Just as fire drills teach us how to respond should an actual fire break out, vaccines expose the immune system to a harmless infectious agent, typically components of a pathogen or a severely weakened pathogen, that trick it into building antibodies that can recognize a real threat.
This strategy of tricking our immune system has been effective against 26 different diseases in humans and eradicated smallpox altogether. We have also helped our furry family members with vaccines against diseases like canine parvovirus in dogs and panleukopenia in cats. Presently, developing a vaccine for COVID-19 is our best hope for ending the ongoing pandemic that has already claimed thousands of lives.
The scramble to develop a vaccine for this novel coronavirus offers us an opportunity to ask an equally important question: Given the effectiveness of vaccines for humans and animals, can we make plant vaccines that protect our food supply from destructive pests?
This is a complicated question, but the short answer is yes. Vaccine-like technologies have already been developed to protect plants from deadly infections. Not only can these tools help reduce billions of dollars worth of crop damage, they pose little risk to human health and could help cut the use of synthetic pesticides.
A primer on plant immune response
Let’s examine those aforementioned complications. Plant vaccines work a bit differently than those designed to protect humans and our pets, primarily because plants possess an innate immune system that functions differently from the adaptive immune system present in humans and other animals.
The plant immune system is made up of two parts: receptors on the outside of cells that identify pathogens or byproducts of infected cells, and proteins within cells that recognize infection-promoting molecules released by pathogens. Activation of either of these mechanisms during an infection results in a strengthening of cell walls, producing enzymes to counter damaging pathogenic enzymes and antimicrobial compounds called defensins and phytoalexins. This process also triggers something known as a “hypersensitive response” that leads to strategic cell death as a way to stymie progress of the infection.
Put simply, the difference between plant and animal immune systems is that instead of organizing a directed assault against a specific pathogen, plants sound a general call to arms. Furthermore, unlike antibodies, the receptors and proteins in plants that recognize signs of infection are genetically coded so that new receptors cannot be produced within a generation to deal with a new pathogen. This is why the plant immune system is innate and animal immune systems are adaptive.
Can vaccinating plants be an effective strategy?
Given these differences in immune systems, plant vaccines would induce a general strengthening of immunity (instead of preparing the plant to deal with a specific threat) by tricking its immune system to activate with a fake infection.
Through a mechanism called defense priming, compounds that bind to immune receptors or proteins in uninfected/unharmed areas of the plant stimulate the accumulation of molecules that amplify cellular signals, increase the number of immune receptors produced, and facilitate changes in DNA packaging to allow for faster access to defense-related genes.
In this heightened defensive state, the cells are able to respond faster and more forcefully to a pathogen. There are already several products available that function in this manner. While not called plant vaccines, these products use pathogen-derived molecules to stimulate defense priming in plants, often as a supplement to normal pesticide and fungicide applications.
One example is the Messenger pesticide, which uses a protein called Harpin from Erwinia amylovora, a pathogen that causes fire blight in apples and pears. Another product uses chitosan: a derivative of chitin, which is found in fungal cell walls and insect exoskeletons. Research shows that pre-treatment of plants with chitosan before fungal infection improved resistance by as much as 68 percent when applied 72 hours before infection. Even when co-treated with the fungus, plants had improved immunity, though the effectiveness of these plant vaccines will vary based on the plant species and pathogen causing infection.
Boosting plant immune systems with these vaccine-like compounds could be a novel way to reduce overall use of pesticides. With many in the public disapproving of synthetic chemical pesticides, plant vaccines could serve as a natural replacement or amendment to current crop protection strategies. At the first sign of infection a farmer could apply a vaccine to the whole field to reduce the spread, while applying pesticides only to the affected area. Notably, many naturally derived plant vaccines are known for their non-toxicity to humans and the environment. Chitosan has even been used in different drug delivery systems for pharmaceutical applications.
While plant vaccines may not completely replace synthetic chemical pesticides, they are another important tool in the arsenal to keep crops happy and healthy, and by extension to ensure our access to a sustainable food supply. As farmers know all too well, pests and pathogens can quickly learn how to defeat individual crop protection tools. Plant vaccines can therefore help us stay one step ahead in the evolutionary arms race against insects and microbes that want to eat our food.
Tautvydas Shuipys is a PhD candidate in the Genetics and Genomics Graduate Program at the University of Florida. Follow him on Twitter @tshuipys