There’s a chance farmers might regain control with glyphosate—but it’ll take work and patience. Researchers from Kansas State University recently discovered the pathway for glyphosate resistance in weeds is one that is unstable and could be decreased. “We have somehow caught it [resistance] in between becoming permanently resistant,” says Bikram Gill, director of Kansas State’s Wheat Genetics Resource Center and co-author of the study. The resistance mechanism isn’t fully integrated back into the plant’s DNA. Because the resistance is unstable, farmers might have a chance to beat it. “What we want to explore is, for example, if we do not apply glyphosate repeatedly or reduce the selection by glyphosate, we can make these ring-structured chromosomes unstable and once again make these plants susceptible to glyphosate,” says Mithila Jugulam, Kansas State weed scientist and co-author of the study.
Researchers recommend best management practices like rotating herbicides and crops. “This may allow evolving resistance to dissipate as we know that [the resistance] is unstable and can be lost in the absence of herbicide selection pressure,” Jugulam adds. How resistance formed. “What we found that was new was how these weeds have evolved resistance to glyphosate in such a short time,” Jugulam says. “If you look at the evolution of glyphosate resistance in Palmer amaranth, based on our research, it appears to have occurred very rapidly.” Glyphosate resistance in Palmer amaranth and other amaranth species not only evolved rapidly, but spread rapidly, too. Nearly every corn-growing state has at least one reported incident of glyphosate-resistant amaranth. “We found that glyphosate-resistant Palmer amaranth plants carry the glyphosate target gene in hundreds of copies,” Jugulam says. “Therefore, even if you applied an amount much higher than the recommended dose of glyphosate, plants would not be killed.” Abnormal DNA is what fueled resistance. “Normally, the genetic material in all organisms—including humans—is found in long, linear DNA molecules,” Gill says. “But when [K-State researchers] looked at 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.” This unusual DNA structure is called extra-chromosomal circular DNA (eccDNA). Each of these structures produces enzymes to tolerate glyphosate when it is sprayed on the plant. “Therefore, the plant is not affected by glyphosate application,” Gill says. “Eventually, we think that these eccDNAs can be incorporated into the linear chromosome. If that happens, then they will become resistant forever.” What’s unfortunate is the eccDNA structures are hereditary. So with each seed produced, which can be in the hundreds of thousands for amaranth species, resistance spreads. It can also be passed to related species, like Palmer amaranth pollinating a waterhemp plant, for example. Double check your weed management strategy—are you following best management practices to beat glyphosate resistance?