Photosystem I-dependent cyclic electron transport is important in controlling Photosystem II activity in leaves under conditions of water stress

Katona, Eva; Neimanis, Spidola; Schönknecht, Gerald; Heber, U.

doi:10.1007/bf00029818

Cited by 69 publications

(36 citation statements)

References 41 publications

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“…In leaves of a C3 plant, light scattering was in the absence of CO2, increased far more by far-red light than by low-intensity red light when the beams were balanced so as to produce comparable excitation of PSII (14). Scattering was decreased not only by CO2 but also by elevated concentra-tions of 02.…”

Section: Role Of Psi-dependent Cyclic Electron Transportmentioning

confidence: 96%

“…The high oxidation status of PSI under conditions in which a greater level of reduction would be expected is made possible by the control of electron flow from PSII (8,11,14,23,24). Control becomes strong when access of CO2 to the photosynthetic apparatus is restricted.…”

Section: Consequences Of the H+/atp Ratio In Atp Synthesismentioning

confidence: 99%

“…Such an increase would replenish electrons lost from the cyclic pathway to NADP as NADPH becomes more rapidly reoxidized by increased carbon assimilation and/or photorespiration. At intermittent levels of light scattering, in the presence of predominantly far-red background illumination, brief flashes of high-intensity red light cause transient increases in light scattering (14). This increase is dependent on the intensity of the far-red background light.…”

Section: Role Of Psi-dependent Cyclic Electron Transportmentioning

confidence: 99%

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Concerning a Dual Function of Coupled Cyclic Electron Transport in Leaves

Heber¹,

Walker²

1992

Plant Physiol.

Self Cite

270

159

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Coupled cyclic electron transport is assigned a role in the protection of leaves against photoinhibition in addition to its role in ATP synthesis. In leaves, as in reconstituted thylakoid systems, cyclic electron transport requires 'poising," i.e. availability of electrons at the reducing side of photosystem I (PSI) and the presence of some oxidized plastoquinone between photosystem 11 (PSII) and PSI. Under self-regulatory poising conditions that are established when carbon dioxide limits photosynthesis at high light intensities, and particularly when stomata are partially or fully closed as a result of water stress, coupled cyclic electron transport controls linear electron transport by helping to establish a proton gradient large enough to decrease PSII activity and electron flow to PSI. This brings electron donation by PSII, and electron consumption by available electron acceptors, into a balance in which PSI becomes more oxidized than it is during fast carbon assimilation. Avoidance of overreduction of the electron transport chain is a prerequisite for the efficient protection of the photosynthetic apparatus against photoinactivation.Despite several decades of intensive research, it is still not entirely clear how in leaves the reactions of the photosynthetic electron transport chain, which yield reductant (in the form of NADPH) and phosphate energy (in form of ATP), are geared to the reactions of stromal enzymes. These use this "assimilatory power' (1) from the chloroplast stroma to the intrathylakoid space of the chloroplasts, and that the free energy of the proton gradient thus formed is used for ATP synthesis, neither the stoichiometry of H+/e coupling nor that of ATP synthesis are known for certain. During linear electron transport from water to NADP, two protons are released inside the thylakoids per H20 oxidized by PSII and at least another two by the subsequent oxidation of the plastoquinol formed as a consequence of the transfer of the two electrons of water to plastoquinone. Unfortunately, it is still not quite clear how the Cyt b/f complex mediates oxidation of plastoquinol. If it passes electrons of this two-electron carrier straight on to PSI, which lifts them to the redox levels of Fd and NADP, the H+/e ratio of linear electron transport is 2. If only one electron from the plastoquinol is transferred to PSI and the other one is shuttled back to plastoquinone in the loop of a Mitchellian Q-cycle, the H+/e ratio is 3. According to Rich (19), Q-cycle coupling is obligatory. Others consider it to be facultative, with decreasing coupling efficiency at increasing light intensities (17, 18). Crowther and Hind (4) proposed a modified Q-cycle, with electron input from the reducing side of PSI. CONSEQUENCES OF THE H+/ATP RATIO

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Section: Role Of Psi-dependent Cyclic Electron Transportmentioning

confidence: 96%

Section: Consequences Of the H+/atp Ratio In Atp Synthesismentioning

confidence: 99%

Section: Role Of Psi-dependent Cyclic Electron Transportmentioning

confidence: 99%

See 1 more Smart Citation

Concerning a Dual Function of Coupled Cyclic Electron Transport in Leaves

Heber¹,

Walker²

1992

Plant Physiol.

Self Cite

270

159

View full text Add to dashboard Cite

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“…Subsequently, the increase in excitation pressure (caused by loss of quenching) at the reaction centers, compounded by the accumulation of reduced electron carriers, would result in increased photodamage (9). Thus, a flexible or dynamic relationship between q E and LEF is essential and indeed has been demonstrated to be substantial (24,(26)(27)(28)(29)(30)(31). For example, when CO 2 levels were lowered from ambient to near 0 ppm, the sensitivity of q E to LEF increased by Ϸ5-fold (23).…”

Section: The Need For Flexibility In Antenna Down-regulationmentioning

confidence: 99%

Modulation of energy-dependent quenching of excitons in antennae of higher plants

Avenson

Cruz

Kramer

2004

Proc. Natl. Acad. Sci. U.S.A.

137

194

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Energy-dependent exciton quenching, or qE, protects the higher plant photosynthetic apparatus from photodamage. Initiation of q E involves protonation of violaxanthin deepoxidase and PsbS, a component of the photosystem II antenna complex, as a result of lumen acidification driven by photosynthetic electron transfer. It has become clear that the response of q E to linear electron flow, termed ''q E sensitivity,'' must be modulated in response to fluctuating environmental conditions. Previously, three mechanisms have been proposed to account for q E modulation: (i) the sensitivity of q E to the lumen pH is altered; (ii) elevated cyclic electron flow around photosystem I increases proton translocation into the lumen; and (iii) lowering the conductivity of the thylakoid ATP synthase to protons (g H؉) allows formation of a larger steady-state proton motive force (pmf). Kinetic analysis of the electrochromic shift of intrinsic thylakoid pigments, a linear indicator of transthylakoid electric field component, suggests that, when CO 2 alone was lowered from 350 ppm to 50 ppm CO 2, modulation of qE sensitivity could be explained solely by changes in conductivity. Lowering both CO 2 (to 50 ppm) and O2 (to 1%) resulted in an additional increase in q E sensitivity that could not be explained by changes in conductivity or cyclic electron flow associated with photosystem I. Evidence is presented for a fourth mechanism, in which changes in q E sensitivity result from variable partitioning of proton motive force into the electric field and pH gradient components. The implications of this mechanism for the storage of proton motive force and the regulation of the light reactions are discussed. P lant chloroplasts convert light energy into two forms usable by the biochemical processes of the plant (1, 2). Redox free energy is stored by linear electron flow (LEF) through photosystem (PS) II, the cytochrome b 6 f complex, PS I, ferredoxin, and finally NADPH. Translocation of protons from the stroma to the lumen is coupled to LEF, resulting in the establishment of transthylakoid proton motive force (pmf ), which drives the synthesis of ATP from ADP and P i at the thylakoid CF o -CF 1 ATP synthase (3). It has become clear that certain redox carriers and the pmf also play regulatory roles in photosynthesis. The redox status of the electron transfer chain regulates a range of processes by means of the thioredoxin system (4) and the plastoquinone pool (5). Meanwhile, the pH component (⌬pH) of pmf regulates the efficiency of light capture by means of protonation of thylakoid lumen proteins (6). The balancing of these two roles governs the development and efficiency of the photochemical machinery, as well as the avoidance of harmful side reactions.The Need for Down-Regulation of the Photosynthetic Apparatus Plants are exposed to widely varying environmental conditions, often resulting in light energy capture that exceeds the capacity of the photosynthetic apparatus (7-10), which in turn can lead to photodamage (11,12). Plants have evolved...

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“…Excessive excitation of PSII during severe water stress and stomatal closure can be avoided through many mechanisms of energy dissipation, including photorespiration (Sharp and Boyer, 1986), cyclic electron transport (Katona et al, 1992), CO, recycling (Stuhlfauth et al, 1990), leaf movement (Ludlow and Bjorkman, 1984), alternative electron acceptors (Osmond, 1995 Carbohydrates and increased nonphotochemical quenching (Valentini et al, 1995) including increases in xanthophyll cycle carotenoids (Demmig-Adams et al, 1989). However, these mechanisms may not entirely make up for the loss in energy dissipation due to increased carboxylation efficiency in elevated [CO,] at these high temperatures.…”

Section: Dlscusslonmentioning

confidence: 99%