Involved in monoterpene (C10) biosynthesis. The major product is beta-myrcene (56%) followed by (E)-beta-ocimene (20%) and minor amounts (less than 5%) of the cyclic monoterpene (-)-limonene, (+)-limonene, 2-carene and tricyclene. (2E)-geranyl diphosphate = beta-myrcene + diphosphate (2E)-geranyl diphosphate = diphosphate + tricyclene (2E)-geranyl diphosphate = (E)-beta-ocimene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Predominantly expressed in flowers but also in leaves, siliques and in stems. By methyl jasmonate. Also induced in response to the caterpillar P.xylostella or P.rapae feeding. 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. Tpsb subfamily.