*Article* **Genome-Wide Identification and Expression Analysis of** *XTH* **Gene Family during Flower-Opening Stages in** *Osmanthus fragrans*

**Yang Yang 1,2,3, Yunfeng Miao 1,3, Shiwei Zhong 1,3, Qiu Fang 1,3, Yiguang Wang 1,3, Bin Dong 1,2,3,\* and Hongbo Zhao 1,2,3,\***


**Abstract:** *Osmanthus fragrans* is an aromatic plant which is widely used in landscaping and garden greening in China. However, the process of flower opening is significantly affected by ambient temperature changes. Cell expansion in petals is the primary factor responsible for flower opening. Xyloglucan endoglycolase/hydrolase (XTH) is a cell-wall-loosening protein involved in cell expansion or cell-wall weakening. Through whole-genome analysis, 38 *OfXTH* genes were identified in *O. fragrans* which belong to the four main phylogenetic groups. The gene structure, chromosomal location, synteny relationship, and cis-acting elements prediction and expression patterns were analyzed on a genome-wide scale. The expression patterns showed that most *OfXTHs* were closely associated with the flower-opening period of *O. fragrans*. At the early flower-opening stage (S1 and S2), transcriptome and qRT-PCR analysis revealed the expression of *OfXTH24*, *27*, *32*, *35* and *36* significantly increased under low ambient temperature (19 ◦C). It is speculated that the five genes might be involved in the regulation of flower opening by responding to ambient temperature changes. Our results provide solid foundation for the functional analysis of *OfXTH* genes and help to explore the mechanism of flower opening responding to ambient temperature in *O. fragrans*.

**Keywords:** *Osmanthus fragrans*; xyloglucan endoglycolase/hydrolase; flower opening; ambient temperature
