Light-induced Change of Configuration of the LHCII-Bound Xanthophyll (Tentatively Assigned to Violaxanthin): A Resonance Raman Study

Probing Biological Nitrogen Fixation in Legumes Using Raman Spectroscopy

Biological nitrogen fixation (BNF) by symbiotic bacteria plays a vital role in sustainable agriculture. However, current quantification methods are often expensive and impractical. This study explores the potential of Raman spectroscopy, a non-invasive technique, for rapid assessment of BNF activity in soybeans. Raman spectra were obtained from soybean plants grown with and without rhizobia bacteria to identify spectral signatures associated with BNF. δN15 isotope ratio mass spectrometry (IRMS) was used to determine actual BNF percentages. Partial least squares regression (PLSR) was employed to develop a model for BNF quantification based on Raman spectra. The model explained 80% of the variation in BNF activity. To enhance the model's specificity for BNF detection regardless of nitrogen availability, a subsequent elastic net (Enet) regularisation strategy was implemented. This approach provided insights into key wavenumbers and biochemicals associated with BNF in soybeans.

Isomerization of carotenoids in photosynthesis and metabolic adaptation

In nature, carotenoids are present as trans- and cis-isomers. Various physical and chemical factors like light, heat, acids, catalytic agents, and photosensitizers can contribute to the isomerization of carotenoids. Living organisms in the process of evolution have developed different mechanisms of adaptation to light stress, which can also involve isomeric forms of carotenoids. Particularly, light stress conditions can enhance isomerization processes. The purpose of this work is to review the recent studies on cis/trans isomerization of carotenoids as well as the role of carotenoid isomers for the light capture, energy transfer, photoprotection in light-harvesting complexes, and reaction centers of the photosynthetic apparatus of plants and other photosynthetic organisms. The review also presents recent studies of carotenoid isomers for the biomedical aspects, showing cis- and trans-isomers differ in bioavailability, antioxidant activity and biological activity, which can be used for therapeutic and prophylactic purposes.

A Possible Mechanism for Aggregation-Induced Chlorophyll Fluorescence Quenching in Light-Harvesting Complex II from the Marine Green Alga Bryopsis corticulans

The light-harvesting complex II of a green alga Bryopsis corticulans (B-LHCII) is peculiar in that it contains siphonein and siphonaxathin as carotenoid (Car). Since the S₁ state of siphonein and siphonaxathin lies substantially higher than the Q_y state of chlorophyll a (Chl a), the Chl a(Q_y)-to-Car(S₁) excitation energy transfer is unfeasible. To understand the photoprotective mechanism of algal photosynthesis, we investigated the influence of temperature on the excitation dynamics of B-LHCII in trimeric and aggregated forms. At room temperature, the aggregated form showed a 10-fold decrease in fluorescence intensity and lifetime than the trimeric form. Upon lowering the temperature, the characteristic 680 nm fluorescence (F-680) of B-LHCII in both forms exhibited systematic intensity enhancement and spectral narrowing; however, only the aggregated form showed a red emission extending over 690-780 nm (F-RE) with pronounced blueshift, lifetime prolongation, and intensity boost. The remarkable T-dependence of F-RE is ascribed to the Chl-Chl charge transfer (CT) species involved directly in the aggregation-induced Chl deactivation. The CT-quenching mechanism, which is considered to be crucial for B. corticulans photoprotection, draws strong support from the positive correlation of the Chl deactivation rate with the CT state population, as revealed by comparing the fluorescence dynamics of B-LHCII with that of the plant LHCII.

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.

Mechanisms shaping the synergism of zeaxanthin and PsbS in photoprotective energy dissipation in the photosynthetic apparatus of plants

Safe operation of photosynthesis is vital to plants and is ensured by the activity of processes protecting chloroplasts against photo-damage. The harmless dissipation of excess excitation energy is considered to be the primary photoprotective mechanism and is most effective in the combined presence of PsbS protein and zeaxanthin, a xanthophyll accumulated in strong light as a result of the xanthophyll cycle. Here we address the problem of specific molecular mechanisms underlying the synergistic effect of zeaxanthin and PsbS. The experiments were conducted with Arabidopsis thaliana, using wild-type plants, mutants lacking PsbS (npq4), and mutants affected in the xanthophyll cycle (npq1), with the application of molecular spectroscopy and imaging techniques. The results lead to the conclusion that PsbS interferes with the formation of densely packed aggregates of thylakoid membrane proteins, thus allowing easy exchange and incorporation of xanthophyll cycle pigments into such structures. It was found that xanthophylls trapped within supramolecular structures, most likely in the interfacial protein region, determine their photophysical properties. The structures formed in the presence of violaxanthin are characterized by minimized dissipation of excitation energy. In contrast, the structures formed in the presence of zeaxanthin show enhanced excitation quenching, thus protecting the system against photo-damage.

Light-Modulated Sunscreen Mechanism in the Retina of the Human Eye

The functioning of the human eye in the extreme range of light intensity requires a combination of the high sensitivity of photoreceptors with their photostability. Here, we identify a regulatory mechanism based on dynamic modulation of light absorption by xanthophylls in the retina, realized by reorientation of pigment molecules induced by trans-cis photoisomerization. We explore this photochemically switchable system using chromatographic analysis coupled with microimaging based on fluorescence lifetime and Raman scattering, showing it at work in both isolated human retina and model lipid membranes. The molecular mechanism underlying xanthophyll reorientation is explained in terms of hydrophobic mismatch using molecular dynamics simulations. Overall, we show that xanthophylls in the human retina act as "molecular blinds", opening and closing on a submillisecond timescale to dynamically control the intensity of light reaching the photoreceptors, thus enabling vision at a very low light intensity and protecting the retina from photodegradation when suddenly exposed to strong light.

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.

Light-induced formation of dimeric LHCII

It emerges from numerous experiments that LHCII, the major photosynthetic antenna complex of plants, can appear not only in the trimeric or monomeric states but also as a dimer. We address the problem whether the dimeric form of the complex is just a simple intermediate element of the trimer-monomer transformation or if it can also be a physiologically relevant molecular organization form? Dimers of LHCII were analyzed with application of native electrophoresis, time-resolved fluorescence spectroscopy, and fluorescence correlation spectroscopy. The results reveal the appearance of two types of LHCII dimers: one formed by the dissociation of one monomer from the trimeric structure and the other formed by association of monomers into a distinctively different molecular organizational form, characterized by a high rate of chlorophyll excitation quenching. The hypothetical structure of such an energy quencher is proposed. The high light-induced LHCII dimerization is discussed as a potential element of the photoprotective response in plants.

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.

Effects of Molecular Symmetry on the Electronic Transitions in Carotenoids

The aim of this work is the verification of symmetry effects on the electronic absorption spectra of carotenoids. The symmetry breaking in cis-β-carotenes and in carotenoids with nonlinear π-electron system is of virtually no effect on the dark transitions in these pigments, in spite of the loss of the inversion center and evident changes in their electronic structure. In the cis isomers, the S2 state couples with the higher excited states and the extent of this coupling depends on the position of the cis bend. A confrontation of symmetry properties of carotenoids with their electronic absorption and IR and Raman spectra shows that they belong to the C1 or C2 but not the C2h symmetry group, as commonly assumed. In these realistic symmetries all the electronic transitions are symmetry-allowed and the absence of some transitions, such as the dark S0 → S1 transition, must have another physical origin. Most likely it is a severe deformation of the carotenoid molecule in the S1 state, unachievable directly from the ground state, which means that the Franck-Condon factors for a vertical S0 → S1 transition are negligible because the final state is massively displaced along the vibrational coordinates. The implications of our findings have an impact on the understanding of the photophysics and functioning of carotenoids.

Light-Driven Reconfiguration of a Xanthophyll Violaxanthin in the Photosynthetic Pigment-Protein Complex LHCII: A Resonance Raman Study

Resonance Raman analysis of the photosynthetic complex LHCII, immobilized in a polyacrylamide gel, reveals that one of the protein-bound xanthophylls, assigned as violaxanthin, undergoes light-induced molecular reconfiguration. The phototransformation is selectively observed in a trimeric structure of the complex and is associated with a pronounced twisting and a trans-cis molecular configuration change of the polyene chain of the carotenoid. Among several spectral effects accompanying the reconfiguration there are ones indicating a carotenoid triplet state. Possible physiological importance of the light-induced violaxanthin reconfiguration as a mechanism associated with making the pigment available for enzymatic deepoxidation in the xanthophyll cycle is discussed.

The xanthophyll cycle pigments, violaxanthin and zeaxanthin, modulate molecular organization of the photosynthetic antenna complex LHCII

The effect of violaxanthin and zeaxanthin, two main carotenoids of the xanthophyll cycle, on molecular organization of LHCII, the principal photosynthetic antenna complex of plants, was studied in a model system based on lipid-protein membranes, by means of analysis of 77 K chlorophyll a fluorescence and "native" electrophoresis. Violaxanthin was found to promote trimeric organization of LHCII, contrary to zeaxanthin which was found to destabilize trimeric structures. Moreover, violaxanthin was found to induce decomposition of oligomeric LHCII structures formed in the lipid phase and characterized by the fluorescence emission band at 715 nm. Both pigments promoted formation of two-component supramolecular structures of LHCII and xanthophylls. The violaxanthin-stabilized structures were composed mostly of LHCII trimers while, the zeaxanthin-stabilized supramolecular structures of LHCII showed more complex organization which depended periodically on the xanthophyll content. The effect of the xanthophyll cycle pigments on molecular organization of LHCII was analyzed based on the results of molecular modeling and discussed in terms of a physiological meaning of this mechanism. Supramolecular structures of LHCII stabilized by violaxanthin, prevent uncontrolled oligomerization of LHCII, potentially leading to excitation quenching, therefore can be considered as structures protecting the photosynthetic apparatus against energy loses at low light intensities.

Nanoscale resolution in infrared imaging of protein-containing lipid membranes

The precise imaging of biomolecular entities contributes to an understanding of the relationship between their structure and function. However, the resolution of conventional infrared microscopic imaging is diffraction limited and does not exceed a few micrometres. Atomic force microscopy, on the other hand, can detect infrared absorption down to the sub-micrometer level. In the present report, we demonstrate that for multi-bilayer lipid samples containing the plant photosynthetic pigment-protein complex LHCII, the resolution of this latter technique can be better than 20 nm. Such a high resolution is attributable to two factors: (i) the relatively high infrared absorption by the complex that is integrated perpendicular to the plane of the multilayer film, and (ii) the distinctly different mechanical properties and thermal conductivity of the lipid and protein components of the sample.

Heat- and light-induced detachment of the light-harvesting antenna complexes of photosystem I in isolated stroma thylakoid membranes

The multisubunit pigment-protein complex of photosystem I (PSI) consists of a core and peripheral light-harvesting antenna (LHCI). PSI is thought to be a rather rigid system and very little is known about its structural and functional flexibility. Recent data, however, suggest LHCI detachment from the PSI supercomplex upon heat and light treatments. Furthermore, it was suggested that the splitting off of LHCI acts as a safety valve for PSI core upon photoinhibition (Alboresi et al., 2009). In this work we analyzed the heat- and light-induced reorganizations in isolated PSI vesicles (stroma membrane vesicles enriched in PSI). Using differential scanning calorimetry we revealed a stepwise disassembly of PSI supercomplex above 50°C. Circular dichroism, sucrose gradient centrifugation and 77K fluorescence experiments identified the sequence of events of PSI destabilization: 3min heating at 60°C or 40min white light illumination at 25°C resulted in pronounced Lhca1/4 detachment from the PSI supercomplex, which is then followed by the degradation of Lhca2/3. The similarity of the main structural effects due to heat and light treatments supports the notion that thermo-optic mechanism, structural changes induced by ultrafast local thermal transients, which has earlier been shown to be responsible for structural changes in the antenna system of photosystem II, can also regulate the assembly and functioning of PSI antenna.

Raman spectroscopy of microbial pigments

Raman spectroscopy is a rapid nondestructive technique providing spectroscopic and structural information on both organic and inorganic molecular compounds. Extensive applications for the method in the characterization of pigments have been found. Due to the high sensitivity of Raman spectroscopy for the detection of chlorophylls, carotenoids, scytonemin, and a range of other pigments found in the microbial world, it is an excellent technique to monitor the presence of such pigments, both in pure cultures and in environmental samples. Miniaturized portable handheld instruments are available; these instruments can be used to detect pigments in microbiological samples of different types and origins under field conditions.

Hierarchical organization and structural flexibility of thylakoid membranes

Chloroplast thylakoid membranes accommodate densely packed protein complexes in ordered, often semi-crystalline arrays and are assembled into highly organized multilamellar systems, an organization warranting a substantial degree of stability. At the same time, they exhibit remarkable structural flexibility, which appears to play important - yet not fully understood - roles in different short-term adaptation mechanisms in response to rapidly changing environmental conditions. In this review I will focus on dynamic features of the hierarchically organized photosynthetic machineries at different levels of structural complexity: (i) isolated light harvesting complexes, (ii) molecular macroassemblies and supercomplexes, (iii) thylakoid membranes and (iv) their multilamellar membrane systems. Special attention will be paid to the most abundant systems, the major light harvesting antenna complex, LHCII, and to grana. Two physical mechanisms, which are less frequently treated in the literature, will receive special attention: (i) thermo-optic mechanism -elementary structural changes elicited by ultrafast local heat transients due to the dissipation of photon energy, which operates both in isolated antenna assemblies and the native thylakoid membranes, regulates important enzymatic functions and appears to play role in light adaptation and photoprotection mechanisms; and (ii) the mechanism by which non-bilayer lipids and lipid phases play key role in the functioning of xanthophyll cycle de-epoxidases and are proposed to regulate the protein-to-lipid ratio in thylakoid membranes and contribute to membrane dynamics. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.

The negative feedback molecular mechanism which regulates excitation level in the plant photosynthetic complex LHCII: towards identification of the energy dissipative state

Overexcitation of the photosynthetic apparatus is potentially dangerous because it can cause oxidative damage. Photoprotection realized via the feedback de-excitation in the pigment-protein light-harvesting complex LHCII, embedded in the chloroplast lipid environment, was studied with use of the steady-state and time-resolved fluorescence spectroscopy techniques. Illumination of LHCII results in the pronounced singlet excitation quenching, demonstrated by decreased quantum yield of the chlorophyll a fluorescence and shortening of the fluorescence lifetimes. Analysis of the 77K chlorophyll a fluorescence emission spectra reveals that the light-driven excitation quenching in LHCII is associated with the intensity increase of the spectral band in the region of 700nm, relative to the principal band at 680nm. The average chlorophyll a fluorescence lifetime at 700nm changes drastically upon temperature decrease: from 1.04ns at 300K to 3.63ns at 77K. The results of the experiments lead us to conclude that: (i) the 700nm band is associated with the inter-trimer interactions which result in the formation of the chlorophyll low-energy states acting as energy traps and non-radiative dissipation centers; (ii) the Arrhenius analysis, supported by the results of the FTIR measurements, suggests that the photo-reaction can be associated with breaking of hydrogen bonds. Possible involvement of photo-isomerization of neoxanthin, reported previously (Biochim. Biophys. Acta 1807 (2011) 1237-1243) in generation of the low-energy traps in LHCII is discussed.

Photosynthetic antenna complex LHCII studied with novel fluorescence techniques

LHCII is the largest light-harvesting pigment-protein complex of plants, comprising more than half of photosynthetically active chlorophyll pigments in biosphere. Understanding relationship between the molecular structure of the complex and photophysical processes that undergo in this pigment-protein complex is an aim of numerous current studies. This chapter addresses possibility of the application of single-molecule fluorescence measurements and fluorescence lifetime imaging microscopy (FLIM) in a study of LHCII.

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.

Modeling of a violaxanthin-chlorophyll b chromophore pair in its LHCII environment using CAM-B3LYP

Collecting energy for photosystem II is facilitated by several pigments, xanthophylls and chlorophylls, embedded in the light harvesting complex II (LHCII). One xanthophyll, violaxanthin (Vio), is loosely bound at a site close to a chlorophyll b (Chl). No final answer has yet been found for the role of this specific xanthophyll. We study the electronic structure of Vio in the presence of Chl and under the influence of the LHCII environment, represented by a point charge field (PCF). We compare the capability of the long range corrected density functional theory (DFT) functional CAM-B3LYP to B3LYP for the modeling of the UV/vis spectrum of the Vio+Chl pair. CAM-B3LYP was reported to allow for a very realistic reproduction of bond length alternation of linear polyenes, which has considerable impact on the carotenoid structure and spectrum. To account for the influence of the LHCII environment, the chromophore geometries are optimized using an ONIOM(DFT/6-31G(d):PM6) scheme. Our calculations show that the energies of the locally excited states are almost unaffected by the presence of the partner chromophore or the PCF. There are, however, indications for excitonic coupling of the Chl Soret band and Vio. We propose that Vio may accept energy from blue-light excited Chl.

Strong light-induced reorganization of pigment-protein complexes of thylakoid membranes in rye (spectroscopic study)

The supramolecular reorganization of LHCII complexes within the thylakoid membrane in Secale cereale leaves under low and high light condition was examined. Rye seedlings were germinated hydroponically in a climate chamber with a 16 h daylight photoperiod, photosynthetic photon flux density (PPFD) of 150 μmo lm(-2)s(-1) and 24/16°C day/night temperature. The influence of pre-illumination of the plants with high light intensity on the PSII antenna complexes was studied by comparison of the structure and function of the LHCII complexes and organization of thylakoid membranes isolated from 10-day-old plants illuminated with low (150 μmo lm(-2)s(-1)) or high (1200 μmo lm(-2)s(-1)) light intensity. Aggregated and trimeric with monomeric forms of LHCII complexes were separated from the whole thylakoid membranes using non-denaturing electrophoresis. Analyses of fluorescence emission spectra of these different LHCII forms showed that the monomer was the most effective aggregating antenna form. Moreover, photoprotection connected with LHCII aggregation was more effective upon LHCII monomers in comparison to trimer aggregation. Light stress induced specific organization of neighboring LHCII complexes, causing an increase in fluorescence yield of the long-wavelength bands (centered at 701 and 734 nm). The changes in the organization of the thylakoid membrane under light stress, observed by analysis of absorbance spectra obtained by Fourier transform infrared spectroscopy, also indicated light-induced LHCII aggregation.

Investigation of the molecular mechanism of the blue-light-specific excitation energy quenching in the plant antenna complex LHCII

Excitation of the major photosynthetic antenna complex of plants, LHCII, with blue light (470nm) provides an advantage to plants, as it gives rise to chlorophyll a fluorescence lifetimes shorter than with excitation with red light (635nm). This difference is particularly pronounced in fluorescence emission wavelengths longer than 715nm. Illumination of LHCII preparation with blue light additionally induces fluorescence quenching, which develops on a minute timescale. This effect is much less efficient when induced by red light, despite the equalized energy absorbed in both the spectral regions. Simultaneous analysis of the fluorescence and photoacoustic signals in LHCII demonstrated that the light-driven fluorescence quenching is not associated with an increase in heat emission. Instead, a reversible light-induced conformational transformation of the protein takes place, as demonstrated by the FTIR technique. These findings are discussed in terms of the blue-light-specific excitation energy quenching in LHCII, which may have photoprotective applications.

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.

Light-driven regulatory mechanisms in the photosynthetic antenna complex LHCII

Protection against strong-light-induced photodamage of the photosynthetic apparatus and entire organisms is a vital activity in plants and is also realized at the molecular level of the antenna complexes. Reported recently, the regulatory mechanisms which operate in the largest plant antenna complex, LHCII (light-harvesting complex II), based on light-driven processes, are briefly reviewed and discussed. Among those processes are the light-induced twisting of the configuration of the LHCII-bound neoxanthin, the light-induced configurational transition of the LHCII-bound violaxanthin, the light-induced trimer–monomer transition in LHCII and the blue-light-induced excitation quenching in LHCII. The physiological importance of the processes reviewed is also discussed with emphasis on the photoprotective excitation quenching and on possible involvement in the regulation of the xanthophyll cycle.

Blue-light-controlled photoprotection in plants at the level of the photosynthetic antenna complex LHCII

Plants have developed several adaptive regulatory mechanisms, operating at all the organization levels, to optimize utilization of light energy and to protect themselves against over-excitation-related damage. We report activity of a previously unknown possible regulatory mechanism that operates at the molecular level of the major photosynthetic pigment-protein complexes of plants, LHCII. This mechanism is driven exclusively by blue light, operates in the trimeric but not in the monomeric complex, and results in singlet excitation quenching leading to thermal energy dissipation. The conclusions are based on single molecule fluorescence lifetime analysis, direct measurements of thermal energy dissipation by photo-thermal spectroscopy, and on fluorescence spectroscopy. Possible molecular mechanisms involved in the blue-light-induced photoprotective effect are discussed, including xanthophyll photo-isomerization and the thermo-optic effect.

Supramolecular organization of the main photosynthetic antenna complex LHCII: a monomolecular layer study

The light-harvesting pigment-protein complex LHCII is a main antenna complex of the photosynthetic apparatus of plants, responsible for collecting light energy and also for photoprotection against overexcitation-induced damage. Realization of both functions depends on molecular organization of the complex. Monolayer technique has been applied to address the problem of supramolecular organization of LHCII. Analysis of the isotherms of compression of monomolecular films formed at the argon-water interface shows that LHCII appears in two phases: one characterized by the specific molecular area characteristic of trimeric and one of monomeric organization of LHCII. Monolayers of LHCII were deposited by means of the Langmuir-Blodgett technique to solid supports and examined by means of AFM, FTIR, fluorescence spectroscopy, and fluorescence lifetime imaging microscopy (FLIM). FTIR analysis shows that organization of the trimers of LHCII within a monolayer is associated with formation of intermolecular hydrogen bonds between neighboring polypeptides. The linear-dichroism FTIR analysis reveals that polypeptide fragments involved in intermolecular interactions are oriented at an angle of 67 degrees with respect to the normal axis to the plane of the layer. Fluorescence and fluorescence lifetime analysis reveal that the organization of LHCII within monolayers is associated with formation of the low-lying excitonic energy levels that can be potentially responsible for excess excitation quenching. FLIM and AFM reveal heterogeneous organization of LHCII monolayers, in particular, formation of ring-like structures. The potential of LHCII to form molecular structures characterized by pigment excitonic interactions is discussed in terms of regulation of the photosynthetic accessory function and photoprotection against overexcitation-induced damage.