Marker gene

"marks" transformed cells

During genetic transformation, the new genes are inserted in only a fraction of the plant cells. Marker genes can be used to identify the tiny number of cells which have successfully acquired the new gene. The marker gene is transferred alongside the gene of interest (the target or beneficial gene).

The most commonly used marker genes at present are antibiotic or herbicide resistance genes. They all work by making the modified cells detoxify substances which would otherwise be fatal to them. For example, a herbicide resistance gene confers tolerance of the herbicide. If the cells come into contact with the corresponding substance following transformation (in the culture medium, for example), only those plants which have acquired the marker gene, and therefore the target gene as well, will survive. Transgenic plants are then grown from these cells.

Metabolic markers are a potential alternative to the controversial antibiotic resistance genes. They enable the plants to grow on unfamiliar culture media or to produce metabolic products which allow only the transgenic cells to grow. However, this method is still in the early stages of development.

The marker genes are generally no longer needed once the transgenic cells have been successfully identified. For this reason a series of research projects are looking at ways of achieving gene transfer without marker genes, or the subsequent removal of the marker genes.