Abstract
Linear electron flow (LEF) from photosystem (PS) II to I and cyclic electron flow (CEF) around PSI co-exist in the chloroplast thylakoid membranes. While LEF predominantly supplies ATP and NADPH for carbon fixation, CEF supplies ATP additionaly and regulates photoprotection. In LEF, light excitations in PSII and PSI are poised by a mechanism called state transition, which reversibly moves light harvesting antenna between the two PSs. Chlamydomonas reinhardtii shows a large capacity of state transitions, which is coupled with the activation of CEF. From in vivo analyses in C. reinhardtii, it was revealed that state transitions exhibit a biphasic control of the photosynthetic electron flow; 1) under moderate environmental disturbance such as changing light conditions, state transitions adjust antenna size of PSI and PSII for maximize LEF, and 2) under a severe stress such as an anaerobic condition, state transitions switch electron flow from linear to cyclic for preventing photodamage. The magnitude of environmental stress is reflected in the extent of reduced plastoquinone pool, which in turn induces state transitions at the required level.
