Written by: Peter Newman
Just when we think we know who the Prime Minister is in Australia you blink your eyes and we have a new one! At the time of writing, Scott Morrison is Prime Minister but this could change by the time you receive this. Just when we thought we understood the mechanism of trifluralin resistance we blink and find another. Earlier in the year, we reported on research by AHRI PhD student Jinyi Chen confirming that a target site mutation that confers resistance to trifluralin. This research is funded by GRDC, and PhD student Jinyi Chen is financially supported by UWA and China Scholarship Council.
Now Jinyi has confirmed that metabolic resistance to trifluralin is also possible. She studied three trifluralin resistant populations of ryegrass and found that one population had both target site and metabolic resistance mechanisms, sometimes in the same individual plant. The other two populations had metabolic resistance only.
The resistant populations could take up and translocate trifluralin just like susceptible ryegrass, but once the trifluralin was in the plant it was metabolised quickly. The exact metabolic resistance mechanism is still to be confirmed, and P450 enzymes are the prime suspect. This will be the subject of ongoing research.
There is never a single Australian Prime Minister for a full term these days and there is never a single resistance mechanism.
Metabolic resistance to herbicides appears to be an ongoing theme amongst resistant Australian weeds these days. Our recent AHRI insight about multiple cross-resistance to pre-emergent herbicides is a testimony to this.
If only target site resistance was at play, mixing and rotating herbicides would go a long way to solving our herbicide resistance problems because cross-resistance across herbicide groups would not exist. But metabolic resistance has the ability to work across herbicide groups and a weed may be resistant to a herbicide before it has even been sprayed with that herbicide. We need to understand what resistance mechanisms are at play so we can work out the best strategies to prolong the life of our herbicides.
If only metabolic resistance was at play we may be able to figure out a way to switch this resistance off using a P450 inhibitor. But unfortunately, more often than not, we find multiple resistance mechanisms to a single herbicide.
No target site resistance detected
The first step in this study was to determine if the known target site resistance mutations in the α-tubulin (e.g. mutations at 202, 239, 243 etc.) were present. The researchers found a 243 target site resistance in one resistant population but two other resistant populations did not have target site mutations, hence metabolic resistance was suspected.
No difference in translocation
The researchers then embarked on a process of elimination. They first used radiolabelled 14C-trifluralin to determine if there was any difference in root uptake of the herbicide between resistant and susceptible populations. The image below shows that trifluralin was translocated equally between the resistant and susceptible populations so they knew that this was not the mechanism.
Enhanced Metabolism is involved
The scientific methodology is pretty tricky to describe here. Given the volatile nature of trifluralin, Jinyi had to overcome significant technical hurdles to study this resistance mechanism. The graph below is the data from this part of the study showing that more trifluralin was metabolised (broken down) in the resistant populations than the susceptible ones. I won’t begin to try to explain this part of the study, and to be perfectly honest I don’t understand it, but we will take the researchers word for it that the resistant plants could metabolise the herbicide before it could reach the target site.
P450s are the prime suspect
Another tricky bit of research by AHRI researcher Roberto Busi gave us a clue that P450s are involved. P450s are a superfamily of enzymes that have many functions in plants including metabolising various molecules. Roberto treated ryegrass with an insecticide called phorate that is known to inhibit P450s and this reversed the metabolic trifluralin resistance. If metabolic trifluralin resistance was the only mechanism we may be able to use this knowledge to work out a way to reverse resistance in the field, but alas, we have target site resistance as well. Bummer!
This will be the subject of future research to determine if P450s are in fact the culprit.
There’s rarely a single resistance mechanism to a particular herbicide and this research confirms that there are multiple mechanisms for trifluralin resistance. If there was a single mechanism we may be able to work out a way around it, but unfortunately nature bats last and evolution is powerful and diverse.
Jinyi Chen, Danica Goggin, Heping Han, Roberto Busi, Qin Yu and Stephen Powles, all from AHRI.