K-State researchers crack pigweed resistance code

kansas state university pigweed researchers
Kansas State University researchers have discovered the mechanism by which weeds develop resistance to glyphosate, an herbicide. Their work could lead to improved weed control strategies and improved production in farm fields and other areas where weeds affect plants and crops. Pictured, left to right, are Mithila Jugulam, Dal-Hoe Koo, Bernd Friebe and Bickram Gill — photo courtesy Kansas State University

A team led by Kansas State University researchers has cracked the genetic code about how Palmer amaranth quickly grows resistant to glyphosate herbicide.

The findings could have broad implications for agriculture and other industries, according to a university news release.

Their work appeared in the March edition of the prestigious “Proceedings of the National Academy of Sciences.”

Although herbicide resistance in itself is not now, what the researchers discovered that was new is how the weeds evolved to tolerate much higher than labeled rates of glyphosate.

“We found that glyphosate-resistant Palmer amaranth plants carry the glyphosate target gene in hundreds of copies,” Mithila Jugulam, K-State weed scientist and co-author of the article, said in the news release. “Therefore, even if you applied an amount much higher than the recommended dose of glyphosate, the plants would not be killed.”

Genetic material in all organisms, including humans, is found in long, linear DNA molecules called chromosomes. But when researchers examined the glyphosate-resistant weeds, they found the glyphosate target gene, along with other genes, actually escaped from the chromosomes and formed a separate self-replicating circular DNA structure.

Scientists refer to this structure as extra-chromosomal circular DNA or eccDNA. Each eccDNA has one copy of the gene that produces an enzyme targeted by glyphosate.

“Because of the presence of hundreds of eccDNAs in each cell, the amount of the enzyme is also abundant,” Bikram Gill, director of KSU’s Wheat Genetics Resource Center, said in the release.”Therefore, the plant is not affected by glyphosate application and thus the weed is resistant to the herbicide.”

Gill said indications are that once a weed has acquired eccDNA, the resistance may evolve as quickly as in one generation.

“We think that the resistance via eccDNA is transitory: It can be passed to the weed’s offspring and other related weed species,” he says. “We have somehow caught it in between becoming permanently resistant. Eventually, we think that these eccDNAs can be incorporated into the linear chromosome. If that happens, then they will become resistant forever.”

Funding for this research was provided in part by grants from the Kansas Wheat Commission; the Kansas Crop Improvement Association; a National Science Foundation grant received through the Wheat Genetics Resource Center; the K-State Department of Agronomy (College of Agriculture); and USDA’s Agricultural Research Service. Kansas State University worked in collaboration with researchers at Clemson University, the USDA Agricultural Research Service in Mississippi and Michigan State University.

Read the full PNAS article at http://www.pnas.org/content/early/2018/03/08/1719354115

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