A new study published in the prestigious journal Proceedings of the National Academy of Sciences takes some of the mystery out of how aggressive weeds like Palmer amaranth and common waterhemp developed resistance to glyphosate herbicides so quickly.

Researchers from Kansas State University have discovered that these pigweeds develop extra-chromosomal circular DNA (eccDNA), genetic material that is not incorporated into the long-strand DNA.  Instead, in these glyphosate-resistant weeds, “the glyphosate target gene, along with other genes actually escaped from the chromosomes and formed a separate, self-replicating circular DNA structure” says says Bikram Gill, director of Kansas State University’s Wheat Genetics Resource Center and study co-author. Dr. Gill’s quote is from a EurekAlert! story about the team’s new paper.

Each cell has hundreds of these eccDNA bundles, and each of these contains its own copy of the gene targeted by glyphosate herbicides.  Thus, the plant is not affected by the glyphosate application.  Plus, eccDNA is easily passed on to a plant’s offspring or other related weeds, so glyphosate resistance can spread in as little as one generation.

Given that eccDNA is known to be unstable, scientists hope that by understanding this mechanism, they will be able to disrupt the development of glyphosate resistance in these common weeds.

There appears to be a window of opportunity here, says Gill, “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.”

In the meantime, this research promotes the current strategy of diversifying the herbicide regimen to avoid spawning resistant weeds.   The absence of “herbicide selection pressure” may result in the breakdown of the eccDNA process, extending the time period during which glyphosate-based herbicides remain effective.

Sources:

Dal-Hoe Koo, William T. Molin, Christopher A. Saski, Jiming Jiang, Karthik Putta, Mithila Jugulam, Bernd Friebe and Bikram S. Gill, “Extrachromosomal circular DNA-based amplification and transmission of herbicide resistance in crop weed Amaranthus palmeri,”  Proceedings of the National Academy of Sciences, published online ahead of print: March 12, 2018.

Kansas State University, “Kansas State University researchers make breakthrough in glyphosate resistance in pigweeds,” EurekAlert!, March 12, 2018.