Short-term and Long-term Adaptation of the Photosynthetic Apparatus: Homeostatic Properties of Thylakoids

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Light-energy conversion in thylakoids is accomplished by cooperative interactions between two photoreactions. The balance between these two photoreactions determines the efficiency of energy conversion. The efficiency of energy conversion is maintained at a high level by at least two regulatory mechanisms: short-term adaptation and long-term adaptation. Short-term adaptation, i.e. the state transition, is a regulatory mechanism that controls the distribution of excitation energy transfer from the light-harvesting antenna complexes, the phycobilisomes (PBS), to the two photosystems. The transfer of energy trapped by PBS to the Chl a of Photosystem I (PS I) or Photosystem II (PS II) is regulated either at the transfer point from the PBS to the two photosystems or at a transfer point between the Chl a of PS II and PS I. Energy transfer from the PBS to PS I increases, and to PS II decreases, when most of PS II centers are closed, and the opposite occurs upon a shift to conditions under which most PS I centers are closed. This regulation occurs in response to the state of balance between the two photoreactions through monitoring of the redox status of electron transport between the two photosystems or through monitoring the electrochemical potential of the membrane around the two photosystem complexes. This regulatory process is called’ short-term adaptation’ because this regulation can occur rather rapidly (usually completed within several minutes or less). ‘Long-term adaptation’ refers to a regulated change in the stoichiometry of PS I and PS II in the thylakoids. The PS LPS II ratio becomes greater-values of 2.0 to 3.0 are typical-when cells are grown under conditions where most PS II centers are closed (e.g., green-rich light). The ratio becomes small, approximately 1.0 , upon a shift to conditions where most of PS I centers are closed (e.g., growth in red-rich light). The PS LPS II ratio is also regulated in response to the redox state of electron transport between two photosystems. Regulation of PS I synthesis appears to be the general pattern in cyanobacteria. Synthesis of PS I complexes apparently is controlled at the assembly level but not at the level of apoprotein synthesis. This regulation seems to occur by controlling Chl a synthesis or transport to the site for PS I assembly. The regulation of PS I synthesis is as rapid as state transition. However, the PS I:PS II ratio changes more slowly over a period of hours or days, and hence this process is referred to as ‘long-term adaptation.’ These short-term and long-term adaptation regulatory responses operate together to produce fine-and coarse-tuning adjustments, respectively, and to balance the activities of the two photoreactions in response to changing photosynthetic environments. The phenomena associated with these responses as well as possible mechanisms for their regulation are discussed.