New genetic vulnerability to herbicide found in nearly 50 sweet and field corn lines

When a sweet corn breeder reached out in 2021 to report severe injury from the herbicide tolpyralate, Marty Williams hoped it was a fluke isolated to a single inbred line.

But two years later, after methodical field, greenhouse, and genetic testing, his new Pest Management Science study not only confirms sensitivity to tolpyralate in 49 sweet corn and field corn lines but also reveals a new genetic vulnerability that may affect corn more generally.

Tolpyralate is a relatively new HPPD-inhibiting herbicide labeled for all types of corn. Typically, corn detoxifies HPPD inhibitors before they can cause injury through the expression of the Nsf1 gene. Corn lines with mutant nsf1 alleles can show sensitivity to HPPD inhibitors, but that wasn't the case with tolpyralate in the lines Williams tested. Instead, his study showed that tolypyralate sensitivity is related to a different gene entirely, explaining why sensitivity was neither expected nor caught during the breeding process.

"Cross-sensitivity to multiple postemergence herbicides, all linked to mutant nsf1 alleles, has been understood for years. Breeders typically screen with a product like nicosulfuron, an ALS inhibitor, because it'll identify (i.e., kill) any inbreds that aren't tolerant to a wide variety of herbicides, including most HPPD inhibitors," said Williams, an ecologist with USDA's Agricultural Research Service and affiliate professor in the Department of Crop Sciences, part of the College of Agricultural, Consumer and Environmental Sciences (ACES) at the University of Illinois Urbana-Champaign.

The original sweet corn line from 2021 had been screened with nicosulfuron, showing no injury and indicating the Nsf1 gene was doing its job. Expecting the same result with tolpyralate was reasonable since no one had reported major crop injury from the new herbicide. So, when tolpyralate injury reared its head, the breeder was baffled.

The unusual case led Williams' team to start hunting for bleached-white corn—the telltale sign of HPPD-inhibitor injury—around the U. of I. farms.

They didn't have to hunt long.

Here and there, among strapping green corn rows, were stunted white stragglers. The team contacted the researchers running trials around the farms to find out what had been sprayed. Tolpyralate, every time.

Faced with a phenomenon that looked less and less like a fluke, Williams' crew embarked on field and greenhouse trials to determine just how widespread tolpyralate sensitivity was. Having easy access to a sweet corn diversity panel, they focused mostly on that group. But they also tested a narrow panel of field corn genotypes.

From the modest screening, the team documented 49 sweet corn (43) and field corn (6) inbreds that suffered moderate to severe injury from tolpyralate. Importantly, the source of the sugary enhancer gene in sweet corn, a parent line for many sweet corn hybrids, was among the most sensitive genotypes, suggesting sensitivity could be even more widespread.

Interestingly, injury was far worse with the addition of atrazine and herbicide adjuvants commonly co-applied with HPPD inhibitors.

"When we applied pure tolpyralate to the sensitive sweet corn inbred, the crop looked fine," Williams said. "But when we added adjuvants recommended by the herbicide label—crop oil or methylated seed oil —we got a severe bleaching response. And when we also included atrazine, which is common with HPPD inhibitors, plant mortality was rapid."

Williams clarified it's not feasible to just remove the adjuvants from the tank. They improve herbicide uptake by weeds and are essential for successful weed control.

"Tolpyralate has agronomic advantages, but obviously, it will have limited utility if it harms the crop," Williams said.

With mounting evidence suggesting nsf1 wasn't to blame for tolpyralate sensitivity, the team then mapped the genome to find the culprit.

"Using the original sensitive sweet corn line to map the trait, we narrowed it down to the region on chromosome 5 near Nsf1. But it's not Nsf1, and there's nothing obvious in the genomic region we identified that easily explains tolpyralate sensitivity. So, while we've mapped the trait, the physiological mechanism remains elusive."

Williams notes that more research is needed to get to the bottom of tolpyralate sensitivity, both in terms of the physiological mechanism and how widespread the trait might be in all types of corn. He said there's potential to develop molecular markers that can identify sensitive corn lines, which would be useful in improving tolerance to tolpyralate.

For now, he wants to raise awareness among corn breeders, growers, and chemical companies working on the next generation of HPPD-inhibitors, especially since this is the first incidence of a genetic vulnerability to a corn herbicide documented in over three decades.

"What we have learned from this research may be helpful beyond tolpyralate itself since several new HPPD inhibitors derived from the same chemical structure are being developed," Williams said. "If we can avoid additional problems in the future, let's do it now."