Over 40,000 species of plants accumulate fructan, p-2-1-and P-2-6-linked polymers of fructose as a storage reserve. Due to their high fructose content, severa1 commercial applications for fructans have been proposed. However, plants that accumulate these polymers are not agronomically suited for large-scale cultivation or processing. This study describes the transformation of a Bacillus amyloliguefaciens SacB gene into maize (Zea mays 1.) callus by particle bombardment. Tissue-specific expression and targeting of the SacB protein to endosperm vacuoles resulted in stable accumulation of high-molecular-weight fructan in mature seeds. Accumulation of fructan in the vacuole had no detectable effect on kernel development or germination. Fructan levels were found to be approximately 9-fold higher in sh, mutants compared to wild-type maize kernels. In contrast to vacuole-targeted expression, starch synthesis and endosperm development in mature seeds containing a cytosolically expressed SacB gene were severely affected. The data demonstrate that hexose resulting from cytosolic SacB activity was not utilized for starch synthesis. Transgenic seeds containing a chimeric SacB gene provide further evidence that the dominant pathway for starch synthesis in maize endosperm is through uridine diphosphoglucose catalyzed by the enzyme sucrose synthase.

Fructans are linear or brancbed polymers containing a single sucrose and repeating fructose residues. An early model for fructan biosyntbesis in bigber plants suggested that partial synthesis of tbe polymer occurred in tbe cell cytosol. Tbe current model suggests tbat synthesis requires the interaction of two separate fructosyltransferases located in the vacuole. Tobacco lines containing a chemically induced promoter, directing expression of the Bacillus amyloliquefaciens SacB gene in tbe present study, provided an opportunity to regulate and target fructan synthesis to tbe cytosol of transgenic plants. Induced expression of tbe gene led to rapid destruction of leaf tissue. Amino acid substitution at a bigbly conserved site (Arg^^^) in tbe SacB gene reduced tbe fructosyltransferase efficiency witbout reducing tbe invertase activity of tbe enzyme. Expression of tbe mutant gene in transgenic tobacco also resulted in leaf damage. Hov^^ever, the appearance of necrotic tissue was greatly delayed. The results suggest tbat tbe pbenotype is due to accumulation of fructan in tbe cytosol. Fructan metabolism in tbe cytosol of potato tubers was also detrimental to development. Tuber size and starcb synthesis was significantly reduced in lines containing tbe untargeted gene. Transgenic tobacco and potato containing tbe SacB gene offer an opportunity to study tbe metabolism of fructan and the effect of accumulation on plant cell development.