ing source leaves in which they would not normally be synthesized. The metabolic function and cellular
location of these induced oligosaccharides are not known. Clearly, more research is also needed in the
area of raffinose family oligosaccharide synthesis in sink tissues.
F. Cyclitols and Galactosylcyclitols
Like the RFOs [64], these metabolites, and especially the galactosylcyclitols, often occur in high amounts
in dry seeds, with certain legume seeds being particularly good sources. Seeds of Vigna angularisL. con-
tain galactosylononitol [65] and seeds of Lens culinarisL. contain a range of these compounds including
galactopinitol A, galactopinitol B, digalactopinitiol, ciceritol, and fagopyritol [66]. In addition to these
cyclitols, seeds of both species contain high levels of RFOs, particularly stachyose. There is now strong
evidence that stachyose synthase, which catalyzes the synthesis of stachyose in the RFO pathway, is also
responsible for the galactosyltransferase reactions producing the galactosylcyclitols [67].
G. Structural Carbohydrates
A key structural component of plant cells that differentiates them from animal cells is the presence of a
rigid, highly structured cell wall. The primary plant cell wall consists largely of cellulose and related het-
eroglycan polymers [68] and can therefore be regarded as a very specialized carbohydrate. The formation
of cell wall material requires a tremendous input of carbon and is therefore one of the most important uses
of photosynthetically fixed carbon in a developing plant tissue. From an agronomic standpoint, the cell
wall gives rise to many important fibers (e.g., cotton) with many industrial applications and also is an im-
portant fiber component of animal feeds.
The principal cell wall component is the carbohydrate cellulose, which is a linear glucose polymer
containing-1,4 linkages. Synthesis of cellulose occurs at the plasma membrane by terminal complexes
consisting of cellulose synthase associated with sucrose synthase subunits. Sucrose delivered to the
plasma membrane is degraded by sucrose synthase to liberate UDPG and fructose. UDPG is then used for
cellulose microfibril chain elongation catalyzed by the enzyme cellulose synthase (UDPG: glucan syn-
thase) [69,70]. The pectins and hemicelluloses of the wall matrix are heterogeneous polysaccharides and,
unlike cellulose, are synthesized mainly in the Golgi apparatus by a series of glycosyltransferase reac-
tions. Because of the complexity and heterogeneity of these carbohydrates, understanding of their path-
ways of biosynthesis is still fragmented.
IV. FUTURE PERSPECTIVES
The advent of molecular biology techniques now allows us to study carbohydrate biochemistry in ways
that before were impossible. With these techniques, we are now able to create transgenic plants with en-
hanced or novel carbohydrate production or to knock out completely genes responsible for carbohydrate
pathway enzymes. With these types of experiments has come the realization that there is a bigger picture
to view in terms of carbohydrates in plants, for it is now becoming quite evident that carbohydrates play
many surprising roles in plant metabolism. We are only just beginning to discover and appreciate the new
and exciting roles that carbohydrates play as sensors of environmental cues, as signal transduction inter-
mediates, and as regulators of gene expression.
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