TRANSGENICS IN CROP IMPROVEMENT

Plants containing introduced DNA are known as transgenic plants or genetically engineered plants. They acquire a new trait from introduced DNA.

The acquired trait is heritable. This principle is called “gain of function”.

Some introduced DNA blocks the expression of certain original genes of the plants. This is called “cassey function”.

The development of transgenic plants is the result of integrated application of r – DNA technology, gene transfer methods & tissue culture techniques.

Transgenic plants in food, fibre, vegetable & fruit crops have led to improved crop yields.

Genes are transferred from entirely irrelevant organisms (notably bacteria & viruses) to crops producing new useful traits in crops.

Genetic transfer facilitates introduction of only desirable genes from donor without cotransfer of any undesirable genes from donor species, which normally occurs by conventional breeding techniques.

The list of plant species that can be transformed by vector mediated (Agrobacterium) and vector – less methods has been growing continuously.

At present transfer capability has been extended to more than 120 species in atleast 35 families.

Monocot species were earlier considered outside the host range of Agrobacterium tumifaciens, which led to the development of direct DNA introduction or vector–less methods for transformation.

Thus most success in development of transgenic plants in monocots have been due to vectorless systems.

However, recently Agrobacterium mediated transfer has been reported in monocot species including the most important food crops like rice (1994), maize (1996) & wheat (1997).

RESISTANCE TO BIOTIC STRESSES.

Genetic transformation has led to the possibility of transforming crops for enhanced resistance to insects and diseasescaused by fungal & bacterial pathogens & viruses. Herbicide resistance in crop plants has led to effective use of herbicides to control weeds.

HERBICIDE RESISTANT PLANTS :

Plants that can tolerate herbicides are called herbicide resistant plants.

Herbicide resistance of crop varieties help us in the effective use of herbicides to control weeds.

So transgenic crop plants with herbicide resistance have been developed by adopting genetic engineering.

GLYPHOSATE RESISTANT PLANTS :

Glyphosate is the active ingradient of many broad spectrum herbicides such as :

• a) Glyphos
• b) Round up
• c) Tumble weed

It enters the plant cells and inhibits the enzyme 5 enol pyruvyl shikimate 5 phosphate synthetase [EPSP Synthetase]

This enzyme is present in chloroplast.

Glyphosate is the competitive inhibitor for EPSP Synthetase.

It inhibits the synthesis of aromatic amino acids from phospho enol pyruvate. Therefore the plants die when glyphosate is added.

It blocks the biosynthesis of tryptophan & phenylalanine. It does not effect man and animals.

The cell line of petunia resistant to glyphosate over produced the EPSP Synthetase.

The mRNA of EPSP Synthesizing gene was isolated from the cell line & cDNA was made from that mRNA, CaMV (cauliflower mosaic virus) promoter was linked to the 5 end of cDNA & cDNA in construct was introduced into Agrobacterium through disarmed Ti Plasmid.

The Agrobacterium were allowed to inject leaf discs of petunia sensitive to glyphosate. The transgenic petunias are resistant to glyphosate by over producing the enzyme EPSP Synthetase.

The EPSP Synthesizing genes of petunia are also transferred to tomato, tobacco, maize, cotton etc.

The soil bacterium Salmonella typhimurium & E.coli are resistant to glyphosate. They produce glyphosate tolerant EPSP Synthetase.

They produce EPSP Synthetase which is normal in activity but poor in affinity to glyphosate.

This enzyme is coded by a gene called “aro A gene”.

The aro A gene transferred to many plants through disarmed Ti plasmids for producing glyphosate resistance in them.

Glyphosate tolerant petunia, potato, tobacco, tomato, cotton etc have been made by this method.

SULPHONYL UREA RESISTANT PLANTS :

Sulphonyl urea tolerant plants have been developed by cDNA technology.

Sulphonyl urea inhibit acetolactate synthetase (ALS) which synthesizes branched chain aminoacids.

Arabidopsis ALS Gene produces ALS which does the usual function but has poor affinity to sulphonyl urea.

The ALS Gene of Arabidopsis is transferred to tobacco through disarmed Ti Plasmid. The transgenic tobacco is 100 times more resistant to sulphonyl urea.

ATRAZENE :

It inhibits QB Protein, of photosystem II in chloroplast. Hence in Atrazene sensitive plants photosynthesis declines & finally the plants die.

A mutant amaranthus hybridus has Atrazene tolerant QB protein & hence the plant show Atrazene resistance.

The gene for QB protein in Amaranthus hybridus was transferred to tobacco & some other plants. The transgenic plants are resistant to Atrazene.

The gene in Maize for glutathione – s – transferase can detoxifythe atrazine & thereby tolerate the Atrazene.

PHOSPHINOTHRICIN (PPT):

PPT is the active ingredient of the commercial herbicide “Basta”.

The enzyme phosphinothricin acetyl transferase in streptomyces hygroscopicus can detoxify the PPT, it is encoded by “bar” gene.

The bar gene of streptomyces hygroscopicus was transferred to tobacco, wheat, maize, soyabeen etc.

The transgenic plants were resistant to PPT.

BROMOXYNIL TOLERANCE :

Here a gene called bxn enables Klebsiella pneumoniae to resist bromoxynil.

The bxn gene codes for the enzyme Nitrilase that can detoxify the Bromoxinyl residues.

The bxn gene was transferred to tomato along with CaMV promoter. The transgenic tomato was resistant to bromoxynil.