Subcellular Location

It has been accepted since the early work of Edelman and Jefford (13) that the vacuole is the probable site of fructan storage in plants. In 1983, this location was verified to be the site of fructan storage in leaves of Hordeum vulgare (37). Soon thereafter, fructan metabolizing enzymes were also established to be compartmentalized in vacuoles of Hordeum leaves (38) and of Helianthus tubers (39). However, there are reports of significant amounts of fructan hydrolases being associated with cell walls. Roots of Cichorium have been documented to have as much as 70-85% of the total inulinase activity tightly bound to cell walls (40). FEH association with cell walls has also been documented in corms of Phleum (41). Until recently, it was thought that fructan degrading enzymes associated with cell walls would be devoid of fructan substrates. However, Livingston and Henson (42) reported the presence of fructans and FEH activity in the apoplastic fluid from oat crowns. They observed that the apoplastic fructan concentration increased during the second phase of cold-hardening treatment and hypothesized that the increase in apoplastic fructan was part of an adaptive response possibly related to freezing tolerance. Fructans have also been documented in the phloem sap and vascular tissues of Agave leaves (43). Fructosyl transferase activity was also present in the vascular tissues. With the understanding that fructans and fructan metabolising enzymes are not located exclusively in vacuoles, new roles for this important carbohydrate undoubtedly will be discovered.

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