Currently, little is known about how the synthesis of nucleotide-sugars is controlled in time or space, and how it relates to the glycosyltransferases that actually make the diverse glycan polymers. What is the limiting factor in wall synthesis? Is it supply of NDP-sugars (as is the case for starch) or glycosyltransferases?

We will divide this section into three topics: sugar flux, role of isoforms, topology and protein complexes.

2.5.2.1 Sugar flux

Although a considerable proportion of cellular sugar ends up in wall polysaccharides, some sugar-derivatives are required for glycoprotein, glycolipid, and glycoside synthesis. In addi­tion, significant amounts of sugars are stored either as large glycans such as starch, small-sized glycans (e. g., raffinose, fructan), or as the disaccharide sucrose. We would like to point out two issues related to flux: 1) growth potential of a cell; 2) whether some wall components compensate for the lack, or reduced amount, of other glycans.

1 New meristematic cells need to expand and grow to their prospective developmental tissue (e. g., leaf cells). What determines the growth potential and the cell’s final size is unclear. Logically, with limited wall precursors the potential for growth is restricted since wall polymers are not made. The underlying mechanism that controls this complex develop­mental process is still unknown and poses a fascinating scientific quest. For example, do transcription factors regulate coordinately the expression of “tissue-fate genes” as well as NDP-sugar biosynthetic genes and genes involved in the supply of carbon? If carbon flux is not limited and all NDP-sugar biosynthetic genes are highly expressed — would the cell be larger? Do young, old, or stressed cells sense sugar availability or sugar status for growth and/or for storage in different ways? What are the ultimate determinates for growth; sugars or sugar-phosphates? Several sugar-sensing (signaling) proteins (and cor­responding genes) have been isolated. It is assumed that sugar sensing (i. e., the interaction between a sugar molecule and a sensor protein) mediates a signal which initiates signal transduction cascades that result in cellular responses such as altered gene expression and enzymatic activities. Sugars as signaling molecules affect the plants at all stages of growth starting from seed germination to seed development. Sugars, like hormones, can act as primary messengers and regulate signals that control the expression of various genes in­volved in sugar-phosphates and wall metabolism. But do NDP-sugars function, in part, as signal molecules? In human cells, UDP-GlcNAc serves as a glucose sensor and moves between the cytosol and the nucleus. A cytosolic and nuclear-localized soluble enzyme, known as OGT, catalyzes the O-linked transfer of GlcNAc from UDP-GlcNAc directly to Ser/Thr of target proteins (503). For example, the O-GlcNAcylation, of the transcription factor Sp1 promotes nuclear localization of Sp1 and its ability to transactivate calmod­ulin (CaM) gene transcription (504). Whether plant cells consist of analogous signals to suppress or activate wall-biosynthetic genes by monitoring levels of NDP-sugars is unknown.

A major task for future research will be to investigate the relationships between isoforms that produce the same nucleotide-sugar, GTs, and sugar-sensing genes. Once the function of wall-related genes becomes known, bioinformatics will be useful in identifying a common set of genes that are coordinately expressed or suppressed to form a specific glycan.