Reactive oxygen signalling: the latest news
Introduction
Several recent reviews have described the biological activities of reactive oxygen species (ROS), placing special emphasis on the signalling role of hydrogen peroxide (H2O2) 1., 2., 3.. Increases in H2O2 concentration that are triggered by either biotic or abiotic stresses had generally been attributed to different mechanisms. However, the results of recent studies on ROS production no longer support such a clear distinction. In particular, H2O2 that is produced by cytosolic membrane-bound NADPH oxidases has been implicated as a signal in a wide range of biotic and abiotic stress responses. These responses include defence reactions against pathogens and herbivores [1], the closure of stomata [4••] and the regulation of cell expansion and plant development (4.••, 5.••; Figure 1). Furthermore, the intracellular sites of ROS production and the chemical identity of a given ROS seem to affect its role in signalling.
Section snippets
Defence responses
During defence responses, ROS are produced by plant cells because of the enhanced enzymatic activities of plasma-membrane-bound NADPH oxidases, cell-wall-bound peroxidases [6] and amine oxidases in the apoplast [7]. Recent studies have identified respiratory burst oxidase homologues (rboh), plant homologues of the catalytic subunit of phagocyte NADPH oxidase (gp91phox), as a source of ROS during the apoplastic oxidative burst [3]. After treatment with cryptogein, an elicitor of defence
The sensing of ROS
ROS produced in different subcellular compartments influence the expression of a large number of genes [2]. This suggests that cells have evolved strategies to utilise ROS as biological signals that control various genetic stress programs. This interpretation is based on the unstated assumption that a given ROS can interact selectively with a target molecule that perceives the increase of ROS concentration, and then translates this information into a change of gene expression. Such a change in
If ROS act as signals, does the intracellular site of ROS generation matter?
In plants, ROS are produced continuously as by-products of various metabolic pathways that are localised in different cellular compartments. The production of ROS in theses different organelles is involved in the regulation of several cellular processes.
Conclusions
During the evolution of organisms that are adapted to aerobic life conditions, ROS seem to have undergone several modifications of their biological activities. The continuous production of ROS, an unavoidable consequence of aerobic metabolic processes such as respiration and photosynthesis, has necessitated the evolution of ROS scavengers in order to minimise the cytotoxic effects of ROS within the cell. At the same time, sensing changes of ROS concentrations that result from metabolic
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
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of special interest
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of outstanding interest
Acknowledgements
Financial support through the Swiss Federal Institute of Technology and the Swiss National Science Foundation is gratefully acknowledged.
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