Photosynthetic light-harvesting antennae are pigment-binding proteins that perform one of the most fundamental tasks on Earth, capturing light and transferring energy that enables life in our biosphere. Adaptation to different light environments led to the evolution of an astonishing diversity of light-harvesting systems. At the same time, several strategies have been developed to optimize the light energy input into photosynthetic membranes in response to fluctuating conditions. The basic feature of these prompt responses is the dynamic nature of antenna complexes, whose function readily adapts to the light available. High-resolution microscopy and spectroscopic studies on membrane dynamics demonstrate the crosstalk between antennae and other thylakoid membrane components. With the increased understanding of light-harvesting mechanisms and their regulation, efforts are focusing on the development of sustainable processes for effective conversion of sunlight into functional bio-products. The major challenge in this approach lies in the application of fundamental discoveries in light-harvesting systems for the improvement of plant or algal photosynthesis. Here, we underline some of the latest fundamental discoveries on the molecular mechanisms and regulation of light harvesting that can potentially be exploited for the optimization of photosynthesis.

• MeSH
• Photosynthesis * physiology   MeSH
• Adaptation, Physiological MeSH
• Plants metabolism   MeSH
• Light-Harvesting Protein Complexes * metabolism   MeSH
• Thylakoids metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Light-Harvesting Protein Complexes * MeSH

Carotenoids represent the first line of defence of photosystems against singlet oxygen (1O2) toxicity, because of their capacity to quench the chlorophyll triplet state (3Chl) through a physical mechanism based on the transfer of triplet excitation (triplet-triplet energy transfer, TTET). In previous works, we showed that the antenna LHCII is characterised by a robust photoprotective mechanism, able to adapt to the removal of individual chlorophylls while maintaining a remarkable capacity for 3Chl quenching. In this work, we investigated the effects on this quenching induced in LHCII by the replacement of the lutein bound at the L1 site with violaxanthin and zeaxanthin. We studied LHCII isolated from the Arabidopsis thaliana mutants lut2-in which lutein is replaced by violaxanthin-and lut2 npq2, in which all xanthophylls are replaced constitutively by zeaxanthin. We characterised the photophysics of these systems via optically detected magnetic resonance (ODMR) and time-resolved electron paramagnetic resonance (TR-EPR). We concluded that, in LHCII, lutein-binding sites have conserved characteristics, and ensure efficient TTET regardless of the identity of the carotenoid accommodated.

• Keywords
• LHCII, ODMR, TR-EPR, TTET, carotenoid, light-harvesting complex II, triplet state,
• MeSH
• Arabidopsis * metabolism   MeSH
• Chlorophyll metabolism   MeSH
• Photosystem II Protein Complex metabolism   MeSH
• Carotenoids metabolism   MeSH
• Lutein * MeSH
• Energy Transfer MeSH
• Light-Harvesting Protein Complexes metabolism   MeSH
• Xanthophylls chemistry   MeSH
• Zeaxanthins metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Chlorophyll MeSH
• Photosystem II Protein Complex MeSH
• Carotenoids MeSH
• Lutein * MeSH
• Light-Harvesting Protein Complexes MeSH
• violaxanthin MeSH Browser
• Xanthophylls MeSH
• Zeaxanthins MeSH

Antenna proteins play a major role in the regulation of light-harvesting in photosynthesis. However, less is known about a possible link between their sizes (oligomerization state) and fluorescence intensity (number of photons emitted). Here, we used a microscopy-based method, Fluorescence Correlation Spectroscopy (FCS), to analyze different antenna proteins at the particle level. The direct comparison indicated that Chromera Light Harvesting (CLH) antenna particles (isolated from Chromera velia) behaved as the monomeric Light Harvesting Complex II (LHCII) (from higher plants), in terms of their radius (based on the diffusion time) and fluorescence yields. FCS data thus indicated a monomeric oligomerization state of algal CLH antenna (at our experimental conditions) that was later confirmed also by biochemical experiments. Additionally, our data provide a proof of concept that the FCS method is well suited to measure proteins sizes (oligomerization state) and fluorescence intensities (photon counts) of antenna proteins per single particle (monomers and oligomers). We proved that antenna monomers (CLH and LHCIIm) are more "quenched" than the corresponding trimers. The FCS measurement thus represents a useful experimental approach that allows studying the role of antenna oligomerization in the mechanism of photoprotection.

• Keywords
• Chromera velia, antenna proteins, fluorescence correlation spectroscopy, light-harvesting, microscopy, photosynthesis, protein diffusion, protein oligomerization,
• MeSH
• Algal Proteins chemistry   metabolism   MeSH
• Fluorescence * MeSH
• Spectrometry, Fluorescence MeSH
• Photosynthesis * MeSH
• Kinetics MeSH
• Protein Multimerization MeSH
• Protein Transport MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Algal Proteins MeSH