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TR: Bt toxins




Clive Newel wrote:

I read this claim recently:
> Toxins used in transgenic food crops are plant-produced Bt
> toxins called cry" toxins and they can be modified for specific
> results, such as to kill only unwanted insects. Also, these Bt
> toxins are only present in plant foliage, not the grain
I have heard the opposite claimed, that the toxins are present in all parts 
of the plant. I have heard that "every mouthful of Bt potato contains a 
dose of Bt pesticide"
Where does the truth lie?

Clive Elwell



The toxins you refer to are proteins expressed by specific genes from the 
common soil bacteria Bacillus thuringiensis.
With modern biotechnology, these genes can be introduced into plants, 
letting the plant express these proteins.
There are hybdreds of different Bt genes identified, each coding for 
different proteins. Each toxin has a very specific mode of action, most 
work on the destructionof the insect (mid) gut, thus preventing the insect 
from further feeding and damage to the plant. The toxins identified have 
been classified with the pre-fix Cry. For exmple, the most used Bt gene at 
the moment in maize is Cry1A(b). The toxins are also very specific 
regarding the insect on which it is active.
Cry1A(b) is toxic to certain lepidopterous species such as European corn 
borer but is not toxic to ladybeetles or bees or other insect genera.
Other known Bt expressed proteins are active against for instance 
mosquitoes, beetles, ants , nematodes etc etc.

Where and how the plant expresses the protein is regulated by the promotor 
used in the gene construct inserted into the plants. By choosing the right 
promotor, protein expression could for instance be limited to the  green 
leaves or to the roots or, if desirable, be expressed in all plant tissues 
at the same time. The first approved Bt maize for instance has protein 
expression limited to the green leaves and pollen and is not present in the 
grains.

If the protein is expressed in all plant parts, it would be true to say 
that you get a mouthful of Bt protein. As the protein is only toxic to 
certain target insect pests and not humans, it is not correct to say that 
you ingest toxins when eating Bt plants.

Edo Lin



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