Mechanisms of Herbicide Resistance
For a herbicide to reach its active site in the plant, it must be taken into the plant and moved in lethal concentrations to the site where it has activity. Once it reaches the target site, it must be able to bind to the active site and stop that particular pathway. For a plant to be resistant, there must be a change that will allow it to avoid one or more of these steps. Theoretically, there could be a change in any one of these necessary steps beginning with uptake of the herbicide into the plant.
Potential mechanisms that could be responsible for resistance:
- Reduced uptake – the herbicide is not taken up in lethal quantities.
- Reduced translocation – the herbicide is not transported to the site in the plant where it has activity.
- Sequestration – the herbicide is physically removed from the target site.
- Target-site mutation – there is a change in the binding site that prevents the herbicide from binding or interacting.
- Metabolism – the herbicide is modified into a nontoxic molecule before it reaches the target site.
The most common mechanisms responsible for resistance are mutations in genes encoding the herbicide target-site (target-site mutations) and genes coding for proteins that regulate the metabolism of the herbicide to a non-lethal molecule. There are cases of resistance that are not due to target-site or metabolism based resistance but the mechanisms have not been well documented. Examples of target-site and metabolism-based resistance will be discussed. There are examples of other resistance mechanisms that are not included in this lesson.