Francesco Saccon

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. However, under stress, the very fuel of PSII, solar photons, can damage its delicate inner components and hamper photosynthesis. In a crucial regulatory strategy in plants, LHCII evolved a flexible design that allows it to switch between light-harvesting and dissipative conformations, thereby safely releasing the excess energy that is absorbed into heat. Several mechanisms have been proposed to explain chlorophyll de-excitation pathways in LHCII, such as chlorophyll–chlorophyll charge transfer states, resonance energy transfer from chlorophylls to a carotenoid S1 state, and chlorophyll–carotenoid reductive energy transfer. This Perspective critically assesses the listed proposals, addressing both the physical mechanism of quenching and the nature of the quenching pigment. These hypotheses are then discussed in the context of state-of-the-art biochemical, physiological, and genetic knowledge to scrutinize their likeliness to occur in the native thylakoid membranes.