Reactive oxygen species (ROS) are an inevitable consequence of aerobic metabolism. The accumulation of ROS, and resulting oxidative stress, appear to underlie a diverse array of human pathologies, from Alzheimer's disease to diabetes. In contrast, the cells of healthy individuals effectively manage ROS through the synthesis of antioxidant compounds, the rapid repair of oxidative damage, and the efficient balancing of cellular reductant pools. The latter of these strategies, redox balancing, is poorly understood, primarily due to the lack of physiologically relevant animal models to study this dynamic process. Our recent work has shown that simple, biologically realistic manipulations of inorganic nitrogen nutrition in Arabidopsis thaliana can rapidly and predictably alter cellular redox status, leading to adaptive changes in the expression and activity of several redox balancing enzymes in the mitochondrial respiratory chain. These results demonstrate that A. thaliana can serve as a unique model for studies of redox balancing and its role in ROS management. Thus, the primary objective of this project is to characterize the global response of A. thaliana cells to nitrogen source-induced changes in cellular redox status. Toward this end, we will utilize genome microarrays to directly examine how the transcriptome of root cells is affected by alterations in cellular redox status (Specific Aim 2). Microarray data will be analyzed using software designed to identify and statistically quantify the coordinated regulation of biochemical pathways, and we are particularly interested in examining the pathways involved in the production, intracellular partitioning, and oxidation of reductant (i.e. potential redox balancing pathways). In addition to studying redox balancing at the level of gene expression, we will also directly examine how nitrogen source affects cellular ROS levels (H2O2), lipid per oxidation, and the size and oxidation state of antioxidant pools (glutathione and ascorbate) (Specific Aim 1). Overall, these studies will allow us to link a physiologically realistic change in cellular redox state to quantitative assessments of oxidative stress and a defined transcriptional response. Ultimately, this work will inform the development of a global, experimentally-supported model of cellular redox balancing, a process which is central to the maintenance of human health. A variety of diseases, from Alzheimer's to diabetes, are related to the oxidative stress that arises from disruptions in the cell's delicate "electron economy". The proposed project will elucidate how healthy cells dynamically adjust their physiology to prevent oxidative stress under changing environmental conditions, using the plant Arabidopsis thaliana as an experimental model.