Differential expression of carotenoid-related genes determines diversified carotenoid coloration in flower petal of Osmanthus fragrans

There are four Osmanthus fragrans cultivar groups, which exhibit significant inter-group differences in flower coloration. In this study, we analyzed carotenoid metabolites and the expression of carotenoid-biosynthetic genes in three O. fragrans cultivars differing substantially in flower coloration: 'Zi Yingui' (butter yellow), 'Jingui' (golden yellow), and 'Chenghong Dangui' (orange-red). The results of high-performance liquid chromatography (HPLC) analysis show that 'Zi Yingui' petals contain small amounts of β-carotene, 'Jingui' petals have high levels of lutein as well as low levels of α-carotene and β-carotene, and 'Chenghong Dangui' petals accumulate considerable concentrations of α-carotene and β-carotene. Reverse transcription-polymerase chain reaction (RT-PCR) analysis of genes in the carotenoid synthesis and degradation pathways indicates that these genes have cultivar-specific modes of expression. OfHYB and OfZEP are more strongly expressed in 'Zi Yingui' and 'Jingui' petals than in 'Chenghong Dangui' petals, resulting in greater β-carotene concentrations in the latter. OfLCYE is most strongly expressed in 'Jingui' petals, leading to lutein accumulation. The overexpression of OfCCD1 and OfCCD4 in 'Zi Yingui' and 'Jingui' is responsible for the near non-existence of α-carotene and β-carotene in petals thereof. Methylation analysis of the promoter region of the OfCCD4 gene in 'Zi Yingui' and 'Chenghong Dangui' shows that 'Chenghong Dangui' has a methylation ratio of 50.18 %, higher than the 43.87 % of 'Zi Yingui'. The results of the present study indicates that O. fragrans cultivars vary in pigment composition and concentrations due to differential expressions of the downstream genes in the carotenoid synthesis pathway as well as the genes in the carotenoid degradation pathway.