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

The centrality of redox regulation and sensing of reactive oxygen species in abiotic and biotic stress acclimatization

During land plant evolution, the number of genes encoding for components of the thiol redox regulatory network and the generator systems of reactive oxygen species (ROS) expanded, tentatively indicating that they have a role in tailored environmental acclimatization. This hypothesis has been validated both experimentally and theoretically during the last few decades. Recent developments of dynamic redox-sensitive GFP (roGFP)-based in vivo sensors for H2O2 and the redox potential of the glutathione pool have paved the way for dissecting the kinetics changes that occur in these crucial parameters in response to environmental stressors. The versatile cellular redox sensory and response regulatory system monitors alterations in redox metabolism and controls the activity of redox target proteins, and thereby affects most, if not all, cellular processes ranging from transcription to translation and metabolism. This review uses examples to describe the role of the redox- and ROS-dependent regulatory network in realising the appropriate responses to diverse environmental stresses. The selected case studies concern different environmental challenges, namely excess excitation energy, the heavy metal cadmium and the metalloid arsenic, nitrogen or phosphate shortages as examples for nutrient deficiency, wounding, and nematode infestation. Each challenge affects the redox-regulatory and ROS network, but our present state of knowledge also points toward pressing questions that remain open in relation to the translation of redox regulation to environmental acclimatization.

Adventitious Root Formation in Plants: The Implication of Hydrogen Peroxide and Nitric Oxide

Adventitious root formation is defined as the formation of new roots on above-ground plant parts and is considered crucial for the survival of a plant under harsh environmental conditions (i.e., flooding, salt stress, and other abiotic stresses) as well as in the nursery industry. Clonal propagation is based on the ability of a plant part to grow and generate a completely new plant, genetically identical to the mother plant, where the plant part came from. Nurseries exploit this potential by multiplying millions of new plants. Most nurseries use cuttings to achieve that, through the induction of adventitious root formation. Many factors have been implicated in the capacity of a cutting to root, with the major role being played by auxins. During the last few decades, intense interest has emerged in the role of other potential rooting co-factors, such as carbohydrates, phenolics, polyamines, and other plant growth regulators, as well as signal molecules, such as reactive oxygen and nitrogen species. Among the latter, hydrogen peroxide and nitric oxide have been found to play significant roles in adventitious root formation. Their production, action, and general implication in rhizogenesis are discussed in this review, in terms of interaction with other molecules and signaling.

Nitric Oxide Acts as a Key Signaling Molecule in Plant Development under Stressful Conditions

Nitric oxide (NO), a colorless gaseous molecule, is a lipophilic free radical that easily diffuses through the plasma membrane. These characteristics make NO an ideal autocrine (i.e., within a single cell) and paracrine (i.e., between adjacent cells) signalling molecule. As a chemical messenger, NO plays a crucial role in plant growth, development, and responses to biotic and abiotic stresses. Furthermore, NO interacts with reactive oxygen species, antioxidants, melatonin, and hydrogen sulfide. It regulates gene expression, modulates phytohormones, and contributes to plant growth and defense mechanisms. In plants, NO is mainly produced via redox pathways. However, nitric oxide synthase, a key enzyme in NO production, has been poorly understood recently in both model and crop plants. In this review, we discuss the pivotal role of NO in signalling and chemical interactions as well as its involvement in the mitigation of biotic and abiotic stress conditions. In the current review, we have discussed various aspects of NO including its biosynthesis, interaction with reactive oxygen species (ROS), melatonin (MEL), hydrogen sulfide, enzymes, phytohormones, and its role in normal and stressful conditions.

Cellular messengers involved in the inhibition of the Arabidopsis primary root growth by bacterial quorum-sensing signal N-decanoyl-L-homoserine lactone

BACKGROUND

N-acyl-homoserine lactones (AHLs) are used as quorum-sensing signals by Gram-negative bacteria, but they can also affect plant growth and disease resistance. N-decanoyl-L-homoserine lactone (C10-HSL) is an AHL that has been shown to inhibit primary root growth in Arabidopsis, but the mechanisms underlying its effects on root architecture are unclear. Here, we investigated the signaling components involved in C10-HSL-mediated inhibition of primary root growth in Arabidopsis, and their interplay, using pharmacological, physiological, and genetic approaches.

RESULTS

Treatment with C10-HSL triggered a transient and immediate increase in the concentrations of cytosolic free Ca²⁺ and reactive oxygen species (ROS), increased the activity of mitogen-activated protein kinase 6 (MPK6), and induced nitric oxide (NO) production in Arabidopsis roots. Inhibitors of Ca²⁺ channels significantly alleviated the inhibitory effect of C10-HSL on primary root growth and reduced the amounts of ROS and NO generated in response to C10-HSL. Inhibition or scavenging of ROS and NO neutralized the inhibitory effect of C10-HSL on primary root growth. In terms of primary root growth, the respiratory burst oxidase homolog mutants and a NO synthase mutant were less sensitive to C10-HSL than wild type. Activation of MPKs, especially MPK6, was required for C10-HSL to inhibit primary root growth. The mpk6 mutant showed reduced sensitivity of primary root growth to C10-HSL, suggesting that MPK6 plays a key role in the inhibition of primary root growth by C10-HSL.

CONCLUSION

Our results indicate that MPK6 acts downstream of ROS and upstream of NO in the response to C10-HSL. Our data also suggest that Ca²⁺, ROS, MPK6, and NO are all involved in the response to C10-HSL, and may participate in the cascade leading to C10-HSL-inhibited primary root growth in Arabidopsis.

Nitric oxide is involved in hydrogen sulfide-induced adventitious rooting in tomato (Solanum lycopersicum)

Nitric oxide (NO) and hydrogen sulfide (H2 S) are signalling molecules that regulate adventitious rooting in plants. However, little is known about the cross-talk between NO and H2 S during adventitious rooting. Tomato (Solanum lycopersicum L.) explants were used to investigate the roles of and relationships between NO and H2 S during rooting. Effects of the NO donor sodium nitroprusside (SNP) and the H2 S donor sodium hydrosulfide (NaHS) on adventitious rooting were dose-dependent, and the greatest biological responses were observed under 25μM SNP and 50μM NaHS. The positive effect of NaHS was reversed by the NO scavenger 2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), indicating that the H2 S-induced response was partially NO-dependent. Peroxidase (POD), polyphenol oxidase (PPO), and superoxide dismutase (SOD) activities significantly increased by SNP and NaHS treatment, and indoleacetic acid oxidase (IAAO) activity and the O2 - and H2 O2 content significantly decreased by SNP and NaHS treatment. SNP and NaHS treatment also increased the content of soluble sugar and protein and indole-3-acetic acid (IAA). cPTIO significantly mitigated the increases in POD, PPO and SOD activity and soluble sugar, protein and IAA content induced by NaHS. SNP and NaHS upregulated the expression of auxin-related genes (ARF4 and ARF16 ), cell cycle-related genes (CYCD3 , CYCA3 and CDKA1 ), and antioxidant-related genes (TPX2 , SOD and POD ); whereas cPTIO significantly inhibited the increase in the expression of these genes induced by NaHS. Overall, these results show that NO may be involved in H2 S-induced adventitious rooting by regulating the activity of rooting-related enzymes, the expression of related genes, and the content of various nutrients.

Cerium-Promoted Ginsenosides Accumulation by Regulating Endogenous Methyl Jasmonate Biosynthesis in Hairy Roots of Panax ginseng

Among rare earth elements, cerium has the unique ability of regulating the growth of plant cells and the biosynthesis of metabolites at different stages of plant development. The signal pathways of Ce3+-mediated ginsenosides biosynthesis in ginseng hairy roots were investigated. At a low concentration, Ce3+ improved the elongation and biomass of hairy roots. The Ce3+-induced accumulation of ginsenosides showed a high correlation with the reactive oxygen species (ROS), as well as the biosynthesis of endogenous methyl jasmonate (MeJA) and ginsenoside key enzyme genes (PgSS, PgSE and PgDDS). At a Ce3+ concentration of 20 mg L-1, the total ginsenoside content was 1.7-fold, and the total ginsenosides yield was 2.7-fold that of the control. Malondialdehyde (MDA) content and the ROS production rate were significantly higher than those of the control. The activity of superoxide dismutase (SOD) was significantly activated within the Ce3+ concentration range of 10 to 30 mg L-1. The activity of catalase (CAT) and peroxidase (POD) strengthened with the increasing concentration of Ce3+ in the range of 20-40 mg L-1. The Ce3+ exposure induced transient production of superoxide anion (O2•-) and hydrogen peroxide (H2O2). Together with the increase in the intracellular MeJA level and enzyme activity for lipoxygenase (LOX), there was an increase in the gene expression level of MeJA biosynthesis including PgLOX, PgAOS and PgJMT. Our results also revealed that Ce3+ did not directly influence PgSS, PgSE and PgDDS activity. We speculated that Ce3+-induced ROS production could enhance the accumulation of ginsenosides in ginseng hairy roots via the direct stimulation of enzyme genes for MeJA biosynthesis. This study demonstrates a potential approach for understanding and improving ginsenoside biosynthesis that is regulated by Ce3+-mediated signal transduction.

NO Network for Plant-Microbe Communication Underground: A Review

Mechanisms governing plant-microbe interaction in the rhizosphere attracted a lot of investigative attention in the last decade. The rhizosphere is not simply a source of nutrients and support for the plants; it is rather an ecosystem teeming with diverse flora and fauna including different groups of microbes that are useful as well as harmful for the plants. Plant-microbe interaction occurs via a highly complex communication network that involves sophisticated machinery for the recognition of friend and foe at both sides. On the other hand, nitric oxide (NO) is a key, signaling molecule involved in plant development and defense. Studies on legume-rhizobia symbiosis suggest the involvement of NO during recognition, root hair curling, development of infection threads, nodule development, and nodule senescence. A similar role of NO is also suggested in the case of plant interaction with the mycorrhizal fungi. Another, insight into the plant-microbe interaction in the rhizosphere comes from the recognition of pathogen-associated molecular patterns (PAMPs)/microbe-associated molecular patterns (MAMPs) by the host plant and thereby NO-mediated activation of the defense signaling cascade. Thus, NO plays a major role in mediating the communication between plants and microbes in the rhizosphere. Interestingly, reports suggesting the role of silicon in increasing the number of nodules, enhancing nitrogen fixation, and also the combined effect of silicon and NO may indicate a possibility of their interaction in mediating microbial communication underground. However, the exact role of NO in mediating plant-microbe interaction remains elusive. Therefore, understanding the role of NO in underground plant physiology is very important, especially in relation to the plant's interaction with the rhizospheric microbiome. This will help devise new strategies for protection against phytopathogens and enhancing plant productivity by promoting symbiotic interaction. This review focuses on the role of NO in plant-microbe communication underground.

Evidence for a role of nitric oxide in iron homeostasis in plants

Nitric oxide (NO), once regarded as a poisonous air pollutant, is now understood as a regulatory molecule essential for several biological functions in plants. In this review, we summarize NO generation in different plant organs and cellular compartments, and also discuss the role of NO in iron (Fe) homeostasis, particularly in Fe-deficient plants. Fe is one of the most limiting essential nutrient elements for plants. Plants often exhibit Fe deficiency symptoms despite sufficient tissue Fe concentrations. NO appears to not only up-regulate Fe uptake mechanisms but also makes Fe more bioavailable for metabolic functions. NO forms complexes with Fe, which can then be delivered into target cells/tissues. NO generated in plants can alleviate oxidative stress by regulating antioxidant defense processes, probably by improving functional Fe status and by inducing post-translational modifications in the enzymes/proteins involved in antioxidant defense responses. It is hypothesized that NO acts in cooperation with transcription factors such as bHLHs, FIT, and IRO to regulate the expression of enzymes and proteins essential for Fe homeostasis. However, further investigations are needed to disentangle the interaction of NO with intracellular target molecules that leads to enhanced internal Fe availability in plants.

Microcutting Redox Profile and Anatomy in Eucalyptus spp. With Distinct Adventitious Rooting Competence

Adventitious root (AR) development takes place in an intricate cellular environment. Reactive oxygen species (ROS) and antioxidant defenses, triggered by wounding in cuttings, can modulate this process. A comparative assessment of biochemical and anatomical parameters at critical rooting stages in hard- (Eucalyptus globulus Labill.) and easy- (Eucalyptus grandis W.Hill ex Maiden) to-root species was carried out. Microcuttings from seedlings were inoculated in auxin-free AR induction medium and, after 96 h, transferred to AR formation medium for a period of 24 h. Samples were collected upon excision (Texc) and at the 5th day post excision (Tform). Delayed xylem development, with less lignification, was recorded in E. globulus, when compared to E. grandis, suggesting lower activity of the cambium layer, an important site for AR development. Superoxide was more densely present around the vascular cylinder at both sampled times, and in greater quantity in E. globulus than E. grandis, declining with time in the former. Hydrogen peroxide was localized primarily along cell walls, more intensely in the primary xylem and phloem, and increased significantly at Tform in E. globulus. Ascorbate peroxidase (APX), superoxide dismutase (SOD), and catalase (CAT) activities were generally higher in E. grandis and varied as a function of time in E. globulus. Soluble guaiacol peroxidase (GPRX) activity increased from Texc to Tform in both species, whereas cell wall-bound GPRX activity increased with time in E. grandis, surpassing E. globulus. Flavonoid content increased with time in E. grandis and was higher than E. globulus at Tform. Principal component analysis showed that species- and time-derived differences contributed to almost 80% of the variance. Overall, data indicate that E. grandis shows higher cambium activity and tighter modulation of redox conditions than E. globulus. These features may influence ROS-based signaling and phytohormone homeostasis of cuttings, thereby impacting on AR development. Besides being players in the realm of AR developmental differences, the specific features herein identified could become potential tools for early clone selection and AR modulation aiming at improved clonal propagation of this forest crop.

Potential use and perspectives of nitric oxide donors in agriculture

Nitric oxide (NO) has emerged in the last 30 years as a key molecule involved in many physiological processes in plants, animals and bacteria. Current research has shown that NO can be delivered via donor molecules. In such cases, the NO release rate is dependent on the chemical structure of the donor itself and on the chemical environment. Despite NO's powerful signaling effect in plants and animals, the application of NO donors in agriculture is currently not implemented and research remains mainly at the experimental level. Technological development in the field of NO donors is rapidly expanding in scope to include controlling seed germination, plant development, ripening and increasing shelf-life of produce. Potential applications in animal production have also been identified. This concise review focuses on the use of donors that have shown potential biotechnological applications in agriculture. Insights are provided into (i) the role of donors in plant production, (ii) the potential use of donors in animal production and (iii) future approaches to explore the use and applications of donors for the benefit of agriculture. © 2016 Society of Chemical Industry.

Carbon Monoxide Is Involved in Hydrogen Gas-Induced Adventitious Root Development in Cucumber under Simulated Drought Stress

Hydrogen gas (H2) and carbon monoxide (CO) are involved in plant growth and developmental processes and may induce plant tolerance to several stresses. However, the independent roles and interaction effect of H2 and CO in adventitious root development under drought conditions have still not received the needed research attention. We hypothesize that there exists crosstalk between H2 and CO during adventitious root development under drought stress. The results of our current study revealed that 50% (v/v) hydrogen-rich water (HRW), 500 μM Hemin (the CO donor) and 30% (w/v) CO aqueous solution apparently promoted the development of adventitious roots in cucumber explants (Cucumis Sativus L.) under drought stress. H2 and CO increased relative water content (RWC), leaf chlorophyll content (chlorophyll a, b, and a+b), and chlorophyll fluorescence parameters [photochemical efficiency of photosystem II (PSII), PSII actual photochemical efficiency and photochemical quench coefficient] under drought condition. When the CO scavenger hemoglobin (Hb) or zinc protoporphyrin IX (ZnPPIX) was added to HRW/CO aqueous solution, the positive effect of HRW/CO aqueous solution on RWC, leaf chlorophyll content, and chlorophyll fluorescence parameters were reversed. Additionally, superoxide dismutases, peroxidase, catalase, and ascorbate peroxidase was significantly increased in the explants treated with HRW and CO aqueous solution under drought stress, thus alleviating oxidative damage, as indicated by decreases in thiobarbituric acid reactive substances (TBARS), hydrogen peroxide (H2O2), and superoxide radical (O2-) levels. H2 and CO also improved the levels of water soluble carbohydrate, total soluble protein, and proline content. However, the above CO/H2-mediated effects were reversed by CO scavenger Hb or CO specific synthetic inhibitor ZnPPIX. Therefore, CO may be involved in H2-induced adventitious rooting under drought stress and alleviate oxidative damage by enhancing RWC, leaf chlorophyll content, chlorophyll fluorescence parameters, metabolic constituent content, activating anti-oxidant enzymes and reducing TBARS, O2-, and H2O2 levels.

Nitric oxide is involved in hydrogen gas-induced cell cycle activation during adventitious root formation in cucumber

BACKGROUND

Adventitious root development is a complex process regulated through a variety of signaling molecules. Hydrogen gas (H2) and nitric oxide (NO), two new signaling molecules are both involved in plant development and stress tolerance.

RESULTS

To investigate the mechanism of adventitious root development induced by hydrogen-rich water (HRW), a combination of fluorescence microscopy and molecular approaches was used to study cell cycle activation and cell cycle-related gene expression in cucumber (Cucumis sativus 'Xinchun 4') explants. The results revealed that the effect of HRW on adventitious root development was dose-dependent, with maximal biological responses at 50 % HRW. HRW treatment increased NO content in a time-dependent fashion. The results also indicated that HRW and NO promoted the G1-to-S transition and up-regulated cell cycle-related genes: CycA (A-type cyclin), CycB (B-type cyclin), CDKA (cyclin-dependent kinase A) and CDKB (cyclin-dependent kinase B) expression. Additionally, target genes related to adventitious rooting were up-regulated by HRW and NO in cucumber explants. While, the responses of HRW-induced adventitious root development and increase of NO content were partially blocked by a specific NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt, NO synthase (NOS)-like enzyme inhibitor N(G) -nitro-L-arginine methylester hydrochloride, or nitrate reductase inhibitors tungstate and NaN3. These chemicals also partially reversed the effect of HRW on cell cycle activation and the transcripts of cell cycle regulatory genes and target genes related adventitious root formation.

CONCLUSIONS

Together, NO may emerge as a downstream signaling molecule in H2-induced adventitious root organogenesis. Additionally, H2 mediated cell cycle activation via NO pathway during adventitious root formation.

SIGNAL MEDIATORS AT INDUCTION OF HEAT RESISTANCE OF WHEAT PLANTLETS BY SHORT-TERM HEATING

The effects of functional interplay of calcium ions, reactive oxygen species (ROS) and nitric oxide (NO) in the cells of wheat plantlets roots (Triticum aestivum L.) at the induction of their heat resistance by a short-term influence of hyperthermia (heating at the temperature of 42 degrees C during 1 minute) have been investigated. The transitional increase of NO and H2O2 content, invoked by heating, was suppressed by the treatment of plantlets with the antagonists of calcium EGTA (chelator of exocellular calcium), lanthanum chloride (blocker of calcium channels of various types) and neomycin (inhibitor of phosphatidylinositol-dependent phospholipase C). The rise of hydrogen peroxide content, caused by hardening, was partially suppressed by the action of inhibitors of nitrate reductase (sodium wolframate) and NO-synthase (N(G)-nitro-L-arginine methyl ester--L-NAME), and the increasing of nitric oxide content was suppressed by the treatment of plants with the antioxidant ionol and with the scavenger of hydrogen peroxide (dimethylthiourea). These compounds and antagonists of calcium also partially removed the effect of the rise of plantlets' heat resistance, invoked by hardening heating. The conclusion on calcium's role in the activation of enzymatic systems, generating reactive oxygen species and nitric oxide, and on the functional interplay of these signal mediators at the induction of heat resistance of plantlets by hardening heating is made.

Nitric Oxide Alleviates Salt Stress Inhibited Photosynthetic Performance by Interacting with Sulfur Assimilation in Mustard

The role of nitric oxide (NO) and sulfur (S) on stomatal responses and photosynthetic performance was studied in mustard (Brassica juncea L.) in presence or absence of salt stress. The combined application of 100 μM NO (as sodium nitroprusside) and 200 mg S kg(-1) soil (S) more prominently influenced stomatal behavior, photosynthetic and growth performance both in the absence and presence of salt stress. The chloroplasts from salt-stressed plants had disorganized chloroplast thylakoids, but combined application of NO and S resulted in well-developed chloroplast thylakoids and properly stacked grana. The leaves from plants receiving NO plus S exhibited lower superoxide ion accumulation under salt stress than the plants receiving NO or S. These plants also exhibited increased activity of ATP-sulfurylase (ATPS), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR) and optimized NO generation that helped in minimizing oxidative stress. The enhanced S-assimilation of these plants receiving NO plus S resulted in increased production of cysteine (Cys) and reduced glutathione (GSH). These findings indicated that NO influenced photosynthesis under salt stress by regulating oxidative stress and its effects on S-assimilation, an antioxidant system and NO generation. The results suggest that NO improves photosynthetic performance of plants grown under salt stress more effectively when plants received S.

Versatile roles of plant NADPH oxidases and emerging concepts

NADPH oxidase (NOX) is a key player in the network of reactive oxygen species (ROS) producing enzymes. It catalyzes the production of superoxide (O2(-)), that in turn regulates a wide range of biological functions in a broad range of organisms. Plant Noxes are known as respiratory burst oxidase homologs (Rbohs) and are homologs of catalytic subunit of mammalian phagocyte gp91(phox). They are unique among other ROS producing mechanisms in plants as they integrate different signal transduction pathways in plants. In recent years, there has been addition of knowledge on various aspects related to its structure, regulatory components and associated mechanisms, and its plethora of biological functions. This update highlights some of the recent developments in the field with particular reference to important members of the plant kingdom.

Nitric oxide, antioxidants and prooxidants in plant defence responses

In plant cells the free radical nitric oxide (NO) interacts both with anti- as well as prooxidants. This review provides a short survey of the central roles of ascorbate and glutathione-the latter alone or in conjunction with S-nitrosoglutathione reductase-in controlling NO bioavailability. Other major topics include the regulation of antioxidant enzymes by NO and the interplay between NO and reactive oxygen species (ROS). Under stress conditions NO regulates antioxidant enzymes at the level of activity and gene expression, which can cause either enhancement or reduction of the cellular redox status. For instance chronic NO production during salt stress induced the antioxidant system thereby increasing salt tolerance in various plants. In contrast, rapid NO accumulation in response to strong stress stimuli was occasionally linked to inhibition of antioxidant enzymes and a subsequent rise in hydrogen peroxide levels. Moreover, during incompatible Arabidopsis thaliana-Pseudomonas syringae interactions ROS burst and cell death progression were shown to be terminated by S-nitrosylation-triggered inhibition of NADPH oxidases, further highlighting the multiple roles of NO during redox-signaling. In chemical reactions between NO and ROS reactive nitrogen species (RNS) arise with characteristics different from their precursors. Recently, peroxynitrite formed by the reaction of NO with superoxide has attracted much attention. We will describe putative functions of this molecule and other NO derivatives in plant cells. Non-symbiotic hemoglobins (nsHb) were proposed to act in NO degradation. Additionally, like other oxidases nsHb is also capable of catalyzing protein nitration through a nitrite- and hydrogen peroxide-dependent process. The physiological significance of the described findings under abiotic and biotic stress conditions will be discussed with a special emphasis on pathogen-induced programmed cell death (PCD).

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.

Endogenous nitric oxide generation in protoplast chloroplasts

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

Roles of reactive oxygen species in methyl jasmonate and nitric oxide-induced tanshinone production in Salvia miltiorrhiza hairy roots

Salvia miltiorrhiza is one of the most popular traditional Chinese medicinal plants for treatment of coronary heart disease. Tanshinones are the main biological active compounds in S. miltiorrhiza. In this study, effects of exogenous methyl jasmonate (MJ) and nitric oxide (NO) on tanshinone production in S. miltiorrhiza hairy roots were investigated and the roles of reactive oxygen species (ROS) in MJ and NO-induced tanshinone production were elucidated further. The results showed that contents of four tanshinone compounds were significantly increased by 100 μM MJ when compared to the control. Application of 100 μM sodium nitroprusside (SNP), a donor of NO, also resulted in a significant increase of tanshinone production. Expression of two key genes encoding 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) and 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR) was up-regulated by MJ and SNP. Generations of O(2)(-) and H(2)O(2) were triggered by MJ, but not by SNP. The increase of tanshinone production and up-regulation of HMGR and DXR expression induced by MJ were significantly inhibited by ROS scavengers, superoxide dismutase (SOD) and catalase (CAT). However, neither SOD nor CAT was able to suppress the SNP-induced increase of tanshinone production and expression of HMGR and DXR gene. In conclusion, tanshinone production was significantly stimulated by MJ and SNP. Of four tanshinone compounds, cryptotanshinone accumulation was most affected by MJ elicitation, while cryptotanshinone and tanshinone IIA accumulation was more affected by SNP elicitation. ROS mediated MJ-induced tanshinone production, but SNP-induced tanshinone production was ROS independent.

Nitric oxide elicitation for secondary metabolite production in cultured plant cells

Nitric oxide (NO) is an important signal molecule in stress responses. Accumulation of secondary metabolites often occurs in plants subjected to stresses including various elicitors or signal molecules. NO has been reported to play important roles in elicitor-induced secondary metabolite production in tissue and cell cultures of medicinal plants. Better understanding of NO role in the biosynthesis of such metabolites is very important for optimizing the commercial production of those pharmaceutically significant secondary metabolites. This paper summarizes progress made on several aspects of NO signal leading to the production of plant secondary metabolites, including various abiotic and biotic elicitors that induce NO production, elicitor-triggered NO generation cascades, the impact of NO on growth development and programmed cell death in medicinal plants, and NO-mediated regulation of the biosynthetic pathways of such metabolites. Cross-talks among NO signaling and reactive oxygen species, salicylic acid, and jasmonic acid are discussed. Some perspectives on the application of NO donors for induction of the secondary metabolite accumulation in plant cultures are also presented.

High temperature limits in vivo pollen tube growth rates by altering diurnal carbohydrate balance in field-grown Gossypium hirsutum pistils

It has recently been reported that high temperature slows in vivo pollen tube growth rates in Gossypium hirsutum pistils under field conditions. Although numerous physical and biochemical pollen-pistil interactions are necessary for in vivo pollen tube growth to occur, studies investigating the influence of heat-induced changes in pistil biochemistry on in vivo pollen tube growth rates are lacking. We hypothesized that high temperature would alter diurnal pistil biochemistry and that pollen tube growth rates would be dependent upon the soluble carbohydrate content of the pistil during pollen tube growth. G. hirsutum seeds were sown on different dates to obtain flowers exposed to contrasting ambient temperatures but at the same developmental stage. Diurnal pistil measurements included carbohydrate balance, glutathione reductase (GR; EC 1.8.1.7), soluble protein, superoxide dismutase (SOD; EC 1.15.1.1), NADPH oxidase (NOX; EC 1.6.3.1), adenosine triphosphate (ATP), and water-soluble calcium. Soluble carbohydrate levels in cotton pistils were as much as 67.5% lower under high temperature conditions (34.6 °C maximum air temperature; August 4, 2009) than under cooler conditions (29.9 °C maximum air temperature; August 14, 2009). Regression analysis revealed that pollen tube growth rates were highly correlated with the soluble carbohydrate content of the pistil during pollen tube growth (r² = 0.932). Higher ambient temperature conditions on August 4 increased GR activity in the pistil only during periods not associated with in vivo pollen tube growth; pistil protein content declined earlier in the day under high temperatures; SOD and NOX were unaffected by either sample date or time of day; pistil ATP and water soluble calcium were unaffected by the warmer temperatures. We conclude that moderate heat stress significantly alters diurnal carbohydrate balance in the pistil and suggest that pollen tube growth rate through the style may be limited by soluble carbohydrate supply in the pistil.

Sodium nitroprusside-mediated alleviation of iron deficiency and modulation of antioxidant responses in maize plants

BACKGROUND AND AIMS

Nitric oxide (NO) has been reported to alleviate Fe-deficiency effects, possibly by enhancing the functional Fe status of plants. This study examines changes in tissue Fe status and oxidative metabolism in Fe-deficient maize (Zea mays L.) plants enriched with NO using sodium nitroprusside (SNP) as a source.

METHODOLOGY

Measurements included changes in concentrations of H(2)O(2), non-protein thiols, levels of lipid peroxidation and activities of superoxide dismutase (SOD) and of the Fe-requiring antioxidant haem enzymes catalase, peroxidase and ascorbate peroxidases. Internal NO in Fe-deficient maize plants was manipulated with SNP and the NO scavenger, methylene blue (MB). A key control was treatment with sodium ferrocyanide (SF), a non-NO-supplying analogue of SNP.

PRINCIPAL RESULTS

SNP but not SF caused re-greening of leaves in Fe-deficient maize plants over 10-20 days, increased in vivo NO content, raised chlorophyll and carotenoid concentrations, promoted growth in dry weight, increased the activities of H(2)O(2)-scavenging haem enzymes and enhanced lipid peroxidation, while decreasing SOD activity and H(2)O(2) concentrations. The NO scavenger, MB, blocked the effects of the SNP. Although SNP and SF each donated Fe and increased active Fe, only SNP increased leaf chlorophyll.

CONCLUSIONS

NO plays a role in Fe nutrition, independently of its effect on total or active Fe status. The most probable mechanism of NO involvement is to increase the intracellular availability of Fe by means of modulating redox. This is likely to be achieved by enhancing the chemical reduction of foliar Fe(III) to Fe(II).

Heat stress-induced limitations to reproductive success in Gossypium hirsutum

Using in vitro systems, numerous authors have cited the sensitivity of pollen tube growth to high temperature as a major cause of low yields for crops with valuable reproductive structures. We investigated the hypothesis that in vivo fertilization efficiency would be negatively affected by heat stress-induced changes in energy reserves and calcium-mediated oxidative status in the pistil. Gossypium hirsutum plants exposed to optimal (30/20 degrees C) or high day temperature (38/20 degrees C) conditions during flowering were analyzed for fertilization efficiency via UV microscopic observation of pollen tube-containing ovules and for soluble carbohydrates, adenosine triphosphate (ATP), calcium, antioxidant enzyme activity and NADPH oxidase (NOX; EC 1.6.3.1) activity in the pistil. Leaf measurements included gas exchange, chlorophyll content, quantum efficiency and ATP content of the subtending leaf on the day of anthesis. In the pistil fertilization efficiency, soluble carbohydrates, ATP content and NOX activity declined significantly, whereas water soluble calcium and glutathione reductase (EC 1.8.1.7) activity increased, and superoxide dismutase (EC 1.15.1.1) activity remained unchanged. In leaves, heat stress decreased photosynthesis, quantum efficiency and chlorophyll content, but increased stomatal conductance. We conclude that decreased source leaf activity either inhibits pollen development, tube growth through the style or guidance to the ovules as a result of an insufficient energy supply to the developing pistil. We further conclude that a calcium-augmented antioxidant response in heat-stressed pistils interferes with enzymatic superoxide production needed for normal pollen tube growth and fertilization of the ovule.

A possible mechanism for the apocynin-induced nitric oxide accumulation in plants

Nitric oxide (NO) is a small, ubiquitous bioactive molecule, postulated as a broad spectrum anti-stress compound. The NADPH oxidase inhibitor apocynin induces the accumulation of endogenous NO in leaves of maize seedlings through a nitric oxide synthase (NOS)-like activity, and confers an augmented tolerance to UV-B-induced oxidative damage. Here we propose a mechanism for the apocynin-induced NO increase in plants. NOS catalyzes the oxidation of arginine to citrulline and NO. It is suggested that apocynin inhibits arginase, the enzyme that hydrolyzes L-arginine to urea and L-ornithine, increasing the arginine availability for arginine-dependent NO synthesis. Superoxide (O(2)(-)) is a strong NO scavenger due to its high reactivity with NO to give peroxynitrite (ONOO(-)). Superoxide is mainly produced by plant NADPH oxidase (pNOX). Inhibition of pNOX by apocynin at relatively high NO concentration, could reduces the formation of O(2)(-) and ONOO(-), increasing the availability of a huge amount of NO. We consider apocynin as a very attractive compound for studying NO-regulated processes in plants since it can replace the use of NO donors and overcome the subsequent technical problems.

The enzymology of nitric oxide in bacterial pathogenesis and resistance

Mammalian NOSs (nitric oxide synthases) are haem-based monoxygenases that oxidize the amino acid arginine to the intracellular signal and protective cytotoxin nitric oxide (NO). Certain strains of mostly Gram-positive bacteria contain homologues of the mammalian NOS catalytic domain that can act as NOSs when suitable reductants are supplied. Crystallographic analyses of bacterial NOSs, with substrates and haem-ligands, have disclosed important features of assembly and active-centre chemistry, both general to the NOS family and specific to the bacterial proteins. The slow reaction profiles and especially stable haem-oxygen species of NOSs derived from bacterial thermophiles have facilitated the study of NOS reaction intermediates. Functionally, bacterial NOSs are distinct from their mammalian counterparts. In certain strains of Streptomyces, they participate in the biosynthetic nitration of plant toxins. In the radiation-resistant bacterium Deinococcus radiodurans, NOSs are also likely to be involved in biosynthetic nitration reactions, but, furthermore, appear to play an important role in the recovery from damage induced by UV radiation.

Nitric oxide: new evidence for novel therapeutic indications

BACKGROUND

Nitric oxide (NO) deficiency is implicated in many pathophysiological processes in mammals. NO is a ubiquitous molecule involved in multiple cellular functions. Uncontrolled or inappropriate production of NO may lead to several disease states including septic shock, rheumatoid and inflammatory arthropathies, and expansion of cerebral damage after stroke. However, to date, there are no therapeutic agents available that can overcome these conditions. Similarly, underproduction of NO by NO synthase or enhanced breakdown of NO also leads to diseases such as hypertension, ischemic conditions, pre-eclampsia, premature delivery, among others. NO donor therapies are indicated in these conditions.

RESULTS

Nitroglycerin and nitrates (NO donors) have been used as therapeutic agents for the past century, particularly to treat vascular disease, and the only significant adverse effects are headaches. NO donors are highly cost-effective and have beneficial effects in multiple body systems. When the body cannot generate NO via NO synthase or due to rapid turnover leading to inadequate amounts of NO available for biological homeostasis, administration of exogenous NO, or prolongation of the actions of endogenous NO, are practical ways to supplement NO.

CONCLUSION

Recipients of such therapy include patients with angina pectoris, coronary artery disease, hypertension, osteoporosis, gastrointestinal motility disorders, pregnancy-related disorders including premature delivery, pre-eclampsia, vulvodynia, and erectile dysfunction in men. Postmenopausal NO deficiency is rectified with hormone replacement therapy, which enhances local production of NO. Declining local NO production secondary to estrogen deficiency in postmenopausal women and perhaps in older men could be one of the reasons for age-related increased incidences of cardiovascular events and sexual dysfunction. Thus, in addition to supplementation of NO compounds in acute situations like alleviating angina and erectile dysfunction, chronic NO therapy is cost-effective in decreasing cardiovascular events, and improving the urogenital system and skeletal health.

Aerobic nitric oxide production by Azospirillum brasilense Sp245 and its influence on root architecture in tomato

The major feature of the plant-growth-promoting bacteria Azospirillum brasilense is its ability to modify plant root architecture. In plants, nitric oxide (NO) mediates indole-3-acetic acid (IAA)-signaling pathways leading to both lateral (LR) and adventitious (AR) root formation. Here, we analyzed aerobic NO production by A. brasilense Sp245 wild type (wt) and its mutants Faj009 (IAA-attenuated) and Faj164 (periplasmic nitrate reductase negative), and its correlation with tomato root-growth-promoting effects. The wt and Faj009 strains produced 120 nmol NO per gram of bacteria in aerated nitrate-containing medium. In contrast, Faj164 produced 5.6 nmol NO per gram of bacteria, indicating that aerobic denitrification could be considered an important source of NO. Inoculation of tomato (Solanum lycopersicum Mill.) seedlings with both wt and Faj009 induced LR and AR development. In contrast, Faj164 mutant was not able to promote LR or AR when seedlings grew in nitrate. When NO was removed with the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), both LR and AR formation were inhibited, providing evidence that NO mediated Azospirillum-induced root branching. These results show that aerobic NO synthesis in A. brasilense could be achieved by different pathways and give evidence for an NO-dependent promoting activity on tomato root branching regardless of bacterial capacity for IAA synthesis.