Identification of the Populus euphratica XTHs Gene Family and the Response of PeXTH7 to Abiotic Stress

<i>Populus euphratica</i> Oliv. serves as a keystone species in desert riparian ecosystems. Owing to its pronounced tolerance to drought and salinity, as well as its robust reproductive capacity, it has become a pioneer species in desert oases. The xyloglucan endotransglucosylase (XET)/hydrolase (<i>XTH</i>) gene family plays a critical role in the remodeling of plant cell walls; however, its potential biological functions in poplar remain poorly understood. In this study, we identified the <i>XTH</i> gene family in <i>P. euphratica</i> and conducted a preliminary functional analysis. A total of 33 <i>PeXTH</i> genes were identified, which were unevenly distributed across the chromosomes, with the highest density observed on chromosome 6. Conserved domain analysis indicated that most members contain the typical GH16 domain associated with xyloglucan endotransglucosylase activity. Phylogenetic analysis classified them into four distinct subgroups, exhibiting evolutionary conservation with the model dicot plant of <i>Arabidopsis thaliana</i>. Notably, the promoter analysis revealed an abundance of ABA-responsive and stress-related cis-elements, suggesting their potential involvement in response to multiple stresses. Under drought stress, <i>PeXTH7</i> (<i>PeuTF07G00088.1</i>) exhibited a distinct expression pattern, with transcript levels significantly increasing with persistent treatment. RT-qPCR results confirmed that <i>PeXTH7</i> is highly expressed in both roots and leaves. Furthermore, subcellular localization assays demonstrated that the PeXTH7 protein localizes to the secretory pathway and the cell wall, implying a role in cell wall dynamic remodeling through the regulation of xyloglucan metabolism. The <i>PeXTH7</i>-overexpressing transgenic lines exhibited a significant increase in root length compared to the wild-type controls. As the first systematic analysis of the <i>XTH</i> gene family in <i>P. euphratica</i>, this study fills an important knowledge gap and provides new insights into the adaptive mechanisms of desert tree species.