CmHY5 functions in apigenin biosynthesis by regulating flavone synthase II expression in chrysanthemum flowers.

The predominant flavones in the ray florets of chrysanthemum flowers are apigenin and its derivatives. CmHY5 participates in apigenin biosynthesis by directly regulating the expression of FNSII-1 in chrysanthemum. Chrysanthemum (Chrysanthemum morifolium) flowers have been used for centuries as functional food and in herbal tea and traditional medicine. The chrysanthemum flower contains significant amounts of the biologically active compound flavones, which has medicinal properties. However, the mechanism regulating flavones biosynthesis in chrysanthemum flowers organs is still unclear. Here, we compared the transcriptomes and metabolomes of different floral organs between two cultivars with contrasting flavone levels in their flowers. We identified 186 flavonoids by metabolome analysis. The predominant flavones in the ray florets of chrysanthemum flowers are apigenin and its derivatives, of which the contents are highly correlated with the expression of flavones synthase II gene CmFNSII-1. We also determined that CmHY5 is a direct upstream regulator of CmFNSII-1 transcription. We showed that CmHY5 RNAi interference lines in chrysanthemum have lower contents of apigenin compared to wild-type chrysanthemum. Our results demonstrated that CmHY5 participates in flavone biosynthesis by directly regulating the expression of FNSII-1 in chrysanthemum.