Catalyzes the conversion of geranylgeranyl diphosphate to the phytoalexin precursor syn-copalyl diphosphate (PubMed:15341631, PubMed:15255861, PubMed:23621683). Required for the biosynthesis of momilactones that exude from roots and act as allelochemicals against lowland weeds in paddy soil (PubMed:23621683). (2E,6E,10E)-geranylgeranyl diphosphate = 9alpha-copalyl diphosphate Induced by UV irradiation (PubMed:15341631, PubMed:15255861). Induced by methyl jasmonate (PubMed:15255861). The Asp-Xaa-Asp-Asp (DXDD) motif is important for the catalytic activity, presumably through binding to Mg(2+). No visible phenotype under normal growth conditions (PubMed:23621683). Mutant plants exhibit increased susceptibility to the rice blast fungus Magnaporthe grisea, and reduced capacity to inhibit germination and growth of lowland weeds in paddy soil, possibly due to decreased levels of momilactones and oryzalexin S (PubMed:23621683). Phytoalexins are diterpenoid secondary metabolites involved in the defense mechanism of the plant and produced in response to attack (by a pathogen, elicitor or UV irradiation). Belongs to the terpene synthase family.