Geranyl diphosphate (GPP), the precursor of many
monoterpene end products, is synthesized in plastids by a condensation of
dimethylallyl diphosphate and
isopentenyl diphosphate (
IPP) in a reaction catalyzed by homodimeric or heterodimeric GPP synthase (GPPS). In the heterodimeric
enzymes, a noncatalytic small subunit (GPPS.SSU) determines the product specificity of the catalytic large subunit, which may be either an active
geranylgeranyl diphosphate synthase (GGPPS) or an inactive GGPPS-like
protein. Here, we show that expression of snapdragon (Antirrhinum majus) GPPS.SSU in tobacco (Nicotiana tabacum) plants increased the total GPPS activity and
monoterpene emission from leaves and flowers, indicating that the introduced catalytically inactive GPPS.SSU found endogenous large subunit partner(s) and formed an active snapdragon/tobacco GPPS in planta. Bimolecular fluorescence complementation and in vitro
enzyme analysis of individual and hybrid
proteins revealed that two of four GGPPS-like candidates from tobacco EST databases encode bona fide GGPPS that can interact with snapdragon GPPS.SSU and form a functional GPPS
enzyme in plastids. The formation of chimeric GPPS in transgenic plants also resulted in leaf
chlorosis, increased
light sensitivity, and
dwarfism due to decreased levels of
chlorophylls,
carotenoids, and
gibberellins. In addition, these transgenic plants had reduced levels of
sesquiterpene emission, suggesting that the export of
isoprenoid intermediates from the plastids into the cytosol was decreased. These results provide genetic evidence that GPPS.SSU modifies the chain length specificity of phylogenetically distant GGPPS and can modulate
IPP flux distribution between GPP and GGPP synthesis in planta.