Bt is toxic to plants

July 2011

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A recently published paper demonstrates the breathtaking assumptions on which commercial GM crops are based.

When genetic transformation of plants was first attempted, it was soon discovered that, while all DNA has a simple four-fold chemical structure which can easily be translated by man into a specific protein, DNA is not just DNA.

Most artificial DNA constructs to date have been copied from bacterial genomes, and multiplied up in a second species of bacterium before being inserted into plant cells. However, plants won't willingly generate a bacterial protein. Genetic engineers found they had to alter their artificial DNA considerably. For example, they have to make the 'copied', DNA chemically plant-like, and often have to trim it down to bare essentials to reduce its interference with the plant's physiology. The novel gene will not do anything unless it is switched on, and so a viral DNA 'promoter', powerful enough to force gene expression in any genomic environment, is routinely added on.

Finally, the engineered construct has to be attached to a 'transfer' DNA sequence at one end to stick it into place. To identify whether or not a cell has been successfully transformed, a 'marker' gene (often an antibiotic-resistance gene) is attached to the other end.

Once these tricks of the genetic engineering trade had been developed, industry homed in on two, single-gene, traits for commercial use: crops which shrug off herbicides, and crops which destroy their own insect pests.

The assumption that herbicide-tolerant plants remain unharmed by the herbicide they're designed to 'tolerate' was recently exploded (see WHAT GLYPHOSATE DOES TO PLANTS – GMFS News archive, December 2010). Similar assumptions of no harm in the 'Bt insecticide-generating plants are, only now, being proven wrong too.

Moreover, the effectiveness of all the smart GM tricks employed for decades are being questioned.

A belated study has revealed that Bt 'Cry1Ac' insecticidal protein expressed in GM maize and cotton commercialised in over 50 countries, is toxic to the plants as well as to insects.

It seems that, finding themselves generating a toxin, the GM plants naturally protect themselves by suppressing its production. The result is that despite the viral promoters, levels of Cry1Ac can be successfully reduced in specific plant tissues, and at certain times during the plants' life.

In practical terms this means that, not only is the plant weakened by the inbuilt toxin, but it is vulnerable to insect attack because the GM insecticide may not be present where the pest is.

(COMMENT Compare this with natural plant defenses which are switched on in response to insect attack so that they're there when and where they're needed: a plant with a single hole in a leaf is a very healthy one which has sent the potential diner packing).

For farmers, the implications are serious. The GM crop is haphazardly protected, and chronic low-level Cry1Ac production provides and ideal situation for the generation of insects resistant to the toxin. The biotech industry's answer to these failings of its Bt crop has been to recommend farmers apply prophylactic insecticidal sprays to avoid resistance developing (i.e. routine extra work and expense which GM growers expected to avoid in the first place).

The study also found that attaching the Bt-gene directly to transfer DNA doesn't necessarily make it end up in the cell: it's just as possible for the desired gene to be lost while the transfer and marker DNA flanking it end up as planned in the plant.

(COMMENT This suggests the interesting possibility of non-random alterations to the invading DNA, which just happen to exclude the genes which produce toxins the plant doesn't want)

What the authors realised was that between the newly discovered toxicity of Cry1Ac and the false-positive transformations arising from the marker genes which have detached from the DNA they were supposed to be marking, GM crops developers have been actively selecting against plants with high, stable Cry1Ac gene expression. The surviving, naturally fittest, plants are the ones which eliminate or control the Bt gene.

The editor of the publishing Journal commented:
“This is a completely unexpected finding. Until this point, if you asked someone in the plant biotechnology community what the Cry1Ac toxin does in plants, they would say it kills insects. No one has yet demonstrated harm to plants as this study has done.” 
The researchers point out that their observations may have been overlooked in the past as most previous studies were aimed at finding plant varieties that can be genetically altered just enough so that they are suitable for cultivation. Indeed, a few minutes with a search engine will tell you that the research on Cry1Ac has focused on which pests it kills, how it kills them, and the pest-resistance management needed. Effects on the plant itself (the thing we are going to eat and farmers are going to sell) have been side-stepped.

Industry has quite evidently biased the research agenda on these crops by its entirely commercial focus. Despite the warnings from the minuscule success rate of transformations with Cry1Ac (all such GM maize, world-wide, is derived from only 6-7 original transformation events), the crops were rushed to market.

It has been established that the negative effects of Cry1Ac on the plant can be reduced and consistency of pesticide generation can be improved “to a certain extent” by gene insertion into the chloroplasts (the green part of the plant cells).

(COMMENT Such GM plants would still, presumably, be vulnerable to insect attack on the non-green roots and seeds.)

Interestingly, in the current Bt cotton and maize plants, which have undergone nuclear transformation, Cry1Ac levels are linked to the chlorophyll (green) content of the tissues. The reasons for this and the mechanism of the toxic effect are unknown.


Besides side-stepping the effects of this BtBt-toxins along with many other transgenic proteins should be suspected allergens until proven otherwise: 

“Bt-toxins are actual and potential allergens for human beings. Field workers exposed to Bt spray experienced allergic skin sensitization and induction of IgE and IgG antibodies to the spray. Recombinant Cry1Ac protoxin was found to be a potent mucosal immunogen, as potent as cholera toxin. A Bt strain that caused severe human necrosis (tissue death) killed mice infected through the nose within 8 hours, from clinical toxic-shock syndrome.”

(Read the whole article - Scientists confirm failures of Bt Crops - on the Institute of Science in Society website) The case for this recommendation to be implemented is now reinforced.

The recent discovery of Bt protein circulating in the tissues of humans exposed to GM food and GM-fed animal products reinforces the need for precautionary action. If you haven't already done so, think about responding to the ACTION ALERT ON Bt – May 2011.

The safety studies for these GM crops grown in more than 50 countries haven't reached the starting post, and things aren't much different for the GM in the pipeline. As this one insightful, independent, study has shown, the simplest and cheapest course to a safe and secure food supply is one undistorted by 'clever' GM tricks.

  • Preeti Rawat it al., 2011, Detrimental effect of expression of Bt endotoxin Cry1Ac on in vitro regeneration, in vivo growth and development of tobacco and cotton transgenics, Indian Academy of Sciences, Journal of Bioscience, June
  • Gene alarm on GM crops, Telegraph, 2.06.11
  • Scientists Confirm Failures of Bt-Crops, Institute of Science in Society Report 26.09.05

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