Aflatoxin-free crop technique could beef up third world food security

Crops all over the world are susceptive to infection by fungi of various Aspergillus species, a fungus that produces secondary metabolites known as aflatoxins.©iStock

A technique that reduces plant contamination from a toxic metabolite could help save millions of tons of crops from this threat to food security especially in the developing nations.

The approach—Host-Induced Gene Silencing (HIGS)—focuses on modifying corn plants to produce small RNA molecules that prevent fungi from producing the highly poisonous chemical aflatoxin.

In eliminating the aflatoxin out of the human food chain, the HIGS approach has the advantage of preventing the fungus from making toxin in the first place while the crop is maturing.

"This corn plant would be like any other," said lead study author Dr Monica Schmidt, assistant professor at the University of Arizona's School of Plant Sciences.

"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."

Current strategies focus on protecting crops only after they have been harvested and stored and include solar-powered fans that suck out air from storage facilities. Other approached have involved sealing crops in huge storage bags to create airless conditions so fungus cannot grow.

Besides the health concerns, the Food and Agricultural Organization (FAO) have estimated that 25% of the world’s crops are wasted each year due to this type of contamination, representing a critical economic loss for crop producers.

Maize, the staple food for many sub-Saharan countries, has been considered as one of the most susceptible crop commodities to aflatoxin contamination.

Study details

Dr Schmidt and her team began by introducing an engineered DNA structure into the corn that in turn passes RNA material into the fungus when it infected the corn plant.

The modified corn plants are then equipped with genetic instructions for small RNA molecules that only affect the edible kernels, not the whole plant. 

Once inside the fungal cells, these RNA molecules find their corresponding target sequences of the fungus' own RNA that code for an enzyme needed for toxin production.

This causes the toxin production to shut down, but does not impact the fungus in any other way. The fungus continues to harmlessly grow and live on the corn.

"The corn is constantly producing that RNA during the entire development of the kernel," Schmidt explained. "When the kernels come in contact with the fungus, the RNA moves over into the fungus."

In total, the team infected corn plants with Aspergillus monitoring their growth for one month.

While untreated control plants had toxin levels between 1,000 and 10,000 parts per billion (PPB), 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," Dr Schmidt said. 

Aflatoxin levels worldwide

The World Health Organisation has categorised aflatoxins as a category one carcinogen, meaning they are a definite cause of cancer.

Despite this, EU regulation on allowed aflatoxin levels was raised from 4 micrograms per kilogram (μg/kg) to 8.10 μg/kg in 2010.

The EU has also set specific limits for certain aflatoxin types such as aflatoxin B1 and for total aflatoxins (B1, B2, G1 and G2) in nuts, dried fruits, cereals and spices.

Limits range from 2-12 μg/kg for B1 and from 4-15 μg/kg for total aflatoxins.

There is also a limit of 0.050 μg/kg for aflatoxin M1 in milk products. In addition, limits of 0.10 μg/kg for B1 and 0.025 μg/kg for M1 have been set for infant foods.

Other studies look to preventative methods, including a European study from 2014 that found up to 70% of aflatoxins were wiped out during preparation and cooking of almond nougat in caramelised sugar.

 

Source: Science Advances

 Published online ahead of print: DOI: 10.1126/sciadv.1602382

“Aflatoxin-free transgenic maize using host-induced gene silencing.”

Authors: Dhiraj Thakare, Jianwei Zhang, Rod Wing, Peter Cotty and Monica Schmidt

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