Chromosomes feature in evolution of resistance

Peter Newman*

Some things are hard to understand.

Mayonnaise on fries.

Lemon in beer.

The latest paper published by the AHRI team!

A truly international team, including Chun Zhang from China, Eric Patterson from the USA, and Qin Yu from AHRI recently published a paper that was well beyond me.

I read it three times to write this article, then gave up and phoned a friend, Dr Todd Gaines to the rescue again (Qin was on holidays!).

Todd explained that the researchers had found a hot spot on the DNA of goosegrass that is essentially a herbicide resistance generator.

That hotspot is just near the telomere on the chromosome. You may have heard in the media over the years that it is the shortening of the telomeres that is responsible for my wrinkles. As we age, the telomeres get shorter, and that is part of the ageing process.

The over-expression of genes, creating 25x the amount of the glyphosate resistant EPSPS enzyme,
occurs near the ‘skipping rope handles’ of the chromosome. 

If a chromosome is a skipping rope, the telomeres are the handles. They protect the ends of the chromosomes.

In this research, the team found that in glyphosate-resistant goosegrass the EPSPS gene (coding for EPSPS enzymes that glyphosate binds to) was translocated to the sub-telomeric region (near the skipping rope handles), and duplicated many times there, leading to glyphosate resistance occurring.

If you’re looking for a simple explanation from a luddite, read more.

Telomeres are sections of DNA found at the ends of chromosomes.

A telomere (skipping rope handle) is like a cap on the end of a chromosome. It is a repetitive sequence of DNA that doesn’t code for anything, it’s just there to match up the ends of the DNA.

The part of the skipping rope just next to the handles is called the sub-telomeric region. It just so happens that the EPSPS gene in glyphosate-resistant goosegrass was located here.

If you can imagine the handle of a skipping rope getting damaged, you can visualise that the rope near the handle might fray a little, and the strands of rope slip on each other.

This is what can happen with DNA. If there is slippage of the DNA, mistakes can happen, and a hotspot of genetic variation occurs. In this case, it caused an over-expression of the EPSPS enzyme, resulting in 25 times as many copies of the DNA code for this enzyme, and therefore 25 times as much EPSPS.

We have reported on enhanced gene expression due to increased EPSPS gene copies in the past, particularly in Palmer amaranth with glyphosate resistance. However, other weeds, including Palmer amaranth don’t necessarily have the EPSPS gene near the skipping rope handle.

Essentially, what happens is instead of the plant producing a normal amount of the EPSPS enzyme, it produces truckloads of the stuff. Glyphosate can still bind to the EPSPS enzyme, and stop it working, but there’s so much of it floating around in the cells of the plant that glyphosate can’t bind to all of it and so there’s still enough left over to enable the plant to function as normal and survive the glyphosate application.

The interesting thing about this research is that the team have provided additional evidence that the sub-telomeric region may be a hotspot for genetic variation for herbicide resistance evolution. This will lead to further research to determine if other genetic variations endowing herbicide resistance are common in this region.

While the team was at it, they also described the entire genome of goosegrass (Eleusine indica), adding to the list of weeds with their genome described at the chromosome level. That rolls off the tongue easy doesn’t it! Just describe another genome while you’re at it, please!


Clearly, this is research at a very high level that very few people on the planet understand. We don’t necessarily know where the research will lead us in the future, but we do know that our scientists are generating new information that is giving us a very deep understanding of herbicide resistance at the genome level, and hopefully, this leads to more weed management solutions in the future.

*Peter Newman is Australian Herbicide Resistance Initiative (AHRI) and WeedSmart western extension agronomist. Mr Newman grew up in Perth studied agricultural science at UWA. He has previously worked in the private industry as an agronomist and as research officer for DAFWA. He joined WeedSmart in 2016 and has been a regular author of AHRI Insight since 2013. He is based in Geraldton, WA.

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