It is now believed that synthesis of starch in the amyloplasts of sink tissues takes place from hexose-
P imported from the cytoplasm [60]. The subsequent enzymatic steps appear to be similar to the series
found in source leaves. Somewhat surprisingly, the regulation of ADPGPPase by 3-PGA and Piappears
to be a constitutive property of this enzyme from all sources studied so far [4,5,7], even though for most
sinks 3-PGA is not a major amyloplast metabolite.
B. Sucrose
Synthesis of sucrose is an important physiological function in some sink tissues—for example, in ripen-
ing fruits, where stored starch is metabolized to sugar during the ripening process. Similar metabolism of
starch or fructans also occurs upon sprouting of perennating organs such as tubers and bulbs. During ger-
mination of seeds, conversion to sucrose of stored carbohydrates, and also other reserves such as wall ma-
terials and oils, occurs in endosperm tissues and cotyledons and is an essential process for growth and
emergence of the embryo. The regulation of sucrose synthesis in sink tissues is under active investigation
in many research laboratories, but comprehensive details of control of sucrose synthesis in sinks are still
lacking.
C. Fructans
There are many reports of fructan accumulation in vegetative sinks of fructan-accumulating plants. In-
deed, fructan accumulation is far more common in vegetative storage sinks than it is in source leaves,
where experimental manipulation must often be used to induce fructan accumulation. Fructans accumu-
late naturally to particularly high levels in overwintering organs such as bulbs and tubers and also in stems
of grasses, possibly in response to environmental cues. In wheat plants, stem fructans form a pool of car-
bon reserves that can be drawn on during grain filling [22]. The exact regulatory mechanisms controlling
fructan accumulation from imported solutes in sinks are not yet known.
D. Polyols
Although sink tissues such as seeds may contain polyols, these are usually only trace components, not ma-
jor carbohydrates. In polyol-translocating species, vegetative tissues such as petioles, stems, and roots
may accumulate polyols, but there is no evidence that this occurs by directly synthesis rather than by sim-
ple import from the phloem. Polyol metabolism in sink tissues is, therefore, poorly understood and needs
to be addressed in further research [61].
E. Raffinose Family Oligosaccharides
Raffinose oligosaccharides are prevalent in seeds of numerous plant species, even those that do not use
these sugars as phloem-mobile compounds. Thus, although the biosynthetic pathway of raffinose
oligosaccharide synthesis is not operative in the source leaves of many plants, it is encoded in the genome
and is expressed in the developing seeds. It is thought that these oligosaccharides, which accumulate dur-
ing seed drying, may allow maintenance of seed cellular membranes during desiccation. Despite their
prevalence and the possibility that they perform an essential metabolic role in the dry seed, the regulatory
mechanisms controlling synthesis of these sugars from imported assimilates in seeds have received rela-
tively little attention. Indeed, current research appears to be focusing on genetics and molecular protocols
for removal of this biochemical pathway from seeds because the raffinose oligosaccharides are major
antinutritional factors in many seed crops used for human consumption. It will be interesting to see what
success these approaches achieve if indeed the raffinose pathway turns out to be necessary for seed
viability.
Raffinose oligosaccharides also accumulate in some vegetative storage tissues, where they may sim-
ply reflect accumulation of imported stachyose [62]. However, there are numerous cases in which these
sugars appear to be synthesized de novo in vegetative tissues including, interestingly, the sugar beet tap-
root. Although the first isolation of galactinol was from this tissue [63] and although raffinose produced
in sugar beet is a major “contaminant,” which interferes with sucrose crystallization, there has been little
study of this de novo biochemical pathway in vegetative tissues. Certain environmental stresses, such as
low temperature, can also result in accumulation of these oligosaccharides in vegetative tissues, includ-
480 PATTANAGUL ET AL.