PARENT SESSION
Posters P2B Light, redox and metabolic regulation: Light Reactions. Abstracts (444-478)

Influence of the redox potential of the quinone acceptor on singlet oxygen production in Photosystem II. Anja Liszkay^*,1, Christian Fufezan¹, Beat Fischer³, Arthur William Rutherford², ¹ Institute of Biology II, Freiburg, Germany³ Department of Envrinmental Microbiology and Molecular Ecotoxicology, Duebendorf, Switerland² Institute of Biology II, Germany

ABSTRACT- Photosynthetic organisms are subjected to photo-oxidative stress when more light energy is absorbed than used in photosynthesis. In the light, highly reactive singlet oxygen can be produced via triplet chlorophyll formation in the reaction centre of photosystem II and in the antenna system. In the antenna triplet chlorophyll is produced directly by excited singlet chlorophyll while in the reaction centre it is formed via charge recombination of the light-induced charge pair. Changes in the midpoint potential of the primary quinone acceptor in photosystem II modulate the pathway of charge recombination in photosystem II and influence the yield of singlet oxygen production. The influence of different herbicides (DCMU and phenolic herbicides) on the midpoint potentials of the primary quinone acceptor QA can be used as a tool to investigate the charge recombination pathways in PSII. While production of hydroxyl and superoxide anion radicals were herbicide-independent, the yield of singlet oxygen with a phenolic herbicide was twice that with an urea herbicide. Photo-generated reactive oxygen species in herbicide-treated photosystem II were investigated by spin-trapping EPR. The yield of singlet oxygen production correlates with the reported influence of these herbicides on the redox properties of the quinone QA and the proposed distribution of pathways of recombination reactions [1]. In addition, we observed an upregulation of gene expression in response to singlet oxygen produced by PSII. A glutathione peroxidase homologous gene from Chlamydomonas is specifically up-regulated by singlet oxygen in the presence of a phenolic herbicide while it is down-regulated in the presence of DCMU. [1] Krieger-Liszkay A, Rutherford AW (1998) Biochemistry 37, 17339-17344.

KEY WORDS: photosystem II, spin trapping EPR, singlet oxygen, gene expression