Q.Give a brief account of herbicide resistance in transgenic plants.

Transgenic crops survive broad-spectrum herbicide spraying either by carrying an insensitive target enzyme or by detoxifying the herbicide.

Broad-spectrum herbicides such as glyphosate kill both weeds and crop plants by inhibiting an essential plant enzyme (glyphosate inhibits EPSPS, an enzyme of the shikimate pathway needed for aromatic amino acid synthesis). Genetic engineers make crops herbicide-tolerant (HT) by introducing one of two kinds of genes via Agrobacterium-mediated or biolistic (gene gun) transformation:

1. An insensitive version of the herbicide's target enzyme -- e.g. a modified EPSPS gene (from a glyphosate-resistant strain of Agrobacterium, CP4) that is not inhibited by glyphosate, so the plant's own aromatic-amino-acid synthesis continues even when sprayed.
2. A herbicide-detoxifying gene -- e.g. the bar (or pat) gene, originally from the soil bacterium Streptomyces, encoding phosphinothricin acetyltransferase, an enzyme that chemically inactivates the herbicide phosphinothricin/glufosinate by acetylating it, before it can harm the plant. …