## 4 Material and Methods
### Accession Numbers  
Sequence data for genes in this article can be found in the GenBank under the following identifiers: _PfFPPS1_ (MK492685), _PIGPPS1.LSU_ (MK492686), _PIGPPS2.LSU_ (MK492687), _PIGPPS.SSU1_ (MK492683), and _PIGPPS.SSU2_ (MK492684).  
**Supplementary Materials:** The following are available online. Figure S1. Amino acid comparison of PtFPPS1, PtGPPS1.LSU, PtGPPS2.LSU, PtGPPS.SSU1, and PtGPPS.SSU2 with FPPS and GPPS from other plants. The first aspartate-rich motif (FARM), the second aspartate-rich motif (SARM) and the two CoxoC motifs are marked with red boxes. The predicted signal peptides of the poplar IDS proteins are underlined. Figure S2. Signal peptide prediction of PtFPPS1, PtGPPS1.LSU, PtGPPS2.LSU, PtGPPS.SSU1, and PtGPPS.SSU2. Signal peptide (cTP) prediction was done using the web-based prediction programs chloro v1.1 ([[http://www.cbcs.dtu.dk/services/ChloroP/](http://www.cbcs.dtu.dk/services/ChloroP/)]([http://www.cbcs.dtu.dk/services/ChloroP/](http://www.cbcs.dtu.dk/services/ChloroP/))) (A), target v1.1 ([[http://www.cbcs.dtu.dk/services/TargetP/](http://www.cbcs.dtu.dk/services/TargetP/)]([http://www.cbcs.dtu.dk/services/TargetP/](http://www.cbcs.dtu.dk/services/TargetP/))) (B), and Predendor v1.04 ([[https://urgi.versailes.inra.fr/predator/](https://urgi.versailes.inra.fr/predator/)]([https://urgi.versailes.inra.fr/predator/](https://urgi.versailes.inra.fr/predator/))) (C). Figure S3. Relative expression of putative _IDS_ genes in undamaged (control) and _Lymuntian digbar_-damaged (herbivory) leaves of _Populus trichocarppa_. Gene expression was analyzed using quantitative RT-PCR. Means and standard errors are shown (n = 5). Asterisks indicate statistical significance in Student's t-tests and from Mann-Whitney Rank Sum Tests, respectively. _PfFPPS1_ (_P_ = 0.151, T = 20.00); _PIGPPS1.LSU_ (_P_ = 0.008, T = 15.00); _PIGPPS2.LSUI_ (_P_ = 0.329, t = 1.040); _PIGPPS.SSU1_ (_P_ = 0.008, T = 15.00); _PIGPPS.SSU2_ (_P_ = 0.796, t = 0.268). Figure S4. PtGPPS.SSU1 and PtGPPS.SSU2 have no IDS activity when tested with HP and DMAPP. The genes were heterologously expressed in _Escherichia coli_ and purified recombinant proteins were incubated with the substrates IPP and DMAPP. Product analysis was done using LC-MS/MS. Enzyme products were identified using authentic standards. An empty vector was expressed under the same conditions, assayed with IPP and DMAPP, and used as negative control. Figure S5. The activity of PtGPPS1.LSU (A) and PtGPPS2.LSU (B) is not influenced by PtGPPS.SSU2. The genes were heterologously expressed in _Escherichia coli_ and purified recombinant proteins were incubated with the substrates IPP and DMAPP. Product analysis was done using LC-MS/MS. Enzyme products were identified using authentic standards.  
**Author Contributions:** Conceptualization, T.G.K.; Investigation, N.D.L., N.P.P., R.N., A.S., S.I., and T.G.K.; Resources, J.G.; Supervision, J.G. and T.G.K.; Writing--original draft, N.D.L. and T.G.K.; and Writing--review and editing, N.D.L., N.P.P., R.N., A.S., S.I., J.G., and T.G.K.  
**Funding:** This research was funded by the Max-Planck Society.  
**Acknowledgments:** We thank [PERSON] for excellent technical assistance and [PERSON] and [PERSON] for rearing the poplar trees.  
**Conflicts of Interest:** The authors declare no conflict of interest.