Herbicide Resistance in Plants

A weed in a general sense a plant that is considered as unwanted plants in farm fields, gardens, parks etc., and generally the term is often used to describe the plants that grow & reproduce aggressively. They are one of the major problems in crop management as they compete with crops for water and nutrients and as a result decrease farming yields and crop production.

Until the emergence of genetically modified crops, selective herbicides ie., herbicides that only kill a specific weed and these herbicides come into contact with a planted crop of different species without harming them. The major drawback in using selective herbicides is each wed requires a different herbicide, which in large volumes is very costly.

One of the most common herbicide is Glyphosate or ((N-(Phosphonomethyl) Glycine) is a broad spectrum herbicide used to kill weeds. Glyphosate kills plants by interfering with the synthesis of amino acids, Phenyl alanine, tyrosine, and tryptophan. It does this by inhibiting the enzyme 5-enol pyruvyl shikimate-3-phosphate synthatase (EPSPS), which catalyzes the reaction of shikimate-3-phosphate & phosphoenol Pyruvate to form 5-enol pyruvyl-shikimate-3-phosphate (ESP). ESP is subsequently dephosphorylated to chorismate an essential precursor in plants for the aromatic acids, i.e., phenylalanine, tyrosine, and tryptophan. These amino acids are used as building blocks in peptides and to produce secondary metabolites such as foliates, ubiquinones and napthoquinones.

Glyphosate has several dissociable hydrogen atoms and it tends to exist as zwitterion where phosphonic hydrogen dissociates ad joins the amine group and is soluble in water to 12g/l at room temperature.

Some micro organisms have a different version of 5-enolpyruvyl-shikimate-3-phosphate synthatase that is resistant to Glyphosate inhibition. The version mostly used in genetically modified groups is isolated from Agrobacterium strain CP4. The CP4 EPSPS gene is engineered for plant expression by fusing the 5’ end of the gene to a chloroplast transit peptide from Petunia EPSPS gene. Transit peptides have an N-terminal presequence which directs them to an organelle such as chloroplast, mitochondrion etc., The transit peptide is required for their transport across the relevant membranes from their transport across relevant membranes from their site of synthesis

This transit peptide is used because it has the ability to deliver bacterial EPSPS to the chloroplasts because chloroplasts synthesize the amino acids Phenyl alanine, Tyrosine & Tryptophan. The naturally occurring EPSPS enzyme from Agrobacterium CP4 can be identified from a screen of micro organism cell extract having very high Glyphosate tolerance kinetic parameters.

After thepre-CP4 EPSPS protein (i.e., the one which contains chloroplast transit peptide amino terminal extension) reaches the chloroplast, and then the transit peptide is cleaved and degraded in the cytoplasm like any other nuclear encoded chloroplast targeted protein. This leaves mature CP4 EPSPS with no chloroplast transit peptide attached.


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