Hydrogen peroxide (H 2 O 2 ) is steadily gaining more attention in the field of molecular biology research. It is a major REDOX (reduction–oxidation reaction) metabolite and at high concentrations induces oxidative damage to biomolecules, which can culminate in cell death. However, at concentrations in the low nanomolar range, H 2 O 2 acts as a signalling molecule and in many aspects, resembles phytohormones. Though its signalling network in plants is much less well characterized than are those of its counterparts in yeast or mammals, accumulating evidence indicates that the role of H 2 O 2 -mediated signalling in plant cells is possibly even more indispensable. In this review, we summarize hydrogen peroxide metabolism in plants, the sources and sinks of this compound and its transport via peroxiporins. We outline H 2 O 2 perception, its direct and indirect effects and known targets in the transcriptional machinery. We focus on the role of H 2 O 2 in plant growth and development and discuss the crosstalk between it and phytohormones. In addition to a literature review, we performed a meta-analysis of available transcriptomics data which provided further evidence for crosstalk between H 2 O 2 and light, nutrient signalling, temperature stress, drought stress and hormonal pathways.
Hydrogen peroxide, a chemical compound discovered by Louis Jacques Thenard a hundred years ago, has properties that could justify classifying it as a phytohormone. In nature, it can be of inorganic origin, for example, via reactions in the atmosphere [ 1 ] but H 2 O 2 from this source has only an indirect effect on living organisms. Thenard was the first to discover not only that H 2 O 2 decomposes into water but also that it can cause skin blistering at a high concentration. However, oxidative stress is not the sole effect of this molecule. It is an evolutionarily conserved signalling molecule and in plants, it has gained attention also for its role in the regulation of growth and development. Indeed, the number of H 2 O 2 -related research articles published each year has doubled since 2008, with Web of Science listing over 3000 plant science publications on this topic in the last five years. In this review, we summarize different aspects of H 2 O 2 -mediated responses in plants, starting with the sources, catabolism and transport of H 2 O 2 . We then describe mechanisms for its perception and discuss its role in plant signalling networks and its effects on plant growth and development.
Hydrogen peroxide H2O2 is a non-radical reactive oxygen species (ROS) and it, like singlet oxygen 1O2 and free radicals such as superoxide anion O2− and hydroxyl radical •OH, is one of the major members of the ROS family [2]. In contrast to other ROS, H2O2 is relatively stable, with a half-life of ms and its level in a plant leaf oscillates around 1 µmol per gram of fresh weight under natural conditions [3]. There are numerous routes, both enzymatic and non-enzymatic, for H2O2 production in plant cells. The key sources include photorespiration, electron transport chains and redox reactions in the apoplast [4,5]. The KEGG (Kyoto Encyclopedia of Genes and Genomes) database lists 150 classes of enzyme that produce or utilize hydrogen peroxide. Of these, only 29 enzymes encoded by 227 genes are annotated in Arabidopsis and the largest enzyme family formed by peroxidases has 75 entries ( , Supplementary tables). However, not all of these enzymes necessarily participate in peroxide metabolism in plants. For instance, a flavin-containing monooxygenase like YUC6 may produce hydrogen peroxide in the absence of its substrate but in vitro experiments indicate that in this case the uncoupled reaction represents less than 4% of the enzyme’s activity [6]. In contrast, mammalian flavin-containing monooxygenases are clearly a source of hydrogen peroxide [7]. The key enzymes that are involved in Arabidopsis H2O2 metabolism reside in the apoplast, peroxisome, chloroplast and mitochondria and they will be described in detail.
Under favourable conditions, the majority of intracellular H2O2 is produced from molecular oxygen by a stepwise reaction via a superoxide anion intermediate which undergoes enzymatic reduction to H2O2. Excessive energy and/or malfunctioning of chloroplast and mitochondrial energetic metabolism are key causes of superoxide anion generation in plant cells. In chloroplasts, superoxide anions are produced when the electron-transport chain of photosystem I is oversaturated by excessive irradiation and electrons are transmitted by the Mehler reaction to oxygen molecules [8]. The resulting superoxide anions are then converted to H2O2. This dismutation step is a pH-dependent non-enzymatic event (for details see for example, [9]) but cells also catalyse the process by means of superoxide dismutase (SOD) in order to rapidly remove the toxic superoxide radical. Besides photosystem I, H2O2 may also originate at the manganese-containing, oxygen-evolving complex which is the donor site of photosystem II and by the reduction of singlet oxygen or superoxide anions by photosynthetic electron transport chain components such as plastoquinol [10]. In seeds and non-photosynthetic parts of plants, the main sources of superoxide anion are coupled with the processes of cell respiration in mitochondria. Electron leakage occurs especially in complexes I, II and III and it is estimated that 1–5% of the oxygen entering the plant respiratory chain is converted into H2O2 [11,12,13]. The Arabidopsis genome encodes eight SOD isozymes which can be divided into three classes according to their metal cofactor (Fe2+, Mn2+, Cu2+). There are three chloroplastic Fe-SODs and two Mn-SODs localized in mitochondria. The Fe-SODs are considered to be the oldest in evolutionary terms but the two classes share structural similarities and can also be found in prokaryotes. In contrast, the Cu/Zn-SOD class, which has three isozymes in Arabidopsis, most likely emerged after oxygen saturated the atmosphere. It is specific to eukaryotes and can be present in different cell compartments [14,15].
The second largest group of H2O2-producing enzymes consists of the respiratory burst oxidases ( ), which are also known as respiratory burst oxidase homologs (RBOHs) based on their homology to mammalian phagocyte NADPH oxidase (nicotinamide adenine dinucleotide phosphate oxidase). RBOHs, together with the type III cell wall peroxidases, are associated with the so-called “oxidative burst,” which is considered to be one of the main responses of plant cells to biotic or abiotic stress [19,20] but is also a crucial part of normal plant growth and development [21]. RBOHs are plasma membrane-localized proteins which oxidize cytosolic NADPH, transferring the released electron to O2 and producing superoxide which is then dismutated. In Arabidopsis, there are ten RBOH genes which are divided into three classes according to their tissue-specificity [22,23]. RBOHs are probably the best studied enzymatic ROS-generating system in plants and different regulatory mechanisms have been described. RBOHs undergo multiple post-translational modifications (PTMs), including S-nitrosylation and phosphorylation, that are required for enzyme activity and are regulated by calcium ions and phosphatidic acid [24,25].
Hydrogen peroxide is an end product of oxidative degradation of amines and polyamine degradation is considered to be an especially important source of hydrogen peroxide in plants (e.g., [26]). Plant polyamines are catabolized by two distinct classes of amine oxidases, the flavin adenine dinucleotide (FAD)-dependent polyamine oxidases and the copper amine oxidases, of which there are, respectively, five and eight putative functional isozymes encoded by the Arabidopsis genome [27]. The copper amine oxidases oxidize primary amino groups, producing ammonia, H2O2 and an aminoaldehyde, whereas the polyamine oxidases oxidize the secondary amino groups and the reaction products depend on the catalytic mechanism and substrate specificity of a given isozyme. All five Arabidopsis polyamine oxidases are reportedly intracellular and oxidize the carbon on the exo-side of the N4 atom of spermine and spermidine to produce 1,3-diaminopropane, H2O2 and an aminoaldehyde [28]. Polyamines play an important role in plant tolerance of abiotic stress and at least part of this tolerance is associated with hydrogen peroxide production (see for example, review [29]). Furthermore, polyamines represent a direct link between H2O2 and hormonal pathways, as it has been shown that cytokinin increases the polyamine content of plants [30].
Peroxisomal enzymes represent a major site of H2O2 production in a plant cell. In Arabidopsis, in addition to SOD and amine oxidases that are present in multiple compartments, peroxisomes contain acyl-CoA oxidases, glycolate oxidases, uricase, sulphite oxidase, aldehyde oxidase and sarcosine oxidase ( ). Xanthine oxidase, which converts xanthine to urate and H2O2, can be also localized in peroxisomes [2] but a putative Arabidopsis homolog that preferentially accepts NAD+ as the electron acceptor [31] reportedly resides in the cytosol. A significant proportion of peroxisomal H2O2 originates during the beta-oxidation of long-chain fatty acids via acyl-CoA oxidase [32], which is an especially important process in germinating seeds that contain glyoxysomes, specialized peroxisome-like organelles. However, in photosynthetic tissues, the role of peroxisomes in H2O2 metabolism is predominantly via photorespiration reactions that may contribute up to 70% of the total production of H2O2 in a plant cell [33,34]. In this reaction, glycolate produced in chloroplasts is converted to glyoxylate by glycolate oxidase in peroxisomes. The Arabidopsis genome contains five genes encoding glycolate oxidase and their combined relative expression in photosynthetic tissues is the highest of all H2O2-producing enzymes ( ). However, the actual levels of H2O2 in peroxisomes are kept in check by catalase and it is estimated that the peroxisomal H2O2 concentration is under 10 μM [35].
Plant cells survive with H2O2 levels that would kill animal cells and the estimated endogenous H2O2 content of plant cells is usually much higher than that found in animals and bacteria [36]. H2O2 accumulation increases the probability of hydroxyl radical production via the Fenton reaction and this would cause significant oxidative damage to cellular structures if it were not for the presence of a highly efficient antioxidant system. Higher plants contain several types of peroxidases, including catalases, ascorbate peroxidases (APX), thiol-specific peroxidases and classical secretory plant peroxidase. Furthermore, non-enzymatic compounds like tocopherols, ascorbic acid and flavonoids and glutathione play significant roles in H2O2 scavenging [37,38]. The plastoquinone and ubiquinone pool also contribute to the ROS scavenging process as illustrated in recent reports [39,40]. In accordance, inhibition of enzymes that maintain the oxidized plastoquinone and ubiquinone pool, plastid terminal oxidases and mitochondrial alternative oxidases, respectively, stimulates H2O2 production [41,42].
Though catalase belongs to the peroxidase family, it is usually considered separately due to its unique ability to convert two molecules of H2O2 into water and molecular oxygen without the need for any reductant. This heme-containing enzyme is first oxidized to a high-valence iron intermediate, which is then reduced by a further reaction with H2O2 [43]. Under specific circumstances, the intermediate may also react with a different substrate and catalase may oxidize donors such as alcohols or phenols. Catalase has a high turnover rate but a low substrate affinity, with a Km value in the millimolar range, a far greater concentration of H2O2 that that expected to be present in the cell [35]. As an illustration, the activity of a single molecule of rice catalase (kcat 80,000; Km 100 mM) [44] would be equivalent to more than 2200% of tobacco APX (kcat 1800; Km 0.022 mM) [45] at 100 mM H2O2 but to only 1% at concentrations below 1 µM H2O2, which would render catalase redundant. Of course, the constants determined in vitro may be misleading; the active form of catalase is a tetramer and it has been shown that, for example, PTMs may significantly affect the kinetics of a multimeric enzyme (e.g., [46]). Nevertheless, even though catalase activity has also been reported in the cytosol and mitochondria, its predominant localization is in peroxisomes, compartments with a high H2O2 concentration where its efficiency should be greatest (e.g., [47]). There are three functionally conserved classes of catalase with different spatial and developmental localizations in plants. For example, in tobacco catalase class I detoxifies H2O2 produced in photorespiration reactions, class II is localized in the vascular system and class III is present predominantly in flowers and fruits [48].
APX and glutathione peroxidases belong to the most important group of intracellular peroxidases [49]. Several types of APX have been described in plants; they include soluble enzymes in the cytosol, chloroplast and mitochondria and membrane-bound peroxidases in peroxisomes, glyoxysomes and thylakoids [50]. APX is the first enzyme in the so-called ascorbate-glutathione cycle, which includes monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase and reduces H2O2 and regenerates ascorbate via NAD(P)H [49]. The Arabidopsis genome encodes seven different APX isozymes and as indicated above, APX may be more important than catalase for H2O2 metabolism. Indeed, it has been shown that in the absence of cytosolic APX1, the entire chloroplastic H2O2-scavenging system in Arabidopsis collapses, H2O2 levels increase and protein oxidation occurs [51]. The thiol-specific peroxidases peroxiredoxins and glutathione peroxidases detoxify a broad spectrum of peroxide substrates [8]. However, recent evidence from S. cerevisiae indicates that this could be a secondary role and that thiol peroxidases perceive and transfer oxidative signals to signalling proteins and regulate transcription [52]. In plants and bacteria, six groups of peroxiredoxins are recognized on the basis of differences in sequence, structure and positions of conserved cysteinyl residues [53].
Peroxidases are by far the most abundant family of enzymes in H2O2 metabolism ( ). These so-called class III peroxidases probably have a correspondingly diverse range of functions, of which only a few, in certain plant species, have been revealed (see for example [54,55] for details). From the point of view of this review, it is important to note that the class III peroxidases participate not only in H2O2 catabolism via oxidation of phenolic compounds but also in producing it via an oxidative cycle using apoplastic reductants. For instance, it has been shown that in Arabidopsis cell culture they contribute to ca. 50% of the H2O2 produced during the oxidative burst in pathogen defence [56]. Class III peroxidases can be found in vacuoles but the majority are apoplastic or associated with cell walls in the apoplast as they play a key role in maintaining cell wall integrity by catalysing its cross-linking and loosening, lignification and suberization [57].
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