EXISTENCE AND FUNCTION OF PETAL COLOUR IDENTITY GENE IN APETALOUS WILD CHRYSANTHEMUM

Abstract

In most chrysanthemums, the capitulum is composed of ray florets and disk florets. The ray floret colour in yellow-flowered cultivars is mainly due to carotenoids. In white-flowered chrysanthemum cultivars (Chrysanthemum morifolium), the ray floret colour is due to the cleavage of carotenoids catalyzed by carotenoid cleavage dioxgenase4 (CmCCD4a). CmCCD4a suppression by RNA interference (RNAi) gives a yellow colour to white ray florets. In wild chrysanthemums, the ray florets are either white or yellow, indicating that ray floret colour probably results from carotenoid cleavage in wild chrysanthemums, as in chrysanthemum cultivars. Southern blot analysis confirmed that wild chrysanthemum species with white ray florets had CmCCD4a orthologs, while those with yellow ray florets did not. This suggests that carotenoid cleavage also contributes to ray floret colour in wild chrysanthemums. CmCCD4a expression is observed to be strictly limited to ray florets. Southern blot analysis showed that Chrysanthemum shiwogiku has CmCCD4a orthologs, although its capitulum has only disk florets and no ray florets. To confirm the function of this CmCCD4a ortholog, C. shiwogiku was interspecifically hybridized with a yellow-flowered cultivar 'Squash' that does not have CmCCD4a. The capitulum of all progenies produced white ray florets. CmCCD4a ortholog fragments were detected in all progenies by polymerase chain reaction. These results showed that the CmCCD4a orthologs present in apetalous C. shiwogiku, and its translated product has an enzymatic function in the ray florets.