Transgenic Crops and Herbicide Resistance

Study Notes

Transgenic crops are genetically modified to exhibit herbicide, biotic, and abiotic stress resistance.
Herbicide tolerance is the most prevalent genetically modified (GM) trait in crops.
It was amongst the first GM traits tested and used commercially.
Common GM traits include herbicide tolerance, insect tolerance, and a combination of both.
Weeds are plants growing in cultivated fields that negatively impact crops.
Methods to control weeds include mechanical (hoeing, hand-pulling), physical (heat, light), biological (living organisms), and chemical (herbicides).
Herbicides account for approximately 60% of pesticides used in the US.
Weeds compete with crops for soil nutrients, impacting crop yield.
Modern agriculture developed effective herbicides to reduce weed impact.

Herbicide resistance is a plant's ability to withstand greater herbicide dosages compared to the wild type, sustaining a near-normal life cycle.
This resistance can be a trait or quality present within a species or a population of plants.

Some commercially important crop plants may lack tolerance to certain herbicides.
Using varied herbicides to target a wide range of weeds may inadvertently harm crops.
Some herbicides lack the desired high toxicity level for weeds while avoiding crop damage.

These herbicides are effective against many weed types.
Their use is restricted to when crops are not susceptible to the herbicide.
Cultivating herbicide-tolerant crops provides the opportunity to apply herbicides more effectively, targeting weeds without harming crops.
Some crops naturally possess resistance to certain herbicides. Herbicide resistance in crops can arise due to mutation and natural selection.

Research during World War II in the UK and US led to the discovery of herbicides.
2,4-D, a modern herbicide, was discovered and synthesized by W.G. Templeman at Imperial Chemical Industries in 1940.
2,4-D was shown to effectively kill broadleaf weeds in cereal crops without harming the cereal crop.

Herbicides are significantly more toxic to plants than animals.
Herbicides act as inhibitors of plant-specific processes.
Plants are autotrophic and produce all their essential macromolecules.
Pathways for producing essential human nutrients (like vitamins and amino acids) are also targets for herbicides, mainly located in the chloroplasts.

The herbicidal activity of herbicides originates from the targeted and specific inhibition of a single enzyme or protein.
Herbicides belong to various chemical families and have 15 broad classes categorized by mechanism of action.
Most herbicides have a single mechanism of action.
Specific enzymes are vulnerable to herbicidal activity.
For example, five chemical families of herbicides target the enzyme acetolactate synthase, an enzyme involved in the biosynthesis of branched-chain amino acids.

Herbicides vary in their chemical properties, leading to differences in toxicity, environmental persistence, and biodegradability.
Herbicides are categorized based on their characteristics: (1) site of uptake into the plant, (2) degree of translocation into the plant, and (3) time of application (pre-planting or post-harvest).

Glyphosate binds and blocks the activity of EPSPS (enolpyruvateshikimate-3-phosphate synthase).
EPSPS is an enzyme critical for the biosynthesis of aromatic amino acids in plants.
Inhibition of EPSPS by glyphosate prevents the plant from producing essential aromatic amino acids needed for protein synthesis.