Tobacco (Nicotiana tabacum) plants strictly adjust the contents of both ATP synthase and cytochrome b 6 f complex to the metabolic demand for ATP and NADPH. While the cytochrome b 6 f complex catalyzes the rate-limiting step of photosynthetic electron flux and thereby controls assimilation, the functional significance of the ATP synthase adjustment is unknown. Here, we reduced ATP synthase accumulation by an antisense approach directed against the essential nuclearencoded g-subunit (AtpC) and by the introduction of point mutations into the translation initiation codon of the plastidencoded atpB gene (encoding the essential b-subunit) via chloroplast transformation. Both strategies yielded transformants with ATP synthase contents ranging from 100 to <10% of wild-type levels. While the accumulation of the components of the linear electron transport chain was largely unaltered, linear electron flux was strongly inhibited due to decreased rates of plastoquinol reoxidation at the cytochrome b 6 f complex (photosynthetic control). Also, nonphotochemical quenching was triggered at very low light intensities, strongly reducing the quantum efficiency of CO 2 fixation. We show evidence that this is due to an increased steady state proton motive force, resulting in strong lumen overacidification, which in turn represses photosynthesis due to photosynthetic control and dissipation of excitation energy in the antenna bed. INTRODUCTIONThe capacity of the photosynthetic light reactions to provide ATP and NADPH must be closely adjusted to their metabolic consumption by the Calvin cycle, the subsequent reactions of dark metabolism such as starch synthesis, and other anabolic pathways within the chloroplast. Upon hyperactivity of the light reactions, the metabolic regeneration of NADP + , ADP, and P i will limit photosynthetic electron transport, resulting in detrimental side reactions. NADP + limitation would result in electron transfer to alternative acceptors, such as O 2 , generating reactive oxygen species. These can damage the photosynthetic apparatus itself and also initiate cell death responses (Kim et al., 2008).Reduced ADP and P i regeneration results in substrate limitation of the thylakoid ATP synthase, reducing proton efflux from the lumen and resulting in an increase of the proton motive force (pmf) across the thylakoid membrane (Takizawa et al., 2008;Kiirats et al., 2009). Under standard growth conditions, the pmf is partitioned into an electrochemical component (DC) and a proton gradient (DpH) in such a way that the pH value of the thylakoid lumen is usually kept between 7.0 and 6.5 (Takizawa et al., 2007). However, in response to short-term imbalances between proton translocation into the lumen by photosynthetic electron transport and use of the pmf for ATP synthesis, the pH of the thylakoid lumen can drop below 6.5. This initiates photoprotective feedback responses such as nonphotochemical quenching (qN), which is the thermal dissipation of excess excitation energy in the photosystem II (PSII) antenna bed in the...

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