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Front. Plant Sci. | doi: 10.3389/fpls.2019.00974

The High Light Response in Arabidopsis Requires the Calcium Sensor Protein CAS, a Target of STN7- and STN8-Mediated Phosphorylation

  • 1Section of Plant Cell Biology, Institute for Cellular and Molecular Botany, University of Bonn, Germany
  • 2Plant Nutrition, Institute of Crop Science and Resource Conservation, University of Bonn, Germany
  • 3Dept. Biology I, Ludwig Maximilian University of Munich, Germany
  • 4Department of Molecular Systems Biology, University of Vienna, Austria
  • 5Independent researcher, Germany
  • 6University of Liège, Belgium

Reversible phosphorylation of thylakoid proteins contributes to photoacclimation responses in photosynthetic organisms, enabling the fine-tuning of light harvesting under changing light conditions and promoting the onset of photoprotective processes. However, the precise functional role of many of the described phosphorylation events on thylakoid proteins remains elusive. The calcium sensor receptor protein (CAS) has previously been indicated as one of the targets of the state transition kinase 8 (STN8). Here we show that in Arabidopsis thaliana, CAS is also phosphorylated by the state transition kinase 7 (STN7), as well as by another, so-far unknown, Ca2+-dependent kinase. Phosphoproteomics analysis and in vitro phosphorylation assays on CAS variants identified the phylogenetically conserved residues Thr-376, Ser-378 and Thr-380 as the major phosphorylation sites of the STN kinases. Spectroscopic analyses of chlorophyll fluorescence emission at 77K further showed that, while the cas mutant is not affected in state transition, it displays a persistent strong excitation of PSI under high light exposure similar to the phenotype observed for mutants of thylakoid-associated phosphatase 38 (tap38/pph1) and proton gradient regulation 5 (pgr5). Together with the observation of a strong concomitant phosphorylation of light harvesting complex II (LHCII) and photosynthetic core proteins under high irradiance in the cas mutant this suggests a role for CAS in the STN7/STN8/TAP38 network of phosphorylation-mediated photoacclimation processes in Arabidopsis.

Keywords: Calcium sensor protein, CAS, State transition, STN7/STN8, Phosphorylation, chloroplast, photoacclimation

Received: 09 Apr 2019; Accepted: 11 Jul 2019.

Edited by:

Yan Lu, Western Michigan University, United States

Reviewed by:

Jean-David Rochaix, Université de Genève, Switzerland
Cornelia Spetea, University of Gothenburg, Sweden
Rikard Fristedt, Vrije Universiteit Amsterdam, Netherlands  

Copyright: © 2019 Cutolo, Parvin, Ruge, Roustan, Weckwerth, Teige, Grieco, Larosa and Vothknecht. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Prof. Ute C. Vothknecht, Section of Plant Cell Biology, Institute for Cellular and Molecular Botany, University of Bonn, Bonn, 53113, North Rhine-Westphalia, Germany, vothknecht@uni-bonn.de