Cloning and functional analysis of the promoters that upregulate carotenogenic gene expression during flower development in Gentiana lutea

The mechanism of white flower formation in Brassica rapa is distinct from that in other Brassica species

KEY MESSAGE

A single nucleotide (G) deletion in the third exon of BraA02.PES2-2 (Bra032957) leads to the conversion of flower color from yellow to white in B. rapa, and knockout mutants of its orthologous genes in B. napus showed white or pale yellow flowers. Brassica rapa (2n = 20, AA) is grown worldwide as an important crop for edible oil and vegetables. The bright yellow flower color and long-lasting flowering period give it aesthetic qualities appealing to countryside tourists. However, the mechanism controlling the accumulation of yellow pigments in B. rapa has not yet been completely revealed. In this study, we characterized the mechanism of white flower formation using a white-flowered natural B. rapa mutant W01. Compared to the petals of yellow-flowered P3246, the petals of W01 have significantly reduced content of yellowish carotenoids. Furthermore, the chromoplasts in white petals of W01 are abnormal with irregularly structured plastoglobules. Genetic analysis indicated that the white flower was controlled by a single recessive gene. By combining BSA-seq with fine mapping, we identified the target gene BraA02.PES2-2 (Bra032957) homologous to AtPES2, which has a single nucleotide (G) deletion in the third exon. Seven homologous PES2 genes including BnaA02.PES2-2 (BnaA02g28340D) and BnaC02.PES2-2 (BnaC02g36410D) were identified in B. napus (2n = 38, AACC), an allotetraploid derived from B. rapa and B. oleracea (2n = 18, CC). Knockout mutants of either one or two of BnaA02.PES2-2 and BnaC02.PES2-2 in the yellow-flowered B. napus cv. Westar by the CRISPR/Cas9 system showed pale-yellow or white flowers. The knock-out mutants of BnaA02.PES2-2 and BnaC02.PES2-2 had fewer esterified carotenoids. These results demonstrated that BraA02.PES2-2 in B. rapa, and BnaA02.PES2-2 and BnaC02.PES2-2 in B. napus play important roles in carotenoids esterification in chromoplasts that contributes to the accumulation of carotenoids in flower petals.

Molecular cloning and characterization of Cinnamoyl-CoA reductase promoter gene from Asarum sieboldii Miq

Asarum sieboldii Miq., a perennial herb of the family Aristolochiaceae, is widely used in China to treat cold, fever, aphthous stomatitis, toothache, gingivitis, and rheumatoid arthritis. Methyleugenol is the most representative pharmacological constituent of this medicinal herb. Cinnamoyl-CoA reductase (CCR), which has been well known for occupying a critical position in the lignin biosynthesis pathway, is also shared with the biosynthesis of methyleugenol. To better understand the regulatory mechanisms of methyleugenol biosynthesis, a 1530-bp long promoter region of the AsCCR1 gene was isolated. PLACE and PlantCARE analysis affirmed the existence of the core promoter elements such as TATA and CAAT boxes, abiotic stress-responsive cis-regulation elements like abscisic acid-responsive element, G-box, and MBS in the isolated sequence. The histochemical assay suggested that it was a constitutive promoter, highly expressed in the root tissue. Moreover, the region of -200 bp to ATG (start codon) was enough to drive the expression of It GUS gene. Treatments with low temperature and high concentration of gibberellin or abscisic acid demonstrated the abiotic stress-induced expression of the AsCCR1 promoter. Overall, this study revealed the isolation and characterization of the AsCCR1 promoter. Moreover, it also provided a candidate gene for molecular breeding in A. sieboldii to enhance its pharmacological potential.

A Comparative Study of Flavonoids and Carotenoids Revealed Metabolite Responses for Various Flower Colorations Between Nicotiana tabacum L. and Nicotiana rustica L

Tobacco is a model plant for studying flower coloration. Flavonoids and carotenoids were reported to contribute to the flower color in many plants. We investigated the mechanism underlying flower color formation in tobacco by comparing the profiling flavonoids and carotenoids between various species Nicotiana tabacum L. and Nicotiana rustica L., as their flowers commonly presented red (pink) and yellow (orange), respectively. The metabolomes were conducted by UPLC-ESI-MS/MS system. The main findings were as follows: (1) A total of 31 flavonoids and 36 carotenoids were identified in all four cultivars involved in N. tabacum and N. rustica. (2) Flavonoids and carotenoids tended to concentrate in the red flowers (N. tabacum) and yellow flowers (N. rustica), respectively. (3) About eight flavonoids and 12 carotenoids were primarily screened out for metabolic biomarkers, such as the robust biomarker involving kaempferol-3-o-rut, quercetin-glu, rutin, lutein, and β-carotene. This is the first research of systematic metabolome involving both flavonoids and carotenoids in tobacco flower coloration. The metabolic mechanism concluded that flavonoids and carotenoids mainly contributed to red (pink) and yellow (orange) colors of the tobacco flowers, respectively. Our finding will provide essential insights into characterizing species and modifying flower color in tobacco breeding through genetic improvement or regulation of featured metabolic synthesis.

A novel gene, CaATHB-12, negatively regulates fruit carotenoid content under cold stress in Capsicum annuum

BACKGROUND

Carotenoids, the secondary metabolites terpenoids, are the largest factors that form the fruit color. Similar to flavonoids, they are not only safe and natural colorants of fruits but also play a role as stress response biomolecules.

METHODS

To study the contribution of the key genes in carotenoids biosynthesis, fruit-color formation, and in response to cold stress, we characterized the key regulatory factor CaATHB-12 from the HD-ZIP I sub-gene family members in pepper.

RESULTS

Cold stress enhanced carotenoid accumulation as compared with the normal condition. CaATHB-12 silencing through virus-induced gene silencing changed the fruit color by regulating the carotenoid contents. CaATHB-12 silencing increased the antioxidant enzyme activities in the fruits of pepper, exposed to cold stress, whereas CaATHB-12 overexpression decreased the activities of antioxidant enzymes in the transgenic Arabidopsis lines, exposed to cold stress, suggesting that CaATHB-12 is involved in the regulation of cold stress in the pepper fruits.

CONCLUSION

Our research will provide insights into the formation of fruit color in pepper and contribution of CaATHB-12 in response to cold stress. Further study should be focused on the interaction between CaATHB-12 and its target gene.

The Genes of CYP, ZEP, and CCD1/4 Play an Important Role in Controlling Carotenoid and Aroma Volatile Apocarotenoid Accumulation of Apricot Fruit

Carotenoids are important coloration molecules and indispensable component of the human diet. And these compounds confer most of the apricot fruit yellow or orange color. In China, fruit of some apricot cultivar present light-yellow color but strong flowery flavor, however, the chemical mechanism remains unknown. Here, carotenoids and aroma volatile apocarotenoids (AVAs) in three skin types of apricot cultivars (orange, yellow, and light-yellow skinned) were determined by HPLC and GC-MS, respectively. And the transcript levels of carotenogenic genes were analyzed by qRT-PCR. The orange-skinned cultivars "Hongyu" and "Danxing" fruit presented the most abundant total carotenoid, β-carotene and specific α-carotene contents, and β-carotene (52-77%) increased to become the dominant carotenoid during fruit ripening. The transcript levels of lycopene β-cyclase (LCYb) and β-carotene hydroxylase (CHYb) sharply increased during ripening. The yellow-skinned cultivars "Sulian No. 2" and "Akeyaleke" fruit contained lower levels of total carotenoids and β-carotene but were rich in phytoene. The light-yellow coloration of "Baixing" and "Luntaixiaobaixing" fruit was attributed to low amounts of total carotenoids, lutein, and neoxanthin and an absence of β-cryptoxanthin, but high level of aroma volatile apocarotenoids (AVAs) such as β-ionone were detected in these cultivars fruit, accompanied by low transcript levels of carotene hydroxylase (CYP) and zeaxanthin epoxidase (ZEP) but high levels of carotenoid cleavage dioxygenase 1 (CCD1) and CCD4. Correlation analysis showed that the expression level of CCD1 negatively correlated with carotenoid accumulation but positively with AVAs production. These collected results suggest that both carotenoid biosynthesis and degradation are important for apricot coloration and aroma formation. CYP, ZEP, CCD1, and CCD4 may be the key regulation points for carotenoid and AVAs accumulation in apricot fruit, which provide important targets for quality-oriented molecular breeding.

Functional Analysis of the Marigold (Tagetes erecta) Lycopene ε-cyclase (TeLCYe) Promoter in Transgenic Tobacco

Lycopene ε-cyclases (LCYEs) are key enzymes in carotenoid biosynthesis converting red lycopene to downstream lutein. The flowers of marigold (Tagetes erecta) have been superior sources to supply lutein. However, the transcriptional regulatory mechanisms of LCYe in lutein synthesis are still unclear in marigold. In this work, the expression pattern of the TeLCYe gene in marigold indicated that TeLCYe mainly expressed in floral organs. To gain a better understanding of the expression and regulatory mechanism of TeLCYe gene, the TeLCYe promoter was isolated, sequenced, and analyzed through bioinformatics tools. The results suggested that the sequence of TeLCYe promoter contained various putative cis-acting elements responsive to exogenous and endogenous factors. The full-length TeLCYe promoter and three 5'-deletion fragments were fused to the GUS reporter gene and transferred into tobacco to test the promoter activities. A strong GUS activity was observed in stems of seedlings, leaves of seedlings, middle stems, top leaves, petals, stamens, and stigmas in transgenic tobacco containing full-length TeLCYe promoter LP0-2086. Deletion of - 910 to - 429 bp 5' to ATG significantly increased the GUS activity in chloroplast-rich tissues and floral organs, while deletion occurring from 1170 to 910 bp upstream ATG decreased the TeLCYe promoter strength in stems of seedlings, leaves of seedlings, top leaves and sepals. Functional characterization of the full-length TeLCYe promoter and its' deletion fragments in stable transgenic tobacco indicated that the LP0-2086 contains some specific cis-acting elements, which might result in the high-level expression of in floral organs, and LP2-910 might contain some specific cis-acting elements which improved GUS activities in vegetable tissues.

Potentiation of concurrent expression of lipogenic genes by novel strong promoters in the oleaginous microalga Phaeodactylum tricornutum

There has been growing interest in using microalgae as production hosts for a wide range of value-added compounds. However, microalgal genetic improvement is impeded by lack of genetic tools to concurrently control multiple genes. Here, we identified two novel strong promoters, designated Pt202 and Pt667, and delineated their potential role on simultaneously driving the expression of key lipogenic genes in Phaeodactylum tricornutum. In silico analyses of the identified promoter sequences predicted the presence of essential core cis elements such as TATA and CAAT boxes. Regulatory role of the promoters was preliminarily assessed by using GUS reporter which demonstrated strong GUS expression. Thereafter, two key lipogenic genes including malic enzyme (PtME) and 5-desaturase (PtD5b), were overexpressed by the two promoters Pt202 and Pt667, respectively, in P. tricornutum. Combinatorial gene overexpression did not impair general physiological performance, meanwhile neutral lipid content was remarkably increased by 2.4-fold. GC-MS analysis of fatty acid methyl esters revealed that eicosapentaenoic acid (EPA; C20:5) was increased significantly. The findings augment a crucial kit to microalgal genetic tools that could facilitate the multiple-gene expression driven by various promoters, and promote microalgae for industrial bioproduction.

Carotenoid Accumulation and Its Contribution to Flower Coloration of Osmanthus fragrans

Among naturally occurring pigments, carotenoids are importantly involved in the photosynthesis of plants and responsible for the coloration of petals and fruits. Osmanthus fragrans Lour., a famous ornamental plant, has many cultivars with different flower color. Petal coloration in O. fragrans mainly depends on the kinds of carotenoids and their contents. To investigate the mechanism of flower coloration in different cultivars, an analysis of phenotypic classification, phytochemistry, as well as the expression of carotenoid metabolism genes based on different groups was performed in the present study. Two main clusters including the orange-red cluster containing Aurantiacus cultivars and the yellowish-white cluster containing the other three cultivar groups were classified using the CIEL∗a∗b∗ system. No significant differences in flavonoid contents were observed between these two clusters. However, carotenoids, especially α-carotene and β-carotene, were found to have crucial roles in the diversity of floral coloration among the different cultivars. Carotenoid compositions in the petals of cultivars from both clusters consisted of α-carotene, β-carotene, α-cryptoxanthin, β-cryptoxanthin, lutein, and zeaxanthin, but carotenoid accumulation patterns during the flowering process were different. The petals of the yellowish-white cultivars exhibited high contents of β-carotene, lutein and α-carotene, whereas the petals of the orange-red cultivars mainly contained β-carotene and α-carotene. The profound diversity in the total carotenoid concentrations in the two clusters was determined by the transcript levels of OfCCD4. Furthermore, the accumulation of upstream products with orange color in orange-red cultivars was partially due to the low expression of OfCHYB, whereas the relatively higher OfCHYB expression in the petals of the yellowish-white cultivars led to higher proportions of lutein, which is yellow. We also found that downregulation of OfLCYE, which encodes 𝜀-ring cyclase, indicated that the carotenoid flux of most cultivars mainly resulted in more β, β-branched products. Additionally, carotenoid biosynthesis in green tissues and petals was compared, revealing the tissue specificity of carotenoid accumulation in O. fragrans. Therefore, the effects of multiple genes on carotenoid accumulation give rise to the colorful O. fragrans.

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.