Specific localization of arabinogalactan-protein epitopes at the surface of maize root hairs

Arabinogalactan-proteins (AGPs) are often localized at plant surfaces. However, their function there is unknown. We have used immunogold/silver and immunofluorescence techniques to study the developmental occurrence of an glucuronic acid (GlcA)-containing AGP epitope and beta-(1 leads to 6)-galactan AGP epitope at the surfaces of maize roots, especially during root hair formation and growth. Both techniques revealed that the GlcA AGP epitope was localized specifically at the surface of maize root hairs of all developmental stages. This epitope was most strongly expressed within the early bulge during initiation of root hairs and within the tip of elongating root hairs, while it was depleted from the tips of growth-terminating root hairs. The pattern of distribution of GlcA epitope was modified by ethrel treatment and water incubation. The beta-(1 leads to 6)-galactan epitope was not specific to the root hairs as it occurred also on the root surface within the root-hair forming zone. Nevertheless, this AGP epitope was similarly regulated during root hair morphogenesis as the GlcA epitope. These results indicate that surfaces of initiating and growing maize root hairs are equipped with AGP epitopes containing GlcA and of beta-(1 leads to 6)-galactan. Possible roles of AGPs on the root and root hair surface are discussed.