Techniques for Genetically Modifying Plants
The following explains some of the techniques used to genetically modify plants. For a more basic introduction to DNA, genes and GM you can look at wikipedia.
Genetic modification involves the introduction into an organism of new genes, usually from an unrelated organism, using artificial laboratory techniques. The intention is to either (most commonly) introduce a new trait into the recipient organism, such as tolerance to a chemical herbicide or resistance to insect attack, or to modify the activity of the organism's own genes, to slow natural decay and extend the life of flowers, for example. A package of DNA is usually transferred in the process which includes:
- The 'gene of interest' (GOI) which gives the desired trait. The gene may come from any other organism but in commercial crops, bacteria are the commonest source of these genes.
- Genes to switch the GOI on and off. These promoter and terminator genes, may also come from a range of organisms but viral, bacterial and plant promoters are the most commonly used in commercialised GM crops.
- A marker gene which helps scientists identify when a plant cell has been successfully genetically modified or 'transformed'. This is necessary because the GM technique is inefficient, with only 1-2% of cells are successfully transformed. To identify these cells and use them to grow into plants, at the end of the modification process the cells are cultured in a medium where the presence of the marker genes means they can survive or produce a colour change, while other non-transformed cell will die or not have the colour mark. Marker genes include antibiotic resistance, herbicide tolerance and fluorescence.
- There may also be remnants of the vector DNA (see below) also inserted.
There are two main methods that are used to genetically modify plants:
- Using the bacterium, Agrobacterium tumifaciens, as a vector for the DNA. Agrobacterium has the ability to infect plants and insert DNA into a plant's genome. It causes crown gall tumours in natural infections. This method has mainly been used to modify broad leaved plants, such as sugar beet and oilseed rape, but is now also being applied to monocot species, such as maize and rice.
- Particle bombardment or biolistics where the DNA to be inserted is coated on minute gold particles and fired into plants cells. This approach is used for monocot plants such as maize and rice.
Both GM techniques are not sufficiently accurate or precise enough to allow the introduction of a gene or gene(s) at one particular site in the genome. Rather, there may be multiple copies inserted, genes may be in the forward or reverse orientation and there may be fragments of genes from the vector also transferred. Deletion, rearrangement and replication of the plant's own genes is also seen. These transformation-induced mutations may occur at the site of insertion or be genome wide. Such effects occur regardless of the source of the gene.
The implications of transformation-induced mutations include:
- the possible disruption of endogenous (i.e. the plant's own) genes and their abnormal functioning - this could affect biochemical pathways and lead to the production of unexpected toxins or anti-nutrients;
- increases or decreases of the activity of endogenous genes through the introduction or disruption of control genes - this could increase or decrease the levels of naturally occurring toxins or allergenic proteins;
- silencing of genes in subsequent generations if multiple copies exist as a result of transgene silencing.