Waterhemp’s built-in detox system complicates control

Waterhemp, a troublesome, hard-to-kill weed, is relatively new to Canada and a major concern for corn and soybean farmers.

It first appeared in Ontario in 2002, spread to Quebec and, in 2016, showed up in a soybean field southeast of Winnipeg.

Waterhemp resists multiple herbicides, adapted aggressively to modern farming, can adapt to different environmental conditions and has rewritten its own script on detoxification mechanisms.

Researchers at the University of Illinois have now shown waterhemp’s true capabilities. It can resist not only herbicides in use but those that have not yet been commercialized.

According to a news release, syncarpic acid-3 (SA3) is the “great-great-grandfather”of the HPPD-inhibiting herbicide Callisto that suppresses annual broadleaf weeds.

“HPPD is an enzyme involved in making vitamins A and E, among others,” said Dean Riechers, professor in the crop sciences department at the university.

“Vitamins A and E are antioxidants, so they are like sunscreens for plants and their green chlorophyll is not damaged. The HPPD inhibitor herbicides knock out HPPD, including vitamins A and E. This is how herbicides work (against weeds).

“The treated plants turn white because their chlorophyll is destroyed without the vitamins’ protection. The bad news is that these herbicides do not work anymore in resistant waterhemp populations, so growers are running out of effective options.”

Riechers said corn can tolerate Callisto and other herbicides because it has a robust detoxification system to neutralize and cordon off the harmful chemical. But corn’s neutralizing system does not work on SA3, the forerunner of Callisto, which is why the chemical has never been used on the crop.

Yet HPPD-resistant waterhemp, with a detoxification system that mimics that of corn, has shown it can also detoxify SA3.

“This is probably the first known example where waterhemp has evolved a detox mechanism that a crop doesn’t have,” said Riechers. “It’s using a completely different mechanism, adding to the complexity of controlling this weed.”

The discovery means the weed could theoretically be resistant to new herbicide products even before they hit the shelves.

“We’ve always known metabolic resistance is dangerous because it could confer resistance to a yet-to-be-discovered herbicide,” he said. “We’ve just shown that this is a reality.”

Riechers and his team traced the biochemical reactions inside resistant waterhemp plants when treated with SA3. According to the media release, detoxification of herbicides and other toxic compounds usually happens in distinct phases. The first involves enzymes known as p450s that remove electrons from the toxic compounds, making them less reactive inside plant cells.

But in resistant waterhemp, the opposite happened. Electrons were added to SA3 molecules. In phase two, enzymes known as GSTs are not activated for Callisto because p450s get the job done efficiently for corn. But for SA3, GSTs were activated.

“Along with the removal of a water molecule in the first phase, the addition of those electrons prepared the phase-two GST enzymes to detoxify SA3,” said Crystal Concepcion, postdoctoral research associate in Riechers’ lab.

“It’s surprising because not only did the phase-one reactions not proceed as expected, we did not anticipate GSTs to be involved for this class of herbicides. This is very rare.”

Riechers said this deviation from standard biochemical detoxification patterns is something truly novel and it is challenging and potentially damaging for farmers.

Corn, soybean and sorghum all use GSTs to metabolize S-metolachlor, a soil-applied herbicide that provides residual weed control. The assumption was the waterhemp used the same enzyme mechanism to detoxify the herbicide. Instead, it relied on p450s and, in fact, the enzyme was 20 times greater in resistant waterhemp than in corn.

Figuring out resistant waterhemp’s game plan has been an ongoing challenge. Research has reinforced the need to rely more on non-chemical control methods so weeds don’t get a chance to go to seed.

“Companies don’t want to develop chemistries and invest 10 to 15 years developing a new herbicide, patent and release it, only to find that waterhemp can figure it out,” said Riechers.

“Plants do have a general way of detoxifying chemicals, some better than others. But now we almost have to ignore what chemistries corn can detoxify. It’s almost a given that waterhemp can follow suit.

“We have to get really creative and go way above and beyond what corn can do because waterhemp has already shown it can go above corn.”

While waterhemp and its cousin palmer amaranth have earned notoriety as broadleaf weeds with an ability to detoxify just about everything, other weeds around the world have similar stubborn herbicide resistance. Annual ryegrass in Western Australia has developed resistance to herbicide groups and, in Europe, blackgrass is one of the most herbicide-resistant weeds in at least 10 countries.

“These are a big headache for farmers,” said Riechers. “We are trying to find out what these plants can do to figure out a way to get around it.”

Part of that approach, he said, is to employ cover cropping, intercropping, tilling and zero-till.

“If winter wheat is planted and harvested in early summer followed by a summer crop, then waterhemp won’t have a chance to go to seed. While winter wheat may not be as profitable, if you plant it every few years you can knock the waterhemp down.

“You’ve got to knock it down and stay ahead of it. It’s a way of being proactive and diversifying farming practices.”

The value of diversifying crop production may become clearer as climate changes, Riechers added.

“Weeds will adapt to weather and waterhemp has adapted very well to weather changes,” he said. “They are inherently diverse because, as male and female plants, they have to outcross and that creates great genetic variability.

“When you plant soybeans, it’s a nice uniform crop with every plant basically the same, all maturing at the same time. Waterhemp is invading and taking advantage of what we are doing to help the crop, basically. But if we rely on herbicides too much, then they can clearly show they can fight back and win.”

Now Riechers and his team are back at the drawing board.

“We have to think of something different that maybe even corn or soybean can’t do at this point. We need to find something that will not be so easily overcome by the weed.

“The next generation of herbicides cannot be volatile, cannot hurt the groundwater, threaten animals and must be inexpensive. We have a limited tool kit right now and the last thing we want to do is waste it and not be able to use anything.”

The research was recently published in the journal New Phytologist.