PARENT SESSION

Symposium S1B Photooxidative stress, photoinhibition
Monday August 30th, 2004 10:20 AM-12:20 PM Room 210A
Chair: Christine Foyer
Co-Chair: Klaus Apel

A manganese mechanism functions in photoinhibition under ultraviolet and visible light. Esa Tyystjärvi^*,1, Marja Hakala¹, Ilona Tuominen¹, Mika Keränen¹, Päivi Sarvikas¹, Taina Tyystjärvi¹, ¹ University of Turku, Department of Biology, TURKU, FINLAND

ABSTRACT- We illuminated leaves in the presence of an inhibitor of the Calvin-Benson cycle, and found that blocking photochemical quenching slightly protects against photoinhibition. Furthermore, when isolated thylakoids were supplied with an electron acceptor of PSI to improve photochemical quenching, photoinhibition was enhanced. We also compared photoinhibition induced by single turnover flashes, fired at 1-18 s intervals, and by continuous light, and found that the photoinhibitory efficiency of light is the same under continuous and flash light, although short flashes cause much fewer electron transfer events per unit time than continuous light. These results indicate that photoinhibition proceeds, at least in part, through a mechanism that is relatively independent of photosynthetic electron flow. We also found that the protective efficiency of non-photochemical quenching is low, indicating that the antenna of PSII is not the main photoreceptor of photoinhibition. To identify the photoreceptor, we studied the action spectrum of photoinhibition and found that it resembles the absorption spectra of manganese(III/IV)-containing model compounds of the oxygen-evolving manganese cluster, with a steep increase from green to ultraviolet-C light. This resemblance led us to study the relationship between manganese and photoinhibition. The results show that the release of a manganese ion to the lumen is the earliest detectable step of photoinhibition both under ultraviolet and visible light, and that the activity of the oxygen-evolving complex is lost before electron transfer activity from P₆₈₀ to Q_A. We conclude that photoinhibition proceeds largely through a mechanism that starts with light absorption by the manganese ions of the oxygen-evolving complex. Release of the excited manganese inactivates electron donation to P₆₈₀, and oxidized P₆₈₀ causes irreversible damage to the reaction center.

KEY WORDS: manganese, ultraviolet, photoinhibition, oxygen-evolving complex