Abstract
The ripening of papaya is a physiological and metabolic process associated with accumulation of carotenoids, alternation of flesh color and flavor, which depending on genotype and external factors such as light and hormone. Transcription factors regulating carotenoid biosynthesis have not been analyzed during papaya fruit ripening. RNA-Seq experiments were implemented using different ripening stages of papaya fruit from two papaya varieties. Cis-elements in lycopene β-cyclase genes (CpCYC-B and CpLCY-B) were identified, and followed by genome-wide analysis to identify transcription factors binding to these cis-elements, resulting in the identification of CpbHLH1 and CpbHLH2, two bHLH genes. The expressions of CpbHLH1/2 were changed during fruit development, coupled with transcript increase of carotenoid biosynthesis-related genes including CpCYC-B, CpLCY-B, CpPDS2, CpZDS, CpLCY-E, and CpCHY-B. Yeast one-hybrid (Y1H) and transient expression assay revealed that CpbHLH1/2 could bind to the promoters of CpCYC-B and CpLCY-B, and regulate their transcriptions. In response to strong light, the results of elevated expression of carotenoid biosynthesis-related genes and the changed expression of CpbHLH1/2 indicated that CpbHLH1/2 were involved in light-mediated mechanisms of regulating critical genes in the carotenoid biosynthesis pathway. Collectively, our findings demonstrated several TF family members participating in the regulation of carotenoid genes and proved that CpbHLH1 and CpbHLH2 individually regulated the transcription of lycopene β-cyclase genes (CpCYC-B and CpLCY-B). This study yielded novel findings on regulatory mechanism of carotenoid biosynthesis during papaya fruit ripening.
Highlights
Carotenoids are important structural and functional pigments existed in all plants, which are important for photosynthesis, plant growth and development
To further analyze the potential transcription factors regulating carotenoid biosynthesis genes involved in papaya fruit ripening, RNA-Seq analysis was implemented using libraries constructed from papaya varieties SunUp and AU9 (GRN, CB, and RP) fruit at different developmental stages
Carotenoid accumulation could be promoted by light, and could be harmed by During papaya fruits ripening, transcription factors play a significant role in carotenoid biosynthesis and enhance carotenoid accumulation (Fig. 1)
Summary
Carotenoids are important structural and functional pigments existed in all plants, which are important for photosynthesis, plant growth and development. Many fleshy fruits, including papaya and tomato, have a dramatic flesh color change resulted from synthesized carotenoids accumulating in chromoplast during fruit ripening. The content of lycopene and β-carotene would have a significant increase from color-break stage to mature fruit, respectively, in red and yellow-fleshed papaya and tomato. Carotenoid biosynthesis pathway is regulated by an genetically programmed mechanism that involves phytohormones and environmental factors such as, light, water, and temperature[3]. In papaya carotenoid biosynthesis pathway, with phytoene synthase (PSY) functioned, phytoene is synthesized by condensing two molecules of geranylgeranyl diphosphate (GGPP) in the plastids.
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