Changes in energy uxes during NPQ in LHCII and PSII-LHCII

PhD ThesisThe non-photochemical quenching of excess energy (NPQ) is a fast molecular adaptation of photosynthetic organisms to variations of sunlight intensities. In plants, the energy-dependent quenching (qE) is the main NPQ component, which promptly protects the thylakoid membrane components by dissipating the excess energy absorbed. While the trigger of this physiological process is known to be thylakoid DpH, the site in the membrane, the structural changes involved and the nature of the quencher pigment are still a subject of debate. In this thesis, I addressed these gaps in our knowledge of qE. The results presented here show that neither minor light harvesting antenna complexes nor reaction cores are sites of qE, which instead takes place entirely in major LHCII trimers. The nature of the change from a light-harvesting to a dissipative state in LHCII and its dependence on the binding of xanthophyll-cycle carotenoids was investigated. Zeaxanthin was found to exert no e ect on the quenching dynamics of single LHCII trimers, disproving its role as a quencher. However, it controls the kinetics of the transition to the quenched state by favouring LHCII aggregation. To determine the nature of the quencher species, transient absorption spectroscopy was applied to isolated LHCII. A mechanism was identi ed whereby chlorophylls donate energy to a carotenoid species, likely a lutein, leading to quick energy dissipation. Overall, this work reveals the self-regulatory nature of photosynthetic light harvesting, showing that in principle only trimeric LHCII and the proton gradient are su cient to enable qE in vivo. The protein PsbS and zeaxanthin exert an allosteric regulation of the process, that, by tuning the degree of antennae sensitivity to the amplitude of the proton gradient, assures a ne control of light harvestin