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Index >> Biotechnology in Agriculture >>Tolerance to Herbicides

Tolerance to Herbicides

	Tolerance to Herbicides A factor that limits the use of herbicides is the fact that many herbicides not only kill the weeds but also harm the main crop they are intended to protect. Secondly, the residues of herbicides in soil may damage the growth of subsequent crops grown on the same field. Herein lies the objective of developing transgenic plants capable of resisting herbicides applied against target weeds. Many transgenic plants exhibiting herbicide tolerance have been sold in the market and over 45 companies around the world are engaged in developing plants capable of resisting herbicides such as triazines, atrazine, paraquat, sulfonylureas, glyphosate, dalapon and likewise, over 48 research institutes around the world have research projects devoted towards herbicide tolerant plants.
In the USA., of the 27.8 million hactares planted with transgenic plants 71.0 per cent is covered with plants (soybean and maize) resistant to "Roundup" herbicide, which in fact is the trade name for glyphosate a widely used herbicide. These transgenic plants carry the gene that neutralizes the herbicide. Two approaches used in developing resistance to herbicides are (1) genetic engineering so that the plant target enzyme is made less sensitive to the herbicide and (2) metabolic inactivation by which the herbici4e molecule is degraded or converted to an inactive form.
Glyphosate is a non-selective herbicide. The enzyme in plants which binds to glyophosphate is 5-enolpyruvyl 3-phosphoshikimate phosphate (EPSP) synthase. Cloning genes into plants which code for a modified EPSP synthase enzyme that has reduced affinity for binding glyphosate has been the strategy for developing transgenic soybeans, cotton and oil seed rape. Similar strategy has been followed for tolerance to sulfonyl ureas, another class of herbicides. In this case, genes that are tolerant to acetolactate synthase (ALS) have been isolated from tobacco and evaluated.
The above two examples serve to illustrate the target enzymes that are being made less sensitive to the herbicide. In another class of herbicides, the bromoxynils, the metabolic inactivation or degradation approach has been used. The enzyme nitrilase is encoded by a gene from the common soil bacterium Klebsiella and this enzyme has the capability to degrade the herbicide. Tolerance to this herbicide in cotton is being investigated by cloning the gene responsible for nitrilase production from Klebsiella into cotton. A second example is the development of resistance to the herbicide, glufosinate by cloning the bar gene for phosphinothricin acetyl transferase from a soil actinomytete into plants.

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