Abstract
Reactive oxygen species (ROS) act not only as toxic compounds, but also as signaling molecules associated with responses to abiotic and biotic stresses in living organisms. The cellular redox state consisting of a delicate balance between ROS-production and ROS-scavenging must be strictly controlled. Ascorbic acid (AsA) is the most abundant water-soluble antioxidant in plants. Biochemical and molecular genetic approaches we have found by that AsA is mainly synthesized via a D-mannose/L-galactose pathway and that the pathway is regulated by light/dark and plastid-derived signals. AsA peroxidase (APX) plays a central role in controlling cellular H_2O_2 levels. We have demonstrated that cytosolic and chloroplastic APXs are required for plant tolerance and response to various types of stresses. Furthermore, using a conditional APX silencing system, we have found that chloroplastic H_2O_2 acts as a specific signal and regulates plant response to stress and hormones. Thus, our findings revealed a central role of AsA metabolism for regulating the cytotoxic effect and the signaling function of ROS in plants.
