Biosynthesis, Compartmentation and Cellular Functions of Glutathione in Plant Cells

Abstract

Glutathione is the most abundant low molecular weight thiol in all plant cells with the only exception of some plant species that produce and accumulate homologous tripeptides to similar levels. The broad range of functions of glutathione in terms of detoxification of heavy metals, xenobiotics and reactive oxygen species (ROS) has been highlighted in numerous reviews before. Glutathione S-conjugates formed during detoxification of electrophilic xenobiotics are immediately sequestered to the vacuole for degradation. This degradation is initiated by cleavage of the two terminal amino acids of glutathione. The cleavage of the γ-peptide bond between glutamate and cysteine involves a specific γ-glutamyl transpeptidase. Other members of this gene family are suggested to be involved in glutathione catabolism in the apoplast and linked to long-distance transport of glutathione. Recent findings on the biosynthesis and compartmentation now begin to illuminate how the biosynthesis of glutathione is regulated at the molecular level and how different subcellular pools of glutathione are interconnected. Glutamate-cysteine ligase (GSH1) is the key regulatory enzyme of glutathione biosynthesis. Redox-dependent modulation of GSH1 activity also makes GSH1 a key factor in cellular redox homeostasis. Current work indicates that the redox state of the cellular glutathione redox buffer can be read out and directly transferred to target proteins by glutaredoxins. In this way glutathione is both, a scavenger for toxic compounds and a sensor for environmental signals which impact on the cellular redox state. This review aims at describing the important recent results on the cellular glutathione homeostasis in plant cells and highlighting the implications for glutathione-based redox sensing and signaling.