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Plant Physiology Preview Published on March 5, 2008; 10.1104/pp.108.117218
OPEN ACCESS ARTICLE
Received February 1, 2008 Ammonia triggers photodamage of photosystem II in the cyanobacterium Synechocystis sp. strain PCC 6803
Institut fur Mikrobiologie und Molekularbiologie, Justus-Liebig-Universitat Giessen, Heinrich-Buff-Ring 26-32, D-35392 Gi essen, Germany; Fachbereich Biologie-Molekulare Pflanzenphysiologie, Philipps-Universitat Marburg, Karl-von-Frisch-Str. 8, D-35032 Marburg, Germany; Institut fur Biochemie, Universitat zu Koln, Zulpicher Str. 47, D-50674 Koln; Lehrstuhl fur Mikrobiologie/Organismische Interaktionen, Eberhard-Karls-Universitat Tubingen, Auf der Morgenstelle 28, D-72076 Tubingen, Germany * Corresponding author; email: karl.forchhammer{at}uni-tuebingen.de.
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, FtsH2-deficient mutants are highly sensitive and loose 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 (OEC), suggesting that the OEC may be a direct target for ammonia toxicity.
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