Photodamage to photosystem II - primary and secondary events

Abstract

High light stress results in a reduction in the photosynthetic capacity of plants. This photoinhibition is targeted to photosystem II and seems to be an inevitable consequence of the complicated redox chemistry involved in the light-driven water-plastoquinone oxidoreduction reaction. Photoinactivation leads to irreversible damage of the reaction centre of photosystem II, in particular the D1 protein. In this paper, we present evidence which indicates that the inhibition of electron transport can be initiated at both the acceptor and donor sides of photosystem II. In the former case, there is a sequential array of events leading to overreduction of the primary quinone acceptor Q A. This, in turn, leads to the formation of a P 680 chlorophyll triplet. The donor-side inhibition is thought to involve the accumulation of the highly oxidizing species P + 680 and Tyr + z. These species will lead to oxidative damage of the D1 protein, triggering it for degradation. During acceptor-side inhibition, the protein is damaged by singlet oxygen formed via a reaction between P 680 triplet and oxygen. Damaged D1 protein is degraded by a serine protease located in the photosystem II complex. Preliminary inhibition and binding studies using the classical serine protease inhibitor diisopropylfluorophosphate suggest that the active serine is located on a 43 kDa polypeptide. Several D1 protein digestion fragments have been identified and a cleavage site between the stromally exposed loop connecting transmembrane helices IV and V is suggested.