Peroxisomal membrane manganese superoxide dismutase: characterization of the isozyme from watermelon (Citrullus lanatus Schrad.) cotyledons

Reactive oxygen species and organellar signaling

The evolution of photosynthesis and its associated metabolic pathways has been crucial to the successful establishment of plants, but has also challenged plant cells in the form of production of reactive oxygen species (ROS). Intriguingly, multiple forms of ROS are generated in virtually every plant cell compartment through diverse pathways. As a result, a sophisticated network of ROS detoxification and signaling that is simultaneously tailored to individual organelles and safeguards the entire cell is necessary. Here we take an organelle-centric view on the principal sources and sinks of ROS across the plant cell and provide insights into the ROS-induced organelle to nucleus retrograde signaling pathways needed for operational readjustments during environmental stresses.

Nitric Oxide (NO) Scaffolds the Peroxisomal Protein-Protein Interaction Network in Higher Plants

The peroxisome is a single-membrane subcellular compartment present in almost all eukaryotic cells from simple protists and fungi to complex organisms such as higher plants and animals. Historically, the name of the peroxisome came from a subcellular structure that contained high levels of hydrogen peroxide (H2O2) and the antioxidant enzyme catalase, which indicated that this organelle had basically an oxidative metabolism. During the last 20 years, it has been shown that plant peroxisomes also contain nitric oxide (NO), a radical molecule than leads to a family of derived molecules designated as reactive nitrogen species (RNS). These reactive species can mediate post-translational modifications (PTMs) of proteins, such as S-nitrosation and tyrosine nitration, thus affecting their function. This review aims to provide a comprehensive overview of how NO could affect peroxisomal metabolism and its internal protein-protein interactions (PPIs). Remarkably, many of the identified NO-target proteins in plant peroxisomes are involved in the metabolism of reactive oxygen species (ROS), either in its generation or its scavenging. Therefore, it is proposed that NO is a molecule with signaling properties with the capacity to modulate the peroxisomal protein-protein network and consequently the peroxisomal functions, especially under adverse environmental conditions.

Plant Peroxisomes: A Factory of Reactive Species

Plant peroxisomes are organelles enclosed by a single membrane whose biochemical composition has the capacity to adapt depending on the plant tissue, developmental stage, as well as internal and external cellular stimuli. Apart from the peroxisomal metabolism of reactive oxygen species (ROS), discovered several decades ago, new molecules with signaling potential, including nitric oxide (NO) and hydrogen sulfide (H2S), have been detected in these organelles in recent years. These molecules generate a family of derived molecules, called reactive nitrogen species (RNS) and reactive sulfur species (RSS), whose peroxisomal metabolism is autoregulated through posttranslational modifications (PTMs) such as S-nitrosation, nitration and persulfidation. The peroxisomal metabolism of these reactive species, which can be weaponized against pathogens, is susceptible to modification in response to external stimuli. This review aims to provide up-to-date information on crosstalk between these reactive species families and peroxisomes, as well as on their cellular environment in light of the well-recognized signaling properties of H2O2, NO and H2S.

Acquiring control: The evolution of ROS-Induced oxidative stress and redox signaling pathways in plant stress responses

Reactive oxygen species (ROS) - the byproducts of aerobic metabolism - influence numerous aspects of the plant life cycle and environmental response mechanisms. In plants, ROS act like a double-edged sword; they play multiple beneficial roles at low concentrations, whereas at high concentrations ROS and related redox-active compounds cause cellular damage through oxidative stress. To examine the dual role of ROS as harmful oxidants and/or crucial cellular signals, this review elaborates that (i) how plants sense and respond to ROS in various subcellular organelles and (ii) the dynamics of subsequent ROS-induced signaling processes. The recent understanding of crosstalk between various cellular compartments in mediating their redox state spatially and temporally is discussed. Emphasis on the beneficial effects of ROS in maintaining cellular energy homeostasis, regulating diverse cellular functions, and activating acclimation responses in plants exposed to abiotic and biotic stresses are described. The comprehensive view of cellular ROS dynamics covering the breadth and versatility of ROS will contribute to understanding the complexity of apparently contradictory ROS roles in plant physiological responses in less than optimum environments.

The activity of superoxide dismutases (SODs) at the early stages of wheat deetiolation

Unbound tetrapyrroles, i.e. protochlorophyllide (Pchlide), chlorophyllide and chlorophylls, bring the risk of reactive oxygen species (ROS) being generated in the initial stages of angiosperm deetiolation due to inefficient usage of the excitation energy for photosynthetic photochemistry. We analyzed the activity of superoxide dismutases (SODs) in etiolated wheat (Triticum aestivum) leaves and at the beginning of their deetiolation. Mn-SOD and three isoforms of Cu/Zn-SODs were identified both in etiolated and greening leaves of T. aestivum. Two Cu/Zn-SODs, denoted as II and III, were found in plastids. The activity of plastidic Cu/Zn-SOD isoforms as well as that of Mn-SOD correlated with cell aging along a monocot leaf, being the highest at leaf tips. Moreover, a high Pchlide content at leaf tips was observed. No correlation between SOD activity and the accumulation of photoactive Pchlide, i.e. Pchlide bound into ternary Pchlide:Pchlide oxidoreductase:NADPH complexes was found. Cu/Zn-SOD I showed the highest activity at the leaf base. A flash of light induced photoreduction of the photoactive Pchlide to chlorophyllide as well as an increase in all the SODs activity which occurred in a minute time-scale. In the case of seedlings that were deetiolated under continuous light of moderate intensity (100 μmol photons m-2 s-1), only some fluctuations in plastidic Cu/Zn-SODs and Mn-SOD within the first four hours of greening were noticed. The activity of SODs is discussed with respect to the assembly of tetrapyrroles within pigment-protein complexes, monitored by fluorescence spectroscopy at 77 K.

The Proteome of Fruit Peroxisomes: Sweet Pepper (Capsicum annuum L.) as a Model

Despite of their economical and nutritional interest, the biology of fruits is still little studied in comparison with reports of other plant organs such as leaves and roots. Accordingly, research at subcellular and molecular levels is necessary not only to understand the physiology of fruits, but also to improve crop qualities. Efforts addressed to gain knowledge of the peroxisome proteome and how it interacts with the overall metabolism of fruits will provide tools to be used in breeding strategies of agricultural species with added value. In this work, special attention will be paid to peroxisomal proteins involved in the metabolism of reactive oxygen species (ROS) due to the relevant role of these compounds at fruit ripening. The proteome of peroxisomes purified from sweet pepper (Capsicum annuum L.) fruit is reported, where an iron-superoxide dismutase (Fe-SOD) was localized in these organelles, besides other antioxidant enzymes such as catalase and a Mn-SOD, as well as enzymes involved in the metabolism of carbohydrates, malate, lipids and fatty acids, amino acids, the glyoxylate cycle and in the potential organelles' movements.

Plant peroxisomes: A nitro-oxidative cocktail

Although peroxisomes are very simple organelles, research on different species has provided us with an understanding of their importance in terms of cell viability. In addition to the significant role played by plant peroxisomes in the metabolism of reactive oxygen species (ROS), data gathered over the last two decades show that these organelles are an endogenous source of nitric oxide (NO) and related molecules called reactive nitrogen species (RNS). Molecules such as NO and H2O2 act as retrograde signals among the different cellular compartments, thus facilitating integral cellular adaptation to physiological and environmental changes. However, under nitro-oxidative conditions, part of this network can be overloaded, possibly leading to cellular damage and even cell death. This review aims to update our knowledge of the ROS/RNS metabolism, whose important role in plant peroxisomes is still underestimated. However, this pioneering approach, in which key elements such as β-oxidation, superoxide dismutase (SOD) and NO have been mainly described in relation to plant peroxisomes, could also be used to explore peroxisomes from other organisms.

Responses of transgenic Arabidopsis plants and recombinant yeast cells expressing a novel durum wheat manganese superoxide dismutase TdMnSOD to various abiotic stresses

In plant cells, the manganese superoxide dismutase (Mn-SOD) plays an elusive role in the response to oxidative stress. In this study, we describe the isolation and functional characterization of a novel Mn-SOD from durum wheat (Triticum turgidum L. subsp. Durum), named TdMnSOD. Molecular phylogeny analysis showed that the durum TdMnSOD exhibited high amino acids sequence identity with other Mn-SOD plants. The three-dimensional structure showed that TdMnSOD forms a homotetramer and each subunit is composed of a predominantly α-helical N-terminal domain and a mixed α/β C-terminal domain. TdMnSOD gene expression analysis showed that this gene was induced by various abiotic stresses in durum wheat. The expression of TdMnSOD enhances tolerance of the transformed yeast cells to salt, osmotic, cold and H2O2-induced oxidative stresses. Moreover, the analysis of TdMnSOD transgenic Arabidopsis plants subjected to different environmental stresses revealed low H2O2 and high proline levels as compared to the wild-type plants. Compared with the non-transformed plants, an increase in the total SOD and two other antioxidant enzyme activities including catalase (CAT) and peroxidases (POD) was observed in the three transgenic lines subjected to abiotic stress. Taken together, these data provide evidence for the involvement of durum wheat TdMnSOD in tolerance to multiple abiotic stresses in crop plants.

ROS Generation in Peroxisomes and its Role in Cell Signaling

In plant cells, as in most eukaryotic organisms, peroxisomes are probably the major sites of intracellular H₂O₂ production, as a result of their essentially oxidative type of metabolism. In recent years, it has become increasingly clear that peroxisomes carry out essential functions in eukaryotic cells. The generation of the important messenger molecule hydrogen peroxide (H₂O₂) by animal and plant peroxisomes and the presence of catalase in these organelles has been known for many years, but the generation of superoxide radicals (O₂^·- ) and the occurrence of the metalloenzyme superoxide dismutase was reported for the first time in peroxisomes from plant origin. Further research showed the presence in plant peroxisomes of a complex battery of antioxidant systems apart from catalase. The evidence available of reactive oxygen species (ROS) production in peroxisomes is presented, and the different antioxidant systems characterized in these organelles and their possible functions are described. Peroxisomes appear to have a ROS-mediated role in abiotic stress situations induced by the heavy metal cadmium (Cd) and the xenobiotic 2,4-D, and also in the oxidative reactions of leaf senescence. The toxicity of Cd and 2,4-D has an effect on the ROS metabolism and speed of movement (dynamics) of peroxisomes. The regulation of ROS production in peroxisomes can take place by post-translational modifications of those proteins involved in their production and/or scavenging. In recent years, different studies have been carried out on the proteome of ROS metabolism in peroxisomes. Diverse evidence obtained indicates that peroxisomes are an important cellular source of different signaling molecules, including ROS, involved in distinct processes of high physiological importance, and might play an important role in the maintenance of cellular redox homeostasis.

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.

Peroxisomes sense and respond to environmental cues by regulating ROS and RNS signalling networks

BACKGROUND

Peroxisomes are highly dynamic, metabolically active organelles that used to be regarded as a sink for H2O2 generated in different organelles. However, peroxisomes are now considered to have a more complex function, containing different metabolic pathways, and they are an important source of reactive oxygen species (ROS), nitric oxide (NO) and reactive nitrogen species (RNS). Over-accumulation of ROS and RNS can give rise oxidative and nitrosative stress, but when produced at low concentrations they can act as signalling molecules.

SCOPE

This review focuses on the production of ROS and RNS in peroxisomes and their regulation by antioxidants. ROS production is associated with metabolic pathways such as photorespiration and fatty acid β-oxidation, and disturbances in any of these processes can be perceived by the cell as an alarm that triggers defence responses. Genetic and pharmacological studies have shown that photorespiratory H2O2 can affect nuclear gene expression, regulating the response to pathogen infection and light intensity. Proteomic studies have shown that peroxisomal proteins are targets for oxidative modification, S-nitrosylation and nitration and have highlighted the importance of these modifications in regulating peroxisomal metabolism and signalling networks. The morphology, size, number and speed of movement of peroxisomes can also change in response to oxidative stress, meaning that an ROS/redox receptor is required. Information available on the production and detection of NO/RNS in peroxisomes is more limited. Peroxisomal homeostasis is critical for maintaining the cellular redox balance and is regulated by ROS, peroxisomal proteases and autophagic processes.

CONCLUSIONS

Peroxisomes play a key role in many aspects of plant development and acclimation to stress conditions. These organelles can sense ROS/redox changes in the cell and thus trigger rapid and specific responses to environmental cues involving changes in peroxisomal dynamics as well as ROS- and NO-dependent signalling networks, although the mechanisms involved have not yet been established. Peroxisomes can therefore be regarded as a highly important decision-making platform in the cell, where ROS and RNS play a determining role.

Physiology of pepper fruit and the metabolism of antioxidants: chloroplasts, mitochondria and peroxisomes

BACKGROUND AND AIMS

Pepper (Capsicum annuum) contains high levels of antioxidants, such as vitamins A and C and flavonoids. However, information on the role of these beneficial compounds in the physiology of pepper fruit remains scarce. Recent studies have shown that antioxidants in ripe pepper fruit play a key role in responses to temperature changes, and the redox state at the time of harvest affects the nutritional value for human consumption. In this paper, the role of antioxidant metabolism of pepper fruit during ripening and in the response to low temperature is addressed, paying particular attention to ascorbate, NADPH and the superoxide dismutase enzymatic system. The participation of chloroplasts, mitochondria and peroxisomes in the ripening process is also investigated.

SCOPE AND RESULTS

Important changes occur at a subcellular level during ripening of pepper fruit. Chloroplasts turn into chromoplasts, with drastic conversion of their metabolism, and the role of the ascorbate-glutathione cycle is essential. In mitochondria from red fruits, higher ascorbate peroxidase (APX) and Mn-SOD activities are involved in avoiding the accumulation of reactive oxygen species in these organelles during ripening. Peroxisomes, whose antioxidant capacity at fruit ripening is substantially affected, display an atypical metabolic pattern during this physiological stage. In spite of these differences observed in the antioxidative metabolism of mitochondria and peroxisomes, proteomic analysis of these organelles, carried out by 2-D electrophoresis and MALDI-TOF/TOF and provided here for the first time, reveals no changes between the antioxidant metabolism from immature (green) and ripe (red) fruits.

CONCLUSIONS

Taken together, the results show that investigation of molecular and enzymatic antioxidants from cell compartments, especially chloroplasts, mitochondria and peroxisomes, is a useful tool to study the physiology of pepper fruit, particularly in the context of expanding their shelf-life after harvest and in maintaining their nutritional value.

Antioxidant systems from Pepper (Capsicum annuum L.): involvement in the response to temperature changes in ripe fruits

Sweet pepper is susceptible to changes in the environmental conditions, especially temperatures below 15 °C. In this work, two sets of pepper fruits (Capsicum annuum L.) which underwent distinct temperature profiles in planta were investigated. Accordingly, two harvesting times corresponding to each set were established: Harvest 1, whose fruits developed and ripened at 14.9 °C as average temperature; and Harvest 2, with average temperature of 12.4 °C. The oxidative metabolism was analyzed in all fruits. Although total ascorbate content did not vary between Harvests, a shift from the reduced to the oxidized form (dehydroascorbate), accompanied by a higher ascorbate peroxidase activity, was observed in Harvest 2 with respect to Harvest 1. Moreover, a decrease of the ascorbate-generating enzymatic system, the γ-galactono-lactone dehydrogenase, was found at Harvest 2. The activity values of the NADP-dependent dehydrogenases analyzed seem to indicate that a lower NADPH synthesis may occur in fruits which underwent lower temperature conditions. In spite of the important changes observed in the oxidative metabolism in fruits subjected to lower temperature, no oxidative stress appears to occur, as indicated by the lipid peroxidation and protein oxidation profiles. Thus, the antioxidative systems of pepper fruits seem to be involved in the response against temperature changes.

Combined proteomic and molecular approaches for cloning and characterization of copper-zinc superoxide dismutase (Cu, Zn-SOD2) from garlic (Allium sativum)

Superoxide dismutases (SODs; EC 1.15.1.1) are key enzymes in the cells protection against oxidant agents. Thus, SODs play a major role in the protection of aerobic organisms against oxygen-mediated damages. Three SOD isoforms were previously identified by zymogram staining from Allium sativum bulbs. The purified Cu, Zn-SOD2 shows an antagonist effect to an anticancer drug and alleviate cytotoxicity inside tumor cells lines B16F0 (mouse melanoma cells) and PAE (porcine aortic endothelial cells). To extend the characterization of Allium SODs and their corresponding genes, a proteomic approach was applied involving two-dimensional gel electrophoresis and LC-MS/MS analyses. From peptide sequence data obtained by mass spectrometry and sequences homologies, primers were defined and a cDNA fragment of 456 bp was amplified by RT-PCR. The cDNA nucleotide sequence analysis revealed an open reading frame coding for 152 residues. The deduced amino acid sequence showed high identity (82-87%) with sequences of Cu, Zn-SODs from other plant species. Molecular analysis was achieved by a protein 3D structural model.

Role of peroxisomes as a source of reactive oxygen species (ROS) signaling molecules

Peroxisomes are very dynamic and metabolically active organelles and are a very important source of reactive oxygen species (ROS), H2O2, O2 (.-) and · OH, which are mainly produced in different metabolic pathways, including fatty acid β-oxidation, photorespiration, nucleic acid and polyamine catabolism, ureide metabolism, etc. ROS were originally associated to oxygen toxicity; however, these reactive species also play a central role in the signaling network regulating essential processes in the cell. Peroxisomes have the capacity to rapidly produce and scavenge H2O2 and O2 (.-) which allows to regulate dynamic changes in ROS levels. This fact and the plasticity of these organelles, which allows adjusting their metabolism depending on different developmental and environmental cues, makes these organelles play a central role in cellular signal transduction. The use of catalase and glycolate oxidase loss-of-function mutants has allowed to study the consequences of changes in the levels of endogenous H2O2 in peroxisomes and has improved our knowledge of the transcriptomic profile of genes regulated by peroxisomal ROS. It is now known that peroxisomal ROS participate in more complex signaling networks involving calcium, hormones, and redox homeostasis which finally determine the response of plants to their environment.

Isozymes of antioxidative enzymes during ripening and storage of ber ( Ziziphus mauritiana Lamk.)

Isozyme profile of antioxidative enzymes viz. superoxide dismutase (SOD), peroxidase (POX), catalase (CAT) and ascorbate peroxidase (APX) was studied during ripening and storage of two cultivars of ber fruit (Ziziphus mauritiana Lamk.) differing in their shelf-lives viz. Umran (shelf-life, 8-9 d) and Kaithali (shelf-life, 4-5 d). The profile revealed that Umran variety exhibited three bands each of SOD and POX while in Kaithali, these enzymes had two isoenzymes throughout ripening. CAT and APX, however, showed two isozymes each during ripening of both the varieties and the pattern remained the same at all the stages of ripening except at the initial stage i.e immature green stage where single CAT isozyme was visible. During storage, one extra band each of SOD and POX present only in Umran got disappeared at later stages of storage, whereas in Kaithali, the pattern remained unchanged. Also, there was no change in the pattern of CAT and APX isozymes during storage of both the varieties. One isozyme of CAT could be considered as ripening related while one isozyme each of SOD and POX could be related to higher shelf life of fruits.

Proteome of plant peroxisomes: new perspectives on the role of these organelles in cell biology

Peroxisomes are cell organelles bounded by a single membrane with a basically oxidative metabolism. Peroxisomes house catalase and H(2)O(2)-producing flavin-oxidases as the main protein constituents. However, since their discovery in early fifties, a number of new enzymes and metabolic pathways have been reported to be also confined to these organelles. Thus, the presence of exo- and endo-peptidases, superoxide dismutases, the enzymes of the plant ascorbate-glutathione cycle plus ascorbate and glutathione, several NADP-dehydrogenases, and also L-arginine-dependent nitric oxide synthase activity has evidenced the relevant role of these organelles in cell physiology. In recent years, the study of new functions of peroxisomes has become a field of intensive research in cell biology, and these organelles have been proposed to be a source of important signal molecules for different transduction pathways. In plants, peroxisomes participate in seed germination, leaf senescence, fruit maturation, response to abiotic and biotic stress, photomorphogenesis, biosynthesis of the plant hormones jasmonic acid and auxin, and in cell signaling by reactive oxygen and nitrogen species (ROS and RNS, respectively). In order to decipher the nature and specific role of the peroxisomal proteins in these processes, several approaches including in vivo and in vitro import assays and generation of mutants have been used. In the last decade, the development of genomics and the report of the first plant genomes provided plant biologists a powerful tool to assign to peroxisomes those proteins which harbored any of the two peroxisomal targeting signals (PTS, either PTS1 or PTS2) described so far. Unfortunately, those molecular approaches could not give any response to those proteins previously localized in plant peroxisomes by classical biochemical and cell biology methods that did not contain any PTS. However, more recently, proteomic studies of highly purified organelles have provided evidence of the presence in peroxisomes of new proteins not previously reported. Thus, the contribution of proteomic approaches to the biology of peroxisomes is essential, not only for elucidation of the mechanisms involved in the import of the PTS1- and PTS2-independent proteins, but also to the understanding of the role of these organelles in the cell physiology of plant growth and development.