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 The weed that represents the biggest threat to Midwestern corn
and soybean production, waterhemp, has outsmarted almost every
kind of herbicide on the market today.
University of Illinois scientists are working to reveal
waterhemp’s tricks. Through years of research, they discovered
the weed can ramp up production of detoxifying enzymes that
neutralize certain herbicides before they can disrupt essential
cellular processes. Metabolic resistance, as this strategy is
known, is just one process by which waterhemp evades herbicides.
Unfortunately, because there may be hundreds of detoxifying
enzymes involved, metabolic resistance is hard to identify and
even harder to combat.
In two recent studies, Illinois researchers explain metabolic
resistance to three commonly used herbicides in waterhemp,
getting closer to finding important genetic cues. Results also
confirm the importance of using a multi-pronged approach to
waterhemp control.
“These waterhemp populations are adapting and evolving
incredible abilities to metabolize everything. It’s bad news,
but at least we understand the mechanisms better. And
ultimately, that understanding could potentially be exploited to
use waterhemp’s metabolic arsenal against itself,” says Dean
Riechers, professor in the Department of Crop Sciences at
Illinois and co-author on both studies. “That’s one interesting
way our research could be directly applied to controlling this
weed.”
Last year, Illinois researchers documented resistance to Group
15 herbicides in waterhemp. This group of herbicides, including
S-metolachlor, targets very-long-chain fatty acid production in
sensitive plants. The researchers suspected it was also a case
of metabolic resistance, and the Illinois team, led by graduate
student Seth Strom, has now confirmed it in a study published in
Pest Management Science.
“We were the first group in the world to show resistance to
Group 15 herbicides in waterhemp, and now we have identified the
mechanism behind it,” Riechers says. “Again, it's not good news
because it means we're running out of herbicides, and in this
case it involves pre-emergence herbicides.”
The study suggests two classes of detoxifying enzymes, known as
GSTs and P450s, appear to neutralize S-metolachlor in resistant
waterhemp.
Group 15 herbicides can be safely used in corn because the crop
uses GSTs to naturally detoxify the chemicals; in other words,
corn has a natural tolerance to these chemicals. Strom’s
research suggests waterhemp is not only able to mimic corn’s
natural detoxification mechanism, but it evolved an additional
way to avoid being harmed by S-metolachlor.
Honing in on the two classes of detoxifying
enzymes is not the end of the story, however. Because plants
have hundreds of enzymes in each class, the researchers have
more work ahead of them to identify the specific genes that are
activated.
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In a separate study, Riechers and another group of
Illinois scientists revealed more of waterhemp’s metabolic secrets.
“We have known for the last 10 years that whenever we see waterhemp
with resistance to an HPPD inhibitor in the field, such as
mesotrione, it has always shown metabolic atrazine resistance, too.
However, it is possible for waterhemp to be resistant to atrazine
and not mesotrione,” Riechers says.
The apparent association between mesotrione and metabolic atrazine
resistance could be coincidental, but given how often the
resistances co-occur, Riechers thought the genes controlling
resistance for the two chemicals might be shared or linked.
In a study published in Weed Science, graduate student Kip Jacobs
demonstrated an overlap in the genes responsible for metabolic
atrazine and mesotrione resistance. Because researchers already knew
the single gene for metabolic atrazine resistance, the results get
them closer to understanding the genes conferring mesotrione
resistance.
“Whenever we find out whether it's two or three or four genes
involved in mesotrione resistance, our results tell us one of them
should be the metabolic atrazine resistance gene,” Riechers says.
“We know which one that is.”
Unfortunately, even if researchers are able to trace each resistance
trait back to the genetic level, that won’t ensure an easy solution
to the problem. Experts say there are no new herbicide
sites-of-action coming into the marketplace, so farmers will need to
consider alternative methods of weed control.
“With metabolic resistance, our predictability is virtually zero. We
have no idea what these populations are resistant to until we get
them under controlled conditions. It’s just another example of how
we need a more integrated system, rather than relying on chemistry
only. We can still use the chemistry, but have to do something in
addition,” says Aaron Hager, associate professor in the Department
of Crop Sciences at Illinois and co-author on the Pest Management
Science study. “We have to rethink how we manage waterhemp long
term.”
The Pest Management Science [DOI: 10.1002/ps.5868] and Weed Science
[DOI: 10.1017/wsc.2020.31] studies are available online or by
request. Funding was provided by Syngenta Crop Protection and the
Weed Science Society of America. The Department of Crop Sciences is
in the College of Agricultural, Consumer and Environmental Sciences
at the University of Illinois.
[Source: Dean Riechers, News writer:
Lauren Quinn] |
