Phosphorylation of photosystem II core proteins prevents undesirable cleavage of D 1 and contributes to the fine‐tuned repair of photosystem II

Summary Photosystem II ( PSII ) is a primary target for light‐induced damage in photosynthetic protein complexes. To avoid photoinhibition, chloroplasts have evolved a repair cycle with efficient degradation of the PSII reaction center protein, D 1, by the proteases F ts H and D eg. Earlier reports have described that phosphorylated D 1 is a poor substrate for proteolysis, suggesting a mechanistic role for protein phosphorylation in PSII quality control, but its precise role remains elusive. STN 8, a protein kinase, plays a central role in this phosphorylation process. To elucidate the relationship between phosphorylation of D 1 and the protease function we assessed in this study the involvement of STN 8, using A rabidopsis thaliana mutants lacking F ts H 2 [ yellow variegated2 ( var2 )] and D eg5/ D eg8 ( deg5 deg8 ). In support of our presumption we found that phosphorylation of D 1 increased more in var2 . Furthermore, the coexistence of var2 and stn8 was shown to recover the delay in degradation of D 1, resulting in mitigation of the high vulnerability to photoinhibition of var2 . Partial D 1 cleavage fragments that depended on D eg proteases tended to increase, with concomitant accumulation of reactive oxygen species in the mutants lacking STN 8. We inferred that the accelerated degradation of D 1 in var2 stn8 presents a tradeoff in that it improved the repair of PSII but simultaneously enhanced oxidative stress. Together, these results suggest that PSII core phosphorylation prevents undesirable cleavage of D 1 by D eg proteases, which causes cytotoxicity, thereby balancing efficient linear electron flow and photo‐oxidative damage. We propose that PSII core phosphorylation contributes to fine‐tuned degradation of D 1.