SORBITOL-6-PHOSPHATE DEHYDROGENASE EXPRESSION IN TRANSGENIC TOBACCO -HIGH AMOUNTS OF SORBITOL LEAD TO NECROTIC LESIONS

We analyzed transgenic tobacco (Nicotiana tabacum L.) expressing Stpd1 , a cDNA encoding sorbitol-6-phosphate dehydrogenase from apple, under the control of a cauliflower mosaic virus 35S promoter. In 125 indepe ndent transformants variable amounts of sorbitol ranging from 0.2 to 1 30 mu mol g(-1) fresh weight were found. Plants that accumulated up to 2 to 3 mu mol g(-1) fresh weight sorbitol were phenotypically normal, with successively slower growth as sorbitol amounts increased. Plants accumulating sorbitol at 3 to 5 mu mol g(-1) fresh weight occasionall y showed regions in which chlorophyll was partially lost, but at highe r sorbitol amounts young leaves of all plants lost chlorophyll in irre gular spots that developed into necrotic lesions. When sorbitol exceed ed 15 to 20 mu mol g(-1) fresh weight, plants were infertile, and at e ven higher sorbitol concentrations the primary regenerants were incapa ble of forming roots in culture or soil. In mature plants sorbitol amo unts varied with age, leaf position, and growth conditions. The appear ance of lesions was correlated with high sorbitol, glucose, fructose, and starch, and low myo-inositol. Supplementing myo-inositol in seedli ngs and young plants prevented lesion formation. Hyperaccumulation of sorbitol, which interferes with inositol biosynthesis, seems to lead t o osmotic imbalance, possibly acting as a signal affecting carbohydrat e allocation and transport.