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. Expressed at highest levels in epicotyls and hypocotyls of 14-day seedlings. Detected at low levels in stem apices and root tips of 14-day seedlings, and in 2-day etiolated seedlings. Not detected in 75-day plants. Up-regulated by brassinosteroids, at a post-transcriptional level. Contains at least one intrachain disulfide bond essential for its enzymatic activity. N-glycosylated; not essential for its enzymatic activity. Belongs to the glycosyl hydrolase 16 family.