CYTOSOLIC EXPRESSION OF THE BACILLUS-AMYLOLIQUEFACIENS SACB PROTEIN INHIBITS TISSUE-DEVELOPMENT IN TRANSGENIC TOBACCO AND POTATO

Fructans are linear or branched polymers containing a single sucrose a nd repeating fructose residues. An early model for fructan biosynthesi s in higher plants suggested that partial synthesis of the polymer occ urred in the cell cytosol. The current model suggests that synthesis r equires 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 the present study, provided an opportunity to regulate and target fruc tan synthesis to the cytosol of transgenic plants. Induced expression of the gene led to rapid destruction of leaf tissue. Amino acid substi tution at a highly conserved site (Arg(331)) in the SacB gene reduced the fructosyltransferase efficiency without reducing the invertase act ivity of the enzyme. Expression of the mutant gene in transgenic tobac co also resulted in leaf damage. However, the appearance of necrotic t issue was greatly delayed. The results suggest that the phenotype is d ue to accumulation of fructan in the cytosol. Fructan metabolism in th e cytosol of potato tubers was also detrimental to development. Tuber size and starch synthesis was significantly reduced in lines containin g the untargeted gene. Transgenic tobacco and potato containing the Sa cB gene offer an opportunity to study the metabolism of fructan and th e effect of accumulation on plant cell development.