# Overexpression of _PtDXS_ Enhances Stress Resistance in Poplars
## 1 Introduction  
Higher plants have two distinct isoprenoid biosynthesis pathways, the chloroplastic methylerythnitolphosphate (MEP) pathway, which is responsible for producing monotereponoids and triterpenoids, and the cytosolic nevalonic acid (MVA) pathway, which is responsible for synthesizing sesquiterpenoids and triterpenes [1; 2; 3]. Both the MEP and MVA pathways provide isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) for isoprenoid biosynthesis [4; 5]. All known isoprenoids are necessary for life [6]. For example, the isoprenoid vitamin A plays a role in human growth and development as well as in immune system maintenance. Isoprenoids are also implicated in defense against biotic and abiotic stresses, intracellular signal transduction and vesicular transport, and formation of cellular and organelle membranes [7; 8; 9; 10; 11].  
In the first important step in the MEP pathway, 1-deoxy-d-xylulose-5-phosphate synthase (DXS) catalyzes the condensation of pyruvate and glyceraldehyde-3-phosphate (G-3-P) to form1-deoxyy-D-xylulose 5-phosphate (DXP) [12]. DXP synthesis requires Mg\({}^{2+}\) or Mn\({}^{2+}\) and thiamine pyrophosphate (TPP) as a cofactor [13; 14]. Next, 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR) catalyzes the conversion of 1-deoxyy-D-xylulose 5-phosphate to 2-C-methyl-D-erythntol-4-phosphate (MEP), which requires NADPH as a cofactor as well as Mg\({}^{2+}\) or Mn\({}^{2+}\)[15].  
_DXS_ genes from _Ginkgo biloba_, _Aquilaria sinensis_, _Arabidopsis thaliana_, conifers, maize, and soybean have been cloned and characterized [16; 17; 18; 19]. Transformation of tomato plants with bacterial _DXS_s resulted in significant increases in carotenoid content [20]. In _DXS_-overexpressing _A. thaliana_, the contents of terpenoids-tocopherols, chlorophyll, carotenoids, and abscisic acid (ABA) were significantly higher than wild types (WT) [21]. In addition, the contents of these terpenoids in _DXS_-underexpressing _A. thaliana_ were significantly lower than WT plants. The chlorophyll content of _DXS_-over- and under-expressing _A. thaliana_ was 130% to 142% and 65% to 84% that of the WT, respectively. In addition, the hypocotyls of _DXS_-over- and under-expressing _A. thaliana_ were significantly shorter and longer than WT plants, respectively. _AtDXS_ from _A. thaliana_ was transferred into laverander, resulting in a significantly higher essential oil content in T0 and T1 transgenic seedlings than in WT plants, but the carotenoid and chlorophyll contents were unaffected [22]. Transfer of a bacterial _DXS_ gene into potato resulted in tuber lengthening and premature flowering, indicating DXS is involved in phenotypic regulation. In addition, the caroten content increased two-fold and that of lycopene by six- to seven-fold compared to the control [21]. Thus, DXS is the rate-limiting enzyme in isoprenoid synthesis and a variety of physiological processes.  
In this work, we report the cloning and characterization of _DXS_ from _Populus trichocarpa_. We also evaluated _PtDXS_ expression in different plant tissues and in the presence of abiotic stresses. Overexpression of _PtDXS_ enhanced the tolerance of poplar to biotic stresses.