The tripeptide glutathione (GSH), is synthesized from the amino acids glutamate, cysteine and glycine by the enzymes gamma-glutamylcysteine synthetase, and glutathione synthetase (Meister, 1983):

Under a variety of environmental stresses (e.g. high light and low temperature, or other conditions (e.g. water deficits) which disrupt membrane-bound electron transport systems), and in response to pathogens (see e.g. Vanacker et al, 1998; Able et al, 1998), plants produce a range of active oxygen species, including superoxide, hydrogen peroxide and the hydroxyl radical (Smirnoff, 1993). Superoxide is converted to hydrogen peroxide by the action of superoxide dismutase [EC 1.15.1.1]. The hydroxyl radical can then form from the reaction of hydrogen peroxide with superoxide, particularly in the presence of iron or other transition state metals (Smirnoff, 1993). These toxic species can also form when plants are exposed to ozone, SO₂, NO and NO₂. They react with membranes, proteins and nucleic acids (Smirnoff, 1993). Glutathione is involved in quenching these free radicals through the ascorbate/GSH cycle (Alscher, 1989; Smirnoff, 1993; 1995; Gossett et al, 1996). Thus, hydrogen peroxide can be detoxified by the action of ascorbate peroxidase, dehydroascorbate reductase, and glutathione reductase:

Strictly speaking, the initial product of the ascorbate peroxidase reaction is monodehydroascorbate, which can then either be reduced back to ascorbate by the action of an NAD(P)H-dependent monodehydroascorbate oxidoreductase [EC 1.6.5.4] (the latter enzyme exists in multiple forms associated with the chloroplast, peroxisomes, mitochondria and plasma membrane (see Berczi and Moller (2000) and references cited therein)), or directly reduced back to ascorbate by an electron from PS-I (Smirnoff, 1995). Any monodehydroascorbate not so reduced disproportionates to ascorbate plus dehydroascorbate (Smirnoff, 1995); it is only the latter which is reduced by glutathione in the reactions depicted above. The enzymes of this cycle are chloroplast localized, and induced by an array of environmental factors (Alscher, 1989; Pastori and Trippi, 1992; Smirnoff, 1995). Overexpression of glutathione reductase leads to an increase in antioxidant capacity and resistance to photoinhibition in poplar (Foyer et al, 1995). Overexpression of FeSOD in poplar chloroplasts can protect PSII from overreduction when intracellular CO₂ is depleted, and from MV-induced photoinhibition (Arisi et al, 1998). However, overproduction of ascorbate peroxidase in the tobacco chloroplast does not appear to provide protection against ozone (Torsethaugen et al, 1997). Studies with transgenic plants over-expressing gamma-glutamylcysteine synthetase suggest that cysteine biosynthesis may be the rate-limiting for foliar glutathione synthesis (Noctor et al, 1996). The reader is referred to Schmidt and Jager (1992) for a review of cysteine biosynthesis in plants.

Oxidative conditions in chloroplasts which overwhelm the above ascorbate peroxidase system are thought to cause Rubisco degradation and oxidative inactivation of other enzymes of the reductive pentose phosphate pathway, resulting in a decrease in NADPH demand by the Calvin cycle. A rise in the NADPH/NADP⁺ ratio might in turn contribute to oxidative damage by NADPH-driven reduction of Fe³⁺ and other transition metals involved in hydroxyl radical formation through the Fenton reaction. It is proposed that release of the flavoenzyme ferredoxin-NADP⁺ reductase (FNR) [EC 1.18.1.2] from the thylakoid to the chloroplast stroma could play an important role in preventing or alleviating the transient accumulation of abnormally high levels of NADPH, therefore decreasing Fe²⁺-mediated hydroxyl radical production (Palatnik et al, 1997).

Although the chloroplast localized ascorbate-glutathione cycle is well characterized, it is now becoming clear that the enzymes of the ascorbate-glutathione cycle are also found in mitochondria and peroxisomes and may represent an important antioxidant protection system against H₂O₂ generated in these organelles (Jimenez et al, 1997; 1998). For a recent discussion of peroxisome antioxidant enzymes and the role of peroxisomal activated oxygen in senescence, see del Rio et al (1998) and Jimenez et al (1998). The enzymes of the ascorbate-glutathione cycle are also found in the apoplast, where they may play a key role in plant-pathogen interactions (Vanacker et al, 1998). The ascorbate-glutathione cycle may also play an important role in detoxifying active oxygen species in roots during re-aeration following hypoxia or anoxia (Biemelt et al, 1998).

Certain plants (notably legumes) synthesize homoglutathione, in which B-alanine is substituted for glycine as the terminal amino acid. Homoglutathione may play an important role in mobilizing protein S in N-deficient soybean plants, serving as the principal transport compound for the export of organic S (Sunarpi and Anderson, 1997). In legumes, homoglutathione may substitute for glutathione in the ascorbate/(homo)glutathione hydrogen peroxide scavenging system of root nodules (Dalton et al, 1998).

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