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 excis ion and continuous illumination, and their sugars were analyzed either 24 or 60 h after induction by both gel permeation and high performanc e liquid chromatography. The earliest accumulated fructan consisted ma inly 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 incu bation contained a higher proportion of (2 --> 6)-linkages than those accumulated early after induction. Stems of field-grown wheat containe d 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' pat hway is proposed in which a (2 --> 1)-specific fructan fructosyl trans ferase and O-6 branching activity produce branched oligomers rich in ( 2 --> 1)-linkages, and in which a fructan exo-hydrolase cleaves 1-link ed terminal-fructosyl units selectively to have phlein-like oligomers resistant to further hydrolysis.