Sévenier et al (116) described the conversion of sugar beet into fructan beet by introduction of single gene encoding 1-sst isolated from jerusalem artichoke. In tap root of sugar beet transformed with 1-sst gene, the stored sucrose is almost totally converted into low molecular weight fructans. In contrast, 1-sst expression in the leaves resulted in only low levels of fructans. Despite the storage carbohydrate having been altered, the expression of the 1-sst gene did not have any visible effect on phenotype and did not affect the growth rate of taproot as observed tinder green house conditions. These researches appear promising in replacing sucrose with fructo oligosaccharides produced in fructan synthesizing beets.
Recent researches now indicate the establishment of two enzyme system SST and FFT in the synthesis of fructans. FFT is a dual-purpose flexible enzyme and can be used both in the polymerization and depolymerization of fructans. SST appears to be different from yeast invertase as the former enzyme could synthesize GF2 at low sucrose concentrations. However, recent reports on the presence of cell wall fructan exohydrolase and fructan in the apoplast of oat may lead to new researches in the fructan accumulating systems. The comparative status of fructans in apoplast of chicory and jerusalem artichoke is still to be investigated. The role of fructan exohydrolase as a survival enzyme, which becomes active during restricted supply of photosynthate, depending upon the physiological state of the plant or due to adverse environmental conditions, has been established. However, the mode of synthesis of only fructose containing oligosaccharides (inulo-n-ose series) during storage of chicory and jerusalem artichoke is still not clear. The work of Sevenier et al (116) has shown the potential of converting sugar beet in to a fructo oligosaccharides accumulating crop by incorporating SST gene. Future work could relate to improving the fructo oligosaccharide content of sucrose storing crops by lowering the sucrose content in the mature tissue. It will be no wonder if in few years time parts of crops raised under sugarcane and sugar beet may give oligosaccharides instead of sucrose. Potential of creating new crops by genetic engineering by introducing fructan synthesizing genes (inulin and levan types) into starch storing crops of variable dependence on abiotic factors like water and temperature could be pursued vigorously in the coming years. These researches in addition will help in decoding the precise physiological role of inulin and levan type fructans. In crops like chicory and jerusalem artichoke, modifying or mutating the FFT gene so as to make it unfunctional could lead to some more valuable crop products. Another important area in the coming years will be on the involvement of signal transduction pathways in the regulation of fructan metabolism. Both from industrial and agricultural point of view attempts to enhance the fructan accumulating phase in tubers/roots with an emphasis to delay the fructan degradation which results in deterioration of inulin quality could be another interesting area of research in subsequent years. Attempts to increase the polymerization of fructose in fructan storing sinks by importing certain fructan synthesising genes could be made.
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