Abstract
Singlet oxygen (1O2) refers to the lowest excited electronic state of molecular oxygen. It easily oxidizes biological molecules and, therefore, is cytotoxic. In plant cells, 1O2 is formed mostly in the light in thylakoid membranes by reaction centers of photosystem II. In high concentrations, 1O2 destroys membranes, proteins and DNA, inhibits protein synthesis in chloroplasts leading to photoinhibition of photosynthesis, and can result in cell death. However, 1O2 also acts as a signal relaying information from chloroplasts to the nucleus, regulating expression of nuclear genes. In spite of its extremely short lifetime, 1O2 can diffuse from the chloroplasts into the cytoplasm and the apoplast. As shown by recent studies, 1O2-activated signaling pathways depend not only on the levels but also on the sites of 1O2 production in chloroplasts, and can activate two types of responses, either acclimation to high light or programmed cell death. 1O2 can be produced in high amounts also in root cells during drought stress. This review summarizes recent advances in research on mechanisms and sites of 1O2 generation in plants, on 1O2-activated pathways of retrograde- and cellular signaling, and on the methods to study 1O2 production in plants.
Highlights
The most effective way to transform the energy of sunlight into chemical energy that has evolved naturally on Earth is oxygenic photosynthesis [1]
This is accompanied by the translocation of electrons through the electron transport chain (ETC) and by transfer of light energy between chlorophyll molecules, which occurs via generation of excited states of chlorophyll
In tobacco Bright Yellow 2 (BY-2) suspension culture cells, 1 O2 was produced in the dark in the apoplast in response to hyperosmotic stress induced by salt or sorbitol, supposedly during peroxidase reactions in cell walls [62,63,64]
Summary
The most effective way to transform the energy of sunlight into chemical energy that has evolved naturally on Earth is oxygenic photosynthesis [1]. Its side-product is molecular oxygen in the ground triplet state (3 O2 ), which is generated during oxidation of water coupled to the activity of photosystem. Oxygenic photosynthesis enabled aerobic respiration necessary for most eukaryotic organisms populating the Earth. The appearance of oxygen in the atmosphere increased the emergence of reactive oxygen species (ROS) which can be harmful, and even dangerous, for cells. Other ROS, such as atomic oxygen or ozone, are not produced in living cells. Solid arrows indicate electron spins (modified from [4]).
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