PHOTOINHIBITION OF PHOTOSYSTEM-2 - INACTIVATION, PROTEIN DAMAGE AND TURNOVER

Even though light is the source of energy for photosynthesis, it can a lso be harmful to plants. Light-induced damage is targetted mainly to Photosystem II and leads to inactivation of electron transport and sub sequent oxidative damage of the reaction centre, in particular to the D1 protein. Inactivation and protein damage can be induced by two diff erent mechanisms, either from the acceptor side or from donor side of P680. The damaged D1 protein is triggered for degradation and digested by at least one serine-type proteinase that is tightly associated wit h the Photosystem II complex itself. The damaged Photosystem II comple x dissociates from the light-harvesting antenna and migrates from appr essed to non-appressed thylakoid regions where a new D1 protein is co- translationally inserted into the partially disassembled Photosystem I I complex. D1 protein phosphorylation probably allows for coordinated biodegradation and biosynthesis of the D1 protein. After religation of cofactors and assembly of subunits, the repaired Photosystem II compl ex can again be found in the appressed membrane regions. Various prote ctive mechanisms and an efficient repair cycle of Photosystem II allow plants to survive light stress.