Depletion of stromal P(i) induces high 'energy-dependent' antenna exciton quenching (q(E)) by decreasing proton conductivity at CF(O)-CF(1) ATP synthase.
ABSTRACT This work tests two models to account for the effects of depletion of stromal inorganic phosphate (P(i)), which results in down-regulation of light capture via the exciton quenching (q(E)) mechanism and has been proposed to act in feedback regulation of the light reactions. In both models, antenna down-regulation is activated by acidification of the lumen, despite the fact that linear electron flow (LEF) (and associated proton flux) is decreased upon P(i) depletion. In one model, an imbalance of ATP or NADPH activates cyclic electron transfer around photosystem I (CEF1), increasing proton influx to the lumen. In the second, the effective conductivity of the CF(O)-CF(1) ATP synthase to protons (g(H)(+)) is decreased, retarding proton efflux from the lumen. Sequestering of P(i) by mannose infiltration increased sensitivities of q(E) and pmf to LEF. The effects were attributable to decreases in g(H)(+), but not to CEF1 and were largely reversed by subsequent P(i) feeding. Rapid recovery of g(H)(+) in the dark suggested that dark-labile metabolic pools are responsible for regulation of the ATP synthase. Overall, these results support models where accumulation of Benson-Calvin cycle intermediates or lowering of stromal P(i) below its K(M)at the ATP synthase, retards proton efflux from the lumen, leading to build-up of pmf and subsequent down-regulation of photosynthetic light capture.
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ABSTRACT: The chloroplast ATP synthase catalyzes the light-driven synthesis of ATP and acts as a key feedback regulatory component of photosynthesis. Arabidopsis possesses two homologues of the regulatory γ subunit of the ATP synthase, encoded by the ATPC1 and ATPC2 genes. Using a series of mutants, we show that both these subunits can support photosynthetic ATP synthesis in vivo with similar specific activities, but that in wild-type plants, only γ(1) is involved in ATP synthesis in photosynthesis. The γ(1)-containing ATP synthase shows classical light-induced redox regulation, whereas the mutant expressing only γ(2)-ATP synthase (gamma exchange-revised ATP synthase, gamera) shows equally high ATP synthase activity in the light and dark. In situ redox titrations demonstrate that the regulatory thiol groups on γ(2)-ATP synthase remain reduced under physiological conditions but can be oxidized by the strong oxidant diamide, implying that the redox potential for the thiol/disulphide transition in γ(2) is substantially higher than that for γ(1). This regulatory difference may be attributed to alterations in the residues near the redox-active thiols. We propose that γ(2)-ATP synthase functions to catalyze ATP hydrolysis-driven proton translocation in nonphotosynthetic plastids, maintaining a sufficient transthylakoid proton gradient to drive protein translocation or other processes. Consistent with this interpretation, ATPC2 is predominantly expressed in the root, whereas modifying its expression results in alteration of root hair development. Phylogenetic analysis suggests that γ(2) originated from ancient gene duplication, resulting in divergent evolution of functionally distinct ATP synthase complexes in dicots and mosses.Proceedings of the National Academy of Sciences 02/2012; 109(9):3293-8. · 9.74 Impact Factor
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ABSTRACT: The term 'photosynthetic control' describes the short- and long-term mechanisms that regulate reactions in the photosynthetic electron transport (PET) chain so that the rate of production of ATP and NADPH is coordinated with the rate of their utilization in metabolism. At low irradiances these mechanisms serve to optimize light use efficiency, while at high irradiances they operate to dissipate excess excitation energy as heat. Similarly, the production of ATP and NADPH in ratios tailored to meet demand is finely tuned by a sophisticated series of controls that prevents the accumulation of high NAD(P)H/NAD(P) ratios and ATP/ADP ratios that would lead to potentially harmful over-reduction and inactivation of PET chain components. In recent years, photosynthetic control has also been extrapolated to the regulation of gene expression because mechanisms that are identical or similar to those that serve to regulate electron flow through the PET chain also coordinate the regulated expression of genes encoding photosynthetic proteins. This requires coordinated gene expression in the chloroplasts, mitochondria, and nuclei, involving complex networks of forward and retrograde signalling pathways. Photosynthetic control operates to control photosynthetic gene expression in response to environmental and metabolic changes. Mining literature data on transcriptome profiles of C(3) and C(4) leaves from plants grown under high atmospheric carbon dioxide (CO(2)) levels compared with those grown with ambient CO(2) reveals that the transition to higher photorespiratory conditions in C(3) plants enhances the expression of genes associated with cyclic electron flow pathways in Arabidopsis thaliana, consistent with the higher ATP requirement (relative to NADPH) of photorespiration.Journal of Experimental Botany 02/2012; 63(4):1637-61. · 5.24 Impact Factor
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ABSTRACT: Chlamydomonas raudensis Ettl UWO241, a natural variant of C. raudensis, is deficient in state transitions. Its habitat, the deepest layer of Lake Bonney in Antarctica, features low irradiance, low temperature, and high salinity. Although psychrophily and low-light acclimation of this green alga has been described, very little information is available on the effect of salinity. Here, we demonstrate that this psychrophile is halotolerant, not halophilic, and it shows energy redistribution between photosystem I and II based on energy spillover under low-salt conditions. Furthermore, we revealed that C. raudensis exhibits higher non-photochemical quenching in comparison with the mesophile Chlamydomonas reinhardtii, when grown with low-salt, which is due to the lower proton conductivity across the thylakoid membrane. Significance of the C. raudensis UWO241 traits found in the low salinity culture are implicated with their natural habitats, including the high salinity and extremely stable light environments.Photosynthesis Research 02/2009; 99(3):195-203. · 3.15 Impact Factor