Involved in the biosynthesis of the pterocarpin phytoalexins. Acts on isoflavones with a 4'-methoxy group on the B-ring, such as biochanin A, formononetin and 2'-hydroxyformononetin. Has a low activity with daidzein and pseudobaptigenin, and no activity with the 7-O-methylated isoflavonoids isoformononetin and prunetin. formononetin + O2 + reduced [NADPH--hemoprotein reductase] = calycosin + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] kcat is 0.028 sec(-1) with formononetin as substrate. kcat is 0.1 sec(-1) with biochanin A as substrate. Optimum pH is 8.0. Expressed constitutively in leaves and stems, but not in roots. Up-regulated by methyl jasmonate and herbivory, but only weakly by elicitor. Belongs to the cytochrome P450 family.