TcCGT1, which is initiated by the spontaneous deprotonation of the substrate. The spacious binding pocket explains the substrate promiscuity, and the binding pose of the substrate determines C‐ or O‐glycosylation activity. Site‐directed mutagenesis at two residues (I94E and G284K) switched C‐ to O‐glycosylation. TcCGT1 is the first plant CGT with a crystal structure and the first flavone 8‐C‐glycosyltransferase