Abstract
Oxygen is a natural acceptor of electrons in the respiratory pathway of aerobic organisms and in many other biochemical reactions. Aerobic metabolism is always associated with the formation of reactive oxygen species (ROS). ROS may damage biomolecules but are also involved in regulatory functions of photosynthetic organisms. This review presents the main properties of ROS, the formation of ROS in the photosynthetic electron transport chain and in the stroma of chloroplasts, and ROS scavenging systems of thylakoid membrane and stroma. Effects of ROS on the photosynthetic apparatus and their roles in redox signaling are discussed.
Highlights
Molecular oxygen (O2) is a natural acceptor of electrons in the respiratory pathway of aerobic organisms and in many other biochemical reactions
All other forms of active oxygen are produced via an electron transfer mechanism. 1O2 and partially reduced forms of oxygen have a higher reactivity towards many organic molecules than the ground state of oxygen and they are collectively called reactive oxygen species (ROS)
The chloroplast stroma is not considered as a significant source of 1O2, disintegration of the antenna complexes under stress conditions and disturbances in Chl synthesis and the accumulation of its precursors may lead to 1O2 production in the stroma [114]
Summary
Molecular oxygen (O2) is a natural acceptor of electrons in the respiratory pathway of aerobic organisms and in many other biochemical reactions. In addition to ROS-scavenging systems, chloroplasts have pathways, like non-photochemical quenching of excitation energy (NPQ), cyclic electron flow and plastid terminal oxidase (PTOX)-mediated chlororespiration, that diminish the appearance of long-lived redox active compounds [6,7,8]. Imbalances between ROS production and scavenging cause changes in the redox state of the cell through a change in the levels of reduced and oxidized forms of antioxidants like ascorbate (AscH2), glutathione and thiol-containing compounds. Besides ROS scavenging and the photochemical quenching of excitation energy, the water–water cycle generates a proton gradient across the thylakoid membrane for both ATP production and the enhancement of NPQ [13]. The review presents the main properties of ROS and their typical reactions, the formation of ROS in the photosynthetic electron transport chain and in the stroma of chloroplasts, and the ROS-scavenging systems of thylakoid membrane and stroma
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