ENHANCEMENT OF SEED-GERMINATION IN HIGH SALINITY BY ENGINEERING MANNITOL EXPRESSION IN ARABIDOPSIS-THALIANA

The bacterial gene mtlD, which encodes mannitol 1-phosphate dehydrogenase (E.C.1.1.1.17), was transformed into Arabidopsis thaliana and expressed under control of the CaMV 35S promoter, MtlD-transformants accumulated mannitol, a sugar alcohol that is not normally found in Arabidopsis, Amounts of soluble carbohydrates, sucrose, glucose, fructose, myo-inositol and mannitol were determined in different tissues of wild-type and transgenic plants. We estimated that less than 1% of the carbon assimilated was converted into mannitol by the transgenic plants. The establishment of individual transformed lines (after self-crossing three times) resulted in high and low mannitol-producing lines which were stably maintained, The presence of mannitol did not alter plant appearance or growth habit. When MtlD-expressing seeds and control seeds (T3 generation) were imbibed with solutions containing NaCl (range 0 to 400 mol m(-3)), transgenic seeds containing mannitol germinated in medium supplemented with up to 400 mol m(-3) NaCl, while control seeds ceased germination at 100 mol m(-3) NaCl, It is doubtful whether the ability to germinate in high salt was a result of an osmotic effect exerted by elevated levels of mannitol, considering that mannitol concentrations were in the mol m(-3) range in seeds. A specific effect of polyols, for example on the integrity of subcellular membranes or enzymes, cannot be excluded.