Repair of UV-B-incduced damage of photosystem II and DNA in Chlamydomonas reinhardtii

Abstract

UV-B (290 - 320 nm) radiation has multiple effects on the functions of plants. For example, UV-B causes the damage to DNA, proteins and lipids, and the inhibition of photosynthesis, especially photosystem II (PSII). We focused on UV-B-induced damage and recovery of PSII activity and DNA in Chlamydomonas reihardtii. The cells were irradiated under 2 Wm-2 at the surface of cell suspension (about 0.07 Wm-2 in average). UV-B radiation resulted in the inhibition of Fv/Fm and oxygen evolution that decreased to 40 - 50% and 20 - 30% of dark control level, respectively after 2 h (about 0.5 kJm-2), and then decreased gradually to about 30% and about 6%, respectively after 4 h (about 1.2 kJm-2). UV-B radiation caused the formation of two photoproducts of DNA, cyclobutane pyrimidine dimers (CPD) and pyrimidine (6-4) pyrimidinone photoproducts (6-4 PP) which were detected using their respective monochlonal antibodies. CPD and 6-4 PP of DNA extracted from the cells were slightly detected upon UV-B radiation for 2 h. Levels of both photoproducts increased significantly after 4 h. When the cell suspension was transferred to the condition under visible light after UV-B exposure for 2 h, the levels of CPD and 6-4 PP decreased monotonously, and restored approximately to the original level after 4 h. Kinetics of the recovery of Fv/Fm showed two phases, that is, the rapid increase during first 30 - 60 min and a limited slow increase after the subsequent illumination for 2 to 4 h. We will discuss the two recovery phases in Fv/Fm.