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PLNT4600
Issues in Agricultural Biotechnology |
January 29, 2007
Bt Toxin (Bacillus
thuringiensis)
Presented by: Meghan K. Rose
Outline:
What is the Bt
Toxin and how is it used
Types of Bt crops
Toxicity level and
mode of action
Effects on Non-target organisms
Invasion and persistence of Bt genes in
the natural environment
Resistance
What is the Bt Toxin and how is it used?
- The Bt toxin (Bt
Cry) a crystalline protein produced when the microorganism Bacillus
thuringiensis sporulates - Toxic to insects
(different strains of the microorganism toxic to different insects)
- Fermented
cultures have been used as a commercial insecticide spray for the past 40
years
- In 1999, 29 million acres of Bt corn, potato and cotton were
grown globally, saving $92 million in pesticide application in the USA
alone
- A very important
insecticide available for use by Certified Organic farmers
http://textbookofbacteriology.net/Anthrax.html
www.learner.org/.../
Bt transgenic Crops
Corn (field and sweet)- Cotton
- Soybean
- Potato
- Tobacco
- Rapeseed
- Rice
- These crops
express short sequences of the Bt gene from Bacillus thuringiensis
and thus can make the protein themselves http://www.tricity.wsu.edu/aenews/April00AENews/Apr00AENews.htm#anchor5338542
-Ubiquitin promoter
-CryA- coding region
-PolyA tail from nopaline
synthase gene
-constructed by PCR
R. Sardana et al. 1996. Construction and rapid testing
of synthetic and modified toxin gene sequences -plants transformed by particle bombardment
CryIA (b&c) by
expression in maize endosperm culture. Plant Cell Reports. Vol 15:9
Toxicity levels and mode
of action
TABLE 1
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No Observable Effect Level for mortality following exposure of rats to purified Bt toxic protein (A) |
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Registered Transgene
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Registrant
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NOEL (mg/kg)
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Digestibility
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Bt CryIA(b) |
Monsanto |
>4000 |
Rapidly degraded |
|
Bt CryIA(b) |
Novartis |
>3280 |
Rapidly degraded |
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Bt CryIA(c) |
Dekalb |
>5000 |
Rapidly degraded |
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Bt Cry9C |
AgrEvo |
>3760 |
Stable |
|
Bt spray |
Several |
>5050 |
Degraded |
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Table Salt |
|
3750 (B) |
|
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Caffeine |
|
25 |
|
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Vitamin A |
|
4.3 (C) |
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2,4-D |
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1 |
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A A)
All information from the EPA Biopesticide Safety Sheets (See endnote 16)
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·
Many household substances are more toxic to rats at a
much lower dose than the Bt toxic protein
·
Humans exposed orally to 1000 mg/day for 3-5 days of Bt have showed no ill effects.
TABLE 2
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Concentration of protein (microgram per gram of wet plant tissue) in various corn tissues and estimated grams of protein per acre of corn. |
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Registered
Transgene
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Whole Plant
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Leaf
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Roots
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Pollen
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Grain
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Grams of
insecticidal protein per acre
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Bt CryIA(b) |
3.65-4.65 |
7.93-10.34 |
NA |
0.09 |
0.18-0.39 |
16.4-20.9 |
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Bt CryIA(b) |
0.6 |
4.4 |
<0.008 (A) |
7.1 |
<0.005 (A) |
2.7 |
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Bt CryIA(c) (B) |
0.22 |
0.10 - 0.26 |
0.03 |
NA |
0.01 |
1.5 |
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Bt Cry9C (B) |
4.7 |
9.5 |
5.6 |
0.1 |
4 |
21.3 |
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Bt spray (C) |
NA (D) |
NA |
NA |
NA |
NA |
14 |
(A) Limit of detection for the Novartis transgene protein was roots,
0.008 µg protein per gram tissue and grain, 0.005 µg/g
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·
Not all
tissues in the plant produce the same level of the protein
http://www.tricity.wsu.edu/aenews/April00AENews/Apr00AENews.htm#anchor5338542
·
Bacillus thuringiensis
o
var. tenebrionis
- Colorado potato beetle, elm leaf beetle larvae
o
var. kurstaki
- caterpillars
o
var. israelensis
- mosquito, black fly, fungus gnat larvae
o
var. aizawai
- wax moth larvae, various caterpillars, especially the diamondback moth caterpillar
http://www.nysaes.cornell.edu/ent/biocontrol/pathogens/bacteria.html
Effects on non-target organisms
- Doesn’t
persist in the digestive tract of mammals
- Has shown some
cases of eye and skin irritation in experiments with rabbits
- Ladybugs and
pollinator bees (beneficial insects) are unharmed as well as spiders,
lacewings, hover flies, beetles, and earthworms. All these insects are
more harmfully affected with the use of chemical pesticides.
- MONARCHS
- We’ve all
heard this argument against Bt corn, here’s what I found
·
Milkweed
leaves dusted with Bt transformed corn pollen and fed to monarch butterfly
larvae caused an increase in mortality rate and a decrease in growth rate.
Compared to larvae fed leaves with non 
transformed pollen - no effect.
·
Does
the amount of pollen on this leaf accurately represent how much pollen would
land on milkweed growing along the edges of fields and in ditches?
·
Monarch
Butterfly numbers are declining. Why?
o Winter storm mortality was high in 03-04 season
o Poor weather during 04 breeding season
o Habitat deterioration in over wintering sites in
Mexico
o Summer breeding habitats and nectar sources in US
being eliminated by herbicides
o WHERE’S Bt?????????????? Didn’t make the
list
·
Other
findings
o
Not all
strains Bt corn are equally toxic to the larvae and adult monarchs
o The only milkweed with enough pollen to cause damage
is adjacent to a corn field (a few meters). However, the EPA says there is not
enough pollen in the surrounding 1 meter to be lethally toxic.
o Pollen is only shed for a short period in the season
·
Solutions??
o Can we eliminate expression of the gene in the
pollen?
o Can we keep the edges of corn fields free of
Milkweed?
o Can we eliminate the varieties of Bt Corn that are
the most toxic?
Invasion and persistence of Bt genes in the
natural environment
Results from the EPA,
http://www.epa.gov/pesticides/biopesticides/pips/are_bt_crops_safe.pdf
·
gene flow
from corn, cotton and potatoes to weedy species do not have “a reasonable
possibility of occurring”
o inability to pollinate wild relatives (differences in
genomes and life cycles)
o Cotton has the possibility of transferring genes
through pollen to wild relatives in Hawaii, Puerto Rico, US
·
horizontal
gene transfer from Bt crops to soil microorganisms is “extremely
rare” and only occurs under artificial circumstances favoring transfer,
not field conditions.
Resistance
- Bt sprays and
crops are very important in agricultural practices in
- A major concern
for organic farmers as Bt sprays is one a few pest management tools at
their disposal
- Currently the
EPA has outlined the following measures to delay the onset of resistance
in an IRM (insect resistance management)
program. Similar standards exist in
- Planting refuge
crops for every Bt crop seeded
This
set up increases the probability of a mating between a Bt resistant insect and
a non-resistant insect leading to non-resistant offspring.
http://www.bt.ucsd.edu/crop_refuge.html
o
Monitoring
target insect changes in susceptibility
o
Grower
education
o
Remedial
action plans by companies in the event resistance does occur
- Failure of a
producer to comply with these regulations may result in the ineligibility
to purchase Bt crops
- Currently in the
field, only the diamondback moth larvae are known to be resistant to Bt
toxin
In Conclusion
There are still some
unknowns about how the Bt gene may affect wild relatives and whether there are
more negatively affected non-target organisms. Overall, through my internet
research I found that the risks associated with Bt crops do not out weigh the
benefits of this environmentally friendly pest management system. Proper
management of Bt crops (through IRM programs) is crucial for preserving their
effectiveness.