Responding to the first known report of waterhemp showing resistance to HPPD (4-hydroxyphenylpyruvate dioxygenase)-inhibiting herbicides, such as Callisto, Impact, and Laudis, weed science researchers at the University of Illinois have identified two unique mechanisms in the plant that have allowed the weed to "get around" these herbicides.
"Waterhemp is very diverse, which you can see in the field. There are red plants, green plants, tall, short, bushy—basically a germplasm pool," says Dean Riechers, a U of I Professor of weed physiology. " If you keep spraying the same herbicide over and over, eventually you're going to find that rare plant that can resist it."
Resistance resembles natural resistance of corn
What the U of I researchers found of great concern in this population was the way in which the waterhemp resisted the herbicide—in much the same way that corn naturally resists HPPD-inhibiting herbicides.
"It mimics corn but also mimics the super bacteria that are resistant to all the antibiotics out there. Weeds are kind of like bacteria in that respect; at least this population is. Whatever active herbicide we throw on it, with the exception of glyphosate, it doesn't work anymore," he said.
The study was prompted in 2009 when a continuous seed corn grower from central Illinois realized the HPPD-inhibiting herbicides he was using were no longer killing waterhemp plants, which by then had grown into a literal mat of weeds across the field, said Riechers.
"It became obvious to the grower that something was wrong, but it probably started years before that," Riechers said, adding that the grower had been planting continuous seed corn every year, using HPPD-inhibiting herbicides for at least eight years in a row.
"Mesotrione and atrazine are normally two very good herbicides that are safe on corn but still kill waterhemp," Riechers said.
Working with Syngenta Crop Protection, the maker of the HPPD-inhibiting herbicide Callisto, the researchers first looked at herbicide target genes in the waterhemp plants, expecting to find signs of a mutation in the plant's HPPD gene sequence, expression, or in reduced herbicide absorption. They were able to establish that none of these measures were behind the resistance.
Instead the researchers found that resistance was due to increased metabolism of mesotrione and atrazine—via P450 enzymes for mesotrione and GST enzymes for atrazine. The faster metabolism of the HPPD-inhibiting herbicides in waterhemp resembles the natural mechanism in corn, where the P450 enzymes confer tolerance to Callisto.
Riechers said there are many P450 and GST genes in plants, possibly hundreds, but they have not yet narrowed down which ones are involved in waterhemp resistance. "But we know they are in those family of metabolism genes," he said.
Occurrences are not isolated
Although the 2009 report was the first to document this type of resistance, Riechers said four or five other locations in the Midwest have since reported similar occurrences.
"It doesn't appear to be isolated because it looks like there are other resistant Amaranthus populations coming up," Riechers said. "The concerning thing is that some of these fields actually did have corn and soybean rotations. They weren't just growing corn, they were rotating, which is what you're supposed to do. But it still became HPPD resistant, and we're not sure how that happened."
Source: University of Illinois