University of Illinois researchers identified a unique mechanism of resistance used by a waterhemp biotype to survive exposure to PPO herbicides. Researchers found that waterhemp has to ability to evolve differently than other resistant weeds.

The PPO herbicides are the most commonly used postemergence herbicides for waterhemp control in fields that are not planted with Roundup Ready soybeans. A specific waterhemp population in Illinois is also resistant to the classes of herbicides that inhibit acetolactate synthase and triazines, leaving only glyphosate as a potential means of postemergence control in soybean. 

Aaron Hager, assistant professor of Extension weed science at the University of Illinois,  points out that there are currently only four herbicide active ingredients for postemergence waterhemp control in soybeans, and three of these belong to one chemical family. The diphenylether herbicides (PPO-inhibitors), such as Ultra Blazer, Flexstar, and Cobra/Phoenix, were once used extensively for waterhemp control in soybeans until being largely displaced by glyphosate.  

The researchers conducted a series of experiments to determine how a population of waterhemp from Western Illinois that was not controlled by postemergence applications of diphenylether herbicides responded to various soil-applied and postemergence herbicides under actual field conditions. 

"It soon became obvious that this waterhemp biotype did in fact demonstrate resistance to various PPO-inhibiting herbicides," Hager says. "After several years of extensive field, greenhouse, and laboratory research, we documented in 2005 that this waterhemp biotype was resistant to not simply one herbicide family but to three different herbicide families: ALS inhibitors, PPO inhibitors, and triazines." 

"Plants have two different forms of the PPO enzyme, one that functions in chloroplasts and one that functions in mitochondria," says Pat Tranel, associate professor of molecular weed science at the U of I. "Typically, these two forms of the enzyme are each encoded by its own gene. What we found in waterhemp was a gene that encoded both forms of the enzyme." 

He notes that this turned out to be the gene where they identified the mutation that resulted in the herbicide resistance.

"That there are two different PPO enzymes in plants may partly explain why resistance to PPO inhibitors is so rare," Tranel says. "A mutation in a gene encoding just one or the other may not be sufficient to confer resistance. Waterhemp got around this problem by having a gene encoding both forms. Thus one mutation yielded two different resistant enzymes." 

Tranel explains that the second novel aspect of the resistance mechanism is the type of mutation. 

"Herbicide-resistance mutations typically involve a single change in the DNA sequence that, in turn, confers a single change in the protein, or enzyme," Tranel says. "In other words, there is a substitution of one amino acid for another in the enzyme, and this is responsible for the insensitivity of the enzyme to the herbicide." 

In this case, however, there was a deletion of an amino acid, rather than a substitution. 

"Prior to our research, a deletion mutation had not been reported as a naturally occurring herbicide-resistance mechanism," Tranel says. "The net result is the same - the plant is resistant due to an altered site of action. However, this unique mutation illustrates the genetic diversity that waterhemp has, and its ability to evolve in response to our attempts to control it."