Tocopherol as singlet oxygen scavenger in photosystem II.

Singlet oxygen is formed in the photosystem II reaction center in the quench of P680 triplets, and the yield is dependent on light intensity and the reduction level of plastoquinone. Singlet oxygen in PS II triggers the degradation of the D1 protein. We investigated the participation of tocopherol as a singlet oxygen scavenger in this system. For this purpose, we inhibited tocopherol biosynthesis at the level of the HPP-dioxygenase in the alga Chlamydomonas reinhardtii under conditions in which plastoquinone did not limit the photosynthesis rate. In the presence of the inhibitor and in high light for 2 h, photosynthesis in vivo and photosystem II was inactivated, the D1 protein was degraded, and the tocopherol pool was depleted and fell below its turnover rate/h. The inhibited system could be fully resuscitated upon the addition of a chemical singlet oxygen quencher (diphenylamine), and partly by synthetic cell wall permeable short chain alpha- and gamma-tocopherol derivatives. We conclude that under conditions of photoinhibition and extensive D1 protein turnover tocopherol has a protective function as a singlet oxygen scavenger.