Control of reactive oxygen species homeostasis in response to environmental stress

Sammanfattning: Plants are exposed to various fluctuations in their environmental conditions - light intensity, temperature, water status - and have to adapt in order to survive. Plant acclimatory responses can include the formation of new tissues, e.g., aerenchyma, or the activation of defense systems, e.g., the ascorbate-glutathione cycle for detoxification of reactive oxygen species (ROS). A prominent ROS is hydrogen peroxide (H2O2), a non-radical molecule formed during the reduction of oxygen. Due to its non-radical nature, H2O2 is more stable than other ROS and this longevity makes it the most abundant ROS in the plant cell and potentially harmful. In spite of this, H2O2 is involved in several signal transduction processes in plant cells, e.g., in the control of stomatal aperture, in plant-symbiont- and in plant-pathogen interactions, also in programmed cell death (PCD). Therefore, it is important for plant cells to maintain a tight control of H2O2 levels. In this study, the role of H2O2 production and -detoxification was studied in different plant processes. First, signaling leading to aerenchyma formation was studied in Arabidopsis thaliana. This plant shows lysigenous aerenchyma formation, a process involving PCD, which meant it was preceded by H2O2 formation. Second, the role of a H2O2 detoxifying enzyme, cytosolic ascorbate peroxidase 2 (APX2) from A. thaliana, in the local and systemic response to excess light stress, was studied by means of the characterization of APX2 knockout mutant lines. Third, antioxidant defense was studied in two types of nitrogen-fixing actinorhizal root nodules with different oxygen metabolism, from Datisca glomerata and Casuarina glauca. Fourth, the role of a plant natriuretic peptide from actinorhizal nodules of Alnus glutinosa, in abiotic stress resistance was studied.