Flavonoid 3'-O-methyltransferase from rice: cDNA cloning, characterization and functional expression.

Plant O-methyltransferases (OMTs) are known to be involved in methylation of plant secondary metabolites, especially phenylpropanoid and flavonoid compounds. An OMT, ROMT-9, was cloned and characterized from rice using a reverse transcriptase polymerase chain reaction (RT-PCR). The blast results for ROMT-9 showed a 73% identity with caffeic acid OMTs from maize and Triticum aestivum. ROMT-9 was expressed in Escherichia coli and its recombinant protein was purified using affinity chromatography. It was then tested for its ability to transfer the methyl group of S-adenosyl-l-methionine to the flavonoid substrates, eriodictyol, luteolin, quercetin, and taxifolin, all of which have a 3'-hydroxyl functional group. The reaction products were analyzed using TLC, HPLC, HPLC/MS, and NMR spectroscopy. The NMR analysis showed that ROMT-9 transferred the methyl group specifically to the 3'-hydroxyl group of quercetin, resulting in the formation of its methoxy derivative. Furthermore, ROMT-9 converted flavonoids containing the 3'-hydroxy functional group such as eriodictyol, luteolin, quercetin and taxifolin into the corresponding methoxy derivatives, suggesting that ROMT-9 is an OMT with strict specificity for the 3'-hydroxy group of flavonoids.