Two Photochemical Systems in Photosynthesis

Duysens, L.N.M.; Amesz, J.; Kamp, B. M.

doi:10.1038/190510a0

Cited by 353 publications

(115 citation statements)

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“…The absorption of sunlight and the conversion of excitation energy to chemical energy in photosynthesis take place in photosystem II (PSII) and photosystem I (PSI) in the thylakoid membrane of chloroplasts [1]. Distinct pigment -protein complexes are contained within PSI and PSII and perform the functions of light absorption and excitation energy transfer to a photochemical reaction centre [2,3].…”

Section: Introductionmentioning

confidence: 99%

Modulation of the light-harvesting chlorophyll antenna size in Chlamydomonas reinhardtii by TLA1 gene over-expression and RNA interference

Mitra

Kirst

Dewez

et al. 2012

Phil. Trans. R. Soc. B

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Truncated light-harvesting antenna 1 (TLA1) is a nuclear gene proposed to regulate the chlorophyll (Chl) antenna size in Chlamydomonas reinhardtii. The Chl antenna size of the photosystems and the chloroplast ultrastructure were manipulated upon TLA1 gene over-expression and RNAi downregulation. The TLA1 over-expressing lines possessed a larger chlorophyll antenna size for both photosystems and contained greater levels of Chl b per cell relative to the wild type. Conversely, TLA1 RNAi transformants had a smaller Chl antenna size for both photosystems and lower levels of Chl b per cell. Western blot analyses of the TLA1 over-expressing and RNAi transformants showed that modulation of TLA1 gene expression was paralleled by modulation in the expression of light-harvesting protein, reaction centre D1 and D2, and VIPP1 genes. Transmission electron microscopy showed that modulation of TLA1 gene expression impacts the organization of thylakoid membranes in the chloroplast. Over-expressing lines showed well-defined grana, whereas RNAi transformants possessed loosely held together and more stroma-exposed thylakoids. Cell fractionation suggested localization of the TLA1 protein in the inner chloroplast envelope and potentially in association with nascent thylakoid membranes, indicating a role in Chl antenna assembly and thylakoid membrane biogenesis. The results provide a mechanistic understanding of the Chl antenna size regulation by the TLA1 gene.

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Section: Introductionmentioning

confidence: 99%

Modulation of the light-harvesting chlorophyll antenna size in Chlamydomonas reinhardtii by TLA1 gene over-expression and RNA interference

Mitra

Kirst

Dewez

et al. 2012

Phil. Trans. R. Soc. B

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“…In oxygenic photosynthesis the minimum quantum requirement for 02 evolution is obtained when, for every photon absorbed by PSII, one, and only one, photon is also absorbed by PSI (3)(4)(5). While the 'Z' scheme (8) predicts a minimum quantum requirement of 8 quanta/02, at subsaturating photon flux densities the minimum quantum requirement observed is seldom less than 10 quanta/02.…”

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Evidence for Cyclic Electron Flow around Photosystem II in Chlorella pyrenoidosa

Falkowski¹,

Fujita²,

Ley³

et al. 1986

Plant Physiol.

112

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ABSTRACrElectron flow around photosystem II was investigated in Chlorelia pyrenoidosa. Using a bare platinum 02 electrode, simultaneou messuremnts were made of steady-state photosynthesis in continuous light, the yield of oxygen (Yo,) produced by a superimposed saturating xenon flash, and the change in fluorescence yield of a weak flash triggered before and 70 microseconds after the saturating flash. Throughout most of the continuous photosynthesis-irradiance curve, normalized 02 flash yields (YoYoN.) and normalized variable fluorescence yields (A+/At') were linearly correlated with a slope of 1.0. As photosynthetic rates reached light saturation, however, the variable fluorescence yields remained relatively constant while 02 flash yields decreased. These results strongly suggest that there is a cyclic electron flow around photosystem II in unpoisoned intact cells at light saturation and supraoptimal light intensities.In oxygenic photosynthesis the minimum quantum requirement for 02 evolution is obtained when, for every photon absorbed by PSII, one, and only one, photon is also absorbed by . While the 'Z' scheme (8) predicts a minimum quantum requirement of 8 quanta/02, at subsaturating photon flux densities the minimum quantum requirement observed is seldom less than 10 quanta/02. The increase from 8 to 10 in quantum requirement is often attributed to cyclic electron flow around PSI (2, 12). As the rate of photon absorption by PSII approaches the capacity ofthe electron transport system, the pools ofelectron carriers between the two photosystems potentially become reduced, resulting in the onset of light saturation of steady-state photosynthesis. Such a situation could lead to cyclic electron flow around PSII and such a cycle has been postulated (1, 7, 1 1), but compelling evidence for the process has been lacking in intact photosynthetic organisms without the addition of inhibitors of electron transport (e.g. [7,9]).Here we report on the effects of continuous photon flux densities on electron flow around PSII in the eucaryotic alga, Chlorella pyrenoidosa, and specifically address the issue of whether cyclic electron flow around PSII ever occurs in vivo. To investigate cyclic electron flow around PSII, two measurements ' Supported by a grant-in-aid for Science Research, 58480016 59127027, to Y. F., and partly carried out at the National Institute for Basic Biology in Okazaki.were made simultaneously on the same sample. The number of PSII traps capable of forming 02 was assayed by measuring the 02 produced in single-turnover saturating flashes (6). At the same time the fraction of QA that remained oxidized was followed by measuring the change in the fluorescence yield of a weak probe flash which was triggered 70 ;ss after the saturating flash (10). MATERIALS AND METHODS Chlorella pyrenoidosa, Emerson strain, was grown in minimal media supplemented with A5 trace metals (14) at 23°C in continuous tungsten light which supplied 3.9 x 1016 quanta cm 2 s-'. The cultures were aerated with 1% CO2 in air, and only ce...

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Action Spectra of O₂ Evolution by Chloroplasts with & without Added Substrate, for Regeneration of O₂ Evolving Ability by Far-red, & for O₂ Uptake

Fork

1963

Plant Physiol.

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Recent studies (3,6,10,12) have shown that photosynthesis requires two photochemical reactions driven by different pigments. Furthermore, it has been suggested (3, 10, 12) that these two light reactions occur in series. Absorption by chlorophyll b and by chloroplhyll a,7, (so-called accessory pigments) are thought to bring about oxidation of water and evolution of O., with a reduction of an acceptor [X, or perhaps plastoquinone (12)]. A second photochemlical step, mediatefl by a long-wavelength form of chlorophyll a, causes reduction of TPN (or an artificial Hill oxidlant) with the electrons coming from reduced X.If some oxidized X were available, O. evolution would be possible without a Hill oxidant and without the functioning of the long-wavelengtlh chlorophyll reaction. (It may be recalled in this regard that a limited O., evolution upon illumination of chloroplasts suspended only in sucrose solutions has been known for over 80 years.) Such a recurring, but small, evolution of 02 by illuminated chloroplasts was also observed in the present study. The assumption that this 02 evolution in the absence of a Hill oxidant is a reflection of the activity of the accessory-pigment reaction can be tested by comparing the action spectra for O. evolution before and after the addition of a Hill oxidant. The action spectrum determined in the absence of an oxidant should have its peaks at wavelengths corresponding to the absorption of chlorophyll b and chlorophyll a,70. In the presence of a Hill oxidant the action spectrum for 02 evolution would be shifted to longer wavelengths and reflect the functioning of both pigment systems. , fig 3) shows a high initial rate (spike) followed by a rapid decrease to a steadystate net 02 evolution after 3 minutes. The subsequent spike after 3 minutes of darkness was smaller than the initial one and smaller also than a later one following 10 minutes of darkness. Figure 3B compares the effect of exposing chloroplasts to the same intensity of 650 myA light under aerobic conditions. Upon illumination, the 02-production spike is seen again. However, the rate of 02 production declines in the light reaching a steadystate net 02 uptake after about three minutes. The chloroplasts show an 02 gulp when the light is turned off. The dark base line, which slowly drifts upwards, is attained again after about three minutes.Chloroplasts retained their ability to evolve 02 70. (upper). Action spectrum for the production of the 02 spike by Swiss chard chloroplasts. The circulating solution used is described in the legend of figure 3. The action spectrum was determined by using equal incident quanta (477 ergs cm-2sec-1 at 650 m,u). At points below 580 mu the light intensities were limited by the output from the monochromator. These points were adjusted upwards appropriately to correspond with other points in the action spectrum. Temp = 19.50, gas phase, N2. 60FIG. 6 (lower). Relative height of the 02 production spike as a function of 650 m,u intensity for chloroplasts used for the action spectrum given i...

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Two Photochemical Systems in Photosynthesis

Cited by 353 publications

References 11 publications

Modulation of the light-harvesting chlorophyll antenna size in Chlamydomonas reinhardtii by TLA1 gene over-expression and RNA interference

Modulation of the light-harvesting chlorophyll antenna size in Chlamydomonas reinhardtii by TLA1 gene over-expression and RNA interference

Evidence for Cyclic Electron Flow around Photosystem II in Chlorella pyrenoidosa

Action Spectra of O₂ Evolution by Chloroplasts with & without Added Substrate, for Regeneration of O₂ Evolving Ability by Far-red, & for O₂ Uptake

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Two Photochemical Systems in Photosynthesis

Cited by 353 publications

References 11 publications

Modulation of the light-harvesting chlorophyll antenna size in Chlamydomonas reinhardtii by TLA1 gene over-expression and RNA interference

Modulation of the light-harvesting chlorophyll antenna size in Chlamydomonas reinhardtii by TLA1 gene over-expression and RNA interference

Evidence for Cyclic Electron Flow around Photosystem II in Chlorella pyrenoidosa

Action Spectra of O2 Evolution by Chloroplasts with & without Added Substrate, for Regeneration of O2 Evolving Ability by Far-red, & for O2 Uptake

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Product

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Action Spectra of O₂ Evolution by Chloroplasts with & without Added Substrate, for Regeneration of O₂ Evolving Ability by Far-red, & for O₂ Uptake