They pop up in farm fields across 22 states, and they've been called the single largest threat to production agriculture that farmers have ever seen. They are "superweeds" -- undesirable plants that can tolerate multiple herbicides, including the popular gylphosate, also known as RoundUp -- and they cost time and money because the only real solution is for farmers to plow them out of the field before they suffocate corn, soybeans or cotton.
In an article in the Nov. 23 issue of the journal TheProceedings of the National Academy of Science
, researchers with Dow AgroSciences and the University of Missouri report on two bacterial enzymes that, when transformed into corn and soybeans, provide robust resistance to the herbicide 2,4-D. The discovery may soon provide Missouri corn and soybean growers a solution to the growing problem of herbicide-resistant weeds.
The spread of herbicide resistance has become an increasing concern for growers because undesired plants, such as Palmer amaranth and tall waterhemp in Missouri, are becoming tolerant to the broad-spectrum herbicide glyphosate, better known as Roundup.
Glyphosate is commonly used for controlling weeds in fields of Roundup Ready corn and soybean hybrids.
An herbicide commonly used to control lawn dandelions, 2,4-D, may be a new option on the horizon for growers who urgently need an alternative to glyphosate, said Zhanyuan Zhang, research associate professor of plant sciences at the University of Missouri. Zhang, director of MU's Plant Transformation Core facility, teamed up with research scientists at Dow AgroSciences, LLC, to engineer soybean plants resistant to 2,4-D.
Using a bioinformatic approach to mine a genetic database, the researchers pinpointed two bacterial enzymes - AAD-1 and AAD-12 - that break down 2,4-D. These enzymes were similar to a third bacterial enzyme that confers resistance to 2,4-D in transgenic cotton. When the new enzymes were inserted into the genome of the model plant Arabidopsis thaliana, the resulting plants showed little to no visible signs of damage when treated with 2,4-D. Plants with only the AAD-12 enzyme also survived applications of two other broadleaf herbicides, triclopyr and fluroxypr.
The herbicide protection of these enzymes was subsequently tested in corn and soybeans. Using robust transformation methods, the corn plant was engineered to produce the enzyme AAD-1, and the soybean plant engineered to produce the enzyme AAD-12.
The resulting plants were then subjected to herbicide treatment under greenhouse and field conditions at several locations and through multiple generations. Under all conditions, the plants exhibited excellent resistance to 2,4-D, even when applied at higher than normal levels. The corn plants exhibited none of the injuries commonly associated with treatment with 2,4-D, such as root malformations, and also were resistant to two grass herbicides. No negative effects on yield or other agronomic traits were observed in the transgenic corn or soybean plants.
Other advantages of 2,4-D include low cost, short environmental persistence, and low toxicity to humans and wildlife.
"Unlike glyphosate, which targets amino acid synthesis, 2,4-D is a hormone regulator. Because it has a different mode of action, 2,4-D is an ideal herbicide to deal with glyphosate-resistant weeds," said Zhang, who carried out the soybean transformation and contributed to some data analysis for the study.
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