Plant breeders are masters of genome tuning, spending years developing crops with the genes to resist disease, drought, and pests. Now, a team of industry and academic researchers in Germany and the United Kingdom has developed a viral vector technology that could accomplish similar feats temporarily by delivering genes to plant cells via an agricultural spray. The tool, recently described in recently described in Nature Plants might one day help growers respond to environmental challenges mid-season by adjusting traits like flowering time or drought tolerance, while avoiding the pitfalls of pesticides—all without altering the crop’s genome.
“It’s a big advance,” says Lee Hickey, a plant breeder and crop geneticist at the University of Queensland in Australia, who was not involved in the study. “Weather is becoming increasingly difficult to forecast, and so this [spray] could provide another tool for farmers to respond to weather changes once that crop has been sown.”
For decades, plant geneticist and biotechnologist Yuri Gleba, a study coauthor, and others have worked to turn plants into protein-producing machines for therapeutic applications. They harness the power of agrobacteria (bacteria adept at inserting genes into plant cells), as well as RNA plant viruses that can move inside plants and express foreign proteins.
Recognizing the crop trait applications, Gleba and colleagues designed a gene delivery system that could one day be scaled up to the level of a farm. “We believe that this is a nice recipe for very quick modulation of plant agronomic performance,” says Gleba, the CEO of Nomad Bioscience GmbH in Halle, Germany.
The system works by packaging a gene inside a DNA strand that’s complementary to the desired RNA viral vector and then inserting it into the agrobacteria. The agrobacteria are then applied onto crops with the same sprayers growers use to apply pesticides. Agrobacteria transfer the vector carrying the gene to plant cells, and the plant’s cellular machinery then transcribes the DNA into RNA; the resulting RNA viral vector moves inside the plant, replicating, and the RNA is translated into proteins that ultimately change the plant’s performance.
In another version of the tool, the team directly sprayed RNA virus particles, which deliver the gene to the plant in RNA form.
For both methods, the change is transient—the inserted gene is expressed without entering the plant’s genome, and the viral vector has a short life. “As a plant allows it to replicate, [the viral vector] goes on and destroys itself by losing some parts,” explains Gleba.
“The beauty is that this technology is not editing the genome,” says Hickey, who wrote an accompanying News & Views article on the spray. As a result, he claims that the seeds or fruit harvested would not be considered “genetically modified,” i.e. a GMO.
Gleba and his team confirmed that each spray works by inserting a gene for green fluorescent protein (GFP) into the viral vector and spraying 28 plant species, including potato, tomato, and wheat. After using the agrobacteria spray, cells in the leaves began to glow—proof that the new gene was being expressed.
The viral particle spray also caused cells to glow, though far less efficiently. In the test plant Nicotiana benthamiana, agrobacteria successfully delivered the viral vectors carrying the GFP gene to as many as 1 in 100 plant cells, compared to no more than about 1 in 10,000 cells for the viral particles alone. (The disparity in success might have something to do with the bacteria being able to swiftly move through the stomata, Gleba speculates.)
The team then tested whether the sprays could deliver genes involved in regulatory circuits that control agricultural traits. Using primarily the agrobacteria spray, they introduced a slew of genes involved in flowering time to Arabidopsis, tobacco, and tomato. The researchers were able to trigger or delay flowering, and even force flowering in a tobacco variety that doesn’t naturally flower until day length shortens.
They also suppressed height in crops, such as tomato, pepper, and pea, and controlled traits related to drought and pest resistance. The large number of known plant-trait genes offer a “huge opportunity to find the solution that is exactly what you want for exactly the crop you want,” says Gleba.
The direct RNA viral particle spray was less efficient in tomato and tobacco, but was the only one that worked with crops such as pea and wheat. Gleba views this RNA-based spray as having the most immediate promise for commercialization, as it may sidestep some of the regulatory hurdles governing the release of DNA into the environment.
But it’s not clear whether regulators or the public will accept the spray in an open field environment anytime soon, even in viral particle form, says plant physiologist Brian Ayre of the University of North Texas, who was not involved in the study. Still, Ayre, who in previous work used a viral vector to control flowering time in cotton, was impressed by the study’s diversity of crops and the “versatility of the different genes delivered.” He sees promise in more confined greenhouse environments like commercial seed production. Breeders too could use the spray to shorten generation times and speed up progress, says Hickey.
Gleba doesn’t plan to commercialize the spray, as his company works primarily in the pharmaceutical sphere, but he expects that others will. As Gleba sees it, this temporary reprogramming of crops is “the next Klondike in agriculture.”