Regulation of Carotenoid Pigmentation in Corollas of Petunia

Abstract

Petunia (Petunia hybrida) is an important ornamental plant, with corolla colors ranging widely from pink to red to purple, owing mainly to anthocyanins. Although there are no bright-yellow-flowered cultivars, some pale-yellow-flowered cultivars accumulate a small amount of carotenoids. To find a key regulatory step that controls carotenoid content in petunia corollas, we compared the expression of carotenoid metabolism genes and carotenoid composition in corollas of white-flowered and pale-yellow-flowered cultivars. Pale yellow corollas tended to have higher expression of biosynthesis genes. The most prominent result was the complete lack of carotenoid cleavage dioxygenase 4a (CCD4a) transcripts in pale yellow corollas. We found two insertions, one in the putative promoter region and the other in the coding region, of the genomic CCD4a sequence of a pale-yellow-flowered cultivar relative to that of a white-flowered cultivar. We consider this the main reason for the lack of CCD4a transcripts. The results suggest that pale yellow corollas have higher carotenoid biosynthesis activity and lower catabolism activity than white corollas. We propose that carotenoid content in petunia corollas is determined by the balance of the degradation and biosynthesis of carotenoids.