By Erik Stokstad
More than a decade ago, a research group used the genome editor CRISPR to put evolution on fast forward, spurring a gene to spread throughout a population of lab-reared fruit flies many times faster than it normally could in nature. Mosquitoes with CRISPR-based “gene drives” came soon after, then mice a few years later—advances that brought a fraught mix of technological promise and ethical complexity. Proponents tout gene drives as a way to prevent insect-borne diseases, wipe out rats and other invasive creatures, and even help prevent extinction of endangered species. But one set of organisms had stood apart from the excitement: plants.
Now, geneticists report that synthetic gene drives can work in flora, too. Circumventing a long-standing hurdle, two teams have independently engineered Arabidopsis thaliana, a small mustard popular for lab work, to carry a genetic payload that is inherited by up to 99% of offspring. Modeling suggests a similar gene could permeate a natural plant population in 10 to 30 generations. “What they’ve achieved is pretty amazing,” says Paul Neve, a weed scientist at the University of Copenhagen. “It is clever and innovative.”
In a pair of papers in Nature Plants last week, the teams described mimicking a natural gene drive involving “selfish” genetic sequences known as toxin-antidote elements. Their success opens the possibility of knocking back weeds that have evolved to resist many herbicides. Or gene drive could transform species to be less troublesome, so they can continue to provide food and habitat for pollinators and other wildlife.
A plant gene drive system could be “really valuable for sustainable weed management,” says Mithila Jugulam, weed physiologist and molecular biologist at Kansas State University. But Todd Gaines, a weed biologist at Colorado State University, cautions, “I could see a lot of headwinds,” including selling farmers on the technology and gaining regulatory approval.
It’s still a long and expensive process to get a genetically modified (GM) crop past U.S. agencies, including the Department of Agriculture, which has a mandate to minimize the risk of new weeds and pests. So, winning approval to release GM weeds? “It’s a horror story in the making,” says University of Illinois Urbana-Champaign weed scientist Patrick Tranel.
Natural gene drives, instances where the rules of inheritance are broken, are rare. Normally, each copy of a gene, called an allele, has a coin-toss chance of being inherited. Some so-called selfish stretches of DNA, however, have evolved ways to cheat their way past other alleles, becoming ever more common in the population even if they don’t enhance the success of the organism.
Pollen and ovules that carry the gene editor survive because of the rescue gene. Those without die when CRISPR destroys their YKT61. To show this could rapidly spread a gene, the team attached a marker gene that turns surviving seeds red. The gene drive was effective, as 97% to 99% of Arabidopsis produced red seeds. And it was stable, lasting for five generations.
In China, a second team took the same approach and achieved similar results. Led by Wenfeng Qian, a synthetic biologist from the Institute of Genetics and Developmental Biology of the Chinese Academy of Sciences and Peking University, it chose a different gene for CRISPR to cut, one required for pollen germination. The group’s red marker showed up in 88% to 99% of seeds for two generations. The moment lab members opened the seed pods was “thrilling,” Qian recalls.
The successes suggest new ways to control weeds that have evolved resistance to multiple herbicides. Take pigweed (Amaranthus palmeri), a plant that can choke off crops such as soybeans and cause allergies in people. Both teams calculate that in 10 to 30 generations their gene drive could saturate any plant population with a gene causing complete sterility. Or it could spread a cargo gene that would make the weed more benign without eliminating it—maybe one making the plant nonallergenic.
Hay says a gene drive aimed at eradicating a weed could be designed so that genetic recombination—the DNA shuffling that happens in reproductive cells—ultimately separates its genetic components and shuts it down, reducing the risk that the fatal gene drive spreads to weeds beyond the farm field. “It’s very important that the technology can be targeted, but not completely destructive,” says Kan Wang, a plant geneticist at Iowa State University. “That’s a part I really appreciate.”
However safe, a synthetic gene drive might have limited appeal for farm applications, Neve says. Waiting a decade to eradicate weeds could be a nonstarter for farmers. The model also assumes farmers would boost the existing weed population by adding 10% more weeds bearing gene drive—requiring a lot of planting and greater plant consumption of water and nutrients. Hay sees gene drive as an add-on to other measures, imagining that farmers would plant a fringe of gene drive–bearing weeds around their fields each year, bit by bit pushing the weed population to zero.
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