**1. Introduction**

Xyloglucan endotransglucosylase/hydrolase (XTH), a cell-wall-relaxation factor, modifies the cellulose–xylan complex structure through catalyzing the breaking and rejoining of xyloglucan molecules in the plant cell wall [1]. XTH belongs to the Glycoside Hydrolase Family 16 (GH16) of the Carbohydrate Active Enzyme Family Database (CAZy). XTH proteins contain a Glyco\_hydro\_16 protein domain and a XET\_C domain [2,3]. Accumulating evidence has demonstrated that XTH proteins were related to the two kinks enzyme activity of xyloglucan transglycosylation (XET) and xyloglucan hydrolysis (XEH) [4]. In *Arabidopsis thaliana*, *XTH* gene family can be divided into four groups, and the different groups have different functions or enzyme activity. For instance, Group I/II and Group IIIB genes show XET activity, while Group IIIA genes show XEH activity [2,5].

Increasing reports showed that XTH family genes play important roles in many processes of plant growth and development. During root development stages, high expression of *SlXTH1* can be increased by XET activity in the hypocotyl elongation zone and affect the hypocotyl growth and cell wall extensibility in tomato [6]. Similarly, *AtXTH21* gene played

**Citation:** Yang, Y.; Miao, Y.; Zhong, S.; Fang, Q.; Wang, Y.; Dong, B.; Zhao, H. Genome-Wide Identification and Expression Analysis of *XTH* Gene Family during Flower-Opening Stages in *Osmanthus fragrans*. *Plants* **2022**, *11*, 1015. https://doi.org/10.3390/ plants11081015

Academic Editors: Aiping Song and Yu Chen

Received: 18 March 2022 Accepted: 2 April 2022 Published: 8 April 2022

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a principal role in the growth of the primary roots by altering the deposition of cellulose and the elongation of the cell wall [7]. In addition, *AtXTH9* was positively correlated with cell elongation of the inflorescence stem [8]. In flowering plants, flower opening is dependent on petal growth, which is closely related to the cell expansion. In *Ipomoea nil*, *InXTH2*, *InXTH3*, and *InXTH4* were upregulated by dark period and their expression was positively correlated with the rate of flower opening [9]. *EgXTH1* from *Eustoma grandiflorum* was involved in cell expansion, and the expression was increased with the flower-opening process compared with the control [10]. In *Rosa hybrida*, *RhXTH1* were relevant to the typical growth in different part of petals, and also increased markedly during petal growth [11]. Conversely, if *XTH* genes are not expressed or inhibited, the petal growth will not continue, thereby effectively inhibiting flower opening [12]. Therefore, this evidence suggests that *XTH* genes are an important regulator, and significantly influence petal cell expansion during the flower-opening process of most species.

*Osmanthus fragrans* is one of the Chinese top ten traditional flowers, known for its strong fragrance, and is widely cultivated as a garden tree in most Asian countries due to its high ornamental value and aromatic scent. However, the changes in ambient temperature significantly affected the opening and longevity of the flower [13]. In our previous research, we found that the flower opening is significantly promoted by low ambient temperature 19 ◦C in *O. fragrans*, and the flower-opening process is accompanied by petal cells significant expansion [14,15]. To clarify whether *OfXTH* genes regulate flower opening in response to ambient temperature, the expression profiles of *OfXTH* genes were analyzed during flower-opening process under different temperature treatment. Our results could deepen understanding of the *OfXTH* gene family and help to explore the mechanism of *O. fragrans* petal expansion responding to ambient temperature changes.
