Regulation of LHCII aggregation by different thylakoid membrane lipids

In the present study the influence of the lipid environment on the organization of the main light-harvesting complex of photosystem II (LHCII) was investigated by 77K fluorescence spectroscopy. Measurements were carried out with a lipid-depleted and highly aggregated LHCII which was supplemented with the different thylakoid membrane lipids. The results show that the thylakoid lipids are able to modulate the spectroscopic properties of the LHCII aggregates and that the extent of the lipid effect depends on both the lipid species and the lipid concentration. Addition of the neutral galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) seems to induce a modification of the disorganized structures of the lipid-depleted LHCII and to support the aggregated state of the complex. In contrast, we found that the anionic lipids sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG) exert a strong disaggregating effect on the isolated LHCII. LHCII disaggregation was partly suppressed under a high proton concentration and in the presence of cations. The strongest suppression was visible at the lowest pH value (pH 5) and the highest Mg(2+) concentration (40 mM) used in the present study. This suggests that the negative charge of the anionic lipids in conjunction with negatively charged domains of the LHCII proteins is responsible for the disaggregation. Additional measurements by photon correlation spectroscopy and sucrose gradient centrifugation, which were used to gain information about the size and molecular mass of the LHCII aggregates, confirmed the results of the fluorescence spectroscopy. LHCII treated with MGDG and DGDG formed an increased number of aggregates with large particle sizes in the micromm-range, whereas the incubation with anionic lipids led to much smaller LHCII particles (around 40 nm in the case of PG) with a homogeneous distribution.

Xanthophyll cycle--a mechanism protecting plants against oxidative stress

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

Ferredoxin:NADP+ oxidoreductase as a target of Cd2+ inhibitory action--biochemical studies

The ferredoxin:NADP+ oxidoreductase (FNR) catalyses the ferredoxin-dependent reduction of NADP+ to NADPH in linear photosynthetic electron transport. The enzyme also transfers electrons from reduced ferredoxin (Fd) or NADPH to the cytochrome b(6)f complex in cyclic electron transport. In vitro, the enzyme catalyses the NADPH-dependent reduction of various substrates, including ferredoxin, the analogue of its redox centre - ferricyanide, and the analogue of quinones, which is dibromothymoquinone. This paper presents results on the cadmium-induced inhibition of FNR. The K(i) value calculated for research condition was 1.72 mM. FNR molecule can bind a large number of cadmium ions, as shown by the application of cadmium-selective electrode, but just one ion remains bound after dialysis. The effect of cadmium binding is significant disturbance in the electron transfer process from flavin adenine dinucleotide (FAD) to dibromothymoqinone, but less interference with the reduction of ferricyanide. However, it caused a strong inhibition of Fd reduction, indicating that Cd-induced changes in the FNR structure disrupt Fd binding. Additionally, the protonation of the thiol groups is shown to be of great importance in the inhibition process. A mechanism for cadmium-caused inhibition is proposed and discussed with respect to the in vitro and in vivo situation.

Responses of Lemna trisulca L. (Duckweed) exposed to low doses of cadmium: thiols, metal binding complexes, and photosynthetic pigments as sensitive biomarkers of ecotoxicity

Lemna species are reported to accumulate a variety of metals from contaminated/polluted sites. Cadmium is a nonessential element for plant metabolism. In this work, we aimed to investigate physiological responses to low doses of cadmium (up to 100 microM). From exposure to the lowest Cd concentration (1 microM) to the highest (100 microM), photosynthetic pigments (Chl a, b, carotenoids) and the ratios of Chl a/b, Chl (a + b)/carotenoids decreased as a function of the Cd dose. The content of soluble proteins decreased in a dose-dependent manner, while total soluble thiols drastically increased. In Cd-treated fronds, the dose-dependent accumulation of a polypeptide with an apparent molecular weight of 24 kDa, as well as the appearance of two smaller polypeptides with molecular weights <6.5 kDa, was observed in sodium dodecyl sulfate polyacrylamide gel electrophoresis. Our results show that in Lemna trisulca, different adaptative mechanisms may be involved in counterbalancing low and high doses of a particular toxicant (cadmium). This feature makes this plant potentially useful material in biomonitoring and phytotoxicity testing.

Low-light-induced violaxanthin de-epoxidation in shortly preheated leaves: uncoupling from Delta pH-dependent nonphotochemical quenching

Plants protect themselves against excessive light by the induction of Delta pH-dependent nonphotochemical quenching (qE) that is associated with de-epoxidation of violaxanthin (V) to zeaxanthin (Z) in thylakoid membranes. In this work, we report that low light (12 micromol photons m(-2) s(-1)) is sufficient for a marked stimulation of the V to Z conversion in shortly preheated wheat leaves (5 min, 40 degrees C), but without a substantial increase in qE. Re-irradiation of these leaves with high light led to a rapid induction of nonphotochemical quenching, implying a potential photoprotective role of low-light-induced Z in preheated leaves. On the contrary to low light conditions, preheated leaves exposed to high light behaved similar to nonheated leaves with respect to the V to Z conversion and qE induction. The obtained results indicate that low-light-induced lumen acidification in preheated leaves is high enough to activate V de-epoxidation, but not sufficiently high to induce the formation of quenching centers.

[Light-induced reduction of protochlorophyllide in angiosperms and chloroplast development]

One of the final reactions of chlorophyll (Chl) biosynthesis, e.g: photoreduction of protochlorophyllide (Pchlid) to chlorophyllide (Chlid) is a light-induced process in Angiosperm plants and it is catalyzed by light-dependent NADPH-Pchlid oxidoreductase (1.3.1.33; LPOR). In darkness, Chl biosynthesis is stopped at the stage of Pchlid formation. Seedlings and plastids develop according to a different pattern than that observed in the light. Moreover, synthesis of some proteins of the photosynthetic apparatus is inhibited. Light triggers the Pchlid photoreduction to Chlid, which induces the cascade of biochemical reactions and structural changes leading to the assembly of thylakoid membranes. In the present paper, the current knowledge on LPOR protein, mechanism of Pchlid to Chlid photoreduction, the role of lipid structure in etioplasts as well as spectral properties of Pchlid in etiolated seedlings and model systems is summarized.

[Reactive oxygen species in plants--production, deactivation and role in signal transduction]

Reactive oxygen species (RFT) are continuously produced in plants as byproducts of many metabolic reactions, connectedwith photosynthesis, photorespiration and cellular respiration. RFT species are highly toxic and rapidly detoxified by various cellular enzymatic and nonenzymatic mechanisms. Oxidative stress occurs when there is a serious imbalance between the production of RFT and antioxidative defense. RFT cause cellular damage by oxidizing biocompounds such as: membrane lipids, proteins and DNA. RFT participate in signal transduction by initialization a chain reaction in response to stress conditions or influence gene expression. An overview of the latest literature is presented in terms of RFT production, antioxidant free radical scavenging and redox signaling in plant cells.

The main thylakoid membrane lipid monogalactosyldiacylglycerol (MGDG) promotes the de-epoxidation of violaxanthin associated with the light-harvesting complex of photosystem II (LHCII)

In higher plants, the major part of the xanthophyll cycle pigment violaxanthin (Vx) is non-covalently bound to the main light-harvesting complex of PSII (LHCII). Under saturating light conditions Vx has to be released from its binding site into the surrounding lipid phase, where it is converted to zeaxanthin (Zx) by the enzyme Vx de-epoxidase (VDE). In the present study we investigated the influence of thylakoid lipids on the de-epoxidation of Vx, which was still associated with the LHCII. We isolated LHCII with different concentrations of native, endogenous lipids and Vx by sucrose gradient centrifugation or successive cation precipitation. Analysis of the different LHCII preparations showed that the concentration of LHCII-associated Vx was correlated with the concentration of the main thylakoid lipid monogalactosyldiacylglycerol (MGDG) associated with the complexes. Decreases in the MGDG content of the LHCII led to a diminished Vx concentration, indicating that a part of the total Vx pool was located in an MGDG phase surrounding the LHCII, whereas another part was bound to the LHCII apoproteins. We further studied the convertibility of LHCII-associated Vx in in-vitro enzyme assays by addition of isolated VDE. We observed an efficient and almost complete Vx conversion in the LHCII fractions containing high amounts of endogenous MGDG. LHCII preparations with low concentrations of MGDG exhibited a strongly reduced Vx de-epoxidation, which could be increased by addition of exogenous, pure MGDG. The de-epoxidation of LHCII-associated Vx was saturated at a much lower concentration of native, endogenous MGDG compared with the concentration of isolated, exogenous MGDG, which is needed for optimal VDE activity in in-vitro assays employing pure isolated Vx.

Identification and characterization of Cd-induced peptides in Egeria densa (water weed): Putative role in Cd detoxification

Egeria densa has ability to grow in heavy metal contaminated and polluted bodies of water. Shoots exposed to Cd at concentrations up to 300microM for 7 days showed a pronounced decrease in chlorophyll a and in total protein concentration. Thiol-containing compounds and low-molecular-weight polypeptides were detected in Cd-treated plant extracts by gel filtration chromatography. Two Cd-binding fractions, a thiol-enriched fraction and a non-thiol fraction with a lower molecular weight were identified in extracts by gel filtration. The main fraction of thiol-containing polypeptide, purified by gel filtration and anion-exchange chromatography had a molecular weight of approximately 10kDa. This peptide was characterized by a broad absorption band specific to mercaptide bonds and Cd-sensitive fluorescence emission of aromatic amino acid residues. Our results indicate that cadmium exposure of plants resulted in both a formation of thiol-enriched cadmium complexing peptides and a synthesis of low-molecular-weight metal chelators. The putative role of these compounds in Cd detoxification is discussed.

Ecophysiological tolerance of Elodea canadensis to nickel exposure

Biological accumulation of nickel and concomitant ecophysiological responses were studied in the leaves of Elodea canadensis treated with different concentrations of Ni (1-50 microM) for 5d. In low concentrations nickel was accumulated mainly in the soluble protein fraction, which correlated with its highest observed accumulation coefficient. In higher concentrations, Ni binding in the non-protein soluble fraction was observed. The effects of increasing nickel concentrations on the accumulation of photosynthetic pigments, gas exchange rates, lipid peroxidation, biosynthesis of thiol-containing compounds and the activity of selected enzymes--markers of oxidative stress were investigated. The appearance of several new polypeptides with apparent molecular weights below 20 kDa, was found by SDS-PAGE in Ni-treated Elodea leaves. Our results indicate that Ni, in concentrations up to 10 microM could induce sub-lethal oxidative stress in Elodea leaves. In response, plants developed detoxification mechanisms including an enhanced biosynthesis of thiol-containing compounds which facilitated Ni accumulation and sequestration in plant tissues effectively. Hence, E. canadensis could be used in the biological removal of Ni from polluted water up to 10 microM concentration.

Developmental stages of delayed-greening leaves inferred from measurements of chlorophyll content and leaf growth

Chlorophyll (Chl) accumulation and leaf growth were analysed in delayed-greening leaves of Theobroma cacao (L.) to examine whether these parameters are correlated during leaf development and can be used as non-destructive indicators of leaf developmental stages. There was a clear correlation between Chl content and leaf relative growth rate (RGR) and between Chl content and percentage of full leaf expansion (%FLE) under different growth conditions. Five distinct developmental phases were defined according to the correlation between these parameters and corroborated by data from the analyses of leaf growth (epidermal cell size and specific leaf area) or photosynthetic properties (maximal PSII efficiency, CO₂ assimilation and non-structural carbohydrate contents). The five phases were characterised by rapid leaf expansion by cell division (I), pronounced cell expansion (II), development of photosynthetic capacity concomitant with reinforcement of leaf structure (III), and maturation (IV and V). The transition from cell division to cell expansion happened uniformly across the leaf lamina between phase I and II; the sink-to-source transition was found between phase III and IV. These results demonstrate coordinated development of photosynthetic machinery and leaf structure in delayed-greening leaves and provide a simple and non-invasive method for estimation of leaf developmental stages in T. cacao.

Copper toxicity in leaves of Elodea canadensis Michx

Elodea canadensis (Canadian waterweed) has an ability to accumulate and bioconcentrate heavy metals. In this work, selected cellular responses for Cu treatment were studied in leaves of E. canadensis. Short term experiments, i.e. 1 week exposure to 0.5, 1, 5, and 10 microM of Cu indicated that concentrations up to 10 microM Cu causes a pronounced accumulation of photosynthetic pigments, a drastic degradation of soluble proteins with molecular weight above 18 kDa and a rapid accumulation of polypeptides with molecular weight below 14 kDa. The connection of these observations with copper detoxification mechanisms in aquatic macrophytes are discussed.

Zinc protects Ceratophyllum demersum L. (free-floating hydrophyte) against reactive oxygen species induced by cadmium

Evidence for Zn protection against Cd-induced reactive oxygen species in the free-floating hydrophyte Ceratophyllum demersum L. is presented in this paper. Metal treatments of 10 micromol/L Cd, 10 Cd micromol/L supplemented with Zn (10, 50, 100 and 200 micromol/L) and Zn-alone treatments of the same concentrations were used. Using 5,5 dimethyl pyrroline-N-oxide as the spin-probe, electron spin resonance spectra indicated a drastic increase in hydroxyl radicals (OH()) in Cd-10 micromol/L treatments, which was closely correlating with the enhanced formation of hydrogen peroxide (H(2)O(2)) and generation of superoxide radical (O(2)(-)) triggered by the oxidation of NADPH. The supplementation of adding Zn (10-200 micromol/L) to the Cd-10 micromol/L treatments significantly decreased the production of free radicals especially by eliminating the precursors of OH() through inhibition of NADPH oxidation. Cd-enhanced ROS production which substantially increased the oxidative products of proteins measured as carbonyls was effectively inhibited by Zn supplementation.

Two isoforms of ferredoxin:NADP(+) oxidoreductase from wheat leaves: purification and initial biochemical characterization

Ferredoxin:NADP(+) oxidoreductase is an enzyme associated with the stromal side of the thylakoid membrane in the chloroplast. It is involved in photosynthetic linear electron transport to produce NADPH and is supposed to play a role in cyclic electron transfer, generating a transmembrane pH gradient allowing ATP production, if photosystem II is non-functional or no NADP(+) is available for reduction. Different FNR isoforms have been described in non-photosynthetic tissues, where the enzyme catalyses the NADPH-dependent reduction of ferredoxin (Fd), necessary for some biosynthetic pathways. Here, we report the isolation and purification of two FNR isoproteins from wheat leaves, called FNR-A and FNR-B. These forms of the enzyme were identified as products of two different genes, as confirmed by mass spectrometry. The molecular masses of FNR-A and FNR-B were 34.3 kDa and 35.5 kDa, respectively. The isoelectric point of both FNR-A and FNR-B was about 5, but FNR-B appeared more acidic (of about 0.2 pH unit) than FNR-A. Both isoenzymes were able to catalyse a NADPH-dependent reduction of dibromothymoquinone and the mixture of isoforms catalysed reduction of cytochrome c in the presence of Fd. For the first time, the pH- and ionic strength dependent oligomerization of FNRs is observed. No other protein was necessary for complex formation. The putative role of the two FNR isoforms in photosynthesis is discussed based on current knowledge of electron transport in chloroplasts.

Metabolic control of the membrane fluidity in Bacillus subtilis during cold adaptation

Membrane fluidity adaptation to the low growth temperature in Bacillus subtilis involves two distinct mechanisms: (1) long-term adaptation accomplished by increasing the ratio of anteiso- to iso-branched fatty acids and (2) rapid desaturation of fatty acid chains in existing phospholipids by induction of fatty acid desaturase after cold shock. In this work we studied the effect of medium composition on cold adaptation of membrane fluidity. Bacillus subtilis was cultivated at optimum (40 degrees C) and low (20 degrees C) temperatures in complex medium with glucose or in mineral medium with either glucose or glycerol. Cold adaptation was characterized by fatty acid analysis and by measuring the midpoint of phospholipid phase transition T(m) (differential scanning calorimetry) and membrane fluidity (DPH fluorescence polarization). Cells cultured and measured at 40 degrees C displayed the same membrane fluidity in all three media despite a markedly different fatty acid composition. The T(m) was surprisingly the highest in the case of a culture grown in complex medium. On the contrary, cultivation at 20 degrees C in the complex medium gave rise to the highest membrane fluidity with concomitant decrease of T(m) by 10.5 degrees C. In mineral media at 20 degrees C the corresponding changes of T(m) were almost negligible. After a temperature shift from 40 to 20 degrees C, the cultures from all three media displayed the same adaptive induction of fatty acid desaturase despite their different membrane fluidity values immediately after cold shock.

Amino sugars: new inhibitors of zeaxanthin epoxidase, a violaxanthin cycle enzyme

The effect of three sugars and their amino derivatives on violaxanthin cycle enzymes activity was investigated in duckweed (Lemna trisulca), a model water-plant. No effect of sugars and amino sugars on violaxanthin de-epoxidase was observed independent of incubation time; however, epoxidation of zeaxanthin to violaxanthin was inhibited. The minimum amino sugar concentrations causing maximum inhibition of zeaxanthin epoxidation have been estimated. Amino sugars but not sugars caused more than a 50% inhibition of zeaxanthin epoxidation in duckweed after a 24h incubation when applied at a concentration of 0.5%. Incubation with amino sugars under a 6d photoperiod enhanced the inhibitory effect. Zeaxanthin epoxidation was completely inhibited under such conditions, whereas only a minor inhibitory effect was observed in sugar treated plants. The strong amino sugar inhibition of zeaxanthin epoxidase activity represents additional evidence for the creation of an unstable carotenoid carbocation in the molecular mechanism of epoxidation.

Protochlorophyll complexes with similar steady-state fluorescence characteristics can differ in fluorescence lifetimes. A model study in Triton X-100

The steady-state and time-resolved fluorescence characteristics of protochlorophyll (Pchl) dissolved in neat Triton X-100 and in Triton X-100 micelles were investigated, and the fluorescence lifetimes of different Pchl spectral forms were studied. Varying the concentration of Pchl or diluting the micellar solutions either with a buffer or with a micellar solution, 631-634, 645-655, 680-692 and above 700 nm emitting Pchl complexes were prepared, the ratios of which varied from one another. The fluorescence decay of the 631-634 nm emitting (monomeric) form had a mono-exponential character with a 5.4-ns fluorescence lifetime. The long-wavelength Pchl complexes (aggregates) had two fluorescence lifetime values within a range of 1.4-3.9 ns and 0.15-0.84 ns, which showed high variability in different environments. Depending on the conditions, either mono- or double-exponential fluorescence decay was found for a fluorescence band at 680-685 nm. These data show that despite their very similar steady-state fluorescence properties, Pchl complexes can differ in fluorescence lifetimes, which may reflect different molecular structures, intrinsic geometries or different molecular interactions. This underlines the importance of complex spectroscopic analysis for a precise description of native and artificial chlorophyllous pigment forms.

Thermotropic phase behaviour of lipid bilayers containing carotenoid pigment canthaxanthin: a differential scanning calorimetry study

In this study we address the problem of the effect of canthaxanthin on the thermotropic properties of lipid membranes formed with lipids which differ in the thickness of their hydrophobic core, size of polar heads or presence of the ester carbonyl group. For all the lipids a decrease in main transition enthalpy has been observed, indicating that canthaxanthin alters the membrane properties in its gel phase. The strongest influence of canthaxanthin on main phase transition and pretransition has been observed for the lipid having the thinnest hydrophobic region. Component analysis indicates a distinct cooperativity change, which most probably colligates with the formation of new thermotropic phases. The effect of canthaxanthin has been almost negligible in the case of phosphatidylethanolamines. The absence of the ester carbonyl group results in different thermotropic behavior, especially for low canthaxanthin concentrations. The effect of canthaxanthin is explained in terms of its organization within the membrane.

Disintegration of the prolamellar body structure at high concentrations of Hg2+

The effects of high concentrations of Hg (2+) (10 (-2) M and 10 (-3) M) were investigated on the ultrastructure and on the light-induced transformation of isolated prolamellar bodies (PLBs) of dark-grown wheat leaves. Our earlier work on wheat leaf homogenates ( , Plant Biology 6, 358 - 368) showed that, depending on the concentration, Hg (2+) reacts with protochlorophyllide, NADPH and the NADPH : protochlorophyllide oxidoreductase (POR, EC 1.3.1.33) enzyme and induces disaggregation of the macrodomain structure of this latter. Spectroscopic analyses confirmed that 15 min incubation with 10 (-2) M Hg (2+) at 4 degrees Celsius completely inhibited the activity of POR also in isolated PLBs. Ultrastructural investigations revealed the loosening of the PLB structure in the Hg (2+)-treated sample, i.e., intensive vesicle formation on the surface of the PLB membranes. The hexagonal geometry of the inner lattice was not disturbed, however, the unit cell size significantly increased. The disruption of the PLB membranes upon irradiation was studied after 40 min incubation with 10 (-3) M Hg (2+) at 4 degrees Celsius and a subsequent irradiation for 40 min at 20 degrees Celsius. Equimolar concentrations (10 (-3) M) of NADPH and Hg (2+) were added to the samples 10 min prior or after the addition of Hg (2+). Our results suggest that Hg (2+) accelerates the disruption of the PLB membranes and that NADPH can only partially prevent this process. These membrane transformations were similar to those observed in the initial steps of the Shibata shift of control samples.