September 30, 2024
Do you remember that Simpsons episode where Homer gets a job promoting his local bowling alley and he goes from reading “Advanced Marketing” to “Beginning Marketing” to looking up “marketing” in the dictionary?
Sometimes we feel that way with herbicide resistance research because there’s never one mechanism of resistance. Just when we think we understand something, it changes.
Doh!
Sometimes a weed will have one mechanism of resistance to a particular herbicide, and sometimes it will have several.
A few years back, AHRI researcher, Dr. Danica Goggin, discovered that a population of ryegrass that was resistant to Sakura® (pyroxasulfone) had the GST mechanism.
Now we have quite a few populations of ryegrass that are resistant to Sakura®, and we are trying to work out if they all have the same mechanism of resistance or if there are multiple mechanisms at work.
Danica tested 10 populations of ryegrass with Sakura® resistance and found that while GST is the main mechanism, there are probably some other things at play. She also confirmed that GST’s are a major contributor to how wheat tolerates Sakura®.
What is this GST resistance?
GST stands for Glutathione-S-transferase. The clue is in the name.
Essentially, these enzymes join a large, glutathione molecule (a mini protein) onto the herbicide, then the molecule gets chopped in half. The herbicide molecule now doesn’t look like a herbicide anymore, which stops it working.
Roberto Busi went on to set up a herbicide resistance testing service at UWA and quickly found a number of Sakura resistant ryegrass populations. Sakura resistance is still relatively rare, but it’s getting easier to find.
From this testing service, Danica took ten populations and tested them to see if GST’s are the main culprit, or if there are other things at play. She also measured wheat.
Wheat survives Sakura with GST’s
In the study, Danica measured the GST activity in the above ground and below ground tissues of wheat and ryegrass. Sakura works on the roots, so it will come as little surprise to learn that she didn’t see much GST activity towards Sakura in the shoots. But in the roots, 7-day old wheat seedlings had 7 to 25 fold GST activity compared to ryegrass.
GST’s in ryegrass
GST activity in resistant ryegrass was higher than the susceptible populations, but only about 2-fold higher.
What about other mechanisms?
Danica and the team measured every other resistance mechanism they could dream up, and found that there may be something else at play involving our good friend P450, but a Random Forest analysis led them to conclude that GST activity is the main mechanism.
What is a random forest analysis?
It sounds like you would travel along a road and select a forest at random, then analyse it. But it’s not as exciting as that.
It actually involves machine-learning that takes all the different things you’ve measured in a bunch of samples and tries to use this to put the samples into the groups that you’ve specified (in this case, susceptible and resistant roots and shoots). It takes out one measurement each time to see how it affects the groupings, and this allows you to determine which of the measurements makes the greatest contribution to the groupings (i.e. in this case, which measurement is most relevant to resistance). It generates a huge number of phylogenetic “trees”, which is where the forest comes from.
Summary
There you have it. Another example of a weed mimicking how a crop plant tolerates a herbicide. As with all of this research, it is likely that other mechanisms will show up in the future, but for now we have a main culprit, which we actually understand pretty well. This knowledge will help us develop strategies to slow down the evolution of resistance to this herbicide, perhaps by mixing and rotating with herbicides that succumb to different resistance mechanisms.
Read the paper here.
Posted in: AHRI Insight, Herbicide resistance mechanisms