INVOLVEMENT OF THE GAMMA GLUTAMYL CYCLE IN THE PLANT CELL REDOX HOMEOSTASIS

The existence of a gamma-glutamyl cycle in plants that is functional to the retrieval of extracellular glutathione (GSH) has been demonstrated in recent literature. In this cycle, glutathione is extruded to the apoplast, and sequentially degraded to its constituent aminoacids by gamma-glutamyl transferase (GGT) and cys-gly dipeptidase (CD) activity. Aminoacids are then taken up and glutathione is reassembled inside the cell. While increased GGT activity has been reported to be associated with oxidative stress conditions, the exact significance of the gamma-glutamyl cycle in plant physiology remains obscure. Arabidopsis thaliana GGT1- knockout mutants exhibit no clear phenotype but a shorter life cycle and premature senescence; extracellular washing fluid analysis and immunodetection in these mutants also indicates increased apoplastic glutathione and ascorbate concentration. We carried out a comparative proteomic analysis on tissues from wildtype and GGT1- knockout Arabidopsis thaliana plants, by means of protein labelling by iTRAQ and LC-MS/MS for simultaneous identification and relative quantification. Results indicate significant upregulation of several cytoplasmic antioxidant and stress-related enzymes (e.g. GSTs, catalases, peroxidases, ascorbate biosynthesis, peroxiredoxins, heat shock proteins). These evidences suggest that apoplastic, cell-wall bound GGT activity is responsible for cell redox coordination and that its silencing evokes a generalised alert response.