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."

[www.pnas.org]