Pesticide alternative helps plants protect themselves from disease

Though it's not as widely publicized as the effects of climate change, crop diseases are one of the biggest threats to food security. According to the Food and Agricultural Organization of the United Nations, roughly 20-40 percent of the world's crops are lost to pests and diseases each year. Scientists have been experimenting with alternatives to conventional pesticides and thanks to a newly developed gene-silencing technique, farmers might be able to strengthen their crops' defense systems without any potential gene-altering fallout.

The study, led by agricultural biotechnologist Neena Mitter at the University of Queensland (UQ), involves the development of BioClay, a spray that makes use of microscopic sheets of clay that contain double-stranded RNA (ribonucleic acid). This is released when a plant is threatened, triggering a process known as RNA interference (RNAi), a natural process that leads to the plant silencing the genes of the harmful virus. What makes this so fascinating is that this plant defense system is highly adaptive. Its responses are not one-size-fits-all but tailored to counter each virus that enters its system based on the information it receives from the latter's genome.

There are two points to note about the UQ team's success. Firstly, while commercial RNA sprays do exist on the market, they are targeted at killing insects, not building up a plant's defence. Secondly, the plant-oriented techniques that have been produced in labs so far last only a few days, which is a problem in the real world because most farmers operate on slim profit margins and would not want to spray their crops so frequently, especially if it is going to cost them. In their experiments with tobacco plants, Mitter and her team found that a single application of the spray stopped the pepper mild mottle virus from wreaking havoc for 20 days. This is the first time that researchers have been able to achieve such a result.

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

There are many factors that make RNAi an attractive solution to corporations and farmers alike. Firstly, while sprays can easily be tailored to fight new viruses or insect infestations, its use is not limited to disease and pest control. Indeed, there are all kinds of other applications – from helping plants get through droughts to changing the colors of flowers and increasing the nutritional value of food crops.

Secondly, compared to developing genetically modified (GMO) crops from scratch, RNAi sprays are potentially far cheaper and less time-intensive to develop. According to a survey conducted by trade association CropLife International, developing new GMO crops could easily cost more than US$100 million and take more than 10 years to develop.

Thirdly, unlike GMO methods, it is a new technique that doesn't alter the plant's genome, which potentially removes it from the bad press and legal regulations that have dogged biotech crops. In addition, from an ecological perspective, unlike conventional pesticides, which don't differentiate between helpful and harmful insects, with RNA sprays, scientists could theoretically refer to DNA data to avoid genetic matches with friendly bugs, such as honeybees. That said, some scientists have pointed out that this is harder than it sounds, since many insect species often share the same important genes.

Another potential advantage is that it should be less harmful to human health since RNA is broken down quickly by enzymes in the saliva and digestive juices.

The question now is how long it will take researchers to get BioClay ready for commercial use. One of the biggest hurdles is cost – manufacturing RNA is not cheap. Guus Bakkeren, a research scientist at Agriculture and Agri-Food Canada, is of the opinion that while the technique could certainly help advance scientific research, it is still too premature to talk of its use in the field.

"In general, these techniques are very valuable for scientific research, especially in cases where the pathogens cannot be easily genetically modified for testing gene functions, such as the obligate biotrophic rust fungi," he says. In the case of large-scale applications, the costs of producing si- or dsRNA molecules keep it from becoming an economically feasible option for now.

On the bright side, biotech startups such as Apse are attempting to lower the production costs of RNA for agricultural applications. If successful, this could turn RNA sprays into a much-needed breakthrough for farmers and developing countries that depend on agriculture for trade.

The results of the study were published in Nature Plants.

Source: University of Queensland