New Genetic Vulnerability to Herbicide Found in Nearly 50 Sweet and Field Corn Lines

Dec 14, 2023

By Lauren Quinn

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 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 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."

The study, “First report of severe tolpyralate sensitivity in corn (Zea mays) discovers a novel genetic factor conferring crop response to an herbicide,” is published in Pest Management Science [DOI: 10.1002/ps.7896]. Authors include Marty Williams, Nicholas Hausman, Ana Saballos, Christopher Landau, Matthew Brooks, Pat Flannery, William Tracy, and Charlie Thompson.

Source : illinois.edu

Video: Turning Plant Defense Into a Management Strategy

Turning Plant Defense Into a Management Strategy

Understanding how a plant responds to stress is one thing.

Using that knowledge to make better management decisions is another.

Systemic acquired resistance, or SAR, is the plant's more direct defense response. When stress or infection occurs, the plant begins signaling throughout its system and preparing defensive compounds.

But if we know that response exists, can we help prepare the plant before the stress arrives?

The answer starts with understanding what triggers the response and what the plant needs to carry it out.

The Trigger and the Fuel

Salicylic acid plays an important role in triggering the SAR pathway.

Think of it as turning the truck on.

The engine may be running, but it still needs fuel to do the work.

In this case, manganese plays an important role in supporting the enzyme systems involved in the plant's defensive response.

This makes manganese status an important part of the conversation. Whether a producer is using tissue testing, sap analysis, or simply scouting for visible deficiency symptoms, the goal is to make sure the plant has adequate manganese available.

Manganese is required in relatively small amounts, but that does not make its role small.

If the plant receives a signal to defend itself but lacks the nutrition needed to support that response, it may struggle to carry out the process efficiently.

The trigger matters.

The fuel matters too.

Prepare Before the Stress Arrives

The best time to think about stress management is before the plant is overwhelmed.

Once a crop is already struggling, management can quickly turn into a game of catch-up.

This is similar to nitrogen management. Once a plant becomes severely deficient, correcting the problem does not necessarily erase the time and yield potential already lost.

Plant defense can work the same way.

Low-rate, targeted approaches designed to support the SAR pathway may fit best ahead of an expected stress event rather than after significant damage has already occurred.

That requires producers to think about predictable stress.

We may not know exactly what the weather will do tomorrow, but we generally know summer heat is coming. We know certain field conditions increase disease pressure. We know a herbicide application can temporarily stress a crop as the plant processes the chemistry.

Even a properly timed and labeled herbicide application can create a temporary response in the plant.

That does not mean the herbicide is bad.

Weeds can create significantly more yield loss than the temporary stress caused by controlling them.

The question is not whether we should eliminate every stressor.

The question is whether we can better prepare the plant to manage necessary and predictable stress.

Not All Stress Is Bad

Stress is a normal part of plant growth.

A perfectly stress-free environment does not exist in the field.

In fact, some stress is necessary for normal plant development. A plant responds to wind, temperature, moisture, sunlight, and countless other environmental signals throughout the season.

The environment is stress.

The plant's job is to manage it.

Problems begin when the stress load becomes greater than the plant's ability to respond.

Extreme heat, drought, high salt concentrations, disease pressure, and even certain management practices can add to that load.

This is where understanding SAR becomes useful.

Instead of waiting until the plant is visibly struggling, producers can begin identifying periods when stress is likely and make management decisions around those windows.

ISR Starts With the Soil

While SAR is a more direct defense response, induced systemic resistance, or ISR, brings the conversation back to soil health.

The longevity of a farm is closely connected to the health of its soil.

Carbon plays a major role because it supports biological life within the soil. Bacteria, fungi, and other organisms interact with plant roots and influence how the plant grows, accesses nutrients, and prepares for stress.

This is why soil health cannot be reduced to one product or one application.

It is a system.

Keeping living plants in the field longer can support biological activity. Cover crops may fit some operations. Better water management can improve soil conditions.