The effect of xyloglucan on spore germination, hyphal length and mycorrhizal colonization of alfalfa plants was studied. The
presence of high concentrations of xyloglucan in the rooting medium inhibited mycorrhizal colonization in plants inoculated with
Glomus mosseae. Intermediate xyloglucan concentrations had no effect on arbuscular mycorrhizal (AM) colonization, but a low
concentration increased mycorrhization of host plants. The effects of these doses on spore germination and hyphal length of G.
mosseae were similar to those observed for mycorrhizal colonization. Production of xyloglucanase was assayed during colonization
by the AM fungus G. mosseae in lettuce and onion. Endoxyloglucanase activity peaked 15 d after inoculation, whereas
exoxyloglucanase activity peaked at 30 and 50 d. Extracts from external mycelia of G. mosseae showed endo- and exoxyloglucanase
activities. Some of the endoxyloglucanase activities detected in AM colonized plant roots may be derived from the AM fungus, as
endoxyloglucanase proteins found in the external mycelia of G. mosseae and in mycorrhizal root extracts showed similar
electrophoretic mobility. These results suggest that xyloglucanase is involved in the process of colonization of plants by G. mosseae.