L3 - Herbicide Tolerance Flashcards
define weed
plant growing in the wrong place at the wrong time
Weeds contribute to what losses
loss in production 13.2% $76bn
What quantitative damage do weeds cause
due to competition with water, light and nutrients, and to antagonism (parasitism and allelopathy)
What qualitative damage do weeds cause
indirect damage due to crop yield reduction, contamination of seeds, slowing of tillage and harvesting practices, and degradation of quality of milk or other animal products
How can weeds be controlled
• Herbicides: but heavy usage has lead to groundwater contamination, death of wildlife species, human/animal illnesses
(Also farmers have to use both broad-spectrum herbicides (before planting) and several narrow-spectrum herbicides (during crop growth))
- Tillage: leaves valuable topsoil exposed to wind and water erosion
- Hand weeding: Very labour intensive, not suitable for large scale
Herbicides contribute how much of the pesticide market
45%
$33bn
N. America has the greatest sales of pesticides T/F
T - 27% of the market
Whats the most common herbicide, whats its LD50 value
Glyphosate
LD50 301 for humans - low mammalian toxicity
(lower the value the more toxic)
What mode of action is taken on grasses and dicots, Roundup is an example of this mode.
•EPSPS inhibitors (grasses & dicots): The enzyme EPSPS is used in the synthesis of the amino acids tryptophan, phenylalanine & tyrosine.
Glyphosate (Roundup) is a systemic EPSPS inhibitor inactivated by soil contact
What are ACCase inhibitors
•ACCase inhibitors (kill grasses): Acetyl coenzyme A carboxylase (ACCase) is part of the first step of lipid synthesis. Thus, ACCase inhibitors affect cell membrane production in the plant meristems
What are ALS inhibitors
•ALS inhibitors (grasses & dicots): The acetolactate synthase (ALS) enzyme is the first step in the synthesis of the branched-chain amino acids (valine, leucine, isoleucine) eventually leading to inhibition of protein synthesis
How do synthetic auxins work?
mimic this plant hormone. They have several points of action on the cell membrane, and are effective in the control of dicot plants. E.g. 2, 4-D
Discuss Photosystem II inhibitors mode of action
reduce e- flow from water to NADPH2+ at the photochemical step in photosynthesis. Cause electrons to accumulate on chlorophyl molecules -> increased oxidation -> death
Discuss Photosystem I inhibitors mode of action
steal e- from the normal pathway through FeS – Fdx – NADP leading to direct discharge of electrons on oxygen
- > reactive oxygen species
- > high levels of oxidation
- > plant death
List all the modes of action - herbicides
- EPSPS inhibitors
- ACCase inhibitors
- ALS inhibitors
- Synthetic auxins
- Photosystem II inhibitors
- Photosystem I inhibitors
When was roundup made by who/when/what is it?
Monsanto
1970
Glyphosate - EPSPS inhibitor
What reaction to EPSP synthase? How does ESPSS inhibitors work?
S3P + phosphenolpyruvate –> ESP
Glyphosate binds to phosphenolpyruvate the enzyme cannot bind
Herbicides can be categorised into what two groups? discuss them
What is the AIM
- Selective herbicides kill weeds but not the crop plant: can be sprayed on a growing crop
- Non-selective herbicides are only usable before the crop emerges
- Easier to develop non-selective herbicides than selective ones
• Aim: make crop plant resistant to a non-selective herbicide. Herbicide will then kill weeds but not the crop plant: herbicide is made to be selective
What are the 4 mechanisms of herbicide resistance/tolerance
- Over-expression of protein which is the target for the herbicide
- Alteration of the site of herbicide action
- Introduction by gene transfer techniques of herbicide detoxification genes from bacteria
- Herbicide detoxification in plants
What gene found were encodes glyphosate resistance? how?
aroA gene
S. typhimurium/ E. coli
Makes glyphosate less able to bind to target protein “Alteration of the site of herbicide action”
psbA encodes resistance to what?
Attrazine
Glufosinate resistance is encoded by what gene? found where? what type of gene is thi?
bar gene
streptomyces
detoxification gene from bacteria
Give an example of Herbicide detoxification in plants
glutathione-S-transferase (GST). This enzyme occurs in plants and renders xenobiotics ineffective by conjugating them to reduced glutathione
How does Soybean 40-3-2 work?
•Resistant to glyphosate (Roundup)
•Contains a mutant form of EPSPS (from CP4 strain of agrobacterium tumefaciens) which does not bind to glyphosate, therefore conferring resistance
- Herbicide tolerance
