Malonyltransferase with broad substrate specificity acting on phenolic glucosides including xenobiotic naphthols. Has activity against flavonoid 7-O-glucosides, flavonoid 3-O-glucosides and naphthol glucosides, and to a lesser extent against coumarin glucosides in vitro. Prefers malonyl-CoA as an acyl donor, but also active with succinyl-CoA and methylmalonyl-CoA, but not with acetyl-CoA. a flavonol 3-O-beta-D-glucoside + malonyl-CoA = a flavonol 3-O-(6-O-malonyl-beta-D-glucoside) + CoA a flavonol 7-O-beta-D-glucoside + malonyl-CoA = a flavonol 7-O-(6-O-malonyl-beta-D-glucoside) + CoA kcat is 11.0 sec(-1) for kaempferol 7-O-glucoside. kcat is 6.8 sec(-1) for kaempferol 3-O-glucoside. kcat is 6.5 sec(-1) for 2-naphthol glucoside. kcat is 8.0 sec(-1) for malonyl-CoA. kcat is 9.3 sec(-1) for succinyl-CoA. Optimum pH is 8.0-8.5. Stable below 40 degrees Celsius and pH 8.0 for 30 min. Monomer. Highly expressed in flower. Also expressed in flower bud, stem, root and leaf. Expressed in 2-week old seedlings. The HXXXXD motif is essential for acyltransferase activity. Belongs to the plant acyltransferase family. Phenolic glucoside malonyltransferase subfamily.