"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