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. Involved in the accumulation of hemicelluloses. Has a high XEH activity and only a slight XET activity in vitro, but the main in planta activity seems to be XET, thus controlling aluminum sensitivity (PubMed:23204407, PubMed:23104861, PubMed:25446234). Acceptor preferences are XXXGol = XXFGol > XXLGol > XLLGol = XLFGol (PubMed:25446234). 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. xyloglucan + H2O = xyloglucan oligosaccharides. KM for xyloglucan as donor substrate is 1.6 mg/ml. KM is quoted in mg/ml, not uM, because XTHs are able to utilise any segment of the polysaccharide chain equally well, not just one site per molecule as with the acceptor. Optimum pH is 5 for the XET activity. Optimum pH is 4.5 - 4.75 for the XEH activity. Interacts with XTH17. The formation of an XTH17-XTH31 dimer may be required for XET activity. Predominantly expressed in root (PubMed:11673616, Ref.1). Weakly expressed in influorescence stems (PubMed:11673616). Expressed in root tips and elongation zones, stems, young leaves, flowers and siliques (PubMed:23204407). Expressed in root, hypocotyl, and etiolated whole seedlings (PubMed:23104861). Expressed in germinating seeds from 24 hours after imbibation, and reaches a maximum level at 72 hours. After 96 hours, it then decreases. Up-regulated by gibberellins (Probable). Not induced by auxin (Ref.1). Down-regulated by aluminum (PubMed:21285327, PubMed:23204407). Contains at least one intrachain disulfide bond essential for its enzymatic activity. Reduced growth, decreased cell wall xyloglucan content and increased aluminum resistance (PubMed:23204407). No visible phenotype under normal growth conditions (PubMed:23104861). In contrast to group 1 and group 2 endotransglucosylase/hydrolase proteins, it may not contain the ligase activity, and may catalyze endohydrolysis xyloglucan polymers only. Belongs to the glycosyl hydrolase 16 family. XTH group 3 subfamily.