PGR5 ensures photosynthetic control to safeguard photosystem I under fluctuating light conditions.

In a plant's natural environment, the intensity of light can change rapidly due to sunflecks, cloudiness and intermittent shading. Fluctuations between high and low illumination phases expose the photosynthetic machinery to rapidly changing signals that can be overlapping or contradictory, and accordingly plants have developed astute acclimation strategies to maintain optimal photosynthetic performance in these conditions. Continuous exposure to high light induces an array of protective mechanisms at anatomical, chemical and molecular levels, but when high light phases are short, such as under fluctuating light conditions, the protective strategies that afford protection to constant high light are not employed by plants. One mechanism that is engaged under both constant and fluctuating high light is the photosynthetic control of the Cyt b 6f complex, which prevents hyper-reduction of the electron transfer chain in order to protect PSI from photodamage. The PGR5 protein was recently shown to play an indispensable role in this protective mechanism. This review revisits the findings of earlier studies into photosynthetic control and places PGR5 within the broader context of photoprotection and light acclimation strategies.