THE VACUOLAR SORTING DOMAIN OF SPORAMIN TRANSPORTS GUS, BUT NOT LEVANSUCRASE, TO THE PLANT VACUOLE

Fructans (polyfructosylsucrose) are synthesized by a number of plants and micro-organisms. Plant fructans are localized in the vacuole and h ave a low degree of polymerization (DP), whereas the fructans synthesi zed by microorganisms are usually much bigger. There is an increasing interest in fructans for both food and non-food applications. In order to accumulate fructans of high DP in the plant vacuole, the levansucr ase protein of Bacillus subtilis was fused to the vacuolar targeting s equence of sporamin and expressed in plants. Transgenic tobacco plants in which this fusion gene is expressed accumulate fructans to levels up to 21% of the d. wt. They showed a reduced translocation of carbohy drates, bleaching of the leaves, stunted growth and increased levels o f hexoses and starch. The levansucrase protein was not translocated to the plant vacuole, but retained in the endomembrane system, even thou gh the same targeting signal was able to translocate the E. coli GUS p rotein to the plant vacuole.