Gamma-Glutamyl Cycle in Plants: Possible Implications in Apoplastic Redox Control and Redox Sensing
There is now increasing evidence of a gamma-glutamyl cycle occurring in plants that closely resembles what happens in animals, which consists of glutathione extrusion to the extracellular space, sequential degradation to its constituent amino acids by gamma-glutamyl transferase (GGT) and Cys-Gly dipeptidase (CD) activity, reuptake by amino acid transporters and glutathione reassembly inside the cell. Here we demonstrate that the GGT bound to the cell wall in Arabidopsis leaves recovers extracellular glutathione that is probably extruded to counteract conditions of oxidative stress due to UV-B exposure. Experiments on barley roots provide evidence of the existence of two different Cys-Gly dipeptidases: one is associated with the cell wall and has a higher affinity for reduced Cys-Gly; the other is presumably bound to the plasma membrane. The different site of CD isoforms with a different specificity for the reduced and oxidized substrate forms implies that reduced glutathione and Cys-Gly must spread through the apoplastic space before they are cleaved by the corresponding enzyme; in so doing, these thiols scan the extracellular space and eventually react with oxidants or sensitive components bound to the plasma membrane acting as sulfur switches. Taken together, these findings suggest that the gamma-glutamyl cycle is implicated in apoplastic redox control and redox sensing.