Patterns of fructan synthesized by onion fructan:fructan 6G-fructosyltransferase expressed in tobacco BY2 cells - is fructan:fructan 1-fructosyltransferase needed in onion?

Abstract

• Fructan:fructan 6G-fructosyltransferase (6G-FFT) has been proposed to be the enzyme essential for the production of neo-series inulin. Transfer of a fructose unit from short chain inulins to the C6 of the glucose residue of sucrose or inulin was proposed to be its most important characteristic. Here, we investigate the activity of 6G-FFT from onion (Allium cepa) more thoroughly. • Tobacco BY2 suspension cultures were employed as an expression system for the fructosyltransferase 6G-FFT. Activity was measured using 1-kestose as a substrate and products were detected using high-performance anion exchange chromatography (HPAEC). • 6G-FFT showed multiple activities. An array of fructans of the inulin series and inulin neo-series were produced. First 1,1-kestotetraose and 1 and 6G-kestotetraose were synthesized, as well as 6G-kestotriose. Prolonged incubation produces a complex fructan series with a higher degree of polymerization. • The fructan pattern observed after incubation of onion 6G-FFT with 1-kestose closely resembles the complex fructan pattern found in onion. These results questions the need for a separate fructan:fructan 1-fructosyltransferase (1-FFT) activity in onion.