7-O-methyltransferase involved in the biosynthesis of isoformononetin. Can use daidzein as substrate, but not medicarpin or 2,7,4'-trihydroxyisoflavanone. a 7-hydroxyisoflavone + S-adenosyl-L-methionine = a 7-methoxyisoflavone + H(+) + S-adenosyl-L-homocysteine Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family. COMT subfamily.