Redox potentials of electron acceptors in photosystem II<sub>α</sub> and II<sub>β</sub>

Thielen, A.P.G.M.; Gorkom, Hans J. van

doi:10.1016/0014-5793(81)80166-4

Cited by 78 publications

(26 citation statements)

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“…One theory is that these herbicides block electron flow between the primary acceptor (Q) and the secondary acceptor (B) by decreasing the redox potential of B, which shifts the equilibrium of Q-B ± QB-to the left (32). Changes in redox potential due to herbicides have yet to be demonstrated (29). A second theory is that B is a quinone free to diffuse in the thylakoid membrane and exchange with the plastoquinone pool (12,26).…”

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confidence: 99%

Changes in [¹⁴C]Atrazine Binding Associated with the Oxidation-Reduction State of the Secondary Quinone Acceptor of Photosystem II

Jursinic¹,

Stemler²

1983

Plant Physiol.

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One hypothesis of triazne-type herbicide action in photosynthetic material is that the herbicide molecule competes with a secondary quinone acceptor, B, for a binding site at the reaction center of photosystem II. The binding affinity of B has been suggested to change with its level of reduction, being most strongly bound in its semiquinone form. Among the major classes of herbicides are those that inhibit electron transport in photosynthesis (21). The mode of action of most of these inhibitors is blockage of electron flow associated with PSII. Diuron and atrazine are two herbicides of this type that have been studied extensively and are believed to act near the same site (22,23).It has been hypothesized that diuron acts on the oxidizing side (7, 10, 25) as well as on the reducing side (9; for a review, see 13) of PSII. The effects on the reducing side correlate with binding of high specificity (30) and occur at much lower concentrations than the oxidizing-side effects. In this work, we have studied the inhibition of electron flow on the reducing side of PSII.'Supported by funds from National Science Foundation grant PCM 8004075 to A. S.The modes of action ofdiuron and atrazine are still unknown. One theory is that these herbicides block electron flow between the primary acceptor (Q) and the secondary acceptor (B) by decreasing the redox potential of B, which shifts the equilibrium of Q-B ± QB-to the left (32). Changes in redox potential due to herbicides have yet to be demonstrated (29). A second theory is that B is a quinone free to diffuse in the thylakoid membrane and exchange with the plastoquinone pool (12,26). This vacates a binding site at the PSII reaction center, which can be filled in a competitive manner by a herbicide molecule. With the herbicide bound, the binding site is not available for B, and normal electron flow past Q is not possible (31). This proposal is quite analogous to the suggested mechanism of action of o-phenanthroline in purple photosynthetic-bacteria (35,36). Support for this theory is based on the kinetics of charge transfer at the reaction center, and on how these kinetics are altered when sufficient time is allowed for the hypothesized exchange of herbicide for the quinone B to take place (31,35,36 (1,6,8,15), caused the affinity for atrazine to significantly decrease. MATERIALS AND METHODSBroken chloroplasts (thylakoids) were isolated from leaves of dwarf peas (Pisum sativum) grown in a laboratory growth chamber and harvested 14 to 21 d after germination. The isolation procedure was as previously described (14), and the reaction medium used was 400 mm saccharose, 50 mm Tes, 10 mm NaCl, 5 mM MgCl2 (pH 7.5

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confidence: 99%

Changes in [¹⁴C]Atrazine Binding Associated with the Oxidation-Reduction State of the Secondary Quinone Acceptor of Photosystem II

Jursinic¹,

Stemler²

1983

Plant Physiol.

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“…Because PSII exists in different forms in cells, we measured two types of heterogeneity, one that determines electron-transport capacity and one that determines the effective size of the antenna system serving individual reaction centers. The electron-transport assay distinguishes between PSII A and PSII X reaction centers (Thielen and Van Gorkom, 1981;Lavergne, 1982aLavergne, , 1982bMelis, 1985;Graan and Ort, 1986;Chylla et al, 1987;Chylla andWhitmarsh, 1989, 1990;Nedbal et al, 1991). PSII A centers turn over at rates of a few hundred electrons per second in saturating light, whereas the turnover of PSII X is 1/10th to 1/1000th that of active centers.…”

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Characterization of Photosystem II Activity and Heterogeneity during the Cell Cycle of the Green AlgaScenedesmus quadricauda1

et al. 1999

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The photosynthetic activity of the green alga Scenedesmus quadricauda was investigated during synchronous growth in light/dark cycles. The rate of O 2 evolution increased 2-fold during the first 3 to 4 h of the light period, remained high for the next 3 to 4 h, and then declined during the last half of the light period. During cell division, which occurred at the beginning of the dark period, the ability of the cells to evolve O 2 was at a minimum. To determine if photosystem II (PSII) controls the photosynthetic capacity of the cells during the cell cycle we measured PSII activity and heterogeneity. Measurements of electron-transport activity revealed two populations of PSII, active centers that contribute to carbon reduction and inactive centers that do not. Measurements of PSII antenna sizes also revealed two populations, PSII ␣ and PSII ␤ , which differ from one another by their antenna size. During the early light period the photosynthetic capacity of the cells doubled, the O 2 -evolving capacity of PSII was nearly constant, the proportion of PSII ␤ centers decreased to nearly zero, and the proportion of inactive PSII centers remained constant. During the period of minimum photosynthetic activity 30% of the PSII centers were insensitive to the inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea, which may be related to reorganization of the thylakoid membrane. We conclude from these results that PSII does not limit the photosynthetic activity of the cells during the first half of the light period. However, the decline in photosynthetic activity observed during the last half of the light period can be accounted for by limited PSII activity.The photosynthetic activity of synchronously grown cells of algae is strongly modulated during the cell cycle. In algal cells synchronized by light/dark periods, the rate of photosynthesis can vary more than 2-fold. The photosynthetic activity reaches a maximum during the early phase of the light period, persists for a few hours, and then steadily declines until the end of the light period, which coincides with the onset of cell division (Sorokin, 1957;Sorokin and Mayers, 1957). Respiratory activity exhibits a similar periodic modulation during the cell cycle (Sorokin and Mayers, 1957). Despite decades of research, the factors that control the photosynthetic activity of a cell during its development have not been identified. Some studies indicate that a component of the electron-transport apparatus of the thylakoid membrane may be rate limiting during the cell cycle (Senger and Bishop, 1967, 1969;Schor et al., 1970;Senger, 1970; Frickel-Faulstich and Senger, 1974; Hesse et al., 1976 Hesse et al., , 1977Mende et al., 1981), whereas other studies point to a limitation in the C-reduction cycle (Walther and Edmunds, 1973;Myers and Graham, 1975). These observations raise the possibility that the site of control may change during the cell cycle.Several studies reveal cell-cycle-dependent modifications of the photosynthetic machinery of the thylakoid membrane that could impose l...

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“…the PSII reducing side heterogeneity (24,25,29,45). This type of heterogeneity was identified by the inability of a small fraction of PSII centers to transfer electrons from Q. to QB.…”

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Activation of a Reserve Pool of Photosystem II in Chlamydomonas reinhardtii Counteracts Photoinhibition

Neale¹,

Melis²

1990

Plant Physiol.

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The effect of strong irradiance (Q000 micromole photons per square meter per second) on PSII heterogeneity in intact cells of Chiamydomonas reinhardtii was investigated. Low light (LL,15 micromole photons per square meter per second) grown C. reinhardtii are photoinhibited upon exposure to strong irradiance, and the loss of photosynthetic functioning is due to damage to PSII. Under physiological growth conditions, PSII is distributed into two pools. The large antenna size (PSIIa) centers account for about 70% of all PSII in the thylakoid membrane and are responsible for plastoquinone reduction (OB-reducing centers). The smaller antenna (PSII) account for the remainder of PSII and exist in astate not yet able to photoreduce plastoquinone (0B-nonreducing centers). The exposure of C. reinhardtii cells to 60 minutes of strong irradiance disabled about half of the primary charge separation between P680 and pheophytin. The PSII,content remained the same or slightly increased during strong-irradiance treatment, whereas the photochemical activity of PSIIa decreased by 80%. Analysis of fluorescence induction transients displayed by intact cells indicated that strong irradiance led to a conversion of PSlI,6 from a 0.-nonreducing to a QO-reducing state. Parallel measurements of the rate of oxygen evolution revealed that photosynthetic electron transport was maintained at high rates, despite the loss of activity by a majority of PSIIa. The results suggest that PSlI, in C. reinhardtii may serve as a reserve pool of PSII that augments photosynthetic electron-transport rates during exposure to strong irradiance and partially compensates for the adverse effect of photoinhibition on PSII,.Light energy drives photosynthesis but excess light is potentially damaging to the photosynthetic apparatus. The latter phenomenon is called photoinhibition and is manifested as a loss in chloroplast electron-transport capacity (for recent reviews see 38, 39). Though plants differ greatly in their sensitivity to photoinhibition, current evidence indicates widespread occurrence of photoinhibition in both terrestrial and aquatic environments (2, 33). In most cases, the loss of PSII function is responsible for decreased electron-transport activity, though damage to the PSI complex has also been reported (39). When isolated spinach thylakoids were illuminated with strong-irradiance (2500 Amol.m-2 s-') at 0C, both the QA2 (320 nm absorbance change) and Pheo (685 nm absorbance change) signal were lowered in parallel (8,9,12), suggesting inhibition of primary photochemistry. The PSII primary charge separation was also inhibited when intact cells of the green alga, Chiamydomonas reinhardtii, were exposed to strong irradiance (12). The rapid loss of the pheophytin photoreduction during photoinhibition has also been detected using EPR difference spectroscopy (46). These results indicated that photoinhibitory damage affected the ability of PSII to form the P680+ Pheo-charge separation.An alternative proposal (22, 37) is that photoinhibition result...

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Redox potentials of electron acceptors in photosystem II_α and II_β

Cited by 78 publications

References 21 publications

Changes in [¹⁴C]Atrazine Binding Associated with the Oxidation-Reduction State of the Secondary Quinone Acceptor of Photosystem II

Changes in [¹⁴C]Atrazine Binding Associated with the Oxidation-Reduction State of the Secondary Quinone Acceptor of Photosystem II

Characterization of Photosystem II Activity and Heterogeneity during the Cell Cycle of the Green AlgaScenedesmus quadricauda1

Activation of a Reserve Pool of Photosystem II in Chlamydomonas reinhardtii Counteracts Photoinhibition

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Redox potentials of electron acceptors in photosystem IIα and IIβ

Cited by 78 publications

References 21 publications

Changes in [14C]Atrazine Binding Associated with the Oxidation-Reduction State of the Secondary Quinone Acceptor of Photosystem II

Changes in [14C]Atrazine Binding Associated with the Oxidation-Reduction State of the Secondary Quinone Acceptor of Photosystem II

Characterization of Photosystem II Activity and Heterogeneity during the Cell Cycle of the Green AlgaScenedesmus quadricauda1

Activation of a Reserve Pool of Photosystem II in Chlamydomonas reinhardtii Counteracts Photoinhibition

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Redox potentials of electron acceptors in photosystem II_α and II_β

Changes in [¹⁴C]Atrazine Binding Associated with the Oxidation-Reduction State of the Secondary Quinone Acceptor of Photosystem II

Changes in [¹⁴C]Atrazine Binding Associated with the Oxidation-Reduction State of the Secondary Quinone Acceptor of Photosystem II