Food Biochemistry and Food Processing (2 edition)

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32 Starch Synthesis in the Potato Tuber 615

ROUTES OF STARCH SYNTHESIS AND

DEGRADATION AND THEIR

REGULATION

With the possible exception of sucrose, starch is the most impor-

tant metabolite of plant carbohydrate metabolism. It is by far the

most dominant storage polysaccharide and is present in all major

organs of most plants, in some instances at very high levels. Due

to the high likelihood of starch turnover, its metabolism is best

considered as the balance between the antagonistic operation of

pathways of synthesis and degradation.

To investigate the regulation of starch synthesis in more detail,

growing potato tubers have been used as a model system. Un-

like many other tissues, the entry of sucrose into metabolism is

relatively simple, in that it is unloaded symplasmically from

the phloem, degraded via sucrose synthase to fructose and

UDPglucose, which are converted to hexose monophosphates by

fructokinase and UDPglucose pyrophosphorylase, respectively

(Geigenberger 2003a). In contrast to sucrose degradation, which

is localized in the cytosol, starch is synthesized predominantly,

if not exclusively, in the plastid. The precise pathway of starch

synthesis depends on the form in which carbon crosses the amy-

loplast membrane (Fig. 32.2). This varies between species and

has been the subject of considerable debate (Keeling et al. 1988,

Hatzfeld and Stitt 1990, Tauberger et al. 2000). Categorical ev-

idence that carbon enters potato tuber,Chenopodium rubrum

suspension cell, maize endosperm, and wheat endosperm amy-

loplasts in the form of hexose monophosphates rather than triose

phosphates was provided by determination of the degree of ran-

domization of radiolabel in glucose units isolated from starch

following incubation of the various tissues with glucose labeled

at the C1 or C6 positions (Keeling et al. 1988, Hatzfeld and

Stitt 1990). The cloning of a hexose monophosphate transporter

from potato and the finding that the cauliflower homolog is

highly specific for glucose-6-phosphate provides strong support

for this theory (Kammerer et al. 1998). Further evidence in sup-

port of glucose-6-phosphate import was provided by studies of

transgenic potato lines in which the activity of the plastidial

isoform of phosphoglucomutase was reduced by antisense inhi-

bition, leading to a large reduction in starch content of the tubers

(Tauberger et al. 2000). These data are in agreement with the

observations that heterotrophic tissues lack plastidial fructose-

1,6-bisphosphatase expression and activity (Entwistle and Rees

1990, Kossmann et al. 1992). The results of recent transgenic and

immunolocalization experiments have indicated that the sub-

strate for uptake is most probably species specific, with clear

evidence for the predominant route of uptake in the developing

potato tuber being in the form of glucose-6-phosphate. By con-

trast, in barley, wheat, oat, and possibly maize, the predominant

form of uptake, at least during early stages of seed endosperm

development, is as ADP-glucose (Neuhaus and Emes 2000).

Irrespective of the route of carbon import, ADP-glucose

pyrophosphorylase (AGPase, EC 2.7.7.27) plays an impor-

tant role in starch synthesis, catalyzing the conversion of

Figure 32.2.Pathways of sucrose to starch conversion in plants: 1 , ADP-glucose transporter; 2 , ATP/ADP translocator; 3 ,

glucose-1-phoshate (Glc1P) translocator; 4 , glucose-6-phosphate (Glc6P) translocator; 5 , cytosolic ADP-glucose pyrophosphorylase

(AGPase); 6 , cytosolic phosphoglucomutase; 7 , plastidial AGPase; 8 , plastidial phosphoglucomutase. In growing potato tubers, incoming

sucrose is degraded by sucrose synthase to fructose and UDPglucose and subsequently converted to fructose-6-phosphate (Fru6P) and

Glc1P by fructokinase and UDPglucose pyrophosphorylase, respectively (not shown in detail). The conversion of Fru6P to Glc6P in the

cytosol is catalyzed by phosphoglucoisomerase, and the cleavage of PPito 2 Piin the plastid is catalyzed by inorganic pyrophosphatase. In

potato tubers, there is now convincing evidence that carbon enters the plastid almost exclusively via the Glc6P translocator, whereas in cereal

endosperm, the predominant form of uptake is as ADPglucose.