A kaleidoscope of photosynthetic antenna proteins and their emerging roles.

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.

Quantifying the long-term interplay between photoprotection and repair mechanisms sustaining photosystem II activity.

The photosystem II reaction centre (RCII) protein subunit D1 is the main target of light-induced damage in the thylakoid membrane. As such, it is constantly replaced with newly synthesised proteins, in a process dubbed the 'D1 repair cycle'. The mechanism of relief of excitation energy pressure on RCII, non-photochemical quenching (NPQ), is activated to prevent damage.

Chlorophyll a de-excitation pathways in the LHCII antenna.

Photosystem II (PSII) uses light energy to split water into protons, electrons, and oxygen, ultimately sustaining heterotrophic life on Earth. The major light harvesting complex in plants (LHCII) is packed with chlorophylls and carotenoids and is the main supplier of excitation energy to PSII reaction centers. The protein scaffold acts as a programmed solvent for the pigments in LHCII, tuning their orientations while at the same time impeding concentration quenching to ensure efficient storage of excitation energy by chlorophylls.

Protection of photosystem I during sudden light stress depends on ferredoxin:NADP(H) reductase abundance and interactions.

Plant tolerance to high light and oxidative stress is increased by overexpression of the photosynthetic enzyme Ferredoxin:NADP(H) reductase (FNR), but the specific mechanism of FNR-mediated protection remains enigmatic. It has also been reported that the localization of this enzyme within the chloroplast is related to its role in stress tolerance. Here, we dissected the impact of FNR content and location on photoinactivation of photosystem I (PSI) and photosystem II (PSII) during high light stress of Arabidopsis (Arabidopsis thaliana).

Regulation of photosynthetic electron flow on dark to light transition by ferredoxin:NADP(H) oxidoreductase interactions.

During photosynthesis, electron transport is necessary for carbon assimilation and must be regulated to minimize free radical damage. There is a longstanding controversy over the role of a critical enzyme in this process (ferredoxin:NADP(H) oxidoreductase, or FNR), and in particular its location within chloroplasts. Here we use immunogold labelling to prove that FNR previously assigned as soluble is in fact membrane associated.

Unraveling the Excited-State Dynamics and Light-Harvesting Functions of Xanthophylls in Light-Harvesting Complex II Using Femtosecond Stimulated Raman Spectroscopy.

Photosynthesis in plants starts with the capture of photons by light-harvesting complexes (LHCs). Structural biology and spectroscopy approaches have led to a map of the architecture and energy transfer pathways between LHC pigments. Still, controversies remain regarding the role of specific carotenoids in light-harvesting and photoprotection, obligating the need for high-resolution techniques capable of identifying excited-state signatures and molecular identities of the various pigments in photosynthetic systems.

A Protein Environment-Modulated Energy Dissipation Channel in LHCII Antenna Complex.

The major light-harvesting complex of photosystem II (LHCII) is the main contributor to sunlight energy harvesting in plants. The flexible design of LHCII underlies a photoprotective mechanism whereby this complex switches to a dissipative state in response to high light stress, allowing the rapid dissipation of excess excitation energy (non-photochemical quenching, NPQ). In this work, we locked single LHCII trimers in a quenched conformation after immobilization of the complexes in polyacrylamide gels to impede protein interactions.

The robustness of the terminal emitter site in major LHCII complexes controls xanthophyll function during photoprotection.

Xanthophylls in light harvesting complexes perform a number of functions ranging from structural support to light-harvesting and photoprotection. In the major light harvesting complex of photosystem II in plants (LHCII), the innermost xanthophyll binding pockets are occupied by lutein molecules. The conservation of these sites within the LHC protein family suggests their importance in LHCII functionality.

Rapid regulation of photosynthetic light harvesting in the absence of minor antenna and reaction centre complexes.

Plants are subject to dramatic fluctuations in the intensity of sunlight throughout the day. When the photosynthetic machinery is exposed to high light, photons are absorbed in excess, potentially leading to oxidative damage of its delicate membrane components. A photoprotective molecular process called non-photochemical quenching (NPQ) is the fastest response carried out in the thylakoid membranes to harmlessly dissipate excess light energy.

Aggregation-Related Nonphotochemical Quenching in the Photosynthetic Membrane.

The photosynthetic apparatus of plants is a robust self-adjustable molecular system, able to function efficiently under varying environmental conditions. Under strong sunlight, it switches into photoprotective mode to avoid overexcitation by safely dissipating the excess absorbed light energy via nonphotochemical quenching (NPQ). Unfortunately, heterogeneous organization and simultaneous occurrence of multiple processes within the thylakoid membrane impede the study of natural NPQ under in vivo conditions; thus, usually artificially prepared antennae have been studied instead.

Spectroscopic Properties of Violaxanthin and Lutein Triplet States in LHCII are Independent of Carotenoid Composition.

Chlorophyll triplet excited states are byproducts of photosynthetic processes that can indirectly harm biological membranes by forming highly reactive oxygen species. A crucial photoprotective mechanism evolved by plants to counter this threat involves the triplet energy transfer from chlorophylls to carotenoid molecules, in which triplet states are not reactive. In the major light-harvesting complex of photosystem II (LHCII), the two central luteins play an important role in the mechanism, but it has been shown that carotenoid triplets are formed even when other carotenoids replace them in their binding sites.

Single-molecule microscopy studies of LHCII enriched in Vio or Zea.

Plants have developed multiple self-regulatory mechanisms to efficiently function under varying sunlight conditions. At high light intensities, non-photochemical quenching (NPQ) is activated on a molecular level, safely dissipating an excess excitation as heat. The exact molecular mechanism for NPQ is still under debate, but it is widely agreed that the direct participation of the carotenoid pigments is involved, one of the proposed candidate being the zeaxanthin.

The causes of altered chlorophyll fluorescence quenching induction in the Arabidopsis mutant lacking all minor antenna complexes.

Non-photochemical quenching (NPQ) of chlorophyll fluorescence is the process by which excess light energy is harmlessly dissipated within the photosynthetic membrane. The fastest component of NPQ, known as energy-dependent quenching (qE), occurs within minutes, but the site and mechanism of qE remain of great debate. Here, the chlorophyll fluorescence of Arabidopsis thaliana wild type (WT) plants was compared to mutants lacking all minor antenna complexes (NoM).