Unbalanced Expression of Structural Genes in Carotenoid Pathway Contributes to the Flower Color Formation of the Osmanthus Cultivar ‘Yanzhi Hong’

Carotenoids are important natural pigments that are responsible for the fruit and flower colors of many plants. The composition and content of carotenoid can greatly influence the color phenotype of plants. However, the regulatory mechanism underling the divergent behaviors of carotenoid accumulation, especially in flower, remains unclear. In this study, a new cultivar Osmanthus fragrans ‘Yanzhi Hong’ was used to study the regulation of carotenoid pigmentation in flower. Liquid chromatograph–mass spectrometer (LC-MS) analysis showed that β-carotene, phytoene, lycopene, γ-carotene, and lutein were the top five pigments enriched in the petals of ‘Yanzhi Hong’. Through transcriptome analysis, we found that the expression of the structural genes in carotenoid pathway was imbalanced: most of the structural genes responsible for lycopene biosynthesis were highly expressed throughout the flower developmental stages, while those for lycopene metabolism kept at a relatively lower level. The downregulation of LYCE, especially at the late developmental stages, suppressed the conversion from lycopene to α-carotene but promoted the accumulation of β-carotene, which had great effect on the carotenoid composition of ‘Yanzhi Hong’. Ethylene response factor (ERF), WRKY, basic helix-loop-helix (bHLH), v-myb avian myeloblastosis viral oncogene homolog (MYB), N-Acetylcysteine (NAC), auxin response factor (ARF), and other transcription factors (TFs) have participated in the flower color regulation of ‘Yanzhi Hong’, which formed co-expression networks with the structural genes and functioned in multiple links of the carotenoid pathway. The results suggested that the cyclization of lycopene is a key link in determining flower color. The modification of the related TFs will break the expression balance between the upstream and downstream genes and greatly influence the carotenoid profile in flowers, which can be further used for creating colorful plant germplasms.