Modification of the Target - The target has been modified for developing resistance against atleast three herbicides (glyphosate, sul­phonylureas and imidazolinones). The transgenic petunia plants resistant to glyphosate (active ingredient of Roundup herbicide) were developed by transfer of a gene for EPSPS (S-enol-pyruvyl- shikimat-3-phosphate synthase), that overproduces this enzyme.

This overexpressing gene was isolated from plants selected for herbicide resistance. In some other cases a gene aroA was isolated from the bacteria Salmonella typhimurium or E. coli and was transferred to tomato and/or tobacco.
Another class of herbicides includes sulphonylurea compounds (active ingredients of Glean & Qust herbicide) and imidazolinones that inhibit the enzyme acetolactate synthase (AlS), which is involved in the bisoynthesis of branched chain amino acids like leucine, isoleucine and valine.

Hoechst, isolated a gene from 'Streptomyces hygroscopicus', which encodes an enzyme, capable of inactivating the herbicide 'Basta'. Transgenic plants with this gene have been produced and field tested, demonstrating effectiveness of this gene for protection against the herbicide 'Basta'.