Exploring the differential mechanisms of carotenoid biosynthesis in the yellow peel and red flesh of papaya
Red-fleshed papaya is a good material to study the different carotenoids accumulation mechanism in the peel and flesh. Although the peel and flesh of papaya closely integrated into one body, the flesh coloration changing from white to red, while the exocarp coloration changing from green to yellow. In this study, the major carotenoids accumulation and the expression patterns of key carotenoid biosynthesis pathway genes in the process of papaya fruit ripening were studied, and the carotenoid biosynthetic pathways in the yellow peel and red flesh of papaya were investigated.
The carotenoid composition in papaya flesh and peel were different. The major carotenoids were lutein and β-carotene in the peel, while lycopene in the flesh. The accumulation of carotenoids, including lycopene, β-carotene, and β-cryptoxanthin were considered to cause the orange-red color of papaya cv. ‘Daqing No.10’ flesh. The color of peel changed from green to yellow because of the fast degradation of chlorophyll and the appearance of carotenoids such as lutein and β-carotene. Thirteen genes that encode enzymes in the carotenoid biosynthetic pathway were detected in papaya fruit transcriptome: two phytoene synthase (PSY1, PSY2), two phytoene desaturase (PDS1, PDS2), one ζ-carotene desaturase (ZDS), four lycopene cyclase (CYCB, LCYB1, LCYB2, LCYE), one β-carotene hydroxylase (CHYB), one carotene ε-monooxygenase (LUT1), one violaxanthin de-epoxidase (VDE), and one zeaxanthin epoxidase (ZEP). The results of RNA-Seq and RT-qPCR showed the expression of carotenoid biosynthetic pathway genes was consistent with the change of carotenoid content. Carotenoid biosynthetic pathways in the yellow peel and red flesh of papaya were analysed based on the major carotenoids accumulation and the expression patterns of key carotenoid biosynthesis pathway genes. There was only a β-branch of carotenoid biosynthesis in the flesh of papaya, while there were both α- and β-branch of carotenoid biosynthesis in papaya peel. In the process of papaya fruit ripening, the α-branch was inhibited and the β-branch was enhanced in the peel.
The differential carotenoid accumulation and biosynthesis pathway genes expression in peel and flesh, lay a foundation for further study and provide further insights to control fruit color and improve fruit quality and appearance.
KeywordsCarica papaya L. Carotenoids Metabolic pathway RNA-Seq Coloration
Color break stage
Chromoplast-specific lycopene β-cyclase
Differentially expressed Genes
Differential gene expression
The flesh of GS
The flesh of CB
The flesh of HY
The flesh of FY
Fragments Per Kilobase of transcript per Million mapped reads
Full yellow stage
Half yellow stage
Kyoto Encyclopedia of Genes and Genomes
The peel of GS
The peel of CB
The peel of HY
The peel of FY
Reverse transcription quantitative PCR
Sequence Read Archive
Ultra-high Performance Liquid Chromatography
Carotenoids are one of the most essential components for human nutrition and health, mainly due to their pro-vitamin A and antioxidant proportion. Humans are not able to synthesize carotenoids and depend entirely on natural sources or dietary supplements. Dietary carotenoids are thought to provide health benefits in decreasing the risk of disease, particularly certain cancers and eye disease [1, 2]. The most common dietary carotenoids are α-carotene, β-carotene, β-cryptoxanthin, lycopene, lutein, and zeaxanthin; α-carotene, β-carotene, and β-cryptoxanthin are precursors of vitamin A, while lycopene, lutein, zeaxanthin have no vitamin A activity but can also act as antioxidants. Besides the nutritional value of carotenoids, they are the main coloring pigments of the yellow, orange, and red colors of fruits: mango (Mangifera indica L.) , tomato (Lycopersicon esculentum Mill.) , orange (Citrus) , banana (Musa spp.) , papaya (Carica papaya L.) , and loquat (Eriobotrya japonica L.) .
The biosynthetic pathway of carotenoids in high plants has been gradually clarified by biochemical, classical genetic and molecular genetic analysis. Plant carotenoids are isoprenoid-derived molecules generally synthesized and located in plastids. Two Geranylgeranyl pyrophosphate (GGPP) molecules condense via phytoene synthase (PSY), forming phytoene; phytoene forms lycopene via phytoene desaturase (PDS) and ζ-carotene desaturase (ZDS); then the biosynthetic pathway of carotenoids splits after biosynthesis of lycopene into the α-branch and the β-branch: ① in α-branch, lycopene cyclase (LCY) catalyzes the cyclization of lycopene to form α-carotene; α-carotene is converted into lutein by β-carotene hydroxylase (CHYB) and carotene ε-monooxygenase (LUT1); ② in β-branch, LCY catalyzes the cyclization of lycopene to form β-carotene, and CHYB catalyzes the conversion of β-carotene to β-cryptoxanthin and β-cryptoxanthin to zeaxanthin [9, 10]. Many carotenoid synthesis genes including PSY, PDS, ZDS, and LCY have been cloned successfully and genetically modified plants with increased carotenoids levels have been obtained such as rice, tomato, potato, and canola [11, 12].
Papaya is an important tropical and sub-tropical fruit crop which is known for its high nutritional values like vitamins A and vitamins C [13, 14]. There are two types of papaya, red-fleshed and yellow-fleshed. The major carotenoid in the pulp of red-fleshed papaya is lycopene, while the major carotenoids in the yellow-fleshed papaya are β-carotene and β-cryptoxanthin [15, 16]. It has been documented that a 2-bp insertion is present in the chromoplast specific lycopene β-cyclase (CYCB) gene resulting in a recessive loss-of-function mutation, so as to accumulate a large amount of lycopene which is responsible for the red flesh . Pigments in the flesh of papaya are known, but pigments in the peel of papaya remain unknown. The color of exocarp turns from green to yellow and the color of the flesh changes from white to red during the fruit ripening process in red-fleshed papaya. The differential accumulation of carotenoids and differential expression of carotenoid synthesis genes were studied in order to understand the different carotenoids accumulation mechanism in the peel and flesh of papaya.
Papaya coloration during papaya fruit ripening
Chlorophyll change and carotenoid accumulation
Almost no chlorophyll, lutein, and zeaxanthin were detected in the flesh, but a large amount of lycopene, some β-carotene, and a small amount of β-cryptoxanthin were detected in the flesh. In the process of fruit ripening, there was a sustained increase in carotenoids (lycopene, β-carotene, β-cryptoxanthin) content. Therefore, the flesh color changed from white to red. Except for lycopene, the other carotenoids (lutein, β-carotene, β-cryptoxanthin, zeaxanthin) content in peels were higher than that in fleshes. This illustrates that exocarp is an important source of carotenoids.
The gene number of all samples
Total Clean Reads No.
Known Gene No.
(Known Gene Ratio)
New Gene No.
All Gene No.
All Reference Gene No.
The pathway enrichment analysis was carried out using the KEGG pathway database. We focused on the analysis of carotenoid biosynthetic pathway. Comparing with FL1, the expression of PDS, ZDS, CYCB (chromoplast specific lycopene β-cyclase), CHYB, and VDE (Violaxanthin de-epoxidase) increased, while the expression of LCYB2 decreased in FL2 (Additional file 1: Figure S1). Also, the expression of most carotenoid biosynthetic pathway genes (PDS, ZDS, CYCB, LUT1, CHYB, VDE, and ZEP (zeaxanthin epoxidase)) in the peel increased with papaya ripening, while the expression of LCYE decreased (Additional file 2: Figure S2). Comparing of carotenoid biosynthetic genes in the peel and flesh of papaya, we found that LCYE was the only gene that had been enriched in FL2 by KEGG pathway enrichment. The expression level of LCYE in FL2 was significantly lower than that in PE2 (Additional file 3: Figure S3). This indicated that the differential expression of LCYE may be the key reason for the difference of carotenoid components between flesh and peel in color break stage. As previously described, the major carotenoid in the peel was lutein, but it was not detected in the flesh. There was no lutein in the flesh because of non-detectable expression of LCYE.
Screening carotenoid biosynthesis related genes in the transcriptome
Phytoene synthase (PSY) is a key regulator in the carotenoid biosynthetic pathway ; Two PSY genes were detected in the transcriptome. The expression of PSY1 increased in the flesh and peel, and no expression of PSY2 was detected, thereby demonstrating that PSY1 was active in the carotenoid biosynthetic pathway in papaya. Similar to papaya, PSY1 plays an important role in tomato fruit tissue pigmentation too . Two PDS genes were detected, and the expression patterns were the same, but the expression abundance between the two genes was different. The expression of PDS1 in FL2 is 394.46, while that of PDS2 is 7.9 (Additional file 4: Table S1). Because the abundant gene transcripts account for more enzymatic activity participating in carotenoid synthesis, PSY1 and PDS1 are the major genes involved in the synthesis of carotenoid in fruit. The expression of two genes (ZDS and CHYB) increased with papaya ripening. Four lycopene cyclase genes were detected: LCYB1, LCYB2, CYCB, and LCYE. LCYB1 and CYCB had been cloned and characterized previously [20, 21]. We didn’t analyze CYCB because there is a 2-bp insertion in CYCB gene in red-fleshed papaya, CYCB is a functional gene in yellow-fleshed papaya, but a pseudo gene in red-fleshed papaya . The expression of the other three lycopene cyclase genes was different. This showed that they may play different roles in carotenoid synthesis.
RT-qPCR analysis of transcription levels of five carotenoid biosynthetic pathway genes
Carotenoid biosynthetic pathway in the yellow peel and red flesh of papaya
Although the flesh and peel of red-fleshed papaya fruit are closely integrated into one body, the coloration of the flesh and peel are quite different: the color of exocarp turning from green to yellow and the color of flesh changing from white to red. So exploring the differential mechanisms of carotenoid biosynthesis in the yellow peel and red flesh of papaya is an interesting research point. The color transformation is directly related to the change of the carotenoid component. Some studies showed that the concentration of carotenoids was significantly higher in the peel than that in the flesh such as Citrus iyo fruit  and loquat fruit . The coloration of loquat peel due to the transient increase of carotenoids and degradation of chlorophylls . In papaya fruit, carotenoids (lycopene, β-carotene, β-cryptoxanthin) accumulates extensively in the flesh, therefore the flesh color changes from white to red; although the content of lutein and β-carotene decreased in the peel, the rapid loss of chlorophyll leads to the appearance of carotenoids causes a color shift from green to yellow. Microscopic observation also proved that the carotenoid composition between the peel and the flesh are different. Yellow round-shaped elements are abundant in peel cells, while larger yellow round-shaped and several red elongated contours crystals present in flesh cells. This result is consistent with previous research that β-carotene is deposited in globular and tubular elements in a lipid-dissolved form whereas lycopene is deposited in a solid crystalline form [23, 24].
The synthesis of carotenoid in peel and flesh of papaya had been analysed base on the distribution of carotenoid components and the expression of carotenoid biosynthesis pathway genes. The increase in the content of β-cryptoxanthin and zeaxanthin was accompanied by the decrease in the content of lutein and β-carotene, indicating that there were both α- and β-branch in the peel and the β-branch was enhanced in the process of papaya fruit ripening. However, there is only a β-branch in the flesh because no lutein and expression of LCYE were detected, LCYE was the only gene that had been enriched in FL2 by KEGG pathway enrichment. It expressed abundantly in the green peel, but hardly expressed in the flesh. LCYE is the key reason for the differential carotenoid accumulation in the flesh and peel of papaya. Except for LCYE, three lycopene cyclases had been detected in papaya fruit transcriptome, LCYB1, CYCB, and LCYB2. LCYB1 has been considered as a chloroplast-specific lycopene β-cyclase as well as CYCB is a chromoplast-specific lycopene β-cyclase [20, 21]. Although the gene of CYCB was named as LCY-β2 in previous studies, CYCB was used in our paper [20, 25]. LCYB1 was supposed to be a chloroplast-specific lycopene β-cyclase, but it had higher expression level in red flesh and the downstream products of lycopene β-cyclase like β-carotene and β-cryptoxanthin were also detected, which means LCYB1 expresses in non-chlorophyllous tissue as well. This is not consistent with previous studies . LCYB2 is a newly discovered gene of lycopene cyclase in papaya, the expression of LCYB2 in peel was higher than that in flesh, and the transcript levels had been decreasing with papaya fruit ripening. However, the function of LCYB2 is still unclear in papaya.
The reason for the conversion from α-branch to β-branch in the peel of papaya during fruit ripening is not clear. One of the key roles of carotenoids in plants is protecting the photosynthetic apparatus against photoinhibition and photodamage . The xanthophyll cycle transforms the excitation energy into heat and thereby prevents the formation of damaging ROS. It is an important photoprotective process in plants . Violaxanthin de-epoxidase (VDE) and zeaxanthin epoxidase (ZEP) are the two enzymes in the xanthophyll cycle. In excess light conditions, VDE catalyzes the conversion of violaxanthin to zeaxanthin via antheraxanthin, whereas ZEP catalyzes the reverse reaction . The expression of ZEP and VDE were up-regulated in color break stage of papaya, this may due to chlorophyll degradation enhanced the activity of the xanthophyll cycle to avoid severe photodamage under strong illumination.
By above knowable, the differential expression of the key carotenoid biosynthesis pathway genes in the peel and flesh of papaya result in the differential accumulation of carotenoids. But little is known about the regulatory mechanisms of the genes involved in ɑ- and β-branch. Transcriptional factors (TFs) play important roles in a variety of cellular and developmental processes, such as fruit ripening, senescence, hormone signalling, and various biotic and abiotic stress responses. For example, several TFs have been reported to be involved in the ripening of banana , tomato , and papaya . Recently, some TFs were found to play essential roles in regulating the carotenoid biosynthesis in plants. An R2R3-MYB represses the transformation of α- and β-branch carotenoids by negatively regulating expression of CrBCH2 and CrNCED5 in flavedo of Citrus reticulate . CpNAC1 is a positive regulator of carotenoid biosynthesis during papaya fruit ripening . The protein of CpNAC2 physically interacts with CpEIN3a, which act as a transcriptional activator of CpPDS4 and CpCHY-b by directly binding to their promoters has been proposed . Several TFs including MYB, ERF, bZIP, bHLH, WRKY, and NAC had been selected according to the expression patterns with papaya fruit ripening in our research (data not given). Some TFs expression decreased in both flesh and peel, while some TFs were gradually increased, paralleling the increase of the carotenoids content; some TFs have higher expression levels in the flesh, and some TFs have much higher expression levels in the peel. The differential expression of these TFs suggests that they may be involved in the regulation of carotenoid biosynthesis genes transcription in flesh or peel. The functions of these TFs should be studied in the future.
Carotenoid biosynthetic pathway in the yellow peel and red flesh of papaya was revealed on the basis of the major carotenoids accumulation and the expression patterns of key carotenoid biosynthesis pathway genes. There is only a β-branch of carotenoid biosynthesis in the flesh of papaya, while there are both α- and β-branch in papaya peel. However, additional genomics wide analysis are required to determine and validate. This work elucidated the differential anabolic pathway of carotenoids in peel and flesh of papaya, and provide further insights to control fruit color and improve fruit quality and appearance.
Papaya fruits (C. papaya cv. ‘Daqing No.10’) in different development stages (GS, Green stage; CB, Color break stage; HY, Half yellow stage; FY, Full yellow stage) were harvested from Longzhijia fruit and vegetable farmers’ specialized cooperatives in November in Zhangzhou, China. The healthy fruits of similar size, shape, and maturity were divided into four groups. Five papaya in the same stage were peeled, seeds were removed, and the peel and flesh were cut into some pieces respectively. The pieces were mixed and weighed, frozen in liquid nitrogen, and stored at − 80 °C. Finally, eight types of samples were obtained, namely FL1 (the flesh of GS), FL2 (the flesh of CB), FL3 (the flesh of HY), FL4 (the flesh of FY), PE1 (the peel of GS), PE2 (the peel of CB), PE3 (the peel of HY), PE4 (the peel of FY). Tissue samples (root, stem, leaf, and flower) were collected for reverse transcription quantitative PCR (RT-qPCR) analysis. For chromoplast observation, small pieces of flesh and peel of FY stage papaya were obtained respectively. All pieces were cut with a sharp razor blade to thin sections. Then the ultrathin sections were put on a clean slide and examined with an Olympus IX51 (Tokyo, Japan).
Spectrophotometric determination of chlorophyll content
Chlorophyll was extracted from 200 mg of ground papaya fruit materials using 25 mL of 80% acetone. The absorbance was determined at 663 nm and 646 nm with a UV-Vis spectrophotometer equipped with 1.0 cm quartz cells. Based on the equation of total chlorophyll concentration CT = 7.18 × A663 + 17.32 × A646, total chlorophyll content was determined .
UPLC analysis of major carotenoids
An modified method for the extraction of major carotenoids from papaya fruit was used in the study . Briefly, the sample (peel or flesh) was lyophilized using a freeze dryer at − 40 °C for two days, followed by mechanical grinding with liquid nitrogen, and stored at − 20 °C. papaya fruit (250 mg) was added into 2.5 mL of acetone (1:10). Ultrasonic-assisted extraction was used to extract carotenoids for 1 h at 240 W at 20 °C, and then centrifuged for 10 min at 10,000 rpm at 4 °C. The liquid supernatant was filtered by 0.22 μm organic filter, and stored at − 20 °C for later use.
Carotenoid pigments analysis was referenced to Yan  and Lee . Chromatography was carried out with a Waters Ultra-high performance liquid chromatography. Samples were measured by a C18 carotenoid column (100 mm × 2.1 i.d., 1.7 μm) from Waters. The eluent was methyl tertiary butyl ether and methanol (V/V = 30:70) with the type of isocratic elution. And each eluent contained 0.01% BHT and 0.05% TEA (triethylamine) as modifiers in order to prevent the degradation of carotenoids on the column. The following rate was 0.5 mL/min, column temperature was 25 °C, and the injection volume was 5 μL, analyzing with UV at 445 nm. The lutein, β-carotene, lycopene standards were obtained from Solarbio (Beijing Solarbio Science & Technology Co., Ltd., Beijing, China). The β-cryptoxanthin standard was from Sigma (USA).
RNA extraction and RNA-Seq
Total RNA was extracted from the papaya fleshes or peels using an RNA extraction kit (Dongsheng Biotech, Guangzhou, China). Total RNA of four samples (PE1, PE2, FL1, FL2) was used to prepare cDNA libraries using the Illumina Dynabeads® mRNA DIRECT™ Kit. Then, the cDNA libraries were used for paired-end 125 sequencing using an Illumina Hiseq2500 at Genedenovo Biotechnology Co., Ltd. (Guangzhou, China). In total, four sets of raw reads were obtained, and all sequencing data were deposited in the NCBI Sequence Read Archive (SRA). The raw reads were filtered to remove “dirty” data, including low-quality reads, adaptor sequences, and rRNA reads, to generate “clean” reads. FPKM (Fragments Per Kilobase of transcript per Million mapped reads) was used to measure the transcript abundance of each gene. Those with a fold-change of ≥2 and a false discovery rate (FDR) < 0.05 were considered significantly differentially expressed genes (DEGs) . All of the genes were annotated using the reference papaya genome database (Carica papaya Version1.181: CpGDB181(JGI)) , NCBI non-redundant (Nr) database, the Kyoto Encyclopedia of genes and genomes (KEGG) pathway database. The KEGG enrichment analysis was performed with a Qvalue cut off of 0.05.
Reverse transcription quantitative PCR analysis
PCR primers used in this study
We sincerely thank Qi Xin Chen (Genedenovo Biotechnology Co., Ltd., Guangzhou, Beijing) for the technical assistance.
Innovation and Technology Foundation in Fujian Agriculture and Forestry University, China (KFA17355A).
Science Foundation for Doctorate Research in Hebei Normal University of Science and Technology, China (2018YB023).
Availability of data and materials
The raw data had been submitted to the NCBI SRA database, the accessions number is SRA688610.
YHS conceived the study and designed the experiments. YHS, FYY, BGL, WWZ, TJ, and LF carried out the experiments and analyzed the data. YHS wrote the manuscript. XJC and RM guided and revised the article. All authors read and approved the final manuscript.
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The authors declare that they have no competing interests.
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- 3.Zhou Y, Tang Y, Tan X, Guo J. Effects of exogenous ABA, GA3 and cell-wall-degrading enzyme activity, carotenoid content in ripening mango fruit. Acta Phytophisiologica Sinica. 1996;22:421–6.Google Scholar
- 6.Davey MW, Bergh IVD, Markham R, Swennen R, Keulemans J. Genetic variability in Musa fruit provitamin a carotenoids, lutein and mineral micronutrient contents. Food Chem. 2009;115(3):806–13.Google Scholar
- 7.Schweiggert RM, Steingass CB, Mora E, Esquivel P, Carle R. Carotenogenesis and physico-chemical characteristics during maturation of red fleshed papaya fruit (Carica papaya L.). Food Res Int. 2011;44(5):1373–80.Google Scholar
- 12.Rosati C, Diretto G, Giuliano G. Biosynthesis and engineering of carotenoids and apocarotenoids in plants: state of the art and future prospects. Biotechnol Genet Eng Rev. 2009;26(1):139–62.Google Scholar
- 13.Krishna K, Paridhavi M, Patel JA. Review on nutritional, medicinal and pharmacological properties of papaya (Carica papaya Linn.). Natural product radiance. 2008;7(4):364–73.Google Scholar
- 14.Yamamoto HY. Biochemistry of the violaxanthin cycle in higher plants. Pure Appl Chem. 1979;51(3):639–48.Google Scholar
- 15.Wall MM. Ascorbic acid, vitamin a, and mineral composition of banana (Musa sp.) and papaya (Carica papaya) cultivars grown in Hawaii. J. Food Compos. Anal. 2006;19:434–45.Google Scholar
- 16.Saengmanee P, Burns P, Chaisan T, Thaipong K, Siriphanich J. Genetic diversity of genes involved in the carotenoid pathway of Carica papaya L. and their expression during fruit ripening. J. Plant Biochem. Biotechnol. 2017;27(1):1–10.Google Scholar
- 22.Wang W, Xu J, Xu C. Developmental changes in external color, pigment content and composition in Citrus iyo fruit. Acta Horticulturae Sinica. 2006;33(3):461–5.Google Scholar
- 27.Huner NPA, Öquist G, Sarhan F. Energy balance and acclimation to light and cold. Trends Plant Sci. 1998;3(6):224–30.Google Scholar
- 30.Fu CC, Han YC, Qi XY, Shan W, Chen JY, Lu WJ, et al. Papaya CpERF9 acts as a transcriptional repressor of cell-wall-modifying genes CpPME1/2 and CpPG5 involved in fruit ripening. Plant Cell Rep. 2016;35(11):1–12.Google Scholar
- 34.Lichtenthaler HK, Wellburn AR. Determination of total carotenoids and chlorophylls a and b of leaf in different solvents. Analysis. 1983;11(5):591–2.Google Scholar
- 35.Yan J, Cai Z, Shen Z, Zhang B, Ruijuan MA, Mingliang YU. Extraction and analytical methods of carotenoids in fruit of yellow flesh peach. Journal of Fruit Science. 2015;32:1267–74.Google Scholar
- 39.Livak KJ, Schmittgen TD. Analysis of relative gene expression data using real-time quantitative PCR and the 2−ΔΔCT method. Methods. 2001;25(4):402–8.Google Scholar
- 40.Naik PS, Chanemougasoundharam A, Khurana SMP, Kalloo G. Genetic manipulation of carotenoid pathway in higher plants. Curr Sci. 2003;85(25):1423–30.Google Scholar
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