Fungi don't like Bt crops

September 2016

Photo Creative Commons

The 'Bt' toxins produced by many commercialised GM crops are based on insecticidal proteins produced by soil bacteria, but have many important differences to the natural substance.

To mention just a few, they're plant proteins with bacterial qualities, or bacterial proteins with plant qualities, depending on your point of view. They're produced in a fully active (toxic) form without the environmental barriers inherent in the natural bacterial versions which need to be exposed to insect gut chemistry to become activated. GM 'Bt' toxins are unnatural in conception, construction, location and action.

Industry marketing has plugged its Bt as an insect-specific toxin which, outside of the pest gut, is just an innocuous protein. Who would have thought it could be harmful to fungi?

Soil fungi are fundamental to soil fertility and plant nutrition. They play key roles in the cycling of nutrients in the soil and supplying them to plants, and in soil water availability.

Such fungi also assist the plant in tolerating pathogens and abiotic stresses.

Most plants have an intimate relationship with specific fungi, which grow around their roots and penetrate into the cells of the root where a mutual exchange of nutrients can proceed.

So what happens when a fungus tries to colonise a GM, Bt-generating, plant?

Some studies have already reported negative effects on the fungi associated with Bt-plant roots.

The latest published science took a detailed look at the whole colonisation process in four strains of GM cotton. It covered spore germination, growth outside the root, root penetration, and the final fungal structure development within the root. It also examined related plant growth.

Negative effects associated with the Bt plants were found at all fungal stages measured: spore germination was reduced; fungal growth in terms of strand length and branching was reduced; the formation of the specialised cells which penetrate into the plants was reduced by up to a third; the intensity of root colonisation was reduced by up to a half; the specialised fungal cells inside the plant which channel in nutrients were reduced in number by up to two-thirds and a greater proportion were found to be degenerated than in the non-GM controls. Above ground, the yields of shoots and leaves of Bt plants were lower.

This last finding is easily explained by the lack of adequate fungal colonisation of the roots and consequent impaired nutrient supply. The mechanisms responsible for inhibition of successful fungal presence in the roots remain to be elucidated. There are, however, many possible, and possibly interacting, avenues to explore.

For example, there's the artificial Bt protein itself which can accumulate to surprisingly high levels (up to 3.8 milligrams per kilogram of GM plant matter have been recorded), and the toxin is exuded by the roots into the soil. Direct toxic effects on fungi haven't been ruled out.

Communication failures between the unnatural plants and their fungal friends are a strong candidate: the spores didn't seem to realise the GM plants were there, and the developing fungi didn't seem very confident about the novel plant they were growing beside and into, nor did they seem too keen to feed off the GM plant.

A host of indirect adverse effects could be at play here, for example: unusual by-products of the Bt plants' disturbed physiology could be toxic or be interfering with communications; Bt protein or other unusual root exudates could be altering the soil microbes around the roots with knock-on effects on micobial-linked nutrients and toxins.

OUR COMMENT 

The soil fungi which have been evolving with plants for some 400 million years clearly aren't any more comfortable with the new man-made versions with their strange bacterial-like qualities than we are. 

If they're not good enough for a fungus are Bt crops good enough for you? 

SOURCE

• Xiu-Hua Chen, et al., 2016, Evidences of inhibited arbuscular mycorrhizal fungal development and colonization in multiple lines of Bt cotton, Agriculture, Ecosystems and Environment 230