Isolation and characterization of carotenoid cleavage dioxygenase 4 genes from different citrus species

Citrus β-carotene hydroxylase 2 (BCH2) participates in xanthophyll synthesis by catalyzing the hydroxylation of β-carotene and compensates for BCH1 in citrus carotenoid metabolism

As an essential horticultural crop, Citrus has carotenoid diversity, which affects its aesthetic and nutritional values. β,β-Xanthophylls are the primary carotenoids accumulated in citrus fruits, and non-heme di-iron carotene hydroxylase (BCH) enzymes are mainly responsible for β,β-xanthophyll synthesis. Previous studies have focused on the hydroxylation of BCH1, but the role of its paralogous gene in citrus, BCH2, remains largely unknown. In this study, we revealed the β-hydroxylation activity of citrus BCH2 (CsBCH2) for the first time through the functional complementation assay using Escherichia coli, although CsBCH2 exhibited a lower activity in hydroxylating β-carotene into β-cryptoxanthin than citrus BCH1 (CsBCH1). Our results showed that overexpression of CsBCH2 in citrus callus increased xanthophyll proportion and plastoglobule size with feedback regulation of carotenogenic gene expression. This study revealed the distinct expression patterns and functional characteristics of two paralogous genes, CsBCH1 and CsBCH2, and illustrated the backup compensatory role of CsBCH2 for CsBCH1 in citrus xanthophyll biosynthesis. The independent function of CsBCH2 and its cooperative function with CsBCH1 in β-cryptoxanthin biosynthesis suggested the potential of CsBCH2 to be employed for expanding the synthetic biology toolkit in carotenoid engineering.

Gardenia carotenoid cleavage dioxygenase 4a is an efficient tool for biotechnological production of crocins in green and non-green plant tissues

Crocins are beneficial antioxidants and potential chemotherapeutics that give raise, together with picrocrocin, to the colour and taste of saffron, the most expensive spice, respectively. Crocins are formed from crocetin dialdehyde that is produced in Crocus sativus from zeaxanthin by the carotenoid cleavage dioxygenase 2L (CsCCD2L), while GjCCD4a from Gardenia jasminoides, another major source of crocins, converted different carotenoids, including zeaxanthin, into crocetin dialdehyde in bacterio. To establish a biotechnological platform for sustainable production of crocins, we investigated the enzymatic activity of GjCCD4a, in comparison with CsCCD2L, in citrus callus engineered by Agrobacterium-mediated supertransformation of multi genes and in transiently transformed Nicotiana benthamiana leaves. We demonstrate that co-expression of GjCCD4a with phytoene synthase and β-carotene hydroxylase genes is an optimal combination for heterologous production of crocetin, crocins and picrocrocin in citrus callus. By profiling apocarotenoids and using in vitro assays, we show that GjCCD4a cleaved β-carotene, in planta, and produced crocetin dialdehyde via C₃₀ β-apocarotenoid intermediate. GjCCD4a also cleaved C₂₇ β-apocarotenoids, providing a new route for C₁₇ -dialdehyde biosynthesis. Callus lines overexpressing GjCCD4a contained higher number of plastoglobuli in chromoplast-like plastids and increased contents in phytoene, C17:0 fatty acid (FA), and C18:1 cis-9 and C22:0 FA esters. GjCCD4a showed a wider substrate specificity and higher efficiency in Nicotiana leaves, leading to the accumulation of up to 1.6 mg/g dry weight crocins. In summary, we established a system for investigating CCD enzymatic activity in planta and an efficient biotechnological platform for crocins production in green and non-green crop tissues/organs.

Comparative transcriptomics of wild and commercial Citrus during early ripening reveals how domestication shaped fruit gene expression

BACKGROUND

Interspecific hybridizations and admixtures were key in Citrus domestication, but very little is known about their impact at the transcriptomic level. To determine the effects of genome introgressions on gene expression, the transcriptomes of the pulp and flavedo of three pure species (citron, pure mandarin and pummelo) and four derived domesticated genetic admixtures (sour orange, sweet orange, lemon and domesticated mandarin) have been analyzed at color break.

RESULTS

Many genes involved in relevant physiological processes for domestication, such sugar/acid metabolism and carotenoid/flavonoid synthesis, were differentially expressed among samples. In the low-sugar, highly acidic species lemon and citron, many genes involved in sugar metabolism, the TCA cycle and GABA shunt displayed a reduced expression, while the P-type ATPase CitPH5 and most subunits of the vacuolar ATPase were overexpressed. The red-colored species and admixtures were generally characterized by the overexpression in the flavedo of specific pivotal genes involved in the carotenoid biosynthesis, including phytoene synthase, ζ-carotene desaturase, β-lycopene cyclase and CCD4b, a carotenoid cleavage dioxygenase. The expression patterns of many genes involved in flavonoid modifications, especially the flavonoid and phenylpropanoid O-methyltransferases showed extreme diversity. However, the most noticeable differential expression was shown by a chalcone synthase gene, which catalyzes a key step in the biosynthesis of flavonoids. This chalcone synthase was exclusively expressed in mandarins and their admixed species, which only expressed the mandarin allele. In addition, comparisons between wild and domesticated mandarins revealed that the major differences between their transcriptomes concentrate in the admixed regions.

CONCLUSION

In this work we present a first study providing broad evidence that the genome introgressions that took place during citrus domestication largely shaped gene expression in their fruits.

SlJMJ7 orchestrates tomato fruit ripening via crosstalk between H3K4me3 and DML2-mediated DNA demethylation

The ripening of fleshy fruits is a unique developmental process that Arabidopsis and rice lack. This process is driven by hormones and transcription factors. However, the critical and early regulators of fruit ripening are still poorly understood. Here, we revealed that SlJMJ7, an H3K4 demethylase, is a critical negative regulator of fruit ripening in tomato. Combined genome-wide transcription, binding sites, histone H3K4me3 and DNA methylation analyses demonstrated that SlJMJ7 regulates a key group of ripening-related genes, including ethylene biosynthesis (ACS2, ACS4 and ACO6), transcriptional regulation (RIN and NOR) and DNA demethylation (DML2) genes, by H3K4me3 demethylation. Moreover, loss of SlJMJ7 function leads to increased H3K4me3 levels, which directly activates ripening-related genes, and to global DML2-mediated DNA hypomethylation in fruit, which indirectly prompts expression of ripening-related genes. Together, these effects lead to accelerated fruit ripening in sljmj7 mutant. Our findings demonstrate that SlJMJ7 acts as a master negative regulator of fruit ripening not only through direct removal of H3K4me3 from multiple key ripening-related factors, but also through crosstalk between histone and DNA demethylation. These findings reveal a novel crosstalk between histone methylation and DNA methylation to regulate gene expression in plant developmental processes.

Modelling of the Citrus CCD4 Family Members: In Silico Analysis of Membrane Binding and Substrate Preference

Coloring is one of the most important characteristics in commercial flowers and fruits, generally due to the accumulation of carotenoid pigments. Enzymes of the CCD4 family in citrus intervene in the generation of β-citraurin, an apocarotenoid responsible for the reddish-orange color of mandarins. Citrus CCD4s enzymes could be capable of interacting with the thylakoid membrane inside chloroplasts. However, to date, this interaction has not been studied in detail. In this work, we present three new complete models of the CCD4 family members (CCD4a, CCD4b, and CCD4c), modeled with a lipid membrane. To identify the preference for substrates, typical carotenoids were inserted in the active site of the receptors and the protein–ligand interaction energy was evaluated. The results show a clear preference of CCD4s for xanthophylls over aliphatic carotenes. Our findings indicate the ability to penetrate the membrane and maintain a stable interaction through the N-terminal α-helical domain, spanning a contact surface of 2250 to 3250 Ų. The orientation and depth of penetration at the membrane surface suggest that CCD4s have the ability to extract carotenoids directly from the membrane through a tunnel consisting mainly of hydrophobic residues that extends up to the catalytic center of the enzyme.

Exploring the Diversity and Regulation of Apocarotenoid Metabolic Pathways in Plants

In plants, carotenoids are subjected to enzyme-catalyzed oxidative cleavage reactions as well as to non-enzymatic degradation processes, which produce various carbonyl products called apocarotenoids. These conversions control carotenoid content in different tissues and give rise to apocarotenoid hormones and signaling molecules, which play important roles in plant growth and development, response to environmental stimuli, and in interactions with surrounding organisms. In addition, carotenoid cleavage gives rise to apocarotenoid pigments and volatiles that contribute to the color and flavor of many flowers and several fruits. Some apocarotenoid pigments, such as crocins and bixin, are widely utilized as colorants and additives in food and cosmetic industry and also have health-promoting properties. Considering the importance of this class of metabolites, investigation of apocarotenoid diversity and regulation has increasingly attracted the attention of plant biologists. Here, we provide an update on the plant apocarotenoid biosynthetic pathway, especially highlighting the diversity of the enzyme carotenoid cleavage dioxygenase 4 (CCD4) from different plant species with respect to substrate specificity and regioselectivity, which contribute to the formation of diverse apocarotenoid volatiles and pigments. In addition, we summarize the regulation of apocarotenoid metabolic pathway at transcriptional, post-translational, and epigenetic levels. Finally, we describe inter- and intraspecies variation in apocarotenoid production observed in many important horticulture crops and depict recent progress in elucidating the genetic basis of the natural variation in the composition and amount of apocarotenoids. We propose that the illustration of biochemical, genetic, and evolutionary background of apocarotenoid diversity would not only accelerate the discovery of unknown biosynthetic and regulatory genes of bioactive apocarotenoids but also enable the identification of genetic variation of causal genes for marker-assisted improvement of aroma and color of fruits and vegetables and CRISPR-based next-generation metabolic engineering of high-value apocarotenoids.

Molecular Characterization, Expression Analysis of Carotenoid, Xanthophyll, Apocarotenoid Pathway Genes, and Carotenoid and Xanthophyll Accumulation in Chelidonium majus L

Chelidonium majus L. is a perennial herbaceous plant that has various medicinal properties. However, the genomic information about its carotenoid biosynthesis pathway (CBP), xanthophyll biosynthesis pathway (XBP), and apocarotenoid biosynthesis pathway (ABP) genes were limited. Thus, the CBP, XBP, and ABP genes of C. majus were identified and analyzed. Among the 15 carotenoid pathway genes identified, 11 full and 4 partial open reading frames were determined. Phylogenetic analysis of these gene sequences showed higher similarity with higher plants. Through 3D structural analysis and multiple alignments, several distinct conserved motifs were identified, including dinucleotide binding motif, carotene binding motif, and aspartate or glutamate residues. Quantitative RT-PCR showed that CBP, XBP, and ABP genes were expressed in a tissue-specific manner; the highest expression levels were achieved in flowers, followed by those in leaves, roots, and stems. The HPLC analysis of the different organs showed the presence of eight different carotenoids. The highest total carotenoid content was found in leaves, followed by that in flowers, stems, and roots. This study provides information on the molecular mechanisms involved in CBP, XBP, and ABP genes, which might help optimize the carotenoid production in C. majus. The results could also be a basis of further studies on the molecular genetics and functional analysis of CBP, XBP, and ABP genes.

Lycopene Accumulation in Cara Cara Red-flesh Navel Orange Is Correlated with Weak Abscisic Acid Catabolism

Lycopene is the main pigment in red-flesh citrus fruits, and its formation is a research hotspot. To explore the basis of lycopene accumulation in red-flesh mutants, we profiled the terpenoid metabolites. Compared with their respective wild types, Cara Cara (Cara) [and Red-Anliu (R-An)] oranges showed increased carotenoid and limonoid aglycone contents and decreased contents of abscisic acid (ABA) catabolites, monoterpenoid volatiles, and sesquiterpenoid volatiles. Cara contained less than half of the amount of ABA glucose ester (ABAGE), the main ABA derivative in oranges. Parallel lower transcript levels of NCED and ABA glucosyltransferase in Cara were detected at the mature green stage. These results document the changes in terpenoid profiles in Cara and show that the red flesh of citrus color mutants is related to weak ABA catabolism, especially ABAGE, and decreased transcript levels of two genes encoding uridine diphosphate (UDP)-glycosyltransferases that catalyze ABAGE biosynthesis.

Ethylene activation of carotenoid biosynthesis by a novel transcription factor CsERF061

Chromoplast-specific lycopene β-cyclase (LCYb2) is a critical carotenogenic enzyme, which controls the massive accumulation of downstream carotenoids, especially provitamin A carotenoids, in citrus. Its regulatory metabolism is largely unknown. Here, we identified a group I ethylene response factor, CsERF061, in citrus by yeast one-hybrid screen with the promoter of LCYb2. The expression of CsERF061 was induced by ethylene. Transcript and protein levels of CsERF061 were increased during fruit development and coloration. CsERF061 is a nucleus-localized transcriptional activator, which directly binds to the promoter of LCYb2 and activates its expression. Overexpression of CsERF061 in citrus calli and tomato fruits enhanced carotenoid accumulation by increasing the expression of key carotenoid pathway genes, and increased the number of chromoplasts needed to sequester the elevated concentrations of carotenoids, which was accompanied by changes in the concentrations of abscisic acid and gibberellin. Electrophoretic mobility shift and dual-luciferase assays verified that CsERF061 activates the promoters of nine other key carotenoid pathway genes, PSY1, PDS, CRTISO, LCYb1, BCH, ZEP, NCED3, CCD1, and CCD4, revealing the multitargeted regulation of CsERF061. Collectively, our findings decipher a novel regulatory network of carotenoid enhancement by CsERF061, induced by ethylene, which will be useful for manipulating carotenoid accumulation in citrus and other plants.

LC-MS-Based Profiling Provides New Insights into Apocarotenoid Biosynthesis and Modifications in Citrus Fruits

Apocarotenoids contribute to fruit color and aroma, which are critical quality and marketability attributes. Previously, we reported that the red peels of citrus fruits, which are characterized by higher expression levels of a carotenoid cleavage dioxygenase 4b (CitCCD4b) gene, accumulate higher levels of β-citraurin and β-citraurinene than yellow peels. Here, we identified and quantified 12 apocarotenoids, either volatile or nonvolatile, in citrus peel using liquid chromatography-mass spectrometry (LC-MS). Our results show that red peels contain also dramatically higher amounts of β-apo-8'-carotenal, crocetin dialdehyde known from saffron, β-citraurol, β-cyclocitral, and 3-OH-β-cyclocitral and up to about 17-fold higher levels of 3-OH-β-cyclocitral glucoside (picrocrocin isomer). The content of these apocarotenoids was also significantly increased in different CitCCD4b-overexpressing transgenic callus lines, compared with corresponding controls. Transient expression of CitCCD4b in Nicotiana benthamiana leaves resulted in a striking increase in the 3-OH-β-cyclocitral level and the accumulation of picrocrocin. Thus, our work reinforces the specific function of CitCCD4b in producing C₁₀ apocarotenoid volatiles and C₃₀ pigments in citrus peel and uncovers its involvement in the biosynthesis of picrocrocin, C₂₀ dialdehyde, and C₃₀ alcohol apocarotenoids, suggesting the potential of this enzyme in metabolic engineering of apocarotenoids and their derivatives.

Building the Synthetic Biology Toolbox with Enzyme Variants to Expand Opportunities for Biofortification of Provitamin A and Other Health-Promoting Carotenoids

Carotenoids are a large class of structures that are important in human health and include both provitamin A and nonprovitamin A compounds. Vitamin A deficiency is a global health problem that can be alleviated by enriching provitamin A carotenoids in a range of food crops. Suitable plants for biofortification are those with high levels of the provitamin A biosynthetic precursor, lycopene, which is enzymatically converted by lycopene β-cyclase (LCYB) to β-carotene, a provitamin A carotenoid. Crops, such as citrus, naturally accumulate high levels of provitamin A and other health-promoting carotenoids. Such plants may have useful genes to expand the synthetic biology toolbox for producing a range of phenotypes, including both high provitamin A crops and crops with unique compositions of health-promoting carotenoids. To examine enzyme variants having different activity levels, we introduced two citrus LCYB alleles into tomato, a plant with fruit rich in lycopene. Overexpression in tomato of the stronger allele of the citrus chromoplast-specific lycopene β-cyclase (CsLCYb2a) produced "golden" transgenic tomato fruits with 9.3-fold increased levels of β-carotene at up to 1.5 mg/g dry weight. The use of the weaker allele, CsLCYb2b, also led to enhanced levels of β-carotene but in the context of a more heterogeneous composition of carotenoids. From a synthetic biology standpoint, these allelic differences have value for producing cultivars with unique carotenoid profiles. Overexpression of the citrus LCYB genes was accompanied by increased expression of other genes encoding carotenoid biosynthetic enzymes and increased size and number of chromoplasts needed to sequester the elevated levels of carotenoids in the transgenic tomato fruits. The overexpression of the citrus LCYB genes also led to a pleiotropic effect on profiles of phytohormones and primary metabolites. Our findings show that enzyme variants are essential synthetic biology parts needed to create a wider range of metabolic engineering products. In this case, strong and weak variants of LCYB proved useful in creating dietary sources to alleviate vitamin A deficiency or, alternatively, to create crops with a heterogeneous composition including provitamin A and healthful, nonprovitamin A carotenoids.

Roles of RIN and ethylene in tomato fruit ripening and ripening-associated traits

RIPENING INHIBITOR (RIN)-deficient fruits generated by CRISPR/Cas9 initiated partial ripening at a similar time to wild-type (WT) fruits but only 10% WT concentrations of carotenoids and ethylene (ET) were synthesized. RIN-deficient fruit never ripened completely, even when supplied with exogenous ET. The low amount of endogenous ET that they did produce was sufficient to enable ripening initiation and this could be suppressed by the ET perception inhibitor 1-MCP. The reduced ET production by RIN-deficient tomatoes was due to an inability to induce autocatalytic system-2 ET synthesis, a characteristic feature of climacteric ripening. Production of volatiles and transcripts of key volatile biosynthetic genes also were greatly reduced in the absence of RIN. By contrast, the initial extent and rates of softening in the absence of RIN were similar to WT fruits, although detailed analysis showed that the expression of some cell wall-modifying enzymes was delayed and others increased in the absence of RIN. These results support a model where RIN and ET, via ERFs, are required for full expression of ripening genes. Ethylene initiates ripening of mature green fruit, upregulates RIN expression and other changes, including system-2 ET production. RIN, ET and other factors are required for completion of the full fruit-ripening programme.

Carotenoid biofortification in crop plants: citius, altius, fortius

Carotenoids are indispensable for human health, required as precursors of vitamin A and efficient antioxidants. However, these plant pigments that play a vital role in photosynthesis are represented at insufficient levels in edible parts of several crops, which creates a need for increasing their content or optimizing their composition through biofortification. In particular, vitamin A deficiency, a severe health problem affecting the lives of millions in developing countries, has triggered the development of a series of high-provitamin A crops, including Golden Rice as the best-known example. Further carotenoid-biofortified crops have been generated by using genetic engineering approaches or through classical breeding. In this review, we depict carotenoid metabolism in plants and provide an update on the development of carotenoid-biofortified plants and their potential to meet needs and expectations. Furthermore, we discuss the possibility of using natural variation for carotenoid biofortification and the potential of gene editing tools. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

RNA-seq Analysis of Salt-Stressed Versus Non Salt-Stressed Transcriptomes of Chenopodium quinoa Landrace R49

Quinoa (Chenopodium quinoa Willd.), a model halophytic crop species, was used to shed light on salt tolerance mechanisms at the transcriptomic level. An RNA-sequencing analysis of genotype R49 at an early vegetative stage was performed by Illumina paired-ends method comparing high salinity and control conditions in a time-course pot experiment. Genome-wide transcriptional salt-induced changes and expression profiling of relevant salt-responsive genes in plants treated or not with 300 mM NaCl were analyzed after 1 h and 5 days. We obtained up to 49 million pairs of short reads with an average length of 101 bp, identifying a total of 2416 differentially expressed genes (DEGs) based on the treatment and time of sampling. In salt-treated vs. control plants, the total number of up-regulated and down-regulated genes was 945 and 1471, respectively. The number of DEGs was higher at 5 days than at 1 h after salt treatment, as reflected in the number of transcription factors, which increased with time. We report a strong transcriptional reprogramming of genes involved in biological processes like oxidation-reduction, response to stress and response to abscisic acid (ABA), and cell wall organization. Transcript analyses by real-time RT- qPCR supported the RNA-seq results and shed light on the contribution of roots and shoots to the overall transcriptional response. In addition, it revealed a time-dependent response in the expression of the analyzed DEGs, including a quick (within 1 h) response for some genes, suggesting a "stress-anticipatory preparedness" in this highly salt-tolerant genotype.

Genuine Carotenoid Profiles in Sweet Orange [Citrus sinensis (L.) Osbeck cv. Navel] Peel and Pulp at Different Maturity Stages

The carotenogenesis in the endocarp and flavedo of Navel oranges over four consecutive maturity stages was assessed by high-performance liquid chromatography-diode array detection-atmospheric pressure chemical ionization-multistage mass spectrometry. After optimization of the extraction method, 77 carotenoids, including 26 monoesters and 33 diesters of violaxanthin, β-citraurin, and antheraxanthin, were characterized. Whereas chloroplast-specific pigments, such as (all-E)-lutein and (all-E)-β-carotene, predominated in the flavedo of green-ripe fruit, a highly complex pattern of xanthophyll esters was found in the mature oranges. Total carotenoid contents of flavedo were approximately 9-fold higher [12 605 μg/100 g of fresh weight (FW)] than those in the endocarp (1354 μg/100 g of FW) at the fully mature stage. The mature endocarp abundantly contained violaxanthin mono- and diesters, in addition to diverse antheraxanthin esters, which were exclusively detected in this fruit fraction. Likewise, β-citraurin esters were found to be unique flavedo constituents of mature fruit. Therefore, they may support the detection of fraudulent use of peel fractions during orange juice production.

Natural Variation in CCD4 Promoter Underpins Species-Specific Evolution of Red Coloration in Citrus Peel

Carotenoids and apocarotenoids act as phytohormones and volatile precursors that influence plant development and confer aesthetic and nutritional value critical to consumer preference. Citrus fruits display considerable natural variation in carotenoid and apocarotenoid pigments. In this study, using an integrated genetic approach we revealed that a 5' cis-regulatory change at CCD4b encoding CAROTENOID CLEAVAGE DIOXYGENASE 4b is a major genetic determinant of natural variation in C₃₀ apocarotenoids responsible for red coloration of citrus peel. Functional analyses demonstrated that in addition the known role in synthesizing β-citraurin, CCD4b is also responsible for the production of another important C₃₀ apocarotenoid pigment, β-citraurinene. Furthermore, analyses of the CCD4b promoter and transcripts from various citrus germplasm accessions established a tight correlation between the presence of a putative 5' cis-regulatory enhancer within an MITE transposon and the enhanced allelic expression of CCD4b in C₃₀ apocarotenoid-rich red-peeled accessions. Phylogenetic analysis provided further evidence that functional diversification of CCD4b and naturally occurring variation of the CCD4b promoter resulted in the stepwise evolution of red peels in mandarins and their hybrids. Taken together, our findings provide new insights into the genetic and evolutionary basis of apocarotenoid diversity in plants, and would facilitate breeding efforts that aim to improve the nutritional and aesthetic value of citrus and perhaps other fruit crops.

SLAF-Based Construction of a High-Density Genetic Map and Its Application in QTL Mapping of Carotenoids Content in Citrus Fruit

Carotenoids are important antioxidant components in the human diet. To develop carotenoid-rich agricultural products by genetic intervention, understanding the genetic basis of carotenoids variation is essential. In this study, we constructed a high-density integrated genetic map with 3817 molecular markers using specific locus amplified fragment (SLAF) sequencing from a C. reticulata × P. trifoliata F₁ pseudotestcross population. A total of 17 significant quantitative trait loci (QTLs) distributed on Chromosomes (Chr) 2, 3, 5, 6, and 9 were detected to determine the carotenoid variation in the population. In particular, three QTL colocalizations for multiple carotenoid constituents were observed on Chr 2, 3, and 9, one of which was located on Chr2:34,654,608-35430715 accounted for 20.1-25.4% of the variation of luteoxanthin, auroxanthin, lutein, violaxanthin, and total carotenoid content. Overall, this study provides a genetic foundation for marker-assisted selection (MAS) breeding of nutritionally enhanced citrus fruit.

Evolutionarily distinct carotenoid cleavage dioxygenases are responsible for crocetin production in Buddleja davidii

Crocetin, one of the few colored apocarotenoids known in nature, is present in flowers and fruits and has long been used medicinally and as a colorant. Saffron is the main source of crocetin, although a few other plants produce lower amounts of this apocarotenoid. Notably, Buddleja davidii accumulates crocetin in its flowers. Recently, a carotenoid dioxygenase cleavage enzyme, CCD2, has been characterized as responsible for crocetin production in Crocus species. We searched for CCD2 homologues in B. davidii and identified several CCD enzymes from the CCD1 and CCD4 subfamilies. Unexpectedly, two out of the three CCD4 enzymes, namely BdCCD4.1 and BdCCD4.3, showed 7,8;7',8' activity in vitro and in vivo over zeaxanthin. In silico analyses of these enzymes and CCD2 allowed the determination of key residues for this activity. Both BdCCD4 genes are highly expressed during flower development and transcripts levels parallel the accumulation of crocins in the petals. Phylogenetic analysis showed that BdCCD4.2 grouped with almost all the characterized CCD4 enzymes, while BdCCD4.1 and BdCCD4.3 form a new sub-cluster together with CCD4 enzymes from certain Lamiales species. The present study indicates that convergent evolution led to the acquisition of 7,8;7',8' apocarotenoid cleavage activity in two separate CCD enzyme families.

Carotenoid Cleavage Oxygenases from Microbes and Photosynthetic Organisms: Features and Functions

Apocarotenoids are carotenoid-derived compounds widespread in all major taxonomic groups, where they play important roles in different physiological processes. In addition, apocarotenoids include compounds with high economic value in food and cosmetics industries. Apocarotenoid biosynthesis starts with the action of carotenoid cleavage dioxygenases (CCDs), a family of non-heme iron enzymes that catalyze the oxidative cleavage of carbon-carbon double bonds in carotenoid backbones through a similar molecular mechanism, generating aldehyde or ketone groups in the cleaving ends. From the identification of the first CCD enzyme in plants, an increasing number of CCDs have been identified in many other species, including microorganisms, proving to be a ubiquitously distributed and evolutionarily conserved enzymatic family. This review focuses on CCDs from plants, algae, fungi, and bacteria, describing recent progress in their functions and regulatory mechanisms in relation to the different roles played by the apocarotenoids in these organisms.

Diversity in the carotenoid profiles and the expression of genes related to carotenoid accumulation among citrus genotypes

Carotenoids are not only important to the plants themselves but also are beneficial to human health. Since citrus fruit is a good source of carotenoids for the human diet, it is important to study carotenoid profiles and the accumulation mechanism in citrus fruit. Thus, in the present paper, we describe the diversity in the carotenoid profiles of fruit among citrus genotypes. In regard to carotenoids, such as β-cryptoxanthin, violaxanthin, lycopene, and β-citraurin, the relationship between the carotenoid profile and the expression of carotenoid-biosynthetic genes is discussed. Finally, recent results of quantitative trait locus (QTL) analyses of carotenoid contents and expression levels of carotenoid-biosynthetic genes in citrus fruit are shown.

Carotenoid metabolism and regulation in horticultural crops

Carotenoids are a diverse group of pigments widely distributed in nature. The vivid yellow, orange, and red colors of many horticultural crops are attributed to the overaccumulation of carotenoids, which contribute to a critical agronomic trait for flowers and an important quality trait for fruits and vegetables. Not only do carotenoids give horticultural crops their visual appeal, they also enhance nutritional value and health benefits for humans. As a result, carotenoid research in horticultural crops has grown exponentially over the last decade. These investigations have advanced our fundamental understanding of carotenoid metabolism and regulation in plants. In this review, we provide an overview of carotenoid biosynthesis, degradation, and accumulation in horticultural crops and highlight recent achievements in our understanding of carotenoid metabolic regulation in vegetables, fruits, and flowers.