Mechanisms shaping the synergism of zeaxanthin and PsbS in photoprotective energy dissipation in the photosynthetic apparatus of plants

Safe operation of photosynthesis is vital to plants and is ensured by the activity of numerous processes protecting chloroplasts against photo-damage. The harmless dissipation of excess excitation energy is believed to be the main photoprotective mechanism and is most effective with the simultaneous presence of PsbS protein and zeaxanthin, a xanthophyll accumulated in strong light as a result of the xanthophyll cycle activity. Here we address the problem of specific molecular mechanisms underlying the synergistic effect of zeaxanthin and PsbS. The experiments were conducted with Arabidopsis thaliana, the wild-type and the mutants lacking PsbS (npq4) and affected in the xanthophyll cycle (npq1), with the application of multiple molecular spectroscopy and imaging techniques. Research results lead to the conclusion that PsbS interferes with the formation of tightly packed aggregates of thylakoid membrane proteins, thus enabling the incorporation of xanthophyll cycle pigments into such structures. It was found that xanthophylls trapped within supramolecular structures, most likely in the interfacial protein region, determine their photophysical properties. The structures formed in the presence of violaxanthin are characterized by minimized dissipation of excitation energy. In contrast, the structures formed in the presence of zeaxanthin show enhanced excitation quenching, thus protecting the system against photo-damage.