Introduction
Weeds are unwanted and useless plants that along with the crop plants. Weeds compete with the crops for light and nutrients, besides harboring various pathogen. So it is estimated that the worlds crops yield is reduced by 10-15% due to the presence of weeds. Herbicides are broad spectrum as they can kill wide range of weeds.
What are Herbicides?
Herbicides are chemicals that are sprays on the garden used to kill weeds. They are generally a last resort for home gardeners. They have several advantages and disadvantages. An ideal herbicide is to posses the following characters:
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- Capable of killing weeds without affecting crop plants.
- Not toxic to animals and microorganisms.
- Rapidly trans located with in the target plant.
- Rapidly degraded in the soil
Effects
When herbicides are sprayed on fields, they cannot distinguish from crops and weeds. Scientists have developed GM crops that are resistant crops. Several classes of herbicides are effective for broad spectrum weed control. Act by inactivating vital enzymes (involved in photosynthesis). To face this problem, herbicide resistant plants are generated.
Herbicide resistant plants
Herbicide resistant plants are the plants having the ability to reduce the herbicide-sensitive target in the plant which binds to the herbicide. Genes for resistance against certain herbicides have been introduced into crop plants so they can thrive even when exposed to herbicides.
Herbicide resistant crops
Objectives:
- Modification of plant enzyme target of herbicidal action to render it insensitive to herbicide.
- Overproduction of the unmodified target protein permitting normal metabolism to occur even in herbicide presence.
- Introduction of an enzyme to degrade herbicide prior to its action.
Examples:
Soybean, Maize, Cotton, tomato.
Mechanism
- Introduction of a gene coding for an herbicide detoxifying enzymes.
- Introduction of gene coding for a of a normal functioning enzymes or over expression of the genes coding for a herbicide target enzyme such that the normal metabolic functioning is still achieved in the plant even though some of the enzyme is inhibited.
- Modification of the herbicide target enzyme is such a way that the herbicide molecular does not bind to it and.
- The more recently described engineering of active herbicide efflux from plant cells.
Glyphosate resistance
Glyposate= ”Roundup”, “Tumbleweed”,=systematic herbicide.
Marketed under the name roundup, glyposate inhibits the enzyme EPSPS(5- enolpyruvylshikimate-3 phophate- involved in chloroplast amino acid synthesis), makes aromatic amino acids. Ex: cotton, corn, soya beans.
Glyfosinate resistance
- Glyfosinate (Basta-the active ingredient being phosphinothricin) mimics the structure of the amino acid glutamine, which blocks the enzyme glutamate synthase.
- Plants receive a gene from the bacterium streptomyces (bar gene) that produce a protein that inactivates the herbicide.
Ex: cotton, corn, rice, sugar beets.
Bromoxynil resistance: A gene encoding the enzyme Bromoxynil nitrilase(BXN) is transferred from klebsiella pneumonia bacteria to plants. Nitrilase inactivates the bromoxynil before it kills the plant. Ex: cotton.
Sulfonylurea: Kills plants by blocking an enzyme needed for synthesis of the amino acids like valine, leucine, and isoleucine. Resistance generated by mutating a gene in tobacco plants (acetolactate synthase), and transferring the mutated gene into crop plants. Ex: cotton.
Glyphsate resistance in crop plants: An over expression gene of EPSPS was detected in petunia. Gene from petunia was isolated and introduced in to other plants. The transgenic plants can tolerate glyphosate 2-4 times higher than that required to kill wild type weed plants.
Glyphosate- tolerant crops: Glyphosate herbicide kills plants by blocking the EPSPS (5-enolpyruvylshikimate-3-phosphate synthase) enzyme. EPSPS is an enzyme involved in the biosynthesis of aromatic amino acids such as, phenylalanine, tyrosine and tryptophan via the shikimate pathway in bacteria, fungi and plants. Not found in the mammalian genome.It is the biological target for the herbicide glyphosate. Glyphosate competitively inhibit EPSPS and plants die due to a lack of aromatic amino acids required for their survival. A version of the enzyme that both was resistant to glyphosate and that was still efficient enough to drive adequate plant growth was identified in an Agrobacterium strain called cp4.This version of enzyme, CP4 EPSPS, is the one that has been engineered into several genetically modifies crops.
Advantages:
- Easy to use,
- Full spectrum weed control,
- Environmentally safe,
- Crop safe,
- Control of larger weeds,
- Less cost.
Disadvantages
The increased use of glyphosate on glyphphosate resistant crops could lead to increases in human health problems. Glyphosate formulated herbicides have been linked to numerous health problems including cancer. Short term health effects include lung congestion and increased breathing rates.
REFERENCE
- https://www.isaaa.org/resources/publications/pocketk/10/default.asp
- https://grdc.com.au/resources-and-publications/groundcover/ground-cover-issue-11/herbicide-resistant-crops
- https://enveurope.springeropen.com/articles/10.1186/s12302-016-0100-y
- https://www.isaaa.org/resources/publications/pocketk/10/default.asp
- https://grdc.com.au/resources-and-publications/groundcover/ground-cover-issue-11/herbicide-resistant-crops
- https://grdc.com.au/resources-and-publications/groundcover/ground-cover-issue-11/herbicide-resistant-crops
- https://enveurope.springeropen.com/articles/10.1186/s12302-016-0100-y
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