Uses dimethylallyl diphosphate and isopentenyl diphosphate to catalyze the cis-prenyl chain elongation and produce the 20 carbon product nerylneryl diphosphate. dimethylallyl diphosphate + 3 isopentenyl diphosphate = 3 diphosphate + nerylneryl diphosphate dimethylallyl diphosphate + isopentenyl diphosphate = diphosphate + neryl diphosphate isopentenyl diphosphate + neryl diphosphate = (2Z,6Z)-farnesyl diphosphate + diphosphate (2Z,6Z)-farnesyl diphosphate + isopentenyl diphosphate = diphosphate + nerylneryl diphosphate Optimum pH is 8.0-8.5. Expressed in stems (PubMed:23134568). Expressed in petiolules (PubMed:25786135). Expressed at low levels in leaf trichomes, old leaf and roots (PubMed:23134568). Belongs to the UPP synthase family.