tosyl donor to create the fructosylsucrose isokestose by the reaction catalyzed by the enzyme sucrose:su-
crosylfructose transferase (SST, reaction 1, Figure 4). The glucose released by the SST reaction is thought
to reenter the general cytoplasmic hexose phosphate pool following phosphorylation (Figure 4). Chain
elongation then proceeds by the reaction catalyzed by another enzyme, fructan:fructosyltransferase (FFT,
reaction 2, Figure 4), which utilizes the fructosylsucrose as a fructose donor to another fructosylsucrose.
Distinct FFT enzymes can be isolated from plant tissues, which can form -1,2 or -2,6 linkages to fruc-
tose or glucose residues of fructosylsucrose formed by the SST reactions [13,14]. Both SST and FFT were
purified to homogeneity and incubation of these enzymes with sucrose resulted in the formation of a fruc-
tan polymer length of up to 20 fructosyl residues.
Both SST and FFT enzymes appear to be localized exclusively in plant vacuoles, where fructan ac-
cumulation occurs [15,16]. Vacuolar fructan synthesis lowers the sucrose concentration in the cell and
prevents sugar-induced feedback inhibition of photosynthesis. Continuous illumination or feeding su-
crose to excised leaves of fructan-accumulating species induces fructan synthesis, suggesting a correla-
tion between high sucrose levels and the induction of fructan synthesis. In leaves, fructan levels are usu-
ally low, but fructans can accumulate in response to environmental conditions that serve to elevate
carbohydrate levels—for example, in response to low temperatures [20]. Experimentally, cereal and grass
leaves can be induced to form large quantities of fructan following excision, which eliminates phloem
transport of sucrose, and continuous illumination, which promotes sucrose synthesis [15,21]. Fructan
pools can also form important reserve sources for use during grain filling or other periods of high sink de-
mand [22]. The genetic machinery for fructan synthesis, therefore, is present in leaves, although the
growth conditions or developmental cues a plant experiences may not always result in activation of this
machinery.
Fructans appear to be a form of readily accessible carbon and are degraded by the action of a fructan
hydrolase,-fructofuranosidase. This enzyme degrades the fructan polymers by removing the terminal
fructose residue, resulting in the release of free fructose. It is thought that fructan metabolism in the vac-
CARBOHYDRATE SYNTHESIS AND CROP METABOLISM 473
Figure 4 Pathway of fructan synthesis in the vacuole of photosynthetic leaf cells. Reaction 1, sucrose su-
crosyl transferase (SST); reaction 2, fructan:fructan fructosyltransferase (FFT).