The role of plant glutathione S-transferases in herbicide metabolism

Glutathione S-transferase (GST) enzymes catalyze the conjugation of reduced glutathione to pesticide substrates, leading to their irreversible detoxification. Glutathione conjugation of xenobiotics is a very well studied Phase II detoxification reaction in plants, and is presumed to be requisite for their transport into the vacuole (Phase III) and possible further catabolism within the vacuole (Phase IV). GST-catalyzed glutathione conjugation is thus critical for removing xenobiotics from the cytosol and preventing them from interacting with their target sites. In plants, the expression of GST genes is regulated by many stimuli, including biotic and abiotic stresses and exposure to xenobiotics, thus indicating an important role for GST proteins in various detoxification processes. Chemicals called herbicide safeners, which protect grass crops from herbicide injury, stimulate herbicide metabolism by increasing the activity of GST enzymes that detoxify herbicides. GST proteins are soluble and their subcellular localization has usually been presumed to be cytosolic. However, immunocytochemical studies in our lab have shown that GST proteins are localized in the cytosol and inside the vacuoles of epidermal and sub-epidermal cells in herbicide safener-treated coleoptiles [Planta 217:831-840]. The majority of immunoreactive GST protein was located in the outer two cell layers of safener-treated coleoptiles, indicating that these cell types may be involved in a novel form of herbicide detoxification mechanism that involves vacuolar accumulation of GST protein and xenobiotic-glutathione conjugates.