Intersubgeneric crosses between white-flowered evergreen azalea species and a yellow-flowered deciduous azalea (Rhododendron japonicum f. flavum) can produce yellow-flowered evergreen azaleas. The petal color of F1 progenies from these crosses is known to fade during petal development due to a high expression of the carotenoid cleavage dioxygenase 4 (CCD4) gene inherited from the evergreen species. In this study, to facilitate the selection of evergreen azalea species as cross parents that have low expression levels of CCD4 in developing petals, CCD4 expression levels were compared among 16 evergreen azalea species belonging to the subgenus Tsutsusi. The identity values of the deduced amino acid sequences of CCD4 were high (from 97% to 100%) among evergreen azalea species. The expression levels of CCD4 genes in petals of evergreen azalea species at the day of anthesis varied widely from 2.04 to 20.47 times relative to that in R. japonicum f. flavum. Among these species, R. amanoi maintained low CCD4 expression levels throughout all petal development stages that were not significantly different from those of R. japonicum f. flavum (JPN). This species is therefore considered a promising breeding material for producing yellow-flowered evergreen azaleas.
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