Singlet Oxygen: Applications in Biosciences and Nanosciences
Chapter 12: Endogenous Singlet Oxygen Photosensitizers in Plants1
Published:27 Jan 2016
Special Collection: 2016 ebook collection
J. B. Arellano and K. R. Naqvi, in Singlet Oxygen: Applications in Biosciences and Nanosciences, ed. S. Nonell, C. Flors, S. Nonell, and C. Flors, The Royal Society of Chemistry, 2016, ch. 12, pp. 239-269.
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Singlet oxygen (1O2), a highly reactive oxygen species, is inherently produced in chloroplasts of plants. Chlorophylls are used by plants to harvest light and to transport the singlet electronic excitation from the antenna complexes to the reaction center (RC) of photosystem I (PSI) and PSII. However, chlorophylls are also efficient photosensitizers of 1O2 when they are isolated, when the excitation energy flow is impaired in the antenna complexes, or when the electron transport in PSII is inhibited. In the last case, chlorophyll triplets are formed, and transfer their electronic excitation to O2. That chlorophylls act as donors of singlet excitation to other chlorophylls or as donors of triplet excitation to carotenoids as well as O2 makes 1O2 a constant threat for plants. However, plants have developed protection mechanisms for dealing with the danger. Several molecular processes work together in chloroplasts to cope with photosensitization of 1O2 and to minimize the resulting damage. Protection utilizes two strategies: to forestall the formation of 1O2 (either by preventing the formation of the would-be sensitizer or through deactivating it by a quencher other than O2), and to quench, by physical or chemical means, any 1O2 that does get formed. Among the photosynthetic complexes, PSII is unique in that its primary electron donor is unprotected by carotenoids and 1O2 oxidizes the pigments of PSII RC; intriguingly the carotenoid oxidation products are signaling molecules that can reprogram gene expression. Finally, the distance over which 1O2 can diffuse in a viscous cellular medium, as found inside chloroplasts, is analyzed.