*Article* **Characterization of the** *XTH* **Gene Family: New Insight to the Roles in Soybean Flooding Tolerance**

**Li Song 1,2,\*, Babu Valliyodan 2, Silvas Prince 2,3, Jinrong Wan <sup>2</sup> and Henry T. Nguyen 2,\***


Received: 26 June 2018; Accepted: 6 September 2018; Published: 11 September 2018

**Abstract:** Xyloglucan endotransglycosylases/hydrolases (XTHs) are a class of enzymes involved in the construction and remodeling of cellulose/xyloglucan crosslinks and play an important role in regulating cell wall extensibility. However, little is known about this class of enzymes in soybean. Here, 61 soybean *XTH* genes (*GmXTH*s) were identified and classified into three subgroups through comparative phylogenetic analysis. Genome duplication greatly contributed to the expansion of *GmXTH* genes in soybean. A conserved amino acid motif responsible for the catalytic activity was identified in all GmXTHs. Further expression analysis revealed that most *GmXTHs* exhibited a distinct organ-specific expression pattern, and the expression level of many *GmXTH* genes was significantly associated with ethylene and flooding stress. To illustrate a possible role of *XTH* genes in regulating stress responses, the *Arabidopsis AtXTH31* gene was overexpressed in soybean. The generated transgenic plants exhibited improved tolerance to flooding stress, with a higher germination rate and longer roots/hypocotyls during the seedling stage and vegetative growth stages. In summary, our combined bioinformatics and gene expression pattern analyses suggest that *GmXTH* genes play a role in regulating soybean stress responses. The enhanced soybean flooding tolerance resulting from the expression of an *Arabidopsis* XTH also supports the role of *XTH* genes in regulating plant flooding stress responses.

**Keywords:** *Glycine max*; *XTH* gene family; transgenic soybean; plant genome; plant hormone; flooding; root plasticity
