Genome-Wide Analyses of the XTH Gene Family in Brachypodium distachyon and Functional Analyses of the Role of BdXTH27 in Root Elongation

Xyloglucan endotransglucosylase/hydrolases (XTHs) are a class of cell wall-associated enzymes involved in the construction and remodeling of cellulose/xyloglucan crosslinks. However, knowledge of this gene family in the model monocot <i>Brachypodium distachyon</i> is limited. A total of 29 <i>BdXTH</i> genes were identified from the whole genome, and these were further divided into three subgroups (Group I/II, Group III, and the Ancestral Group) through evolutionary analysis. Gene structure and protein motif analyses indicate that closely clustered <i>BdXTH</i> genes are relatively conserved within each group. A highly conserved amino acid domain (DEIDFEFLG) responsible for catalytic activity was identified in all BdXTH proteins. We detected three pairs of segmentally duplicated <i>BdXTH</i> genes and five groups of tandemly duplicated <i>BdXTH</i> genes, which played vital roles in the expansion of the <i>BdXTH</i> gene family. <i>Cis</i>-elements related to hormones, growth, and abiotic stress responses were identified in the promoters of each <i>BdXTH</i> gene, and when roots were treated with two abiotic stresses (salinity and drought) and four plant hormones (IAA, auxin; GA3, gibberellin; ABA, abscisic acid; and BR, brassinolide), the expression levels of many <i>BdXTH</i> genes changed significantly. Transcriptional analyses of the <i>BdXTH</i> genes in 38 tissue samples from the publicly available RNA-seq data indicated that most <i>BdXTH</i> genes have distinct expression patterns in different tissues and at different growth stages. Overexpressing the <i>BdXTH27</i> gene in <i>Brachypodium</i> led to reduced root length in transgenic plants, which exhibited higher cellulose levels but lower hemicellulose levels compared to wild-type plants. Our results provide valuable information for further elucidation of the biological functions of <i>BdXTH</i> genes in the model grass <i>B. distachyon</i>.