DNA is a water-soluble molecule, aided by tens of proteins to helping in the packaging and re-packing during RNA and DNA synthesis. Proteins are also soluble and if they are too hydrophobic, either a portion will be embedded in membranes or hydrophobic domains will assemble together with other hydrophobic proteins to maintain their solubility. On the other hand, plant polysaccharides provide us with a challenge in terms of understanding how such massive amounts of insoluble or gel-like structures are made inside the Golgi apparatus without hindering other cellular processes. Bacteria, for example, form insoluble lipo-glycans in stages: UDP-sugars made in the cytosol contribute to synthesis of short side chain glycans attached to the inner membrane. These short glycans are made facing the cytosol. The side chains are then flipped and transferred through the outer membrane to the outside of the cell, where specific enzymes cut and assemble the side chains to form complete glycan structures. This process, by analogy, is very similar to the synthesis of core N-linked glycoproteins. But how are pectins and hemicelluloses made in plants? Are Golgi — synthesized polysaccharides made completely in the Golgi, or are they made in smaller fragments that are assembled together at the wall, as is the case in lipo-glycan synthesis? If the entire pectic polymers are made in the Golgi, then are they sequestered or packed temporarily with or by proteins to help with the challenge posed by their physical properties (i. e., solubility, size, etc.)? Further questions will arise. Is HG made inside the lumen (thus making a jelly-like lumen) or perhaps made in specific Golgi stacks, designated only for the wall (not glycoprotein biosynthesis)? The latter scenario could be attractive in light of the fact that unlike humans and fungi, one plant cell consists of hundreds of Golgi stacks. It is possible that an entire “designated Golgi” is moving with its packed glycan(s) to the wall to “unload” the insoluble matrix.

While many of the putative GTs are Type II membrane proteins (i. e., predicted to have their catalytic domain facing the lumen), some GTs, such as mannan synthase, have multi domains that span the membranes. Where is the catalytic domain of such Golgi-enzymes facing? Furthermore, if a glycan is fully made in the Golgi, is it made in one subcompartment of the Golgi, for example, cis-Golgi, or is it initiated in the cis — and further modified in the medial — Golgi (like glycoproteins)? Certainly, understanding where each wall biosynthetic enzyme functions and where its catalytic domain faces is essential to address the above questions.

In addition to location and topology, it is puzzling how polysaccharides are made in the small Golgi cisternae (estimated to be smaller than 20 x 200 nm). The same cisternae are temporarily packed not only with wall-glycans but also with numerous Golgi-resident proteins and large numbers of secretory proteins. If the average size of the catalytic domain of GTs and membrane-bound NDP-sugar biosynthetic enzymes is 30-40 kDa, they can “touch” each other if inserted opposite to each other. Given the low quantity of these metabolic enzymes and the potential solubility problems of some glycans, one can wonder if synthesis of a specific Golgi-glycan is done in a complex of enzymes, as is the case in the synthesis of cellulose.