Photosynthetic Control and Its Role in Protection of Photosystem I against Photoinhibition.

This review addresses photosynthetic control as a protective mechanism that prevents photoinhibition of photosystem I under conditions of imbalance between CO assimilation during the Calvin-Benson-Bassham cycle and light reactions in the thylakoid photosynthetic apparatus. We discuss the pathways of photosystem I photoinhibition and describe protective mechanisms that prevent photodamage of photosystem I. We propose a hypothesis regarding the influence of photosynthetic control on formation of reactive oxygen species in photosystem I. pH-sensitivity of plastoquinol oxidation at the quinol-oxidizing (Qo) site of the cytochrome complex is analyzed, and function of two proton-conducting channels that release protons into the thylakoid lumen from the cytochrome complex is described. We examine impact of photosynthetic control on the functioning of the cytochrome complex itself, and propose a hypothesis regarding the preferential activation of photosynthetic control in the thylakoid grana, which ensures operation of the cyclic electron transport around photosystem I as a main protective mechanism.