# Overexpression of _PtDXS_ Enhances Stress Resistance in Poplars
## 3 Discussion  
The enzymes of the MEP pathway produce precursor compounds. DXS is the rate-limiting enzyme in the MEP pathway. We characterized and evaluated the expression of _DXS_ from \(P\). Figure 8: _M. troglodytte_ feeding assay. (**A**) Leaves of WT and _PtDXS_-transgenic plants after initiating to be eaten by first instar larvae. (**B**) Leaves of WT and _PtDXS_-transgenic plants after after initiating to be eaten by third instar larvae. (**C**) Leaves of WT and _PtDXS_-transgenic plants after after initiating to be eaten by fifth instar larvae. (**D**) Quantitative data for leaves of WT and _PtDXS_-transgenic plants. Three independent experiments were performed; asterisks represent significant differences relative to WT plants (Student’s \(t\)-test; ***, and \(p<0.001\), respectively). Figure 7: Necrotic symptoms of WT and transgenic plants infected with _S. populiperda._ (**A**) Leaves of WT and transgenic plants after inoculation. (**B**) Plaques on WT and transgenic plants. Three independent experiments were performed; three independent experiments were performed; asterisks represent significant differences relative to WT plants (Student’s \(t\)-test; ***, and ***, \(p<0.01\), and \(p<0.001\), respectively). trichocarpa. PtDXS_ encoded a protein with TPP-binding and DRAG domains. The amino acid sequence of PtDXS (XP_024463484) showed a high level of similarity with those of _Alpinia officinarum_ (AEK69518.1), _Hevera brasiliensis_ (ABD92702.1), _Ricinus communis_ (XP_015573388.1), and _Theobroma cacao_ (XP_017975597.1). Type-I and -II DXSs may play an important role in plant primary and secondary metabolism, respectively, and type-III DXSs are commonly found in plant genomes, but the functions of their encoded enzymes are unknown [32]. PtDXS is a type-I DXS and plays an important role in basic life processes. DXS expression is related to flower and fruit development, and circadian rhythm. In addition, the expression patterns of _DXS_ genes in different families (such as Type-I, -II, and -III _DXS_s) may vary in the same species. The expression level of _DXS_ in rose petals increased from the bud to blooming stages. Subsequently, the expression level of _DXS_ decreased as flowers decayed. Thus, _DXS_ is related to flower development [33]. The circadian clock regulates the expression level of _DXS_ in jasmine petal. Tomato _SIDXS2_ is highly expressed in flower [34] and _DXS_ from _Dendrobium candidum_ in the protocorm [35]. In addition, the expression level of _DXS_ differs according to the stage of fruit development [36]. _DXS_ expression is highest at the early stage of leaf development [37] in plants whose leaves are the major organs of terpenoid synthesis. The expression of _DXS_s in different types of the same species may be the same or different. The expression of _DXS_ in _Aquilaria sinensis ([PERSON].) Spreng_ was high in stems, leaves and roots [16]. _DXS1_ from _Salvia milliorrhiza_ was expressed in leaves, stems, and roots, and was highest in leaves and lowest in root. However, expression of _DXS2_ from _S. milliorrhiza_ was highest in root [38]. In this study, _PtDXS_ was constitutively expressed in mature and young leaves, upper and lower regions of stems, petioles, and roots of poplar. _PtDXS_ expression was highest in young leaves, followed by mature leaves, and was lowest in root; this is in agreement with a previous report [39]. Type-I _DXSs_ are expressed in all tissues, except root, of _M. truncatula, Lycopersicon esculentum_, and _Nicotiana tabacum_[40], which is consistent with our results. _PtDXS_ plays an important role in primary metabolism and growth and development. Hormones, light, circadian rhythm, developmental degree, and sucrose concentration regulate the expression of _DXS_genes [41]. Treatment of _Dendrobium officinal_ with ABA, salicylic acid (SA), jasmonic acid (JA), and brassinosteroids resulted in increased _DXS_ expression in the protocorm [35].