Involved in pterocarpan phytoalexin biosynthesis. Catalyzes the last step in the biosynthesis of the phytoalexin medicarpin, and thereby contributes to plant defense reactions. Dirigent proteins impart stereoselectivity on the phenoxy radical-coupling reaction, yielding optically active lignans from two molecules of coniferyl alcohol in the biosynthesis of lignans, flavonolignans, and alkaloids and thus plays a central role in plant secondary metabolism 