Future crops could make their own pesticides

By Robert Arnason

A U.S. government scientist is studying a natural herbicide released into the soil from the roots of sorghum plants

It’s not rocket science.

It’s probably harder than rocket science.

A team of U.S. Department of Agriculture scientists are trying to design crops that kill weeds by exuding a natural herbicide from their roots.

Scott Baerson, a molecular biologist with the U.S. Department of Agriculture, has identified the genes that allow sorghum to produce a natural herbicide called sorgoleone. Baerson, who works for the USDA in Oxford, Mississippi, has been studying the potential of “pesticide-producing” crops for more than 20 years. | USDA photo

In other words, they’re creating oilseed and cereal crops that can control weeds — on their own.

“It’s a complicated thing, because we’re trying to make plants produce something that is poisonous, and probably even poisonous to them,” said Scott Baerson, a molecular biologist in Oxford, Mississippi. “This is not straightforward project. It’s not going to be pop a gene in and (eureka).”

Baerson works at the Natural Products Utilization Research unit for the USDA, a group of about 15 scientists studying pest management tools that are less toxic and environmentally benign.

Since 2000, Baerson has been studying sorgoleone, a natural herbicide released into the soil from the roots of sorghum plants. The chemical suppresses nearby plants, helping sorghum out-compete its rivals.

Such a phenomenon is common in the world of plants.

There’s even a scientific word for it: allelopathy.

“Some plants affect the development of neighbouring plants by releasing secondary metabolites into their environment. This… is a potential tool for weed management within the framework of sustainable agriculture,” says a 2018 paper published in nature.com.

Barley, for instance, produces allelochemicals to stifle weeds and other plants. Certain barley cultivars release a higher volume of weed-fighting chemicals. Plant breeders could potentially develop varieties that are more allelopathic and the barley could then be seeded as a cover crop for weed control.

Baerson became interested in sorgoleone more than 20 years ago because it controls a large number of species, from broadleaf to grassy weeds.

The plan was straightforward:

- Step one: Study the sorghum genes and enzymes involved in exuding sorgoleone.
- Step two: Having identified the genetic package, move it into a test crop.
- Step three: Develop corn, soybean or wheat cultivars that produce their own herbicide and have all the other desirable traits, like big yields.

The process sounds simple but it’s highly complex. Multiple genes and enzymes play a role in releasing sorgoleone into the soil.

“This biochemical process only occurs at the root hair cells, which hasn’t made it any easier to work on,” Baerson said.

After years of research, the USDA scientists identified the genes and enzymes that trigger and control the release of sorgoleone. By 2018, they had transferred the genetic package into rice.

“The rice plants that produce sorgoleone should require less herbicides to control weeds,” the USDA Agricultural Research Service said in news release. “Growers (could) spend less on buying and applying chemicals — a major part of their overhead cost.”

Baerson and his colleagues are still testing the genetically modified rice plants to determine if they have the same weed-fighting ability as sorghum.

“(We’re) working with seedlings, seeing if they are actually producing (sorgoleone),” he said.

“I wish I had more exciting updates on it…. But we don’t know.”

Baerson is also collaborating with University of Wisconsin plant scientists to move the sorghum genetics into corn, soybeans and wheat.

He’s studied sorgoleone for more than two decades but Baerson is still seeking an answer to a key question: how can sorghum release a weed-killer into the soil but be immune to that herbicide?

“We’re investigating the (way) that sorghum plants avoid what’s called auto-toxicity…. That’s a big complication.”

One possibility is that sorghum behaves like a fisherman with a leaky boat. The fisherman bails faster than water flows into the boat, so he doesn’t sink.

“That’s actually how a lot of fungi survive… from the (toxic) compounds that they’re producing,” he said. “They’re secreting it and it wants to come back in. But it’s a one-way highway, moving it out faster than it comes in.”

Baerson and his fellow scientists are making headway but the marathon is far from over. A commercial crop that produces its own herbicide is years away.

There’s no chance it will happen in the next five years, Baerson said.

In the next decade? Maybe not.

Baerson hopes it happens before he retires.

“I know at the minimum, we’ll have all the tools together… for people to succeed at this kind of work in the future…. Like medical science, we just keep building one brick at a time and eventually we reach these goals.”

If they’re able to pull it off, the technology could be a game changer.

It won’t eliminate the need for herbicides, but it should “significantly reduce the amount” applied to cropland, Baerson said.

He compared the technology to self-driving cars.

Manufacturers may have sorted out the basics of autonomous cars, but it’s not the end of the story. They will continue to refine and improve the technology in the next 10 to 20 years.

Similarly, creating a pesticide-producing crop is the first step.

Much more research will be needed to perfect the technology so it performs consistently in the real world.