Resistance Against Stress Drought Salt Heat Freezing, Numer of Genes, Proline and Betaines, Stress Tolerance, Product of Gene

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	Resistance against stress (drought, salt, heat, freezing, etc.) A number of genes are now known to be responsible for providing resistance against stresses such as heat, cold, salt and heavy metals. Stress tolerance in plants can be achieved in two ways: (i) improving protection from stress as done in oxidative stress, which is protected by super-oxide dismutases and other systems; (ii) reducing sensitivity to stress, as done in improving salt tolerance by an osmolyte and chilling tolerance by increasing the level of polyunsaturated fatty acids. Studies are also being conducted on metabolites like proline and betaines that are implicated in stress tolerance.
The products of genes, whose expression is induced by stresses like drought, salt and freezing can be broadly classified in two groups: (i) Proteins that protect cells from dehydration; these include enzymes that are involved in the production of osmoprotectants, late embryogenesis abundant (LEA) proteins, antifreeze proteins, chaperones and detoxification enzymes. (ii) Proteins that are involved in inducing transcription of stress responsive genes; these include transcription factors, protein kinases, and the enzymes involved in phosphoinositide metabolism. Genes encoding proteins belonging to both these groups have been utilized and will be utilized in future for the production of stress resistant transgenic plants.
For instance, resistance against chilling was introduced into tobacco plants, by introducing a gene for ‘glycerol phosphate acyl transferase’ enzyme from Arabidopsis (Arabidopsis is resistant to chilling). This enzyme, encoded by nuclear genome and later transported to chloroplast, determines the level of unsaturation of fatty acids in the phosphatidyl-glycerol of chloroplast membranes. The plants with high proportion of cis-unsaturated fatty acids (e.g. spinach, Arabidopsis) are resistant to chilling, and so are the transgenic tobacco plants carrying the gene for the above enzyme.
It has also been shown that a cis-acting promoter element, called dehydration response element (DRE) plays an important role in regulating gene expression in response to stresses like drought, salt and freezing. The transcription factors designated as DREB IA and DRE2B (DREB = DRE binding protein) specifically bind to DRE and activate transcription of genes containing the DRE sequence in Arabidopsis. The cDNAs for both these proteins have been isolated. Each of these cDNAs, along with CaMV35S promoter, was used for production of transgenic plants which gave strong constitutive expression of stress inducible genes (rd29A, kinl, cor6.6/kin2, cor47/rd17, cor15a and erd10) and were tolerant to stresses like drought, salt and freezing.
	Another cDNA encoding CRT/DRE binding protein (CBFI), when similarly used for the production also led to increased freezing tolerance. Both in case of transgene DREB1A and CBF 1, stress tolerance was associated with growth retardation under normal non-stress conditions. However, another stress inducible promoter rd29A was later used to drive DREB1A, so that the transgenic plants produced with this promoter exhibited stress tolerance and improved growth under non-stressed conditions.