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.
Unanticipated and wide reaching effects on the health of our soils
It is increasingly apparent that Bt crops are a failing technology, unable to withstand the pests they are designed to ward off. Burkina Faso, the first African nation to cultivate Bt cotton has now pulled it from the market due to the low quality of cotton it produces. India is seeing farmers revert to conventional varieties due to dire failures in Bt cotton yields that have pushed farmers further into debt and suicide.
Admitting Bt crop failures, the GM industry have since marketed crops with multiple Bt toxins in an effort to delay their futility. However, as this latest study, performed by researchers in Huazong Agricultural University under the Chinese Ministry of Agriculture shows, there are many ways in which Bt crops can have unanticipated and wide reaching effects on the health of our soils and wider environment, some of which go beyond the inability of Bt toxins to target pests.
Some problems seen with Bt crops – including the spread of crop disease and rise of secondary pest attacks – may well be exacerbated by the damage to soil biology.
Living soil is essential to life, moderating climate, storing and recycling water and nutrients, biodegrading pollutants, with humans utterly dependent on its survival for food production, and also serves as an important mitigating factor in an era of unpredictable climate change.
The sacred properties of soil have long been recognised by human cultures, but this knowledge has been eroded by the advent of industrial agriculture that has resulted in declining soil health. In the last 40 years, we have lost an estimated third of all arable land to soil infertility, reflected in plateauing and even declining crop yield gains in the last decade, despite continued increases in economic investments.
This study is the latest warning that a decisive shift from industrial / GMO practices to sustainable, agroecological methods is needed to undo the damage, and ensure food security and health of people and planet for the future.
AM fungi act as ‘underground highways' supplying nutrients, information
AM fungi form associations with an estimated 94% of plant species, an ancient mutualism believed to have originated some 450 million years ago that facilitated the colonisation of land by early plants.
They are a vital component of the living soil, supplying plants with key nutrients including phosphate, as well as water, supporting plants' ability to deal with biotic and abiotic stresses such as drought, salinity, disease and pest attacks. In return, plants supply sugars to the fungi.
While mushrooms are familiar to us all, scientists are continuing to reveal the intricacies and complexities of this relationship hidden underground, performed by a vast network of fungal threads, the mycelial network, that make up most of the fungal body. Interacting with many plants, they work as a communication web, or underground 'highway' through which plants can relay information, such as the presence of pests to its plant neighbours.
Recent findings tell of the remarkable ability of AM fungi to relay warnings of aphid pest attacks from one pea plant to its neighbours, allowing them to produce the necessary aphid-deterring chemicals to protect themselves. A 2016 study has introduced the concept that trees are not only in competition with each other for nutrients, but can share them. Carbon is supplied to neighbouring trees, even those of a different species, via AM fungal networks.
Indeed the soil, and mycorrhizae themselves, are important mediators of the wider ecosystem function, with mycorrhizal biodiversity correlating with ecosystem variability, nutrient capture and plant productivity.
Harvesting the potential for AM fungi to improve food production and maintain the health of the environment is an ongoing field of research and has great potential to mitigate some of abiotic and biotic stresses that limit yield.
Examples of recent gains in the field include a study citing a 20% increase in cassava yields in Colombian fields. Low phosphate bioavailability in tropical soils suggests an opportunity for using AM fungi to improve yields in such regions. Fivefold differences in rice growth was obtained when the soils were inoculated with different isolates of one AM fungi species.
Such studies raise questions on the extent population genetic diversity and species diversity in crop growth, and also whether such effects are direct or indirect. Care must also be given in understanding how introduction of AM fungi to soils will affect existing fungal populations as well as the wider ecological soil and plant community.
What is already understood is that any loss to mycorrhizal biodiversity and abundance is a threat to the stability, productivity and sustainability of crop cultivation. Further, effects on climate change cannot be disentangled from the dynamics of the soil; the soil contains more carbon than plants and air combined.
Whole ecosystems – including agro-ecosystems – must be protected in order to nullify this threat, and that includes reversing the roll out of GM crops and their associated pesticides.
AM fungal damage by Bt toxins contributing to yield failures?
The latest evidence that Bt crops damage this symbiotic relationship may provide an explanation for the failure of the GM crops to thrive. They are increasingly failing to yield, to effectively ward of pests as they are designed to do, and to resist plant attacks, drought and other stresses. These problems are afflicting both Bt cotton as well as maize in various regions of the world.
While this study by Chen et al., is the first to show a direct effect of Bt crops on the early stages of the AM fungal life cycle, it builds on previous reports of altered relationships between Bt crops and AM fungi, including reduced colonisation of crop roots by AM fungi, as well as lower density of fungi spores in agricultural plots.
The spread of previously unseen diseases such as root rot in cotton plants in India may well be, at least in part, the result of damaged AM fungal symbiosis. Root rot was not seen until the introduction of Bt cotton in India in 2002-2003, affecting 2-3% of crops in one region of Andhra Pradesh, which spread to 40% by 2008.
Studies in bean plants have shown that AM fungi are able to inhibit disease progression by the same fungal pathogen (Rhizoctonia solani) responsible for root rot in cotton. Bt Bringal (aubergine), piloted in Bangledesh over the last couple of seasons has also been a complete failure according to reports citing a variety of problems afflicting the crop including fungal infections (including root and stem rot, wilt, leaf spot and fruit spot), viruses and pest attacks.
Farmers have therefore had to use additional pesticides, increasing costs and pollution of food and land.
The failure of reductionist approaches
As highlighted in various contexts, the reductionist approach to food production is proviing top be a failure, with the science clouded by short-term fixes and economic incentives.
Even Monsanto now realise that maintaining a healthy soil is key – if only for them to investigate ways in which they can manipulate individual factors in the 'creation' of microbial fertilizers. As ever, the warping of science to suit ideology means that addressing the issue from a holistic point of view is again lost to the pursuit of patentable intellectual property and corporate profit..
We move one step at a time, addressing each crop stress singularly, only for it to have knock on effects on all other interrelated factors. Increasing tolerance to one stress through genetic modification or chemical manipulation will not solve the multiple stressors that likely impact a crop at any one time.
AM fungi and the rest of the soil community, on the other hand, should be harnessed on a 'whole ecosystem' basis for their ability to improve resistance to multiple stresses, from nutrient depletion to climate change, many of which are difficult for us to predict.
Originally Posted: theecologist.org