Genetic variation in soybean photosynthetic electron transport capacity is related to plastocyanin concentration in the chloroplast

Fifteen ancestral genotypes of United States soybean cultivars were screened for differences in photosynthetic electron transport capacity using isolated thylakoid membranes. Plants were grown in controlled environment chambers under high or low irradiance conditions. Thylakoid membranes were isolated from mature leaves. Photosynthetic electron transport was assayed as uncoupled Hill activity using 2,6-dichlorophenolindophenol (DCIP). Soybean electron transport activity was dependent on genotype and growth irradiance and ranged from 6 to 91 mmol DCIP reduced [mol chlorophyll]^–1 s^–1. Soybean plastocyanin pool size ranged from 0.1 to 1.3 mol plastocyanin [mol Photosystem I]^–1. In contrast, barley and spinach electron transport activities were 140 and 170 mmol DCIP reduced [mol chlorophyll]^–1 s^–1, respectively, with plastocyanin pool sizes of 3 to 4 mol plastocyanin [mol Photosystem I]^–1. No significant differences in the concentrations of Photosystem II, plastoquinone, cytochrome b₆f complexes, or Photosystem I were observed. Thus, genetic differences in electron transport activity were correlated with plastocyanin pool size. The results suggested that plastocyanin pool size can vary significantly and may limit photosynthetic electron transport capacity in certain species such as soybean. Soybean plastocyanin consisted of two isoforms with apparent molecular masses of 14 and 11 kDa, whereas barley and spinach plastocyanins each consisted of single polypeptides of 8 and 12 kDa, respectively.Abbreviations DAP days after planting - DCIP 2,6-dichlorophenolindophenol - LiDS lithium dodecyl sulfate - PPFD photosynthetic photon flux density (mol photons m^–2 s^–1) - PS I Photosystem I - PS II Photosystem II - P700 reaction center of Photosystem I The US Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

Effect of growth irradiance on plastocyanin levels in barley

Plastocyanin levels in barley (Hordeum vulgare cv Boone) were found to be dependent on growth irradiance. An immunochemical assay was developed and used to measure the plastocyanin content of isolated thylakoid membranes. Barley grown under 600 mole photons m^–2s^–1 contained two- to four-fold greater quantities of plastocyanin per unit chlorophyll compared with plants grown under 60 mole photons m^–2s^–1. The plastocyanin/Photosystem I ratio was found to be 2 to 3 under high irradiance compared with 0.5 to 1.5 under low irradiance. The reduced plastocyanin pool size in low light plants contributed to a two-fold reduction in photosynthetic electron transport activity. Plastocyanin levels increased upon transfer of low light plants to high irradiance conditions. In contrast, plastocyanin levels were not affected in plants transferred from high to low irradiance, suggesting that plastocyanin is not involved in the acclimation of photosynthesis to shade.Abbreviations: BSA bovine serum albumin - chl chlorophyll - cyt cytochrome - DCIP 2,6-dichlorophenolindophenol - PS I Photosystem I - PS II Photosystem II - P700 reaction center of Photosystem I - TBS 20 mM Tris-HCl pH 7.5, 500 mM NaCl - TTBS 20 mM Tris-HCl pH 7.5, 500 mM NaCl, 0.5% (w/v) polyoxyethylenesorbitan monolaurate (Tween-20)     

Chloroplast ultrastructure and thylakoid polypeptide composition are affected by different salt concentrations in the halophytic plant Arthrocnemum macrostachyum

The effect of different external salt concentrations, from 0 mM to 1030 mM NaCl, on photosynthetic complexes and chloroplast ultrastructure in the halophyte Arthrocnemum macrostachyum was studied. Photosystem II, but not Photosystem I or cytochrome b6/f, was affected by salt treatment. We found that the PsbQ protein was never expressed, whereas the amounts of PsbP and PsbO were influenced by salt in a complex way. Analyses of Photosystem II intrinsic proteins showed an uneven degradation of subunits with a loss of about 50% of centres in the 0 mM NaCl treated sample. Also the shape of chloroplasts, as well as the organization of thylakoid membranes were affected by NaCl concentration, with many grana containing few thylakoids at 1030 mM NaCl and thicker grana and numerous swollen thylakoids at 0 mM NaCl. The PsbQ protein was found to be depleted also in thylakoids from other halophytes.     

Photosynthetic functions and thylakoid membrane polypeptide composition in light-sensitive mutants of Chlamydomonas reinhardii

In this paper we compared the pigment composition, photochemical activity, chloroplast ultrastructure, thylakoid membrane polypeptide composition and ribosomal content of wild-type and seven light-sensitive mutants of Chlamydomonas reinhardii.All the mutants had low chlorophyll and carotenoid content compared to wild-type. Mutants lts-30 and lts-135 were also characterized by a complete absence of visible carotenoids, while mutant lts-19 was fully deficient in chlorophylls.In most mutants, the chloroplast fragment could not carry out any DCIP photoreduction and O₂ evolution was also blocked. The PSI/P₇₀₀/activity was decreased in most cases.The mutant strains contained mostly single lamellae in their plastids, that is the stacking capacity of the thylakoid membranes was very decreased or fully absent. In most cases the number of lamellae was also very low.The relative amounts of 70 S ribosomes were decreased in all of the mutants. The thylakoid membranes showed anomalies in the region of 24 000–30 000 dalton polypeptides. The common characteristic for them was the relatively higher amount of the 30 000 dalton polypeptide and considerably decreased level of the 27 000 and 24 000 dalton polypeptides relative to the wild-type. These polypeptides were probably constituents of the chlorophyll-protein complex II which has been suggested to be the light harvesting pigment complex for PSII. The polypeptide of 30 000 daltons is the precursor for the LHCP apoprotein (24 000 dalton protein). It may be that the lighstimulated conversion of this precursor into LHCP apoprotein was blocked in our pigment-deficient mutants.Abbreviations CPI Chlorophyll-protein complex I - PSI Photosystem I - PSII Photosystem II - LHCP Light-harvesting pigment complex - DCIP 2,6-dichlorophenolindophenol - RuDPC-ase Ribulose-1,5-biphosphate-carboxylase - SDS Sodium dodecyl sulfate - LIDS Lithium dodecyl sulfate - PAG Polyacrylamide gel - TKM buffer 25 mM Tris-HCl, pH 7.S; 25 mM KCl; 25 mM Mg acetate     

Effects of canopy shade on the lipid composition of soybean leaves

The effect of canopy shade on leaf lipid composition was examined in soybeans ( Glycine max cv. Young) grown under field conditions. Expanding leaves were tagged at 50, 58 and 65 days after planting (DAP) in plots with either a high (10 plants m⁻¹ row) or low (1 plant m⁻¹ row) plant density. At 92 DAP, light conditions ranged from a pho-tosynthetic photon flux density (PPFD) of 87% of full sun with a far-red/red (735 nm/645 nm) ratio of 0.9 at upper canopy leaves to extreme shade where the PPFD was 10% of full sun with a far-red/red ratio greater than 6. Highly shaded leaves in the high plant density treatment accumulated triacylglycerol (TG) up to 25% of total leaf lipid, a 2.4-fold increase in TG on a chlorophyll basis compared to leaves in the upper canopy. Although total polar lipid content was reduced up to 50% in shaded leaves, shade had little affect on the lipid content or composition of thylakoid membranes. Shade did not affect leaf chlorophyll content. Therefore, the changes in leaf lipid composition were not related to senescence. These findings suggest that conditions of low irradiance and/or a high FR/R ratio cause a shift in carbon metabolism toward the accumulation of TG, a storage lipid. Eighteen-carbon fatty acid desaturation was also affected in highly shaded leaves where a reduction in linolenic acid (18:3) content was accompanied by a proportional increase in oleic (18:1) and linoleic (18:2) acids.     

Concentration-dependent effects of salicylaldoxime on chloroplast reactions

Salicylaldoxime has been found to have a variety of concentration-dependent effects on chloroplast activities. At low concentrations (< 10 mM), salicylaldoxime reversibly inhibits all reactions which involve Photosystem II. Since the DCMU-insensitive silicomolybdate Hill reaction is also inhibited, one site of inhibition is definitely located before the DCMU-sensitive site, possibly before the photoact. The inhibition kinetics and the response of chloroplast fluorescence may indicate another site in the DCMU-sensitive region. At almost exactly the same concentrations (< 10 mM), salicylaldoxime uncouples phosphorylation reversibly, whether it is supported by Photosystem II or by Photosystem I. At higher concentrations (approx. 20 mM) salicylaldoxime inhibits Photosystem II irreversibly, uncouples irreversibly, and begins to cause changes in chloroplast light scattering which could be manifestations of membrane damage. At very high concentrations (approx. 45 mM) salicylaldoxime irreversibly inhibits Photosystem I activity in the region of plastocyanin. This is indicated by the ability of salicylaldoxime to inhibit the photooxidation of cytochrome f but not the photooxidation of P-700.     

Localization of the reaction side of plastocyanin from immunological and kinetic studies with inside-out thylakoid vesicles

(1) The effect of four active antisera against plastocyanin on Photosystem I-driven electron transport and phosphorylation was investigated in spinach chloroplasts. Partial inhibition of electron transport and stimulation of plastocyanin-dependent phosphorylation were sometimes observed after adding amounts of antibodies which were in large excess and not related to the plastocyanin content of the chloroplasts. This indicates effects of the antibodies on the membrane. (2) The antibodies against plastocyanin neither directly nor indirectly agglutinated unbroken chloroplast membranes. (3) The plastocyanin content of right-side-out and inside-out thylakoid vesicles isolated by aqueous polymer two-phase partition from chloroplasts disrupted by Yeda press treatment was determined by quantitative rocket electroimmunodiffusion. Right-side-out vesicles retained about 25%, inside-out vesicles none of the original amount of plastocyanin. (4) The effect of externally added plastocyanin on the reduction of P-700 was studied by monitoring the absorbance changes at 703 nm after a long flash. In inside-out vesicles P-700 was reduced by the added plastocyanin but not in right-side-out vesicles and class II chloroplasts. These results provide strong evidence for a function of plastocyanin at the internal side of the thylakoid membrane.     

Polylysine-enhanced effectiveness of plastocyanin in photosystem I

1. Chloroplast fragments from either Chlamydomonas reinhardi or spinach, which lack plastocyanin, or from Euglena gracilis depleted of cytochrome c552, require a large excess of exogenously added plastocyanin or cytochrome c552 to restore Photosystem I activity.2. In the presence of a small amount of polylysine, Photosystem I activity of chloroplast fragments is stimulated greatly by plastocyanin or cytochrome c552, and the reaction is saturated at a lower concentration of these proteins. Higher concentrations of polylysine inhibit Photosystem I activity; the inhibition is not reversed by plastocyanin or cytochrome c552.3. Salt protects chloroplast fragments from stimulation by polylysine plus plastocyanin or cytochrome c552, and also reverses this stimulation.4. The data suggest that polylysine, at low concentration, enhances binding of plastocyanin or cytochrome c552 to chloroplast membranes, thereby increasing the effective concentration at their site of function. The total inhibition of Photosystem I activity, independent of the presence of plastocyanin or cytochrome c552, at higher polylysine concentrations is similar probably to that observed previously in chloroplasts which retain their plastocyanin.     

Transient kinetics of the electron transfer between P-700, plastocyanin and cytochrome f in chloroplasts suspended in fluid media at sub-zero temperatures

The kinetics of redox changes of P-700, plastocyanin and cytochrome f in chloroplasts suspended in a fluid medium at sub-zero temperatures have been studied following excitation of the chloroplasts with either a single-turnover flash, a series of flashes or continuous light. The results show that: (1) The kinetics of reduction of P-700⁺ and those of oxidation of plastocyanin are consistent with a bimolecular reaction between these two components as previously suggested (Olsen, L.F., Cox, R.P. and Barber, J. (1980) FEBS Lett. 122, 13–16). (2) Cytochrome f shows heterogeneity with respect to its kinetics of oxidation by Photosystem I. (3) In contrast to the situation when plastoquinol is the electron donor, reduction of cytochrome f by electrons derived from diaminodurene occurs with sigmoidal kinetics that shows a good fit to an apparent equilibrium constant of 12 between the cytochrome and P-700. (4) The rate of electron transfer from plastoquinol to Photosystem I depends on the redox state of the plastoquinone pool. (5) In relation to current ideas about the lateral heterogeneity of Photosystem I and Photosystem II in the thylakoid membrane, the results are consistent with the function of plastocyanin as a mobile carrier of electrons in the intrathylakoid space.     

Range of photosynthetic control of postillumination P700+ reduction rate in sunflower leaves

The kinetics of the postillumination reduction of P700⁺ which reflects the rate constant for plastoquinol (PQH₂) oxidation was recorded in sunflower leaves at different photon absorption densities (PAD), CO₂ and O₂ concentrations. The P700 oxidation state was calculated from the leaf transmittance at 830 nm logged at 50 s intervals. The P700⁺ dark reduction kinetics were fitted with two exponents with time constants of 6.5 and about 45 ms at atmospheric CO₂ and O₂ concentrations. The time constant of the fast component, which is the major contributor to the linear electron transport rate (ETR), did not change over the range of PADs of 14.5 to 134 nmol cm^-2 s^-1 in 21% O₂, but it increased up to 40 ms under severe limitation of ETR at low O₂ and CO₂. The acceptor side of Photosystem I (PS I) became reduced in correlation with the downregulation of the PQH₂ oxidation rate constant. It is concluded that thylakoid pH-related downregulation of the PQH₂ oxidation rate constant (photosynthetic control) is not present under normal atmospheric conditions but appears under severe limitation of the availability of electron acceptors. The measured range of photosynthetic control fits with the maximum variation of ETR under natural stress in C₃ plants. Increasing the carboxylase/oxygenase specificity would lead to higher reduction of the PS I acceptor side under stress.Abbreviations Cyt b ₆ f cytochrome b ₆ f complex - C_w cell-wall CO₂ concentration, M - ETR electron transport rate - Fd ferredoxin - FNR ferredoxin-NADP reductase - FRL far-red light - PC plastocyanin - PAD photon absorption density nmol cm^-2 s^-1 - PFD photon flux density nmol cm^-2 s^-1 - PS I Photosystem I complex - PQ plastoquinon - PQH₂ plastoquinol - PS II Photosystem II complex - P700 Photosystem I donor pigment, reduced - S830 830 nm signal (D830, difference of S830 from the dark level) - WL white light - Y_l maximum quantum yield of PS I electron transport, rel. un     

Ontogenetic Change of the PSI- and PSII-Distribution in the Thylakoid System of Euglena gracilis

The thylakoid membranes of isolated Euglena chloroplasts wereseparated into two fractions by aqueous two-phase-partitioning(mixture of dextran 500 and poly(ethylene glycol) 4000) followingpress disruption. These two fractions differ in many respectsduring most of the cell cycle of this alga in comparison withthe thylakoid characteristics of higher plants or green algae.The amount of thylakoid membranes with separation characteristicscomparable with inside-out-vesicles of higher plant chloroplastschanges depending on the cell cycle stage of Euglena gracilis.Photosystems II and I are not restricted to one fraction. Boththylakoid membrane fractions evolve oxygen photosynthetically.When chloroplast differentiation in Euglena gracilis is complete(i.e. at the end of the light-time) the composition and thephotosynthetic efficiency of the two thylakoid fractions aregenerally equal. Photosystem I-related LHCI is present in bothfractions. Photosystem II-related CP29, however, was only detectedin unfractionated thylakoid membranes. The implications forthylakoid organization in Euglena chloroplasts are discussed. Key words: Euglena gracilis, photosystem I, photosystem II, stacking, thylakoids     

Short-term responses of Photosystem I to heat stress

When 23°C-grown potato leaves (Solanum tuberosum L.) were exposed for 15 min to elevated temperatures in weak light, a dramatic and preferential inactivation of Photosystem (PS) II was observed at temperatures higher than about 38°C. In vivo photoacoustic measurements indicated that, concomitantly with the loss of PS II activity, heat stress induced a marked gas-uptake activity both in far-red light (>715 nm) exciting only PS I and in broadband light (350–600 nm) exciting PS I and PS II. In view of its suppression by nitrogen gas and oxygen and its stimulation by high carbon-dioxide concentrations, the bulk of the photoacoustically measured gas uptake by heat-stressed leaves was ascribed to rapid carbon-dioxide solubilization in response to light-modulated stroma alkalization coupled to PS I-driven electron transport. Heat-induced gas uptake was observed to be insensitive to the PS II inhibitor diuron, sensitive to the plastocyanin inhibitor HgCl₂ and saturated at a rather high photon flux density of around 1200 E m^–2 s^–1. Upon transition from far-red light to darkness, the oxidized reaction center P700⁺ of PS I was re-reduced very slowly in control leaves (with a half time t_1/2 higher than 500 ms), as measured by leaf absorbance changes at around 820 nm. Heat stress caused a spectacular acceleration of the postillumination P700⁺ reduction, with t_1/2 falling to a value lower than 50 ms (after leaf exposure to 48°C). The decreased t_1/2 was sensitive to HgCl₂ and insensitive to diuron, methyl viologen (an electron acceptor of PS I competing with the endogenous acceptor ferredoxin) and anaerobiosis. This acceleration of the P700⁺ reduction was very rapidly induced by heat treatment (within less than 5 min) and persisted even after prolonged irradiation of the leaves with far-red light. After heat stress, the plastoquinone pool exhibited reduction in darkness as indicated by the increase in the apparent Fo level of chlorophyll fluorescence which could be quenched by far-red light. Application (for 1 min) of far-red light to heat-pretreated leaves also induced a reversible quenching of the maximal fluorescence level Fm, suggesting formation of a pH gradient in far-red light. Taken together, the presented data indicate that PS I responded to the heat-induced loss of PS II photochemical activity by catalyzing an electron flow from stromal reductants. Heat-stress-induced PS I electron transport independent of PS II seems to constitute a protective mechanism since block of this electron pathway in anaerobiosis was observed to result in a dramatic photoinactivation of PS I.Abbreviations PFD photon flux density - PS Photosystem - Apt and Aox amplitude of the photothermal and photobaric components of the photoacoustic signal, respectively - P700 reaction center pigment of PS I - Fo and Fm initial and maximal levels of chlorophyll fluorescence, respectively - Fv=Fm Fo-variable chlorophyll fluorescence - Q_A primary (stable) electron acceptor of PS II - DCMU (diuron) 3-(3,4-dichlorophenyl)-1,1-dimethylurea - Cyt cytochrome     

Phosphofructokinase isozymes of Morris hepatomas

The spin-lattice relaxation rate of solvent protons in suspensions of chloroplast thylakoid membranes undergoes a large transient depression following illumination in white light. This change appears to require the presence of chelatable paramagnetic ions; it is absent in chloroplasts exposed to 1 mm EDTA during the homogenization step of the isolation procedure, but reappears when 50 μm MnCl₂ is added to these suspensions. Conditions that inhibit light-induced R₁ changes are (i) anaerobiosis, (ii) inhibition of plastocyanin function byHg⁺²/CN, and (iii) the presence of superoxide dismutase. These observations suggest that chemical oxidation of nonfunctional Mn (II) by superoxide ion, which is generated under aerobic conditions by autooxidizable acceptors of Photosystem I, is responsible for the phenomenon. This interpretation was confirmed by experiments involving superoxide generation in the dark, using the NADPH-driven diaphorase activity of ferredoxin-NADP-reductase with benzylviologen as an autooxidizable acceptor.     

Genetic engineering of thylakoid protein complexes by chloroplast transformation in Chlamydomonas reinhardtii

Chloroplast transformation of Chlamydomonas reinhardtii has developed into a powerful tool for studying the structure, function and assembly of thylakoid protein complexes in a eukaryotic organism. In this article we review the progress that is being made in the development of procedures for efficient chloroplast transformation. This focuses on the development of selectable markers and the use of Chlamydomonas mutants, individually lacking thylakoid protein complexes, as recipients. Chloroplast transformation has now been used to engineer all four major thylakoid protein complexes, photosystem II, photosystem I, cytochrome b ₆/f and ATP synthase. These results are discussed with an emphasis on new insights into assembly and function of these complexes in chloroplasts as compared with their prokaryotic counterparts.Abbreviations ENDOR electron nuclear double resonance - ESEEM electron spin echo envelope modulation - LHC light harvesting complex - PSI Photosystem I - PS II Photosystem II - P₆₈₀ primary electron donor in PS II - P₇₀₀ primary electron donor in PS I     

Photosystem I polypeptides

Photosystem I mediates light-induced electron transport from reduced plastocyanin in the thylakoid lumen to oxidized ferredoxin in the stroma. Photosystem I is located in the stroma lamellae of the thylakoid system and consists of a peripheral light-harvesting pigment-protein complex and a core complex carrying the electron transfer components and additional antenna pigments. The core complex consists of 11 different polypeptide subunits, five of which are chloroplast encoded and six of which are encoded by nuclear genes. The structure and function of the different subunits of the photosystem 1 core complex is discussed.     

Molecular organization of oxygen-evolution system in chloroplast

The main function of Photosystem II in chloroplast is to oxidize water molecules to produce oxygen. Strong oxidant produced by photoreaction at Photosystem II reaction center derives electrons from water and the electrons are transferred via Photosystem I to NADP⁺. The components required for water oxidation in Photosystem II were identified and their molecular properties as well as their roles in the oxygen evolution process were elucidated. The entity of the oxygen evolution system is a supramolecular complex of Photosystem II in the thylakoid membrane where reaction center binding polypeptides, three extrinsic polypeptides, managenese atoms, Ca²⁺ and Cl⁻ ions are the essential components, and they constitute a specific catalytic domain for water oxidation. Recipient of the Botanical Society Award for Young Scientists, 1988.     

The influence of high levels of brief or prolonged supplementary far-red illumination during growth on the photosynthetic characteristics, composition and morphology of Pisum sativum chloroplasts

Abstract. Peas were grown in controlled environments (12h white fluorescent light. ∼47 μmol photons m^-2 s 1/12 dark, 25 °C), using (1) 15-min far-red illumination at the end of each photoperiod (brief FR) to simulate the increase in the far-red/red ratio near the end of the day, and (2) high levels of supplementary far-red light (red:far-red ratio=0.04) during the entire photoperiod (long-term FR) to simulate extreme shade conditions under a plant canopy. Brief FR illumination led to marked morphological effects attributable to phytochrome regulation, namely, an increase in internodal length, but a decrease in leaflet area, chloroplast size and chlorophyll content per chloroplast compared with the control. Significantly, brief FR illumination had little or no effect on the amounts of the major chloroplast components (ribulose 1.5-biphosphate carboxylase, adenosine triphosphate synthase, cytochrome b/f complex and Photosystem II) relative to chlorophyll or Photosystem I, and the leaf photosynthetic capacities per unit chlorophyll were similar. In contrast, supplementing high levels of far-red light during the entire photoperiod not only led to the phytochrome effects above, but there was also a marked increase in leaf photosynthetic capacity per unit chlorophyll. due to increased amounts of the major chloroplast components relative to chlorophyll or Photosystem I. We hypothesize that supplementary far-red light, absorbed by Photosystem I, induced an increase in the major chloroplast components by a photosynthetic feedback mechanism. In fully greened leaves, we propose that the two photosystems themselves, rather than phytochrome, may be the predominent sensors of light quantity in triggering modulations of the stoichiometries of chloroplast components, which in turn lead to varying photosynthetic capacities.     

Contrasting effects of plastocyanin on the photoreduction and photooxidation of cytochrome f in chloroplasts

Removal of plastocyanin from Photosystem I subchloroplast particles had no effect on the Photosystem I photooxidation of cytochrome f. Chloroplasts depleted of plastocyanin by sonication lost the ability to reduce cytochrome f in Photosystem II light. Addition of plastocyanin restored the photoreduction of cytochrome f. These results are consistent with a plastocyanin site on the reducing side of cytochrome f.     

Excitation energy transfer and charge separation in the isolated Photosystem II reaction center

The nature of excitation energy transfer and charge separation in isolated Photosystem II reaction centers is an area of considerable interest and controversy. Excitation energy transfer from accessory chlorophyll a to the primary electron donor P680 takes place in tens of picoseconds, although there is some evidence that thermal equilibration of the excitation between P680 and a subset of the accessory chlorophyll a occurs on a 100-fs timescale. The intrinsic rate for charge separation at low temperature is accepted to be ca. (2 ps)^–1, and is based on several measurements using different experimental techniques. This rate is in good agreement with estimates based on larger sized particles, and is similar to the rate observed with bacterial reaction centers. However, near room temperature there is considerable disagreement as to the observed rate for charge separation, with several experiments pointing to a ca. (3 ps)^–1 rate, and others to a ca. (20 ps)^-1 rate. These processes and the experiments used to measure them will be reviewed.Abbreviations Chl chlorophyll - FWHM full-width at half-maximum - Pheo pheophytin - PS II Photosystem II - P680 primary electron donor of the Photosystem II reaction center - RC reaction center The US Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

The structure and function of the chloroplast photosynthetic membrane — a model for the domain organization

Recent work on the domain organization of the thylakoid is reviewed and a model for the thylakoid of higher plants is presented. According to this model the thylakoid membrane is divided into three main domains: the stroma lamellae, the grana margins and the grana core (partitions). These have different biochemical compositions and have specialized functions. Linear electron transport occurs in the grana while cyclic electron transport is restricted to the stroma lamellae. This model is based on the following results and considerations. (1) There is no good candidate for a long-range mobile redox carrier between PS II in the grana and PS I in the stroma lamellae. The lateral diffusion of plastoquinone and plastocyanin is severely restricted by macromolecular crowding in the membrane and the lumen respectively. (2) There is an excess of 14±18% chlorophyll associated with PS I over that of PS II. This excess is assumed to be localized in the stroma lamellae where PS I drives cyclic electron transport. (3) For several plant species, the stroma lamellae account for 20±3% of the thylakoid membrane and the grana (including the appressed regions, margins and end membranes) for the remaining 80%. The amount of stroma lamellae (20%) corresponds to the excess (14–18%) of chlorophyll associated with PS I. (4) The model predicts a quantum requirement of about 10 quanta per oxygen molecule evolved, which is in good agreement with experimentally observed values. (5) There are at least two pools of each of the following components: PS I, PS II, cytochrome bf complex, plastocyanin, ATP synthase and plastoquinone. One pool is in the grana and the other in the stroma compartments. So far, it has been demonstrated that the PS I, PS II and cytochrome bf complexes each differ in their respective pools.Abbreviations PS I and PS II Photosystem I and II - P 700 reaction center of PS I - LHC II light-harvesting complex II