The building blocks for polysaccharide synthesis are nucleotide sugars (NDP-sugars). The sugar moieties in NDP-sugars are incorporated into growing polysaccharide polymers by glycosyltransferases (GTs). A major contributor to glycan diversity is the number of NDP — sugars that an organism produces. In the plant kingdom, 30 different NDP-sugars have been identified (403). It is estimated that over 50 enzymes are directly involved in the synthesis of NDP-sugars in plants. To date only 22 NDP-sugar biosynthetic genes have been functionally identified [see (404) and text below] (Table 5.6). While it is widely accepted that NDP-sugars are the precursors for cell wall polysaccharides, glycoproteins, and glycolipids, it should be kept in mind that in addition to NDP-sugars, lipid-bound sugars are also sugar donors for the synthesis of glycans (for example, synthesis of the N-glycan core of glycoproteins in eukaryotes and addition of galacturonic acid to Rhizobium lipopolysaccharides via a prenyl phosphate-galacturonic acid donor substrate) (405, 406). Whether the initiation of plant cell wall polysaccharide synthesis is mediated by a core glycan that requires lipid-bound sugars at the ER, remains to be established.

Relatively few NDP-sugars are made inside the ER and Golgi apparatus where glycans are made. Most NDP-sugars are produced in the cytosol. Thus, specific NDP-sugar trans­porters exist to facilitate the import of NDP-sugars from the cytosol into the correct lumen of the endomembrane where GTs reside. It is predicted that ~20 NDP-sugar transporters exist in plants of which functionally only six have been characterized (A. Orellana, personal communication). These transporters are localized to the ER and Golgi apparatus (93, 407). While some of the NDP-sugar transporters are specific, the relatively low number of trans­porters would suggest that some NDP-sugar transporters may accept several NDP-sugars. In addition to the need for GTs, NDP-sugars, and their transporters for wall polysaccharide synthesis, some wall polysaccharides (i. e. pectins and hemicelluloses) are also modified by acetyl and methyl groups. Thus, diverse acetyltransferases and methyltransferases are also required. Little is known about their substrate specificity, as none has been biochemically purified. Basic questions such as what controls the degree and number of methyl modifi­cations on a specific glycan remain elusive. Hence, no wall-related acetyl — or methyltrans — ferases have been functionally cloned. The methyltransferases (MetTs) and acetyltransferases (AceTs) generally utilize S-adenosyl-L-methionine (SAM) and acetyl-CoA as methyl and acetyl donors, respectively (229, 285, 335, 337). A recent study in A. Orellana’s laboratory led to the biochemical identification of a SAM transporter activity in the Golgi apparatus of pea (A. Orellana, personal communication). Beyond synthesis, plant glycans undergo further modification, including degradation and remodeling by specific glycosidases and esterases. Due to space limitations, transporters and glycan modifying enzymes will not be summarized in this review, rather, the reader is referred to a recent review (246).

Wall biogenesis is a complex cellular event similar to an assembly line. It requires the supply of a wide range of precursors targeted to different subcellular locations for a process that begins in one subcompartment and continues in other subcompartments as the glycans are synthesized and modified. During this process the concerted action of a range of cytosolic,

Table 5.6 NDP-sugar biosynthetic genes
Mutant, isoforms (putative?), locus (aa) Cell location Sloppy, At5g52560 614 UGIcPP#1 470 At5g17310 469 UGIcPP#2 At3g03250 SuSyl, At5g20830 808 Mito SuSy2, At3g43190 807 Cyto SuSy3, At5g49190 808 Golgi At4g02280 809 ADG1, Aps1, 520 Chlo At5g48300 476 Chlo Aps2, At1 g05610? 521 Chlo Арі 1, ADG2 523 At5g19220 521 Apl2, At1g27680 520 Apl3, At4g39210 Apl4, At2g21 590 GalK, At3g06580 496 UGE1, At1g12780 351 Cyto UGE2,At4g23920 350 Cyto UGE3, At1 g63180 351 Cyto Rhd1,UGE4, At1g64440 348 Cyto-Golgi UGE5, At1g10960 351 associated Urs1, At1 g78570 669? Cyto At3g14790 667 Rhm2, mum4, At1g53500 NRS/er, At1 g63000 301 Ugd1, At5g39320 480 Ugd2, At3g29360? 480 Ugd3, At5g1 5490 480 Ugd4, At1g26570 481
Enzyme U DP-sugar PPase UDP-GIc PPase Sucrose synthase ADP-GIc PPase	Activity Sugar-1 — P + UTP U DP-sugar + РРІ Glc-1 — P + UTP+* UDP-GIc+ РРІ	Syn Sloppy UGlc PPase
Sucrose + UDP UDP-GIc + Frc
SuSy
Glc-1 — P + ATP ADP-GIc + РРІ	AGPase small sub AGPase large sub LS
Gal + ATP Gal-1 — P + ADP UDP-GIc UDP-Gal	Gall, GalK UGE
Gal К UDP-GIc 4-epimerase UDP-Rha synthase UDP-Rha epimerase/reductase UDP-GIc dehydrogenase
UDP-GIc + NAD(P)H ^UDP-Rha
URS Rhm NRSer UGlcDH UGD
UDP-4keto 6deoxyGlc + NAD(P)H -^UDP-Rha UDP-GIc + 2NAD —)>UDP-GlcA + 2NADH

UGlcAEI, At2g45310 UGIcAE2, Gae6, At3g23820 UGIcAE3, Gael, At4g30440 UGIcAE4, At4g00110 UGIcAE5, Gae2 At1g02000? UGIcAE6, Gae5, At4g12250?

Uxs1, At3g53520 Uxs2, At3g62830 Uxs4, At2g47650 Uxs3, At5g59290 Uxs5, At3g46440 Uxs6, At2g28760

AXS1, At1 g08200 At2g27860

Aral, At4g16130

Mur4, UXE1, At1g30620 UXE2, At2g34850?

Uxe3, At3g34850 Uxe4, At5g44480?

Cyt1, At2g39770 At4g30570?

GMD1, At5g66280?

GMD2, mur1, At3g51160

Ger1, At1g73250 Ger2, At1 g1 7890

Gme1, At5g28840

kdsA1, At1 g79500 kdsA2, At5g09730

kdsB, At1 g53000

ER, and Golgi enzymes, as well as ER, Golgi, vesicular and plasma membrane proteins is required to facilitate the production of one type of glycan. Therefore, knowledge regarding the catalytic topology of each membrane enzyme and the cellular machinery that partitions, regulates, and traffics each protein and the corresponding glycan-intermediates, to their correct subcompartments must be understood to truly comprehend wall assembly and synthesis.