Reactive Oxygen Species (ROS) are ubiquitous metabolites in all aerobic organisms. Traditionally ROS have been considered as harmful, accidental byproducts of cellular functions involving electron transport chains or electron transfer. However, it is now recognized that controlled production of ROS has significant signaling functions, for example, in pathogen defense, in the regulation of stomatal closure, or in cell-to-cell signaling. ROS formation in subcellular compartments is critical to act as “alarm” signal in the response to stress, and the concept of ROS as primarily signaling substances has emerged. The involvement of ROS in several developmental and inducible processes implies that there must be coordinated function of signaling network(s) that govern ROS responses and subsequent processes. The air pollutant ozone can be used as a useful tool to elucidate the function of apoplastic ROS: O3 degrades in cell wall into various ROS which are interpreted as ROS with signaling function inducing downstream responses. We have used ozone as a tool in mutant screens and transcript profiling-reverse genetics to identify genes involved in processes related to the signaling function of ROS. We review here our recent findings in the elucidation of apoplastic ROS sensing, signaling, and interaction with various symplastic components. 1. Introduction Reactive Oxygen Species (ROS) are ubiquitous metabolites in all aerobic organisms. ROS include superoxide (？ ), hydrogen peroxide (H2O2), hydroxyl radical (？OH), singlet oxygen (1O2), and the air pollutant ozone (O3). Traditionally ROS have been considered as harmful, accidental byproducts of cellular functions involving electron transport chains or electron transfer, where electrons are accidentally passed to molecular oxygen. Consequently, cells have effective enzymatic and chemical antioxidative defenses to remove these ROS [1, 2]. The most prominent sources of ROS in plant cells are thought to be chloroplasts and mitochondria, which both contain electron transfer chains involving multiple components. Processes that take place in peroxisomes and cytoplasm also produce ROS and the apoplast can be an important site for ROS generation, although the quantity of ROS produced is several-fold less than that of the organelles. However, it is now recognized that controlled production of ROS, for example, by the plasma membrane-localized NADPH oxidases, which produce in a controlled fashion ？ to the apoplastic space, has significant signaling functions [3–5]. ROS formation in other subcellular compartments, for example,
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