Circadian regulation of the PhCCD1 carotenoid cleavage dioxygenase controls emission of beta-ionone, a fragrance volatile of petunia flowers

Carotenoid biosynthesis genes in rice: structural analysis, genome-wide expression profiling and phylogenetic analysis

Carotenoids, important lipid-soluble antioxidants in photosynthetic tissues, are known to be completely absent in rice endosperm. Many studies, involving transgenic manipulations of carotenoid biosynthesis genes, have been performed to get carotenoid-enriched rice grain. Study of genes involved in their biosynthesis can provide further information regarding the abundance/absence of carotenoids in different tissues. We have identified 16 and 34 carotenoid biosynthesis genes in rice and Populus genomes, respectively. A detailed analysis of the domain structure of carotenoid biosynthesis enzymes in rice, Populus and Arabidopsis has shown that highly conserved catalytic domains, along with other domains, are present in these proteins. Phylogenetic analysis of rice genes with Arabidopsis and other characterized carotenoid biosynthesis genes has revealed that homologous genes exist in these plants, and the duplicated gene copies probably adopt new functions. Expression of rice and Populus genes has been analyzed by full-length cDNA- and EST-based expression profiling. In rice, this analysis was complemented by real-time PCR, microarray and signature-based expression profiling, which reveal that carotenoid biosynthesis genes are highly expressed in light-grown tissues, have differential expression pattern during vegetative/reproductive development and are responsive to stress.

Cloning and functional characterization of carotenoid cleavage dioxygenase 4 genes

Although a number of plant carotenoid cleavage dioxygenase (CCD) genes have been functionally characterized in different plant species, little is known about the biochemical role and enzymatic activities of members of the subclass 4 (CCD4). To gain insight into their biological function, CCD4 genes were isolated from apple (Malus x domestica, MdCCD4), chrysanthemum (Chrysanthemum x morifolium, CmCCD4a), rose (Rosa x damascena, RdCCD4), and osmanthus (Osmanthus fragrans, OfCCD4), and were expressed, together with AtCCD4, in Escherichia coli. In vivo assays showed that CmCCD4a and MdCCD4 cleaved beta-carotene well to yield beta-ionone, while OfCCD4, RdCCD4, and AtCCD4 were almost inactive towards this substrate. No cleavage products were found for any of the five CCD4 genes when they were co-expressed in E. coli strains that accumulated cis-zeta-carotene and lycopene. In vitro assays, however, demonstrated the breakdown of 8'-apo-beta-caroten-8'-al by AtCCD4 and RdCCD4 to beta-ionone, while this apocarotenal was almost not degraded by OfCCD4, CmCCD4a, and MdCCD4. Sequence analysis of genomic clones of CCD4 genes revealed that RdCCD4, like AtCCD4, contains no intron, while MdCCD, OfCCD4, and CmCCD4a contain introns. These results indicate that plants produce at least two different forms of CCD4 proteins. Although CCD4 enzymes cleave their substrates at the same position (9,10 and 9',10'), they might have different biochemical functions as they accept different (apo)-carotenoid substrates, show various expression patterns, and are genomically differently organized.

RNA interference-mediated repression of MtCCD1 in mycorrhizal roots of Medicago truncatula causes accumulation of C27 apocarotenoids, shedding light on the functional role of CCD1

Tailoring carotenoids by plant carotenoid cleavage dioxygenases (CCDs) generates various bioactive apocarotenoids. Recombinant CCD1 has been shown to catalyze symmetrical cleavage of C(40) carotenoid substrates at 9,10 and 9',10' positions. The actual substrate(s) of the enzyme in planta, however, is still unknown. In this study, we have carried out RNA interference (RNAi)-mediated repression of a Medicago truncatula CCD1 gene in hairy roots colonized by the arbuscular mycorrhizal (AM) fungus Glomus intraradices. As a consequence, the normal AM-mediated accumulation of apocarotenoids (C(13) cyclohexenone and C(14) mycorradicin derivatives) was differentially modified. Mycorradicin derivatives were strongly reduced to 3% to 6% of the controls, while the cyclohexenone derivatives were only reduced to 30% to 47%. Concomitantly, a yellow-orange color appeared in RNAi roots. Based on ultraviolet light spectra and mass spectrometry analyses, the new compounds are C(27) apocarotenoic acid derivatives. These metabolic alterations did not lead to major changes in molecular markers of the AM symbiosis, although a moderate shift to more degenerating arbuscules was observed in RNAi roots. The unexpected outcome of the RNAi approach suggests C(27) apocarotenoids as the major substrates of CCD1 in mycorrhizal root cells. Moreover, literature data implicate C(27) apocarotenoid cleavage as the general functional role of CCD1 in planta. A revised scheme of plant carotenoid cleavage in two consecutive steps is proposed, in which CCD1 catalyzes only the second step in the cytosol (C(27)-->C(14)+C(13)), while the first step (C(40)-->C(27)+C(13)) may be catalyzed by CCD7 and/or CCD4 inside plastids.

Microbial Production of C13-Norisoprenoids and Other Aroma Compounds via Carotenoid Cleavage

Carotenoids are important precursors of a variety of compounds: the C(20)-retinoids, the C(15)-phytohormones, and the C(9)- to C(13)-aromas. Among the last type, C(13)-carotenoid-derived compounds (norterpenoids/norisoprenoids) such as ionones and damascones, constitute an essential aroma note in tea, grapes, roses, tobacco, and wine. Extraction of carotenoid-derived aroma compounds from plant sources is not economically realistic or considerably expensive. The biotechnological production of aroma compounds represents a feasible alternative and offers the production of enantiomerically pure molecules which can be labeled as "natural." To date, research in the production of ionones or the C(10)-compound, safranal, has mainly been focused on plant dioxygenases that cleave carotenoids in the positions between carbons 9 and 10 (9'-10') or 7 and 8 (7'-8'), respectively. Although relatively little is known about the microbial conversion of carotenoids into compounds with aroma due to the well known advantages of manipulating microorganisms, the aim of this work is to review the current state of the research in microbial production of norisoprenoids and other aroma compounds derived from carotenoid cleavage.

Functional characterization of FaCCD1: a carotenoid cleavage dioxygenase from strawberry involved in lutein degradation during fruit ripening

A gene encoding a carotenoid cleavage dioxygenase class 1 enzyme (FaCCD1) was identified among a strawberry fruit expressed sequence tag collection. The full-length cDNA was isolated, and the expression profiles along fruit receptacle development and ripening, determined by quantitative real time polymerase chain reaction, showed that FaCCD1 is a ripening-related gene that reaches its maximal level of expression in the red fully ripe stage. FaCCD1 was expressed in Escherichia coli, and the products formed by the recombinant protein through oxidative cleavage of carotenoids were identified by liquid chromatography-mass spectrometry and gas chromatography-mass spectrometry analyses. The FaCCD1 protein cleaves zeaxanthin, lutein, and beta-apo-8'-carotenal in vitro. Although beta-carotene is not a good substrate for FaCCD1 in vitro, the expression of FaCCD1 in an engineered carotenoid-producing E. coli strain caused the degradation of beta-carotene in vivo. Additionally, the carotenoid profile in strawberry was analyzed by high-performance liquid chromatography-photodiode detection, and a correlation between the increase of the expression level of FaCCD1 during ripening and the decrease of the lutein content suggests that lutein could constitute the main natural substrate of FaCCD1 activity in vivo.

Cloning and characterisation of a maize carotenoid cleavage dioxygenase (ZmCCD1) and its involvement in the biosynthesis of apocarotenoids with various roles in mutualistic and parasitic interactions

Colonisation of maize roots by arbuscular mycorrhizal (AM) fungi leads to the accumulation of apocarotenoids (cyclohexenone and mycorradicin derivatives). Other root apocarotenoids (strigolactones) are involved in signalling during early steps of the AM symbiosis but also in stimulation of germination of parasitic plant seeds. Both apocarotenoid classes are predicted to originate from cleavage of a carotenoid substrate by a carotenoid cleavage dioxygenase (CCD), but the precursors and cleavage enzymes are unknown. A Zea mays CCD (ZmCCD1) was cloned by RT-PCR and characterised by expression in carotenoid accumulating E. coli strains and analysis of cleavage products using GC-MS. ZmCCD1 efficiently cleaves carotenoids at the 9, 10 position and displays 78% amino acid identity to Arabidopsis thaliana CCD1 having similar properties. ZmCCD1 transcript levels were shown to be elevated upon root colonisation by AM fungi. Mycorrhization led to a decrease in seed germination of the parasitic plant Striga hermonthica as examined in a bioassay. ZmCCD1 is proposed to be involved in cyclohexenone and mycorradicin formation in mycorrhizal maize roots but not in strigolactone formation.

Cytosolic and plastoglobule-targeted carotenoid dioxygenases from Crocus sativus are both involved in beta-ionone release

Saffron, the processed stigma of Crocus sativus, is characterized by the presence of several apocarotenoids that contribute to the color, flavor, and aroma of the spice. However, little is known about the synthesis of aroma compounds during the development of the C. sativus stigma. The developing stigma is nearly odorless, but before and at anthesis, the aromatic compound beta-ionone becomes the principal norisoprenoid volatile in the stigma. In this study, four carotenoid cleavage dioxygenase (CCD) genes, CsCCD1a, CsCCD1b, CsCCD4a, and CsCCD4b, were isolated from C. sativus. Expression analysis showed that CsCCD1a was constitutively expressed, CsCCD1b was unique to the stigma tissue, but only CsCCD4a and -b had expression patterns consistent with the highest levels of beta-carotene and emission of beta-ionone derived during the stigma development. The CsCCD4 enzymes were localized in plastids and more specifically were present in the plastoglobules. The enzymatic activities of CsCCD1a, CsCCD1b, and CsCCD4 enzymes were determined by Escherichia coli expression, and subsequent analysis of the volatile products was generated by GC/MS. The four CCDs fell in two phylogenetically divergent dioxygenase classes, but all could cleave beta-carotene at the 9,10(9',10') positions to yield beta-ionone. The data obtained suggest that all four C. sativus CCD enzymes may contribute in different ways to the production of beta-ionone. In addition, the location and precise timing of beta-ionone synthesis, together with its known activity as a fragrance and insect attractant, suggest that this volatile may have a role in Crocus pollination.

An investigation of carotenoid biosynthesis in Coffea canephora and Coffea arabica

Carotenoids are essential components of the photosynthetic apparatus in a wide range of organisms. They participate in the adaptation of plastids to changing environmental light conditions and prevent photo-oxidative damage of the photosynthetic apparatus by detoxifying reactive oxygen species. We identified eight cDNAs from the carotenoid biosynthetic pathway (PSY, PDS, ZDS, PTOX, LCY-E, CRTR-B, ZEP and VDE) and two cDNA encoding carotenoid cleavage dioxygenase family members (NCED3 and CCD1) in Coffea canephora. We also obtained cDNA encoding several different fibrillin proteins involved in carotenoid sequestration (FIB). Expression of the coffee carotenoid genes was determined in leaf, branch and flower tissues using quantitative RT-PCR. Expression analysis of these genes in leaf tissue from osmotically stressed plants was also carried out. These experiments showed that the transcript levels of PTOX, CRTR-B, NCED3, CCD1 and FIB1 increased under these stress conditions, while LCY-E decreased, indicating that the metabolic flux towards the xanthophyll cycle branch of the carotenoid biosynthetic pathway may be favoured in leaves under drought conditions. Functional analysis of CcCRTR-B using an in vivo method employing Escherichia coli strains engineered to make carotenoids confirmed that the beta-carotene hydroxylase activity of CcCRTR-B generates beta-cryptoxanthin and zeaxanthin from beta-carotene. A similar approach was also used to show that CcCCD1 encoded a functional 9,10(9'10') carotenoid cleavage dioxygenase, and thus that this enzyme is capable of forming one or more apocarotenoids in vivo. Finally, high-performance liquid chromatography analysis of coffee leaves revealed the presence of alpha-carotene and suggests that Coffea arabica may have higher levels of alpha-carotene than C. canephora.

Carotenoid metabolism: biosynthesis, regulation, and beyond

Carotenoids are indispensable to plants and play a critical role in human nutrition and health. Significant progress has been made in our understanding of carotenoid metabolism in plants. The biosynthetic pathway has been extensively studied. Nearly all the genes encoding the biosynthetic enzymes have been isolated and characterized from various organisms. In recent years, there is an increasing body of work on the signaling pathways and plastid development, which might provide global control of carotenoid biosynthesis and accumulation. Herein, we will highlight recent progress on the biosynthesis, regulation, and metabolic engineering of carotenoids in plants, as well as the future research towards elucidating the regulatory mechanisms and metabolic network that control carotenoid metabolism.

Identification of bacterial carotenoid cleavage dioxygenase homologues that cleave the interphenyl alpha,beta double bond of stilbene derivatives via a monooxygenase reaction

Carotenoid cleavage oxygenases (CCOs), which are also referred to as carotenoid cleavage dioxygenases (CCDs) are a new class of nonheme iron-type enzymes that oxidatively cleave double bonds in the conjugated carbon chain of carotenoids. The oxidative cleavage mechanism of these enzymes is not clear, and both monooxygenase and dioxygenase mechanisms have been proposed for different carotenoid cleavage enzymes. CCOs have been described from plants, animals, fungi, and cyanobacteria, but little is known about their distribution and activities in bacteria other than cyanobacteria. We surveyed bacterial genome sequences for CCO homologues and report the characterization of CCO homologues that were identified in Novosphingobium aromaticivorans DSM 12444 (NOV1 and NOV2) and in Bradyrhizobium sp. (BRA-J and BRA-S). In vitro and in vivo assays with carotenoid and stilbene compounds were used to investigate the cleavage activities of the recombinant enzymes. The NOV enzymes cleaved the interphenyl alpha-beta double bond of stilbenes that had an oxygen functional group at the 4' carbon atom (e.g., resveratrol, piceatannol, and rhaponticin) to the corresponding aldehyde products. Carotenoids and apocarotenoids were not substrates for these enzymes. The two homologous enzymes from Bradyrhizobium sp. did not possess carotenoid or stilbene cleavage oxygenase activities, but showed activity with farnesol. To investigate whether the oxidative cleavage of stilbenes proceeds via a monooxygenase or dioxygenase reaction, oxygen-labeling studies were conducted with NOV2. Our labeling studies show that the double-bond cleavage of stilbenes occurs via a monooxygenase reaction mechanism.

Mutagenesis in Petunia x hybrida Vilm. and isolation of a novel morphological mutant

Traditionally, mutagenesis has been used to introduce novel genetic variability in ornamental crops. More recently, it has become a powerful tool in gene discovery and functional analyses in reverse genetics approaches. The present work aimed to compare the efficiency of physical and chemical agents in generating mutant populations of petunia. We have indirectly evaluated the genomic damage by analyzing developmental characteristics of the plantlets derived from treated seeds employing gamma radiation at 0, 20, 40, 60, 80 and 100 Gy and the alkylating agent ethyl-methanesulfonate (EMS) at 0, 0.05, 0.1, 0.15, 0.2 and 0.25% (v/v). Gamma rays and EMS caused developmental defects and decreased seedling viability in plants obtained from the mutagenized seeds. High mutagen doses reduced in approximately 44% the number of plants with primary leaves at 15 days after sowing (DAS) and decreased seedling survival rates to 55% (gamma) and 28% (EMS), in comparison to untreated controls. Seedling height decrease was proportional to increasing EMS dosage, whereas 40 and 60 Gy of gamma irradiation caused the most significant reduction in height. Moderate DNA damage allowing a high saturation of mutant alleles in the genome and the generation of viable plants for reverse genetics studies was correlated to the biological parameter LD50, the dose required to kill half of the tested population. It corresponded to 100 Gy for gamma radiation and 0.1% for EMS treatment. The optimized mutagen treatments were used to develop petunia mutant populations (M1 and M2) and novel morphological mutants were identified.

In vitro characterization of a carotenoid cleavage dioxygenase from Nostoc sp. PCC 7120 reveals a novel cleavage pattern, cytosolic localization and induction by highlight

Carotenoid oxygenases catalyse the cleavage of C-C double bonds forming apocarotenoids, a diverse group of compounds, including retinoids and the precursors of some phytohormones. Some apocarotenoids, like beta-ionone (C(13)), are ecologically important volatiles released by plants and cyanobacteria. In this work, we elucidated the activity of the Nostoccarotenoid cleavage dioxygenase (NosCCD, previously named NSC1) using synthetic and cyanobacterial substrates. NosCCD converted bicyclic and monocyclic xanthophylls, including myxoxanthophylls, glycosylated carotenoids that are essential for thylakoid and cell wall structure. The products identified revealed two different cleavage patterns. The first is observed with bicyclic xanthophylls and is identical with that of plant orthologues, while the second is novel and occurs upon cleavage of monocyclic substrates at the C9-C10 and C7'-C8' double bonds. These properties enable the enzyme to produce a plenitude of different C(10) and C(13) apocarotenoids. Expression analyses indicated a role of NosCCD in response to highlight stress. Western blot analyses of Nostoc cells revealed NosCCD as a soluble enzyme in the cytosol, which also accomodates NosCCD substrates. Incubation of the corresponding fraction with synthetic substrates revealed the activity of the native enzyme and confirmed its induction by highlight.

The carotenoid cleavage dioxygenase 1 enzyme has broad substrate specificity, cleaving multiple carotenoids at two different bond positions

In many organisms, various enzymes mediate site-specific carotenoid cleavage to generate biologically active apocarotenoids. These carotenoid-derived products include provitamin A, hormones, and flavor and fragrance molecules. In plants, the CCD1 enzyme cleaves carotenoids at 9,10 (9',10') bonds to generate multiple apocarotenoid products. Here we systematically analyzed volatile apocarotenoids generated by maize CCD1 (ZmCCD1) from multiple carotenoid substrates. ZmCCD1 did not cleave geranylgeranyl diphosphate or phytoene but did cleave other linear and cyclic carotenoids, producing volatiles derived from 9,10 (9',10') bond cleavage. Additionally the Arabidopsis, maize, and tomato CCD1 enzymes all cleaved lycopene to generate 6-methyl-5-hepten-2-one. 6-Methyl-5-hepten-2-one, an important flavor volatile in tomato, was produced by cleavage of the 5,6 or 5',6' bond positions of lycopene but not geranylgeranyl diphosphate, zeta-carotene, or phytoene. In vitro, ZmCCD1 cleaved linear and cyclic carotenoids with equal efficiency. Based on the pattern of apocarotenoid volatiles produced, we propose that CCD1 recognizes its cleavage site based on the saturation status between carbons 7 and 8 (7' and 8') and carbons 11 and 12 (11' and 12') as well as the methyl groups on carbons 5, 9, and 13 (5', 9', and 13').

Reverse genetics of floral scent: application of tobacco rattle virus-based gene silencing in Petunia

Floral fragrance is responsible for attracting pollinators as well as repelling pathogens and pests. As such, it is of immense biological importance. Molecular dissection of the mechanisms underlying scent production would benefit from the use of model plant systems with big floral organs that generate an array of volatiles and that are amenable to methods of forward and reverse genetics. One candidate is petunia (Petunia hybrida), which has emerged as a convenient model system, and both RNAi and overexpression approaches using transgenes have been harnessed for the study of floral volatiles. Virus-induced gene silencing (VIGS) is characterized by a simple inoculation procedure and rapid results relative to transgenesis. Here, we demonstrate the applicability of the tobacco rattle virus-based VIGS system to studies of floral scent. Suppression of the anthocyanin pathway via chalcone synthase silencing was used as a reporter, allowing easy visual identification of anthocyaninless silenced flowers/tissues with no effect on the level of volatile emissions. Use of tobacco rattle virus constructs containing target genes involved in phenylpropanoid volatile production, fused to the chalcone synthase reporter, allowed simple identification of flowers with suppressed activity of the target genes. The applicability of VIGS was exemplified with genes encoding S-adenosyl-l-methionine:benzoic acid/salicylic acid carboxyl methyltransferase, phenylacetaldehyde synthase, and the myb transcription factor ODORANT1. Because this high-throughput reverse-genetics approach was applicable to both structural and regulatory genes responsible for volatile production, it is expected to be highly instrumental for large-scale scanning and functional characterization of novel scent genes.

Diurnal regulation of scent emission in rose flowers

Previous studies have shown diurnal oscillation of scent emission in rose flowers with a peak during the day (Helsper in Planta 207:88-95, 1998; Picone in Planta 219:468-478, 2004). Here, we studied the regulation of scent production and emission in Rosa hybrida cv. Fragrant Cloud during the daily cycle and focused on two terpenoid compounds, germacrene D and geranyl acetate, whose biosynthetic genes have been characterized by us previously. The emission of geranyl acetate oscillated during the daily light/dark cycle with a peak early in the light period. A similar daily fluctuation was found in the endogenous level of this compound and in the expression of its biosynthetic gene, alcohol acetyl transferase (RhAAT). The rhythmic expression of RhAAT continued under conditions of constant light or darkness, indicating regulation by the endogenous circadian clock. However, the accumulation and emission of geranyl acetate ceased under continuous light. Our results suggest that geranyl acetate production is limited by the level of its substrate geraniol, which is suppressed under constant light conditions. The emission of germacrene D also oscillated during the daily cycle with a peak early in the light period. However, the endogenous level of this compound and the expression of its biosynthetic gene germacrene D synthase (RhGDS) were constant throughout the day. The diurnal oscillation of germacrene D emission ceased under continuous light, suggesting direct regulation by light. Our results demonstrate the complexity of the diurnal regulation of scent emission: although the daily emission of most scent compounds is synchronized, various independently evolved mechanisms control the production, accumulation and release of different volatiles.

Retinal biosynthesis in fungi: characterization of the carotenoid oxygenase CarX from Fusarium fujikuroi

The car gene cluster of the ascomycete Fusarium fujikuroi encodes two enzymes responsible for torulene biosynthesis (CarRA and CarB), an opsin-like protein (CarO), and a putative carotenoid cleaving enzyme (CarX). It was presumed that CarX catalyzes the formation of the major carotenoid in F. fujikuroi, neurosporaxanthin, a cleavage product of torulene. However, targeted deletion of carX did not impede neurosporaxanthin biosynthesis. On the contrary, DeltacarX mutants showed a significant increase in the total carotenoid content, indicating an involvement of CarX in the regulation of the pathway. In this work, we investigated the enzymatic activity of CarX. The expression of the enzyme in beta-carotene-accumulating Escherichia coli cells led to the formation of the opsin chromophore retinal. The identity of the product was proven by high-performance liquid chromatography and gas chromatography-mass spectrometry. Subsequent in vitro assays with heterologously expressed and purified CarX confirmed its beta-carotene-cleaving activity and revealed its capability to produce retinal also from other substrates, such as gamma-carotene, torulene, and beta-apo-8'-carotenal. Our data indicate that the occurrence of at least one beta-ionone ring in the substrate is required for the cleavage reaction and that the cleavage site is determined by the distance to the beta-ionone ring. CarX represents the first retinal-synthesizing enzyme reported in the fungal kingdom so far. It seems likely that the formed retinal is involved in the regulation of the carotenoid biosynthetic pathway via a negative feedback mechanism.

Production of a recombinant carotenoid cleavage dioxygenase from grape and enzyme assay in water-miscible organic solvents

A recombinant carotenoid cleavage dioxygenase from Vitis vinifera L. was produced by Escherichia coli as a fusion with the glutathione-S-transferase (GST) protein under different bacterial growth conditions. The enzyme production was monitored by a GST assay. Addition of Triton X-100 prior to bacterial cell disruption doubled the release of soluble protein. A simple spectrophotometric enzyme assay was developed to measure carotenoid cleavage activity using lutein as substrate. Enzyme activity showed a 26-fold increase with the addition of 10% (v/v) acetone in the reaction mixture.

Dissecting the biosynthetic pathway for the bypass1 root-derived signal

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

Circadian rhythms of isoprene biosynthesis in grey poplar leaves

Isoprene (2-methyl-1,3-butadiene) emission varies diurnally in different species. In poplar (Populus spp.), it has recently been shown that the gene encoding the synthesizing enzyme for isoprene, isoprene synthase (ISPS), displays diurnal variation in expression. Working on shoot cultures of Grey poplar (Populus x canescens) placed under a different light regime in phytochambers, we showed that these variations in PcISPS gene expression, measured by quantitative real-time polymerase chain reaction, are not only due to day-night changes, but also are linked to an internal circadian clock. Measurement of additional selected isoprenoid genes revealed that phytoene synthase (carotenoid pathway) displays similar fluctuations, whereas 1-deoxy-d-xylulose 5-phosphate reductoisomerase, possibly the first committed enzyme of the 1-deoxy-d-xylulose 5-phosphate pathway, only shows light regulation. On the protein level, it appeared that PcISPS activity and protein content became reduced under constant darkness, whereas under constant light, activity and protein content of this enzyme were kept high. In contrast, isoprene emission rates under continuous irradiation displayed circadian changes as is the case for gene expression of PcISPS. Furthermore, binding assays with Arabidopsis (Arabidopsis thaliana) late elongated hypocotyl, a transcription factor of Arabidopsis involved in circadian regulation, clearly revealed the presence of circadian-determining regulatory elements in the promoter region of PcISPS.

A proteomics dissection of Arabidopsis thaliana vacuoles isolated from cell culture

To better understand the mechanisms governing cellular traffic, storage of various metabolites, and their ultimate degradation, Arabidopsis thaliana vacuole proteomes were established. To this aim, a procedure was developed to prepare highly purified vacuoles from protoplasts isolated from Arabidopsis cell cultures using Ficoll density gradients. Based on the specific activity of the vacuolar marker alpha-mannosidase, the enrichment factor of the vacuoles was estimated at approximately 42-fold with an average yield of 2.1%. Absence of significant contamination by other cellular compartments was validated by Western blot using antibodies raised against specific markers of chloroplasts, mitochondria, plasma membrane, and endoplasmic reticulum. Based on these results, vacuole preparations showed the necessary degree of purity for proteomics study. Therefore, a proteomics approach was developed to identify the protein components present in both the membrane and soluble fractions of the Arabidopsis cell vacuoles. This approach includes the following: (i) a mild oxidation step leading to the transformation of cysteine residues into cysteic acid and methionine to methionine sulfoxide, (ii) an in-solution proteolytic digestion of very hydrophobic proteins, and (iii) a prefractionation of proteins by short migration by SDS-PAGE followed by analysis by liquid chromatography coupled to tandem mass spectrometry. This procedure allowed the identification of more than 650 proteins, two-thirds of which copurify with the membrane hydrophobic fraction and one-third of which copurifies with the soluble fraction. Among the 416 proteins identified from the membrane fraction, 195 were considered integral membrane proteins based on the presence of one or more predicted transmembrane domains, and 110 transporters and related proteins were identified (91 putative transporters and 19 proteins related to the V-ATPase pump). With regard to function, about 20% of the proteins identified were known previously to be associated with vacuolar activities. The proteins identified are involved in ion and metabolite transport (26%), stress response (9%), signal transduction (7%), and metabolism (6%) or have been described to be involved in typical vacuolar activities, such as protein and sugar hydrolysis. The subcellular localization of several putative vacuolar proteins was confirmed by transient expression of green fluorescent protein fusion constructs.

The rice HIGH-TILLERING DWARF1 encoding an ortholog of Arabidopsis MAX3 is required for negative regulation of the outgrowth of axillary buds

Rice tillering is an important agronomic trait for grain production. The HIGH-TILLERING DWARF1 (HTD1) gene encodes an ortholog of Arabidopsis MAX3. Complementation analyses for HTD1 confirm that the defect in HTD1 is responsible for both high-tillering and dwarf phenotypes in the htd1 mutant. The rescue of the Arabidopsis max3 mutant phenotype by the introduction of Pro(35S):HTD1 indicates HTD1 is a carotenoid cleavage dioxygenase that has the same function as MAX3 in synthesis of a carotenoid-derived signal molecule. The HTD1 gene is expressed in both shoot and root tissues. By evaluating Pro(HTD1):GUS expression, we found that the HTD1 gene is mainly expressed in vascular bundle tissues throughout the plant. Auxin induction of HTD1 expression suggests that auxin may regulate rice tillering partly through upregulation of HTD1 gene transcription. Restoration of dwarf phenotype after the removal of axillary buds indicates that the dwarfism of the htd1 mutant may be a consequence of excessive tiller production. In addition, the expression of HTD1, D3 and OsCCD8a in the htd1 and d3 mutants suggests a feedback mechanism may exist for the synthesis and perception of the carotenoid-derived signal in rice. Characterization of MAX genes in Arabidopsis, and identification of their orthologs in pea, petunia and rice indicates the existence of a conserved mechanism for shoot-branching regulation in both monocots and dicots.

Carotenoid cleavage dioxygenase (CmCCD4a) contributes to white color formation in chrysanthemum petals

The white petals of chrysanthemum (Chrysanthemum morifolium Ramat.) are believed to contain a factor that inhibits the accumulation of carotenoids. To find this factor, we performed polymerase chain reaction-Select subtraction screening and obtained a clone expressed differentially in white and yellow petals. The deduced amino acid sequence of the protein (designated CmCCD4a) encoded by the clone was highly homologous to the sequence of carotenoid cleavage dioxygenase. All the white-flowered chrysanthemum cultivars tested showed high levels of CmCCD4a transcript in their petals, whereas most of the yellow-flowered cultivars showed extremely low levels. Expression of CmCCD4a was strictly limited to flower petals and was not detected in other organs, such as the root, stem, or leaf. White petals turned yellow after the RNAi construct of CmCCD4a was introduced. These results indicate that in white petals of chrysanthemums, carotenoids are synthesized but are subsequently degraded into colorless compounds, which results in the white color.

Retinal is formed from apo-carotenoids in Nostoc sp. PCC7120: in vitro characterization of an apo-carotenoid oxygenase

The sensory rhodopsin from Anabaena (Nostoc) sp. PCC7120 is the first cyanobacterial retinylidene protein identified. Here, we report on NosACO (Nostoc apo-carotenoid oxygenase), encoded by the ORF (open reading frame) all4284, as the candidate responsible for the formation of the required chromophore, retinal. In contrast with the enzymes from animals, NosACO converts beta-apo-carotenals instead of beta-carotene into retinal in vitro. The identity of the enzymatic products was proven by HPLC and gas chromatography-MS. NosACO exhibits a wide substrate specificity with respect to chain lengths and functional end-groups, converting beta-apo-carotenals, (3R)-3-hydroxy-beta-apo-carotenals and the corresponding alcohols into retinal and (3R)-3-hydroxyretinal respectively. However, kinetic analyses revealed very divergent Km and Vmax values. On the basis of the crystal structure of SynACO (Synechocystis sp. PCC6803 apo-carotenoid oxygenase), a related enzyme showing similar enzymatic activity, we designed a homology model of the native NosACO. The deduced structure explains the absence of beta-carotene-cleavage activity and indicates that NosACO is a monotopic membrane protein. Accordingly, NosACO could be readily reconstituted into liposomes. To localize SynACO in vivo, a Synechocystis knock-out strain was generated expressing SynACO as the sole carotenoid oxygenase. Western-blot analyses showed that the main portion of SynACO occurred in a membrane-bound form.

A 9-cis-epoxycarotenoid dioxygenase inhibitor for use in the elucidation of abscisic acid action mechanisms

The plant hormone abscisic acid (ABA) accumulates in response to drought stress and confers stress tolerance to plants. 9-cis-Epoxycarotenoid dioxygenase (NCED), the key regulatory enzyme in the ABA biosynthesis pathway, plays an important role in ABA accumulation. Treatment of plants with abamine, the first NCED inhibitor identified, inhibits ABA accumulation. On the basis of structure-activity relationship studies of abamine, we identified an inhibitor of ABA accumulation more potent than abamine and named it abamineSG. An important structural feature of abamineSG is a three-carbon linker between the methyl ester and the nitrogen atom. Treatment of osmotically stressed plants with 100 microM abamineSG inhibited ABA accumulation by 77% as compared to the control, whereas abamine inhibited the accumulation by 35%. The expression of AB A-responsive genes and ABA catabolic genes was strongly inhibited in abamineSG-treated plants under osmotic stress. AbamineSG is a competitive inhibitor of the enzyme NCED, with a K(i) of 18.5 microM. Although the growth of Arabidopsis seedlings was inhibited by abamine at high concentrations (>50 microM), an effect that was unrelated to the inhibition of ABA biosynthesis, seedling growth was not affected by 100 microM abamineSG. These results suggest that abamineSG is a more potent and specific inhibitor of ABA biosynthesis than abamine.

Plant carotenoid cleavage oxygenases and their apocarotenoid products

The oxidative cleavage of carotenoids leads to the production of apocarotenoids and is catalyzed by a family of carotenoid cleavage dioxygenases (CCDs). CCDs often exhibit substrate promiscuity, which probably contributes to the diversity of apocarotenoids found in nature. Biologically and commercially important apocarotenoids include the phytohormone abscisic acid, the visual and signaling molecules retinal and retinoic acid, and the aromatic volatile beta-ionone. Unexpected properties associated with the CCD catalytic products emphasize their role in many aspects of plant growth and development. For instance, CCD7 and CCD8 produce a novel, graft-transmissible hormone that controls axillary shoot growth in plants. Here, CCDs are discussed according to their roles in the biosynthesis of these products. Recent studies regarding their mechanism of action are also addressed.

Characterization of three members of the Arabidopsis carotenoid cleavage dioxygenase family demonstrates the divergent roles of this multifunctional enzyme family

Arabidopsis thaliana has nine genes that constitute a family of putative carotenoid cleavage dioxygenases (CCDs). While five members of the family are believed to be involved in synthesis of the phytohormone abscisic acid, the functions of the other four enzymes are less clear. Recently two of the enzymes, CCD7/MAX3 and CCD8/MAX4, have been implicated in synthesis of a novel apocarotenoid hormone that controls lateral shoot growth. Here, we report on the molecular and genetic interactions between CCD1, CCD7/MAX3 and CCD8/MAX4. CCD1 distinguishes itself from other reported CCDs as being the only member not targeted to the plastid. Unlike ccd7/max3 and ccd8/max4, both characterized as having highly branched phenotypes, ccd1 loss-of-function mutants are indistinguishable from wild-type plants. Thus, even though CCD1 has similar enzymatic activity to CCD7/MAX3, it does not have a role in synthesis of the lateral shoot growth inhibitor. Rather, it may have a role in synthesis of apocarotenoid flavor and aroma volatiles, especially in maturing seeds where loss of function leads to significantly higher carotenoid levels.

The carotenase AtCCD1 from Arabidopsis thaliana is a dioxygenase

Apocarotenoids resulting from the oxidative cleavage of carotenoids serve as important signaling and accessory molecules in a variety of biological processes. The enzymes catalyzing these reactions are referred to as carotenases or carotenoid oxygenases. Whether they act according to a monooxygenase mechanism, requiring two oxygens from different sources, or a dioxygenase mechanism is still a topic of controversy. In this study, we utilized the readily available beta-apo-8'-carotenal as a substrate for the heterologously expressed AtCCD1 protein from Arabidopsis thaliana to investigate the oxidative cleavage mechanism of the 9,10 double bond of carotenoids. Beta-ionone and a C(17)-dialdehyde were detected as products by gas and liquid chromatography-mass spectrometry as well as NMR analysis. Labeling experiments using H(2)(18)O or (18) O(2) showed that the oxygen in the keto-group of beta-ionone is derived solely from molecular dioxygen. When experiments were performed in an (18)O(2)-enriched atmosphere, a substantial fraction of the C(17)-dialdehyde contained labeled oxygen. The results unambiguously demonstrate a dioxygenase mechanism for the carotenase AtCCD1 from A. thaliana.

Study on the mutagenicity and antimutagenicity of beta-ionone in the Salmonella/microsome assay

beta-Ionone (BIO) is a degraded (C(13)) sesquiterpenoid compound found in a variety of edible and aromatic plants. BIO and other ionone derivatives have been used in fragrance products and as flavoring food additives. In this study we investigated the mutagenic and antimutagenic activities of BIO using the Salmonella/microsome assay. Mutagenicity was evaluated by two tests with Salmonella typhimurium strains TA100, TA98, TA97a and TA1535, without and with addition of S9 mixture. A first assay was performed by the plate incorporation procedure and a confirmation test by the pre-incubation method. In either test, no increase in the number of his(+) revertant colonies over the negative (solvent) control values was noted with any of the four tester strains thereby indicating that BIO was not genotoxic in the Salmonella assay. Antimutagenic activity was investigated by testing (by the plate incorporation method) different non-toxic doses of BIO against one or more non-toxic doses of direct-acting (sodium azide: SA, 4-nitroquinoline-N-oxide: 4-NQNO, 2-nitrofluorene: 2-NF and nitro-o-phenylenediamine: NPD) as well as indirect-acting (cyclophosphamide: CP, benzo[a]pyrene: B[a]P, aflatoxin B1: AFB1, 2-aminoanthracene: 2-AA, and 2-aminofluorene: 2-AF) mutagens. BIO did not alter the effects of any direct-acting mutagen or B[a]P and 2-AF. Mutagenic effects of AFB1 and CP, however, were markedly and dose-dependently antagonized by BIO. It has been reported that, in the rat liver, activation of B[a]P and 2-AF depend on CYP1A1 activity, and that CYP2B subfamily is involved in the metabolic activation of CP and AFB1. It has also been described that BIO is a potent inhibitor of CYP2B1/2 and a weaker inhibitor of CYP1A1. Therefore, antagonism of CP-and AFB1-induced mutagenic effects by BIO could have been mediated-at least in part-by the inhibition of CYP2B enzymes.

A carotenoid cleavage dioxygenase from Vitis vinifera L.: functional characterization and expression during grape berry development in relation to C13-norisoprenoid accumulation

A potential Carotenoid Cleavage Dioxygenase (CCD) gene was identified among a Vitis vinifera L. EST collection and a full-length cDNA (VvCCD1) was isolated. Recombinant expression of VvCCD1 confirmed that the gene encoded a functional CCD. Experimental evidence was obtained that VvCCD1 cleaves zeaxanthin symmetrically yielding 3-hydroxy-beta-ionone, a C(13)-norisoprenoidic compound, and a C(14)-dialdehyde. Expression of the gene was studied by real-time PCR at different developmental stages of grape berries from Muscat of Alexandria and Shiraz cultivars. A significant induction of the gene expression approaching véraison was observed in both cultivars. In parallel, the C(13)-norisoprenoid level increased from véraison to maturity in both cultivars.

Ethylene-regulated floral volatile synthesis in petunia corollas

In many flowering plants, such as petunia (Petunia x hybrida), ethylene produced in floral organs after pollination elicits a series of physiological and biochemical events, ultimately leading to senescence of petals and successful fertilization. Here, we demonstrate, using transgenic ethylene insensitive (44568) and Mitchell Diploid petunias, that multiple components of emission of volatile organic compounds (VOCs) are regulated by ethylene. Expression of benzoic acid/salicylic acid carboxyl methyltransferase (PhBSMT1 and 2) mRNA is temporally and spatially down-regulated in floral organs in a manner consistent with current models for post-pollination ethylene synthesis in petunia corollas. Emission of methylbenzoate and other VOCs after pollination and exogenous ethylene treatment parallels a reduction in PhBSMT1 and 2 mRNA levels. Under cyclic light conditions (day/night), PhBSMT mRNA levels are rhythmic and precede emission of methylbenzoate by approximately 6 h. When shifted into constant dark or light conditions, PhBSMT mRNA levels and subsequent methylbenzoate emission correspondingly decrease or increase to minimum or maximum levels observed during normal conditions, thus suggesting that light may be a more critical influence on cyclic emission of methylbenzoate than a circadian clock. Transgenic PhBSMT RNAi flowers with reduced PhBSMT mRNA levels show a 75% to 99% decrease in methylbenzoate emission, with minimal changes in other petunia VOCs. These results implicate PhBSMT1 and 2 as genes responsible for synthesis of methylbenzoate in petunia.

Carotenoid pigmentation affects the volatile composition of tomato and watermelon fruits, as revealed by comparative genetic analyses

Tomato near-isogenic lines differing in fruit carotenogenesis genes accumulated different aroma volatiles, in a strikingly similar fashion as compared to watermelon cultivars differing in fruit color. The major volatile norisoprenoids present in lycopene-containing tomatoes and watermelons were noncyclic, such as geranial, neral, 6-methyl-5-hepten-2-one, 2,6-dimethylhept-5-1-al, 2,3-epoxygeranial, (E,E)-pseudoionone, geranyl acetone, and farnesyl acetone, seemingly derived from lycopene and other noncyclic tetraterpenoids. Beta-ionone, dihydroactinodiolide, and beta-cyclocitral were prominent in both tomato and watermelon fruits containing beta-carotene. Alpha-ionone was detected only in an orange-fleshed tomato mutant that accumulates delta-carotene. A yellow flesh (r) mutant tomato bearing a nonfunctional psy1 gene and the yellow-fleshed watermelon Early Moonbeam, almost devoid of carotenoid fruit pigments, also lacked norisoprenoid derivatives and geranial. This study provides evidence, based on comparative genetics, that carotenoid pigmentation patterns have profound effects on the norisoprene and monoterpene aroma volatile compositions of tomato and watermelon and that in these fruits geranial (trans-citral) is apparently derived from lycopene in vivo.

The Decreased apical dominance1/Petunia hybrida CAROTENOID CLEAVAGE DIOXYGENASE8 gene affects branch production and plays a role in leaf senescence, root growth, and flower development

Carotenoids and carotenoid cleavage products play an important and integral role in plant development. The Decreased apical dominance1 (Dad1)/PhCCD8 gene of petunia (Petunia hybrida) encodes a hypothetical carotenoid cleavage dioxygenase (CCD) and ortholog of the MORE AXILLARY GROWTH4 (MAX4)/AtCCD8 gene. The dad1-1 mutant allele was inactivated by insertion of an unusual transposon (Dad-one transposon), and the dad1-3 allele is a revertant allele of dad1-1. Consistent with its role in producing a graft-transmissible compound that can alter branching, the Dad1/PhCCD8 gene is expressed in root and shoot tissue. This expression is upregulated in the stems of the dad1-1, dad2, and dad3 increased branching mutants, indicating feedback regulation of the gene in this tissue. However, this feedback regulation does not affect the root expression of Dad1/PhCCD8. Overexpression of Dad1/PhCCD8 in the dad1-1 mutant complemented the mutant phenotype, and RNA interference in the wild type resulted in an increased branching phenotype. Other differences in phenotype associated with the loss of Dad1/PhCCD8 function included altered timing of axillary meristem development, delayed leaf senescence, smaller flowers, reduced internode length, and reduced root growth. These data indicate that the substrate(s) and/or product(s) of the Dad1/PhCCD8 enzyme are mobile signal molecules with diverse roles in plant development.

The tomato carotenoid cleavage dioxygenase 1 genes contribute to the formation of the flavor volatiles beta-ionone, pseudoionone, and geranylacetone

Volatile terpenoid compounds, potentially derived from carotenoids, are important components of flavor and aroma in many fruits, vegetables and ornamentals. Despite their importance, little is known about the enzymes that generate these volatiles. The tomato genome contains two closely related genes potentially encoding carotenoid cleavage dioxygenases, LeCCD1A and LeCCD1B. A quantitative reverse transcriptase-polymerase chain reaction analysis revealed that one of these two genes, LeCCD1B, is highly expressed in ripening fruit (4 days post-breaker), where it constitutes 0.11% of total RNA. Unlike the related neoxanthin cleavage dioxygenases, import assays using pea chloroplasts showed that the LeCCD1 proteins are not plastid-localized. The biochemical functions of the LeCCD1 proteins were determined by bacterial expression and in vitro assays, where it was shown that they symmetrically cleave multiple carotenoid substrates at the 9,10 (9',10') positions to produce a C14 dialdehyde and two C13 cyclohexones that vary depending on the substrate. The potential roles of the LeCCD1 genes in vivo were assessed in transgenic tomato plants constitutively expressing the LeCCD1B gene in reverse orientation. This over-expression of the antisense transcript led to 87-93% reductions in mRNA levels of both LeCCD1A and LeCCD1B in the leaves and fruits of selected lines. Transgenic plants exhibited no obvious morphological alterations. High-performance liquid chromatography analysis showed no significant modification in the carotenoid content of fruit tissue. However, volatile analysis showed a > or =50% decrease in beta-ionone (a beta-carotene-derived C13 cyclohexone) and a > or =60% decrease in geranylacetone (a C13 acyclic product likely derived from a lycopene precursor) in selected lines, implicating the LeCCD1 genes in the formation of these important flavor volatiles in vivo.