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 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.”
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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.
The New Phytologist article is available at https://doi.org/10.1111/nph.17708.
The Plant and Cell Physiology article is available at https://doi.org/10.1093/pcp/pcab132.
Both projects were funded in part by Syngenta.
The Department of Crop Sciences is in the College of Agricultural,
Consumer and Environmental Sciences at the University of Illinois at
Urbana Champaign.
[Source: Dean Riechers
News writer: Lauren Quinn] |
