The role of glycine in determining the rate of glutathione synthesis in poplar. Possible implications for glutathione production during stress

Abstract

The terminal step of glutathione (GSH) synthesis is the condensation of γ‐glutamyl‐cysteine (γ‐EC) with glycine. Relatively little information exists concerning the importance of photorespiratory glycine in determining the rate of conversion of γ‐EC to GSH. Consequently, the effect of exogenous glycine and of illumination on foliar contents of γ‐EC and GSH was studied in excised leaves and leaf discs from untransformed poplar (Populus tremula×P. alba) and poplar overexpressing γ‐glutamylcysteine synthetase (γ‐ECS; EC 6.3.2.2). Poplars strongly overexpressing γ‐ECS (ggs28) had enhanced levels of γ‐EC and GSH compared to untransformed poplars. The relationship between γ‐EC and GSH contents in ggs28 was light dependent. In illuminated leaves, GSH contents were up to 50‐fold higher than γ‐EC. On darkening, γ‐EC accumulated markedly and GSH declined, so that the GSH:γ‐EC ratio was close to 1. These dark‐induced changes were prevented by supplying glycine through the petiole or by incubation of leaf discs on glycine. Dark accumulation of γ‐EC in leaf discs from untransformed poplar was also prevented by supplying glycine. Supplying cysteine in the dark to discs from untransformed poplar and ggs28 increased γ‐EC levels markedly but GSH levels only slightly. Subsequent illumination caused γ‐EC to decrease and GSH to increase. Supplying glycine in concert with cysteine had similar effects to illumination. The data suggest that photorespiratory glycine is essential for GSH synthesis, especially under stress conditions, where increased amounts of GSH are required.

Kinetic analysis of maize glutathione S-transferase I catalysing the detoxification from chloroacetanilide herbicides

“…It has been demonstrated that GSH conjugation for a variety of herbicides is the major resistance and selectivity factor in plants and provides a tool to control weeds in agronomic crops (Marrs 1996;Rossini et al 1997;Noctor et al 1997;Cummins et al 1999). The phi class isoenzyme GST I from maize has been the subject of intense research because of its ability to detoxify a wide range of xenobiotics (McGonigle et al 2000).…”

“…Plant GSTs gained particular attention with respect to the detoxification from herbicides that belong mainly to two classes, chloroacetanilides and triazines (Marrs 1996;Rossini et al 1997;Noctor et al 1997;Cummins et al 1999). These herbicides are widely used for the control of annual grasses and broad-leaf weeds in a variety of major crops such as maize and soybeans (Sharp 1988).…”

Salinity-induced glutathione synthesis in Brassica napus

“…Recent studies support the notion that cysteine availability in plants also plays an important role in determining cellular GSH concentrations through the kinetic restriction of the reaction catalysed by c-ECS (Noctor et al 1997). Environmental conditions that stimulate synthesis of GSH will therefore place an increased demand upon S-assimilation into cysteine.…”

Glutathione homeostasis and redox-regulation by sulfhydryl groups

“…Further evidence for glycine as a limiting factor in the dark came from experiments with transgenic poplars with increased GSH biosynthesis in either the cytosol or the chloroplasts due to expression of bacterial GSH1 in either compartment. In both cases GSH-accumulation could be enhanced by feeding glycine or switching on the light to enhance glycine biosynthesis through photorespiration (Noctor et al 1997(Noctor et al , 1999. These limitations, however, only become relevant under experimental conditions with enhanced flux through the GSH biosynthesis pathway, independent of whether this was achieved by molecular engineering of cysteine biosynthesis (Blaszczyk et al 1999;Harms et al 2000) or GSH biosynthesis (Noctor et al 1997) or whether cysteine production was increased through feeding of H 2 S (Buwalda et al 1990).…”

“…These values clearly show already how the redox state can differ between cytosol and chloroplasts and how light-induced changes of stromal pH might directly affect the reduction ability of glutathione. The correct interpretation of the redox state is even further complicated by the fact that the glutathione level itself can exhibit diurnal fluctuations (Schupp and Rennenberg 1988;Noctor et al 1997Noctor et al , 1998b, undergo lightdependent changes that might be linked to vernalization Yanagida et al 2004), or concentrations might change depending on the developmental state of cells along the Arabidopsis root axis (Sa´nchez-Ferna´ndez et al 1997;Fricker et al 2000). Nevertheless, from the considerations outlined above, it is obvious that the cellular glutathione redox-buffer allows finetuning of the cellular redox-environment under normal conditions as well as under conditions of external environmental stress.…”