Chloroplast Acclimation, Photodamage and Repair Reactions of Photosystem-II in the Model Green Alga Dunaliella salina

Photosynthesis is a light-driven process that sustains virtually all life on earth. However, light energy in excess of what is required for the saturation of photosynthesis causes photo-oxidative damage to the D1/32 kD (psbA gene product) reaction center protein of photosystem II. If not corrected, such photodamage could cause inhibition of photosynthesis and plant growth. Through evolution, organisms of oxygenic photosynthesis devised a unique repair mechanism by which to recover from this frequently occurring photo-oxidative damage. The repair process entails selective degradation and replacement of the photodamaged D1 reaction center protein in the thylakoid membrane of photosynthesis. In this chapter, up-to-date information about the photosystem II (PSII) damage and repair cycle in the model unicellular green alga Dunaliella salina Teod. [Eukaryota; Viridiplantae; Chlorophyta; Chlorophyceae; Chlamydomonadales; Dunaliellaceae; Dunaliella] is presented. The chapter first examines the physiology of cellular and chloroplast acclimation to irradiance. Examples of environmental and biotic factors that modulate the PSII photodamage are also provided. Lastly, the temporal sequence of events and current knowledge on the molecular mechanism of the PSII repair process are summarized in detail. Emphasis is placed on the role of the ELIP/Cbr protein and of the carotenoid zeaxanthin during D1 turnover and PSII repair. This chapter also examines the role of the chloroplast-localized heat-shock protein 70B (HSP70B) in facilitating the disassembly of photodamaged PSII and insertion of a de novo synthesized D1 protein in the PSII reaction center complex.