Herbicide Resistance in crops Introduction Herbicide tolerance is

Herbicide Resistance in crops

Introduction • Herbicide tolerance is the predominant GM trait. • It was one of the first GM trait to be tested in the Field, and subsequently for commercial production. • Most frequent transgenic traits in commercial plantings are as follows. ü Herbicide tolerance ü Insect tolerance ü Herbicide tolerance + Insect resistance

WHAT ARE WEEDS? • All the plants in the cultivated field giving harm rather than benefit. • Different ways to fight weeds: • Mechanical: Hoeing, Hand Plucking • Physical: heat and Light • Biological : Use of living organism • Chemical: Use of herbicides

The use of herbicides in modern agriculture • Herbicides are used to control Unwanted plants. • About 60% of pesticides used in U. S. are herbicides. • Weeds Compete with crops for soil nutrients. • Thus weeds are one of the three classes of biotic stress that have a major impact on the proportion of world crop yield available for human consumption. • Modern agriculture has developed range of effective Herbicides to tackle the effect of weed on crop yield.

What is herbicide resistance in plants? • The herbicide resistance is the ability, trait or quality of a population of plants within a species of plant cells in culture, to withstand a particular herbicide at a dosage that is substantially greater than the wild type of that plant is able to withstand, with a near normal life cycle.

Problems in application of herbicides • Lack of tolerance to the chemical by one or more of the famous world crops • Use of multiple types of herbicides to broaden the spectrum of the affected weeds, which in turn increase the possibility that the crop is injured also. • Lack of high toxicity to weeds while crops are not affected

Broad-spectrum herbicides • Active against wide range of weeds • Only use when crop is not itself vulnerable to herbicide. • The development of herbicide tolerant crops offers the opportunity to spray the crops at most effective time to kill the weed species without damaging the crops. • Some of the crop plants are naturally resistant to certain herbicides, and that tolerant strains may appear through the processes of mutation and natural selection.

First herbicide • the first major breakthrough was the result of research conducted in both the UK and the US during the Second World War into the potential use of herbicides in war. • The first modern herbicide, 2, 4 -D, was first discovered and synthesized by W. G. Templeman at Imperial Chemical Industries. • In 1940, he showed that "Growth substances applied appropriately would kill certain broad-leaved weeds in cereals without harming the crops”.

What types of compounds are herbicides ? Properties of herbicides • Herbicides are much more toxic to the plants than to the animals. • Inhibitors of plant specific processes. • Plants are autotrophic, they synthesize all their macromolecular components denovo. Ø Example: The compounds that are essential in human Diet (vitamins, amino acids etc ) are synthesized by Plants, and these biosynthetic pathways are therefore possible targets for herbicides. These pathways are located in Chloroplast.

• Herbicidal activity of herbicides results from the specific inhibition of a single enzyme/protein. • Belong to a wide range of different chemical families. 15 broad classes of mode of activity. • Most herbicides have one mode of action. • Certain enzymes are vulnerable to herbicidal activity. ØExample: 5 chemical families of herbicides target the Enzyme acetolactate synthase (ALS). ALS catalyze first reaction in biosynthetic pathway of branched chain Amino acids.

• The widely varied chemical properties of herbicides means that they also differ in their toxicity, Environmental persistence and biodegradability. • Herbicides are also classified according to their 1) Site of uptake into plant 2) Degree of translocation into plant 3) Time of application (preplanting or preharvesting).

Case study • Glyphosate (N-(phosphonomethyl)glycine) is a broad-spectrum systemic herbicide used to kill weeds, especially annual broadleaf weeds and grasses known to compete with commercial crops grown around the globe. • It was discovered to be a herbicide by Monsanto chemist in 1970. Monsanto brought it to market in the 1970 s under the trade name "Roundup”.

• Glyphosate's mode of action is to inhibit an enzyme involved in the synthesis of the aromatic amino acids tyrosine, tryptophan and phenylalanine. • It is absorbed through foliage and translocated to growing points. Because of this mode of action, it is only effective on actively growing plants; it is not effective as a pre-emergence herbicide.

• Glyphosate kills plants by interfering with the synthesis of the aromatic amino acids phenylanine, tyrosine and tryptophan. • • It does this by inhibiting the enzyme 5 enolpyruvylshikimate-3 -phosphate synthase (EPSPS), which catalyzes the reaction of shikimate-3 -phosphate (S 3 P) and phosphoenolpyruvate to form 5 -enolpyruvylshikimate-3 -phosphate (ESP).

• ESP is subsequently dephosphorylated to chorismate, an essential precursor for the amino acids mentioned before. • These amino acids are used in protein synthesis and to produce secondary metabolites such as folates, ubiquinones and naphthoquinone. • The commercially important enzyme that glyphosate inhibits, EPSPS, is found only in plants and micro-organisms. • EPSPS is not present in animals, which instead obtain aromatic amino acids from their diet. • However, glyphosate has also been shown to inhibit other plant enzymes, and also has been found to affect animal enzyme.

Mode Of Action Of Glyphosate § Glyphosate specifically binds and blocks the activity of EPSPS (enolpyruvateshikimate-3 -phosphate synthase). § EPSPS is an enzyme of the aromatic amino acid biosynthetic pathway. § Glyphosate inhibition of EPSPS prevents the plant from synthesizing the aromatic amino acids essential for protein synthesis. 17

• Some crops have been genetically engineered to be resistant to glyphosate (i. e. , "Roundup Ready", also created by Monsanto Company). • Such crops allow farmers to use glyphosate as a post-emergence herbicide against both broadleaf and cereal weeds, • but the development of similar resistance in some weed species is emerging as a costly problem. So was the first "Roundup Ready" crop.

Environmental impact of herbicide tolerant crops • The adoption of herbicide tolerant crops could have positive or negative environmental consequences. • A number of different single gene strategies using accessible genes are possible trait itself can be used as selectable marker. • In USA proportion of herbicide tolerant soya bean from 17% in 1997 to 68% in 2001. • Herbicide tolerant cotton expanded from 10% in 1997 to 56% in 2001.

THANKS