Probing resistant genes
In hand, even the experts can’t tell the difference between weed seeds that are resistant or susceptible to glyphosate.
Under high-powered molecular biology machines and instruments, however, the differences between plants begin to take on a different magnitude.
At her lab at the University of Arkansas, Nilda Burgos, a Division of Agriculture weed scientist, checks the genes of Palmer amaranth populations gathered from three different regions of the state. The project is part of a multi-disciplinary effort to look at what one researcher called the “biggest threat to agriculture since the boll weevil.”
• Researchers are looking at the genetics of weed resistance.
• What makes a weed resistant? Less gene diversity.
• The goal is to understand how weeds escape glyphosate.
Weed resistance is forcing agriculture in the South to take a second look at how crops are produced. To deal with it in the short term, Extension and chemical companies alike are recommending rotation of modes of action as well as crops. Many farmers in the Mid-South have broken out the hoes, while others have gone back to cultivating, a practice they parked at the edge of the field almost 15 years ago when glyphosate supposedly made weed control by plowing a thing of the past.
In the long term, however, researchers, Extension and those in the private sector are looking for ways to “manage” weed resistance.
A genetic profile
Through the lens of the microscope, Burgos and Amy Lawton-Ruth of Clemson University are developing a genetic profile of what makes a weed resistant.
The target for glyphosate is known as EPSPS. Glyphosate-resistant crops such as those bearing the Roundup Ready trademark are genetically engineered to express an EPSPS gene that is resistant to glyphosate. The glyphosate-resistant EPSPS gene carries a mutation in the glyphosate-binding site that prevents glyphosate binding. Some glyphosate-resistant weed populations evolved because repeated applications of glyphosate selected for individuals carrying a mutation. This is one of the ways plants fight the herbicide. It’s currently being investigated in the Arkansas populations.
In some cases, herbicide-resistant plants overcome the herbicide by producing high amounts of the herbicide target. This mechanism, also referred to as target-site amplification, is demonstrated in a Palmer amaranth population from Georgia. It boils down to achieving resistance “by just sheer numbers,” Burgos says. “If you have a lot of functional enzymes that are resisting glyphosate, you can’t inhibit all of them.”
In her work, Burgos is currently sequencing the EPSPS gene of not only the resistant plants, but also the susceptible populations — i.e., those weeds that respond to glyphosate. So far, Burgos has detected significant differences between susceptible and resistant plants, segmenting resistance in Arkansas by low, medium and high levels. “We think we do have different mechanisms, even within the same population, but is it a mutation in the target site, or target site amplification or translocation?” she asks. The goal is to understand how different populations escape glyphosate and come up with “alternative control methods.”
“The frequency of mutation is different from susceptible to resistant weeds,” she says. “If you are applying glyphosate repeatedly, and there are escapes that are able to produce seed, the population composition would change.
“The data is showing that the resistant population has a lower frequency of genetic differences — less variations at the target site when compared to a susceptible population.”
Out in the greenhouse, Burgos demonstrates. The pigweed seeds growing in trays come from 40 different locations across Arkansas. She and her colleagues in the Extension service and graduate students harvested pigweeds, bagged them, planted seed from each plant in trays and treated them with glyphosate. The seeds are numbered according to the location where the plant was growing in Arkansas fields. A quick glance at the progeny from one plant in Chicot County helps to explain the reason farmers are having a difficult time controlling weed resistance. From the same plant, some of the weed seedlings die from exposure to glyphosate while others, after a short injury period, spring back to life.
“Selection for pigweed resistance started the first time you applied glyphosate,” Burgos says. “It’s possible to kill them all, but Palmer emerges continually throughout the season. The late-emerging plants escape glyphosate and, if left to mature, will reset the seed back.”
Getting a good handle on pigweed is elusive at best because of its continual emergence throughout the growing season, Burgos says. “Because glyphosate doesn’t have soil activity, the escapes that produce seed change the dynamics to where they’re emerging later in the season to avoid the onslaught of chemicals. These mutations impart resistence.
“Before you know it, the dynamics of the population have changed, and you’re dealing with a bigger problem,” Burgos says. “That’s what farmers are now facing.”
As to the thought that farmers can switch to a silver bullet and solve the problem, Burgos goes back to the selection metaphor. “Overuse of another chemical that doesn’t begin with a ‘g’ will only select for and create more resistance problems.”
This article published in the November, 2010 edition of MID-SOUTH FARMER.