Ascorbate and glutathione: the heart of the redox hub.

The discovery that there is a close relationship between ascorbate and glutathione dates from soon after the characterization of the chemical formulae of the two molecules (Szent-Györgyi, 1931; Hopkins and Morgan, 1936). Similarly, it has long been known that thylakoids can generate hydrogen peroxide (H2O2; Mehler, 1951). Following the discovery of superoxide and superoxide dismutase in mammalian cells (McCord and Fridovich, 1969) and their subsequent discovery and study in chloroplasts (Allen and Hall, 1973; Asada et al., 1974), it became obvious that systems must exist to metabolize H2O2 produced in this organelle. Once it was accepted that the peroxisomal enzyme, catalase, was not found in the chloroplast, a predominant idea was that chloroplast H2O2 must escape to be metabolized by this enzyme or by “guaiacol-type” peroxidases for which no specific substrate had been identified. However, the discovery of thiol-disulfide exchange as a significant mechanism of enzyme regulation led to the finding that H2O2 could inactivate photosynthetic metabolism and activate respiratory pathways (Kaiser, 1979; Charles and Halliwell, 1980). Within the context of these exciting developments in oxygen biochemistry, Barry Halliwell, a newly appointed young lecturer at Kings College, London, sought to resolve the issue of how H2O2 was metabolized in chloroplasts by assigning his first Ph.D. student, Christine Foyer, to this task. Based on an initial hypothesis that ascorbate and glutathione had the potential to act in detoxification, it was shown that both metabolites and enzymes linking NADPH, glutathione, and ascorbate were found in isolated chloroplast preparations (Foyer and Halliwell, 1976, 1977), and a simple metabolic scheme was proposed (Fig. 1). Even at that time, it was considered that ascorbate oxidase could act as a terminal oxidase and, thus, as a sink for reducing power. However, no specific ascorbateor glutathione-dependent peroxidase had been identified in plants. The proposed role of ascorbate and glutathione in H2O2 metabolism in chloroplasts led to the successful identification of thylakoid-bound and soluble stromal ascorbate peroxidase (APX; Groden and Beck, 1979; Kelly and Latzko, 1979). It was subsequently shown that ascorbate could also be regenerated in the chloroplast by other mechanisms depending on ferredoxin or NADPH (Asada, 1999). Now known as the ascorbate-glutathione or “FoyerHalliwell-Asada” pathway, the resulting scheme is recognized to be a key player in H2O2 metabolism in both animals and plants. Components of this pathway have been shown to be present in animals and in the plant cell, cytosol, mitochondria, and peroxisomes as well as the chloroplast (Edwards et al., 1991; Mittler and Zilinskas, 1991; Jiménez et al., 1997). Here, we explore current concepts on the functions of ascorbate and glutathione in H2O2 metabolism and signaling, but also in the wider contexts of plant development and environmental responses. We pay particular attention to studies in which the status of ascorbate and glutathione themselves has been manipulated. Such changes may be linked to or independent of modified activity of dependent enzymes, just as the activity of ascorbateor glutathionedependent components may be modified without sustained, marked changes in ascorbate or glutathione status. Among key current questions is the nature of the mechanisms that link changes in ascorbate and glutathione status to downstream signaling, and we discuss these in the light of recent advances, notably information generated from genetically based studies of Arabidopsis (Arabidopsis thaliana).