Type III polyketide synthase involved preferentially in the biosynthesis of 4-hydroxycoumarin from salicoyl-CoA. Can also use benzoyl-CoA and malonyl-CoA to produce 3,5-dihydroxybiphenyl as a major product and benzoyldiacetic acid lactone as a minor side product. Can also use m-hydroxybenzoyl-CoA as substrate, producing m-hydroxybenzoyl diacetic acid lactone as a derailment product. No activity with p-hydroxybenzoyl-CoA, CoA-linked cinnamic acids or acetyl-CoA. 2-hydroxybenzoyl-CoA + malonyl-CoA = 4-hydroxycoumarin + CO2 + 2 CoA kcat is 1.02 min(-1) with benzoyl-CoA as substrate. kcat is 2.22 min(-1) with salicoyl-CoA as substrate. Optimum pH is 6.5-7.0. Optimum temperature is 35 degrees Celsius. Homodimer. Up-regulated by elicitor. Belongs to the thiolase-like superfamily. Chalcone/stilbene synthases family.