Chapter 16: MOA Part 2: Amino Acid Synthesis Inhibitors & Nitrogen Metabolism Inhibitors
16.5 Herbicide Resistance to ALS Inhibitors
At the time of this chapter’s development, there were 49 different weed species in the US alone that have confirmed resistance. In fact, just five years after the commercial release of the first SU herbicide, chlorosulfuron (Glean), DuPont removed this product from the market due to the rapid expansion of herbicide resistant kochia.
So why did resistance to ALS inhibitors appear so rapidly compared to other herbicide MOAs? The fact that the ALS gene is nuclear encoded has significant implications with regard to the spread of the resistance gene. It has been suggested that the natural mutation rate conferring ALS resistance might be as high as 10^-6 for some weed species (probably kochia). The other major factors contributing to the rapid spread of resistance are:
- very strong herbicide selection pressure
- dominant trait resulting in highly resistant heterozygous individuals
- pollen mediated gene flow allowing for resistance in plants that have never been exposed to the herbicide
- no fitness penalty associated with the resistance gene.
Single point mutations are sufficient to affect the binding of one or several ALS inhibitors, resulting in herbicide resistance. There are four amino acids that are the most common sites for these mutations; alanine 122 (A122), proline 197 (P197), tryptophan 574 (W574), and serine 653 (S653).
A website has been established and is maintained by a consortium of public and private weed researchers to document and catalog the latest information on herbicide resistant weeds world-wide. The International Survey of Herbicide Resistant Weeds is a collaborative effort between weed scientists in over 80 countries and is located at www.weedscience.org . The consortium’s main aim is to maintain scientific accuracy in the reporting of herbicide resistant weeds globally. This collaborative effort is supported by government, academic, and industry weed scientists worldwide. This project is funded by the Global Herbicide Resistance Action Committee and CropLife International.
- Ian Heap. Director of the International Survey of Herbicide-Resistant Weeds.
IanHeap@weedscience.org - Mark Peterson. Chairman of the Herbicide Resistance Action Committee.
mapeterson@dow.com - Michael Horak. Chairman of the North American Herbicide Resistance Action Committee.
michael.j.horak@monsanto.com