Genome-Wide Identification of the XTH Gene Family in Hydrangea macrophylla and Functional Characterization of HmXTH6 in Aluminum Accumulation

The xyloglucan endotransglucosylase/hydrolase (XTH) gene family plays key roles in plant cell wall remodeling and aluminum (Al) stress responses. However, a systematic genome-wide identification and functional characterization of the XTH gene family in Hydrangea macrophylla has not yet been reported. In this study, a total of 29 HmXTH genes were identified from the H. macrophylla genome. Phylogenetic analysis classified these genes into three distinct subgroups all of which contained the conserved catalytic motif DEIDFEFLG. Gene structure analysis revealed that most HmXTH genes possessed two conserved domains Glyco_hydro_16 and XET_C. Promoter analysis identified an enrichment of cis-acting elements associated with stress responses, hormone signaling, growth and development binding sites, while expression profiling demonstrated tissue-enriched patterns. Under Al stress, HmXTH6 was the only gene significantly upregulated across all three tissues (roots, stems, and leaves), suggesting its potential involvement in Al accumulation. Virus-induced gene silencing revealed that silencing HmXTH6 significantly decreased Al content by 86.8% and weakened Al signals in cell wall of leaves compared to controls, confirming its pivotal role in Al sequestration. These findings lay a theoretical foundation for elucidating the molecular mechanisms underlying Al hyperaccumulation.