The molecule that protects against aflatoxin

Researchers at the University of Arizona have found a promising way to prevent the loss of millions of tons of crops to a fungus each year, offering the potential to dramatically improve food security, especially in developing countries.

The team’s approach uses transgenic corn plants that produce small RNA molecules that prevent fungi from producing aflatoxin, highly toxic substances that can render an entire harvest unsafe for human consumption even in small amounts.

Crops all over the world are susceptive to infection by fungi of various Aspergillus species, a fungus that produces secondary metabolites known as aflatoxins. These compounds have been implicated in stunting children’s growth, increasing the risk for liver cancer and making people more susceptible to diseases such as HIV and malaria. The results of the study, published in Science Advances, showed that transgenic corn plants infected with the fungus suppressed toxin levels below detectable limits.

Unlike in the US, where crops intended for human consumption are tested for aflatoxin and incinerated once levels approach 20 parts/billion (equivalent to one drop of water in a 22,000-gallon pool), no testing is available in many developing parts of the world, especially in Africa, where millions of people depend on consuming what they harvest. There, toxin levels up to 100,000 parts/billion have been measured, said study leader Monica Schmidt, an assistant professor in the UA’s School of Plant Sciences and a member of the UA’s BIO5 Institute.

“Aflatoxin is one of the most potent toxins on the planet,” Schmidt said. “Usually it won’t kill a person outright, but it can make you very sick.”

Funded by the Bill and Melinda Gates Foundation, Schmidt and her team set out to study whether a naturally occurring biological mechanism called RNA interference could be used as a weapon against the Aspergillus toxin. That approach, called Host-Induced Gene Silencing, or HIGS, builds on previous work by other researchers who discovered that during the infectious process, the host plant and the fungus exchange small nucleic acid molecules.

The HIGS approach has a distinctive advantage over existing efforts to keep aflatoxin out of the human food chain, because it prevents the fungus from making toxin in the first place while the crop is growing in the field, as opposed to protecting crops only after they have been harvested and stored. Such approaches include solar-powered fans that suck out air from storage facilities or sealing crops in huge storage bags creating airless conditions so fungus cannot grow.

In their experiments, the team infected corn plants with Aspergillus and let them grow for one month. While untreated control plants were found to harbour toxin levels between 1000 and 10,000/billion, toxin levels were undetectable in the transgenic plants.

“The detection limit is not zero, but low enough for the corn to be safe to eat,” Schmidt said.

“This corn plant would be like any other,” she said. “The only trait that sets it apart is its ability to shut down the toxin production. It shouldn’t have any other effects, but obviously, a lot of downstream testing will be required before it could be grown in the fields.”

Image caption: Despite being infected with Aspergillus fungus, this transgenic corn cob is not contaminated with aflatoxin, thanks to its defence line of RNA molecules that shut down the toxin production in the fungus. Photo: Monica Schmidt.