In plant cells, as in all other living cells, most of metabolism takes place in a reducing environment in subcellular compartments, such as the cytosol and the chloroplast stroma, because the many enzymes that have critical thiol groups as part of their catalytic
John T. Hancock (ed.), Methods in Molecular Biology, Redox-Mediated Signal Transduction, vol. 476 © 2008 Humana Press, a part of Springer Science + Business Media, Totowa, NJ DOI: 10.1007/978-1-59745-129-1_5
sites require protection from oxidation (1). However, some subcellular compartments, in particular the endoplasmic reticulum (2), may be more oxidizing to promote folding ofproteins via the formation of disulfide bridges (3). When oxidative stress occurs, the reducing state of the cell can be overwhelmed, resulting in cell death (4). Despite the potential for oxidative damage to plant cells, transitory changes in reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), superoxide anion (O2-), and singlet oxygen (:O2) occur, which have been suggested to initiate signaling for defence against pathogens (5), acclimation to a range of abiotic stresses (1), and developmental processes, such as secondary root hair formation and xylem differentiation (6). Changes in ROS may be associated with a transitory decrease in the redox status of some subcellular compartments, such as the apoplast during induction of defence against pathogens and the chloroplast stroma during exposure of plants to high light (7, 8). However, it has been argued that such changes in ROS levels associated with signaling responses are limited to discrete parts of the cell contained by a reducing environment that may be refractory to changes in its redox status (4, 9).
In subcellular compartments, such as the cytosol, the presence of millimolar amounts of reduced glutathione (GSH) have been suggested to act as a cellular redox buffer keeping many proteins cysteine moieties in their reduced state (e.g., Noctor et al. 2002; Mullineaux and Rausch 2005) (10, 11). This view is undergoing modification as a result of recent research of authors who suggest that a better way of viewing the role of GSH and its oxidized form, glutathione disulphide (GSSG), both directly and as an enzyme co-factor, is to ensure that redox active protein thiols are kept in specific oxidation states and prevented from entering terminal, irreversible oxidation states (3). Given the millimolar concentration of GSH in plant cells (10), it is of considerable importance to many biologists to have reliable methods for measuring the in vivo redox potential of the glutathione redox couple since it is likely to impact on the functioning of many key proteins under conditions that promote oxidative stress.
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