Chlorophyll-protein Complexes Research Articles

Stark fluorescence spectroscopy reveals two emitting sites in the dissipative state of FCP antennas

Diatoms are characterized by very efficient photoprotective mechanisms where the excess energy is dissipated as heat in the main antenna system constituted by fucoxanthin–chlorophyll (Chl) protein complexes (FCPs). We performed Stark fluorescence spectroscopy on FCPs in their light-harvesting and energy dissipating states. Our results show that two distinct emitting bands are created upon induction of energy dissipation in FCPa and possibly in FCPb. More specifically one band is characterized by broad red shifted emission above 700nm and bears strong similarity with a red shifted band that we detected in the dissipative state of the major light-harvesting complex II (LHCII) of plants [26]. We discuss the results in the light of different mechanisms proposed to be responsible for photosynthetic photoprotection.

Comparative transcriptome analysis of energy-rich Arabidopsis thaliana under dark and light conditions

Overexpression of Arabidopsis thaliana purple acid phosphatases AtPAP2 in Arabidopsis can promote plant growth. The overexpression (OE) lines flower earlier, grow faster, and contain more ATP, sucrose and glucose. The seed yields and silique numbers of OE lines are also more than the control lines (Sun et al., 2012a). In this study, we compared the leaf transcriptomes of 20-d-old transgenic and wild-type Arabidopsis grown under long day (16h/8h) condition. Total RNAs were collected at three time points: end of night (t=0 hr), one hour after light was turned on (t=1 hr), and eight hours after light was turned on (t=8 hr). AtPAP2 is dually targeted to chloroplasts and mitochondria. To study the RNA encoded by chloroplast and mitochondrial genomes, ribosomal RNAs were removed before sequencing. Approximate 65 million clean reads (~6Gbp) were obtained by Illumina HiSeqTM 2000 sequencing from each library. In total, after assembly 29,435 transcripts are detected from the six libraries. More genes are suppressed in OE leaves (vs. WT) at all the three time points, 1,623 (downregulated) versus 945 (up-regulated), 1,908 (down-regulated) versus 712 (up-regulated) and 1,642 (down-regulated) versus 824 (up-regulated) at t=0, 1 and 8 hours, respectively. Expression profiles of light-induced transcripts based on K-means clustering were analyzed. There are significant changes in the transcription levels of various components of photosystems, light-harvesting chlorophyll protein complexes (LHC), respiratory complexes, RNA polymerases and ribosomes. Our data provide systemic portraits of global changes in Arabidopsis transcriptome exerted by dark to light transition (external energy input) and high internal energy status. Comparative transcriptome analysis of energy-rich Arabidopsis thaliana under dark and light conditions Chao LIANG and Boon Leong LIM* vechao@hku.hk School of Biological Sciences, the University of Hong Kong, Hong Kong, China Fig. 1 Growth phenotypes of four-wk-old Arabidopsis under long day (16hr light/8hr dark)) conditions. WT, wild type; T-DNA, AtPAP2 mutant line; OE, AtPAP2 over-expression lines (Sun et al., 2012b). Arabidopsis thaliana ecotypes Columbia (Col-0) (wild type: WT), AtPAP2 overexpressors (OE) in Col-0 background were used in this study (Sun et al. 2012b) . Arabidopsis seeds of WT and OE7 lines were surface sterilized and plated on Murashige and Skoog medium supplemented with 2% (w/v) sucrose for 10 days before the seedling were transferred to soil under 16 hours light (22°C) / 8 hours dark (18°C) period in a growth chamber at a light intensity of 120–150 μmol m-2 s-1. Leaves of 20-day-old plants without bolting were immediately frozen in liquid nitrogen for RNA extraction. Leaves were harvested at three different time points: t=0 hr (end of night), t=1 hr (one hour after light turn on) and t=8 hr (eight hours after light turn on) respectively. Flowchart Total RNA Removal of Ribosome RNA Raw Data Allignment with reference gene and genome file (SOAP aligner/SOAP2) Clean Reads QC Expression level(RPKM) Cluster analysisc Gene ontology analysisc KEGG pathway analysis Background Methodology

Transcriptome profiling of peanut gynophores revealed global reprogramming of gene expression during early pod development in darkness

BackgroundAfter the zygote divides few times, the development of peanut pre-globular embryo and fruit is arrested under white or red light. Embryo development could be resumed in dark condition after gynophore is buried in soil. It is interesting to study the mechanisms of gynophore development and pod formation in peanut.ResultsIn this study, transcriptome analysis of peanut gynophore was performed using Illumina HiSeq™ 2000 to understand the mechanisms of geocarpy. More than 13 million short sequences were assembled into 72527 unigenes with average size of 394 bp. A large number of genes that were not identified previously in peanut EST projects were identified in this study, including most genes involved in plant circadian rhythm, intra-cellular transportation, plant spliceosome, eukaryotes basal transcription factors, genes encoding ribosomal proteins, brassinosteriod biosynthesis, light-harvesting chlorophyll protein complex, phenylpropanoid biosynthesis and TCA cycle. RNA-seq based gene expression profiling results showed that before and after gynophore soil penetration, the transcriptional level of a large number of genes changed significantly. Genes encoding key enzymes for hormone metabolism, signaling, photosynthesis, light signaling, cell division and growth, carbon and nitrogen metabolism as well as genes involved in stress responses were high lighted.ConclusionsTranscriptome analysis of peanut gynophore generated a large number of unigenes which provide useful information for gene cloning and expression study. Digital gene expression study suggested that gynophores experience global changes and reprogram from light to dark grown condition to resume embryo and fruit development.

Evolutionary Changes in Chlorophyllide a Oxygenase (CAO) Structure Contribute to the Acquisition of a New Light-harvesting Complex in Micromonas*♦

Chlorophyll b is found in photosynthetic prokaryotes and primary and secondary endosymbionts, although their light-harvesting systems are quite different. Chlorophyll b is synthesized from chlorophyll a by chlorophyllide a oxygenase (CAO), which is a Rieske-mononuclear iron oxygenase. Comparison of the amino acid sequences of CAO among photosynthetic organisms elucidated changes in the domain structures of CAO during evolution. However, the evolutionary relationship between the light-harvesting system and the domain structure of CAO remains unclear. To elucidate this relationship, we investigated the CAO structure and the pigment composition of chlorophyll-protein complexes in the prasinophyte Micromonas. The Micromonas CAO is composed of two genes, MpCAO1 and MpCAO2, that possess Rieske and mononuclear iron-binding motifs, respectively. Only when both genes were introduced into the chlorophyll b-less Arabidopsis mutant (ch1-1) was chlorophyll b accumulated, indicating that cooperation between the two subunits is required to synthesize chlorophyll b. Although Micromonas has a characteristic light-harvesting system in which chlorophyll b is incorporated into the core antennas of reaction centers, chlorophyll b was also incorporated into the core antennas of reaction centers of the Arabidopsis transformants that contained the two Micromonas CAO proteins. Based on these results, we discuss the evolutionary relationship between the structures of CAO and light-harvesting systems.

Organization and functionality of chlorophyll–protein complexes in thylakoid membranes isolated from Pb-treated Secale cereale

In this study, Secale cereale seedlings cultivated under 0 (control), 2 or 5mM Pb(NO3)2 concentrations were used to examine alterations in the organization and functionality of chlorophyll–protein complexes in thylakoid membranes under Pb ion stress. The studies were conducted on whole leaves of rye seedlings or thylakoid membranes isolated from Pb-treated and control plants. Using non-denaturing electrophoresis, it was assessed that increasing Pb concentrations resulted in an increase in the value of the ratio of the content of LHCII oligomers (mainly trimers) to the content of LHCII monomers. The parameters of chlorophyll fluorescence induction (qp and qn) indicated that the change in the LHCII supramolecular organization in the presence of Pb ions was connected with an increase in non-photochemical fluorescence quenching. Quantification of photosynthetic pigments showed that both Pb concentrations decreased the content of chlorophyll a, chlorophyll b, and carotenoids. The changes in the pigment content led to a significant reduction in light absorption by antenna complexes. However, the absorption spectra showed that red light was preferentially absorbed by antenna complexes in thylakoid membranes isolated from the Pb-treated plants. Examination of fluorescence emission spectra revealed that Pb ions decreased the fluorescence quantum yield of PSII. The emission spectra of thylakoids indicated a relative increase in the intensity of fluorescence emission from the trimeric and aggregated forms of the LHCII complexes in comparison to the intensity of fluorescence emission from PSI antenna complexes under excitation at 440nm. Simultaneously, under excitation at 470nm, we observed a rise in fluorescence intensity from the LHCII trimer after addition of 5mM Pb as well as a decrease in fluorescence intensity from the LHCII aggregates and PSI core and LHCI antenna complexes under both Pb concentrations. Pb treatments also reduced excitation energy absorbed by chlorophyll b and carotenoids within antenna complexes and transferred to chlorophyll a species emitting at 680nm.

THE EFFECT OF INTERMITTENT LIGHT ON CHLOROPLAST DEVELOPMENT IN A GREENING PIGMENT MUTANT OF THE GREEN ALGA SCENEDESMUS OBLIQUUS

ABSTRACT Pigment mutant C-2A' from Scenedesmus obliquus which greens only in the light was submitted to intermittent light conditions (2 min white light (60 W/m2)/60 min darkness). Under these conditions only 10% of the total chlorophyll was formed compared to cells under continuous light, whereas chlorophyll b remained at the initial trace level. All chlorophyll-protein complexes of the light-harvesting complex were missing, while those related to PSII reaction centers increased. The capacities of PSI and PSII per chlorophyll were higher in cells partially greened under intermittent light than in those under continuous light. Transformation of cells from intermittent to continuous light caused a drastic increase in chlorophylls a and b above the normal level, establishing a chlorophyll a/b ratio and the light-harvesting complex pattern found in wild-type cells.

MgTPPS/block copolymers complexes for enhanced stability and photoactivity

Inspired by the chlorophyll–protein complex in photosynthesis, two types of micelles were formed by the noncovalent complexes between magnesium porphyrins and block copolymers. A series of electrostatic micelles were prepared from magnesium tetrakis(4-sulfonatophenyl) porphyrin (MgTPPS) and poly(ethylene glycol)-block-poly(L-lysine). Meanwhile, the coordination micelles were fabricated from MgTPPS and poly(ethylene glycol)-block-poly(4-vinylpyridine). Here, we aim to analyze the origin of the enhanced stability of MgTPPS, particularly focusing on the photoactivity of MgTPPS in the micelles. The electrostatic micelles had better hydrolytic stability due to the electrostatic repulsion interaction and the coordination micelles showed high photostability, benefiting from axial coordination. The electrostatic micelles can promote the generation of singlet oxygen due to their swollen micellar cores. While the coordination micelles displayed better electron transfer ability owing to the lower HOMO of MgTPPS induced by axial coordination. These results may provide a novel approach to understand the amazing properties of the natural chlorophyll–protein complex and also have important implications, such as, in artificial photosynthetic reactions and photodynamic therapy.

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.

Photosystem II Function and Dynamics in Three Widely Used Arabidopsis thaliana Accessions

Columbia-0 (Col-0), Wassilewskija-4 (Ws-4), and Landsberg erecta-0 (Ler-0) are used as background lines for many public Arabidopsis mutant collections, and for investigation in laboratory conditions of plant processes, including photosynthesis and response to high-intensity light (HL). The photosystem II (PSII) complex is sensitive to HL and requires repair to sustain its function. PSII repair is a multistep process controlled by numerous factors, including protein phosphorylation and thylakoid membrane stacking. Here we have characterized the function and dynamics of PSII complex under growth-light and HL conditions. Ws-4 displayed 30% more thylakoid lipids per chlorophyll and 40% less chlorophyll per carotenoid than Col-0 and Ler-0. There were no large differences in thylakoid stacking, photoprotection and relative levels of photosynthetic complexes among the three accessions. An increased efficiency of PSII closure was found in Ws-4 following illumination with saturation flashes or continuous light. Phosphorylation of the PSII D1/D2 proteins was reduced by 50% in Ws-4 as compared to Col-0 and Ler-0. An increase in abundance of the responsible STN8 kinase in response to HL treatment was found in all three accessions, but Ws-4 displayed 50% lower levels than Col-0 and Ler-0. Despite this, the HL treatment caused in Ws-4 the lagest extent of PSII inactivation, disassembly, D1 protein degradation, and the largest decrease in the size of stacked thylakoids. The dilution of chlorophyll-protein complexes with additional lipids and carotenoids in Ws-4 may represent a mechanism to facilitate lateral protein traffic in the membrane, thus compensating for the lack of a full complement of STN8 kinase. Nevertheless, additional PSII damage occurs in Ws-4, which exceeds the D1 protein synthesis capacity, thus leading to enhanced photoinhibition. Our findings are valuable for selection of appropriate background line for PSII characterization in Arabidopsis mutants, and also provide the first insights into natural variation of PSII protein phosphorylation.

Spectral Hole Burning, Recovery, and Thermocycling in Chlorophyll–Protein Complexes: Distributions of Barriers on the Protein Energy Landscape

Chlorophyll-protein complexes are ideal model systems for protein energy landscape research. Here pigments, used in optical spectroscopy experiments as sensitive probes to local dynamics, are built into protein by Nature (in a large variety of local environments; without extraneous chemical manipulations or genetic engineering). Distributions of the tunneling parameter, λ, and/or protein energy landscape barrier heights, V, have been determined for (the lowest energy state of) the CP43 core antenna complex of photosystem II. We demonstrate that spectral hole burning (SHB) and hole recovery (HR) measurements are capable of delivering important information on protein energy landscape properties and spectral diffusion mechanism details. In particular, we show that tunneling rather than barrier hopping is responsible for both persistent SHB and subsequent HR at 5-12 K, which allows us to estimate the md(2) parameter of the tunneling entities as ~1.0 × 10(-46) kg·m(2). The subdistributions of λ actually contributing to the nonsaturated spectral holes (and affecting their recovery) differ from the respective full true distributions. In the case of the full λ-distribution being uniform (or the barrier height distribution ~1/√V, a model which has been widely employed in theories of amorphous solids at low temperatures and in HR analysis), the difference is qualitative, with λ subdistributions probed in the HR experiments being highly asymmetrical, and barrier V subdistributions deviating significantly from ~1/√V. Thus, the distribution of λ for the protein energy landscape tier directly probed by SHB is likely Gaussian and not uniform. Additionally, a Gaussian distribution of barriers, with parameters incompatible with those of the landscape tier directly probed by SHB, contributes to the thermocycling results.

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

BackgroundThe 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.ResultsOur 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.ConclusionsBased 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.

A NUP98-HOXD13 leukemic fusion gene leads to impaired class switch recombination and antibody production

Myelodysplastic syndrome is a clonal process characterized by ineffective hematopoiesis and progression to acute leukemia. Although many myelodysplastic syndrome and leukemic patients have compromised immunity, the role of underlying mutations in regulating immune function is poorly understood. Recent studies show that NUP98-HOXD13 (NHD13) fusion gene results in myelodysplastic syndrome and impairs lymphocyte differentiation in transgenic mice. In our studies, we sought to elucidate the mechanism by which NHD13 affects B-lymphocyte development and function. Based on our preliminary findings that transgenic mice had increased levels of IgM and reduced IgG1 and IgE, we hypothesized that the fusion gene might impair class switch recombination (CSR). Mice were immunologically challenged with dinitrophenol. NHD13 mice showed a marked reduction in B-lymphocyte differentiation in their bone marrow and spleen following dinitrophenol stimulation and had reduced production of dinitrophenol-specific antibodies. Spleen follicles from these mice were small and hypocellular, indicating failure of clonal expansion. When isolated NHD13 B lymphocytes were stimulated in vitro using Escherichia coli lipopolysaccharide or lipopolysaccharide + interleukin-4, they failed to undergo sufficient CSR and proliferation. Taken together, our findings show that expression of NUP98-HOXD13 impairs CSR and reduces the antibody-mediated immune response, in addition to its role in leukemia. Further delineation of the NUP98-HOXD13 transgene may reveal novel pathways involved in CSR.

Water-soluble chlorophyll protein (WSCP) of Arabidopsis is expressed in the gynoecium and developing silique

Water-soluble chlorophyll protein (WSCP) has been found in many Brassicaceae, most often in leaves. In many cases, its expression is stress-induced, therefore, it is thought to be involved in some stress response. In this work, recombinant WSCP from Arabidopsis thaliana (AtWSCP) is found to form chlorophyll-protein complexes in vitro that share many properties with recombinant or native WSCP from Brassica oleracea, BoWSCP, including an unusual heat resistance up to 100°C in aqueous solution. A polyclonal antibody raised against the recombinant apoprotein is used to identify plant tissues expressing AtWSCP. The only plant organs containing significant amounts of AtWSCP are the gynoecium in open flowers and the septum of developing siliques, specifically the transmission tract. In fully grown but still green siliques, the protein has almost disappeared. Possible implications for AtWSCP functions are discussed.

Isolation and Characterization of Homodimeric Type-I Reaction Center Complex from Candidatus Chloracidobacterium thermophilum, an Aerobic Chlorophototroph

The recently discovered thermophilic acidobacterium Candidatus Chloracidobacterium thermophilum is the first aerobic chlorophototroph that has a type-I, homodimeric reaction center (RC). This organism and its type-I RCs were initially detected by the occurrence of pscA gene sequences, which encode the core subunit of the RC complex, in metagenomic sequence data derived from hot spring microbial mats. Here, we report the isolation and initial biochemical characterization of the type-I RC from Ca. C. thermophilum. After removal of chlorosomes, crude membranes were solubilized with 0.1% (w/v) n-dodecyl β-D-maltoside, and the RC complex was purified by ion-exchange chromatography. The RC complex comprised only two polypeptides: the reaction center core protein PscA and a 22-kDa carotenoid-binding protein denoted CbpC. The absorption spectrum showed a large, broad absorbance band centered at ∼483 nm from carotenoids as well as smaller Q(y) absorption bands at 672 and 812 nm from chlorophyll a and bacteriochlorophyll a, respectively. The light-induced difference spectra of whole cells, membranes, and the isolated RC showed maximal bleaching at 840 nm, which is attributed to the special pair and which we denote as P840. Making it unique among homodimeric type-I RCs, the isolated RC was photoactive in the presence of oxygen. Analyses by optical spectroscopy, chromatography, and mass spectrometry revealed that the RC complex contained 10.3 bacteriochlorophyll a(P), 6.4 chlorophyll a(PD), and 1.6 Zn-bacteriochlorophyll a(P)' molecules per P840 (12.8:8.0:2.0). The possible functions of the Zn-bacteriochlorophyll a(P)' molecules and the carotenoid-binding protein are discussed.

Effects of the Distributions of Energy or Charge Transfer Rates on Spectral Hole Burning in Pigment–Protein Complexes at Low Temperatures

Effects of the distributions of excitation energy transfer (EET) rates (homogeneous line widths) on the nonphotochemical (resonant) spectral hole burning (SHB) processes in photosynthetic chlorophyll-protein complexes (reaction center [RC] and CP43 antenna of Photosystem II from spinach) are considered. It is demonstrated that inclusion of such a distribution results in somewhat more dispersive hole burning kinetics. More importantly, however, inclusion of the EET rate distributions strongly affects the dependence of the hole width on the fractional hole depth. Different types of line width distributions have been explored, including those resulting from Förster type EET between weakly interacting pigments as well as Gaussian ones, which may be a reasonable approximation for those resulting, for instance, from so-called extended Förster models. For Gaussian line width distributions, it is possible to determine the parameters of both line width and tunneling parameter distributions from SHB data without a priori knowledge of any of them. Concerning more realistic asymmetric distributions, we demonstrate, using the simple example of CP43 antenna, that one can use SHB modeling to estimate electrostatic couplings between pigments and support or exclude assignment of certain pigment(s) to a particular state.

Conservation of Spin Polarization during Triplet–Triplet Energy Transfer in Reconstituted Peridinin–Chlorophyll–Protein Complexes

Peridinin-chlorophyll-protein (PCP) complexes, where the N-terminal domain of native PCP from Amphidinium carterae has been reconstituted with different chlorophyll (Chl) species, have been investigated by time-resolved EPR in order to elucidate the details of the triplet-triplet energy transfer (TTET) mechanism. This spectroscopic approach exploits the concept of spin conservation during TTET, which leads to recognizable spin-polarization effects in the observed time-resolved EPR spectra. The spin polarization produced at the acceptor site (peridinin) depends on the initial polarization of the donor (chlorophyll) and on the relative geometric arrangement of the donor-acceptor spin axes. A variation of the donor triplet state properties in terms of population probabilities or triplet spin axis directions, as produced by replacement of chlorophyll a (Chl a) with non-native chlorophyll species (ZnChl a and BacterioChl a) in the reconstituted complexes, is unambiguously reflected in the polarization pattern of the carotenoid triplet state. For the first time, in the present investigation spin-polarization conservation has been shown to occur among natural cofactors in protein complexes during the TTET process. Proving the validity of the assumption of spin conservation adopted in the EPR spectral analysis, the results reinforce the hypothesis that in PCP proteins peridinin 614, according to X-ray nomenclature (Hofmann, E.; et al. Science 1996, 272, 1788-1791), is the carotenoid of election in the photoprotection mechanism based on TTET.

Photoinhibition: molecular mechanisms and physiological significance

Photoinhibition: molecular mechanisms and physiological significance

Impact of moderate Fe excess under Cd stress on the photosynthetic performance of poplar (Populus jacquemontiana var. glauca cv. Kopeczkii)

Cadmium interference with Fe nutrition has a strong impact on the development and efficiency of the photosynthetic apparatus. To shed more light on the interaction between Fe and Cd, it was studied how iron given in moderate excess under Cd stress affects the development and functioning of chlorophyll-protein complexes. Poplar plants grown in hydroponics up to four-leaf stage were treated with 10 μM Cd(NO₃)₂ in the presence of 50 μM Fe([III])-citrate as iron supply (5xFe + Cad) for two weeks. Though leaf area growth was inhibited similarly to that of Cad (10 μM Cd(NO₃)₂ + 10 μM Fe([III])-citrate) plants, chlorophyll content, ¹⁴CO₂ fixation and quenching parameters calculated from PAM fluorescence induction measurements were control-like in 5xFe+Cad leaves. Increased chloroplast iron content (measured photometrically by the bathophenanthroline disulfonate method) without changes in the iron and cadmium content of leaves (determined by inductively coupled plasma mass spectrometry) pointed out that a key factor in the observed protection of photosynthesis is the iron-excess-induced redistribution of iron in the leaf. However, the chlorophyll a/b ratio and the chlorophyll-protein pattern obtained by Deriphat PAGE remained similar to that of Cad leaves. The decreased amount of PSII core and PSI in mature and developing leaves, respectively, refers to developmental stage-dependent remodelling of thylakoids in the presence of Cd. The results underline not only the beneficial effect of iron excess under Cd stress, but also refer to the importance of a proper Fe/Cd ratio and light environment to avoid its possible harmful effects.

Light stress photodynamics of chlorophyll-binding proteins in Arabidopsis thaliana thylakoid membranes revealed by high-resolution mass spectrometric studies

In higher plants the light energy is captured by the photosynthetic pigments that are bound to photosystem I and II and their light-harvesting complex (LHC) subunits. In this study, we examined the photodynamic changes within chlorophyll-protein complexes in the thylakoid membrane of Arabidopsis thaliana leaves adapted to low light and subsequently exposed to light stress. Chlorophyll-protein complexes were isolated using sucrose density gradient centrifugation and blue-native polyacrylamid gel electrophoresis (BN-PAGE). Proteome analysis was performed using SDS-PAGE, HPLC and high resolution mass spectrometry. We identified several rarely expressed and stress-induced chlorophyll-binding proteins, showed changes in localization of early light-induced protein family and LHC protein family members between different photosynthetic complexes and assembled/disassembled subcomplexes under light stress conditions and discuss their role in a variety of light stress-related processes.

Molecular operation of metals into the function and state of photosystem II

Action sites of different metals in the electron transport reactions of Photosystem II (PS II) evaluated by delayed fluorescence in the ms range (ms DF) and pigment-pigment, pigment-protein and protein-protein interaction states by electrophoretic measurements are presented. The main targets for the metals action were shown to be:(i) Cd(2+), Ni(2+), Co(2+)-Y(z) or CaMn(4)-cluster on the donor site with dependence on pH;(ii) Ni(2+), Co(2+), Zn(2+), Al(3+), Mn(2+) between Q(A) and Q(B) on the acceptor site; effect of Al(3+) and Mn(2+) is observed only in acidic pH. Investigated metals bring about monomerization of oligomeric and dimeric chlorophyll-protein complexes (CPC) and destabilization of protein-protein interactions. Molecular mechanisms of metals interference with the structure of PS II are discussed.