Sesquiterpene cyclase that converts farnesyl diphosphate to the volatile beta-selinene and the corresponding nonvolatile antibiotic derivative beta-costic acid, especially in roots upon infection by rootworm larvae (D.balteata) and fungal pathogens (e.g. Fusarium spp.). (2E,6E)-farnesyl diphosphate = (+)-beta-selinene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Monomer. Accumulates strongly following fungal elicitation (e.g. Fusarium spp., C.heterostrophus, F.verticillioides, R.microsporus and A.parasiticus). The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family. The allele found in cv. Missouri 17 (AC K7WDC8) encodes an active enzyme while the allele found in cv. B73 contains a frame shift mutation.