Contribution made by the carotenoid cleavage dioxygenase 4 gene to yellow colour fade in azalea petals

Carotenoid content, composition, and the expression patterns of carotenoid biosynthesis and cleavage genes during petal development were compared among a yellow-flowered deciduous azalea (Rhododendron japonicum f. flavum), a white-flowered evergreen azalea (‘Miyamasatsuki’), and their progeny, to determine the factors that cause reduction in carotenoid content as the petals of the progeny develop. During the early, green petal flowering stage, total carotenoid contents were 31.27 μg g⁻¹ Fresh Weight (F. W.) in R. japonicum f. flavum, 17.84 μg g⁻¹ F. W. in ‘Miyamasatsuki’, and 42.18 μg g⁻¹ F. W. in their progeny. During subsequent flower development, total carotenoid contents remained similar to the green petal stage for R. japonicum f. flavum. However, the content decreased in ‘Miyamasatsuki’ and their progeny at one day before anthesis, and became less than 3 μg g⁻¹ F. W. during the later stages. The expression levels of PSY and PDS increased significantly in R. japonicum f. flavum than in ‘Miyamasatsuki’ as the flowers developed. Their expressions in the progeny were mid-way between both parents. The expression level of CCD4 was significantly higher in ‘Miyamasatsuki’ and the progeny than in R. japonicum f. flavum for all development stages. This result suggested that the high expression level of CCD4, which was inherited from ‘Miyamasatsuki’, was the main factor controlling the reduction in carotenoid content in the progeny.