Genome-Wide Analysis of NAC Transcription Factors and Characterization of the Cold Stress Response in Sweet Osmanthus

Sweet osmanthus (Osmanthus fragrans) is an evergreen aromatic woody tree widely used in landscaping. However, O. fragrans is very sensitive to cold stress, which seriously limits its cultivation and application in regions of low temperature. The NAC transcription factor gene family which has been characterized in many plant species could participate in multiple processes and play critical roles in plant response to external stress. However, little is known about this family in O. fragrans, an important woody ornamental plant. Here, the structural characteristics, chromosomal location, evolutionary relationship, cold stress expression patterns, and subcellular localization analysis of NAC family genes in O. fragrans were analyzed on a genome-wide scale. A total of 119 O. fragrans NAC genes were identified, and gene structure analysis showed that most OfNACs contained two introns, suggesting that structural diversity of the O. fragrans NAC family is relatively low. The phylogeny evolutionary tree showed that the NACs of O. fragrans were classified into 13 branches. The transcriptome data suggested that each branch members had different expression patterns in various tissues and a number of OfNACs showed tissue-specific expression profiles. The quantitative real-time PCR and subcellular localization results indicated that expression patterns of most branch '5B' members were significantly induced by cold stress and they were mainly located in the nucleus, especially OfNAC49 and 59, indicating that branch '5B' members could have important roles in cold tolerance in O. fragrans. Remarkably, expression levels of many branch '5B' genes were also significantly changed under waterlogging stress, suggesting that this branch might be widely involved in tolerance to various abiotic stresses. Our study should supply useful gene resources for the cold resistance breeding of O. fragrans.