Switching off photoprotection of photosystem I - a novel tool for gradual PSI photoinhibition.

Photosystem I (PSI) has evolved in anaerobic atmospheric conditions and until today remains susceptible to oxygen. To minimize the probability of damaging side reactions, plants have evolved sophisticated mechanisms to control electron transfer, and PSI becomes inhibited only when malfunctions of these regulatory mechanisms occur. Because of the complicated induction of PSI photoinhibition, a detailed investigation into the process and following reactions are still largely missing. Here, we introduce the theoretical framework and a novel method for an easy and controlled induction of PSI photoinhibition in vivo. The method mimics the PSI damage mechanisms of fluctuating light-sensitive mutant plants (stn7, pgr5) which cannot control electron donation to PSI. Because PSII and PSI have different light absorption properties, electrons accumulate in the intersystem electron transfer chain (ETC), if PSII is preferentially excited. A saturating light pulse given upon an over-reduced ETC leads to the saturation of PSI electron acceptors, ultimately leading to the production of reactive oxygen species and photoinhibition of PSI. By adjusting the time of the light treatment, PSI can be gradually photoinhibited, providing a novel tool to holistically investigate the PSI photoinhibition phenomenon.