Abstract
Light harvesting for oxygenic photosynthesis is regulated to prevent the formation of harmful photoproducts by activation of photoprotective mechanisms safely dissipating the energy absorbed in excess. Lumen acidification is the trigger for the formation of quenching states in pigment binding complexes. With the aim to uncover the photoprotective functional states responsible for excess energy dissipation in green algae and mosses, we compared the fluorescence dynamic properties of the light-harvesting complex stress-related (LHCSR1) protein, which is essential for fast and reversible regulation of light use efficiency in lower plants, as compared to the major LHCII antenna protein, which mainly fulfills light harvesting function. Both LHCII and LHCSR1 had a chlorophyll fluorescence yield and lifetime strongly dependent on detergent concentration but the transition from long- to short-living states was far more complete and fast in the latter. Low pH and zeaxanthin binding enhanced the relative amplitude of quenched states in LHCSR1, which were characterized by the presence of 80 ps fluorescence decay components with a red-shifted emission spectrum. We suggest that energy dissipation occurs in the chloroplast by the activation of 80 ps quenching sites in LHCSR1 which spill over excitons from the photosystem II antenna system.
Highlights
Eukaryotic photosynthetic organisms harvest photons using chlorophyll- and carotenoid-binding proteins called light-harvesting complexes (LHCs)[1,2,3,4]
We have studied the optical properties of P.p.LHCSR1 produced by overexpression in tobacco in either its Vio- or Zea-binding forms and purified either directly from dark-adapted leaves or upon incubation at low pH which activates Vio de-epoxidation
By using steady-state and time-resolved spectroscopic methods, we show that acidification of the medium and protein-protein interactions induce fluorescence quenching in P.p.LHCSR1 to a far higher extent compared to the homologous trimeric LHCII antenna protein, leading to the formation of a red-shifted spectral component with lifetime below 100 ps, which can act as a suitable quencher for LHC complexes
Summary
Eukaryotic photosynthetic organisms harvest photons using chlorophyll- and carotenoid-binding proteins called light-harvesting complexes (LHCs)[1,2,3,4]. Besides direct protonation of PSBS/LHCSR proteins, additional regulation of NPQ activity is provided by the xanthophyll cycle, consisting in reversible epoxidation of zeaxanthin (Zea) in plants and algae, or diatoxanthin in diatoms[30, 31]. LHCSR1 from P. patens has been reported to host a Zea-dependent quenching mechanism triggered at low pH involving energy transfer from chlorophyll to the carotenoid S1 state[29]. By using steady-state and time-resolved spectroscopic methods, we show that acidification of the medium and protein-protein interactions induce fluorescence quenching in P.p.LHCSR1 to a far higher extent compared to the homologous trimeric LHCII antenna protein, leading to the formation of a red-shifted spectral component with lifetime below 100 ps, which can act as a suitable quencher for LHC complexes. The binding of Zea further decreased fluorescence lifetime of this pigment-protein complex and enhanced the sensitivity of quenching to low pH
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