It’s a story suited for a Hollywood horror film, yet it’s also a
tenet of evolutionary biology.
Introduce a toxin to a system, and you
inevitably select for resistant survivors. These few individuals gain a
reproductive advantage and multiply; sometimes they can’t be stopped
with even the most potent chemicals.
For years, this general plot line made headlines in the fields of
antibiotic resistance and cancer research. More recently, plants have
become a common protagonist. Weeds around the world are developing
resistance to glyphosate—one of the most common herbicides on the
market—and like bacteria and tumor cells, many plants can also withstand
multiple other toxins, each with unique molecular targets.
In January, a hair-raising infestation of the kochia
shrub was confirmed in Alberta, Canada. Originally introduced to desert
climates as forage for cattle, the tenacious weed can now survive
glyphosate, which targets an enzyme involved in the biosynthesis of
aromatic compounds. It can also withstand chemicals that inhibit the ALS
enzyme, involved in the production of amino acids.
At least 2,000 acres
are now impacted, and “we expect more cases will be confirmed after a
field survey this fall,” says Hugh Beckie of Agriculture and Agri-Food
Canada, the government department
that manages farming policies.
The United States are also being taken by storm. Palmer amaranth
recently developed resistance to the same two classes of chemicals in
Tennessee. Since 2009, the tall, spindly weed has swept across 1 million acres of cropland, causing some farmers to abandon their fields. And in California, a plant named hairy fleabane recently crept into vineyards.
It is now able to withstand both glyphosate and Paraquat
that hijacks photons from proteins involved in photosynthesis.
Worldwide, 23 weed species
have developed glyphosate resistance, and at least 10 of these have
also developed resistance to other herbicides, according to the
International Survey of Herbicide Resistant Weeds.
And Bill Freese, of
the Center for Food Safety
in Washington, DC, believes these numbers underestimate the problem. In
order for a weed to be listed as resistant, it must survive four times
the concentration used to kill susceptible plants. “Some weeds tolerate
lower levels of glyphosate, and these also have a big impact in the
field,” he says.
Weed infestations are more of a nuisance than a monstrosity—but they
are biting into farmer’s pocketbooks. In Alabama, 61 percent of soybean
fields are infested with glyphosate-resistant Palmer amaranth, costing
farmers $71 million a year in lost yields, and 80 percent of the state’s
cotton is also infested, with losses now totaling $10.9 million.
Selecting for super-weeds
It’s not uncommon for bacteria to multiply every 20 minutes, but
plants have a much longer life cycle and thus a slower rate of
evolution. This makes herbicide resistance deceptively improbable.
Kochia scoparia Wikimedia Commons, WildBoar
The chance that a single mutation will confer herbicide resistance is
1 in 100,000, making the likelihood of a double resistant mutant less than 1 in 10 billion
Early industry-sponsored research suggested resistance to glyphosate
was particularly unlikely because large mutations in the herbicide’s
target, the EPSPS enzyme, would render it dysfunctional, killing the
plant before it could reproduce.
“The claims made were naïve, and resistant weeds have indeed developed,” says David Mortensen
a weed scientist at Pennsylvania State University. “When a chemical is
applied to such a wide area—to nearly all soybean and cotton, and a big
percentage of corn—the selection pressure is too intense.”
Indeed, glyphosate use has increased dramatically, from the 4 million
or so pounds that were applied to corn in 2000 to 65 million pounds
last year, with use on cotton and soy fields also climbing. Much of this
increase can be attributed to the incorporation of genetically
engineered crops that are unaffected by glyphosate,
which is sold by St.
Louis-based Monsanto under the brand name Roundup. To help farmers
spray glyphosate directly over fields without harming crops, Monsanto
released Roundup Ready soy and canola in 1996. Genetically engineered
cotton and corn soon followed, and by 2001, the GE crops spanned
millions of acres. This is when resistant weeds made their debut.
“Glyphosate has been around since the 1970s, but resistant weeds
didn’t become a serious problem until the herbicide was packaged with
genetically engineered crops,” says Mortensen.
Another strategy—used by plants and bacteria, alike—is to overexpress
targeted enzymes, so that some can still function properly even while
others are destroyed by the chemical. Glyphosate-resistant waterhemp,
Palmer amaranth, kochia, and Italian ryegrass, for example, all
overexpress the EPSPS gene that the herbicide targets.
Yet instead of
simply upregulating gene expression, glyphosate-resistant plants make multiple copies
of the EPSPS gene. “This is very hard to do—it’s much less likely than a point mutation,” says Tranel.
The rise of the resistant
If the situation wasn’t bad enough already, it appears to be
snowballing. Weeds in nine different countries have independently
developed resistance to multiple modes of action. Some stubborn
survivors can now survive most of the chemicals used by farmers, and the
infestations are spreading.
Last year, for example, farmers in Iowa reported infestations of
waterhemp in their corn and soy fields. The weed has now encroached on
500 acres, and continues to survive treatments of glyphosate and six
additional chemicals. The case is a rare example of a weed developing
resistance to three chemical classes, each with a unique molecular
target. Even more impressive, a biotype of Rigid Ryegrass growing in
Victoria, Australia, is now resistant to four chemical classes.
about 10 acres are impacted so far, but the weeds are predicted to
Despite the seemingly small odds of a plant evolving resistance to
multiple herbicides, the dramatic increase in glyphosate-resistant
weeds, which now infest more than 17 million acres nationwide, has made
this possibility exponentially more likely.
“We don’t need a single
plant to undergo two unlikely adaptations—we just need one event to
happen in a biotype that already has glyphosate resistance,” says
The next wave of genetically modified seeds will be the ultimate
experiment, says Freese. Monsanto is developing crops that can be
simultaneously sprayed with glyphosate and the herbicide dicamba. Dow
AgroSciences plans to market genetically engineered corn seeds resistant
to both glyphosate and the herbicide 2,4-D
; the US Department of
Agriculture is reviewing the technology this summer. “Just as we had an
increase in glyphosate [use] after Roundup Ready crops were released, we
may soon see a huge increase in 2,4-D,” Freese says. “In our view, this
will also lead to weeds with multiple resistance to 2,4-D and
Correction (May 24): The odds of a plant developing double
resistance to the chemicals mentioned in this story are more like 1 in
10 billion. The original version stated “less than 1 in a trillion,”
which reflects a less conservative number.
The Scientist regrets the error.