Evidence for the de novo synthesis of Fructan by enzymes from higher plants: a Reappraisal of the SST/FFT model

SUMMARY For the following reasons, it is argued that the enzymological evidnce is insufficient to sustain the SST/FFT model for fructan systhesis. (i) The original model was speculative. It was formulated without in vitro evidence for the synthesis of inulin of degree of polymerization greater than 3 (DP >3), and without published evidence for the existence of a key enzyme (SST). The model is shown to be untestable in vitro. Subsequently, studies have not domonstrated the synthesis of inulin in vitro , except for trisaccharide. (ii) Most data cited in support of the model are concerned with the synthesis of the smallest fructans (trisaccharides) by crude enzyme extracts. Much of these data can be explained in terms of artifactual fructosyl trasfer by invertase, a common contaminant of plant enzyme preparations. (iii) There are few reports of the de novo enzymatic synthesis of fructan DP >3 in plants. Where it has been reported, it was eatalyzed by crude or partially purified enzymes. Hence the number and properties of the enzymes involved is not known, providing no evidence for distinct SST and FFT enzymes. (iv) Assays for FFT require preformed fructan substates. FFT transfers fructose between preformed fructan molecules without net synthesis of fructan. Hence measurement of FFT is not evidence for net fructan synthesis. (v) Invertase can mimic the FFT reaction. (vi) The enzymes of fructan biosynthesis have not been purified and characterized in any plant species. Where functional separation of SST and FFT activities has been achieved. Reconstitution experiments have not been performed demostrating de novo fructan synthesis by the recombined activities. (vi) The SST/FFT model explains the synthesis of the linear β‐2,1‐linked homologous series of fructans. The model is inadequate to explain the synthesis of other linkage types and branched fructans. (vii) There is no demonstration of the enzymatic de novo synthesis of the full complement of tissue fructan by SST/FFT or otherwise, for any plant species. Although were is a substantial body of evidence which is consistent with the SST/FFT model, it is concluded that the hypothesis is flawed and that the enzymological data from all species do not validate the model. Enzyme systems from grasses are described which overcome some of these difficulties.