Defense Against Water Stress Accumulation of Osmoprotectants Like Glycine Betaine, Osmoprotectants, Osmolyte, Ammonium Compounds

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	Defense against water stress: Accumulation of osmoprotectants like glycine betaine Resistance against abiotic stresses like drought, salinity and cold (all leading to dehydration) is achieved in plants by the accumulation of low molecular weight non toxic compounds, collectively described as osmoprotectants. These compounds help plants in two ways: (i) by acting as a cytoplasmic osmolyte, thereby facilitating water intake and water retention and (ii) by protecting and stabilizing macromolecules from damage induced by high salt levels. Some of these osmoprotectants include sugars, alcohols, proline and quaternary ammonium compounds (e.g. glycine betaine).
Glycine betaine is a very effective. osmolyte which accumulates during water stress due to high salinity, drought and low temperature in bacteria, cyanobacteria and in higher plants (particularly members of Chenopodiaceae and Poaceae, which are generally drought resistant). Several crops like potato, rice, tomato and tobacco do not accumulate glycine betaine, but can be made to accumulate it by manipulating glycine betaine biosynthetic pathway through transgenesis. Glycine betaine is obtained in two steps as follows:

Biosynthesis of choline, which is a raw material for glycine betaine is under feedback control (not feedback inhibition), so that if choline is utilized by glycine betaine pathway introduced through genetic engineering, further choline uptake can be stimulated to allow synthesis of more glycine betaine. The two enzymes involved in the above glycine betaine biosynthetic pathway include the following: (i) choline dehydrogenase (CDH) in E. coli and choline monoxygenase in spinach; each of which converts choline into betaine aldehyde; and (ii) betaine aldehyde dehydrogenase (BADH), which converts betaine aldehyde into glycine betaine. Bacterial CDH is the most useful enzyme, since it is able to catalyze not only the oxidation of choline to betaine aldehyde, but also the conversion of betaine aldehyde to glycine betaine. Tobacco lacks both the steps in glycine betaine biosynthetic pathway.
	Transgenic tobacco plants carrying E. coli betAgene encoding CDH, were produced and were found to possess an 80% increase in salt tolerance (a report in 1996) at a salt (NaCl) concentration of 300 mM. Betaine aldehyde dehydrogenase (BADH) gene from E. coli. Was also used for production of transgenic plants exhibiting increased production of glycine betaine and imparting salt tolerance. Several other genes induced in response to salt stress in plants have been identified. These include RAB, salt and osmotingenes.

The functions of these genes are not as clearly understood as those of the glycine betaine biosynthetic pathway. Transgenic plants carrying E. coli mannitol-I-phosphate dehydrogenase gene have also been produced, but a rather high concentration of mannitol (100 mM) is needed to withstand high NaCI concentration.

This concentration of mannitol can be deleterious to the plant. In view of the above, glycine betaine is considered to be one of the most efficient osmoprotectant. Low concentration of this osmolyte gives substantial protective effect in plants.