New mutations for herbicide resistance rare

Most resistance seems to come from pre-existing genetic conditions

The researchers started with a single seed of Amaranthus hypochondriacus, their test plant, and grew generations of the plant to produce the seeds they needed for their experiments.

After exposing more than 70 million seeds to a soil-based herbicide, researchers at the University of Illinois were not able to find a single herbicide-resistant mutant.

Though preliminary, the findings suggest that the mutation rate in amaranth is very low, and that low-level herbicide application contributes little — if anything — to the onset of new mutations conferring resistance, researchers say.

The study is reported in the journal Weed Science.

Any major stress that does not kill a plant can contribute to genetic mutations in its seeds and pollen, said University of Illinois crop sciences professor, Patrick Tranel, who led the new research. Even the ultraviolet light in sunlight can stress a plant and increase the likelihood of mutations in its offspring, he said. Such mutations increase genetic diversity and species’ survival.

“Resistance to herbicides comes from genetic variation in a population,” Tranel said. “If an individual weed has the right mutation that allows it to survive a particular herbicide, that individual will survive and pass the trait to its progeny.”

The relative contribution of new mutations to the problem of herbicide resistance is poorly understood, Tranel said.

He and his colleagues hoped to determine the baseline mutation rate for a plant of the genus Amaranthus, a group that includes waterhemp, Palmer amaranth and other problematic agricultural weeds.

They also wanted to test whether herbicide applications that failed to kill the plant increased that baseline rate.

The researchers started with a single seed of Amaranthus hypochondriacus, and eventually tested 70 million.

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