Immunolocalization of β-(1→4) and β-(1→6)-D-galactan epitopes in the cell wall and Golgi stacks of developing flax root tissues

Most cell wall components are carbohydrate including the major matrix polysaccharides, pectins and hemicelluloses, and the arabinogalactan-protein proteoglycans. Both types of molecules are assembled in the Golgi apparatus and transported in secretory vesicles to the cell surface. We have employed antibodies specific to beta-(1-->6) and beta-(1-->4)-D-galactans, present in plant cell wall polysaccharides, in conjunction with immunofluorescence and electron microscopy to determine the location of the galactan-containing components in the cell wall and Golgi stacks of flax root tip tissues. Immunofluorescence data show that beta-(1-->4)-D-galactan epitopes are restricted to peripheral cells of the root cap. These epitopes are not expressed in meristematic and columella cells. In contrast, beta-(1-->6)-D-galactan epitopes are found in all cell types of flax roots. Immunogold labeling experiments show that both epitopes are specifically located within the wall immediately adjacent to the plasma membrane. They are also detected in Golgi cisternae and secretory vesicles, which indicates the involvement of the Golgi apparatus in their synthesis and transport. These findings demonstrate that the synthesis and localization of beta-(1-->4)-D-galactan epitopes are highly regulated in developing flax roots and that different beta-linked D-galactans associated with cell wall polysaccharides are expressed in a cell type-specific manner.