The ascorbate-glutathione cycle in the cytosolic and chloroplastic fractions of drought-tolerant and drought-sensitive poplars

Abstract

Summary To determine whether a drought-tolerant Populus euramericana clone (Dorskamp) exhibits a more efficient reactive oxygen species (ROS)-scavenging system than a drought-sensitive one (Luisa Avanzo), we determined the activity of two enzymes of the ascorbate-glutathione cycle. Because ROS were mainly generated in illuminated chloroplasts, cytosolic and chloroplastic ascorbate peroxidase (AP) and glutathione reductase (GR) were followed in seedlings exposed for 12 h to control or 100 mmol/L mannitol (corresponding to -0.224 MPa). Whatever the treatment, the activities of plastidial ascorbate peroxidase (AP) and glutathione reductase (GR) were lower than those of cytosolic fractions. Mannitol treatment did not affect plastidial GR activity of Dorskamp, but increased cytosolic AP activity. Although ROS were not produced in 12 h stressed Luisa Avanzo, decreases in cytosolic AP and GR activities and increases in plastidial AP activity occurred. Changes in apparent Km values of the two enzymes were observed in both subcellular compartments of each stressed clone. When AP and GR of both clones were incubated in the presence of mannitol in vitro, the percent of inhibition of plastidial AP and GR was the highest in Luisa Avanzo. Whatever the clones, the GR isoforms of cytosolic and plastidial fractions were differently responsive to ROS-generated in vitro and to inhibitors of catalase and AP. The consequences of the efficiency of the ascorbate-glutathione cycle in each subcellular compartment and of the differential reactivity of AP and GR towards mannitol and ROS are discussed for each clone with regard to their drought responses.