oligosaccharides is that the transfer of galactose residues to sucrose probably does not occur in the pho-
tosynthetic cell where sucrose is synthesized [37–39].
As indicated in Figure 7, the synthesis of raffinose family oligosaccharides is now believed to take
place in two separate leaf cell types: the photosynthetic mesophyll cell and the modified phloem com-
panion cell, or intermediary cell, which is characteristically found in leaves in which these oligosaccha-
rides are synthesized [40,41]. As far as is known, production of sucrose in leaves in these plants occurs
in much the same fashion as in other plant species. In the cucurbit vine crops, sucrose synthesis does not
appear to be light regulated [42], which suggests that SPS is not controlled by protein phosphorylation.
The actual mechanisms controlling sucrose production in raffinose oligosaccharide–synthesizing plants
have not been elucidated. There is evidence, however, that sucrose synthesis occurs within the cytoplasm
of the photosynthetic cells [37,43].
In plants that synthesize the raffinose family oligosaccharides, sucrose is used as a phloem-mobile
and a storage carbohydrate. However, it must also be used as the sucrose backbone for the synthesis of
the raffinose oligosaccharides. The way in which mobile or storage sucrose pools are kept separated from
metabolizable sucrose pools is not clear, but compartmentation within the two different cell types in-
volved in raffinose oligosaccharide biosynthesis may be occurring.
To further complicate partitioning in these leaves, carbon must also be diverted awayfrom the su-
crose biosynthetic pathway to allow formation of the galactose donor, galactinol. In some plants, this may
476 PATTANAGUL ET AL.Figure 7 Pathway of raffinose family oligosaccharide biosynthesis in leaf tissues. Reaction 1, galactinol syn-
thase (GS); reaction 2, raffinose synthase (RS); reaction 3, stachyose synthase (SS).