Species differences in SPS regulation are now being reported. Interestingly, the type of SPS regula-
tion seen in a given plant appears to be correlated with the type of carbohydrate stored diurnally in its
leaves. For example, in maize and spinach, which accumulate both sucrose and starch as temporary stor-
age reserves, SPS is subject to allosteric control via Glu-1-P and Piand also appears to be under phos-
phorylation control. In contrast, in soybean, in which starch alone is accumulated as a storage carbohy-
drate, there appears to be no such regulation of SPS [12,13].
C. Fructans
In some plant species, water-soluble polymers known as fructans accumulate as carbohydrate storage
products. Fructans, as the name implies, are linear and branched polymers of fructose; in leaves, they are
derived from photosynthetically produced sucrose. Fructans have as their core starting component a sin-
gle molecule of sucrose, to which chains of fructose residues are attached. The type of linkage between
adjacent fructose residues, as well as the point of attachment of the fructose chains to the sucrose
molecule, determines the type of fructan accumulated in a given plant [15–17].
Fructans have some physiological significance and a variety of applications. Small fructans have a
sweet taste, whereas longer fructan chains form emulsions with a fat-like texture and a neutral taste. The
human digestive system does not contain enzymes capable of hydrolyzing fructans; therefore, there is
strong interest in the food industry in developing fructans for use as low-calorie food ingredients [18]. In
plants, other than being a major reserve carbohydrate, fructans have been implicated as protective agents
imparting tolerance to water deficit and low temperatures [19].
- Fructan Structure
Three major classes of fructans are found in agronomically important crop plants—the isokestose or in-
ulin series, the kestose or phlein series, and the neokestose series, each of which is named for its charac-
teristic trisaccharide sucrosyl-fructose [15–17]. In the isokestose series, which is synthesized in members
of the Asteraceae such as Jerusalem artichoke (Helianthus tuberosusL.), fructose residues are attached to
the fructosyl residue of sucrose in nonreducing -2,1-linkages. Fructans of the isokestose series, there-
fore, have the general form
Glu-1,2-Fru-1, (2-Fru-1)n, 2-Fru
↑
Sucrosewherenmaxis approximately 35.
Fructans of the kestose series, which are common in many temperate grass species including wheat
and barley, consist of fructose residues joined by -2,6 linkages and have the general form
Glu-1,2-Fru-6, (2-Fru-6)n, 2-Fru
↑
Sucrosewherenmaxis approximately 250.
Fructans of the neokestose series, which have been isolated from asparagus (Asparagus officinalis
L.), have fructose residues joined to both the glucose and the fructose residues of sucrose and have the
general form
Fru-2, (1-Fru-2)m-1-Fru-2, 6-Glu-1,2-Fru-1, (2-Fru-1)n, 2-Fru
↑
Sucrosewheremmaxandnmaxare each approximately 10 residues.
Branched fructans also occur in nature, and frequently there is more than one fructan series in the
same plant.
- Fructan Synthesis in Leaves
The biosynthetic pathway leading to fructan synthesis differs substantially from that leading to sucrose or
starch synthesis in that the fructosyl donor is not a sugar nucleotide. Instead, sucrose itself acts as the fruc-
472 PATTANAGUL ET AL.