ADP-Glc is the major precursor for starch synthesis. Starch is a polymer of a-D-Glc consisting of two types of molecules: amylose (linear a-1,4-linked glucose) and amylopectin in which one Glc in every 20 or so residues on an amylose-like structure is branched by an a(1- 6)-linkage connected to an a-1,4-linked chain. Although starch synthesis is not related to pectin synthesis, the information available about the regulation of the synthesis of the starch precursor, ADP-Glc, can be used as a paradigm when considering the synthesis and regulation of other NDP-sugars.

Starch, the main carbon storage form in plants, is made in plastids of photosyn­thetic and non-photosynthetic tissues. Adenosine 5′-diphosphate:glucose pyrophospho — rylase (ADPGlc:PPase) catalyzes the first and rate limiting step in starch biosynthesis (i. e., the conversion of Glc-1-P and ATP to ADP-Glc and pyrophosphate (436). In cereal en­dosperms, two distinct ADPGlc PPases exist, one is found in the cytosol and the other in plastids (437-439). By contrast, ADPGlc PPase is exclusively located in plastids of leaves of both mono — and dicotyledonous plants, as well as in heterotrophic organs of dicotyledonous plants. Plant ADPGlc PPase is composed of two types of subunits (small and large) and is allosterically regulated by 3-phosphoglycerate and phosphate. The Arabidopsis genome con­sists of six ADPGlc PPase-encoding genes (two small subunits, ApS1 and ApS2; and four large subunits, ApL1-ApL4). Based on recombinant enzyme activities, mRNA expression and the fact that recombinant Aps2 has no ADPGlc PPase activity, it has been proposed (440, 441) that ApS1 is the main catalytic isoform responsible for ADPGlc PPase activity in all tissues of Arabidopsis. The authors suggested that each isoform of the large subunits plays a regulatory role. The large subunit, ApL1 is expressed in source tissues, whereas ApL3 and ApL4 are the main isoforms expressed in sink tissues. Thus, in source tissues, ADPGlc PPase could be regulated by the 3-phosphoglycerate/phosphate ratios, while in sink tissues; the enzyme would be dependent on the availability of substrates for starch synthesis.

In cereal endosperm, on the other hand, a different regulation of starch synthesis may operate. It appears that the transport of ADP-Glc from the cytosol into the plastid is the limiting factor. This became clear during the characterization of a plastidial ADPGlc trans­porter (HvNstl) barley mutant with low-starch content (442). The mutant accumulates high levels of ADP-Glc in the developing endosperm indicating that cytosolic pool of ADP­Glc is not under metabolic control in this tissue. Lastly, leaves overexpressing SuSy showed a large increase in the levels of both ADP-Glc and starch, compared with WT leaves, while leaves overexpressing antisense SuSy accumulated low amounts of both ADP-Glc and starch (438). The above findings, which originated in the Pozueta-Romero’s laboratory, show that in source leaves ADP-Glc produced by SuSy (outside the chloroplast) is directly linked to,

and appears to control starch biosynthesis. This implies that SuSy, but not ADPGlc:PPase, controls the level of ADP-Glc in the cytosol in source leaves (438, 439).

More recently, a new enzyme activitywas identified in Arabidopsis thatmustbe considered to better evaluate the metabolic fate of ADP-Glc in the cytosol. Recombinant Arabidopsis At5g18200 has ADP-Glc phosphorylase activity (please note it is not a PPase). The enzyme is capable of transferring AMP from ADP-Glc onto either Pi or Gal-1-P (443) as shown in the scheme below:

ADP-Glc + Pi ^ Glc-1-P + ADP ADP-Glc + Gal-1-P ^ Glc-1-P + ADP-Gal

Unlike the human and fungal GalT enzyme, which transfers UMP from UDP-Glc onto Gal — 1-P forming Glc-1-P and UDP-Gal, the ADP-Glc phosphorylase cannot utilize UDP-Glc as a donor substrate (443).

We put forward that the above-described distinct regulatory role of ADPGlc PPase and SuSy are examples that highlight the possibility that different isoforms of nucleotide-sugar biosynthetic enzymes may have distinct roles in plants and that different plant species may regulate the same metabolic pathway in different ways.