CHLOROPHYLL a/b-BINDING PROTEINS

Chloroplast history clarified by the criterion of light-harvesting complex

Bacterial essence of mitochondria and chloroplasts was initially proclaimed in general outline. Later, the remarkable insight gave way to an elaborate hypothesis. Finally, it took shape of a theory confirmed by molecular biology data. In particular, the rrn operon, which is the key phylogeny marker, locates chloroplasts on the tree of Cyanobacteria. Chloroplast ancestry and diversity can be also traced with the rpoС and psbA genes, rbc operon, and other molecular criteria of prime importance. Another criterion, also highly reliable, is light-harvesting complex (LHC). LHC pigment and protein moieties specify light acclimation strategies in evolutionary retrospect and modern biosphere. The onset of symbiosis between eukaryotic host and pre-chloroplast, as well as further mutual adjustment of partners depended on physiological competence of LHC. In this review, the criterion of LHC is applied to the origin and diversity of chloroplasts. In particular, ancient cyanobacterium possessing tandem antenna (encoded by the cbp genes and the pbp genes, correspondingly), and defined as a prochlorophyte, is argued to be chloroplast ancestor.

Rapid identification and characterization of genetic loci for defective kernel in bread wheat

BACKGROUND

Wheat is a momentous crop and feeds billions of people in the world. The improvement of wheat yield is very important to ensure world food security. Normal development of grain is the essential guarantee for wheat yield formation. The genetic study of grain phenotype and identification of key genes for grain filling are of great significance upon dissecting the molecular mechanism of wheat grain morphogenesis and yield potential.

RESULTS

Here we identified a pair of defective kernel (Dek) isogenic lines, BL31 and BL33, with plump and shrunken mature grains, respectively, and constructed a genetic population from the BL31/BL33 cross. Ten chromosomes had higher frequency of polymorphic single nucleotide polymorphism (SNP) markers between BL31 and BL33 using Wheat660K chip. Totally 783 simple sequence repeat (SSR) markers were chosen from the above chromosomes and 15 of these were integrated into two linkage groups using the genetic population. Genetic mapping identified three QTL, QDek.caas-3BS.1, QDek.caas-3BS.2 and QDek.caas-4AL, explaining 14.78-18.17%, 16.61-21.83% and 19.08-28.19% of phenotypic variances, respectively. Additionally, five polymorphic SNPs from Wheat660K were successfully converted into cleaved amplified polymorphic sequence (CAPS) markers and enriched the target regions of the above QTL. Biochemical analyses revealed that BL33 has significantly higher grain sucrose contents at filling stages and lower mature grain starch contents than BL31, indicating that the Dek QTL may be involved in carbohydrate metabolism. As such, the candidate genes for each QTL were predicated according to International Wheat Genome Sequence Consortium (IWGSC) RefSeq v1.0.

CONCLUSIONS

Three major QTL for Dek were identified and their causal genes were predicted, laying a foundation to conduct fine mapping and dissect the regulatory mechanism underlying Dek trait in wheat.

RNA-seq analyses of Arabidopsis thaliana seedlings after exposure to blue-light phototropic stimuli in microgravity

PREMISE

Plants synthesize information from multiple environmental stimuli when determining their direction of growth. Gravity, being ubiquitous on Earth, plays a major role in determining the direction of growth and overall architecture of the plant. Here, we utilized the microgravity environment on board the International Space Station (ISS) to identify genes involved influencing growth and development of phototropically stimulated seedlings of Arabidopsis thaliana.

METHODS

Seedlings were grown on the ISS, and RNA was extracted from 7 samples (pools of 10-15 plants) grown in microgravity (μg) or Earth gravity conditions (1-g). Transcriptomic analyses via RNA sequencing (RNA-seq) of differential gene expression was performed using the HISAT2-Stringtie-DESeq2 RNASeq pipeline. Differentially expressed genes were further characterized by using Pathway Analysis and enrichment for Gene Ontology classifications.

RESULTS

For 296 genes that were found significantly differentially expressed between plants in microgravity compared to 1-g controls, Pathway Analysis identified eight molecular pathways that were significantly affected by reduced gravity conditions. Specifically, light-associated pathways (e.g., photosynthesis-antenna proteins, photosynthesis, porphyrin, and chlorophyll metabolism) were significantly downregulated in microgravity.

CONCLUSIONS

Gene expression in A. thaliana seedlings grown in microgravity was significantly altered compared to that of the 1-g control. Understanding how plants grow in conditions of microgravity not only aids in our understanding of how plants grow and respond to the environment but will also help to efficiently grow plants during long-range space missions.

Shotgun Quantitative Proteomic Analysis of Proteins Responding to Drought Stress in Brassica rapa L. (Inbred Line "Chiifu")

Through a comparative shotgun quantitative proteomics analysis in Brassica rapa (inbred line Chiifu), total of 3,009 nonredundant proteins were identified with a false discovery rate of 0.01 in 3-week-old plants subjected to dehydration treatment for 0, 24, and 48 h, plants subjected to drought stress. Ribulose-bisphosphate carboxylases, chlorophyll a/b-binding protein, and light harvesting complex in photosystem II were highly abundant proteins in the leaves and accounted for 9%, 2%, and 4%, respectively, of the total identified proteins. Comparative analysis of the treatments enabled detection of 440 differentially expressed proteins during dehydration. The results of clustering analysis, gene ontology (GO) enrichment analysis, and analysis of composite expression profiles of functional categories for the differentially expressed proteins indicated that drought stress reduced the levels of proteins associated with photosynthesis and increased the levels of proteins involved in catabolic processes and stress responses. We observed enhanced expression of many proteins involved in osmotic stress responses and proteins with antioxidant activities. Based on previously reported molecular functions, we propose that the following five differentially expressed proteins could provide target genes for engineering drought resistance in plants: annexin, phospholipase D delta, sDNA-binding transcriptional regulator, auxin-responsive GH3 family protein, and TRAF-like family protein.

Physiological and iTRAQ-Based Proteomic Analyses Reveal the Function of Spermidine on Improving Drought Tolerance in White Clover

Endogenous spermidine interacting with phytohormones may be involved in the regulation of differentially expressed proteins (DEPs) associated with drought tolerance in white clover. Plants treated with or without spermidine (50 μM) were subjected to 20% PEG 6000 nutrient solution to induce drought stress (50% leaf-relative water content). The results showed that increased endogenous spermidine induced by exogenous spermidine altered endogenous phytohormones in association with improved drought tolerance, as demonstrated by the delay in water-deficit development, improved photosynthesis and water use efficiency, and lower oxidative damage. As compared to untreated plants, Spd-treated plants maintained a higher abundance of DEPs under drought stress involved in (1) protein biosynthesis (ribosomal and chaperone proteins); (2) amino acids synthesis; (3) the carbon and energy metabolism; (4) antioxidant and stress defense (ascorbate peroxidase, glutathione peroxidase, and dehydrins); and (5) GA and ABA signaling pathways (gibberellin receptor GID1, ABA-responsive protein 17, and ABA stress ripening protein). Thus, the findings of proteome could explain the Spd-induced physiological effects associated with drought tolerance. The analysis of functional protein-protein networks further proved that the alteration of endogenous spermidine and phytohormones induced the interaction among ribosome, photosynthesis, carbon metabolism, and amino acid biosynthesis. These differences could contribute to improved drought tolerance.

Proteins associated with heat-induced leaf senescence in creeping bentgrass as affected by foliar application of nitrogen, cytokinins, and an ethylene inhibitor

Heat stress causes premature leaf senescence in cool-season grass species. The objective of this study was to identify proteins regulated by nitrogen, cytokinins, and ethylene inhibitor in relation to heat-induced leaf senescence in creeping bentgrass (Agrostis stolonifera). Plants (cv. Penncross) were foliar sprayed with 18 mM carbonyldiamide (N source), 25 μM aminoethoxyvinylglycine (AVG, ethylene inhibitor), 25 μM zeatin riboside (ZR, cytokinin), or a water control, and then exposed to 20/15°C (day/night) or 35/30°C (heat stress) in growth chambers. All treatments suppressed heat-induced leaf senescence, as shown by higher turf quality and chlorophyll content, and lower electrolyte leakage in treated plants compared to the untreated control. A total of 49 proteins were responsive to N, AVG, or ZR under heat stress. The abundance of proteins in photosynthesis increased, with ribulose-1,5-bisphosphate carboxylase/oxygenase affected by all three treatments, chlorophyll a/b-binding protein by AVG and N or Rubisco activase by AVG. Proteins for amino acid metabolism were upregulated, including alanine aminotransferase by three treatments and ferredoxin-dependent glutamate synthase by AVG and N. Upregulated proteins also included catalase by AVG and N and heat shock protein by ZR. Exogenous applications of AVG, ZR, or N downregulated proteins in respiration (enolase, glyceraldehyde 3-phosphate dehydrogenase, and succinate dehygrogenase) under heat stress. Alleviation of heat-induced senescence by N, AVG, or ZR was associated with enhanced protein abundance in photosynthesis and amino acid metabolism and stress defense systems (heat shock protection and antioxidants), as well as suppression of those imparting respiration metabolism.

Photoinhibition of photosystem I in a pea mutant with altered LHCII organization

Comparative analysis of in vivo chlorophyll fluorescence imaging revealed that photosystem II (PSII) photochemical efficiency (Fv/Fm) of leaves of the Costata 2/133 pea mutant with altered pigment composition and decreased level of oligomerization of the light harvesting chlorophyll a/b-protein complexes (LHCII) of PSII (Dobrikova et al., 2000; Ivanov et al., 2005) did not differ from that of WT. In contrast, photosystem I (PSI) activity of the Costata 2/133 mutant measured by the far-red (FR) light inducible P700 (P700(+)) signal exhibited 39% lower steady state level of P700(+), a 2.2-fold higher intersystem electron pool size (e(-)/P700) and higher rate of P700(+) re-reduction, which indicate an increased capacity for PSI cyclic electron transfer (CET) in the Costata 2/133 mutant than WT. The mutant also exhibited a limited capacity for state transitions. The lower level of oxidizable P700 (P700(+)) is consistent with a lower amount of PSI related chlorophyll protein complexes and lower abundance of the PsaA/PsaB heterodimer, PsaD and Lhca1 polypeptides in Costata 2/133 mutant. Exposure of WT and the Costata 2/133 mutant to high light stress resulted in a comparable photoinhibition of PSII measured in vivo, although the decrease of Fv/Fm was modestly higher in the mutant plants. However, under the same photoinhibitory conditions PSI photochemistry (P700(+)) measured as ΔA820-860 was inhibited to a greater extent (50%) in the Costata 2/133 mutant than in the WT (22%). This was accompanied by a 50% faster re-reduction rate of P700(+) in the dark indicating a higher capacity for CET around PSI in high light treated mutant leaves. The role of chloroplast thylakoid organization on the stability of the PSI complex and its susceptibility to high light stress is discussed.

Non-destructive measurement of chlorophyll b : a ratios and identification of photosynthetic pathways in grasses by reflectance spectroscopy

Equations for non-destructive determination of chlorophyll b : a ratios in grasses were developed from reflectance spectra of intact leaves of barley (Hordeum vulgare L.) and two barley mutants: clorina f2, which lacks chlorophyll b and clorina f104, which has a low chlorophyll b content. These plants enabled separation of effects of chlorophyll composition on reflectance spectra due to differential light absorption by chlorophylls a and b and to measure the effects of chlorophyll b on the contribution of fluorescence emitted by chlorophyll a to the reflectance spectra. Indices developed from these data were then tested on growth chamber-grown leaves from six C₃ and 17 C₄ grass species (7 NAD-ME and 10 NADP-ME subtypes). We used the chlorophyll b : a ratio because the data were less skewed than the chlorophyll a : b ratio. The best index for determination of the chlorophyll b : a ratio utilised wavelengths affected by chlorophyll absorbance: [R₆₂₆ - 0.5 (R₆₀₃ + R₆₄₇)]/[R₅₅₂- R₆₂₆]. The chlorophyll b : a ratio was significantly lower in the C₄ than C₃ grasses, but was not sufficient in itself to separate these two functional groups. However, because of differences in fluorescence characteristics, C₃ and C₄ species could be distinguished by an index based on wavelengths affected by chlorophyll fluorescence: [R_{696 to 709}/R_{545 to 567}].

A Bloch equation approach to intensity dependent optical spectra of light harvesting complex II: excitation dependence of light harvesting complex II pump-probe spectra

On the basis of the recent progress in the resolution of the structure of the antenna light harvesting complex II (LHC II) of the photosystem II, we propose a microscopically motivated theory to predict excitation intensity-dependent spectra. We show that optical Bloch equations provide the means to include all 2( N ) excited states of an oligomer complex of N coupled two-level systems and analyze the effects of Pauli Blocking and exciton-exciton annihilation on pump-probe spectra. We use LHC Bloch equations for 14 Coulomb coupled two-level systems, which describe the S (0) and S (1) level of every chlorophyll molecule. All parameter introduced into the Hamiltonian are based on microscopic structure and a quantum chemical model. The derived Bloch equations describe not only linear absorption but also the intensity dependence of optical spectra in a regime where the interplay of Pauli Blocking effects as well as exciton-exciton annihilation effects are important. As an example, pump-probe spectra are discussed. The observed saturation of the spectra for high intensities can be viewed as a relaxation channel blockade on short time scales due to Pauli blocking. The theoretical investigation is useful for the interpretation of the experimental data, if the experimental conditions exceed the low intensity pump limit and effects like strong Pauli Blocking and exciton-exciton annihilation need to be considered. These effects become important when multiple excitations are generated by the pump pulse in the complex.

Nonperturbative theory for the optical response to strong light of the light harvesting complex II of plants: Saturation of the fluorescence quantum yield

Recent progress in resolution of the structure of the light harvesting complex II provides the basis for theoretical predictions on nonlinear optical properties from microscopic calculations. An approach to absorption and fluorescence is presented within the framework of Bloch equations using a correlation expansion of relevant many particle interactions. The equations derived within the framework of this theory are applied to describe fluorescence saturation phenomena. The experimentally observed decrease of the normalized fluorescence quantum yield from 1 to 0.0001 upon increasing the intensity of laser pulse excitation at 645 nm by five orders of magnitude [R Schödel et al., Biophys. J. 71, 3370 (1996)] is explained by Pauli blocking effects of optical excitation and excitation energy transfer.

Relationship between the organization of the PS II super complex and the functions of the photosynthetic apparatus

The chlorophyll fluorescence and the photosynthetic oxygen evolution (flash-induced oxygen yield patterns and oxygen bursts under continuous irradiation) were investigated in the thylakoid membranes with different stoichiometry and organization of the chlorophyll-protein complexes. Data show that the alteration in the organization of the photosystem II (PS II) super complex, i.e. the amount and the organization of the light-harvesting chlorophyll a/b protein complex (LHCII), which strongly modifies the electric properties of the membranes, influences both the energy redistribution between the two photosystems and the oxygen production reaction. The decrease of surface electric parameters (charge density and dipole moments), associated with increased degree of LHCII oligomerization, correlates with the strong reduction of the energy transfer from PS II to PSI. In the studied pea thylakoid membranes (wild types Borec, Auralia and their mutants Coeruleovireus 2/16, Costata2/133, Chlorotica XV/1422) with enhanced degree of oligomerization of LHCII was observed: (i) an increase of the S(0) populations of PS II in darkness; (ii) an increase of the misses; (iii) an alteration of the decay kinetics of the oxygen bursts under continuous irradiation. There is a strict correlation between the degree of LHCII oligomerization in the investigated pea mutants and the ratio of functionally active PS II alpha to PS II beta centers, while in thylakoid membranes without oligomeric structure of LHCII (Chlorina f2 barley mutant) the PS II alpha centers are not registered.

New Insights into the Nature and Phylogeny of Prasinophyte Antenna Proteins: Ostreococcus tauri, a Case Study

The basal position of the Mamiellales (Prasinophyceae) within the green lineage makes these unicellular organisms key to elucidating early stages in the evolution of chlorophyll a/b-binding light-harvesting complexes (LHCs). Here, we unveil the complete and unexpected diversity of Lhc proteins in Ostreococcus tauri, a member of the Mamiellales order, based on results from complete genome sequencing. Like Mantoniella squamata, O. tauri possesses a number of genes encoding an unusual prasinophyte-specific Lhc protein type herein designated "Lhcp". Biochemical characterization of the complexes revealed that these polypeptides, which bind chlorophylls a, b, and a chlorophyll c-like pigment (Mg-2,4-divinyl-phaeoporphyrin a5 monomethyl ester) as well as a number of unusual carotenoids, are likely predominant. They are retrieved to some extent in both reaction center I (RCI)- and RCII-enriched fractions, suggesting a possible association to both photosystems. However, in sharp contrast to previous reports on LHCs of M. squamata, O. tauri also possesses other LHC subpopulations, including LHCI proteins (encoded by five distinct Lhca genes) and the minor LHCII polypeptides, CP26 and CP29. Using an antibody against plant Lhca2, we unambiguously show that LHCI proteins are present not only in O. tauri, in which they are likely associated to RCI, but also in other Mamiellales, including M. squamata. With the exception of Lhcp genes, all the identified Lhc genes are present in single copy only. Overall, the discovery of LHCI proteins in these prasinophytes, combined with the lack of the major LHCII polypeptides found in higher plants or other green algae, supports the hypothesis that the latter proteins appeared subsequent to LHCI proteins. The major LHC of prasinophytes might have arisen prior to the LHCII of other chlorophyll a/b-containing organisms, possibly by divergence of a LHCI gene precursor. However, the discovery in O. tauri of CP26-like proteins, phylogenetically placed at the base of the major LHCII protein clades, yields new insight to the origin of these antenna proteins, which have evolved separately in higher plants and green algae. Its diverse but numerically limited suite of Lhc genes renders O. tauri an exceptional model system for future research on the evolution and function of LHC components.

Ion transport across model lipid membranes containing light-harvesting complex II: an effect of light

The effect of light on proton transport across lipid membranes of small unilamellar liposomes containing incorporated major light-harvesting pigment-protein complex of Photosystem II (LHCII) has been studied with the application of pH-sensitive dyes entrapped inside vesicles. Proton permeability coefficient for LHCII-modified membranes was found to be about twice as high as in the case of the control pure lipid vesicles. Illumination of the samples with light absorbed by the LHCII-bound photosynthetic pigments considerably affects the kinetics of proton transport: it increases the rate and decreases the steady-state level of proton gradient across the membranes. The effect was interpreted in terms of heat-induced conformational changes of LHCII molecular structures that affect proton buffering capacity of this protein. Both the control and the LHCII-modified lipid membranes have been found to be practically impermeable to Ca(++) ions, as demonstrated by fluorescence of liposome-entrapped calcium-sensitive probe calcium crimson. The slight differences in the proton transport across the LHCII-containing membranes under the presence of Ca(++) suggest calcium binding to this antenna protein.

Role of LHCII organization in the interaction of substituted 1,4-anthraquinones with thylakoid membranes

The chlorophyll fluorescence, photochemical activity and surface electric properties of thylakoid membranes with different stoichiometry of pigment-protein complexes and organization of the light-harvesting chlorophyll a/b protein complex of photosystem II (LHCII) were studied in the presence of substituted 1,4-anthraquinones. Data show strong dependence of the quenching of the chlorophyll fluorescence on the structural organization of LHCII. The increase of the LHCII oligomerization, which is associated with significant reduction of the transmembrane electric charge asymmetry and electric polarizability of the membrane, correlates with enhanced quenching effect of substituted 1,4-athraquinones. Crucial for the large quinone-induced changes in the membrane electric dipole moments is the structure of the quinone molecule. The strongest reduction in the values of the dipole moments is observed after interaction of thylakoids with 3-chloro-9-hydroxy-1,4-anthraquinone (TF33) which has the highest quenching efficiency. The quinone induced changes in the photochemical activity of photosystem II (PSII) correlate with the total amount of the supramolecular LHCII-PSII complex and depend on the number of substituents in the 1,4-anthraquinone molecule.

Degradation of the main Photosystem II light-harvesting complex

Many factors trigger the degradation of proteins, including changes in environmental conditions, genetic mutations, and limitations in the availability of cofactors. Despite the importance for viability, still very little is known about protein degradation and its regulation. The degradation of the most abundant membrane protein on Earth, the light-harvesting complex of Photosystem II (LHC II), is highly regulated under different environmental conditions, e.g. light stress, to prevent photochemical damage of the reaction center. However, despite major effort to identify the protease/proteases involved in the degradation of the apoproteins of LHC II the molecular details of this important process remain obscure. LHC II belongs to the family of chlorophyll a/b binding proteins (CAB proteins) and is located in the thylakoid membrane of the plant chloroplast. The results of biochemical experiments to isolate and characterize the protease degrading LHC II are summarized here and compared to our own recent finding indicating that a metalloprotease of the FtsH family is involved in this process.

Functional implications of pigments bound to a cyanobacterial cytochrome b6f complex

A highly purified cytochrome b(6)f complex from the cyanobacterium Synechocystis sp. PCC 6803 selectively binds one chlorophyll a and one carotenoid in analogy to the recent published structure from two other b(6)f complexes. The unknown function of these pigments was elucidated by spectroscopy and site-directed mutagenesis. Low-temperature redox difference spectroscopy showed red shifts in the chlorophyll and carotenoid spectra upon reduction of cytochrome b(6), which indicates coupling of these pigments with the heme groups and thereby with the electron transport. This is supported by the correlated kinetics of these redox reactions and also by the distinct orientation of the chlorophyll molecule with respect to the heme cofactors as shown by linear dichroism spectroscopy. The specific role of the carotenoid echinenone for the cytochrome b(6)f complex of Synechocystis 6803 was elucidated by a mutant lacking the last step of echinenone biosynthesis. The isolated mutant complex preferentially contained a carotenoid with 0, 1 or 2 hydroxyl groups (most likely 9-cis isomers of beta-carotene, a monohydroxy carotenoid and zeaxanthin, respectively) instead. This indicates a substantial role of the carotenoid - possibly for strucure and assembly - and a specificity of its binding site which is different from those in most other oxygenic photosynthetic organisms. In summary, both pigments are probably involved in the structure, but may also contribute to the dynamics of the cytochrome b(6)f complex.

The three isoforms of the light-harvesting complex II: spectroscopic features, trimer formation, and functional roles

The major light-harvesting complex (LHC-II) of higher plants plays a crucial role in capturing light energy for photosynthesis and in regulating the flow of energy within the photosynthetic apparatus. Native LHC-II isolated from plant tissue consists of three isoforms, Lhcb1, Lhcb2, and Lhcb3, which form homo- and heterotrimers. All three isoforms are highly conserved among different species, suggesting distinct functional roles. We produced the three LHC-II isoforms by heterologous expression of the polypeptide in Escherichia coli and in vitro refolding with purified pigments. Although Lhcb1 and Lhcb2 are very similar in polypeptide sequence and pigment content, Lhcb3 is clearly different because it lacks an N-terminal phosphorylation site and has a higher chlorophyll a/b ratio, suggesting the absence of one chlorophyll b. Low temperature absorption and fluorescence emission spectra of the pure isoforms revealed small but significant differences in pigment organization. The oligomeric state of the pure isoforms and of their permutations was investigated by native gel electrophoresis, sucrose density gradient centrifugation, and SDS-PAGE. Lhcb1 and Lhcb2 formed trimeric complexes by themselves and with one another, but Lhcb3 was able to do so only in combination with one or both of the other isoforms. We conclude that the main role of Lhcb1 and Lhcb2 is in the adaptation of photosynthesis to different light regimes. The most likely role of Lhcb3 is as an intermediary in light energy transfer from the main Lhcb1/Lhcb2 antenna to the photosystem II core.

A novel role of water-soluble chlorophyll proteins in the transitory storage of chorophyllide

All chlorophyll (Chl)-binding proteins involved in photosynthesis of higher plants are hydrophobic membrane proteins integrated into the thylakoids. However, a different category of Chl-binding proteins, the so-called water-soluble Chl proteins (WSCPs), was found in members of the Brassicaceae, Polygonaceae, Chenopodiaceae, and Amaranthaceae families. WSCPs from different plant species bind Chl a and Chl b in different ratios. Some members of the WSCP family are induced after drought and heat stress as well as leaf detachment. It has been proposed that this group of proteins might have a physiological function in the Chl degradation pathway. We demonstrate here that a protein that shared sequence homology to WSCPs accumulated in etiolated barley (Hordeum vulgare) seedlings exposed to light for 2 h. The novel 22-kD protein was attached to the outer envelope of barley etiochloroplasts, and import of the 27-kD precursor was light dependent and induced after feeding the isolated plastids the tetrapyrrole precursor 5-aminolevulinic acid. HPLC analyses and spectroscopic pigment measurements of acetone-extracted pigments showed that the 22-kD protein is complexed with chlorophyllide. We propose a novel role of WSCPs as pigment carriers operating during light-induced chloroplast development.

Effect of macromolecules and triton X-100 on the triplet of aggregated chlorophyll in aqueous solution

Chlorophyll a (Chla) in aqueous solution (2-6% acetone) is present as mono- and dihydrated aggregated forms which are characterized by specific ground state absorption spectra. The amount of dihydrated form is larger in the presence of macromolecules, such as bovine serum albumin (BSA), lysozime and polyvinyl alcohol (PVA), increasing from BSA to lysozime and PVA. Chla in aqueous acetone with and without macromolecules is characterized by low fluorescence and the absence of triplet-triplet (T-T) absorption. The ratio of dihydrated to monohydrated forms is significantly influenced by triton X-100. For lower triton X-100 concentrations, i.e. smaller than the critical micelle concentration of 0.26 mM (cmc), dihydrated forms are converted into monohydrated in both aqueous acetone and the presence of BSA or lysozime. In the presence of PVA dihydrated forms appeared to be resistant to triton X-100 action. Moreover, for triton X-100 concentrations of 2-3 times higher than cmc the amount of these forms is increased with time. T-T absorption of both mono- and dihydrated Chla aggregates was not detected in the presence of [triton X-100] < cmc. The lack of T-T absorption in aqueous acetone solution as well as in the presence of macromolecules implies that the triplet lifetime of the chlorophyll aggregates is short (tauT<10 ns) and/or the quantum yield of intersystem crossing is small (<5 x 10(-3)). The Chla monomers start to be formed as solubilized in the micelle for [triton X-100] larger than cmc, showing substantial fluorescence and T-T absorption.

Plants lacking the main light-harvesting complex retain photosystem II macro-organization

Photosystem II (PSII) is a key component of photosynthesis, the process of converting sunlight into the chemical energy of life. In plant cells, it forms a unique oligomeric macrostructure in membranes of the chloroplasts. Several light-harvesting antenna complexes are organized precisely in the PSII macrostructure-the major trimeric complexes (LHCII) that bind 70% of PSII chlorophyll and three minor monomeric complexes-which together form PSII supercomplexes. The antenna complexes are essential for collecting sunlight and regulating photosynthesis, but the relationship between these functions and their molecular architecture is unresolved. Here we report that antisense Arabidopsis plants lacking the proteins that form LHCII trimers have PSII supercomplexes with almost identical abundance and structure to those found in wild-type plants. The place of LHCII is taken by a normally minor and monomeric complex, CP26, which is synthesized in large amounts and organized into trimers. Trimerization is clearly not a specific attribute of LHCII. Our results highlight the importance of the PSII macrostructure: in the absence of one of its main components, another protein is recruited to allow it to assemble and function.

Xanthophyll biosynthetic mutants of Arabidopsis thaliana: altered nonphotochemical quenching of chlorophyll fluorescence is due to changes in Photosystem II antenna size and stability

Xanthophylls (oxygen derivatives of carotenes) are essential components of the plant photosynthetic apparatus. Lutein, the most abundant xanthophyll, is attached primarily to the bulk antenna complex, light-harvesting complex (LHC) II. We have used mutations in Arabidopsis thaliana that selectively eliminate (and substitute) specific xanthophylls in order to study their function(s) in vivo. These include two lutein-deficient mutants, lut1 and lut2, the epoxy xanthophyll-deficient aba1 mutant and the lut2aba1 double mutant. Photosystem stoichiometry, antenna sizes and xanthophyll cycle activity have been related to alterations in nonphotochemical quenching of chlorophyll fluorescence (NPQ). Nondenaturing polyacrylamide gel electrophoresis indicates reduced stability of trimeric LHC II in the absence of lutein (and/or epoxy xanthophylls). Photosystem (antenna) size and stoichiometry is altered in all mutants relative to wild type (WT). Maximal DeltapH-dependent NPQ (qE) is reduced in the following order: WT>aba1>lut1 approximately lut2>lut2aba1, paralleling reduction in Photosystem (PS) II antenna size. Finally, light-activation of NPQ shows that zeaxanthin and antheraxanthin present constitutively in lut mutants are not qE active, and hence, the same can be inferred of the lutein they replace. Thus, a direct involvement of lutein in the mechanism of qE is unlikely. Rather, altered NPQ in xanthophyll biosynthetic mutants is explained by disturbed macro-organization of LHC II and reduced PS II-antenna size in the absence of the optimal, wild-type xanthophyll composition. These data suggest the evolutionary conservation of lutein content in plants was selected for due to its unique ability to optimize antenna structure, stability and macro-organization for efficient regulation of light-harvesting under natural environmental conditions.

Design, synthesis and properties of synthetic chlorophyll proteins

A chemoselective method is described for coupling chlorophyll derivatives with an aldehyde group to synthetic peptides or proteins modified with an aminoxyacetyl group at the epsilon-amino group of a lysine residue. Three template-assembled antiparallel four-helix bundles were synthesized for the ligation of one or two chlorophylls. This was achieved by coupling unprotected peptides to cysteine residues of a cyclic decapeptide by thioether formation. The amphiphilic helices were designed to form a hydrophobic pocket for the chlorophyll derivatives. Chlorophyll derivatives Zn-methyl-pheophorbide b and Zn-methyl-pyropheophorbide d were used. The aldehyde group of these chlorophyll derivatives was ligated to the modified lysine group to form an oxime bond. The peptide-chlorophyll conjugates were characterized by electrospray mass spectrometry, analytical HPLC, and UV/visible spectroscopy. Two four-helix bundle chlorophyll conjugates were further characterized by size-exclusion chromatography, circular dichroism, and resonance Raman spectroscopy.

Stable insertion of the early light-induced proteins into etioplast membranes requires chlorophyll a

Etiolated plant seedlings exposed to light respond by transient accumulation of the nucleus-encoded, plastid-located early light-inducible proteins (Elips). These proteins are distant relatives of the light-harvesting chlorophyll a/b-binding gene family and bind pigments with unusual characteristics. To investigate whether accumulation of Elips in plastid membranes is post-translationally regulated by pigments, reconstitution studies were performed, where in vitro transcribed and translated low molecular mass Elip precursors of barley were combined with lysed barley etioplasts complemented with various compositions of isolated pigments. We showed that the membrane insertion of Elips, as proven by protease protection assays and washes with a chaotropic salt or alkali, depended strictly on chlorophyll a but not on chlorophyll b or xanthophyll zeaxanthin. The amount of inserted Elips increased almost linearly with the chlorophyll a concentration, and the insertion efficiency was not significantly influenced by a light intensity between 1 and 1,000 micromol x m(-2) x s(-1). In contrast, in vitro import of Elip precursors into greening plastids was enhanced by high intensity light. Thus, we conclude that although chlorophylls bound to Elips seem to not be involved in light harvesting, they are crucial for a stable insertion of these proteins into the plastid membrane.

Carotenoid-to-chlorophyll energy transfer in recombinant major light-harvesting complex (LHCII) of higher plants. I. Femtosecond transient absorption measurements

The energy transfer kinetics from carotenoids to chlorophylls and among chlorophylls has been measured by femtosecond transient absorption kinetics in a monomeric unit of the major light-harvesting complex (LHCII) from higher plants. The samples were reconstituted complexes with different carotenoid contents. The kinetics was measured both in the carotenoid absorption region and in the chlorophyll Q(y) region using two different excitation wavelengths suitable for selective excitation of the carotenoids. Analysis of the data shows that the overwhelming part of the energy transfer from the carotenoids occurs directly from the initially excited S(2) state of the carotenoids. Only a small part (<20%) may possibly take an S(1) pathway. All the S(2) energy transfer from carotenoids to chlorophylls occurs with time constants <100 fs. We have been able to differentiate among the three carotenoids, two luteins and neoxanthin, which have transfer times of approximately 50 and 75 fs for the two luteins, and approximately 90 fs for neoxanthin. About 50% of the energy absorbed by carotenoids is initially transferred directly to chlorophyll b (Chl b), while the rest is transferred to Chl a. Neoxanthin almost exclusively transfers to Chl b. Due to various complex effects discussed in the paper, such as a specific coupling of Chl b and Chl a excited states, the percentage of direct Chl b transfer thus is somewhat lower than estimated by us previously for LHCII from Arabidopsis thaliana. (Connelly, J. P., M. G. Müller, R. Bassi, R. Croce, and A. R. Holzwarth. 1997. Biochemistry. 36:281). We can distinguish three different Chls b receiving energy directly from carotenoids. We propose as a new mechanism that the carotenoid-to-Chl b transfer occurs to a large part via the B(x) state of Chl b and to the Q(x) state, while the transfer to Chl a occurs only via the Q(x) state. We find no compelling evidence in favor of a substantial S(1) transfer path of the carotenoids, although some transfer via the S(1) state of neoxanthin can not be entirely excluded. The S(1) lifetimes of the two luteins were determined to be 15 ps and 3.9 ps. A detailed quantitative analysis and kinetic model of the processes described here will be presented in a separate paper.

Resolution and identification of the protein components of the photosystem II antenna system of higher plants by reversed-phase liquid chromatography with electrospray-mass spectrometric detection

Reversed-phase liquid chromatography (RPLC) was interfaced to mass spectrometry (MS) with an electrospray ion (ESI) source for the separation and accurate molecular mass determination of the individual intrinsic membrane proteins that comprise the photosystem II (PS II) major light-harvesting complex (LHC II) and minor (CP24, CP26 and CP29) antenna system, whose molecular masses range between 22,000 and 29,000. PS II is a supramolecular complex intrinsic of the thylacoid membrane, which plays the important role in photosynthesis of capturing solar energy, and transferring it to photochemical reaction centers where energy conversion occurs. The protein components of the PS II major and minor antenna systems were extracted from spinach thylacoid membranes and separated using a butyl-silica column eluted by an acetonitrile gradient in 0.05% (v/v) aqueous trifluoroacetic acid. On-line electrospray MS allowed accurate molecular mass determination and identification of the protein components of PS II major and minor antenna system. The proposed RPLC-ESI-MS method holds several advantages over sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the conventional technique for studying membrane proteins, including a better protein separation, mass accuracy, speed and efficiency.

Effect of light-harvesting complex II on ion transport across model lipid membranes

The effect of the incorporation of the major light-harvesting complex of photosystem II (LHCII) to planar bilayer lipid membranes (BLMs) formed from soybean asolectin and unilamellar small liposomes formed from egg-yolk phosphatidylcholine on ion transport across the lipid bilayer has been studied. The specific conductivity of the BLM rises from 5.2 +/- 0.8 x 10(-9) up to 510 x 10(-9) O(-1) cm(-2) upon the incorporation of LHCII. The conductivity of the membrane with LHCII depends upon the ionic strength of the bathing solution and is higher by a factor of five when the KCl concentration increases from 0.02 to 0.22 M. Such a strong effect has not been observed in the same system without LHCII. The liposome model is also applied to analyse the effect of LHCII on the bilayer permeability to protons. Unilamellar liposomes with a diameter less than 50 nm have been prepared, containing (trapped inside) Neutral Red, a pigment sensitive to proton concentration. A gradient of protons on the membrane is generated by the acidification of the liposome suspension and spectral changes of Neutral Red are recorded in time, reflecting the penetration of protons into the internal space of liposomes. Two components of proton permeation across liposome membranes are observed: a fast one (proceeding within seconds) and a slow one (operating on the time scale of minutes). The rate of both components of proton transport across LHCII-containing membranes is higher than for liposomes alone. The enhancement effect of LHCII on the ion transport across the lipid membrane is discussed in terms of aggregation of the pigment-protein complexes. The possible physiological importance of such an effect in controlling ion permeability across the thylakoid membrane is discussed.

Relevance of the chlorophyll phytyl chain on lamellar phase formation and organisation

A series of modified chlorophylls (chlorophyll a, pyrochlorophyll a, Zn-pheophytin a and Zn-pheophorbide a) have been inserted into lamellar phases of sodium bis-(2-ethylhexyl)-sulfosuccinate (AOT). The role played by the different functional groups in affecting the bilayer formation and organisation has been investigated by means of the NMR quadrupolar splitting technique. Evidence is reported for the first time on the capacity of the phytyl chain of the chlorophylls to anchor the tetrapyrroles into the bilayer, favouring at the same time the regular formation of the lamellae.

Assignment of the lowest Qy-state and spectral dynamics of the CP29 chlorophyll a/b antenna complex of green plants: a hole-burning study

Low-temperature absorption, fluorescence and persistent non-photochemical hole-burned spectra are reported for the CP29 chlorophyll (Chl) a/b antenna complex of photosystem II of green plants. The absorption-origin band of the lowest Qy-state lies at 678.2 nm and carries a width of approximately 130 cm-1 that is dominated by inhomogeneous broadening at low temperatures. Its absorption intensity is equivalent to that of one of the six Chl a molecules of CP29. The absence of a significant satellite hole structure produced by hole burning, within the absorption band of the lowest state, indicates that the associated Chl a molecule is weakly coupled to the other Chl and, therefore, that the lowest-energy state is highly localized on a single Chl a molecule. The electron-phonon coupling of the 678.2 nm state is weak with a Huang-Rhys factor S of 0.5 and a peak phonon frequency (omega m) of approximately 20 cm-1. These values give a Stokes shift (2S omega m) in good agreement with the measured positions of the absorption band at 678.2 nm and a fluorescence-origin band at 679.1 nm. Zero-phonon holes associated with the lowest state have a width of approximately 0.05 cm-1 at 4.2 K, corresponding to a total effective dephasing time of approximately 400 ps. The temperature dependence of the zero-phonon holewidth indicates that this time constant is dominated at temperatures below 8 K by pure dephasing/spectral diffusion due to coupling of the optical transition to the glass-like two-level systems of the protein. Zero-phonon hole-widths obtained for the Chl b bands at 638.5 and 650.0 nm, at 4.2 K, lead to lower limits of 900 +/- 150 fs and 4.2 +/- 0.3 ps, respectively, for the Chl b-->Chl a energy-transfer times. Downward energy transfer from the Chl a state(s) at 665.0 nm occurs in 5.3 +/- 0.6 ps at 4.2 K.

The N-terminal domain of the light-harvesting chlorophyll a/b-binding protein complex (LHCII) is essential for its acclimative proteolysis

Variations in the amount of the light-harvesting chlorophyll a/b-binding protein complex (LHCII) is essential for regulation of the uptake of light into photosystem II. An endogenous proteolytic system was found to be involved in the degradation of LHCII in response to elevated light intensities and the proteolysis was shown to be under tight regulation [Yang, D.-H. et al. (1998) Plant Physiol. 118, 827-834]. In this study, the substrate specificity and recognition site towards the protease were examined using reconstituted wild-type and mutant recombinant LHCII. The results show that the LHCII apoprotein and the monomeric form of the holoprotein are targeted for proteolysis while the trimeric form is not. The N-terminal domain of LHCII was found to be essential for recognition by the regulatory protease and the involvement of the N-end rule pathway is discussed.

Chlorophyll a self-assembly in polar solvent-water mixtures

The conversion of chlorophyll a (Chl a) monomers into large aggregates in six polar solvents upon addition of water has been studied by means of absorption, fluorescence spectroscopy and fluorescence lifetime measurements for the purpose of elucidating the various environmental factors promoting Chl a self-assembly and determining the type of its organization. Two empirical solvent parameter scales were used for quantitative characterization of the different solvation properties of the solvents and their mixtures with water. The mole fractions of water f1/2 giving rise to the midpoint values of the relative fluorescence quantum yield were determined for each solvent, and then various solvent-water mixture parameters for the f1/2 values were compared. On the basis of their comparison, it is concluded that the hydrogen-bonding ability and the dipole-dipole interactions (function of the dielectric constant) of the solvent-water mixtures are those that promote Chl a self-assembly. The influence of the different nature of the non-aqueous solvents on the Chl aggregation is manifested by both the different water contents required to induce Chl monomer-->aggregate transition and the formation of two types of aggregates at the completion of the transition: species absorbing at 740-760 nm (in methanol, ethanol, acetonitrile, acetone) and at 667-670 nm (in pyridine and tetrahydrofuran). It is concluded that the type of Chl organization depends on the coordination ability and the polarizability (function of the index of refraction) of the organic solvent. The ordering of the solvents with respect to the f1/2 values--methanol < ethanol < acetonitrile < acetone < pyridine < tetrahydrofuran--yielded a typical lyotropic (Hofmeister) series. On the basis of this solvent ordering and the disparate effects of the two groups of solvents on the Chl a aggregate organization, it is pointed out that the mechanism of Chl a self-assembly in aqueous media can be considered a manifestation of the Hofmeister effect, as displayed in the lipid-phase behavior (Koynova et al., Eur. J. Biophys. 25, 261-274, 1997). It relates to the solvent ability to modify the bulk structure and to distribute unevenly between the Chl-water interface and bulk liquid.

The AtCAO gene, encoding chlorophyll a oxygenase, is required for chlorophyll b synthesis in Arabidopsis thaliana

Chlorophyll b is synthesized from chlorophyll a and is found in the light-harvesting complexes of prochlorophytes, green algae, and both nonvascular and vascular plants. We have used conserved motifs from the chlorophyll a oxygenase (CAO) gene from Chlamydomonas reinhardtii to isolate a homologue from Arabidopsis thaliana. This gene, AtCAO, is mutated in both leaky and null chlorina1 alleles, and DNA sequence changes cosegregate with the mutant phenotype. AtCAO mRNA levels are higher in three different mutants that have reduced levels of chlorophyll b, suggesting that plants that do not have sufficient chlorophyll b up-regulate AtCAO gene expression. Additionally, AtCAO mRNA levels decrease in plants that are grown under dim-light conditions. We have also found that the six major Lhcb proteins do not accumulate in the null ch1-3 allele.

Expression of a higher plant light-harvesting chlorophyll a/b-binding protein in Synechocystis sp. PCC 6803

A chimeric lhcb gene, coding for Lhcb, a higher plant chlorophyll a/b-binding light-harvesting complex of photosystem II (LHCII), was constructed using the Synechocystis sp. PCC 6803 psbA3 promoter and a modified lhcb gene from pea. This construct drives synthesis of full-length, mature Lhcb under the control of the strong psbA3 promoter that usually drives expression of the D1 protein of photosystem II. This chimeric gene was transformed into a photosystem I-less/chlL(-) Synechocystis sp. PCC 6803 strain that is unable to synthesize chlorophyll in darkness. In the resulting strain, a high level of lhcb transcript was detected and transcript accumulation was enhanced by addition of exogenous Zn-chlorophyllide b. The chimeric lhcb gene was translated to produce full-length Lhcb as demonstrated by pulse-labeling: a new radioactively labeled band of a size corresponding to full-length Lhcb was visible on autoradiograms. Using Triton X-114 phase fractionation, this labeled protein band was found to partition to the phase containing integral membrane proteins, indicating that the pulse-labeled Lhcb is readily integrated into the membrane. However, Lhcb was rapidly degraded and did not accumulate in thylakoid membranes to levels that were detectable other than by pulse labeling. Upon immunological detection with LHCII antibodies, a small protein (approximately 8 kDa) was found specifically in the lhcb-containing mutant. We interpret this protein to be a degradation product of the full-length Lhcb. This fragment was stabilized by supplementing cells with xanthophylls, which incorporated into thylakoid membranes only in the mutant carrying lhcb. The lutein/chlorophyll ratio of thylakoids of this mutant was about 1 : 10. These results indicate that in this cyanobacterial system Lhcb is synthesized, integrated into the membrane, and then degraded to a approximately 8 kDa fragment that is stabilized by pigment binding and does not require the presence of chlorophyll b.

Quenching of chlorophyll fluorescence by triplets in solubilized light-harvesting complex II (LHCII)

The quenching of chlorophyll fluorescence by triplets in solubilized trimeric light harvesting complexes was analyzed by comparative pump-probe experiments that monitor with weak 2-ns probe pulses the fluorescence yield and changes of optical density, DeltaOD, induced by 2-ns pump pulses. By using a special array for the measurement of the probe fluorescence (Schödel R., F. Hillman, T. Schrötter, K.-D. Irrgang, J. Voight, and G. Biophys. J. 71:3370-3380) the emission caused by the pump pulses could be drastically reduced so that even at highest pump pulse intensities, IP, no significant interference with the signal due to the probe pulse was observed. The data obtained reveal: a) at a fixed time delay of 50 ns between pump and probe pulse the fluorescence yield of the latter drastically decreased with increasing IP, b) the recovery of the fluorescence yield in the microseconds time domain exhibits kinetics which are dependent on IP, c) DeltaOD at 507 nm induced by the pump pulse and monitored by the probe pulse with a delay of 50 ns (reflecting carotenoid triplets) increases with IP without reaching a saturation level at highest IP values, d) an analogous feature is observed for the bleaching at 675 nm but it becomes significant only at very high IP values, e) the relaxation of DeltaOD at 507 nm occurs via a monophasic kinetics at all IP values whereas DeltaOD at 675 nm measured under the same conditions is characterized by a biphasic kinetics with tau values of about 1 microseconds and 7-9 microseconds. The latter corresponds with the monoexponential decay kinetics of DeltaOD at 507 nm. Based on a Stern-Volmer plot, the time-dependent fluorescence quenching is compared with the relaxation kinetics of triplets. It is shown that the fluorescence data can be consistently described by a quenching due to triplets.

Isolation of pigment-binding early light-inducible proteins from pea

The early light-inducible proteins (ELIPs) in chloroplasts possess a high sequence homology with the chlorophyll a/b-binding proteins but differ from those proteins by their substoichiometric and transient appearance. In the present study ELIPs of pea were isolated by a two-step purification strategy: perfusion chromatography in combination with preparative isoelectric focussing. Two heterogeneous populations of ELIPs were obtained after chromatographic separation of solubilized thylakoid membranes using a weak anion exchange column. One of these populations contained ELIPs in a free form providing the first isolation of these proteins. To prove whether the isolated and pure forms of ELIP bind pigments, spectroscopic and chromatographic analysis were performed. Absorption spectra and TLC revealed the presence of chlorophyll a and lutein. Measurements of steady-state fluorescence emission spectra at 77 K exhibited a major peak at 674 nm typical for chlorophyll a bound to the protein matrix. The action spectrum of the fluorescence emission measured at 674 nm showed several peaks originating mainly from chlorophyll a. It is proposed that ELIPs are transient chlorophyll-binding proteins not involved in light-harvesting but functioning as scavengers for chlorophyll molecules during turnover of pigment-binding proteins.

The phospholipid-deficient pho1 mutant of Arabidopsis thaliana is affected in the organization, but not in the light acclimation, of the thylakoid membrane

The pho1 mutant of Arabidopsis has been shown to respond to the phosphate deficiency in the leaves by decreasing the amount of phosphatidylglycerol (PG). PG is thought to be of crucial importance for the organization and function of the thylakoid membrane. This prompted us to ask what the consequences of the PG deficiency may be in the pho1 mutant when grown under low or high light. While in the wild-type, the lipid pattern was almost insensitive to changes in the growth light, PG was reduced to 45% under low light in the mutant, and it decreased further to 35% under high light. Concomitantly, sulfoquinovosyl diacylglycerol (SQDG) and to a lesser extent digalactosyl diacylglycerol (DGDG) increased. The SQDG increase correlated with increased amounts of the SQD1 protein, an indicator for an actively mediated process. Despite of alterations in the ultrastructure, mutant thylakoids showed virtually no effects on photosynthetic electron transfer, O2 evolution and excitation energy allocation to the reaction centers. Our results support the idea that PG deficiency can at least partially be compensated for by the anionic lipid SQDG and the not charged lipid DGDG. This seems to be an important strategy to maintain an optimal thylakoid lipid milieu for vital processes, such as photosynthesis, under a restricted phosphate availability.