Gibberellins are derived from the diterpene precursor geranylgeranyl diphophosphate (GGPP). GGPP is converted to ent-kaurene, which contains the basic structure of gibberellins, in the plastids by the combined actions of copalyl diphosphate synthase (CPS) and ent-kaurene synthase (KS). Generally, geranylgeranyl diphosphate synthase (GGPS) is suggested to use isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) as substrates to generate the GGPP that is used by CPS.

In this chapter we will provide data to show that actually the activity of geranyl diphosphate synthase (GPS) is required in both tomato (Solanum lycopersicum) and Arabidopsis thaliana for the biosynthesis of gibberellins. This finding indicates that GGPS uses GPP and IPP as substrates to produce the GGPP precursor for gibberellin biosynthesis. We will also argue that the pool of GGPP that is used for the biosynthesis of gibberellins is different from those GGPP pools that are used for other terpenoid-based molecules. Through analysis of Arabidopsis microarray data we attempt to predict which member of the GGPS gene family is actually involved in gibberellin precursor biosynthesis.