[ashwathi]

The techniques of gene transfer have reflected many successful examples in variety of fields. Some of the examples in the field of agricultural biotechnology are:

Resistance to herbicides: It is important that plants do not get affected on the application of herbicides. In order to install this property of resistance, different methods has been approached through means of gene transfer:
Production of target molecules: The target molecules which are resistant to action of herbidicdes are identified. The gene responsible for such target molecules are incorporated into the plant genome so that the plants produce such target molecules expressing resistance to herbicides. Eg: the gene aroA isolated from Salmonella typhimurium or E.coli is known to produce an enzyme EPSPS which is resistant to the glyphospate herbicide. Transgenic tomato and tobacco plants produced by transfer of this gene were seen to exhibit herbicide resistance.
Detoxification of herbicides: Certain plants produce enzymes which has the ability to detoxify the herbicide action. The introduction of the respective genes can induce herbicide resistance in plants. Main example for this is detoxification of atrazine herbicide by maize plants producing enzyme glutathione-S-transferase.

Resistance to insects:
In the case of developing insect resistant plants, the process involves transfer of insect resistant genes. The major breakthrough in developing resistance to insects was obtained from Bacillus thuringiensis. A gene called as ‘cry genes’ produces a protein called as cry protein which are largely responsible for insect resistance. In the bacterium, these proteins produced exhibit resistance to insects. The cry protein ingested by insects undergoes modification to release toxins in the gut region resulting in the lysis of insects. These genes have been successfully transferred to the genome of plants like tobacco, potato and tomato. Such transgenic plants are found to exhibit resistance to insects like Manduca sexta and Heliothis virescens. Over a period of time, it was found that insects were developing resistance against such proteins. Combating to this, development of transgenic plants expressing alternate form of cry protein but maintaining its toxicity was introduced. The modified cry genes showed greater levels of expression and toxicity.

Virus resistance:
Several strategies have been followed for developing virus resistance in plants. It includes expression of coat proteins, developing cDNA from satellite RNA, degradation of viral genome, antisense RNA approach, and production of viral specific RNAase.
Of these, the technique involving expression of coat protein has met with high level of efficiency. The transgenic plants produced were integrated with genes responsible for producing viral coat proteins. This helped to reduce viral replication thus developing viral resistance.

Bacterial resistance:
In developing resistance against bacteria, different approaches targeting the growth and development of bacteria or bringing about degradation of toxins released have been applied. Of this the most successful phenomenon was that of artificial cell death. In response to certain stimuli the cell of certain organisms produces proteins which bring about the death of cells. Such mechanism is known as programmed cell death. In transgenic plants this capability of cell death was modified so that the infected cells gets targeted and trigger a response of artificial cell death leading to the death of infectious agent. This phenomenon is found to be active against fungi infection also.

Drought resistance:
Several genes has been identified which show increased response to abiotic stress like abscisic acid, osmotic stress etc. Some of the genes involved are:
Rab which respond to abscisic acid; SalT responsive against salt stress and proBA and proC involved in proline biosynthesis.
The genes responsible for proline biosynthesis when isolated and expressed in transgenic plants resulted in high expression and consequent development of resistance to osmotic pressure was indicated. This resistance was possible due to the deposition of proline in the cells which helps in regulating water activity of the cell. Certain other genes like mtl1D from E.coli have been seen to express mannitol accumulation in cells which aid in plant growth under non favourable conditions.

Improving seeds quality:
Certain seeds produced by native plants have been found to be deficient in some important nutrient lowering the seed quality. With the help of genetic engineering, the seed quality can be increased so as to produce economically important seeds. The basis of this is, introduction of genes into the genome corresponding to the production of missing nutrient or modification of endogenous genes of plants to produce seeds of high quality. Eg:- In peas, the seed produced has lesser sulphur containing amino acids. Such amino acids are rich in sunflower seeds. The integration of gene encoding for such amino acids in peas seeds and expression in transgenic tobacco, led to the production of seeds with the respective amino acids in adequate amounts.