Chapter 12: Endogenous Singlet Oxygen Photosensitizers in Plants¹

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.