Herbicides ONveg in The Grower Resistance Weed Management

How Do Weeds Resist Glyphosate

Dr. Stephen Powles, Australian Herbicide Resistance Initiative (AHRI)

Reprinted with permission in ONvegetables in The Grower, December 2013

If you ever find yourself in the situation where you are catering for a group of people, and you are wondering how much food to prepare, the best thing to do is to prepare a little extra, just in case. The last thing that you want to do is run out.

Believe it or not, this is how some weeds resist glyphosate. They make an extra-large batch of the enzyme that glyphosate binds to, just in case. This way, if the weed is sprayed with glyphosate that inhibits some of the enzyme, there is still enough left for the plant to function and survive. This mechanism is known as ‘Gene Amplification’ and was discovered by Dr. Todd Gaines along with a large team of scientists from around the world.

There are currently six known mechanisms of glyphosate resistance and several more are suspected.

The table below lists the six known mechanisms of glyphosate resistance.

Mechanism Weeds Strength of Resistance
Target site 106 mutation Ryegrass 2 to 3 fold
Target site 102 + 106 mutations Goosegrass >100 fold
Gene amplification Amaranthus (Palmer amaranth), Ryegrass, Kochia 6 to 40 fold
Vacuole sequestration Conyza (fleabane), Ryegrass 7 to 11 fold
Reduced cell uptake Amaranthus (Palmer amaranth), Johnsongrass  
Hypersensitive (source leaf) Giant ragweed  

There are several other mechanisms of glyphosate resistance currently being researched but are yet to be confirmed.

Q. How many weed scientists does it take to identify a glyphosate resistance mechanism?

A. 18.

No, this is not a bad joke, it is reality. The research effort led by Dr. Todd Gaines with Colorado State University, AHRI and Bayer CropScience Germany, involved collaborating with 17 other scientists around the world to identify a new glyphosate resistance mechanism. This gives some indication of how complex these new findings are. There is a considerable global effort to better understand glyphosate resistance.

The mechanism discovered in this research is called ‘Gene Amplification’ because the plant produces many copies of the gene that codes for the EPSPS enzyme.

Glyphosate kills plants by inhibiting the enzyme EPSPS. This research discovered that America’s biggest problem weed, Pigweed (Palmer amaranth), developed resistance by producing a lot more of this enzyme. This research identified Pigweed with 5 fold to 160 fold more copies of the EPSPS gene.  More copies of the gene resulted in more EPSPS enzyme activity. The effect of additional EPSPS genes is additive, and additional copies of the gene infer higher levels of resistance.  Put simply, more copies of the EPSPS gene = more EPSPS activity = higher levels of resistance to glyphosate.

Glyphosate can still bind and inhibit some of the EPSPS enzyme produced by the plant, but the plant survives because there is enough EPSPS enzyme left over to do its job and keep the plant alive.

This resistance mechanism has now also been confirmed in ryegrass and Kochia species (confirmed resistant in Western Canada). These weed species are the world champions of developing resistance to herbicides. It comes as no surprise that these weed species are each able to develop several different mechanisms of resistance to glyphosate.



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