Flavonoid B-ring-specific O-methyltransferase with a preference for flavones > dihydroflavones > flavonols that possess at least two B-ring hydroxyl groups. Active with tricetin, 5-hydroxyferulic acid, luteolin, quercitin, eriodictyol, quercetagetin, taxifolin, gossypetin and myricetin. No activity with naringenin, apigenin, kaempferol, 7,8-dihydroxy- or 5,7,8-trihydroxy flavones, chlorogenic acid, gallic acid or daphnetin. Catalyzes the sequential O-methylation of tricetin via 3'-O-methyltricetin, 3',5'-O-methyltricetin to 3',4',5'-O-trimethyltricetin. May also be involved in S lignin biosynthesis. 3 S-adenosyl-L-methionine + tricetin = 3',4',5'-O-trimethyltricetin + 3 H(+) + 3 S-adenosyl-L-homocysteine Homodimer. The monomer is fully active and dimerization is not required for sequential methylation. Expressed in roots, stems and leaves. OMT1 has a very low activity with phenylpropanoids (5-hydroxyferulic acid and caffeic acid) while OMT2 has a good activity with them. Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family. COMT subfamily.