Optimum production of fructan will be specific for each transgenic plant species, but the cytosol should not necessarily be ruled out as a compartment for accumulating high levels of polymer. A major obstacle must be overcome before this prospect becomes reality. That obstacle is clearly demonstrated in the altered development of maize endosperm expressing a cytosol-targeted SacB gene. Continued work is necessary to establish the mechanism of altered development. Through the use of EST library sequencing coupled to DNA array analysis, differences in gene expression or repression may provide clues to the regulation of endosperm development. Identifying subsets of altered gene expression in the transgenic line, compared to wild-type controls, will perhaps lead back to the genes involved in regulating the signaling pathways. Bypassing the signaling pathways may eventually be used to the delay onset of PCD nearer to its timing in wild-type seeds. Lengthening the time of cell viability may then lead to increased levels of novel polymer synthesis in transgenic seeds. Understanding this pathway will also help in characterizing many of the factors, which regulate conversion of sucrose to starch in wild-type kernels.
Expressing a bacterial fructosyltransferase in distinct cellular compartments led to dramatically different seed phenotypes in maize. It is often difficult to express the phenotypes associated with cytosolic targeting in words, and certain Figures do not completely capture the profound changes in transgenic tissues expressing the bacterial gene. Cytosolic-targeting of the SacB enzyme was also reported in potato tubers , Expression in tubers also led to severe reduction in starch synthesis. Without question, the effect of altered carbohydrate metabolism on plant development is most clearly illustrated by attempts to induce microtubers from transgenic potato lines. Figure 9 shows that microtuber induction was not possible using transgenic stem sections incubated on media containing high levels of sucrose. The numerous differences in gene expression between these two completely different arrangements of plant tissue are rather obvious. It is nothing less than astounding that a single addition to the genome is the difference between formation of a microtuber or the unrelated structure which forms on transgenic stems. Understanding the mechanism behind this would be even more remarkable.
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