The coupled photocycle of phenyl-p-benzoquinone and Light-Harvesting Complex II (LHCII) within the biohybrid system

The combination of trimeric form of the light-harvesting complex II (LHCII₃), a porous graphite electrode (GE), and the application of phenyl-p-benzoquinone (PPBQ), the quinone derivative, allow the construction of a new type of biohybrid photoactive system. The Chl fluorescence decay and voltammetric analyzes revealed that PPBQ impacts LHCII₃ proportionally to accessible quenching sites and that PPBQ forms redox complexes with Chl in both ground and excited states. As a result, photocurrent generation is directly dependent on PPBQ-induced quenching of Chl fluorescence. Since PPBQ also undergoes photoactivation, the action of GE-LHCII₃-PPBQ depends on the mutual coupling of LHCII₃ and PPBQ photocycles. The GE-LHCII₃-PPBQ generates a photocurrent of up to 4.5 µA and exhibits considerable stability during operation. The three-dimensional arrangement of graphite scraps in GE builds an active electrode surface and stabilizes LHCII₃ in its native form in low-density multilayers. The results indicate the future usability of such designed photoactive device.

The SnRK2.10 kinase mitigates the adverse effects of salinity by protecting photosynthetic machinery

SNF1-Related protein kinases Type 2 (SnRK2) are plant-specific enzymes widely distributed across the plant kingdom. They are key players controlling abscisic acid (ABA)-dependent and ABA-independent signaling pathways in the plant response to osmotic stress. Here we established that SnRK2.4 and SnRK2.10, ABA-nonactivated kinases, are activated in Arabidopsis thaliana rosettes during the early response to salt stress and contribute to leaf growth retardation under prolonged salinity but act by maintaining different salt-triggered mechanisms. Under salinity, snrk2.10 insertion mutants were impaired in the reconstruction and rearrangement of damaged core and antenna protein complexes in photosystem II (PSII), which led to stronger non-photochemical quenching, lower maximal quantum yield of PSII, and lower adaptation of the photosynthetic apparatus to high light intensity. The observed effects were likely caused by disturbed accumulation and phosphorylation status of the main PSII core and antenna proteins. Finally, we found a higher accumulation of reactive oxygen species (ROS) in the snrk2.10 mutant leaves under a few-day-long exposure to salinity which also could contribute to the stronger damage of the photosynthetic apparatus and cause other deleterious effects affecting plant growth. We found that the snrk2.4 mutant plants did not display substantial changes in photosynthesis. Overall, our results indicate that SnRK2.10 is activated in leaves shortly after plant exposure to salinity and contributes to salt stress tolerance by maintaining efficient photosynthesis and preventing oxidative damage.

A Phage Display-Identified Short Peptide Capable of Hydrolyzing Calcium Pyrophosphate Crystals-The Etiological Factor of Chondrocalcinosis

Chondrocalcinosis is a metabolic disease caused by the presence of calcium pyrophosphate dihydrate crystals in the synovial fluid. The goal of our endeavor was to find out whether short peptides could be used as a dissolving factor for such crystals. In order to identify peptides able to dissolve crystals of calcium pyrophosphate, we screened through a random library of peptides using a phage display. The first screening was designed to select phages able to bind the acidic part of alendronic acid (pyrophosphate analog). The second was a catalytic assay in the presence of crystals. The best-performing peptides were subsequently chemically synthesized and rechecked for catalytic properties. One peptide, named R25, turned out to possess some hydrolytic activity toward crystals. Its catalysis is Mg²⁺-dependent and also works against soluble species of pyrophosphate.

Too rigid to fold: Carotenoid-dependent decrease in thylakoid fluidity hampers the formation of chloroplast grana

In chloroplasts of land plants, the thylakoid network is organized into appressed regions called grana stacks and loosely arranged parallel stroma thylakoids. Many factors determining such intricate structural arrangements have been identified so far, including various thylakoid-embedded proteins, and polar lipids that build the thylakoid matrix. Although carotenoids are important components of proteins and the lipid phase of chloroplast membranes, their role in determining the thylakoid network structure remains elusive. We studied 2D and 3D thylakoid network organization in carotenoid-deficient mutants (ccr1-1, lut5-1, szl1-1, and szl1-1npq1-2) of Arabidopsis (Arabidopsis thaliana) to reveal the structural role of carotenoids in the formation and dynamics of the internal chloroplast membrane system. The most significant structural aberrations took place in chloroplasts of the szl1-1 and szl1-1npq1-2 plants. Increased lutein/carotene ratio in these mutants impaired the formation of grana, resulting in a significant decrease in the number of thylakoids used to build a particular stack. Further, combined biochemical and biophysical analyses revealed that hampered grana folding was related to decreased thylakoid membrane fluidity and significant changes in the amount, organization, and phosphorylation status of photosystem (PS) II (PSII) supercomplexes in the szl1-1 and szl1-1npq1-2 plants. Such changes resulted from a synergistic effect of lutein overaccumulation in the lipid matrix and a decreased level of carotenes bound with PS core complexes. Moreover, more rigid membrane in the lutein overaccumulating plants led to binding of Rubisco to the thylakoid surface, additionally providing steric hindrance for the dynamic changes in the level of membrane folding.

In vivo creation of plasmid pCRT01 and its use for the construction of carotenoid-producing Paracoccus spp. strains that grow efficiently on industrial wastes

Background

Carotenoids are natural tetraterpene pigments widely utilized in the food, pharmaceutical and cosmetic industries. Currently, chemical synthesis of these compounds outperforms their production in Escherichia coli or yeast due to the limited efficiency of the latter. The use of natural microbial carotenoid producers, such as bacteria of the genus Paracoccus (Alphaproteobacteria), may help to optimize this process. In order to couple the ability to synthesize these pigments with the metabolic versatility of this genus, we explored the possibility of introducing carotenoid synthesis genes into strains capable of efficient growth on simple low-cost media.

Results

We constructed two carotenoid-producing strains of Paracoccus carrying a new plasmid, pCRT01, which contains the carotenoid synthesis gene locus crt from Paracoccus marcusii OS22. The plasmid was created in vivo via illegitimate recombination between crt-carrying vector pABW1 and a natural “paracoccal” plasmid pAMI2. Consequently, the obtained fusion replicon is stably maintained in the bacterial population without the need for antibiotic selection. The introduction of pCRT01 into fast-growing “colorless” strains of Paracoccus aminophilus and Paracoccus kondratievae converted them into efficient producers of a range of both carotenes and xanthophylls. The exact profile of the produced pigments was dependent on the strain genetic background. To reduce the cost of carotenoid production in this system, we tested the growth and pigment synthesis efficiency of the two strains on various simple media, including raw industrial effluent (coal-fired power plant flue gas desulfurization wastewater) supplemented with molasses, an industrial by-product rich in sucrose.

Conclusions

We demonstrated a new approach for the construction of carotenoid-producing bacterial strains which relies on a single plasmid-mediated transfer of a pigment synthesis gene locus between Paracoccus strains. This strategy facilitates screening for producer strains in terms of synthesis efficiency, pigment profile and ability to grow on low-cost industrial waste-based media, which should increase the cost-effectiveness of microbial production of carotenoids.

Small Cyclic Peptide for Pyrophosphate Dependent Ligation in Prebiotic Environments

All life on Earth uses one universal biochemistry stemming from one universal common ancestor of all known living organisms. One of the most striking features of this universal biochemistry is its utter dependence on phosphate group transfer between biochemical molecules. Both nucleic acid and peptide biological synthesis relies heavily on phosphate group transfer. Such dependents strongly indicate very early incorporation of phosphate chemistry in the origin of life. Perhaps as early as prebiotic soup stage. We report here on a short cyclic peptide, c(RPDDHR), designed rationally for pyrophosphate interaction, which is able to create a new amide bond dependent on the presence of pyrophosphate. We believe this result to be a first step in the exploration of Phosphate Transfer Catalysts that must have been present and active in prebiotic soup and must have laid down foundations for the universal bioenergetics.

Specific Composition of Lipid Phases Allows Retaining an Optimal Thylakoid Membrane Fluidity in Plant Response to Low-Temperature Treatment

Thylakoid membranes isolated from leaves of two plant species, the chilling tolerant (CT) pea and chilling sensitive (CS) runner bean, were assessed for the composition of lipids, carotenoids as well as for the arrangement of photosynthetic complexes. The response to stress conditions was investigated in dark-chilled and subsequently photo-activated detached leaves of pea and bean. Thylakoids of both species have a similar level of monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), but different sulfoquinovosyldiacylglycerol to phosphatidylglycerol (PG) ratio. In pea thylakoid fraction, the MGDG, DGDG and PG, have a higher double bond index (DBI), whereas bean thylakoids contain higher levels of high melting point PG. Furthermore, the lutein to the β-carotene ratio is higher in bean thylakoids. Smaller protein/lipid ratio in pea than in bean thylakoids suggests different lipid-protein interactions in both species. The differences between species are also reflected by the course of temperature-dependent plots of chlorophyll fluorescence pointing various temperatures of the lipid phase transitions of pea and bean thylakoids. Our results showed higher fluidity of the thylakoid membrane network in pea than in bean in optimal temperature conditions. Dark-chilling decreases the photochemical activity and induces significant degradation of MGDG in bean but not in pea leaves. Similarly, substantial changes in the arrangement of photosynthetic complexes with increase in LHCII phosphorylation and disturbances of the thylakoid structure take place in bean thylakoids only. Changes in the physical properties of bean thylakoids are manifested by the conversion of a three-phase temperature-dependent plot to a one-phase plot. Subsequent photo-activation of chilled bean leaves caused a partial restoration of the photochemistry and of membrane physical properties, but not of the photosynthetic complexes arrangement nor the thylakoid network structure. Summarizing, the composition of the thylakoid lipid matrix of CT pea allows retaining the optimal fluidity of its chloroplast membranes under low temperatures. In contrast, the fluidity of CS bean thylakoids is drastically changed, leading to the reorganization of the supramolecular structure of the photosynthetic complexes and finally results in structural remodeling of the CS bean thylakoid network.

Galactolipid deficiency disturbs spatial arrangement of the thylakoid network in Arabidopsis thaliana plants

The chloroplast thylakoid network is a dynamic structure which, through possible rearrangements, plays a crucial role in regulation of photosynthesis. Although the importance of the main components of the thylakoid membrane matrix, galactolipids, in the formation of the network of internal plastid membrane was found before, the structural role of monogalactosyldiacylglycerol (MGDG) and digalactosylidacylglycerol (DGDG) is still largely unknown. We elucidated detailed structural modifications of the thylakoid membrane system in Arabidopsis thaliana MGDG- and DGDG-deficient mutants. An altered MGDG/DGDG ratio was structurally reflected by formation of smaller grana, local changes in grana stacking repeat distance, and significant changes in the spatial organization of the thylakoid network compared with wild-type plants. The decrease of the MGDG level impaired the formation of the typical helical grana structure and resulted in a 'helical-dichotomic' arrangement. DGDG deficiency did not affect spatial grana organization but changed the shape of the thylakoid membrane network in situ from lens like into a flattened shape. Such structural disturbances were accompanied by altered composition of carotenoid and chlorophyll-protein complexes, which eventually led to the decreased photosynthetic efficiency of MGDG- and DGDG-deficient plants.

Electrochemical characterization of LHCII on graphite electrodes - Potential-dependent photoactivation and arrangement of complexes

Light-dependent electrochemical properties of the light harvesting complexes of Photosystem II (LHCII) and the corresponding interactions with screen-printed graphite electrodes (GEs) are determined. No exogenous soluble redox mediators are used. LHCII isolated from spinach leaves are immobilized on GE by physical adsorption and through interactions with glutaraldehyde. Importantly, the insertion of LHCII into the pores of a GE is achieved by subjecting the electrode to specific potentials. Both trimeric and aggregated forms of LHCII located within the graphite layer retain their native structures. Voltammetric current peaks centred at ca. -230 and + 50 mV vs. Ag/AgCl (+94 and + 374 mV vs. NHE) limit the investigation of the reduction and oxidation processes of immobilized LHCII. An anodic photocurrent is generated in the LHCII-GE proportional to light intensity and can reach a value of 150 nA/cm². Light-dependent charge separation in LHCII followed by electron transfer to the GE occurs only at potentials of above -200 mV vs. Ag/AgCl (+124 mV vs. NHE). Our results illustrate the importance of the structural proximity of LHCII and GE for photocurrent generation.

Dark-chilling and subsequent photo-activation modulate expression and induce reversible association of chloroplast lipoxygenase with thylakoid membrane in runner bean (Phaseolus coccineus L.)

Lipoxygenases (LOXs) are non-haem iron-containing dioxygenases that catalyse oxygenation of polyunsaturated fatty acids. This reaction is the first step in biosynthesis of oxylipins, which play important and diverse roles in stress response. In this study, we identified four LOX genes (PcLOXA, B, C, D) in chilling-sensitive runner bean (Phaseolus coccineus L.) plant and analyzed their expression patterns during long term dark-chilling (4 °C) stress and during day/night (21ºC/4 °C) temperature fluctuations. Three of the four identified LOX genes, namely PcLOXA, PcLOXB and PcLOXD, were induced by wounding stress, while only the PcLOXA was induced by dark-chilling of both detached (wounded) leaves and whole plants. We identified PcLOXA as a chloroplast-targeted LOX protein and investigated its expression during chilling stress in terms of abundance, localization inside chloroplasts and interactions with the thylakoid membranes. The analysis by immunogold electron microscopy has shown that more than 60% of detectable PcLOXA protein was associated with thylakoids, and dark-chilling of leaves resulted in increased amounts of this protein detected within grana margins of thylakoids. This effect was reversible under subsequent photo-activation of chilled leaves. PcLOXA binding to thylakoids is not mediated by the posttranslational modification but rather is based on direct interactions of the protein with membrane lipids; the binding strength increases under dark-chilling conditions.

A chloroplast "wake up" mechanism: Illumination with weak light activates the photosynthetic antenna function in dark-adapted plants

The efficient and fluent operation of photosynthesis in plants relies on activity of pigment-protein complexes called antenna, absorbing light and transferring excitations toward the reaction centers. Here we show, based on the results of the fluorescence lifetime imaging analyses of single chloroplasts, that pigment-protein complexes, in dark-adapted plants, are not able to act effectively as photosynthetic antennas, due to pronounced, adverse excitation quenching. It appeared that the antenna function could be activated by a short (on a minute timescale) illumination with light of relatively low intensity, substantially below the photosynthesis saturation threshold. The low-light-induced activation of the antenna function was attributed to phosphorylation of the major accessory light-harvesting complex LHCII, based on the fact that such a mechanism was not observed in the stn7 Arabidopsis thaliana mutant, with impaired LHCII phosphorylation. It is proposed that the protein phosphorylation-controlled change in the LHCII clustering ability provides mechanistic background for this regulatory process.

Dark-chilling induces substantial structural changes and modifies galactolipid and carotenoid composition during chloroplast biogenesis in cucumber (Cucumis sativus L.) cotyledons

Plants in a temperate climate are often subject to different environmental factors, chilling stress among them, which influence the growth especially during early stages of plant development. Chloroplasts are one of the first organelles affected by the chilling stress. Therefore the proper biogenesis of chloroplasts in early stages of plant growth is crucial for undertaking the photosynthetic activity. In this paper, the analysis of the cotyledon chloroplast biogenesis at different levels of plastid organization was performed in cucumber, one of the most popular chilling sensitive crops. Influence of low temperature on the ultrastructure was manifested by partial recrystallization of the prolamellar body, the formation of elongated grana thylakoids and a change of the prolamellar body structure from the compacted "closed" type to a more loose "open" type. Structural changes are strongly correlated with galactolipid and carotenoid content. Substantial changes in the galactolipid and the carotenoid composition in dark-chilled plants, especially a decrease of the monogalactosyldiacylglycerol to digalactosyldiacylglycerol ratio (MGDG/DGDG) and an increased level of lutein, responsible for a decrease in membrane fluidity, were registered together with a slower adaptation to higher light intensity and an increased level of non-photochemical reactions. Changes in the grana thylakoid fluidity, of their structure and photosynthetic efficiency in developing chloroplasts of dark-chilled plants, without significant changes in the PSI/PSII ratio, could distort the balance of photosystem rearrangements and be one of the reasons of cucumber sensitivity to chilling.

Overlapping toxic effect of long term thallium exposure on white mustard (Sinapis alba L.) photosynthetic activity

BACKGROUND

Heavy metal exposure affect plant productivity by interfering, directly and indirectly, with photosynthetic reactions. The toxic effect of heavy metals on photosynthetic reactions has been reported in wide-ranging studies, however there is paucity of data in the literature concerning thallium (Tl) toxicity. Thallium is ubiquitous natural trace element and is considered the most toxic of heavy metals; however, some plant species, such as white mustard (Sinapis alba L.) are able to accumulate thallium at very high concentrations. In this study we identified the main sites of the photosynthetic process inhibited either directly or indirectly by thallium, and elucidated possible detoxification mechanisms in S. alba.

RESULTS

We studied the toxicity of thallium in white mustard (S. alba) growing plants and demonstrated that tolerance of plants to thallium (the root test) decreased with the increasing Tl(I) ions concentration in culture media. The root growth of plants exposed to Tl at 100 μg L(-1) for 4 weeks was similar to that in control plants, while in plants grown with Tl at 1,000 μg L(-1) root growth was strongly inhibited. In leaves, toxic effect became gradually visible in response to increasing concentration of Tl (100 - 1,000 μg L(-1)) with discoloration spreading around main vascular bundles of the leaf blade; whereas leaf margins remained green. Subsequent structural analyses using chlorophyll fluorescence, microscopy, and pigment and protein analysis have revealed different effects of varying Tl concentrations on leaf tissue. At lower concentration partial rearrangement of the photosynthetic complexes was observed without significant changes in the chloroplast structure and the pigment and protein levels. At higher concentrations, the decrease of PSI and PSII quantum yields and massive oxidation of pigments was observed in discolored leaf areas, which contained high amount of Tl. Substantial decline of the photosystem core proteins and disorder of the photosynthetic complexes were responsible for disappearance of the chloroplast grana.

CONCLUSIONS

Based on the presented results we postulate two phases of thallium toxicity on photosynthesis: the non-destructive phase at early stages of toxicant accumulation and the destructive phase that is restricted to the discolored leaf areas containing high toxicant content. There was no distinct border between the two phases of thallium toxicity in leaves and the degree of toxicity was proportional to the migration rate of the toxicant outside the vascular bundles. The three-fold (nearly linear) increase of Tl(I) concentration was observed in damaged tissue and the damage appears to be associated with the presence of the oxidized form of thallium - Tl(III).

Three-Dimensional Visualization of the Tubular-Lamellar Transformation of the Internal Plastid Membrane Network during Runner Bean Chloroplast Biogenesis

Chloroplast biogenesis is a complex process that is integrated with plant development, leading to fully differentiated and functionally mature plastids. In this work, we used electron tomography and confocal microscopy to reconstruct the process of structural membrane transformation during the etioplast-to-chloroplast transition in runner bean (Phaseolus coccineus). During chloroplast development, the regular tubular network of paracrystalline prolamellar bodies (PLBs) and the flattened porous membranes of prothylakoids develop into the chloroplast thylakoids. Three-dimensional reconstruction is required to provide us with a more complete understanding of this transformation. We provide spatial models of the bean chloroplast biogenesis that allow such reconstruction of the internal membranes of the developing chloroplast and visualize the transformation from the tubular arrangement to the linear system of parallel lamellae. We prove that the tubular structure of the PLB transforms directly to flat slats, without dispersion to vesicles. We demonstrate that the grana/stroma thylakoid connections have a helical character starting from the early stages of appressed membrane formation. Moreover, we point out the importance of particular chlorophyll-protein complex components in the membrane stacking during the biogenesis. The main stages of chloroplast internal membrane biogenesis are presented in a movie that shows the time development of the chloroplast biogenesis as a dynamic model of this process.

Potato Annexin STANN1 Promotes Drought Tolerance and Mitigates Light Stress in Transgenic Solanum tuberosum L. Plants

Annexins are a family of calcium- and membrane-binding proteins that are important for plant tolerance to adverse environmental conditions. Annexins function to counteract oxidative stress, maintain cell redox homeostasis, and enhance drought tolerance. In the present study, an endogenous annexin, STANN1, was overexpressed to determine whether crop yields could be improved in potato (Solanum tuberosum L.) during drought. Nine potential potato annexins were identified and their expression characterized in response to drought treatment. STANN1 mRNA was constitutively expressed at a high level and drought treatment strongly increased transcription levels. Therefore, STANN1 was selected for overexpression analysis. Under drought conditions, transgenic potato plants ectopically expressing STANN1 were more tolerant to water deficit in the root zone, preserved more water in green tissues, maintained chloroplast functions, and had higher accumulation of chlorophyll b and xanthophylls (especially zeaxanthin) than wild type (WT). Drought-induced reductions in the maximum efficiency and the electron transport rate of photosystem II (PSII), as well as the quantum yield of photosynthesis, were less pronounced in transgenic plants overexpressing STANN1 than in the WT. This conferred more efficient non-photochemical energy dissipation in the outer antennae of PSII and probably more efficient protection of reaction centers against photooxidative damage in transgenic plants under drought conditions. Consequently, these plants were able to maintain effective photosynthesis during drought, which resulted in greater productivity than WT plants despite water scarcity. Although the mechanisms underlying this stress protection are not yet clear, annexin-mediated photoprotection is probably linked to protection against light-induced oxidative stress.

A reaction center-dependent photoprotection mechanism in a highly robust photosystem II from an extremophilic red alga, Cyanidioschyzon merolae

Members of the rhodophytan order Cyanidiales are unique among phototrophs in their ability to live in extremely low pH levels and moderately high temperatures. The photosynthetic apparatus of the red alga Cyanidioschyzon merolae represents an intermediate type between cyanobacteria and higher plants, suggesting that this alga may provide the evolutionary link between prokaryotic and eukaryotic phototrophs. Although we now have a detailed structural model of photosystem II (PSII) from cyanobacteria at an atomic resolution, no corresponding structure of the eukaryotic PSII complex has been published to date. Here we report the isolation and characterization of a highly active and robust dimeric PSII complex from C. merolae. We show that this complex is highly stable across a range of extreme light, temperature, and pH conditions. By measuring fluorescence quenching properties of the isolated C. merolae PSII complex, we provide the first direct evidence of pH-dependent non-photochemical quenching in the red algal PSII reaction center. This type of quenching, together with high zeaxanthin content, appears to underlie photoprotection mechanisms that are efficiently employed by this robust natural water-splitting complex under excess irradiance. In order to provide structural details of this eukaryotic form of PSII, we have employed electron microscopy and single particle analyses to obtain a 17 Å map of the C. merolae PSII dimer in which we locate the position of the protein mass corresponding to the additional extrinsic protein stabilizing the oxygen-evolving complex, PsbQ'. We conclude that this lumenal subunit is present in the vicinity of the CP43 protein, close to the membrane plane.

Molecular architecture of plant thylakoids under physiological and light stress conditions: a study of lipid-light-harvesting complex II model membranes

In this study, we analyzed multibilayer lipid-protein membranes composed of the photosynthetic light-harvesting complex II (LHCII; isolated from spinach [Spinacia oleracea]) and the plant lipids monogalcatosyldiacylglycerol and digalactosyldiacylglycerol. Two types of pigment-protein complexes were analyzed: those isolated from dark-adapted leaves (LHCII) and those from leaves preilluminated with high-intensity light (LHCII-HL). The LHCII-HL complexes were found to be partially phosphorylated and contained zeaxanthin. The results of the x-ray diffraction, infrared imaging microscopy, confocal laser scanning microscopy, and transmission electron microscopy revealed that lipid-LHCII membranes assemble into planar multibilayers, in contrast with the lipid-LHCII-HL membranes, which form less ordered structures. In both systems, the protein formed supramolecular structures. In the case of LHCII-HL, these structures spanned the multibilayer membranes and were perpendicular to the membrane plane, whereas in LHCII, the structures were lamellar and within the plane of the membranes. Lamellar aggregates of LHCII-HL have been shown, by fluorescence lifetime imaging microscopy, to be particularly active in excitation energy quenching. Both types of structures were stabilized by intermolecular hydrogen bonds. We conclude that the formation of trans-layer, rivet-like structures of LHCII is an important determinant underlying the spontaneous formation and stabilization of the thylakoid grana structures, since the lamellar aggregates are well suited to dissipate excess energy upon overexcitation.

Molecular architecture of plant thylakoids under physiological and light stress conditions: a study of lipid-light-harvesting complex II model membranes

In this study, we analyzed multibilayer lipid-protein membranes composed of the photosynthetic light-harvesting complex II (LHCII; isolated from spinach [Spinacia oleracea]) and the plant lipids monogalcatosyldiacylglycerol and digalactosyldiacylglycerol. Two types of pigment-protein complexes were analyzed: those isolated from dark-adapted leaves (LHCII) and those from leaves preilluminated with high-intensity light (LHCII-HL). The LHCII-HL complexes were found to be partially phosphorylated and contained zeaxanthin. The results of the x-ray diffraction, infrared imaging microscopy, confocal laser scanning microscopy, and transmission electron microscopy revealed that lipid-LHCII membranes assemble into planar multibilayers, in contrast with the lipid-LHCII-HL membranes, which form less ordered structures. In both systems, the protein formed supramolecular structures. In the case of LHCII-HL, these structures spanned the multibilayer membranes and were perpendicular to the membrane plane, whereas in LHCII, the structures were lamellar and within the plane of the membranes. Lamellar aggregates of LHCII-HL have been shown, by fluorescence lifetime imaging microscopy, to be particularly active in excitation energy quenching. Both types of structures were stabilized by intermolecular hydrogen bonds. We conclude that the formation of trans-layer, rivet-like structures of LHCII is an important determinant underlying the spontaneous formation and stabilization of the thylakoid grana structures, since the lamellar aggregates are well suited to dissipate excess energy upon overexcitation.

Correlation between spatial (3D) structure of pea and bean thylakoid membranes and arrangement of chlorophyll-protein complexes

BACKGROUND

The thylakoid system in plant chloroplasts is organized into two distinct domains: grana arranged in stacks of appressed membranes and non-appressed membranes consisting of stroma thylakoids and margins of granal stacks. It is argued that the reason for the development of appressed membranes in plants is that their photosynthetic apparatus need to cope with and survive ever-changing environmental conditions. It is not known however, why different plant species have different arrangements of grana within their chloroplasts. It is important to elucidate whether a different arrangement and distribution of appressed and non-appressed thylakoids in chloroplasts are linked with different qualitative and/or quantitative organization of chlorophyll-protein (CP) complexes in the thylakoid membranes and whether this arrangement influences the photosynthetic efficiency.

RESULTS

Our results from TEM and in situ CLSM strongly indicate the existence of different arrangements of pea and bean thylakoid membranes. In pea, larger appressed thylakoids are regularly arranged within chloroplasts as uniformly distributed red fluorescent bodies, while irregular appressed thylakoid membranes within bean chloroplasts correspond to smaller and less distinguished fluorescent areas in CLSM images. 3D models of pea chloroplasts show a distinct spatial separation of stacked thylakoids from stromal spaces whereas spatial division of stroma and thylakoid areas in bean chloroplasts are more complex. Structural differences influenced the PSII photochemistry, however without significant changes in photosynthetic efficiency. Qualitative and quantitative analysis of chlorophyll-protein complexes as well as spectroscopic investigations indicated a similar proportion between PSI and PSII core complexes in pea and bean thylakoids, but higher abundance of LHCII antenna in pea ones. Furthermore, distinct differences in size and arrangements of LHCII-PSII and LHCI-PSI supercomplexes between species are suggested.

CONCLUSIONS

Based on proteomic and spectroscopic investigations we postulate that the differences in the chloroplast structure between the analyzed species are a consequence of quantitative proportions between the individual CP complexes and its arrangement inside membranes. Such a structure of membranes induced the formation of large stacked domains in pea, or smaller heterogeneous regions in bean thylakoids. Presented 3D models of chloroplasts showed that stacked areas are noticeably irregular with variable thickness, merging with each other and not always parallel to each other.

Chloroplast biogenesis - correlation between structure and function

Chloroplast biogenesis is a multistage process leading to fully differentiated and functionally mature plastids. Complex analysis of chloroplast biogenesis was performed on the structural and functional level of its organization during the photoperiodic plant growth after initial growth of seedlings in the darkness. We correlated, at the same time intervals, the structure of etioplasts transforming into mature chloroplasts with the changes in the photosynthetic protein levels (selected core and antenna proteins of PSI and PSII) and with the function of the photosynthetic apparatus in two plant species: bean (Phaseolus vulgaris L.) and pea (Pisum sativum L). We selected these plant species since we demonstrated previously that the mature chloroplasts differ in the thylakoid organization. We showed that the protein biosynthesis as well as photosynthetic complexes formation proceeds gradually in both plants in spite of periods of darkness. We found that both steady structural differentiation of the bean chloroplast and reformation of prolamellar bodies in pea were accompanied by a gradual increase of the photochemical activity in both species. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

3-D modelling of chloroplast structure under (Mg2+) magnesium ion treatment. Relationship between thylakoid membrane arrangement and stacking

We performed for the first time three-dimensional (3D) modelling of the entire chloroplast structure. Stacks of optical slices obtained by confocal laser scanning microscope (CLSM) provided a basis for construction of 3D images of individual chloroplasts. We selected pea (Pisum sativum) and bean (Phaseolus vulgaris) chloroplasts since we found that they differ in thylakoid organization. Pea chloroplasts contain large distinctly separated appressed domains while less distinguished appressed regions are present in bean chloroplasts. Different magnesium ion treatments were used to study thylakoid membrane stacking and arrangement. In pea chloroplasts, as demonstrated by 3D modelling, the increase of magnesium ion concentration changed the degree of membrane appression from wrinkled continuous surface to many distinguished stacked areas and significant increase of the inter-grana area. On the other hand 3D models of bean chloroplasts exhibited similar but less pronounced tendencies towards formation of appressed regions. Additionally, we studied arrangements of thylakoid membranes and chlorophyll-protein complexes by various spectroscopic methods, Fourier-transform infrared spectroscopy (FTIR) among others. Based on microscopic and spectroscopic data we suggested that the range of chloroplast structure alterations under magnesium ions treatment is a consequence of the arrangement of supercomplexes. Moreover, we showed that stacking processes always affect the structural changes of chloroplast as a whole.

Two isoforms of ferredoxin:NADP(+) oxidoreductase from wheat leaves: purification and initial biochemical characterization

Ferredoxin:NADP(+) oxidoreductase is an enzyme associated with the stromal side of the thylakoid membrane in the chloroplast. It is involved in photosynthetic linear electron transport to produce NADPH and is supposed to play a role in cyclic electron transfer, generating a transmembrane pH gradient allowing ATP production, if photosystem II is non-functional or no NADP(+) is available for reduction. Different FNR isoforms have been described in non-photosynthetic tissues, where the enzyme catalyses the NADPH-dependent reduction of ferredoxin (Fd), necessary for some biosynthetic pathways. Here, we report the isolation and purification of two FNR isoproteins from wheat leaves, called FNR-A and FNR-B. These forms of the enzyme were identified as products of two different genes, as confirmed by mass spectrometry. The molecular masses of FNR-A and FNR-B were 34.3 kDa and 35.5 kDa, respectively. The isoelectric point of both FNR-A and FNR-B was about 5, but FNR-B appeared more acidic (of about 0.2 pH unit) than FNR-A. Both isoenzymes were able to catalyse a NADPH-dependent reduction of dibromothymoquinone and the mixture of isoforms catalysed reduction of cytochrome c in the presence of Fd. For the first time, the pH- and ionic strength dependent oligomerization of FNRs is observed. No other protein was necessary for complex formation. The putative role of the two FNR isoforms in photosynthesis is discussed based on current knowledge of electron transport in chloroplasts.

Contrasting effect of dark-chilling on chloroplast structure and arrangement of chlorophyll-protein complexes in pea and tomato: plants with a different susceptibility to non-freezing temperature

The effect of dark-chilling and subsequent photoactivation on chloroplast structure and arrangements of chlorophyll-protein complexes in thylakoid membranes was studied in chilling-tolerant (CT) pea and in chilling-sensitive (CS) tomato. Dark-chilling did not influence chlorophyll content and Chl a/b ratio in thylakoids of both species. A decline of Chl a fluorescence intensity and an increase of the ratio of fluorescence intensities of PSI and PSII at 120 K was observed after dark-chilling in thylakoids isolated from tomato, but not from pea leaves. Chilling of pea leaves induced an increase of the relative contribution of LHCII and PSII fluorescence. A substantial decrease of the LHCII/PSII fluorescence accompanied by an increase of that from LHCI/PSI was observed in thylakoids from chilled tomato leaves; both were attenuated by photoactivation. Chlorophyll fluorescence of bright grana discs in chloroplasts from dark-chilled leaves, detected by confocal laser scanning microscopy, was more condensed in pea but significantly dispersed in tomato, compared with control samples. The chloroplast images from transmission-electron microscopy revealed that dark-chilling induced an increase of the degree of grana stacking only in pea chloroplasts. Analyses of O-J-D-I-P fluorescence induction curves in leaves of CS tomato before and after recovery from chilling indicate changes in electron transport rates at acceptor- and donor side of PS II and an increase in antenna size. In CT pea leaves these effects were absent, except for a small but irreversible effect on PSII activity and antenna size. Thus, the differences in chloroplast structure between CS and CT plants, induced by dark-chilling are a consequence of different thylakoid supercomplexes rearrangements.

Light-dependent reversal of dark-chilling induced changes in chloroplast structure and arrangement of chlorophyll-protein complexes in bean thylakoid membranes

Changes in chloroplast structure and rearrangement of chlorophyll-protein (CP) complexes were investigated in detached leaves of bean (Phaseolus vulgaris L. cv. Eureka), a chilling-sensitive plant, during 5-day dark-chilling at 1 degrees C and subsequent 3-h photoactivation under white light (200 mumol photons m(-2) s(-1)) at 22 degrees C. Although, no change in chlorophyll (Chl) content and Chl a/b ratio in all samples was observed, overall fluorescence intensity of fluorescence emission and excitation spectra of thylakoid membranes isolated from dark-chilled leaves decreased to about 50%, and remained after photoactivation at 70% of that of the control sample. Concomitantly, the ratio between fluorescence intensities of PSI and PSII (F736/F681) at 120 K increased 1.5-fold upon chilling, and was fully reversed after photoactivation. Moreover, chilling stress seems to induce a decrease of the relative contribution of LHCII fluorescence to the thylakoid emission spectra at 120 K, and an increase of that from LHCI and PSI, correlated with a decrease of stability of LHCI-PSI and LHCII trimers, shown by mild-denaturing electrophoresis. These effects were reversed to a large extent after photoactivation, with the exception of LHCII, which remained partly in the aggregated form. In view of these data, it is likely that dark-chilling stress induces partial disassembly of CP complexes, not completely restorable upon photoactivation. These data are further supported by confocal laser scanning fluorescence microscopy, which showed that regular grana arrangement observed in chloroplasts isolated from control leaves was destroyed by dark-chilling stress, and was partially reconstructed after photoactivation. In line with this, Chl a fluorescence spectra of leaf discs demonstrated that dark-chilling caused a decrease of the quantum yield PSII photochemistry (F(v)/F(m)) by almost 40% in 5 days. Complete restoration of the photochemical activity of PSII required 9 h post-chilling photoactivation, while only 3 h were needed to reconstruct thylakoid membrane organization and chloroplast structure. The latter demonstrated that the long-term dark-chilled bean leaves started to suffer from photoinhibition after transfer to moderate irradiance and temperature conditions, delaying the recovery of PSII photochemistry, independently of photo-induced reconstruction of PSII complexes.

The action of oxygen on chlorophyll fluorescence quenching and absorption spectra in pea thylakoid membranes under the steady-state conditions

The effect of oxygen concentration on both absorption and chlorophyll fluorescence spectra was investigated in isolated pea thylakoids at weak actinic light under the steady-state conditions. Upon the rise of oxygen concentration from anaerobiosis up to 412 microM a gradual absorbance increase around both 437 and 670 nm was observed, suggesting the disaggregation of LHCII and destacking of thylakoids. Simultaneously, an increase in oxygen concentration resulted in a decline in the Chl fluorescence at 680 nm to about 60% of the initial value. The plot of normalized Chl fluorescence quenching, F(-O(2))/F(+O(2)), showed discontinuity above 275 microM O(2), revealing two phases of quenching, at both lower and higher oxygen concentrations. The inhibition of photosystem II by DCMU or atrazine as well as that of cyt b(6)f by myxothiazol attenuated the oxygen-induced quenching events observed above 275 microM O(2), but did not modify the first phase of oxygen action. These data imply that the oxygen mediated Chl fluorescence quenching is partially independent on non-cyclic electron flow. The second phase of oxygen-induced decline in Chl fluorescence is diminished in thylakoids with poisoned PSII and cyt b(6)f activities and treated with rotenone or N-ethylmaleimide to inhibit NAD(P)H-plastoquinone dehydrogenase. The data suggest that under weak light and high oxygen concentration the Chl fluorescence quenching results from interactions between oxygen and PSI, cyt b(6)f and Ndh. On the contrary, inhibition of non-cyclic electron flow by antimycin A or uncoupling of thylakoids by carbonyl cyanide m-chlorophenyl hydrazone did not modify the steady-state oxygen effect on Chl fluorescence quenching. The addition of NADH protected thylakoids against oxygen-induced Chl fluorescence quenching, whereas in the presence of exogenic duroquinone the decrease in Chl fluorescence to one half of the initial level did not result from the oxygen effect, probably due to oxygen action as a weak electron acceptor from PQ pool and an insufficient non-photochemical quencher. The data indicate that mechanism of oxygen-induced Chl fluorescence quenching depends significantly on oxygen concentration and is related to both structural rearrangement of thylakoids and the direct oxygen reduction by photosynthetic complexes.

Monitoring the mitochondrial transmembrane potential with the JC-1 fluorochrome in programmed cell death during mesophyll leaf senescence

Analysis of the mitochondrial transmembrane potential (Delta Psi(m)) with the help of the JC-1 fluorochrome (5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolcarbocyanine iodide) during mesophyll leaf senescence was performed in order to determine whether a reduction of Delta Psi(m) takes place during mesophyll senescence and whether plant mitochondria, like mammalian ones, might be involved in the induction of programmed cell death. Fluorescence analysis of mesophyll protoplasts of Pisum sativum in a confocal microscope, fluorescent spectra analysis and time dependence of fluorescence intensity of monomers and of J-aggregates revealed that JC-1 is incorporated and accumulated specifically in plant mitochondria. Analysis of Delta Psi(m) during mesophyll protoplast senescence revealed that two subpopulations of mitochondria which differ in Delta Psi(m) exist in all analyzed stages of leaf senescence. The first subpopulation contains mitochondria with red fluorescence of J-aggregates due to an unperturbed high Delta Psi(m). The second subpopulation comprises mitochondria with green fluorescence of monomers due to a low Delta Psi(m), proving total depolarization of mitochondrial membranes. Fluorescence analysis demonstrated that even in the latest analyzed stages of leaf senescence, mitochondria with a high Delta Psi(m) still exist. Fluorometric measurements revealed that the fluorescence intensity of J-aggregates decreases with the age of plants, which indicates that a reduction of Delta Psi(m) during the mesophyll senescence process takes place; however, it does not take place within the whole population of mitochondria of the same protoplast. The reason of this can be due to a dramatic reorganization of mitochondria in mesophyll cells and the appearance of large mitochondria with local heterogeneity of Delta Psi(m) in the oldest analyzed stages. All mitochondria in every stage of senescence maintained their membrane organization even when their size, distribution, and spatial organization in protoplasts changed dramatically. We stated that the reduction of Delta Psi(m) does not directly induce programmed cell death in mesophyll cells, as opposed to animal apoptosis.

Conformational rearrangements in light-harvesting complex II accompanying light-induced chlorophyll a fluorescence quenching

Light-induced chlorophyll a (Chl a) fluorescence quenching was studied in light-harvesting complex of photosystem II (LHCII). Fluorescence intensity decreased by ca. 20% in the course of 20 min illumination (412 nm, 36 micromol m(-2) s(-1)) and was totally reversible within 30 min dark adaptation. The pronounced quenching was observed only in LHCII in an aggregated form and exclusively in the presence of molecular oxygen. Structural rearrangement of LHCII correlated to the quenching was monitored by measuring changes in UV-Visible light absorption spectra, and by measuring Fourier-transform infrared spectroscopy (FTIR) in the Amide I region of the protein (1600-1700 cm(-1)). The light-induced structural rearrangement of LHCII was interpreted as a partial disaggregation of the complex based on the decrease in the light scattering signal and the characteristic features observed in the FTIR spectra: the relative increase in the intensity of the band at 1653 cm(-1), corresponding to a protein in the alpha-helical structure at the expense of the band centered at 1621 cm(-1), characteristic of aggregated forms. The fact that the light-driven isomerization of the all-trans violaxanthin to the 13-cis form was not observed under the non-oxygenic conditions coincided with the lack of large-scale conformational reorganization of LHCII. The kinetics of this large-scale structural effect does not correspond to the light-induced fluorescence quenching, in contrast to the kinetics of structural changes in LHCII observable at low oxygen concentrations. Photo-conversion of 5% of the pool of all-trans violaxanthin to 9-cis isomer was observed under such conditions. Possible involvement of the violaxanthin isomerization in the process of structural rearrangements and excitation quenching in LHCII is discussed.

Peroxidative reactions attenuate oxygen effect on spectroscopic properties of isolated chloroplasts

Steady-state absorption and fluorescence excitation spectra have been measured at 25 degrees C in order to elucidate the differences between isolated chloroplasts from pea (chilling-sensitive plant) and bean (chilling-tolerant plant) and their response to oxygen treatment. A weaker light harvesting in bean in comparison with pea chloroplasts is related to higher free fatty acids level and extended peroxidation activities of bean chloroplasts. Peroxidation of free fatty acids in bean chloroplasts results in an accumulation of oxygenated forms of fatty acids demonstrated by a large negative band around 400 nm in absorption difference spectra, while the excitation spectra are not significantly altered. Similar changes have been observed in the lipase-treated pea chloroplasts. In contrast, in both pea and bean chloroplasts exhibiting no peroxidation due to antimycin A treatment, oxygen induces a pronounced absorbance increase in the regions around 435, 470 and 674 nm indicating the chloroplast swelling. A decline of chlorophyll fluorescence excitation caused by oxygen, may result from a decrease in energy transfer from antennae complexes to chlorophyll species emitting at both 680 and 740 nm. The oxygen-induced changes are partially reversed upon restoration of anaerobic conditions. The presented data show for the first time, that in contrast to pea chloroplasts the peroxidation abolishes an oxygen-induced decrease in light harvesting in bean chloroplasts, i.e., a chilling-sensitive plant.

The stimulatory effect of alloxan diabetes on citrulline formation in rabbit liver mitochondria

The effect of alloxan diabetes on citrulline formation from NH4Cl and bicarbonate was studied in rabbit liver mitochondria incubated with glutamate or succinate as respiratory substrate, as well as with exogenous ATP in the presence of uncoupler and oligomycin. In contrast to ornithine transcarbamoylase, the activity of carbamoyl-phosphate synthetase (ammonia) was higher in mitochondria from diabetic animals than in those from normal ones. In diabetic rabbits the rates of citrulline synthesis were stimulated under all conditions studied. In contrast, levels of N-acetylglutamate, an activator of carbamoyl-phosphate synthetase (ammonia), were significantly increased only in the presence of glutamate, while the highest rates of citrulline formation occurred in uncoupled mitochondria incubated with exogenous ATP as energy source. Treatment of animals with alloxan resulted in an increase of both the intramitochondrial ATP level and the rate of adenine nucleotide translocation across the mitochondrial membrane. The results indicate that the stimulation of citrulline formation in liver mitochondria of diabetic rabbits is mainly due to an increase in carbamoyl-phosphate synthetase (ammonia) activity and an elevation of content of intramitochondrial ATP, a substrate of this enzyme.