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1. Introduction {#sec1-ijms-16-20468} =============== Gene expression characterization at the steady-state mRNA level is an important step in many plant biological processes. Northern blotting \[[@B1-ijms-16-20468],[@B2-ijms-16-20468],[@B3-ijms-16-20468],[@B4-ijms-16-20468]\], is not an optimal method for gene expression quantification anymore, because it requires a large amount of RNA and is limited to detecting high-abundance transcripts. Therefore, reverse transcription-quantitative polymerase chain reaction (RT-qPCR), which can detect low-abundance mRNAs and exhibits high specificity, sensitivity, and stability \[[@B5-ijms-16-20468],[@B6-ijms-16-20468],[@B7-ijms-16-20468]\], has become one of the most commonly used techniques \[[@B8-ijms-16-20468]\]. However, RT-qPCR has certain disadvantages; for example, reference genes are important to correct the experimental error that occurs during RNA extraction, and cDNA preparation, but the normalized results are highly dependent on the expression stability of the reference genes selected \[[@B9-ijms-16-20468]\]. The use of inappropriate reference genes can result in biased target gene expression profiles; thus, using suitable reference genes for data normalization is a prerequisite for any RT-qPCR study \[[@B10-ijms-16-20468]\]. The expression of an ideal reference gene should not vary between different tissues or cells under investigation, nor in response to any experimental treatment \[[@B11-ijms-16-20468]\]. However, extensive transcriptomic data mining and gene expression analysis studies have reported that the reliability of these endogenous controls can be influenced by various conditions \[[@B12-ijms-16-20468],[@B13-ijms-16-20468],[@B14-ijms-16-20468],[@B15-ijms-16-20468],[@B16-ijms-16-20468],[@B17-ijms-16-20468]\]. Therefore, to obtain accurate and reliable target gene expression profiles, it is important to identify the best reference genes for use in each experimental set-up and verify these in each individual experiment before performing gene expression normalization. In common plant species, including *Arabidopsis*, rice, soybean, and petunia, genes associated with basic cellular processes such as those related to ubiquitin degradation, polyubiquitin, ubiquitin-conjugating enzymes, and ubiquitin ligases (collectively known as *UBQ*), as well elongation factors (EFs) \[[@B18-ijms-16-20468],[@B19-ijms-16-20468],[@B20-ijms-16-20468],[@B21-ijms-16-20468]\], have been identified and used for RT-qPCR \[[@B6-ijms-16-20468],[@B12-ijms-16-20468],[@B13-ijms-16-20468],[@B19-ijms-16-20468]\]. Nevertheless, few reference genes have been identified in forest trees, which dominate much of the terrestrial landscape on earth. Members of the genus *Populus* are used as a model forest species for diverse studies because of their amenability to experimental and genetic manipulation \[[@B22-ijms-16-20468],[@B23-ijms-16-20468],[@B24-ijms-16-20468],[@B25-ijms-16-20468]\]. Studies in *Populus* focus mainly on improving their ability to respond to environmental stress while maintaining their high-speed growth capacity because there is strong demand for their cultivation in highly saline and arid soils in many parts of the world \[[@B26-ijms-16-20468],[@B27-ijms-16-20468]\]. *Populus euphratica* Oliv., which is native to desert regions ranging from western China to North Africa, is the only arboreal species established in the world's largest shifting-sand desert, the Taklimakan Desert in China \[[@B28-ijms-16-20468]\]. The most significant characteristic of *P. euphratica* is its extraordinary adaptation to salt stress \[[@B28-ijms-16-20468],[@B29-ijms-16-20468],[@B30-ijms-16-20468]\]. This species can survive concentrations of NaCl in nutrient solution up to 450 mM while maintaining higher growth and photosynthetic rates than other poplar species \[[@B31-ijms-16-20468],[@B32-ijms-16-20468],[@B33-ijms-16-20468]\]. Previous studies of *P. euphratica* have mainly focused on analyses of salt-responsive genes \[[@B26-ijms-16-20468],[@B30-ijms-16-20468],[@B34-ijms-16-20468],[@B35-ijms-16-20468]\], miRNAs \[[@B36-ijms-16-20468],[@B37-ijms-16-20468]\], or salt-related transcriptome sequencing \[[@B26-ijms-16-20468],[@B38-ijms-16-20468],[@B39-ijms-16-20468]\]. In these studies, the genes up-regulated or down-regulated by salt stress are important and should be identified using validated reference genes by RT-qPCR. Here, we evaluated the expression stability in *P. euphratica* leaves, stems,