Abstract
BackgroundCarotenoids and flavonoids are important secondary metabolites in plants, which exert multiple bioactivities and benefits to human health. Although the genes that encode carotenogenesis and flavonoid biosynthetic enzymes are well characterized, the transcriptional regulatory mechanisms that are related to the pathway genes remain to be investigated. In this study, ‘Cara cara’ navel orange (CNO) fruit at four development stages were used to identify the key genes and TFs for carotenoids and flavonoids accumulation.ResultsIn this study, CNO was used to investigate the profiles of carotenoids and flavonoids by a combination of metabolomic and transcriptomic analyses. The important stage for the accumulation of the major carotenoid, lycopene was found to be at 120 days after florescence (DAF). The transcripts of five carotenogenesis genes were highly correlated with lycopene contents, and 16, 40, 48, 24 and 18 transcription factors (TFs) were predicted to potentially bind 1-deoxy-D-xylulose-5-phosphate synthase (DXS1), deoxyxylulose 5-phosphate reductoisomerase (DXR), geranylgeranyl diphosphate synthase (GGPPS2), phytoene synthase (PSY1) and lycopene β-cyclase (LCYB) promoters, respectively. Narirutin was the most abundant flavonoid in the flesh at the early stages, 60 DAF was the most important stage for the accumulation of flavonoids, and 17, 22, 14, 25, 24 and 16 TFs could potentially bind phenylalanine ammonia-lyase (PAL-1 and PAL-4), 4-Coumarate-CoA ligase (4CL-2 and 4CL-5), chalcone synthase (CHS-1) and chalcone isomerase (CHI) promoters, respectively. Furthermore, both sets of 15 candidate TFs might regulate at least three key genes and contribute to carotenoids/flavonoids accumulation in CNO fruit. Finally, a hierarchical model for the regulatory network among the pathway genes and TFs was proposed.ConclusionsCollectively, our results suggest that DXS1, DXR, GGPPS2, PSY1 and LCYB genes were the most important genes for carotenoids accumulation, while PAL-1, PAL-4, 4CL-2, 4CL-5, CHS-1 and CHI for flavonoids biosynthesis. A total of 24 TFs were postulated as co-regulators in both pathways directly, which might play important roles in carotenoids and flavonoids accumulation in CNO fruit.
Highlights
Carotenoids and flavonoids are important secondary metabolites in plants, which exert multiple bioactivities and benefits to human health
Lutein was found at 180 days after florescence (DAF) and its level increased at 210 DAF; βcryptoxanthin and β-carotene were detected at 150 DAF and tended to increase from 150 to 210 DAF; violaxanthin was detected at 120 DAF, significantly increased from 150 to 180 DAF (Additional file 1: Table S1)
Carotenoids and flavonoids are important metabolites in citrus fruit, and lycopene was found to be the major compound contributing to the red flesh in cara’ navel orange (CNO), while narirutin as the dominant flavonoid at the early stage
Summary
Carotenoids and flavonoids are important secondary metabolites in plants, which exert multiple bioactivities and benefits to human health. Citrus is one of the most important fruits in the world, and is rich in various primary and secondary metabolites, such as amino acids, organic acids, soluble sugars, flavonoids, volatile terpenoids, limonoids, coumarins and carotenoids [1,2,3,4,5]. Some metabolites, including flavonoids, limonoids, organic acids and soluble sugars, were reported to contribute to fruit taste [3, 4], whilst terpenoids and carotenoids were associated with fruit aroma and color [2, 6]. Carotenoids contribute to the eye-attracting fruit flesh color, and exhibit various bioactivities in human body, such as prevention of cancer and other diseases [7] and acting as the precursors for vitamin A [8]. A high concentration of lycopene in citrus fruit improves the fruit quality but is beneficial to human health
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