induced phytotoxicity and its role in protein nitration

Nitric oxide (NO) is recognized as a biological messenger in various tissues to regulate diverse range of physiological process including growth, development and response to abiotic and biotic factors. The NO emission from plants is known since the 1970s, and there is copious information on the multiple effects of exogenously applied NO on different physiological and biochemical processes of plants. Heavy metal toxicity is one of the major abiotic stresses leading to hazardous effects in plants and its toxicity is based on chemical and physical property. A common consequence of heavy metal toxicity is the uncontrolled and excessive accumulation of reactive oxygen species (ROS) which leads to peroxidation of lipids, oxidation of protein, inactivation of enzymes, DNA damage and/or interact with other vital constituents of plant cells. Recently, an increasing number of articles have reported the effects of exogenous NO on alleviating heavy metal toxicity in plants but knowledge of physiological mechanisms of NO in alleviating heavy metal toxicity is quite limited, and some results contradict one another. Therefore, to help clarify the roles of NO in heavy metal tolerance, it is important to review and discuss the recent advances on this area of research. NO can provoke both beneficial and harmful effects, which depend on the concentration and location of NO in the plant cells. NO alleviates the harmfulness of the ROS, and reacts with other target molecules, and regulates the expression of stress responsive genes under various stress conditions. This manuscript includes, the latest advances in understanding the effects of endogenous NO on heavy metal toxicity and the mechanisms and role of NO as an antioxidant as well as in protein nitration are highlighted.

The nitric oxide donor sodium nitroprusside regulates polyamine and proline metabolism in leaves of Medicago truncatula plants

Nitric oxide (NO), polyamines, and proline have all been suggested to play key roles in a wide spectrum of physiological processes and abiotic stress responses. Although exogenous application of polyamines has been shown to induce NO production, the effect of NO on polyamine biosynthesis has not yet been elucidated. Several reports exist that demonstrate the protective action of sodium nitroprusside (SNP), a widely used NO donor, which acts as a signal molecule in plants responsible for the regulation of the expression of many defense-related enzymes. This study attempted to provide a novel insight into the effects of application of low (100 μΜ) and high (2.5 mM) concentrations of SNP on the biosynthesis of two major abiotic stress response-related metabolites, polyamines and proline, in mature (40 day) and senescing (65 day) Medicago truncatula plants. Physiological data showed that long-term (24 h), higher SNP concentration resulted in decreased photosynthetic rate and stomatal conductance followed by intracellular putrescine and proline accumulation, as a result of an increase in biosynthetic arginine decarboxylase (ADC) and Δ(1) -pyrroline-5-carboxylate synthetase (P5CS) enzymatic activity, respectively. Further analysis of polyamine oxidase (PAO)/diamine oxidase (DAO) polyamine catabolic enzymes indicated that DAO enzymatic activity increased significantly in correlation with putrescine accumulation, whereas PAO activity, involved in spermidine/spermine degradation, increased slightly. Moreover, transcriptional analysis of polyamine and proline metabolism genes (P5CS, P5CR, ADC, SPMS, SPDS, SAMDC, PAO, DAO) further supported the obtained data and revealed a complex SNP concentration-, time-, and developmental stage-dependent mechanism controlling endogenous proline and polyamine metabolite production. This is the first report to provide a global analysis leading to a better understanding of the role of the widely used NO donor SNP in the regulation of key stress-related metabolic pathways.

Importance of nitric oxide in cadmium stress tolerance in crop plants

Cadmium (Cd(2+)) is a widespread heavy metal pollutant in the environment with a long biological half-life, originating mainly from industrial processes and phosphate fertilizers. It is easily taken up by plants, resulting in toxicity symptoms, such as chlorosis, wilting, growth reduction, and cell death. This cellular toxicity might result from interactions with vital metabolic pathways, carboxyl or thiol groups of proteins and reactive oxygen species (ROS) burst in plants. Plant exposure even to low concentrations of Cd may lead to cell death but the mechanism of its toxicity is still debatable. Therefore, exploring various ways to improve crop productivity and/or alleviate Cd stress effects is one of the major areas of concern. Nitric oxide (NO) is a hydrophobic gaseous molecule involved in various physiological processes such as germination, root growth, stomatal closure, control of the flowering timing etc. NO also functions as cell signaling molecule in plants and play important roles in the regulation of plant responses to both abiotic and biotic stress conditions. At the molecular level, NO signaling includes protein modification by binding to critical cysteine residues, heme or iron-sulfur centers and tyrosine residue nitration via peroxynitrite formation (ONOO(-)), mobilization of secondary messengers (Ca(2+), cyclic GMP and cyclic ADP-Rib) and modulation of protein kinase activities. Significant research had been done to understand the NO biosynthesis and signaling in plants under stress, but several questions still need to be answered. The present review is focused specifically on the importance of NO as Cd stress modulator in crop plants.

Nitric oxide (NO) counteracts cadmium induced cytotoxic processes mediated by reactive oxygen species (ROS) in Brassica juncea: cross-talk between ROS, NO and antioxidant responses

Research on NO in plants has achieved huge attention in recent years mainly due to its function in plant growth and development under biotic and abiotic stresses. In the present study, we investigated Cd induced NO generation and its relationship to ROS and antioxidant regulation in Brassica juncea. Cd accumulated rapidly in roots and caused oxidative stress as indicated by increased level of lipid peroxidation and H2O2 thus, inhibiting the overall plant growth. It significantly decreased the root length, leaf water content and photosynthetic pigments. A rapid induction in intracellular NO was observed at initial exposures and low concentrations of Cd. A 2.74-fold increase in intracellular NO was recorded in roots treated with 25 μM Cd than control. NO effects on Malondialdehyde (MDA) content and on antioxidant system was investigated by using sodium nitroprusside (SNP), a NO donor and a scavenger, [2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethylinidazoline-1-oxyl-3-oxide] (cPTIO). Roots pretreated with 5 mM SNP for 6 h when exposed to 25 μM Cd for 24 h reduced the level of proline, non-protein thiols, SOD, APX and CAT in comparison to only Cd treatments. However, this effect was almost blocked by 100 μM cPTIO pretreatment to roots for 1 h. This ameliorating effect of NO was specific because cPTIO completely reversed the effect in the presence of Cd. Thus, the present study report that NO strongly counteracts Cd induced ROS mediated cytotoxicity in B. juncea by controlling antioxidant metabolism as the related studies are not well reported in this species.

Arbuscular mycorrhizal fungal inoculation increases phenolic synthesis in clover roots via hydrogen peroxide, salicylic acid and nitric oxide signaling pathways

Arbuscular mycorrhizal fungi can increase the host resistance to pathogens via promoted phenolic synthesis, however, the signaling pathway responsible for it still remains unclear. In this study, in order to reveal the signaling molecules involved in this process, we inoculated Trifolium repense L. with an arbuscular mycorrhizal fungus (AMF), Glomus mosseae, and monitored the contents of phenolics and signaling molecules (hydrogen peroxide (H(2)O(2)), salicylic acid (SA), and nitric oxide (NO)) in roots, measured the activities of l-phenylalanine ammonia-lyase (PAL) and nitric oxide synthase (NOS), and the expression of pal and chs genes. Results demonstrated that AMF colonization promoted the phenolic synthesis, in parallel with the increase in related enzyme activity and gene expression. Meanwhile, the accumulation of all three signaling molecules was also up-regulated by AMF. This study suggested that AMF increased the phenolic synthesis in roots probably via signaling pathways of H(2)O(2), SA and NO in a signaling cascade.

Effect of elevated CO₂ on phosphorus nutrition of phosphate-deficient Arabidopsis thaliana (L.) Heynh under different nitrogen forms

Phosphorus (P) nutrition is always a key issue regarding plants responses to elevated CO(2). Yet it is unclear of how elevated CO(2) affects P uptake under different nitrogen (N) forms. This study investigated the influence of elevated CO(2) (800 µl l(-1)) on P uptake and utilization by Arabidopsis grown in pH-buffered phosphate (P)-deficient (0.5 µM) hydroponic culture supplying with 2mM nitrate (NO(3)(-)) or ammonium (NH(4)(+)). After 7 d treatment, elevated CO(2) enhanced the biomass production of both NO(3)(-)- and NH(4) (+)-fed plants but decreased the P amount absorbed per weight of roots and the P concentration in the shoots of plants supplied with NH(4)(+). In comparison, elevated CO(2) increased the amount of P absorbed per weight of roots, as well as the P concentration in plants and alleviated P deficiency-induced symptoms of plants supplied with NO(3)(-). Elevated CO(2) also increased the root/shoot ratio, total root surface area, and acid phosphatase activity, and enhanced the expression of genes or transcriptional factors involving in P uptake, allocation and remobilization in P deficient plants. Furthermore, elevated CO(2) increased the nitric oxide (NO) level in roots of NO(3)(-)-fed plants but decreased it in NH(4)(+)-fed plants. NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) inhibited plant P acquisition by roots under elevated CO(2). Considering all of these findings, this study concluded that a combination of elevated CO(2) and NO(3)(-) nutrition can induce a set of plant adaptive strategies to improve P status from P-deficient soluble sources and that NO may be a signalling molecule that controls these processes.

Nitric oxide enhances development of lateral roots in tomato (Solanum lycopersicum L.) under elevated carbon dioxide

Elevated carbon dioxide (CO₂) has been shown to enhance the growth and development of plants, especially of roots. Amongst them, lateral roots play an important role in nutrient uptake, and thus alleviate the nutrient limitation to plant growth under elevated CO₂. This paper examined the mechanism underlying CO₂ elevation-induced lateral root formation in tomato. The endogenous nitric oxide (NO) in roots was detected by the specific probe 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate (DAF-FM DA). We suggest that CO₂ elevation-induced NO accumulation was important for lateral root formation. Elevated CO₂ significantly increased the activity of nitric oxide synthase in roots, but not nitrate reductase activity. Moreover, the pharmacological evidence showed that nitric oxide synthase rather than nitrate reductase was responsible for CO₂ elevation-induced NO accumulation. Elevated CO₂ enhanced the activity of nitric oxide synthase and promoted production of NO, which was involved in lateral root formation in tomato under elevated CO₂.

Creating a bio-hybrid signal transduction pathway: opening a new channel of communication between cells and machines

Manipulation of signal transduction pathways presents a viable mechanism to interface cells with electronics. In this work, we present a two-step signal transduction pathway involving cellular and electronic transduction elements. In order to circumvent many of the conventional difficulties encountered when harnessing chemical signalling for the purpose of electronics communication, gaseous nitric oxide (NO) was selected as the signalling molecule. By genetic engineering of the nitric oxide synthase protein eNOS and insertion of light-oxygen-voltage (LOV) domains, we have created a photoactive version of the protein. The novel chimeric eNOS was found to be capable of producing NO in response to excitation by visible light. By coupling these mutant cells to a surface modified platinum electrode, it was possible to convert an optical signal into a chemical one, followed by subsequent conversion of the chemical signal into an electrical output.

FNOCT as a fluorescent probe for in vivo localization of nitric oxide distribution in tobacco roots

The nitric oxide-specific fluorescent probe Fluorescent Nitric Oxide Cheletropic Trap (FNOCT) 8a was applied to intact tobacco (Nicotiana tabacum cv. Samsun) roots to detect sites of nitric oxide formation and NO distribution. Three week old tobacco seedlings were gently removed from the sand culture pots with intact roots and transferred to small Petri dishes, whose base was replaced by a thin coverslip. Intact roots were subjected to FNOCT 8a to localize NO-dependent fluorescence in these roots; controls with an exogenous NO donor confirmed the presence and distribution of the probe in the roots. To confirm the NO-dependent fluorescence, roots were incubated with the three different NO scavengers cPTIO {2-(4-Carboxyphenyl)-4,4,5,5-tetramethylimidazoline-L-oxyl-3-oxide}, methylene blue and sodium diethyl dithiocarbamate (DCC) followed by incubation with FNOCT 8a. Methylene blue and DCC were able to completely quench NO-dependent fluorescence, cPTIO quenched partially. The roots were incubated in the presence of NaNO₂ and NaNO₃, which are substrates for nitrite:nitric oxide reductase (NI-NOR) and plasma membrane-bound nitrate reductase (PM-NR), respectively. The NO-dependent fluorescence was more or less same at the root tips upon treatment with NaNO₂, while the overall fluorescence was reduced in the presence of NaNO. Fluorescence from the living roots was visualized by inverted confocal laser scanning microscope (CLSM) using UV laser (excitation 360 nm and emission 408 nm). A specialized apparatus has been devised by the authors for analysis of intact roots as described in the methods section of this paper. Intact roots were chosen for microscopic observation rather than incised roots to avoid production of NO due to stress or physical injury.

RETRACTED: Nitric oxide and flavonoids are systemically induced by UV-B in maize leaves

This article has been retracted: please see Elsevier Policy on Article Withdrawal (https://www.elsevier.com/about/our-business/policies/article-withdrawal). This article has been retracted at the request of Cristina Lombardo, Lorenzo Lamattina, Raul Cassia. Several figures in the article by Tossi et al appear to have been intentionally manipulated and, therefore, representing results that are not accurate. The specific concerns are 1) the NO/-UVB panel in Fig. 1B is an apparent duplication of the Fig. 4 NO/PC panel; 2) the Flavonoid/UVB panel in Fig. 1B is an apparent duplication of the Fig. 4 Flavonoid/U panel; and 3), many of the RT-PCR bands in Fig. 5 are apparently identical. The apparent duplications of the panels in Fig. 1B and Fig. 4 appears to have been done intentionally. The brightness of the published Fig. 1B NO/-UVB panel was decreased and rotated 180 degrees relative to the NO/PC panel in Fig. 4. The two images are identical when the brightness of Fig. 1B is enhanced and the Fig. 4 panel rotated 180 degrees as shown in the attachment. Likewise, Fig. 1B Flavonoid/UVB panel was manipulated to disguise it from the Flavonoid/U panel in Fig. 4. We thank Dr Elisabeth Bik for drawing the irregularities to the authors' attention.

Effects of exogenous nitric oxide in wheat seedlings under chilling stress

The effects of nitric oxide (NO) on chilling tolerance (contents of hydrogen peroxide (H2O2) and superoxide anion (O2–) and lipid peroxidation level (malondialdehyde, MDA)) and the activities of antioxidant enzymes (superoxide dismutase (SOD), peroxidase (POX) and catalase (CAT)) were investigated in the leaves of wheat ( Triticum aestivum L.) exposed to chilling. NO treatment was carried out through spraying of sodium nitroprusside (SNP), which is a donor of NO. To do this, SNP concentrations of 0.1 and 1 mM were applied on the leaves of 11-day plants and the plants were then exposed to chilling conditions (5/2°C) for 3 days. The chilling stress treatment increased both the activities of antioxidant enzymes and the levels of MDA, H2O2 and O2–. Similarly, NO treatment enhanced SOD, POX and CAT activities under chilling stress, whereas it decreased H2O2 and O2.– contents as well as MDA level. The most effective concentration was determined as 0.1 mM SNP. Exogenous SNP application as a donor of NO was found to have an important ameliorative effect on cold tolerance of seedling exposed to chilling stress by stimulating antioxidant enzyme activity.

Carbon monoxide enhances the chilling tolerance of recalcitrant Baccaurea ramiflora seeds via nitric oxide-mediated glutathione homeostasis

Both carbon monoxide (CO) and nitric oxide (NO) play fundamental roles in plant responses to environmental stress. Glutathione (GSH) homeostasis through the glutathione-ascorbate cycle regulates the cellular redox status and protects the plant from damage due to reactive oxygen species (ROS) or reactive nitrogen species (RNS). Most recalcitrant seeds are sensitive to chilling stress, but the roles of and cross talk among CO, NO, ROS, and GSH in recalcitrant seeds under low temperature are not well understood. Here, we report that the germination of recalcitrant Baccaurea ramiflora seeds shows sensitivity to chilling stress, but application of exogenous CO or NO markedly increased GSH accumulation, enhanced the activities of antioxidant enzymes involved in the glutathione-ascorbate cycle, decreased the content of H(2)O(2) and RNS, and improved the tolerance of seeds to low-temperature stress. Compared to orthodox seeds such as maize, only transient accumulation of CO and NO was induced and only a moderate increase in GSH was shown in the recalcitrant B. ramiflora seeds. Exogenous CO or NO treatment further increased the GSH accumulation and S-nitrosoglutathione reductase (GSNOR) activity in B. ramiflora seeds under chilling stress. In contrast, suppressing CO or NO generation, removing GSH, or blocking GSNOR activity resulted in increases in ROS and RNS and impaired the germination of CO- or NO-induced seeds under chilling stress. Based on these results, we propose that CO acts as a novel regulator to improve the tolerance of recalcitrant seeds to low temperatures through NO-mediated glutathione homeostasis.

Overexpression of a Medicago truncatula stress-associated protein gene (MtSAP1) leads to nitric oxide accumulation and confers osmotic and salt stress tolerance in transgenic tobacco

The impact of Medicago truncatula stress-associated protein gene (MtSAP1) overexpression has been investigated in Nicotiana tabacum transgenic seedlings. Under optimal conditions, transgenic lines overexpressing MtSAP1 revealed better plant development and higher chlorophyll content as compared to wild type seedlings. Interestingly, transgenic lines showed a stronger accumulation of nitric oxide (NO), a signaling molecule involved in growth and development processes. This NO production seemed to be partially nitrate reductase dependent. Due to the fact that NO has been also reported to play a role in tolerance acquisition of plants to abiotic stresses, the responses of MtSAP1 overexpressors to osmotic and salt stress have been studied. Compared to the wild type, transgenic lines were less affected in their growth and development. Moreover, NO content in MtSAP1 overexpressors was always higher than that detected in wild seedlings under stress conditions. It seems that this better tolerance induced by MtSAP1 overexpression could be associated with this higher NO production that would enable seedlings to reach a high protection level to prepare them to cope with abiotic stresses.

Nitric oxide production mediates oligogalacturonide-triggered immunity and resistance to Botrytis cinerea in Arabidopsis thaliana

Nitric oxide (NO) regulates a wide range of plant processes from development to environmental adaptation. In this study, we investigated the production and/or function of NO in Arabidopsis thaliana leaf discs and plants elicited by oligogalacturonides (OGs) and challenged with Botrytis cinerea. We provided evidence that OGs triggered a fast and long lasting NO production which was Ca(2+) dependent and involved nitrate reductase (NR). Accordingly, OGs triggered an increase of both NR activity and transcript accumulation. NO production was also sensitive to the mammalian NO synthase inhibitor L-NAME. Intriguingly, we showed that L-NAME affected NO production by interfering with NR activity, thus questioning the mechanisms of how this compound impairs NO synthesis in plants. We further demonstrated that NO modulates RBOHD-mediated reactive oxygen species (ROS) production and participates in the regulation of OG-responsive genes such as anionic peroxidase (PER4) and a β-1,3-glucanase. Mutant plants impaired in PER4 and β-1,3-glucanase, as well as Col-0 plants treated with the NO scavenger cPTIO, were more susceptible to B. cinerea. Taken together, our investigation deciphers part of the mechanisms linking NO production, NO-induced effects and basal resistance to B. cinerea.

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.

Thiol synthetases of legumes: immunogold localization and differential gene regulation by phytohormones

In plants and other organisms, glutathione (GSH) biosynthesis is catalysed sequentially by γ-glutamylcysteine synthetase (γECS) and glutathione synthetase (GSHS). In legumes, homoglutathione (hGSH) can replace GSH and is synthesized by γECS and a specific homoglutathione synthetase (hGSHS). The subcellular localization of the enzymes was examined by electron microscopy in several legumes and gene expression was analysed in Lotus japonicus plants treated for 1-48 h with 50 μM of hormones. Immunogold localization studies revealed that γECS is confined to chloroplasts and plastids, whereas hGSHS is also in the cytosol. Addition of hormones caused differential expression of thiol synthetases in roots. After 24-48 h, abscisic and salicylic acids downregulated GSHS whereas jasmonic acid upregulated it. Cytokinins and polyamines activated GSHS but not γECS or hGSHS. Jasmonic acid elicited a coordinated response of the three genes and auxin induced both hGSHS expression and activity. Results show that the thiol biosynthetic pathway is compartmentalized in legumes. Moreover, the similar response profiles of the GSH and hGSH contents in roots of non-nodulated and nodulated plants to the various hormonal treatments indicate that thiol homeostasis is independent of the nitrogen source of the plants. The differential regulation of the three mRNA levels, hGSHS activity, and thiol contents by hormones indicates a fine control of thiol biosynthesis at multiple levels and strongly suggests that GSH and hGSH play distinct roles in plant development and stress responses.

Exogenous nitric oxide donor protects Artemisia annua from oxidative stress generated by boron and aluminium toxicity

Nitric oxide (NO) is an important signal molecule modulating the response of plants to environmental stress. Here we report the effects of boron (B) and aluminium (Al) contamination in soil, carried out with or without application of exogenous SNP (NO donor), on various plant processes in Artemisia annua, including changes in artemisinin content. The addition of B or Al to soil medium significantly reduced the yield and growth of plants and lowered the values of net photosynthetic rate, stomatal conductance, internal CO(2) concentration and total chlorophyll content. The follow-up treatment of NO donor favoured growth and improved the photosynthetic efficiency in stressed as well as non-stressed plants. Artemisinin content was enhanced by 24.6% and 43.8% at 1mmole of soil-applied B or Al. When SNP was applied at 2mmole concentration together with either 1mmole of B and/or Al, it further stimulated artemisinin biosynthesis compared to the control. Application of B+Al+SNP proved to be the best treatment combination for the artemisinin content in Artemisia annua leaves.