Lichtschutzfunktion von Anthocyanen in Blättern Höherer Pflanzen: Abschirmung und antioxidative Wirkung

Anthocyane werden in Blättern häufig gebildet, wenn die Umweltbedingungen die Photosyntheseaktivität hemmen und bei hohen Lichtstärken das Risiko eines zunehmenden Auftretens von reaktiven Sauerstoffspezies (ROS) besteht. Die roten Pigmente werden dann vor allem in exponierten Gewebeabschnitten der Blätter eingelagert. Eine mögliche Lichtschutzfunktion der Anthocyane könnte einerseits darauf beruhen, dass sie das Mesophyll gegen photosynthetisch aktive Strahlung abschirmen. Andererseits könnten Anthocyane auch durch ihre antioxidative Wirkung den Photosyntheseapparat vor einer Deaktivierung durch ROS schützen. Basierend auf Chlorophyllfluoreszenz konnte mit Blättern der grünen und roten Varietät von Berberis thunbergii die Abschirmung quantifiziert und mit Messungen der photosynthetischen Sauerstofffreisetzung sowie durch die Analyse der Photosynthesepigmente bestätigt werden. Weiterhin konnte mit Blättern der Modellpflanze Arabidopsis thaliana nachgewiesen werden, dass Anthocyane durch antioxidative Wirkung und Abschirmung photooxidativen Stress verhindern

Relative sensitivity of DNA and photosystem II in Ulva intestinalis (Chlorophyta) under natural solar irradiation

High intensities of sunlight can result in DNA and photosystem II (PSII) damage. However, the relative sensitivity of both these targets under natural sunlight and especially over a long period has not been studied in algae so far. Although DNA damage is highly specifically induced by ultraviolet-B radiation (UVB, 280-315 nm), PSII is inactivated by a broad spectrum. The green macroalga Ulva intestinalis is an appropriate and interesting study organism with which to investigate the relative importance of the 2 different targets of sunlight because this alga contains no UV-screening protective pigments, although it is exposed to strong solar irradiation in its natural habitat. This entails a high risk of DNA damage. Therefore, diel time courses and long-term development of DNA damage and the optimal quantum yield of PSII (F-v/F-m) were studied in situ. F-v/F-m was extremely reduced at noon, but a fast recovery was observed in the afternoon. As dark-adapted basal fluorescence (F-o) of PSII was substantially decreased during the day, non-photochemical quenching is suggested to be a key photoprotective strategy in U. intestinalis. In contrast, even in samples with strongly reduced F-v/F-m, only very low DNA damage was found, irrespective of the accumulated UVB dose. We propose that efficient photoreactivation driven by natural sunlight balances the induction of dimers. This leads to a higher UVB tolerance of DNA than that observed in algae under experimental UVB irradition. In this field study, U. intestinalis suffered more from photoinhibition than from DNA damage

Glycolytic Shunts Replenish the Calvin-Benson-Bassham Cycle as Anaplerotic Reactions in Cyanobacteria

The recent discovery of the Entner-Doudoroff (ED) pathway as a third glycolytic route beside Embden-Meyerhof-Parnas (EMP) and oxidative pentose phosphate (OPP) pathway in oxygenic photoautotrophs requires a revision of their central carbohydrate metabolism. In this study, unexpectedly, we observed that deletion of the ED pathway alone, and even more pronounced in combination with other glycolytic routes, diminished photoautotrophic growth in continuous light in the cyanobacterium Synechocystis sp. PCC 6803. Furthermore, we found that the ED pathway is required for optimal glycogen catabolism in parallel to an operating Calvin–Benson–Bassham (CBB) cycle. It is counter-intuitive that glycolytic routes, which are a reverse to the CBB cycle and do not provide any additional biosynthetic intermediates, are important under photoautotrophic conditions. However, observations on the ability to reactivate an arrested CBB cycle revealed that they form glycolytic shunts that tap the cellular carbohydrate reservoir to replenish the cycle. Taken together, our results suggest that the classical view of the CBB cycle as an autocatalytic, completely autonomous cycle that exclusively relies on its own enzymes and CO2 fixation to regenerate ribulose-1,5-bisphosphate for Rubisco is an oversimplification. We propose that in common with other known autocatalytic cycles, the CBB cycle likewise relies on anaplerotic reactions to compensate for the depletion of intermediates, particularly in transition states and under fluctuating light conditions that are common in nature