RNAi may hold key to glyphosate resistance

VANCOUVER — Weed scientists have worried for years about the lack of new technologies and chemistries needed to fight herbicide resistant weeds.

However, something new is now on the horizon.

Monsanto has discovered that it is possible to kill glyphosate resistant weeds with a biological technique called RNA interference (RNAi).

Doug Sammons, a senior fellow with Monsanto who studies the management of glyphosate resistant weeds, said greenhouse and field tests indicate a mixture of glyphosate and an RNAi biological molecule, can kill glyphosate resistant Palmer amaranth.

“The combination of RNA plus glyphosate, results in the (weeds) being controlled,” Sammons told the Weed Science Society of America annual meeting held in Vancouver Feb. 3-6.

Sammons showed photos of patches of glyphosate resistant Palmer amaranth from field trials conducted last year.

Glyphosate alone failed to kill the weed In one of the patches.

“(But) when we include our trigger (RNAi) with our glyphosate, we get a bare ground plot,” he said.

He also showed photos from greenhouse trials where a combination of RNAi and glyphosate killed weeds that were previously resistant to glyphosate.

Sammons said the technology uses a plant enzyme called EPSPS, which is needed to synthesize amino acids.

Glyphosate binds to the EPSPS enzyme in susceptible weeds, obstructing plant growth and development and eventually killing the weed.

Resistant weeds such as Palmer amaranth overcome glyphosate by manufacturing additional copies of the EPSPS enzyme.

“In resistant Palmer amaranth, there’s more enzyme than there is glyphosate, generally,” Sammons said.

Monsanto’s website describes RNAi as a way to turn down or shut off the expression of certain genes, which suppresses the production of a specific protein in an organism.

In the case of resistant weeds, Monsanto is using RNAi to shut down their production of the EPSPS enzyme. Glyphosate is once again effective if the weed produces fewer copies of EPSPS.

“Silencing creates an opportunity for the herbicide to work again,” Sammons said in an interview at the conference.

“So it’s going to make the herbicide more active on the target that’s selected.”

He said tests indicate that RNAi by itself can moderately inhibit weed growth, but Monsanto is focused on using RNAi in combination with herbicides.

“(In tests), by 19 days it was pretty clear that the sensitive plants were individually stunted, so it looked like the dose of RNAi was giving us a little bit of… activity by itself,” he said.

Sammons said weed scientists are curious about RNAi for two reasons.

“One is, there are some problems to solve. The second is because there’s been such a shortage of new areas (for weed control) discovered in the last 20 years…. This is really a new opportunity.”

Steve Duke, a U.S. Department of Agriculture weed scientist and glyphosate expert in Mississippi, said government and university scientists are also studying the potential of RNAi.

“It’s exciting, cutting edge concept for managing pests, and the EPA (Environmental Protection Agency) is looking at it very carefully, in terms of what risks there might be on non-target organisms,” he said.

“But it’s still early in the game, as far as sorting out if it’s going to be economical, efficacious and safe enough for widespread use…. But the nice thing about the technology is you can fine-tune the product for a specific weed species or a group of weeds.”

Many scientists believe RNAi has tremendous potential to combat weed resistance to glyphosate and other herbicides, but others worry about off target effects.

Adam Davis, a USDA weed scientist in Urbana, Illinois, said the risks might outweigh the benefits.

“RNAi could be very powerful, but I have to admit that I’m also concerned about the potential for unintended consequences,” he said.

“Potentially, there might be some be some evolutionary change that reduces its selectivity and effects some non-target organisms.”

Sammons said Monsanto is studying if RNAi has off target gene activity and off species activity. So far they haven’t found any evidence, he added.

He also said it’s true that RNAi may provoke biological changes in weeds that cause resistance, but the technology also offers a potential solution.

“Every time we come up with a new herbicide, the plants respond. So should I say that’s not going to happen? No. There will be a consequence, but we’re not clear what it will look like,” he said.

“Every consequence has an origin and has a gene. Now we can go back after it (with RNAi).”

Sammons said using RNAi to combat herbicide resistance is at the early stages of development at Monsanto.

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