Cytosolic expression of the Bacillus amyloliquefaciens SacB protein inhibits tissue development in transgenic tobacco and potato

Abstract

Fructans are linear or branched polymers containing a single sucrose and repeating fructose residues. An early model for fructan biosynthesis in higher plants suggested that partial synthesis of the polymer occurred in the cell cytosol. The current model suggests that 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 the present study, provided an opportunity to regulate and target fructan synthesis to the cytosol of transgenic plants. Induced expression of the gene led to rapid destruction of leaf tissue. Amino acid substitution at a highly conserved site (Arg³³¹) in the SacB gene reduced the fructosyltransferase efficiency without reducing the invertase activity of the enzyme. Expression of the mutant gene in transgenic tobacco also resulted in leaf damage. However, the appearance of necrotic tissue was greatly delayed. The results suggest that the phenotype is due to accumulation of fructan in the cytosol. Fructan metabolism in the cytosol of potato tubers was also detrimental to development. Tuber size and starch synthesis was significantly reduced in lines containing the untargeted gene. Transgenic tobacco and potato containing the SacB gene offer an opportunity to study the metabolism of fructan and the effect of accumulation on plant cell development.