A study of GMO cotton varieties shows they disrupt an important beneficial soil fungus, writes Eva Sirinathsinghji, apparently due to the Bt insecticide they are engineered to express. Disruption caused by the transgenic cotton to mycorrhizal fungi, and the wider soil ecosystem, may underlie the low yields and poor pest resistance now endemic among Bt GM crops.

A new study finds that transgenic cotton genetically modified to express a Bt (Bacillus thuringiensis) insect toxin inhibits the development of the beneficial soil organism Rhizophagus irregularis, a common arbuscular mycorrhizal (AM) fungus.

The study, which examined three separate genetically modified (GM) cotton lines and three non-GM lines, also found that the GM varieties disrupt the ability of the fungus to form a symbiotic association with the GM crop.

The fungus, when grown with GM cotton, displays fewer reproductive spores, fewer associations with GM cotton roots and increased fungal degeneration.

While the specific mechanism requires further study, the statistical analysis carried out in the study demonstrates that the "Bt-trait significantly contributes to the inhibition of pre-symbiotic development and AM fungal colonization, which might be attributed to either Bt toxin toxicity or interference of signal perception between AM fungi and the hosts."

Analysing three commercialised Bt cotton crops grown in China, the investigators found a reduction of fungal colonisation of roots by 44.4%, 25.0% and 51.3% for each line when compared with their isogenic parental controls.

Branching of the AM fungi was also significantly reduced, with shortened hyphae and reduced arbuscule frequency – tuft-like structures that help colonise roots on the Bt lines, which were reduced by up to 68.2% for one of the lines. This was consistent with a significant reduction in shoot biomass (Bt lines having a biomass of 0.34, 0.33 and 0.30 grams for each line) compared to controls (0.27, 0.27 and 0.25 g).

Crucially, the GM cotton plants appeared to suffer from their reduced ability to associate with the mycorrhizal fungus: they showed reduced shoot growth when compared with non-GM parental lines.

This highlights the importance of maintaining a living, healthy soil – which our industrial agricultural practices are destroying.