Weed goes off script to resist herbicides

Date:
October 5, 2021
Source:
University of Illinois College of Agricultural, Consumer and
Environmental Sciences
Summary:
Cementing waterhemp's reputation as a hard-to-kill weed in corn
and soybean production systems, researchers have now documented
the weed deviating from standard detoxification strategies to
resist an herbicide that has never been commercialized.



FULL STORY ========================================================================== Cementing waterhemp's reputation as a hard-to-kill weed in corn and
soybean production systems, University of Illinois researchers have now documented the weed deviating from standard detoxification strategies
to resist an herbicide that has never been commercialized.


==========================================================================
The chemical in question, syncarpic acid-3 (SA3), is the great-great grandfather of the HPPD-inhibiting herbicide Callisto. SA3 has never been
used in corn because it has the rather unfortunate effect of killing the
crop along with the weeds. 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 systems don't work on SA3.

Weeds like waterhemp typically evolve detoxification systems that
mimic corn's.

That's why it's especially surprising that HPPD-resistant waterhemp can detoxify SA3.

"This is probably the first known example where waterhemp has evolved
a detox mechanism that a crop doesn't have. It's using a completely
different mechanism, adding to the complexity of controlling this weed,"
says Dean Riechers, professor in the Department of Crop Sciences at U
of I and co-author on a new study in New Phytologist.

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

"We've always known metabolic resistance is dangerous because it could
confer resistance to a yet-to-be-discovered herbicide. We've just shown
that this is a reality," Riechers says. "Companies don't want to invest
10-15 years in developing a new herbicide, patent and release it, and
find it doesn't work on day one. Our research reinforces that we need
to rely more on non-chemical control methods and make sure weeds don't
go to seed." Riechers and postdoctoral associate Crystal Concepcion
traced the biochemical reactions inside resistant waterhemp plants when
treated with SA3.



========================================================================== Detoxification of herbicides and other toxic compounds usually happens
in distinct phases. The first involves a group of enzymes known as p450s
that remove electrons from toxic compounds, making them less reactive
inside plant cells. But in resistant waterhemp, the opposite happened: electrons were added to SA3 molecules.

Phase-two enzymes known as GSTs are normally not activated for Callisto
because p450s get the job done so quickly and efficiently in corn. But
for SA3, GSTs did the heavy lifting of detoxification.

"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," Concepcion says. "It's surprising because not only did the phase-one reactions not proceed as expected, we didn't even anticipate GSTs to
be involved for this class of herbicides. We don't see corn preparing
chemicals for attack by GSTs. This is very, very rare for herbicides."
Riechers says this deviation from standard biochemical detoxification
patterns represents something truly novel and potentially damaging for
crop producers.

"It's definitely challenging," he says.

The research group is on a roll with unexpected findings.



========================================================================== Scientists have known for years that corn, soybeans, and sorghum use GSTs
to metabolize S-metolachlor, a soil-applied herbicide offering residual
weed control. Therefore, they assumed waterhemp used the same mechanism
to detoxify the chemical. But in a recent paper, published in Plant and
Cell Physiology, Riechers' research team documented another example of waterhemp going off script.

"In this case, we were thinking it was GSTs all the way. But the data
told us otherwise. The metabolomics approach we took informed us that
GSTs aren't the main mechanism to detoxify S-metolachlor in resistant waterhemp. It's actually p450s," Riechers says.

Last year, Riechers worked with former doctoral student Seth Strom,
extension weed scientist and crop sciences professor Aaron Hager, and
others to show waterhemp employs both p450s and GSTs in detoxifying
Group 15 herbicides. But when they dug deeper in the new Plant and
Cell Physiology study, the researchers found GST enzyme activity was
detectable in both resistant and sensitive waterhemp but much lower than
in corn. In contrast, p450 activity in resistant waterhemp was 20 times
greater than in the crop and in sensitive waterhemp.

"Studying resistance to soil-applied herbicides like S-metolachlor
can be challenging, especially in waterhemp where there were not any
templates or previous methods to follow. Developing methods to understand S-metolachlor resistance was worth every minute knowing that results
could eventually help provide solutions for growers," says Strom, now
a field R&D scientist at Syngenta Crop Protection.

Both studies demonstrate that waterhemp is done relying on corn
for detoxification cues, and is evolving its own ways of conquering
herbicides.

========================================================================== Story Source: Materials provided by University_of_Illinois_College_of_Agricultural,_Consumer and_Environmental_Sciences. Original written by Lauren Quinn. Note:
Content may be edited for style and length.


========================================================================== Journal References:
1. Jeanaflor Crystal T. Concepcion, Shiv S. Kaundun, James A. Morris,
Sarah‐Jane Hutchings, Seth A. Strom, Anatoli V. Lygin, Dean E.

Riechers. Resistance to a nonselective 4‐hydroxyphenylpyruvate
dioxygenase‐inhibiting herbicide via novel
reduction-dehydration- glutathione conjugation in Amaranthus
tuberculatus. New Phytologist, 2021; DOI: 10.1111/nph.17708
2. Seth A Strom, Aaron G Hager, Jeanaflor Crystal T Concepcion,
Nicholas J
Seiter, Adam S Davis, James A Morris, Shiv S Kaundun, Dean
E Riechers.

Metabolic Pathways for S-Metolachlor Detoxification Differ Between
Tolerant Corn and Multiple-Resistant Waterhemp. Plant and Cell
Physiology, 2021; DOI: 10.1093/pcp/pcab132 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211005175352.htm

--- up 4 weeks, 5 days, 8 hours, 25 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)