A natural variation of flavone synthase <scp>II</scp> gene enhances flavone accumulation and confers drought adaptation in chrysanthemum

Flavones, a key group of flavonoids, play a significant role in plant adaptation to ecological niches and are valuable medicinal resources. However, the genetic basis underlying their contribution to ecological adaptation remains largely unknown. Here, using metabolite-based genome-wide association study, we report that the natural variation of flavone contents in Chrysanthemum indicum, a wild chrysanthemum and medicinal herb, is mainly determined by a recently duplicated flavone synthase II gene CiFNSII-1.2. Enzymatic assays and molecular dynamics simulations reveal that the key amino acid residues 246^th and 261^th confer the higher enzymatic activity of CiFNSII-1.2 compared with its ancestral form. These residues act as critical modulators, regulating the flexibility of the external entrance and contributing to the enzyme's improved functionality. Transgenic evaluation demonstrate that CiFNSII-1.2 contributes to flavone accumulation and drought adaptation. Our findings provide insights into the biochemical and evolutionary role of flavones in facilitating adaptation to drought-prone habitats in chrysanthemum.