Engineering of ubiquinone biosynthesis using the yeast coq2 gene confers oxidative stress tolerance in transgenic tobacco

Antioxidants of Non-Enzymatic Nature: Their Function in Higher Plant Cells and the Ways of Boosting Their Biosynthesis

Plants are exposed to a variety of abiotic and biotic stresses leading to increased formation of reactive oxygen species (ROS) in plant cells. ROS are capable of oxidizing proteins, pigments, lipids, nucleic acids, and other cell molecules, disrupting their functional activity. During the process of evolution, numerous antioxidant systems were formed in plants, including antioxidant enzymes and low molecular weight non-enzymatic antioxidants. Antioxidant systems perform neutralization of ROS and therefore prevent oxidative damage of cell components. In the present review, we focus on the biosynthesis of non-enzymatic antioxidants in higher plants cells such as ascorbic acid (vitamin C), glutathione, flavonoids, isoprenoids, carotenoids, tocopherol (vitamin E), ubiquinone, and plastoquinone. Their functioning and their reactivity with respect to individual ROS will be described. This review is also devoted to the modern genetic engineering methods, which are widely used to change the quantitative and qualitative content of the non-enzymatic antioxidants in cultivated plants. These methods allow various plant lines with given properties to be obtained in a rather short time. The most successful approaches for plant transgenesis and plant genome editing for the enhancement of biosynthesis and the content of these antioxidants are discussed.

A Trifolium repens flavodoxin-like quinone reductase 1 (TrFQR1) improves plant adaptability to high temperature associated with oxidative homeostasis and lipids remodeling

Maintenance of stable mitochondrial respiratory chains could enhance adaptability to high temperature, but the potential mechanism was not elucidated clearly in plants. In this study, we identified and isolated a TrFQR1 gene encoding the flavodoxin-like quinone reductase 1 (TrFQR1) located in mitochondria of leguminous white clover (Trifolium repens). Phylogenetic analysis indicated that amino acid sequences of FQR1 in various plant species showed a high degree of similarities. Ectopic expression of TrFQR1 protected yeast (Saccharomyces cerevisiae) from heat damage and toxic levels of benzoquinone, phenanthraquinone and hydroquinone. Transgenic Arabidopsis thaliana and white clover overexpressing TrFQR1 exhibited significantly lower oxidative damage and better photosynthetic capacity and growth than wild-type in response to high-temperature stress, whereas AtFQR1-RNAi A. thaliana showed more severe oxidative damage and growth retardation under heat stress. TrFQR1-transgenic white clover also maintained better respiratory electron transport chain than wild-type plants, as manifested by significantly higher mitochondrial complex II and III activities, alternative oxidase activity, NAD(P)H content, and coenzyme Q10 content in response to heat stress. In addition, overexpression of TrFQR1 enhanced the accumulation of lipids including phosphatidylglycerol, monogalactosyl diacylglycerol, sulfoquinovosyl diacylglycerol and cardiolipin as important compositions of bilayers involved in dynamic membrane assembly in mitochondria or chloroplasts positively associated with heat tolerance. TrFQR1-transgenic white clover also exhibited higher lipids saturation level and phosphatidylcholine:phosphatidylethanolamine ratio, which could be beneficial to membrane stability and integrity during a prolonged period of heat stress. The current study proves that TrFQR1 is essential for heat tolerance associated with mitochondrial respiratory chain, cellular reactive oxygen species homeostasis, and lipids remodeling in plants. TrFQR1 could be selected as a key candidate marker gene to screen heat-tolerant genotypes or develop heat-tolerant crops via molecular-based breeding.

Genome-Wide Identification and Expression Analysis of UBiA Family Genes Associated with Abiotic Stress in Sunflowers (Helianthus annuus L.)

The UBiA genes encode a large class of isopentenyltransferases, which are involved in the synthesis of secondary metabolites such as chlorophyll and vitamin E. They performed important functions in the whole plant's growth and development. Current studies on UBiA genes were not comprehensive enough, especially for sunflower UBiA genes. In this study, 10 HaUBiAs were identified by domain analysis these HaUBiAs had five major conserved domains and were unevenly distributed on six chromosomes. By constructing phylogenetic trees, 119 UBiA genes were found in 12 species with different evolutionary levels and divided into five major groups, which contained seven conserved motifs and eight UBiA subsuper family domains. Tissue expression analysis showed that HaUBiAs were highly expressed in the roots, leaves, and seeds. By using promoter analysis, the cis-elements of UBiA genes were mainly in hormone signaling and stress responses. The qRT-PCR results showed that HaUBiA1 and HaUBiA5 responded strongly to abiotic stresses. Under ABA and MeJA treatments, HaUBiA1 significantly upregulated, while HaUBiA5 significantly decreased. Under cold stress, the expression of UBiA1 was significantly upregulated in the roots and stems, while UBiA5 expression was increased only in the leaves. Under anaerobic induction, UBiA1 and UBiA5 were both upregulated in the roots, stems and leaves. In summary, this study systematically classified the UBiA family and identified two abiotic stress candidate genes in the sunflower. It expands the understanding of the UBiA family and provides a theoretical basis for future abiotic stress studies in sunflowers.

The Transcriptional Responses of Ectomycorrhizal Fungus, Cenococcum geophilum, to Drought Stress

With global warming, drought has become one of the major environmental pressures that threaten the development of global agricultural and forestry production. Cenococcum geophilum (C. geophilum) is one of the most common ectomycorrhizal fungi in nature, which can form mycorrhiza with a large variety of host trees of more than 200 tree species from 40 genera of both angiosperms and gymnosperms. In this study, six C. geophilum strains with different drought tolerance were selected to analyze their molecular responses to drought stress with treatment of 10% polyethylene glycol. Our results showed that drought-sensitive strains absorbed Na and K ions to regulate osmotic pressure and up-regulated peroxisome pathway genes to promote the activity of antioxidant enzymes to alleviate drought stress. However, drought-tolerant strains responded to drought stress by up-regulating the functional genes involved in the ubiquinone and other terpenoid-quinone biosynthesis and sphingolipid metabolism pathways. The results provided a foundation for studying the mechanism of C. geophilum response to drought stress.

How plants synthesize coenzyme Q

Coenzyme Q (CoQ) is a conserved redox-active lipid that has a wide distribution across the domains of life. CoQ plays a key role in the oxidative electron transfer chain and serves as a crucial antioxidant in cellular membranes. Our understanding of CoQ biosynthesis in eukaryotes has come mostly from studies of yeast. Recently, significant advances have been made in understanding CoQ biosynthesis in plants. Unique mitochondrial flavin-dependent monooxygenase and benzenoid ring precursor biosynthetic pathways have been discovered, providing new insights into the diversity of CoQ biosynthetic pathways and the evolution of phototrophic eukaryotes. We summarize research progress on CoQ biosynthesis and regulation in plants and recent efforts to increase the CoQ content in plant foods.

Kaempferol as a precursor for ubiquinone (coenzyme Q) biosynthesis: An atypical node between specialized metabolism and primary metabolism

Ubiquinone (coenzyme Q) is a vital respiratory cofactor and liposoluble antioxidant. Studies have shown that plants derive approximately a quarter of 4-hydroxybenzoate, which serves as the direct ring precursor of ubiquinone, from the catabolism of kaempferol. Biochemical and genetic evidence suggests that the release of 4-hydroxybenzoate from kaempferol is catalyzed by heme-dependent peroxidases and that 3-O-glycosylations of kaempferol act as a negative regulator of this process. These findings not only represent an atypical instance of primary metabolite being derived from specialized metabolism but also raise the question as to whether ubiquinone contributes to the ROS scavenging and signaling functions already established for flavonols.

A dedicated flavin-dependent monooxygenase catalyzes the hydroxylation of demethoxyubiquinone into ubiquinone (coenzyme Q) in Arabidopsis

Ubiquinone (Coenzyme Q) is a vital respiratory cofactor and liposoluble antioxidant. In plants, it is not known how the C-6 hydroxylation of demethoxyubiquinone, the penultimate step in ubiquinone biosynthesis, is catalyzed. The combination of cross-species gene network modeling along with mining of embryo-defective mutant databases of Arabidopsis thaliana identified the embryo lethal locus EMB2421 (At1g24340) as a top candidate for the missing plant demethoxyubiquinone hydroxylase. In marked contrast with prototypical eukaryotic demethoxyubiquinone hydroxylases, the catalytic mechanism of which depends on a carboxylate-bridged di-iron domain, At1g24340 is homologous to FAD-dependent oxidoreductases that instead use NAD(P)H as an electron donor. Complementation assays in Saccharomyces cerevisiae and Escherichia coli demonstrated that At1g24340 encodes a functional demethoxyubiquinone hydroxylase and that the enzyme displays strict specificity for the C-6 position of the benzoquinone ring. Laser-scanning confocal microscopy also showed that GFP-tagged At1g24340 is targeted to mitochondria. Silencing of At1g24340 resulted in 40 to 74% decrease in ubiquinone content and de novo ubiquinone biosynthesis. Consistent with the role of At1g24340 as a benzenoid ring modification enzyme, this metabolic blockage could not be bypassed by supplementation with 4-hydroxybenzoate, the immediate precursor of ubiquinone's ring. Unlike in yeast, in Arabidopsis overexpression of demethoxyubiquinone hydroxylase did not boost ubiquinone content. Phylogenetic reconstructions indicated that plant demethoxyubiquinone hydroxylase is most closely related to prokaryotic monooxygenases that act on halogenated aromatics and likely descends from an event of horizontal gene transfer between a green alga and a bacterium.

Engineering high coenzyme Q10 tomato

Coenzyme Q (CoQ) is vital for energy metabolism in living organisms. In humans, CoQ₁₀ deficiency causes diseases and must be replenished via diet; however, CoQ content in plant foods is primarily low. Here, we report the breeding of high CoQ₁₀ tomato lines by expressing four enzymes with a fruit-specific promoter, which modifies the chloroplast chorismate pathway, enhances cytosolic isoprenoid biosynthesis, and up-regulates the first two reactions in mitochondrion that construct the CoQ₁₀ polyisoprenoid tail. We show that, while the level of the aromatic precursor could be markedly elevated, head group prenylation is the key to increasing the final CoQ₁₀ yield. In the HUCD lines expressing all four transgenes, the highest CoQ₁₀ content (0.15 mg/g dry weight) shows a seven-fold increase from the wild-type level and reaches an extraordinarily rich CoQ₁₀ food grade. Overviewing the changes in other terpenoids by transcriptome and metabolic analyses reveals variable contents of carotenoids and α-tocopherol in the HUCD lines. In addition to the enigmatic relations among different terpenoid pathways, high CoQ₁₀ plants maintaining substantial levels of either vitamin can be selected. Our investigation paves the way for the development of CoQ₁₀-enriched crops as dietary supplements.

3-O-glycosylation of kaempferol restricts the supply of the benzenoid precursor of ubiquinone (Coenzyme Q) in Arabidopsis thaliana

Ubiquinone (Coenzyme Q) is a vital respiratory cofactor and antioxidant in eukaryotes. The recent discovery that kaempferol serves as a precursor for ubiquinone's benzenoid moiety both challenges the conventional view of flavonoids as specialized metabolites, and offers new prospects for engineering ubiquinone in plants. Here, we present evidence that Arabidopsis thaliana mutants lacking kaempferol 3-O-rhamnosyltransferase (ugt78d1) and kaempferol 3-O-glucosyltransferase (ugt78d2) activities display increased de novo biosynthesis of ubiquinone and increased ubiquinone content. These data are congruent with the proposed model that unprotected C-3 hydroxyl of kaempferol triggers the oxidative release of its B-ring as 4-hydroxybenzoate, which in turn is incorporated into ubiquinone. Ubiquinone content in the ugt78d1/ugt78d2 double knockout represented 160% of wild-type level, matching that achieved via exogenous feeding of 4-hydroxybenzoate to wild-type plants. This suggests that 4-hydroxybenzoate is no longer limiting ubiquinone biosynthesis in the ugt78d1/ugt78d2 plants. Evidence is also shown that the glucosylation of 4-hydroxybenzoate as well as the conversion of the immediate precursor of kaempferol, dihydrokaempferol, into dihydroquercetin do not compete with ubiquinone biosynthesis in A. thaliana.

Arabidopsis 4-COUMAROYL-COA LIGASE 8 contributes to the biosynthesis of the benzenoid ring of coenzyme Q in peroxisomes

Plants have evolved the ability to derive the benzenoid moiety of the respiratory cofactor and antioxidant, ubiquinone (coenzyme Q), either from the β-oxidative metabolism of p-coumarate or from the peroxidative cleavage of kaempferol. Here, isotopic feeding assays, gene co-expression analysis and reverse genetics identified Arabidopsis 4-COUMARATE-COA LIGASE 8 (4-CL8; At5g38120) as a contributor to the β-oxidation of p-coumarate for ubiquinone biosynthesis. The enzyme is part of the same clade (V) of acyl-activating enzymes than At4g19010, a p-coumarate CoA ligase known to play a central role in the conversion of p-coumarate into 4-hydroxybenzoate. A 4-cl8 T-DNA knockout displayed a 20% decrease in ubiquinone content compared with wild-type plants, while 4-CL8 overexpression boosted ubiquinone content up to 150% of the control level. Similarly, the isotopic enrichment of ubiquinone's ring was decreased by 28% in the 4-cl8 knockout as compared with wild-type controls when Phe-[Ring-13C6] was fed to the plants. This metabolic blockage could be bypassed via the exogenous supply of 4-hydroxybenzoate, the product of p-coumarate β-oxidation. Arabidopsis 4-CL8 displays a canonical peroxisomal targeting sequence type 1, and confocal microscopy experiments using fused fluorescent reporters demonstrated that this enzyme is imported into peroxisomes. Time course feeding assays using Phe-[Ring-13C6] in a series of Arabidopsis single and double knockouts blocked in the β-oxidative metabolism of p-coumarate (4-cl8; at4g19010; at4g19010 × 4-cl8), flavonol biosynthesis (flavanone-3-hydroxylase), or both (at4g19010 × flavanone-3-hydroxylase) indicated that continuous high light treatments (500 µE m-2 s-1; 24 h) markedly stimulated the de novo biosynthesis of ubiquinone independently of kaempferol catabolism.

SmPPT, a 4-hydroxybenzoate polyprenyl diphosphate transferase gene involved in ubiquinone biosynthesis, confers salt tolerance in Salvia miltiorrhiza

SmPPT, which encodes 4-hydroxybenzoate polyprenyl diphosphate transferase involved in ubiquinone biosynthesis, confers salt tolerance to S. miltiorrhiza through enhancing the activities of POD and CAT to scavenge ROS. Ubiquinone (UQ), also known as coenzyme Q (CoQ), is a key electron transporter in the mitochondrial respiratory system. UQ is composed of a benzene quinone ring and a polyisoprenoid side chain. Attachment of polyisoprenoid side chain to the benzene quinone ring is a rate-limiting step catalyzed by 4-hydroxybenzoate polyprenyl diphosphate transferase (PPT). So far, only a few plant PPT-encoding genes have been functionally analyzed. Through genome-wide analysis and subsequent molecular cloning, a PPT-encoding gene, termed SmPPT, was identified from an economically and academically important medicinal model plant, Salvia miltiorrhiza. SmPPT contained many putative cis-elements associated with abiotic stresses in the promoter region and were responsive to PEG-6000 and methyl jasmonate treatments. The deduced SmPPT protein contains the PT_UbiA conserved domain of polyprenyl diphosphate transferase and an N-terminal mitochondria transit peptide. Transient expression assay of SmPPT-GFP fusion protein showed that SmPPT was mainly localized in the mitochondria. SmPPT could functionally complement coq2 mutation and catalyzed UQ6 production in yeast cells. Overexpression of SmPPT increased UQ production and enhanced salt tolerance in S. miltiorrhiza. Under salinity stress conditions, transgenic plants accumulated less H₂O₂ and malondialdehyde and exhibited higher peroxidase (POD) and catalase (CAT) activities compared with wild-type plants. It indicates that SmPPT confers salt tolerance to S. miltiorrhiza at least partially through enhancing the activities of POD and CAT to scavenge ROS.

Functional Analysis of Polyprenyl Diphosphate Synthase Genes Involved in Plastoquinone and Ubiquinone Biosynthesis in Salvia miltiorrhiza

Polyprenyl diphosphate synthase (PPS) plays important roles in the biosynthesis of functionally important plastoquinone (PQ) and ubiquinone (UQ). However, only few plant PPS genes have been functionally characterized. Through genome-wide analysis, two PPS genes, termed SmPPS1 and SmPPS2, were identified from Salvia miltiorrhiza, an economically significant Traditional Chinese Medicine material and an emerging model medicinal plant. SmPPS1 and SmPPS2 belonged to different phylogenetic subgroups of plant trans-long-chain prenyltransferases and exhibited differential tissue expression and light-induced expression patterns. Computational prediction and transient expression assays showed that SmPPS1 was localized in the chloroplasts, whereas SmPPS2 was mainly localized in the mitochondria. SmPPS2, but not SmPPS1, could functionally complement the coq1 mutation in yeast cells and catalyzed the production of UQ-9 and UQ-10. Consistently, both UQ-9 and UQ-10 were detected in S. miltiorrhiza plants. Overexpression of SmPPS2 caused significant UQ accumulation in S. miltiorrhiza transgenics, whereas down-regulation resulted in decreased UQ content. Differently, SmPPS1 overexpression significantly elevated PQ-9 content in S. miltiorrhiza. Transgenic lines showing a down-regulation of SmPPS1 expression exhibited decreased PQ-9 level, abnormal chloroplast and trichome development, and varied leaf bleaching phenotypes. These results suggest that SmPPS1 is involved in PQ-9 biosynthesis, whereas SmPPS2 is involved in UQ-9 and UQ-10 biosynthesis.

Gene network modules associated with abiotic stress response in tolerant rice genotypes identified by transcriptome meta-analysis

Abiotic stress tolerance is a complex trait regulated by multiple genes and gene networks in plants. A range of abiotic stresses are known to limit rice productivity. Meta-transcriptomics has emerged as a powerful approach to decipher stress-associated molecular network in model crops. However, retaining specificity of gene expression in tolerant and susceptible genotypes during meta-transcriptome analysis is important for understanding genotype-dependent stress tolerance mechanisms. Addressing this aspect, we describe here "abiotic stress tolerant" (ASTR) genes and networks specifically and differentially expressing in tolerant rice genotypes in response to different abiotic stress conditions. We identified 6,956 ASTR genes, key hub regulatory genes, transcription factors, and functional modules having significant association with abiotic stress-related ontologies and cis-motifs. Out of the 6956 ASTR genes, 73 were co-located within the boundary of previously identified abiotic stress trait-related quantitative trait loci. Functional annotation of 14 uncharacterized ASTR genes is proposed using multiple computational methods. Around 65% of the top ASTR genes were found to be differentially expressed in at least one of the tolerant genotypes under different stress conditions (cold, salt, drought, or heat) from publicly available RNAseq data comparison. The candidate ASTR genes specifically associated with tolerance could be utilized for engineering rice and possibly other crops for broad-spectrum tolerance to abiotic stresses.

Phytophthora infestans Dihydroorotate Dehydrogenase Is a Potential Target for Chemical Control - A Comparison With the Enzyme From Solanum tuberosum

The oomycete Phytophthora infestans is the causal agent of tomato and potato late blight, a disease that causes tremendous economic losses in the production of solanaceous crops. The similarities between oomycetes and the apicomplexa led us to hypothesize that dihydroorotate dehydrogenase (DHODH), the enzyme catalyzing the fourth step in pyrimidine biosynthetic pathway, and a validated drug target in treatment of malaria, could be a potential target for controlling P. infestans growth. In eukaryotes, class 2 DHODHs are mitochondrially associated ubiquinone-linked enzymes that catalyze the fourth, and only redox step of de novo pyrimidine biosynthesis. We characterized the enzymes from both the pathogen and a host, Solanum tuberosum. Plant DHODHs are known to be class 2 enzymes. Sequence analysis suggested that the pathogen enzyme (PiDHODHs) also belongs to this class. We confirmed the mitochondrial localization of GFP-PiDHODH showing colocalization with mCherry-labeled ATPase in a transgenic pathogen. N-terminally truncated versions of the two DHODHs were overproduced in E. coli, purified, and kinetically characterized. StDHODH exhibited a apparent specific activity of 41 ± 1 μmol min^-1 mg^-1, a k_cat^app of 30 ± 1 s^-1, and a K_m^app of 20 ± 1 μM for L-dihydroorotate, and a K_m^app= 30 ± 3 μM for decylubiquinone (Qd). PiDHODH exhibited an apparent specific activity of 104 ± 1 μmol min^-1 mg^-1, a k_cat^app of 75 ± 1 s^-1, and a K_m^app of 57 ± 3 μM for L-dihydroorotate, and a K_m^app of 15 ± 1 μM for Qd. The two enzymes exhibited different activities with different quinones and napthoquinone derivatives, and different sensitivities to compounds known to cause inhibition of DHODHs from other organisms. The IC₅₀ for A77 1726, a nanomolar inhibitor of human DHODH, was 2.9 ± 0.6 mM for StDHODH, and 79 ± 1 μM for PiDHODH. In vivo, 0.5 mM A77 1726 decreased mycelial growth by approximately 50%, after 92 h. Collectively, our findings suggest that the PiDHODH could be a target for selective inhibitors and we provide a biochemical background for the development of compounds that could be helpful for the control of the pathogen, opening the way to protein crystallization.

'Hidden' Terpenoids in Plants: Their Biosynthesis, Localization and Ecological Roles

Terpenoids are the largest group of plant specialized (secondary) metabolites. These naturally occurring chemical compounds are highly diverse in chemical structure. Although there have been many excellent studies of terpenoids, most have focused on compounds built solely of isoprene units. Plants, however, also contain many 'atypical' terpenoids, such as glycosylated volatile terpenes and composite-type terpenoids, the latter of which are synthesized by the coupling of isoprene units on aromatic compounds. This mini review describes these 'hidden' terpenoids, providing an overview of their biosynthesis, localization, and biological and ecological activities.

Methods for Structural and Functional Analyses of Intramembrane Prenyltransferases in the UbiA Superfamily

The UbiA superfamily is a group of intramembrane prenyltransferases that generate lipophilic compounds essential in biological membranes. These compounds, which include various quinones, hemes, chlorophylls, and vitamin E, participate in electron transport and function as antioxidants, as well as acting as structural lipids of microbial cell walls and membranes. Prenyltransferases producing these compounds are involved in important physiological processes and human diseases. These UbiA superfamily members differ significantly in their enzymatic activities and substrate selectivities. This chapter describes examples of methods that can be used to group these intramembrane enzymes, analyze their activity, and screen and crystallize homolog proteins for structure determination. Recent structures of two archaeal homologs are compared with structures of soluble prenyltransferases to show distinct mechanisms used by the UbiA superfamily to control enzymatic activity in membranes.

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.

Coenzyme Q10 production in plants: current status and future prospects

Coenzyme Q10 (CoQ10) or Ubiquinone10 (UQ10), an isoprenylated benzoquinone, is well-known for its role as an electron carrier in aerobic respiration. It is a sole representative of lipid soluble antioxidant that is synthesized in our body. In recent years, it has been found to be associated with a range of patho-physiological conditions and its oral administration has also reported to be of therapeutic value in a wide spectrum of chronic diseases. Additionally, as an antioxidant, it has been widely used as an ingredient in dietary supplements, neutraceuticals, and functional foods as well as in anti-aging creams. Since its limited dietary uptake and decrease in its endogenous synthesis in the body with age and under various diseases states warrants its adequate supply from an external source. To meet its growing demand for pharmaceutical, cosmetic and food industries, there is a great interest in the commercial production of CoQ10. Various synthetic and fermentation of microbial natural producers and their mutated strains have been developed for its commercial production. Although, microbial production is the major industrial source of CoQ10 but due to low yield and high production cost, other cost-effective and alternative sources need to be explored. Plants, being photosynthetic, producing high biomass and the engineering of pathways for producing CoQ10 directly in food crops will eliminate the additional step for purification and thus could be used as an ideal and cost-effective alternative to chemical synthesis and microbial production of CoQ10. A better understanding of CoQ10 biosynthetic enzymes and their regulation in model systems like E. coli and yeast has led to the use of metabolic engineering to enhance CoQ10 production not only in microbes but also in plants. The plant-based CoQ10 production has emerged as a cost-effective and environment-friendly approach capable of supplying CoQ10 in ample amounts. The current strategies, progress and constraints of CoQ10 production in plants are discussed in this review.

Metabolic engineering for the production of prenylated polyphenols in transgenic legume plants using bacterial and plant prenyltransferases

Prenylated polyphenols are secondary metabolites beneficial for human health because of their various biological activities. Metabolic engineering was performed using Streptomyces and Sophora flavescens prenyltransferase genes to produce prenylated polyphenols in transgenic legume plants. Three Streptomyces genes, NphB, SCO7190, and NovQ, whose gene products have broad substrate specificity, were overexpressed in a model legume, Lotus japonicus, in the cytosol, plastids or mitochondria with modification to induce the protein localization. Two plant genes, N8DT and G6DT, from Sophora flavescens whose gene products show narrow substrate specificity were also overexpressed in Lotus japonicus. Prenylated polyphenols were undetectable in these plants; however, supplementation of a flavonoid substrate resulted in the production of prenylated polyphenols such as 7-O-geranylgenistein, 6-dimethylallylnaringenin, 6-dimethylallylgenistein, 8-dimethylallynaringenin, and 6-dimethylallylgenistein in transgenic plants. Although transformants with the native NovQ did not produce prenylated polyphenols, modification of its codon usage led to the production of 6-dimethylallylnaringenin and 6-dimethylallylgenistein in transformants following naringenin supplementation. Prenylated polyphenols were not produced in mitochondrial-targeted transformants even under substrate feeding. SCO7190 was also expressed in soybean, and dimethylallylapigenin and dimethylallyldaidzein were produced by supplementing naringenin. This study demonstrated the potential for the production of novel prenylated polyphenols in transgenic plants. In particular, the enzymatic properties of prenyltransferases seemed to be altered in transgenic plants in a host species-dependent manner.

Analysis of CoQ10 in cultivated tobacco by a high-performance liquid chromatography-ultraviolet method

Coenzyme Q (CoQ) is a naturally occurring lipid-soluble quinone that performs multiple functions in all living cells and has become a popular antioxidant supplement, a coadjuvant in the treatment of heart disease, and the object of study for treating neurodegenerative disorders. Although there are many tools for CoQ analysis of microbial and animal samples, there have been relatively few reports of methods for CoQ analysis of green plants. This work describes a method for the routine analysis of coenzyme Q(10) in green leaf tissue of cultivated Nicotiana tabacum (tobacco) using high-performance liquid chromatography (HPLC) with UV detection. The method was applied to the analysis of CoQ(10) in N. tabacum 'KY14' leaves at different stalk positions representing young lanceolate to senescing leaves, and it was found that CoQ(10) increased as leaf position changed down the stalk from 18.69 to 82.68 μg/g fw. The method was also used to observe CoQ(10) in N. tabacum 'NC55' and N. tabacum 'TN90LC' leaves over time, finding that CoQ(10) leaf content remained relatively stable from 3 to 6 weeks but increased in both cultivars at 8 weeks. This method will likely be useful in the analysis of CoQ(10) in the green leaves of other plant species.

A topological map of the compartmentalized Arabidopsis thaliana leaf metabolome

BACKGROUND

The extensive subcellular compartmentalization of metabolites and metabolism in eukaryotic cells is widely acknowledged and represents a key factor of metabolic activity and functionality. In striking contrast, the knowledge of actual compartmental distribution of metabolites from experimental studies is surprisingly low. However, a precise knowledge of, possibly all, metabolites and their subcellular distributions remains a key prerequisite for the understanding of any cellular function.

METHODOLOGY/PRINCIPAL FINDINGS

Here we describe results for the subcellular distribution of 1,117 polar and 2,804 lipophilic mass spectrometric features associated to known and unknown compounds from leaves of the model plant Arabidopsis thaliana. Using an optimized non-aqueous fractionation protocol in conjunction with GC/MS- and LC/MS-based metabolite profiling, 81.5% of the metabolic data could be associated to one of three subcellular compartments: the cytosol (including the mitochondria), vacuole, or plastids. Statistical analysis using a marker-'free' approach revealed that 18.5% of these metabolites show intermediate distributions, which can either be explained by transport processes or by additional subcellular compartments.

CONCLUSION/SIGNIFICANCE

Next to a functional and conceptual workflow for the efficient, highly resolved metabolite analysis of the fractionated Arabidopsis thaliana leaf metabolome, a detailed survey of the subcellular distribution of several compounds, in the graphical format of a topological map, is provided. This complex data set therefore does not only contain a rich repository of metabolic information, but due to thorough validation and testing by statistical methods, represents an initial step in the analysis of metabolite dynamics and fluxes within and between subcellular compartments.

Two solanesyl diphosphate synthases with different subcellular localizations and their respective physiological roles in Oryza sativa

Long chain prenyl diphosphates are crucial biosynthetic precursors of ubiquinone (UQ) in many organisms, ranging from bacteria to humans, as well as precursors of plastoquinone in photosynthetic organisms. The cloning and characterization of two solanesyl diphosphate synthase genes, OsSPS1 and OsSPS2, in Oryza sativa is reported here. OsSPS1 was highly expressed in root tissue whereas OsSPS2 was found to be high in both leaves and roots. Enzymatic characterization using recombinant proteins showed that both OsSPS1 and OsSPS2 could produce solanesyl diphosphates as their final product, while OsSPS1 showed stronger activity than OsSPS2. However, an important biological difference was observed between the two genes: OsSPS1 complemented the yeast coq1 disruptant, which does not form UQ, whereas OsSPS2 only very weakly complemented the growth defect of the coq1 mutant. HPLC analyses showed that both OsSPS1 and OsSPS2 yeast transformants produced UQ9 instead of UQ6, which is the native yeast UQ. According to the complementation study, the UQ9 levels in OsSPS2 transformants were much lower than that of OsSPS1. Green fluorescent protein fusion analyses showed that OsSPS1 localized to mitochondria, while OsSPS2 localized to plastids. This suggests that OsSPS1 is involved in the supply of solanesyl diphosphate for ubiquinone-9 biosynthesis in mitochondria, whereas OsSPS2 is involved in providing solanesyl diphosphate for plastoquinone-9 formation. These findings indicate that O. sativa has a different mechanism for the supply of isoprenoid precursors in UQ biosynthesis from Arabidopsis thaliana, in which SPS1 provides a prenyl moiety for UQ9 at the endoplasmic reticulum.

Think outside the box: selenium volatilization altered by a broccoli gene in the ubiquinone biosynthetic pathway

Selenium metabolism has been an area of active research because of the essentiality as well as toxicity of selenium to animals and humans. Biologically based selenium volatilization has been a particular area of interest for its potential in making detoxification of selenium pollution highly effective. Recently, we have isolated a broccoli BoCOQ5-2 methyltransferase gene involved in the ubiquinone biosynthetic pathway and found that it promoted selenium volatilization in both bacteria and plants. The identification of BoCOQ5-2 methyltransferase as a facilitator of selenium volatilization showed that selenium metabolism is regulated by other metabolic processes outside of the selenium/sulfur metabolic pathway. The interplay between ubiquinone and selenium metabolisms is possible through the protective function of ubiquinone against oxidative stresses induced by selenium. This observation could lead to new approaches to enhance selenium phytoremediation.

Monoterpene engineering in a woody plant Eucalyptus camaldulensis using a limonene synthase cDNA

Metabolic engineering aimed at monoterpene production has become an intensive research topic in recent years, although most studies have been limited to herbal plants including model plants such as Arabidopsis. The genus Eucalyptus includes commercially important woody plants in terms of essential oil production and the pulp industry. This study attempted to modify the production of monoterpenes, which are major components of Eucalyptus essential oil, by introducing two expression constructs containing Perilla frutescens limonene synthase (PFLS) cDNA, whose gene products were designed to be localized in either the plastid or cytosol, into Eucalyptus camaldulensis. The expression of the plastid-type and cytosol-type PFLS cDNA in transgenic E. camaldulensis was confirmed by real-time polymerase chain reaction (PCR). Gas chromatography with a flame ionization detector analyses of leaf extracts revealed that the plastidic and cytosolic expression of PFLS yielded 2.6- and 4.5-times more limonene than that accumulated in wild-type E. camaldulensis, respectively, while the ectopic expression of PFLS had only a small effect on the emission of limonene from the leaves of E. camaldulensis. Surprisingly, the high level of PFLS in Eucalyptus was accompanied by a synergistic increase in the production of 1,8-cineole and alpha-pinene, two major components of Eucalyptus monoterpenes. This genetic engineering of monoterpenes demonstrated a new potential for molecular breeding in woody plants.

Involvement of a broccoli COQ5 methyltransferase in the production of volatile selenium compounds

Selenium (Se) is an essential micronutrient for animals and humans but becomes toxic at high dosage. Biologically based Se volatilization, which converts Se into volatile compounds, provides an important means for cleanup of Se-polluted environments. To identify novel genes whose products are involved in Se volatilization from plants, a broccoli (Brassica oleracea var italica) cDNA encoding COQ5 methyltransferase (BoCOQ5-2) in the ubiquinone biosynthetic pathway was isolated. Its function was authenticated by complementing a yeast coq5 mutant and by detecting increased cellular ubiquinone levels in the BoCOQ5-2-transformed bacteria. BoCOQ5-2 was found to promote Se volatilization in both bacteria and transgenic Arabidopsis (Arabidopsis thaliana) plants. Bacteria expressing BoCOQ5-2 produced an over 160-fold increase in volatile Se compounds when they were exposed to selenate. Consequently, the BoCOQ5-2-transformed bacteria had dramatically enhanced tolerance to selenate and a reduced level of Se accumulation. Transgenic Arabidopsis expressing BoCOQ5-2 volatilized three times more Se than the vector-only control plants when treated with selenite and exhibited an increased tolerance to Se. In addition, the BoCOQ5-2 transgenic plants suppressed the generation of reactive oxygen species induced by selenite. BoCOQ5-2 represents, to our knowledge, the first plant enzyme that is not known to be directly involved in sulfur/Se metabolism yet was found to mediate Se volatilization. This discovery opens up new prospects regarding our understanding of the complete metabolism of Se and may lead to ways to modify Se-accumulator plants with increased efficiency for phytoremediation of Se-contaminated environments.

Functional characterization of LePGT1, a membrane-bound prenyltransferase involved in the geranylation of p-hydroxybenzoic acid

The AS-PT (aromatic substrate prenyltransferase) family plays a critical role in the biosynthesis of important quinone compounds such as ubiquinone and plastoquinone, although biochemical characterizations of AS-PTs have rarely been carried out because most members are membrane-bound enzymes with multiple transmembrane alpha-helices. PPTs [PHB (p-hydroxybenzoic acid) prenyltransferases] are a large subfamily of AS-PTs involved in ubiquinone and naphthoquinone biosynthesis. LePGT1 [Lithospermum erythrorhizon PHB geranyltransferase] is the regulatory enzyme for the biosynthesis of shikonin, a naphthoquinone pigment, and was utilized in the present study as a representative of membrane-type AS-PTs to clarify the function of this enzyme family at the molecular level. Site-directed mutagenesis of LePGT1 with a yeast expression system indicated three out of six conserved aspartate residues to be critical to the enzymatic activity. A detailed kinetic analysis of mutant enzymes revealed the amino acid residues responsible for substrate binding were also identified. Contrary to ubiquinone biosynthetic PPTs, such as UBIA in Escherichia coli which accepts many prenyl substrates of different chain lengths, LePGT1 can utilize only geranyl diphosphate as its prenyl substrate. Thus the substrate specificity was analysed using chimeric enzymes derived from LePGT1 and UBIA. In vitro and in vivo analyses of the chimeras suggested that the determinant region for this specificity was within 130 amino acids of the N-terminal. A 3D (three-dimensional) molecular model of the substrate-binding site consistent with these biochemical findings was generated.

Plants utilize isoprene emission as a thermotolerance mechanism

Isoprene is a volatile compound emitted from leaves of many plant species in large quantities, which has an impact on atmospheric chemistry due to its massive global emission rate (5 x 10(14) carbon g year(-1)) and its high reactivity with the OH radical, resulting in an increase in the half-life of methane. Isoprene emission is strongly induced by the increase in isoprene synthase activity in plastids at high temperature in the day time, which is regulated at its gene expression level in leaves, while the physiological meaning of isoprene emission for plants has not been clearly demonstrated. In this study, we have functionally overexpressed Populus alba isoprene synthase in Arabidopsis to observe isoprene emission from transgenic plants. A striking difference was observed when both transgenic and wild-type plants were treated with heat at 60 degrees C for 2.5 h, i.e. transformants revealed clear heat tolerance compared with the wild type. High isoprene emission and a decrease in the leaf surface temperature were observed in transgenic plants under heat stress treatment. In contrast, neither strong light nor drought treatments showed an apparent difference. These data suggest that isoprene emission plays a crucial role in a heat protection mechanism in plants.

Phenotypes and fed-batch fermentation of ubiquinone-overproducing fission yeast using ppt1 gene

Ubiquinone (UQ), a component of the electron transfer system in many organisms, has been widely used for pharmaceuticals and cosmetics. In this study, we cloned and overexpressed the full-length ppt1 (MTppt1) gene, which encodes p-hydroxybenzoate:polyprenyltransferase and ERppt1 gene, which was modified to be localized on endoplasmic reticulum in fission yeast. The yeast MTppt1 and ERppt1 transgenic lines showed about 3.7 and 5.1 times increment in UQ content and the recombinant yeasts with a higher UQ level are more resistant to H(2)O(2), Cu(2+) and NaCl, and interestingly their growth was also faster than the wild type at lower temperature. For large-scale cultivation, the direct feedback control of glucose using an on-line ethanol concentration monitor for ubiquinone production of yeast ERppt1 by high-cell-density fermentation was investigated and the fermentation parameters (e.g., dissolved oxygen, pH, ethanol concentration, oxygen uptake rate, carbon dioxide evolution rate and respiration quotient) were also discussed. After 90 h cultures, the yeast dry cell weight reached 57 gl(-1) and the ubiquinone yield reached 23 mgl(-1). In addition, plasmid stability was maintained at high level throughout the fermentation.

Heterologous expression of a mammalian ABC transporter in plant and its application to phytoremediation

Mammalian ATP-binding cassette (ABC) transporters involved in the multidrug-resistance of cancer cells can efflux cytotoxic compounds that show a wide variety of chemical structures and biological activities. Human multidrug resistance-associated protein (hMRP1) is one of the most intensively studied ABC transporters and many substrates have been identified, including both organic and inorganic compounds. In an attempt at novel 'transport engineering' using hMRP1 as a molecular pump, we established transgenic tobacco plants that showed clear resistance to cadmium and daunorubicin, although they were not resistant to etoposide, another known substrate of hMRP1. When expressed in tobacco cells, hMRP1 protein was localized at vacuolar membrane, while members of the MRP family are localized at plasma membrane in mammalian cells to reduce the cellular accumulation of various drugs. Thus, the hMRP1-expressing tobacco cells were able to take up these substrates across the tonoplast and sequestrate them in the vacuolar matrix. These results suggest that it may be possible to use the transgenic tobacco in phytoremediation, where a single transformation with an ABC transporter with broad substrate specificity should be effective for extracting various environmental pollutants including both organic and inorganic compounds, and accumulate them in the plant body. This should be advantageous for the remediation of a complex polluted environment, which is commonly found in the real world.

Functional Characterization of OsPPT1, Which Encodes p-Hydroxybenzoate Polyprenyltransferase Involved in Ubiquinone Biosynthesis in Oryza sativa

Prenylation of the aromatic intermediate p-hydroxybenzoate (PHB) is a critical step in ubiquinone (UQ) biosynthesis. The enzyme that catalyzes this prenylation reaction is p-hydroxybenzoate polyprenyltransferase (PPT), which substitutes an aromatic proton at the m-position of PHB with a prenyl chain provided by polyprenyl diphosphate synthase. The rice genome contains three PPT candidates that share significant similarity with the yeast PPT (COQ2 gene), and the rice gene showing the highest similarity to COQ2 was isolated by reverse transcription-PCR and designated OsPPT1a. The deduced amino acid sequence of OsPPT1a contained a putative mitochondrial sorting signal at the N-terminus and conserved domains for putative substrate-binding sites typical of PPT protein family members. The subcellular localization of OsPPT1a protein was shown to be mainly in mitochondria based on studies using a green fluorescent protein-PPT fusion. A yeast complementation study revealed that OsPPT1a expression successfully recovered the growth defect of the coq2 mutant. A prenyltransferase assay using recombinant protein showed that OsPPT1a accepted prenyl diphosphates of various chain lengths as prenyl donors, whereas it showed strict substrate specificity for the aromatic substrate PHB as a prenyl acceptor. The apparent K (m) values for geranyl diphosphate and PHB were 59.7 and 6.04 microM, respectively. The requirement by OsPPT1a and COQ2 for divalent cations was also studied, with Mg2+ found to produce the highest enzyme activity. Northern analysis showed that OsPPT1a mRNA was accumulated in all tissues of O. sativa. These results suggest that OsPPT1a is a functional PPT involved in UQ biosynthesis in O. sativa.

Metabolic engineering of coenzyme Q by modification of isoprenoid side chain in plant

Coenzyme Q (CoQ), an electron transfer molecule in the respiratory chain and a lipid-soluble antioxidant, is present in almost all organisms. Most cereal crops produce CoQ9, which has nine isoprene units. CoQ10, with 10 isoprene units, is a very popular food supplement. Here, we report the genetic engineering of rice to produce CoQ10 using the gene for decaprenyl diphosphate synthase (DdsA). The production of CoQ9 was almost completely replaced with that of CoQ10, despite the presence of endogenous CoQ9 synthesis. DdsA designed to express at the mitochondria increased accumulation of total CoQ amount in seeds.

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.

COQ9, a new gene required for the biosynthesis of coenzyme Q in Saccharomyces cerevisiae

Currently, eight genes are known to be involved in coenzyme Q6 biosynthesis in Saccharomyces cerevisiae. Here, we report a new gene designated COQ9 that is also required for the biosynthesis of this lipoid quinone. The respiratory-deficient pet mutant C92 was found to be deficient in coenzyme Q and to have low mitochondrial NADH-cytochrome c reductase activity, which could be restored by addition of coenzyme Q2. The mutant was used to clone COQ9, corresponding to reading frame YLR201c on chromosome XII. The respiratory defect of C92 is complemented by COQ9 and suppressed by COQ8/ABC1. The latter gene has been shown to be required for coenzyme Q biosynthesis in yeast and bacteria. Suppression by COQ8/ABC1 of C92, but not other coq9 mutants tested, has been related to an increase in the mitochondrial concentration of several enzymes of the pathway. Coq9p may either catalyze a reaction in the coenzyme Q biosynthetic pathway or have a regulatory role similar to that proposed for Coq8p.

Transporters of secondary metabolites

The membrane transport of plant secondary metabolites is a newly developing research area. Recent progress in genome and expressed sequence tag (EST) databases has revealed that many transporters and channels exist in plant genome. Studies of the genetic sequences that encode these proteins, and of phenotypes caused by the mutation of these sequences, have been used to characterize the membrane transport of plant secondary metabolites. Such studies have clarified that membrane transport is fairly specific and highly regulated for each secondary metabolite. Not only genes that are involved in the biosynthesis of secondary metabolites but also genes that are involved in their transport will be important for systematic metabolic engineering aimed at increasing the productivity of valuable secondary metabolites in planta.