Fructo-oligosaccharide content and fructosyltransferase activity during growth of onion bulbs

Abstract

The accumulation of fructo-oligosaccharides and the activities of fructosyltransferase (sucrose:sucrose 1(F)-fructosyltransferase (SST), 1(F)-fructosyltransferase (1(F)-FT) and 6(G)-fructosyltransferase (6(G)-FT)) in the bulbs of three onion cultivars were investigated from June to September 1993. The total fructo-oligosaccharide content increased from June until August, then decreased in September, except in one cultivar. The levels of neokestose and its related tetrasaccharides (1(F),6(G)-di-β-D-fructofuranosyl sucrose and 6(G)(1-β-D-fructofuranosyl)2sucrose) were higher than those of 1-kestose and nystose throughout growth. The activities of 6(G)-FT, 1(F)-FT and SST were high in June and July, then decreased; SST activity was very low in September. The activity ratios of 6(G)-FT to 1(F)-FT varied between 1.86 and 2.65 over the growth period. Two trisaccharides, three tetrasaccharides and four pentasaccharides were identified, together with a mixture of hexa- and heptasaccharides, all of which were synthesized in vitro from 0.1 M sucrose by an enzyme preparation of onion bulbs harvested in August. Octa- and nonasaccharides other than the saccharides formed from sucrose were also synthesized from 0.1 M 1-kestose or 0.1 M neokestose. All the saccharides produced from sucrose, 1-kestose or neokestose by the crude enzyme prepared from onion bulbs were identical to the saccharides occurring naturally in onion bulbs.