New ‘medicine’ could vaccinate plants against devastating viruses

Few things are scarier to a pumpkin farmer than the letters CMV. They stand for cucumber mosaic virus, a pathogen that lays waste to entire fields of pumpkins, cucumbers, and melons. Now, researchers have found a way to quickly develop vaccines that could eventually protect crops from viral pathogens.

“This is a really neat finding,” says Anna Whitfield, a plant pathologist at North Carolina State University in Raleigh who was not involved in the study. Viruses are an ever-evolving threat to global food security, Whitfield says, and the new technique might help farmers keep up with the constantly changing pathogens.

When a virus infects a plant cell, it often releases RNA—either in the form of messenger RNA or double-stranded RNA—which travels through the cell, helping the virus replicate. Defense proteins inside the plant cell recognize these viral RNAs, and enzymes that act like tiny scissors slice them apart. Some of the resulting bits and pieces of RNA, called small interfering RNAs (siRNAs), team up with a group of proteins called the Argonaute complex. The siRNAs serve as identifiers that lead the Argonaute complex to RNA on the virus genome, which the Argonaute complex and other proteins then destroy.

The tactic is deadly, but not always efficient. Of the many thousands of various siRNAs made by the plant, very few have the right chemical properties to fight the viral RNA. Biochemist Sven-Erik Behrens at Martin Luther University Halle-Wittenberg in Germany and his colleagues set out to streamline the process.

They developed molecular tests to identify which siRNAs are efficient at fighting viruses. In laboratory experiments with tobacco plants, they showed they could pick the winners and use them as a vaccine against the tomato bushy stunt virus, which slows growth and damages leaves in tobacco plants. The best siRNA, sprayed on the leaves, protected 90% of the plants, the team reports this month in the journal Nucleic Acids Research .

There are other ways to predict which siRNAs might be effective against a plant virus, but most of these are computer models that do not always work as expected, Behrens says.

An exciting part of the study is that the team simply sprayed the siRNAs on the plant or rubbed them onto the leaves, Whitfield says. This is much simpler and faster than genetically engineering a plant for viral resistance, allowing scientists and farmers to keep up with the quick evolution of viral pathogens.

The researchers are now working on finding the most efficient and cost-effective way of administering the vaccine to plants, such as a spray that uses nanoparticles to deliver the siRNAs. They’re also trying to identify siRNAs that would fight economically important viruses such as CMV or the zucchini yellow mosaic virus, on plants from melons to tomatoes.