Except for cellulose and callose, all of the other plant polysaccharides appear to be syn­thesized in the Golgi or to pass through the Golgi en route to the cell wall. The available experimental results do not currently lead to an unambiguous picture of how the diverse wall polysaccharides are synthesized in, and travel through the Golgi to the wall. Immunoelec — tron microscopy using antibodies to XG, HG, and RGI showed that these polysaccharides are found in Golgi vesicles but not in the ER (106,234). Different types of Golgi cisternae contain different sets of glycosyltransferases. Thus, the functional organization of the biosynthetic pathways of complex polysaccharides is consistent with these molecules being processed in a cis-to-trans direction like the N-linked glycans. RG-I and HG polysaccharides appear to be synthesized in cis — and medial-cisternae (235). Methylesterification of the carboxyl groups of the galacturonic acid residues in the polygalacturonic acid domains occurs mostly in medial cisternae, and arabinose-containing side chains of the polygalacturonic acid domains are added to the nascent polygalacturonic acid/rhamnogalacturonan-I molecules in the trans — cisternae. In root tip cortical parenchyma cells, anti-RG-I and the anti-XG antibodies are shown to bind to complementary subsets of Golgi cisternae, and several lines of indirect evidence suggest that these complex polysaccharides may also exit from different cisternae (224). On the other hand, xyloglucan and polygalacturonic acid/rhamnogalacturonan-I can be synthesized concomitantly within the same Golgi stack.

O-linked arabinosylation of the hydroxyproline residues of extensin occurs in cis — cisternae, and the glycosylated proteins pass through all cisternae before they are packed into secretory vesicles in the monensin-sensitive, trans-Golgi network (224). The (3-1,4-linked D-glucosyl backbone of xyloglucan is synthesized in trans-cisternae, and the terminal fucosyl residues on the trisaccharide side chains of xyloglucan are partly added in the trans-cisternae, and partly in the trans-Golgi network (235). It has been shown by immuno-electron mi­croscopy using anti-a-L-Fuc-(1^2)-D-Gal antibodies, that fucosylated XyG first appears in the lumen of the trans-Golgi and trans-Golgi network before vesicle mediated secretion to the cell wall (224, 400). This activity appeared to be spatially distinct from galactosyl — and xylosyltransferase activity (401, 402).