Insect Resistance Flashcards
(14 cards)
How much of the world’s crops are lost to prests
Estimated 5-20% of world wide crops lost to pests
Monocultures contribute to this problem
Crops are sprayed with insecticides 6-8 times per season, costing $5 billion a year
What natural resistance mechanisms do plants have for pests?
- Physical barriers
- Toxic secondary metabolites (nicotine)
- Primary gene producers (digestive enzyme inhibitors)
What are digestive enzyme inhibitors?
Affect a broad range of insect species
Certain plants are able to produce inhibitors to insect gut proteases, stops digestion in insects, starve, die
E.g. CpTI
When was CpTI expressed in tobacco plants successfully?
1987, resistant to corn earworm and other tobacco pests
What is another example of a proteinase inhibitor?
Potato proteinase inhibitor II protects rice against pink stem borer
What are some problems for commercialisation of insect resistant plants?
- Affect human digestion?: Insects have simpler digestive systems than humans, CpTI will be inactivated by pepsin in our stomachs
- Resistance?: CpTI binds to protease active site, non-deleterious mutations could impact the effectiveness of these inhibitors
- Affect pollinators?: Only affects insects that eat the plant
What is the Bt toxin?
- From Bacillus thuringiensis strains, which produce crystalline proteins on sporulation which are toxic to insect larvae
- Bind to receptors in mid-gut, opening ion channels, bursting epithelial cells via osmosis
- Also kills insect larvae
- Receptors NOT found in animals
- However, effect is short lived and can be washed off plants, not efficient from an agricultural perspective
How can the short lived effect of the Bt toxin be counteracted?
By producing transgenic plants which express the Bt gene, does not have to be physically applied.
What are the issues of transgenic Bt expressing tobacco?
- Low expression of the Bt gene
Could be due to: - Codon usage bias, which effects tRNA and translation
- Mismatch in GC content between bacteria and plants
- Cryptic splice sites, bacterial genes may accidentally resemble plant splice sites
How would insects evolve resistance to Bt toxins?
- Receptors mutate and no longer bind to the toxin
- To prevent resistance, Bt genes and other genes can be stacked
E.g.: - Bollgard II has cry1Ac + cry2Ab
- Yieldgard plus has cry1Ab + cry3Bb1
What are non-Bt refuges?
Integrated pest management
- Utilises refuge areas planted with non-transgenic plants
- Reduces selective pressure
- More likely for resistant insect to breed with non resistant ect. as most insects in Bt crop are dead
What are the benefits of Bt toxin?
- Increased yields (10% increase in cotton in china)
- Increased opportunity for beneficial insects
- Less pesticide use (up to 50% less in cotton in china)
- Better disease resistance (to viruses and fungi)
From 1996-2008 how much pesticides were saved from being used?
8.4%
Reduction of ~356 million kg of active ingredients
How much cotton in the US is Bt?
Approx 90% of cotton in the US is Bt,
Estimated 10% improved yield
