Sites of biosynthesis of carotenoids in Capsicum chromoplasts

Transcriptional Regulation of Ripening in Chili Pepper Fruits (Capsicum spp.)

Chili peppers represent a very important horticultural crop that is cultivated and commercialized worldwide. The ripening process makes the fruit palatable, desirable, and attractive, thus increasing its quality and nutritional value. This process includes visual changes, such as fruit coloration, flavor, aroma, and texture. Fruit ripening involves a sequence of physiological, biochemical, and molecular changes that must be finely regulated at the transcriptional level. In this review, we integrate current knowledge about the transcription factors involved in the regulation of different stages of the chili pepper ripening process.

Regulation of Carotenoid Biosynthesis in Photosynthetic Organs

A substantial proportion of the dazzling diversity of colors displayed by living organisms throughout the tree of life is determined by the presence of carotenoids, which most often provide distinctive yellow, orange and red hues. These metabolites play fundamental roles in nature that extend far beyond their importance as pigments. In photosynthetic lineages, carotenoids are essential to sustain life, since they have been exploited to maximize light harvesting and protect the photosynthetic machinery from photooxidative stress. Consequently, photosynthetic organisms have evolved several mechanisms that adjust the carotenoid metabolism to efficiently cope with constantly fluctuating light environments. This chapter will focus on the current knowledge concerning the regulation of the carotenoid biosynthetic pathway in leaves, which are the primary photosynthetic organs of most land plants.

Biochemistry and molecular biology of carotenoid biosynthesis in chili peppers (Capsicum spp.)

Capsicum species produce fruits that synthesize and accumulate carotenoid pigments, which are responsible for the fruits' yellow, orange and red colors. Chili peppers have been used as an experimental model for studying the biochemical and molecular aspects of carotenoid biosynthesis. Most reports refer to the characterization of carotenoids and content determination in chili pepper fruits from different species, cultivars, varieties or genotypes. The types and levels of carotenoids differ between different chili pepper fruits, and they are also influenced by environmental conditions. Yellow-orange colors of chili pepper fruits are mainly due to the accumulation of α- and β-carotene, zeaxanthin, lutein and β-cryptoxanthin. Carotenoids such as capsanthin, capsorubin and capsanthin-5,6-epoxide confer the red colors. Chromoplasts are the sites of carotenoid pigment synthesis and storage. According to the most accepted theory, the synthesis of carotenoids in chili peppers is controlled by three loci: c1, c2 and y. Several enzymes participating in carotenoid biosynthesis in chili pepper fruits have been isolated and characterized, and the corresponding gene sequences have been reported. However, there is currently limited information on the molecular mechanisms that regulate this biosynthetic pathway. Approaches to gain more knowledge of the regulation of carotenoid biosynthesis are discussed.

The carotenoid biosynthetic pathway: thinking in all dimensions

The carotenoid biosynthetic pathway serves manifold roles in plants related to photosynthesis, photoprotection, development, stress hormones, and various volatiles and signaling apocarotenoids. The pathway also produces compounds that impact human nutrition and metabolic products that contribute to fragrance and flavor of food and non-food crops. It is no surprise that the pathway has been a target of metabolic engineering, most prominently in the case of Golden Rice. The future success and predictability of metabolic engineering of carotenoids rests in the ability to target carotenoids for specific physiological purposes as well as to simultaneously modify carotenoids along with other desired traits. Here, we ask whether predictive metabolic engineering of the carotenoid pathway is indeed possible. Despite a long history of research on the pathway, at this point in time we can only describe the pathway as a parts list and have almost no knowledge of the location of the complete pathway, how it is assembled, and whether there exists any trafficking of the enzymes or the carotenoids themselves. We discuss the current state of knowledge regarding the "complete" pathway and make the argument that predictive metabolic engineering of the carotenoid pathway (and other pathways) will require investigation of the three dimensional state of the pathway as it may exist in plastids of different ultrastructures. Along with this message we point out the need to develop new types of visualization tools and resources that better reflect the dynamic nature of biosynthetic pathways.

Synergistic interactions between carotene ring hydroxylases drive lutein formation in plant carotenoid biosynthesis

Plant carotenoids play essential roles in photosynthesis, photoprotection, and as precursors to apocarotenoids. The plastid-localized carotenoid biosynthetic pathway is mediated by well-defined nucleus-encoded enzymes. However, there is a major gap in understanding the nature of protein interactions and pathway complexes needed to mediate carotenogenesis. In this study, we focused on carotene ring hydroxylation, which is performed by two structurally distinct classes of enzymes, the P450 CYP97A and CYP97C hydroxylases and the nonheme diiron HYD enzymes. The CYP97A and HYD enzymes both function in the hydroxylation of β-rings in carotenes, but we show that they are not functionally interchangeable. The formation of lutein, which involves hydroxylation of both β- and ε-rings, was shown to require the coexpression of CYP97A and CYP97C enzymes. These enzymes were also demonstrated to interact in vivo and in vitro, as determined using bimolecular fluorescence complementation and a pull-down assay, respectively. We discuss the role of specific hydroxylase enzyme interactions in promoting pathway flux and preventing the formation of pathway dead ends. These findings will facilitate efforts to manipulate carotenoid content and composition for improving plant adaptation to climate change and/or for enhancing nutritionally important carotenoids in food crops.

Plastid localization of the key carotenoid enzyme phytoene synthase is altered by isozyme, allelic variation, and activity

Plant carotenoids have unique physiological roles related to specific plastid suborganellar locations. Carotenoid metabolic engineering could enhance plant adaptation to climate change and improve food security and nutritional value. However, lack of fundamental knowledge on carotenoid pathway localization limits targeted engineering. Phytoene synthase (PSY), a major rate-controlling carotenoid enzyme, is represented by multiple isozymes residing at unknown plastid sites. In maize (Zea mays), the three isozymes were transiently expressed and found either in plastoglobuli or in stroma and thylakoid membranes. PSY1, with one to two residue modifications of naturally occurring functional variants, exhibited altered localization, associated with distorted plastid shape and formation of a fibril phenotype. Mutating the active site of the enzyme reversed this phenotype. Discovery of differential PSY locations, linked with activity and isozyme type, advances the engineering potential for modifying carotenoid biosynthesis.

Carotenoid biosynthesis: Isolation and characterization of a bifunctional enzyme catalyzing the synthesis of phytoene

Phytoene is the first C(40) intermediate in the biogenesis of carotenoids. It is formed by two enzyme activities, catalyzing (i) the coupling of two molecules of geranylgeranyl diphosphate to yield prephytoene diphosphate and (ii) the conversion of prephytoene diphosphate into phytoene. We show now, with Capsicum chromoplast stroma, that the overall activity resides in a single protein, which has been purified to homogeneity by affinity chromatography. The monomeric structure and the molecular size (M(r) 47,500) were demonstrated by NaDodSO(4)/PAGE and glycerol gradient centrifugation. Further characterization was achieved by using specific antibodies which allowed immunofractionation and immunoprecipitation of the enzymatic activity from chromoplast stroma. The two reactions followed conventional Michaelis-Menten kinetics, with K(m) values of 0.30 muM and 0.27 muM, respectively, for geranylgeranyl diphosphate and prephytoene diphosphate. The activity of the enzyme depends strictly upon the presence of Mn(2+). This selectivity may be one of the factors regulating the competition with potentially rival enzymes converting geranylgeranyl diphosphate into other plastid terpenoids. The two enzymatic reactions were inhibited by inorganic pyrophosphate and by the arginine-specific reagent hydroxyphenylglyoxal. In no instance were the two reactions kinetically uncoupled. These properties strongly suggest that the same enzyme catalyzes the two consecutive reactions, and we propose to name it phytoene synthase.

Phytoene desaturase is present in a large protein complex in the plastid membrane

Phytoene desaturase (PDS; EC 1.14.99.-) represents one of the key enzymes in the carotenoid biosynthetic pathway and is present in nearly all types of plastids in plants. To further characterize PDS, we isolated the PDS cDNA from cauliflower (BoPDS) and confirmed its function by heterologous expression in a strain of Escherichia coli containing a carotenoid-producing plasmid. The BoPDS cDNA encodes a predicted mature protein of approximately 55 kDa. In comparison with PDS from a few other plant species, BoPDS exhibited a high enzyme activity in E. coli, and its expression in plastids was independent of carotenoid levels. Plastids were purified from tissues of different plant species including cauliflower curds, tomato fruits, carrot roots and Arabidopsis leaves. By employing both Blue Native PAGE and SDS-PAGE approaches in conjunction with Western blot analysis, it was found that PDS in these plants existed in two forms. The plastid membrane form was present in a large protein complex of approximately 350 kDa, whereas the stroma version was in an approximately 660 kDa complex.

Biogenesis, molecular regulation and function of plant isoprenoids

Isoprenoids represent the oldest class of known low molecular-mass natural products synthesized by plants. Their biogenesis in plastids, mitochondria and the endoplasmic reticulum-cytosol proceed invariably from the C5 building blocks, isopentenyl diphosphate and/or dimethylallyl diphosphate according to complex and reiterated mechanisms. Compounds derived from the pathway exhibit a diverse spectrum of biological functions. This review centers on advances obtained in the field based on combined use of biochemical, molecular biology and genetic approaches. The function and evolutionary implications of this metabolism are discussed in relation with seminal informations gathered from distantly but related organisms.

Relationship between the structural and functional changes of the photosynthetic apparatus during chloroplast-chromoplast transition in flower bud of Lilium longiflorum

The relationship between the structural and functional changes of the photosynthetic apparatus in the flower bud of Lilium longiflorum during chloroplast-chromoplast transition was examined. Compared with green petals, there was a five-fold increase of the carotenoid content in yellow petals, whereas the chlorophyll content decreased five-fold. Absorption and emission fluorescence spectra of chromoplasts indicated that newly synthesized carotenoids were not associated with photosystem II (PSII) photochemistry. The maximum quantum yield in the remaining PSII reaction centers remained constant during the chromoplast formation, whereas the photosynthetic electron transport beyond PSII became inhibited, as indicated by a marked decrease of the O2 evolution capacity, of the photochemical quenching of chlorophyll-alpha fluorescence and of the operational quantum yield of photosynthetic electron transport. Deconvoluted fluorescence emission spectra indicated a more rapid degradation of photosystem I (PSI) complexes than of PSII during chromoplast formation. Compared with green petals, the spillover between PSII and PSI decreased by approximately 40% in yellow petals. Our results indicate that during chloroplast-chromoplast transition in the flower bud of L. longiflorum, PSII integrity was preserved longer than the rest of the photosynthetic apparatus.

Phytoene desaturase is localized exclusively in the chloroplast and up-regulated at the mRNA level during accumulation of secondary carotenoids in Haematococcus pluvialis (Volvocales, chlorophyceae)

The unicellular green alga Haematococcus pluvialis Flotow is known for its massive accumulation of ketocarotenoids under various stress conditions. Therefore, this microalga is one of the favored organisms for biotechnological production of these antioxidative compounds. Astaxanthin makes up the main part of the secondary carotenoids and is accumulated mostly in an esterified form in extraplastidic lipid vesicles. We have studied phytoene desaturase, an early enzyme of the carotenoid biosynthetic pathway. The increase in the phytoene desaturase protein levels that occurs following induction is accompanied by a corresponding increase of its mRNA during the accumulation period, indicating that phytoene desaturase is regulated at the mRNA level. We also investigated the localization of the enzyme by western-blot analysis of cell fractions and by immunogold labeling of ultrathin sections for electron microscopy. In spite of the fact that secondary carotenoids accumulate outside the chloroplast, no extra pathway specific for secondary carotenoid biosynthesis in H. pluvialis was found, at least at this early stage in the biosynthesis. A transport process of carotenoids from the site of biosynthesis (chloroplast) to the site of accumulation (cytoplasmatic located lipid vesicles) is implicated.

Biochemistry and molecular biology of chromoplast development

Plant cells contain a unique class of organelles, designated the plastids, which distinguish them from animal cells. According to the largely accepted endosymbiotic theory of evolution, plastids are descendants of prokaryotes. This process requires several adaptative changes which involve the maintenance and the expression of part of the plastid genome, as well as the integration of the plastid activity to the cellular metabolism. This is illustrated by the diversity of plastids encountered in plant cells. For instance, in tissues undergoing color changes, i.e., flowers and fruits, the chromoplasts produce and accumulate excess carotenoids. In this paper we attempt to review the basic aspects of chromoplast development.

Characterization and molecular cloning of a flavoprotein catalyzing the synthesis of phytofluene and zeta-carotene in Capsicum chromoplasts

In plants, zeta-carotene is the first visible carotenoid formed in the biosynthetic pathway through the following two-step desaturation reaction: phytoene-->phytofluene--> zeta-carotene. Using Capsicum annuum chromoplast membranes and the reconstitution system previously described [Camara, B., Bardat, F. & Monéger, R. (1982) Eur. J. Biochem. 127, 255-258], we have attempted to purify the desaturase(s) catalyzing these reactions. The two activities were coincidental during all the purification procedures. Only a single polypeptide with 56 +/- 2 kDa was detected by SDS/PAGE of all active fractions. The enzyme contained protein-bound FAD. Antibodies raised against the purified polypeptide selectively precipitated the phytoene and the phytofluene desaturase activities, thus demonstrating that the enzyme is a bifunctional flavoprotein. The antibodies were used to isolate a full-length cDNA clone from which was deduced the primary structure of the desaturase which contains a characteristic dinucleotide-binding site. Overexpression of the cDNA in Escherichia coli allowed the production of a recombinant desaturase which had all the properties of the chromoplast desaturase. The phytoene/phytofluene desaturase mRNA levels were extremely low in green fruits and increased slightly before detectable carotenoid synthesis and remained constant throughout ripening. However, the desaturase activity and protein levels were found to increase significantly during the chloroplast to chromoplast transition in C. annuum fruits.

Nucleotide sequence, organization, and nature of the protein products of the carotenoid biosynthesis gene cluster of Rhodobacter capsulatus

Carotenoid pigments are essential for the protection of both photosynthetic and non-photosynthetic tissues from photooxidative damage. Although carotenoid biosynthesis has been studied in many organisms from bacteria to higher plants, little is known about carotenoid biosynthetic enzymes, or the nature and regulation of the genes encoding them. We report here the first DNA sequence of carotenoid genes from any organism. We have determined the complete nucleotide sequence (11,039 bp) of a gene cluster encoding seven of the eight previously known carotenoid genes (crtA, B, C, D, E, F, I) and a new gene, designated crtK, from Rhodobacter capsulatus, a purple non-sulfur photosynthetic bacterium. The 5' flanking regions of crtA, I, D and E contain a highly conserved palindromic sequence homologous to the consensus binding site for a variety of prokaryotic DNA-binding regulatory proteins. This putative regulatory palindrome is also found 5' to the puc operon, encoding the light-harvesting II antenna polypeptides. Escherichia coli-like sigma 70 promoter sequences are located 5' to crtI and crtD, suggesting for the first time that such promoters may exist in purple photosynthetic bacteria. The crt genes form a minimum of four distinct operons, crtA, crtIBK, crtDC and crtEF, based on inversions of transcriptional orientation within the gene cluster. Possible rho-independent transcription terminators are located 3' to crtI, B, K, C and F. The 3' end of crtA may overlap transcription initiation signals for a downstream gene required for bacteriochlorophyll biosynthesis. We have also observed two regions of exceptional amino acid homology between CrtI and CrtD, both of which are dehydrogenases.

Purification of geranylgeranyl diphosphate synthase from Phycomyces blakesleanus

Geranylgeranyl diphosphate synthase has been purified to homogeneity from the carotene-overproducing strain M1 of Phycomyces blakesleanus. Usually two activity peaks with molecular weights of 60,000 and 30,000 eluted on gel exclusion chromatography, suggesting that the enzyme consists of two subunits, with a tendency to dissociate. With homogeneous protein, a single-staining band with molecular weight of 30,000 appeared on sodium dodecyl sulfate gel electrophoresis, confirming a subunit molecular weight of 30,000. Only isopentenyl diphosphate and farnesyl diphosphate were accepted by this enzyme for geranylgeranyl diphosphate formation. The smaller allylic compounds, dimethylallyl and geranyl diphosphate, were utilized at less than 1/20th the rate of farnesyl diphosphate. Michaelis constants of 9 microM for isopentenyl diphosphate and 60 microM for farnesyl diphosphate were found. The isoelectric point is 4.8.

Biosynthesis of monoterpene hydrocarbons by isolated chromoplasts from daffodil flowers

Incubation studies using [1-14C]isopentenyl diphosphate and [1-2H2]isopentenyl diphosphate as substrates revealed that isolated chromoplasts from flowers of Narcissus pseudonarcissus L. are able to synthesize monoterpene hydrocarbons and linalool in high yields. The enzymes involved are soluble in the chromoplast stroma. It is hypothesized that in the plant cell plastids are the site of monoterpene biosynthesis, whereas the formation of sesquiterpenes may be restricted to the cytoplasm/endoplasmic reticulum.

Carotenoid and chlorophyll biosynthesis in isolated plastids from mustard seedling cotyledons (Sinapis alba L.) during etioplast-chloroplast conversion

Etioplasts and etiochloroplasts, isolated from seedlings of white mustard (Sinapis alba L.) grown in continuous far-red light, and chloroplasts isolated from cotyledons and primary leaves of white-light-grown seedlings exhibit high prenyl-lipid-forming activities. Only the etioplasts and etiochloroplasts, and to a much lesser extent chloroplasts from cotyledons are capable of forming carotenes from isopentenyl diphosphate as substrate, whereas in chloroplasts from primary leaves no such activities could be detected. By subfractionation experiments, it could be demonstrated that the phytoene-synthase complex in etioplasts and etiochloroplasts is present in a soluble form in the stroma, whereas the subsequent enzymes, i.e. the dehydrogenase, cis-trans isomerase and cyclase are bound to both membrane fractions, the prolamellar bodies/prothylakoids and the envelopes. In good agreement with previous results using isolated chromoplasts and chloroplasts, it is concluded that the phytoene-synthase complex may change its topology from a peripheral membrane protein in non-green plastids to a tightly membrane-associated protein in chloroplasts. This change is apparently paralleled by altered functional properties which render the complex undetectable in isolated chloroplasts. Further experiments concerning the reduction of chlorophyll a containing a geranylgeranyl side chain to chlorophyll a indicate that the light-induced etioplast-chloroplast conversion is accompanied by a certain reorganization of the polyprenoid-forming enzymatic equipment.

Monoterpene formation by leucoplasts of Citrofortunella mitis and Citrus unshiu : Steps and conditions of biosynthesis

Leucoplasts of immature calamondin and satsuma fruits were incubated with [1-(14)C] isopentenyl pyrophosphate under various conditions. Optimal incorporation of the tracer into geranyl pyrophosphate and monoterpene hydrocarbons occurred in the presence of exogenous dimethylallyl pyrophosphate and Mn(2+) which was more effective than Mg(2+). The dependence of dimethylallyl pyrophosphate showed that about 10 moles were required for 1 mole of isopentenyl pyrophosphate for the best recovery in monoterpene hydrocarbon biosynthesis. A time-course incorporation of isopentenyl pyrophosphate revealed that the C10 hydrocarbon elaboration was dependent on the geranyl pyrophosphate production and at no time neryl pyrophosphate was synthesized by leucoplasts. The amount of labelled farnesyl pyrophosphate was rather low whatever the conditions used in the experiments and sesquiterpene hydrocarbon biosynthesis was never observed.

Solubilization and reconstitution of the membrane-bound carotenogenic enzymes from daffodil chromoplasts

The membrane-bound carotenogenic enzymes of daffodil (Narcissus pseudonarcissus) chromoplast membranes, i.e. dehydrogenation, cis-trans isomerization and cyclization activities, were solubilized in an enzymatically inactive form using the zwitterionic detergent 3-[(3-cholamidopropyl)-dimethylamino]-1-propanesulfonate (Chaps). Full enzymatic activities were regained upon reconstitution of the solubilized proteins into liposomes. These preparations converted radiolabelled cis-phytoene into beta-carotene in a high yield. The reconstituted enzymatic sequence behaved as a tight 'assembly line'. In the enzymatic sequence of the reconstituted system the cis-trans isomerization reaction occurred on the stage of cis-phytofluene after a dehydrogenation of cis-phytoene.

Carotenoid biosynthesis by Aphanocapsa homogenates coupled to a phytoene-generating system from Phycomyces blakesleeanus

The ability of a cell extract from the C5 carB10(-) (phytoene-accumulating) strain of Phycomyces blakesleeanus to convert [2-(14)C]mevalonic acid into phytoene has been exploited in a coupled assay with a cell homogenate from the cyanobacterium Aphanocapsa 6714 (Pasteur Culture Collection]. Incubations containing both cell extracts convert [2-(14)C]mevalonic acid into unsaturated carotenes and xanthophylls of Aphanocapsa, via the [(14)C]phytoene formed by the C5 preparation. The major advantage of this experimental approach is that it simplifies and shortens the assay procedure, since it eliminates the necessity to initially prepare [(14)C]phytoene and then add it to the incubation with an emulsifying agent.

Demonstration and solubilization of S-adenosylmethionine: γ-tocopherol methyltransferase from Capsicum chromoplasts

Through the use of Capsicum chromoplast membranes, we report for the first time the direct methylation of γ-tocopherol into α-tocopherol in the presence of S-adenosylmethionine. Furthermore the S-adenosylmethionine: γ-tocopherol methyltransferase activity has been solubilized. On a protein basis, the activity recovered in the soluble preparation was higher than that bound to the membranes.

Prenyl lipid formation in spinach chloroplasts and in a cell-free system of Synechococcus (Cyanobacteria): polyprenols, chlorophylls, and fatty acid prenyl esters

Isolated chloroplasts from spinach leaf cells, chloroplast subfractions, and a cell-free system of the cyanobacterium Synechococcus CCAP 6312 incorporated [1-(14)C]isopentenyl pyrophosphate in high yields into prenyl lipids. Products were polyprenols (C20, C45) chlorophylls, quinoid compounds, and fatty acid prenyl esters; prenyl pyrophosphates occurred in trace amounts, and carotenes were only formed to a limited extent in the Synechococcus system. The formation of fatty acid prenyl esters, which is described here for the first time, was found to occur in two different ways in the chloroplast system; by an acyl-CoA: polyprenol acyltransferase reaction associated with the envelope membranes and by a transesterification reaction from chlorophyll associated with the thylakoids. Endogenous fatty acid prenyl esters made up about 3% by weight of total lipids in spinach chloroplasts and were also found to be natural constituents of the cyanobacterial cells.

Prenylation and methylation reactions in phylloquinone (vitamin K1) synthesis in Capsicum annuum plastids

The biosynthesis of phylloquinone (vitamin K1) was examined using Capsicum fruit chloroplasts and chromoplasts (apparently phylloquinone free). In both cases, the synthesis of phylloquinone from α-naphthoquinone, dihydro-α-naphthoquinone, 1,4-dihydroxy-2-naphthoic acid (as precursors of the ring moiety) and (S)-adenosyl-L-methionine was achieved. In the presence of phytylpyrophosphate, the biosynthesis of phylloquinone in both organelles is particularly enhanced when 1,4-dihydroxy-2-naphthoic acid is used.