Identification of the NLP Gene Family in Populus euphratica and Its Expression Analysis Under Drought Stress

NIN-like protein (NLP) transcription factors are key regulators of plant nitrate signaling and stress responses. Although extensively studied in Arabidopsis thaliana and various crops, it has rarely been reported in woody plants, particularly in drought-tolerant tree species. In this study, 10 PeNLP genes were identified in the drought-tolerant tree Populus euphratica Oliv. through comparative genomics. These genes were unevenly distributed across seven chromosomes, and the gene-family expansion was mainly driven by whole-genome duplication (WGD). Analysis of conserved domains showed that PeNLPs contained 4–10 characteristic motifs, and most members possessed the typical RWP-RK and PB1-related domains. Collinearity analysis identified 18 NLP orthologous gene pairs between P. euphratica and its relatives (Populus pruinosa and Salix sinopurpurea), which exceeded the 15 pairs detected between P. euphratica and A. thaliana, indicating that the NLP family is more conserved within the Salicaceae. Phylogenetic analysis divided PeNLPs into three subfamilies, and their promoter regions harbored diverse cis-acting elements associated with hormone signaling, environmental stress, growth, and light response. Transcriptome and qRT-PCR analyses further demonstrated that PeNLPs were generally downregulated under drought stress. Overall, this study systematically characterized the evolution, structure, and drought responsiveness of the PeNLPs, providing a theoretical basis and genetic resources for improving nitrogen use efficiency and drought resistance in trees.