Role of glutathione in adaptation and signalling during chilling and cold acclimation in plants

Glutathione is an important component of the ascorbate-glutathione cycle, w hich is involved in the regulation of hydrogen peroxide (H2O2) concentratio ns in plants. During chilling and cold acclimation, i.e. exposure to temper atures between 0 and 15 degreesC, H2O2 may accumulate. Excess electrons fro m the photosynthetic and respiratory electron transport chains can be used for the reduction of oxygen, thus producing superoxide radicals (O-2(.-)); these are subsequently transformed to H2O2 via superoxide dismutase (SOD; E C 1.15.1.1). During the removal of excess H2O2, reduced glutathione (GSH) i s converted to its oxidised form (GSSG), and GSH is regenerated by the acti vity of NADPH-dependent glutathione reductase (GR; EC 1.6.4.2). At low non- freezing temperatures, high GSH content and GR activity were detected in se veral plant species, indicating a possible contribution to chilling toleran ce and cold acclimation. Changes in H2O2 concentration, and GSH/GSSG ratio alter the redox state of the cells and may activate special defence mechani sms through a redox signalling chain. The finding that several defence gene s have antioxidant responsive elements or GSSG binding sites in their regul atory regions supports the idea that redox signalling is involved in regula ting gene expression in response to low temperature.