Battling the tenacious weeds

Evolution is weeds’ best friend. Whenever a new herbicide is introduced, weeds mutate to adapt accordingly and thrive. Now, a group of scientists reckon that acknowledging and investigating their survival skills could throw up some clues on how to control them, writes Carl Zimmer

Depending on your point of view, barnyardgrass is a nightmare or a marvel. 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 per cent. Many barnyardgrass plants are resistant to glyphosate, an herbicide that many farmers currently rely on. 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 per cent 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, Caicedo and her colleagues hope to find some new clues on 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 has 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 . “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. 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 fertilised 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,” 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. 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 an international survey. Weeds became resistant through evolution. Compared with 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 but Glyphosate-resistance is now rampant. Twenty-four species of weeds have evolved it, and they are expanding their range around the world. Some researchers have argued that weeds could be foiled by applying two herbicides at once. But, in the journal Trends in Genetics, a team of scientists present another reason to worry about new crops: weeds can become resistant to more than one herbicide at once.

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,” Ellstrand said. “And we’re just getting to it now.”