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Proline (Pro) accumulation has been correlated with tolerance to drought and salinity stresses in plants. Therefore, overproduction of Pro in plants may lead to increased tolerance against these abiotic stresses. To test this possibility, we overexpressed in tobacco the mothbean delta-pyrroline-5-carboxylate synthetase, a bifunctional enzyme able to catalyze the conversion of glutamate to delta-pyrroline-5-carboxylate, which is then reduced to Pro. The transgenic plants produced a high level of the enzyme and synthesized 10- to 18-fold more Pro than control plants. These results suggest that activity of the first enzyme of the pathway is the rate-limiting factor in Pro synthesis. Exogenous supply of nitrogen further enhanced Pro production. The osmotic potentials of leaf sap from transgenic plants were less decreased under water-stress conditions compared to those of control plants. Overproduction of Pro also enhanced root biomass and flower development in transgenic plants under drought-stress conditions. These data demonstrated that Pro acts as an osmoprotectant and that overproduction of Pro results in the increased tolerance to osmotic stress in plants.
Kishor et al. (Tue,) studied this question.