Reactive oxygen species (ROS) formed from molecular oxygen, and likely inter-conversion pathways that may occur in plants. Singlet oxygen ( 1 O 2 ) is a highly reactive form of di-oxygen (O 2 ) in which one of the un-paired electrons of ground-state di-oxygen is promoted to an orbital of higher energy. The superoxide radical (O − 2 ) , hydrogen peroxide (H 2 O 2 ) and the highly reactive hydroxyl radical (OH) are formed by one-electron reductions of molecular di-oxygen. Cellular defences such as superoxide dismutase (SOD), catalase and peroxidase serve to scavenge O − 2 and hydrogen peroxide (H 2 O 2 ) , thereby preventing their participation in the formation of hydroxyl radical (OH) via the iron catalysed Haber-Weiss reaction. A plasma membrane NADPH oxidase has been proposed to catalyse the initial formation of superoxide from molecular oxygen, although alternate mechanisms have also been suggested (Bolwell et al. , 2002). 

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... and the flavonoids gallocatechin ( R f 0.23) (Figure 4) and rutin ( R f 0.04) (Buschmann et al. , 2000c). These data indicate that the cassava storage root contains a range of easily oxidised compounds that could participate as electron donors in enzymatic or non- enzymatic oxidation reactions during the post-harvest period. The major initial sources of ROS in plant systems are O •− 2 and H 2 O 2 , which may be produced in a ‘con- trolled’ manner via the oxidative burst or as a result of ‘leakage’ from electron transport chains. Dismuta- tion of O •− 2 to H 2 O 2 and O 2 may occur spontaneously or may be catalysed by the enzyme SOD (Figure 1). Our data indicates the presence of four SOD isoforms in the cassava storage root (Figure 5). No obvious changes in isoform pattern or intensity were observed over a 5-day post-harvest time course, suggesting that regulation of SOD isoforms does not play a significant role in the development of PPD. In addition, a cassava root Cu/Zn SOD cDNA clone, MecCuZnSOD (GenBank accession number AF426273) isolated from a root PPD-related cDNA library, was expressed at similar levels in storage root, leaf and petiole (Figure 6). Increases in enzyme activity of both peroxidase and catalase, the two key enzymes involved in turnover of H 2 O 2 , occur during PPD (Beeching et al. , 1998). We have isolated and characterised cassava storage root catalase and peroxidase cDNA clones, Mec- CAT1 (GenBank accession number AF170272) and MecPX2 (AY033386), which were up-regulated during PPD (Figure 7). The MecCAT1 transcript was predominantly expressed in the storage root, although expression was also detected in leaf tissue (Figure 6). Interestingly, MecPX2 showed storage root-specific expression with no expression detected in petiole or leaf. Given the apparent root specificity of this peroxidase clone and its rapid transcript up-regulation in response to injury of the roots, isolation of the cognate promoter may be of interest for further studies. Peroxidase and catalase activities were localised in the cassava root during PPD by light microscopy and/or tissue printing techniques. Catalase activity was distributed throughout the root parenchyma (Figure 2Ei–iv). Increased activity in less ...

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... refer to reactive molecules that result from the reduction of molecular di-oxygen. Whilst molecular di-oxygen is relatively non-reactive and non-toxic, it becomes reactive once its electron structure is altered. Although many ROS are oxygen radicals, i.e. contain an unpaired electron, several, such as singlet oxygen and hydrogen peroxide, are not. The main forms of ROS known to exist in plants and the major interconversion pathways between them are summarised in Figure 1. Secondary radicals such as the alkoxyl radical (RO • ), peroxyl radical (ROO • ) and organic hydroperoxides (ROOH) may be formed in planta by lipid peroxidation initiated either enzymatically (e.g. lipoxygenase) or by ROS such as superoxide or the hydroxyl radical (Larson, 1995). Although neither O ι 2 − nor H 2 O 2 is highly reactive at physiological concentrations, toxicity in vivo arises from their role as substrates in the iron catalysed Haber-Weiss reaction. The hydroxy radical (OH ι ) formed via this reaction and its derivatives are amongst the most reactive species known, and react indiscriminately with cellular macromolecules causing lipid peroxidation, protein denaturation and deoxyribonucleic acid (DNA) damage. In plant systems, the major initial sources of ROS during normal metabolism are the production of superoxide (O •− ) and hydrogen peroxide (H O ) via electron ‘leakage’ from electron transport chains (e.g. photosystems I and II, the mitochondrial electron transport chain, electron transport chains located in the ER, membrane, peroxisomes and nuclear en- velope), and the production of singlet oxygen 1 O 2 during photosynthesis by transfer of excitation energy from triplet chlorophyll to molecular di-oxygen (Bartosz, 1997). Under stress conditions, increased ROS formation often occurs through perturbation of such metabolic paths, and cellular damage arising from environmental stresses is often caused by oxygen radicals. A large body of research has focused on the controlled production of ROS in plant defence, partic- ularly of superoxide and hydrogen peroxide, and especially during the hypersensitive response (HR) and systemic acquired resistance (SAR). This ‘oxidative burst’ is defined as a rapid production of superoxide and/or hydrogen peroxide in response to external stim- uli (Wojtaszek, 1997). The ROS generated during the oxidative burst play several complex and overlapping roles in facilitating plant defence. Essentially, these may be summarised as (1) cell wall strengthening, (2) induction of defence related genes, and (3) triggering of host cell death. An oxidative burst during defence responses to bacterial, viral and nematode pathogens or elicitors, host cell wall derived elicitors such as oligogalacturonide, wounding, mechanical stress ...