CYCLITOL PRODUCTION IN TRANSGENIC TOBACCO

High levels of cyclic sugar alcohols (cyclitols) correlate with tolera nce to osmotic stress in a number of plant species. A gene encoding a cyclitol biosynthesis enzyme from a halophyte, Mesembryanthemum crysta llinum has been introduced into tobacco. The gene, Imt1, encodes a myo -inositol O-methyl transferase that, in M. crystallinum, catalyzes the first step in the stress-induced accumulation of the cyclitol pinitol . Tobacco transformed with the Imt1 cDNA under the control of the CaMV 35S promoter appeared phenotypically normal and exhibited IMT1 enzyme activity. Transformants accumulated a carbohydrate product not detect able in non-transformed control plants. This product was identified by HPLC and NMR as ononitol (1-D-4-O-methyl myo-inositol). Ononitol was a major carbohydrate constituent in leaf tissue of plants expressing t he Imt1 gene, accumulating to up to 25% the level of sucrose in transf ormant seedlings. The identification of ononitol as the IMT1 product a nd the specific accumulation of this compound in transformed tobacco s upport a role for ononitol as a stable intermediate in pinitol biosynt hesis and indicate that an epimerization activity lacking in tobacco i s responsible for the conversion of ononitol to pinitol in M. crystall inum. The production of ononitol in tobacco indicates that plant carbo hydrate metabolism is flexible and can accommodate the synthesis and a ccumulation of non-endogenous metabolites. The transgenic system descr ibed here will serve as a useful model to test the ability of cyclitol s such as ononitol to confer tolerance to environmental stress in a no rmally glycophytic plant.