Generation, Decay and Deleterious Action of ROS
Photosynthetic organisms growing under variable environmental conditions are often exposed to different types of stress like harmful irradiation (UV-B or high-intensity visible light), heat, cold, high salt concentration and also infection of the organisms with pathogens (viruses, bacteria) (Gruissem et al., 2012). Under these circumstances, the balance between oxidants and antioxidants within the cells is disturbed. This imbalance leads to enhanced population of ROS including singlet oxygen (1AgO2), superoxide radicals (O 2* or HO 2), hydrogen peroxide (H2O2) and hydroxyl radicals (HO*). Other highly reactive oxygen species like atomic oxygen or ozone are either not formed or play a role only under very special physiological conditions and will not be considered here. In this sense, the term ROS is used in a restricted manner. In addition to ROS, also reactive nitrogen- and sulfur-based species play an essential role in oxidative stress (OS) developed within the cells (Fryer et al., 2002; Benson, 2002; Blokhina and Fagerstedt, 2010). However, this interesting subject is beyond the scope of this book.
Under optimal conditions, only small amounts of ROS are generated in different cell compartments. However, exposure to stress can lead to a drastic increase of ROS production and sometimes to inhibition of cell defense systems (Desikan et al., 2001; Nishimura and Dangl, 2010). Plants developed a network of reaction cascades interacting with ROS controlled by ROS and controlling ROS. The elucidation of this interaction network is aimed to be conducted in the following two chapters (chap. 3 and chap. 4) (Biel et al., 2009; Schmitt et al., 2014a).
Rapid transient ROS generation can be observed and is called “oxidative burst” (Bolwell et al., 2002). In this case, a high ROS content is attained within time periods from several minutes up to hours. Oxidative bursts occur during many plant cell processes, especially photosynthesis, dark respiration and photorespiration. Studies using advanced imaging techniques, e.g. a luciferase reporter gene expressed under the control of a rapid ROS response promoter in plants (Miller et al., 2009), or a new H2O2/redox state-GFP sensor in zebrafish (Niethammer et al., 2009; see chapter 3.2, "Monitoring of ROS"), revealed that the initial ROS burst triggers a cascade of cell-to-cell communication events that result in formation of a ROS wave. This wave is able to propagate throughout different tissues, thereby carrying the signal over long distances (Mittler et al., 2011). Recently, the auto-propagating nature of the ROS wave was experimentally demonstrated. Miller et al. (2009) used local application of catalase or an NADPH oxidase inhibitor to show that a ROS wave triggered by different stimuli can be blocked at distances of up to 5-8 cm from the site of signal origin. The signal requires the presence of the NADPH oxidase (the product of the RbohD gene) and spreads throughout the plant in both the upper and lower directions.