Waterhemp Resists Soil-Applied Herbicides With One Gene Threatening Corn Belt Crop Production

Waterhemp, a weed that plagues corn and soybean fields across the Midwest, has developed resistance to seven classes of herbicides, leaving farmers with few options to control it.

Now, researchers at the University of Illinois Urbana-Champaign have discovered that a single gene is responsible for one of the most troublesome forms of resistance: the ability to survive soil-applied herbicides.

The gene behind the resistance

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The researchers, led by Dean Riechers, a professor in the Department of Crop Sciences, part of the College of Agricultural, Consumer and Environmental Sciences (ACES) at University of Illinois, focused on S-metolachlor, an active ingredient in Dual Magnum and Dual II Magnum, two popular herbicides that belong to the class of VLCFA-inhibiting herbicides.

These herbicides work by preventing the synthesis of very long chain fatty acids (VLCFAs), which are essential for plant growth and development.

The researchers had previously shown that waterhemp can detoxify S-metolachlor with the help of P450s, a large family of enzymes that can modify toxic compounds and make them less harmful.

However, they did not know which P450s were involved, or how they were regulated.

To find out, they conducted a series of genetic experiments, crossing resistant and sensitive waterhemp plants and analyzing their offspring.

They found that a single major gene, located on chromosome 15, controlled the resistance to S-metolachlor.

This gene, which they named CYP81A12, encodes a P450 enzyme that can metabolize S-metolachlor and reduce its toxicity.

The researchers also found that the gene was dominant, meaning that only one copy of it was enough to confer resistance.

This makes it easier for the trait to spread in the population, as any offspring of a resistant and a sensitive plant will inherit the resistance.

Also Read: North American Waterhemp Now a Pesky Weed that Reduces Crop Yields, Research Reveals Modern Agriculture to Blame

The genetic mechanism of waterhemp's resistance to soil-applied herbicide

The discovery of the gene behind the resistance has important implications for weed management, according to the researchers.

According to the researchers, knowing the genetic basis of resistance can help develop molecular markers to detect it in the field, and also provide insights into the evolution and spread of resistance.

However, they also warn that the resistance to S-metolachlor is not the only problem that waterhemp poses.

The weed can also resist other herbicides with different modes of action, such as atrazine, glyphosate, and 2,4-D.

Moreover, waterhemp can produce thousands of seeds that can remain viable in the soil for years, and can also cross-pollinate with other waterhemp plants, creating new combinations of resistance genes.

Therefore, the researchers suggested that farmers should not rely solely on chemical control, but also use other methods, such as crop rotation, cover crops, tillage, and hand-weeding, to reduce the weed pressure and prevent the emergence of new resistance mechanisms.

The study, "A single dominant gene confers non-target-site resistance to S-metolachlor in waterhemp (Amaranthus tuberculatus)", is published in Weed Science.

The authors are Dylan Kerr, Patrick Tranel, and Dean Riechers from the University of Illinois. The research was funded by the USDA National Institute of Food and Agriculture.