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 OfNAC s 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 OfNAC s 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 .

中文翻译：

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NAC 转录因子的全基因组分析和甜桂花冷应激反应的表征

桂花（Osmanthus fragrans）是一种常绿芳香木本树，广泛用于园林绿化。然而，香椿对冷胁迫非常敏感，严重限制了其在低温地区的栽培和应用。NAC 转录因子基因家族已在许多植物物种中得到表征，可以参与多个过程并在植物对外部胁迫的响应中发挥关键作用。然而，在重要的木本观赏植物 O. fragrans 中，人们对这个家族知之甚少。在这里，在全基因组范围内分析了香茅 NAC 家族基因的结构特征、染色体位置、进化关系、冷应激表达模式和亚细胞定位分析。共鉴定出 119 个 O. fragrans NAC 基因，基因结构分析表明，大多数OfNAC含有两个内含子，表明O. fragrans NAC家族的结构多样性相对较低。系统发育进化树表明，香椿的NACs分为13个分支。转录组数据表明，每个分支成员在各种组织中具有不同的表达模式，并且许多 OfNAC 显​​示出组织特异性表达谱。实时定量PCR和亚细胞定位结果表明，大多数分支'5B'成员的表达模式受冷胁迫显着诱导，主要位于细胞核中，尤其是NAC49和59，表明分支'5B'成员可能具有O. fragrans 耐寒性中的重要作用。值得注意的是，许多分支'5B'基因的表达水平在涝渍胁迫下也发生显着变化，表明该分支可能广泛参与对各种非生物胁迫的耐受。本研究可为香茅的抗寒育种提供有用的基因资源。