## 1 Introduction  
Plant specialized metabolites provide protection against herbivores not only by acting as toxins and feeding deterrents but also by serving as defensive signals. Many plant species, for example, emit complex volatile blends upon herbivory that can attract herbivore enemies [1] or signal impending danger to distant parts of the same plant or neighboring plant [2]. Herbivory-induced plant volatile blends are often complex and possess dozens of substances from diverse classes of natural compounds,including terpenes, green leaf volatiles, aromatic compounds, and nitrogen-containing amino acid derivatives [3]. Although their qualitative and quantitative composition differs between plant species, herbivory-induced volatile blends are often dominated by monoterpenes and sesquiterpenes [1]. One of the most important enzyme groups in plant terpene biosynthesis is the _trans_-isoprenyl diphosphate synthases (_trans_-IDSs), which belong to the family of prenyl transferases [4; 5; 6]. IDS enzymes catalyze condensations of the basic C\({}_{5}\) terpene building blocks, isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), to generate C\({}_{10}\), C\({}_{15}\), C\({}_{20}\), and larger prenyl diphosphates [4; 7; 8]. These monoterpene, sesquiterpene, and diterpene precursors, geranyl diphosphate (GPP), (_E_,_E_)-farnesyl diphosphate (FPP), and (_E_,\(E\),_E_)-geranyl dighosphate (GGPP), respectively, are produced by short-chain _trans_-IDS enzymes that are designated as geranyl diphosphate synthases (GPPSs), farnesyl diphosphate synthases (FPPSs), and geranylgeranyl diphosphate synthases (GGPPSs) according to their major product. Longer prenyl diphosphates such as geranylfarnesyl diphosphate (GFPP) or solanesyl diphosphate are formed by medium-chain and long-chain IDSs [9]. Crystal structures of _trans_-IDS proteins from bacteria, fungi, animals, and plants have revealed a common tertiary structure consisting of 13 \(\alpha\)-helices [10; 11; 12; 13; 14]. Ten of these \(\alpha\)-helices surround a large central cavity and the size and geometric structure of this pocket are thought to be critical for product specificity, with large amino acids at the bottom blocking further chain elongation of the prenyl diphosphate in the active site [7]. The entrance to the pocket is flanked by two highly conserved DD(xx)2/4D motifs named the first aspartate-rich motif (FARM) and the second aspartate-rich motif (SARM) [10]. The aspartate residues of the FARM and SARM motifs were shown to bind divalent metal ions such as magnesium (Mg\({}^{2+}\)) or manganese (Mn\({}^{2+}\)) that in turn bind the diphosphate of the homo-allylic substrate [15]. In plants, _trans_-IDSs usually occur as dimers or as tetramers [14]. While FPPSs and GGPPSs act solely as homodimers, GPPSs can be active both as homo- and heterodimers. The heterodimers consist of a large subunit (LSU) possessing the two DD(xx)2/4D motifs required for catalysis and a catalytically inactive small subunit (SSU) that modulates the catalytic efficiency and chain length specificity of the protein complex [16; 17; 18; 19]. The amino acid motif CxxxC, which is located upstream of the FARM motif and highly conserved among the small and large subunits of heterodimeric plant GPPSs, has been shown to play a critical role in the physical interactions between the two subunits [17]. In previous studies, we investigated the formation and ecological functions of herbivory-induced volatiles emitted from different poplar species [20; 21; 22; 23; 24]. The western balsam poplar (_Populus trichocarpa_), for example, releases more than 50 volatiles when damaged by gypsy moth larvae (_Lymantria dispar_) [20]. Some of the emitted compounds have been shown to deter attacking caterpillars, while others are attractive to a generalist parasitoid [22; 24]. The most prominent components of the _P.