Molecular Basis for Chemical Evolution of Flavones to Flavonols and Anthocyanins in Land Plants

During the course of evolution of land plants, different classes of flavonoids, including flavonols and anthocyanins, sequentially emerged, facilitating adaptation to the harsh terrestrial environment. Flavanone 3β-hydroxylase (F3H), an enzyme functioning in flavonol and anthocyanin biosynthesis and a member of the 2-oxoglutarate-dependent dioxygenase (2-ODD) family, catalyzes the hydroxylation of (2S)-flavanones to dihydroflavonols, but its origin and evolution remain elusive. Here, we demonstrate that functional flavone synthase Is (FNS Is) are widely distributed in the primitive land plants liverworts and evolutionarily connected to seed plant F3Hs. We identified and characterized a set of 2-ODD enzymes from several liverwort species and plants in various evolutionary clades of the plant kingdom. The bifunctional enzyme FNS I/F2H emerged in liverworts, and FNS I/F3H evolved in Physcomitrium (Physcomitrella) patens and Selaginella moellendorffii, suggesting that they represent the functional transition forms between canonical FNS Is and F3Hs. The functional transition from FNS Is to F3Hs provides a molecular basis for the chemical evolution of flavones to flavonols and anthocyanins, which contributes to the acquisition of a broader spectrum of flavonoids in seed plants and facilitates their adaptation to the terrestrial ecosystem.