About 15% of flowering plant species store fructans as a
reserve in at least one of their organs during their life
cycle (Hendry 1993). Fructan, in general is a term used
for any carbohydrate in which fructosyl-fructose links
constitute the majority of the glycosidic bonds. Fructans
are linear or branched fructose polymers, which are either
b 2 ® 1 linked inulins or b 2 ® 6 linked levans. Dicotyledonous
species store inulin type fructans consisting of
linear b (2 ® 1) fructofuranosyl units. More complex and
branched type fructans are common in monocots (Vijin
and Smeekens 1999).
2. Synthesis of fructans
Because fructans are synthesized from sucrose by
repeated fructosyl transfer from a fructosyl donor, both
inulins and levans usually, but not always, have a terminal
glucose unit. The enzyme generally considered to be
involved in plant fructan synthesis is sucrose-sucrose
fructosyl transferase (EC 2×4×1×99) which catalyses the
transfer of a fructose molecule from one sucrose molecule
to another, leading to kestose formation (glucosyl-1, 2
fructosyl-1, 2 fructose). Chain elongation is mediated by
either 1F
– or 6F
fructan–fructan–fructosyl (EC 2×4×1×100)
transferase leading to inulin and levans respectively.