Abstract: The hardwickii（Cucumis sativus var. hardwickii）is recognized as the wild ancestor of cultivated cucumber，and plays an important role in cucumber genetics and cultivar improvement. In this study，a total of 77 NAC transcription factors were identified with methods of bioinformatics based on sequencing hardwickii genome. The NAC transcription factors were highly conserved. The length of sequences varied and ranged from 151 to 653 amino acids，and unevenly distributed in seven chromosomes. The theoretical isoelectric point（pI）ranged from 4.54 to 9.10. Most of NAC transcription factors were located in the cell nucleus and plasma membrane. Phylogenetic analysis showed that NAC proteins could be classified into six subfamilies，of which the third subfamily exhibited structural diversity. Evolutionary analysis indicated that segmental and tandem duplication events had occurred under the purifying selection pressure，while no recent whole-genome duplication（WGD）events were found in NAC transcription factors. Tissue-specific expression analysis showed that most of genes were highly expressed in root，followed by the female flowers and leaves. In addition，12 up-regulated NAC genes and 3 down-regulated NAC genes were identified under low temperature treatment via RT-PCR. Among which，HdNAC32 and HdANC47，a pair of segmental duplication genes，showed opposite expression patterns. Thus the conclusion was drawn out that NAC transcription factors play a critical role in the transcriptional regulation of cold tolerance in hardwickii under cold stress.

Key words: cucumber, hardwickii, NAC transcription factor, cold stress, gene expression