Antioxidants in photosynthesis and human nutrition

Salt stress inhibits photosystems II and I in cyanobacteria

Recent studies of responses of cyanobacterial cells to salt stress have revealed that the NaCl-induced decline in the photosynthetic activities of photosystems II and I involves rapid and slow changes. The rapid decreases in the activities of both photosystems, which occur within a few minutes, are reversible and are associated with osmotic effects, which induce the efflux of water from the cytosol through water channels and rapidly increase intracellular concentrations of salts. Slower decreases in activity, which occur within hours, are irreversible and are associated with ionic effects that are due to the influx of Na(+) and Cl(-) ions through K(+)(Na(+)) channels and, probably, Cl(-) channels, with resultant dissociation of extrinsic proteins from photosystems. In combination with light stress, salt stress significantly stimulates photoinhibition by inhibiting repair of photodamaged photosystem II. Tolerance of photosystems to salt stress can be enhanced by genetically engineered increases in the unsaturation of fatty acids in membrane lipids and by intracellular synthesis of compatible solutes, such as glucosylglycerol and glycinebetaine. In this review, we summarize recent progress in research on the effects of salt stress on photosynthesis in cyanobacteria.

A photoactive carotenoid protein acting as light intensity sensor

Intense sunlight is dangerous for photosynthetic organisms. Cyanobacteria, like plants, protect themselves from light-induced stress by dissipating excess absorbed energy as heat. Recently, it was discovered that a soluble orange carotenoid protein, the OCP, is essential for this photoprotective mechanism. Here we show that the OCP is also a member of the family of photoactive proteins; it is a unique example of a photoactive protein containing a carotenoid as the photoresponsive chromophore. Upon illumination with blue-green light, the OCP undergoes a reversible transformation from its dark stable orange form to a red "active" form. The red form is essential for the induction of the photoprotective mechanism. The illumination induces structural changes affecting both the carotenoid and the protein. Thus, the OCP is a photoactive protein that senses light intensity and triggers photoprotection.

Feeding sustains photosynthetic quantum yield of a scleractinian coral during thermal stress

Thermal resistance of the coral-zooxanthellae symbiosis has been associated with chronic photoinhibition, increased antioxidant activity and protein repair involving high demands of nitrogen and energy. While the relative importance of heterotrophy as a source of nutrients and energy for cnidarian hosts, and as a means of nitrogen acquisition for their zooxanthellae, is well documented, the effect of feeding on the thermal sensitivity of the symbiotic association has been so far overlooked. Here we examine the effect of zooplankton feeding versus starvation on the bleaching susceptibility and photosynthetic activity of photosystem II (PSII) of zooxanthellae in the scleractinian coral Stylophora pistillata in response to thermal stress (daily temperature rises of 2-3 degrees C) over 10 days, employing pulse-amplitude-modulated chlorophyll fluorometry. Fed and starved corals displayed a decrease in daily maximum potential quantum yield (F (v)/F (m)) of PSII, effective quantum yield (F/F (m)') and relative electron transport rates over the course of 10 days. However after 10 days of exposure to elevated temperature, F (v)/F (m) of fed corals was still 50-70% higher than F (v)/F (m) of starved corals. Starved corals showed strong signs of chronic photoinhibition, which was reflected in a significant decline in nocturnal recovery rates of PSII relative to fed corals. This was paralleled by the progressive inability to dissipate excess excitation energy via non-photochemical quenching (NPQ). After 10 days, NPQ of starved corals had decreased by about 80% relative to fed corals. Feeding treatment had no significant effect on chlorophyll a and c (2) concentrations and zooxanthellae densities, but the mitotic indices were significantly lower in starved than in fed corals. Collectively the results indicate that exogenous food may reduce the photophysiological damage of zooxanthellae that typically leads to bleaching and could therefore play an important role in mediating the thermal resistance of some corals.

Influence of daily collection and culture medium recycling on the growth and beta-carotene yield of Dunaliella salina

The halophilic green alga Dunaliella salina has the potential to be cultivated for beta-carotene-rich biomass, however, open-air systems need to be further improved in order to become more competitive and more economical, rather than leave the major beta-carotene consuming market derived from artificially synthesis. A set of daily collection ratios was designed and scaled up with the aim to harvest cell biomass and beta-carotene from D. salina at logarithmic phase; the yields were comparable to the normal culture without daily removal of culture. Daily collection of 1/7.5 volume of algal culture was found to be appropriate to keep the balance between the cell biomass and beta-carotene accumulation. Light intensity as one of the important factors would affect both cell growth and beta-carotene content synchronously. Further, the method of recycling 1/7.5 volume of culture after removal of algae cells was developed in order to decrease input cost for the effective production of beta-carotene, and both the resulting yields of the cell biomass and beta-carotene gained an advantage over those from the normal D. salina culture.

Iodide accumulation provides kelp with an inorganic antioxidant impacting atmospheric chemistry

Brown algae of the Laminariales (kelps) are the strongest accumulators of iodine among living organisms. They represent a major pump in the global biogeochemical cycle of iodine and, in particular, the major source of iodocarbons in the coastal atmosphere. Nevertheless, the chemical state and biological significance of accumulated iodine have remained unknown to this date. Using x-ray absorption spectroscopy, we show that the accumulated form is iodide, which readily scavenges a variety of reactive oxygen species (ROS). We propose here that its biological role is that of an inorganic antioxidant, the first to be described in a living system. Upon oxidative stress, iodide is effluxed. On the thallus surface and in the apoplast, iodide detoxifies both aqueous oxidants and ozone, the latter resulting in the release of high levels of molecular iodine and the consequent formation of hygroscopic iodine oxides leading to particles, which are precursors to cloud condensation nuclei. In a complementary set of experiments using a heterologous system, iodide was found to effectively scavenge ROS in human blood cells.

Self-regulation of photoinduced electron transfer by a molecular nonlinear transducer

Organisms must adapt to survive, necessitating regulation of molecular and subcellular processes. Green plant photosynthesis responds to potentially damaging light levels by downregulating the fraction of excitation energy that drives electron transfer. Achieving adaptive, self-regulating behaviour in synthetic molecules is a critical challenge that must be met if the promises of nanotechnology are to be realized. Here we report a molecular pentad consisting of two light-gathering antennas, a porphyrin electron donor, a fullerene electron acceptor and a photochromic control moiety. At low white-light levels, the molecule undergoes photoinduced electron transfer with a quantum yield of 82%. As the light intensity increases, photoisomerization of the photochrome leads to quenching of the porphyrin excited state, reducing the quantum yield to as low as 27%. This self-regulating molecule modifies its function according to the level of environmental light, mimicking the non-photochemical quenching mechanism for photoprotection found in plants.

Identification of a mechanism of photoprotective energy dissipation in higher plants

Under conditions of excess sunlight the efficient light-harvesting antenna found in the chloroplast membranes of plants is rapidly and reversibly switched into a photoprotected quenched state in which potentially harmful absorbed energy is dissipated as heat, a process measured as the non-photochemical quenching of chlorophyll fluorescence or qE. Although the biological significance of qE is established, the molecular mechanisms involved are not. LHCII, the main light-harvesting complex, has an inbuilt capability to undergo transformation into a dissipative state by conformational change and it was suggested that this provides a molecular basis for qE, but it is not known if such events occur in vivo or how energy is dissipated in this state. The transition into the dissipative state is associated with a twist in the configuration of the LHCII-bound carotenoid neoxanthin, identified using resonance Raman spectroscopy. Applying this technique to study isolated chloroplasts and whole leaves, we show here that the same change in neoxanthin configuration occurs in vivo, to an extent consistent with the magnitude of energy dissipation. Femtosecond transient absorption spectroscopy, performed on purified LHCII in the dissipative state, shows that energy is transferred from chlorophyll a to a low-lying carotenoid excited state, identified as one of the two luteins (lutein 1) in LHCII. Hence, it is experimentally demonstrated that a change in conformation of LHCII occurs in vivo, which opens a channel for energy dissipation by transfer to a bound carotenoid. We suggest that this is the principal mechanism of photoprotection.

Carotenoids and flavonoids contribute to nutritional protection against skin damage from sunlight

The concept of photoprotection by dietary means is gaining momentum. Plant constituents such as carotenoids and flavonoids are involved in protection against excess light in plants and contribute to the prevention of UV damage in humans. As micronutrients, they are ingested with the diet and are distributed into light-exposed tissues, such as skin or the eye where they provide systemic photoprotection. beta-Carotene and lycopene prevent UV-induced erythema formation. Likewise, dietary flavanols exhibit photoprotection. After about 10-12 weeks of dietary intervention, a decrease in the sensitivity toward UV-induced erythema was observed in volunteers. Dietary micronutrients may contribute to life-long protection against harmful UV radiation.

Manipulation of phytoene levels in tomato fruit: effects on isoprenoids, plastids, and intermediary metabolism

In tomato (Solanum lycopersicum), phytoene synthase-1 (PSY-1) is the key biosynthetic enzyme responsible for the synthesis of fruit carotenoids. To further our understanding of carotenoid formation in tomato fruit, we characterized the effect of constitutive expression of an additional tomato Psy-1 gene product. A quantitative data set defining levels of carotenoid/isoprenoid gene expression, enzyme activities, and metabolites was generated from fruit that showed the greatest perturbation in carotenoid content. Transcriptional upregulation, resulting in increased enzyme activities and metabolites, occurred only in the case of Psy-1, Psy-2, and lycopene cyclase B. For reactions involving 1-deoxy-d-xylulose5-phosphate synthase, geranylgeranyl diphosphate synthase, phytoene desaturase, zeta-carotene desaturase, carotene isomerase, and lycopene beta-cyclase, there were no correlations between gene expression, enzyme activities, and metabolites. Perturbations in carotenoid composition were associated with changes in plastid type and with chromoplast-like structures arising prematurely during fruit development. The levels of >120 known metabolites were determined. Comparison with the wild type illustrated that key metabolites (sucrose, glucose/fructose, and Glu) and sectors of intermediary metabolism (e.g., tricarboxylic [corrected] acid cycle intermediates and fatty acids) in the Psy-1 transgenic mature green fruit resembled changes in metabolism associated with fruit ripening. General fruit developmental and ripening properties, such as ethylene production and fruit firmness, were unaffected. Therefore, it appears that the changes to pigmentation, plastid type, and metabolism associated with Psy-1 overexpression are not connected with the ripening process.

Enhancing the carotenoid content of Brassica napus seeds by downregulating lycopene epsilon cyclase

The accumulation of carotenoids in higher plants is regulated by the environment, tissue type and developmental stage. In Brassica napus leaves, beta-carotene and lutein were the main carotenoids present while petals primarily accumulated lutein and violaxanthin. Carotenoid accumulation in seeds was developmentally regulated with the highest levels detected at 35-40 days post anthesis. The carotenoid biosynthesis pathway branches after the formation of lycopene. One branch forms carotenoids with two beta rings such as beta-carotene, zeaxanthin and violaxanthin, while the other introduces both beta- and epsilon-rings in lycopene to form alpha-carotene and lutein. By reducing the expression of lycopene epsilon-cyclase (epsilon-CYC) using RNAi, we investigated altering carotenoid accumulation in seeds of B. napus. Transgenic seeds expressing this construct had increased levels of beta-carotene, zeaxanthin, violaxanthin and, unexpectedly, lutein. The higher total carotenoid content resulting from reduction of epsilon-CYC expression in seeds suggests that this gene is a rate-limiting step in the carotenoid biosynthesis pathway. epsilon-CYC activity and carotenoid production may also be related to fatty acid biosynthesis in seeds as transgenic seeds showed an overall decrease in total fatty acid content and minor changes in the proportions of various fatty acids.

Revisiting the ancient concept of botanical therapeutics

Mixtures of interacting compounds produced by plants may provide important combination therapies that simultaneously affect multiple pharmacological targets and provide clinical efficacy beyond the reach of single compound-based drugs. Developing innovative scientific methods for discovery, validation, characterization and standardization of these multicomponent botanical therapeutics is essential to their acceptance into mainstream medicine.

[Systemic photoprotection through carotenoids]

Nutritional supplements are increasingly used to protect human skin against environmentally-induced damage, most importantly as a consequence of ultraviolet radiation exposure. beta-carotene is a major constituent of comercially available products administered for systemic photoprotection. Studies on the systemic use of beta-carotene provide evidence that 15-30 mg/d over a period of about 10-12 wk produces a protective effect against UV-induced erythema. Similar effects have been attributed to mixtures of carotenoids or after long-term intake of dietary products rich in carotenoids. Supplementation with carotenoids contributes to basal protection of the skin but is not sufficent to obtain complete protection against severe UV irradiation.

Elevated zeaxanthin bound to oligomeric LHCII enhances the resistance of Arabidopsis to photooxidative stress by a lipid-protective, antioxidant mechanism

The xanthophyll cycle has a major role in protecting plants from photooxidative stress, although the mechanism of its action is unclear. Here, we have investigated Arabidopsis plants overexpressing a gene encoding beta-carotene hydroxylase, containing nearly three times the amount of xanthophyll cycle carotenoids present in the wild-type. In high light at low temperature wild-type plants exhibited symptoms of severe oxidative stress: lipid peroxidation, chlorophyll bleaching, and photoinhibition. In transformed plants, which accumulate over twice as much zeaxanthin as the wild-type, these symptoms were significantly ameliorated. The capacity of non-photochemical quenching is not significantly different in transformed plants compared with wild-type and therefore an enhancement of this process cannot be the cause of the stress tolerant phenotype. Rather, it is concluded that it results from the antioxidant effect of zeaxanthin. 80-90% of violaxanthin and zeaxanthin in wild-type and transformed plants was localized to an oligomeric LHCII fraction prepared from thylakoid membranes. The binding of these pigments in intact membranes was confirmed by resonance Raman spectroscopy. Based on the structural model of LHCII, we suggest that the protein/lipid interface is the active site for the antioxidant activity of zeaxanthin, which mediates stress tolerance by the protection of bound lipids.

Development of an Automated Cylindrical Ion Trap Mass Spectrometer for the Determination of Atmospheric Volatile Organic Compounds

Volatile organic compounds released from the biosphere are known to have a large impact on atmospheric chemistry. Field instruments for the detection of these trace gases are often limited by the lack of instrument portability and the inability to distinguish compounds of interest from background or other interfering compounds. We have developed an automated sampling and preconcentration system, coupled to a lightweight, low-power cylindrical ion trap mass spectrometer. The instrument was evaluated by measuring isoprene concentrations during a field campaign at the University of Michigan Biological Station PROPHET lab. Isoprene was preconcentrated by sampling directly into a short capillary column precooled without the aid of cryogens. The capillary column was then rapidly heated by moving the column to a preheated region to obtain fast separation of isoprene from other components, followed by detection with a cylindrical ion trap. This combination yielded a detection limit of approximately 80 ppt (parts per trillion) for isoprene with a measurement frequency of one sample every 11 min. The data obtained by the automated sampling and preconcentration system during the PROPHET 2005 campaign were compared to those of other field instruments measuring isoprene at this site in an intercomparison exercise. The intercomparisons suggest the new inlet system, when coupled with this ion trap detector, provides a viable field instrument for the fast, precise, and quantitative determination of isoprene and other trace gases over a variety of atmospheric conditions.

Metabolic engineering of ketocarotenoids biosynthesis in the unicelullar microalga Chlamydomonas reinhardtii

Most higher plants and microalgae are not able to synthesize ketocarotenoids. In this study the unicellular chlorophyte Chlamydomonas reinhardtii has been genetically engineered with the beta-carotene ketolase cDNA from Haematococcus pluvialis, bkt1 (GeneBank accession no. X86782), involved in the synthesis of astaxanthin, to obtain a transgenic microalga able to synthesize ketocarotenoids. The expression of bkt1 was driven by the Chlamydomonas constitutive promoter of the rubisco small subunit (RbcS2) and the resulting protein was directed to the chloroplast by the Chlamydomonas transit peptide sequences of Rubisco small subunit (RbcS2) or Ferredoxin (Fd). In all transformants containing the bkt1 gene fused to the RbcS2 or the Fd transit peptides a new pigment with the typical ketocarotenoid spectrum was detected. Surprisingly this ketocarotenoid was not astaxanthin nor canthaxanthin. The ketocarotenoid was identified on the basis of its mass spectrum as 3,3'-dihydroxy-beta,epsilon-carotene-4-one (4-keto-lutein) or its isomer ketozeaxanthin.

Characterization of a beta-carotene hydroxylase of Adonis aestivalis and its expression in Arabidopsis thaliana

Carotenoids are plant secondary metabolites that comprise two main groups: carotenes and xanthophylls. The latter group includes zeaxanthin which is synthesized by beta-carotene hydroxylase catalyzing the hydroxylation of the beta-rings of beta-carotene molecules. To develop tools to alter carotenoid biosynthesis in plants, we isolated a cDNA clone encoding a candidate beta-carotene hydroxylase, CrtH1, from the flower petals of Adonis aestivalis. CrtH1 protein has homology to beta-carotene hydroxylases from other organisms, and possesses the four histidine motifs conserved in this family of enzymes. Sequence analysis predicted the presence of a putative plastid transit peptide at the amino terminus and four transmembrane helical regions. Southern-blot analysis showed CrtH1 to be encoded by a multicopy gene family with at least three members in A. aestivalis. Analysis of CrtH1 transcript abundance by Northern blotting indicates it is highly expressed in flower petals, roots and stems, with relatively low expression in leaves and developing seeds. CrtH1 was able to catalyze the formation of zeaxanthin and its intermediate precursor beta-cryptoxanthin from beta-carotene in functional assays conducted in E. coli. Expression of CrtH1 in Arabidopsis thaliana wild type and a mutant deficient for endogenous beta-carotene hydroxylases enhanced the biosynthesis of violaxanthin in the seeds.

Photosynthesis, chlorophyll fluorescence and spectral reflectance in Sphagnum moss at varying water contents

Moss samples from the Fluxnet-Canada western peatland flux station in the Boreal Region of Alberta were measured in the laboratory to obtain the net photosynthesis rate and chlorophyll fluorescence of the moss under controlled environmental conditions, including the regulation of moss water content, simultaneously with measurements of moss spectral reflectance. One objective was to test whether the photochemical reflectance index (PRI) detected changes in moss photosynthetic light-use efficiency that were consistent with short-term (minutes to hours) changes in xanthophyll cycle pigments and associated changes in non-photochemical quenching (NPQ), as recorded by chlorophyll fluorescence. The rate of net photosynthesis was strongly inhibited by water content at values exceeding approximately 9 (fresh weight/dry weight) and declined as the water content fell below values of approximately 8. Chlorophyll fluorescence measurements of maximum photosystem II efficiency generally remained high until the water content was reduced from the maximum of about 20 to values of approximately 10-11, and then declined with further reductions in moss water content. A significant linear decline in NPQ was observed as moss water content was reduced from maximum to low water content values. There was a strong negative correlation between changes in NPQ and PRI. These data suggest that PRI measurements are a good proxy for short-term shifts in photosynthetic activity in Sphagnum moss. A second objective was to test how accurately the water band index (WBI, ratio of reflectance at 900 and 970 nm) recorded changes in moss water content during controlled laboratory studies. Strong linear relationships occurred between changes in moss water content and the WBI, although the slopes of the linear relationships were significantly different among sample replicates. Therefore, WBI appeared to be a useful tool to determine sample-specific water content without destructive measurements.

The Arabidopsis aba4-1 mutant reveals a specific function for neoxanthin in protection against photooxidative stress

The aba4-1 mutant completely lacks neoxanthin but retains all other xanthophyll species. The missing neoxanthin in light-harvesting complex (Lhc) proteins is compensated for by higher levels of violaxanthin, albeit with lower capacity for photoprotection compared with proteins with wild-type levels of neoxanthin. Detached leaves of aba4-1 were more sensitive to oxidative stress than the wild type when exposed to high light and incubated in a solution of photosensitizer agents. Both treatments caused more rapid pigment bleaching and lipid oxidation in aba4-1 than wild-type plants, suggesting that neoxanthin acts as an antioxidant within the photosystem II (PSII) supercomplex in thylakoids. While neoxanthin-depleted Lhc proteins and leaves had similar sensitivity as the wild type to hydrogen peroxide and singlet oxygen, they were more sensitive to superoxide anions. aba4-1 intact plants were not more sensitive than the wild type to high-light stress, indicating the existence of compensatory mechanisms of photoprotection involving the accumulation of zeaxanthin. However, the aba4-1 npq1 double mutant, lacking zeaxanthin and neoxanthin, underwent stronger PSII photoinhibition and more extensive oxidation of pigments than the npq1 mutant, which still contains neoxanthin. We conclude that neoxanthin preserves PSII from photoinactivation and protects membrane lipids from photooxidation by reactive oxygen species. Neoxanthin appears particularly active against superoxide anions produced by the Mehler's reaction, whose rate is known to be enhanced in abiotic stress conditions.

Outdoor cultivation of microalgae for carotenoid production: current state and perspectives

Microalgae are a major natural source for a vast array of valuable compounds, including a diversity of pigments, for which these photosynthetic microorganisms represent an almost exclusive biological resource. Yellow, orange, and red carotenoids have an industrial use in food products and cosmetics as vitamin supplements and health food products and as feed additives for poultry, livestock, fish, and crustaceans. The growing worldwide market value of carotenoids is projected to reach over US$1,000 million by the end of the decade. The nutraceutical boom has also integrated carotenoids mainly on the claim of their proven antioxidant properties. Recently established benefits in human health open new uses for some carotenoids, especially lutein, an effective agent for the prevention and treatment of a variety of degenerative diseases. Consumers' demand for natural products favors development of pigments from biological sources, thus increasing opportunities for microalgae. The biotechnology of microalgae has gained considerable progress and relevance in recent decades, with carotenoid production representing one of its most successful domains. In this paper, we review the most relevant features of microalgal biotechnology related to the production of different carotenoids outdoors, with a main focus on beta-carotene from Dunaliella, astaxanthin from Haematococcus, and lutein from chlorophycean strains. We compare the current state of the corresponding production technologies, based on either open-pond systems or closed photobioreactors. The potential of scientific and technological advances for improvements in yield and reduction in production costs for carotenoids from microalgae is also discussed.

Carotenoid biosynthesis structural genes in carrot (Daucus carota): isolation, sequence-characterization, single nucleotide polymorphism (SNP) markers and genome mapping

Carotenoid pigments are important components of the human diet and carrots are the main dietary sources of the vitamin A precursors alpha- and beta-carotene. Carotenoids play essential biological roles in plants and the genes coding for the carotenoid pathway enzymes are evolutionarily conserved, but little information exists about these genes for carrot. In this study, we utilized published carrot sequences as well as heterologous PCR approaches with primers derived from sequence information of other plant species to isolate 24 putative genes coding for carotenoid biosynthesis enzymes in carrot. Twenty-two of these genes were placed on the carrot genetic linkage map developed from a cross between orange-rooted and white-rooted carrot. The carotenoid genes were distributed in eight of the nine linkage groups in the carrot genome recommending their use for merging maps. Two genes co-localized with a genomic region spanning one of the most significant quantitative trait loci (QTL) for carotenoid accumulation. Carotenoid biosynthesis cDNAs linked to root color mutations and to QTL for carotenoid accumulation may suggest a functional role for them as candidate genes. RACE PCR and reverse transcriptase PCR were used to amplify the full-length transcript for twenty expressed carotenoid biosynthesis genes and sequences were submitted to GenBank. The cloning and sequence information of these genes is useful for PCR-based expression studies and may point toward transgenic approaches to manipulate carotenoid content in carrot.

Dissecting the biosynthetic pathway for the bypass1 root-derived signal

The Arabidopsis BYPASS1 (BPS1) gene is required for normal root and shoot development. In bps1 mutants, grafting and root excision experiments have shown that mutant roots produce a transmissible signal that is capable of arresting shoot development. In addition, we previously showed that growth of bps1 mutants on the carotenoid biosynthesis inhibitor fluridone resulted in partial rescue of both leaf and root defects. These observations suggest that a single mobile carotenoid-derived signal affects both root and shoot development. Here, we describe further characterization of the bps1 root-derived signal using genetic and biosynthetic inhibitor approaches. We characterized leaf and root development in double mutants that combined the bps1 mutant with mutants that have known defects in genes encoding carotenoid processing enzymes or defects in responses to carotenoid-derived abscisic acid. Our studies indicate that the mobile signal is neither abscisic acid nor the MAX-dependent hormone that regulates shoot branching, and that production of the signal does not require the activity of any single carotenoid cleavage dioxygenase. In addition, our studies with CPTA, a lycopene cyclase inhibitor, show that signal production requires synthesis of beta-carotene and its derivatives. Furthermore, we show a direct requirement for carotenoids as signal precursors, as the GUN plastid-to-nucleus signaling pathway is not required for phenotypic rescue. Together, our results suggest that bps1 roots produce a novel mobile carotenoid-derived signaling compound.

Metabolite profiling of plant carotenoids using the matrix-assisted laser desorption ionization time-of-flight mass spectrometry

Although modern MS has facilitated the advent of metabolomics, some natural products such as carotenoids are not readily compatible to detection by MS. In the present article, we describe how matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI/TOF-MS) can be utilized to acquire mass spectra of carotenoids effectively. The procedure is sensitive (pmole range), reduces 'spot to spot' variation and provides high mass accuracy, thus aiding identification. The technique has been applied in vivo to the analysis of carotenoids in isolated plant cells and in vitro to three applications: (i) to show compatibility with purification methods such as LC, TLC and HPLC; (ii) for the rapid identification and quantification (by isotope dilution) of carotenoids present in crude extracts from plant tissues and whole cells; (iii) simultaneous semi-quantitative determination of carotenoids metabolites (m/z values) in crude plant extracts. Multivariate analysis of the recorded m/z values shows the effectiveness of the procedure in distinguishing genotypes from each other. In addition, the utility of the technique has been demonstrated on two mutant tomato populations, to determine alterations in carotenoid content, and a comparison made with traditional HPLC-photodiode array analysis. These data show that MALDI/TOF-MS can be used to rapidly profile, identify and quantify plant carotenoids reproducibly, as well as detecting other metabolites (m/z) in complex biological systems.

Alpha-tocopherol may influence cellular signaling by modulating jasmonic acid levels in plants

Most studies on the function of tocopherols in plants have focused on their photo-protective and antioxidant properties, and it has been recently suggested, though not yet demonstrated, that they may also play a role in cellular signaling. By using vte1 mutants of Arabidopsis thaliana, with an insertion in the promoter region of the gene encoding tocopherol cyclase, we demonstrate here for the first time that tocopherol deficiency may alter endogenous phytohormone levels in plants, thereby reducing plant growth and triggering anthocyanin accumulation in leaves. In plants grown under a combination of high light and low temperature conditions to induce anthocyanin accumulation, we evaluated age-dependent changes in tocopherols, indicators of photo-oxidative stress, phytohormone levels, plant growth and anthocyanin levels in wild type and vte1 mutants. These mutants showed lower tocopherol levels, reduced growth and enhanced anthocyanin accumulation compared with the wild type, while both the maximum and relative efficiencies of PSII, chlorophylls, and carotenoids were not significantly altered. Analyses of phytohormone levels revealed that reduced growth and enhanced anthocyanin accumulation in tocopherol-deficient plants were preceded by increased jasmonic acid levels. This is the first study suggesting a direct effect of tocopherols on phytohormones levels in plants and will undoubtedly help us to better understand the multiple functions tocopherols play in plants, as well as the cellular signaling mechanisms responsible for the phenotypes thus far described in tocopherol-deficient plants.

Outdoor cultivation of lutein-rich cells of Muriellopsis sp. in open ponds

The growth performance of the chlorophycean microalga Muriellopsis sp. outdoors in open tanks agitated with a paddlewheel and its ability to accumulate carotenoids have been evaluated throughout the year. The cells grown in the open system had free lutein as the main carotenoid, with violaxanthin, beta-carotene, and neoxanthin also present. Lutein content of the dry biomass ranged from 0.4 to 0.6%, depending on the growth and environmental conditions. In addition, the biomass of Muriellopsis sp. had a high content in both protein and lipids with about half of the fatty acids being of the polyunsaturated type, with alpha-linolenic acid accounting for almost 30% of the total fatty acids. The effect of determinant parameters on the performance of the cultures in open tanks was evaluated. Operating conditions that allow the maintenance of productive cultures were established under semicontinuous regime for 9 months throughout the year. Biomass and lutein yields in the open system were not far from those in closed tubular photobioreactors, and reached productivity values of 20 g dry biomass, containing around 100 mg lutein m(-2) day(-1) in summer. The outdoor culture of Muriellopsis sp. in open ponds thus represents a real alternative to established systems for the production of lutein.

α‐Tocopherol: A Multifaceted Molecule in Plants

alpha-Tocopherol, which belongs to the vitamin E group of compounds, is a lipophilic antioxidant that has a number of functions in plants. Synthesized from homogentisic acid and isopentenyl diphosphate in the chloroplast envelope, alpha-tocopherol is essential to maintain the integrity of photosynthetic membranes and plays a major role in photo- and antioxidant protection. alpha-Tocopherol scavenges lipid peroxy radicals, thereby preventing the propagation of lipid peroxidation, and protects lipids and other membrane components by physically quenching and reacting chemically with singlet oxygen. Moreover, given that alpha-tocopherol increases membrane rigidity, its concentration, together with that of the other membrane components, may be regulated to afford adequate fluidity for membrane function. Furthermore, recent studies on tocopherol-deficient plants indicate that alpha-tocopherol may affect cellular signaling in plants. Evidence thus far indicates that the effects of this compound in plant cellular signaling may be linked to the control of redox homeostasis. alpha-Tocopherol may influence cellular signaling by controlling the propagation of lipid peroxidation in chloroplasts, therefore modulating the formation of oxylipins such as the phytohormone jasmonic acid.

The cauliflower Or gene encodes a DnaJ cysteine-rich domain-containing protein that mediates high levels of beta-carotene accumulation

Despite recent progress in our understanding of carotenogenesis in plants, the mechanisms that govern overall carotenoid accumulation remain largely unknown. The Orange (Or) gene mutation in cauliflower (Brassica oleracea var botrytis) confers the accumulation of high levels of beta-carotene in various tissues normally devoid of carotenoids. Using positional cloning, we isolated the gene representing Or and verified it by functional complementation in wild-type cauliflower. Or encodes a plastid-associated protein containing a DnaJ Cys-rich domain. The Or gene mutation is due to the insertion of a long terminal repeat retrotransposon in the Or allele. Or appears to be plant specific and is highly conserved among divergent plant species. Analyses of the gene, the gene product, and the cytological effects of the Or transgene suggest that the functional role of Or is associated with a cellular process that triggers the differentiation of proplastids or other noncolored plastids into chromoplasts for carotenoid accumulation. Moreover, we demonstrate that Or can be used as a novel genetic tool to induce carotenoid accumulation in a major staple food crop. We show here that controlling the formation of chromoplasts is an important mechanism by which carotenoid accumulation is regulated in plants.

A two-photon excitation study on the role of carotenoid dark states in the regulation of plant photosynthesis

Plants are exposed to sun light intensities that vary rapidly over several orders of magnitude during a typical day. It is known that the regulation of photosynthetic activity under these circumstances is essential for the survival and fitness of natural and gene modified plants. A quick balancing between utilization and dissipation of absorbed light energy ensures optimized levels of CO(2) fixation and protection from photo damage by excessive light-irradiation. Despite intensive investigations the biophysical mechanisms of these regulation processes are still poorly understood. Potentially involved singlet states of carotenoids are optically "dark" and so far it was impossible to investigate their role directly in living plants by conventional absorption or fluorescence spectroscopy. Here, we show by selective two-photon excitation of the carotenoid dark states in plant that a dominant part of the regulation is correlated with a substantial change in the energy transfer between these states and the chlorophylls (Chl). The results support a considerable role of the molecular gear shift model in which a reversible and step-wise enzymatic modification of the electronic structure of xanthophyll carotenoids enables a switching between carotenoid-to-Chl light-harvesting and Chl-to-carotenoid quenching. The shifting can be observed in real time in any plant. Treatment with the xanthophyll cycle inhibitor dithiothreitol slowed down both the light adaptation and the carotenoid-Chl energy flow changes to the same extent. Based on these results, we propose a biophysical quenching model in which both carotenoid dark states and radical cations contribute to the dissipation of excessive excitation energy.

Down co-regulation of light absorption, photochemistry, and carboxylation in Fe-deficient plants growing in different environments

The regulation of photosynthesis through changes in light absorption, photochemistry, and carboxylation efficiency has been studied in plants grown in different environments. Iron deficiency was induced in sugar beet (Beta vulgaris L.) by growing plants hydroponically in controlled growth chambers in the absence of Fe in the nutrient solution. Pear (Pyrus communis L.) and peach (Prunus persica L. Batsch) trees were grown in field conditions on calcareous soils, in orchards with Fe deficiency-chlorosis. Gas exchange parameters were measured in situ with actual ambient conditions. Iron deficiency decreased photosynthetic and transpiration rates, instantaneous transpiration efficiencies and stomatal conductances, and increased sub-stomatal CO(2) concentrations in the three species investigated. Photosynthesis versus CO(2) sub-stomatal concentration response curves and chlorophyll fluorescence quenching analysis revealed a non-stomatal limitation of photosynthetic rates under Fe deficiency in the three species investigated. Light absorption, photosystem II, and Rubisco carboxylation efficiencies were down-regulated in response to Fe deficiency in a coordinated manner, optimizing the use of the remaining photosynthetic pigments, electron transport carriers, and Rubisco.

Genetic control of fruit vitamin C contents

An F(1) progeny derived from a cross between the apple (Malus x domestica) cultivars Telamon and Braeburn was used to identify quantitative trait loci (QTL) linked to the vitamin C (l-ascorbate [l-AA]) contents of fruit skin and flesh (cortex) tissues. We identified up to three highly significant QTLs for both the mean l-AA and the mean total l-AA contents of fruit flesh on both parental genetic linkage maps, confirming the quantitative nature of these traits. These QTLs account for up to a maximum of 60% of the total population variation observed in the progeny, and with a maximal individual contribution of 31% per QTL. QTLs common to both parents were identified on linkage groups (LGs) 6, 10, and 11 of the Malus reference map, while each parent also had additional unique QTLs on other LGs. Interestingly, one strong QTL on LG-17 of the Telamon linkage map colocalized with a highly significant QTL associated with flesh browning, and a minor QTL for dehydroascorbate content, supporting earlier work that links fruit l-AA contents with the susceptibility of hardfruit to postharvest browning. We also found significant minor QTLs for skin l-AA and total l-AA (l-AA + dehydroascorbate) contents in Telamon. Currently, little is known about the genetic determinants underlying tissue l-AA homeostasis, but the presence of major, highly significant QTL in both these apple genotypes under field conditions suggests the existence of common control mechanisms, allelic heterozygosity, and helps outline strategies and the potential for the molecular breeding of these traits.

A chromoplast-specific carotenoid biosynthesis pathway is revealed by cloning of the tomato white-flower locus

Carotenoids and their oxygenated derivatives xanthophylls play essential roles in the pigmentation of flowers and fruits. Wild-type tomato (Solanum lycopersicum) flowers are intensely yellow due to accumulation of the xanthophylls neoxanthin and violaxanthin. To study the regulation of xanthophyll biosynthesis, we analyzed the mutant white-flower (wf). It was found that the recessive wf phenotype is caused by mutations in a flower-specific beta-ring carotene hyroxylase gene (CrtR-b2). Two deletions and one exon-skipping mutation in different CrtR-b2 wf alleles abolish carotenoid biosynthesis in flowers but not leaves, where the homologous CrtR-b1 is constitutively expressed. A second beta-carotene hydroxylase enzyme as well as flower- and fruit-specific geranylgeranyl diphosphate synthase, phytoene synthase, and lycopene beta-cyclase together define a carotenoid biosynthesis pathway active in chromoplasts only, underscoring the crucial role of gene duplication in specialized plant metabolic pathways. We hypothesize that this pathway in tomato was initially selected during evolution to enhance flower coloration and only later recruited to enhance fruit pigmentation. The elimination of beta-carotene hydroxylation in wf petals results in an 80% reduction in total carotenoid concentration, possibly caused by the inability of petals to store high concentrations of carotenoids other than xanthophylls and by degradation of beta-carotene, which accumulates as a result of the wf mutation but is not due to altered expression of genes in the biosynthetic pathway.

Content of redox-active compounds (ie, antioxidants) in foods consumed in the United States

BACKGROUND

Supplements containing ascorbic acid, alpha-tocopherol, or beta-carotene do not protect against oxidative stress-related diseases in most randomized intervention trials. We suggest that other redox-active phytochemicals may be more effective and that a combination of different redox-active compounds (ie, antioxidants or reductants) may be needed for proper protection against oxidative damage.

OBJECTIVE

We aimed to generate a ranked food table with values for total content of redox-active compounds to test this alternative antioxidant hypothesis.

DESIGN

An assay that measures the total concentration of redox-active compounds above a certain cutoff reduction potential was used to analyze 1113 food samples obtained from the US Department of Agriculture National Food and Nutrient Analysis Program.

RESULTS

Large variations in the content of antioxidants were observed in different foods and food categories. The food groups spices and herbs, nuts and seeds, berries, and fruit and vegetables all contained foods with very high antioxidant contents. Most food categories also contained products almost devoid of antioxidants. Of the 50 food products highest in antioxidant concentrations, 13 were spices, 8 were in the fruit and vegetables category, 5 were berries, 5 were chocolate-based, 5 were breakfast cereals, and 4 were nuts or seeds. On the basis of typical serving sizes, blackberries, walnuts, strawberries, artichokes, cranberries, brewed coffee, raspberries, pecans, blueberries, ground cloves, grape juice, and unsweetened baking chocolate were at the top of the ranked list.

CONCLUSION

This ranked antioxidant food table provides a useful tool for investigations into the possible health benefit of dietary antioxidants.

Long-term ingestion of high flavanol cocoa provides photoprotection against UV-induced erythema and improves skin condition in women

Dietary antioxidants contribute to endogenous photoprotection and are important for the maintenance of skin health. In the present study, 2 groups of women consumed either a high flavanol (326 mg/d) or low flavanol (27 mg/d) cocoa powder dissolved in 100 mL water for 12 wk. Epicatechin (61 mg/d) and catechin (20 mg/d) were the major flavanol monomers in the high flavanol drink, whereas the low flavanol drink contained 6.6 mg epicatechin and 1.6 mg catechin as the daily dose. Photoprotection and indicators of skin condition were assayed before and during the intervention. Following exposure of selected skin areas to 1.25 x minimal erythemal dose (MED) of radiation from a solar simulator, UV-induced erythema was significantly decreased in the high flavanol group, by 15 and 25%, after 6 and 12 wk of treatment, respectively, whereas no change occurred in the low flavanol group. The ingestion of high flavanol cocoa led to increases in blood flow of cutaneous and subcutaneous tissues, and to increases in skin density and skin hydration. Skin thickness was elevated from 1.11 +/- 0.11 mm at wk 0 to 1.24 +/- 0.13 mm at wk 12; transepidermal water loss was diminished from 8.7 +/- 3.7 to 6.3 +/- 2.2 g/(h x m2) within the same time frame. Neither of these variables was affected in the low flavanol cocoa group. Evaluation of the skin surface showed a significant decrease of skin roughness and scaling in the high flavanol cocoa group compared with those at wk 12. Dietary flavanols from cocoa contribute to endogenous photoprotection, improve dermal blood circulation, and affect cosmetically relevant skin surface and hydration variables.

A new paradigm for the action of reactive oxygen species in the photoinhibition of photosystem II

Inhibition of the activity of photosystem II (PSII) under strong light is referred to as photoinhibition. This phenomenon is due to the imbalance between the rate of photodamage to PSII and the rate of the repair of damaged PSII. Photodamage is initiated by the direct effects of light on the oxygen-evolving complex and, thus, photodamage to PSII is unavoidable. Studies of the effects of oxidative stress on photodamage and subsequent repair have revealed that reactive oxygen species (ROS) act primarily by inhibiting the repair of photodamaged PSII and not by damaging PSII directly. Thus, strong light has two distinct effects on PSII; it damages PSII directly and it inhibits the repair of PSII via production of ROS. Investigations of the ROS-induced inhibition of repair have demonstrated that ROS suppress the synthesis de novo of proteins and, in particular, of the D1 protein, that are required for the repair of PSII. Moreover, a primary target for inhibition by ROS appears to be the elongation step of translation. Inhibition of the repair of PSII by ROS is accelerated by the deceleration of the Calvin cycle that occurs when the availability of CO(2) is limited. In this review, we present a new paradigm for the action of ROS in photoinhibition.

Winter acclimation of PsbS and related proteins in the evergreen Arctostaphylos uva-ursi as influenced by altitude and light environment

The evergreen groundcover bearberry (Arctostaphylos uva-ursi [L.] Sprengel) was characterized over two successive years (2002-2004) from both sun-exposed and shaded sites at a montane ponderosa pine and subalpine forest community of 1900- and 2800-m-high altitudes, respectively. During summer, photosynthetic capacities and pre-dawn photosystem II (PSII) efficiency were similarly high in all four populations, and in winter, only the sun-exposed and shaded populations at 2800 m exhibited complete down-regulation of photosynthetic oxygen evolution capacity and consistent sustained down-regulation of PSII efficiency. This photosynthetic down-regulation at high altitude involved a substantial decrease in PSII components [pheophytin, D1 protein, oxygen evolving complex ([OEC)], a strong up-regulation of several anti-early-light-inducible protein (Elip)- and anti-high-light-inducible protein (Hlip)-reactive bands and a warm-sustained retention of zeaxanthin and antheraxanthin (Z + A). PsbS, the protein modulating the rapid engagement and disengagement of Z +A in energy dissipation, exhibited its most pronounced winter increases in the shade at 1900 m, and thus apparently assumes a greater role in providing rapidly reversible zeaxanthin-dependent photoprotection during winter when light becomes excessive in the shaded population, which remains photosynthetically active. It is attractive to hypothesize that PsbS relatives (Elips/Hlips) may be involved in sustained zeaxanthin-dependent photoprotection under the more extreme winter conditions at 2800 m.

Natural genetic variation for improving crop quality

The narrow genetic basis of many crops combined with restrictions on the commercial use of genetically modified plants, has led to a surge of interest in exploring natural biodiversity as a source of novel alleles to improve the productivity, adaptation, quality and nutritional value of crops. Genetic methodologies have been applied to natural variation to improve quality aspects that are associated with the chemical composition of agricultural products. A future challenge in this emerging field is to integrate metabolic, phenotypic and genomic databases to allow a wider view of the plant metabolome and the application of this knowledge within genomics-assisted breeding.

Adaptation and acclimation of photosynthetic microorganisms to permanently cold environments

Persistently cold environments constitute one of our world's largest ecosystems, and microorganisms dominate the biomass and metabolic activity in these extreme environments. The stress of low temperatures on life is exacerbated in organisms that rely on photoautrophic production of organic carbon and energy sources. Phototrophic organisms must coordinate temperature-independent reactions of light absorption and photochemistry with temperature-dependent processes of electron transport and utilization of energy sources through growth and metabolism. Despite this conundrum, phototrophic microorganisms thrive in all cold ecosystems described and (together with chemoautrophs) provide the base of autotrophic production in low-temperature food webs. Psychrophilic (organisms with a requirement for low growth temperatures) and psychrotolerant (organisms tolerant of low growth temperatures) photoautotrophs rely on low-temperature acclimative and adaptive strategies that have been described for other low-temperature-adapted heterotrophic organisms, such as cold-active proteins and maintenance of membrane fluidity. In addition, photoautrophic organisms possess other strategies to balance the absorption of light and the transduction of light energy to stored chemical energy products (NADPH and ATP) with downstream consumption of photosynthetically derived energy products at low temperatures. Lastly, differential adaptive and acclimative mechanisms exist in phototrophic microorganisms residing in low-temperature environments that are exposed to constant low-light environments versus high-light- and high-UV-exposed phototrophic assemblages.

Excited-State Dynamics of Carotenoids in Light-Harvesting Complexes. 1. Exploring the Relationship between the S1 and S* States

Dispersed transient absorption spectra collected at variable excitation intensities in combination with time-resolved signals were used to explore the underlying connectivity of the electronic excited-state manifold of the carotenoid rhodopin glucoside in the light-harvesting 2 complex isolated from Rhodopseudomonas acidophila. We find that the S state, which was recently identified as an excited state in carotenoids bound in bacterial light-harvesting complexes, exhibits a different response to the increase of excitation intensity than the S(1) state, which suggests that the models used so far to describe the excited states of carotenoids are incomplete. We propose two new models that can describe both the time-resolved and the intensity-dependent data; the first postulates that S(1) and S* are not populated in parallel after the decay of the initially excited S(2) state but instead result from the excitation of distinct ground-state subpopulations. The second model introduces a resonantly enhanced light-induced transition during excitation, which promotes population to higher-lying excited states that favors the formation of S* over S(1). Multiwavelength target analysis of the time-resolved and excitation-intensity dependence measurements were used to characterize the involved states and their responses. We show that both proposed models adequately fit the measured data, although it is not possible to determine which model is most apt. The physical origins and implications of both models are explored.