Molecular Analysis of Photoprotection of Photosynthesis

Abstract

Plants have diverse defense mechanisms against high light stress. Plants can reduce absorption of light energy through chloroplast avoidance and antenna size reduction. However, the capacity of the avoidance and the antenna size reduction for protection is limited, so that plants often absorb more energy than they can use. Therefore, plants need mechanisms to deal with this excess absorbed light energy, such as harmless thermal dissipation by feedback de-excitation. The transthylakoid pH gradient, xanthophyll cycle, PsbS, and other light-harvesting complex proteins are required for this thermal dissipation. In addition, alternative electron transport allows electrons to pass to acceptors other than CO2, thereby relieving overreduction of electron transport components in high light conditions. To detoxify reactive oxygen species that are inevitably produced during high light stress, plants have antioxidants including carotenoids, ascorbate, and tocopherols. In spite of these photoprotective mechanisms, photodamage may still occur, and efficient repair of damaged systems could be a photoprotective mechanism. In this chapter, recently published molecular genetics studies on each step of photoprotection have been reviewed. Genes required for each defense mechanism that have been identified thus far are introduced, and cloned genes that can possibly be related to photoprotection are discussed.