Monoterpene synthase converting geranyl diphosphate to (-)-beta-pinene and (-)-alpha-pinene in a 94:6 ratio. (2E)-geranyl diphosphate = (1S,5S)-beta-pinene + diphosphate Binds 3 Mg(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Expressed in young leaves. Detected in mature leaves, inflorescences and stems, but not in roots. Circadian-regulation. Expression peaked after 9 hours in light. Down-regulated by wounding and elicitor treatment. 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.