AbstractProtein extract of germinating nasturtium (Tropaeolum majus) seeds containing xyloglucan endotransglycosylase (xyloglucan xyloglucosyl transferase, EC 2.4.1.207, abbreviated XET) exhibited the heterotransglycosylating activity with donor/acceptor substrate pair xyloglucan/sulphorhodamine labelled pustulooligosaccharides (XG/PUOS-SR) in a dot blot assay. The heterotransglycosylating activity was confirmed by the substrate-product changes during transglycosylation by HPLC size-exclusion chromatography. Another donor substrate capable of being coupled with PUOS-SR was cellulose, probably owing to its structural similarity to xyloglucan. Surprisingly, microscopic comparison of the incorporation of the labelled xyloglucan nonasaccharide XGO9-SR (specific substrate for XET) and PUOS-SR into the cell wall structures clearly showed differences in their binding to specific cell structures: the primary cell wall and the plasma membrane. These findings indicate the existence in nasturtium of XETs with different localisation, substrate specificity and, probably, function.