Genome-Wide Analysis of the NAC Transcription Factor Gene Family Reveals Differential Expression Patterns and Cold-Stress Responses in the Woody Plant Prunus mume

NAC transcription factors (TFs) participate in multiple biological processes, including biotic and abiotic stress responses, signal transduction and development. Cold stress can adversely impact plant growth and development, thereby limiting agricultural productivity. Prunus mume, an excellent horticultural crop, is widely cultivated in Asian countries. Its flower can tolerate freezing-stress in the early spring. To investigate the putative NAC genes responsible for cold-stress, we identified and analyzed 113 high-confidence PmNAC genes and characterized them by bioinformatics tools and expression profiles. These PmNACs were clustered into 14 sub-families and distributed on eight chromosomes and scaffolds, with the highest number located on chromosome 3. Duplicated events resulted in a large gene family; 15 and 8 pairs of PmNACs were the result of tandem and segmental duplicates, respectively. Moreover, three membrane-bound proteins (PmNAC59 / 66 / 73) and three miRNA-targeted genes (PmNAC40 / 41/ 83) were identified. Most PmNAC genes presented tissue-specific and time-specific expression patterns. Sixteen PmNACs (PmNAC11/19/20/23/ 41/ 48/58/7 4 / 7 5/76/78/79 / 85 / 86/103/111) exhibited down-regulation during flower bud opening and are, therefore, putative candidates for dormancy and cold-tolerance. Seventeen genes (PmNAC11/12/17/21/29 / 42/30/48/ 59/66/73/75/85/86/93/99/111) were highly expressed in stem during winter and are putative candidates for freezing resistance. The cold-stress response pattern of 15 putative PmNACs was observed under 4 degrees C at different treatment times. The expression of 10 genes (PmNAC11/20/23/ 40 / 42/48 / 57 /60/66/86) was upregulated, while 5 genes (PmNAC59 / 61/ 82 / 85 / 107) were significantly inhibited. The putative candidates, thus identified, have the potential for breeding the cold-tolerant horticultural plants. This study increases our understanding of functions of the NAC gene family in cold tolerance, thereby potentially intensifying the molecular breeding programs of woody plants.