An ␣-l-fucosidase (EC 3.2.1.51) able to release the t-fucosyl residue from the side chain of xyloglucan oligosaccharides has been detected in the leaves of Arabidopsis plants. Moreover, an ␣-l-fucosidase with similar substrate specificity was purified from cabbage (Brassica oleracea) leaves to render a single band on SDS-PAGE. Two peptide sequences were obtained from this protein band, and they were used to identify an Arabidopsis gene coding for an ␣-fucosidase that we propose to call AtFXG1. In addition, an Arabidopsis gene with homology with known ␣-l-fucosidases has been also found, and we proposed to name it as AtFUC1. Both AtFXG1 and ATFUC1 were heterologously expressed in Pichia pastoris cells and the ␣-l-fucosidase activities secreted to the culture medium. The ␣-l-fucosidase encoded by AtFXG1 was active against the oligosaccharides from xyloglucan XXFG as well as against 2Ј-fucosyl-lactitol but not against p-nitrophenyl-␣-lfucopyranoside. However, the AtFUC1 heterologously expressed was active only against 2Ј-fucosyl-lactitol. Thus, the former must be related to xyloglucan metabolism.Plant cell walls are built by two independent networks, cellulose microfibers cross-linked by xyloglucan chains and cross-linked pectins, both networks contributing to their mechanical and functional properties (Roberts, 2001). The xyloglucan-cellulose complex has been considered as the network responsible for controlling the rate of cell expansion, xyloglucan being the load-bearing component in the primary cell walls because of its proposed cross-linking of the cellulose microfibers (Fry, 1989). It consists of a linearattached to the OH-6 of -glucosyl residues. In addition to its structural role, xyloglucan may act as a source of signaling molecules. Oligosaccharides derived from xyloglucan have been found to be formed in vivo (Fry, 1986), and they have been shown to regulate auxin-induced Fry, 1988, 1990) and acid pH-induced (Lorences et al., 1990) growth, the t-Fuc being necessary for the regulatory effect of the XXFG, a nonasaccharide derived from xyloglucan (Fry et al., 1993). The Fuc-deficient mur mutants of Arabidopsis (Reiter et al., 1993) showed dwarf growth habit and fragile cell walls.Enzymes that modify xyloglucan oligosaccharides have been detected in plant cell walls (Fry, 1995). An ␣-fucosidase that removes the ␣-l-fucosyl residue from XXFG has been purified from pea (Pisum sativum) epicotyls (Farkas et al., 1991; Augur et al., 1993) and has been cloned (Augur et al., 1995). Furthermore, cDNAs encoding ␣-l-fucosidase have been also isolated from human (Fukushima et al., 1985;Occhiodoro et al., 1989) and rat (Fisher and Aronson, 1989) livers. However, no ␣-l-fucosidase has yet been identified in Arabidopsis. Although some sequences homologous to pea ␣-l-fucosidase have been found in leguminous plants, there is no clear candidate for this activity in the whole genome of Arabidopsis.Thus, our aim in the present paper has been to look for an Arabidopsis gene(s) encoding for ␣-fucosidase activity. Two differ...