Light-harvesting Complex Of Photosystem Research Articles

Ratio of Intensities of Blue and Red Light at Cultivation Influences Photosynthetic Light Reactions, Respiration, Growth, and Reflectance Indices in Lettuce.

Simple SummaryIllumination is an important factor for plant life because light is the basis of photosynthesis and productivity, the regulator of physiological processes, and a potential cause of damage. The development of LED technology has contributed to increasing the efficiency of illumination during plant cultivation through the use of light sources with narrow spectral bands. However, the characteristics of influence of light sources with different spectra on specific species of agricultural plants require further investigation. In the present work, we analyzed the influence of two variants of LED illumination (with increased intensity of red or blue light) on physiological processes in lettuce. These variants were selected because they corresponded to two maximums of photosynthetic light absorption. It was shown that, under the increased intensity of the blue light, both respiration and cyclic electron flow were stimulated; theseprocesses are known to be related to stress changes in plants. In contrast, under the increased intensity of the red light, linear electron flow was stimulated, a process that is related to plant productivity, and the biomass during cultivation was increased. The reflectance of leaves was also dependent on the variant of illumination. In the future, our results can be used to increase the efficiency of lettuce cultivation.LED illumination can have a narrow spectral band; its intensity and time regime are regulated within a wide range. These characteristics are the potential basis for the use of a combination of LEDs for plant cultivation because light is the energy source that is used by plants as well as the regulator of photosynthesis, and the regulator of other physiological processes (e.g., plant development), and can cause plant damage under certain stress conditions. As a result, analyzing the influence of light spectra on physiological and growth characteristics during cultivation of different plant species is an important problem. In the present work, we investigated the influence of two variants of LED illumination (red light at an increased intensity, the “red” variant, and blue light at an increased intensity, the “blue” variant) on the parameters of photosynthetic dark and light reactions, respiration rate, leaf reflectance indices, and biomass, among other factors in lettuce (Lactuca sativa L.). The same light intensity (about 180 µmol m−2s−1) was used in both variants. It was shown that the blue illumination variant increased the dark respiration rate (35–130%) and cyclic electron flow around photosystem I (18–26% at the maximal intensity of the actinic light) in comparison to the red variant; the effects were dependent on the duration of cultivation. In contrast, the blue variant decreased the rate of the photosynthetic linear electron flow (13–26%) and various plant growth parameters, such as final biomass (about 40%). Some reflectance indices (e.g., the Zarco-Tejada and Miller Index, an index that is related to the core sizes and light-harvesting complex of photosystem I), were also strongly dependent on the illumination variant. Thus, our results show that the red illumination variant contributes a great deal to lettuce growth; in contrast, the blue variant contributes to stress changes, including the activation of cyclic electron flow around photosystem I.

Membrane-dependent heterogeneity of LHCII characterized using single-molecule spectroscopy

In green plants, light harvesting complex of Photosystem II (LHCII) absorbs and transports excitation energy toward the photosynthetic reaction centers and serves as a site for energy-dependent nonphotochemical quenching (qE), the photoprotective dissipation of energy as heat. LHCII is thought to activate dissipation through conformational changes that change the photophysical behaviors. Understanding this balance requires a characterization of how the conformations of LHCII, and thus its photophysics, are influenced by individual factors within the membrane environment. Here, we used ensemble and single-molecule fluorescence to characterize the excited-state lifetimes and switching kinetics of LHCII embedded in nanodisc- and liposome-based model membranes of various sizes and lipid compositions. As the membrane area decreased, the quenched population and the rate of conformational dynamics both increased because of interactions with other proteins, the aqueous solution, and/or disordered lipids. Although the conformational states and dynamics were similar in both thylakoid and asolectin lipids, photodegradation increased with thylakoid lipids, likely because of their charge and pressure properties. Collectively, these findings demonstrate the ability of membrane environments to tune the conformations and photophysics of LHCII.

The Mechanism of Non-Photochemical Quenching in Plants: Localization and Driving Forces.

Non-photochemical chlorophyll fluorescence quenching (NPQ) remains one of the most studied topics of the 21st century in photosynthesis research. Over the past 30 years, profound knowledge has been obtained on the molecular mechanism of NPQ in higher plants. First, the largely overlooked significance of NPQ in protecting the reaction center of photosystem II (RCII) against damage, and the ways to assess its effectiveness are highlighted. Then, the key in vivo signals that can monitor the life of the major NPQ component, qE, are presented. Finally, recent knowledge on the site of qE and the possible molecular events that transmit ΔpH into the conformational change in the major LHCII [the major trimeric light harvesting complex of photosystem II (PSII)] antenna complex are discussed. Recently, number of reports on Arabidopsis mutants lacking various antenna components of PSII confirmed that the in vivo site of qE rests within the major trimeric LHCII complex. Experiments on biochemistry, spectroscopy, microscopy and molecular modeling suggest an interplay between thylakoid membrane geometry and the dynamics of LHCII, the PsbS (PSII subunit S) protein and thylakoid lipids. The molecular basis for the qE-related conformational change in the thylakoid membrane, including the possible onset of a hydrophobic mismatch between LHCII and lipids, potentiated by PsbS protein, begins to unfold.

The robustness of the terminal emitter site in major LHCII complexes controls xanthophyll function during photoprotection

Xanthophylls in light harvesting complexes perform a number of functions ranging from structural support to light-harvesting and photoprotection. In the major light harvesting complex of photosystem II in plants (LHCII), the innermost xanthophyll binding pockets are occupied by lutein molecules. The conservation of these sites within the LHC protein family suggests their importance in LHCII functionality. In the present work, we induced the photoprotective switch in LHCII isolated from the Arabidopsis mutant npq1lut2, where the lutein molecules are exchanged with violaxanthin. Despite the differences in the energetics of the pigments and the impairment of chlorophyll fluorescence quenching in vivo, we show that isolated complexes containing violaxanthin are still able to induce the quenching switch to a similar extent to wild type LHCII monomers. Moreover, the same spectroscopic changes take place, which suggest the involvement of the terminal emitter site (L1) in energy dissipation in both complexes. These results indicate the robust nature of the L1 xanthophyll binding domain in LHCII, where protein structural cues are the major determinant of the function of the bound carotenoid.

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.

Thylakoid membrane unstacking increases LHCII thermal stability and lipid phase fluidity.

Thylakoids are highly protein-enriched membranes that harbor a number of multicomponent photosynthetic complexes. Similarly to other biological membranes the protein constituents are heterogeneously distributed laterally in the plane of the membrane, however the specific segregation into stacked (grana patches) and unstacked (stroma lamellae) membrane layers is a unique feature of the thylakoid. Both the lateral and the vertical arrangements of the integral membrane proteins within the three-dimensional thylakoid ultrastructure are thought to have important physiological function. In this work we explore the role of membrane stacking for the thermal stability of the photosynthetic complexes in thylakoid membranes. By means of circular dichroism and differential scanning calorimetry we demonstrate that the thermal stability of the monomeric and trimeric forms of the major light harvesting complex of photosystem II (LHCII) increases upon unstacking. This effect was suggested to be due to the detachment of LHCII from photosystem II and consequent attachment to photosystem I subunits and/or the fluidization of the lipid matrix upon unstacking. The changes in the physical properties of the protein and lipid membrane components upon unstacking result in strongly reduced photosystem II excitation energy utilization.

The influence of light regime on the growth data and pigment composition of the plant Gentiana lutea cultured in vitro

New technologies of reintroduction of plant species presuppose implementing both traditional and biotechnological methods for obtaining certain planting materials. However, plants cultivated in vitro exist in specific conditions that lead to changes in their structural and functional state. This explains why it is hard for them to adapt to ex vitro and in situ conditions. Therefore, there is a need for the development of a multistage method of cultivating in vitro plants that would make the influence on their adaptive mechanism in ex vitro and in situ conditions possible. One of its stages is the optimization of the light regime of cultivation which can both initiate the change of the state of the photosynthetic apparatus of plants and increase their bioproductivity stimulating the work of their protective system. This work studies changes in the morphogenesis, growth data and pigment composition of the rare species of Gentiana lutea L. of three populations in the Ukrainian Carpathian (mountains Pozhyzhevska and Sheshul-Pavlyk, plateau Lemska) in vitro focusing particularly on the cultivation light regime. The research has proved the inefficiency of using fluorescent lamps of daylight lamps (LD) type as source of illumination because the low intensity of luminous flux in the area of photosynthetically active radiation (PAR), as well as high proportion of wavelength of blue (400–500 nm) and green (500–600 nm) range in the spectrum cause specific reactions of photomorphogenesis, which, despite the high content of pigments in plastids, lead to poor development of root systems, stretching the stems, formation of small leaves with thin leaflet plate, generally low productivity and low adaptive potential of G. lutea plants to ex vitro and in situ conditions. Complement of cold white light lamps to the fluorescent lamps LD type in the ratio of 1 : 1 enables one to increase the intensity of illumination in the field of PAR and raise the fraction of wavelength of red range (600–700 nm). Such light conditions both improve the bio-productivity of G. lutea plants of all three populations cultured in vitro in comparison to the LD type regimen, reducing the content of chlorophyll b and carotenoids in light-harvesting complexes of photosystems and facilitate an increase in the microclonal multiplication factor without using higher concentrations of exogenous growth regulators,which significantly reduces the cost of the process of obtaining planting materials. It was proved that a combination of LD type lamps, cold white light lamps and phytolamps in the ratio 1 : 1 : 0.6 should be used on the final stages of preparation of the planting material of G. lutea before transferring it to ex vitro and in situ conditions. This relates to the fact that the increase of the wavelength of the red range results in the widening of the active surface of the leaves, rise in the content of photosynthetic pigments, and the noticeable growth of the aboveground and underground parts of the plants. The article assumes that the use of such illumination mode will ensure a faster transition of cultured in vitro G. lutea plants from heterotrophic to autotrophic nutrition, improving their adaptive potential and enabling easier adaptation to non-sterile ex vitro and in situ conditions.

Energizing the light harvesting antenna: Insight from CP29

How do plants cope with excess light energy? Crop health and stress tolerance are governed by molecular photoprotective mechanisms. Protective exciton quenching in plants is activated by membrane energization, via unclear conformational changes in proteins called antennas. Here we show that pH and salt gradients stimulate the response of such an antenna under low and high energization by all-atom Molecular Dynamics Simulations. Novel insight establishes that helix-5 (H5) conformation in CP29 from spinach is regulated by chemiosmotic factors. This is selectively correlated with the chl-614 macrocycle deformation and interactions with nearby pigments, that could suggest a role in plant photoprotection. Adding to the significance of our findings, H5 domain is conserved among five antennas (LHCB1–5). These results suggest that light harvesting complexes of Photosystem II, one of the most abundant proteins on earth, can sense chemiosmotic gradients via their H5 domains in an upgraded role from a solar detector to also a chemiosmotic sensor.

Deficiency and toxicity of nanomolar copper in low irradiance—A physiological and metalloproteomic study in the aquatic plant Ceratophyllum demersum

Essential trace elements (Cu2+, Zn2+, etc) lead to toxic effects above a certain threshold, which is a major environmental problem in many areas of the world. Here, environmentally relevant sub-micromolar concentrations of Cu2+ and simulations of natural light and temperature cycles were applied to the aquatic macrophyte Ceratophyllum demersum a s a model for plant shoots. In this low irradiance study resembling non‐summer conditions, growth was optimal in the range 7.5–35nM Cu, while PSII activity (Fv/Fm) was maximal around 7.5nM Cu. Damage to the light harvesting complex of photosystem II (LHCII) was the first target of Cu toxicity (>50nM Cu) where Cu replaced Mg in the LHCII-trimers. This was associated with a subsequent decrease of Chl a as well as heat dissipation (NPQ). The growth rate was decreased from the first week of Cu deficiency. Plastocyanin malfunction due to the lack of Cu that is needed for its active centre was the likely cause of diminished electron flow through PSII (ΦPSII). The pigment decrease added to the damage in the photosynthetic light reactions. These mechanisms ultimately resulted in decrease of starch and oxygen production.

Mechanism of interaction of Al3+ with the proteins composition of photosystem II.

The inhibitory effect of Al3+on photosystem II (PSII) electron transport was investigated using several biophysical and biochemical techniques such as oxygen evolution, chlorophyll fluorescence induction and emission, SDS-polyacrylamide and native green gel electrophoresis, and FTIR spectroscopy. In order to understand the mechanism of its inhibitory action, we have analyzed the interaction of this toxic cation with proteins subunits of PSII submembrane fractions isolated from spinach. Our results show that Al 3+, especially above 3 mM, strongly inhibits oxygen evolution and affects the advancement of the S states of the Mn4O5Ca cluster. This inhibition was due to the release of the extrinsic polypeptides and the disorganization of the Mn4O5Ca cluster associated with the oxygen evolving complex (OEC) of PSII. This fact was accompanied by a significant decline of maximum quantum yield of PSII (Fv/Fm) together with a strong damping of the chlorophyll a fluorescence induction. The energy transfer from light harvesting antenna to reaction centers of PSII was impaired following the alteration of the light harvesting complex of photosystem II (LHCII). The latter result was revealed by the drop of chlorophyll fluorescence emission spectra at low temperature (77 K), increase of F0 and confirmed by the native green gel electrophoresis. FTIR measurements indicated that the interaction of Al 3+ with the intrinsic and extrinsic polypeptides of PSII induces major alterations of the protein secondary structure leading to conformational changes. This was reflected by a major reduction of α-helix with an increase of β-sheet and random coil structures in Al 3+-PSII complexes. These structural changes are closely related with the functional alteration of PSII activity revealed by the inhibition of the electron transport chain of PSII.

Effects of far red light on the induction changes of prompt and delayed fluorescence and the redox state of P700 in Scenedesmus quadricauda

The spectral composition of sunlight in natural aquatic habitats depends on the depth of submergence and the solar time. Changes in the fractional amount of far-red light (FRL) in the total photon flux are associated with the redistribution of light energy absorbed by the two photosystems of photosynthesis. Experiments with a culture of Scenedesmus quadricauda revealed the influence of FRL preillumination on photosynthetic electron transport and associated processes in microalgae. The plant efficiency analyzer M-PEA-2 was used to measure simultaneously the induction curves of prompt fluorescence, delayed fluorescence, and the redox transients of chlorophyll P700. The results are discussed in terms of the action of FRL preillumination on cyclic and noncyclic electron flows with account for relocation of the mobile light-harvesting complex of photosystem II during reversible State 1–State 2 transitions. Essential similarities and substantial distinctions have been revealed in the effects of FRL on photosynthetic electron flows in microalgae and higher plant leaves.

Self-assembly of the light-harvesting complex of photosystem II (LHCII) on alkanethiol-modified gold electrodes

A light-harvesting complex of photosystem II (LHCII), isolated from spinach, was immobilized onto a gold electrode modified with self-assembled monolayers (SAMs) of alkanethiols, NH2–(CH2) n –SH, n = 2, 6, 8, 11; HOOC–(CH2)7–SH; and CH3–(CH2)7–SH; and a bare electrode. The extent of LHCII complex adsorption according to surface treatment decreased in the order amino groups > carboxylic acid groups > methyl groups and increased with the methylene chain length in NH2–(CH2) n –SH. Interestingly, the photocurrent density depended on the terminal group and the methylene chain length in NH2–(CH2) n –SH and decreased in the order amino groups > methyl groups > carboxylic acid groups. An efficient photocurrent response of the LHCII complex on SAMs of NH2–(CH2) n –SH, n = 8 was observed upon illumination at 680 nm. These results indicated that the LHCII complexes were well organized on the cationic surfaces of the gold electrodes modified with amino alkanethiols. The quantum yield depended on the methylene chain length (n), where the maximum photocurrent response was observed at n = 8, which corresponded to a distance of 1.7 nm between the terminal amino group in NH2–(CH2)8–SH and the gold surface.

AtFtsH heterocomplex-mediated degradation of apoproteins of the major light harvesting complex of photosystem II (LHCII) in response to stresses

Chloroplastic heterocomplex consisting of AtFtsH1, 2, 5 and 8 proteases, integrally bound to thylakoid membrane was shown to play a critical role in degradation of photodamaged PsbA molecules, inherent to photosystem II (PSII) repair cycle and in plastid development. As no one thylakoid bound apoproteins besides PsbA has been identified as target for the heterocomplex-mediated degradation we investigated the significance of this protease complex in degradation of apoproteins of the major light harvesting complex of photosystem II (LHCII) in response to various stressing conditions and in stress-related changes in overall composition of LHCII trimers of PSII-enriched membranes (BBY particles). To reach this goal a combination of approaches was applied based on immunoblotting, in vitro degradation and non-denaturing isoelectrofocusing. Exposure of Arabidopsis thaliana leaves to desiccation, cold and high irradiance led to a step-wise disappearance of Lhcb1 and Lhcb2, while Lhcb3 level remained unchanged, except for high irradiance which caused significant Lhcb3 decrease. Furthermore, it was demonstrated that stress-dependent disappearance of Lhcb1–3 is a proteolytic phenomenon for which a metalloprotease is responsible. No changes in Lhcb1–3 level were observed due to exposition of var1-1 mutant leaves to the three stresses clearly pointing to the involvement of AtFtsH heterocomplex in the desiccation, cold and high irradiance-dependent degradation of Lhcb1 and Lhcb2 and in high irradiance-dependent degradation of Lhcb3. Non-denaturing isoelectrofocusing analyses revealed that AtFtsH heterocomplex-dependent differential Lhcb1–3 disappearance behaviour following desiccation stress was accompanied by modulations in abundances of individual LHCII trimers of BBY particles and that LHCII of var1-1 resisted the modulations.

Photosynthetic acclimation responses of maize seedlings grown under artificial laboratory light gradients mimicking natural canopy conditions

In this study we assessed the ability of the C4 plant maize to perform long-term photosynthetic acclimation in an artificial light quality system previously used for analyzing short-term and long-term acclimation responses (LTR) in C3 plants. We aimed to test if this light system could be used as a tool for analyzing redox-regulated acclimation processes in maize seedlings. Photosynthetic parameters obtained from maize samples harvested in the field were used as control. The results indicated that field grown maize performed a pronounced LTR with significant differences between the top and the bottom levels of the plant stand corresponding to the strong light gradients occurring in it. We compared these data to results obtained from maize seedlings grown under artificial light sources preferentially exciting either photosystem II or photosystem I. In C3 plants, this light system induces redox signals within the photosynthetic electron transport chain which trigger state transitions and differential phosphorylation of LHCII (light harvesting complexes of photosystem II). The LTR to these redox signals induces changes in the accumulation of plastid psaA transcripts, in chlorophyll (Chl) fluorescence values F\\rm s/F\\rm m, in Chl a/b ratios and in transient starch accumulation in C3 plants. Maize seedlings grown in this light system exhibited a pronounced ability to perform both short-term and long-term acclimation at the level of psaA transcripts, Chl fluorescence values F\\rm s/F\\rm m and Chl a/b ratios. Interestingly, maize seedlings did not exhibit redox-controlled variations of starch accumulation probably because of its specific differences in energy metabolism. In summary, the artificial laboratory light system was found to be well-suited to mimic field light conditions and provides a physiological tool for studying the molecular regulation of the LTR of maize in more detail.

Light intensity dependent expression of Lhca gene family encoding LHCI in Dunaliella salina

In order to compare the expression pattern and function of Lhca gene families, theDsLhca genes have been cloned, and two novel genes have been isolated by screening aDunaliella salina cDNA library with a DNA probe coding for the conserved first transmembrane helix of this protein. The number of exons in the different DsLhca genes was slightly conserved. The deduced pre-proteins of the two novel gene had molecular masses of 24.7 and 30.7 kDa, and were assigned into light-harvesting complex of photosystem I (LHCI) Type I and Type III of higher plants in phylogenetic tree, respectively. The DsLhca1 and DsLhca3 responded to the light intensity divergently from DsLhca2. It indicates that changes in the protein composition of LHCI, depending on the growth conditions, will thus result in an adaptation state of the antenna produced to perform better under different environmental conditions. Key words: Dunaliella salina, Lhca gene family, light-harvesting complex of photosystem I (LHCI), environment stress response.

Conformational switching explains the intrinsic multifunctionality of plant light-harvesting complexes

The light-harvesting complexes of photosystem I and II (Lhcas and Lhcbs) of plants display a high structural homology and similar pigment content and organization. Yet, the spectroscopic properties of these complexes, and accordingly their functionality, differ substantially. This difference is primarily due to the charge-transfer (CT) character of a chlorophyll dimer in all Lhcas, which mixes with the excitonic states of these complexes, whereas this CT character is generally absent in Lhcbs. By means of single-molecule spectroscopy near room temperature, we demonstrate that the presence or absence of such a CT state in Lhcas and Lhcbs can occasionally be reversed; i.e., these complexes are able to interconvert conformationally to quasi-stable spectral states that resemble the Lhcs of the other photosystem. The high structural similarity of all the Lhca and Lhcb proteins suggests that the stable conformational states that give rise to the mixed CT-excitonic state are similar for all these proteins, and similarly for the conformations that involve no CT state. This indicates that the specific functions related to Lhca and Lhcb complexes are realized by different stable conformations of a single generic protein structure. We propose that this functionality is modulated and controlled by the protein environment.

Identification of a specific fucoxanthin-chlorophyll protein in the light harvesting complex of photosystem I in the diatom Cyclotella meneghiniana

Thylakoids of the diatom Cyclotella meneghiniana were separated by discontinuous gradient centrifugation into photosystem (PS) I, PSII, and fucoxanthin-chlorophyll protein (FCP) fractions. FCPs are homologue to light harvesting complexes of higher plants with similar function in e.g. brown algae and diatoms. Still, it is unclear if FCP complexes are specifically associated with either PSI or PSII, or if FCP complexes function as one antenna for both photosystems. However, a trimeric FCP complex, FCPa, and a higher FCP oligomer, FCPb, have been described for C. meneghiniana, already. In this study, biochemical and spectroscopical evidences are provided that reveal a different subset of associated Fcp polypeptides within the isolated photosystem complexes. Whereas the PSII associated Fcp antenna resembles FCPa since it contains Fcp2 and Fcp6, at least three different Fcp polypeptides are associated with PSI. By re-solubilisation and a further purification step Fcp polypeptides were partially removed from PSI and both fractions were analysed again by biochemical and spectroscopical means, as well as by HPLC. Thereby a protein related to Fcp4 and a so far undescribed 17 kDa Fcp were found to be strongly coupled to PSI, whereas presumably Fcp5, a subunit of the FCPb complex, is only loosely bound to the PSI core. Thus, an association of FCPb and PSI is assumed.

Crystal structure of plant light-harvesting complex shows the active, energy-transmitting state.

Plants dissipate excess excitation energy as heat by non-photochemical quenching (NPQ). NPQ has been thought to resemble in vitro aggregation quenching of the major antenna complex, light harvesting complex of photosystem II (LHC-II). Both processes are widely believed to involve a conformational change that creates a quenching centre of two neighbouring pigments within the complex. Using recombinant LHC-II lacking the pigments implicated in quenching, we show that they have no particular role. Single crystals of LHC-II emit strong, orientation-dependent fluorescence with an emission maximum at 680 nm. The average lifetime of the main 680 nm crystal emission at 100 K is 1.31 ns, but only 0.39 ns for LHC-II aggregates under identical conditions. The strong emission and comparatively long fluorescence lifetimes of single LHC-II crystals indicate that the complex is unquenched, and that therefore the crystal structure shows the active, energy-transmitting state of LHC-II. We conclude that quenching of excitation energy in the light-harvesting antenna is due to the molecular interaction with external pigments in vitro or other pigment-protein complexes such as PsbS in vivo, and does not require a conformational change within the complex.

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.

Construction of a chloroplast protein interaction network and functional mining of photosynthetic proteins in Arabidopsis thaliana

Chloroplast is a typical plant cell organelle where photosynthesis takes place. In this study, a total of 1,808 chloroplast core proteins in Arabidopsis thaliana were reliably identified by combining the results of previously published studies and our own predictions. We then constructed a chloroplast protein interaction network primarily based on these core protein interactions. The network had 22,925 protein interaction pairs which involved 2,214 proteins. A total of 160 previously uncharacterized proteins were annotated in this network. The subunits of the photosynthetic complexes were modularized, and the functional relationships among photosystem I (PSI), photosystem II (PSII), light harvesting complex of photosystem I (LHC I) and light harvesting complex of photosystem I (LHC II) could be deduced from the predicted protein interactions in this network. We further confirmed an interaction between an unknown protein AT1G52220 and a photosynthetic subunit PSI-D2 by yeast two-hybrid analysis. Our chloroplast protein interaction network should be useful for functional mining of photosynthetic proteins and investigation of chloroplast-related functions at the systems biology level in Arabidopsis.