3-O-methyltransferase involved in the phytoalexin pisatin biosynthesis. Can use (+)-6a-hydroxymaackiain, (+)-maackiain and with a lower activity (+)-medicarpin and 2,7,4'-trihydroxyisoflavanone as substrates, but not (-)-6a-hydroxymaackiain, daidzein, formononetin or isoliquiritigenin. (+)-6a-hydroxymaackiain + S-adenosyl-L-methionine = (+)-pisatin + 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.