Generation and scavenging of reactive oxygen species in chloroplasts: a submolecular approach

The specific traits of oxygen chemistry are examined as a basis of the step-wise electron acceptance by O2, giving rise to incompletely reduced highly reactive oxygen species (ROS) comprising both free radical (O2radical dot−, OHradical dot, HO2radical dot) and non-radical (molecular) forms (H2O2). The reversal of electron spin of oxygen, induced by energy transfer from triplet state chlorophyll, is considered as a generator of another non-radical ROS, singlet oxygen (1O2). The view that ROS are primarily agents of damage is questioned by data proving their beneficial role and particularly their signaling function. In this context the importance of the equilibrium between ROS production and scavenging, i.e. the regulation of ROS is emphasized. Particular attention is given to the chloroplast, a major source of ROS, which harbors ROS-producing centers (triplet chlorophyll, ETC in PSI and PSII) and a diversified ROS-scavenging network (antioxidants, SOD, APX–glutathione cycle, and a thioredoxin system). The electron carriers for ROS generation are specified. These include Fe-S clusters and ferredoxin in PSI, and QA and QB in PSII. The mechanisms and the molecular targets of damaging effect of ROS in chloroplasts are described. The photoprotective role of ROS formation is explained in terms of the use of oxygen as an electron acceptor, as an alternative to NADP, in high-light conditions producing excess electron flow, overreduction of the ETC and overproduction of NADPH. Adequate scavenging of ROS essentially contributes to photoprotection. Stress factors limiting CO2 fixation and hence decreasing NADPH utilization and NADP regeneration exacerbate high-light stress; ROS formation and scavenging become unbalanced, and oxidative damage may occur. The water–water cycle in chloroplasts and photorespiration as alternative mechanisms of ROS scavenging, NADP regeneration, and dissipation of excess excitation energy are briefly examined. The defensive role of avoidance of ROS generation through alternative oxidases, reducing O2 directly to water, is also described. The signaling function of ROS is illustrated by data proving the high-light induced systemic induction of APX genes and acclimation phenomena mediated by H2O2. The use of transgenic approaches for modeling ROS-tolerant plants is discussed.