# Overexpression of _PtDXS_ Enhances Stress Resistance in Poplars
## 3 Discussion
In _Aquilaria sinensis_, the expression of _AsDXS1_ is regulated by physical and chemical factors, but that of _AsDXS2_ and _AsDXS3_ is regulated only by physical and chemical factors. These three genes were regulated by methyl jasmonate, but at different times [16]. Treatment with 200 mM NaCl resulted in _PtDXS_ expression 18-fold higher than that in untreated plants. Treatment with 200 \(\upmu\)M ABA significantly upregulated the expression of _PtDXS_ from 3 to 48 h, with a peak at 12 h. Treatment with 2 mM H\({}_{2}\)O\({}_{2}\) resulted in peak expression of _PtDXS_ at 48 h, when it was 18-fold that of the control. The expression of _PtDXS_ decreased during the first 2 days of treatment with 10% PEG6000, and increased to a peak at 7 days. Thus, we speculate that _PtDXS_ may facilitate development and improvement of poplar resistance to salt and drought stresses. Recombinant PtDXS was capable of catalyzing the formation of DXP from G-3-P and pyruvate. The color complementation method has been used to assess the function of DXS in _Amomum villosum_ and _Camptotheca acuminata_[42]. As reported by [PERSON] [43], _E. coli_ strains lacking genes related to carotenoid synthesis do not produce lycopene. The result of a colorimetric assay indicated that _PtDXS_ encodes a functional protein, which increased the accumulation of \(\beta\)-carotene via the MEP pathway. Therefore, _PtDXS_ could help to demonstrate an increasing metabolic flux to the synthesis of isoprene compounds according to the color complementation. Genes may be co-expressed with other upstream or downstream genes in their metabolic pathways. Upregulation of the expression of potato _DXS_ in _A. thaliana_ increased the expression of downstream _GGPPS_ genes, increasing the carotenoid content. In addition, upregulation of phytoene synthase (_PSY_) expression increased the \(\beta\)-carotene content of transgenic plants [44]. In this study, the expression levels of the MEP-related genes _DXR_, _HDS_, _HDR_, _MCT_, and _CMK_ as well as downstream genes were increased significantly in the transgenic plants. By contrast, the expression levels of the MVA-related genes _HMGS_, _HMGR_, and _MVD_ were decreased. These data suggest that the MEP pathway may play a leading role in terpenoid biosynthesis, while the MVA pathway plays an auxiliary role. DXS is related to terpenoid synthesis and regulates the generation of downstream products. As the first key enzyme in terpenoid biosynthesis, overexpression or inhibition of DXS can cause changes in downstream metabolites. The carotenoid, chlorophyll, and ABA contents of _Arabidopsis_ harboring exogenous _DXS_ increased [44; 45]. However, the ABA and GA contents of _Arabidopsis_ decreased following transfer of _DXS1_ from _Solanum tuberosum_[44]. The expression of _DXS_ from _Nicotiana tabucum_ was positively related to the carotenoid content of fruit, and silencing of _SIDXS2_ reduced the \(\beta\)-carotene content [34]. In this study, the transgenic plants had significantly greater ABA and GA contents than the WT. Thus, the GA and ABA contents are correlated with the expression level of _DXS_, and thus DXS plays an important role in their biosynthesis. Infection of the root of wheat, maize, and rice by mycorrhizal fungi alters the expression levels of _DXS_s [46]. Mycorrhizal fungi infect the roots of _Medicago truncatula_, which increases the concentration of MtDXS2 [23]. Potato late blight is related to the expression level of _DXS_, and a decrease in _DXS_ expression results in decreased levels of terpenoids, which are related to disease resistance [44]. In this study, the transgenic plants showed enhanced resistance to _S. populiperda_ infection as well as reduced feeding by _M. troglodyta_. We cloned full-length _PtDXS_, which is related to isoprenoid biosynthesis, from _P. trichocarpa_. We also assessed the expression pattern of _PtDXS_ in various tissues and under abiotic stresses and analyzed the expression levels of MEP- and MVA-related genes in the transgenic and WT plants.