Genomic insights into the evolution of flavonoid biosynthesis and O-methyltransferase and glucosyltransferase in Chrysanthemum indicum

Flavonoids are a class of secondary metabolites widely distributed in plants. Regiospecific modification by methylation and glycosylation determines flavonoid diversity. A rare flavone glycoside, diosmin (luteolin-4′-methoxyl-7-O-glucosyl-rhamnoside), occurs in Chrysanthemum indicum. How Chrysanthemum plants evolve new biosynthetic capacities remains elusive. Here, we assemble a 3.11-Gb high-quality C. indicum genome with a contig N50 value of 4.39 Mb and annotate 50,606 protein-coding genes. One (CiCOMT10) of the tandemly repeated O-methyltransferase genes undergoes neofunctionalization, preferentially transferring the methyl group to the 4′-hydroxyl group of luteolin with ortho-substituents to form diosmetin. In addition, CiUGT11 (UGT88B3) specifically glucosylates 7-OH group of diosmetin. Next, we construct a one-pot cascade biocatalyst system by combining CiCOMT10, CiUGT11, and our previously identified rhamnosyltransferase, effectively producing diosmin with over 80% conversion from luteolin. This study clarifies the role of transferases in flavonoid diversity and provides important gene elements essential for producing rare flavone. [Display omitted] •Genome of Chrysanthemum indicum is successfully assembled at the chromosome level•The specific 4′-O-methyltransferase might be newly emerged in Chrysanthemum plants•Identification of methyltransferase and glucosyltransferase completes diosmin pathway•Diosmin can be synthesized from luteolin through a multi-enzyme cascade reaction Diosmin is a pharmaceutically active flavone glycoside found in Chrysanthemum indicum; how a plant synthesizes diosmin remains to be determined. Deng et al. identify that the positional-specific 4′-O-methyltransferase and 7-O-glucosyltransferase, in combination with a rhamnosyltransferase, enable the reconstruction of diosmin biosynthetic pathway from luteolin via in vitro enzymatic cascade catalysis.