Henry Grabar has had enough of the battle with the Kestrel. All he wanted was to build a small garden in Brooklyn—a little peace amidst the cacophony of city life. But his nascent garden was soon dominated by a plant with beet red leaves. The fastest growing plant he had ever seen could grow up to 10 feet tall and grow as thick as a corn field. Even with herbicide it was almost impossible to kill.
Invasive species of plants and weeds do not only destroy gardens. Weeds reduce crop yields at an average annual cost of $33 billion, and control measures can raise another $6 billion. Herbicides are a defense, but they come with their own baggage. Weeds quickly develop resistance to chemicals, and the resulting products can be a hard sell to many consumers.
Weeds often seem to have the upper hand. Can we take it away?
Two recent studies say yes. Using a technology called synthetic gene drive, the teams spliced genetic fragments into the mustard plant popular in laboratory studies. Gene drives, previously verified in fruit flies, mosquitoes and mice, break the rules of inheritance and allow “selfish” genes to spread rapidly across the species.
But making the gene drives work in plants has been a headache, in part because of the way they repair their DNA. New studies have found a clever solution that leads to roughly 99% of the synthetic genetic load spreading to subsequent generations, as opposed to nature’s 50%. Computer models suggest that gene drives could spread throughout a plant population in about 10 to 30 generations.
By suppressing natural evolution, gene drives could add genes that make weeds more vulnerable to herbicides or reduce their pollination and numbers. Beneficial genes can also spread across crops – essentially speeding up the practice of crossing for desirable traits.
“Imagine a future where crop-robbing agricultural weeds or biodiversity-threatening invasive plants could be kept on a genetic leash,” wrote Paul Neve of the University of Copenhagen and Luke Barrett of CSIRO Agriculture and Food in Australia, who were not involved in the study . .
Table of Contents
Toggle50/50
Heredity is a coin toss for most species. Half of an offspring’s genetic material comes from each parent.
Gene drives torpedo this hereditary rule. Developed around a decade ago, the technology relies on CRISPR – a gene-editing tool – to spread a new gene in a population, beating the 50/50 odds. In insects and mammals, a gene can reproduce about 80 percent of the time, displacing an inherited trait by generations and irreversibly changing an entire species.
Although it may seem somewhat nefarious, gene drives are designed for good. The main subject of research is the control of disease-carrying mosquitoes by genetically modifying the males to make them sterile. Once released, they outcompete their natural counterparts, reducing the number of free-living mosquitoes and subsequently reducing the risk of many diseases. In indoor cages, the gene drives completely suppressed the insect population within a year. Small-scale field tests are underway.
Gene drives also intrigued plant scientists, but initial efforts in plants failed.
The technology relies on CRISPR, which cuts DNA to insert, delete or swap genetic letters. Cells that sense damage to their DNA activate internal molecular “repairmen” to splice genes back together and accept the gene drives and their genetic cargo.
Plants are different. Their cells also have a DNA repair mechanism, but it is only partially similar to that of insects or mice. Inserting a classical gene drive into plants can cause genetic mutations at the target site and even induce resistance to the gene drive in a kind of cellular civil war.
What does not kill you makes you stronger
As a solution, both new studies used a system called “toxin-antidote.” Compared to previous gene units, it does not rely on canonical DNA repair.
The teams used the self-pollinating mustard plant for the study. A darling of plant science research, its genome is well known, and because the plant is self-pollinating, the experiment is easier to handle. To create the gene drive, they developed a CRISPR-based method to destroy a gene that is critical for survival, called a “torpedo.” No pollen without a gene can live on. The second construct, the “antidote”, carried a copy of the same gene but with modifications to make it resistant to destruction by CRISPR.
They examined two different genetic burdens. One study looked at a gene that is essential for both male and female reproductive cells in plants. The second targeted a gene that disrupts pollen production.
Here’s the clever part: As the plant is pollinated, the offspring can inherit either the toxin, the antidote, or both. Only those with the antidote survive – plants that inherit the toxin die quickly. As a result, the system acted as a gene drive, with plants carrying the CRISPR-resistant gene taking over the population. The gene drives were highly efficient and passed down the generations about 99 percent of the time. And the researchers saw no signs of evolutionary adaptation — known as resistance — against the new genetic makeup.
Computer modeling showed that the gene drive could overtake a single plant species in 10 to 30 generations. That’s impressive, according to Neve and Barrett. Artificial genetic changes often do not take hold in wild plants – the plants tend to die. New gene drives suggest they could potentially last longer in the field, fight invasive species or grow more resilient, pest-resistant crops that pass beneficial traits down the generations.
Despite their promise, gene drives remain controversial because of their potential to change an entire species. Scientists are still debating the ecological impacts. There is also concern that gene drives may jump to unintended targets. For now, studies have suggested genetic “brakes” to keep gene drives in check. Most studies are conducted under carefully controlled laboratory conditions, and in malaria, potential unintended consequences are carefully considered before gene-carrying mosquitoes are released into the wild.
Even when the science works, the road to regulatory and societal approval can hit hurdles. Selling the technology to farmers can be difficult. And CRISPRed plants as a food source could also be tainted by the negative perception of genetically modified organisms (GMOs).
For now, the teams are looking for a more acceptable everyday use – weed control. There are still a few kinks to work out. Gene drives only work if they can spread, so the ideal use is in plants that pollinate others, rather than self-pollinating ones like those in the studies. Still, the results are a proof of concept that the powerful technology can work in plants—though it may still be a while before it helps Henry with his pinworm problem.
Image credit: Anthony Wade/Unsplash
