Differences in fructan accumulated in induced and field-grown wheat plants: an elongation-trimming pathway for their synthesis



Mature blades of wheat seedlings were induced to form fructan by excision and continuous illumination, and their sugars were analyzed either 24 or 60 h after induction by both gel permeation and high performance liquid chromatography. The earliest accumulated fructan consisted mainly of 1-kestose, nystose, bifurcose, and short oligomers rich in (2→1)-linkages and branch point residues. After several days, fructan included more than fifty compounds, among which one-half of the total were heptamers or larger. Oligomers accumulated during long-term incubation contained a higher proportion of (2→6)-linkages than those accumulated early after induction. Stems of field-grown wheat contained about the same amount of fructan as blades of induced seedlings, but with larger proportions of phlein oligomers initiating with 6-kestose, phlein-like oligomers initiating with bifurcose, and other branched oligomers enriched in (2→6)-linkages. An ‘elongation-trimming’ pathway is proposed in which a (2→1)-specific fructan fructosyl transferase and O-6 branching activity produce branched oligomers rich in (2→1)-linkages, and in which a fructan exo-hydrolase cleaves 1-linked terminal-fructosyl units selectively to have phlein-like oligomers resistant to further hydrolysis.