Isopentenyl diphosphate isomerase exerts limited control over terpenoid biosynthesis in two woody plant species

Terpenoid biosynthesis involves linear prenyl diphosphate intermediates of various chain lengths. These are constructed from two C5 precursors, the starter unit dimethylallyl diphosphate (DMADP) and the extender unit, isopentenyl diphosphate (IDP). Isopentenyl diphosphate isomerase (IDI) alters the DMADP:IDP ratio and may furnish a specific blend of C5 precursors appropriate for the length of intermediates being formed in each cellular compartment. We studied IDI in two woody plant species, Norway spruce (Picea abies) and gray poplar (Populus x canescens), whose major terpenoid specialized metabolites are of different sizes. While the catalytic parameters of IDI from each species measured in vitro were in line with the different C5 precursor demands, the DMADP:IDP ratios of both species in vivo did not differ. Moreover, though IDI silencing in both spruce and poplar increased IDP content and significantly decreased the DMADP:IDP ratio, it caused few significant alterations in the content of downstream terpenoid pathway intermediates or products. Taken together, these results suggest that IDI exercises a limited control over the relative amounts of different size terpenoid products. Nevertheless, the elevated IDP content of both transgenic spruce and poplar lines was associated with dramatically increased emission of isoprenol and isoprenyl acetate. Feeding experiments with cultured poplar plants indicated that these metabolites were derived directly from IDP, and their formation could serve as a metabolic mechanism to reduce high intracellular accumulation of IDP. Such a mechanism can be considered analogous to the formation of isoprene as a way to reduce high concentrations of DMADP.