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Bonardi, Vera (2006): Molecular-genetic characterization of thylakoid protein phosphorylation in Arabidopsis thaliana. Dissertation, LMU München: Faculty of Biology
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Abstract

Plants respond to changes in illumination conditions by modifying the thylakoid proteins post-translationally and by reorganizing the photosynthetic machinery. However, the mechanisms that characterize the short-term and the long-term responses are different. In the first case, the organism reacts to rapid illumination changes via phosphorylation of photosystem II (PSII) and light-harvesting (LHCII) proteins. Phosphorylation of PSII is thought to be relevant for PSII turnover, whereas LHCII phosphorylation is required for state transitions, which ensure the redistribution of the excitation energy between the two photosystems. Long-term imbalances in the energy distribution elicit changes in the composition and stoichiometry of the photosynthetic apparatus (photosynthetic acclimation). Two types of thylakoid protein kinases have been previously associated with LHCII phosphorylation, the TAK (thylakoid-associated kinase) proteins in Arabidopsis thaliana and Stt7 in Chlamydomonas reinhardtii. This work shows that the TAK proteins (TAK1, TAK2, and TAK3) are neither involved in LHCII phosphorylation nor in state transitions. In addition, evidences are provided that exclude any role of TAK2 and TAK3 in the photosynthetic electron flow. In Arabidopsis, two Stt7-like proteins exist, STN7 and STN8. Loss of STN7 blocks both LHCII phosphorylation and state transitions, indicating that this protein is a genuine Stt7 homolog. In contrast, STN8 is required for the quantitative phosphorylation of PSII core proteins. PSII activity under high-intensity light is affected only slightly in stn8 mutants, and D1 turnover is indistinguishable from the wild-type (WT), implying that reversible protein phosphorylation is not essential for PSII repair. Functional characterization of stn7 mutants showed that STN7 is not only associated with the short term response, but it is also required for the adaptation to long-term illumination changes including light-quality-induced changes in the mRNA expression of nuclear and plastid genes for photosynthetic proteins. This indicates that short-term and long-term photosynthetic adaptations are coupled and that phosphorylation of LHCII, or of an unknown substrate of STN7, is crucial for the control of photosynthetic gene expression and readjustment of photosystem stoichiometry.