New steps in the regulation of photosynthesis: the influence of CF0-CF1 ATP synthase conductivity on the sensitivity of antenna down-regulation

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A 'standard' model has been proposed for the regulation of photosynthesis wherein acidification of the thylakoid lumen is the key regulatory intermediate which activates processes which dissipated excitation energy from the pigment beds associated with photosystem (PS) II, i.e. non-photochemical exciton quenching (NPQ). While this model may qualitatively account for downregulation when ADP and ATP are limiting, it runs into problems under stressed conditions, especially when photosynthesis is limited by the availability of PS I electron acceptors. In this case, the ETC may well become over-reduced before the lumen could be sufficiently acidified to initiate NPQ, leading to photoinhibition. Since plants are well-adapted to such conditions, it is clear that the simple model must be modified. Using new spectroscopic tools developed in our laboratory, we have investigated the relationship between pmf, the rate of ATP synthesis, and antenna downregulation in intact tobacco leaves. We show that the relationship between NPQ and electron transfer is modulated by the conductivity of the ATP synthase to protons as well as the parsing of pmf into Dy and DpH components. Under low [CO2] or under environmental stresses, the conductivity of the ATP synthase is significantly reduced while the Dy component of pmf is suppressed, leading to a greater sensitivity of NPQ to electron transfer. We hypothesize that modulation of both pmf parsing and ATP synthase conductivity contribute to down-regulation. This work was supported by US Department of Energy Grant DE-FG03-98ER20299.