Introduction

A major part of our present knowledge on the processes involved in glutathione metabolism originates from research performed by animal and human biochemists and physiologists up to the late 1970s (Meister, 1981; Meister & Anderson, 1983). Until that time interest of plant biochemists and physiologists in research on glutathione was rather low. Apparently, it was assumed that experiments on glutathione metabolism performed with animal tissues had provided answers that are also valid for plants. This situation changed entirely about a decade ago, when it became obvious that glutathione is an important factor in stress physiology of plants (Smith, Polle & Rennenberg, 1990; Rennenberg & Brunold, 1994). Glutathione was found to be an antioxidant, both as a constituent of the chemical defence system and as a substrate of the enzymatic defense system in the cytoplasm and the chloroplasts of the cells (Polle & Rennenberg, 1993, 1994). Glutathione was found to counteract heat, cold and drought stress (Smith et al, 1990). Glutathione was identified as a substrate for the conjugation of xenobiotics (Rennenberg & Lamoureux, 1990) and for the synthesis of phytochelatins, i.e. poly(γ-glutamylcysteinyl)-glycines involved in metal homeostasis and metal detoxification (Rennenberg & Brunold, 1994). Glutathione was found to be involved in signal transduction in plant-pathogen interactions (Rennenberg & Brunold, 1994). Glutathione was found to be a storage form of reduced sulphur, compensating stress from excess sulphur in plants (Rennenberg, 1984; De Kok, 1990).