Herbicide Tolerance

Question 1

How much of global crop production is lost to weed infestation?

Answer 1

~10% of global crop production lost to weed infestation, despite $30-40 billion spent on 100 different herbicides

Question 2

What do herbicides do?

Answer 2

Inhibit essential plant processes (e.g. photosynthesis, amino acid synthesis)
Herbicides are not selective

Question 3

What HT crop has the largest cover?

Answer 3

HT soybean accounts for 37.8% of global GM planting
HT maize is 18.8%

Question 4

Why are herbicide resistant crops rapidly produced?

Answer 4

• Much known about the biochemical pathways targeted
• Genetic resources available
• Single gene mechanisms possible (simple to engineer)

Question 5

What are the strategies for herbicide tolerance?

Answer 5

• Increased levels of target enzyme
• Introduced mutant enzyme resistance
• Introduce new enzyme which detoxifies the herbicide

Question 6

What is glyphosate?

Answer 6

• Most widely used herbicide
• Low toxicity to animals, short half life in soil due to microbial activity
• Effective against 76/78 of worst weeds
• Competitive inhibitor of EPSP synthase, inhibits amino acid synthesis in chloroplasts

Question 7

What have been the tests into whether a plant can become glyphosate resistant?

Answer 7

• Pet CTP: overexpressed petunia EPSP synthase, 40x increase in EPSP activity, plants tolerant to 4x the normal level of glyphosate. However not sufficient resistance
• E.coli mutant EPSP: EPSP is mutant, so does not bind to glyphosate, also lowered PEP activity, did not lead to glyphosate resistance
• A. tumefaciens mutant: retained affinity for PEP, successfully glyphosate resistant.

Question 8

What were Monsanto’s successful herbicide resistant crops?

Answer 8

Round Up Ready soybean and oilseed rape grown commercially in 1995.
Cotton, maize, alfalfa and sugar beet now commercialised.

Question 9

What is glyphosate detoxification?

Answer 9

Bacterial glyphosate oxidoreductase (GOX) metabolises glyphosate to glyoxylate and AMPA (not toxic)
Included in RR maize, soy and rapeseed

Question 10

What is glufosinate / phosphinothricin?

Answer 10

• Found in a variety of major crops
• Functions on glutamate synthase cycle
• Blocks conversion of glutamate to glutamine, increases ammonia levels (toxic)
• Also inhibits photosynthesis

Question 11

What is glufosinate / phosphinothricin detoxification?

Answer 11

• LIBERTY LINK TOLERANCE
• Streptomycin bacteria produces antibiotic compound bialaphos, which is converted to PPT
• The bacteria is resistant due to the BAR gene
• So BAR gene is transformed into plants, making them resistant to PPT

Question 12

What does SmartStax offer?

Answer 12

An eight-gene stacked combo
Monsanto and Dow AgroSciences:
2010, corn with 8 stacked GE genes
Resistant to multiple pests and herbicides (contains Liberty Link)

Question 13

What are the concerns over herbicide tolerant plants?

Answer 13

• Some concerns over the safety of Monsanto products (e.g. agent orange defoliant contaminated with dioxin)
• Active ingredient in round up was considered carcinogenic
• 2015 - WHO said that glyphosate was probably carcinogenic
• European Food Safety Authority ruled that glyphosate did not present a risk