CHAPTER FIVE Characterization of a T-DNA Inserted STN8 Kinase Mutant of Oryza sativa L. Krishna Nath 1 , Sujata R. Mishra 1 , Ismayil S. Zulfugarov 1 , Sharif-Ar-Raffi 1 , Chin-Bum Lee 2 , Gynheung An 3 , and Choon-Hwan Lee 1 Abstract In Arabidopsis two Stt7-like proteins exist, STN7 and STN8. Loss of STN8 kinase blocks photosystem II (PSII) core protein phosphorylation whereas STN7 is required for LHC II phosphoryla- tion. In this work, a T-DNA inserted stn8 kinase mutant line of rice (Oryza sativa L.) was selected and characterized to understand the role of STN8 kinase in rice. The mutant plants were slightly shorter than wildtype (WT). The transcript level was observed only in photosynthetic organs in WT, but not in the mutant. Inactivation of photochemi- cal efficiency under high light stress was more severe in the stn8 mutant than in WT. Recovery of PSII activity was also slower in the mutant. However, in the presence of lincomycin changes in photochemical efficiency and its recovery process of mutants was similar to WT. The phosphorylation of the PSII core proteins in the stn8 kinase mutant was abolished without changing LHC II phos- phorylation indicating that STN8 kinase in rice is required for PSII core phosphorylation but not for LHC II phosphorylation and state transition. The discrepancy observed in stn8 mutant of a model monocot plant, rice, from those reported in a model dicot plant, Arabidopsis, is discussed. Keywords D1 protein, photochemical efficiency, PSII core phosphorylation, STN8 kinase, T-DNA Introduction Light is the ultimate substrate in photosynthesis, but it can be harmful and leads to oxidative dam- age of the photosynthetic apparatus. Illumination changes elicit modifications of thylakoid proteins and repair and reorganization of the photosynthetic machinery. This process involves short term phos- phorylation of the photosystem II (PSII) core and light harvesting complex II (LHCII) proteins. The PSII repair cycle is regulated by the phosphoryla- tion of PSII core proteins, primarily the D1 pro- tein, whose degradation is proposed to occur only after its prior dephosphorylation (Rintamäki et al. 1996). A homologue of Stt7 called Stt7.2 exists in 1 Department of Biological Sciences, Pusan National University, Busan 609-735, Korea 2 Department of Molecular Biology, Dong-eui University, Busan, 614–714, Korea 3 Division of Life Sciences, Pohang University of Science and Technology, Pohang 790-784, Korea J.F. Allen, E. Gantt, J.H. Golbeck, and B. Osmond (eds.), Photosynthesis. Energy from the Sun: 14th International Congress on Photosynthesis, 1307–1311. © 2008 Springer.