Gamma glutamyl transferase activity partecipates to the salvage of extracellular glutathione and redox control in the root apoplast.

In animal systems, the gamma-glutamyl cycle enables the recovery of extracellular glutathione by its degradation catalized by a membrane-bound enzyme gamma-glutamyl transferase (GGT). Only recently this enzyme has gained attention also in plant science, and its subcellular localization has been ascribed to the membrane/cell wall compartment. The significance of a specific expression in different organs/tissues of different isoforms in Arabidopsis has however provided no insight about GGT physiological functions in plants. In this work we demonstrate, by immunohistochemical analysis, that GGT is almost exclusively localized in the cell wall of barley roots. We also show enzymehistochemically that GGT activity is high in the root tip and cortex. By the use of GGT activity inhibitors and diamide treatment, we describe variations in thiol content in roots and growth solution that are compatible with the existence of a gamma-glutamyl cycle in roots which assists the recovery of extracellular glutathione and may participate to the control of the apoplastic redox state. The ecophysiological implications of this findings will be discussed.