The photoprotective dilemma of a chloroplast: to avoid high light or to quench the fire?

The safe and smooth functioning of photosynthesis in plants is ensured by the operation of numerous regulatory mechanisms that adjust the density of excitation resulting from photon absorption to the capabilities of the photosynthetic apparatus. Such mechanisms include the movement of chloroplasts inside cells and the quenching of electronic excitations in the pigment-protein complexes. Here, we address the problem of a possible cause-and-effect relationship between these two mechanisms. Both the light-induced chloroplast movements and quenching of chlorophyll excitations were analyzed simultaneously with the application of fluorescence lifetime imaging microscopy of Arabidopsis thaliana leaves, wild-type and impaired in chloroplast movements or photoprotective excitation quenching. The results show that both regulatory mechanisms operate over a relatively wide range of light intensities. By contrast, impaired chloroplast translocations have no effect on photoprotection at the molecular level, indicating the direction of information flow in the coupling of these two regulatory mechanisms: from the photosynthetic apparatus to the cellular level. The results show also that the presence of the xanthophyll zeaxanthin is necessary and sufficient for the full development of photoprotective quenching of excessive chlorophyll excitations in plants.

Automated summary

The safe and smooth functioning of photosynthesis in plants is ensured by various regulatory mechanisms, including chloroplast movements and quenching of electronic excitations. A study using fluorescence lifetime imaging microscopy of Arabidopsis thaliana leaves found that these mechanisms operate over a wide range of light intensities. Impaired chloroplast movements do not affect photoprotection at the molecular level, indicating the one-way information flow from the photosynthetic apparatus to the cellular level. The presence of zeaxanthin is necessary and sufficient for the full development of photoprotective quenching in plants.