Monoterpenes, together with sesquiterpenes, play varying roles in plant interactions with the environment depending on the organs where they are produced; these compounds likely function to attract pollinators when they are present in flowers, and can protect plants against herbivores or microbes when produced in leaves or in roots. It is well-known that GPP (Geranyl diphosphate), the common precursor of monoterpenes, is biosynthesized by GPPS (Geranyl diphosphate synthase). To date, two types of GPPS (GPPS, EC2.5.1.1) have been reported from plants: homomeric GPPS and heteromeric GPPS. Heteromeric GPPS is composed of one inactive small subunit (SSU, SSUs can be divided into two types: SSU I and SSU II) and one large subunit (LSU). In the Arabidopsis genome, there are 12 GGPPS paralogs and 1 homomeric GPPS gene (At2g34630). A previous in vitro study characterized a heteromeric G(G)PS composed of GGPPS12 (also known as AtSSU, encoded by At4g38460 and belonging to the type II small subunit type) and GGPPS11 (also known as AtLSU, encoded by At4g36810). However, there is controversy as to which type of GPPS is involved in monoterpene biosynthesis in Arabidopsis. This controversy stems from the fact that both homomeric and heteromeric GPPS enzyme complexes catalyze the condensation of DMAPP and IPP to form GPP when assayed in vitro. In the study reported here,  we comprehensively analyzed transgenic GGPPS12, GGPPS11 and GPPS Arabidopsis plants and provide solid genetic evidence to support the supposition that heteromeric G(G)PPS, and not homomeric GPPS, is involved in monoterpene biosynthesis, at least in flowers of Arabidopsis.