The Biological Functions of Glutathione Revisited in Arabidopsis Transgenic Plants with Altered Glutathione Levels

Abstract

A functional analysis of the role of glutathione in protecting plants from environmental stress was undertaken by studying Arabidopsis that had been genetically modified to have altered glutathione levels. The steady-state glutathione concentration in Arabidopsis plants was modified by expressing the cDNA for ␥-glutamyl-cysteine synthetase (GSH1) in both the sense and antisense orientation. The resulting plants had glutathione levels that ranged between 3% and 200% of the level in wild-type plants. Arabidopsis plants with low glutathione levels were hypersensitive to Cd due to the limited capacity of these plants to make phytochelatins. Plants with the lowest levels of reduced glutathione (10% of wild type) were sensitive to as little as 5 m Cd, whereas those with 50% wild-type levels required higher Cd concentrations to inhibit growth. Elevating glutathione levels did not increase metal resistance. It is interesting that the plants with low glutathione levels were also less able to accumulate anthocyanins supporting a role for glutathione S-transferases for anthocyanin formation or for the vacuolar localization and therefore accumulation of these compounds. Plants with less than 5% of wild-type glutathione levels were smaller and more sensitive to environmental stress but otherwise grew normally.Glutathione (GSH), the tripeptide ␥-glutamylcysteinyl-Gly, is the major source of non-protein thiols in most plant cells (Bergmann and Rennenberg, 1993). The chemical reactivity of the thiol group of glutathione makes it particularly suitable to serve a broad range of biochemical functions in all organisms. It has an oxidation reduction potential of Ϫ0.23 V that allows it to act as an effective electron acceptor and donor for numerous biological reactions. The nucleophilic nature of the thiol group also is important in the formation of mercaptide bonds with metals and for reacting with select electrophiles. This reactivity, along with the relative stability and high water solubility of GSH, makes it an ideal biochemical to protect plants against stress including oxidative stress, heavy metals, and certain exogenous and endogenous organic chemicals.

A Mechanism of Nonphotochemical Energy Dissipation, Independent from PsbS, Revealed by a Conformational Change in the Antenna Protein CP26

“…In control conditions, all genotypes showed a similar growth rate ( Figure 1D). In high light conditions ( Figure 1B), older leaves at the periphery of the rosettes of wild-type plants showed bleaching of chlorophyll accompanied by reddening of leaves as a result of accumulation of the purple flavonoid anthocyanin, an indicator of stress in Arabidopsis (Dixon and Paiva, 1995;Xiang et al, 2001). The younger leaves at the center of rosettes, however, were normally green and did not accumulate anthocyanins nor show other stress effects.…”

Enhanced tolerance to and accumulation of mercury, but not arsenic, in plants overexpressing two enzymes required for thiol peptide synthesis

“…GSH provides heavy metal resistance to plants, but also serves to protect plants from other environmental stresses, such as low temperatures (Badiani et al, 1997;Kocsy et al, 2001) or xenobiotics (Xiang and Oliver, 1998). In addition, GSH is required for the synthesis of many plant protectants, including the anthocyanins (Xiang et al, 2001). To test the potential of increased ECS and GS levels to enhance resistance to cadmium, Indian mustard and Arabidopsis were genetically modified to overexpress the bacterial ECS (Zhu et al, 1999a), Arabidopsis ECS (Xiang et al, 2001), and bacterial GS proteins (Zhu et al, 1999a).…”

“…In addition, GSH is required for the synthesis of many plant protectants, including the anthocyanins (Xiang et al, 2001). To test the potential of increased ECS and GS levels to enhance resistance to cadmium, Indian mustard and Arabidopsis were genetically modified to overexpress the bacterial ECS (Zhu et al, 1999a), Arabidopsis ECS (Xiang et al, 2001), and bacterial GS proteins (Zhu et al, 1999a). Indian mustard transgenic lines overexpressing GS were able to increase the biosynthesis of both GSH and PCs, weakly enhancing Cd(II) tolerance (Zhu et al, 1999a).…”

Aγ-Glutamyl Transpeptidase-Independent Pathway of Glutathione Catabolism to Glutamate via 5-Oxoproline in Arabidopsis

“…Toxic xenobiotics, including herbicides, are conjugated with GSH by glutathione S-transferases and sequestered into the vacuole (Marrs, 1996). GSH is also involved in controlling cell size and root development by regulating the cell cycle (Vernoux et al, 2000;Xiang et al, 2001). GSH exists in high concentration in plant tissues and acts as a substrate for central biochemicals, particularly Cys, and Cys availability for protein synthesis and as a precursor for metabolites may be related to the rate of GSH degradation (Leustek et al, 2000).…”