We report on the homo- and hetero-transglycosylation activities of the HvXET3 and HvXET4 xyloglucan xyloglucosyl transferases (XET; EC 2.4.1.207) from barley (Hordeum vulgareL.), and the visualisation of these activities in young barley roots using Alexa Fluor 488-labelled oligosaccharides. We discover that these isozymes catalyse the transglycosylation reactions with the chemically defined donor and acceptor substrates, specifically with the xyloglucan donor and the penta-galacturonide [alpha(1-4)GalAp](5)acceptor - the homogalacturonan (pectin) fragment. This activity is supported by 3D molecular models of HvXET3 and HvXET4 with the docked XXXG donor and [alpha(1-4)GalAp](5)acceptor substrates at the -4 to +5 subsites in the active sites. Comparative sequence analyses of barley isoforms and seed-localised TmXET6.3 from nasturtium (Tropaeolum majusL.) permitted the engineering of mutants of TmXET6.3 that could catalyse the hetero-transglycosylation reaction with the xyloglucan/[alpha(1-4)GalAp](5)substrate pair, while wild-type TmXET6.3 lacked this activity. Expression data obtained by real-time quantitative polymerase chain reaction ofHvXETtranscripts and a clustered heatmap of expression profiles of the gene family revealed thatHvXET3andHvXET6co-expressed but did not share the monophyletic origin. Conversely,HvXET3andHvXET4shared this relationship, when we examined the evolutionary history of 419 glycoside hydrolase 16 family members, spanning monocots, eudicots and a basal Angiosperm. The discovered hetero-transglycosylation activity in HvXET3 and HvXET4 with the xyloglucan/[alpha(1-4)GalAp](5)substrate pair is discussed against the background of roles of xyloglucan-pectin heteropolymers and how they may participate in spatial patterns of cell wall formation and re-modelling, and affect the structural features of walls.