SALINITY AND DROUGHT TOLERANCE OF MANNITOL-ACCUMULATING TRANSGENIC TOBACCO

Tobacco plants (Nicotiana tabacum L.) were transformed with a mannitol -1-phosphate dehydrogenase gene resulting in mannitol accumulation. Ex periments were conducted to determine whether mannitol provides salt a nd/or drought stress protection through osmotic adjustment. Non-stress ed transgenic plants were 20-25% smaller than non-stressed, non-transf ormed (wild-type) plants in both salinity and drought experiments. How ever, salt stress reduced dry weight in wild-type plants by 44%, but d id not reduce the dry weight of transgenic plants. Transgenic plants a djusted osmotically by 0.57 MPa, whereas wild-type plants did not adju st osmotically in response to salt stress. Calculations of solute cont ribution to osmotic adjustment showed that mannitol contributed only 0 .003-0.004 MPa to the 0.2 MPa difference in full turgor osmotic potent ial (pi(o)) between salt-stressed transgenic and wild-type plants. Ass uming a cytoplasmic location for mannitol and that the cytoplasm const ituted 5% of the total water volume, mannitol accounted for only 30-40 % of the change in pi(o) of the cytoplasm. Inositol, a naturally occur ring polyol in tobacco, accumulated in response to salt stress in both transgenic and wild-type plants, and was 3-fold more abundant than ma nnitol in transgenic plants. Drought stress reduced the leaf relative water content, leaf expansion, and dry weight of transgenic and wild-t ype plants. However, pi(o) was not significantly reduced by drought st ress in transgenic or wild-type plants, despite an increase in non-str uctural carbohydrates and mannitol in droughted plants. We conclude th at (1) mannitol was a relatively minor osmolyte in transgenic tobacco, but may have indirectly enhanced osmotic adjustment and salt toleranc e; (2) inositol cannot substitute for mannitol in this role; (3) slowe r growth of the transgenic plants, and not the presence of mannitol pe r se, may have been the cause of greater salt tolerance, and (4) manni tol accumulation was enhanced by drought stress but did not affect pi( o) or drought tolerance.