Degradation of Glutathione and Glutathione-Conjugates in Plants.

Glutathione (GSH) is a ubiquitous, abundant, and indispensable thiol for plants that participates in various biological processes, such as the scavenging of oxidative stress, redox signaling, storage and transport of sulfur, detoxification of harmful substances, and the metabolisms of several compounds; therefore, knowledge of GSH metabolism is essential for plant science. Nevertheless, elucidation of GSH degradation is insufficient, which has hampered our understanding of plant life. Over the last five decades, the idea of the γ-glutamyl cycle has been dominant in GSH studies, and the exoenzyme γ-glutamyl transpeptidase (GGT) has been regarded as the major GSH degradation enzyme. However, recent studies have shown that GSH is degraded in cells by cytosolic enzymes such as γ-glutamyl cyclotransferase (GGCT) or γ-glutamyl peptidase (GGP). Meanwhile, a portion of GSH is degraded after conjugation with other molecules, which has also been found to be carried out by vacuolar GGT, GGP, or phytochelatin synthase (PCS). These findings are now displacing the assumption of the γ-glutamyl cycle, and a novel overview of the plant GSH degradation pathways is essential. This review aims to build a foundation for future studies by summarizing the current understanding of GSH/GS-conjugate degradation and presenting its overall picture.