Figure: 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 Bikram Gill.

Kansas State University (K-State) researchers have discovered the mechanism by which weeds develop resistance to the popular herbicide glyphosate. The research team found how Palmer amaranth and common waterhemp evolved resistance to glyphosate in such a short time. Based on research, they discovered that glyphosate resistance in Palmer amaranth occurred very rapidly.

They found that glyphosate-resistant Palmer amaranth plants carry the glyphosate target gene in hundreds of copies. Mithila Jugulam, a K-State weed scientist and co-author of the PNAS article added, "Even if you applied an amount much higher than the recommended dose of glyphosate, the plants would not be killed."

The chromosome experts on the team 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 structure is called extra-chromosomal circular DNA (eccDNA). Each eccDNA has one copy of the gene that produces an enzyme that is the target for glyphosate. Because there are hundreds of eccDNAs in each cell, there is also an abundant amount of enzyme. Therefore, the plant is not affected by glyphosate application and the weed is resistant to the herbicide.

Jugulan said, "Glyphosate has a lot of good characteristics as an herbicide. The recommendations that K-State and many others are promoting is ‘do not abuse glyphosate.' Use the recommended integrated weed management strategies so that we do not lose the option of using glyphosate for the sustainability of our agriculture."

For more, read the K-State Research and Extension News. The full article can be accessed on the website for the Proceedings of the National Academy of Sciences.