The GENOMES UNCOUPLED4 (GUN4) protein stimulates chlorophyll biosynthesis by activating Mg-chelatase, the enzyme that commits protoporphyrin IX to chlorophyll biosynthesis. This stimulation depends on GUN4 binding the ChlH subunit of Mg-chelatase and the porphyrin substrate and product of Mg-chelatase. After binding porphyrins, GUN4 associates more stably with chloroplast membranes and was proposed to promote interactions between ChlH and chloroplast membranesthe site of Mg-chelatase activity. GUN4 was also proposed to attenuate the production of reactive oxygen species (ROS) by binding and shielding light-exposed porphyrins from collisions with O 2 . To test these proposals, we first engineered Arabidopsis thaliana plants that express only porphyrin binding-deficient forms of GUN4. Using these transgenic plants and particular mutants, we found that the porphyrin binding activity of GUN4 and Mg-chelatase contribute to the accumulation of chlorophyll, GUN4, and Mg-chelatase subunits. Also, we found that the porphyrin binding activity of GUN4 and Mgchelatase affect the associations of GUN4 and ChlH with chloroplast membranes and have various effects on the expression of ROS-inducible genes. Based on our findings, we conclude that ChlH and GUN4 use distinct mechanisms to associate with chloroplast membranes and that mutant alleles of GUN4 and Mg-chelatase genes cause sensitivity to intense light by a mechanism that is potentially complex.

Cyclic electron flow (CEF) around photosystem I is thought to balance the ATP/NADPH energy budget of photosynthesis, requiring that its rate be finely regulated. The mechanisms of this regulation are not well understood. We observed that mutants that exhibited constitutively high rates of CEF also showed elevated production of H 2 O 2 . We thus tested the hypothesis that CEF can be activated by H 2 O 2 in vivo. CEF was strongly increased by H 2 O 2 both by infiltration or in situ production by chloroplastlocalized glycolate oxidase, implying that H 2 O 2 can activate CEF either directly by redox modulation of key enzymes, or indirectly by affecting other photosynthetic processes. CEF appeared with a half time of about 20 min after exposure to H 2 O 2 , suggesting activation of previously expressed CEF-related machinery. H 2 O 2 -dependent CEF was not sensitive to antimycin A or loss of PGR5, indicating that increased CEF probably does not involve the PGR5-PGRL1 associated pathway. In contrast, the rise in CEF was not observed in a mutant deficient in the chloroplast NADPH:PQ reductase (NDH), supporting the involvement of this complex in CEF activated by H 2 O 2 . We propose that H 2 O 2 is a missing link between environmental stress, metabolism, and redox regulation of CEF in higher plants. I n oxygenic photosynthesis, linear electron flow (LEF) is the process by which light energy is captured to drive the extraction of electrons and protons from water and transfer them through a system of electron carriers to reduce NADPH. LEF is coupled to proton translocation into the thylakoid lumen, generating an electrochemical gradient of protons ðΔμ H +Þ or proton motive force (pmf). The pmf drives the synthesis of ATP to power the reactions of the Calvin-Benson-Bassham (CBB) cycle and other essential metabolic processes in the chloroplast. The pmf is also a key regulator of photosynthesis in that it activates the photoprotective q E response to dissipate excess light energy and downregulates electron transfer by controlling the rate of oxidation of plastoquinol at the cytochrome b 6 f complex (b 6 f), thus preventing the buildup of reduced intermediates (1, 2).LEF results in the transfer or deposition into the lumen of three protons for each electron transferred through PSII, plastoquinone (PQ), b 6 f, plastocyanin, and photosystem I (PSI) to ferredoxin (Fd). The synthesis of one ATP is thought to require the passage of 4.67 protons through the ATP synthase, so that LEF should produce a ratio of ATP/NADPH of about 1.33; this ratio is too low to sustain the CBB cycle or supply ATP required for translation, protein transport, or other ATP-dependent processes (3). In addition, the relative demands for ATP and NADPH can change dramatically depending on environmental, developmental, and other factors, leading to rapid energy imbalances that require dynamical regulation of ATP/NADPH balance.Several alternative electron flow pathways in the chloroplast have been proposed to augment ATP production, thus balancing the ATP/NADPH ...

Crop canopies create environments of highly fluctuating light intensities. In such environments, photoprotective mechanisms and their relaxation kinetics have been hypothesized to limit photosynthetic efficiency and therefore crop yield potential. Here, we show that overexpression of the Arabidopsis thylakoid K+/H+ antiporter KEA3 accelerates the relaxation of photoprotective energy-dependent quenching after transitions from high to low light in Arabidopsis and tobacco. This, in turn, enhances PSII quantum efficiency in both organisms, supporting that in wild-type plants, residual light energy quenching following a high to low light transition represents a limitation to photosynthetic efficiency in fluctuating light. This finding underscores the potential of accelerating quenching relaxation as a building block for improving photosynthetic efficiency in the field. Additionally, by overexpressing natural KEA3 variants with modification to the C-terminus, we show that KEA3 activity is regulated by a mechanism involving its lumen-localized C-terminus, which lowers KEA3 activity in high light. This regulatory mechanism fine-tunes the balance between photoprotective energy dissipation in high light and maximum quantum yield in low light, likely to be critical for efficient photosynthesis in fluctuating light conditions.

Cyclic electron flow around photosystem I (CEF) is critical for balancing the photosynthetic energy budget of the chloroplast by generating ATP without net production of NADPH. We demonstrate that the chloroplast NADPH dehydrogenase complex, a homolog to respiratory Complex I, pumps approximately two protons from the chloroplast stroma to the lumen per electron transferred from ferredoxin to plastoquinone, effectively increasing the efficiency of ATP production via CEF by 2-fold compared with CEF pathways involving non-proton-pumping plastoquinone reductases. By virtue of this proton-pumping stoichiometry, we hypothesize that NADPH dehydrogenase not only efficiently contributes to ATP production but operates near thermodynamic reversibility, with potentially important consequences for remediating mismatches in the thylakoid energy budget.

In wild type plants, decreasing CO2 lowers the activity of the chloroplast ATP synthase, slowing proton efflux from the thylakoid lumen resulting in buildup of thylakoid proton motive force (pmf). The resulting acidification of the lumen regulates both light harvesting, via the qE mechanism, and photosynthetic electron transfer through the cytochrome b6f complex. Here, we show that the cfq mutant of Arabidopsis, harboring single point mutation in its γ-subunit of the chloroplast ATP synthase, increases the specific activity of the ATP synthase and disables its down-regulation under low CO2. The increased thylakoid proton conductivity (gH+) in cfq results in decreased pmf and lumen acidification, preventing full activation of qE and more rapid electron transfer through the b6f complex, particularly under low CO2 and fluctuating light. These conditions favor the accumulation of electrons on the acceptor side of PSI, and result in severe loss of PSI activity. Comparing the current results with previous work on the pgr5 mutant suggests a general mechanism where increased PSI photodamage in both mutants is caused by loss of pmf, rather than inhibition of CEF per se. Overall, our results support a critical role for ATP synthase regulation in maintaining photosynthetic control of electron transfer to prevent photodamage.

Plants use sunlight as their primary energy source. During photosynthesis, absorbed light energy generates reducing power by driving electron transfer reactions. These are coupled to the transfer of protons into the thylakoid lumen, generating a proton motive force (pmf) required for ATP synthesis. Sudden alterations in light availability have to be met by regulatory mechanisms to avoid the over-accumulation of reactive intermediates and maximize energy efficiency. Here, the acidification of the lumen, as an intermediate product of photosynthesis, plays an important role by regulating photosynthesis in response to excitation energy levels. Recent findings reveal pmf regulation and the modulation of its composition as key determinants for efficient photosynthesis, plant growth, and survival in fluctuating light environments.

Photosynthesis captures light energy to produce ATP and NADPH. These molecules are consumed in the conversion of CO 2 to sugar, photorespiration, and NO 3 2 assimilation. The production and consumption of ATP and NADPH must be balanced to prevent photoinhibition or photodamage. This balancing may occur via cyclic electron flow around photosystem I (CEF), which increases ATP/NADPH production during photosynthetic electron transport; however, it is not clear under what conditions CEF changes with ATP/NADPH demand. Measurements of chlorophyll fluorescence and dark interval relaxation kinetics were used to determine the contribution of CEF in balancing ATP/NADPH in hydroponically grown Arabidopsis (Arabidopsis thaliana) supplied different forms of nitrogen (nitrate versus ammonium) under changes in atmospheric CO 2 and oxygen. Measurements of CEF were made under low and high light and compared with ATP/NADPH demand estimated from CO 2 gas exchange. Under low light, contributions of CEF did not shift despite an up to 17% change in modeled ATP/NADPH demand. Under high light, CEF increased under photorespiratory conditions (high oxygen and low CO 2 ), consistent with a primary role in energy balancing. However, nitrogen form had little impact on rates of CEF under high or low light. We conclude that, according to modeled ATP/ NADPH demand, CEF responded to energy demand under high light but not low light. These findings suggest that other mechanisms, such as the malate valve and the Mehler reaction, were able to maintain energy balance when electron flow was low but that CEF was required under higher flow.Photosynthesis must balance both the amount of energy harvested by the light reactions and how it is stored to match metabolic demands. Light energy is harvested by the photosynthetic antenna complexes and stored by the electron and proton transfer complexes as ATP and NADPH. It is used primarily to meet the energy demands for assimilating carbon (from CO 2 ) and nitrogen (from NO 3 2 and NH 4 + ; Keeling et al., 1976;Edwards and Walker, 1983;Miller et al., 2007). These processes require different ratios of ATP and NADPH, requiring a finely balanced output of energy in these forms. For example, if ATP were to be consumed at a greater rate than NADPH, electron transport would rapidly become limiting by the lack of NADP + , decreasing rates of proton translocation and ATP regeneration. Alternatively, if NADPH were consumed faster than ATP, proton translocation through ATP synthase would be reduced due to limiting ADP and the difference in pH between lumen and stroma would increase, restricting plastoquinol oxidation at the cytochrome b 6 f complex and initiating nonphotochemical quenching (Kanazawa and Kramer, 2002). The stoichiometric balancing of ATP and NADPH must occur rapidly, because pool sizes of ATP and NADPH are relatively small and fluxes through primary metabolism are large (Noctor and Foyer, 2000;Avenson et al., 2005;Cruz et al., 2005;Amthor, 2010).The balancing of ATP and NADPH supply is further complicated...

The chloroplast must regulate supply of reducing equivalents and ATP to meet rapid changes in downstream metabolic demands. Cyclic electron flow around photosystem I (CEF) is proposed to balance the ATP/NADPH budget by using reducing equivalents to drive plastoquinone reduction, leading to the generation of proton motive force and subsequent ATP synthesis. While high rates of CEF have been observed in vivo, isolated thylakoids show only very slow rates, suggesting that the activity of a key complex is lost or down-regulated upon isolation. We show that isolation of thylakoids while in the continuous presence of reduced thiol reductant dithiothreitol (DTT), but not oxidized DTT, maintains high CEF activity through an antimycin A sensitive ferredoxin:quinone reductase (FQR). Maintaining low concentrations (~2 mM) of reduced DTT while modulating the concentration of oxidized DTT leads to reversible activation/inactivation of CEF with an apparent midpoint potential of -306 mV (±10 mV) and n=2, consistent with redox modulation of a thiol/disulfide couple and thioredoxin-mediated regulation of the plastoquinone reductase involved in the antimycin A-sensitive pathway, possibly at the level of the PGRL1 protein. Based on proposed differences in regulatory modes, we propose that the FQR and NADPH:plastoquinone oxidoreductase (NDH) pathways for CEF are activated under different conditions and fulfill different roles in chloroplast energy balance.