Methyltransferase involved in the phytoalexin pisatin biosynthesis. Has both 3- and 4'-O-methyltransferase activities. Can use (+)-6a-hydroxymaackiain, 2,7,4'-trihydroxyisoflavanone and with much less activity (+)-medicarpin as substrates, but not (-)-6a-hydroxymaackiain, daidzein, formononetin or isoliquiritigenin. May be involved in formononetin biosynthesis. (+)-6a-hydroxymaackiain + S-adenosyl-L-methionine = (+)-pisatin + H(+) + S-adenosyl-L-homocysteine a 4'-hydroxyisoflavone + S-adenosyl-L-methionine = a 4'-methoxyisoflavone + H(+) + S-adenosyl-L-homocysteine Up-regulated by copper. Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family. COMT subfamily.