Haemoglobin modulates salicylate and jasmonate/ethylene-mediated resistance mechanisms against pathogens

Protein S-nitrosylation in photosynthetic organisms: A comprehensive overview with future perspectives

BACKGROUND

The free radical nitric oxide (NO) and derivative reactive nitrogen species (RNS) play essential roles in cellular redox regulation mainly through protein S-nitrosylation, a redox post-translational modification in which specific cysteines are converted to nitrosothiols.

SCOPE OF VIEW

This review aims to discuss the current state of knowledge, as well as future perspectives, regarding protein S-nitrosylation in photosynthetic organisms.

MAJOR CONCLUSIONS

NO, synthesized by plants from different sources (nitrite, arginine), provides directly or indirectly the nitroso moiety of nitrosothiols. Biosynthesis, reactivity and scavenging systems of NO/RNS, determine the NO-based signaling including the rate of protein nitrosylation. Denitrosylation reactions compete with nitrosylation in setting the levels of nitrosylated proteins in vivo.

GENERAL SIGNIFICANCE

Based on a combination of proteomic, biochemical and genetic approaches, protein nitrosylation is emerging as a pervasive player in cell signaling networks. Specificity of protein nitrosylation and integration among different post-translational modifications are among the major challenges for future experimental studies in the redox biology field. This article is part of a Special Issue entitled: Plant Proteomics--a bridge between fundamental processes and crop production, edited by Dr. Hans-Peter Mock.

Reactive Nitrogen Species in Mitochondria and Their Implications in Plant Energy Status and Hypoxic Stress Tolerance

Hypoxic and anoxic conditions result in the energy crisis that leads to cell damage. Since mitochondria are the primary organelles for energy production, the support of these organelles in a functional state is an important task during oxygen deprivation. Plant mitochondria adapted the strategy to survive under hypoxia by keeping electron transport operative even without oxygen via the use of nitrite as a terminal electrons acceptor. The process of nitrite reduction to nitric oxide (NO) in the mitochondrial electron transport chain recycles NADH and leads to a limited rate of ATP production. The produced ATP alongside with the ATP generated by fermentation supports the processes of transcription and translation required for hypoxic survival and recovery of plants. Non-symbiotic hemoglobins (called phytoglobins in plants) scavenge NO and thus contribute to regeneration of NAD(+) and nitrate required for the operation of anaerobic energy metabolism. This overall operation represents an important strategy of biochemical adaptation that results in the improvement of energy status and thereby in protection of plants in the conditions of hypoxic stress.

A JAZ Protein in Astragalus sinicus Interacts with a Leghemoglobin through the TIFY Domain and Is Involved in Nodule Development and Nitrogen Fixation

Leghemoglobins (Lbs) play an important role in legumes-rhizobia symbiosis. Lbs bind O2 and protect nitrogenase activity from damage by O2 in nodules, therefore, they are regarded as a marker of active nitrogen fixation in nodules. Additionally, Lbs are involved in the nitric oxide (NO) signaling pathway, acting as a NO scavenger during nodule development and nitrogen fixation. However, regulators responsible for Lb expression and modulation of Lb activity have not been characterized. In our previous work, a Jasmonate-Zim-domain (JAZ) protein interacting with a Lb (AsB2510) in Astragalus sinicus was identified and designated AsJAZ1. In this study, the interaction between AsJAZ1 and AsB2510 was verified using a yeast two-hybrid system and in vitro Glutathione S-transferase (GST) pull-down assays, resulting in identification of the interaction domain as a TIFY (previously known as zinc-finger protein expressed in inflorescence meristem, ZIM) domain. TIFY domain is named after the most conserved amino acids within the domain. Bimolecular fluorescence complementation (BiFC) was used to confirm the interaction between AsJAZ1 and AsB2510 in tobacco cells, demonstrating that AsJAZ1-AsB2510 interaction was localized to the cell membrane and cytoplasm. Furthermore, the expression patterns and the symbiotic phenotypes of AsJAZ1 were investigated. Knockdown of AsJAZ1 expression via RNA interference led to decreased number of nodules, abnormal development of bacteroids, accumulation of poly-x-hydroxybutyrate (PHB) and loss of nitrogenase activity. Taken together, our results suggest that AsJAZ1 interacts with AsB2510 and participates in nodule development and nitrogen fixation. Our results provide novel insights into the functions of Lbs or JAZ proteins during legume-rhizobia symbiosis.

Control of resin production in Araucaria angustifolia, an ancient South American conifer

Araucaria angustifolia is an ancient slow-growing conifer that characterises parts of the Southern Atlantic Forest biome, currently listed as a critically endangered species. The species also produces bark resin, although the factors controlling its resinosis are largely unknown. To better understand this defence-related process, we examined the resin exudation response of A. angustifolia upon treatment with well-known chemical stimulators used in fast-growing conifers producing both bark and wood resin, such as Pinus elliottii. The initial hypothesis was that A. angustifolia would display significant differences in the regulation of resinosis. The effect of Ethrel(®) (ET - ethylene precursor), salicylic acid (SA), jasmonic acid (JA), sulphuric acid (SuA) and sodium nitroprusside (SNP - nitric oxide donor) on resin yield and composition in young plants of A. angustifolia was examined. In at least one of the concentrations tested, and frequently in more than one, an aqueous glycerol solution applied on fresh wound sites of the stem with one or more of the adjuvants examined promoted an increase in resin yield, as well as monoterpene concentration (α-pinene, β-pinene, camphene and limonene). Higher yields and longer exudation periods were observed with JA and ET, another feature shared with Pinus resinosis. The results suggest that resinosis control is similar in Araucaria and Pinus. In addition, A. angustifolia resin may be a relevant source of valuable terpene chemicals, whose production may be increased by using stimulating pastes containing the identified adjuvants.

Transcriptional profile of sweet orange in response to chitosan and salicylic acid

Background

Resistance inducers have been used in annual crops as an alternative for disease control. Wood perennial fruit trees, such as those of the citrus species, are candidates for treatment with resistance inducers, such as salicylic acid (SA) and chitosan (CHI). However, the involved mechanisms in resistance induced by elicitors in citrus are currently few known.

Results

In the present manuscript, we report information regarding the transcriptional changes observed in sweet orange in response to exogenous applications of SA and CHI using RNA-seq technology. More genes were induced by SA treatment than by CHI treatment. In total, 1,425 differentially expressed genes (DEGs) were identified following treatment with SA, including the important genes WRKY50, PR2, and PR9, which are known to participate in the salicylic acid signaling pathway, and genes involved in ethylene/Jasmonic acid biosynthesis (ACS12, AP2 domain-containing transcription factor, and OPR3). In addition, SA treatment promoted the induction of a subset of genes involved in several metabolic processes, such as redox states and secondary metabolism, which are associated with biotic stress. For CHI treatment, there were 640 DEGs, many of them involved in secondary metabolism. For both SA and CHI treatments, the auxin pathway genes were repressed, but SA treatment promoted induction in the ethylene and jasmonate acid pathway genes, in addition to repressing the abscisic acid pathway genes. Chitosan treatment altered some hormone metabolism pathways. The DEGs were validated by quantitative Real-Time PCR (qRT-PCR), and the results were consistent with the RNA-seq data, with a high correlation between the two analyses.

Conclusions

We expanded the available information regarding induced defense by elicitors in a species of Citrus that is susceptible to various diseases and identified the molecular mechanisms by which this defense might be mediated.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1440-5) contains supplementary material, which is available to authorized users.

Involvement of nitric oxide in the jasmonate-dependent basal defense against root-knot nematode in tomato plants

Jasmonic acid (JA) and nitric oxide (NO) are well-characterized signaling molecules in plant defense responses. However, their roles in plant defense against root-knot nematode (RKN, Meloidogyne incognita) infection are largely unknown. In this study, we found that the transcript levels of the JA- and NO-related biosynthetic and signaling component genes were induced after RKN infection. Application of exogenous JA and sodium nitroprusside (SNP; a NO donor) significantly decreased the number of egg masses in tomato roots after RKN infection and partially alleviated RKN-induced decreases in plant fresh weight and net photosynthetic rate. These molecules also alleviated RKN-induced increases in root electrolyte leakage and membrane peroxidation. Importantly, NO scavenger partially inhibited JA-induced RKN defense. The pharmacological inhibition of JA biosynthesis significantly increased the plants' susceptibility to RKNs, which was effectively alleviated by SNP application, showing that NO may be involved in the JA-dependent RKN defense pathway. Furthermore, both JA and SNP induced increases in protease inhibitor 2 (PI2) gene expression after RKN infestation. Silencing of PI2 compromised both JA- and SNP-induced RKN defense responses, suggesting that the PI2 gene mediates JA- and NO-induced defense against RKNs. This work will be important for deepening the understanding of the mechanisms involved in basal defense against RKN attack in plants.

Nitric oxide: a multitasked signaling gas in plants

Nitric oxide (NO) is a gaseous reactive oxygen species (ROS) that has evolved as a signaling hormone in many physiological processes in animals. In plants it has been demonstrated to be a crucial regulator of development, acting as a signaling molecule present at each step of the plant life cycle. NO has also been implicated as a signal in biotic and abiotic responses of plants to the environment. Remarkably, despite this plethora of effects and functional relationships, the fundamental knowledge of NO production, sensing, and transduction in plants remains largely unknown or inadequately characterized. In this review we cover the current understanding of NO production, perception, and action in different physiological scenarios. We especially address the issues of enzymatic and chemical generation of NO in plants, NO sensing and downstream signaling, namely the putative cGMP and Ca(2+) pathways, ion-channel activity modulation, gene expression regulation, and the interface with other ROS, which can have a profound effect on both NO accumulation and function. We also focus on the importance of NO in cell-cell communication during developmental processes and sexual reproduction, namely in pollen tube guidance and embryo sac fertilization, pathogen defense, and responses to abiotic stress.

Hypoxia induces stem and leaf nitric oxide (NO) emission from poplar seedlings

Hypoxia leads to NO formation in poplar roots. Additionally, either NO or a NO derivative is transported from the roots to the shoot causing NO emission from aboveground plant organs. Nitric oxide (NO) is involved in the response of plants to various forms of stress including hypoxia. It also seems to play an important role in stomatal closure during stress exposure. In this study, we investigated the formation of NO in roots of intact poplar (Populus × canescens) plants in response to hypoxia, as well as its dependence on nitrate availability. We further addressed the question if root hypoxia triggers NO emission from aboveground plant parts, i.e., stems and leaves of young poplar trees. Our results indicate that NO is formed in poplar roots in response to hypoxia and that this production depends on the availability of nitrate and its conversion product nitrite. As long as nitrate was available in the nutrient solution, NO emission of roots occurred; in the range of the nitrate concentrations (10-100 µM) tested, NO emission was widely independent on nitrate concentration. However, the time period in which NO was emitted and the total amount of NO emitted strongly depended on the nitrate concentration of the solution. Hypoxia also led to increased NO emissions from the leaves and stems of the trees. There was a tight correlation between leaf and stem NO emission of hypoxia-treated plants. We propose that NO is produced by nitrate reductase in the roots and either NO itself, a metabolic NO precursor, or a NO derivative is transported in the xylem sap of the trees from the roots to the shoot thereby mediating NO emission from aboveground parts of the plant.

THB1, a truncated hemoglobin, modulates nitric oxide levels and nitrate reductase activity

Hemoglobins are ubiquitous proteins that sense, store and transport oxygen, but the physiological processes in which they are implicated is currently expanding. Recent examples of previously unknown hemoglobin functions, which include scavenging of the signaling molecule nitric oxide (NO), illustrate how the implication of hemoglobins in different cell signaling processes is only starting to be unraveled. The extent and diversity of the hemoglobin protein family suggest that hemoglobins have diverged and have potentially evolved specialized functions in certain organisms. A unique model organism to study this functional diversity at the cellular level is the green alga Chlamydomonas reinhardtii because, among other reasons, it contains an unusually high number of a particular type of hemoglobins known as truncated hemoglobins (THB1-THB12). Here, we reveal a cell signaling function for a truncated hemoglobin of Chlamydomonas that affects the nitrogen assimilation pathway by simultaneously modulating NO levels and nitrate reductase (NR) activity. First, we found that THB1 and THB2 expression is modulated by the nitrogen source and depends on NIT2, a transcription factor required for nitrate assimilation genes expression. Furthermore, THB1 is highly expressed in the presence of NO and is able to convert NO into nitrate in vitro. Finally, THB1 is maintained on its active and reduced form by NR, and in vivo lower expression of THB1 results in increased NR activity. Thus, THB1 plays a dual role in NO detoxification and in the modulation of NR activity. This mechanism can partly explain how NO inhibits NR post-translationally.

Systematic analysis of phloem-feeding insect-induced transcriptional reprogramming in Arabidopsis highlights common features and reveals distinct responses to specialist and generalist insects

Phloem-feeding insects (PFIs), of which aphids are the largest group, are major agricultural pests causing extensive damage to crop plants. In contrast to chewing insects, the nature of the plant response to PFIs remains poorly characterized. Scrutiny of the literature concerning transcriptional responses of model and crop plant species to PFIs reveals surprisingly little consensus with respect to the transcripts showing altered abundance following infestation. Nevertheless, core features of the transcriptional response to PFIs can be defined in Arabidopsis thaliana. This comparison of the PFI-associated transcriptional response observed in A. thaliana infested by the generalists Myzus persicae and Bemisia tabaci with the specialist Brevicoryne brassicae highlights the importance of calcium-dependent and receptor kinase-associated signalling. We discuss these findings within the context of the complex cross-talk between the different hormones regulating basal immune response mechanisms in plants. We identify PFI-responsive genes, highlighting the importance of cell wall-associated kinases in plant-PFI interactions, as well as the significant role of kinases containing the domain of unknown function 26. A common feature of plant-PFI interaction is enhanced abundance of transcripts encoding WRKY transcription factors. However, significant divergence was observed with respect to secondary metabolism dependent upon the insect attacker. Transcripts encoding enzymes and proteins associated with glucosinolate metabolism were decreased following attack by the generalist M. persicae but not by the specialist B. brassicae. This analysis provides a comprehensive overview of the molecular patterns associated with the plant response to PFIs and suggests that plants recognize and respond to perturbations in the cell wall occurring during PFI infestation.

Free radical-mediated systemic immunity in plants

Systemic acquired resistance (SAR) is a form of defense that protects plants against a broad-spectrum of secondary infections by related or unrelated pathogens. SAR related research has witnessed considerable progress in recent years and a number of chemical signals and proteins contributing to SAR have been identified. All of these diverse constituents share their requirement for the phytohormone salicylic acid, an essential downstream component of the SAR pathway. However, recent work demonstrating the essential parallel functioning of nitric oxide (NO)-derived and reactive oxygen species (ROS)-derived signaling together with SA provides important new insights in the overlapping pathways leading to SAR. This review discusses the potential significance of branched pathways and the relative contributions of NO/ROS-derived and SA-derived pathways in SAR.

Hemoglobin Control of Cell Survival/Death Decision Regulates in Vitro Plant Embryogenesis

Programmed cell death (PCD) in multicellular organisms is a vital process in growth, development, and stress responses that contributes to the formation of tissues and organs. Although numerous studies have defined the molecular participants in apoptotic and PCD cascades, successful identification of early master regulators that target specific cells to live or die is limited. Using Zea mays somatic embryogenesis as a model system, we report that the expressions of two plant hemoglobin (Hb) genes (ZmHb1 and ZmHb2) regulate the cell survival/death decision that influences somatic embryogenesis through their cell-specific localization patterns. Suppression of either of the two ZmHbs is sufficient to induce PCD through a pathway initiated by elevated NO and Zn²⁺ levels and mediated by production of reactive oxygen species. The effect of the death program on the fate of the developing embryos is dependent on the localization patterns of the two ZmHbs. During somatic embryogenesis, ZmHb2 transcripts are restricted to a few cells anchoring the embryos to the subtending embryogenic tissue, whereas ZmHb1 transcripts extend to several embryonic domains. Suppression of ZmHb2 induces PCD in the anchoring cells, allowing the embryos to develop further, whereas suppression of ZmHb1 results in massive PCD, leading to abortion. We conclude that regulation of the expression of these ZmHbs has the capability to determine the developmental fate of the embryogenic tissue during somatic embryogenesis through their effect on PCD. This unique regulation might have implications for development and differentiation in other species.

Scavenging iron: a novel mechanism of plant immunity activation by microbial siderophores

Siderophores are specific ferric iron chelators synthesized by virtually all microorganisms in response to iron deficiency. We have previously shown that they promote infection by the phytopathogenic enterobacteria Dickeya dadantii and Erwinia amylovora. Siderophores also have the ability to activate plant immunity. We have used complete Arabidopsis transcriptome microarrays to investigate the global transcriptional modifications in roots and leaves of Arabidopsis (Arabidopsis thaliana) plants after leaf treatment with the siderophore deferrioxamine (DFO). Physiological relevance of these transcriptional modifications was validated experimentally. Immunity and heavy-metal homeostasis were the major processes affected by DFO. These two physiological responses could be activated by a synthetic iron chelator ethylenediamine-di(o-hydroxyphenylacetic) acid, indicating that siderophores eliciting activities rely on their strong iron-chelating capacity. DFO was able to protect Arabidopsis against the pathogenic bacterium Pseudomonas syringae pv tomato DC3000. Siderophore treatment caused local modifications of iron distribution in leaf cells visible by ferrocyanide and diaminobenzidine-H₂O₂ staining. Metal quantifications showed that DFO causes a transient iron and zinc uptake at the root level, which is presumably mediated by the metal transporter iron regulated transporter1 (IRT1). Defense gene expression and callose deposition in response to DFO were compromised in an irt1 mutant. Consistently, plant susceptibility to D. dadantii was increased in the irt1 mutant. Our work shows that iron scavenging is a unique mechanism of immunity activation in plants. It highlights the strong relationship between heavy-metal homeostasis and immunity.

An assessment of the biotechnological use of hemoglobin modulation in cereals

Non-symbiotic hemoglobin (nsHb) genes are ubiquitous in plants, but their biological functions have mostly been studied in model plant species rather than in crops. nsHb influences cell signaling and metabolism by modulating the levels of nitric oxide (NO). Class 1 nsHb is upregulated under hypoxia and is involved in various biotic and abiotic stress responses. Ectopic overexpression of nsHb in Arabidopsis thaliana accelerates development, whilst targeted overexpression in seeds can increase seed yield. Such observations suggest that manipulating nsHb could be a valid biotechnological target. We studied the effects of overexpression of class 1 nsHb in the monocotyledonous crop plant barley (Hordeum vulgare cv. Golden Promise). nsHb was shown to be involved in NO metabolism in barley, as ectopic overexpression reduced the amount of NO released during hypoxia. Further, as in Arabidopsis, nsHb overexpression compromised basal resistance toward pathogens in barley. However, unlike Arabidopsis, nsHb ectopic overexpression delayed growth and development in barley, and seed specific overexpression reduced seed yield. Thus, nsHb overexpression in barley does not seem to be an efficient strategy for increasing yield in cereal crops. These findings highlight the necessity for using actual crop plants rather than laboratory model plants when assessing the effects of biotechnological approaches to crop improvement.

Evolution, three-dimensional model and localization of truncated hemoglobin PttTrHb of hybrid aspen

Thus far, research on plant hemoglobins (Hbs) has mainly concentrated on symbiotic and non-symbiotic Hbs, and information on truncated Hbs (TrHbs) is scarce. The aim of this study was to examine the origin, structure and localization of the truncated Hb (PttTrHb) of hybrid aspen (Populus tremula L. × tremuloides Michx.), the model system of tree biology. Additionally, we studied the PttTrHb expression in relation to non-symbiotic class1 Hb gene (PttHb1) using RNAi-silenced hybrid aspen lines. Both the phylogenetic analysis and the three-dimensional (3D) model of PttTrHb supported the view that plant TrHbs evolved vertically from a bacterial TrHb. The 3D model suggested that PttTrHb adopts a 2-on-2 sandwich of α-helices and has a Bacillus subtilis -like ligand-binding pocket in which E11Gln and B10Tyr form hydrogen bonds to a ligand. However, due to differences in tunnel cavity and gate residue (E7Ala), it might not show similar ligand-binding kinetics as in Bs-HbO (E7Thr). The immunolocalization showed that PttTrHb protein was present in roots, stems as well as leaves of in vitro -grown hybrid aspens. In mature organs, PttTrHb was predominantly found in the vascular bundles and specifically at the site of lateral root formation, overlapping consistently with areas of nitric oxide (NO) production in plants. Furthermore, the NO donor sodium nitroprusside treatment increased the amount of PttTrHb in stems. The observed PttTrHb localization suggests that PttTrHb plays a role in the NO metabolism.

Plant hemoglobin participation in cell fate determination

Plant hemoglobins (Hbs) have been identified as master regulators in determining the developmental fate of specific cells during maize embryogenesis. Whether an embryogenic cell lives or undergoes programmed cell death (PCD) is modulated by Hbs, through their tight interactions with nitric oxide (NO) and auxin. During maize embryogenesis, Hb-suppressing cells accumulate NO, are depleted of auxin, and are committed to die. We propose that Hbs control cell fate by regulating NO and auxin homeostasis, and that this type of mechanism may influence other hormonal responses modulating plant behavior during development and stress conditions.

NO homeostasis is a key regulator of early nitrate perception and root elongation in maize

Crop plant development is strongly dependent on nitrogen availability in the soil and on the efficiency of its recruitment by roots. For this reason, the understanding of the molecular events underlying root adaptation to nitrogen fluctuations is a primary goal to develop biotechnological tools for sustainable agriculture. However, knowledge about molecular responses to nitrogen availability is derived mainly from the study of model species. Nitric oxide (NO) has been recently proposed to be implicated in plant responses to environmental stresses, but its exact role in the response of plants to nutritional stress is still under evaluation. In this work, the role of NO production by maize roots after nitrate perception was investigated by focusing on the regulation of transcription of genes involved in NO homeostasis and by measuring NO production in roots. Moreover, its involvement in the root growth response to nitrate was also investigated. The results provide evidence that NO is produced by nitrate reductase as an early response to nitrate supply and that the coordinated induction of non-symbiotic haemoglobins (nsHbs) could finely regulate the NO steady state. This mechanism seems to be implicated on the modulation of the root elongation in response to nitrate perception. Moreover, an improved agar-plate system for growing maize seedlings was developed. This system, which allows localized treatments to be performed on specific root portions, gave the opportunity to discern between localized and systemic effects of nitrate supply to roots.

Leucaena leucocephala leachate compromised membrane integrity, respiration and antioxidative defence of water hyacinth leaf tissues

BACKGROUND

Water hyacinth is an invasive aquatic weed in many regions of the world. In this study, the bioherbicidal potential of allelopathic plant Leucaena leucocephala against water hyacinth was investigated using a leaf disc assay.

RESULTS

L. leucocephala leachate enhanced electrolyte leakage from water hyacinth leaf discs in a concentration-dependent manner. Control experiments eliminated the possibilities that increased membrane permeability in the leachate-treated leaf discs was due to pH or osmotic effects of the leachate. Thus, the loss of membrane stability in the leachate-treated leaf discs was likely due to phytotoxins detected in the leachate, namely mimosine and phenolic constituents. Decline in tissue respiration was detected in leachate-treated water hyacinth leaf discs. This suggests that the L. leucocephala leachate may contain compounds which acted as respiratory inhibitors. Enhanced reactive oxygen species production coincided with inhibition of catalase and ascorbate peroxidase activities in the leachate-treated water hyacinth leaf tissues. The injurious effects of L. leucocephala leachate on water hyacinth leaf discs probably involved direct inhibition of antioxidant enzymes in addition to direct involvement of some allelochemicals in reactive oxygen species formation.

CONCLUSION

In summary, the toxic effects of L. leucocephala leachate on water hyacinth leaf discs likely lay in its ability to effectively compromise the membrane integrity, tissue respiration and antioxidant defence of the latter.

The responses of Vitreoscilla hemoglobin-expressing hybrid aspen (Populus tremula × tremuloides) exposed to 24-h herbivory: expression of hemoglobin and stress-related genes in exposed and nonorthostichous leaves

The responses of transcriptome and phenolic compounds were determined with Populus tremula L. × Populus tremuloides Michx. expressing the hemoglobin (Hb) of Vitreoscilla (VHb) and non-transformant (wt) line. After 24-h exposure of leaves to Conistra vaccinii L., the transcript levels of endogenous non-symbiotic class 1 Hb (PttHb1) and truncated Hb (PttTrHb) genes were modestly reduced and increased, respectively, in both wt and VHb-expressing line. Besides the herbivory exposed leaves showing the most significant transcriptome changes, alterations were also detected in the transcriptome of nonorthostichous leaves positioned directly above the exposed leaves. Both wt and VHb-expressing line displayed similar herbivory-induced effects on gene expression, although the extent of responses was more pronounced in the wt than in the VHb-expressing line. The contents of phenolic compounds were not altered due to herbivory and they were alike in the wt and VHb-expressing line. In addition, we determined the relative growth rates (RGRs) of Orthosia gothica L., Ectropis crepuscularia Denis & Schiff. and Orgyia antiqua L. larvae, and found no variation in the RGRs between the lines. Thus, VHb-expressing P. tremula × tremuloides lines showed to be comparable with wt in regards to the food quality of leaves.

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 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.

The role of nitric oxide and hemoglobin in plant development and morphogenesis

Plant morphogenesis is regulated endogenously through phytohormones and other chemical signals, which may act either locally or distant from their place of synthesis. Nitric oxide (NO) is formed by a number of controlled processes in plant cells. It is a central signaling molecule with several effects on control of plant growth and development, such as shoot and root architecture. All plants are able to express non-symbiotic hemoglobins at low concentration. Their function is generally not related to oxygen transport or storage; instead they effectively oxidize NO to NO(3)(-) and thereby control the local cellular NO concentration. In this review, we analyze available data on the role of NO and plant hemoglobins in morphogenetic processes in plants. The comparison of the data suggests that hemoglobin gene expression in plants modulates development and morphogenesis of organs, such as roots and shoots, through the localized control of NO, and that hemoglobin gene expression should always be considered a modulating factor in processes controlled directly or indirectly by NO in plants.

Hemoglobin regulation of plant embryogenesis and plant pathogen interaction

Plant hemoglobins are ubiquitous molecules involved in several aspects of plant development and stress responses. Studies on the functional aspects of plant hemoglobins at the cellular level in these processes are limited, despite their ability to scavenge nitric oxide (NO), an important signal molecule interfering with hormone synthesis and sensitivity. This mini-review summarizes current knowledge on plant hemoglobins, analyzes their participation in plant pathogen interaction and embryogenesis and proposes a possible model centering on jasmonic acid (JA) as a downstream component of hemoglobin responses.

Current methods for detecting ethylene in plants

BACKGROUND

In view of ethylene's critical developmental and physiological roles the gaseous hormone remains an active research topic for plant biologists. Progress has been made to understand the ethylene biosynthesis pathway and the mechanisms of perception and action. Still numerous questions need to be answered and findings to be validated. Monitoring gas production will very often complete the picture of any ethylene research topic. Therefore the search for suitable ethylene measuring methods for various plant samples either in the field, greenhouses, laboratories or storage facilities is strongly motivated.

SCOPE

This review presents an update of the current methods for ethylene monitoring in plants. It focuses on the three most-used methods - gas chromatography detection, electrochemical sensing and optical detection - and compares them in terms of sensitivity, selectivity, time response and price. Guidelines are provided for proper selection and application of the described sensor methodologies and some specific applications are illustrated of laser-based detector for monitoring ethylene given off by Arabidopsis thaliana upon various nutritional treatments.

CONCLUSIONS

Each method has its advantages and limitations. The choice for the suitable ethylene sensor needs careful consideration and is driven by the requirements for a specific application.

Modulating protein function through reversible oxidation: Redox-mediated processes in plants revealed through proteomics

It has been clearly demonstrated that plants redox control can be exerted over virtually every cellular metabolic pathway affecting metabolic homeostasis and energy balance. Therefore, a tight link exists between cellular/compartmental steady-state redox level and cellular metabolism. Proteomics offers a powerful new way to characterize the response and regulation of protein oxidation in different cell types and in relation to cellular metabolism. Compelling evidence revealed in proteomics studies suggests the integration of the redox network with other cellular signaling pathways such as Ca(2+) and/or protein phosphorylation, jasmonic, salicylic, abscisic acids, ethylene, and other phytohormones. Here we review progress in using the various proteomics techniques and approaches to answer biological questions arising from redox signaling and from changes in redox status of the cell. The focus is on reversible redox protein modifications and on three main processes, namely oxidative and nitrosative stress, defense against pathogens, cellular redox response and regulation, drawing on examples from plant redox proteomics studies.

Integrating nitric oxide into salicylic acid and jasmonic acid/ ethylene plant defense pathways

Plant defense against pests and pathogens is known to be conferred by either salicylic acid (SA) or jasmonic acid (JA)/ethylene (ET) pathways, depending on infection or herbivore-grazing strategy. It is well attested that SA and JA/ET pathways are mutually antagonistic allowing defense responses to be tailored to particular biotic stresses. Nitric oxide (NO) has emerged as a major signal influencing resistance mediated by both signaling pathways but no attempt has been made to integrate NO into established SA/JA/ET interactions. NO has been shown to act as an inducer or suppressor of signaling along each pathway. NO will initiate SA biosynthesis and nitrosylate key cysteines on TGA-class transcription factors to aid in the initiation of SA-dependent gene expression. Against this, S-nitrosylation of NONEXPRESSOR OF PATHOGENESIS-RELATED PROTEINS1 (NPR1) will promote the NPR1 oligomerization within the cytoplasm to reduce TGA activation. In JA biosynthesis, NO will initiate the expression of JA biosynthetic enzymes, presumably to over-come any antagonistic effects of SA on JA-mediated transcription. NO will also initiate the expression of ET biosynthetic genes but a suppressive role is also observed in the S-nitrosylation and inhibition of S-adenosylmethionine transferases which provides methyl groups for ET production. Based on these data a model for NO action is proposed but we have also highlighted the need to understand when and how inductive and suppressive steps are used.

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

BACKGROUND AND AIMS

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

SCOPE AND CONCLUSIONS

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

Hemoglobin: a nitric-oxide dioxygenase

Members of the hemoglobin superfamily efficiently catalyze nitric-oxide dioxygenation, and when paired with native electron donors, function as NO dioxygenases (NODs). Indeed, the NOD function has emerged as a more common and ancient function than the well-known role in O2 transport-storage. Novel hemoglobins possessing a NOD function continue to be discovered in diverse life forms. Unique hemoglobin structures evolved, in part, for catalysis with different electron donors. The mechanism of NOD catalysis by representative single domain hemoglobins and multidomain flavohemoglobin occurs through a multistep mechanism involving O2 migration to the heme pocket, O2 binding-reduction, NO migration, radical-radical coupling, O-atom rearrangement, nitrate release, and heme iron re-reduction. Unraveling the physiological functions of multiple NODs with varying expression in organisms and the complexity of NO as both a poison and signaling molecule remain grand challenges for the NO field. NOD knockout organisms and cells expressing recombinant NODs are helping to advance our understanding of NO actions in microbial infection, plant senescence, cancer, mitochondrial function, iron metabolism, and tissue O2 homeostasis. NOD inhibitors are being pursued for therapeutic applications as antibiotics and antitumor agents. Transgenic NOD-expressing plants, fish, algae, and microbes are being developed for agriculture, aquaculture, and industry.

Haemoglobin modulates NO emission and hyponasty under hypoxia-related stress in Arabidopsis thaliana

Nitric oxide (NO) and ethylene are signalling molecules that are synthesized in response to oxygen depletion. Non-symbiotic plant haemoglobins (Hbs) have been demonstrated to act in roots under oxygen depletion to scavenge NO. Using Arabidopsis thaliana plants, the online emission of NO or ethylene was directly quantified under normoxia, hypoxia (0.1-1.0% O(2)), or full anoxia. The production of both gases was increased with reduced expression of either of the Hb genes GLB1 or GLB2, whereas NO emission decreased in plants overexpressing these genes. NO emission in plants with reduced Hb gene expression represented a major loss of nitrogen equivalent to 0.2mM nitrate per 24h under hypoxic conditions. Hb gene expression was greatly enhanced in flooded roots, suggesting induction by reduced oxygen diffusion. The function could be to limit loss of nitrogen under NO emission. NO reacts with thiols to form S-nitrosylated compounds, and it is demonstrated that hypoxia substantially increased the content of S-nitrosylated compounds. A parallel up-regulation of Hb gene expression in the normoxic shoots of the flooded plants may reflect signal transmission from root to shoot via ethylene and a role for Hb in the shoots. Hb gene expression was correlated with ethylene-induced upward leaf movement (hyponastic growth) but not with hypocotyl growth, which was Hb independent. Taken together the data suggest that Hb can influence flood-induced hyponasty via ethylene-dependent and, possibly, ethylene-independent pathways.