The most abundant non-protein thiol compound in plants is the tripeptide glutathione (GSH,(Rennenberg 1997). Additionally structural homologues of GSH are found in plant tissues. In bean, soybean and some other leguminous plants (Leguminosae), homoglutathione (hGSH, was detected in considerable amounts (Klapheck 1988). Beside GSH, substantial amounts of hydroxymethyl-glutathione were detected in several species of the Gramineae family, for example in wheat, barley and rice (Klapheck et al. 1992, Zopes et al. 1993). Some other thiol peptides are also known (Meuwly et al. 1995). In most plant tissues, GSH is predominantly present in its reduced form, but the oxidized, disulphide form of glutathione (GSSG) can also be detected. The regeneration of GSH from GSSG is catalysed by the glutathione reductase enzyme (GR, EC 1.6. 4.2.), which maintains the high GSH/GSSG ratio in plant cells (De Kok and Stulen 1993).

GSH and its homologues exhibit high chemical reactivity due to the sulphydryl group of their cysteine moiety. They are involved in various metabolic processes and are essential components of antioxidative and detoxification systems in plant cells. GSH can react both as a reducing agent and as a strong nucleophile, participating in the elimination of reactive oxygen species (ROS) via thiol-disulphide redox reactions, and in detoxification of various xenobiotics by conjugation reactions, respectively (De Kok and Stulen 1993, Mauch and Dudler 1993, Kömives et al. 1998, Noctor et al. 1998). The accumulation of GSH has been observed in various plants exposed to a wide range of abiotic stress effects. Several lines of evidence suggest that GSH and its homologues play major regulatory roles in biochemical-