COVID-19 has tragically given many people a crash course in the importance of antibodies, the proteins that attack pathogens and are produced by the sophisticated immune systems of humans and other animals. Now, researchers at a UK plant research institute have found a way to equip plants with an antibody-based defense to a specific threat, which could speed up the creation of crops resistant to any type of virus, bacteria or emergent fungus.
“It’s a really creative and bold approach,” says Jeff Dangl, a plant immunologist at the University of North Carolina, Chapel Hill. Roger Innes, a plant geneticist at Indiana University, Bloomington, adds: “This would be much, much faster than standard plant breeding and hopefully much more effective.”
The strategy is to inoculate an alpaca or other camel relative with a protein from the plant pathogen to be attacked, purify the unusually small antibodies they produce, and engineer the corresponding gene segment for them into a plant’s immune gene. In a proof of concept described today in Sciencethis approach equipped a model plant species with immunity against a modified version of a virus that infects potatoes and related crops.
Farmers lose many billions of dollars to plant diseases each year, and emerging pathogens pose new threats to food security in the developing world. Plants have evolved their own multifaceted immune system, initiated by cellular receptors that recognize general features of pathogens, such as a bacterial cell wall, as well as intracellular receptors for molecules secreted by specific pathogens. If a plant cell detects these molecules, it can trigger its own death to save the rest of the plant. But plant pathogens often evolve and evade those receptors.
A long-standing dream in plant biotechnology is to create genes designed to resist diseases that can occur as soon as pathogens emerge. One approach is to edit the gene for a plant immune receptor, altering the shape of the protein so that it recognizes a particular pathogen molecule. This requires specific knowledge of both the receptor and its target in the pathogen.
Instead, Sophien Kamoun, a molecular biologist at the Sainsbury Laboratory, and her colleagues harnessed an animal immune system to help make the receptor modifications. During an infection with a new pathogen, animals produce billions of subtly different antibodies, ultimately selecting and mass-producing those that best attack the invader.
Camelids, which include alpacas, camels and llamas, are workhorses for antibody design because their immune systems create compact versions, called nanobodies, encoded by small genes. As proof-of-principle for the new plant defense strategy, Kamoun’s group turned to two standard camelid nanobodies that do not recognize pathogenic proteins, but rather two different fluorescent molecules, including one called green fluorescent protein (GFP). The team chose these nanobodies to detect test viruses, in this case a potato virus, engineered to produce fluorescent proteins.
Jiorgos Kourelis, a postdoc in Kamoun’s lab, first reported the nanobody-targeting GFP gene to the gene for an intracellular immune receptor in the tobacco relative. Nicotiana benthamiana. In a follow-up demonstration, she repeated the feat with the nanobody gene recognizing the other glowing protein. It took several tries and adjustments to create plants that did not generate autoimmune responses due to the modified receptors, which would have stunted growth and impaired fertility.
Next, Clémence Marchal, also a postdoc in Kamoun’s lab, investigated how well plants with the nanobody-enhanced receptors detected the altered potato viruses. Marchal found that the plants mounted a vigorous immune response (the patches of self-destructive cells were visible to the naked eye) and experienced almost no viral replication, while the leaves of control plants suffered from infection.
Plant breeders often “stack” resistance genes into plant varieties to add protection against several diseases at once. In the team’s experiment, plants that received genes for both types of nanobody were protected against either virus. “The exciting part of this technology is that we have the potential to create tailor-made resistance genes and keep up with a pathogen,” says Kamoun.
Since then, the group has engineered a culture to produce nanobodies that detect actual pathogen molecules, though Kamoun refuses to identify the plant before the team has tested whether it resists pathogen attack. The Sainsbury Laboratory has filed patent applications around the world on the strategy, including in Europe, where public opposition to genetic engineering means it is unlikely to be commercialized any time soon. But Kamoun says there is commercial interest from elsewhere.
Dangl and others are optimistic that the nanobody approach should work in crops. “This technology is a potential game changer,” he says. Ksenia Krasileva, a geneticist at the University of California, Berkeley, says that nanobody fusion with plant immune receptors opens up a vast body of biomedical knowledge for plant scientists. “Now we can take all that research and translate it to save crops. Here we have a perfect melting point.”
