Generation of superoxide anion and localization of CuZn-superoxide dismutase in the vascular tissue of spinach hypocotyls: their association with lignification

CuZn-superoxide dismutase is differentially modified in localization and expression by three abiotic stresses in miniature rose bushes

Three abiotic stresses, copper application (CS), mechanical rubbing (MS) and water deprivation (WS) applied on miniature rose bushes specifically activate the expression of the CuZn-Superoxide dismutase (SOD). The Cu/Zn-SOD protein immunodetected in the 4th internode was shown engaged in lignification in phloem, cambium and xylem cells. The SOD occurrence was detailed in the vessel associated cells (VACs), using immunogold labeling observed in transmission electron microscopy. The enzyme was detected in mitochondria, plastids, Golgi vesicles, endoplasmic reticulum and plasma membrane. In addition, in pit-fields without plasmodesmata linking vessel associated cells to vessels, the abiotic stresses increased the transfer apparatus volume. The content in unmethylatedpectins increased in wall ingrowths after CS and MS, but not in WS. In addition to the different localization, the SOD was differentially overexpressed according to the applied stress: an isoform detected at 17 kDa under CuSO4 application, two isoforms respectively detected at 20 and 17 kDa under MS and detected at 17 and 15 kDa under WS. Notably, the only 17 kDa isoform was detected in plasma membrane vesicles from plants submitted to the three stresses. Thus, by increasing the transfer apparatus development, the key role of VACs was emphasized in establishing an adaptative response to abiotic stresses, in miniature rose bushes. Additionally, it has been observed that the differential SOD localization under such stresses sustained the regulatory function of VACs in the transitory sink function of xylem.

Superoxide Production under Soft X-ray Irradiation of Liquid Water

Soft X-rays behave like particles with high linear energy transfer, as they deposit a large amount of their energy in the nanometric range, triggered by inner-shell ionization. In water, this can lead to the formation of a doubly ionized water molecule (H₂O²⁺) and the emission of two secondary electrons (photoelectron and Auger electron). Our focus lies on detecting and quantifying the superoxide (HO₂°) production via the direct pathway, i.e., from the reaction between the dissociation product of H₂O²⁺, i.e., the oxygen atom (∼4 fs), and the °OH radicals present in the secondary electron tracks. The HO₂° yield for 1620 eV photons, via this reaction pathway, was found to be 0.005 (±0.0007) μmol/J (formed within the ∼ps range). Experiments were also performed to determine the yield of HO₂° production via another (indirect) pathway, involving solvated electrons. The indirect HO₂° yield, measured experimentally as a function of photon energy (from 1700 to 350 eV), resulted in a steep decrease at around 1280 eV and a minimum close to zero at 800 eV. This behavior in contradiction with the theoretical prediction reveals the complexity hidden in the intratrack reactions.

Prevention of Chilling Injury in Pomegranates Revisited: Pre- and Post-Harvest Factors, Mode of Actions, and Technologies Involved

The storage life of pomegranate fruit (Punica granatum L.) is limited by decay, chilling injury, weight loss, and husk scald. In particular, chilling injury (CI) limits pomegranate long-term storage at chilling temperatures. CI manifests as skin browning that expands randomly with surface spots, albedo brown discoloration, and changes in aril colors from red to brown discoloration during handling or storage (6-8 weeks) at <5-7 °C. Since CI symptoms affect external and internal appearance, it significantly reduces pomegranate fruit marketability. Several postharvest treatments have been proposed to prevent CI, including atmospheric modifications (MA), heat treatments (HT), coatings, use of polyamines (PAs), salicylic acid (SA), jasmonates (JA), melatonin and glycine betaine (GB), among others. There is no complete understanding of the etiology and biochemistry of CI, however, a hypothetical model proposed herein indicates that oxidative stress plays a key role, which alters cell membrane functionality and integrity and alters protein/enzyme biosynthesis associated with chilling injury symptoms. This review discusses the hypothesized mechanism of CI based on recent research, its association to postharvest treatments, and their possible targets. It also indicates that the proposed mode of action model can be used to combine treatments in a hurdle synergistic or additive approach or as the basis for novel technological developments.

Study on Characteristics and Lignification Mechanism of Postharvest Banana Fruit during Chilling Injury

The banana is prone to chilling injury (CI) at low temperature and showing a series of chilling symptoms, such as peel browning, etc. Lignification is a response to abiotic stress and senescence, which is an important manifestation of fruits and vegetables during chilling exposure. However, little is known about the lignification of bananas during low-temperature storage. Our study explored the characteristics and lignification mechanism of banana fruits during low-temperature storage by analyzing the changes of chilling symptoms, oxidative stress, cell wall metabolism, microstructures, and gene expression related to lignification. The results showed that CI inhibited post-ripening by effecting the degradation of the cell wall and starch and accelerated senescence by increasing O^2- and H₂O₂ content. For lignification, Phenylalanine ammonia-lyase (PAL) might start the phenylpropanoid pathway of lignin synthesis. Cinnamoyl-CoA reductase 4 (CCR4), cinnamyl alcohol dehydrogenase 2 (CAD2), and 4-coumarate--CoA ligase like 7 (4CL7) were up-regulated to promote the lignin monomer's synthesis. Peroxidase 1 (POD1) and Laccase 3 (LAC3) were up-regulated to promote the oxidative polymerization of lignin monomers. These results suggest that changes of the cell wall structure and cell wall metabolism, as well as lignification, are involved in the senescence and quality deterioration of the banana after chilling injury.

Regulation of PaRBOH1-mediated ROS production in Norway spruce by Ca2+ binding and phosphorylation

Plant respiratory burst oxidase homologs (RBOHs) are plasma membrane-localized NADPH oxidases that generate superoxide anion radicals, which then dismutate to H₂O₂, into the apoplast using cytoplasmic NADPH as an electron donor. PaRBOH1 is the most highly expressed RBOH gene in developing xylem as well as in a lignin-forming cell culture of Norway spruce (Picea abies L. Karst.). Since no previous information about regulation of gymnosperm RBOHs exist, our aim was to resolve how PaRBOH1 is regulated with a focus on phosphorylation. The N-terminal part of PaRBOH1 was found to contain several putative phosphorylation sites and a four-times repeated motif with similarities to the Botrytis-induced kinase 1 target site in Arabidopsis AtRBOHD. Phosphorylation was indicated for six of the sites in in vitro kinase assays using 15 amino-acid-long peptides for each of the predicted phosphotarget site in the presence of protein extracts of developing xylem. Serine and threonine residues showing positive response in the peptide assays were individually mutated to alanine (kinase-inactive) or to aspartate (phosphomimic), and the wild type PaRBOH1 and the mutated constructs transfected to human kidney embryogenic (HEK293T) cells with a low endogenous level of extracellular ROS production. ROS-producing assays with HEK cells showed that Ca²⁺ and phosphorylation synergistically activate the enzyme and identified several serine and threonine residues that are likely to be phosphorylated including a novel phosphorylation site not characterized in other plant species. These were further investigated with a phosphoproteomic study. Results of Norway spruce, the first gymnosperm species studied in relation to RBOH regulation, show that regulation of RBOH activity is conserved among seed plants.

Interference Expression of StMSD Inhibited the Deposition of Suberin and Lignin at Wounds of Potato Tubers by Reducing the Production of H2O2

Superoxide dismutase (SOD) actively participates in the wound stress of plants. However, whether StMSD mediates the generation of H₂O₂ and the deposition of suberin polyphenolic and lignin at potato tuber wounds is elusive. In this study, we developed the StMSD interference expression of potato plants and tubers by Agrobacterium tumefaciens-mediated transformation. The StSOD expression showed a marked downregulation in StMSD-interference tubers, especially StCSD2 and StCSD3. The content of O₂^•− exhibited a noticeable increase together with the inhibition in H₂O₂ accumulation. Moreover, the gene expression levels of StPAL (phenylalanine ammonia-lyase) and StC4H (cinnamate-4-hydroxylase) were downregulated in StMSD-interference tubers, and less suberin polyphenolic and lignin depositions at the wounds were observed. Taken together, the interference expression of StMSD can result in less suberin polyphenolic and lignin deposition by inhibiting the disproportionation of O₂^•− to H₂O₂ and restraining phenylpropanoid metabolism in tubers.

Rhizobial HmuSpSym as a heme-binding factor is required for optimal symbiosis between Mesorhizobium amorphae CCNWGS0123 and Robinia pseudoacacia

Nitrogen-fixing root nodules are formed by symbiotic association of legume hosts with rhizobia in nitrogen-deprived soils. Successful symbiosis is regulated by signals from both legume hosts and their rhizobial partners. HmuS is a heme degrading factor widely distributed in bacteria, but little is known about the role of rhizobial hmuS in symbiosis with legumes. Here, we found that inactivation of hmuS_pSym in the symbiotic plasmid of Mesorhizobium amorphae CCNWGS0123 disrupted rhizobial infection, primordium formation, and nitrogen fixation in symbiosis with Robinia pseudoacacia. Although there was no difference in bacteroids differentiation, infected plant cells were shrunken and bacteroids were disintegrated in nodules of plants infected by the ΔhmuS_pSym mutant strain. The balance of defence reaction was also impaired in ΔhmuS_pSym strain-infected root nodules. hmuS_pSym was strongly expressed in the nitrogen-fixation zone of mature nodules. Furthermore, the HmuS_pSym protein could bind to heme but not degrade it. Inactivation of hmuS_pSym led to significantly decreased expression levels of oxygen-sensing related genes in nodules. In summary, hmuS_pSym of M. amorphae CCNWGS0123 plays an essential role in nodule development and maintenance of bacteroid survival within R. pseudoacacia cells, possibly through heme-binding in symbiosis.

Quantitative Analysis for ROS-Producing Activity and Regulation of Plant NADPH Oxidases in HEK293T Cells

Reactive oxygen species (ROS) produced by plant NADPH oxidases, respiratory burst oxidase homologs (RBOHs), play key roles in biotic and abiotic stress responses and development in plants. While properly controlled amounts of ROS function as signaling molecules, excessive accumulation of ROS can cause undesirable side effects due to their ability to oxidize DNA, lipids, and proteins. To limit the damaging consequences of unrestricted ROS accumulation, RBOH activity is tightly controlled by post-translational modifications (PTMs) and protein-protein interactions. In order to analyze these elaborate regulatory mechanisms, it is crucial to quantitatively assess the ROS-producing activity of RBOHs. Given the high endogenous ROS generation in plants, however, it can be challenging in plant cells to measure ROS production derived from specific RBOHs and to analyze the contribution of regulatory events for their activation and inactivation. Here we describe human embryonic kidney 293T (HEK293T) cells as a heterologous expression system and a useful tool to quantitatively monitor ROS production by RBOHs. This system permits the reconstitution of regulatory events to dissect the effects of Ca²⁺, phosphorylation, and protein-protein interactions on RBOH-dependent ROS production.

Glycinebetaine mitigates tomato chilling stress by maintaining high-cyclic electron flow rate of photosystem I and stability of photosystem II

Glycinebetaine alleviates chilling stress by protecting photosystems I and II in BADH-transgenic and GB-treated tomato plants, which can be an effective strategy for improving crop chilling tolerance. Tomato (Solanum lycopersicum) is one of the most cultivated vegetables in the world, but is highly susceptible to chilling stress and does not naturally accumulate glycinebetaine (GB), one of the most effective stress protectants. The protective mechanisms of GB on photosystem I (PSI) and photosystem II (PSII) against chilling stress, however, remain poorly understood. Here, we address this problem through exogenous GB application and generation of transgenic tomatoes (Moneymaker) with a gene encoding betaine aldehyde dehydrogenase (BADH), which is the key enzyme in the synthesis of GB, from spinach. Our results demonstrated that GB can protect chloroplast ultramicrostructure, alleviate PSII photoinhibition and maintain PSII stability under chilling stress. More importantly, GB increased the electron transfer between Q_A and Q_B and the redox potential of Q_B and maintained a high rate of cyclic electron flow around PSI, contributing to reduced production of reactive oxygen species, thereby mitigating PSI photodamage under chilling stress. Our results highlight the novel roles of GB in enhancing chilling tolerance via the protection of PSI and PSII in BADH transgenic and GB-treated tomato plants under chilling stress. Thus, introducing GB-biosynthetic pathway into tomato and exogenous GB application are effective strategies for improving chilling tolerance.

Using Hyper as a molecular probe to visualize hydrogen peroxide in living plant cells: An updated method

Reactive oxygen species (ROS) are highly reactive reduced oxygen molecules that play a myriad of roles in animal and plant cells. In plant cells the production of ROS results from aerobic metabolism during respiration and photosynthesis. Therefore mitochondria, chloroplasts, and peroxisomes constitute an important source of ROS. However, ROS can also be produced in response to many physiological stimuli such as pathogen attack, hormone signaling, abiotic stresses or during cell wall organization and plant morphogenesis. The study of ROS in plant cells has been limited to biochemical assays and use of fluorescent probes, however, the irreversible oxidation of the fluorescent dyes prevents the visualization of dynamic changes. We have previously reported that Hyper 1 is a biosensor for H₂O₂ and consists of a circularly permutated YFP (cpYFP) inserted into the regulatory domain of the Escherichia coli hydrogen peroxide (H₂O₂) sensor protein OxyR rendering it an H₂O₂-specific quantitative probe (Bilan & Belousov, 2018; Hernandez-Barrera et al., 2015). Herein we describe an updated protocol for using the improved new version of Hyper 2 and Hyper 3 as a dynamic biosensor for H₂O₂ in Arabidopsis with virtually unlimited potential to detect H₂O₂ throughout the plant and under a broad range of developmental and environmental conditions (Bilan et al., 2013).

Use of Ore-Derived Humic Acids With Diverse Chemistries to Elucidate Structure-Activity Relationships (SAR) of Humic Acids in Plant Phenotypic Expression

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Plant Copper Metalloenzymes As Prospects for New Metabolism Involving Aromatic Compounds

Copper is an important transition metal cofactor in plant metabolism, which enables diverse biocatalysis in aerobic environments. Multiple classes of plant metalloenzymes evolved and underwent genetic expansions during the evolution of terrestrial plants and, to date, several representatives of these copper enzyme classes have characterized mechanisms. In this review, we give an updated overview of chemistry, structure, mechanism, function and phylogenetic distribution of plant copper metalloenzymes with an emphasis on biosynthesis of aromatic compounds such as phenylpropanoids (lignin, lignan, flavonoids) and cyclic peptides with macrocyclizations via aromatic amino acids. We also review a recent addition to plant copper enzymology in a copper-dependent peptide cyclase called the BURP domain. Given growing plant genetic resources, a large pool of copper biocatalysts remains to be characterized from plants as plant genomes contain on average more than 70 copper enzyme genes. A major challenge in characterization of copper biocatalysts from plant genomes is the identification of endogenous substrates and catalyzed reactions. We highlight some recent and future trends in filling these knowledge gaps in plant metabolism and the potential for genomic discovery of copper-based enzymology from plants.

Spatial distribution of apoplastic antioxidative constituents in maize root

Apoplastic antioxidative constituents (enzymes, primary and secondary metabolites, ROS) from different root zones of hydroponically grown maize (Zea mays L.) were investigated using a noninvasive isolation procedure: filter strip method. Filter strips were placed at specific positions on the root surface: apical zone (tip) and basal zone (base) to absorb apoplastic fluid. Three major classes of low-weight metabolites (organic acids, sugars, and phenolics) have been identified by HPLC-ECD. The longitudinal distribution of sugars and organic acids had the same pattern: higher concentration in the tip than the base, while it was vice versa for phenolics. The specific activities of guaiacol peroxidase, superoxide dismutase, and ascorbate peroxidase were higher in the apoplastic fluid from the root base than the tip, and their different isoforms were separated by isoelectric focusing. Electron paramagnetic resonance (EPR) spectroscopy coupled with the spin-trapping method using DEPMPO showed a persistent generation of hydroxyl radical in the root tip. In vivo EPR imaging of the whole maize root with membrane-permeable and impermeable aminoxyl spin-probes, enabling real-time detection of ROS formation within and outside the membranes, demonstrated ROS accumulation on the root surface, while endodermis and central cylinder were ROS free. For the first time in plant research, 2D EPR images enabled the direct demonstration of site-specific free radical production along the root. Highly sensitive analytical techniques combined with the filter strips, as a non-invasive tool, have increased our knowledge of metabolic processes occurring in the apoplast and their spatial-temporal changes in small regions of the intact root tissue.

Molecular mechanisms for magnesium-deficiency-induced leaf vein lignification, enlargement and cracking in Citrus sinensis revealed by RNA-Seq

Citrus sinensis (L.) Osbeck seedlings were fertigated with nutrient solution containing 2 [magnesium (Mg)-sufficiency] or 0 mM (Mg-deficiency) Mg(NO3)2 for 16 weeks. Thereafter, RNA-Seq was used to investigate Mg-deficiency-responsive genes in the veins of upper and lower leaves in order to understand the molecular mechanisms for Mg-deficiency-induced vein lignification, enlargement and cracking, which appeared only in the lower leaves. In this study, 3065 upregulated and 1220 downregulated, and 1390 upregulated and 375 downregulated genes were identified in Mg-deficiency veins of lower leaves (MDVLL) vs Mg-sufficiency veins of lower leaves (MSVLL) and Mg-deficiency veins of upper leaves (MDVUL) vs Mg-sufficiency veins of upper leaves (MSVUL), respectively. There were 1473 common differentially expressed genes (DEGs) between MDVLL vs MSVLL and MDVUL vs MSVUL, 1463 of which displayed the same expression trend. Magnesium-deficiency-induced lignification, enlargement and cracking in veins of lower leaves might be related to the following factors: (i) numerous transciption factors and genes involved in lignin biosynthesis pathways, regulation of cell cycle and cell wall metabolism were upregulated; and (ii) reactive oxygen species, phytohormone and cell wall integrity signalings were activated. Conjoint analysis of proteome and transcriptome indicated that there were 287 and 56 common elements between DEGs and differentially abundant proteins (DAPs) identified in MDVLL vs MSVLL and MDVUL vs MSVUL, respectively, and that among these common elements, the abundances of 198 and 55 DAPs matched well with the transcript levels of the corresponding DEGs in MDVLL vs MSVLL and MDVUL vs MSVUL, respectively, indicating the existence of concordances between protein and transcript levels.

Reactive Oxygen Species and Antioxidants in Postharvest Vegetables and Fruits

Reducing oxidative species to non- or less-reactive matter is the principal function of an antioxidant. Plant-based food is the main external source of antioxidants that helps protect our cells from oxidative damage. During postharvest storage and distribution, fruits and vegetables often increase ROS production that is quenched by depleting their antioxidant pools to protect their cells, which may leave none for humans. ROS are molecules produced from oxygen metabolism; some of the most widely analyzed ROS in plants are singlet oxygen, superoxide, hydrogen peroxide, and hydroxyl radicals. ROS concentration and lifetime are determined by the availability and composition of the antioxidant system that includes enzymatic components such as SOD, CAT, and APX and nonenzymatic components such as vitamins, polyphenols, and carotenoid. Depending on its concentration in the cell, ROS can either be harmful or beneficial. At high concentrations, ROS can damage various kinds of biomolecules such as lipids, proteins, DNA, and RNA, whereas at low or moderate concentrations, ROS can act as second messengers in the intracellular signaling cascade that mediates various plant responses. Novel postharvest methods are sought to maintain fruit and vegetable quality, including minimizing ROS while preserving their antioxidant content.

Salt Solution Treatments Trigger Antioxidant Defense Response against Gray Mold Disease in Table Grapes

To obtain a clear understanding of the mode of action of potassium bicarbonate (PB), sodium silicate (SSi) and calcium chelate (CCh) solutions (1%) in inducing resistance to gray mold disease in table grapes, enzymatic and nonenzymatic investigations were carried out. In particular, changes in the activity of the enzymes superoxide dismutase (SOD), ascorbate peroxidase (APX) and peroxidase (POD), total phenolic content and total flavonoid content were studied. As indirect action, PB, SSi and CCh reduced the incidence of gray mold by 43%, 50% and 41%, respectively. The highest activity of SOD was detected at 48 h in SSi-treated tissue, PB-treated tissue and CCh-treated tissue, and it was 1.7-, 1.4- and 1.2-fold higher, respectively, compared to the control. The APX activity was significantly higher in SSi-treated tissue than in the control at 24, 48 and 72 h and showed an increase in activity 2-fold for all times. Additionally, PB, SSi and CCh increased the activity of POD by 1.4-, 1.2- and 2.7-fold at 48 h posttreatment, respectively. The results showed that CCh was the most pronounced salt to increase both total phenol and flavonoid contents by 1.3 and 2.1, respectively. Additionally, the three tested salts induced an increase in total phenols and total flavonoids at 48 h posttreatment. The obtained result is one more movement towards an overall understanding of the mechanism by which salt solutions act as antimicrobial agents against gray mold of table grapes.

Detection of superoxide dismutase (Cu-Zn) isoenzymes in leaves and pseudobulbs of Bulbophyllum morphologlorum Kraenzl orchid by comparative proteomic analysis

Typically, biological systems are protected from the toxic effect of free radicals by antioxidant defense. Extracts from orchids have been reported to show high levels of exogenous antioxidant activity including Bulbophyllum orchids but so far, there have been no reports on antioxidant enzymes. Therefore, differences in protein expression from leaves and pseudobulbs of Bulbophyllum morphologlorum Kraenzl and Dendrobium Sonia Earsakul were studied using two-dimensional gel electrophoresis and mass spectrometry (LC/MS/MS). Interestingly, the largest group of these stress response proteins were associated with antioxidant defense and temperature stress, including superoxide dismutase (Cu–Zn) and heat shock protein 70. The high expression of this antioxidant enzyme from Bulbophyllum morphologlorum Kraenzl was confirmed by activity staining on native-PAGE, and the two Cu/Zn-SODs isoenzymes were identified as Cu/Zn-SOD 1 and Cu/Zn-SOD 2 by LC/MS/MS. The results suggested that Bulbophyllum orchid can be a potential plant source for medicines and natural antioxidant supplements.

Allies or Enemies: The Role of Reactive Oxygen Species in Developmental Processes of Black Cottonwood (Populus trichocarpa)

In contrast to aboveground organs (stems and leaves), developmental events and their regulation in underground organs, such as pioneer and fine roots, are quite poorly understood. The objective of the current study was to achieve a better understanding of the physiological and molecular role of reactive oxygen species (ROS) and ROS-related enzymes in the process of stem and pioneer root development in black cottonwood (Populus trichocarpa), as well as in the senescence of leaves and fine roots. Results of a transcriptomic analysis revealed that primary/secondary growth and senescence are accompanied by substantial changes in the expression of genes related to oxidative stress metabolism. We observed that some mechanisms common for above- and under-ground organs, e.g., the expression of superoxide dismutase (SOD) genes and SOD activity, declined during stems' and pioneer roots' development. Moreover, the localization of hydrogen peroxide (H₂O₂) and superoxide (O₂•^-) in the primary and secondary xylem of stems and pioneer roots confirms their involvement in xylem cell wall lignification and the induction of programmed cell death (PCD). H₂O₂ and O₂•^- in senescing fine roots were present in the same locations as demonstrated previously for ATG8 (AuTophaGy-related) proteins, implying their participation in cell degradation during senescence, while O₂•^- in older leaves was also localized similarly to ATG8 in chloroplasts, suggesting their role in chlorophagy. ROS and ROS-related enzymes play an integral role in the lignification of xylem cell walls in Populus trichocarpa, as well as the induction of PCD during xylogenesis and senescence.

CRISPR/Cas9 knockout of leghemoglobin genes in Lotus japonicus uncovers their synergistic roles in symbiotic nitrogen fixation

Legume nodules contain high concentrations of leghemoglobins (Lbs) encoded by several genes. The reason for this multiplicity is unknown. CRISPR/Cas9 technology was used to generate stable mutants of the three Lbs of Lotus japonicus. The phenotypes were characterized at the physiological, biochemical and molecular levels. Nodules of the triple mutants were examined by electron microscopy and subjected to RNA-sequencing (RNA-seq) analysis. Complementation studies revealed that Lbs function synergistically to maintain optimal N₂ fixation. The nodules of the triple mutants overproduced superoxide radicals and hydrogen peroxide, which was probably linked to activation of NADPH oxidases and changes in superoxide dismutase isoforms expression. The mutant nodules showed major ultrastructural alterations, including vacuolization, accumulation of poly-β-hydroxybutyrate and disruption of mitochondria. RNA-seq of c. 20 000 genes revealed significant changes in expression of carbon and nitrogen metabolism genes, transcription factors, and proteinases. Lb-deficient nodules had c. 30-50-fold less heme but similar transcript levels of heme biosynthetic genes, suggesting a post-translational regulatory mechanism of heme synthesis. We conclude that Lbs act additively in nodules and that the lack of Lbs results in early nodule senescence. Our observations also provide insight into the reprogramming of the gene expression network associated with Lb deficiency, probably as a result of uncontrolled intracellular free O₂ concentration.

Photoinhibition of Photosystem I Provides Oxidative Protection During Imbalanced Photosynthetic Electron Transport in Arabidopsis thaliana

Photosynthesis involves the conversion of sunlight energy into stored chemical energy, which is achieved through electron transport along a series of redox reactions. Excess photosynthetic electron transport might be dangerous due to the risk of molecular oxygen reduction, generating reactive oxygen species (ROS) over-accumulation. Avoiding excess ROS production requires the rate of electron transport to be coordinated with the capacity of electron acceptors in the chloroplast stroma. Imbalance between the donor and acceptor sides of photosystem I (PSI) can lead to inactivation, which is called PSI photoinhibition. We used a light-inducible PSI photoinhibition system in Arabidopsis thaliana to resolve the time dynamics of inhibition and to investigate its impact on ROS production and turnover. The oxidation state of the PSI reaction center and rates of CO2 fixation both indicated strong and rapid PSI photoinhibition upon donor side/acceptor side imbalance, while the rate of inhibition eased during prolonged imbalance. PSI photoinhibition was not associated with any major changes in ROS accumulation or antioxidant activity; however, a lower level of lipid oxidation correlated with lower abundance of chloroplast lipoxygenase in PSI-inhibited leaves. The results of this study suggest that rapid activation of PSI photoinhibition under severe photosynthetic imbalance protects the chloroplast from over-reduction and excess ROS formation.

On the Origin and Fate of Reactive Oxygen Species in Plant Cell Compartments

Reactive oxygen species (ROS) have been recognized as important signaling compounds of major importance in a number of developmental and physiological processes in plants. The existence of cellular compartments enables efficient redox compartmentalization and ensures proper functioning of ROS-dependent signaling pathways. Similar to other organisms, the production of individual ROS in plant cells is highly localized and regulated by compartment-specific enzyme pathways on transcriptional and post-translational level. ROS metabolism and signaling in specific compartments are greatly affected by their chemical interactions with other reactive radical species, ROS scavengers and antioxidant enzymes. A dysregulation of the redox status, as a consequence of induced ROS generation or decreased capacity of their removal, occurs in plants exposed to diverse stress conditions. During stress condition, strong induction of ROS-generating systems or attenuated ROS scavenging can lead to oxidative or nitrosative stress conditions, associated with potential damaging modifications of cell biomolecules. Here, we present an overview of compartment-specific pathways of ROS production and degradation and mechanisms of ROS homeostasis control within plant cell compartments.

Orchestration of Cu-Zn SOD and class III peroxidase with upstream interplay between NADPH oxidase and PM H+-ATPase mediates root growth in Vigna radiata (L.) Wilczek

Post-germination plant growth depends on the regulation of reactive oxygen species (ROS) metabolism, spatiotemporal pH changes and Ca⁺² homeostasis, whose potential integration has been studied during Vigna radiata (L.) Wilczek root growth. The dissipation of proton (H⁺) gradients across plasma membrane (PM) by CCCP (protonophore) and the inhibition of PM H⁺-ATPase by sodium orthovanadate repressed SOD (superoxide dismutase; EC 1.15.1.1) activity as revealed by spectrophotometric and native PAGE assay results. Similar results derived from treatment with DPI (NADPH oxidase inhibitor) and Tiron (O₂- scavenger) denote a functional synchronization of SOD, PM H⁺-ATPase and NOX, as the latter two enzymes are substrate sources for SOD (H⁺ and O₂-, respectively) and are involved in a feed-forward loop. After SOD inactivation, a decline in apoplastic H₂O₂ content was observed in each treatment group, emerging as a possible cause of the diminution of class III peroxidase (Prx; EC 1.11.1.7), which utilizes H₂O₂ as a substrate. In agreement with the pivotal role of Ca⁺² in PM H⁺-ATPase and NOX activation, Ca⁺² homeostasis antagonists, i.e., LaCl₃ (Ca⁺² channel inhibitor), EGTA (Ca⁺² chelator) and LiCl (endosomal Ca⁺² release blocker), inhibited both SOD and Prx. Finally, a drastic reduction in apoplastic OH (hydroxyl radical) concentrations (induced by each treatment, leading to Prx inhibition) was observed via fluorometric analysis. A consequential inhibition of root growth observed under each treatment denotes the importance of the orchestrated functioning of PM H⁺-ATPase, NOX, Cu-Zn SOD and Prx during root growth. A working model demonstrating postulated enzymatic synchronization with an intervening role of Ca⁺² is proposed.

Identification of novel superoxide dismutase isoenzymes in the olive (Olea europaea L.) pollen

BACKGROUND

Among antioxidant enzymes, the superoxide dismutase (SOD) family is a major actor in catalysing the disproportionation of superoxide. Apart from its role as antioxidant, these enzymes have a role in cell signalling, and Cu,Zn-SOD proteins are also major pollen allergens. In order to deepen our understanding of the SOD isoenzymes present in olive pollen and to analyse the molecular variability of the pollen Cu,Zn-SOD family, we carried out biochemical, transcriptomic and localization studies of pollen grains from different olive cultivars and other allergenic species.

RESULTS

Olive pollen showed a high rate of total SOD activity in all cultivars assayed, which did not correlate with pollen viability. Mass spectrometry analysis together with activity assays and Western blotting experiments enabled us to identify new forms of Cu,Zn-SOD enzyme (including chloroplastidic and peroxisomal forms) as well as differentially expressed Mn-, Fe- and Cu,Zn-SOD isoenzymes among the pollen of different olive cultivars and allergenic species. Ultrastructural localization of Cu,Zn-SOD revealed its plastidial localization in the pollen grain. We also identified the occurrence of a shorter form of one of the cytosolic Cu,Zn-SOD enzymes, likely as the result of alternative splicing. This shorter enzyme showed lower SOD activity as compared to the full length form.

CONCLUSIONS

The presence of multiple SOD isoenzymes in the olive pollen could be related to the need of finely tuning the ROS metabolism during the transition from its quiescent condition at maturity to a highly metabolically active state at germination.

Reciprocal Interactions between Cadmium-Induced Cell Wall Responses and Oxidative Stress in Plants

Cadmium (Cd) pollution renders many soils across the world unsuited or unsafe for food- or feed-orientated agriculture. The main mechanism of Cd phytotoxicity is the induction of oxidative stress, amongst others through the depletion of glutathione. Oxidative stress can damage lipids, proteins, and nucleic acids, leading to growth inhibition or even cell death. The plant cell has a variety of tools to defend itself against Cd stress. First and foremost, cell walls might prevent Cd from entering and damaging the protoplast. Both the primary and secondary cell wall have an array of defensive mechanisms that can be adapted to cope with Cd. Pectin, which contains most of the negative charges within the primary cell wall, can sequester Cd very effectively. In the secondary cell wall, lignification can serve to immobilize Cd and create a tougher barrier for entry. Changes in cell wall composition are, however, dependent on nutrients and conversely might affect their uptake. Additionally, the role of ascorbate (AsA) as most important apoplastic antioxidant is of considerable interest, due to the fact that oxidative stress is a major mechanism underlying Cd toxicity, and that AsA biosynthesis shares several links with cell wall construction. In this review, modifications of the plant cell wall in response to Cd exposure are discussed. Focus lies on pectin in the primary cell wall, lignification in the secondary cell wall and the importance of AsA in the apoplast. Regarding lignification, we attempt to answer the question whether increased lignification is merely a consequence of Cd toxicity, or rather an elicited defense response. We propose a model for lignification as defense response, with a central role for hydrogen peroxide as substrate and signaling molecule.

Xylogenesis in zinnia (Zinnia elegans) cell cultures: unravelling the regulatory steps in a complex developmental programmed cell death event

MAIN CONCLUSION

Physiological and molecular studies support the view that xylogenesis can largely be determined as a specific form of vacuolar programmed cell death (PCD). The studies in xylogenic zinnia cell culture have led to many breakthroughs in xylogenesis research and provided a background for investigations in other experimental models in vitro and in planta . This review discusses the most essential earlier and recent findings on the regulation of xylem elements differentiation and PCD in zinnia and other xylogenic systems. Xylogenesis (the formation of water conducting vascular tissue) is a paradigm of plant developmental PCD. The xylem vessels are composed of fused tracheary elements (TEs)-dead, hollow cells with patterned lignified secondary cell walls. They result from the differentiation of the procambium and cambium cells and undergo cell death to become functional post-mortem. The TE differentiation proceeds through a well-coordinated sequence of events in which differentiation and the programmed cellular demise are intimately connected. For years a classical experimental model for studies on xylogenesis was the xylogenic zinnia (Zinnia elegans) cell culture derived from leaf mesophyll cells that, upon induction by cytokinin and auxin, transdifferentiate into TEs. This cell system has been proven very efficient for investigations on the regulatory components of xylem differentiation which has led to many discoveries on the mechanisms of xylogenesis. The knowledge gained from this system has potentiated studies in other xylogenic cultures in vitro and in planta. The present review summarises the previous and latest findings on the hormonal and biochemical signalling, metabolic pathways and molecular and gene determinants underlying the regulation of xylem vessels differentiation in zinnia cell culture. Highlighted are breakthroughs achieved through the use of xylogenic systems from other species and newly introduced tools and analytical approaches to study the processes. The mutual dependence between PCD signalling and the differentiation cascade in the program of TE development is discussed.

Extra-Cellular But Extra-Ordinarily Important for Cells: Apoplastic Reactive Oxygen Species Metabolism

Reactive oxygen species (ROS), by their very nature, are highly reactive, and it is no surprise that they can cause damage to organic molecules. In cells, ROS are produced as byproducts of many metabolic reactions, but plants are prepared for this ROS output. Even though extracellular ROS generation constitutes only a minor part of a cell's total ROS level, this fraction is of extraordinary importance. In an active apoplastic ROS burst, it is mainly the respiratory burst oxidases and peroxidases that are engaged, and defects of these enzymes can affect plant development and stress responses. It must be highlighted that there are also other less well-known enzymatic or non-enzymatic ROS sources. There is a need for ROS detoxification in the apoplast, and almost all cellular antioxidants are present in this space, but the activity of antioxidant enzymes and the concentration of low-mass antioxidants is very low. The low antioxidant efficiency in the apoplast allows ROS to accumulate easily, which is a condition for ROS signaling. Therefore, the apoplastic ROS/antioxidant homeostasis is actively engaged in the reception and reaction to many biotic and abiotic stresses.

Modulation of superoxide dismutase (SOD) isozymes by organ development and high long-term salinity in the halophyte Cakile maritima

Superoxide dismutase (SOD) activity catalyzes the disproportionation of superoxide radicals into hydrogen peroxide and oxygen. This enzyme is considered to be a first line of defense for controlling the production of reactive oxygen species (ROS). In this study, the number and type of SOD isozymes were identified in the principal organs (roots, stems, leaves, flowers, and seeds) of Cakile maritima. We also analyzed the way in which the activity of these SOD isozymes is modulated during development and under high long-term salinity (400 mM NaCl) stress conditions. The data indicate that this plant contains a total of ten SOD isozymes: two Mn-SODs, one Fe-SOD, and seven CuZn-SODs, with the Fe-SOD being the most prominent isozyme in the different organs analyzed. Moreover, the modulation of SOD isozymes, particularly CuZn-SODs, was only detected during development and under severe salinity stress conditions. These data suggest that, in C. maritima, the occurrence of these CuZn-SODs in roots and leaves plays an adaptive role since this CuZn-SOD isozyme might replace the diminished Fe-SOD activity under salinity stress to overcome this adverse environmental condition.

Natural Variation of Cold Deacclimation Correlates with Variation of Cold-Acclimation of the Plastid Antioxidant System in Arabidopsis thaliana Accessions

Temperature variations impact on the balance between photosynthetic electron transport and electron-consuming assimilation reactions and transiently increase generation of reactive oxygen species (ROS). Previous studies demonstrated that the expression of C-repeat binding factors (CBFs), which activate cold acclimation reactions, respond to chloroplast ROS signals and that cold deacclimation is partly halted for days after the transfer of acclimated plants to optimal growth conditions in four Arabidopsis accessions from cold-continental habitats. We hypothesized that these accessions differ from others in the regulation of the plastid antioxidant system (PAS). In the present study, we compared the expression intensity of the 12 most prominent PAS genes for peroxidases, superoxide dismutase and low molecular weight antioxidant regenerating enzymes in 10 Arabidopsis accessions with regulation of CBF and COR (cold regulated genes) transcript levels and cold-regulated metabolite levels prior to cold, after 2 week long cold acclimation and during the first 3 days of deacclimation. In the accessions with prolonged activation of cold responses, by trend, weaker induction of various cold-inducible PAS genes and stronger decreases in the expression of negatively cold-regulated PAS genes were observed. Low PAS gene expression delayed the post-cold decrease in H2O2 levels after transfer of the plants from cold to optimal growth conditions. We conclude that weaker expression of various PAS genes in the cold is an adapted strategy of the Arabidopsis accessions N14, N13, Ms-0, and Kas-1 to avoid full inactivation of cold-responses in the first days after the end of the cold period.

Dry Priming of Maize Seeds Reduces Aluminum Stress

Aluminum (Al) toxicity is directly related to acidic soils and substantially limits maize yield. Earlier studies using hormones and other substances to treat the seeds of various crops have been carried out with the aim of inducing tolerance to abiotic stress, especially chilling, drought and salinity. However, more studies regarding the effects of seed treatments on the induction of Al tolerance are necessary. In this study, two independent experiments were performed to determine the effect of ascorbic acid (AsA) seed treatment on the tolerance response of maize to acidic soil and Al stress. In the first experiment (greenhouse), the AsA seed treatment was tested in B73 (Al-sensitive genotype). This study demonstrates the potential of AsA for use as a pre-sowing seed treatment (seed priming) because this metabolite increased root and shoot growth under acidic and Al stress conditions. In the second test, the evidence from field experiments using an Al-sensitive genotype (Mo17) and an Al-tolerant genotype (DA) suggested that prior AsA seed treatment increased the growth of both genotypes. Enhanced productivity was observed for DA under Al stress after priming the seeds. Furthermore, the AsA treatment decreased the activity of oxidative stress-related enzymes in the DA genotype. In this study, remarkable effects using AsA seed treatment in maize were observed, demonstrating the potential future use of AsA in seed priming.

Molecular cloning and characterization of two manganese superoxide dismutases from Miscanthus × giganteus

Six MnSOD genes were isolated from five Miscanthus species. MgMnSOD1 functions in mitochondria and MgMnSOD1 seems to be the main MnSOD gene involved in stress response of M. × giganteus. Miscanthus × giganteus is a promising biomass energy crop with advantages of vigorous growth, high yield, low fertilizer and pesticide inputs. However, poor overwinter ability limits its widespread cultivation. Moreover, narrow genetic base may increase the risk of susceptibility to diseases and pests. Manganese superoxide dismutase (MnSOD), an important antioxidant enzyme involved in stress tolerance is able to protect plant cells from accumulated reactive oxygen species by converting superoxide to peroxide and oxygen. In many plants, overexpression of MnSOD has shown the ability to enhance the resistance to various stresses. This article describes the studies performed in an attempt to elucidate the molecular and enzymatic properties of MnSODs in M. × giganteus. MnSOD genes from M. × giganteus (MgMnSOD1, MgMnSOD2), M. lutarioriparia (MlMnSOD), M. sacchariflora (MsaMnSOD), M. sinensis (MsiMnSOD), and M. floridulus (MfMnSOD) were cloned and sequenced. The sequence analysis and expression patterns of MgMnSOD1 and MgMnSOD2 suggest that they were orthologous genes which were inherited from the two parents, M. sacchariflora and M. sinensis, respectively. In addition, MgMnSOD1 is predicted to be the main MnSOD gene involved in stress response of M. × giganteus. The activity of purified recombinant MgMnSOD1 was 1854.79 ± 39.98 U mg(-1) (mean ± SD). Further enzymatic assays revealed that the protein exhibited an outstanding thermal stability. MgMnSOD1 is predicted to be targeted to mitochondria and involved in removing the superoxide radical generated by respiration. The presence and sequences of other SOD isozymes transcripts were also investigated in this study.

Monoclonal antibody-based analysis of cell wall remodeling during xylogenesis

Xylogenesis, a process by which woody tissues are formed, entails qualitative and quantitative changes in the cell wall. However, the molecular events that underlie these changes are not completely understood. Previously, we have isolated two monoclonal antibodies, referred to as XD3 and XD27, by subtractive screening of a phage-display library of antibodies raised against a wall fraction of Zinnia elegans xylogenic culture cells. Here we report the biochemical and immunohistochemical characterization of those antibodies. The antibody XD3 recognized (1→4)-β-D-galactan in pectin fraction. During xylogenesis, the XD3 epitope was localized to the primary wall of tracheary-element precursor cells, which undergo substantial cell elongation, and was absent from mature tracheary elements. XD27 recognized an arabinogalactan protein that was bound strongly to a germin-like protein. The XD27 epitope was localized to pre-lignified secondary walls of tracheary elements. Thus these cell-wall-directed monoclonal antibodies revealed two molecular events during xylogenesis. The biological significance of these events is discussed in relation to current views of the plant cell wall.

Spatio-temporal relief from hypoxia and production of reactive oxygen species during bud burst in grapevine (Vitis vinifera)

BACKGROUND AND AIMS

Plants regulate cellular oxygen partial pressures (pO2), together with reduction/oxidation (redox) state in order to manage rapid developmental transitions such as bud burst after a period of quiescence. However, our understanding of pO2 regulation in complex meristematic organs such as buds is incomplete and, in particular, lacks spatial resolution.

METHODS

The gradients in pO2 from the outer scales to the primary meristem complex were measured in grapevine (Vitis vinifera) buds, together with respiratory CO2 production rates and the accumulation of superoxide and hydrogen peroxide, from ecodormancy through the first 72 h preceding bud burst, triggered by the transition from low to ambient temperatures.

KEY RESULTS

Steep internal pO2 gradients were measured in dormant buds with values as low as 2·5 kPa found in the core of the bud prior to bud burst. Respiratory CO2 production rates increased soon after the transition from low to ambient temperatures and the bud tissues gradually became oxygenated in a patterned process. Within 3 h of the transition to ambient temperatures, superoxide accumulation was observed in the cambial meristem, co-localizing with lignified cellulose associated with pro-vascular tissues. Thereafter, superoxide accumulated in other areas subtending the apical meristem complex, in the absence of significant hydrogen peroxide accumulation, except in the cambial meristem. By 72 h, the internal pO2 gradient showed a biphasic profile, where the minimum pO2 was external to the core of the bud complex.

CONCLUSIONS

Spatial and temporal control of the tissue oxygen environment occurs within quiescent buds, and the transition from quiescence to bud burst is accompanied by a regulated relaxation of the hypoxic state and accumulation of reactive oxygen species within the developing cambium and vascular tissues of the heterotrophic grapevine buds.

Spatial and temporal regulation of the metabolism of reactive oxygen and nitrogen species during the early development of pepper (Capsicum annuum) seedlings

BACKGROUND AND AIMS

The development of seedlings involves many morphological, physiological and biochemical processes, which are controlled by many factors. Some reactive oxygen and nitrogen species (ROS and RNS, respectively) are implicated as signal molecules in physiological and phytopathological processes. Pepper (Capsicum annuum) is a very important crop and the goal of this work was to provide a framework of the behaviour of the key elements in the metabolism of ROS and RNS in the main organs of pepper during its development.

METHODS

The main seedling organs (roots, hypocotyls and green cotyledons) of pepper seedlings were analysed 7, 10 and 14 d after germination. Activity and gene expression of the main enzymatic antioxidants (catalase, ascorbate-glutathione cycle enzymes), NADP-generating dehydrogenases and S-nitrosoglutathione reductase were determined. Cellular distribution of nitric oxide ((·)NO), superoxide radical (O2 (·-)) and peroxynitrite (ONOO(-)) was investigated using confocal laser scanning microscopy.

KEY RESULTS

The metabolism of ROS and RNS during pepper seedling development was highly regulated and showed significant plasticity, which was co-ordinated among the main seedling organs, resulting in correct development. Catalase showed higher activity in the aerial parts of the seedling (hypocotyls and green cotyledons) whereas roots of 7-d-old seedlings contained higher activity of the enzymatic components of the ascorbate glutathione cycle, NADP-isocitrate dehydrogenase and NADP-malic enzyme.

CONCLUSIONS

There is differential regulation of the metabolism of ROS, nitric oxide and NADP dehydrogenases in the different plant organs during seedling development in pepper in the absence of stress. The metabolism of ROS and RNS seems to contribute significantly to plant development since their components are involved directly or indirectly in many metabolic pathways. Thus, specific molecules such as H2O2 and NO have implications for signalling, and their temporal and spatial regulation contributes to the success of seedling establishment.

The cell biology of lignification in higher plants

BACKGROUND

Lignin is a polyphenolic polymer that strengthens and waterproofs the cell wall of specialized plant cell types. Lignification is part of the normal differentiation programme and functioning of specific cell types, but can also be triggered as a response to various biotic and abiotic stresses in cells that would not otherwise be lignifying.

SCOPE

Cell wall lignification exhibits specific characteristics depending on the cell type being considered. These characteristics include the timing of lignification during cell differentiation, the palette of associated enzymes and substrates, the sub-cellular deposition sites, the monomeric composition and the cellular autonomy for lignin monomer production. This review provides an overview of the current understanding of lignin biosynthesis and polymerization at the cell biology level.

CONCLUSIONS

The lignification process ranges from full autonomy to complete co-operation depending on the cell type. The different roles of lignin for the function of each specific plant cell type are clearly illustrated by the multiple phenotypic defects exhibited by knock-out mutants in lignin synthesis, which may explain why no general mechanism for lignification has yet been defined. The range of phenotypic effects observed include altered xylem sap transport, loss of mechanical support, reduced seed protection and dispersion, and/or increased pest and disease susceptibility.

Expression of SOD and APX genes positively regulates secondary cell wall biosynthesis and promotes plant growth and yield in Arabidopsis under salt stress

Abiotic stresses cause accumulation of reactive oxygen species (ROS), such as hydrogen peroxide (H2O2) in plants. Sophisticated mechanisms are required to maintain optimum level of H2O2 that acts as signalling molecule regulating adaptive response to salt stress. CuZn-superoxide dismutase (CuZn-SOD) and ascorbate peroxidase (APX) constitute first line of defence against oxidative stress. In the present study, PaSOD and RaAPX genes from Potentilla atrosanguinea and Rheum australe, respectively were overexpressed individually as well as in combination in Arabidopsis thaliana. Interestingly, PaSOD and dual transgenic lines exhibit enhanced lignin deposition in their vascular bundles with altered S:G ratio under salt stress. RNA-seq analysis revealed that expression of PaSOD gene in single and dual transgenics positively regulates expression of lignin biosynthesis genes and transcription factors (NACs, MYBs, C3Hs and WRKY), leading to enhanced and ectopic deposition of lignin in vascular tissues with larger xylem fibres and alters S:G ratio, as well. In addition, transgenic plants exhibit growth promotion, higher biomass production and increased yield under salt stress as compared to wild type plants. Our results suggest that in dual transgenics, ROS generated during salt stress gets converted into H2O2 by SOD and its optimum level was maintained by APX. This basal level of H2O2 acts as messenger for transcriptional activation of lignin biosynthesis in vascular tissue, which provides mechanical strength to plants. These findings reveal an important role of PaSOD and RaAPX in enhancing salt tolerance of transgenic Arabidopsis via increased accumulation of compatible solutes and by regulating lignin biosynthesis.

Quantification of the antioxidant activity in salt-stressed tissues

Biochemical methods available for the measurement of antioxidant activity in salt-stressed tissues are reviewed, outlining the most important advantages and shortcomings of the methods. Here we consider commonly used methods for measuring total antioxidant capacity and phenolic content, ABTS and Folin-Ciocalteu's procedure, respectively. Moreover, we presented assays for determination of antioxidant enzymes activities: superoxide dismutase, catalase, and ascorbate peroxidase. This choice of methods enables us to elucidate a full profile of antioxidant activities, evaluating their effectiveness against various reactive oxygen species produced during salt stress.

The accumulation of endogenous proline induces changes in gene expression of several antioxidant enzymes in leaves of transgenic Swingle citrumelo

Plant exposure to abiotic stresses leads to an accumulation of reactive oxygen species with the concomitant increase in antioxidant defense mechanisms. Previous studies showed that exogenous application of proline mitigate the deleterious effects caused by oxidative stress due to its ability to increase the activity of antioxidant enzymes. However, there are no reports of the effects of high endogenous accumulation of proline in the transcriptional pattern of antioxidant enzymes genes under normal conditions of water supply or in response to water deficit. Here, we show that isoforms of four antioxidant enzymes genes (Ascorbate peroxidase-APX, Catalase-CAT, Superoxide dismutase-SOD and Glutathione reductase-GR) were differentially regulated in leaves of Swingle citrumelo transgenic plants with high endogenous proline accumulation submitted to water deficits and also under normal water supply condition. Proline per se caused a two-fold change in the transcription activity of APX1, APXcl, CAT2 and Cu/ZnSOD2, while during water deficit proline influenced mRNAs levels in APXs and Cu/ZnSODs isoforms, MnSODmit and GRcl. This study adds new information on the role of proline during drought conditions and, more important, without the potential confounding effects imposed by water deficiency. We showed that, in addition to its known effects on diverse plant physiological and biochemical processes, high endogenous proline can also acts as a regulatory/signalling molecule capable of altering the transcript levels of stress-related genes.

Using hyper as a molecular probe to visualize hydrogen peroxide in living plant cells: a method with virtually unlimited potential in plant biology

Reactive oxygen species (ROS) are highly reactive reduced oxygen molecules that play a myriad of roles in animal and plant cells. In plant cells, the production of ROS occurs as a result of aerobic metabolism during respiration and photosynthesis. Therefore mitochondria, chloroplasts, and peroxisomes constitute an important source of ROS. However, they can be produced in response to many physiological stimuli such as pathogen attack, hormone signaling, abiotic stresses, or during cell wall organization and plant morphogenesis. Monitoring ROS in plant cells has been limited to biochemical assays and use of fluorescent probes, however, the irreversible oxidation of the fluorescent dyes make it impossible to visualize dynamic changes of ROS. Hyper is a recently developed live cell probe for H2O2 and consists of a circularly permutated YFP (cpYFP) inserted into the regulatory domain of the Escherichia coli hydrogen peroxide (H2O2) sensor OxyR rendering it a H2O2 specific ratiometric, and therefore quantitative probe. Herein, we describe a protocol for using Hyper as a dynamic probe for H2O2 in Arabidopsis with virtually unlimited potential to detect H2O2 throughout the plant and under a broad range of developmental and environmental conditions.

Expression of Mn-SOD, iNOS and eNOS mRNAs in osteoblasts from the maxilla of osteopetrotic mice

Active oxygens and free radicals are involved in the metabolism and clinical conditions of tissues; however, little is known about the localisation and expression levels of associated enzymes. The expressions of active oxygens, free radicals and associated enzymes are reported to be site-specific; therefore, the expression states of free radical enzymes differ between sites, even within the same cell. In particular, there has been no report concerning the catabolic enzymes of active oxygens in osteoblasts of the maxilla, other than normal osteoblasts that were weakly positive by immunohistochemical staining. We conducted this study to elucidate the relationship between osteodystrophy and radical-associated enzymes by investigating mRNAs of enzymes associated with active oxygens and free radicals using osteoblasts from the maxilla of normal and osteopetrotic model (op/op) mice. In op/op mouse maxilla osteoblasts, mRNAs of Mn-SOD, i-NOS and e-NOS were strongly positive. Mn-SOD and iNOS enzymes were considered to be highly expressed in osteoblasts of the narrowed medullary cavity of this bone.

Overexpression of GSH1 gene mimics transcriptional response to low temperature during seed vernalization treatment of Arabidopsis

Keeping imbibed seeds at low temperatures for a certain period, so-called seed vernalization (SV) treatment, promotes seed germination and subsequent flowering in various plants. Vernalization-promoting flowering requires GSH. However, we show here that increased GSH biosynthesis partially mimics SV treatment in Arabidopsis thaliana. SV treatment (keeping imbibed seeds at 4°C for 24 h) induced a specific pattern of gene expression and promoted subsequent flowering in WT A. thaliana. A similar pattern was observed at 22°C in transgenic (35S-GSH1) plants overexpressing the γ-glutamylcysteine synthetase gene GSH1, coding for an enzyme limiting GSH biosynthesis, under the control of the cauliflower mosaic virus 35S promoter. This pattern of gene expression was further strengthened at 4°C and indistinguishable from the WT pattern at 4°C. However, flowering in 35S-GSH1 plants was less responsive to SV treatment than in WT plants. There was a difference in the transcript behavior of the flowering repressor FLC between WT and 35S-GSH1 plants. Unlike other genes responsive to SV treatment, the SV-dependent decrease in FLC in WT plants was reversed in 35S-GSH1 plants. SV treatment increased the GSSG level in WT seeds while its level was high in 35S-GSH1 plants, even at a non-vernalizing temperature. Taking into consideration that low temperatures stimulate GSH biosynthesis and cause oxidative stress, GSSG is considered to trigger a low-temperature response, although enhanced GSH synthesis was not enough to completely mimic the SV treatment.

The Ustilago maydis effector Pep1 suppresses plant immunity by inhibition of host peroxidase activity

The corn smut Ustilago maydis establishes a biotrophic interaction with its host plant maize. This interaction requires efficient suppression of plant immune responses, which is attributed to secreted effector proteins. Previously we identified Pep1 (Protein essential during penetration-1) as a secreted effector with an essential role for U. maydis virulence. pep1 deletion mutants induce strong defense responses leading to an early block in pathogenic development of the fungus. Using cytological and functional assays we show that Pep1 functions as an inhibitor of plant peroxidases. At sites of Δpep1 mutant penetrations, H₂O₂ strongly accumulated in the cell walls, coinciding with a transcriptional induction of the secreted maize peroxidase POX12. Pep1 protein effectively inhibited the peroxidase driven oxidative burst and thereby suppresses the early immune responses of maize. Moreover, Pep1 directly inhibits peroxidases in vitro in a concentration-dependent manner. Using fluorescence complementation assays, we observed a direct interaction of Pep1 and the maize peroxidase POX12 in vivo. Functional relevance of this interaction was demonstrated by partial complementation of the Δpep1 mutant defect by virus induced gene silencing of maize POX12. We conclude that Pep1 acts as a potent suppressor of early plant defenses by inhibition of peroxidase activity. Thus, it represents a novel strategy for establishing a biotrophic interaction.

Aluminum toxicity recovery processes in root apices. Possible association with oxidative stress

Al inhibits root apex elongation with concomitant morphological injuries such as ruptures punctuated by the regions stained with Evans blue. The recovery can be investigated by transfer of Al-injured roots to a solution lacking Al. In the Al-injured root apex, superoxide anion, H(2)O(2), Al, and lignin accumulate. During the recovery process, the central cylinder elongates leaving the region stained with Evans blue without marked disappearance. The obvious function of the region is not clear but may trigger the elongation of central cylinder during the recovery process. Thus the function of the region stained with Evans blue might be derived from the programmed cell-like idea. Oxidative stress concerns events induced under Al toxicity and the recovery process. The superoxide anion is primarily formed by plasma membrane-associated NADPH oxidase and is dismuted to H(2)O(2) and O(2) by superoxide dismutase. H(2)O(2) provides the electrons for the polymerization of phenolics to lignin, which causes the stiffening of the cell wall. The distortion of the cell wall caused by lignin may induce the breaking and tearing of cells, which results in the formation of ruptures at the rhizodermis and outer cortex layers. The production of superoxide anion, H(2)O(2), and lignin was reduced during the recovery process and thereby the elongation of the central cylinder may be induced.

Effects of Pb and Ni stress on oxidative stress parameters in three moss species

Antioxidative responses of the mosses Hypnum plumaeforme, Thuidium cymbifolium, and Brachythecium piligerum to short-term Pb and Ni stress were investigated. Both Pb and Ni treatment increased the formation of reactive oxygen species and lipid peroxidation, decreased superoxide dismutase (SOD), and catalase (CAT) activities in H. plumaeforme and T. cymbifolium. However, SOD activity in B. piligerum was increased under 10mM Pb stress and Ni increased CAT activity in B. piligerum under 1mM Ni stress. Peroxidase (POX) activity in the three mosses was increased by Pb and Ni exposure, indicating that POX plays an important role in preventing heavy metal-induced oxidative stress. The accumulation of O(2)(-) and H(2)O(2) in mosses is related to the decline in SOD and CAT activities. B. piligerum is the most sensitive and T. cymbifolium is the most tolerant species to Pb and Ni stress among the three bryophytes.

Changes to the proteome and targeted metabolites of xylem sap in Brassica oleracea in response to salt stress

Root-to-shoot signalling via xylem sap is an important mechanism by which plants respond to stress. This signalling could be mediated by alteration in the concentrations of inorganic and/or organic molecules. The effect of salt stress on the contents of xylem sap in Brassica olarecea has been analysed by mass spectrometry in order to quantify these changes. Subcellular location of arabinogalactan proteins (AGPs) by immunogold labelling and peroxidase isozymes was also analysed by isoelectrofocusing. The xylem sap metabolome analysis demonstrated the presence of many organic compounds such as sugars, organic acids and amino acids. Of these, amino acid concentrations, particularly that of glutamine, the major amino acid in the sap, were substantially reduced by salt stress. The xylem sap proteome analysis demonstrated the accumulation of enzymes involved in xylem differentiation and lignification, such as cystein proteinases, acid peroxidases, and a putative hydroxycinnamoyl-CoA:shikimate hydroxycinnamoyl transferase under salt stress. The peroxidase isozyme pattern showed that salt stress induced a high accumulation of an acid isoform. These results suggest that xylem differentiation and lignification is induced by salt stress. The combination of different methods to analyse the xylem sap composition provides new insights into mechanisms in plant development and signalling under salt stress.

Effect of oligogalacturonides on root length, extracellular alkalinization and O₂⁻-accumulation in alfalfa

The effects of an oligogalacturonic acid (OGA) pool on root length of intact alfalfa seedlings (Medicago sativa L.), on extracellular pH and on both extracellular and intracellular O₂⁻ dynamics were examined in this study. Lower OGA concentrations (25, 50 and 75 μg mL⁻¹)promoted root length, but 50 μg mL⁻¹ had a stronger effect in promoting growth, while the higher OGA concentration (100 μg mL⁻¹)had no significant effect. Extracellular alkalinization was tested only at concentrations higher than 50 μg mL⁻¹ OGA, showing that the response is determined not only by the specific size of OGA, but also by the concentration of OGA. The promoting effect of OGA on root growth at 25, 50 and 75 μg mL⁻¹ OGA concentrations in alfalfa root appeared to be unrelated to extracellular alkalinization. A possible explanation could be the induction of an O₂⁻ burst at non-toxic levels, which could drive directly or indirectly several processes associated with root elongation in 25, 50 and 75 μg mL⁻¹ OGA-treated seedlings. Analyses using confocal microscopy showed that the increase in the O₂⁻ generation, mainly in the epidermal cells, induced by 50 μg mL⁻¹ OGA could be related to the promoting effect on root growth. The combination of OGA with DPI allowed us to demonstrate that there are different O₂⁻-generating sources in the epidermal cells of the meristematic zone, likely NADPH oxidase and oxidases or oxido-reductase enzymes, insensitive to DPI, that maintain detectable O₂⁻ accumulation at 60 and 120 min of treatment. These results suggest that OGA induce an oxidative burst by several O₂⁻-generating sources in the active growth zones.

BYPASS1: synthesis of the mobile root-derived signal requires active root growth and arrests early leaf development

BACKGROUND

The Arabidopsis bypass1 (bps1) mutant root produces a biologically active mobile compound that induces shoot growth arrest. However it is unknown whether the root retains the capacity to synthesize the mobile compound, or if only shoots of young seedlings are sensitive. It is also unknown how this compound induces arrest of shoot growth. This study investigated both of these questions using genetic, inhibitor, reporter gene, and morphological approaches.

RESULTS

Production of the bps1 root-synthesized mobile compound was found to require active root growth. Inhibition of postembryonic root growth, by depleting glutathione either genetically or chemically, allowed seedlings to escape shoot arrest. However, the treatments were not completely effective, as the first leaf pair remained radialized, but elongated. This result indicated that the embryonic root transiently synthesized a small amount of the mobile substance. In addition, providing glutathione later in vegetative development caused shoot growth arrest to be reinstated, revealing that these late-arising roots were still capable of producing the mobile substance, and that the older vegetative leaves were still responsive. To gain insight into how leaf development responds to the mobile signal, leaf development was followed morphologically and using the CYCB1,1::GUS marker for G2/M phase cells. We found that arrest of leaf growth is a fully penetrant phenotype, and a dramatic decrease in G2/M phase cells was coincident with arrest. Analyses of stress phenotypes found that late in development, bps1 cotyledons produced necrotic lesions, however neither hydrogen peroxide nor superoxide were abundant as leaves underwent arrest.

CONCLUSIONS

bps1 roots appear to require active growth in order to produce the mobile bps1 signal, but the potential for this compound's synthesis is present both early and late during vegetative development. This prolonged capacity to synthesize and respond to the mobile compound is consistent with a possible role for the mobile compound in linking shoot growth to subterranean conditions. The specific growth-related responses in the shoot indicated that the mobile substance prevents full activation of cell division in leaves, although whether cell division is a direct response remains to be determined.

Comparative proteomics analysis of differentially expressed proteins in soybean cell wall during flooding stress

Flooding is a major problem for soybean crop as it reduces the growth and grain yield. To investigate the function of the soybean cell wall in the response to flooding stress, cell wall proteins were analyzed. Cell wall proteins from roots and hypocotyls of soybeans, which were germinated for 2 days and subjected to 2 days of flooding, were purified, separated by two-dimensional polyacrylamide gel electrophoresis and stained with Coomassie brilliant blue. Sixteen out of 204 cell wall proteins showed responses to flooding stress. Of these, two lipoxygenases, four germin-like protein precursors, three stem 28/31 kDa glycoprotein precursors, and one superoxide dismutase [Cu-Zn] were downregulated. A copper amine oxidase was found to have shifted from the basic to acidic zone following flooding stress. Based on these results, it was confirmed by the lignin staining that the lignification was suppressed in the root of soybean under the flooding stress. These results suggest that the roots and hypocotyls of soybean caused the suppression of lignification through decrease of these proteins by downregulation of reactive oxygen species and jasmonate biosynthesis under flooding stress.

Scavenging of reactive oxygen species in apoplastic and symplastic areas of rolled leaves in Ctenanthe setosa under drought stress

The correspondence among apoplastic and symplastic antioxidant status, stomatal conductance and water potential was investigated during leaf rolling in Ctenanthe setosa (Rosc.) Eichler (Marantaceae) under drought stress. Apoplastic and symplastic extractions of leaf and petiole were performed at different visual leaf rolling scores from 1 to 4 (1 is unrolled, 4 is tightly rolled and the others are intermediate form). In the leaf symplast, the highest changes were found in catalase (CAT) and guaiacol peroxidase (GPX) activities when compared to score 1 during leaf rolling. No significant change was observed in superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities in the symplast of leaf during the rolling. The same phenomenon was also present in the symplast of petiole except APX activity. In the leaf apoplast, the highest increase occurred in APX and GPX activities, whilst a slight increase in CAT and SOD activities. In the apoplast of petiole, the highest increment was found only in GPX activity, while there were small increases in SOD, APX and CAT activities. Hydrogen peroxide content increased up to score 3 in the apoplast and symplast of leaf and petiole but then slightly decreased. Also, superoxide production increased in the leaf and petiole apoplast but its quantity in the apoplast was much more than that of the symplast. On the other hand, NAD(P)H oxidase activity increased in the leaf but no change was observed in the petiole. In conclusion, as a result of water deficit during leaf rolling antioxidant enzymes are induced to scavenging of ROS produced in symplast and apoplast.