Quercetin was converted to quercetin 3-O-(6-O-malonyl)-beta-D-glucosidc through the regioselective glucosylation at 3-position of quercetin and the following regioselective malonylation at 6-position of sugar moiety of quercetin 3-O-beta-D-glucoside with cultured Nicotiana tabacum cells. On the other hand, N. tabacum cells glucosylated epicatechin and catechin to the corresponding 3'-, 5-, and 7-0-beta-D-glucosides, respectively.