## 5 Conclusions  
This study investigated the functional characteristics of GbHMGS1 through overexpression of the _GbHMGS1_ gene in a hybrid poplar. Through subcellular localization assays, we found that the GbHMGS1 protein is localized in the cytoplasm. The results reveal that 31 metabolites were significantly upregulated in the transgenic poplar group compared with the CK poplar control group. Specifically, the lanosterol (triterpeneoid), dehydrodiabetic acid (diterpenoid), and phytol (diterpenoid) contents were increased in the transgenic _Populus_ plants compared with their counterpart plants. Hence, the overexpression of _GbHMGS1_ substantially improved the metabolite contents in the transgenic poplar group. This study provides a basis for a more in-depth understanding of the corresponding biosynthesis pathways and the regulation of the accumulation of metabolites. **Supplementary Materials:** The following are available online at [[https://www.mdpi.com/article/10.3390/f120509595/s1](https://www.mdpi.com/article/10.3390/f120509595/s1)]([https://www.mdpi.com/article/10.3390/f120509595/s1](https://www.mdpi.com/article/10.3390/f120509595/s1)), **Figure S1:** Total ion chromatographs (TICs) of metabolites extracted from _Populus_, **Figure S2:** Metabolomic profiling of the leaves from _HMGS1_-overexpressing and CK plants. The partial least squares discriminant analysis (PLS-DA) (a) and orthogonal projections to latent structures (OPLS-DA) (b) scores obtained from the analysis of metabolites in the leaves of _HMGS1_-overexpressing and CK plants are shown, **Table S1:** Statistical analysis of adventitious root development and plant height between CK and transgenic _Populus_ (45 days), **Supplementary data S1:** Full-length _GbHMGS1_ and _GbHMGS2_, **Supplementary data S2:** Two hundred seventy metabolites were annotated using the information within the Fiehn database. The raw data were log2-transformed in the table. The QC samples of these metabolites changed little and were very uniform. **Author Contributions:** [PERSON] and [PERSON] designed the experiments. [PERSON] wrote the paper, performed the experiments, and analyzed the data with contributions from [PERSON], [PERSON], and [PERSON], [PERSON] and [PERSON] helped complete the experiments. [PERSON] and [PERSON] revised the manuscript. All authors have read and agreed to the published version of the manuscript. **Funding:** This study was supported by the National Natural Science Foundation of China (31971689), the Special Fund for Forest Scientific Research in Public Welfare (201504105), the Agricultural Science and Technology Independent Innovation Funds of Jiangsu Province (CX(16)1005), the National Key Research and Development Program of China (2017 YFD0600700), the Postgraduate Research and Practice Innovation Program of Jiangsu Province (KYCX18_0954), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). **Role of the Funding Source:** The funding bodies provided the financial support to the research projects, but funders were not involved in the experiment design, data analysis, or preparation of the manuscript. **Conflicts of Interest:** The authors declare no conflict of interest. * [PERSON] et al. (1995) [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON] [PERSON] Isolation and characterization of a cDNA encoding _Arabidopsis thaliana_ 3-hydroxy-3-methylglutaryl-coenzyme A synthase. _Gene_**1995**, _167_, 197-201. [CrossRef]
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