Herbicide-Tolerant Crops

A herbicide-tolerant crop is a crop that can survive treatment with a particular weed-killing chemical at doses used to control susceptible weeds. Genetic engineering gives the plant this ability by introducing or modifying genes so that the herbicide either cannot effectively block an essential plant enzyme or is rendered less harmful within the plant.

The two main engineering strategies

Herbicides often kill plants by interfering with essential biochemical reactions. Engineers can protect these reactions or enable the crop to detoxify the chemical.

• Target-site insensitivity involves supplying an enzyme that performs its normal biological function but is less sensitive to the herbicide.
• Herbicide detoxification involves introducing a gene encoding an enzyme that converts the herbicide into a less harmful compound.
• The resulting tolerance is specific to the herbicide or herbicide group targeted by the engineered trait; it does not make the crop tolerant to every weedkiller.

How the principal examples work

• Glyphosate inhibits EPSPS, an enzyme in the shikimate pathway that is needed for synthesising aromatic amino acids.
• A gene encoding a glyphosate-insensitive EPSPS, such as CP4 EPSPS from an Agrobacterium strain, can be introduced into a crop. The introduced enzyme maintains the pathway even when glyphosate inhibits the plant's sensitive enzyme.
• Glufosinate inhibits glutamine synthetase, disrupting nitrogen metabolism and causing harmful ammonia accumulation.
• The bacterial bar or pat genes encode phosphinothricin acetyltransferase. This enzyme acetylates and inactivates glufosinate, protecting the crop from its action.

Introducing and establishing the trait

The selected gene is assembled into a gene construct with regulatory sequences that enable its expression in plant cells. Where necessary, targeting sequences direct the protein to the cellular compartment in which it must function.

• The construct can be delivered through Agrobacterium-mediated transformation, which uses a modified bacterial DNA-transfer system, or through particle bombardment, which delivers DNA-coated microscopic particles into plant cells.
• Transformed cells are selected and regenerated into whole plants using tissue culture.
• Plants are tested for gene expression, herbicide tolerance and stable inheritance; the trait can then be transferred into suitable crop varieties through breeding.
• The engineered trait protects the crop, not neighbouring weeds. Repeated reliance on the same herbicide can select for resistant weeds, so tolerance does not remove the need for integrated weed management.