Australian researchers have developed a gene-silencing spray that can modify plants without actually changing their DNA. That means you could make a crop more resistant to viruses or drought, without having to genetically modify them.

The team hopes that the spray will be used to usher in new, environmentally friendly crop production in the future by enabling crops to weather droughts or grow in harsher conditions without solely relying on genetically modified organisms (GMOs).

"We believe it offers a step change in environmentally sustainable crop protection," team member Neena Mitter from the University of Queensland told Michael Le Page at New Scientist.

Other researchers have developed similar sprays in the past, but the team says their spray was able to protect tobacco plants from a virus for 20 days with a single application, which is the longest result so far.

So how does it work? Well, the spray, known as BioClay, doesn't actively change the DNA of the plants that it touches. Instead, it manipulates a built-in defence system used by plant cells to fight off viral infection, which is capable of temporarily shutting down gene expression.

"When viruses invade cells, the cells cut up some of the viral RNAs to make short pieces of double-stranded RNAs, which they use to recognise and destroy any RNAs with matching sequences," Le Page reports.

This process is known as RNA interference (or simply RNAi), and when the RNAi recognises an RNA strand, it attaches to it and degrades it.

As you might remember from high school biology, DNA is translated into RNA, which then goes out into the cell and makes a protein. But if that RNA is broken down, no protein is made, and the DNA is essentially turned off - it can't trigger any change in the plant. 

Scientists can hijack this process by using the RNA of the gene they want to silence in place of RNAi. When they do this, no virus is actually introduced, but this tricks the plant into thinking its own RNA is a virus, which triggers the defence system and turns off the gene's expression without actually editing the DNA sequence. 

"Once BioClay is applied, the plant 'thinks' it is being attacked by a disease or pest insect, and responds by protecting itself from the targeted pest or disease," says Mitter in a press release.

"A single spray of BioClay protects the plant and then degrades, reducing the risk to the environment or human health."

In the past, sprays that have used the same approach have only lasted for a few days at a time, because the RNAs are quick to break down. And it's too expensive for farmers to have to re-apply a spray every couple of days.

To make the effect last longer, the team took RNA from a tobacco plant that would usually make the crop susceptible to a virus, and combined them with positively-charged clay nanoparticles.

These clay nanoparticles are made of stacked sheets of minerals that protect the negatively charged RNAs and gradually break down to release them to the cell, which prolongs the gene-silencing effect.

When sprayed over, say, a field, the clay will eventually break down, releasing the RNAs, which are absorbed by the plants and kick off the interference, effectively modifying the plant without actually modifying its DNA.

And although early sprays similar to this one cost upwards of US$100,000 per gram, it's possible to make them for as little as US$2 per gram, the team says.

While the hope is that the spray will offer a way to modify crops without changing them at their core, sprays like these do still pose problems.

For example, if we start to use them all of the time, plants might evolve a natural defence against them. There's also a problem with 'non-targets' - plants that don't need editing - also getting misted with the spray, with potentially adverse effects.

There's also likely to be a debate over what we consider GMOs to be in the future.

Does the spray change the plants enough to justify a GMO classification? Or will organic farmers be able to classify gene-silenced plants as "all natural"?  

John Killmer, from the biotech startup Apse, who wasn't involved in the study, but is working on a similar type of spray, doesn't seem too worried about the potential set-backs.

"I have had organic growers call me up and tell me to hurry up with the technology," he told New Scientist.

We'll have to wait to see how everything pans out, but the development of the spray could offer a way for farmers to increase their crop yields by making plants hardier, without messing about with their DNA.

The research was published in Nature Plants.