Role of the PsbS protein in Chlamydomonas reinhardtii
Plant Biochemistry and Biophysics
Final Report Abstract
Heat dissipation of excess excitation energy (NPQ) under high light (HL) is an important photoprotective mechanism active in all photosynthetic organisms, which aims at the minimization of photo-oxidative damage by reactive oxygen species. In land plants, the photosystem II (PSII) protein PsbS plays a key role in NPQ regulation. PsbS acts as sensor of the lumen pH and controls conformational changes in the PSII antenna, which are required for NPQ activation. In the green alga Chlamydomonas reinhardtii, LHCSR proteins serve as pH-sensor and trigger NPQ activation. Here, NPQ activation is only possible after several hours of HL acclimation of the cells, which initiates the synthesis of LHCSR proteins. The role of PsbS in C. reinhardtii is still unclear. At the start of the project, it was known that PsbS is only transiently expressed during HL acclimation and that PsbS has no direct essential function in NPQ. In this project, we investigated the possible function of PsbS and the regulation of the PsbS amount in C. reinhardtii in more detail. We studied the HL acclimation of C. reinhardtii cells in response to different stress conditions in comparison with the dynamics of PsbS accumulation, and further investigated the factors that control the degradation of PsbS. PsbS degradation was found to depend on light and on the action of Deg and FtsH proteases. The PsbS degradation machinery was further shown to be specific for the PsbS protein of C. reinhardtii since the Arabidopsis PsbS protein was not degraded. Exposure to different stress conditions such as fluctuating HL or combined salt and HL stress, resulted in increased accumulation of PsbS in parallel with an increase of the amount of zeaxanthin, a xanthophyll known to serve photoprotective functions in chloroplast. The loss of PsbS did not impair the NPQ capacity but resulted in reduced zeaxanthin amounts and an altered lipid composition. These findings support a photoprotective role of PsbS in C. reinhardtii independent of NPQ, but likely related to the modification of the properties or the reorganization of the thylakoid membrane.
Publications
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Combined high light and salt stress enhances accumulation of PsbS and zeaxanthin in Chlamydomonas reinhardtii. Physiologia Plantarum, 176(2).
Hemker, Fritz; Zielasek, Fabian & Jahns, Peter
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Intervening dark periods negatively affect the photosynthetic performance of Chlamydomonas reinhardtii during growth under fluctuating high light. Plant, Cell & Environment, 47(11), 4246-4258.
Hemker, Fritz; Ammelburger, Nicolas & Jahns, Peter
