Arabinogalactan proteins (AGPs) are abundant plant cell surface proteoglycans widely distributed in plant species. Since high concentrations of beta-glucosyl Yariv reagent (beta glcY), which binds selectively to AGPs, inhibited cell division of protoplast-regenerated cells of the liverwort Marchantia polymorpha L. (Shibaya and Sugawara in Physiol Plant 130:271-279, 2007), we investigated the mechanism underlying the inability of the cells to divide normally by staining nuclei, cell walls and beta-1,3-glucan. Microscopic observation showed that the diameter of regenerated cells cultured with beta glcY was about 2.8-fold larger than that of cells cultured without beta glcY. The cells cultured with beta glcY were remarkably multinucleated. These results indicated that beta glcY did not inhibit mitosis but induced multinucleation. In the regenerated cells cultured with low concentrations of beta glcY (5 and 1 mu g ml(-1)), the cell plate was stained strongly by beta glcY, suggesting abundant AGPs in the forming cell plate. In these cell plates, beta-1,3-glucan was barely detectable or not detected. In multinucleated cells, cell plate-like fragments, which could not reach the cell wall, were frequently observed and they were also stained strongly by beta glcY. Our results indicated that AGPs might have an important role in cell plate formation, and perturbation of AGPs with beta glcY might result in remarkable multinucleation in protoplast-regenerated cells of M. polymorpha.