Accumulation of trehalose within transgenic chloroplasts confers drought tolerance

Yeast trehalose phosphate synthase(TPS1) gene was introduced into the tobacco chloroplast ornuclear genomes to study resultant phenotypes. PCR and Southern blots confirmedstable integration of TPS1 into the chloroplast genomes ofT1, T2 and T3 transgenic plants. Northern blotanalysis of transgenic plants showed that the chloroplast transformantexpressed169-fold more TPS1 transcript than the best survivingnuclear transgenic plant. Although both the chloroplast and nuclear transgenicplants showed significant TPS1 enzyme activity, no significant trehaloseaccumulation was observed in T0/T1 nuclear transgenicplants whereas chloroplast transgenic plants showed 15–25 fold higheraccumulation of trehalose than the best surviving nuclear transgenic plants.Nuclear transgenic plants (T0) that showed even small amounts oftrehalose accumulation showed stunted phenotype, sterility and otherpleiotropiceffects whereas chloroplast transgenic plants (T1, T2,T3) showed normal growth and no pleiotropic effects. Transgenicchloroplast thylakoid membranes showed high integrity under osmotic stress asevidenced by retention of chlorophyll even when grown in 6% PEG whereaschloroplasts in untransformed plants were bleached. After 7 hrdrying, chloroplast transgenic seedlings (T1, T3)successfully rehydrated while control plants died. There was no differencebetween control and transgenic plants in water loss during dehydration butdehydrated leaves from transgenic plants (not watered for 24 days) recoveredupon rehydration turning green while control leaves dried out. Theseobservations suggest that trehalose functions by protecting biologicalmembranesrather than regulating water potential. In order to prevent escape of droughttolerance trait to weeds and associated pleiotropic traits to related crops, itmay be desirable to engineer crop plants for drought tolerance via thechloroplast genome instead of the nuclear genome.