1. Departments of Molecular Biology and Plant Biology, University of Geneva, 30, Quai Ernest Ansermet, 1211 Geneva, Switzerland
2. CEA Cadarache, DSV, DEVM, Laboratoire d'Ecophysiologie de la Photosynthèse, UMR 6191 CNRS-CEA, Aix Marseille II, F-3108 Saint-Paul-Durance, France

Correspondence to: Jean-David Rochaix¹ Correspondence and requests for materials should be addressed to J.-D.R. (Email: jean-david.rochaix@molbio.unige.ch).

Abstract

Photosynthetic organisms are able to adjust to changing light conditions through state transitions, a process that involves the redistribution of light excitation energy between photosystem II (PSII) and photosystem I (PSI)^{1, 2}. Balancing of the light absorption capacity of these two photosystems is achieved through the reversible association of the major antenna complex (LHCII) between PSII and PSI (ref. 3). Excess stimulation of PSII relative to PSI leads to the reduction of the plastoquinone pool and the activation of a kinase^{4, 5}; the phosphorylation of LHCII; and the displacement of LHCII from PSII to PSI (state 2). Oxidation of the plastoquinone pool by excess stimulation of PSI reverses this process (state 1). The Chlamydomonas thylakoid-associated Ser-Thr kinase Stt7, which is required for state transitions, has an orthologue named STN7 in Arabidopsis⁶. Here we show that loss of STN7 blocks state transitions and LHCII phosphorylation. In stn7 mutant plants the plastoquinone pool is more reduced and growth is impaired under changing light conditions, indicating that STN7, and probably state transitions, have an important role in response to environmental changes.

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