Characterization of xyloglucan-specific fucosyltransferase activity in Golgi-enriched microsomal preparations from wheat seedlings

Xyloglucan (XyG) is a major hemicellulosic polymer in primary cell walls of dicotyledonous plants but represents only a minor constituent of cell walls from graminaceous monocotyledons (Poaceae). Our current information on XyG biosynthesis in vitro comes exclusively from studies on dicotyledonous plants. While XyG has been reported in grass cell walls, there are no studies of XyG biosynthesis in vitro in grasses. In this report, we investigated XyG structure and biosynthesis in etiolated wheat seedlings and showed that their walls contain small amounts (4-14%) of XyG. Furthermore, structural analysis using electrospray ionization mass spectrometry (ESI-MS) and high pH anion exchange chromatography (HPAEC) revealed that wheat XyG may be of XXGGG-type. Interestingly, detergent extracts from root microsomes were able to fucosylate tamarind XyG in vitro in a similar way as fucosyltransferase activity from Arabidopsis thaliana (AtFUT1) and pea (PsFUT1). Endoglucanase digestion of the [14C]fucosylated-tamarind XyG formed by the wheat fucosyltransferase activity released radiolabeled oligosaccharides that co-eluted with authentic fucoslyated XyG oligosaccharides (XXFG and XLFG). Although wheat fucosyltransferase activity was low, it appeared to be specific to XyG and required divalent ions (Mg2+ or Mn2+) for full activity. Together, these results suggest that the XyG fucosylation mechanism is conserved between monocots and dicots.