Waterhemp’s resistance mechanism to herbicides identified

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.

Dean Riechers, a U of I Professor of weed physiology, along with other collaborators at the U of I, recently published a paper describing the two distinct metabolic detoxification mechanisms that confer resistance to mesotrione (Callisto) and atrazine (Aatrex) in an Illinois waterhemp population.

"Waterhemp is very diverse, which you can see in the field. There are red plants, green plants, tall, short, bushy-basically a germplasm pool. If you keep spraying the same herbicide over and over, eventually you're going to find that rare plant that can resist it," said Riechers.

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.

"In the pharmaceutical industry, doctors know that you can't keep recommending the same ampicillin prescription. You might have a limited time you can use that, and then you have to use something different," Riechers said.

"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.

The HPPD enzyme helps protect the plant's chlorophyll by producing protective compounds such as Vitamins A and E. Rong Ma, a doctoral student in weed science and herbicide physiology with Riechers at the U of I and one of the researchers on the study, explained that by inhibiting HPPD, chlorophyll is degraded and new leaves will show bleaching. HPPD-inhibitors are systemic herbicides and will continue to move toward new tissue in the plant, eventually killing it.

Older waterhemp growth in the study initially did show bleaching from Callisto, however new emerging leaves recovered and turned green due to the increased metabolism of the herbicides, Ma explained. She added that the corn in the central Illinois field study was also very short due to the waterhemp taking away needed water and nutrients.

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."

What are the implications for growers now that this resistance has emerged?

Ma said one of the first things growers should consider is not using the same herbicide mode of action repeatedly. "For example, don't use HPPD-inhibiting herbicides alone for several years in a row because it is then easier for weeds to develop resistance," she said.

"That's just from the chemical standpoint, but growers could also use tillage because there's no resistance to tillage," Riechers said.

"Farmers use no-till systems, often plant in narrow rows, and for the most part have gotten away from tillage for weed management. In a way we have aided waterhemp in becoming a problem by not using tillage, using the same chemical over and over, and by not rotating crops."

The U of I researchers are now looking at new ways to overcome the resistance mechanism by blocking P450 enzymes in the weed while it is still small by using a different chemical inhibitor.

"We found waterhemp resistance to mesotrione due to P450s so now we are trying to use P450 inhibitors combined with mesotrione to see if the two will work together to inhibit waterhemp resistance. But we don't want to see any injury to corn," Ma said.

In her recent field study at the original resistance site, she noted some bleaching on corn using a P450 inhibitor and an HPPD-inhibiting herbicide together, but Ma said the corn "catches up," and shows no difference as it grows. "As corn continues to grow, it seems to have enough P450s to overcome the herbicide," she said.

Riechers added that if they can find inhibitors of these herbicide metabolism pathways in waterhemp, this would be a possible way to get around resistance and still have it be safe for corn. "If it's metabolizing it, in theory we can block the metabolism and we should be able to use the herbicide again to control resistant waterhemp," he said.