Picture yourself as a city slicker driving to work. Between your house and the freeway there are four sets of traffic lights.
On this particular day all of the traffic lights are stuck on red. The city is in chaos with no-one able to get to work. You then spend the rest of your life, in your car, at a red traffic light.
Perish the thought!
This is how 2,4-D resistance works in wild radish.
The traffic lights are transporters that allow 2,4-D to move from cell to cell, and the freeway is the phloem – the plant veins that move sugars and other products around the plant.
AHRI researcher, Dr Danica Goggin, has spent the last three years studying the 2,4-D resistance mechanism in wild radish. She used radioactive 2,4-D to determine that the herbicide was getting stuck in the treated leaf of resistant plants. The 2,4-D couldn’t get to the growing point to kill the plant.
On the edge of plant cells there are proteins called ABCB transporters that ‘push’ some compounds from one cell to the next. It’s not yet confirmed, but Danica strongly suspects that in resistant plants these transporters (traffic lights) have been modified and no longer move 2,4-D from cell to cell. This means that 2,4-D can’t reach the phloem (freeway), so it stays in the treated leaf where it’s ineffective.
In other words, 2,4-D becomes like a contact herbicide, affecting only the leaves it hits.
The herbicide 2,4-D has been used for almost 70 years and we’ve only just worked out how it actually kills weeds! Given that we didn’t really know how 2,4-D worked, it was impossible to think that we could understand the resistance mechanism.
Fortunately, international research has now defined the mode of action of auxinic herbicides such as 2,4-D (stay tuned for a future edition of AHRI insight), paving the way for Dr Danica Goggin from AHRI (with ARC Linkage funding, partnered with Nufarm) to understand the resistance mechanism.
Danica used radioactive 2,4-D (14C labelled) to measure the movement of 2,4-D. Ten tiny droplets of radioactive 2,4-D were placed on individual leaves of plants from one population known to be susceptible to 2,4-D and two resistant populations – one from Wongan Hills and another from Eneabba in Western Australia. An autoradiograph (photo of radioactive 2,4-D) was taken 24 hours after treating the leaves. The results were amazing.
2,4-D was translocated throughout the susceptible plants in 24 hours (quite amazing to see just how quickly herbicides are translocated!). All of the 2,4-D stayed in the treated leaf of the resistant plants. These leaves curled up in response to the 2,4-D, but the rest of the plant was unaffected.
To kill a weed, 2,4-D must be moved to the growing point of the plant. The growing point of young wild radish is the bit where new leaves emerge.
Why is 2,4-D staying in the treated leaf?
At first vacuole sequestration was suspected.
The vacuole has many jobs in plant cells, one of which is to act as a rubbish bin where toxic compounds can be stored. Some resistant plants have adapted to ‘pump’ herbicides into the vacuole where they are trapped and are ineffective. However, with some tricky science, this option was ruled out. Danica suspects that ABCB auxin transporters could be to blame for 2,4-D resistance, but this is yet to be confirmed.
What are ABCB transporters?
They are proteins that are located on cell membranes. ABCB transporters help move certain compounds across the cell membrane. Some compounds can simply diffuse across the cell membrane but others need help. This is called active transport. The ABCB transporter recognises the compound (in this case 2,4-D) and binds to it, then pushes it across the cell membrane with a “power stroke” energised by a special chemical (called ATP). It’s a bit like an air lock in Star Wars. Door opens, storm troopers move in, door closes behind them, then next door opens.
Why does 2,4-D need an ABCB transporter?
Why can’t it just diffuse across the membrane? The pH inside the cell is about neutral. At this pH, 2,4-D loses a H+ ion and becomes negatively charged. With this negative charge it cannot diffuse across the membrane. This is called the acid trap. So it needs help from a transporter to push it out of the cell.
So 2,4-D in resistant plants can’t get to the phloem (freeway)
In normal, susceptible plants, 2,4-D moves from cell to cell using ABCB transporters. It then moves into the phloem where it’s translocated throughout the plant and can reach the growing point. In resistant plants, it simply can’t get to the phloem. It’s likely that 2,4-D sprayed onto resistant plants only makes it as far as the top layer or two of cells in the leaf.
Is there cross-resistance to other auxinic herbicides?
Yes. One of the 2,4-D resistant wild radish populations in this study also proved to be cross-resistant to other auxinic herbicides including MCPA (LVE Agritone), dicamba (Kamba) and mecoprop (Mecopropylamine). The other population of 2,4-D resistant wild radish was cross-resistant only to MCPA. However, we don’t yet know whether the resistance mechanism for these other herbicides is the same as for 2,4-D.
Are there other resistance mechanisms?
Probably. In this study, Danica identified that there was no difference in leaf uptake of 2,4-D, nor did the resistant plants detoxify the 2,4-D. But there may well be other resistance mechanisms in other populations.
It’s a good thing it took our weeds a long time to become resistant to 2,4-D because we only just worked out how 2,4-D actually works! This research tells us that 2,4-D acts a little like a contact herbicide when sprayed onto 2,4-D resistant wild radish. This research will help us understand how to get the best out of auxinic herbicides in the future and could possibly help scientists develop new auxinic herbicides.
For now though, we can mentally picture 2,4-D stuck at the traffic lights, unable to get to the freeway, and use this image to help us choose the best possible weed management with the current tools that we have.
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