Catalyzes xyloglucan endohydrolysis (XEH) and/or endotransglycosylation (XET). Cleaves and religates xyloglucan polymers, an essential constituent of the primary cell wall, and thereby participates in cell wall construction of growing tissues. breaks a beta-(1->4) bond in the backbone of a xyloglucan and transfers the xyloglucanyl segment on to O-4 of the non-reducing terminal glucose residue of an acceptor, which can be a xyloglucan or an oligosaccharide of xyloglucan. Interacts with XTH31. The formation of an XTH17-XTH31 dimer may be required for XET activity. Root specific (PubMed:11673616). Expressed in all cell types in the elongating and differentiating region of the root (PubMed:15659443). Up-regulated by auxin and brassinolide (PubMed:11673616). Down-regulated by aluminum. Contains at least one intrachain disulfide bond essential for its enzymatic activity. Knockdown mutants have shorter roots, decreased cell wall xyloglucan content and increased aluminum resistance. Belongs to the glycosyl hydrolase 16 family. XTH group 2 subfamily.