Extracellular glutathione recycling by γ-glutamyl transferase in barley root tip exposed to cadmium

• A considerable amount of extracellular glutathione is detectable in intact root tips.
• Low Cd concentrations activated, while high ones inhibited the activity of GGT.
• Depletion of eGSH inhibited GGT activity, whereas its elevated level activated it.
• Elevated eGSH triggered ROS production and caused root growth inhibition.
• The toxicity of the Cd–GSH complex was more serious than the toxicity of Cd alone.

A considerable amount of extracellular glutathione (eGSH) is detectable in intact root tips, but even the immersion of roots into water markedly decreased its detectability, suggesting its localization into the rhizosphere. While in water-treated seedlings the amount of eGSH was rapidly restored during the recovery period, in Cd-treated roots its amount was restored much more slowly. Enhanced γ-glutamyl transferase (GGT) activity was detected in 15 μM Cd-treated root tips 1 h after the end of short-term treatment, reaching its maximum level 2 h after the treatment. Thereafter it decreased to the level of control roots. By contrast, higher Cd concentrations inhibited the activity of GGT and more strongly inhibited the restoration of eGSH pool in root tips. Depletion of eGSH was accompanied by inhibition of GGT activity, whereas its elevated level slightly activated it. We showed that elevated eGSH, induced either by exogenously applied GSH or by GGT activity inhibition, triggered enhanced ROS generation and marked root growth inhibition in both control and Cd-treated seedlings. In addition, the toxicity of the Cd–GSH complex was more serious than the toxicity of Cd alone. Our results indicated that just recycling of eGSH or/and Cd/eGSH complex by GGT but not its level in the apoplast plays role in the sensing of toxic compounds and in the activation of defence responses. On the other hand, above a certain threshold concentration both eGSH and Cd/GSH complex due to their high toxicity can impair defence responses, causing detrimental damages in root tissues.