A (1 -> 3,6)-beta-D-galactosyl epitope containing uronic acids associated with bioactive pectins occurs in discrete cell wall domains in hypocotyl and root tissues of flax seedlings

We have used a well-characterized antibody specific for an epitope consisti ng of (1-->3,6)-beta-D-galactosyl residues with terminal glucuronic or 4-O- methylglucuronic acids of a bioactive pectin and immunocytochemistry to inv estigate its secretion and wall distribution in the hypocotyl and root tiss ues of flax seedlings. Our results show that this antigenic epitope is asso ciated with flax pectins and is expressed by all the cells of the hypocotyl and root tissues. In the hypocotyl, it is abundant in the primary wall of epidermal cells as well as in the secondary wall of fiber cells, and is rel atively less abundant in parenchyma cell walls. In contrast, the epitope is not detected in the middle lamellae and cell junction regions. In the root tip cells, immunogold electron microscopy shows that the cell walls of per ipheral, columella, meristematic, cortical, and epidermal cells contain sig nificant amounts of this epitope and that the distribution patterns are dis tinct. Together, these findings show that the antigenic epitope occurs in d iscrete domains of the wall implying a strict spatial regulation of the epi tope-containing molecules. The results also show that, in root cells, the e pitope is present within Golgi cisternae and is predominantly assembled in the trans and the trans-Golgi network compartments.