TaLEA (Tamarix androssowii late embryogenesis abundant gene, DQ663481) gene was introduced into Populus simonii × Populus nigra by Agrobacterium tumefaciens-mediated transformation with the aim of improving salt-tolerance. Among the 15 transgenic lines, one showed a dwarf phenotype (dwf1). Under the same growth conditions, dwf1 height was significantly reduced compared with the wild-type and the other transgenic lines. The mechanisms underlying this effect were investigated in mutant and wild-type plants using a label-free quantitative proteomics approach. Among proteins that identified, 99 were significantly altered. With the exception of proteins with unidentified or unclassified functions, these proteins were classified into eight groups based on gene product subcellular localization and biological process (metabolism, stress, protein synthesis and degradation, transcriptional regulation, cell fate, transportation, cell wall, and cytoskeleton). Differential expression patterns were identified for key enzymes involved in major metabolic pathways such as the Calvin cycle and glycolysis, thus indicating the interplay of complex molecular events in generation of the dwf1 mutant. Overall, the differentially expressed proteins in dwf1 might provide some useful insights into the dwarf formation.