Carotene desaturation is linked to a respiratory redox pathway in Narcissus pseudonarcissus chromoplast membranes. Involvement of a 23-kDa oxygen-evolving-complex-like protein

Carotenoid-carbohydrate crosstalk: evidence for genetic and physiological interactions in storage tissues across crop species

Carotenoids play essential roles in photosynthesis, photoprotection, and human health. Efforts to increase carotenoid content in several staple crops have been successful through both conventional selection and genetic engineering methods. Interestingly, in some cases, altering carotenoid content has had unexpected effects on other aspects of plant metabolism, impacting traits like sugar content, dry matter percentage, fatty acid content, stress tolerance, and phytohormone concentrations. Studies across several diverse crop species have identified negative correlations between carotenoid and starch contents, as well as positive correlations between carotenoids and soluble sugars. Collectively, these reports suggest a metabolic interaction between carotenoids and carbohydrates. We synthesize evidence pointing to four hypothesized mechanisms: (1) direct competition for precursors; (2) physical interactions in plastids; (3) influences of sugar or apocarotenoid signaling networks; and (4) nonmechanistic population or statistical sources of correlations. Though the carotenoid biosynthesis pathway is well understood, the regulation and interactions of carotenoids, especially in nonphotosynthetic tissues, remain unclear. This topic represents an underexplored interplay between primary and secondary metabolism where further research is needed.

Mycobacterium tuberculosis Central Metabolism Is Key Regulator of Macrophage Pyroptosis and Host Immunity

Metabolic dysregulation in Mycobacterium tuberculosis results in increased macrophage apoptosis or pyroptosis. However, mechanistic links between Mycobacterium virulence and bacterial metabolic plasticity remain ill defined. In this study, we screened random transposon insertions of M. bovis BCG to identify mutants that induce pyroptotic death of the infected macrophage. Analysis of the transposon insertion sites identified a panel of fdr (functioning death repressor) genes, which were shown in some cases to encode functions central to Mycobacterium metabolism. In-depth studies of one fdr gene, fdr8 (BCG3787/Rv3727), demonstrated its important role in the maintenance of M. tuberculosis and M. bovis BCG redox balance in reductive stress conditions in the host. Our studies expand the subset of known Mycobacterium genes linking bacterial metabolic plasticity to virulence and also reveal that the broad induction of pyroptosis by an intracellular bacterial pathogen is linked to enhanced cellular immunity in vivo.

Chromoplast differentiation in bell pepper (Capsicum annuum) fruits

We report here a detailed analysis of the proteome adjustments that accompany chromoplast differentiation from chloroplasts during bell pepper (Capsicum annuum) fruit ripening. While the two photosystems are disassembled and their constituents degraded, the cytochrome b₆ f complex, the ATPase complex, and Calvin cycle enzymes are maintained at high levels up to fully mature chromoplasts. This is also true for ferredoxin (Fd) and Fd-dependent NADP reductase, suggesting that ferredoxin retains a central role in the chromoplasts' redox metabolism. There is a significant increase in the amount of enzymes of the typical metabolism of heterotrophic plastids, such as the oxidative pentose phosphate pathway (OPPP) and amino acid and fatty acid biosynthesis. Enzymes of chlorophyll catabolism and carotenoid biosynthesis increase in abundance, supporting the pigment reorganization that goes together with chromoplast differentiation. The majority of plastid encoded proteins decline but constituents of the plastid ribosome and AccD increase in abundance. Furthermore, the amount of plastid terminal oxidase (PTOX) remains unchanged despite a significant increase in phytoene desaturase (PDS) levels, suggesting that the electrons from phytoene desaturation are consumed by another oxidase. This may be a particularity of non-climacteric fruits such as bell pepper that lack a respiratory burst at the onset of fruit ripening.

Electron transport pathways in isolated chromoplasts from Narcissus pseudonarcissus L

During daffodil flower development, chloroplasts differentiate into photosynthetically inactive chromoplasts having lost functional photosynthetic reaction centers. Chromoplasts exhibit a respiratory activity reducing oxygen to water and generating ATP. Immunoblots revealed the presence of the plastid terminal oxidase (PTOX), the NAD(P)H dehydrogenase (NDH) complex, the cytochrome b₆ f complex, ATP synthase and several isoforms of ferredoxin-NADP⁺ oxidoreductase (FNR), and ferredoxin (Fd). Fluorescence spectroscopy allowed the detection of chlorophyll a in the cytochrome b₆ f complex. Here we characterize the electron transport pathway of chromorespiration by using specific inhibitors for the NDH complex, the cytochrome b₆ f complex, FNR and redox-inactive Fd in which the iron was replaced by gallium. Our data suggest an electron flow via two separate pathways, both reducing plastoquinone (PQ) and using PTOX as oxidase. The first oxidizes NADPH via FNR, Fd and cytochrome b_h of the cytochrome b₆ f complex, and does not result in the pumping of protons across the membrane. In the second, electron transport takes place via the NDH complex using both NADH and NADPH as electron donor. FNR and Fd are not involved in this pathway. The NDH complex is responsible for the generation of the proton gradient. We propose a model for chromorespiration that may also be relevant for the understanding of chlororespiration and for the characterization of the electron input from Fd to the cytochrome b₆ f complex during cyclic electron transport in chloroplasts.

RNA-seq highlights parallel and contrasting patterns in the evolution of the nuclear genome of fully mycoheterotrophic plants

BACKGROUND

While photosynthesis is the most notable trait of plants, several lineages of plants (so-called full heterotrophs) have adapted to obtain organic compounds from other sources. The switch to heterotrophy leads to profound changes at the morphological, physiological and genomic levels.

RESULTS

Here, we characterize the transcriptomes of three species representing two lineages of mycoheterotrophic plants: orchids (Epipogium aphyllum and Epipogium roseum) and Ericaceae (Hypopitys monotropa). Comparative analysis is used to highlight the parallelism between distantly related fully heterotrophic plants. In both lineages, we observed genome-wide elimination of nuclear genes that encode proteins related to photosynthesis, while systems associated with protein import to plastids as well as plastid transcription and translation remain active. Genes encoding components of plastid ribosomes that have been lost from the plastid genomes have not been transferred to the nuclear genomes; instead, some of the encoded proteins have been substituted by homologs. The nuclear genes of both Epipogium species accumulated nucleotide substitutions twice as rapidly as their photosynthetic relatives; in contrast, no increase in the substitution rate was observed in H. monotropa.

CONCLUSIONS

Full heterotrophy leads to profound changes in nuclear gene content. The observed increase in the rate of nucleotide substitutions is lineage specific, rather than a universal phenomenon among non-photosynthetic plants.

Plant-type phytoene desaturase: Functional evaluation of structural implications

Phytoene desaturase (PDS) is an essential plant carotenoid biosynthetic enzyme and a prominent target of certain inhibitors, such as norflurazon, acting as bleaching herbicides. PDS catalyzes the introduction of two double bonds into 15-cis-phytoene, yielding 9,15,9'-tri-cis-ζ-carotene via the intermediate 9,15-di-cis-phytofluene. We present the necessary data to scrutinize functional implications inferred from the recently resolved crystal structure of Oryza sativa PDS in a complex with norflurazon. Using dynamic mathematical modeling of reaction time courses, we support the relevance of homotetrameric assembly of the enzyme observed in crystallo by providing evidence for substrate channeling of the intermediate phytofluene between individual subunits at membrane surfaces. Kinetic investigations are compatible with an ordered ping-pong bi-bi kinetic mechanism in which the carotene and the quinone electron acceptor successively occupy the same catalytic site. The mutagenesis of a conserved arginine that forms a hydrogen bond with norflurazon, the latter competing with plastoquinone, corroborates the possibility of engineering herbicide resistance, however, at the expense of diminished catalytic activity. This mutagenesis also supports a "flavin only" mechanism of carotene desaturation not requiring charged residues in the active site. Evidence for the role of the central 15-cis double bond of phytoene in determining regio-specificity of carotene desaturation is presented.

PTOX Mediates Novel Pathways of Electron Transport in Etioplasts of Arabidopsis

The immutans (im) variegation mutant of Arabidopsis defines the gene for PTOX (plastid terminal oxidase), a versatile plastoquinol oxidase in chloroplast membranes. In this report we used im to gain insight into the function of PTOX in etioplasts of dark-grown seedlings. We discovered that PTOX helps control the redox state of the plastoquinone (PQ) pool in these organelles, and that it plays an essential role in etioplast metabolism by participating in the desaturation reactions of carotenogenesis and in one or more redox pathways mediated by PGR5 (PROTON GRADIENT REGULATION 5) and NDH (NAD(P)H dehydrogenase), both of which are central players in cyclic electron transport. We propose that these elements couple PTOX with electron flow from NAD(P)H to oxygen, and by analogy to chlororespiration (in chloroplasts) and chromorespiration (in chromoplasts), we suggest that they define a respiratory process in etioplasts that we have termed "etiorespiration". We further show that the redox state of the PQ pool in etioplasts might control chlorophyll biosynthesis, perhaps by participating in mechanisms of retrograde (plastid-to-nucleus) signaling that coordinate biosynthetic and photoprotective activities required to poise the etioplast for light development. We conclude that PTOX is an important component of metabolism and redox sensing in etioplasts.

Biosynthesis of Carotenoids in Plants: Enzymes and Color

Carotenoids are the most important biocolor isoprenoids responsible for yellow, orange and red colors found in nature. In plants, they are synthesized in plastids of photosynthetic and sink organs and are essential molecules for photosynthesis, photo-oxidative damage protection and phytohormone synthesis. Carotenoids also play important roles in human health and nutrition acting as vitamin A precursors and antioxidants. Biochemical and biophysical approaches in different plants models have provided significant advances in understanding the structural and functional roles of carotenoids in plants as well as the key points of regulation in their biosynthesis. To date, different plant models have been used to characterize the key genes and their regulation, which has increased the knowledge of the carotenoid metabolic pathway in plants. In this chapter a description of each step in the carotenoid synthesis pathway is presented and discussed.

Plastoquinone and Ubiquinone in Plants: Biosynthesis, Physiological Function and Metabolic Engineering

Plastoquinone (PQ) and ubiquinone (UQ) are two important prenylquinones, functioning as electron transporters in the electron transport chain of oxygenic photosynthesis and the aerobic respiratory chain, respectively, and play indispensable roles in plant growth and development through participating in the biosynthesis and metabolism of important chemical compounds, acting as antioxidants, being involved in plant response to stress, and regulating gene expression and cell signal transduction. UQ, particularly UQ₁₀, has also been widely used in people's life. It is effective in treating cardiovascular diseases, chronic gingivitis and periodontitis, and shows favorable impact on cancer treatment and human reproductive health. PQ and UQ are made up of an active benzoquinone ring attached to a polyisoprenoid side chain. Biosynthesis of PQ and UQ is very complicated with more than thirty five enzymes involved. Their synthetic pathways can be generally divided into two stages. The first stage leads to the biosynthesis of precursors of benzene quinone ring and prenyl side chain. The benzene quinone ring for UQ is synthesized from tyrosine or phenylalanine, whereas the ring for PQ is derived from tyrosine. The prenyl side chains of PQ and UQ are derived from glyceraldehyde 3-phosphate and pyruvate through the 2-C-methyl-D-erythritol 4-phosphate pathway and/or acetyl-CoA and acetoacetyl-CoA through the mevalonate pathway. The second stage includes the condensation of ring and side chain and subsequent modification. Homogentisate solanesyltransferase, 4-hydroxybenzoate polyprenyl diphosphate transferase and a series of benzene quinone ring modification enzymes are involved in this stage. PQ exists in plants, while UQ widely presents in plants, animals and microbes. Many enzymes and their encoding genes involved in PQ and UQ biosynthesis have been intensively studied recently. Metabolic engineering of UQ₁₀ in plants, such as rice and tobacco, has also been tested. In this review, we summarize and discuss recent research progresses in the biosynthetic pathways of PQ and UQ and enzymes and their encoding genes involved in side chain elongation and in the second stage of PQ and UQ biosynthesis. Physiological functions of PQ and UQ played in plants as well as the practical application and metabolic engineering of PQ and UQ are also included.

Phytoene Desaturase from Oryza sativa: Oligomeric Assembly, Membrane Association and Preliminary 3D-Analysis

Recombinant phytoene desaturase (PDS-His₆) from rice was purified to near-homogeneity and shown to be enzymatically active in a biphasic, liposome-based assay system. The protein contains FAD as the sole protein-bound redox-cofactor. Benzoquinones, not replaceable by molecular oxygen, serve as a final electron acceptor defining PDS as a 15-cis-phytoene (donor):plastoquinone oxidoreductase. The herbicidal PDS-inhibitor norflurazon is capable of arresting the reaction by stabilizing the intermediary FAD_red, while an excess of the quinone acceptor relieves this blockage, indicating competition. The enzyme requires its homo-oligomeric association for activity. The sum of data collected through gel permeation chromatography, non-denaturing polyacrylamide electrophoresis, chemical cross-linking, mass spectrometry and electron microscopy techniques indicate that the high-order oligomers formed in solution are the basis for an active preparation. Of these, a tetramer consisting of dimers represents the active unit. This is corroborated by our preliminary X-ray structural analysis that also revealed similarities of the protein fold with the sequence-inhomologous bacterial phytoene desaturase CRTI and other oxidoreductases of the GR2-family of flavoproteins. This points to an evolutionary relatedness of CRTI and PDS yielding different carotene desaturation sequences based on homologous protein folds.

Respiratory processes in non-photosynthetic plastids

Chlororespiration is a respiratory process located in chloroplast thylakoids which consists in an electron transport chain from NAD(P)H to oxygen. This respiratory chain involves the NAD(P)H dehydrogenase complex, the plastoquinone pool and the plastid terminal oxidase (PTOX), and it probably acts as a safety valve to prevent the over-reduction of the photosynthetic machinery in stress conditions. The existence of a similar respiratory activity in non-photosynthetic plastids has been less studied. Recently, it has been reported that tomato fruit chromoplasts present an oxygen consumption activity linked to ATP synthesis. Etioplasts and amyloplasts contain several electron carriers and some subunits of the ATP synthase, so they could harbor a similar respiratory process. This review provides an update on the study about respiratory processes in chromoplasts, identifying the major gaps that need to be addressed in future research. It also reviews the proteomic data of etioplasts and amyloplasts, which suggest the presence of a respiratory electron transport chain in these plastids.

Carotenoid metabolism in plants

Carotenoids are mostly C40 terpenoids, a class of hydrocarbons that participate in various biological processes in plants, such as photosynthesis, photomorphogenesis, photoprotection, and development. Carotenoids also serve as precursors for two plant hormones and a diverse set of apocarotenoids. They are colorants and critical components of the human diet as antioxidants and provitamin A. In this review, we summarize current knowledge of the genes and enzymes involved in carotenoid metabolism and describe recent progress in understanding the regulatory mechanisms underlying carotenoid accumulation. The importance of the specific location of carotenoid enzyme metabolons and plastid types as well as of carotenoid-derived signals is discussed.

Functional and molecular characterization of plastid terminal oxidase from rice (Oryza sativa)

The plastid terminal oxidase (PTOX) is a plastohydroquinone:oxygen oxidoreductase that shares structural similarities with alternative oxidases (AOX). Multiple roles have been attributed to PTOX, such as involvement in carotene desaturation, a safety valve function, participation in the processes of chlororespiration and setting the redox poise for cyclic electron transport. We have investigated a homogenously pure MBP fusion of PTOX. The protein forms a homo-tetrameric complex containing 2 Fe per monomer and is very specific for the plastoquinone head-group. The reaction kinetics were investigated in a soluble monophasic system using chemically reduced decyl-plastoquinone (DPQ) as the model substrate and, in addition, in a biphasic (liposomal) system in which DPQ was reduced with DT-diaphorase. While PTOX did not detectably produce reactive oxygen species in the monophasic system, their formation was observed by room temperature EPR in the biphasic system in a [DPQH₂] and pH-dependent manner. This is probably the result of the higher concentration of DPQ achieved within the partial volume of the lipid bilayer and a higher Km observed with PTOX-membrane associates which is ≈47mM compared to the monophasic system where a Km of ≈74μM was determined. With liposomes and at the basic stromal pH of photosynthetically active chloroplasts, PTOX was antioxidant at low [DPQH₂] gaining prooxidant properties with increasing quinol concentrations. It is concluded that in vivo, PTOX can act as a safety valve when the steady state [PQH₂] is low while a certain amount of ROS is formed at high light intensities.

Disruption of the homogentisate solanesyltransferase gene results in albino and dwarf phenotypes and root, trichome and stomata defects in Arabidopsis thaliana

Homogentisate solanesyltransferase (HST) plays an important role in plastoquinone (PQ) biosynthesis and acts as the electron acceptor in the carotenoids and abscisic acid (ABA) biosynthesis pathways. We isolated and identified a T-DNA insertion mutant of the HST gene that displayed the albino and dwarf phenotypes. PCR analyses and functional complementation also confirmed that the mutant phenotypes were caused by disruption of the HST gene. The mutants also had some developmental defects, including trichome development and stomata closure defects. Chloroplast development was also arrested and chlorophyll (Chl) was almost absent. Developmental defects in the chloroplasts were consistent with the SDS-PAGE result and the RNAi transgenic phenotype. Exogenous gibberellin (GA) could partially rescue the dwarf phenotype and the root development defects and exogenous ABA could rescue the stomata closure defects. Further analysis showed that ABA and GA levels were both very low in the pds2-1 mutants, which suggested that biosynthesis inhibition by GAs and ABA contributed to the pds2-1 mutants' phenotypes. An early flowering phenotype was found in pds2-1 mutants, which showed that disruption of the HST gene promoted flowering by partially regulating plant hormones. RNA-sequencing showed that disruption of the HST gene resulted in expression changes to many of the genes involved in flowering time regulation and in the biosynthesis of PQ, Chl, GAs, ABA and carotenoids. These results suggest that HST is essential for chloroplast development, hormone biosynthesis, pigment accumulation and plant development.

An ATP synthase harboring an atypical γ-subunit is involved in ATP synthesis in tomato fruit chromoplasts

Chromoplasts are non-photosynthetic plastids specialized in the synthesis and accumulation of carotenoids. During fruit ripening, chloroplasts differentiate into photosynthetically inactive chromoplasts in a process characterized by the degradation of the thylakoid membranes, and by the active synthesis and accumulation of carotenoids. This transition renders chromoplasts unable to photochemically synthesize ATP, and therefore these organelles need to obtain the ATP required for anabolic processes through alternative sources. It is widely accepted that the ATP used for biosynthetic processes in non-photosynthetic plastids is imported from the cytosol or is obtained through glycolysis. In this work, however, we show that isolated tomato (Solanum lycopersicum) fruit chromoplasts are able to synthesize ATP de novo through a respiratory pathway using NADPH as an electron donor. We also report the involvement of a plastidial ATP synthase harboring an atypical γ-subunit induced during ripening, which lacks the regulatory dithiol domain present in plant and algae chloroplast γ-subunits. Silencing of this atypical γ-subunit during fruit ripening impairs the capacity of isolated chromoplast to synthesize ATP de novo. We propose that the replacement of the γ-subunit present in tomato leaf and green fruit chloroplasts by the atypical γ-subunit lacking the dithiol domain during fruit ripening reflects evolutionary changes, which allow the operation of chromoplast ATP synthase under the particular physiological conditions found in this organelle.

The Mechanism of Variegation in immutans Provides Insight into Chloroplast Biogenesis

The immutans (im) variegation mutant of Arabidopsis has green and white-sectored leaves due to the absence of fully functional plastid terminal oxidase (PTOX), a plastoquinol oxidase in thylakoid membranes. PTOX appears to be at the nexus of a growing number of biochemical pathways in the plastid, including carotenoid biosynthesis, PSI cyclic electron flow, and chlororespiration. During the early steps of chloroplast biogenesis, PTOX serves as an alternate electron sink and is a prime determinant of the redox poise of the developing photosynthetic apparatus. Whereas a lack of PTOX causes the formation of photooxidized plastids in the white sectors of im, compensating mechanisms allow the green sectors to escape the effects of the mutation. This manuscript provides an update on PTOX, the mechanism of im variegation, and findings about im compensatory mechanisms.

On the structure and function of the phytoene desaturase CRTI from Pantoea ananatis, a membrane-peripheral and FAD-dependent oxidase/isomerase

CRTI-type phytoene desaturases prevailing in bacteria and fungi can form lycopene directly from phytoene while plants employ two distinct desaturases and two cis-tans isomerases for the same purpose. This property renders CRTI a valuable gene to engineer provitamin A-formation to help combat vitamin A malnutrition, such as with Golden Rice. To understand the biochemical processes involved, recombinant CRTI was produced and obtained in homogeneous form that shows high enzymatic activity with the lipophilic substrate phytoene contained in phosphatidyl-choline (PC) liposome membranes. The first crystal structure of apo-CRTI reveals that CRTI belongs to the flavoprotein superfamily comprising protoporphyrinogen IX oxidoreductase and monoamine oxidase. CRTI is a membrane-peripheral oxidoreductase which utilizes FAD as the sole redox-active cofactor. Oxygen, replaceable by quinones in its absence, is needed as the terminal electron acceptor. FAD, besides its catalytic role also displays a structural function by enabling the formation of enzymatically active CRTI membrane associates. Under anaerobic conditions the enzyme can act as a carotene cis-trans isomerase. In silico-docking experiments yielded information on substrate binding sites, potential catalytic residues and is in favor of single half-site recognition of the symmetrical C(40) hydrocarbon substrate.

Developmental defects in mutants of the PsbP domain protein 5 in Arabidopsis thaliana

Plants contain an extensive family of PsbP-related proteins termed PsbP-like (PPL) and PsbP domain (PPD) proteins, which are localized to the thylakoid lumen. The founding member of this family, PsbP, is an established component of the Photosystem II (PS II) enzyme, and the PPL proteins have also been functionally linked to other photosynthetic processes. However, the functions of the remaining seven PPD proteins are unknown. To elucidate the function of the PPD5 protein (At5g11450) in Arabidopsis, we have characterized a mutant T-DNA insertion line (SALK_061118) as well as several RNAi lines designed to suppress the expression of this gene. The functions of the photosynthetic electron transfer reactions are largely unaltered in the ppd5 mutants, except for a modest though significant decrease in NADPH dehydrogenase (NDH) activity. Interestingly, these mutants show striking plant developmental and morphological defects. Relative to the wild-type Col-0 plants, the ppd5 mutants exhibit both increased lateral root branching and defects associated with axillary bud formation. These defects include the formation of additional rosettes originating from axils at the base of the plant as well as aerial rosettes formed at the axils of the first few nodes of the shoot. The root-branching phenotype is chemically complemented by treatment with the synthetic strigolactone, GR24. We propose that the developmental defects observed in the ppd5 mutants are related to a deficiency in strigolactone biosynthesis.

The regulation of carotenoid pigmentation in flowers

Carotenoids fulfill many processes that are essential for normal growth and development in plants, but they are also responsible for the breathtaking variety of red-to-yellow colors we see in flowers and fruits. Although such visual diversity helps to attract pollinators and encourages herbivores to distribute seeds, humans also benefit from the aesthetic properties of flowers and an entire floriculture industry has developed on the basis that new and attractive varieties can be produced. Over the last decade, much has been learned about the impact of carotenoid metabolism on flower color development and the molecular basis of flower color. A number of different regulatory mechanisms have been described ranging from the transcriptional regulation of genes involved in carotenoid synthesis to the control of carotenoid storage in sink organs. This means we can now explain many of the natural colorful varieties we see around us and also engineer plants to produce flowers with novel and exciting varieties that are not provided by nature.

An orange ripening mutant links plastid NAD(P)H dehydrogenase complex activity to central and specialized metabolism during tomato fruit maturation

In higher plants, the plastidial NADH dehydrogenase (Ndh) complex supports nonphotochemical electron fluxes from stromal electron donors to plastoquinones. Ndh functions in chloroplasts are not clearly established; however, its activity was linked to the prevention of the overreduction of stroma, especially under stress conditions. Here, we show by the characterization of Orr(Ds), a dominant transposon-tagged tomato (Solanum lycopersicum) mutant deficient in the NDH-M subunit, that this complex is also essential for the fruit ripening process. Alteration to the NDH complex in fruit changed the climacteric, ripening-associated metabolites and transcripts as well as fruit shelf life. Metabolic processes in chromoplasts of ripening tomato fruit were affected in Orr(Ds), as mutant fruit were yellow-orange and accumulated substantially less total carotenoids, mainly beta-carotene and lutein. The changes in carotenoids were largely influenced by environmental conditions and accompanied by modifications in levels of other fruit antioxidants, namely, flavonoids and tocopherols. In contrast with the pigmentation phenotype in mature mutant fruit, Orr(Ds) leaves and green fruits did not display a visible phenotype but exhibited reduced Ndh complex quantity and activity. This study therefore paves the way for further studies on the role of electron transport and redox reactions in the regulation of fruit ripening and its associated metabolism.

The pds2 mutation is a lesion in the Arabidopsis homogentisate solanesyltransferase gene involved in plastoquinone biosynthesis

Plastoquinone plays critical roles in photosynthesis, chlororespiration and carotenoid biosynthesis. The previously isolated pds2 mutant from Arabidopsis was deficient in tocopherol and plastoquinone accumulation, and the biochemical phenotype of this mutant could not be reversed by externally applied homogentisate, suggesting a later step in tocopherol and/or plastoquinone biosynthesis had been disrupted. Recently, the protein encoded by At3g11950 (AtHST) was shown to condense homogentisate with solanesyl diphosphate (SDP), the substrate for plastoquinone synthesis, but not phytyl diphosphate (PDP), the substrate for tocopherol biosynthesis. We have sequenced the AtHST allele in the pds2 mutant background and identified an in-frame 6 bp (2 aa) deletion in the gene. The pds2 mutation could be functionally complemented by constitutive expression of AtHST, demonstrating that the molecular basis for the pds2 mutation is this 6 bp-lesion in the AtHST gene. Confocal microscopy of EGFP tagged AtHST suggested that AtHST is localized to the chloroplast envelope, supporting the hypothesis that plastoquinone synthesis occurs in the plastid.

Why is golden rice golden (yellow) instead of red?

The endosperm of Golden Rice (Oryza sativa) is yellow due to the accumulation of beta-carotene (provitamin A) and xanthophylls. The product of the two carotenoid biosynthesis transgenes used in Golden Rice, phytoene synthase (PSY) and the bacterial carotene desaturase (CRTI), is lycopene, which has a red color. The absence of lycopene in Golden Rice shows that the pathway proceeds beyond the transgenic end point and thus that the endogenous pathway must also be acting. By using TaqMan real-time PCR, we show in wild-type rice endosperm the mRNA expression of the relevant carotenoid biosynthetic enzymes encoding phytoene desaturase, zeta-carotene desaturase, carotene cis-trans-isomerase, beta-lycopene cyclase, and beta-carotene hydroxylase; only PSY mRNA was virtually absent. We show that the transgenic phenotype is not due to up-regulation of expression of the endogenous rice pathway in response to the transgenes, as was suggested to be the case in tomato (Lycopersicon esculentum) fruit, where CRTI expression resulted in a similar carotenoid phenomenon. This means that beta-carotene and xanthophyll formation in Golden Rice relies on the activity of constitutively expressed intrinsic rice genes (carotene cis-trans-isomerase, alpha/beta-lycopene cyclase, beta-carotene hydroxylase). PSY needs to be supplemented and the need for the CrtI transgene in Golden Rice is presumably due to insufficient activity of the phytoene desaturase and/or zeta-carotene desaturase enzyme in endosperm. The effect of CRTI expression was also investigated in leaves of transgenic rice and Arabidopsis (Arabidopsis thaliana). Here, again, the mRNA levels of intrinsic carotenogenic enzymes remained unaffected; nevertheless, the carotenoid pattern changed, showing a decrease in lutein, while the beta-carotene-derived xanthophylls increased. This shift correlated with CRTI-expression and is most likely governed at the enzyme level by lycopene-cis-trans-isomerism. Possible implications are discussed.

A study in scarlet: enzymes of ketocarotenoid biosynthesis in the flowers of Adonis aestivalis

The red ketocarotenoid astaxanthin (3,3'-dihydroxy-4,4'-diketo-beta,beta-carotene) is widely used as an additive in feed for the pigmentation of fish and crustaceans and is frequently included in human nutritional supplements as well. There is considerable interest in developing a plant-based biological production process for this valuable carotenoid. Adonis aestivalis (Ranunculaceae) is unusual among plants in synthesizing and accumulating large amounts of astaxanthin and other ketocarotenoids. The formation of astaxanthin requires only the addition of a carbonyl at the number 4 carbon of each beta-ring of zeaxanthin (3,3'-dihydroxy-beta,beta-carotene), a carotenoid typically present in the green tissues of higher plants. We screened an A. aestivalis flower library to identify cDNAs that might encode the enzyme that catalyzes the addition of the carbonyls. Two closely related cDNAs selected in this screen were found to specify polypeptides similar in sequence to plant beta-carotene 3-hydroxylases, enzymes that convert beta-carotene (beta,beta-carotene) into zeaxanthin. The Adonis enzymes, however, exhibited neither 4-ketolase nor 3-hydroxylase activity when presented with beta-carotene as the substrate in Escherichia coli. Instead, the products of the Adonis cDNAs were found to modify beta-rings in two distinctly different ways: desaturation at the 3,4 position and hydroxylation of the number 4 carbon. The 4-hydroxylated carotenoids formed in E. coli were slowly metabolized to yield compounds with ketocarotenoid-like absorption spectra. It is proposed that a 3,4-desaturation subsequent to 4-hydroxylation of the beta-ring leads to the formation of a 4-keto-beta-ring via an indirect and unexpected route: a keto-enol tautomerization.

Function of the 23 kDa extrinsic protein of Photosystem II as a manganese binding protein and its role in photoactivation

The function of the extrinsic 23 kDa protein of Photosystem II (PSII) was studied with respect to Mn binding and its ability to supply Mn to PSII during photoactivation, i.e. the light-dependent assembly of the tetramanganese cluster. The extrinsic proteins and the Mn cluster were removed by TRIS treatment from PSII-enriched membrane fragments and purified by anion exchange chromatography. Room temperature EPR spectra of the purified 23 kDa protein demonstrated the presence of Mn. Photoactivation was successful with low Mn concentrations when the 23 kDa protein was present, while in its absence a higher Mn concentration was needed to reach the same level of oxygen evolution activity. In addition, the rate of photoactivation was significantly accelerated in the presence of the 23 kDa protein. It is proposed that the 23 kDa protein plays an important role in providing Mn during the process of PSII assembly and that it acquires Mn during the light-induced turnover of D1 in the PSII damage-repair cycle and delivers Mn to repaired PSII.

Analysis in vitro of the enzyme CRTISO establishes a poly-cis-carotenoid biosynthesis pathway in plants

Most enzymes in the central pathway of carotenoid biosynthesis in plants have been identified and studied at the molecular level. However, the specificity and role of cis-trans-isomerization of carotenoids, which occurs in vivo during carotene biosynthesis, remained unresolved. We have previously cloned from tomato (Solanum lycopersicum) the CrtISO gene, which encodes a carotene cis-trans-isomerase. To study the biochemical properties of the enzyme, we developed an enzymatic in vitro assay in which a purified tomato CRTISO polypeptide overexpressed in Escherichia coli cells is active in the presence of an E. coli lysate that includes membranes. We show that CRTISO is an authentic carotene isomerase. Its catalytic activity of cis-to-trans isomerization requires redox-active components, suggesting that isomerization is achieved by a reversible redox reaction acting at specific double bonds. Our data demonstrate that CRTISO isomerizes adjacent cis-double bonds at C7 and C9 pairwise into the trans-configuration, but is incapable of isomerizing single cis-double bonds at C9 and C9'. We conclude that CRTISO functions in the carotenoid biosynthesis pathway in parallel with zeta-carotene desaturation, by converting 7,9,9'-tri-cis-neurosporene to 9'-cis-neurosporene and 7'9'-di-cis-lycopene into all-trans-lycopene. These results establish that in plants carotene desaturation to lycopene proceeds via cis-carotene intermediates.

Chlororespiration

Chlororespiration has been defined as a respiratory electron transport chain (ETC) in interaction with the photosynthetic ETC in thylakoid membranes of chloroplasts. The existence of chlororespiration has been disputed during the last decade, with the initial evidence mainly obtained with intact algal cells being possibly explained by redox interactions between chloroplasts and mitochondria. The discovery in higher-plant chloroplasts of a plastid-encoded NAD(P)H-dehydrogenase (Ndh) complex, homologous to the bacterial complex I, and of a nuclear-encoded plastid terminal oxidase (PTOX), homologous to the plant mitochondrial alternative oxidase, brought molecular support to the concept of chlororespiration. The functionality of these proteins in non-photochemical reduction and oxidation of plastoquinones (PQs), respectively, has recently been demonstrated. In thylakoids of mature chloroplasts, chlororespiration appears to be a relatively minor pathway compared to linear photosynthetic electron flow from H2O to NADP+. However, chlororespiration might play a role in the regulation of photosynthesis by modulating the activity of cyclic electron flow around photosystem I (PS I). In non-photosynthetic plastids, chlororespiratory electron carriers are more abundant and may play a significant bioenergetic role.

The Arabidopsis immutans mutation affects plastid differentiation and the morphogenesis of white and green sectors in variegated plants

The immutans (im) variegation mutant of Arabidopsis has green and white leaf sectors due to the action of a nuclear recessive gene, IMMUTANS (IM). This gene encodes the IM protein, which is a chloroplast homolog of the mitochondrial alternative oxidase. Because the white sectors of im accumulate the noncolored carotenoid, phytoene, IM likely serves as a redox component in phytoene desaturation. In this paper, we show that IM has a global impact on plant growth and development and is required for the differentiation of multiple plastid types, including chloroplasts, amyloplasts, and etioplasts. IM promoter activity and IM mRNAs are also expressed ubiquitously in Arabidopsis. IM transcript levels correlate with carotenoid accumulation in some, but not all, tissues. This suggests that IM function is not limited to carotenogenesis. Leaf anatomy is radically altered in the green and white sectors of im: Mesophyll cell sizes are dramatically enlarged in the green sectors and palisade cells fail to expand in the white sectors. The green im sectors also have significantly higher than normal rates of O(2) evolution and elevated chlorophyll a/b ratios, typical of those found in "sun" leaves. We conclude that the changes in structure and photosynthetic function of the green leaf sectors are part of an adaptive mechanism that attempts to compensate for a lack of photosynthesis in the white leaf sectors, while maximizing the ability of the plant to avoid photodamage.

Carotenoid Biosynthesis and Biotechnological Application

A survey is given on the carotenoid biosynthetic pathway leading to beta-carotene and its oxidation products in bacteria and plants. This includes the synthesis of prenyl pyrophosphates via the mevalonate or the 1-deoxyxylulose-5-phosphate pathways as well as the reaction sequences of carotenoid formation and interconversion together with the properties of the enzymes involved. Biotechnological application of this knowledge resulted in the development of heterologous carotenoid production systems using bacteria and fungi with metabolic engineered precursor supply and crop plants with manipulated carotenoid biosynthesis. The recent developments in engineering crops with increased carotenoid contents are covered.

A plastid terminal oxidase comes to light: implications for carotenoid biosynthesis and chlororespiration

Inactivation of a plastid located quinone-oxygen oxidoreductase gene in the immutans Arabidopsis mutant leads to a photobleached phenotype because of a lack of photoprotective carotenoids. Inactivation of the corresponding gene in the ghost tomato mutant leads to a similar phenotype in leaves and to carotenoid deficiency in petals and ripe fruits. This plastid terminal oxidase (the first to be cloned and biochemically characterized) resembles the mitochondrial cyanide-insensitive alternative oxidase. Here, we propose a model integrating this novel oxidase as a component of an electron transport chain associated to carotenoid desaturation, as well as to a respiratory activity within plastids.

A plastid terminal oxidase associated with carotenoid desaturation during chromoplast differentiation

The Arabidopsis IMMUTANS gene encodes a plastid homolog of the mitochondrial alternative oxidase, which is associated with phytoene desaturation. Upon expression in Escherichia coli, this protein confers a detectable cyanide-resistant electron transport to isolated membranes. In this assay this activity is sensitive to n-propyl-gallate, an inhibitor of the alternative oxidase. This protein appears to be a plastid terminal oxidase (PTOX) that is functionally equivalent to a quinol:oxygen oxidoreductase. This protein was immunodetected in achlorophyllous pepper (Capsicum annuum) chromoplast membranes, and a corresponding cDNA was cloned from pepper and tomato (Lycopersicum esculentum) fruits. Genomic analysis suggests the presence of a single gene in these organisms, the expression of which parallels phytoene desaturase and zeta-carotene desaturase gene expression during fruit ripening. Furthermore, this PTOX gene is impaired in the tomato ghost mutant, which accumulates phytoene in leaves and fruits. These data show that PTOX also participates in carotenoid desaturation in chromoplasts in addition to its role during early chloroplast development.

Two Arabidopsis thaliana carotene desaturases, phytoene desaturase and zeta-carotene desaturase, expressed in Escherichia coli, catalyze a poly-cis pathway to yield pro-lycopene

We have expressed in Escerichia coli the enzymes geranylgeranyl diphosphate synthase and phytoene synthase, from the soil bacterium Erwinia stewartii, and the two carotene desaturases phytoene desaturase and carotene zeta-carotene desaturase from Arabidopsis thaliana. We show that pro-lycopene (7,9,7',9'-tetra-cis)-lycopene is the main end product of the plant desaturation pathway in these cells. In addition, light is required in this system. Whereas in the dark mainly zeta-carotene, the phytoene desaturase product, accumulates, illumination leads to activation of this intermediate caused by its photoisomerization. zeta-Carotene then meets the stereospecific requirements of zeta-carotene desaturase and pro-lycopene is formed. In contrast, a strain of E. coli carrying geranylgeranyl diphosphate synthase, phytoene desaturase and the bacterial carotene desaturase CrtI, which mediates lycopene formation from phytoene, does not require light, nor is a poly-cis-lycopene species formed. The stereoselectivity of the plant-type desaturation pathway expressed in E. coli is the same as previously shown with chromoplast membranes. As the phytoene desaturase and zeta-carotene desaturase used originate from a system not capable of developing chromoplasts, this indicates that the poly-cis pathway of carotene desaturation may have a wider occurrence than initially believed.

GENES AND ENZYMES OF CAROTENOID BIOSYNTHESIS IN PLANTS

Carotenoids are integral and essential components of the photosynthetic membranes in all plants. Within the past few years, genes encoding nearly all of the enzymes required for the biosynthesis of these indispensable pigments have been identified. This review focuses on recent findings as to the structure and function of these genes and the enzymes they encode. Three topics of current interest are also discussed: the source of isopentenyl pyrophosphate for carotenoid biosynthesis, the progress and possibilities of metabolic engineering of plants to alter carotenoid content and composition, and the compartmentation and association of the carotenogenic enzymes. A speculative schematic model of carotenogenic enzyme complexes is presented to help frame and provoke insightful questions leading to future experimentation.

Induced beta-carotene synthesis driven by triacylglycerol deposition in the unicellular alga dunaliella bardawil

Under stress conditions such as high light intensity or nutrient starvation, cells of the unicellular alga Dunaliella bardawil overproduce beta-carotene, which is accumulated in the plastids in newly formed triacylglycerol droplets. We report here that the formation of these sequestering structures and beta-carotene are interdependent. When the synthesis of triacylglycerol is blocked, the overproduction of beta-carotene is also inhibited. During overproduction of beta-carotene no up-regulation of phytoene synthase or phytoene desaturase is observed on the transcriptional or translational level, whereas at the same time acetyl-CoA carboxylase, the key regulatory enzyme of acyl lipid biosynthesis, is increased, at least in its enzymatic activity. We conclude that under normal conditions the carotenogenic pathway is not maximally active and may be appreciably stimulated in the presence of sequestering structures, creating a plastid-localized sink for the end product of the carotenoid biosynthetic pathway.

Chloroplast import of four carotenoid biosynthetic enzymes in vitro reveals differential fates prior to membrane binding and oligomeric assembly

The precursor proteins of the carotenogenic enzymes geranylgeranyl diphosphate synthase, phytoene synthase, phytoene desaturase and lycopene cyclase were imported into isolated pea chloroplasts. Geranylgeranyl diphosphate synthase remained soluble in the stroma in a free form and phytoene synthase associated to thylakoid membranes upon import, both as expected. Surprisingly, phytoene desaturase and lycopene cyclase, which strongly depend on membrane association for enzymatic activity, also remained soluble in the chloroplast stroma. The soluble forms of these enzymes were, however, still competent for membrane-association, e.g. with protein-free liposomal membranes. Indeed the soluble forms of phytoene synthase, phytoene desaturase and lycopene cyclase occurred as ATP- and cold-sensitive high-molecular-mass complexes. Gel-filtration experiments and blue native-PAGE plus autoradiography and western blot analysis indicated a participation of the chloroplast 60-kDa chaperonin (Cpn60) in the soluble high-molecular-mass complexes of imported carotenogenic enzymes. Finally, it was inferred that a membrane-bound regulatory factor plays a decisive role in membrane-binding.

Redox chains in chloroplast envelope membranes: spectroscopic evidence for the presence of electron carriers, including iron-sulfur centers

We have shown that envelope membranes from spinach chloroplasts contain (i) semiquinone and flavosemiquinone radicals, (ii) a series of iron-containing electron-transfer centers, and (iii) flavins (mostly FAD) loosely associated with proteins. In contrast, we were unable to detect any cytochrome in spinach chloroplast envelope membranes. In addition to a high spin [1Fe]3+ type protein associated with an EPR signal at g = 4.3, we observed two iron-sulfur centers, a [4Fe-4S]1+ and a [2Fe-2S]1+, associated with features, respectively, at g = 1.921 and g = 1.935, which were detected after reduction by NADPH and NADH, respectively. The [4Fe-4S] center, but not the [2Fe-2S] center, was also reduced by dithionite or 5-deazaflavin/oxalate. An unusual Fe-S center, named X, associated with a signal at g = 2.057, was also detected, which was reduced by dithionite but not by NADH or NADPH. Extremely fast spin-relaxation rates of flavin- and quinone-free radicals suggest their close proximity to the [4Fe-4S] cluster or the high-spin [1Fe]3+ center. Envelope membranes probably contain enzymatic activities involved in the formation and reduction of semiquinone radicals (quinol oxidase, NADPH-quinone, and NADPH-semiquinone reductases). The physiological significance of our results is discussed with respect to (i) the presence of desaturase activities in envelope membranes and (ii) the mechanisms involved in the export of protons to the cytosol, which partially regulate the stromal pH during photosynthesis. The characterization of such a wide variety of electron carriers in envelope membranes opens new fields of research on the functions of this membrane system within the plant cell.