These results challenge the suggestion that NPQ, induced by PGR5-dependent cyclic electron transfer, would have a key role in regulation of electron transfer from PSII to PSI. It is conceivable that the PGR5-PGRL1 complex senses the redox state of PSI electron acceptors and accordingly exerts feedback-regulation on photosynthetic light reactions, by tuning the resistance of proton translocation via ATP synthase by a mechanism that remains to be characterized. A photon of light strikes the antenna of photosystem II (PSII) and reaches the reaction center. As electrons move through the proteins that reside between PSII and PSI, they lose energy. Trace the movement of electrons in noncyclic electron flow. Leaves from 5-weeks-old plants were used for the experiments. To study the redox state PSII acceptor side, the redox state of QA was estimated by normalizing fluorescence to the maximal fluorescence (F′/Fm). The net reaction (A) produces stoichiometric amounts of ATP, NADPH, and oxygen. Though the two photosystems in the light-dependent reactions got their name in the series, they were discovered, but the photosystem II (PS II) comes first in the path in the electron flow and then the photosystem I (PSI). We tested this hypothesis and demonstrated (Figure 1) that despite the absence of NPQ, the npq4 mutant perfectly oxidizes P700, even more efficiently than WT. The amount of PSI is adjusted according to the acceptor side limitation of PSI, which in turn is dependent on the light intensity, the capacity of carbon metabolism and the amount of active PSII (Munekage et al., 2002, 2008; Suorsa et al., 2012; Tikkanen et al., 2014). A light-dependent reaction or light reaction is a chemical reaction that takes place in the presence of light. Representative data is shown with three different plants measured of each lineage. Sci. doi: 10.1073/pnas.1110189108, Joliot, P., and Joliot, A. How is this energy used to synthesize NADPH? The electrons are finally transferred to a copper-containing protein called plastocyanin (Pc) before being accepted by PSI. U.S.A. 108, 13317–13322. Biophys. This component of NPQ is called qE, which is nearly exclusively responsible for induction of NPQ upon increase in light intensity. Natl. No. A Mn cluster, called the oxygen evolving complex, OEC, is directly involved in the oxidation of watre. (2012). Biophys. This may suggest that in the absence of PGR5-provided ΔpH and resistance against proton extrusion from the lumen, the NDH-1-dependent cyclic is enhanced. Photosystem II photoinhibition-repair cycle protects Photosystem I from irreversible damage. Moreover, the fact that oxidation of P700 is facilitated in the npq4 mutant indicates that in the absence of the PSBS protein, PSI has more excitation energy to oxidize P700 as compared to WT. Nevertheless, when the increase in light intensity is strong enough, the pgr5 mutant can induce a reasonably high NPQ that in WT occurs concomitantly with oxidation of P700 (Figure 2A). This predominant type of electron transport in plants is also called the Z-scheme. As the electrons flow between PSI and PSII, they lose energy to translocate H+ ions from the stroma into the thylakoid lumen. Be sure to include the electron carriers plastoquinone and plastocyanin. © 2021 (Science Facts). Novel insights into plant light-harvesting complex II phosphorylation and ‘state transitions’. Biophys. (A)(P/Pm), (B) (F′/Fm), (C) (1–Fm′/Fm) in WT (black dots), pgr5 (blue dots), and npq4 (purple dots) during a steep increase in actinic light intensity. doi: 10.1016/j.pbi.2013.03.011, Roach, T., and Krieger-Liszkay, A. Chlorophyll a fluorescence and signal from oxidized P700 (Klughammer and Schreiber, 1994, 2008) were detected with Dual-PAM-100 (Heinz Walz). PSII has a redox potential of up to 1.2 V (8, 9), required to abstract electrons from water (Fig. (2013). Based on this paradox, it has also been proposed that PGR5 simply prevents the leaking of protons from the lumen to chloroplast stroma by a still uncharacterized mechanism (Avenson et al., 2005). The ΔpH generated by PSII and the Q cycle is released by ATP synthase in a reaction utilizing the proton motive force. doi: 10.1023/A:1024908829819, Kanazawa, A., and Kramer, D. M. (2002). Chloroplast protein phosphorylation couples plastoquinone redox state to distribution of excitation energy between photosystems. In PSI, the electron is excited again and passed down the second leg of the electron transport chain to a final electron acceptor. This indicates that in the absence of PSBS protein-dependent NPQ, the photochemical capacity of the both photosystems in improved. The oxygen gas exits the leaf through the stomata and is released into the environment. The process results in making NADPH electrons that are removed from the inside and passed through PSII and PSI before ending up in NADPH. Cyclic photophosphorylation does not involve PSII and produces only the ATP, stopping the production of NADPH. The Z-Scheme illustrated below follows electrons taken from water (absorbed through roots) into photosystem II (PSII), which will replace those missing from PSI. 16, 307–314. In WT, NPQ prevents the over-reduction of Plastoquinone (PQ) pool and slows down the Cyt b6f leading to oxidation of plastocyanin (PC) and photosystem I (PSI). On the other hand, the pgr5 mutant is not only deficient in induction of strong NPQ but it also lacks the capability to oxidize P700 upon increase in light intensity. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). In this process, light energy is used for the preparation of ATP from ADP. Front. The cytochrome b6f complex at the crossroad of photosynthetic electron transport pathways. To this end, the two NPQ mutants pgr5 and npq4, the latter lacking the PSBS protein, were characterized. For each lineage, leaves from three different plants were analyzed and SD was calculated with formula ∑ (x−x¯)2/(n−1). The concomitant reduction of the PQ pool and oxidation of P700 strongly suggest that the electron transfer is controlled by Cyt b6f. The role of the PsbS protein in the protection of photosystems I and II against high light in Arabidopsis thaliana. (2012). This obviously has not been considered to be a problem because the simultaneously induced energy-dependent NPQ is believed to selectively downregulate PSII, thus leading to PSII limitation of electron transfer, and consequent oxidation of PSI. Natl. It has been proposed that the deficiency of pgr5 in generation of ΔpH upon increase in light intensity is due to incomplete cycling of electrons from PSI acceptor side back to the PQ pool by a putative FQR (Munekage et al., 2002). Using light energy, PSII acts first to channel an electron through a series of acceptors that drive a proton pump to generate ATP, before passing the electron on to PSI. Induction of NPQ (1–Fm′/Fm) was calculated by reversing the maximal fluorescence from a light-adapted leaf (Fm′) normalized to the maximal fluorescence of a dark-adapted leaf (Fm). Moreover, it has been reported that PSBS mutants can oxidize P700 upon increase in light intensity (Grieco et al., 2012; Roach and Krieger-Liszkay, 2012) indicating that the PSBS-dependent mechanism does not specifically downregulate PSII, but rather affects both photosystems. Photosystem I releases electrons, which results in the formation of an NADPH molecule. Acta 1827, 420–426. Theory of fluorescence induction in photosystem II: derivation of analytical expressions in a model including exciton-radical-pair equilibrium and restricted energy transfer between photosynthetic units. The pgr5 mutant is severely deficient in induction of NPQ when the increase in light intensity is not extreme (Figure 1C). Redox state of PSI was determined by normalizing the signal of oxidized P700 to the signal of maximal proportion of oxidized P700 (P/Pm). doi: 10.1016/j.bbabio.2012.09.011, Stiehl, H. H., and Witt, H. T. (1969). Planta 192, 261–268. The electrons flow from PSII to PSI and lose energy to translocate the H + ions from the stroma into the thylakoid lumen. Despite the induction of NPQ, the pgr5 mutant cannot oxidize P700 (Figures 1A and 2A), which further confirms that NPQ is not a mechanism to control the electron flow to PSI. B 24, 1588–1598. Photosynth. The second photosystem, PSII, absorbs maximum light of wavelength 680 nm and thus, called P680. doi: 10.1016/j.molcel.2012.11.030, Joliot, P., and Johnson, G. N. (2011). Plants absorb energy from the sun during photosynthesis. There are two types of photosystems found in most plants: photosystem I (PSI) and photosystem II (PSII). Integrative regulatory network of plant thylakoid energy transduction. Moreover, it is shown that the pgr5 mutant can induce NPQ in very high light, but still remains deficient in P700 oxidation. Instead, the results presented here are in line with our recent suggestion that both PSII and PSI function under the same light harvesting machinery regulated by ΔpH and the PSBS protein (Tikkanen and Aro, 2014; Grieco et al., 2015). Biochim. Solar energy is converted into chemical form by photosynthetic light reactions, which in plants and green algae take place in the thylakoid membrane inside the chloroplasts. doi: 10.1038/35000131, Mekala, N. R., Suorsa, M., Rantala, M., Aro, E. M., and Tikkanen, M. (2015). PQ accepts electrons from PSII and the electrons are then transferred to PSI via Cyt b 6 f and PC. doi: 10.1073/pnas.182427499, Klughammer, C., and Schreiber, U. Sometimes plants follow an alternative path of electron transport called cyclic photophosphorylation. This type of electron transport involving both PSI and PSII is called non-cyclic photophosphorylation. This process allows the absorption of another photon by the PSI antenna pigments. Detached leaves with petioles submerged in tap water were incubated 10 min in darkness before the measurements. doi: 10.1016/S0006-3495(95)80429-7, Li, X. P., Bjorkman, O., Shih, C., Grossman, A. R., Rosenquist, M., Jansson, S., and Niyogi, K. K. (2000). The flow of electrons from PSII to PSI is called _____ electron flow because electrons … In addition, npq4 reached a still higher level of F′/Fm than pgr5 at the high light intensities (661, 1595 μmol photons m-2 s-1), during which the QA begun to return to its oxidized state (Figure 1B). Biochim. 19 – Photophosphorylation at Cram.com. Plant Physiol. Plants actively avoid state-transitions upon changes in light intensity - role of light-harvesting complex II protein dephosphorylation in high light. Study Flashcards On Biochemistry ch. Trends Plant Sci. Quantitative treatment of the function of plastoquinone in phostosynthesis. doi: 10.1074/jbc.M113.453225, Kramer, D. M., and Evans, J. R. (2011). Cell. Thylakoid protein phosphorylation in higher plant chloroplasts optimizes electron transfer under fluctuating light. A transmembrane protein called ATP synthase helps the H+ ions to return to the stroma, producing ATP from ADP + Pi. ATP is created when hydrogen ions are pumped into the inner space (lumen) of the thylakoid. This challenges the idea that NPQ is required for oxidation of P700 in high light. The mechanisms involved in such a regulation of electron flow have, however, remained elusive. Sci. Limitation of electron flow to PSI upon increase in light intensity has been shown to be crucial for protection of PSI against photodamage (Munekage et al., 2002; Suorsa et al., 2012). Regulating the proton budget of higher plant photosynthesis. Acta 1807, 935–944. 2) PSII to PSI The first set of electrons from PSII move along the ETC towards PSI, releasing energy as they move. doi: 10.1104/pp.112.206466, Hertle, A. P., Blunder, T., Wunder, T., Pesaresi, P., Pribil, M., Armbruster, U., et al. But what happens to the light energy that is absorbed? An improved method, using saturating light-pulses, for the determination of photosystem I quantum yield via P700+-absorbance changes at 830nm. The noncyclic pathway begins with PSII; electrons move from H2O through PS II to PS I and then on to NADP+. Those hydrogen atoms, plus the ones produced by splitting water, accumulate in the thylakoid lumen and will be used synthesize ATP in a later step. (1994). Let’s trace the path of electrons in more detail, starting when they're excited by light energy in PSII. Traditionally, the PGR5 protein is linked to the cyclic electron flow around PSI (CET) via putative Ferredoxin (FD) -PQ oxidoreductase (FQR; Munekage et al., 2002). Steady-state phosphorylation of light-harvesting complex II proteins preserves photosystem I under fluctuating white light. Plant Physiol. Absence of the Proton Gradient Regulation 5 (PGR5) protein from plant chloroplasts prevents the induction of strong trans-thylakoid proton gradient (ΔpH) and consequently also the thermal dissipation of excess energy (NPQ). Diagram the flow of electrons during linear electron transport from water to NADPH. The light energy is trapped by a pigment in the chloroplast and converted into chemical energy. Due to the impaired control of electron transfer via Cyt b6f and low thermal dissipation of excess excitation energy, PSI of the pgr5 mutant is sensitive for photoinhibition and has reduced amount of PSI (Munekage et al., 2002). Nature 65, 441–445. However, based on the behavior of mutants disturbed in the distribution of excitation energy from the LHCII system to PSII and PSI (Tikkanen et al., 2010, 2011; Grieco et al., 2012), it seems highly likely that NPQ downregulates both photosystems to similar extent. 81, 163–183. Indeed, only in a specific case of the stn7 mutant when energy distribution from the LHCII system to PSI is impaired, the relaxation of NPQ in low light selectively affects only PSII leading to high reduction of the PQ pool (Tikkanen et al., 2010, 2011; Grieco et al., 2012). Acta 1817, 2158–2165. A pigment-binding protein essential for regulation of photosynthetic light harvesting. trace the path of a carbon dioxide molecule from the point at which it enters a. On the contrary, the oxidation of P700 at high light is even enhanced in the npq4 mutant as compared to wild type (WT), indicating that in the absence of PSBS-dependent NPQ the excitation energy transfer to PSI is enhanced. Detached leaves were incubated in darkness 10 min before measurements. In growth light or light intensities below the growth light, WT, npq4, and pgr5 can keep the intersystem electron transfer chain optimally oxidized. To evaluate electron donors to photosystem I (PSI) and possible path-ways of cyclic electron ﬂow (CEF1) in isolated bundle sheath strands of maize (Zea mays L.), an NADP-ME species, light-induced redox kinetics of the reaction center chlorophyll of PSI (P700) were followed under aerobic conditions. In vivo modulation of nonphotochemical exciton quenching (NPQ) by regulation of the chloroplast ATP synthase. The differential reduction pattern of pgr5 and npq4 recurred during the steep increase of actinic light intensity (Figure 2B). The process results in making NADPH electrons that are removed from the inside and passed through PSII and PSI before ending up in NADPH. Ans. They are passed from PSII to PSI through an electron transport chain and are transmitted to NADP Plus....NADPH. Opin. Electron transfer from PSII to PSI takes place via the intersystem electron transfer chain composed of Plastoquinone (PQ), Cytochrome b 6 f (Cyt b 6 f), and Plastocyanin (PC). 165, 1604–1617. Biochim. Acta 1837, 210–215. All rights reserved. 4). doi: 10.1515/znb-1969-1219, Suorsa, M., Jarvi, S., Grieco, M., Nurmi, M., Pietrzykowska, M., Rantala, M., et al. The light-dependent reaction takes place within specialized membrane discs of the chloroplast, known as thylakoid. Articles. 168, 721–734. Biophys. Therefore, we assume that there is a still uncharacterized PGR5-dependent mechanism that controls proton translocation across the thylakoid membrane and allows synchronized induction of NPQ together with Cyt b6f-dependent mechanism to control electron flow to PSI. Let’s summarize the flow of electrons from water through the Z-scheme. Copyright © 2015 Tikkanen, Rantala and Aro. Photosystem I releases electrons, which results in … A., Kanazawa, A., and Kramer, D. M. (2005). Use your flowchart to answer the following questions: (a) Trace the path of electrons through PS II and PS L (b) Trace the path of energy through the light reactions. NPQ was almost non-existent in the npq4 mutant, whereas pgr5 was capable of inducing a relatively high NPQ compared to npq4 especially in very high light (Figures 1C and 2C). Safe and efficient function of the photosynthetic light reactions is based on synchronized function of the light-driven enzymes photosystem II (PSII) and photosystem I (PSI), the former splitting water to protons and electrons and the latter using electrons to reduce NADP to NADPH. PQ accepts electrons from PSII and the electrons are then transferred to PSI via Cyt b6f and PC. Your email address will not be published. Photosynthesis - Photosynthesis - The pathway of electrons: The general features of a widely accepted mechanism for photoelectron transfer, in which two light reactions (light reaction I and light reaction II) occur during the transfer of electrons from water to carbon dioxide, were proposed by Robert Hill and Fay Bendall in 1960. oxidation of water by photosystem II (PSII). Trends Plant Sci. (2013). doi: 10.1104/pp.15.00488, Munekage, Y. N., Genty, B., and Peltier, G. (2008). This energy is used to move hydrogen atoms from the stromal side of the membrane to the thylakoid lumen. LHCII is an antenna of both photosystems after long-term acclimation. Although the fluorescence normalized to the maximal fluorescence does not linearly correlate with [QA-] due to the antenna connectivity (Lavergne and Trissl, 1995; Joliot and Joliot, 2003), the parameter F′/Fm is the best parameter for the mutants with severely altered behaviour of both Fm′ and F′. B. The electron transfer reactions in Cyt b6f are coupled to transfer of protons from chloroplast stroma to thylakoid lumen (Q cycle). 99, 155–160. This process of ATP synthesis is known as photophosphorylation, as light provides the energy to carry out the procedure. 49, 511–523. Cell 132, 273–285. The other 5 G3P molecules are used to produce RuBP. Z. Naturforsch. This regulation is based on phosphorylation of thylakoid proteins and required to maintain the functional balance between PSII and PSI upon changes in light quality (Allen et al., 1981; Mekala et al., 2015) and quantity (Mekala et al., 2015). Acad. To clarify the differential roles of ΔpH-dependent control of Cyt b6f and NPQ in regulation of electron flow from PSII to PSI at high light, we investigated WT, pgr5, and npq4 with respect to the redox state of PSI, redox state of PSII electron acceptors and induction of NPQ upon changes in illumination conditions. This is done by utilizing 3 molecules of ATP. The reaction center of PSII (called P680) delivers its high-energy electrons, one at a time, to a primary electron acceptor called pheophytin (Ph), and then sequentially to two bound plastoquinones Q A and Q B. doi: 10.1104/pp.110.166652, Kukuczka, B., Magneschi, L., Petroutsos, D., Steinbeck, J., Bald, T., Powikrowska, M., et al. Res. 288, 13156–13163. doi: 10.1016/j.cell.2007.12.028, Grieco, M., Suorsa, M., Jajoo, A., Tikkanen, M., and Aro, E. M. (2015). First, the redox state of the PQ pool in WT remains relatively stable despite the induction or relaxation of NPQ. Research was supported by the Academy of Finland (projects 271832, 273870, and 260094). Chem. Required fields are marked *. doi: 10.1016/j.bbabio.2012.12.009, Keywords: regulation of photosynthetic electron transfer chain, cyclic electron transfer, thermal dissipation, NPQ, trans-thylakoid proton gradient, control of Cyt b6f, P700 oxidation, Citation: Tikkanen M, Rantala S and Aro E-M (2015) Electron flow from PSII to PSI under high light is controlled by PGR5 but not by PSBS. This model, however, is paradoxical since it states that the slowdown of electron transfer (occurring in the Cyt b6f complex) results from acceleration of electron transfer via Cyt b6f complex (in CET). FIGURE 1. Saturating pulse was applied in every 1 min with a non-gradual increase in the actinic light intensity (0, 127, and 1953 μmol photons m-2 s-1). During this process, usable molecules like ATP and NADPH are generated as energy-carriers. Also known as thylakoid pgr5 and npq4 recurred during the steep increase of light! Movement of hydrogen ions flow through ATP synthase via chemiosmosis to form molecules of ATP from ADP Pi! J. R. ( 2013 ) supplies electrons to the sugar G3P 5 G3P molecules used! Stiehl, H. T. ( 1969 ) carriers plastoquinone and plastocyanin this energy is used to hydrogen! Molecule called P700 entire process occurs in three significant steps or stages they have lost energy before arriving PSI... Chlorophyll to respond to another photon the 1964 experiment occurring in the formation of ΔpH-dependent.! The path of an electron transport pathway to generate ATP thus, called the evolving... Lots of chlorophylls, and Joliot, a ( 2011 ) ) what the. 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