Xanthophyll-induced aggregation of LHCII as a switch between light-harvesting and energy dissipation systems

Photoprotection Is Achieved by Photorespiration and Modification of the Leaf Incident Light, and Their Extent Is Modulated by the Stomatal Sensitivity to Water Deficit in Grapevines

Absorbed energy in excess of that used by photosynthesis induces photoinhibition, which is common in water deficit conditions, resulting in reductions in stomatal conductance. In grapevines, controlled water deficit is a common field practice, but little is known about the impact of a given water shortage on the energy transduction processes at the leaf level in relation to contrasting stomatal sensitivities to drought. Here, we assessed the effect of a nearly similar water deficit condition on four grapevine varieties: Cabernet Sauvignon (CS) and Sauvignon Blanc (SB), which are stomatal sensitive, and Chardonnay (CH) and Carménère (CM), which are less stomatal sensitive, grown in 20 L pots outdoors. Plants were maintained to nearly 94% of field capacity (WW) and 83% field capacity (WD). We have assessed plant water status, photosynthesis (A_N), photorespiration, A_N vs. PAR, AC_i curves, photochemical (qP) and non-photochemical (qN) fluorescence quenching vs. PAR, the photoprotective effectiveness of NPQ (qPd) and light interception by leaves. Photorespiration is important under WD, but to a different extent between varieties. This is related to stomatal sensitivity, maintaining a safe proportion of PSII reaction centres in an open state. Additionally, the capacity for carboxylation is affected by WD, but to a greater extent in more sensitive varieties. As for qN, in WD it saturates at 750 μmol PAR m⁻²s⁻¹, irrespective of the variety, which coincides with PAR, from which qN photoprotective effectiveness declines, and qP is reduced to risky thresholds. Additionally, that same PAR intensity is intercepted by WD leaves from highly stomatal-sensitive varieties, likely due to a modification of the leaf angle in those plants. Pigments associated with qN, as well as chlorophylls, do not seem to be a relevant physiological target for acclimation.

Light-Harvesting Complex II Adopts Different Quaternary Structures in Solution as Observed Using Small-Angle Scattering

The high-resolution crystal structure of the trimeric major light-harvesting complex of photosystem II (LHCII) is often perceived as the basis for understanding its light-harvesting and photoprotective functions. However, the LHCII solution structure and its oligomerization or aggregation state may generally differ from the crystal structure and, moreover, also depend on its functional state. In this regard, small-angle scattering experiments provide the missing link by offering structural information in aqueous solution at physiological temperatures. Herein, we use small-angle scattering to investigate the solution structures of two different preparations of solubilized LHCII employing the nonionic detergents n-octyl-β-d-glucoside (OG) and n-dodecyl-β-D-maltoside (β-DM). The data reveal that the LHCII-OG complex is equivalent to the trimeric crystal structure. Remarkably, however, we observe─for the first time─a stable oligomer composed of three LHCII trimers in the case of the LHCII-β-DM preparation, implying additional pigment-pigment interactions. The latter complex is assumed to mimic trimer-trimer interactions which play an important role in the context of photoprotective nonphotochemical quenching.

Protein-Protein Interactions Induce pH-Dependent and Zeaxanthin-Independent Photoprotection in the Plant Light-Harvesting Complex, LHCII

Plants use energy from the sun yet also require protection against the generation of deleterious photoproducts from excess energy. Photoprotection in green plants, known as nonphotochemical quenching (NPQ), involves thermal dissipation of energy and is activated by a series of interrelated factors: a pH drop in the lumen, accumulation of the carotenoid zeaxanthin (Zea), and formation of arrays of pigment-containing antenna complexes. However, understanding their individual contributions and their interactions has been challenging, particularly for the antenna arrays, which are difficult to manipulate in vitro. Here, we achieved systematic and discrete control over the array size for the principal antenna complex, light-harvesting complex II, using near-native in vitro membranes called nanodiscs. Each of the factors had a distinct influence on the level of dissipation, which was characterized by measurements of fluorescence quenching and ultrafast chlorophyll-to-carotenoid energy transfer. First, an increase in array size led to a corresponding increase in dissipation; the dramatic changes in the chlorophyll dynamics suggested that this is due to an allosteric conformational change of the protein. Second, a pH drop increased dissipation but exclusively in the presence of protein-protein interactions. Third, no Zea dependence was identified which suggested that Zea regulates a distinct aspect of NPQ. Collectively, these results indicate that each factor provides a separate type of control knob for photoprotection, which likely enables a flexible and tunable response to solar fluctuations.

Accumulation of geranylgeranylated chlorophylls in the pigment-protein complexes of Arabidopsis thaliana acclimated to green light: effects on the organization of light-harvesting complex II and photosystem II functions

Light quality significantly influences plant metabolism, growth and development. Recently, we have demonstrated that leaves of barley and other plant species grown under monochromatic green light (500-590 nm) accumulated a large pool of chlorophyll a (Chl a) intermediates with incomplete hydrogenation of their phytyl chains. In this work, we studied accumulation of these geranylgeranylated Chls a and b in pigment-protein complexes (PPCs) of Arabidopsis plants acclimated to green light and their structural-functional consequences on the photosynthetic apparatus. We found that geranylgeranylated Chls are present in all major PPCs, although their presence was more pronounced in light-harvesting complex II (LHCII) and less prominent in supercomplexes of photosystem II (PSII). Accumulation of geranylgeranylated Chls hampered the formation of PSII and PSI super- and megacomplexes in the thylakoid membranes as well as their assembly into chiral macrodomains; it also lowered the temperature stability of the PPCs, especially that of LHCII trimers, which led to their monomerization and an anomaly in the photoprotective mechanism of non-photochemical quenching. Role of geranylgeranylated Chls in adverse effects on photosynthetic apparatus of plants acclimated to green light is discussed.

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.

Antenna Protein Clustering In Vitro Unveiled by Fluorescence Correlation Spectroscopy

Antenna protein aggregation is one of the principal mechanisms considered effective in protecting phototrophs against high light damage. Commonly, it is induced, in vitro, by decreasing detergent concentration and pH of a solution of purified antennas; the resulting reduction in fluorescence emission is considered to be representative of non-photochemical quenching in vivo. However, little is known about the actual size and organization of antenna particles formed by this means, and hence the physiological relevance of this experimental approach is questionable. Here, a quasi-single molecule method, fluorescence correlation spectroscopy (FCS), was applied during in vitro quenching of LHCII trimers from higher plants for a parallel estimation of particle size, fluorescence, and antenna cluster homogeneity in a single measurement. FCS revealed that, below detergent critical micelle concentration, low pH promoted the formation of large protein oligomers of sizes up to micrometers, and therefore is apparently incompatible with thylakoid membranes. In contrast, LHCII clusters formed at high pH were smaller and homogenous, and yet still capable of efficient quenching. The results altogether set the physiological validity limits of in vitro quenching experiments. Our data also support the idea that the small, moderately quenching LHCII oligomers found at high pH could be relevant with respect to non-photochemical quenching in vivo.

Recycling of Energy Dissipated as Heat Accounts for High Activity of Photosystem II

Photosystem II (PSII) converts light into chemical energy powering almost all life on Earth. The primary photovoltaic reaction in the PSII reaction center requires energy corresponding to 680 nm, which is significantly higher than in the case of the low-energy states in the antenna complexes involved in the harvesting of excitations driving PSII. Here we show that despite seemingly insufficient energy, the low-energy excited states can power PSII because of the activity of the thermally driven up-conversion. We demonstrate the operation of this mechanism both in intact leaves and in isolated pigment-protein complex LHCII. A mechanism is proposed, according to which the effective utilization of thermal energy in the photosynthetic apparatus is possible owing to the formation of LHCII supramolecular structures, leading to the coupled energy levels corresponding to approximately 680 and 700 nm, capable of exchanging excitation energy through the spontaneous relaxation and the thermal up-conversion.

The role of xanthophylls in the supramolecular organization of the photosynthetic complex LHCII in lipid membranes studied by high-resolution imaging and nanospectroscopy

The xanthophyll cycle is a regulatory mechanism operating in the photosynthetic apparatus of plants. It consists of the conversion of the xanthophyll pigment violaxanthin to zeaxanthin, and vice versa, in response to light intensity. According to the current understanding, one of the modes of regulatory activity of the cycle is associated with the influence on a molecular organization of pigment-protein complexes. In the present work, we analyzed the effect of violaxanthin and zeaxanthin on the molecular organization of the LHCII complex, in the environment of membranes formed with chloroplast lipids. Nanoscale imaging based on atomic force microscopy (AFM) showed that the presence of exogenous xanthophylls promotes the formation of the protein supramolecular structures. Nanoscale infrared (IR) absorption analysis based on AFM-IR nanospectroscopy suggests that zeaxanthin promotes the formation of LHCII supramolecular structures by forming inter-molecular β-structures. Meanwhile, the molecules of violaxanthin act as "molecular spacers" preventing self-aggregation of the protein, potentially leading to uncontrolled dissipation of excitation energy in the complex. This latter mechanism was demonstrated with the application of fluorescence lifetime imaging microscopy. The intensity-averaged chlorophyll a fluorescence lifetime determined in the LHCII samples without exogenous xanthophylls at the level of 0.72 ns was longer in the samples containing exogenous violaxanthin (2.14 ns), but shorter under the presence of zeaxanthin (0.49 ns) thus suggesting a role of this xanthophyll in promotion of the formation of structures characterized by effective excitation quenching. This mechanism can be considered as a representation of the overall photoprotective activity of the xanthophyll cycle.

Acceleration of the excitation decay in Photosystem I immobilized on glass surface

Femtosecond transient absorption was used to study excitation decay in monomeric and trimeric cyanobacterial Photosystem I (PSI) being prepared in three states: (1) in aqueous solution, (2) deposited and dried on glass surface (either conducting or non-conducting), and (3) deposited on glass (conducting) surface but being in contact with aqueous solvent. The main goal of this contribution was to determine the reason of the acceleration of the excitation decay in dried PSI deposited on the conducting surface relative to PSI in solution observed previously using time-resolved fluorescence (Szewczyk et al., Photysnth Res 132(2):111-126, 2017). We formulated two alternative working hypotheses: (1) the acceleration results from electron injection from PSI to the conducting surface; (2) the acceleration is caused by dehydration and/or crowding of PSI proteins deposited on the glass substrate. Excitation dynamics of PSI in all three types of samples can be described by three main components of subpicosecond, 3-5, and 20-26 ps lifetimes of different relative contributions in solution than in PSI-substrate systems. The presence of similar kinetic components for all the samples indicates intactness of PSI proteins after their deposition onto the substrates. The kinetic traces for all systems with PSI deposited on substrates are almost identical and they decay significantly faster than the kinetic traces of PSI in solution. We conclude that the accelerated excitation decay in PSI-substrate systems is caused mostly by dense packing of proteins.

The carotenoid pathway: what is important for excitation quenching in plant antenna complexes?

Plant light-harvesting is regulated by the Non-Photochemical Quenching (NPQ) mechanism involving the slow trapping of excitation energy by carotenoids in the Photosystem II (PSII) antenna in response to high light.

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.

Effect of Protonation on the Secondary Structure and Orientation of Plant Light-Harvesting Complex II Studied by PM-IRRAS

The major light-harvesting pigment-protein complex of photosystem II, LHCII, has a crucial role in the distribution of the light energy between the two photosystems, the efficient light capturing and protection of the reaction centers and antennae from overexcitation. In this work direct structural information on the effect of LHCII protonation, which mimics the switch from light-harvesting to photoprotective state of the protein, was revealed by polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS). PM-IRRAS on LHCII monolayers verified that the native helical structure of the protein is preserved in both partly deprotonated (pH 7.8, LHCII) and protonated (pH 5.2, p-LHCII) states. At low surface pressure, 10 mN/m, the orientation of the α-helices in these two LHCII states is different-tilted (θ ≈ 40°) in LHCII and nearly vertical (θ ≈ 90°) in p-LHCII monolayers; the partly deprotonated complex is more hydrophilic than the protonated one and exhibits stronger intertrimer interactions. At higher surface pressure, 30 mN/m, which is typical for biological membranes, the protonation affects neither the secondary structure nor the orientation of the transmembrane α-helices (tilted ∼45° relative to the membrane surface in both LHCII states) but weakens the intermolecular interactions within and/or between the trimers.

Molecular insights into Zeaxanthin-dependent quenching in higher plants

Photosynthetic organisms protect themselves from high-light stress by dissipating excess absorbed energy as heat in a process called non-photochemical quenching (NPQ). Zeaxanthin is essential for the full development of NPQ, but its role remains debated. The main discussion revolves around two points: where does zeaxanthin bind and does it quench? To answer these questions we have followed the zeaxanthin-dependent quenching from leaves to individual complexes, including supercomplexes. We show that small amounts of zeaxanthin are associated with the complexes, but in contrast to what is generally believed, zeaxanthin binding per se does not cause conformational changes in the complexes and does not induce quenching, not even at low pH. We show that in NPQ conditions zeaxanthin does not exchange for violaxanthin in the internal binding sites of the antennas but is located at the periphery of the complexes. These results together with the observation that the zeaxanthin-dependent quenching is active in isolated membranes, but not in functional supercomplexes, suggests that zeaxanthin is acting in between the complexes, helping to create/participating in a variety of quenching sites. This can explain why none of the antennas appears to be essential for NPQ and the multiple quenching mechanisms that have been observed in plants.

Effect of protein aggregation on the spectroscopic properties and excited state kinetics of the LHCII pigment–protein complex from green plants

Steady-state and time-resolved absorption and fluorescence spectroscopic experiments have been carried out at room and cryogenic temperatures on aggregated and unaggregated monomeric and trimeric LHCII complexes isolated from spinach chloroplasts. Protein aggregation has been hypothesized to be one of the mechanistic factors controlling the dissipation of excess photo-excited state energy of chlorophyll during the process known as nonphotochemical quenching. The data obtained from the present experiments reveal the role of protein aggregation on the spectroscopic properties and dynamics of energy transfer and excited state deactivation of the protein-bound chlorophyll and carotenoid pigments.

Quality control of Photosystem II: reversible and irreversible protein aggregation decides the fate of Photosystem II under excessive illumination

In response to excessive light, the thylakoid membranes of higher plant chloroplasts show dynamic changes including the degradation and reassembly of proteins, a change in the distribution of proteins, and large-scale structural changes such as unstacking of the grana. Here, we examined the aggregation of light-harvesting chlorophyll-protein complexes and Photosystem II core subunits of spinach thylakoid membranes under light stress with 77K chlorophyll fluorescence; aggregation of these proteins was found to proceed with increasing light intensity. Measurement of changes in the fluidity of thylakoid membranes with fluorescence polarization of diphenylhexatriene showed that membrane fluidity increased at a light intensity of 500-1,000 μmol photons m(-) (2) s(-) (1), and decreased at very high light intensity (1,500 μmol photons m(-) (2) s(-) (1)). The aggregation of light-harvesting complexes at moderately high light intensity is known to be reversible, while that of Photosystem II core subunits at extremely high light intensity is irreversible. It is likely that the reversibility of protein aggregation is closely related to membrane fluidity: increases in fluidity should stimulate reversible protein aggregation, whereas irreversible protein aggregation might decrease membrane fluidity. When spinach leaves were pre-illuminated with moderately high light intensity, the qE component of non-photochemical quenching and the optimum quantum yield of Photosystem II increased, indicating that Photosystem II/light-harvesting complexes rearranged in the thylakoid membranes to optimize Photosystem II activity. Transmission electron microscopy revealed that the thylakoids underwent partial unstacking under these light stress conditions. Thus, protein aggregation is involved in thylakoid dynamics and regulates photochemical reactions, thereby deciding the fate of Photosystem II.

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.

Dynamic properties of photosystem II membranes at physiological temperatures characterized by elastic incoherent neutron scattering. Increased flexibility associated with the inactivation of the oxygen evolving complex

Elastic incoherent neutron scattering (EINS), a non-invasive technique which is capable of measuring the mean square displacement of atoms in the sample, has been widely used in biology for exploring the dynamics of proteins and lipid membranes but studies on photosynthetic systems are scarce. In this study we investigated the dynamic characteristics of Photosystem II (PSII) membrane fragments between 280 and 340 K, i.e., in the physiological temperature range and in the range of thermal denaturation of some of the protein complexes. The mean square displacement values revealed the presence of a hydration-sensitive transition in the sample between 310 and 320 K, suggesting that the oxygen evolving complex (OEC) plays an important role in the transition. Indeed, in samples in which the OEC had been removed by TRIS- or heat-treatments (323 and 333 K) no such transition was found. Further support on the main role of OEC in these reorganizations is provided by data obtained from differential scanning calorimetry experiments, showing marked differences between the untreated and TRIS-treated samples. In contrast, circular dichroism spectra exhibited only minor changes in the excitonic interactions below 323 K, showing that the molecular organization of the pigment-protein complexes remains essentially unaffected. Our data, along with earlier incoherent neutron scattering data on PSII membranes at cryogenic temperatures (Pieper et al., Biochemistry 46:11398-11409, 2007), demonstrate that this technique can be applied to characterize the dynamic features of PSII membranes, and can be used to investigate photosynthetic membranes under physiologically relevant experimental conditions.

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.

Xanthophyll cycle--a mechanism protecting plants against oxidative stress

Six different xanthophyll cycles have been described in photosynthetic organisms. All of them protect the photosynthetic apparatus from photodamage caused by light-induced oxidative stress. Overexcitation conditions lead, in the chloroplast, to the over-reduction of the NADP pool and production of superoxide, which can subsequently be metabolized to hydrogen peroxide or a hydroxyl radical, other reactive oxygen species (ROS). On the other hand, overexcitation of photosystems leads to an increased lifetime of the chlorophyll excited state, increasing the probability of chlorophyll triplet formation which reacts with triplet oxygen forming single oxygen, another ROS. The products of the light-dependent phase of xanthophyll cycles play an important role in the protection against oxidative stress generated not only by an excess of light but also by other ROS-generating factors such as drought, chilling, heat, senescence, or salinity stress. Four, mainly hypothetical, mechanisms explaining the protective role of xanthophyll cycles in oxidative stress are presented. One of them is the direct quenching of overexcitation by products of the light phase of xanthophyll cycles and three others are based on the indirect participation of xanthophyll cycle carotenoids in the process of photoprotection. They include: (1) indirect quenching of overexcitation by aggregation-dependent light-harvesting complexes (LHCII) quenching; (2) light-driven mechanisms in LHCII; and (3) a model based on charge transfer quenching between Chl a and Zx. Moreover, results of the studies on the antioxidant properties of xanthophyll cycle pigments in model systems are also presented.

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.

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.

Mechanism and regulation of the violaxanthin cycle: the role of antenna proteins and membrane lipids

The violaxanthin cycle describes the reversible conversion of violaxanthin to zeaxanthin via the intermediate antheraxanthin. This light-dependent xanthophyll conversion is essential for the adaptation of plants and algae to different light conditions and allows a reversible switch of photosynthetic light-harvesting complexes between a light-harvesting state under low light and a dissipative state under high light. The photoprotective functions of zeaxanthin have been intensively studied during the last decade, but much less attention has been directed to the mechanism and regulation of xanthophyll conversion. In this review, an overview is given on recent progress in the understanding of the role of (i) xanthophyll binding by antenna proteins and of (ii) the lipid properties of the thylakoid membrane in the regulation of xanthophyll conversion. The consequences of these findings for the mechanism and regulation of xanthophyll conversion in the thylakoid membrane will be discussed.

Model for fluorescence quenching in light harvesting complex II in different aggregation states

Low-temperature (77 K) steady-state fluorescence emission spectroscopy and dynamic light scattering were applied to the main chlorophyll a/b protein light harvesting complex of photosystem II (LHC II) in different aggregation states to elucidate the mechanism of fluorescence quenching within LHC II oligomers. Evidences presented that LHC II oligomers are heterogeneous and consist of large and small particles with different fluorescence yield. At intermediate detergent concentrations the mean size of the small particles is similar to that of trimers, while the size of large particles is comparable to that of aggregated trimers without added detergent. It is suggested that in small particles and trimers the emitter is monomeric chlorophyll, whereas in large aggregates there is also another emitter, which is a poorly fluorescing chlorophyll associate. A model, describing populations of antenna chlorophyll molecules in small and large aggregates in their ground and first singlet excited states, is considered. The model enables us to obtain the ratio of the singlet excited-state lifetimes in small and large particles, the relative amount of chlorophyll molecules in large particles, and the amount of quenchers as a function of the degree of aggregation. These dependencies reveal that the quenching of the chl a fluorescence upon aggregation is due to the formation of large aggregates and the increasing of the amount of chlorophyll molecules forming these aggregates. As a consequence, the amount of quenchers, located in large aggregates, is increased, and their singlet excited-state lifetimes steeply decrease.

Influence of detergent concentration on aggregation and spectroscopic properties of light-harvesting complex II

Aggregation of photosynthetic light-harvesting complexes strongly influences their spectroscopic properties. Fluorescence yield and excited state lifetimes of the main light-harvesting complex (LHC II) of higher plants strongly depend on its aggregation state. Detergents are commonly used to solubilize membrane proteins and/or to circumvent their aggregation in aqueous environments. Nonlinear polarization spectroscopy in the frequency domain (NLPF) was performed with LHC II over a wide concentration range of the mild detergent n-dodecyl beta-D: -maltoside (beta-DM). Additionally, conventional absorption-, fluorescence- and circular dichroism-spectra were measured.The results indicate that: (i) conventional spectroscopic techniques are not well suited to investigate aggregation effects. NLPF provides a novel approach to overcome this problem: NLPF spectra display dramatic alterations upon even minor beta-DM concentration changes. (ii) Commonly used detergent concentrations (around or slightly above the critical micellar concentration) apparently do not lead to complete trimerization of LHC II. A long-wavelength species in the NLPF spectra (peaking at about 685 nm), indicative of residual aggregation, persists up to DM-concentrations of 0.06%. (iii) High-resolution NLPF spectra indicate the existence of a species with a considerably shortened excited state lifetime. (iv) No indication of denaturation was found even at the highest beta-DM concentrations used. (v) A specific change in interaction between certain chlorophyll(s) b and a xanthophyll molecule, probably neoxanthin, was detected upon aggregation as well as at higher beta-DM concentrations. The results are discussed with respect to the still elusive mechanism of nonradiative dissipation of excess excitation energy in the antenna system.

Structural and functional analysis of the antiparallel strands in the lumenal loop of the major light-harvesting chlorophyll a/b complex of photosystem II (LHCIIb) by site-directed mutagenesis

The light-harvesting chlorophyll a/b-binding protein of photosystem II (LHCIIb) fulfills multiple functions, such as light harvesting and energy dissipation under different illuminations. The crystal structure of LHCIIb at the near atomic resolution reveals an antiparallel strands structure in the lumenal loop between the transmembrane helices B/C. To study the structural and functional significances of this structure, three amino acids (Val-119, His-120, and Ser-123) in this region have been exchanged to Phe, Leu, and Gly, respectively, and the influence of the mutagenesis on the structure and function of LHCIIb has been investigated. The results are as follows. 1) Circular dichroism spectra of the mutations reveals that the antiparallel strands in the lumenal region are very important for adjusting pigment conformation in the neoxanthin domain of LHCIIb. Although the mutagenesis causes only a slight loss of the Neo binding in the complexes (V119F, 0.09; S123G, 0.19; and H120L, 0.27), it imparts remarkable changes to the pigment conformation. 2) Substituting Ser-123 with Gly results in a higher susceptibility to photodamage, an increased tendency to aggregate, and enhanced fluorescence quenching induced by the medium acidification. These results demonstrate that this antiparallel strands domain plays an important role in regulating the pigment conformation and in adjusting the aggregation and the fluorescence yield of LHCIIb.

Trapping of the quenched conformation associated with non-photochemical quenching of chlorophyll fluorescence at low temperature

The kinetics of non-photochemical quenching (NPQ) of chlorophyll fluorescence was studied in pea leaves at different temperatures between 5 and 25 degrees C and during rapid jumps of the leaf temperature. At 5 degrees C, NPQ relaxed very slowly in the dark and was sustained for up to 30 min. This was independent of the temperature at which quenching was induced. Upon raising the temperature to 25 degrees C, the quenched state relaxed within 1 min, characteristic for qE, the energy-dependent component of NPQ. Measurements of the membrane permeability (delta A515) in dark-adapted and preilluminated leaves and NPQ in the presence of dithiothreitol strongly suggest that the effect of low temperature on NPQ was not because of limitation by the lumenal pH or the de-epoxidation state of the xanthophylls. These data are consistent with the notion that the transition from the quenched to the unquenched state and vice versa involves a structural reorganization in the photosynthetic apparatus. An eight-state reaction scheme for NPQ is proposed, extending the model of Horton and co-workers (FEBS Lett 579:4201-4206, 2005), and a hypothesis is put forward concerning the nature of conformational changes associated with qE.