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 (By similarity). Required for cell wall modification during the development of tracheary elements. 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. Expressed in 7 day old seedlings, roots, hypocotyls, rosette leaves, internodes between nodes bearing axillary shoots, nodes bearing flowers, flower buds, anthers and siliques. Accumulates in the immature tracheary elements of rosette leaves. Expressed in differentiating vasculature of the root, the hypocotyls, and the flower filaments, as well as in the anthers and the inner subepidermal layer of mature siliques. Contains at least one intrachain disulfide bond essential for its enzymatic activity. In xth27-1 and xth27-2, short-shaped tracheary elements in tertiary veins, reduced number of tertiary veins in the first leaf, and yellow lesion-mimic spots in mature rosette leaves. 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.