Damage and repair of Photosystem II under exposure to UV radiation

Abstract

The ultraviolet component of sunlight is highly detrimental for the photosynthetic apparatus both in the UV-B (280-320 nm) and in the UV-A (320-400 nm) region. By characterizing the mechanism of UV-A induced damage in isolated spinach thylakoid preparations we have shown that it impairs electron transport mainly in the PSII complex, with much less effect on PSI activity. Within PSII, the main action site of UV-A is the Mn cluster of water oxidation with additional effects on the QB binding pocket. This damaging mechanism is basically the same as induced by UV-B. Under natural conditions the lower damaging efficiency of UV-A is compensated by its higher intensity leading to similar impact on photosynthesis as exerted by UV-B. By using 308 nm laser flashes to illuminate PSII centers which are synchronized to various oxidation states of the water-splitting complex, we have shown that the UV-B sensitivity of PSII depends on the redox state of the Mn cluster (low in S0 and S1, high in S2 and S3), which is explained by direct UV absorption within the catalytic site. We have also demonstrated that in the absence of protein synthesis activity, UV-B and PAR inactivates PSII via independent mechanisms without synergistic interaction. However, in intact cyanobacterial cells, PAR provides protection against UV-B damage by enhancing the efficiency of protein repair. Activation of the repair process includes enhanced expression of the psbA and psbD genes, which can also be induced by artificially generated oxygen free radicals indicating that reactive oxygen species are involved in the signal transduction pathway.