Australian Herbicide Resistance Initiative (AHRI)

Glyphosate resistance and plant fitness

Heping Han

Things that affect your fitness:
Christmas – go on, have one more serving of trifle, it is only wafer thin!
Television – for example, spending five full days watching the Boxing Day test match cricket.
Each on their own are not too bad, but put them together and we have a problem.
Things that affect plant fitness:
The TIPS mutation that causes high-level glyphosate resistance. The TIPS mutation is a double mutation of the glyphosate target site, the 102 and the 106 mutation. The 106 mutation has been found in a number of species and has no fitness penalty associated with it, but the 102 + 106 mutation comes with a big fitness penalty.
AHRI researcher Heping Han and others with GRDC support recently found a large fitness penalty in glyphosate-resistant crowsfoot grass. We have previously reported on this glyphosate resistance here. At the time we could see by looking at the pictures of the plants that the ones with the homozygous TIPS mutation were severely stunted. The researchers have now further investigated this and found that the homozygous TIPS plants have 50% reduction in seed set and this escalates to 85% in plants that are in competition with a rice crop.
Most of the known glyphosate resistance mechanisms cause fairly low level, 4 to 8-fold, resistance. The TIPS mutation causes huge, 140 to 180-fold resistance with some plants surviving more than 57 L/ha glyphosate. Will this fitness penalty help keep the TIPS mutation at bay? Unfortunately, not. There is a twist in the tail of this story.

The glyphosate-resistant Crowsfoot grass (Eleusine indica) in this study was from a palm nursery in Malaysia and was the topic of Adam Jalaludin’s PhD at AHRI. This Crowsfoot grass is not only resistant to glyphosate, it’s also resistant to paraquat and glufosinate as well. Nasty customer! You can read about this triple knockdown resistance here.

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The mutations 

The table below describes the four different biotypes of crowsfoot grass that were compared in this study. Essentially they studied a susceptible Crowsfoot grass (wild-type) compared to a biotype with just the 106 mutation, another with both the 102 and 106 mutation (RR TIPS), and a cross between the two (Rr TIPS).

What does homozygous/heterozygous mean? – Quick Refresher 

A diploid species (like humans, ryegrass, crowsfoot grass, etc.) has two matching sets of chromosomes. When a particular gene is identical on each chromosome we call it homozygous. When a particular gene (eg. the gene for glyphosate resistance) is only found on one chromosome only, it’s called heterozygous.

A compound heterozygote is where there’s one mutation on one chromosome and a different mutation on the other. In this case, one chromosome has the 106 mutation, and the other has the 102 + 106 (TIPS) mutation.

Fitness

The photo below shows just how unfit the grass with the homozygous TIPS mutation is. These plants have not been treated with any herbicide. The reduced growth of the homozygous TIPS plants is simply as a result of the TIPS mutation. This photo also shows that the single 106 mutation has no effect on plant fitness.

Biomass and seed set 

The data below shows the scale of the effect of the homozygous TIPS mutation on biomass and seed set. These plants were grown in the absence of crop competition, and with no herbicide applied.

Only the homozygous TIPS plants have a fitness penalty – this is the twist in the story 

This is a major bummer! Only the plants with two copies of the TIPS mutation have the fitness penalty. These are the plants with the RR TIPS in the graph below. The Rr TIPS plants have one copy of the TIPS mutation and one copy of the 106 mutation. This is what’s called a compound heterozygote. These plants don’t have the fitness penalty. These Rr plants have high-level glyphosate-resistance and no fitness penalty. This is the important message out of this study.

If all plants with the TIPS mutation had a fitness penalty, then we could potentially eradicate these extremely resistant plants from a population over time due to the big fitness penalty. But unfortunately, we have plants (Rr) that are highly resistant that have no fitness penalty. This would be a nightmare if it happened in ryegrass.

Now throw in some crop competition 

The graphs below show the effect of crop competition when the weeds were sown in competition with rice plants. The crop reduced the growth of all of the crowsfoot grass, but the ones with the homozygous TIPS mutation (black bars) suffered a lot more than the susceptible (grey bars) and plants with the 106 mutation (white bars).

Summary 

Yes, there’s a big fitness penalty for glyphosate-resistant crowsfoot grass with the homozygous TIPS mutation.

BUT

Plants with the heterozygous TIPS mutation are also very glyphosate-resistant, but have no fitness penalty.

If resistant weeds have a fitness penalty we can develop strategies to eradicate these plants as they will struggle to compete with the susceptible weeds and the crop. But unfortunately, not all of our highly resistant weeds have a fitness penalty.

If this TIPS mutation can happen in one weed species it can happen in others. Most of our glyphosate-resistant weeds to date in Australia have fairly low-level resistance. This is likely to change in the future as we select for other resistance mechanisms. Studies like this one will help us understand and prepare for these future resistance problems when they inevitably occur.
Enjoy the Boxing Day test match?, and go easy on the trifle!

Merry Christmas from the AHRI team ??? 

The research team

This paper was a collaboration between AHRI researchers Heping Han, Qin Yu, and Steve Powles, as well as past AHRI researchers Adam Jalaludin and Martin Vila-Aiub. You can view the paper below:

A double EPSPS gene mutation endowing glyphosate resistance shows a remarkably high resistance cost

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