Drought Resistance -

A number of such genes have been identified; isolated, cloned and expressed in plants, which are potential sources of resistance to abiotic stresses. These genes include Rab (responsive to abscisic acid) and SalT (induced in response to salt stress) genes of rice; genes for enzymes involved in proline biosynthesis in bacteria (proBA and proC in E. coli) and plants, spinach genes involved in betaine synthesis, etc.

In plants, proline is preferentially produced from ornithine under normal conditions. However, under stress it is made directly from glutamate, the first two reactions of the pathway being catalyzed by a single enzyme ∆¹-pyrroline­ 5-carboxylate synthetase (P5CS). The gene encoding P5CS has been isolated from soybean and mothbean, and cloned.

But it should be kept in mind that accumulation of proline is only one of the factors, which enable plants to sustain growth under water stress. Other factors also allow plants to overcome osmotic stress.

For example, expression of E. coli gene mtl1D in plants leads to mannitol accumulation and some degree of enhanced growth under stress. An important aspect of such manipulations, however, remains that the basal metabolism of the plant should be able to sustain a high rate of accumulation of the concerned osmolytes without too much of a 'cost' to the plants.