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Cyclic electron flow around photosystem I is essential for photosynthesis
... accel rys. com) electrons return from PSI to the intersystem ETC via the illusive ferredoxin-plastoquinone reductase (FQR) (Bendall and Manasse 1995;Joliot and Joliot 2002;Munekage et al. 2002Munekage et al. , 2004Munekage et al. , 2008Puthiyaveetil et al. 2016); this route of electron transfer may be termed as a 'short' cycle (CEF1). There are good reasons to believe, based on biochemical experiments, that the illusive FQR can be identified with the electron transfer mediators PGR5 and PGRL-1 bound to PSI (DalCorso et al. 2008). ...
... At the Q i center, according to modified Q cycle, the PQ molecule accepts one electron from Cyt b 6 and serves as the recipient of the second electron coming from the acceptor side of PSI through the cyclic electron transfer chain: PSI → Fd → FQR → c n → (PQ) i . Here, FQR and c n denote the illusive Fd-quinone reductase (Munekage et al. 2004) and c-type cytochrome (Kurisu et al. 2003;Stroebel et al. 2003), respectively. The fully reduced PQH 2 molecule dissociates from Q i and then can bind to the vacant center Q o . ...
... The unique heme c n exists in the Cytb 6 f, but it is absent in the Cytbc 1 complexes. According to the modified Q-cycle, heme c n may participate in cyclic route of electron flow around PSI, mediating the reduction of PQ bound to Q i (Kurisu et al. 2003;Stroebel et al. 2003;Munekage et al. 2004;Shikanai 2007). ...
In oxygenic photosynthetic systems, the cytochrome b6f (Cytb6f) complex (plastoquinol:plastocyanin oxidoreductase) is a heart of the hub that provides connectivity between photosystems (PS) II and I. In this review, the structure and function of the Cytb6f complex are briefly outlined, being focused on the mechanisms of a bifurcated (two-electron) oxidation of plastoquinol (PQH2). In plant chloroplasts, under a wide range of experimental conditions (pH and temperature), a diffusion of PQH2 from PSII to the Cytb6f does not limit the intersystem electron transport. The overall rate of PQH2 turnover is determined mainly by the first step of the bifurcated oxidation of PQH2 at the catalytic site Qo, i.e., the reaction of electron transfer from PQH2 to the Fe2S2 cluster of the high-potential Rieske iron–sulfur protein (ISP). This point has been supported by the quantum chemical analysis of PQH2 oxidation within the framework of a model system including the Fe2S2 cluster of the ISP and surrounding amino acids, the low-potential heme b6L, Glu78 and 2,3,5-trimethylbenzoquinol (the tail-less analog of PQH2). Other structure–function relationships and mechanisms of electron transport regulation of oxygenic photosynthesis associated with the Cytb6f complex are briefly outlined: pH-dependent control of the intersystem electron transport and the regulatory balance between the operation of linear and cyclic electron transfer chains.
... The inter-play between long-distance assimilate movement via the phloem (Münch, 1930) and the movement of H 2 O within the xylem via transpiration (Dixon and Joy, 1894) is still poorly understood (Windt et al., 2006;Nikinmaa et al., 2013). Although studies with woody species using MRI or theoretical modeling have suggested that transpiration rate affects export (Windt et al., 2006;Nikinmaa et al., 2013), there has been very little experimental data linking H 2 O and CO 2 exchanges with the mobility of H 2 O and assimilates via translocation using intact herbaceous plants (Johnson et al., 1992). ...
... The increased relative export during the morning hours of the medium photosynthetic rate experiment under B and RB could be due to activation of CRY. This could induce cyclic electron flow (Figure 8-1; Finazzi et al., 2002;Munekage et al., 2004;Shapiguzov et al., 2010) increasing ATP production needed in apoplastic phloem loading or expression levels of important enzymes and transporters facilitating export (Figures 8-2, 8-5, 8-6). ...
... Interestingly, O wavelengths are major spectral components provided by high pressure sodium luminaries that have been utilized in research and commercial controlled environments. The high absorption of B and O light by pigments and molecules associated with the photosystems (e.g., carotenoids), are involved in energy trapping and therefore may increase cyclic energy transduction facilitating phloem loading and export in the light (Figure 8-1; Finazzi et al., 2002;Munekage et al., 2004;Shapiguzov et al., 2010). ...
Translocation of assimilates is a fundamental process involving carbon and water balance affecting source/sink relationships. Diurnal patterns of CO2 exchange, translocation (carbon export), and transpiration of an intact tomato source leaf were determined during ¹⁴CO2 steady-state labeling under different wavelengths at three pre-set photosynthetic rates. Daily patterns showed that photosynthesis and export were supported by all wavelengths of light tested including orange and green. Export in the light, under all wavelengths was always higher than that at night. Export in the light varied from 65–83% of the total daily carbon fixed, depending on light intensity. Photosynthesis and export were highly correlated under all wavelengths (r = 0.90–0.96). Export as a percentage of photosynthesis (relative export) decreased as photosynthesis increased by increasing light intensity under all wavelengths. These data indicate an upper limit for export under all spectral conditions. Interestingly, only at the medium photosynthetic rate, relative export under the blue and the orange light-emitting diodes (LEDs) were higher than under white and red-white LEDs. Stomatal conductance, transpiration rates, and water-use-efficiency showed similar daily patterns under all wavelengths. Illuminating tomato leaves with different spectral quality resulted in similar carbon export rates, but stomatal conductance and transpiration rates varied due to wavelength specific control of stomatal function. Thus, we caution that the link between transpiration and C-export may be more complex than previously thought. In summary, these data indicate that orange and green LEDs, not simply the traditionally used red and blue LEDs, should be considered and tested when designing lighting systems for optimizing source leaf strength during plant production in controlled environment systems. In addition, knowledge related to the interplay between water and C-movement within a plant and how they are affected by environmental stimuli, is needed to develop a better understanding of source/sink relationships.
... CET cycles electrons from the photosystem I (PSI) acceptor side back to plastoquinone (PQ), cytb 6 f and plastocyanin (PC) thereby forming Δ pH and ATP without net NADPH production, modulating the ATP/NADPH ratio to meet metabolic demand 5,6 . CET plays an important role during induction; it allows electrons to be recycled since nicotinamide adenine dinucleotide phosphate (NADP + ) is limiting and thereby allows Δ pH generation for downregulation of PSII by non-photochemical quenching (NPQ), protection of PSI from overreduction by slowing the turnover of the cytb 6 f complex and for the additional ATP synthesis required for autocatalysis [7][8][9] . ...
... major contributor to CET in Arabidopsis and spinach, and mutants lacking either protein show lower Δ pH levels, reduced electron transfer capacity in moderate and high light and increased susceptibility to photo-oxidative stress particularly in fluctuating light [7][8][9] . Debate continues as to the exact role played by PGR5/PGRL1 in CET. ...
... Debate continues as to the exact role played by PGR5/PGRL1 in CET. In one view, PGR5/PGRL1 is suggested to be an antimycin A-sensitive ferredoxin-PQ oxidoreductase 8 . Alternatively, it has been proposed that PGR5/PGRL1 may influence CET and indeed LET via regulation of the cytb 6 f and ATP synthase complexes 7,10 . ...
Upon transition of plants from darkness to light the initiation of photosynthetic linear electron transfer (LET) from H2O to NADP+ precedes the activation of CO2 fixation, creating a lag period where cyclic electron transfer (CET) around photosystem I (PSI) has an important protective role. CET generates ΔpH without net reduced NADPH formation, preventing overreduction of PSI via regulation of the cytochrome b 6 f (cytb 6 f) complex and protecting PSII from overexcitation by inducing non-photochemical quenching. The dark-to-light transition also provokes increased phosphorylation of light-harvesting complex II (LHCII). However, the relationship between LHCII phosphorylation and regulation of the LET/CET balance is not understood. Here, we show that the dark-to-light changes in LHCII phosphorylation profoundly alter thylakoid membrane architecture and the macromolecular organization of the photosynthetic complexes, without significantly affecting the antenna size of either photosystem. The grana diameter and number of membrane layers per grana are decreased in the light while the number of grana per chloroplast is increased, creating a larger contact area between grana and stromal lamellae. We show that these changes in thylakoid stacking regulate the balance between LET and CET pathways. Smaller grana promote more efficient LET by reducing the diffusion distance for the mobile electron carriers plastoquinone and plastocyanin, whereas larger grana enhance the partition of the granal and stromal lamellae plastoquinone pools, enhancing the efficiency of CET and thus photoprotection by non-photochemical quenching.
... Therefore, we hypothesize that CEF plays a crucial role in protecting PSI against photoinhibition at low temperature stress via Cyt b 6 /f-dependent " photosynthetic control ". The CEF-dependent generation of pmf has two main roles: one is linked to ATP synthesis and balances ATP/NADPH ratio (Avenson et al., 2005;Yamori et al., 2011Yamori et al., , 2015Nishikawa et al., 2012;Walker et al., 2014;Huang et al., 2015), the other is dependent on lumen acidification and favors photoprotection for PSI and PSII (Munekage et al., 2002Munekage et al., , 2004Takahashi et al., 2009;Huang et al., 2011Huang et al., , 2012Huang et al., , 2016cHuang et al., , 2017Suorsa et al., 2012Suorsa et al., , 2016Tikkanen et al., , 2015Chaux et al., 2015;). However, the plasticity in roles of CEF at contrasting temperatures is little known. ...
... Meanwhile, values for Y(I) under all light intensities were significantly higher at 25 °C when compared with 4 °C (Fig. 1A). With the increase in light intensity, the fraction of P700 that is in the oxidized state relative to the overall P700 [Y(ND)] gradually increased (Fig. 1B), as expected from previous results reported in wild-type plants (Munekage et al., 2002Munekage et al., , 2004Kono et al., 2014). Furthermore, under all light intensities, values for Y(ND) were significantly higher at 4 °C than 25 °C, indicating that more P700 is in the oxidized state at 4 °C. ...
... The same value of CEF led to a much higher NPQ at 4 °C. The induction of NPQ mainly relies on the PGR5-CEF-dependent generation of ΔpH (Munekage et al., 2002Munekage et al., , 2004Takahashi et al., 2009;Kono et al., 2014;Sato et al., 2014), the value of NPQ is regarded as an indicator of the ΔpH level. ...
... Με ηδκ ελαίνεζδ ηάπμζςκ εκδείλεςκ πενί ηδξ ζφκδεζδξ ηδξ ηοηθζηήξ νμήξ δθεηηνμκίςκ ιε ηδκ ηαεήθςζδ άκεναηα (Heber et al., 1978; Furbank & Horton, 1987), ιζα πθδεχνα πεζναιαηζηχκ δεδμιέκςκ οπμδείηκοε πςξ δ ηοηθζηή νμή είκαζ ζπεηζηά αζήιακηδ, απμοζία ηαηαπμκήζεςκ ηαζ πςξ ζπεδυκ υθμ ημ ATP πμο πνεζάγεηαζ βζα ηδκ ακαβςβή ημο CO 2 πανάβεηαζ απυ ηδ βναιιζηή νμή (Genty et al., 1990; Harbinson et al., 1990; Herbert et al., 1990; Bendall & Manasse, 1995). Νευηενεξ ιεθέηεξ ςζηυζμ έπμοκ ηαηαδείλεζ υηζ, οπυ μπμζεζδήπμηε ζοκεήηεξ, δ ηοηθζηή νμή δθεηηνμκίςκ είκαζ απαναίηδηδ βζα ηδκ μιαθή ακάπηολδ αθθά ηαζ ηδ νφειζζδ ηδξ βναιιζηήξ νμήξ δθεηηνμκίςκ ηςκ C 3 θοηχκ (Munekage et al., 2004). Ο πνχημξ νυθμξ πμο πνμηάεδηε βζα ηδκ ηοηθζηή νμή δθεηηνμκίςκ είκαζ αοηυξ ηδξ θςημπνμζηαζίαξ (Heber & Walker, 1992; Bendall & Manasse, 1995; Niyogi, 2000). ...
... Πένα υιςξ απυ ηδκ πνυζθαηδ απμιυκςζδ εκυξ πανυιμζμο οπενζοιπθυημο ζηδκ Chlamydomonas reinhardtii (Iwai et al., 2010) δεκ οπάνπμοκ ακαημιζηά δεδμιέκα πμο κα ιπμνμφκ κα ζηδνίλμοκ ημ ιμκηέθμ αοηυ ζηδκ πενίπηςζδ ηςκ ακχηενςκ θοηχκ (Dekker & Boekema, 2005). Ακηίεεηα, πθήεμξ δεδμιέκςκ ζοβηθίκμοκ πνμξ ηδκ άπμρδ πςξ δ ηοηθζηή νμή δθεηηνμκίςκ βφνς απυ ημ PSI πναβιαημπμζείηαζ ηονίςξ ιέζς δφμ πανάθθδθςκ μδχκ, ηαηά ηζξ μπμίεξ έπμοιε ακαβςβή ηδξ πθαζημηζκυκδξ είηε απυ ηδκ Fd ιε ηδ δζαιεζμθάαδζδ ιζαξ ιδ ακαβκςνζζιέκδξ ακαβςβάζδξ ηδξ θεννεδμλίκδξπθαζημηζκυκδξ (FQR) (Arnon et al., 1954; Tagawa et al., 1963; Heber et al., 1978; Miyake et al., 1995; Munekage et al., 2002 Munekage et al., , 2004, πήια 4) είηε απυ ημ NADPH ιε ηδ δζαιεζμθάαδζδ ηδξ αθοδνμβμκάζδξ ημο NADPH (NDH) (Burrows et al., 1998; Shikanai et al., 1998; Joët et al., 2001; Munekage et al., 2004; Munné-Bosch et al., 2005; Johnson, 2005(Sazanov et al., 1995), ηάηζ πμο υπςξ θαίκεηαζ ακηακαηθά ηαζ ζηδ παιδθή απυδμζδ ακαβςβήξ ηδξ NDH απυ δθεηηνυκζα ημο PSI (Munekage et al., 2004). Δπζπθέμκ, μζ ανκδηζηέξ επζπηχζεζξ ηδξ απεκενβμπμίδζδξ ηςκ βμκζδίςκ πμο ηςδζημπμζμφκ ηδκ NDH ζηδκ ζηακυηδηα θςημπνμζηαζίαξ εκυξ θοημφ, είηε είκαζ αιεθδηέεξ, είηε εηδδθχκμκηαζ ηάης απυ ζζπονυηενα ηαηαπμκδηζηέξ ζοκεήηεξ ζε ζφβηνζζδ ιε ηζξ ακηίζημζπεξ επζπηχζεζξ πμο έπεζ δ δζαημπή ηδξ Fd-ελανηχιεκδξ ηοηθζηήξ νμήξ (Munekage et al., 2002Munekage et al., , 2004 Munné-Bosch et al., 2005). ...
... Πένα υιςξ απυ ηδκ πνυζθαηδ απμιυκςζδ εκυξ πανυιμζμο οπενζοιπθυημο ζηδκ Chlamydomonas reinhardtii (Iwai et al., 2010) δεκ οπάνπμοκ ακαημιζηά δεδμιέκα πμο κα ιπμνμφκ κα ζηδνίλμοκ ημ ιμκηέθμ αοηυ ζηδκ πενίπηςζδ ηςκ ακχηενςκ θοηχκ (Dekker & Boekema, 2005). Ακηίεεηα, πθήεμξ δεδμιέκςκ ζοβηθίκμοκ πνμξ ηδκ άπμρδ πςξ δ ηοηθζηή νμή δθεηηνμκίςκ βφνς απυ ημ PSI πναβιαημπμζείηαζ ηονίςξ ιέζς δφμ πανάθθδθςκ μδχκ, ηαηά ηζξ μπμίεξ έπμοιε ακαβςβή ηδξ πθαζημηζκυκδξ είηε απυ ηδκ Fd ιε ηδ δζαιεζμθάαδζδ ιζαξ ιδ ακαβκςνζζιέκδξ ακαβςβάζδξ ηδξ θεννεδμλίκδξπθαζημηζκυκδξ (FQR) (Arnon et al., 1954; Tagawa et al., 1963; Heber et al., 1978; Miyake et al., 1995; Munekage et al., 2002 Munekage et al., , 2004, πήια 4) είηε απυ ημ NADPH ιε ηδ δζαιεζμθάαδζδ ηδξ αθοδνμβμκάζδξ ημο NADPH (NDH) (Burrows et al., 1998; Shikanai et al., 1998; Joët et al., 2001; Munekage et al., 2004; Munné-Bosch et al., 2005; Johnson, 2005(Sazanov et al., 1995), ηάηζ πμο υπςξ θαίκεηαζ ακηακαηθά ηαζ ζηδ παιδθή απυδμζδ ακαβςβήξ ηδξ NDH απυ δθεηηνυκζα ημο PSI (Munekage et al., 2004). Δπζπθέμκ, μζ ανκδηζηέξ επζπηχζεζξ ηδξ απεκενβμπμίδζδξ ηςκ βμκζδίςκ πμο ηςδζημπμζμφκ ηδκ NDH ζηδκ ζηακυηδηα θςημπνμζηαζίαξ εκυξ θοημφ, είηε είκαζ αιεθδηέεξ, είηε εηδδθχκμκηαζ ηάης απυ ζζπονυηενα ηαηαπμκδηζηέξ ζοκεήηεξ ζε ζφβηνζζδ ιε ηζξ ακηίζημζπεξ επζπηχζεζξ πμο έπεζ δ δζαημπή ηδξ Fd-ελανηχιεκδξ ηοηθζηήξ νμήξ (Munekage et al., 2002Munekage et al., , 2004 Munné-Bosch et al., 2005). ...
Despite its significant contribution to the net carbon gain of plants and its distinct functional properties, stem photosynthesis has not yet received adequate scientific attention. For this reason, a combination of anatomical and physiological methods was used to characterize the photosynthetic machinery of the green petioles and pedicels of the monocotyledonous geophyte Zantedeschia aethiopica and the green stems of the dicotyledonous semi-woody species Dianthus caryophyllus, in comparison to the corresponding leaves. Both the green petioles/pedicels of Z. aethiopica and the green stems of D. caryophyllus possess all the anatomical prerequisites of an actively photosynthesizing organ i.e. considerable number of stomata with typical underlying substomatal chambers, chlorenchyma cells which are similar to the leaf palisade chlorenchyma cells and considerable amount of both intercellular spaces and palisade free cell walls. Yet, the palisade cells of Z. aethiopica petioles/pedicels show a peculiar arrangement with their long axis parallel to the longitudinal organ axis. Furthermore, petiole/pedicel photosynthetic characteristics resemble those of leaves under adversity i.e. reduced Rubisco activity/content, high photorespiration rates and significant cyclic electron flow around PSI. It is concluded that these are innate attributes of petiole/pedicel photosynthesis serving particular functions like the increased nitrogen fixing activity of the species, the qualitative adjustment of the petiole/pedicel amino acid content, the active decarboxylation of C4-organic acids and the rapid induction of non-photochemical quenching.
Stem photosynthesis in D. caryophyllus was more efficient than leaf photosynthesis, as a result of the greater rates of stem C3 cycle and a possible organ-specific variation of the specificity factor of Rubisco. In general, D. caryophyllus stems display a photosynthetic pattern of optimal carbon assimilation in the expense of photoprotection. It could be hypothesized that this kind of adaptation could be due to the vertical orientation of stems, which results in lower incident light intensities in vivo and may include the use of C4-organic acids coming up with the transpiration stream as an additional carbon source.
... Cyclic electron transport around PSI supplements ATP synthesis in photosynthesis (Shikanai 2007). In angiosperms, PSI cyclic electron transport consists of two partly redundant pathways (Munekage et al. 2004). The main pathway depends on PROTON GRADIENT REGULATION 5 (PGR5)/PGR5-like Photosynthetic Phenotype 1 (PGRL1) proteins (Munekage et al. 2002, DalCorso et al. 2008) and is sensitive to antimycin A (Hertle et al. 2013, Sugimoto et al. 2013). ...
... An alternative, antimycin A-insensitive, cyclic route of electrons depends on the chloroplast NADH dehydrogenaselike (NDH) complex (Shikanai et al. 1998, Hashimoto et al. 2003). Both pathways contribute to the formation of proton motive force and consequently ATP synthesis (Munekage et al. 2004, Wang et al. 2015). The existence of chloroplast NDH was originally noticed upon the discovery of 11 ndh genes in the plastid genomes of tobacco (Nicotiana tabacum) and liverwort (Marchantia polymorpha). ...
... Recently, the higher order of association of the PSI supercomplex with the core NDH?PSI supercomplex has been reported (Yadav et al. 2016). Even the mutants completely defective in the accumulation of chloroplast NDH did not show any mutant phenotypes in the plant growth in Arabidopsis (Munekage et al. 2004). In the mutant background of pgr5 defective in the main pathway of PSI cyclic electron transport, however, the supercomplex formation via Lhca6 was required for the normal plant growth (Peng et al. 2009). ...
The light-harvesting complex I (LHCI) proteins in Arabidopsis thaliana are encoded by six genes. Major LHCI proteins (Lhca1–Lhca4) harvest light energy and transfer the resulting excitation energy to the PSI core by forming a PSI supercomplex. In contrast, the minor LHCI proteins Lhca5 and Lhca6 contribute to supercomplex formation between the PSI supercomplex and the chloroplast NADH dehydrogenase-like (NDH) complex, although Lhca5 is also solely associated with the PSI supercomplex. Lhca6 was branched from Lhca2 during the evolution of land plants. In this study, we focused on the molecular evolution involved in the transition from a major LHCI, Lhca2, to the linker protein Lhca6. To elucidate the domains of Lhca6 responsible for linker function, we systematically swapped domains between the two LHCI proteins. To overcome problems due to the low stability of chimeric proteins, we employed sensitive methods to evaluate supercomplex formation: we monitored NDH activity by using Chl fluorescence analysis and detected NDH–PSI supercomplex formation by using protein blot analysis in the form of two-dimensional blue-native (BN)/SDS–PAGE. The stromal loop of Lhca6 was shown to be necessary and sufficient for linker function. Chimeric Lhca6, in which the stromal loop was substituted by that of Lhca2, was not functional as a linker and was detected at the position of the PSI supercomplex in the BN–polyacrylamide gel. The stromal loop of Lhca6 is likely to be necessary for the interaction with chloroplast NDH, rather than for the association with the PSI supercomplex.
... Because light fluctuation is a potent stress factor for PSI (, Sejima et al. 2014, Yamori et al. 2016), plants should have mechanisms to cope with the light fluctuation. The cyclic electron flow around PSI (CEF-PSI;Strand et al. 2016), the NADH dehydrogenase-like complex-dependent pathway (NDH-mediated CEF;Shikanai et al. 1998, Shikanai 2016) and the PGR5–PGRL1-mediated pathway (PGR5-mediated CEF;Munekage et al. 2002, Munekage et al. 2004, Hertle et al. 2013, Labs et al. 2016) have been proposed to protect PSI from the fluctuating growth light in A. thaliana (Suorsa et al. 2012) and in rice (Yamori et al. 2016). The results of the experiments employing short-term FL-treatments indicated that the PGR5-mediated CEF-PSI and the water–water cycle (WWC), also known as the Mehler–ascorbate peroxidase pathway (Asada 1999), were essential for protection of PSI (, Allahverdiyeva et al. 2015, Kono and Terashima 2014). ...
... For A. thaliana, however, it was reported that even the complete disruption of plastid ndh genes did not affect photosynthesis (Hashimoto et al. 2003). It was also shown that the phenotype of an NDH-deficient mutant, pgr5?crr4-3, of A. thaliana was apparent only when PGR5 was absent (Munekage et al. 2004). These findings imply a minor role for the NDH-mediated CEF-PSI. ...
... These findings imply a minor role for the NDH-mediated CEF-PSI. It has actually been suggested that, for C 3 plants in general, the PGR5-mediated CEF-PSI, rather than the NDH-mediated CEF-PSI, is the main player (Munekage et al. 2004, Okegawa et al. 2008, Wang et al. 2015). We also suggested the major role of the PGR5-mediated CEF-PSI in protection against FL-induced PSI photoinhibition (, Kono and Terashima 2016). ...
It has been reported that PSI photoinhibition is induced even in wild-type plants of Arabidopsis thaliana, rice and other species by exposure of leaves to fluctuating light (FL) for a few hours. Because plants are exposed to FL in nature, they must possess protective mechanisms against the FL-induced photodamage. Here, using A. thaliana grown at various irradiances, we examined PSI photoprotection by far-red (FR) light at intensities comparable with those observed in nature. Dark-treated leaves were illuminated by red FL alternating high/low light at 1,200/30 µmol m(-2) s(-1) for 800 ms/10 s. By this FL treatment without FR light for 120 min, the level of photo-oxidizable P700 was decreased by 30% even in the plants grown at high irradiances. The addition of continuous FR light during the FL suppressed this damage almost completely. With FR light, P700 was kept in a more oxidized state in both low- and high-light phases. The protective effect of FR light was diminished more in mutants of the NADH dehydrogenase-like complex (NDH)-mediated cyclic electron flow around PSI (CEF-PSI) than in the PGR5 (proton gradient regulation 5)-mediated CEF-PSI, indicating that the NDH-mediated CEF-PSI would be a major contributor to PSI photoprotection in the presence of FR light. We also confirmed that PSI photoinhibition decreased with the increase in growth irradiance in A. thaliana and field-grown plants, and that this PSI photodamage was largely suppressed by addition of FR light. These results clearly indicate that the most effective PSI protection is realized in the presence of FR light.
... Another specific fluorescence transient is the so-called post-illumination fluorescence rise, which is manifested as a transient decrease of the fluorescence yield after switching off the actinic light followed by its recovery in the dark (Shikanai et al. 1998). This effect is related to the backflow of electrons from stromal electron carriers to the PQ pool in the dark, but not related to NPQ phenomena (Munekage et al. 2004), and reflects the NDH-induced reduction of the PQ pool in the dark (Yamori et al. 2015). A related fluorescence wave phenomenon also occurs after single turnover flash illumination in cyanobacteria (Deák et al. 2014) and various microalgae (Krishna et al. 2019, Mohammad Aslam et al. 2022, Patil et al. 2022a under anaerobic conditions, which arises from transient oxidation of the PQ pool due to more efficient electron extraction by PSI than electron inflow from PSII, followed by re-reduction of the PQ pool from stromal components via the NDH1 (cyanobacteria) or NDH2 complexes (green algae). ...
... Although the post-illumination fluorescence rise could not be unequivocally identified at single-cell level, the low-wave phenomenon was visible in individual green cells after a saturation pulse in the dark following the actinic illumination [in agreement with previous studies obtained in bulk cultures (Larcher and Neuner 1989, Xyländer and Hagen 2002, Tsuyama et al. 2004, Fratamico et al. 2016], but not in red cells (Figs. 1,2). In contrast to the post-illumination fluorescence rise, which is related to NDH-mediated reduction of the PQ pool (Munekage et al. 2004, Yamori et al. 2015, the low wave phenomenon is assigned to transient NPQ due to transthylakoid proton gradient formation via cyclic electron flow around PSI (Fratamico et al. 2016). Therefore, we checked the effect of nigericin that eliminates the proton gradient. ...
... In addition to energizing ATP synthesis, lumen acidification depending on the formation of proton motive force also triggers the thermal dissipation of excessively absorbed light energy from PSII 14 and down-regulates the activity of the cytochrome b 6 f (cyt b 6 f) complex to prevent overloading of the electrons toward PSI 15 . In angiosperms, PSI cyclic electron flow consists of two pathways depending on PROTON GRADIENT REGULATION 5 (PGR5)/PGR5-like Photosynthetic Phenotype 1 (PGRL1) proteins and the NDH complex [16][17][18] . In Arabidopsis thaliana (Arabidopsis) mutants deficient in NDH activity, the size of the proton motive force is slightly lower than in wild-type (WT) plants 19 . ...
... Despite the contribution of the chloroplast NDH to proton motive force being smaller than PGR5/PGRL1, the contribution of the chloroplast NDH complex appears under low-light conditions 20,21 , fluctuating light intensity 22 , at low temperatures 23 , and during induction of photosynthesis 24 . Double mutants that are defective in both of these cyclic electron flow pathways demonstrate a severely lowgrowth phenotype, indicating PSI cyclic electron flow to be required for efficient photosynthesis 16 . ...
Chloroplast NADH dehydrogenase-like (NDH) complex is structurally related to mitochondrial Complex I and forms a supercomplex with two copies of Photosystem I (the NDH-PSI supercomplex) via linker proteins Lhca5 and Lhca6. The latter was acquired relatively recently in a common ancestor of angiosperms. Here we show that NDH-dependent Cyclic Electron Flow 5 (NDF5) is an NDH assembly factor in Arabidopsis. NDF5 initiates the assembly of NDH subunits (PnsB2 and PnsB3) and Lhca6, suggesting that they form a contact site with Lhca6. Our analysis of the NDF5 ortholog in Physcomitrella and angiosperm genomes reveals the subunit PnsB2 to be newly acquired via tandem gene duplication of NDF5 at some point in the evolution of angiosperms. Another Lhca6 contact subunit, PnsB3, has evolved from a protein unrelated to NDH. The structure of the largest photosynthetic electron transport chain complex has become more complicated by acquiring novel subunits and supercomplex formation with PSI.
... The CBB requires 1.5 ATP/NADPH, which given the 4.67 H + /ATP ratio inferred by the structure of the chloroplast ATP synthase indicates a ATP shortfall of roughly 0.32 ATP/NADPH from LET alone 71,72 . The situation is complicated by the multitude of other metabolic process in the stroma consuming ATP and NADPH in different ratios 71 ; however, the flexibility in the provision of ATP relative to NADPH provided by CET appears to be crucial to plant fitness 73 . Thus, the high Y(NA) phenotype of stn7 under LL and tap38 under HL could be explained by a shortfall in ATP due to lower CET that in turn leads to a reduced CBB activity. ...
... Here we provide further evidence for this view showing that under HL ΔETR(I) is higher in both stn7 and WT, where grana are larger relative to tap38. It is striking that the symptoms of tap38 in HL are a milder version of those reported for pgr5 mutant, which is compromised in the main CET pathway 73 . Similar to tap38, pgr5 shows lower LET and lower ΔETR(I) in HL, together with reduced ΔpH, lower Y(ND), increased Y(NA) and PSI photoinhibition 22,58 . ...
TAP38/STN7-dependent (de)phosphorylation of light-harvesting complex II (LHCII) regulates the relative excitation rates of photosystems I and II (PSI, PSII) (state transitions) and the size of the thylakoid grana stacks (dynamic thylakoid stacking). Yet, it remains unclear how changing grana size benefits photosynthesis and whether these two regulatory mechanisms function independently. Here, by comparing Arabidopsis wild-type, stn7 and tap38 plants with the psal mutant, which undergoes dynamic thylakoid stacking but lacks state transitions, we explain their distinct roles. Under low light, smaller grana increase the rate of PSI reduction and photosynthesis by reducing the diffusion distance for plastoquinol; however, this beneficial effect is only apparent when PSI/PSII excitation balance is maintained by state transitions or far-red light. Under high light, the larger grana slow plastoquinol diffusion and lower the equilibrium constant between plastocyanin and PSI, maximizing photosynthesis by avoiding PSI photoinhibition. Loss of state transitions in low light or maintenance of smaller grana in high light also both bring about a decrease in cyclic electron transfer and over-reduction of the PSI acceptor side. These results demonstrate that state transitions and dynamic thylakoid stacking work synergistically to regulate photosynthesis in variable light.
... However, some studies indicate that this mechanism does not contribute to a significant transport of electrons in several species such as grape (Flexas et al. 1999), tobacco (Ruuska et al. 2000), and tomato (Haupt-Herting and Fock 2002). CEF is considered a crucial mechanism for preventing PSI from excess light stress by alleviating the over-reduction of PSI acceptor side (Munekage et al. 2002(Munekage et al. , 2004(Munekage et al. , 2008Huang et al. 2011). Drought, with a subsequent lack of CO 2 for carbon fixation due to stomatal closure, is one of the environmental conditions triggering CEF in several plant species (Golding and Johnson 2003;Golding et al. 2004;Rumeau et al. 2007;Munekage et al. 2008;Kohzuma et al. 2009;Johnson 2011). ...
... Similar results were shown by Hernández-Fuentes et al. (2015), where CEF increased under natural drought conditions in the field at low elevations. CEF has been documented as a crucial mechanism for preventing PSI from excess light stress through alleviating the over-reduction of PSI acceptor side (Munekage et al. 2002(Munekage et al. , 2004(Munekage et al. , 2008Huang et al. 2011). In higher plants, there are at least two pathways for CEF that may operate in parallel or differentially in different tissues or metabolic states (Strand et al. 2015); one of these pathway involves an antimycin A-sensitive PQ reductase (Moss and Bendall 1984;Shikanai 2014). ...
The central Chilean Andes are located in a Mediterranean-type climate zone, characterized by dry summers and high irradiance. This creates a contrasting elevational gradient because higher elevations get more solid precipitation and lower temperatures, resulting in higher soil humidity along the growing season compared with severe drought at lower elevations. Therefore, species with wide elevational distributions, such as Phacelia secunda, must have developed specific adaptations to cope with contrasting severity of drought stress-induced photoinhibition at different elevations. We hypothesize that P. secunda from lower elevation, is more tolerant to drought stress-induced photo-damage than plants from high elevation. This higher tolerance will be associated to a higher diversity of photoprotective strategies in plants that naturally suffers severe drought every growing season. To test this hypothesis, plants from 2700 and 3600 m in the central Chilean Andes were grown under the common garden and then subjected to water restriction. We measured stress indicators, photochemistry of PSII and PSI and estimate alternative electron sinks. Drought affected P. secunda photosynthetic performance differentially depending on the elevation of provenance. Plants from lower elevation exhibited higher drought tolerance than higher elevation ones. This was related to higher levels of heat dissipation and alternative electron sinks exhibited by plants from lower elevation under drought stress. We concluded that plants naturally subjected to recurrent drought are better adapted to respond to drought stress using additional photochemical photoprotective mechanisms and confirm the role of alternative electron sinks ameliorating photodamage.
... This is induced by acidification of the thylakoid lumen (Müller et al. 2001). It has been shown that CEF can sustain a large proton gradient across the thylakoid membrane under chilling stress (Munekage et al. 2004). In the present study, we observed that CEF significantly increased when night temperatures were low (i.e., at 10 or 4°C). ...
... This could lead to NADPH accumulation and PSI photoinhibition as described above. Previous studies have indicated that CEF can protect PSI against chilling stress by alleviating overreduction of the PSI acceptor side and by balancing the NADP + /NADPH ratio (Munekage et al. 2002(Munekage et al. , 2004Yamori and Shikanai 2016). In our study, YNA under moderate and high PAR was maintained at a low level after exposure to night chilling at 10 or 4°C. ...
The objective of this experiment was to study the effects of night chilling on the photosynthetic characteristics of cotton (Gossypium hirsutum L.) at a boll-forming stage. The results suggest that overreduction of PSII after night chilling (≤ 10°C) led to excess excitation energy in cotton leaves. The night chilling (compared to 22°C) reduced PSI acceptor side limitation under moderate and high light intensity and increased maximum photooxidizable P700. This suggests that in contrast to PSII, PSI was protected from photoinhibition due to night chilling. However, PSII activity and linear electron transport were not significantly affected by the 30/16°C treatment. In addition, the night chilling (≤ 10°C) increased the quantum yield of cyclic electron transport. This suggests that cyclic electron transport around PSI might be important to prevent photoinhibition of PSI and PSII in cotton under night chilling stress.
... NPQ induction requires a trans-thylakoid proton gradient (DpH) (Munekage et al. 2004;Ruban 2016), which is accelerated by cyclic electron flow (CEF) around PSI (Wang et al. 2006;Takahashi et al. 2009;Shikanai 2014). When CEF activity was inhibited, high light and high temperature inhibited DpH and NPQ severely in tomato (Solanum lycopersicum L.) (Lu et al. 2017). ...
... The over-reduction of the PSI acceptor side could result in the PSI photoinhibition (Munekage et al. 2002). In this study, Y(NA) did not increase in the combined stress, indicating electrons were transported from the PSI acceptor side to plastoquinone though CEF (Munekage et al. 2004). Besides, the level of Y(ND) increased, indicating that the stressed conditions induced a larger fraction of oxidised P700 (Morgan-Kiss et al. 2002;Agrawal et al. 2016). ...
Changes in cyclic electron flow (CEF) around PSI activity after exposing grape (Vitis vinifera L.) seedling leaves to the combined stress of high temperature (HT) and high light (HL) were investigated. The PSII potential quantum efficiency (Fv/Fm) decreased significantly under exposure to HT, and this decrease was greater when HT was combined with HL, whereas the PSI activity maintained stable. HT enhanced CEF mediated by NAD(P)H dehydrogenase remarkably. Compared with the control leaves, the half-Time of P700⁺ re-reduction decreased during the HT treatment∼ this decrease was even more pronounced under the combined stress, implying significantly enhanced CEF as a result of the treatment. However, the heat-induced increase in nonphotochemical quenching (NPQ) was greater under HL, accompanied by a greater enhancement in high-energy state quenching. These results suggest that the combined stress of HT and HL resulted in severe PSII photoinhibition, whereas CEF showed plasticity in its response to environmental stress and played an important role in PSII and PSI photoprotection through accelerating generation of the thylakoid proton gradient and the induction of NPQ.
... PSI and PSII together with the light harvesting complex (LHC) and cytochrome b 6 /f (Cytb 6 f) complex mediate linear electron transport across the thylakoid membrane leading to the reduction of NADP + to NADPH. Except of these four major complexes, there are also other protein components in the thylakoid membrane, such as the PROTON GRADIENT REGULATION 5 (PGR5) complex, PGR5-LIKE1 (PGRL1) complex and NAD(P)H dehydrogenase (NDH) complex, which mediate cyclic electron transport (Kramer et al. 2004;Munekage et al. 2004;Shikanai 2016). Both linear and cyclic electron transport pathway are coupled to the translocation of protons across the thylakoid membrane, which leads to the generation of a transmembrane ΔpH gradient utilized by ATP synthase to produce ATP (Albertsson 2001;Nelson and Yocum 2006). ...
... Over the last years it became obvious that PSI is undoubtedly a key protein in the regulation of electron flow. As PSI is involved in linear electron flow and two types of cyclic electron flow, it is important for modulation of the ATP:NADPH production and for the prevention of photo-oxidative damage under stress conditions (Burrows et al. 1998;Shikanai et al. 1998;Munekage et al. 2004). PSI uses light energy to mediate electron transfer from plastocyanin to ferredoxin, which then transports electrons from PSI to FNR in linear electron flow or to either Cytb 6 f complex or NDH in cyclic electron flow. ...
In nature, plants are continuously exposed to varying environmental conditions. They have developed a wide range of adaptive mechanisms, which ensure their survival and maintenance of stable photosynthetic performance. Photosynthesis is delicately regulated at the level of the thylakoid membrane of chloroplasts and the regulatory mechanisms include a reversible formation of a large variety of specific protein-protein complexes, supercomplexes or even larger assemblies known as megacomplexes. Revealing their structures is crucial for better understanding of their function and relevance in photosynthesis. Here we focus our attention on the isolation and a structural characterization of various large protein supercomplexes and megacomplexes, which involve Photosystem II and Photosystem I, the key constituents of photosynthetic apparatus. The photosystems are often attached to other protein complexes in thylakoid membranes such as light harvesting complexes, cytochrome b 6 f complex, and NAD(P)H dehydrogenase. Structural models of individual supercomplexes and megacomplexes provide essential details of their architecture, which allow us to discuss their function as well as physiological significance.
... The rpo genes are relatively fast-evolving sequences that have been used as markers in phylogenetic studies (Krawczyk and Sawicki 2013). In addition, a photosynthesis-related gene, ndhB, was discovered in S. baicalensis and S. dorogostaiskii, which is associated with photosystem I to form a super-complex that mediates cyclic electron transport (Munekage et al. 2004). ...
Four species of Saussurea, namely S. involucrata, S. orgaadayi, S. bogedaensis, and S. dorogostaiskii, are known as the “snow lotus,” which are used as traditional medicines in China (Xinjiang), Kyrgyzstan, Mongolia, and Russia (Southern Siberia). These species are threatened globally, because of illegal harvesting and climate change. Furthermore, the taxonomic classification and identification of these threatened species remain unclear owing to limited research. The misidentification of medicinal species can sometimes be harmful to health. Therefore, the phylogenetic and genomic features of these species need to be confirmed. In this study, we sequenced five complete chloroplast genomes and seven nuclear ITS regions of four snow lotus species and other Saussurea species. We further explored their genetic variety, selective pressure at the sequence level, and phylogenetic relationships using the chloroplast genome, nuclear partial DNA sequences, and morphological features. Plastome of the snow lotus species has a conserved structure and gene content similar to most Saussurea species. Two intergenic regions (ndhJ–ndhK and ndhD-psaC) show significantly high diversity among chloroplast regions. Thus, ITS and these markers are suitable for identifying snow lotus species. In addition, we characterized 43 simple sequence repeats that may be useful in future population genetic studies. Analysis of the selection signatures identified three genes (rpoA, ndhB, and ycf2) that underwent positive selection. These genes may play important roles in the adaptation of the snow lotus species to alpine environments. S. dorogostaiskii is close to S. baicalensis and exhibits slightly different adaptation from others. The taxonomic position of the snow lotus species, confirmed by morphological and molecular evidence, is as follows: (i) S. involucrata has been excluded from the Mongolian flora due to misidentification as S. orgaadayi or S. bogedaensis for a long time; (ii) S. dorogostaiskii belongs to section Pycnocephala subgenus Saussurea, whereas other the snow lotus species belong to section Amphilaena subgenus Amphilaena; and (iii) S. krasnoborovii is synonymous of S. dorogostaiskii. This study clarified the speciation and lineage diversification of the snow lotus species in Central Asia and Southern Siberia.
... According to Kawashima et al., the contribution of the PGR5 and NDH pathways to ΔpH formation was experimentally estimated at 30% and 5%, respectively 12 . Although the critical function of both the complexes in CET remains under debate, studies have suggested that PGR5/PGRL1 is involved in LET function through the regulation of the cytochrome (Cyt) b 6 f complex and ATP synthetase complex under moderate light conditions in an Antimycin A-sensitive manner [13][14][15] . ...
Cyclic electron transfer (CET) predominates when NADP⁺ is at basal levels, early in photosynthetic induction; however, the mechanism underlying the subsequent supply of NADP⁺ to fully drive steady-state linear electron transfer remains unclear. Here, we investigated whether CET is involved in de novo NADP⁺ supply in Arabidopsis thaliana and measured chloroplastic NADP dynamics to evaluate responsiveness to variable light, photochemical inhibitors, darkness, and CET activity. The sum of oxidized and reduced forms shows that levels of NADP and NAD increase and decrease, respectively, in response to light; levels of NADP and NAD decrease and increase in the dark, respectively. Moreover, consistent with the pH change in the stroma, the pH preference of chloroplast NAD⁺ phosphorylation and NADP⁺ dephosphorylation is alkaline and weakly acidic, respectively. Furthermore, CET is correlated with upregulation of light-responsive NADP level increases and downregulation of dark-responsive NADP level reductions. These findings are consistent with CET helping to regulate NADP pool size via stromal pH regulation under fluctuating light conditions.
... On a physiological level, the most determining impact of NC2 mutation was the enhanced cyclic electron transport activities, which might be the synergistic results from enhanced PQ supply 54,55 , up-regulated NDH complex, and reduced PSII activities 56,57 . CEF plays an essential role in balancing the ATP/NADPH ratio (promoting the supply of additional ATP required by the high-energy consuming process, e.g., protein synthesis and materials transport) and preventing over-reduction damage of PSI and PSII in plants and algae for both short-term acclimation and long-term adaptation [58][59][60] . Previously, changes in shikimate pathway genes and related metabolites were observed in photoautotrophs with regulated CEF activities. ...
Photosynthesis can be impaired by combined high light and high temperature (HLHT) stress. Obtaining HLHT tolerant photoautotrophs is laborious and time-consuming, and in most cases the underlying molecular mechanisms remain unclear. Here, we increase the mutation rates of cyanobacterium Synechococcus elongatus PCC 7942 by three orders of magnitude through combinatory perturbations of the genetic fidelity machinery and cultivation environment. Utilizing the hypermutation system, we isolate Synechococcus mutants with improved HLHT tolerance and identify genome mutations contributing to the adaptation process. A specific mutation located in the upstream non-coding region of the gene encoding a shikimate kinase results in enhanced expression of this gene. Overexpression of the shikimate kinase encoding gene in both Synechococcus and Synechocystis leads to improved HLHT tolerance. Transcriptome analysis indicates that the mutation remodels the photosynthetic chain and metabolism network in Synechococcus. Thus, mutations identified by the hypermutation system are useful for engineering cyanobacteria with improved HLHT tolerance. Cyanobacteria mutants with improved tolerance to combined high light and high temperature (HLHT) are rarely reported. Here, the authors use a hypermutation system for adaptive laboratory evolution and identify a mutant with improved HLHT tolerance by enhancing expression of shikimate kinase.
... Elimination of PGR5-dependent CEF in AntA-treated maize discs led to the nearly complete prevention of F0' increase, i.e., over-reduction of intersystem electron transport chain in cold-stressed plants (Savitch et al. 2011). PGR5-dependent CEF is dominant in C3 plants, especially under stressful conditions, such as drought, high light, or low CO2, and is essential for inducing qE, thus determining its significance for photoprotection and preventing PSI from photodamage (Munekage et al. 2002(Munekage et al. , 2004DalCorso et al. 2008). ...
Haberlea rhodopensis Friv. is unique with its ability to survive desiccation to an air-dry state during periods of extreme drought and freezing temperatures. To understand its survival strategies, it is important to examine the protective mechanisms not only during desiccation but also during rehydration. We investigated the involvement of alternative cyclic electron pathways during the recovery of photosynthetic functions after freezing-induced desiccation. Using electron transport inhibitors, the role of PGR5-dependent and NDH-dependent PSI-cyclic electron flows and plastid terminal oxidase were assessed during rehydration of desiccated leaves. Recovery of PSII and PSI, the capacity of PSI-driven cyclic electron flow, the redox state of plastoquinone pool, and the intersystem electron pool were analyzed. Data showed that the effect of alternative flows is more pronounced in the first hours of rehydration. In addition, the NDH-dependent cyclic pathway played a more determining role in the recovery of PSI than in the recovery of PSII.
... In CET, electrons are transported via PSI to ferredoxin (Fd) and back to the plastoquinone (PQ) pool and PSI complex, generating a proton gradient across the thylakoid membrane for ATP synthesis without NADPH production. PSI-mediated CET is essential for balancing the changing demands for ATP/NADPH under various physiological conditions 4 and for regulating photosynthetic machineries in response to various environmental conditions [1][2][3]5,6 . ...
Chloroplast NADH dehydrogenase-like (NDH) complex is composed of at least 29 subunits and plays an important role in mediating photosystem I (PSI) cyclic electron transport (CET)1–3. It associates with PSI to form the PSI-NDH supercomplex to fulfill its function. Here we report cryo-electron microscopy structure of a PSI-NDH supercomplex from barley (Hordeum vulgare) at an overall resolution of 4.4 Å and local resolutions of 3.40 Å–3.88 Å for the PSI-LHCI and NDH sub-complexes. The result reveals that PSI-NDH is composed of two copies of PSI-LHCI and one NDH complex. Two monomeric LHCI proteins, Lhca5 and Lhca6, mediate the binding of two PSI complexes to NDH. Ten plant chloroplast specific NDH subunits are observed and their exact positions as well as their interactions with other subunits in NDH are elucidated. Taken together, this study provides a structural basis for further investigations on the functions and regulation of the PSI-NDH-dependent CET.
... for CO2 fixation 5,6 . In addition to its role in cyanobacteria, PS-CI is also important in specific cell types that are common to some crop plants and critical for crop yield 7,8 . ...
Photosynthesis and respiration rely upon a proton gradient to produce ATP. In photosynthesis, the Respiratory Complex I homologue, Photosynthetic Complex I (PS CI) is proposed to couple ferredoxin oxidation and plastoquinone reduction to proton pumping across thylakoid membranes, and is fundamental to bioenergetics in photosynthetic bacteria and some higher plant cell types. However, little is known about the PS CI molecular mechanism and attempts to understand its function have previously been frustrated by its large size and high lability. Here, we overcome these challenges by pushing the limits in sample size and spectroscopic sensitivity, to determine arguably the most important property of any electron transport enzyme, the reduction potentials of its cofactors, in this case the iron sulphur clusters of PS CI, and unambiguously assign them to the structure using double electron electron resonance (DEER). We have thus determined the bioenergetics of the electron transfer relay and provide insight into the mechanism of PS CI, laying the foundations for understanding of how this important bioenergetic complex functions.
... The latter is thought to be the primary CEF route in plants. PGR5/PGRL1 was discovered in mutagenized Arabidopsis plants that presented distinctly low ΔpH values across the thylakoid membrane and low steady-state qE values; thus, the name "proton gradient regulation" has been used 5,6 . Moreover, PGRL1 has been proposed to be the elusive ferredoxin (Fd)-PQ reductase in Arabidopsis 7 . ...
In plants and algae, PGR5-dependent cyclic electron flow (CEF) is an important regulator of acclimation to fluctuating environments, but how PGR5 participates in CEF is unclear. In this work, we analyzed two PGR5s in cucumber ( Cucumis sativus L.) under different conditions and found that CsPGR5a played the dominant role in PGR5-dependent CEF. The results of yeast two-hybrid, biomolecular fluorescence complementation (BiFC), blue native PAGE, and coimmunoprecipitation (CoIP) assays showed that PGR5a interacted with PetC, Lhcb3, and PsaH. Furthermore, the intensity of the interactions was dynamic during state transitions, and the abundance of PGR5 attached to cyt b 6 f decreased during the transition from state 1 to state 2, which revealed that the function of PGR5a is related to the state transition. We proposed that PGR5 is a small mobile protein that functions when attached to protein complexes.
Two PGR5s are present in some species of algae and higher plants, and CsPGR5a plays the dominant role in PGR5-dependent cyclic electron flow in cucumber. PGR5 is a small and mobile protein that functions when attached to protein complexes. In this study, the function of PGR5 was found to be partially related to the state transition.
... On the contrary, pgr5-containing plant lines showed a marked difference in ECS relaxation kinetics, revealing a significantly reduced (~30% drop) capability of pgr5, npq4-1 pgr5 and ΔSTeM in generating pmf, when acclimated to moderate-light conditions (50 μmol photons m −2 s −1 ). These findings are in line to what previously reported 22,23,28 . An identical analysis was performed after 4 h exposure to 500 μmol photons m −2 s −1 white light, and comparable differences between pgr5-containg plants and Col-0 were observed (Fig. S1A,B). ...
Although light is essential for photosynthesis, when in excess, it may damage the photosynthetic apparatus, leading to a phenomenon known as photoinhibition. Photoinhibition was thought as a light-induced damage to photosystem II; however, it is now clear that even photosystem I may become very vulnerable to light. One main characteristic of light induced damage to photosystem II (PSII) is the increased turnover of the reaction center protein, D1: when rate of degradation exceeds the rate of synthesis, loss of PSII activity is observed. With respect to photosystem I (PSI), an excess of electrons, instead of an excess of light, may be very dangerous. Plants possess a number of mechanisms able to prevent, or limit, such damages by safe thermal dissipation of light energy (non-photochemical quenching, NPQ), slowing-down of electron transfer through the intersystem transport chain (photosynthesis-control, PSC) in co-operation with the Proton Gradient Regulation (PGR) proteins, PGR5 and PGRL1, collectively called as short-term photoprotection mechanisms, and the redistribution of light between photosystems, called state transitions (responsible of fluorescence quenching at PSII, qT), is superimposed to these short term photoprotective mechanisms. In this manuscript we have generated a number of higher order mutants by crossing genotypes carrying defects in each of the short-term photoprotection mechanisms, with the final aim to obtain a direct comparison of their role and efficiency in photoprotection. We found that mutants carrying a defect in the ΔpH-dependent photosynthesis-control are characterized by photoinhibition of both photosystems, irrespectively of whether PSBS-dependent NPQ or state transitions defects were present or not in the same individual, demonstrating the primary role of PSC in photoprotection. Moreover, mutants with a limited capability to develop a strong PSBS-dependent NPQ, were characterized by a high turnover of the D1 protein and high values of Y(NO), which might reflect energy quenching processes occurring within the PSII reaction center.
... Among them, cyclic electron transfer around photosystem I (PSI CET) is an important mechanism that balances the ATP/ NADPH ratio required for the Calvin-Benson cycle and reduces the ROS production 4,5 . The NDH-1 complex is a key component of this pathway in most photosynthetic organisms and its inactivation leads to growth defects and even cell death under environmental stresses 6,7 . ...
NDH-1 is a key component of the cyclic-electron-transfer around photosystem I (PSI CET) pathway, an important antioxidant mechanism for efficient photosynthesis. Here, we report a 3.2-Å-resolution cryo-EM structure of the ferredoxin (Fd)-NDH-1L complex from the cyanobacterium Thermosynechococcus elongatus. The structure reveals three β-carotene and fifteen lipid molecules in the membrane arm of NDH-1L. Regulatory oxygenic photosynthesis-specific (OPS) subunits NdhV, NdhS and NdhO are close to the Fd-binding site whilst NdhL is adjacent to the plastoquinone (PQ) cavity, and they play different roles in PSI CET under high-light stress. NdhV assists in the binding of Fd to NDH-1L and accelerates PSI CET in response to short-term high-light exposure. In contrast, prolonged high-light irradiation switches on the expression and assembly of the NDH-1MS complex, which likely contains no NdhO to further accelerate PSI CET and reduce ROS production. We propose that this hierarchical mechanism is necessary for the survival of cyanobacteria in an aerobic environment. NDH-1 is a key component of the cyclic-electron-transfer around photosystem I pathway, an antioxidant mechanism for efficient photosynthesis. Here, authors report a cryo-EM structure of the ferredoxin (Fd)-NDH-1L complex from the cyanobacterium Thermosynechococcus elongatus.
... In addition, photosynthetic organisms that are exposed to environmental stress usually exhibit increased demand for ATP 15 , therefore, CEF is crucial for the quick response of phototrophs to the changing environment. Furthermore, CEF plays an essential role in photoprotection by generating a large transmembrane proton gradient, thereby inducing energy dependent quenching to dissipate excessively absorbed energy 9,[16][17][18][19] . ...
NAD(P)H dehydrogenase-like (NDH) complex NDH-1L of cyanobacteria plays a crucial role in cyclic electron flow (CEF) around photosystem I and respiration processes. NDH-1L couples the electron transport from ferredoxin (Fd) to plastoquinone (PQ) and proton pumping from cytoplasm to the lumen that drives the ATP production. NDH-1L-dependent CEF increases the ATP/NADPH ratio, and is therefore pivotal for oxygenic phototrophs to function under stress. Here we report two structures of NDH-1L from Thermosynechococcus elongatus BP-1, in complex with one Fd and an endogenous PQ, respectively. Our structures represent the complete model of cyanobacterial NDH-1L, revealing the binding manner of NDH-1L with Fd and PQ, as well as the structural elements crucial for proper functioning of the NDH-1L complex. Together, our data provides deep insights into the electron transport from Fd to PQ, and its coupling with proton translocation in NDH-1L. NAD(P)H dehydrogenase-like complex NDH-1L couples the electron transport from ferredoxin (Fd) to plastoquinone (PQ) and proton pumping from cytoplasm to the lumen. Here authors report two structures of NDH-1L from Thermosynechococcus elongatus BP-1, in complex with one Fd and an endogenous PQ, respectively.
... Once the formation of ΔpH under high light was impaired, the excess electrons from PSII can easily lead to over-reduction of PSI and thus cause PSI photoinhibition (Munekage et al. 2002(Munekage et al. , 2004Suorsa et al. 2012;Takagi et al. 2017). However, the decrease in ΔpH under moderate heat stress was not accompanied with stronger PSI photoinhibition under fluctuating light. ...
Moderate heat stress is usually accompanied with fluctuating light in summer. Although either heat stress or fluctuating light can cause photoinhibition of photosystems I and II (PSI and PSII), it is unclear whether moderate heat stress accelerate photoinhibition under fluctuating light. Here, we measured chlorophyll fluorescence, P700 redox state and the electrochromic shift signal under fluctuating light at 25°C and 42°C for tobacco leaves. We found that (1) the thylakoid proton conductance was significantly enhanced at 42°C, leading to a decline in trans-thylakoid proton gradient (ΔpH); (2) this low ΔpH at 42°C did not decrease donor-side limitation of PSI and thermal energy dissipation in PSII; (3) the activation of cyclic electron flow (CEF) around PSI was elevated at 42°C; and (4) the moderate heat stress did not accelerate photoinhibition of PSI and PSII under fluctuating light. These results strongly indicate that under moderate heat stress the stimulation of CEF protects PSI under fluctuating light in tobacco leaves.
... When energy absorbed by the photosynthetic system exceeds the rate of CO 2 assimilation, ROS is abundantly produced, making timely management of ROS levels essential. ROS levels in photosynthetic organisms can be controlled by avoiding excess production of ROS through mechanisms such as cyclic electron flow, non-photochemical quenching, and the water-water cycle (1)(2)(3) or scavenging ROS that are inevitably generated. For example, superoxide dismutase and catalase are universally distributed across species and can detoxify superoxide and hydrogen peroxide, respectively. ...
As an adaptation to periodic fluctuations of environmental light, photosynthetic organisms have evolved a circadian clock. The gene expression of ROS scavenging enzymes is regulated by circadian clock genes, which has been considered to help deal with the diurnal photogeneration of reactive oxygen species (ROS). Although a series of recent discoveries have suggested the correlation between circadian clock control and ROS scavenging mechanisms, circadian rhythms of ROS stress tolerance have not been experimentally demonstrated to date. In the present work, we constructed a novel assay using methyl viologen (MV) which generates ROS under light irradiation and experimentally verified the circadian rhythms of ROS stress tolerance in photosynthetic cells of cyanobacterium Synechococcus elongatus PCC7942, a standard model species for the investigation of circadian clock. Here, we report that ROS generated by MV treatment causes damage to stroma components and not to the photosynthetic electron transportation chain, leading to reduced cell viability. The degree of decrease in cell viability was dependent on the subjective time at which ROS stress was applied. Thus, ROS stress tolerance was shown to exhibit circadian rhythms. Notably, rhythms of ROS stress tolerance disappeared in mutant cells lacking the essential clock genes. Further, ROS stress tolerance showed irregular behaviors under irregular light/dark cycles that mismatched the subjective time. These results clearly show that the antioxidant ability in the stroma varies periodically under the control of clock genes. This is the first demonstration of ROS stress tolerance in cyanobacterial cells at a phenotypic level showing circadian oscillation.
... In addition to enabling the flux of electrons from the photosystem into other metabolic pathways, FNRs might also avert this efflux of electrons and mediate the return of Fd-bound electrons to the plastoquinone pool of the photosynthetic system. By doing so, FNRs contribute to the so-called cyclic electron flow, which is essential for balancing the ATP/NADPH ratio generated by the photosystems [389,[391][392][393][394]. ...
Hydrogen (H2) production by microbial dark fermentation has attracted widespread attention as a promising alternative to fossil fuels. However, a major known drawback of this method is that H2 production yields are still too low to make the process economically competitive. Therefore, to enhance the economics of the microbial dark fermentation it is necessary to improve the sugar-to-H2 yield. In this thesis, we set out to achieve a proof of principle that H2 yields can be increased by metabolic engineering of one of the current top H2-producing microbes (i.e. Thermococcus kodakarensis). By introducing a set of genes from a thermophilic bacterium into Thermococcus, we succeeded in producing more H2 from starch, and meanwhile improving the growth rate of the microbe. Our results show that higher H2 production rates can indeed be obtained, though future research is still required. Moreover, we also show that T. kodakarensis is polyploid, which means that it does not contain just a single, but multiple copies of its genome.
... For simplicity of notation, we will assume = = k 1 B in the following. In Fig. 1b,c we provide a schematic description of the working principles of the cytochrome b f 6 complex under the so called cyclic electron flow (CEF) conditions, a situation that turns out to be simpler to model and to simulate, while keeping all the interesting features of the Q-cycle mechanism [28][29][30][31] . Under CEF, electrons transferred on the P-side to PSI are reinjected into the b f 6 complex from the N-side, thus effectively moving in a closed loop contrary to the conventional Linear Electron Flow (LEF) picture, in which electrons are eventually loaded on a NADPH molecule. ...
The Q-cycle mechanism entering the electron and proton transport chain in oxygenic photosynthesis is an example of how biological processes can be efficiently investigated with elementary microscopic models. Here we address the problem of energy transport across the cellular membrane from an open quantum system theoretical perspective. We model the cytochrome b6f protein complex under cyclic electron flow conditions starting from a simplified kinetic model, which is hereby revisited in terms of a Markovian quantum master equation formulation and spin-boson Hamiltonian treatment. We apply this model to theoretically demonstrate an optimal thermodynamic efficiency of the Q-cycle around ambient and physiologically relevant temperature conditions. Furthermore, we determine the quantum yield of this complex biochemical process after setting the electrochemical potentials to values well established in the literature. The present work suggests that the theory of quantum open systems can successfully push forward our theoretical understanding of complex biological systems working close to the quantum/classical boundary.
... The rapid activation of NPQ largely dissipates the excess light energy absorbed during sunflecks (Tausz et al. 2005, Watling et al. 1997. The activation of NPQ is accompanied by a high proton gradient across the thylakoid membrane (ΔpH) (Munekage et al. 2002(Munekage et al. , 2004. This ΔpH-dependent quenching (qE) can prevent overreduction of the plastoquinone pool (Yamori 2016). ...
Chromolaena odorata is a noxious invasive perennial herb in tropics and subtropics throughout the world. However,
photosynthetic induction of this invader is not well understood. Here, we measured the induction of gas exchange and
chlorophyll fluorescence in young and mature leaves of Chromolaena odorata. During photosynthetic induction, the young
leaves exhibited higher biochemical and total limitations to photosynthesis than that of the mature leaves. Photosynthetic
induction in this invader was affected by a mix of biochemical and stomatal limitations. Under a strong photosynthetic
photon flux density, nonphotochemical quenching was rapidly activated to dissipate excessive light energy in both young
and mature leaves. Furthermore, the induction of photosynthetic electron flow was faster than that of net photosynthetic
rate for both young and mature leaves. The rapid activation of nonphotochemical quenching can dissipate excess light
energy and regulate photosynthetic electron flow during photosynthetic induction, especially in the young leaves.
... In this scenario larger grana would have an advantage in terms of more efficient CET compared to smaller grana but only when the redox pressure on the PQ pool from PSII is high. Larger grana are realised not only in high light, where CET may be important for regulation of qE and photosynthetic control, but also in darkness and shade induced by neighbouring plants (PSI excitation, Fig. 2) where the sudden increases in intensity from canopy sunflecks or the morning dark-light transition necessitate the rapid activation of photoprotective mechanisms [50,51,[95][96][97]. ...
The higher plant chloroplast thylakoid membrane system performs the light-dependent reactions of photosynthesis. These provide the ATP and NADPH required for the fixation of CO2 into biomass by the Calvin-Benson cycle and a range of other metabolic reactions in the stroma. Land plants are frequently challenged by fluctuations in their environment, such as light, nutrient and water availability, which can create a mismatch between the amounts of ATP and NADPH produced and the amounts required by the downstream metabolism. Left unchecked, such imbalances can lead to the production of reactive oxygen species that damage the plant and harm productivity. Fortunately, plants have evolved a complex range of regulatory processes to avoid or minimize such deleterious effects by controlling the efficiency of light harvesting and electron transfer in the thylakoid membrane. Generally the regulation of the light reactions has been studied and conceptualised at the microscopic level of protein-protein and protein-ligand interactions, however in recent years dynamic changes in the thylakoid macrostructure itself have been recognised to play a significant role in regulating light harvesting and electron transfer. Here we review the evidence for the involvement of macrostructural changes in photosynthetic regulation and review the techniques that brought this evidence to light.
... Moreover, the expression of petE2 and pgr5 was also upregulated. Both genes encode key players in the cyclic electron flow around PSI, which dissipates the ΔpH across the thylakoid [43][44][45] , and it is essential for the photoprotection of the thylakoids 46 . These transcriptomics data are consistent with those of other plants infected with powdery mildew, in which several photosynthesis-related genes were found to be upregulated 23,47,48 . ...
The cucurbit powdery mildew elicited by Podosphaera xanthii is one of the most important limiting factors in cucurbit production. Our knowledge of the genetic and molecular bases underlying the physiological processes governing this disease is very limited. We used RNA-sequencing to identify differentially expressed genes in leaves of Cucumis melo upon inoculation with P. xanthii, using RNA samples obtained at different time points during the early stages of infection and their corresponding uninfected controls. In parallel, melon plants were phenotypically characterized using imaging techniques. We found a high number of differentially expressed genes (DEGs) in infected plants, which allowed for the identification of many plant processes that were dysregulated by the infection. Among those, genes involved in photosynthesis and related processes were found to be upregulated, whereas genes involved in secondary metabolism pathways, such as phenylpropanoid biosynthesis, were downregulated. These changes in gene expression could be functionally validated by chlorophyll fluorescence imaging and blue-green fluorescence imaging analyses, which corroborated the alterations in photosynthetic activity and the suppression of phenolic compound biosynthesis. The powdery mildew disease in melon is a consequence of a complex and multifaceted process that involves the dysregulation of many plant pathways such as primary and secondary metabolism.
... CEF not only builds up ΔpH across thylakoid, which can activate NPQ and stabilize OEC, but also prevents PSI from excess light stress by alleviating the over-reduction of the PSI acceptor side (Huang et al., 2011;Munekage et al., 2002Munekage et al., , 2004Munekage et al., 2008;Yamori and Shikanai, 2016). Therefore, CEF can play an important role in protecting both PSI and PSII against drought stress (Huang et al., 2012). ...
The effect of drought on the prompt chlorophyll a fluorescence (PF) transient (OJIP), delayed chlorophyll a fluorescence (DF), modulated 820-nm reflection (MR), energy conversion efficiencies in photosystems (PS) I and II, and cyclic electron flow (CEF) activity in two maize hybrids with contrasting drought tolerance was investigated. Our aim was to identify the target site of drought stress on the photosynthetic electron transport chain and investigate the relevance of the CEF pathway to the drought tolerance of maize plants. The OJIP analysis showed that drought stress, depending on its duration, decreased FP, increased FJ, and induced a pronounced K-band and a positive L-band. Moreover, OJIP parameters, including PIABS, RC/CSO, TRO/ABS, and ETO/TRO, were significantly reduced. The DF analysis showed that the values of I1 and I2 in the induction curve and L1 and L2 derived from the decay curve decreased progressively with the duration of drought stress. The MR analysis showed that drought stress inactivated both the fast decrease and slow increase phases of the MR transient, resulting in a gradual decrease in both VPSI and VPSII-PSI. The energy conversion analysis showed that drought stress decreased the PSI photochemical quantum yield Y(I) and PSII photochemical quantum yield Y(II). Compared to the tolerant hybrid, the drought-induced changes in the sensitive hybrid were stronger and appeared at an earlier treatment stage. The CEF activity analysis showed that the CEF pathway under drought stress operated for a longer time in the tolerant hybrid than that in the sensitive hybrid. The above results indicate that drought stress damaged the donor and acceptor sides of PSII, the PSII reaction center and the acceptor side of PSI and decreased the efficiency of both PSI and PSII and the capacity of electron transfer. The CEF pathway might play an important role in the tolerance of the maize photosynthetic electron transport chain to drought stress.
... Nevertheless, it is possible that these AET occur at low light intensities as reported for other groups of microalgae to balance ATP production (Mullineaux, et al. 2004, Munekage et al. 2004, Yamori et al. 2015. ...
The ecological success and also the susceptibility of corals to bleaching have been attributed to their obligate relationship with Symbiodinium and its functional diversity. Deeper understanding of this symbiosis is essential to enable the existence of reef ecosystems. This thesis aimed to determine how the environment and the host exert control over the proteome of Symbiodinium and how this is related to the maintenance of symbiosis.
Firstly, it was explored how the proteome related to the photoacclimation strategies of Symbiodinium, and its regulation under different light environments. An initial assessment of the proteome regulation using shotgun proteomics over Symbiodinium type C1 cultured at two experimental photon fluxes displayed differences in proteins associated with light harvesting, electron transport, carbon fixation and protein modification. Subsequent two-dimensional gel electrophoresis (2-DE) analysis between three Symbiodinium types (A1, A13 and C1) cultured at the two previous photon fluxes revealed unique proteome differences across these types and between both experimental light treatments. Database searches of the isoelectric point (pI) and the molecular weight (MW) tagged these proteins also associated with the same processes matched by previous shotgun analysis over C1 type. These results are in agreement with previous hypothesis on different photoacclimation strategies displayed among different types of Symbiodinium. These strategies comprised differential proteomic modifications in the photosynthetic unit (PSU) of the experimental types of Symbiodinium to balance ATP production and achieve homeostasis at different photon flux environments.
The host control effect and regulation of the proteome and physiology of Symbiodinium ex- hospite was studied using two host release factors (HRFs) a free aminoacid mix (FAA) and the host tissue (HT) homogenate from the coral Pocillopora damicornis. To comprehend the relation between the proteome and physiology various parameters related with the photosynthate production and translocation of Symbiodinium were measured including: glucose released, excitation pressure over photosystem two (PSII), total protein, total glucose and chlorophyll a concentration in the presence and absence of the HRFs. The measured parameters showed variability among types. In general, the parameters directly related to photochemistry: Chl a and excitation pressure over PSII were controlled by photon flux and inversely correlated with each other. Protein and glucose also correlated with each other and were controlled by host factors although HT and FAA had an antagonistic effect of stimulation and inhibition of these parameters respectively. The principal component analysis (PCA) evidenced the existence of a general mechanism of glucose release which correlated with protein expression. The HRFs control over the proteome of Symbiodinium explored by SDS-PAGE analysis displayed different bands with differential expression among types and treatments. Some of these proteins may be related with the stimulation and inhibition of the mechanism of translocation of glucose that were found. Identification of these proteins would probably clarify the physiology behind the translocation of glucose from the Symbiodinium cell to its coral host.
The proteome response and physiology of Symbiodinium was assessed by the interaction of different light environments with different temperatures. Growth patterns and photochemical measurements confirmed unique acclimation processes to light and temperature conditions across the three Symbiodinium types. These acclimation strategies appeared regulated by the differential expression of proteins involved in photosynthesis, protein modification, energy metabolism and cell maintenance. This differential expression of the proteome related to these processes seemed associated with the activation of alternative electron transport pathways to balance ATP production, regulate energy metabolism and fuel the cell at the different experimental light and temperature conditions. The results obtained provide a broader view on how the differential regulation of the proteome mediates the physiological plasticity across types of Symbiodinium to photoacclimate to different light enviroments, to acclimate to high temperature and to respond to stressful conditions determining the functional diversity existing in this genus of coral symbionts.
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... Fine-tuning LEF and CEF maintains the ATP/NADPH equilibrium and efficient carbon assimilation (2,3). CEF also plays an important role in photoprotection (4,5) as it maintains the necessary ΔpH across the thylakoid membrane to allow energydependent nonphotochemical quenching and to control the rate limiting step of LEF (6). The dynamic tuning between LEF and CEF is therefore essential for efficient photosynthesis. ...
Significance
To optimize photosynthetic performance and minimize photooxidative damage, photosynthetic organisms evolved to efficiently balance light energy absorption and electron transport with cellular energy requirements under constantly changing light conditions. The regulation of linear electron flow (LEF) and cyclic electron flow (CEF) contributes to this fine-tuning. Here we present a model of the formation and structural molecular organization of a CEF-performing photosystem I (PSI)–light harvesting complex I (LHCI)–cytochrome (cyt) b 6 f supercomplex from the green alga Chlamydomonas reinhardtii . Such a structural arrangement could modulate the distinct operation of LEF and CEF to optimize light energy utilization, despite the same individual structural units contributing to these two different functional modes.
... smaller half-times t 1/2 ) for all studied SA concentrations (Table 2) suggesting an enhanced capacity for cyclic electron transport compared to the control rice plants. It is known that the cyclic electron flow is essential for efficient photosynthesis [ 20 ]. ...
The impact of exogenous application of different concentrations of salicylic acid (10, 50 and 100 µM) through the rooting medium on the plant growth, the pigment content and the photochemical activities of both photosystem I and photosystem II was investigated. Data revealed that the observed alterations strongly depend on the concentration of applied salicylic acid, as 10 µM is the optimal concentration for the growth and the functional activity of photosynthetic apparatus of rice plants under non-stress conditions. In addition, the concentrations of salicylic acid lower than 100 µM had no effect on the energy transfer between the chlorophyll-protein complexes in thylakoid membranes.
... Fd transports electrons to the Cyt b 6 f complex via many pathways (Hanke and Mulo 2013) and continuous CET generates a proton gradient that drives ATP synthesis (Munekage et al. 2004). The involvement of two proteins -PGR5 (Munekage et al. 2002) andPGRL1 (DalCorso et al. 2008) have been shown to act in a regulatory capacity rather than a direct electron mediator from Fd to Cyt b 6 f complex in higher plants Mulo 2013, Hertle et al. 2013). ...
Ferredoxin (Fd) is a small soluble iron-sulfur protein essential in almost all oxygenic photosynthetic organisms. It contains a single [2Fe-2S] cluster coordinated by four cysteine ligands. It accepts electrons from the stromal surface of PSI and facilitates transfer to a myriad of acceptors involved in diverse metabolic processes, including generation of NADPH via Fd-NADP-reductase, cyclic electron transport for ATP synthesis, nitrate reduction, nitrite reductase, sulfite reduction, hydrogenase and other reductive reactions. Fd serves as the central hub for these diverse cellular reactions and is integral to complex cellular metabolic networks. We describe advances on the central role of Fd and its evolutionary role from cyanobacteria to algae/plants. We compare structural diversity of Fd partners to understand this orchestrating role and shed light on how Fd dynamically partitions between competing partner proteins to enable the optimum transfer of PSI-derived electrons to support cell growth and metabolism.
... Fd transports electrons to the Cyt b 6 f complex via many pathways (Hanke and Mulo 2013) and continuous CET generates a proton gradient that drives ATP synthesis (Munekage et al. 2004). The involvement of two proteins -PGR5 (Munekage et al. 2002) andPGRL1 (DalCorso et al. 2008) have been shown to act in a regulatory capacity rather than a direct electron mediator from Fd to Cyt b 6 f complex in higher plants Mulo 2013, Hertle et al. 2013). ...
... Although the pgr5 mutant shows a wild-type-like phenotype under constant light conditions, the pgr5 crr2 double mutant, which also lacks another CET route mediated by the NAD(P)H dehydrogenase (NDH)-like complex, revealed a stunted phenotype and impaired photosynthetic parameters. Thus, the PSI CET is important not only for C 4 plants but also for C 3 plants (Munekage et al. 2004). Yet another protein, PGR5-LIKE PHOTO-SYNTHETIC PHENOTYPE1 (PGRL1), has been characterized and shown to interact with PGR5 and with PSI (Dal Corso et al. 2008). ...
Photosynthesis is amongst the plant cell functions that are highly sensitive to any type of changes. Sun and shade conditions are prevalent in fields as well as dense forests. Dense forests face extreme sun and shade conditions, and plants adapt themselves accordingly. Sun flecks cause changes in plant metabolic processes. In the field, plants have to face high light intensity and survive under such conditions. Sun and shade type of plants develops a respective type of chloroplasts which help plants to survive and perform photosynthesis under adverse conditions. PSII and Rubisco behave differently under different sun and shade conditions. In this review, morphological, physiological, and biochemical changes under conditions of sun (high light) and shade (low light) on the process of photosynthesis, as well as the tolerance and adaptive mechanisms involved for the same, were summarized.
... However, excessive electron flows may also be rerouted toward photorespiration and other alternative electron sinks, like the Mehler reaction or nitrate reduction. Cyclic electron flow, a pathway possibly linked to chlororespiration, may be activated too and, by balancing energy and electron flows, may prevent ROS generation (Munekage et al., 2004). Later, during the acclimation step, general cell metabolism is modified leading, in particular, to the production of compounds with antioxidant properties, such ...
The understorey origin of coffee trees and the strong plasticity of Coffea arabica leaves in relation to contrasting light environments have been largely shown. The adaptability of coffee leaves to changes in light was tested under controlled conditions by increasing the illumination rate on C. arabica var. Naryelis seedlings acclimated to low light conditions and observing leaf responses at three different developmental stages (juvenile, growing and mature). Only mature leaves proved capable of adapting to new light conditions. In these leaves, different major mechanisms were found to contribute to maintaining a good photosynthetic level. With increased illumination, a high photosynthetic response was conserved thanks to fast nitrogen remobilization, as indicated by SPAD values and the photorespiration rate. Efficient photoprotection was accompanied by a great ability to export sucrose, which prevented excessive inhibition of the Calvin cycle by hexose accumulation. In contrast, in younger leaves, increased illumination caused photodamage, observable even after 9 days of treatment. One major finding was that young coffee leaves rely on the accumulation of chlorogenic acids, powerful antioxidant phenolic compounds, to deal with the accumulation of reactive oxygen species rather than on antioxidant enzymes. Due to a lack of efficient photoprotection, a poor ability to export sucrose and inadequate antioxidant protection, younger leaves seemed to be unable to cope with increased illumination. In these leaves, an absence of induced antioxidant enzyme activity was accompanied, in growing leaves, by an absence of antioxidant synthesis or, in juvenile leaves, inefficient synthesis of flavonoids because located in some epidermis cells. These observations showed that coffee leaves, at the beginning of their development, are not equipped to withstand quick switches to higher light levels. Our results confirm that coffee trees, even selected for full sunlight conditions, remain shade plants possessing leaves able to adapt to higher light levels only when mature.
... The ndhG is one of the 11 NADH dehydrogenase genes, and the ndhG subunit is associated with nuclear-encoded subunits to form the NADH dehydrogenase-like complex in angiosperm chloroplasts. This protein complex associates with photosystem I and then forms a supercomplex, which mediates cyclic electron transport [22], produces ATP to balance the ATP/NADPH ratio, and facilitates chlororespiration [23]. Therefore, the selection values identified in C. chinensis indicate positive selection for elements of the photosystemchlororespiration system. ...
The Coptis chinensis Franch. is an important medicinal plant from the Ranunculales. We used next generation sequencing technology to determine the complete chloroplast genome of C. chinensis . This genome is 155,484 bp long with 38.17% GC content. Two 26,758 bp long inverted repeats separated the genome into a typical quadripartite structure. The C. chinensis chloroplast genome consists of 128 gene loci, including eight rRNA gene loci, 28 tRNA gene loci, and 92 protein-coding gene loci. Most of the SSRs in C. chinensis are poly-A/T. The numbers of mononucleotide SSRs in C. chinensis and other Ranunculaceae species are fewer than those in Berberidaceae species, while the number of dinucleotide SSRs is greater than that in the Berberidaceae. C. chinensis diverged from other Ranunculaceae species an estimated 81 million years ago (Mya). The divergence between Ranunculaceae and Berberidaceae was ~ 111 Mya, while the Ranunculales and Magnoliaceae shared a common ancestor during the Jurassic, ~ 153 Mya. Position 104 of the C. chinensis ndhG protein was identified as a positively selected site, indicating possible selection for the photosystem-chlororespiration system in C. chinensis . In summary, the complete sequencing and annotation of the C. chinensis chloroplast genome will facilitate future studies on this important medicinal species.
... Thus, several genes of the PETC are upregulated in the low temperature condition to compensate for the decreased electron flow by HEP. Furthermore, upregulation of LHCA proteins that belong to PSI also suggests increased cyclic electron flow from PSI for an additional energy source 42 . ...
Temperature is a critical environmental factor that affects microalgal growth. However, microalgal coping mechanisms for temperature variations are unclear. Here, we determined changes in transcriptome, total carbohydrate, total fatty acid methyl ester, and fatty acid composition of Tetraselmis sp. KCTC12432BP, a strain with a broad temperature tolerance range, to elucidate the tolerance mechanisms in response to large temperature variations. Owing to unavailability of genome sequence information, de novo transcriptome assembly coupled with BLAST analysis was performed using strand specific RNA-seq data. This resulted in 26,245 protein-coding transcripts, of which 83.7% could be annotated to putative functions. We identified more than 681 genes differentially expressed, suggesting an organelle-specific response to temperature variation. Among these, the genes related to the photosynthetic electron transfer chain, which are localized in the plastid thylakoid membrane, were upregulated at low temperature. However, the transcripts related to the electron transport chain and biosynthesis of phosphatidylethanolamine localized in mitochondria were upregulated at high temperature. These results show that the low energy uptake by repressed photosynthesis under low and high temperature conditions is compensated by different mechanisms, including photosystem I and mitochondrial oxidative phosphorylation, respectively. This study illustrates that microalgae tolerate different temperature conditions through organelle specific mechanisms.
We have analyzed the effect of salinity on photosystem II (PSII) photochemistry and plastoquinone (PQ) pool in halophytic Mesembryanthemum crystallinum plants. Under prolonged salinity conditions (7 or 10 days of 0.4 M NaCl treatment) we noted an enlarged pool of open PSII reaction centers and increased energy conservation efficiency, as envisaged by parameters of the fast and slow kinetics of chlorophyll a fluorescence. Measurements of oxygen evolution, using 2,6-dichloro-1,4-benzoquinone as an electron acceptor, showed stimulation of the PSII activity due to salinity. In salt-acclimated plants (10 days of NaCl treatment), the improved PSII performance was associated with an increase in the size of the photochemically active PQ pool and the extent of its reduction. This was accompanied by a rise in the NADP⁺/NADPH ratio. The presented data suggest that a redistribution of PQ molecules between photochemically active and non-active fractions and a change of the redox state of the photochemically active PQ pool indicate and regulate the acclimation of the photosynthetic apparatus to salinity.
Cyclic electron transport/flow (CET/CEF) in chloroplasts is a regulatory mechanism crucial for optimization of plant photosynthetic efficiency. CET is catalyzed by a membrane-embedded NAD(P)H dehydrogenase-like (NDH) complex containing at least 29 protein subunits and associating with photosystem I (PSI) to form the NDH-PSI supercomplex. Here we report the 3.9 angstrom resolution structure of Arabidopsis thaliana NDH-PSI (AtNDH-PSI) supercomplex. We have constructed structural models for 26 AtNDH subunits, among which 11 subunits are unique to chloroplast and stabilize the core part of NDH complex. In the supercomplex, one NDH can bind up to two PSI-LHCI complexes at both sides of its membrane arm. Two minor LHCIs, Lhca5 and Lhca6, each present in one PSI-LHCI, interact with NDH and contribute to the supercomplex formation and stabilization. Our results showed structural details of the supercomplex assembly and provide molecular basis for further investigation of the regulatory mechanism of CEF in plants.
Light is the prerequisite for photosynthesis. However, excess light flux higher than the light-saturation point gives rise to photoinhibition or photodamage. To efficiently utilize the excess energy under the light saturation is a long-standing issue of photosynthesis. Herein, we found an electron drainage channel using artificial redox shuttle as a nongenetic tool to direct excessive electron transfer from chloroplast of microalgae (Chlorella pyrenoidosa) to extracellular redox reactions. Guiding the excess electrons to the outside cell enhanced water oxidation activity of photosystem II by 2.6-fold and increased the light saturation point by 7.1-fold. Intrinsic quantum yield and electron transfer rate of photosystems (PSII and PSI) were also in response to an increased light flux, due to the liberation of the initial photoinhibition. The electrons drained from photosynthesis served as the reducing equivalents for extracellular synthesis of chemicals. This work sheds light on the nature of photosynthetic electron transportation and distribution in a light-saturated state of microalgae through a nongenetic drainage of electron for extracellular chemical synthesis.
Key points
• An electron drainage channel was developed using artificial redox mediator to liberate the photoinhibition of microalgal photosynthesis.
• Guiding the excess photosynthetic electrons to the outside cell enhanced water oxidation activity of photosystem II and increased the light saturation point.
• Electrons from photosynthesis with the reducing power can be used for extracellular synthesis of chemicals.
Photosynthesis and respiration rely upon a proton gradient to produce ATP. In photosynthesis, the Respiratory Complex I homologue, Photosynthetic Complex I (PS-CI) is proposed to couple ferredoxin oxidation and plastoquinone reduction to proton pumping across thylakoid membranes. However, little is known about the PS-CI molecular mechanism and attempts to understand its function have previously been frustrated by its large size and high lability. Here, we overcome these challenges by pushing the limits in sample size and spectroscopic sensitivity, to determine arguably the most important property of any electron transport enzyme – the reduction potentials of its cofactors, in this case the iron-sulphur clusters of PS-CI (N0, N1 and N2), and unambiguously assign them to the structure using double electron-electron resonance. We have thus determined the bioenergetics of the electron transfer relay and provide insight into the mechanism of PS-CI, laying the foundations for understanding of how this important bioenergetic complex functions.
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 (NDH), 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 two-fold compared to CEF pathways involving non-proton-pumping plastoquinone reductases. By virtue of this proton-pumping stoichiometry, we hypothesise that NDH not only efficiently contributes to ATP production, but operates near thermodynamic reversibility, with potentially important consequences for remediating mismatches in the thylakoid energy budget.
Cyanobacteria are photosynthetic prokaryotes and widely used for photosynthetic research as model organisms. Partly due to their prokaryotic nature, however, estimation of photosynthesis by chlorophyll fluorescence measurements is sometimes problematic in cyanobacteria. For example, plastoquinone pool is reduced in the dark-acclimated samples in many cyanobacterial species so that conventional protocol developed for land plants cannot be directly applied for cyanobacteria. Even for the estimation of the simplest chlorophyll fluorescence parameter, Fv/Fm, some additional protocol such as addition of DCMU or illumination of weak blue light is necessary. In this review, those problems in the measurements of chlorophyll fluorescence in cyanobacteria are introduced, and solutions to those problems are given.
Our study investigated the physiological and biochemical basis for the effects of exogenous phenolic acids on the function of the photosynthetic apparatus and photosynthetic electron transport rate in strawberry seedlings. Potted seedlings of the strawberry (Fragaria × ananassa Duch.) were used. Syringic acid inhibited net photosynthetic rate and water-use efficiency decreased. Additionally, primary quinone electron acceptor of the PSII reaction centre, the PSII reaction centre and the oxygen evolving complex were also impaired. Both the maximum quantum yield of the PSII primary photochemistry and the performance index on absorption basis were depressed, resulting in reduced function of the photosynthetic electron transport chain. Otherwise, low phthalic acid concentrations enhanced photosynthetic capacity, while high concentrations showed opposite effects. Syringic acid exhibited a higher toxic effect than that of phthalic acid which was more evident at higher concentrations.
The plant ferredoxin-like protein (PFLP) gene, cloned from sweet peppers predicted as an electron carrier in photosynthesis, shows high homology to the Fd-I sequence of Arabidopsis thaliana, Lycopersicon esculentum, Oryza sativa and Spinacia oleracea. Most of pflp related studies focused on anti-pathogenic effects, while less understanding for the effects in photosynthesis with physiological aspects, such as photosynthesis rate, and levels of carbohydrate metabolites. This project focuses on the effects of pflp overexpression on photosynthesis by physiological evaluations of carbon assimilation with significant higher levels of carbohydrates with higher photosynthesis efficiency. In this report, two independent transgenic lines of rice plants (designated as pflp-1 and pflp-2) were generated from non-transgenic TNG67 rice plant (WT). Both transgenic pflp rice plants exhibited enhanced photosynthesis efficiency, and gas exchange rates of photosynthesis were 1.3- and 1.2-fold higher for pflp-1 and pflp-2 than WT respectively. Significantly higher electron transport rates of pflp rice plants were observed. Moreover, photosynthetic products, such as fructose, glucose, sucrose and starch contents of pflp transgenic lines were increased accordingly. Molecular evidences of carbohydrate metabolism related genes activities (osHXK5, osHXK6, osAGPL3, osAGPS2α, osSPS, ospFBPase, oscFBPase, and osSBPase) in transgenic lines were higher than those of WT. For performance of crop production, 1000-grain weight for pflp-1 and pflp-2 rice plants were 52.9 and 41.1 g that were both significantly higher than 31.6 g for WT, and panicles weights were 1.4- and 1.2-fold higher than WT. Panicle number, tiller number per plants for pflp rice plants were all significantly higher compared with those of WT where there was no significant difference observed between two pflp rice plants. Taken altogether; this study demonstrated that constitutive pflp expression can improve rice production by enhancing the capacity of photosynthetic carbon assimilation.
We describe a new member of the class of mutants in Arabidopsis exhibiting high rates of cyclic electron flow around photosystem I (CEF), a light-driven process that produces ATP but not NADPH. High cyclic electron flow 2 (hcef2) shows strongly increased CEF activity through the NADPH dehydrogenase complex (NDH), accompanied by increases in thylakoid proton motive force (pmf), activation of the photoprotective qE response, and the accumulation of H2O2. Surprisingly, hcef2 was mapped to a non-sense mutation in the TADA1 (tRNA adenosine deaminase arginine) locus, coding for a plastid targeted tRNA editing enzyme required for efficient codon recognition. Comparison of protein content from representative thylakoid complexes, the cytochrome bf complex, and the ATP synthase, suggests that inefficient translation of hcef2 leads to compromised complex assembly or stability leading to alterations in stoichiometries of major thylakoid complexes as well as their constituent subunits. Altered subunit stoichiometries for photosystem I, ratios and properties of cytochrome bf hemes, and the decay kinetics of the flash-induced thylakoid electric field suggest that these defect lead to accumulation of H2O2 in hcef2, which we have previously shown leads to activation of NDH-related CEF. We observed similar increases in CEF, as well as increases in H2O2 accumulation, in other translation defective mutants. This suggests that loss of coordination in plastid protein levels lead to imbalances in photosynthetic energy balance that leads to an increase in CEF. These results taken together with a large body of previous observations, support a general model in which processes that lead to imbalances in chloroplast energetics result in the production of H2O2, which in turn activates CEF. This activation could be from either H2O2 acting as a redox signal, or by a secondary effect from H2O2 inducing a deficit in ATP.
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