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Activation of Cyclic Electron Flow by Hydrogen Peroxide in Vivo

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Activation of Cyclic Electron Flow by Hydrogen Peroxide in Vivo Activation of cyclic electron flow by hydrogen peroxide in vivo Deserah D. Stranda,b, Aaron K. Livingstonc,d,1, Mio Satoh-Cruza, John E. Froehlicha,e, Veronica G. Maurinof, and David M. Kramera,b,e,2 aPlant Research Laboratory and Departments of bPlant Biology and eBiochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48824; cSchool of Molecular Biosciences and dInstitute for Biological Chemistry, Washington State University, Pullman, WA 99164; and fInstitute of Developmental and Molecular Biology of Plants, Plant Molecular Physiology and Biotechnology Group, Heinrich-Heine-Universität, Cluster of Excellence on Plant Sciences (CEPLAS), 40225 Düsseldorf, Germany Edited by Bob B. Buchanan, University of California, Berkeley, CA, and approved March 9, 2015 (received for review September 23, 2014) Cyclic electron flow (CEF) around photosystem I is thought to most important and complicated of these pathways is cyclic elec- balance the ATP/NADPH energy budget of photosynthesis, re- tron flow around photosystem I (CEF), in which electron flow quiring that its rate be finely regulated. The mechanisms of this from the acceptor side of PSI is shunted back into the PQ pool, regulation are not well understood. We observed that mutants generating additional pmf that can power ATP production without that exhibited constitutively high rates of CEF also showed net NADPH production. There are several proposed CEF path- elevated production of H2O2. We thus tested the hypothesis that ways that may operate under different conditions or in different CEF can be activated by H2O2 in vivo. CEF was strongly increased species (reviewed in refs. 2 and 3). In higher-plant chloroplasts, by H2O2 both by infiltration or in situ production by chloroplast- the most studied routes of CEF are the antimycin A-sensitive localized glycolate oxidase, implying that H2O2 can activate CEF pathway, which involves a complex of two CEF-related proteins, either directly by redox modulation of key enzymes, or indirectly PGR5 (Proton Gradient Regulation 5) and PGRL1 (PGR5-like 1), by affecting other photosynthetic processes. CEF appeared with a directly reducing the quinone pool (4–7), the respiratory complex half time of about 20 min after exposure to H2O2, suggesting activa- I analog, the NADPH dehydrogenase (NDH) complex (8–10), tion of previously expressed CEF-related machinery. H2O2-dependent which oxidizes Fd or NAD(P)H to reduce plastoquinone (8, 11), CEF was not sensitive to antimycin A or loss of PGR5, indicating that PLANT BIOLOGY and through the Qi site of b6f (12, 13). Different CEF mechanisms increased CEF probably does not involve the PGR5-PGRL1 associated seem to operate in other species. In Chlamydomonas, for example, pathway. In contrast, the rise in CEF was not observed in a mutant CEF seems to be conducted by a supercomplex of PSI, b6f,andthe deficient in the chloroplast NADPH:PQ reductase (NDH), supporting PGRL1 protein (14, 15), and the involvement of PGR5 has recently the involvement of this complex in CEF activated by H O .Wepro- 2 2 been described as important for CEF under hypoxia (16, 17). pose that H O is a missing link between environmental stress, me- 2 2 Regardless of the mechanism of CEF, the overall process must tabolism, and redox regulation of CEF in higher plants. bewellregulatedtoproperlybalancetheproductionofATPto match the demands of metabolism. The mechanism of this regu- cyclic electron flow | photosynthesis | reactive oxygen species | stress | lation is not known, but many general models have been proposed. hydrogen peroxide Perhaps the most widely cited regulatory model is the antenna state transition, which was previously shown to be correlated with n oxygenic photosynthesis, linear electron flow (LEF) is the activation of CEF in Chlamydomonas reinhardtii (14, 18) and favor Iprocess by which light energy is captured to drive the extraction – the formation of the PSI b6f supercomplex (14). However, it was of electrons and protons from water and transfer them through a recently shown that state transitions are not required for CEF system of electron carriers to reduce NADPH. LEF is coupled to activation, supporting models that include redox control (15–17, proton translocation into the thylakoid lumen, generating an 19–22). Other possible regulatory mechanisms include sensing of ðΔμ~ Þ electrochemical gradient of protons H + or proton motive ATP/ADP ratios (23, 24), the redox status of NAD(P)H or Fd force (pmf). The pmf drives the synthesis of ATP to power the (25), various CBB metabolic intermediates (reviewed in ref. 26), – – reactions of the Calvin Benson Bassham (CBB) cycle and other calcium signaling (15, 27), phosphorylation of CEF-related pro- essential metabolic processes in the chloroplast. The pmf is also a teins (27), and the reactive oxygen species H2O2 (26–29). key regulator of photosynthesis in that it activates the photo- protective qE response to dissipate excess light energy and down- Significance regulates electron transfer by controlling the rate of oxidation of plastoquinol at the cytochrome b6f complex (b6f), thus preventing the buildup of reduced intermediates (1, 2). Cyclic electron flow around photosystem I (CEF) is critical for LEF results in the transfer or deposition into the lumen of balancing the energy budget of photosynthesis, but its regu- lation is not well understood. Our results provide evidence that three protons for each electron transferred through PSII, plas- hydrogen peroxide, which is produced as a result of imbalances toquinone (PQ), b f, plastocyanin, and photosystem I (PSI) to 6 in chloroplast redox state, acts as a signaling agent to activate ferredoxin (Fd). The synthesis of one ATP is thought to require CEF in higher plants in vivo. 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 Author contributions: D.D.S., A.K.L., M.S.-C., J.E.F., V.G.M., and D.M.K. designed research; ratio is too low to sustain the CBB cycle or supply ATP required D.D.S., A.K.L., M.S.-C., and J.E.F. performed research; D.D.S., A.K.L., M.S.-C., and J.E.F. for translation, protein transport, or other ATP-dependent pro- analyzed data; and D.D.S., A.K.L., and D.M.K. wrote the paper. cesses (3). In addition, the relative demands for ATP and NADPH The authors declare no conflict of interest. can change dramatically depending on environmental, develop- This article is a PNAS Direct Submission. mental, and other factors, leading to rapid energy imbalances that Freely available online through the PNAS open access option. require dynamical regulation of ATP/NADPH balance. 1Present address: Department of Biology, Portland Community College, Portland, OR 97219. Several alternative electron flow pathways in the chloroplast 2To whom correspondence should be addressed. Email: [email protected]. have been proposed to augment ATP production, thus balancing This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. the ATP/NADPH budget of the chloroplast (2, 3). Perhaps the 1073/pnas.1418223112/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1418223112 PNAS Early Edition | 1of6 Downloaded by guest on September 25, 2021 One possibility is that CEF may be at least partly regulated by H2O2 (26), which is produced by the light reactions of photo- synthesis and already known to regulate other cellular processes such as plant growth, development, and defense (30–32). Based on in vitro studies, it was previously proposed that H2O2 could activate CEF or chlororespiration by modifying the NDH com- plex (27). It has also been shown that H2O2 can increase the expression of the NDH complex (29) and may further affect the accumulation of photosynthetic metabolites, indirectly activating CEF (26). Consistent with this possibility, H2O2 is a well-docu- mented signaling molecule (33), possibly through its ability to oxidize thiols (34, 35). Furthermore, H2O2 is expected to be produced under many conditions that initiate CEF [e.g., under a deficit of ATP, when electrons should accumulate in the PSI acceptor pools, leading to superoxide production that can be converted to H O by superoxide dismutase (36)]. Fig. 1. The hcef1 mutant shows elevated H2O2 accumulation. Leaf H2O2 2 2 content was determined using the Amplex Red assays and expressed as This study aims to test the hypothesis that CEF can be initi- resorufin fluorescence against a standard curve. Results were normalized to ated in vivo by H O using a combination of in vivo spectroscopy 2 2 the average Col-0 H2O2 content. Data are presented on a per-leaf area basis. and genetic modifications to selectively and rapidly initiate H2O2 Mean ± SD, n = 3. (Inset) Qualitative H2O2 accumulation measured by DAB production in the chloroplast. staining of representative leaves of Col-0 (A) and hcef1 (B). Both assays were performed on leaves from fully mature rosettes as described in Livingston Results et al. (37). H2O2 Accumulation in the High-CEF Mutant hcef1. The Arabidopsis mutant hcef1 (37), which is deficient in chloroplastic fructose 1,6-bisphosphase (FBPase) activity and displays constitutively by the decay kinetics of the electrochomic shift (ECS) signal (43, 45). In GO5, gH+ was ∼30% lower than in Col-0 (Fig. S2E), high CEF rates, was tested for increased H2O2 accumulation. When measured by the Amplex Red assay (Fig. 1) hcef1 showed implying that, although the ATP synthase activity was somewhat decreased in the mutant, it could not by itself explain the large three times as much H2O2 as the wild-type Columbia-0 (Col-0) (2.99 ± 0.17 and 1.00 ± 0.13, respectively, n = 3, P > 0.001, increase in light-induced pmf, suggesting that CEF contributes Student’s t test). This result was confirmed by staining leaves with substantially to pmf in GO5 (see discussion in refs.
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