Identification and Characterization of trans -Isopentenyl Diphosphate Synthases Involved in Herbivory-Induced Volatile Terpene Formation in Populus trichocarpa

In response to insect herbivory, poplar releases a blend of volatiles that plays important roles in plant defense. Although the volatile bouquet is highly complex and comprises several classes of compounds, it is dominated by mono- and sesquiterpenes. The most common precursors for mono- and sesquiterpenes, geranyl diphosphate (GPP) and ( )-farnesyl diphosphate (FPP), respectively, are in general produced by homodimeric or heterodimeric -isopentenyl diphosphate synthases ( -IDSs) that belong to the family of prenyltransferases. To understand the molecular basis of herbivory-induced terpene formation in poplar, we investigated the gene family in the western balsam poplar . Sequence comparisons suggested that this species possesses a single FPP synthase gene ( ) and four genes encoding two large subunits ( and ) and two small subunits ( and ) of GPP synthases. Transcript accumulation of and was significantly upregulated upon leaf herbivory, while the expression of , , and was not influenced by the herbivore treatment. Heterologous expression and biochemical characterization of recombinant PtFPPS1, PtGPPS1.LSU, and PtGPPS2.LSU confirmed their respective IDS activities. Recombinant PtGPPS.SSU1 and PtGPPS.SSU2, however, had no enzymatic activity on their own, but PtGPPS.SSU1 enhanced the GPP synthase activities of PtGPPS1.LSU and PtGPPS2.LSU in vitro. Altogether, our data suggest that PtGPPS1.LSU and PtGPPS2.LSU in combination with PtGPPS.SSU1 may provide the substrate for herbivory-induced monoterpene formation in . The sole FPP synthase PtFPPS1 likely produces FPP for both primary and specialized metabolism in this plant species.