Light Acclimation Protein1, conserved in oxygenic phototrophs, regulates ion homeostasis and non-photochemical quenching induction in chloroplasts

Shinji Masudaa, Ryoichi Satob
aCenter for Biological Resources & Informatics, Tokyo Institute of Technology, Yokohama, Japan; bGraduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, Yokohama, Japan

Plants have mechanisms to acclimate to high light conditions by extinction of excess light energy. These mechanisms are important for plants to efficiently perform photosynthesis, and have to be controlled accurately. However, detailed mechanisms of how plants regulate systems by which excess light energy can be diminished are still largely unknown.

Energy quenching (qE) in the chloroplast thylakoid membrane is one of the avoidance responses and has been well studied. qE is induced when light energy is excess, and dissipate the excess light energy as heat [1]. Excellent genetic mutant screening had been performed to identify qE induction mechanism [2,3], which showed that qE is regulated by proton gradient across the thylakoid membrane (ΔpH) [4]. To further find genes associated with light acclimation response, we searched for genes that retain following properties, 1) predicted to localize in chloroplast, 2) co-expressed with qE-related genes, and 3) conserved in oxygenic phototrophs including cyanobacteria, green algae, and higher plants. Under these conditions, we found an Arabidopsis ORF, designated as Light Acclimation Protein1 (LAP1), as a novel chloroplast membrane protein responsible for light acclimation response in Arabidopsis. Arabidopsis LAP1 localizes on chloroplast thylakoid and envelope membranes, and lap1 mutant showed a pale green phenotype with high non-photochemical quenching (NPQ) values compared with those in wild type. A double mutant lacking lap1 and npq4, encoding a component necessary for NPQ induction caused by ΔpH, showed low NPQ induction as observed in a single npq4 mutant. A mutational loss of LAP1 ortholog in the cyanobacterium Synechocystis sp. PCC6803 results in alteration for light-induced proton efflux activity into the medium. These results indicated that LAP1 controls ion homeostasis in chloroplast and cyanobacteria, which is particularly important for light-acclimation response in higher plants. The exact function of LAP1 will be discussed based on our computational simulation system for photosynthetic electron transfer in chloroplast [5].


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