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ENVIRONMENTAL STRESS AND ADAPTATION TO STRESS

Ammonia Triggers Photodamage of Photosystem II in the Cyanobacterium Synechocystis sp. Strain PCC 6803^1,[OA]

Institut für Mikrobiologie und Molekularbiologie, Justus-Liebig-Universität Giessen, D–35392 Giessen, Germany (M.D., N.K., K.F.); Fachbereich Biologie-Molekulare Pflanzenphysiologie, Philipps-Universität Marburg, D–35032 Marburg, Germany (A.B.); Institut für Biochemie, Universität zu Köln, D–50674 Cologne, Germany (K.M., J.N.); and Lehrstuhl für Mikrobiologie/Organismische Interaktionen, Eberhard-Karls-Universität Tübingen, D–72076 Tuebingen, Germany (K.F.)

Ammonia has long been known to be toxic for many photosynthetic organisms; however, the target for its toxicity remains elusive. Here, we show that in the cyanobacterium Synechocystis sp. strain PCC 6803, ammonia triggers a rapid photodamage of photosystem II (PSII). Whereas wild-type cells can cope with this damage by turning on the FtsH2-dependent PSII repair cycle, the FtsH2-deficient mutant is highly sensitive and loses PSII activity at millimolar concentration of ammonia. Ammonia-triggered PSII destruction is light dependent and occurs already at low photon fluence rates. Experiments with monochromatic light showed that ammonia-promoted PSII photoinhibition is executed by wavebands known to directly destroy the manganese cluster in the PSII oxygen-evolving complex, suggesting that the oxygen-evolving complex may be a direct target for ammonia toxicity.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Karl Forchhammer (karl.forchhammer{at}uni-tuebingen.de).

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www.plantphysiol.org/cgi/doi/10.1104/pp.108.117218

^* Corresponding author; e-mail karl.forchhammer{at}uni-tuebingen.de.

Received February 1, 2008; accepted February 25, 2008; published March 5, 2008.