1. A xyloglucan-derived nonasaccharide ([3H]XG9; Glc4,Xyl3,Gal,Fuc) was neither taken up by cultured plant cells nor appreciably hydrolysed by them, but a proportion of it became incorporated into extracellular polymers in all cultures tested (Spinacia, Daucus, Rosa, Acer, Capsicum, Zea and Festuca). 2. In Spinacia these polymers were soluble in 20% (w/v) trichloroacetic acid, had apparent Mr 20,000-30,000, were able to bind reversibly to cellulose powder and were susceptible to hydrolysis by endo-beta-(1→4)-D-glucanase, indicating that they were xyloglucans. 3. The linkage formed between [3H]XG9 and the xyloglucan was alkali-stable and glucanase-labile, indicating that the reaction responsible for the incorporation was a transglycosylation. 4. The reducing terminus of the XG9 moiety remained reducing (convertible into [3H]glucitol by NaBH4) after incorporation into the polymer, showing that the XG9 was the glycosyl acceptor and the polysaccharide the donor. 5. The results provide the first evidence that polymeric xyloglucans are subject in vivo to cleavage followed by transfer of the cut end on the other xyloglucan-related molecules. 6. Similar endotransglycosylation reactions could occur within the primary cell wall, between pairs of high-Mr structural xyloglucan molecules. Such a reaction would provide a mechanism for reversible wall loosening and may thus be relevant to our understanding of plant cell growth.

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