Characterization of Drought Response of Transgenic Tomato Plants Expressing Spermidine Synthase Under Constitutive or Fruit/Ethylene-specific Promoters

We have generated transgenic tomato plants expressing yeast spermidine synthase (ySpdSyn) under the control of either a constitutive (CaMV 35S) or a fruit/ethylene-specific (E8) promoter. Since polyamines have been implicated in imparting tolerance to water stress, growth and physiology of transgenic and wild-type (WT) plants was quantified after water stress and re-watering under the greenhouse environments. Under well-watered conditions, transgenic tomato plants constitutively expressing ySpdSyn accumulated 54% more spermidine than WT plants whereas putrescine and spermine contents remained unaltered. Plants expressing ySpdSyn under E8 promoter did not show any change in polyamine contents. Transgenic plants were slightly smaller than WT plants, whereas dry weight, leaf chlorophyll, photosynthesis, transpiration, leaf relative water content and electrolyte leakage remained unaltered. Water was withheld from four weeks old WT and transgenic plants for 7, 14, 18 or 26 days. Both WT and transgenic plants were able to recover after 7 days of water stress, but not thereafter as plants reached permanent wilting point. Transgenic plants maintained higher leaf relative water content (RWC) during short period of stress although other measurements were not different between transgenic and WT plants. Transgenic leaves with higher RWC also showed 30% more spermidine levels compared to WT. These results suggest that although spermidine helps maintained higher relative water content in transgenic compared to WT plants, this was not sufficient to overcome extended osmotic stress. Correlation of plant morphological and physiological responses with endogenous polyamine levels will be discussed. Taken together, our data indicate only a slight role of spermidine in tolerating osmotic stress in tomato plants grown under greenhouse conditions.