When a sweet corn breeder came in 2021 to report severe damage from the herbicide tolpyralate, Marty Williams hoped it was a fluke isolated from an inbred line. But two years later, after methodical field, greenhouse, and genetic testing, his new Pest Management Science study not only confirmed the sensitivity to tolpyralate in 49 sweet corn and corn lines in the field , but also revealed a new genetic vulnerability that could affect corn in general.
Tolpyralate is a relatively new HPPD-inhibiting herbicide labeled for all corn varieties. Normally, maize detoxifies HPPD-inhibitors before they can cause damage, by expressing the Nsf1 gene. Maize lines with mutant nsf1 alleles may show sensitivity to HPPD-inhibitors, but that was not the case with tolpyralate in the lines tested by Williams. However, his study showed that the sensitivity to tolypyralate is related to a different gene entirely, which explains why the sensitivity is not expected or caught during the breeding process.
Cross-sensitivity to several postemergence herbicides, all linked to mutant nsf1 alleles, has been understood for years. Breeders often screen with a product such as nicosulfuron, an ALS-inhibitor, because it will identify (ie, kill) any inbreds that are intolerant to various herbicides, including most HPPD -inhibitor, said Williams, an ecologist with USDAs 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 was screened with nicosulfuron, showing no damage and showing that the Nsf1 gene is doing its job. Expecting the same result with tolpyralate is reasonable, since no one has reported serious plant damage from the new herbicide. So, when the tolpyralate lesion reared its head, the breeder was confused.
The unusual case led the Williams team to begin hunting for bleached-white corn, the sign of HPPD-inhibitor injury, around U. of I. farms.
They don’t have to hunt for long.
Here and there, among the strapping green rows of corn, there were stunted, white stragglers. The team contacted researchers who were running tests around the fields to find out what had been sprayed. Tolpyralate, every time.
Faced with an event that didn’t look much like a fluke, the Williams crew began field and greenhouse trials to determine how widespread tolpyralate sensitivity was. With easy access to a sweet corn diversity panel, they target most of that group. But they also tested a narrow panel of corn genotypes in the field.
From the moderate screening, the team documented 49 sweet corn (43) and field corn (6) inbreds that suffered moderate to severe damage from tolpyralate. Importantly, the source of the sugary enhancer gene in sweet corn, a parent line for many sweet corn hybrids, is one of the most sensitive genotypes, suggesting that sensitivity may be more widespread.
Interestingly, the damage is worse with the addition of atrazine and herbicide adjuvants that are often applied to HPPD-inhibitors.
When we applied pure tolpyrate to the sensitive sweet corn inbred, the plant looked great, said Williams. But when we added the adjuvants recommended on the herbicide label crop oil or methylated seed oil we got a severe bleaching response. And when we also included atrazine, which is common among HPPD-inhibitors, the plant died quickly.
Williams clarified that it is not possible to simply remove the adjuvants from the tank. They improve herbicide uptake by weeds and are essential for successful weed control.
Tolpyralate has agronomic advantages, but it clearly has limited utility if it harms the crop, Williams said.
With increasing evidence suggesting nsf1 Unable to blame for tolpyralate sensitivity, the team then mapped the genome to find the cause.
Using the original sensitive sweet corn line to map the trait, we mapped it to a region on chromosome 5 near Nsf1. But it is not Nsf1, and there is no clear genomic region that we have identified that readily explains tolpyralate sensitivity. So, while we map behavior, the physiological mechanism remains elusive.
Williams says that more research is needed to get the bottom of the sensitivity to tolpyralate, in terms of the physiological mechanism and how widespread the behavior is in all types of corn. He said there is potential to develop molecular markers that can identify sensitive lines of corn, which could be useful in developing tolerance to tolpyralate.
Currently, he wants to raise the awareness of corn breeders, growers, and chemical companies working on the next generation of HPPD-inhibitors, especially since this is the first incident of a genetic vulnerability in a corn herbicide that has been documented for more than three decades.
What we learn from this research may help beyond tolpyralate itself, because many new HPPD-inhibitors derived from the same chemical structure are being developed, Williams said. If we can avoid further problems in the future, let’s do it now. “
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