Comparison of petunia and calibrachoa in carotenoid pigmentation of corollas

Petunia (Petunia hybrida) is an important ornamental plant with a wide range of corolla colors. Although pale-yellow-flowered cultivars, with a low amount of carotenoids in their corollas, are now available, no deep-yellow-flowered cultivars exist. To find why petunia cannot accumulate enough carotenoids to have deep-yellow flowers, we compared carotenoid profiles and expression of carotenoid metabolic genes between pale-yellow-flowered petunia and deep-yellow-flowered calibrachoa (Calibrachoa hybrida), a close relative. The carotenoid contents and the ratios of esterified xanthophylls to total xanthophylls in petunia corollas were significantly lower than those in calibrachoa, despite similar carotenoid components. A lower esterification rate of trans-xanthophylls than of cis-xanthophylls in petunia suggests that petunia xanthophyll esterase (XES) has low substrate specificity for trans-xanthophylls, which are more abundant than cis-xanthophylls in petunia corolla. The expression of genes encoding key enzymes of carotenoid biosynthesis was lower and that of a carotenoid catabolic gene was higher in petunia. XES expression was significantly lower in petunia. The results suggest that low biosynthetic activity, high cleavage activity, and low esterification activity cause low carotenoid accumulation in petunia corollas.