| dc.description.abstract | The 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 harvesting | en_US |
