lunes, junio 04, 2012

The revenge of weeds

Revenge of the Weeds

Plant pests are evolving to outsmart common herbicides, costing farmers crops and money.

By Amy Coombs | May 20, 2012

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—a chemical 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.


Conyza bonariensis (horseweed) seed heads and flowersWikimedia Commons, Rickjpelleg


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. Only about 10 acres are impacted so far, but the weeds are predicted to spread.

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 Mortensen.

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 glyphosate. ”

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.

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