<b>Differential FeS cluster photodamage plays a critical role in regulating excess electron flow through photosystem I</b>

Photosynthesis is one of the most fundamental processes in biology, providing energy and oxygen to sustain life on Earth. Photosynthetic light reactions are particularly vulnerable to environmental stress, including fluctuating light conditions due to cloudiness andf sunfleck etc. Depending on environmental and metabolic conditions, the sustenance of CO2 fixation rates and optimal metabolism requires diverting the photosynthetic electron flow towards alternative electron transport pathways that divert electrons from linear electron transfer (LET) pathway to molecular oxygen (O2) and quinones or back from PSI to cytochrome b6f (Cytb6f), instead of reaching NADP+.Here, we studied the photoinhibition of photosystem I and II under changing light conditions from constant growth light (120 µmol photons m-2 s-1) to high light (850 µmol photons m-2 s-1) for different times. We observed a loss of P700 oxidation capacity (decrease of Pm) and the damage of FeSX and FeSA/B clusters from the GL-exposed and moderately HL-treated WT and pgr5 mutants of Arabidopsis thaliana. Our results show a photodamage to the PSI FeSA/B clusters, which in WT increased with duration of HL (up to 43% in 120 min), but without noticeable effects on P700 photooxidation capacity (Pm) or the function of the FeSX clusters. The pgr5 mutant showed a difference from WT both in growth light (GL) and during the HL exposure. We provide evidence for sequential damage to PSI FeS clusters under high light (HL), and subsequent slow recovery under low light in Arabidopsis thaliana. In GL, pgr5 exhibited a significantly lower capacity for P700 photooxidation and FeSA/B reduction than WT, and during the HL treatment, there was a severe damage to P700 photooxidation and to both the FeSA/B and FeSX cluster reduction capacities, ultimately leading a release of most of the Fd from the thylakoid membrane. Sequential damage of PSI redox cofactors under HL is shown to affect the delivery of electrons to different pathways from PSI. Overall, our results suggest that PSI FeS clusters in angiosperm chloroplasts are susceptible to over-reduction and photodamage in high light. The PSI FeS cluster damage may occur frequently in high light similar to that of PSII damage, but is not observable by gas exchange or P700 spectroscopic methods. All figures with their raw data sets are present here.