Characterization of the reversible state of photoinhibition occurring in vitro under anaerobic conditions

Photoinhibition of Photosystem II

Photoinhibition of Photosystem II (PSII) is the light-induced loss of PSII electron-transfer activity. Although photoinhibition has been studied for a long time, there is no consensus about its mechanism. On one hand, production of singlet oxygen ((1)O(2)) by PSII has promoted models in which this reactive oxygen species (ROS) is considered to act as the agent of photoinhibitory damage. These chemistry-based models have often not taken into account the photophysical features of photoinhibition-like light response and action spectrum. On the other hand, models that reproduce these basic photophysical features of the reaction have not considered the importance of data about ROS. In this chapter, it is shown that the evidence behind the chemistry-based models and the photophysically oriented models can be brought together to build a mechanism that confirms with all types of experimental data. A working hypothesis is proposed, starting with inhibition of the manganese complex by light. Inability of the manganese complex to reduce the primary donor promotes recombination between the oxidized primary donor and Q(A), the first stable quinone acceptor of PSII. (1)O(2) production due to this recombination may inhibit protein synthesis or spread the photoinhibitory damage to another PSII center. The production of (1)O(2) is transient because loss of activity of the oxygen-evolving complex induces an increase in the redox potential of Q(A), which lowers (1)O(2) production.

Application of low temperatures during photoinhibition allows characterization of individual steps in photodamage and the repair of photosystem II

Recent investigations of photoinhibition have revealed that photodamage to photosystem II (PSII) involves two temporally separated steps: the first is the inactivation of the oxygen-evolving complex by light that has been absorbed by the manganese cluster and the second is the impairment of the photochemical reaction center by light that has been absorbed by chlorophyll. Our studies of photoinhibition in Synechocystis sp. PCC 6803 at various temperatures demonstrated that the first step in photodamage is not completed at low temperatures, such as 10 degrees C. Further investigations suggested that an intermediate state, which is stabilized at low temperatures, might exist at the first stage of photodamage. The repair of PSII involves many steps, including degradation and removal of the D1 protein, synthesis de novo of the precursor to the D1 protein, assembly of the PSII complex, and processing of the precursor to the D1 protein. Detailed analysis of photodamage and repair at various temperatures has demonstrated that, among these steps, only the synthesis of the precursor to D1 appears to proceed at low temperatures. Investigations of photoinhibition at low temperatures have also indicated that prolonged exposure of cyanobacterial cells or plant leaves to strong light diminishes their ability to repair PSII. Such non-repairable photoinhibition is caused by inhibition of the processing of the precursor to the D1 protein after prolonged illumination with strong light at low temperatures.

Effects of bicarbonate and oxygen concentration on photoinhibition of thylakoid membranes

The effects of bicarbonate and oxygen on photoinhibition of thylakoid membranes were investigated by varying their concentrations independently of each other. A pretreatment of the thylakoid suspension which lowered the bicarbonate concentration of the medium without affecting its oxygen content, increased the degree of photoinhibition upon illumination. This showed that the normal bicarbonate content of a thylakoid suspension, originating from dissolved carbondioxide from the air, protects against photoinhibition. The resistance against photoinhibition was further increased by addition of extra NaHCO3 up to about 5 mM. The normal oxygen content can be decreased profoundly without affecting the degree of photoinhibition; in contrast, even small changes from the normal bicarbonate content affected photoinhibition.At oxygen concentrations approximately below 25 μM, added NaHCO3 not only did not protect, but caused a more severe PS 2 inactivation. This was due to a blockage by added NaHCO3 of the recovery from a reversible photoinhibited state.Furthermore, it is shown that if the bicarbonate ions bound to high-affinity sites in PS 2 were replaced by formate ions, the thylakoid membranes became less susceptible to photoinhibition under normal oxygen tension.