occur within the photosynthetic cell, probably through the conversion of UDP glucose to uridine diphos-
phogalactose (UDPGal), the galactose donor used by galactinol synthase (GS, reaction 1, Figure 7).
Galactinol and sucrose then cross into the intermediary cell, via the abundant plasmodesmata that inter-
connect these cells with the photosynthetic cells, where raffinose oligosaccharide synthesis takes place
via the operation of raffinose synthase (RS, reaction 2) and stachyose synthase (SS, reaction 3).
In other plant species, there is evidence that galactinol synthesis may also take place within the in-
termediary cell [38]. In this case, as indicated in Figure 7, sucrose alone may leave the photosynthetic cell,
to be used both as the sucrose moiety of the raffinose sugars and for the synthesis of galactinol.
Metabolism of sucrose may take place via sucrose synthase (reaction 4), which yields UDPG, from which
UDPGal could be synthesized. In squash leaves, immunological data indicate the presence in the inter-
mediary cells of both stachyose synthase (SS, reaction 3) and galactinol synthase (GS, reaction 1), but the
complete details concerning the location of the biosynthetic enzymes in this pathway remain to be estab-
lished. In fact, because these oligosaccharides can also serve a storage function in plant tissues, it is likely
to prove that the entire pathway leading to raffinose oligosaccharide synthesis occurs both in the photo-
synthetic cells, where they are used for storage, and in the intermediary cell, where they are used for trans-
port [38,44–47].
Although raffinose and stachyose are synthesized via the preceding reactions, there is evidence in-
dicating that verbascose and higher degree of polymerization (DP) raffinose family oligosaccharides
(RFO) do not use galactinol as a galactosyl donor. Cold-induced RFO accumulation in Ajuga reptansL.
is associated with an increase in the activity of a novel vacuolar enzyme, galactan:galactan galactosyl-
transferase (GGT) [45,46]. This enzyme catalyzes galactosyl transfer from one raffinose family oligosac-
charide to another, resulting in the formation of galactosides one higher and one lower in degree of poly-
merization than the two starting substrates [47], as shown in Figure 8.
The regulation of the raffinose pathway in source leaves is not fully understood. From preliminary
reports, the key regulating enzyme in the pathway would appear to be galactinol synthase (GS, reaction
1, Figure 7). This enzyme catalyzes the first committed step in the biosynthesis of RFOs and is therefore
potentially a good metabolic control point. GS activity has been shown to increase in response to illumi-
nation and to changes in photoassimilate export rate [48], and levels of GS messenger RNA (mRNA) also
increased when plants were exposed to cold and desiccation, a condition that also induces RFO accumu-
lation [49].
Stachyose synthase (SS, reaction 3, Figure 7), which has higher activity in fruiting than in vegetative
plants [50], may also be under some form of metabolic control. During the dark period, export of
stachyose in muskmelon declines and the plant becomes predominantly a sucrose transporter [51], an ob-
servation suggesting that some form of light regulation of these enzymes is a possibility. In this context,
CARBOHYDRATE SYNTHESIS AND CROP METABOLISM 477
Figure 8 Pathway for production of high-molecular-weight (high DP) raffinose family oligosaccharides.