Putting genes into plants

TRADITIONAL BREEDING leads to gene flow from the donor plant into the recipient and results in transfer of more than one gene.

Pollen is moved from one plant to another and in the process a lot of undesired traits also get transferred. For e.g., a breeder may want to introduce drought tolerance into one plant from another.

Using cross-pollination he could get a plant that is tolerant to drought but also develops large leaves. This is not desirable as it would lead to loss of water from a larger surface area. Since genes have a tendency to get transferred in groups, the gene for drought tolerance may be close to that for large leaves and both genes may be transferred together.

In contrast, the new molecular biology techniques permit identification and transfer of specific traits in a precise and verifiable manner.

Due to its specificity it is also possible to transfer more than one trait at a time with this technique.

To create a biotech plant, four basic steps have to be followed to ensure you get a plant with only the trait that is desired.Preparing the DNA

The desired gene from the DNA of the source (plant or bacteria) is cut using specific enzymes called restriction enzymes. This gene is `glued' to a plasmid. Plasmids are DNA fragments occurring outside the nucleus of bacteria but are very much a part of bacteria and occur naturally. Now, the plasmid contains the transferred gene as well as a gene that is resistant to a particular antibiotic which is a flag or marker to identify the cells that have taken up the new gene.

This plasmid containing the gene of interest is then put into a host bacterium. The bacteria carrying this modified plasmid are allowed to multiply. After the mixing process, only some plasmids and bacteria will contain the gene that was introduced. This is where the marker gene helps in identification of the presence of desired gene.

Since the bacteria are grown in the presence of an antibiotic, only those bacteria containing the useful gene will grow as they also contain the gene for resistance to an antibiotic. The bacteria without the desired gene do not survive.

This antibiotic usually has no medicinal role - it removes any remote risk of resistance to medicinal antibiotics being transferred to disease-causing bacteria. Inserting modified plasmid into plant. The modified plasmid can be inserted into plants by one of the following methods:

- using Agrobacterium, a common soil bacterium that can enter plant cells and transfers the desired gene on the plasmid into the plant along with its DNA.

- using a gene gun - the DNA is fired directly into plant tissue, carried on microscopic high velocity pellets of gold or tungsten. Using the marker gene, it is easy to identify which cells have taken up the desired gene.

The untransformed cells are discarded and a whole plant is grown from a single cell. As cells in that plant will contain the required gene. This is known as tissue culture.

Evaluating the result

The plant containing the new gene has to be tested for its validity because the position of insertion of the desired gene affects its function in the modified plant.

This is done by checking for the expression of the gene, which is controlled by gene switches called `promoters.'

These switches regulate gene expression and are operated by specific stimuli. Promoters are why coloured petals show their colour, but the leaves stay green - and yet each set of cells in the petals and the leaves have identical DNA. Thus a plant with the desired gene is relatively quickly developed using biotechnology.

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Section  : Science & Tech
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