H2O2 and Formation of !AgO2 and Other Reactive Species like HO
Even at high light, no substantial amounts of :AgO2 and H2O2 are accumulated, when the electron flow through the pseudocyclic ETC (vide supra) increases and sufficient amounts of NADP+ are present in the cell. At high light intensity and conditions of saturating CO2 assimilation, the rate of electron flow increases. This leads to its redistribution, i.e. the rate of electron flow to NADP+ decreases and, concomitantly, the rate of electron transfer through the pseudocyclic electron transport increases (Asada, 1999).
H2O2 can also participate in the control of :AgO2 formation, when an excess of H2O2 induces oxidation of the primary electron acceptor of PS II, thus leading to activation of the electron transport. As a result, production of :AgO2 is diminished due to reduced probability of 3Chl population. Accordingly, pseudocyclic electron transport can function as a relaxation system to permit a decline of :AgO2 generation (Galvez- Valdivieso and Mullineaux, 2010). Such effects can result in autoinhi- bited reaction patterns and lead to spatiotemporal oscillations of the ROS distribution e.g. ROS waves.
The steady-state level of cellular H2O2 depends on the redox status of the cell (Karpinski et al., 2003; Mateo et al., 2006). Light-induced ROS generation in plants is mainly determined by the physiological state of the PA (Foyer and Shigeoka, 2011; Asada, 1999). Under physiological conditions, the H2O2 content in the cell is usually less than 1 pM. At elevated concentration, H2O2 inhibits several enzymes by oxidative crosslinking of pairs of cysteine residues. At about 10 pM, H2O2 inhibits CO2 fixation by 50%, which is mainly due to the oxidation of SH groups of Benson-Bassham-Calvin cycle enzymes (Foyer and Shigeoka, 2011). H2O2 can block the protein synthesis in the process of PS II repair (Nishiyama et al., 2001, 2004, 2011; Murata et al., 2012). This effect of H2O2 has been analyzed in the cyanobacterium Synechocystis sp. PCC 6803. It was shown that the translation machinery is inactivated with the elongation factor G (EF-G) being the primary target (see chap. 4.1). Due to that oxidation the protein de novo synthesis is completely blocked via the stop of protein translation. This process has been studied in deep detail and it is understood today mainly as a protective mechanism that avoids an expensive de novo synthesis of proteins in a highly oxidizing environment. Further details on this general type of H2O2 signalling are found in chapter 4.
Elimination of H2O2 is tightly associated with scavenging of other ROS in plant cells. Both, H2O2 production and removal are precisely regulated and coordinated in the same or in different cellular compartments (Karpinski et al., 2003; Foyer and Noctor, 2005; Mateo et al., 2006; Slesak et al., 2007; Pfannschmidt et al., 2009). The mechanisms of H2O2 scavenging are regulated by both, non-enzymatic and enzymatic antioxidants.
The biological toxicity of H2O2 appears through oxidation of SH groups and can be enhanced, if metal catalysts like Fe2+ and Cu2+ take part in this process (Fenton reaction) (see above and Figure 2). The enzyme myeloperoxidase (MPO) can transform H2O2 to hypochloric acid (HOCl), which has high reactivity and can oxidize cysteine residues by forming sulfenic acids (Dickinson and Chang, 2011) (see Figure 55).
Thus, H2O2 takes part in formation of reactive species like HO^ via several pathways.
Figure 55. Formation of hypochloric acid and HO from H2O2 and it's detoxification by enzymes.
Both O 2 and H2O2 are capable to initiate the peroxidation of lipids, but
since HO * is more reactive than H2O2, the initiation of lipid peroxidation is mainly mediated by HO' (Bhattacharjee, 2012; Miller et al., 2009).
Different defense systems have been developed to protect cells from deleterious effects of ROS. The underlying response mechanisms are either leading to diminished generation or enhanced scavenging of ROS. De novo synthesis of antioxidant enzymes (SOD, catalase, ascorbate peroxidase, glutathione reductase) and/or activation of their precursor forms take place and low-molecular antioxidants (ascorbate, glutathione, tocopherols, flavonoids) are also accumulated (Foyer and Noctor, 2005; Hung et al., 2005).
The antioxidant defense system contains many components (Pradedo- va et al., 2011). Essentially, three different types are involved: i) sys- tems/compounds preventing ROS generation, primarily by chelating
transition metals which catalyze HO * radical formation, ii) radical scavenging by antioxidant enzymes and metabolites, and iii) components involved in repair mechanisms. Treatment of mature leaves of wheat plants with H2O2 was shown to activate leaf catalase (Sairam and Srivastava, 2000).