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Looking for Ways to Beat the Weeds
New York Times, July 16, 2013

Depending on your point of view, barnyardgrass is a nightmare or a marvel.

A cotton field was sprayed with residual herbicides before planting to counter glyphosate-resistant weeds.

That’s because it’s a supremely triumphant weed. Barnyardgrass can swoop in to fields and outcompete planted crops. It is particularly devastating on rice farms, where losses sometimes reach 100 percent. It has evolved resistance to a number of herbicides that farmers rely on to control weeds. Even when farmers think they have rid a field of barnyardgrass, they may not have actually won the battle. Each weed can produce up to a million seeds, which nestle into the soil, waiting for a chance to regrow.

Barnyardgrass is but one of many kinds of weeds found around the world. All told, they result in a 10 percent reduction in the productivity of crops. In the United States alone, they cause an estimated $33 billion in losses each year. Herbicides can reduce the toll, but within a few years of the introduction of a new chemical, weeds evolve resistance to it.

For decades, farmers have responded to resistant weeds by turning to a new herbicide. But a number of scientists argue that we need to get off this treadmill. They argue that we can find more effective ways to fight weeds by appreciating how well they’ve done at our expense.

“They’re amazingly successful plants. They’ve evolved to take advantage of us,” said Ana L. Caicedo of the University of Massachusetts, who recently published a review of weed evolution in the journal Heredity. By according weeds the respect they deserve, Dr. Caicedo and her colleagues hope to find some new clues to how to control them.

Barnyardgrass, for example, has changed dramatically from its non-weed ancestors. They originally grew on dry land, for example, and were thus poorly suited to the flooded fields where rice grew. The weeds have evolved a tolerance to waterlogged soils.

Barnyardgrass has also evolved into a master deceiver. It is known as a crop mimic because it has evolved to look just like rice. The base of the leaves have turned from pink to green, for example, and the leaves have become narrower. Blending into a rice field, the plants escape the notice of farmers trying to weed them out.

“The biotech folks would have no clue about how to make one plant look like another plant,” said R. Ford Denison, an evolutionary biologist at the University of Minnesota and the author of “Darwinian Agriculture” (Princeton University Press, 2012). “And yet a thousand years of selection on a small patch of the earth was enough to give it crop mimicry — and flood tolerance.”

This remarkable kind of transformation has been going on since the dawn of civilization. “They’ve been evolving alongside us for thousands of years, since we started agriculture,” said Paul Neve of the University of Warwick in England. Once humans began to farm, they created a new habitat that some plants were able to successfully invade.

Scientists have documented three different ways that plants evolve into weeds. Many species, such as barnyardgrass, evolved from wild ancestors. Biologists have found that certain traits make it easier for wild species to become weeds. They already grow fast, for example, and make lots of seeds.

Parasitic plants are especially well-suited to the weedy life. They wrap around other plants and send their roots into their hosts’ tissues. Rather than making their own food, parasitic plants steal nutrients from their hosts. The parasitic weeds that invade farm fields have not evolved major differences from the ones that attack wild plants.

In other cases, weeds evolved from the union of wild plants and crops. In the 1970s, for example, wild beets in Europe released pollen that fertilized sugar beets growing on farms miles away.

Crops can even turn into weeds. “We domesticated a plant from the wild, and somehow it de-domesticated itself — which I think is pretty exciting,” Dr. Caicedo said.

Among these crops gone wild is a weed known as red rice. A key step in the domestication of rice was breeding plants that held onto their seeds when farmers harvested them. Red rice evolved fragile seeds that broke off and fell to the ground.

The name red rice comes from the russet tinge that the plant evolves as it becomes a weed. Dr. Caicedo and her colleagues suspect the color is produced from a pigment that helps the seeds go dormant — a trait that’s good for a weed but bad for a crop.

“If you’re a farmer, you want your seed to start growing when you plant it,” Dr. Caicedo explained. When weeds produce seeds, on the other hand, some sprout quickly while others go into suspended animation. Those dormant weeds create a seed bank that can sprout later, when conditions may be better for them. “It’s a fantastic trait for a weed to have. You’re hedging your bets,” Dr. Caicedo said.

These de-domesticated weeds don’t simply go back in time to regain the same DNA as their wild ancestors, scientists are finding. Instead, they have acquired new mutations to different genes. “You’ve got a new bag of genetic tricks,” said Norman Ellstrand of the University of California, Riverside.

Once plants become weeds, they keep evolving. New mutations allow some of them to have more offspring than others. Foxtail, for example, evolved to crawl along the ground, where it wouldn’t be destroyed by combine blades.

The past century brought a slew of weed-killing chemicals. They helped boost agricultural productivity, although they also caused environmental damage. And it didn’t take long for them to become less effective at killing weeds. Farmers responded by increasing their dose, but the weeds became even more resistant. Eventually, they had to abandon the old herbicides and turn to new ones. Today 217 species of weeds are resistant to at least one herbicide, according to the International Survey of Herbicide Resistant Weeds.

Weeds became resistant through evolution. Compared with complex traits like dormancy and mimicry, resistance can be quite simple to evolve. In some cases, a weed needs just one mutation to blunt the effect of herbicides.

In the 1970s, there was great hope for a new herbicide called glyphosate (sold by Monsanto as Roundup). Early studies revealed no resistance evolving in weeds, raising hopes that, at last, farmers had escaped from evolution. In the 1980s, Monsanto increased glyphosate’s popularity by introducing genetically modified crops carrying a gene that gave them resistance to the herbicide. Instead of using several different herbicides, many farmers could now use just one.

Some critics predicted that interbreeding between genetically modified crops and wild plants would create “superweeds” — hybrid plants carrying the resistance gene. So far, though, only a few cases have been documented.

And yet the weeds have become resistant to glyphosate anyway. They did so the old-fashioned way: through evolution.

“It’s easy to say, ‘We’ve used it for years and it never developed resistance,” Dr. Ellstrand said. He argues that the reason was that farmers applied glyphosate to relatively little farmland. As they applied it to more and more acreage, they raised the evolutionary reward for mutations that allowed weeds to resist glyphosate. “That ups the selection pressure tremendously,” he said.

Glyphosate-resistance is now rampant. Twenty-four species of weeds have evolved it, and they are expanding their range around the world. Earlier this year, the agricultural consulting firm Stratus reported that half of American farms had glyphosate-resistant weeds in 2012, up from 34 percent the year before.

Monsanto has developed crops that can resist an older herbicide called dicamba. Dow, meanwhile, has developed crops resistant to a different herbicide called 2,4D. The environmental impact of the products is now being evaluated by the Agriculture Department.

Some researchers have argued that weeds could be foiled by combining two resistance genes in one plant, so that farmers could apply two herbicides at once. The odds of a weed having resistance to both chemicals would be tiny.

David Mortensen, a weed biologist at Penn State, rejects these claims. He notes that some weeds are already resistant to dicamba, and others to 2,4D. In the journal Trends in Genetics, a team of French and American weed scientists present another reason to worry about these new crops: weeds can become resistant to more than one herbicide at once. Spraying with one chemical can drive the evolution of an all-purpose stress response system, which can defend the weed against other chemicals.

If farmers plant a large area with the new crops, Dr. Mortensen predicted, they will drive the evolution of a new generation of resistant weeds.

“I am quite certain that this is a short-lived solution,” he said.

Dr. Mortensen and his colleagues are investigating controlling weeds by planting crops like winter rye that can kill weeds by blocking sunlight and releasing toxins. “You want to spread the selection pressure across a number of things that you’re doing, so that the selection pressure is not riding on one tactic,” he said.

To some extent, evolution-guided strategies are not new. Scientists have explored them for battling other enemies, like bacteria that evolve resistance to antibiotics.

“We should be looking at this more carefully,” Dr. Ellstrand said. “And we’re just getting to it now.”

Copyright 2013 The New York Times Company. Reproduced with permission.
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