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APS206: Biotechnology & Food Security > Lecture 9: GM and insect resistance: Bacillus thuringiensis (Bt) > Flashcards

Lecture 9: GM and insect resistance: Bacillus thuringiensis (Bt) Flashcards

1
Q

B.t. =

A

Bacillus thuringiensis

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2
Q

Bacillus thuringiensis is an

A

aerobic gram-positive spore forming bacterium

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3
Q

Bacillus thuringiensis produces:

A

a range of insect toxins known as delta-endotoxins during sporulation

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4
Q

Bacillus thuringiensis affects insects by

A

the crystals produces within the spore dissolves in the insect gut, they are then broken down by the insect digestive system to form the active toxin, and then disrupt the membranes of the gut causing death

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5
Q

how is bacillus thuringiensis used in the field:

A

toxins are extracted fro bacteria & applied to fields as a spray

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6
Q

Natural Bt crystals contain

A

over 21 different toxins

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7
Q

delta-endotoxins how do they work:

A

enzyme in insect gut interacts and causes inactive toxin to be cleaved –> forming active toxin

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8
Q

benefits of Bt toxins over chemical pesticides:

A
  • Specificity
  • -highly specific for insect pests
  • very limited range of activities
  • effective against Lepidoptera & Coleoptera (and some Diptera)
  • Readily degraded in the environment
  • well understood
  • many conventional pesticides are highly toxic
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9
Q

Conventional Bt toxin usage (non-GM):

A

The protein in free form has been applied to fields in massive amounts

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10
Q

Bt targets:

A

highly effective against major pests including diamondback moth (Plutella xylostella), tobacco hornworm & Colorado beetle

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11
Q

advantages to Bt toxin:

A
  • No toxic effects against mammals
  • Public assurances that it was ‘impossible’ to develop since banteria & host coexisted for millions of years (NONSENSE)
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12
Q

disadvantages of Bt toxin:

A
  • Nonsense regarding impossible chance of resistance as were providing a very sturgeon selective pressure (high exposure)
  • rather too readily broken down (requires repeated spraying)
  • toxin must come into contact with target organism
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13
Q

Genetic modification of crops to express Bt toxin- ADVANTAGES

A
  • no need to spray crops repeatedly so reduces insecticide application
  • all parts of the plant can contain toxin (depends on promoter) - useful for protection against root pests
  • highly effective
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14
Q

Genetic modification of crops to express Bt toxin - DISADVANTAGES

A
  • selection pressure for insect resistance is higher
  • -toxin present all the time
  • -originally only single Bt toxins used
  • Bt gene may be transferred to other plants (may creat super-weeds)
  • impact on non-target organisms potentially higher
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15
Q

does it work? example of plant where GM plants expressing Bt toxin are protected against target insect pests

A

Chrysanthemum expressing Bt toxin is protected against beet armyworm larvae

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16
Q

Bollgard in cotton ???

A
  • Bt cotton commercialised by Monsanto as Bollgard
  • Cotton is one of the most heavily sprayed crops grown in the US & insect protection is the highest variable cost associated with cotton growing
17
Q

Bollgard in cotton protects against

A
  • Tobacco budworm (Heliothis virescens)

- Pink bollworm (Pectinophora gossypiella)

18
Q

how many toxins does Bollgard 1 contain?

A

a single Bt toxin (cry1A)

19
Q

how many toxins does Bollgard || contain?

A

2 Bt toxins (cry1A + cry2Ab) (STACKED)

20
Q

how many toxins does Bollgard || contain?

A

2 Bt toxins (cry1A + cry2Ab) (STACKED - to avoid resistance)

21
Q

New studies suggest use of Bt _____ yield

A

STABILISES

  • Monsanto study
  • Strip trials - triple stacked maize vs isogenic control
  • field trials 2005-2009
  • if the conventional variety ‘struggle’d the GM variety tended to do better
  • GM Bt stabilises yield
22
Q

Does GM Bt reduce spray insecticide usage?

A
  • 80 commercial fields growing in Arizona growing non-transgenic, Bt and Bt+herbicide resistant rice were studied
  • there was no significant difference in yield between the crop types
  • Significantly more insecticide was applied to non-transgenic crops in comparison with transgenic crops.
  • CATTANEO ET AL (2006)
23
Q

Does the use of GM Bt plants alter biodiversity?

A

Comparison of ant and beetle density & Species richness between uncultivated fields and cultivated fields
-no add on effect altering biodiversity

24
Q

effect of planting Bt crops on non-Bt crop farmers?

A
  • Planting of Bt crops can help non-Bt farmers, but can also not.
  • example of not: Mirid Bug Outbreak Bt Cotton in China to neighbouring fields and thrived
  • LU ET AL (2010)
25
Problems with Bt: RESISTANCE
- commercial over-application of Bt sprays (NOT GM) has lead to field resistance in Diamondback moth (attacks brassicas) - Resistance to Bt has been observed in lab & field - Cross-resistance develops (resistance to one toxin makes organism resistant to other toxins --> Zhao et al (2005)
26
Problems with Bt: EFFECTIVENESS
- Bollgard protection is not complete (not a 'magic bullet' - initial highly-publicised failure as selected cotton variety was susceptible to drought - resistance increasing
27
Problems with Bt: IMPACT ON NON-TARGET ORGANISMS
- highly-publicised impact impact of pollen from Bt maize on Monarch butterfly (now largely discounted) - toxin persists in the environment in plant material
28
Resistance in field isolates of Helicoverpa sea (corn ear worm) to Bt cotton expressing Cry1A
-No difference in the Lethal Concentration (50%) (LC50) between lab and field strains in 1992-1993. -Significant differences in 2002- 2004. BUT • cross-resistance to cry2A limited • controlled by insecticide use • not responsible for any crop failures-- Tabashnik et al (2008)
29
Does wind dispersed pollen (60m+) from Bt maize have a negative impact on non-target butterfly larvae conditions?
- yes & no - part of the problem is that pollen from any plant (GM or not) has an effect on the larvae - the effect varies between plant varieties
30
plant material containing Bt can ___ in the environment
PERSIST | -get into water ways
31
Why do so many studies not compare like-with-like regarding GM and Bt crops
- access to GM plants is restricted by contracts - GM licensing takes a long time therefore non-GM varieties (planted on a large scale) are always newer than the GM varieties - The debate is heated!
32
solutions to problems of resistance?
- use as part of Integrated Pest Management Scheme - Use multiple toxins (stacking) (e.g. Bollard 2) - rotate crops - plant mosaics - Use refuges to maintain sensitive population
33
problems with refuges to maintain sensitive population
- unpopular with farmers - Biology uncertain (although seems to be working) - compliance poor (but improving); in USA 29% of US farmers broke rules in 2000, in 2001 only 13% - seed companies are meant to be the primary inspectors (clear conflict of interest) - not effective in many countries particularly if farmers retain seed

APS206: Biotechnology & Food Security (19 decks)

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