High levan accumulation in transgenic tobacco plants expressing the Gluconacetobacter diazotrophicus levansucrase gene

Bacterial levansucrase (EC 2.4.1.10) converts sucrose into non-linear levan consisting of long β(2,6)-linked fructosyl chains with β(2,1) branches. Bacterial levan has wide food and non-food applications, but its production in industrial reactors is costly and low yielding. Here, we report the constitutive expression of Gluconacetobacter diazotrophicus levansucrase (LsdA) fused to the vacuolar targeting pre-pro-peptide of onion sucrose:sucrose 1-fructosyltransferase (1-SST) in tobacco, a crop that does not naturally produce fructans. In the transgenic plants, levan with degree of polymerization above 104 fructosyl units was detected in leaves, stem, root, and flowers, but not in seeds. High levan accumulation in leaves led to gradual phenotypic alterations that increased with plant age through the flowering stage. In the transgenic lines, the fructan content in mature leaves varied from 10 to 70% of total dry weight. No oligofructans were stored in the plant organs, although the in vitro reaction of transgenic LsdA with sucrose yielded β(2,1)-linked FOS and levan. Transgenic lines with levan representing up to 30 mg g−1 of fresh leaf weight produced viable seeds and the polymer accumulation remained stable in the tested T1 and T2 progenies. The lsdA-expressing tobacco represents an alternative source of highly polymerized levan.