14. Growing crops

Question 1

Describe the discovery of Bt toxin. 6

Answer 1

1. The bacterium Bacillus thuringiensis (Bt) produces spores, containing a crystal like structure made of protein
2. this protein, Cry, is very toxic to insects and was killing of silkworms (Bt toxin)
3. bacteria produces spores which are eaten by insects
4. spores grow into bacteria inside insect’s gut
5. prototoxin is cleaved in mid gut into acitve toxin
6. it binds to a receptor and increases permeability of gut so ions leave and insects starve

Question 2

how specific is Bt toxin? 4

Answer 2

1. only kills lepidoptera (moths and butterflies), diptera (flies) and coleoptera (beetles)
2. Different toxin affects different insects, can target subsets of insects
3. one strain (subspecies) may contain several different forms of the toxin
4. other animals incl humans can’t cleave it so not affected - highly specific

Question 3

how is bt toxin used as a chemical insecticide spray? 6

Answer 3

1. this is classed as an organic pesticide so hot research topic
2. possible to grow b. thuringiensis
3. dry out and spray on fields, initially leaves active bt toxin on crop
4. not that effective, doesn’t last long as crystals are released and toxins broken down
5. effective protection lasts a few days
6. unlikely that spray will land on same leaf as insect

Question 4

how is bt used as an organic pesticide? 6

Answer 4

1. gene cloned into another bacterium called pseudomonas fluorescens via gm
2. toxin is stable, even after spraying
3. bacteria are dead, not released into environment
4. effective but not very efficient
5. most widely used form of insect control in some crops
6. long period of safe use (GM!)

Question 5

how do we express bt toxin in plants? 5

Answer 5

1. this is more efficient and gene had already been safely and successfully cloned
2. place bacterial gene in front of 35s promotor that gives high levels of expression in all cells
3. nearly all cotton, maize etc have been/will be transformed this way in USA
4. if larvae feed on transgenic plants, they ingest toxin and die
5. now must prevent resistance arising as ideas spread

Question 6

What are the advantages of bt corn? 4

Answer 6

1. more effective pest control
   2, biggest maize pest, the european corn borer, make a hole in the maize plant
   3.this gives pathogens eg fungi an opportunity to enter plant and cause disease
2. bt toxin stops this, so yields are higher

Question 7

How is bt toxin currently used? 3

Answer 7

1. growth has increased almost 10x
2. 20 years since intro of this technique, and now up to 90% of some crops use this or a similar method
3. some now use 2 toxins to reduce chance of resistance

Question 8

Describe methods of conventional weed control. 6

Answer 8

1. plants are autotrophic and synthesise their own aromatic amino acids, these pathways are potential herbicide targets
2. other plant specific pathways are also targeted eg photosynthesis
3. herbicides must be selective, kill weeds without damaging crop
4. it is relatively easy to do this is species are different eg. monocots and dicots, but difficult if similar eg. wild vs crop oats
5. we use a lot of herbicides
6. it is difficult to find new herbicides

Question 9

What are the problems with conventional herbicides? 5

Answer 9

1. require more than one herbicide and several applications
2. often have poor environmental properties eg persistence in environment and non-target toxicity
3. many eg. atrizine have been banned due to our increasing health standards
4. used as pre-emergence sprays before crop grows, just in case weeds are going to grow
5. more resistance arising, broad based

Question 10

what are roundup ready crops?3

Answer 10

1. produced by monsanto
2. own the rights to herbicide glyphosate
3. widely available from garden centres in the uk

Question 11

describe aromatic amino acid synthesis in plants. 4

Answer 11

1. important in plants
2. precursors for other things
3. the shikimate pathway is blocked by glyphosate
4. the products are metabolised easily by microoganisms

Question 12

Do you know the shikimate pathway?

Answer 12

do you?

Question 13

what are the advantages of glyphosate? 6

Answer 13

1. borad based, kills everything
2. no plant naturally resistant, but more and more becoming resistant
3. relatively environmentally friendly compared to traditional control methods, doesn’t persist and breaks down on contact with water
4. very specific inhibitors of EPSP synthase
5. animals, incl, humans, don’t make aromatic amino acids and lack this pathway
6. therefore, not toxic to humans/animals

Question 14

How do we find genes that confer resistance to glyphosate? 5

Answer 14

1. salmonella sysnthesise aromatic amino acids
2. easy to find mutants
3. will not grow in presence of glyphosate
4. AroA mutants will grow on glyphosate
5. AroA mutants have altered EPSP gene

Question 15

What is the dual resistance strategy for herbicides? 5

Answer 15

1. resistant gene alone not efficient as glyphosate can accumulate in cell and inhibit cell if it has CP4 gene
2. glyphosate oxidase (GOX) was isolated from ochrobactrum anthropi
3. this causes metabolism of glyphosate into harmless intermediate, glycine
4. in oilseed rape (canada), monsanto use both agrobacterium CP4 resistance gene and GOX
5. Resistant and aids glyphosate breakdown

Question 16

How do we engineer plants for glyphosate resistance? 4

Answer 16

1. whole plant must be made resistant
2. place mutant AroA gene in front of a strong promoter eg. CAMV 35S, whichis expressed everywhere
3. herbicide still inhibits plant gene, but the mutant bacterial gene still works
4. Every cell has the gene expressed and gives resistance to the herbicide, systemic

Question 17

How do we grow glyphosate resistant plants? 5

Answer 17

1. grow field of genetically engineered crop containing mutant AroA gene
2. spray with roundup/glyphosate
3. May be used pre or post emergence
4. Resistant crop grows normally, all weeds are killed
5. In USA, approx 90% of soybean plants are resistant to glyphosate/alternatives

Question 18

How did we find better glyphosate resistance genes? 3

Answer 18

1. resistance genes from e.coli and salmonella had decreased affinity for the substrate (km)
2. found gene cp4 from agrobactrium that is resistant and retains the low Km
3. used in most dicot crops eg. soybean, cotton, oilseed rape

Question 19

Describe glyphosate resistant soybeans. 3

Answer 19

1. been use din usa for 20 years with no bad side effects
2. UK worried it is too efficient
3. worried all weeds will die, then other, dependent animals eg birds will die

Question 20

What are the disadvantages of using transgenic soybean? 6

Answer 20

1. farmers pay premium to monsanto
2. no increase in yield
3. may still have some toxicity
4. reliance on one herbicide
5. spread of glyphosate resistance, many cases reported
6. large company, monopoly

Question 21

What are the advantages of transgenic soybean? 6

Answer 21

1. more flexible, pre or post emergence use, and only needs to be sprayed when needed
2. decreased tillage, less erosion
3. increase in glyphosate application offset by decrease in application of other herbicides
4. about 10% decrease in total herbicide applications
5. on average, other herbicides at least 3x more toxic and persist in environment 2x as long
6. overall, a farmer saves money on herbicides